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Publication numberUS3710101 A
Publication typeGrant
Publication dateJan 9, 1973
Filing dateOct 6, 1970
Priority dateOct 6, 1970
Also published asCA930078A, CA930078A1, DE2149344A1
Publication numberUS 3710101 A, US 3710101A, US-A-3710101, US3710101 A, US3710101A
InventorsKeeffe T O, P Malmberg
Original AssigneeWestinghouse Electric Corp
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Apparatus and method for alignment of members to electron beams
US 3710101 A
Abstract
Apparatus for exposing precisely located areas of a member to an electron beam wherein the member is precisely located by means of registration indicia thereon which cooperate with an alignment beam of electrons. Suitable control means are provided to be operated by the alignment beam to cause it to coincide with the registration indicia with great precision. The method enables repeated aligning of selected areas of members exactly to a patterned electron beam within a fraction of a micron.
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Description  (OCR text may contain errors)

United States Patent 1191 OKeeffe et al. 1 1 Jan. 9, 1973 s41 APPARATUS AND METHOD FOR 3,326,176 6/1967 Sibley ..250/49.5 T ALIGNMENT OF MEMBERS T0 3,5l9,873 7/1970 O'Keeffe 250/495 T ELECTRON BEAMS {75] Inventors: Terence W. OKeeife; Paul R. 5"'f'f j ""f "gg' Bohrchelt Malmberg both of Pittsburgh Pa. ms an xammer um Attorney-F. Shapoe and C. L. Menzemer [73] 'Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa. [57] ABSTRACT [22] Filed: Oct. 6, 1970 Apparatus for exposing precisely located areas of a [2]] Appl. No; 78,505 member to an electron beam wherein themember is I prec1sely located by means of reglstration 1nd1c1a thereon which cooperate with an alignment beam of [52] 250/495 250/ 49'5 B electrons. Suitable control means are provided to be [51] Ill. Cl ..H01J 37/00 operamd y the alignment beam to cause it to coin Fleld of Search T, cide the registration indicia gr precision. The method enables repeated aligning of selected [56] References cued areas of members exactly to a patterned electron UNITED STATES PATENTS beam within a fraction of a micron.

3,236,707 2/1966 Lins ..250/49.5 10 Claims, 12 Drawing Figures PATENTEDJAII 9191s 3 710 1 01 SHEET 1 [IF 6 FIG. I.

PATENIEUJAN 9 m 3.710.101

' SHEET 2 OF 6 SIZE ERROR FIG. 2.

PATENTEDJAN 9197s sum 3 [IF 6 0.0. RESPONSE t -AC. MODULATION FIG. 5A.

FIG. 5B

SIGNAL SHEET u or 6 X-POSITION Y-POSITION SIGNAL APPARATUS AND METHOD FOR ALIGNMENT OF MEMBERS TO ELECTRON BEAMS RELATED APPLICATIONS This invention is closely related to and an improve-.

ment upon US. application Ser. No. 869,229, filed Oct. 24, 1969, of which the present inventors are coinventors with others.

PRIOR ART It has been proposed to employ both a scanning electron microscope and a photocathode means to produce a desired pattern on members such as silicon wafers coated with an electron resist. One of the problems in both of these techniques is that of precisely aligning the member in a position such that the electron beam from either the scanning electron microscope or the photocathode impinges on precisely located areas of the member, particularly where successive exposures to the electron beam are desired. The resolution of the electron microscope and the photocathode is feasible to 0.5 micron and less. In order to take the maximum advantage of this capability, one of the problems of producing satisfactory successive exposures of a member to the electron beam is that of aligning each exposure with a precision of at least 0.5 micron with respect to the previous electron beam exposure. In processing semiconductor wafers into integrated circuit devices, a minimum of from 6 to 10 separate electron resist layers would be commonly applied, each being exposed to a particular electron beam pattern and then the wafer processed to remove selected portions of the electron beam treated electron resist and then the exposed areas of the wafer are chemically treated after which a new layer of electron resist is applied and similarly processed. Unless the wafer can be aligned each time with an accuracy or precision of at least 0.5 micron with respect to the initial electron beam, the inherent'potential accuracy, the economies and technical advantages of electron beam photocathode technology will be lost.

Since optical systems have an ultimate resolution limit of one micron at best, and in practice 3 to 5 microns is the best realizable precision for a plurality of masks, the electron beam system is almost an order of magnitude superior.

In addition, it is desirable that the time for aligning the wafer with respect to the electron beam to this order of precision be of the order ofa second or two at the most in order that a commercially economical processing system be realized.

SUMMARY OF INVENTION This invention is particularly adapted to the exposure of a member to an electron beam pattern derived from a photocathode wherein at least one alignment electron beam of a predetermined cross-sectional shape cooperates with registration indicia or marks on the member so as to enable precise positioning of the wafer with respect to the photocathode and its patterned beam of electrons. As disclosed in more detail in application Ser. No. 869,229, previously referred to, a planar surfaced photocathode is disposed in spaced relationship to a member, such for example as a semiconductor wafer coated with an electron resist, so

that when the photocathode is exposed to a source of radiation, for instance ultraviolet light, the

photocathode will emit a patterned beam of electrons which are caused to impinge on the semiconductor wafer so that the electron resist is rendered preferentially soluble in predetermined areas defined by the electron beam pattern. The photocathode and member are surrounded by a focusing magnet and two sets of electromagnet coils with mutually perpendicular axes for shifting the point of impingement on the member of electrons driven by a potential applied between the photocathode and the member.

Suitable mechanical stops or other positioning means cooperating with the members are employed to roughly position the member with respect to the electron beam, that is, within a few mils or less of the desired exactness. At least one, and preferably two, registration indicia or marks are applied to the member or during the first electron beam exposure. These marks are produced by applying a coating of an insulating oxide to a selected portion of the member, such oxide being of a substantial thickness. In this oxide there is provided a well of a selected shape with the thickness of the oxide at the bottom of the well being of the order of a micron or less, whereas the oxide surrounding the well area is appreciably thicker. Over this oxide area, there is applied a thin layer of an electrically conductive material, for example, aluminum of a thickness of less than one micron to permit electrons at a potential of the order of 10 kilovolts to penetrate the applied metal into the underlying oxide. The well may have a gross dimension of the order of 10 mils by 10 mils and may be any one of various geometrical shapes, of which a square 12 mils by 12 mils is an example. By applying a direct-current potential between the metal coating over the oxide and the underlying silicon wafer, a current will flow when an aligning electron beam impinges in the well area in proportion to the area of the well imp inged by the electron beam. The flow of electrical current reaches a peak value when the entire well is flooded by the electron beam. Consequently, when an alignment electron beam of precisely the cross-sec tional shape of the well is completely centered in the well a peak current flow is obtained which peak can be readily detected, either visually on a meter or electrically, as compared to the current flow when the electron beam is as much as a fraction of a micron out of coincidence with the well. The electrical current flow may be applied to suitable electronic amplifiers and servo control mechanisms to shift the member with respect to the alignment electron beam to precisely position them so that the alignment electron beam coincides with the well within a fraction ofa micron.

Once this precise alignment has been obtained, then the photocathode can be energized to project on the For a better understanding of the nature and objects of the present invention, reference should be had to the following detailed description of the drawings, wherein:

FIG. 1 is a fragmentary cross-sectional view of the photocathode-member exposure apparatus.

FIG. 2 is a plan view of a wafer on line II-II of FIG.

FIG. 3 is a fragmentary view in perspective of a member with relation to the aligning electron beam;

FIG. 4A is a schematic view of the movement of the aligning electron beam over the registration indicia;

FIG. 4B is a curve of the direct-current response;

FIG.- 4C is a curve of the alternating-current response;

FIG. 4D is a curve of the phase detector output as a result of the passage of the electron beam over the registration of the indicia;

FIGS. 5, 6 and 7 are plan views of three different indicia configurations, while FIGS. 5A, 6A and 7A are the respective direct-current output curves as an alignment electron beam passes thereover.

FIG. 8 is a block diagram of an automatic alignment circuit; and

FIG. 9 is a modified showing of a portion of the block diagram of FIG. 8.

DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. 1 of the drawing, there is illustrated apparatus 10 for the exposure of a member to an electron beam from a photocathode. Apparatus 10 comprises a hermetically sealed casing 12 ofa nonmagnetic metal or alloy provided with suitable removable end caps 13 and 14 to enable the photocathode and members to be introduced into the apparatus. An outlet 16 is provided in casing 12 for connection to a vacuum pump to enable a suitable vacuum to be produced within the apparatus for effective electron beam operation.

Supported from and disposed within the casing 12-13-14 is apparatus 20 for positioning a photocathode in a functionally effective relation to a member to be exposed to electron beams therefrom.

The apparatus 20 comprises a tubular support 22 which is preferably composed of a highly insulating material such as a glass, which may be partially metalized, to provide a means for holding the photocathode and the member in a cooperatively functional position. Disposed about the right hand of the tube 22 is a suitable gasket means 24 disposed in an external welled flange 25 of a deep dished member 26 for holding it in a fixed position. The dished member 26 is provided at its bottom with a flange 28 having a large central aperture 29 for receiving and holding a member support 30 in position thereagainst. A resilient clamp or latch (not shown) is provided to hold support 30 in such position. The member support 30 comprises a central depression 31 in which is placed a member 32, for example a silicon wafer, to be treated by an electron beam. Aperture 27 in member 26 enable the evacuation of the space between 26 and 30.

At the opposite or left hand end of the tubular member 22 is disposed a cylindrical gasket or packing 34 which fits tightly into an external welled flange portion 35 of a second deep dished member 36. The member 36 comprises at the bottom of the dished portion a flange 38 with a large central aperture 39 with means being provided whereby a photocathode 40 may be positioned and held firmly in place against the flange 38. Deep dished members 26 and 36 are of a nonmagnetic metal.

The photocathode 40 may comprise a disc of quartz or other material transparent to ultraviolet light or whatever radiation is being employed for activating the photocathode. The surface of the photocathode within the opening 39 is substantially planar and parallel to the surface of member 32 exposed through aperture 29. The photocathode surface comprises a selected pattern of opaque material areas 42 which prevent the ultravioletradiation passing therethroughfln between the relatively opaque areas 42 are areas 44 comprising a material which will emit electrons when exposed to suitable ultraviolet or other radiation. The opaque layer areas 42 may comprise a layer of titanium oxides of from 600 to 1,000 angstroms in thickness while the electron emissive material at areas 44 may comprise a film of palladium or gold ofa thickness of from 10 to 40 angstroms. A source of high potential 46 is applied to the members 26 and 36 which are electrically conducting so that the member 36 is at a potential of -10 kilovolts, for example, with respect to member 26. This potential difference is conducted to and impressed on the palladium or gold layer of the photocathode and on the member 32. This difference in potential will cause any electrons emitted by the palladium coating 44 to be directed to and accelerated toward the member 32.,A

source 50 of ultraviolet radiation such as a mercury lamp, provided with a reflector 52, if desired, is so disposed as to illuminate the back of the cathode 40 and this radiation passes through all areas except the opaque areas 42 whereby to cause electrons to be emitted only at areas 44. Focusing electromagnets 54 encircle the tube 22 to cause electrons emitted at areas 44 to spiral to a focus at the surface of the member 32. X and Y directional electromagnets or Helmholtz coils 56 and 58, at right angles to each other, are disposed about the casing 12-13-14 to enable the electron beam from areas 44 to be appropriately shifted over the surface of member 32 by suitable energization of the coils 56 and 58, so as to cause the electron beam pattern to impinge on selected areas of the member 32.

Referring to FIG. 2 of the drawing, there is shown in more detail the silicon wafer 32 as it is positioned in a depression 31 on support 30. The wafer 32 is provided with a flat 60 which rests on pins 61 and 62 provided in the support 30. A pin 63 is provided above the diameter parallel to the flat 60 in order that the wafer may be located by the pins 61, 62, 63 with an accuracy of the order of one mil orv better. A movable pin 65 provided with a compression spring 67 is provided approximately 120 from pin 63 to push and retain the wafer 32 firmly in position against pins 61, 62 and 63.

The wafer 32 is provided with a first conductive area 64 which comes in contact with a contact finger 66 such as a spring clip, when the wafer is in position while on the side opposite there is located a second conductive areas 68 with which another electric contact finger 70 engages. The conductive area 64 contains theregistration indicia 72 while the area 68 contains the indicia 74.

Referring to FIG. 3, there is shown a magnified portion partly in cross-section and in greaterdetail, of the metalized area 64 surrounding indicia 72. Metalized area 68 about indicia 74 will be essentially similar in arrangement and operation thereto. The wafer 32 is provided with an oxide layer 80 of a substantial thickness above about one micron which oxide layer is reduced in thickness to provide a well 82 of a selected shape for the indicia 72 at which the oxide is less than one micron in thickness and preferably of the order of 2000 to 5000 angstroms. These thicknesses apply to an aligning electron beam of kilovolts. With higher or lower potentials, the thicknesses will be proportionately changed. The well 82 can be reasonably precisely formed by etching out the oxide through a resist coating having an opening of desired shape or by subjecting the oxide coating to an electron beam of selected shape on a spot so that it is more soluble in a solvent by reason of the BEER effect, such that on subjecting the oxide to a solvent for silica the portion exposed to the electron beam is dissolved to the right depth while the rest of the oxide is much thicker. A layer of aluminum 84, of the order of less than 1 micron, for example 200 angstroms in thickness is deposited, as by evaporation in a vacuum, over the oxide layer 80 including the well 82 so as to provide a depressed well area coating 86. The width W of the well forming the indicia 72 as well as the height is formed to a desired degree of squareness. The spring contact finger 66, insulatedly fastened to support 30, pressing on and in good electrical contact with aluminum layer 84 is connected by a conductor 87 to a source of current 88, illustrated as comprising a direct-current battery, and includes in the circuit a current flow indicator 90 which may comprise an ammeter, a cathode ray tube or other equivalent means. The conductor 87 is connected from the opposite pole of the current source 88 to the bottom of the wafer in any suitable manner. To provide for an electrical connector, the support 30 may comprise a metal in direct contact with wafer 32, or a metal contact touching the wafer.

An aligning electron beam 92 having a square crosssection reasonably close in area to the cross-sectional area of the well 86, is projected from the photocathode 40 and caused to traverse over the portion 64. By reason of the relatively close positioning of the wafer 32 by the pins 61, 62, 63 and 65, the electron beam 92 will ordinarily impinge on at least a part of the well forming the indicia 72. By appropriate control of the focusing magnets 54, and deflection coils 56 and 58, as will be detailed hereinafter, both aligning electron beams may be made to coincide almost exactly with their respective indicia 72 and 74.

Suitable means may be provided for causing the electron beam 92 to move back and forth or to circle in a small radius with respect to the indicia 72 and this gives rise to output signals which enable exact coincidence thereof to be realized. Referring to FIGS. 4A, 4B, 4C and 4D of the drawings, there is illustrated at 4A the cross-sectional shape of a square indicia well 72 while in 48 there is illustrated the direct-current flow as indicated in the indicator means 90 as the square aligning electron beam 92 traverses the indicia 72 in the X direction from left to right, there being no movement in the Y direction. It will be noted that at the point in which the aligning electron beam precisely fills the entire square 72 from vertical side to vertical side the current will be at a maximum, on an oscilloscope this will be evident as a high point 0. At any other position, the indicator will indicate a lesser current. In tests made with apparatus wherein the indicia comprise the square 12 mils by 12 mils on a side, it was quite easy to position the maximum current point within less than 0.5 micron, and usually within 0.3 micron.

In practice, the electron beam 92 may be traversed first from left to right until a peak current flow point such as 0 is located, then the electron beam 92 is traversed up and down in the Y direction, while maintaining the X direction at this peak current point until a second peak current output point is located in the up and down direction, as shown at the right of FIG. 4A. The intersection of these two peak points constitutes the precise geometric center of the indicia 72 and this point may be found repeatedly with a precision of less than 0.5 micron, and often within about 0.2 micron.

FIG. 4C indicates the response when the aligning electron beam 92 oscillates in the X-direction at a suitable frequency during its traverse over the indicia mark 72, the current response assumes a pattern shown in the curve with the coincidence of the geometrical center of the aligning electron beam 92 and indicia 72 being defined by the drop to a minimum at V which has a level above the zero current base corresponding to twice the modulation frequency of the alternating current. When the alternating-current response is fed to a phase detector, the current output curve is that illustrated in FIG. 4D wherein the coincidence of the geometrical center of aligning electron beam 92 and indicia 72 is a zero current point.

Thus in any of FIGS. 4B, 4C or 4D, the current output response curves as the aligning electron beam reaches the center of alignment of the registration mark in the X direction exhibits a pronounced configuration from which it is quite readily ascertainable that the precise center of the mark has been reached. Similar curves will be obtained when the electron beam traverses the mark 72 vertically in the Y direction instead of horizontally or in the X direction. The coincidence of the points indicating the center of the mark 72 indicates the precise centering of the mark 72 with respect to the aligning electron beam. As a consequence, the beam 92 is properly positioned with respect to the X and Y direction.

The indicia mark 72 may be one of any various other shapes such for example as those shown in 5A, 6A and 7A. The current response curve 58 for the configuration in SA has a cusp-shaped point which has some advantages for determining the center point most exactly. A series of concentric circles shown in FIG. 6A produces a current output curve of the type shown in 6B.

A registration mark comprising a tapered arm cross as shown in FIG. 7A with arms 6 mils from end to end, when traversed by an electron beam of similar cross section shape gave the electrical signal curves in the X and Y directions, as shown in FIG. 7B. It is evident that it would be easy to position the alignment electron beam in the X position within a fraction of a micron of the exact geometric center in view of the relative great peaking change of the signal curve within the central ten micron dimension portion thereof.

Strictly speaking, it is not necessary that the alignment electron beam 92 be exactly centered in the indicia mark 72. The benefits of the invention will be obtained if the alignment electron beam produces the same curve of current response each time it passes over the indicia mark 72 with the peak of the curve being observed at the same point at all times, so that the position of the member 32 is determinable within 0.5 micron or better.

The current flow as indicated in the means 90 may be manually observed, for example the means 90 may be a sensitive ammeter or a cathode ray tube current trace. The electron beam or the wafer may be moved relative to each other, either by mechanically operating suitable micrometer adjustment control knobs so as to move the entire member holder 30 within the support 28, or by moving the photocathode 40 itself, until the peak reading is attained in the X and Y direction. Readily controllable means to effectuate the coincidence of the electron beam to the indicia marks would comprise applying correcting electrical currents by a suitable potentiometer to the coils 56 and 58 in order to adjust the position of the aligning electron beam 92.

Manual aligning of the member 32 with respect to the aligning electron beams may be employed for one of a kind devices or development projects.

Manual means for commercial or multiple member runs, however, are not only time consuming but are subject to possible human errors in the visual judgment of the operator of the peak of the current response curves such as shown in FIGS. 4B or 4C, or the zero current point at 4D. Consequently, it is highly desirable to employ suitable electronically controlled and operated alignment means responsive to the flow of current in the conductor 87. Not only can the peak response position be ascertained more rapidly, but this is done more accurately with the same exact point indicated each time alignment is carried out on the same member 32 by electronic controls. Suitable electronic devices to do so are applicable here and several illustrative examples of suitable electronic devices and circuitry will be disclosed herein.

It will be appreciated that while the member 32 is accurately positioned in the X and Y directions with respect to indicia 72, the member 32 may not be in the exact angular position about 72 as a center, and it probably will require a slight rotational adjustment by a small angle of the electron beam pattern with respect to member 32 for complete exactness of coincidence of the entire patterned electron beam from the photocathode with the selected areas on member 32 to be exposed thereto.

To accomplish this latter rotational adjustment, a second aligning electron beam is directed at the registration indicia 74 which is on the opposite edge of the member 32 from indicia 72. If the signal generated by the second aligning beam and the indicia 74 is applied to provide a correction to the focusing magnets 54, this signal can also be employed at the same time to send correcting currents into the coils of the magnets 54 so as to enlarge or reduce the separation of the electron beams until they coincide almost exactly with their respective registration indicia 72 and 74.

In order to provide an electronically controlled servo mechanism to automatically align the member 32 in an exact position, in one embodiment of the invention, sinusoidal signals in quadrature are applied to coils 56 and 58 to impart a small circular scan to the aligning electron beam. This results in an alternating-current signal (such as in FIG. 4C) being generated in the conductor 87 at indicia 72 and a corresponding signal in the conductor associated with indicia 74. The signals are amplified and then synchronously rectified by dual detectors working in phase quadrature. Since two dual detectors are used, there are total of four error signals produced.

From the first dual detector there are derived two error signals, namely for the mismatch along the X and Y perpendicular axes between the aligning electron beam 92 to indicia mark 72 The X and Y error signals each are employed to operate separate integrators or servo mechanisms which provide signals to correct and hold the position of beam 92 on indicia 72, via power amplifiers which drive the deflection coils 56 and 58.

Rotation and size error signals are derived from the second dual detector for the mismatch of the other aligning electron beam with indicia 74. These last error signals are also at mutually perpendicular axes, namely along the line joining 72 and 74, and the perpendicular to this line. These error signals are used to operate separate servo mechanisms or integrators which control the current in coils 54, so that one servo mechanism rotates the entire electron beam pattern about 72 as a center until the second aligning beam has its center on the line joining 72 and 74, while the other servo mechanism or integrator changes the current gradient in coils 54 so that this moves the second aligning electron beam either in or out along the line joining 72 to 74 until the second aligning electron beam is actually centered in indicia 74.

Because of the inherently accurate size control of the entire electron beam image, it has been found that often no significant magnification error is present and the fourth error correction need not be made.

Referring to FIG. 8 of the drawings, there'is illustrated in a block diagram the electronic apparatus for adjusting the aligning electron beams with respect to the marks. 72 and 74 and thereby aligning the member 32 in an exact position with respect thereto and thereby exactly positioning on member 32 the entire electron beam pattern from photocathode 40. Briefly, the X and Y directional signals produced in the conductor 87 are conveyed to a preamplifier which amplified signal is then conveyed to a tuned amplifier 102 whose output passes to a phase adjustor 104 and a dual phase detector 106. A gated oscillator 108 impresses a signal through conductors 110 and 112 via conductors 111 and 113, on the dual phase detector106 whose outputs comprise X-error signals in conductor 114 and Y-error signals in conductor 116, which pass through gate 118 to integrators 124 and 128. The integrators 124 and 128 have direct-current outputs which are modulated alternating current from the oscillator 108 via leads 110 and 112, and then pass to and adjacent the controls in power units (not shown) of the type customarily used to control the magnetic coils, in this case the Helmholtz coils 56 and 58.

Similarly, the alternating-current signal from the conductor 89 at the registration indicia 74 is preamplified at and passed to the tuned amplifier 142 from which it passes to the phase adjustor 144, thence to dual phase detector 146. Two outputs from dual phase detector 146 are produced, one via conductor 148 is a 6 error signal which passes through gate 152 to control a motor-driven precision potentiometer 154 which effects the rotational control of the electron beam pattern by appropriately increasing or decreasing the current to the coils 54, while the other output via conductor 150 and gate 152 controls the size of the electron beam pattern through a motor driven gang potentiometer 155 which adjusts the main focus field.

The error signals in conductors 114, 116, 162 and 163 are cross-fed electronically into a four input delayed null detector 156 whose output is conveyed by lead 164 to a set-reset flip-flop 166. The operation of the flip-flop is initiated by the start means 170, whereupon current begins to flow via lead 167 to a conductor 168 which energizes the ultraviolet source 60 to cause electron beams to be emitted from photocathode 40,

including the two aligning electron beams. Likewise, current from 167 passes through conductor 169 to the gate of oscillator 108 which feeds sinusoidal signals in quadrature through lines 110 and 112 to the X and Y controls 126 and 130, respectively, which causes the entire electron beam pattern, including the aligning electron beams, to oscillate in a circle of, for example, 6 microns diameter at a frequency of 45 Hertz. Once the aligning electron beams are centered in indicia marks 72 and 74 by operation of the integrators 124 and 128 and potentiometers 154 and 155, the error signals passing through leads 158, 160, 162 and 163 reach a zero value (as illustrated in FIG. 4D), as determined by the null detector 156 which thereupon produces a signal which passes through lead 164 to the flip-flop 166 which then terminate the operation of the gated oscillator 108 and closes gates 118 and 152. Then the time sequence of the electron beam exposure of the full area of member 32 begins until a suitable exposure of the electron resist on the entire member 32 takes place. A high inherent pattern fidelity is attained so that no correction to scale has been found necessary in practice.

Referring to FIG. 9, there is shown a simplified block diagram of a portion of the electronic alignment controls. The output of the integrating amplifiers 124 and 128 control the deflection amplifier, while potentiometers control orientation and size of the electron beam pattern.

The photocathode 40 has been generating an electron beam from all the emissive areas 44, but the alignment period is so brief that the electron resist on all the areas of the member 32 has not been appreciably affected. A period of from 3 to seconds is usually adequate to produce a sufficient electron beam treat ment of the electron resist to cause it to be properly differentially soluble in selected solvents.

It will be appreciated that the X, Y and rotational or theta error signal outputs may be applied to precision motor driven means for moving the support 30 as is shown in the previously referred to patent application Ser. No. 869,229. The output from the servos can control the operation of the X, Y and 0 direction motors while the output from the size control servo operates a potentiometer which controls the current in the field coils 54 to determine the size of the alignment electron beam pattern.

The alignment electron beams are preferably derived from the photocathode 40 by providing thereon an area 44 at points related to and corresponding to the marks 72 and 74.

In the event that manual means are employed for alignment of the member to the photocathode, or for other reasons, it is possible to irradiate only the selective areas of the photocathode from which the aligning electron beam is to be generated, with a small spot ultraviolet light, leaving the rest of the photocathode unirradiated. For example, a light pipe comprising a quartz rod may be employed to convey a small beam of ultraviolet light solely to the point where the aligning electron beam (or beams) is to be generated on the photocathode. The ultraviolet source in this last case would be something other than the source 50. In some cases a mask can be moved in to cover the back of the photocathode 40, leaving only two small openings for the ultraviolet light to impinge upon the areas where an aligning electron beam is to be generated.

While two indicia marks cooperating with two aligning electron beams are disclosed in the specific embodiment and excellent results are obtained when they are placed at opposite sides of member 32, the indicia marks 72 and 74 may be disposed at other positions. More than two indicia are usable, but no particular benefits will be obtained as compared to only two indicia. A single mark in the shape of a cross mark such as in FIG. 7 can be employed, with rotation employed after X and Y axis centering is secured until a notching aligning electron beam indicates that the member 32 is rotated so that the arms of the cross in the electron beam is positioned precisely in the arms of the cross mark on the member. However, the accuracy of the rotational adjustment cannot be as exact as with two indicia placed on the opposite side of the wafer.

The registration indicia 72 and 74 may be placed upon the member 32 either as a separate operation or as a part of the first treatment of the member 32 with an electron beam pattern from a first photocathode. To accomplish this last, a silicon wafer is oxidized over its entire surface, an organic electron resist is applied over the oxide on one face of the wafer, and after removing sufficient oxide form the bottom of the wafer so that electrical contact may be made to the conductive wafer surface, it is placed in the wafer support member 26 and simply exposed to an electron beam from the photocathode, without attempting any special alignment. The photocathode has at least one pair, but may have a plurality of pairs, of aligning electron beams projected from the photocathode at the same time. When the electron resist is then treated to remove it preferentially, at least one pair of alignment apertures points are also produced. Upon applying a solvent for silica, such as a hydrogen fluoride solution, the silica is removed to the bare silicon at these alignment aperture points, thereby leaving wells in the oxide. Thereafter, all the organic electron resist is removed. Re-oxidation of the silicon to produce a second coating of oxide is carried out to provide a thinner layer of oxide in each of the pairs of alignment wells. The operator may select any one pair of the wells for use in subsequent aligning procedures, and after masking the rest of the wafer 32 will evaporate or otherwise apply a conductive layer 64 over the area adjacent this selected pair of holes. When the so treated wafer is placed in the support 30, and

into the apparatus 10, the second photocathode will have one or more pairs of aligning electron beams, at least one of which corresponds to this pair of marks so selected earlier. The second photocathode pattern must be aligned with the first photocathode pattern and therefore an automatic electronic alignment means such as is shown in FIG. 8 is employed.

As the member 32 is treated several successive times with electron beams, chemically etched, oxides removed in part, and/or oxidized, the original pair of alignment marks 72 and 74 may deteriorate. A second pair of registration marks may be selected and used because they are superior. Each photocathode may project a series of pairs of aligning electron beams, only one pair being in use at any one time. Alternatively, at the third or fourth or later electron beam treatment, a new pair or pairs of registration indicia are created by the third or fourth photocathode by applying a new and different pair of aligning electron beams upon the electron resist, concurrently with the projection of the aligning electron beams actually used to orient and align the member 32 with reference to the registration marks made at the first electron beam exposure. During subsequent processing, the new pair of registration marks are made effective by providing new wells and applying a conductive coating over the new pair of wells while the original marks are either eliminated or simply have the conductive layer 64 removed. At the subsequent processing in apparatus 10, the photocathode will project aligning electron beams complementary to the position of the new pair toward such new registration indicia and the electronic means will properly orient the electron pattern by reference to these new indicia.

We claim as our invention:

'1. In the process of aligning an electron beam pattern projected from a photocathode with selected areas of a member with a high degree of precision by reference to at least two relatively widely separated registration indicia of predetermined shape formed on the member, the steps comprising:

a. causing separated aligning electron beams to be projected from the photocathode for each registration indicia on the member, each aligning electron beam being of substantially the same cross sectional shape as its respective registration indicia,

b. causing an electrical signal to be developed at each indicia proportional to the amount of the area of the registration indicia impinged by its respective aligning electron beam,

c. moving the alignment electron beam with respect to its respective registration indicia so that the electrical signal therefrom varies,

d. positioning one aligning electron beam at the point where the electrical signal indicates the optimum coincidence of the alignment electron beam with its respective registration indicia, and

e. using the position of said one aligning electron beam with its respective registration indicia as a fixed center, electromagnetically moving the second aligning electron beam both in an arc and radially with respect to said center until the resulting electrical signal indicates optimum coincidence of said second alignment electron beam with its respective registration indicia whereby both the shape and area of the photocathode pattern corresponds precisely to the selected area of the member. I

2. The process of claim 1 wherein the electrical signals derived from the two indicia when the aligning electron beams impinge thereon operate electromagnetic means for shifting each of the electron beams with respect to the indicia so as to center each aligning electron beam in the indicia and coincide in their crosssections. a

3. The process of claim 2 wherein the electrical signals derived from the indicia when the aligning electron beams impinge thereon control an electromagnetic means to shift one electron beam in the X and Y directions and the other electron beam and the photocathode electron beam is rotated in the 0 direction, using the first registration indicia as a center, with respect to the indicia.

4. The process of claim 3 wherein the relative motion of the aligning electron beam to the registration indicia is terminated and the alignment electron beam is held at the point of maximum coincidence thereof.

5. In apparatus for exposing precisely located areas of a member to an electron beam pattern, a photocathode source of a beam of electrons including at least two separated alignment beam portions of predetermined cross-sectional shape, means for positioning the member in a spaced relation to the photocathode source of the beam of electrons, means for applying a potential between the member and the source whereby electrons from the photocathode source are directed to and impinge on the member, electromagnetic means for directing the beam of electrons from the source so as to cause the beam of electrons to impinge on a selected portion of the member close to the desired precisely located area, the member having at least two separated registration indicia in its surface, each registration indicia being of a shape closely similar to the cross-sectional shape of the alignment beam portion, the registration indicia giving rise to an electrical signal when an alignment beam portion impinges thereon, the electrical signal varying with the amount of the area of impingement, the electromagnetic means directing each of the alignment beam portions into the vicinity of a respective registration indicia on the member and into approximate coincidence thereof, and electronic circuit means responsive to said electrical signals to effect a fine adjustment of the position of each of the respective alignment beam portions to the registration indicia of the member so as to cause the electron beam portions to substantially coincide with the respective registration indicia whereby the beam of electrons from the photocathode is precisely located and oriented with respect to the member so that precisely located areas thereof can then be exposed to the electron beam from the photocathode.

6. The apparatus of claim 5, wherein the means to produce electrical signals proportional to the degree of coincidence of the alignment electron beams with their respective registration indicia includes a first registration indicia producing electrical signals upon impingement of the aligning electron beam thereon which affect the electronic circuit means to actuate the electromagnet means to move that aligning electron beam in X and Y directions with respect to the first registration indicia so as to cause the first aligning electron beam to coincide substantially exactly with the registration indicia, and the second registration indicia producing electrical signals to affect the electronic means to actuate the electromagnet means to rotate the second aligning electronic beam in an are about the first registration indicia as a center whereby to cause the second electron beam to coincide substantially exactly with the second registration indicia, thereby effecting substantial exactness of impingement of the entire source electron beam on the desired areas of the member.

7. The apparatus of claim 6, wherein means are provided for oscillating the alignment electron beams so as to provide for varying amounts of coincidence of the alignment electron beams with their respective registration indicia, thereby producing varying electrical signals related to the amount of coincidence, the resulting variable electrical signals controlling the actuation of the means to effect a fine adjustment so as to cause the alignment electron beams to approach and locate a point of optimum coincidence with their respective registration indicia.

8. ln apparatus for exposing precisely located areas of a member to an electron beam pattern, a photocathode source of a beam of electrons including at least one alignment beam portion of predetermined cross-sectional shape, means for positioning the member in a spaced relation to the source of the beam of electrons, means for applying a potential between the member and the source whereby electrons from the source are directed to and impinge on the member, electromagnetic means for directing the beam of electrons from the source so as to cause the beam of electrons to impinge on a selected portion of the member close to the desired precisely located area, the member having at least one registration indicia in its surface of a shape closely similar to the cross-sectional shape of the alignment beam portion, the electromagnetic means directing the alignment beam portion into the vicinity of the registration indicia on the member and into approximate coincidence thereof, means for oscillating the alignment electron beam with respect to the registration indicia to produce varying amounts of coincidence thereof, means for generating a variable electrical signal proportional to such coincidence, means to effect a fine adjustment of the position of the alignment beam to the registration indicia of the member so as to cause the electron beam to substantially coincide with the registration indicia, and means operable by the variable electrical signal for controlling the means to effect the fine adjustment whereby to control the means for oscillating the alignment electron beam to cause the alignment electron beam to approach and locate a point of optimum coincidence with its associated registration indicia whereby the member is precisely located with respect to the beam of electrons from the photocathode and the precisely located areas thereof can then be exposed to the electron beam from the photocathode.

9. The apparatus of claim 8, wherein the fine adjustment means terminates the operation of the oscillating means at the time of maximum coincidence of the alignment electron beams with their registration indicia.

10. In an apparatus for precisely aligning a patterned beam of electron beam radiation which includes two aligning radiation beams, with a selected area on a member having two registration area marks thereon complementary to the aligning beams, means for directing the point of impingement of the patterned electron beam of radiation on the member, electrical circuit means including signal generators associated with each of the marks producing signals varying with the degree of coincidence of each beam to its registration mark, the circuit including electrically operable means to cause the beam of radiation to move in a controlled path, an oscillation generator connected to said electrically operable means to cause the aligning beams to move in such predetermined path with respect to the registration marks, whereby a variable signal is produced, electronic means responsive to the variable signals to produce error signals, and servo mechanisms responsive to the error signals to said electrically operable means to cause each of the aligning beams to approach coincidence with their respective registration marks, and means in said circuit means to terminate the operation of the oscillator generator when coincidence of the aligning radiation beam to their respective registration marks is reached.

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Referenced by
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Classifications
U.S. Classification250/311, 250/491.1, 250/492.2, 148/DIG.102, 250/440.11
International ClassificationH01J37/304, G01N25/48, H01L21/00
Cooperative ClassificationH01J37/3045, Y10S148/102, H01L21/00, G01N25/4813
European ClassificationH01L21/00, G01N25/48A2, H01J37/304B