CA2206212A1 - Phase shifting diffraction interferometer - Google Patents
Phase shifting diffraction interferometerInfo
- Publication number
- CA2206212A1 CA2206212A1 CA002206212A CA2206212A CA2206212A1 CA 2206212 A1 CA2206212 A1 CA 2206212A1 CA 002206212 A CA002206212 A CA 002206212A CA 2206212 A CA2206212 A CA 2206212A CA 2206212 A1 CA2206212 A1 CA 2206212A1
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- Prior art keywords
- light
- polarized
- coherent
- retroreflector
- component
- Prior art date
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Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B11/00—Measuring arrangements characterised by the use of optical techniques
- G01B11/24—Measuring arrangements characterised by the use of optical techniques for measuring contours or curvatures
- G01B11/255—Measuring arrangements characterised by the use of optical techniques for measuring contours or curvatures for measuring radius of curvature
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B11/00—Measuring arrangements characterised by the use of optical techniques
- G01B11/24—Measuring arrangements characterised by the use of optical techniques for measuring contours or curvatures
- G01B11/2441—Measuring arrangements characterised by the use of optical techniques for measuring contours or curvatures using interferometry
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B9/00—Measuring instruments characterised by the use of optical techniques
- G01B9/02—Interferometers
- G01B9/02001—Interferometers characterised by controlling or generating intrinsic radiation properties
- G01B9/02012—Interferometers characterised by controlling or generating intrinsic radiation properties using temporal intensity variation
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B9/00—Measuring instruments characterised by the use of optical techniques
- G01B9/02—Interferometers
- G01B9/02034—Interferometers characterised by particularly shaped beams or wavefronts
- G01B9/02038—Shaping the wavefront, e.g. generating a spherical wavefront
- G01B9/02039—Shaping the wavefront, e.g. generating a spherical wavefront by matching the wavefront with a particular object surface shape
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J9/00—Measuring optical phase difference; Determining degree of coherence; Measuring optical wavelength
- G01J9/02—Measuring optical phase difference; Determining degree of coherence; Measuring optical wavelength by interferometric methods
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B2290/00—Aspects of interferometers not specifically covered by any group under G01B9/02
- G01B2290/30—Grating as beam-splitter
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B2290/00—Aspects of interferometers not specifically covered by any group under G01B9/02
- G01B2290/70—Using polarization in the interferometer
Abstract
An interferometer system in which a linearly polarized beam (10) from a light source (12) passes through a density filter (14). The beam (10) then passes through a half-wave retardation plate (16) which produces two orthogonally polarized beams. The angular orientation of the half-wave retardation plate (16) is used to adjust the relative intensity between the vertical and horizontal components of polarization. The polarized beams are then split by a polarization beamsplitter (22). The transmitted beam (20) is then reflected by a fixed retroreflector (24) back through the polarization beamsplitter (22) to a turning mirror (26). The reflected beam (18) is reflected, by a retroreflector (28), mounted on a piezoelectric translator (30) back through the beamsplitter (22) to the same mirror so that it is parallel to the other beam but laterally displaced from it. It diverges and is coincident with the reference beam (48) which was diffracted by the aperture (42). The measurement beam (46) and the reference beam (48) interfere to produce a fringe pattern that represents a contour map of optical path difference between the wavefront from the optic (50) and a perfect spherical wavefront.
Description
. ~ CA 02206212 1997-0~-27 W.096/~7221 - PCTrUS95/15274 PHASE SHIFTING DIFFRACTION INTERFEROMETER
The United States Government has rights in this invention pursuant to Contract No. W-7405-ENG48 between the United States Department of Energy and the University of California for the operation of Lawrence Livermore National Laboratory.
BACKGROUND OF THE INVENTION
Field of the Invention The present invention relates to interferometry, and more specifically, it relates to high accuracy diffractioh interferometry.
Description of Related Art Interferometry is the preferred method to measure the performance of optical elements and systems. In this method the wavefront of light reflected from or transmitted by the optic to be tested is interfered with the wavefront from a reference surface, to produce an inlelrerence fringe pattern. These inlelrerence fringes are then analyzed to ascertain the performance of the optic. For high performance imaging systems, such as those found in lithographic steppers used to make integrated circuits, this interrerollletric measurement must be made to ever increasing accuracy. The accuracy, however, is limited by how well the reference surface is charac~ri7e~l- Reference surfaces are typically no better than A /50, where A, is the wavelength of visible light, and thus are the limiting factor in fabricating higher performance optical syslell s.
Ther~rore the fabrication of high accuracy optical systems, such as those needed for e,ctrell-e ultraviolet projection lithography which require an accuracy of A /1000, are impossible to qualify with confidence using existing intelferolnetry.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a phase shifting diffraction inlerferometer having A /1000 accuracy.
An interferometer is disclosed which has the capability of measuring optical elements and systems with an accuracy of ~ /1000 -CA 02206212 1997-0~-27 W O9~117221 PCTrUS95/15274 where ~ is the wavelength of visible light. Whereas current inlerrero-l-eters employ a reference surface, which inherently limits the accuracy of the measurement to about ~ /50, this interferometer uses an essentially perfect spherical reference wavefront generated by the funcl~mental process of diffraction. This interferometer is adjustable to give unity fringe visibility, which maximizes the signal-to-noise, and has the means to introduce a controlled prescribed relative phase shift between the reference wavefront and the wavefront from the optics under test, which permits analysis of the inlelrerence fringe pattern using standard phase extraction algorithms.
The interferometer described in this disclosure has the ability to achieve this extremely high accuracy by eliminating the reference surface and substituting an essentially perfect spherical reference wavefront generated by the fundamental process of diffraction.
Whereas current inlelrerometers illuminate the optic to be tested with an aberrated wavefront which also limits the accuracy of the measurement, this interferometer uses an essentially perfect spherical measurement wavefront generated by the fundamental process of diffraction. The invention maximizes the signal-to-noise and permits analysis of the inlelrerellce fringe pattern using standard phase extraction algorithms.
BRIEF DF.~CRIPTION OF THE DRAWINGS
Pigure 1 shows an embodiment of the phase shifting diffraction inle,rerometer.
Figure 2A shows an embodiment of the intel~rometer plate shown in figure 1.
Figure 2B shows an embodiment of the intelreroll.eter plate shown in figure 1.
Figure 3 shows an embodiment of the interrerollleter plate shown in figure 1.
Figure 4 shows an embodiment of the phase shifting diffraction interferometer.
Figure 5 shows a magnified view of the fiber end shown in figure 4.
Figure 6 shows the elongated end of the fiber shown in figure 4.
: ~ CA 02206212 1997-0~-27 Figure 7 shows an embodiment of the phase shifting diffraction interferometer utilizing coherence length.
DETAILED DESCRIPIION OF THE rNVENTION
- The interferometer system is shown figure 1 in an embodiment that most easily illustrates the essence of the idea, but is 5 not necess~rily the preferred embodiment or the only embodiment. It consists of four main parts: the beam conditioning optics; the inlelrerometer; the detection system; and the computer ~yslem.
The major components of the beam conditioning optics are the light source, intensity and contrast controls, and the phase shifting 10 module. Referring to figure 1, a linearly polarized collimated beam 10 from a light source 12 (e.g., a laser) passes through a variable neutral density filter 14 which is used to control the light level to the intel~rometer. The beam 10 then passes through a half-wave retardation plate 16 which produces two orthogonally pol~ri7e-1 beams, 15 beam 18 having vertical (out of plane) pol~ri7~tion and beam 20 having horizontal (in plane) polarization, both beams within beam 10. The angular orientation of the half-wave retardation plate 16 is used to adjust the relative intensity between the vertical and horizontal components of pol~ri7Ation. Beams 18 and 20 having these two 20 pol~ri7~tion components are then split by a polarization beamsplitter 22 so that the hori_ontal polarization (beam 20) is transmitted while the vertical pol~ri7~tion (beam 18) is reflected. The transmitted beam 20 is then reflected by a fixed relroreflector 24 back through the pol~ri7~tion beamsplitter 22 to a turning mirror 26. The reflected beam 18 is reflected 25 by a relrorellector 28, mounted on a piezoelectric translator (PZT) 30, back ~rough the polarization beamsplitter 22 to the same turning mirror 26 so that is it parallel to the other beam but laterally displaced from it. Applying a voltage to the PZT 30 translates the relrore~ector 28 thus shifting the relative phase between the two orthogonally polarized 30 beams. The two beams pass through a pol~ri7er 32 (axis at 45~) to give them the same polAri7~tion and are brought to focus, with a microscope objective 34, on the inlelrerollleter plate 36.
Referring to figure 2A, the inter~rometer plate 36 comprises a glass substrate 38 coated with a highly reflecting, low 35 tran~mi~sion, metallic film 40, through which a circular aperture 42 has been etched. The diameter of the aperture 42 is on the order of the CA 02206212 1997-0~-27 - ~096/17221 PCTrUS95/15274 wavelength of the source 12. Metallic film 40 may typically comprise aluminum having a thickness of 65 nanometers. In the embodiment shown in figure 2B, the il~lelrerometer plate 36 comprises a glass substrate 38 coated with a highly reflecting, low transmission, metallic 5 film 40, through which a circular aperture 42 has been etched, and over which a second partially transmitting metallic film 44, without an aperture, has been coated. The diameter of the aperture 42 is on the order of the wavelength of the source 12. Metallic film 40 may typically comprises aluminum having a thickness of 65 nanometers and metallic 10 film 44 may typically comprises aluminum having a thickness of 26 nanometers. In both embodiments of the interferometer plate 36, the metallic film 40 and the second metallic film 44 may comprise materials other than aluminum, for example, chromium, gold, nickel, silicon and silver. The thicknesses of the film would vary depending on the 15 m~t~ri~l used. This plate serves to generate the point source measurement beam, using one of the beams focused on the plate, to illuminate the optic under test. It simultaneously serves to generate the point source refere..ce beam, using the other focused beam. Because the measurement beam and the reference beam are generated by diffraction, 20 they both comprise a perfect spherical wavefront over some finite solid angle.
Beams 18 and 20 are focused on the inlelrerometer plate 36 so that they are both incident on the aperture 42 in the metallic film 40.
Both beams are diffracted by the aperture 42, producing two spherical 25 wavefronts that diverge as they leave the aperture. Each wavefront is perfectly spherical over a finite angular range (defined by the diameter of the aperture relative to the wavelength) centered about the illumination direction defined by the lateral separation of the beams as they entered the microscope objective. As shown in figure 1, measurement beam 46 30 illllmin~stes the optic S0 under test. This optic typically comprises a concave mirror and is positioned to focus the measurement beam 46 back onto the inlelrerolneter plate 36. Due to the finite extent of the optic 50 and aberrations therein, the focused measurement beam 46 is much larger than the aperture 42 in the metallic film 40, so nearly all of 35 the focused measurement beam 46 is reflected by the film 40 in the embodiment of the interferometer plate 36 shown in figure 2A. In embodiment of the interferometer plate 36 shown in figure 2B, focused ; CA 02206212 1997-0~-27 W O 96/17221 PCTrUS95/15274 measurement beam 46 is reflected by the film 44. It diverges and is coincident with the reference beam 48 which was diffracted by the aperture 42. The measurement beam 46 and the reference beam 48 inlerrere to produce a fringe pattern that represents a contour map of 5 optical path difference between the wavefront from the optic 50 and a perfect spherical wavefront.
The detection system consists of an imaging system to image the optic under test onto a CCD array camera. The imaging system comprises a lens 52, an aperture 54 for spatially filtering the 10 il~lel~ing beams, and a lens 56. The aperture 54 is large enough so that it does not diffract the beam focused through it. A typical size is 250 micrometers. The size of the lenses 52, 56, depends on the size of the optic under test.
The coincident measurement beam 46 and reference beam 15 48, diverging from the inlerrerometer plate, are collected by a spatial filter imaging system which images the surface of the optic 50 under test onto a screen (not shown) or onto a charge coupled device (CCD) camera 58. This guarantees that the phase of the illlel~e~ g wavefronls at each pixel in the CCD camera 58 has a one-to-one colres~ondence with a 20 unique point on the optic 50. It also minimizes effects of edge diffraction from the optic. Aperture 54, at the intermediate focus of the i. leire,ing beams, is used to filter out any light not coming from the immediate area around the aperture in the inlerrerol,leter plate 36. The CCD
camera 58 captures a series of inlelrerence patterns and transfers them-to 25 the computer ~yslem 60.
The computer system consists of a computer having a monitor and software to control the light level and contrast of the inle~rere~ce pattern, software to translate the PZT and thus shift the relative phase between the measurement and reference beams, software 30 to c~lc~ te the phase at each pixel using the transferred inl~lrerence patterns, and software to display the resultant phase map. This software for analyzing the inlelrerellce pattern read into the computer can presently be supplied by several companies. Zygo Corporation produces "Metro Pro" software. Phase Shift Technology produces "Optic Code 35 Analysis Software". WYKO Corporation produces "WISP" software.
The series of inle,rere-.ce patterns that are transferred to the computer are captured as the PZT shifts the relative phase of the , , CA 02206212 1997-0~-27 W O 96tl7221 PCT~US95/15274 interfering beams by 2 7~ radians. The interference patterns are analyzed to determine the proper settings for the light level and contrast of the inlelrerence fringes. This information is used by the computer to adjust the position of the neutral density filter 14 and half-wave retardation 5 plate 16. A second series of inlelrerel~ce patterns are then captured and analyzed to determine the phase at each pixel. This is typically displayed as a contour or 3D plot of the phase. This phase map cGlres~onds to the deviation of the surface of the optic from a perfect sphere.
This inlelreroll,eter is unique in that:
10 a. The measurement beam is generated by diffraction and is a perfect spherical wave over some finite solid angle. The solid angle is defined by the size of the aperture in the interferometer plate relative to the wavelength of light from the source. Smaller apertures produce larger solid angles.
15 b. The reference beam is generated by diffraction and is a perfect spherical wave over the same solid angle as the measurement beam .
c No reference surface is required for this inle~rerometer.
Reference surfaces are a major source of error in inlelrerometry and ultimately limit the accuracy that can be achieved.
20 d. The measurement beam reflected from the optic under test is imaged back onto the aperture of the interferometer plate, giving exact coincidence with the reference beam. This is the ideal condition for achieving the highest degree of accuracy.
e. The relative phase between the measurement and reference 25 beams can be shifted in a controlled way. This permits a series of inlelrerence patterns to be analyzed to dçtermine the phase at each pixel position with the highest degree of accuracy.
f. The relative intensities of the measurement and reference beams can be adjusted (with the half-wave retardation plate) to give maximum 30 possible contrast. This produces the greatest signal-t~noise, necessary for achieving the highest degree of accuracy.
Referring to figure 3, inle~relometer plate 37 may be useful in certain circumstances depending on the optic under test. Here beams 18 and 20 each pass through a prism 17, 19 respectively, and are focused 35 by lens 34 onto two distinct apertures: aperture 41 and aperture 43 in metallic film 45 of the interferometer plate 37. A tilt is introduced between beam 20 and beam 18 by the prisms. The usable metallic films CA 02206212 1997-0~-27 W O 96~17221 PCT~US95tlS274 are the same as described for interferometer plate 36. Apertures 41 and 43 are typically separated from each other by 10 to 500 micrometers. The typical angle between the two focused beams is in the range of 10 to 60 degrees. In the embodiment shown on figure 3, beam 18 passes through 5 prism 17 and is focused by lens 34 onto aperture 43 in intelrerometer plate 37. The resulting diffracted measurement beam 47 is then reflected from test optic 51 to the reflective area surrounding aperture 41 of inlel~rolneter plate 37. This beam is reflected along the same path as reference beam 49 to produce an inle~ference pattern therebetween. This - 10 embodiment is used to test optical surfaces that are concave with respect to the inlerrerometer plate 37.
Another distinctive form of this interferometer system, based on fiber optic technology, operates on the principles described supra. Referring to figure 4, light source 12 is focused by lens 62 onto the 15 end of a single mode fiber 64. A variable fiber coupler 66 splits the beam into two fibers: fiber 68 and fiber 72. The ratio of light split into each fiber is chosen for maximum fringe contrast. Fiber 68 is used for the measurement beam 70. Fiber 72, transmitting rererence beam 74, is wrapped around a PZT cylinder 76 that expands with an applied voltage.
20 This stresses the fiber 72, changing its refractive index, and introduces a phase shift relative to the measurement beam 70 from fiber 68. The measurement beam 70 leaving the end of the fiber is diffracted, producing a perfect spherical wavefront over some finite solid angle.
The solid angle is defined by the size of the fiber core relative to the 25 wavelength of light from the source 12. This spherical measurement beam 70 illuminates the optic 78 under test, which focuses the beam onto the end of the fiber for the rerelence beam. Figure 5 shows a magnified view of the exit portion of fiber 72. Measurement beam 70 is focused onto reflective metallic film 73 on the end of fiber 72, and is 30 reflected along the same path as rererence beam 74. The rest of the system is as in figure 1. This embodiment is used to test positive lenses.
Referring to figure 6, it may be necessary to shrink the core diameter of fibers 68 and 72 to match the aperture size for the intelferometer. This is accomplished by heating and pulling the fiber.
35 For example, fiber 72, having cladding 84 and core 80 is heated and pulled to produced a stretched portion 82 with a core 81 having a diameter that is smaller than the unstretched core portion 80. Metallic ~ - ~
. CA 02206212 1997-0~-27 W O9~/17221- PCTrUS95/15274 film 73 is coated after heating and stretching and can comprise any of the films described supra. One embodiment of the metallic film comprises aluminum having a thickness of 26 nanometers. Single mode fibers typically have core diameters of about 4 micrometers. The measurement beam is reflected from the end of the fiber while the reference beam is transmitted through the film and diffracted, giving a perfect spherical wavefront over the same finite solid angle as the measurement wavefront. The solid angle is defined by the size of the fiber core relative to the wavelength of light from the source 12. The imaging, computer systems, data acquisition and analysis are the same as described above. This fiber optic approach has all the advantages of the system described above as well as the flexibility to independently move the measurement and reference fibers to any positions to suit the inlel~erollletric measurement configuration. If light source 12 comprises a short coherence length, the fiber lengths 68 and 72 must be adjusted so that the optical path lengths from variable fiber coupler 66 to the end of fiber 72 for both the measurement beam 70 and reference beam 74 are equal.
Another embodiment of the invention is shown in figure 7. In this embodiment light source 12 has a short coherence length and beams 18 and 20 are reflected back through the polarization beamsplitter 22 so they are coincident and collinear. Retroreflector 24 is positioned such that optical path length ACD is equal to optical path length ABDED.
In other word, re~orenector 24 is moved to a position where the round-trip beam path difference between the two relrorenectors is equal to the round-trip path between the inlelrerc.-,eter plate and the optic under test. The optical path lengths of the inler[el;ng measurement and rererellce beams are then the same. As in figure 1, PZT 30 can be attached to either relroreflector. This condition produces high contrast fringes in the il.lel~rence pattern and eliminates any extraneous inlelrerel~ce due to light from the measurement beam spreading into the reference beam. This configuration also eliminates any extraneous il.telrerence due to light from the reference beam spreading into the measurement beam.
Another embodiment of the invention is shown in figure 8. This embodiment is the same as in Figure 7, except that intelrerollleter plate 36 is replaced by single mode optical fiber 86. The . CA 02206212 1997-0~-27 W O 96/17221 PCTrUS95/lS274 far end of single mode optical fiber 86 serves the same purpose as inlerrerometer-plate 36, and is shown in detail in Figure 9. This embodiment is most useful when measuring optics such that their conjugates coincide; for example, as when measuring a concave mirror 5 at its center of curvature.
Figure 9 shows the details at the far end of single mode optical fiber 86. It is embedded in substrate 88. The surface of the structure is polished flat and smooth and coated with thin reflecting metal layer 90 that both reflects and transmits approximately 40% of the 10 light from the single mode optical fiber 86.
Another embodiment of the invention is shown in Figure 10. This embodiment is simil~r to Figure 8 except that beams 18 and 20 are reflected back through polarization beamsplitter 22 so they are parallel to and displaced from each other. Reflected beam 18 now is 15 reflected from turning mirror 27, passes through polarizer 33 and is brought to a focus with microscope objective 35, on a second single mode optical fiber 87. The measurement beam 89, leaving the end of single mode optical fiber 87, is diffracted, producing a perfect spherical wavefront over some finite solid angle. After passing through the optic 20 92 under test, aberrations are imparted to measurement beam 89.
Focused measurement beam 89 is reflected by reflecting metal layer 90 on the surhce of single mode optical fiber 86. It diverges and is coincident with rererel~ce beam 48 that is diffracted, producing a perfect spherical wavefront over some finite solid angle. For inlelrerence to 25 take place as described previously, the length of single mode optical fibers 86 and 87 are equal and rel~orerlector 24 is positioned such that the optical path length ACD is equal to optical path length ABF + GH.
Another requirement for inter~rence to take place is that the polari_ation of reference beam 48 and measurement beam 89 be 30 identical. This is accomplished by determining the pGlarization state of each beam, and then physically manipulating the fibers until the measurement beam 89 and the rererence beam 48 have identical pol~ri7~tion states. This embodiment is most useful when measuring optics such that their conjugates are spatially distinct, for example, as 35 when measuring an imaging system.
. . CA 02206212 1997-0~-27 - W O 96/17221 PCTrUS95/15274 Although only those embodiments for the measurement of concave mirrors and positive lenses were described, other embodiments exist for measurement of convex mirrors and negative lenses Changes and modifications in the specifically described 5 embodiments can be carried out without departing from the scope of the invention, which is intended to be limited by the scope of the appended claims.
The United States Government has rights in this invention pursuant to Contract No. W-7405-ENG48 between the United States Department of Energy and the University of California for the operation of Lawrence Livermore National Laboratory.
BACKGROUND OF THE INVENTION
Field of the Invention The present invention relates to interferometry, and more specifically, it relates to high accuracy diffractioh interferometry.
Description of Related Art Interferometry is the preferred method to measure the performance of optical elements and systems. In this method the wavefront of light reflected from or transmitted by the optic to be tested is interfered with the wavefront from a reference surface, to produce an inlelrerence fringe pattern. These inlelrerence fringes are then analyzed to ascertain the performance of the optic. For high performance imaging systems, such as those found in lithographic steppers used to make integrated circuits, this interrerollletric measurement must be made to ever increasing accuracy. The accuracy, however, is limited by how well the reference surface is charac~ri7e~l- Reference surfaces are typically no better than A /50, where A, is the wavelength of visible light, and thus are the limiting factor in fabricating higher performance optical syslell s.
Ther~rore the fabrication of high accuracy optical systems, such as those needed for e,ctrell-e ultraviolet projection lithography which require an accuracy of A /1000, are impossible to qualify with confidence using existing intelferolnetry.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a phase shifting diffraction inlerferometer having A /1000 accuracy.
An interferometer is disclosed which has the capability of measuring optical elements and systems with an accuracy of ~ /1000 -CA 02206212 1997-0~-27 W O9~117221 PCTrUS95/15274 where ~ is the wavelength of visible light. Whereas current inlerrero-l-eters employ a reference surface, which inherently limits the accuracy of the measurement to about ~ /50, this interferometer uses an essentially perfect spherical reference wavefront generated by the funcl~mental process of diffraction. This interferometer is adjustable to give unity fringe visibility, which maximizes the signal-to-noise, and has the means to introduce a controlled prescribed relative phase shift between the reference wavefront and the wavefront from the optics under test, which permits analysis of the inlelrerence fringe pattern using standard phase extraction algorithms.
The interferometer described in this disclosure has the ability to achieve this extremely high accuracy by eliminating the reference surface and substituting an essentially perfect spherical reference wavefront generated by the fundamental process of diffraction.
Whereas current inlelrerometers illuminate the optic to be tested with an aberrated wavefront which also limits the accuracy of the measurement, this interferometer uses an essentially perfect spherical measurement wavefront generated by the fundamental process of diffraction. The invention maximizes the signal-to-noise and permits analysis of the inlelrerellce fringe pattern using standard phase extraction algorithms.
BRIEF DF.~CRIPTION OF THE DRAWINGS
Pigure 1 shows an embodiment of the phase shifting diffraction inle,rerometer.
Figure 2A shows an embodiment of the intel~rometer plate shown in figure 1.
Figure 2B shows an embodiment of the intelreroll.eter plate shown in figure 1.
Figure 3 shows an embodiment of the interrerollleter plate shown in figure 1.
Figure 4 shows an embodiment of the phase shifting diffraction interferometer.
Figure 5 shows a magnified view of the fiber end shown in figure 4.
Figure 6 shows the elongated end of the fiber shown in figure 4.
: ~ CA 02206212 1997-0~-27 Figure 7 shows an embodiment of the phase shifting diffraction interferometer utilizing coherence length.
DETAILED DESCRIPIION OF THE rNVENTION
- The interferometer system is shown figure 1 in an embodiment that most easily illustrates the essence of the idea, but is 5 not necess~rily the preferred embodiment or the only embodiment. It consists of four main parts: the beam conditioning optics; the inlelrerometer; the detection system; and the computer ~yslem.
The major components of the beam conditioning optics are the light source, intensity and contrast controls, and the phase shifting 10 module. Referring to figure 1, a linearly polarized collimated beam 10 from a light source 12 (e.g., a laser) passes through a variable neutral density filter 14 which is used to control the light level to the intel~rometer. The beam 10 then passes through a half-wave retardation plate 16 which produces two orthogonally pol~ri7e-1 beams, 15 beam 18 having vertical (out of plane) pol~ri7~tion and beam 20 having horizontal (in plane) polarization, both beams within beam 10. The angular orientation of the half-wave retardation plate 16 is used to adjust the relative intensity between the vertical and horizontal components of pol~ri7Ation. Beams 18 and 20 having these two 20 pol~ri7~tion components are then split by a polarization beamsplitter 22 so that the hori_ontal polarization (beam 20) is transmitted while the vertical pol~ri7~tion (beam 18) is reflected. The transmitted beam 20 is then reflected by a fixed relroreflector 24 back through the pol~ri7~tion beamsplitter 22 to a turning mirror 26. The reflected beam 18 is reflected 25 by a relrorellector 28, mounted on a piezoelectric translator (PZT) 30, back ~rough the polarization beamsplitter 22 to the same turning mirror 26 so that is it parallel to the other beam but laterally displaced from it. Applying a voltage to the PZT 30 translates the relrore~ector 28 thus shifting the relative phase between the two orthogonally polarized 30 beams. The two beams pass through a pol~ri7er 32 (axis at 45~) to give them the same polAri7~tion and are brought to focus, with a microscope objective 34, on the inlelrerollleter plate 36.
Referring to figure 2A, the inter~rometer plate 36 comprises a glass substrate 38 coated with a highly reflecting, low 35 tran~mi~sion, metallic film 40, through which a circular aperture 42 has been etched. The diameter of the aperture 42 is on the order of the CA 02206212 1997-0~-27 - ~096/17221 PCTrUS95/15274 wavelength of the source 12. Metallic film 40 may typically comprise aluminum having a thickness of 65 nanometers. In the embodiment shown in figure 2B, the il~lelrerometer plate 36 comprises a glass substrate 38 coated with a highly reflecting, low transmission, metallic 5 film 40, through which a circular aperture 42 has been etched, and over which a second partially transmitting metallic film 44, without an aperture, has been coated. The diameter of the aperture 42 is on the order of the wavelength of the source 12. Metallic film 40 may typically comprises aluminum having a thickness of 65 nanometers and metallic 10 film 44 may typically comprises aluminum having a thickness of 26 nanometers. In both embodiments of the interferometer plate 36, the metallic film 40 and the second metallic film 44 may comprise materials other than aluminum, for example, chromium, gold, nickel, silicon and silver. The thicknesses of the film would vary depending on the 15 m~t~ri~l used. This plate serves to generate the point source measurement beam, using one of the beams focused on the plate, to illuminate the optic under test. It simultaneously serves to generate the point source refere..ce beam, using the other focused beam. Because the measurement beam and the reference beam are generated by diffraction, 20 they both comprise a perfect spherical wavefront over some finite solid angle.
Beams 18 and 20 are focused on the inlelrerometer plate 36 so that they are both incident on the aperture 42 in the metallic film 40.
Both beams are diffracted by the aperture 42, producing two spherical 25 wavefronts that diverge as they leave the aperture. Each wavefront is perfectly spherical over a finite angular range (defined by the diameter of the aperture relative to the wavelength) centered about the illumination direction defined by the lateral separation of the beams as they entered the microscope objective. As shown in figure 1, measurement beam 46 30 illllmin~stes the optic S0 under test. This optic typically comprises a concave mirror and is positioned to focus the measurement beam 46 back onto the inlelrerolneter plate 36. Due to the finite extent of the optic 50 and aberrations therein, the focused measurement beam 46 is much larger than the aperture 42 in the metallic film 40, so nearly all of 35 the focused measurement beam 46 is reflected by the film 40 in the embodiment of the interferometer plate 36 shown in figure 2A. In embodiment of the interferometer plate 36 shown in figure 2B, focused ; CA 02206212 1997-0~-27 W O 96/17221 PCTrUS95/15274 measurement beam 46 is reflected by the film 44. It diverges and is coincident with the reference beam 48 which was diffracted by the aperture 42. The measurement beam 46 and the reference beam 48 inlerrere to produce a fringe pattern that represents a contour map of 5 optical path difference between the wavefront from the optic 50 and a perfect spherical wavefront.
The detection system consists of an imaging system to image the optic under test onto a CCD array camera. The imaging system comprises a lens 52, an aperture 54 for spatially filtering the 10 il~lel~ing beams, and a lens 56. The aperture 54 is large enough so that it does not diffract the beam focused through it. A typical size is 250 micrometers. The size of the lenses 52, 56, depends on the size of the optic under test.
The coincident measurement beam 46 and reference beam 15 48, diverging from the inlerrerometer plate, are collected by a spatial filter imaging system which images the surface of the optic 50 under test onto a screen (not shown) or onto a charge coupled device (CCD) camera 58. This guarantees that the phase of the illlel~e~ g wavefronls at each pixel in the CCD camera 58 has a one-to-one colres~ondence with a 20 unique point on the optic 50. It also minimizes effects of edge diffraction from the optic. Aperture 54, at the intermediate focus of the i. leire,ing beams, is used to filter out any light not coming from the immediate area around the aperture in the inlerrerol,leter plate 36. The CCD
camera 58 captures a series of inlelrerence patterns and transfers them-to 25 the computer ~yslem 60.
The computer system consists of a computer having a monitor and software to control the light level and contrast of the inle~rere~ce pattern, software to translate the PZT and thus shift the relative phase between the measurement and reference beams, software 30 to c~lc~ te the phase at each pixel using the transferred inl~lrerence patterns, and software to display the resultant phase map. This software for analyzing the inlelrerellce pattern read into the computer can presently be supplied by several companies. Zygo Corporation produces "Metro Pro" software. Phase Shift Technology produces "Optic Code 35 Analysis Software". WYKO Corporation produces "WISP" software.
The series of inle,rere-.ce patterns that are transferred to the computer are captured as the PZT shifts the relative phase of the , , CA 02206212 1997-0~-27 W O 96tl7221 PCT~US95/15274 interfering beams by 2 7~ radians. The interference patterns are analyzed to determine the proper settings for the light level and contrast of the inlelrerence fringes. This information is used by the computer to adjust the position of the neutral density filter 14 and half-wave retardation 5 plate 16. A second series of inlelrerel~ce patterns are then captured and analyzed to determine the phase at each pixel. This is typically displayed as a contour or 3D plot of the phase. This phase map cGlres~onds to the deviation of the surface of the optic from a perfect sphere.
This inlelreroll,eter is unique in that:
10 a. The measurement beam is generated by diffraction and is a perfect spherical wave over some finite solid angle. The solid angle is defined by the size of the aperture in the interferometer plate relative to the wavelength of light from the source. Smaller apertures produce larger solid angles.
15 b. The reference beam is generated by diffraction and is a perfect spherical wave over the same solid angle as the measurement beam .
c No reference surface is required for this inle~rerometer.
Reference surfaces are a major source of error in inlelrerometry and ultimately limit the accuracy that can be achieved.
20 d. The measurement beam reflected from the optic under test is imaged back onto the aperture of the interferometer plate, giving exact coincidence with the reference beam. This is the ideal condition for achieving the highest degree of accuracy.
e. The relative phase between the measurement and reference 25 beams can be shifted in a controlled way. This permits a series of inlelrerence patterns to be analyzed to dçtermine the phase at each pixel position with the highest degree of accuracy.
f. The relative intensities of the measurement and reference beams can be adjusted (with the half-wave retardation plate) to give maximum 30 possible contrast. This produces the greatest signal-t~noise, necessary for achieving the highest degree of accuracy.
Referring to figure 3, inle~relometer plate 37 may be useful in certain circumstances depending on the optic under test. Here beams 18 and 20 each pass through a prism 17, 19 respectively, and are focused 35 by lens 34 onto two distinct apertures: aperture 41 and aperture 43 in metallic film 45 of the interferometer plate 37. A tilt is introduced between beam 20 and beam 18 by the prisms. The usable metallic films CA 02206212 1997-0~-27 W O 96~17221 PCT~US95tlS274 are the same as described for interferometer plate 36. Apertures 41 and 43 are typically separated from each other by 10 to 500 micrometers. The typical angle between the two focused beams is in the range of 10 to 60 degrees. In the embodiment shown on figure 3, beam 18 passes through 5 prism 17 and is focused by lens 34 onto aperture 43 in intelrerometer plate 37. The resulting diffracted measurement beam 47 is then reflected from test optic 51 to the reflective area surrounding aperture 41 of inlel~rolneter plate 37. This beam is reflected along the same path as reference beam 49 to produce an inle~ference pattern therebetween. This - 10 embodiment is used to test optical surfaces that are concave with respect to the inlerrerometer plate 37.
Another distinctive form of this interferometer system, based on fiber optic technology, operates on the principles described supra. Referring to figure 4, light source 12 is focused by lens 62 onto the 15 end of a single mode fiber 64. A variable fiber coupler 66 splits the beam into two fibers: fiber 68 and fiber 72. The ratio of light split into each fiber is chosen for maximum fringe contrast. Fiber 68 is used for the measurement beam 70. Fiber 72, transmitting rererence beam 74, is wrapped around a PZT cylinder 76 that expands with an applied voltage.
20 This stresses the fiber 72, changing its refractive index, and introduces a phase shift relative to the measurement beam 70 from fiber 68. The measurement beam 70 leaving the end of the fiber is diffracted, producing a perfect spherical wavefront over some finite solid angle.
The solid angle is defined by the size of the fiber core relative to the 25 wavelength of light from the source 12. This spherical measurement beam 70 illuminates the optic 78 under test, which focuses the beam onto the end of the fiber for the rerelence beam. Figure 5 shows a magnified view of the exit portion of fiber 72. Measurement beam 70 is focused onto reflective metallic film 73 on the end of fiber 72, and is 30 reflected along the same path as rererence beam 74. The rest of the system is as in figure 1. This embodiment is used to test positive lenses.
Referring to figure 6, it may be necessary to shrink the core diameter of fibers 68 and 72 to match the aperture size for the intelferometer. This is accomplished by heating and pulling the fiber.
35 For example, fiber 72, having cladding 84 and core 80 is heated and pulled to produced a stretched portion 82 with a core 81 having a diameter that is smaller than the unstretched core portion 80. Metallic ~ - ~
. CA 02206212 1997-0~-27 W O9~/17221- PCTrUS95/15274 film 73 is coated after heating and stretching and can comprise any of the films described supra. One embodiment of the metallic film comprises aluminum having a thickness of 26 nanometers. Single mode fibers typically have core diameters of about 4 micrometers. The measurement beam is reflected from the end of the fiber while the reference beam is transmitted through the film and diffracted, giving a perfect spherical wavefront over the same finite solid angle as the measurement wavefront. The solid angle is defined by the size of the fiber core relative to the wavelength of light from the source 12. The imaging, computer systems, data acquisition and analysis are the same as described above. This fiber optic approach has all the advantages of the system described above as well as the flexibility to independently move the measurement and reference fibers to any positions to suit the inlel~erollletric measurement configuration. If light source 12 comprises a short coherence length, the fiber lengths 68 and 72 must be adjusted so that the optical path lengths from variable fiber coupler 66 to the end of fiber 72 for both the measurement beam 70 and reference beam 74 are equal.
Another embodiment of the invention is shown in figure 7. In this embodiment light source 12 has a short coherence length and beams 18 and 20 are reflected back through the polarization beamsplitter 22 so they are coincident and collinear. Retroreflector 24 is positioned such that optical path length ACD is equal to optical path length ABDED.
In other word, re~orenector 24 is moved to a position where the round-trip beam path difference between the two relrorenectors is equal to the round-trip path between the inlelrerc.-,eter plate and the optic under test. The optical path lengths of the inler[el;ng measurement and rererellce beams are then the same. As in figure 1, PZT 30 can be attached to either relroreflector. This condition produces high contrast fringes in the il.lel~rence pattern and eliminates any extraneous inlelrerel~ce due to light from the measurement beam spreading into the reference beam. This configuration also eliminates any extraneous il.telrerence due to light from the reference beam spreading into the measurement beam.
Another embodiment of the invention is shown in figure 8. This embodiment is the same as in Figure 7, except that intelrerollleter plate 36 is replaced by single mode optical fiber 86. The . CA 02206212 1997-0~-27 W O 96/17221 PCTrUS95/lS274 far end of single mode optical fiber 86 serves the same purpose as inlerrerometer-plate 36, and is shown in detail in Figure 9. This embodiment is most useful when measuring optics such that their conjugates coincide; for example, as when measuring a concave mirror 5 at its center of curvature.
Figure 9 shows the details at the far end of single mode optical fiber 86. It is embedded in substrate 88. The surface of the structure is polished flat and smooth and coated with thin reflecting metal layer 90 that both reflects and transmits approximately 40% of the 10 light from the single mode optical fiber 86.
Another embodiment of the invention is shown in Figure 10. This embodiment is simil~r to Figure 8 except that beams 18 and 20 are reflected back through polarization beamsplitter 22 so they are parallel to and displaced from each other. Reflected beam 18 now is 15 reflected from turning mirror 27, passes through polarizer 33 and is brought to a focus with microscope objective 35, on a second single mode optical fiber 87. The measurement beam 89, leaving the end of single mode optical fiber 87, is diffracted, producing a perfect spherical wavefront over some finite solid angle. After passing through the optic 20 92 under test, aberrations are imparted to measurement beam 89.
Focused measurement beam 89 is reflected by reflecting metal layer 90 on the surhce of single mode optical fiber 86. It diverges and is coincident with rererel~ce beam 48 that is diffracted, producing a perfect spherical wavefront over some finite solid angle. For inlelrerence to 25 take place as described previously, the length of single mode optical fibers 86 and 87 are equal and rel~orerlector 24 is positioned such that the optical path length ACD is equal to optical path length ABF + GH.
Another requirement for inter~rence to take place is that the polari_ation of reference beam 48 and measurement beam 89 be 30 identical. This is accomplished by determining the pGlarization state of each beam, and then physically manipulating the fibers until the measurement beam 89 and the rererence beam 48 have identical pol~ri7~tion states. This embodiment is most useful when measuring optics such that their conjugates are spatially distinct, for example, as 35 when measuring an imaging system.
. . CA 02206212 1997-0~-27 - W O 96/17221 PCTrUS95/15274 Although only those embodiments for the measurement of concave mirrors and positive lenses were described, other embodiments exist for measurement of convex mirrors and negative lenses Changes and modifications in the specifically described 5 embodiments can be carried out without departing from the scope of the invention, which is intended to be limited by the scope of the appended claims.
Claims (24)
1. A phase shifting diffraction interferometer, comprising:
means for separating a linearly polarized, collimated and coherent beam of light into two parallel, spatially separated, orthogonally polarized coherent beams of light;
means for introducing a phase shift between said two parallel, spatially separated, orthogonally polarized coherent beams of light;
a polarizer to orient said two parallel, spatially separated, orthogonally polarized coherent beams of light into two parallel, spatially separated, identically polarized coherent beams of light;
means for focusing said two parallel, spatially separated, identically polarized coherent beams of light to a focal point;
an interferometer plate comprising:
a glass substrate;
a highly reflective metallic film adherent to said glass substrate; and at least one circular aperture through said glass substrate and said highly reflective metallic film, wherein said circular aperture is placed at said focal point and has a diameter of about the size of the wavelength of said coherent beam of light, wherein said interferometer plate diffracts said two parallel, spatially separated, identically polarized coherent beams of light at said focal point to produce a measurement beam and a reference beam; and means for focusing said measurement beam onto said aperture and said highly reflective metallic film, wherein said measurement beam and said reference beam combine to form an interference pattern.
means for separating a linearly polarized, collimated and coherent beam of light into two parallel, spatially separated, orthogonally polarized coherent beams of light;
means for introducing a phase shift between said two parallel, spatially separated, orthogonally polarized coherent beams of light;
a polarizer to orient said two parallel, spatially separated, orthogonally polarized coherent beams of light into two parallel, spatially separated, identically polarized coherent beams of light;
means for focusing said two parallel, spatially separated, identically polarized coherent beams of light to a focal point;
an interferometer plate comprising:
a glass substrate;
a highly reflective metallic film adherent to said glass substrate; and at least one circular aperture through said glass substrate and said highly reflective metallic film, wherein said circular aperture is placed at said focal point and has a diameter of about the size of the wavelength of said coherent beam of light, wherein said interferometer plate diffracts said two parallel, spatially separated, identically polarized coherent beams of light at said focal point to produce a measurement beam and a reference beam; and means for focusing said measurement beam onto said aperture and said highly reflective metallic film, wherein said measurement beam and said reference beam combine to form an interference pattern.
2. A phase shifting diffraction interferometer, comprising:
a light source for producing a linearly polarized, collimated and coherent beam of light;
a single mode fiber having a first end and a second end;
means for focusing said coherent beam of light into said first end of said single mode fiber;
a variable fiber optic splitter fixedly and optically connected to said second end of said single mode fiber, wherein said variable fiber splitter comprises a first fiber optic and a second fiber optic, wherein said first fiber optic is for transmitting a measurement beam, wherein said second fiber is for transmitting a reference beam. said first fiber optic and said second fiber optic both comprising an output end having a reduced diameter portion comprising a core having a diameter of about the wavelength of said coherent beam of light, said output end of said second fiber optic comprising a highly reflecting metallic film;
a piezoelectric (PZT) cylinder, wherein said second fiber optic is wound around said PZT cylinder, wherein said second fiber optic expands as a voltage is applied to expand said PZT cylinder causing a change in the refractive index of said second fiber optic, thereby introducing a phase shift between light traveling in said first fiber optic and said second fiber optic; and means for focusing said reference beam onto said core and said highly reflecting metallic film of said reduced diameter portion of said second fiber optic;
wherein said measurement beam and said reference beam combine to form an interference pattern.
a light source for producing a linearly polarized, collimated and coherent beam of light;
a single mode fiber having a first end and a second end;
means for focusing said coherent beam of light into said first end of said single mode fiber;
a variable fiber optic splitter fixedly and optically connected to said second end of said single mode fiber, wherein said variable fiber splitter comprises a first fiber optic and a second fiber optic, wherein said first fiber optic is for transmitting a measurement beam, wherein said second fiber is for transmitting a reference beam. said first fiber optic and said second fiber optic both comprising an output end having a reduced diameter portion comprising a core having a diameter of about the wavelength of said coherent beam of light, said output end of said second fiber optic comprising a highly reflecting metallic film;
a piezoelectric (PZT) cylinder, wherein said second fiber optic is wound around said PZT cylinder, wherein said second fiber optic expands as a voltage is applied to expand said PZT cylinder causing a change in the refractive index of said second fiber optic, thereby introducing a phase shift between light traveling in said first fiber optic and said second fiber optic; and means for focusing said reference beam onto said core and said highly reflecting metallic film of said reduced diameter portion of said second fiber optic;
wherein said measurement beam and said reference beam combine to form an interference pattern.
3. A phase shifting diffraction interferometer, comprising:
means for producing a linearly polarized, collimated and coherent beam of light;
means for producing a measurement beam and a reference beam from said coherent beam of light, wherein said means for producing a measurement beam and a reference beam comprise a highly reflecting metallic film;
means for introducing a phase shift between said measurement beam and said reference beam; and means for focusing said measurement beam onto said highly reflecting metallic film;
wherein said measurement beam and said reference beam combine to form an interference pattern.
means for producing a linearly polarized, collimated and coherent beam of light;
means for producing a measurement beam and a reference beam from said coherent beam of light, wherein said means for producing a measurement beam and a reference beam comprise a highly reflecting metallic film;
means for introducing a phase shift between said measurement beam and said reference beam; and means for focusing said measurement beam onto said highly reflecting metallic film;
wherein said measurement beam and said reference beam combine to form an interference pattern.
4. A method for producing an interference pattern, comprising:
producing a linearly polarized, collimated and coherent beam of light;
producing a measurement beam and a reference beam from said coherent beam of light, wherein said producing means comprise a highly reflecting metallic film;
introducing a phase shift between said measurement beam and said reference beam; and focusing said measurement beam onto said highly reflecting metallic film;
wherein said measurement beam and said reference beam combine to form an interference pattern.
producing a linearly polarized, collimated and coherent beam of light;
producing a measurement beam and a reference beam from said coherent beam of light, wherein said producing means comprise a highly reflecting metallic film;
introducing a phase shift between said measurement beam and said reference beam; and focusing said measurement beam onto said highly reflecting metallic film;
wherein said measurement beam and said reference beam combine to form an interference pattern.
5. The phase shifting diffraction interferometer of claim 1, wherein said means for separating a linearly polarized, collimated and coherent beam of light into two parallel, spatially separated, orthogonally polarized coherent beams of light comprise:
a light source for producing a linearly polarized, collimated and coherent beam of light;
a variable neutral density filter for controlling the intensity of said linearly polarized, collimated and coherent beam of light;
a half-wave retardation plate for producing, within said linearly polarized, collimated and coherent beam of light, two orthogonally polarized components of light comprising a vertical component and a horizontal component, wherein the angular orientation of said half-wave retardation plate is used to adjust relative intensity between said vertical component and said horizontal component;
a polarization beamsplitter optically positioned to transmit said horizontal component to produce a first transmitted horizontally polarized component, wherein said polarization beamsplitter is optically positioned to reflect said vertical component to produce a first reflected vertically polarized component;
a first retroreflector optically positioned to laterally reflect said first transmitted horizontally polarized component back into said polarization beamsplitter for transmission therethrough to produce a horizontally polarized beam; and a second retroreflector optically positioned to laterally reflect said first reflected vertically polarized component back into said polarization beamsplitter for reflection therefrom to produce a vertically polarized beam;
wherein said vertically polarized beam and said horizontally polarized beam together comprise two parallel, spatially separated, orthogonally polarized coherent beams of light.
a light source for producing a linearly polarized, collimated and coherent beam of light;
a variable neutral density filter for controlling the intensity of said linearly polarized, collimated and coherent beam of light;
a half-wave retardation plate for producing, within said linearly polarized, collimated and coherent beam of light, two orthogonally polarized components of light comprising a vertical component and a horizontal component, wherein the angular orientation of said half-wave retardation plate is used to adjust relative intensity between said vertical component and said horizontal component;
a polarization beamsplitter optically positioned to transmit said horizontal component to produce a first transmitted horizontally polarized component, wherein said polarization beamsplitter is optically positioned to reflect said vertical component to produce a first reflected vertically polarized component;
a first retroreflector optically positioned to laterally reflect said first transmitted horizontally polarized component back into said polarization beamsplitter for transmission therethrough to produce a horizontally polarized beam; and a second retroreflector optically positioned to laterally reflect said first reflected vertically polarized component back into said polarization beamsplitter for reflection therefrom to produce a vertically polarized beam;
wherein said vertically polarized beam and said horizontally polarized beam together comprise two parallel, spatially separated, orthogonally polarized coherent beams of light.
6. The phase shifting diffraction interferometer of claim 5, wherein said means for introducing a phase shift between said two parallel, spatially separated, orthogonally polarized coherent beams of light comprises a piezoelectric translator (PZT) mounted on a retroreflector selected from a group consisting of said first retroreflector and said second retroreflector, wherein said PZT translates said retroreflector when a voltage is applied to said PZT, thus shifting the relative phase between said two parallel, spatially separated, orthogonally polarized beams of light.
7. The phase shifting diffraction interferometer of claim 6, wherein said interferometer plate further comprises a partially reflective metallic film adherent to said highly reflective metallic film, wherein said partially reflective metallic film covers said highly reflective metallic film and said aperture.
8. The phase shifting diffraction interferometer of claim 7, wherein said means for focusing said measurement beam onto said aperture and said highly reflective metallic film comprises an optic to be tested.
9. The phase shifting diffraction interferometer of claim 8, further comprising an imaging system for imaging said interference pattern, said imaging system comprising a spatial filter comprising a first lens, an aperture and a second lens, wherein said spatial filter is positioned to transmit said interference pattern, wherein said aperture is large enough that it does not diffract said interference pattern.
10. The phase shifting diffraction interferometer of claim 9, wherein said imaging system further comprises a screen for displaying said interference pattern after it is transmitted through said spatial filter.
11. The phase shifting diffraction interferometer of claim 9, wherein said imaging system further comprises a charge coupled display camera and monitor for displaying said interference pattern after it is transmitted through said spatial filter.
12. The phase shifting diffraction interferometer of claim 9, further comprising a computer system having a central processing unit, memory and software to: read said interference pattern from said CCD
camera, control the intensity and contrast of said interference pattern, translate said PZT, calculate the phase at each pixel and display a resultant phase map.
camera, control the intensity and contrast of said interference pattern, translate said PZT, calculate the phase at each pixel and display a resultant phase map.
13. The phase shifting diffraction interferometer of claim 8, wherein said means for focusing said two parallel, spatially separated, identically polarized coherent beams of light to a focal point comprises a prism in each beam of said two beams, wherein said prisms refract said two beams away from each other, said means for focusing said two parallel, spatially separated, identically polarized coherent beams of light further comprising a lens.
14. The phase shifting diffraction interferometer of claim 1, wherein said at least one circular aperture comprises two apertures, wherein said lens focuses said two parallel, spatially separated, identically polarized coherent beams of light onto separate apertures of said two apertures.
15. A phase shifting diffraction interferometer comprising:
means for separating a linearly polarized, collimated and spatially coherent beam of light into two orthogonally polarized spatially coherent components of light;
means for introducing a phase shift between said two orthogonally polarized spatially coherent components of light;
a polarizer to orient said two orthogonally polarized spatially coherent components of light into two identically polarized spatially coherent components of light;
means for focusing said two identically polarized spatially coherent components of light to a focal point;
an interferometer plate comprising:
a glass substrate;
a highly reflective metallic film adherent to said glass substrate; and at least one circular aperture through said glass substrate and said highly reflective metallic film, wherein said circular aperture is placed at said focal point and has a diameter of about the size of the wavelength of said linearly polarized, collimated and spatially coherent beam of light, wherein said interferometer plate diffracts said two identically polarized, spatially coherent components of light at said focal point to produce a measurement beam and a reference beam; and means for focusing said measurement beam onto said aperture and said highly reflective metallic film, wherein said measurement beam and said reference beam combine to form an interference pattern.
means for separating a linearly polarized, collimated and spatially coherent beam of light into two orthogonally polarized spatially coherent components of light;
means for introducing a phase shift between said two orthogonally polarized spatially coherent components of light;
a polarizer to orient said two orthogonally polarized spatially coherent components of light into two identically polarized spatially coherent components of light;
means for focusing said two identically polarized spatially coherent components of light to a focal point;
an interferometer plate comprising:
a glass substrate;
a highly reflective metallic film adherent to said glass substrate; and at least one circular aperture through said glass substrate and said highly reflective metallic film, wherein said circular aperture is placed at said focal point and has a diameter of about the size of the wavelength of said linearly polarized, collimated and spatially coherent beam of light, wherein said interferometer plate diffracts said two identically polarized, spatially coherent components of light at said focal point to produce a measurement beam and a reference beam; and means for focusing said measurement beam onto said aperture and said highly reflective metallic film, wherein said measurement beam and said reference beam combine to form an interference pattern.
16. The phase shifting diffraction interferometer of claim 15, wherein said separating means comprise:
a spatially coherent, temporally incoherent light source for producing said beam of light;
a variable neutral density filter for controlling the intensity of said beam of light;
a half-wave retardation plate for producing within said beam of light a vertical component and a horizontal component, wherein the angular orientation of said plate is used to adjust relative intensity between said vertical component and said horizontal component;
a polarization beamsplitter optically positioned to transmit said horizontal component to produce a first transmitted component, wherein said polarization beamsplitter is optically positioned to reflect said vertical component to produce a first reflected component;
a first retroreflector optically positioned to laterally reflect said first transmitted component back into said beamsplitter for transmission therethrough to produce a horizontally polarized beam;
and a second retroreflector optically positioned to laterally reflect said first reflected component back into said beamsplitter for reflection therefrom to produce a vertically polarized beam;
wherein said vertically polarized beam and said horizontally polarized beam are colinear.
a spatially coherent, temporally incoherent light source for producing said beam of light;
a variable neutral density filter for controlling the intensity of said beam of light;
a half-wave retardation plate for producing within said beam of light a vertical component and a horizontal component, wherein the angular orientation of said plate is used to adjust relative intensity between said vertical component and said horizontal component;
a polarization beamsplitter optically positioned to transmit said horizontal component to produce a first transmitted component, wherein said polarization beamsplitter is optically positioned to reflect said vertical component to produce a first reflected component;
a first retroreflector optically positioned to laterally reflect said first transmitted component back into said beamsplitter for transmission therethrough to produce a horizontally polarized beam;
and a second retroreflector optically positioned to laterally reflect said first reflected component back into said beamsplitter for reflection therefrom to produce a vertically polarized beam;
wherein said vertically polarized beam and said horizontally polarized beam are colinear.
17. The phase shifting diffraction interferometer of claim 16, wherein a retroreflector selected from a group consisting of said first retroreflector and said second retroreflector is optically positioned so that the beam path from said first reflected component to said optic to be tested and back to said aperture is equal to the beam path from said first transmitted component to said aperture of said interferometer plate.
18. A phase shifting diffraction interferometer, comprising:
means for producing, within a collimated and spatially coherent beam of light, two orthogonally polarized components of light comprising a vertical component and a horizontal component;
means for introducing a phase shift between said vertical component and said horizontal component, to produce a phase shifted beam;
means for producing a measurement beam and a reference beam from said phase shifted beam;
means for combining said measurement beam and said reference beam to form an interference pattern.
means for producing, within a collimated and spatially coherent beam of light, two orthogonally polarized components of light comprising a vertical component and a horizontal component;
means for introducing a phase shift between said vertical component and said horizontal component, to produce a phase shifted beam;
means for producing a measurement beam and a reference beam from said phase shifted beam;
means for combining said measurement beam and said reference beam to form an interference pattern.
19. The phase shifting diffraction interferometer of claim 18, wherein said means for producing, within a collimated and spatially coherent beam of light, two orthogonally polarized components of light comprising a vertical component and a horizontal component, comprise:
a light source for producing a linearly polarized, collimated and spatially coherent beam of light; and a half-wave retardation plate for producing, within said linearly polarized, collimated and spatially coherent beam of light, two orthogonally polarized components of light comprising a vertical component and a horizontal component, wherein the angular orientation of said half-wave retardation plate is used to adjust relative intensity between said vertical component and said horizontal component.
a light source for producing a linearly polarized, collimated and spatially coherent beam of light; and a half-wave retardation plate for producing, within said linearly polarized, collimated and spatially coherent beam of light, two orthogonally polarized components of light comprising a vertical component and a horizontal component, wherein the angular orientation of said half-wave retardation plate is used to adjust relative intensity between said vertical component and said horizontal component.
20. The phase shifting diffraction interferometer of claim 19, wherein said means for introducing a phase shift between said vertical component and said horizontal component comprise:
a polarization beamsplitter optically positioned for reflecting said vertical component to produce a first reflected vertically polarized component, wherein said polarization beamsplitter is optically positioned for transmitting said horizontal component to produce a first transmitted horizontally polarized component;
a first retroreflector optically positioned to laterally reflect said first transmitted horizontally polarized component back into said polarization beamsplitter for transmission therethrough to produce a horizontally polarized beam;
a second retroreflector optically positioned to laterally reflect said first reflected vertically polarized component back into said polarization beamsplitter for reflection therefrom to produce a vertically polarized beam;
wherein said vertically polarized beam and said horizontally polarized beam are colinear, and wherein said means for introducing a phase shift comprise a piezoelectric translator (PZT) mounted on a PZT reflector selected from a group consisting of said first retroreflector and said second retroreflector.
a polarization beamsplitter optically positioned for reflecting said vertical component to produce a first reflected vertically polarized component, wherein said polarization beamsplitter is optically positioned for transmitting said horizontal component to produce a first transmitted horizontally polarized component;
a first retroreflector optically positioned to laterally reflect said first transmitted horizontally polarized component back into said polarization beamsplitter for transmission therethrough to produce a horizontally polarized beam;
a second retroreflector optically positioned to laterally reflect said first reflected vertically polarized component back into said polarization beamsplitter for reflection therefrom to produce a vertically polarized beam;
wherein said vertically polarized beam and said horizontally polarized beam are colinear, and wherein said means for introducing a phase shift comprise a piezoelectric translator (PZT) mounted on a PZT reflector selected from a group consisting of said first retroreflector and said second retroreflector.
21. The phase shifting diffraction interferometer of claim 20, wherein said means for producing a measurement beam and a reference beam from said phase shifted beam comprise:
a single mode fiber comprising an input end and an output end, wherein said output end is embedded in a substrate, said output end further comprising a thin reflecting metal layer;
means for focusing said phase shifted beam into said input end of said single mode fiber;
wherein said first retroreflector and said second retroreflector are positioned such that the round trip beam path difference between said first retroreflector and said second retroreflector is equal to the round trip beam path between said output end of said single mode fiber and an optic under test.
a single mode fiber comprising an input end and an output end, wherein said output end is embedded in a substrate, said output end further comprising a thin reflecting metal layer;
means for focusing said phase shifted beam into said input end of said single mode fiber;
wherein said first retroreflector and said second retroreflector are positioned such that the round trip beam path difference between said first retroreflector and said second retroreflector is equal to the round trip beam path between said output end of said single mode fiber and an optic under test.
22. A phase shifting diffraction interferometer, comprising:
means for separating a linearly polarized, collimated and spatially coherent beam of light into two parallel, spatially separated, orthogonally polarized coherent beams of light comprising a vertically polarized beam and a horizontally polarized beam;
means for introducing a phase shift between said vertically polarized beam and said horizontally polarized beam;
a first single mode fiber comprising a first input end and a first output end, wherein said first output end is embedded in a substrate, said first output end further comprising a thin reflecting metal layer;
a second single mode fiber comprising a second input end and a second output end;
wherein said first single mode fiber and said second single mode fiber are the same length, means for focussing said horizontally polarized beam into said first single mode fiber, wherein a reference beam diverges from said first output end;
means for focussing said vertically polarized beam into said second single mode fiber, wherein a signal beam diverges from said second output end; and means for combining said measurement beam and said reference beam to form an interference pattern.
means for separating a linearly polarized, collimated and spatially coherent beam of light into two parallel, spatially separated, orthogonally polarized coherent beams of light comprising a vertically polarized beam and a horizontally polarized beam;
means for introducing a phase shift between said vertically polarized beam and said horizontally polarized beam;
a first single mode fiber comprising a first input end and a first output end, wherein said first output end is embedded in a substrate, said first output end further comprising a thin reflecting metal layer;
a second single mode fiber comprising a second input end and a second output end;
wherein said first single mode fiber and said second single mode fiber are the same length, means for focussing said horizontally polarized beam into said first single mode fiber, wherein a reference beam diverges from said first output end;
means for focussing said vertically polarized beam into said second single mode fiber, wherein a signal beam diverges from said second output end; and means for combining said measurement beam and said reference beam to form an interference pattern.
23. The phase shifting diffraction interferometer of claim 22, wherein said means for introducing a phase shift between said vertically polarized beam and said horizontally polarized beam comprise a piezoelectric translator (PZT) mounted on a PZT reflector selected from a group consisting of said first retroreflector and said second retroreflector, wherein said PZT translates said PZT reflector when a voltage is applied to said PZT, thus shifting the relative phase between said vertically polarized beam and said horizontally polarized beam.
24. A phase shifting diffraction interferometer, comprising:
means for producing two orthogonally polarized components of light;
means for introducing a phase shift on one polarization component of said two orthogonally polarized components of light;
means for producing a spherical wavefront from said two orthogonally polarized components of light, wherein said spherical wavefront comprises a reference beam and a measurement beam; and means for combining said reference beam and said measurement beam to produce an interference pattern
means for producing two orthogonally polarized components of light;
means for introducing a phase shift on one polarization component of said two orthogonally polarized components of light;
means for producing a spherical wavefront from said two orthogonally polarized components of light, wherein said spherical wavefront comprises a reference beam and a measurement beam; and means for combining said reference beam and said measurement beam to produce an interference pattern
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US08/345,878 US5548403A (en) | 1994-11-28 | 1994-11-28 | Phase shifting diffraction interferometer |
| US08/345,878 | 1994-11-28 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2206212A1 true CA2206212A1 (en) | 1996-06-06 |
Family
ID=23356893
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002206212A Abandoned CA2206212A1 (en) | 1994-11-28 | 1995-11-21 | Phase shifting diffraction interferometer |
Country Status (7)
| Country | Link |
|---|---|
| US (2) | US5548403A (en) |
| EP (1) | EP0795116A4 (en) |
| JP (1) | JP4130222B2 (en) |
| KR (1) | KR100225923B1 (en) |
| AU (1) | AU4244496A (en) |
| CA (1) | CA2206212A1 (en) |
| WO (1) | WO1996017221A1 (en) |
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-
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- 1995-11-21 WO PCT/US1995/015274 patent/WO1996017221A1/en not_active Application Discontinuation
- 1995-11-21 KR KR1019970703617A patent/KR100225923B1/en not_active IP Right Cessation
- 1995-11-21 EP EP95940819A patent/EP0795116A4/en not_active Withdrawn
- 1995-11-21 AU AU42444/96A patent/AU4244496A/en not_active Abandoned
- 1995-11-21 JP JP51893296A patent/JP4130222B2/en not_active Expired - Fee Related
- 1995-11-21 CA CA002206212A patent/CA2206212A1/en not_active Abandoned
-
1997
- 1997-05-23 US US08/862,758 patent/US5933236A/en not_active Expired - Lifetime
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN104949630A (en) * | 2014-03-25 | 2015-09-30 | 南京理工大学 | Point diffraction interference device with adjustable large-numerical-aperture fringe contrast |
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Also Published As
| Publication number | Publication date |
|---|---|
| US5548403A (en) | 1996-08-20 |
| WO1996017221A1 (en) | 1996-06-06 |
| AU4244496A (en) | 1996-06-19 |
| US5933236A (en) | 1999-08-03 |
| JP4130222B2 (en) | 2008-08-06 |
| JPH10510054A (en) | 1998-09-29 |
| EP0795116A1 (en) | 1997-09-17 |
| KR100225923B1 (en) | 1999-10-15 |
| EP0795116A4 (en) | 2000-01-12 |
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| FZDE | Discontinued |