US3187739A

US3187739A - Method and apparatus for shaping crystals

Info

Publication number: US3187739A
Authority: US
Inventors: Fresne Eugene R Du
Original assignee: General Dynamics Corp
Current assignee: General Dynamics Corp
Priority date: 1963-12-27
Filing date: 1963-12-27
Publication date: 1965-06-08
Anticipated expiration: 1982-06-08

Description

June 8, 1965 E. R. DU FRESNE METHOD AND APPARATUS FOR SHAPING CRYSTALS Filed Dec. 27, 1965 REFRIGERATION APPARATUS United States Patent 3,187,739 METHOD AND APPARATUS FOR SHAPING CRYSTALS Eugene R. Du Fresne, Chicago, Ill., assignor to General Dynamics Corporation, New York, N.Y., a corporation of Delaware Filed Dec. 27, 1963, Ser. No. 333,837 9 Claims. (Cl. 12523) This invention relates to a method of treating crystals, and more particularly to a method of distortion-free shaping of crystals having a diamond crystalline lattice structure.

In commercially producing useful crystals, it is necessary to cut large crystals or monocrystalline ingots to the desired shapes. In the usual production of crystals for semiconductor and other uses relatively large monocrystalline ingots are prepared by growing crystals in barlike configuration. The crystalline ingot is then sliced to produce useful wafers.

Various types of apparatus have been used to cut crystal ingots into desired shapes. Crystal cutting saws, often using a diamond cutting edge, have been employed for the slicing process. Acid carrying cutting wheels and electrolytic etch methods have also been employed. However, none of these methods of cutting semiconductor crystals have proved entirely satisfactory. Methods such as these inherently necessitate a high amount of waste of crystalline material. In this connection these sawing operations accomplish separation by cutting a kerf in the crystal between the material being separated and the remainder of the mother crystal. The crystal material in the kerf is wasted and lost from production.

Furthermore, in producing crystals it is generally important that the crystal wafer produced be smooth and substantially distortion-free. Processes such as sawing and the like produce distorted areas in the crystals, i.e. in the new surfaces of the crystal portions cut by the saw. These distorted surfaces must then be removed, as by etching, to reach the underlying undistorted crystals, resulting in still further waste of useful crystal material. It is desirable to provide crystal shaping methods which eliminate these disadvantages.

It is an object of the present invention to provide an improved method of and apparatus for shaping crystals. Another object is to provide a method of and apparatus for shaping crystals into sections having distortion-free surfaces. A further object of the invention is to provide an economical method of cutting semiconductor crystals which eliminates the waste previously inherent in crystal cutting operations and produces distortion-free crystals in a substantially one-step operation. These and other objects of the invention are more clearly set forth in the following description and in the accompanying drawing in which there is diagrammatically disclosed apparatus suitable for carrying out the method of the invention.

The invention provides a method of distortion-free shaping of crystals having a diamond crystalline lattice structure which comprises lowering the temperature of such a crystal to a sufliciently low value and then cleaving the crystal Preferably, an external force is applied to the surface of the crystal to smoothly cleave the crystal along a predetermined crystallographic plane. The external force is applied preferably along a line of direction that lies in the predetermined plane to assure the production of crystal sections having distortion-free surfaces, i.e. the surfaces that previously lay along the plane of cleavage.

When an external force is applied to crystals of certain materials, such as carbon in diamond crystalline form, at room temperature, the crystals do not fracture irregularly but actually split along definite, well-defined 3,187,739 Patented June 8, 1965 planes. This phenomenon of cleavage has been known for a considerable length of time and has been used in shaping diamonds. However, the number of crystals which exhibit this phenomenon is quite limited. Many crystal-s fracture and produce sections having hackly surfaces. Other crystals are dis-located along slip planes instead of being cleaved, as desired. Still other crystals deform by twinning.

It has been found that the tendency of some crystals to split smoothly is greatly enhanced by significantly reducing the temperature of the crystal to a temperature substantially below the freezing point of water before applying the external cleavage force. Crystals which cannot be cleaved at room temperature may be cleaved reasonably well at a lowered temperature whereas crystals which can be cleaved only with difficulty at room temperature can be easily cleaved at lower temperatures. The cleavage temperature is a function both of the crystal structure and of the specific nature of the crystalline substance itself.

Crystals of different materials have different cleavage tendencies. Experimentation shows that there is some interrelationship between cleavage tendencies and crystal lattice structure. It has been found that materials having a crystalline lattice structure known as the diamond struc ture can be cleaved at low temperatures.

There is no sharp threshold temperature at which the cleavage characteristics of a given material suddenly change. Generally, when cleavage is attempted at a temperature substantially below the freezing point of water, some improvement is shown. At a temperature of about C., there is usually a significant improvement in the cleavage tendency of crystals having a diamond crystalline lattice structure compared to their cleavage tendency at room temperature. In general, the lower the temperature used, the more improvement there is in cleavage tendency. At a temperature of about '200 C., cleavage tendency of such crystals is even further enhanced over their cleavage tendency at about 75 C.

Different elements and diiferent compounds cleave along different crystallographic planes. The systems of planes, along which cleavage commonly occurs, are known by their Miller indices as the planes, planes and the {111} planes. The specific crystallographic plane along which a crystal cleaves is dictated by the individual structure of the crystal itself, and, in general, by the type of crystal lattice the compound has; Diamonds cleave along the {111} planes as do other crystals having the general crystalline lattice of diamonds.

In order to best promote cleavage of a crystal, it is important to apply the external force to the crystal along the expected cleavage plane. In general, the more precisely the applied force is aligned with an edge of the cleavage plane at the surface of the crystal, the smoother will be the cleavage that takes place. In crystals which are more difficult to cleave, the smoothness with which cleavage occurs is further enhanced by not only applying the external force at an edge of the desired plane of cleavage, but als 9 b y moving the instrument, with which the force is applied, in a line of direction that lies in the cleavage plane,

so as to trace out the plane. In crystals which can be fairly readily cleaved, the need for extremely accurate alignment of the external force is unnecessary to obtain smooth splitting of the crystals; however, lowering the temperature often increases the cleavage tendency to such an extent that cleavage occurs even without accurate alignment of the external force, thus providing a further advantage for the improved method.

Low temperatures not only increase the tendency of other crystallographic planes.' This forestallinglofiumdesirable plastic deformation in crystals being operated upon results in further reduction of the waste accompanying the production of single crystals and is another important advantage of the improved method. 7

Silicon is a good example of a common crystalline material whichcan be cleaved with great precision at low temperatures. At room temperature, silicon often splits leaving a conchoidal surface of separation. At a temperature of about 75 C., silicon crystals can be cleaved neatly along smooth,'distortion-free crystalline surfaces.

Silicon has a lattice structure commonly known as the diamond structure and is indicative of materials of this general crystal'type. This class of crystals improves substantially in cleavage tendencyat lower temperatures. Crystalline germainium, another of the materials of this class, shows considerable improvement in cleavage tendency along plane {111} at low temperature, the resultantcrystal sections having extremely smooth, distortion-free surfaces. 7

Various suitable means may be used to apply external force to the crystal to promote the cleavage. One apparatus is diagrammatically shown in the accompanying drawing which provides suitable results. It is to be understood that this particular device is shown primarily for purposes of description and that the invention is to be in no way considered limited to use of a device. of the specific type illustrated.

' A cryostat 5 is shown which includes an insulated chamher 7 that is held at extermely low temperatures as. by suitable refrigeration apparatus while operations are carried on therewithin. A cylindrical anvil 8 is rotatably mounted within'the chamber 7 and supports one end of a crystal ingot 9. while smaller crystal pieces are being oleaved therefrom. The crystal ingot 9 shown'has an elongated shaped and is exemplary of the monocrystalline germanium and the like which is grown in a specific direction. The crystal 9' is suitably connected at its other end to a feed screw 11 as by an oil-based putty which is malleable at room temperature but which becomes rigid at the temperatures within the cryostat, providing firm support for the crystal. engaged in a sleeve 12 in a side wall of the cryostat 5 to allow its manipulation by a knob 13 outside the cryostat.

Theexternal force used to cleave the crystal is applied, in the illustrated apparatus, by a cleavage tool 15 which has an upper head 17 and a chisel-shaped blade 19 made of a material such as beryllium copper. The tool 15 is mounted in'a' suitable sleeve 21 set in the top wall of the are grown so that the {111} plane. is perpendicular to,

the longitudinal axis of the crystaLonly small variations in the blade angle are usually necessary.

Although the crystal mounting means is illustrated in this manner, it should also be noted that various other means for mounting crystals for cutting are well known in the art and can be used in carrying out the improved method. Any suitable adjustable holder may be used which rigidly supports the crystal. Likewise, any of the well known means for positioning crystals so that desired crystallographic planes are oriented in the desired direction may also be employed. a 7

Example I e A silicon crystal is etched by a sodium hydroxide solution in order to develop its {111} crystalline planes and is then positioned in a cryostat. A mixture of solid carbon'dioxide andacetone is introduced into the cryostat by The feed screw 11 is threadedly 4 refrigeration apparatus to cool the crystal to a temperature of about 78 C.

A beryllium copper cleavage tool, having a chisel edge of approximately the same breadth as the crystal, is care fully positioned so that the edge is aligned with a line in the desired {111} crystalline plane. A sharp force is applied to the crystal by hitting the head of the tool with a hammer. The silicon crystal is cleaved at the desired {111} plane producing a thin crystalline wafer of desired size.

Visual examination of the crystalline wafer shows that the cleavage surface is quite smooth. X-ray difiraction examination of the crystal shows it to be distortion-free. The crystal is considered well-suited for use in semiconductor applications.

Example II The procedure of Example I is repeated, substituting liquid nitrogen for the Dry Ice-acetone mixture. Several crystal wafers are cut from the end of the silicon ingot. Each time the blade edge of cleavage tool is not precisely aligned with the desired {111} crystallographic planes but the tool is only generally aligned therewith.

Each of the crystal wafers produced has smooth cleavage surfaces and, under examination, is found to be distortion-free. At the lower cleaving temperatures provided by liquid nitrogen, the cleavage tendency of the silicon crystal is sufficiently increased so that precise alignment of the cutting tool is no longer a prerequisite.

Example III A monocrystal'of germanium, grown so that its {111} crystallographic planes are perpendicular to its longitudinal axis, is suitably secured to a feed screw in a cryostat so that the free end of the crystalline ingot is firmly supported. The refrigeration apparatus is not used so that the cryostat remains at room temperature. A cleavage tool is positioned in alignment with the {111} crystallographic planes in the germanium crystal. Several crystal wafers are split from the germanium crystal ingot by striking the tool with a hammer.

Examination of the crystal wafers shows that they have conchoidal surfaces of varying smoothness where the splitting occurred. These surfaces are not fiat and do not closely follow the {111} plane. These crystals cut are not considered desirable for use in semiconductor applications.

The cryostat is now refrigerated with a mixture of acetone and solid carbon dioxide. The operation is repeated after the cryostat has reached a temperature of about '75 C. The cleavage tool is carefully positioned so that its :blade edge is exactly aligned with the intersection of a {111} plane and the crystal surface. The positioning of the cleavage tool is alsocarefully adjusted so that the direction of motion of the tool will be parallel to a {111} plane so that, as the blade edge of the cleavage tool is moved in cleaving operation, the surface generated will itself be a {111} plane.

The head of the cleavage tool is struck a sharp blow with a hammer and a single crystal segment is split from the major crystal. Several crystal wafers are cleaved in this manner.

The crystal wafers produced have smooth surfaces that follow the {111} crystalline planes. Their surfaces are perfectly flat and mirror smooth, and examination shows them to be distortion-free. These crystal Wafers are considered suitable for semiconductor applications.

Example IV The procedure of Example 111 i'srepeated using liquid nitrogen instead of the acetone-solid carbon dioxide mixture so'that the temperature of the crystal is lowered to about l C. The cleavage tool is only generally aligned and struck a sharp blow, splitting a segment from the crystal. The process is repeated several times.

Examination of the crystal segments shows that cleavage has occurred resulting in smooth surfaces. Examination shows the resultant crystals have distortion-free surfaces.

The invention provides an improved method of producing single crystals from polycrystalline materials and of shaping large crystals. Distortion-free semiconductor crystals can be economically and easily produced by the method in simple one-step operations. The low production cost of crystals afforded by the invention is unobtainable by the prior art methods. Thus, the invention provides a significant advance in the art.

Various of the features of the invention are set forth in the following claims:

What is claimed is:

1. A method of shaping a crystal having a diamond crystalline lattice structure comprising cooling a crystal having a diamond crystalline lattice structure to a temperature substantially below the freezing point of water and cleaving the crystal along a predetermined plane.

2. A method of shaping a crystal having a diamond crystalline lattice structure comprising cooling a crystal having a diamond crystalline lattice structure to a temperature substantially below the freezing point of water, and applying an external force to the crystal along a predetermined line so as to cause cleavage of the crystal along a predetermined plane.

3. The method of claim 2 wherein the semiconductor crystal is silicon.

4. The method of claim 2 wherein the semiconductor crystal is germanium.

5. A method of shaping a crystal having a diamond crystalline lattice structure comprising cooling a crystal having a diamond crystalline lattice structure to a temperature substantially below the freezing point of water, aligning a cutting tool with a predetermined line on the crystal, and applying an external force to the cold crystal through said tool so as cause cleavage along a predetermined plane. I

6. A method of shaping a crystal having a diamond crystalline lattice structure comprising cooling a crystal having a diamond crystalline lattice structure to a temperature of at least about 75 C., and applying an external force to the crystal along a predetermined line so as to cause cleavage of the crystal along a predetermined plane.

7. A method of shaping a crystal having a diamond crystalline lattice structure comprising cooling a crystal having a diamond crystalline lattice structure to a temperature substantially below the freezing point of water, positioning a tool having a blade section so that the blade section lies in the extension of a predetermined crystallographic plane through the crystal and striking the tool to cause it to move in said plane and transmit the sharp blow to the crystal whereby cleavage of the crystal is accomplished along said plane.

8. Apparatus for shaping crystals which apparatus comprises cryogenic means including a chamber adapted to be maintained at a low temperature, means for positioning and rigidly supporting a crystalline ingot in'said chamber in a desired alignment, means for maintaining the temperature of said chamber below at least about C., and means connected to said cryogenic means for applying an external force to the ingot at a predetermined location therein so as to cause cleavage of the crystal.

9. Apparatus for shaping crystals which apparatus comprises cryogenic means including a chamber adapted to be maintained at a low temperature, means for positioning and rigidly supporting a crystalline ingot in said chamber in a desired alignment, means for maintaining the temperature of said chamber below at least about 75 C., and cleavage means connected to said cryogenic means, said cleavage means including a rigid cleavage tool having a chisel point for applying an external force to the crystal along an edge of a predetermined plane in the surface of the crystal and also including a part extending without said chamber for actuating said cleavage means.

References Cited by the Examiner UNITED STATES PATENTS 847,823 3/07 Reynolds 62--46-5 2,203,937 6/40 Barley 83-15 2,218,541 10/40 Kronquest 83-15 3,063,260 11/62 Dennis 6244() ANDREW R. JUHASZ, Primary Examiner.

ROBERT A. OLEARY, Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,187,739 June 8, 1965 Eugene R. Du Fresne It is hereby certified that error appears in the above numbered patent reqliring correction and that the said Letters Patent should read as correctedbelow.

Column 3, line 15, for "germainium" read germanium column 5, line 39, for "as cause cleavage along" read as to cause cleavage of the crystal along Signed and sealed this 23rd day of November 1965.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Alli-sting Officer Commissioner of Patents

Claims

6. A METHOD OF SHAPING A CRYSTAL HAVING A DIAMOND CRYSTALLINE LATTICE STRUCTURE COMPRISING COOLING A CRYSTAL HAVING A DIAMOND CRYSTALLINE LATTICE STRUCTURE TO A TEMPERATURE OF AT LEAST ABOUT -75*C., AND APPLYING AN EXTERNAL FORCE TO THE CRYSTAL ALONG A PREDETERMINED LINE SO AS TO CAUSE CLEAVAGE OF THE CRYSTAL ALONG A PREDETERMINED PLANE.