US 3842544 A
Description (OCR text may contain errors)
United States Patent 11 1 Paola [451 Oct. 22, 1974 l FIXTURE FOR LAPPING AND POLISHING SEMICONDUCTOR WAFERS  Inventor: Carl Ralph Paola, Westfield, NJ.
 Assignee: Bell Telephone Laboratories,
Incorporated, Murray Hill, NJ.
 Filed: Nov. 15, 1973  Appl. N0.: 415,946
 US. Cl 51/129, 51/232, 51/281  lnt. Cl B24b 7/00, B24b 47/02, B24b 1/00  Field of Search 51/124, 129, 131, 237, 51/132, 281, 281 SF; 156/17  References Cited UNITED STATES PATENTS 2,883,803 4/1959 Stead 51/131 2.983.086 5/1961 La Chapelle 51/237 R 3,090,169 5/1963 Bocttcher Reisman et al. 51/384 X Dayetal. ..51/131 Primary ExaminerOthell M. Simpson Attorney, Agent, or FirmP. V. D, Wilde; W. L. Keefauver  ABSTRACT A fixture for lapping and polishing semiconductor materials is described. Fluid-bearing principles are utilized to support the fixture above the surface of a lapping or polishing wheel. Use of such an arrangement results in a high degree of flatness and parallelism and permits only the surface of the semiconductor material to be in contact with the lapping or polishing wheel, thereby reducing the possibility of transfer of contamination from the fixture to the semiconducting material.
8 Claims, 1 Drawing Figure FIXTURE FOR LAPPING AND POLISHING SEMICONDUCTOR WAFERS BACKGROUND OF THE INVENTION 1. Field of the Invention The invention is concerned with fixtures used for polishing and lapping semiconductor materials and, in particular, for polishing and lapping high purity materials, such as gallium arsenide and gallium phosphide wafers, to achieve a high degree of flatness and parallelism and to maintain exceptionally high purity.
2. Description of the Prior Art Work holder assemblies or fixtures that are commonly used today in the semiconductor industry to lap and/or polish semiconductors wafers have two common features: (1) a solid cylindrical piston, which is usually a metal or an inert ceramic such as Al O to which the wafer is secured; and (2) a flanged hollow cylindrical sleeve, which vertically guides the solid piston. The wafer is secured to the end of the piston, usually with wax, and the piston is then inserted into the sleeve such that the surface of the wafer to be mechanically lapped or polished is in contact with a rotating planar lapping or polishing wheel. In general, the primary purpose of the flanged portion of the sleeve is to maintain the perpendicularity of the piston (with mounted wafer) in order to prevent nonparallel lapping or polishing of the wafer. As a result, however, the
flanged portion of the fixture is also in contact with the polishing or lapping wheel during operation. Due to the contact between the flanged portion and the wheel, contaminating impurities from the flanged portion can be transferred during the lapping or polishing operation to the wafer. While the contaminating impurities usually tolerated in processing semiconductor wafers, present use of gallium arsenide having an exceptionally low doping level (e.g., impurity concentrations of from atoms/cm to 10 atoms/cm requires that all potential sources of contamination be eliminated or controlled.
Attempts have been made to overcome the impurity transfer problem by using three standoff pods, having approximately the same composition as the wafer and mounted 120 apart symmetrically around the bottom surface of the flanged portion. However, nonparallel lapping or polishing of the wafer has been observed to occur, due to the difficulty in achieving coplanarity between the surfaces of the pods and the surface of the wafer.
SUMMARY OF THE INVENTION In accordance with the invention, contamination of low-impurity level gallium arsenide or gallium phosphide and nonparallel lapping and polishing is reduced by use of a novel fixture for lapping and polishing. Fluid-bearing principles are utilized to support the fixture above the surface of the lapping or polishing wheel. Such an arrangement ensures that only the surface of the wafer to be lapped or polished is in contact with the lapping or polishing wheel. Furthermore, the flatness of the lapped or polished surface achieved is at least equal to that achieved using present fixtures.
BRIEF DESCRIPTION OF THE DRAWING The FIGURE is a perspective view, partly in section, of a fixture for polishing or lapping semiconductor wafers in accordance with the invention.
DETAILED DESCRIPTION OF THE INVENTION The Drawing is discussed in terms of the preferred embodiment. Other designs of the fixture utilizing fluid-bearing principles may also be evisioned, such as fixtures for lapping or polishing several wafers at a time, etc.
The FIGURE shows a fixture or work holder assembly 10 for polishing and lapping semiconductor wafers. As is well-known from the prior art, such a fixture comprises a (1) disk-shaped pad 12 having a flanged portion or pad 12a and a hollow cylindrical sleeved portion 12b surrounding an opening in the pad, and (2) a solid cylindrical piston member 11 slidably mounted within the sleeve portion, which vertically guides the solid piston. A wafer 13 is secured to the bottom of the piston by an adhesive 14, such as wax. The bottom surface of the flanged portion defines a plane that is parallel to the plane of the wafer that is to result upon polishing. During the lapping orpolishing operation, the bottom surface of the wafer contacts a wheel or disk 15, which is rotated (by means not shown) at some controlled velocity, parallel to the bottom surface of the flanged portion. In order to avoid non-uniform wear of the wheel during the lapping or polishing operation, an arm 16, one end of which encircles the sleeve portion and the other end of which is attached to oscillating means (not shown), may optionally be employed to move the fixture back and forth across the rotating wheel. As is well-known, an abrasive, such as a slurry of an oxide or a carbide, is employed during the lapping or polishing operation to either reduce the thickness of the wafer or to give a smoother finish to the wafer surface, respectively. Alternatively, it may be preferable to use a diamond-impregnated polytetrafluroethylene coated wheel, as described in H. Christensen, Ser. No. 295,423, filed Oct. 6, 1972, or a commerically available diamond impregnated high purity metal wheel (e.g., nickel).
In accordance with the invention and departing from the prior art, the flanged portion includes a plenum chamber 17 into which a fluid is introduced by inlet means 18. A hose 19 connects the inlet means to an external supply of the fluid (not shown).
In the inventive fixture, the fluid flows into the plenum chamber at a controlled pressure. The fluid may be a gas, such as air, oxygen, nitrogen, etc., or a liquid, such as water. A series of apertures 20, the dimensions of which are exaggerated here for purposes of illustration, are symmetrically disposed about the bottom of the flanged portion. The apertures permit the fluid to be introduced such that the pressure profile under the flanged portion is uniformly distributed between the surface of the flanged portion and the rotating wheel, thereby lifting the pad above the wheel to create a uniform gap in order to reduce the possibility of contamination transfer from the flanged portion to the wafer. The small size of the apertures (e.g., about 0.01 inches in diameter) serves to increase the velocity of the fluid as the fluid enters the gap between the surfaces defined by the flanged portion and the lapping or polishing wheel. The high velocity fluid then exhausts to atmosphere pressure. While the inventive results may be obtained using a flat surface across the bottom of the flanged portion, it is preferable to employ a series of grooves 21 and outlet means 22 symmetrically disposed about the bottom surface of the flanged portion and radiating outward for purposes of stabilization. Such stabilization aids in ensuring flatness in the lapped or polished wafer.
The high velocity fluid, lifting the pad creates an effective spring constant, which is a measure of the stiffness of the fixture, that is, the pounds of force per inch the fixture can accommodate without becoming unstable or without causing closure of the gap. The dimension of the gap is determined by the pressure beneath the fixture, which is a function of l) the mass and geometry of the fixture, (2) the fluid inlet pressure, and (3) the exhaust velocity of the fluid.
In order to avoid contamination transfer from the flanged portion to the wafer as a result of contact between the abrasive particles and the flanged portion, the gap must be at least a certain minimum value. The minimum value is dictated by the size of the abrasive particles used for polishing or lapping. Since the particle sizes may have a distribution about some nominal value, it is preferred to maintain the gap at a value equal to at least 2.5 times the nominal particle size. That value is believed to be a safe margin in preventing any contact between the abrasive and the flanged portion.
It has been observed that particles larger than 1 micrometer (about 0.04 mils) may tend to be trapped on the surface of the material being lapped or polished and thus may serve as a contaminating impurity. Accordingly, it may be necessary to ensure that the fluid is filtered so that particles no larger than 1 micrometer are carried along by the fluid.
Due to the necessity for maintaining a low level of impurities in the semiconductive wafer, the fluid must also be of high purity. For gases employed as the fluid, high purity is conveniently achieved in using the same level of purity as in the wafer preparation-typically 99.9999 percent pure. For liquids employed as the fluid, high purity is conveniently achieved by using electronic grade fluids or, in the case of water, 18 megohm water filtered to remove bacteria (e.g., 0.3 micrometer filter).
EXAMPLE A fixture substantially as shown in the FIGURE was constructed. The overall flange diameter was 4 inches and there were 32 apertures having a diameter of 0.0 1 inches. The apertures were positioned at both ends of 16 stabilization grooves spaced 225 apart. The grooves had a width of 0.020 inches and a length of 0.82 inches and radiated out from the center of the fixture. Eight additional grooves, offset from the 16 grooves by 1 125 and spaced 45 apart and having a width of 0.020 inches and a depth of 0.020 inches, served as outlet means. The cylindrical piston had a diameter of about 1.6 inches. Dry filtered air at an inlet pressure of psig was observed to produce a gap of between 0.015 inches and 0.020 inches.
To demonstrate the flatness that can be achieved with the inventive fixture, GaP wafers (1 inch diameter) were lapped on a diamond-impregnated (0.3 micrometers) polytetrafluoroethylene-coated wheel having a diameter of 10 inches and rotated at 75 rpm. Dry
filtered (l micrometer filter) air at 1 psig and 10 psig and distilled deionized filtered (0.45 micrometer filter) water at 15 psig inlet pressures were variously employed as the fluid. In all cases, a flatness of less than $0.025 mils across the wafer surface, as measured by optical interferometry, was achieved. This value is to be contrasted with a value of about 10.1 mil, achieved with a fixture conventionally used in lapping GaP and GaAs wafers.
What is claimed is:
1. Apparatus for removing material from a major surface of a semiconductor wafer comprising:
a. an abrasive disk;
b. means for rotating the disk;
c. a work holder assembly mounted above the surface of the disk; and
(1. means for moving the assembly reciprocally across the surface of the disk, the work holder assembly comprising:
a. a disk-shaped pad having:
1. at least one opening through its thickness;
2. a sleeve portion around the opening;.and
3. a surface, at least a portion of which defines a plane parallel to the plane of the wafer that is to result upon removal of the material; and
b. a cylindrical piston, to which the wafer may be secured, slidably mounted in the sleeve portion so as to bring the major surface of the wafer in contact with the abrasive disk, characterized in that the pad includes means for introducing a fluid between the surface of the pad and the rotating abrasive disk at a pressure sufficient to maintain the pad above the disk at a gap equal to at least 2.5 times the particle size of the abrasive, the means for introducing the fluid including:
a. a plenum chamber within the pad;
b. an opening in the pad for introducing the fluid into the plenum chamber; and
c. a multiplicity of openings in the bottom surface of the pad for introducing the fluid between the surface of the pad and the disk.
2. The apparatus of claim 1 in which the bottom surface of the pad additionally comprises groove means for stabilizing the work holder assembly as the fluid exhausts.
3. Method for controllably removing material from a major surface of a semiconductor wafer comprising:
a. mounting the wafer on a portion of a surface of a work holder assembly, leaving a substantial portion of the surface of the work holder assembly exposed, the wafer being mounted so as to expose the major surface of the wafer from which material is to be removed to a rotating abrasive containing material removing disk;
rotating disk; and introducing a fluid between the surfaces of the work holder assembly and the rotating disk at a pressure sufficient to maintain the surface of the work holder assembly controllably spaced from the rotating disk, whereby a high degree of parallelism and flatness of the wafer is achieved and a high degree of purity of the wafer is maintained.
4. The method of claim 3 in which the fluid is introduced between the bottom surface of the work holder assembly and the rotating disk from a plenum chamber in the work holder assembly.
contacting the exposed surface of the wafer by said assembly is controllably spaced from the rotating disk at a gap equal to at least 2.5 times the particle size of the abrasive.
8. The method of claim 3 in which the surface of the rotating disk comprises a diamond-impregnated polytetrafluoroethylene coating.