METHOD OF PRODUCING WAFERS

Abstract: There is disclosed a method of providing an elongated stock; producing a wafer comprising the steps of processing one end face of an elongated stock to form a first flat surface; cutting through one end portion of the stock transversely by a cutter to provide a slice of a predetermined thickness having opposite side faces defined respectively by the first flat surface and a cut surface subjected to the cutting; and processing the cut surface of the slice to form a second flat surface, using the first flat surface as a reference surface, thereby providing the wafer.

Description: The invention relates to a method for producing a flat large size semiconductor wafer from a stock of silicon or gallium arsenide. US-A-23 82 257 discribes a method of cutting of accurately oriented blanks for use in the manufacture of oscillators or resonators from bars or the like of quartz or other suitable crystals exhibiting the phenomenon of piezo-electricity. The cutting of slices is performed in order to ensure the maintenance of orientation when slices of blank thickness dimensions are cut directly from the original lumps or boulders. The main purpose of said known process is to provide for the maintenance of the original orientation of the quartz crystal. This orientation is maintained throughout the cutting operations. When one face has been properly corrected, then the other face may be corrected with reference to that by bringing it into parallelism thereto. The cutting surface has to lie absolutely in the desired orientation plane. US-A-40 84 354 discloses a process for slicing boules of single crystal material into wafers by grinding so that the longitudinal boule axis corresponds to the crystallographic orientation axis of the boule. The boule is mounted in a fixture and is aligned so that the common longitudinal axis and crystallographic orientation axis is perpendicular to the saw blade. The boule is then rotated while maintaining the orientation of the combined common axis and engaged against an inner diameter rotating saw blade for a time which is sufficient for the blade to slice through the boule and form a wafer. This wafer is polished without the conventional lapping step to form a wafer having a surface that is substantially flat and parallel. The material of said known wafer is a single crystal material such as gadolinium gallium garnet (GGG) and sapphire. This wafer material is suitable for being polished without an intermediate lapping step. In conventional manufacture processes of wafers of a semiconductor material such as silicon or gallium arsenide, a rod-like stock is successively cut into slices to produce the wafers. Generally, the rod-like stock is cut by a cutter of the type having a thin cutter blade of an annular shape having an inner peripheral cutting edge as disclosed in the before-mentioned US-A-40 84 354. More specifically the rod-like stock is introduced into the annular cutter blade in a direction perpendicular to the annular cutter blade. Then the rod-like stock is transversely moved relative to the annular cutter blade which rotates at a high speed about an axis thereof so that the rod-like stock is brought into cutting engagement with the inner peripheral cutting edge and is cut to produce a slice serving as a wafer. The wafer thus cut from the rod-like stock is slightly warped due to the inherent inaccuracy of the above-mentioned cutter, a variation in accuracy of attachment of the annular cutter blade and a variation in balance of cutting resistances offered respectively by opposite inner marginal portions of the annular cutter blade around the inner peripheral cutting edge. It has been desired to eliminate such warp. The degree of warp depends on the material of which the wafer is made and the thickness and diameter of the wafer. Generally such warp is on the order of several micrometers to several tens of micrometers. The warped wafer sliced from the rod-like stock has curved opposite side faces. Even if it is desired to process one of the opposite side faces of the warped wafer into a flat surface using the other side face as a reference surface, this cannot be done since the opposite side faces are curved. The warp of the wafers which develops when the wafer is cut from the rod-like stock by the annular cutter blade adversely affects the dimensional accuracy of the final product. This problem is serious, particularly where the semiconductor wafer has a relatively large diameter with a highly dense circuitry fabricated thereon to provide an integrated circuit. Thus the warp of the wafer presents an obstacle to a highly dense integrated circuit. It is therefore an object of this invention to provide a method of producing a flat large size semiconductor wafer from a stock of silicon or gallium arsenide, of which warp is reduced to a minimum to markedly enhance the flatness of the wafer. According to a first solution of the present invention (claim 1) there is provided a method for producing said wafer comprising the following steps: a) processing one end face of said stock to form a first flat surface having a predetermined roughness not more than 0.2 µm and a predetermined flatness not more than ± 1.0 µm, b) cutting through the end portion of said stock defined by said flat surface transversely by a cutter to provide a large diameter slice of a predetermined thickness having opposite side faces defined respectively by said first flat surface and a cut surface resulting from said cutting, c) processing said cut surface of said slice to form a second flat surface parallel to said first flat surface using said first flat surface as a reference surface and d) chemically etching both surfaces of the semiconductor wafer for improving flatness and consecutively improving the warp of the semiconductor wafer to a value not more than 3 µm. The object of the invention is also solved by features a) to d) of claim 7. Further embodiments are included in claims 2 to 6. There is prepared an ingot or stock of a material such as silicon monocrystal in the form of a rod of a circular cross-section, and one end face of the rod-like ingot is processed into a flat surface with high accuracy by polishing, grinding or cutting. Then, the rod-like stock is cut by a cutter of the type having a thin cutter blade of an annular shape having an inner peripheral cutting edge around an inner periphery thereof. More specifically, the rod-like stock is introduced into the annular cutter blade in a direction perpendicular to the annular cutter blade, and then is transversely moved relative to the annular cutter blade which rotates at a high speed about an axis thereof, so that the rod-like stock is brought into cutting engagement with the inner peripheral cutting edge and is cut transversely to produce a slice of a predetermined thickness to provide a wafer. The slice has opposite side faces defined respectively by the aforesaid flattened surface and a cut surface subjected to the cutting by the annular cutter blade. The cut surface of the slice is not flat but is curved for the reasons mentioned above. The cut surface of the slice is then subjected to flattening as by polishing, grinding or cutting, using the aforesaid flattened surface as a reference surface. The opposite flat side faces of the resultant wafer are parallel. The flattening of the cut surface is carried out, for example, by mounting the slice on a vacuum chuck which holds the slice against movement by suction, and then processing the cut surface by a diamond tool into a flat surface. Thus, since one flat side face of the slice is used as a reference surface for effecting the flatting of the other side face of the slice, the other side face of the slice can be processed into a high flatness. In this manner, the wafer having substantially no warp and hence substantially no curved side faces is obtained. However, in the event that the wafer thus obtained has a slight warp due to either a difference in the amount of the flattening operation between the opposite side faces of the slice or a difference in the amount of strain of the opposite side faces developing during the flattening operation, such a slight warp is removed by stress-relieving annealing or chemical etching, thereby improving the flatness of the opposite side faces of the wafer. For carrying out the above processings to obtain the wafer having the opposite flat side faces, the cutter of the type described having the inner peripheral cutting edge and capable of a surface cutting, a surface grinding or a surface polishing may be arranged in series with a surface cutting machine, a surface grinding machine or a surface polishing machine so as to provide a processing line. Also, one or more of washing or rinsing steps may be arranged along the processing line so as to remove chips or cuttings produced from the stock and the slice, in which case one or more of drying steps may be provided along the processing line. The invention will now be illustrated by way of the following Example: There was prepared an ingot of silicon monocrystal in the form of a cross-sectionally circular rod having a diameter of 150 mm. One end face of the rod-like ingot was ground by a diamond cutter into a flat surface having a surface roughness of 0.2 µm and a flatness of ± 1 µm. Then, the rod-like ingot was transversely cut therethrough at one end portion thereof by a cutter to provide a slice of a thickness of 0.785 mm having opposite side faces defined respectively by the flat ground surface and a cut surface subjected to the cutting, the cutter having an annular cutter blade having around an inner periphery an inner peripheral cutting edge having a diameter of 235 mm and a thickness of 0.29 mm, the annular cutter blade being rotated about its axis to cut the ingot at a rate of 60 mm per minute. Then, the slice was mounted on and fixed to a vacuum chuck including a porous body of alumina, with the ground flat surface held agianst a flat mounting surface of the porous body by suction. Then, the cut surface of the slice was ground by a diamond tool into a flat surface, thereby providing a wafer having the flat opposite side faces. Thus, since the ground flat surface of the slice was held against the flat mounting surface of the porous body of the vacuum chuck, the ground surface served as a reference surface for flattening the cut surface of the slice. Then, each of the opposite flat side faces of the wafer thus obtained was chemically etched by an amount of 5 µm, using a mixed acid composed of hydrofluoric acid, nitric acid and acetic acid, so that the total amount reduced by the etching was 10 µm. Then, the wafer thus etched was rinsed with pure water and thereafter was dried. Then, the warp of the wafer thus obtained was measured. The warp of the wafer was not more than 3 µm. For comparison purposes, 500 comparative wafers were prepared. Each comparative wafer was prepared by cutting an ingot made of the same material and having the same diameter as the above ingot, by the same cutter to produce a slice of the same thickness as the above slice, and then polishing the opposite side faces of the slice, subjected to the cutting, by a lapping machine to reduce the thickness of the slice by an amount of 40 µm, and then etching the opposite side faces of the slice by the above-mentioned mixed acid to reduce the thickness of the slice by an amount of 10 µm. 500 comparative wafers thus obtained were measured to determine their warp. The maximum warp was 17 µm while the minimum warp was 6 µm, and the average warp was 9 µm. As is clear from the above, with the method according to the present invention, the warp of the wafer is kept to an extremely small value.

Claims: A method of producing a flat large size semiconductor wafer from a stock of silicon or gallium arsenide, the method comprising the steps of: a) processing one end face of said stock to form a first flat surface having a predetermined roughness not more than 0.2 µm and a predetermined flatness not more than ± 1.0 µm, b) cutting through the end portion of said stock defined by said flat surface transversely by a cutter to provide a large diameter slice of a predetermined thickness having opposite side faces defined respectively by said first flat surface and a cut surface resulting from said cutting, c) processing said cut surface of said slice to form a second flat surface parallel to said first flat surface using said first flat surface as a reference surface and d) chemically etching both surfaces of the semiconductor wafer for improving flatness and consecutively improving the warp of the semiconductor wafer to a value not more than 3 µm. A method according to claim 1, wherein step b) is performed by an annular cutter blade saw. A method according to claim 1 or 2, wherein for performing step c) the slice is mounted on and fixed to a vacuum chuck with the first flat surface held against a flat mounting surface of the vacuum chuck by suction. A method according to any one of the preceding claims, wherein in step c) said cut surface is ground by a diamond tool to form said second flat surface. A method according to any one of the preceding claims, wherein the wafer is rinsed with pure water and dried after step d). A method according to any one of the preceding claims, wherein in step d) a portion of 5 µm in thickness is removed from each of the first and second flat surfaces by chemical etching. A method of producing a flat large size semiconductor wafer from a stock of silicon or gallium arsenide, the method comprising the steps of: a) processing one end face of said stock to form a first flat surface having a predetermined roughness not more than 0.2 µm and a predetermined flatness not more than ± 1.0 µm, b) cutting through the end portion of said stock defined by said flat surface transversely by a cutter to provide a large diameter slice of a predetermined thickness having opposite side faces defined respectively by said first flat surface and a cut surface resulting from said cutting, c) processing said cut surface of said slice to form a second flat surface parallel to said first flat surface using said first flat surface as a reference surface and d) stress-relieving annealing the semiconductor wafer for improving flatness of said first and second flat surface and consecutively improving the warp of the semiconductor wafer.

Inventors: SAITOU, YUICHI
Applicants: MITSUBISHI MATERIALS CORPORATION, MITSUBISHI MATERIALS SILICON CORPORATION
Application year: 1986
Main classification: C30B33
IPC codes: B28D5, C30B33, B28D1, C30B25