US3749499A

US3749499A - Process and apparatus for centering unground semiconductor rods

Info

Publication number: US3749499A
Application number: US00212265A
Authority: US
Inventors: T Reichard; T Bever
Original assignee: Monsanto Co
Current assignee: SunEdison Inc
Priority date: 1971-12-27
Filing date: 1971-12-27
Publication date: 1973-07-31
Anticipated expiration: 1990-07-31

Abstract

A process and apparatus for obtaining a cylinder of a specified diameter from an unground rod of varying diameters which includes focusing a beam of collimated light on the rod, masking the beam to control its width and to control the amount of light passing each side of the rod, and continuously manipulating and positioning the rod within the beam so as to move the centerline of the above cylinder into the same plane as that containing the centerline of the focussed beam.

Description

[111 3,749,499 [451 July 31,1973

l,85l,l73 3/1932 Hall 356/164 PROCESS AND APPARATUS FOR CENTERING UNGROUND SEMICONDUCTOR RODS Primary Examiner-Ronald L. Wibert Assistant Examiner-V. P. McGraw AuorneyHarold R. Patton [75] Inventors: Thomas E. Reichnrol, Kirkwood;

Thomas L. Bevel, Florissant, both of Mo.

[73] Assignee: Monsanto Company, St. Louis, Mo.

Filed: Dec. 27, 1971 ABSTRACT [2]] Appl' No: 212,265 A process and apparatus for obtaining a cylinder of a specified diameter from an unground rod of varying diameters which includes focusing a beam of collimated [52] US. 356/150, 356/l54, 356/159,

356/167 light on the rod, masking the beam to control its width Int. G01!) 1/00, GOlb 11/00 [58] Field of Search....................

[51 and to control the amount of light passing each side of the rod, and continuously manipulating and positioning 51/323 the rod within the beam so as to move the centerline of the above cylinder into the same plane as that containing the centerline of the focussed beam.

References Cited UNITED STATES PATENTS 3,470,384 Bodnas 356/168 12 Claims, 6 Drawing Figures PATENIEDJULBI I975 3.749.499

sum 1 0F 3 FIG. I.

PATENIEL Jul 31 I975 SHEET 2 OF 3 FIG. 3.

a n w w I25 30 [I9 I05 m W ww w m FIG.4.

FIG. 2.

PATENIEIIJIIIBI I915 3'.74 9,499

SHEET 3 OF 3 I I FINAL INITIAL POSITION POSITION I CLI l I I To I 1 I58 /I54 44 l I I 55 -46 I UL : ULZ ULII DMIN UL2 I I T i I l I I I I48 I I44 g I I I --cLI I CLI I FIG. 5. FIG. 6.

PROCESS AND APPARATUS FOR CENTERING UNGROUND SEMICONDUCTOR RODS FIELD OF THE INVENTION This invention relates generally to the preparation of 5 BACKGROUND The Czochralski process for pulling semiconductor crystals from a melt is well known in the art and has been used for many years to grow single crystals of silicon, germanium, and III-V intermetallic compounds. In this process, a seed crystal is dipped into a molten charge of semiconductive material and then slowly withdrawn at a controlled rate to produce semiconductor rods having diameters typically ranging from onehalf to 4 inches. Although a number of rod diameter control processes have been employed in the past in an attempt to maintain substantially constant diameters in the pulled crystal rods, these monocrystalline rods nevertheless have diameter fluctuations throughout their lengths as a result of uncontrollable environmental changes in the crystal pulling process, such as changes in the melt level, temperature gradients in the melt and in the crystal pulling chamber and other environmental changes which cannot, as a practical matter, be maintained constant throughout the Czochralski process. Consequently, the crystal rods once pulled must be ground down to a chosen uniform diameter using a grinding lathe.

In this grinding process, it is desirable to increase yields and minimize semiconductive material waste by reducing the crystal losses which accompany the process of grinding the rods down to specified diameters. Therefore, it is preferrable to be able to place the crystal rod in the grinding lathe in such a position as to grind the rod down to a cylinder of a desired specified diameter with a minimum of undersize losses. When any length of the unground rod is undersize or has a diameter less than the specified diameter, then this much of the rod cannot be used for making wafers of the above specified diameter. However, the term undersize is used with reference to a specific longitudinal centerline of the unground rod and with reference to the cylinder within the unground rod having this centerline and having the specified diameter. For each unground semiconductor rod from which it is desired to obtain the maximum length of a cylinder at a specified diameter, there are two cases. In the first case, there is no undersize in the unground rod, and within the entire length of the rod to be ground, it is possible to obtain a perfect cylinder whose diameter is smaller than any single diameter ofthe unground rod. In this case, it only becomes necessary to establish the centerline of this cylinder so that the unground rod can be rotated in a grinder lathe about this centerline axis of rotation to grind the rod down to the above cylinder with the specified diameter.

In the second case, the prescribed or predetermined diameter of the cylinder to be ground from the unground rod is larger than some of the many different diameters of the unground rod, and where the latter is true then the unground rod is undersize with respect to the cylinder of the prescribed diameter. Therefore, in the latter case the selection of the centerline of the cylinder of prescribed diameter becomes very important, because there will be a different amount of undersize loss for each centerline selected. Thus, in this second case it is desirable to select the centerline of the unground rod for which undersize losses are at a minimum.

in the past, skilled operators have tried to establish this centerline within the unground rod either by visual inspection of the rod or by using various types of centering templates for making center-point measurements on each end of the rod. The visual inspection technique has not proved entirely satisfactory, since it is impossible with the human eye to locate points on the ends of the unground rod which lie on the above centerline axis. The centering template technique has been used to satisfactorily locate the approximate center points of the area on each end of the unground rod. However, the line which connects these two points is not necessarily the above identified desired centerline axis of rotation.

THE INVENTION In accordance with the present invention, a process and apparatus are provided wherein an optical masking system is employed to locate the centerline axis of the cylinder within the unground semiconductor rod having either no undersize losses or a minimum of undersize losses. In practicing the present invention, the unground semiconductor rod may be manipulated and positioned within a focussed collimated light beam, and the width of this beam and the amounts of light passing each side of the unground rod may be continuously controlled by masking same during the manipulation and positioning of the rod. This beam has a centerline in the same plane as that of the desired centerline sought to be established for the unground semiconductor rod, so that by continuously manipulating and positioning the rod on a support member during the reduction in light beam width, the undersize losses in the unground rod, if any, may be determined. Upon observation of these undersize losses during the manipulation and positioning of the rod, the optimum centerline corresponding to a minimum of undersize losses can be established with a high degree of accuracy. Once this centerline is established, bearings may be removably secured to the ends of the unground rod for rotation in a grinding lathe.

Accordingly, an object of the present invention is to provide a new and useful process for rapidly and accurately centering semiconductor rods in preparation for grinding same to a specified diameter with an absolute minimum of semiconductive material waste.

Another object of this invention is to provide a new and useful rod centering apparatus to facilitate the easy manipulation and handling of an unground semiconductor rod while securing thereto rotational bearings on each end of the rod adaptable for rotation in a grinding lathe.

Another object of this invention is to provide a new and useful process and apparatus of the type described which is easy to construct and operate, reliable in performance and relatively inexpensive in its construction, upkeep and operation.

A further object of this invention is to provide a rod centlx ring process and apparatus of the type described which is operative to accommodate desired centering of semiconductor rods having a variety of shapes, such as flats, bows, dips, S-curves, hooks and the like.

These and other objects and features of the invention will become apparent in the following description of a preferred embodiment of the invention.

DRAWINGS FIG. 1 is a prospective view, partially cut away, of the rod centering apparatus embodying the invention;

FIG. 2 is a cross-section view taken along lines 2-2 of FIG. 1;

FIG. 3 is a greatly enlarged cross section view of the central portion of the turntable assembly in FIG. 2 and is shown with the rod centering bearing thrust into adhesive contact with the semiconductor rod;

FIG. 4 is an enlarged view of the end portion of the semiconductor rod having the adhesive bearing secured thereto on the desired rod centerline;

FIG. 5 is a mask and rod outline of the unground rod in its collimated light masking pattern for case one above where there are no undersize losses, and

FIG. 6 is a mask and rod outline of the unground rod in its collimated light masking pattern for case two above where the unground rod is to be centered for maximum obtainable rod length and a minimum of undersize material losses.

DETAILED DESCRIPTION Referring now to FIG. 1, there is shown, in partially broken away perspective view, rod centering apparatus designated generally as 10 and which includes a means to be described for focusing a beam 12 of collimated light on the semiconductor rod to be centered. This beam 12 originates within a light box 14 having a slit 16 therein through which a divergent light beam 17 passes, and a portion of the beam 17 passes through a Fresnel lens I8 mounted adjacent the opening 19 in the assembly housing 20. The collimated light beam 12 is interrupted by the unground semiconductor rod 22 and split into a pair of parallel collimated beams 24 and 26 which are projected onto a luminescent screen 28. As will be described in detail below, the semiconductor rod 22 is manipulated within the light beam 12 and on lower and upper

rotary turntables

30 and 32, respectively, in order to center the semiconductor rod 22 in preparation for a subsequent grinding operation. The manipulation and positioning of the unground semiconductor rod 22 can be accomplished by means of adjusting screws 34, and in accordance with the present invention the centerline CL of the semiconductor rod 22 is brought into the same plane with that of the centerline CL of the

masking members

44 and 46 for the collimated beam 12 as well as the centerline CL, of the light beam l7. The vertical axis of rotation for the

turntables

30 and 32 also lies in this vertical plane which is parallel to the collimated light beam 12.

The box housing 14 for the light source includes a light bulb 36 which is mounted as shown on a support bracket 38, and the spring biased screws 40 may be adjusted to raise and lower and tilt the support plate 42 and provide an adjustment for the light bulb 36 with respect to the slit 16. A cooling fan 39 is located beneath the bulb 36 to prevent overheating in the box housing 14.

The apparatus 10 also includes an adjustable light shield having the pair of

masking members

44 and 46 vertically suspended as shown adjacent the opening 19, and these

masking members

44 and 46 may be simultaneously moved in a direction perpendicular to the collimated light passing therebetween in order to vary the width of this beam and the amount oflight which passes each side of the semiconductor rod 22. The masking member 44 is fixedly mounted on the back of the coarse thread nut 70 and the masking member 46 is fixedly mounted on the hanger 58, and the

masking members

44 and 46 move toward and away from the center ofthe beam I2 with the movement ofnut 70 and hanger 58 to be described.

The light shield assembly is securely mounted to the housing 20 by means of a pair of support brackets 48, each having a

bearing arm

50 and 52 extending therefrom as shown for receiving an upper guide rail 54. The upper guide rail 54 carries a pair of

hanger members

56 and 58 which are free to slide thereon, and a pair of

lower guide rails

60 and 62 are fixedly mounted as shown between the

hanger members

56 and 58 and beneath the upper guide rail 54.

A mask adjusting thread screw 64 is threaded through an opening in the fixed support arm 66 and is received at one end in a shaft coupler 67. This screw 64 moves from right to left as viewed from the front of assembly 10 and through screen 28 to thereby move the hanger 58 from right to left, sliding on upper guide rail 54. The shaft coupler 67 also receives the thread screw which extends through the opening 68 in the hanger 58 and is threaded through the center of the coarse thread nut 70 which carries the masking member 44. The thread screw 65 is keyed to the hanger 58 so that it can rotate but not move through the hanger 58 from left to right as viewed from the front of assembly It). The coarse thread nut 70 has a pair of

openings

72 and 74 therein through which the

lower guide rails

60 and 62 pass, so that the thread nut 70 is moved on thread screw 65 horizontally from left to right on the guide rails 60 and 62 by turning the adjusting screw 64. This movement simultaneously moves hanger 58 and masking member 46 from right to left, and this movement will be at the same rate as that of hanger 58 if the thread pitch of screw 65 is twice that of screw 64. Therefore, by turning the knob 76, the nut 70 and hanger 68 may be simultaneously moved toward or away from the center of the light beam 12 at the same rate.

The lower turntable 30 is mounted on the bottom wall 78 of the apparatus 10, and the upper turntable 32 is mounted on the support bracket 80 which is held in position by a pair of

vertical guide rails

82 and 84. The bracket 80 may be raised or lowered from the position shown in FIG. 1 by means of a pulley assembly which includes a pair of weights 86 suspended from a pair of

ropes

88 and 90. These ropes are movable around the

pulley arms

92 and 94 which are mounted as shown on the back wall of the assembly 10. The turntable assembly portion of the apparatus 10 can be seen more clearly by referring to the cross section views of FIGS. 2 and 3 which illustrate the rotatable mounting assemblies of both the lower and

upper turntables

30 and 32.

Referring now to FIG. 2, the lower turntable 30 and associated mounting assembly therewith will be described. The upper and lower turntable assemblies are substantially identical, and therefore only the lower turntable 30 and its related mounting assembly will be described herein with reference to FIG. 2. The lower turntable 30 is mounted for rotation about a preselected centerline axis by means of ball-bearings which include an inner race member 95 secured to the turntable 30 and an outer race member 96 secured to a bearing housing 98. The turntable 30 may be freely rotated 360 by means of these ballbearings, and the outer bearing housing 98 is in turn securely clamped to the bottom wall 78 of the assembly by means of a clamp ring 107. This outer clamp ring 107 has its inner peripheral edge in contact with the bottom wall of the outer race member 96 and thereby firmly clamps the outer race member 96 to the bottom wall 78 of the assembly 10. An inner clamp ring or spacer 106 is positioned in contact with the bottom wall of the inner race member 95, and by tightening a lock nut 102 against the spacer 106, the inner race member 95 is held securely against the turntable 30. Note that the lock nut 102 does not contact the outer clamp ring 107, there being an air gap between the latter two members. The spacer 106, therefore, keeps the lock nut 102 separated from the outer clamp ring 107.

The unground semiconductor rod 22 is initially brought to rest on a rubber pad 105 on the upper surface 108 of the rotary turntable 30. The turntable 30 carries on its outer edge, and at 90 spacings, four adjustable screw brackets 110 which threadedly engage, respectively, the four previously identified adjusting screws 34. Thus, in centering the rod 22, an operator may freely rotate the turntable 30 to any one of four different 90 positions and make from one to four adjustments of the screws 34 at each of these 90 positions in order to move the semiconductor rod 22 in the X and Y directions as desired on the rubber pad 105. The adjusting screws 34 each carry swivel pads 112 on the tips thereof, and these pads are brought into resilient contact as shown with the ends of the semiconductor rod 22 being centered.

An air cylinder 114 is secured to the lower end of the turntable 30 by means of a screw 115, and another smaller screw 117 is slidably positioned centrally of the screw 115 and is free to move vertically through the screw 115 so as to move the entire chuck 118 upwardly toward the bottom of the semi-conductor rod 22. The smaller screw 117 may be connected at its lower end to any member (not shown) suitable for efficiently transmitting air pressure to it from the air cylinder 114.

The chuck 118 has a recess in the upper portion thereof for receiving a resilient pad 120 which has an opening in its center for receiving a so-called hex-ball member 119. The rod centering bearing 125 is mounted on this hex-ball member I19 which is in the general shape of a sphere, but has hexagonal outer sections extending from its top as shown in FIG. 2 down to a stem 121 which extends into the narrow central cylindrical recess of the chuck 118. The bearing 125 has its inner recess contoured to the'shape of the hex-ball member 119, so that the bearing 125 may be rotary driven by a similar hex-ball type member (not shown) once the bearing 125 is secured to the end of the rod 22.

Referring now to FIGS. 2, 3 and 4 a pair of rollers, such as

spheres

132 and 134, are held in place as shown adjacent the outer edge of the bearing 125 by means of an O-ring seal 136, and these rollers facilitate the firm and smooth transmittal of vertical motion from the chuck 118 to the bearing 125.

After the rod 22 has been centered in accordance with the present invention, air pressure is introduced by any suitable means into the air cylinder 114 to force the inner screw 117 upwardly through its surrounding outer screw member 115 to thereby move the chuck 118 vertically toward the bottom of the rod 22. Once the adhesive 128 on the upper surface of the bearing 125 comes into contact with the rod surface, then the bearing 125 is free to move against the lower resilient pad 120. Similarly, the upper resilient pad 122 gives with the movement of the upper bearing 126. This movement permits the adhesive 128 to assume the contour of the bottom end of the semiconductor rod 22, and during this upward movement of the bearing 125, this bearing is guided upwardly by the

rollers

132 and 134. If for some reason the rod 22 has been sawed at an angle such that its surface 130 is not in a plane parallel to the plane of the surface of the adhesive 128, then this does not prevent accomplishing a good secure adhesive bond between the rod 22 and adhesive 128 when the bearing 125 is thrust upwardly into contact with the lower surface 130 of the rod 22.

The structure in FIG. 3 differs from that shown in FIG. 2 in that a spherical centering ball 124 rather than a hex-ball is used to transmit the vertical force to the bearing 126, and the centering ball 124 has a stem 127 thereon which extends into the narrow cylindrical vertical slot of the chuck 118. Thus, as shown in FIG. 4, the bearing 126 which is secured to the surface 130 of the semiconductor rod 22 has a hemispherical recess 129 therein which is free to rotate on a spherical or hemispherical member of the same shape. Thus, the rod 22 is provided with a bearing 126 on one end thereof which is freely rotatable in one end of a grinding machine and a bearing 125 secured to the other end thereof which has a hexagonally shaped inner recess. Such recess enables the bearing 125 to be driven by a drive shaft in the other end of the grinding machine and having either an end or a member attached to its end with a contoured hexagonal shape.

Referring now to FIG. 5, the present invention is illustrated for the above described case one wherein there is no single diameter of the unground rod 22 smaller than the target diameter TD to which the rod 22 will be ground. For this case, an operator will place the rod 22 on the

turntables

30 and 32 as described above and continuously rotate these turntables while making the appropriate adjustments of the eight screws 34, four at the bottom and four at the top, at each position. A vernier or scale (not shown) is positioned adjacent the edges of the

blades

44 and 46 and is set at the target diameter TD, so that the spacing between these blades can be observed with respect to the target diameter. The rod 22 is moved in the X and Y directions and simultaneously rotated while the edges of the

mask

44 and 46 are moved inwardly until light passing the rod 22 is just closed off at

locations

140, 142 and 144 along the edges of the rod 22. These points are thus lined up with the edges of the

mask

44 and 46. Thus, for case one above, there is a rod centering alignment dimension T which will be removed during the subsequent grinding process in order to grind the rod 22 down to the target diameter, TD. The centerline CL, of the cylinder having the target diameter TD is, of course, the centerline for a cylinder with a maximum diameter, D which is the maximum diameter rod that could be obtained from the unground rod 22. Therefore, if the object in the subsequent grinding process is to obtain a cylinder of maximum diameter, D from the unground rod 22, then the grinding machine will be set to grind the rod 22 at this diameter.

Referring now to FIG. 6 there is illustrated the above described case two where the unground rod 22 has undersized regions therein with diameters less than the target diameter TD. In this case, the spacing of the

blades

44 and 46 is reduced to the target dimension TD and no further; and for this spacing, light will pass by the edges of the semiconductor rod 22 and be received on the luminescent screen 28 (FIG. 1). In initially positioning the rod 22, assume that it is moved to a first position with respect to the

masks

44 and 46 so that the edges of the rod 22 visually intersect the right hand mask 46 edge at

points

146 and 148 and intersect the left hand mask 44 edge at

points

150 and 152. For this position, there is an undersize loss UL observed on the right hand edge of the rod 22 and an undersize loss UL, on the left hand side of the rod 22 which is within the undersize loss UL,. Therefore, if the rod 22 is centered at the above position, the length of rod UL would be undersized with respect to the target diameter TD and therefore would be lost in a subsequent grinding operation using this initial centerline.

In order to reduce this undersize loss UL the operator will, while simultaneously rotating the rod 22 to each of the 90 positions (or intermediate positions)of screws 34 on the

turntables

30 and 32, move the rod 22 (as viewed in FIG. 6) to the right with respect to the fixed

masks

44 and 46 until the rod 22 now intersects the mask at

points

154 and 156 on its right hand edge corresponding to a new, smaller undersize loss UL For ease of illustration in FIG. 6 of the drawings, there are shown two sets of mask edges and a single rod. But this should not be confused with any movement of the mask edges 44 and 46, since these edges remain fixed during the rod centering process while the rod 22 is moved to the right. However, in order to best illustrate this relative motion and a reduction in undersize losses, the above movement of the rod 22 is represented by the

new points

154 and 156 of intersection of the right hand edge of the rod 22 with right hand mask 46 and new points of

intersection

158 and 160 with the left hand mask 44. This new position of rod 22 corresponds to the new undersize losses UL between points 1154 and 156 and UL between points I58 and 160, so that the new undersize loss UL on the left hand side of the rod 22 is now approximately equal to UL At this point in the rod centering process, the operator can tell that any further shifting of the rod 22 to the right, while decreasing UL will now begin to increase UL Thus, a point of minimum undersize loss has been reached and corresponds to the centerline CL,; and index marks or bearings, such as the

bearings

125 and 126 illustrated in FIG. 2 and 3 may be applied to the ends of the rod 22 in preparation for subsequent rotation and grinding about the centerline.

If, of course, the alignment objective should be to obtain a complete cylinder having its length equal to that of the unground rod and a maximum diameter for such length, then this diameter would be the minimum diameter, D,,,,,,, as shown in FIG. 6. For this centering an operator will further reduce the masked width of the light beam and position the rod 22 until the mask edges 44 and 46 just meet the edges of the rod 22 at points [62 and 1164 thereon at which all light passing by the rod edges is masked or closed off.

Various modifications may be made in the above described embodiments of our invention without departing from the true scope thereof. For example, the

masks

44 and 46 could be located between the rod and the operator instead of between the light source and the rod; and in addition, these masks could be fixed in either of the above positions rather than moveable if only one specified target diameter is of interest. Furthermore, these masks, either fixed or movable could be made of a transparent material having either a straight line indicating the target diameter or a contoured line corresponding to a desired rotational shape other than a straight cylinder. Or, in the alternative, the mask could be partly transparent, so that the complete rod outline shows on a screen, both over and under a target line. Additionally, the mask could be colored with various colors corresponding to undersize and oversize portions of the rod.

It should be further pointed out that the present invention is not limited to a rod centering process for use only with grinders, but, if desired, the present rod centering process can be used in preperation of rods for various types of lapping or sanding machine tools which rotate a centered object about a particular rotational axis of the machine tool. Furthermore, the invention is not limited to the formation of rods having cylindrical shapes, and may be utilized in preparing rods for a grinding operation wherein the said rods are cut to various other shapes. It will also be understood and appreciated by those skilled in the art that the present invention may be fully automated by, for example, the use of photo detectors in a closed-loop servo system wherein the photo detectors sense the light passing on each side of the rod and in response thereto generate a differential signal. Such signal may be processed in a closed-loop servo system to provide the mechanical X-Y positioning movement necessary to position the rod on the turntables. Accordingly, it will be understood that the present invention is not limited to the above described preferred embodiment thereof.

We claim:

1. A process for locating a centerline for a selected geometrical shape within an irregularly shaped object to be ground about said centerline to a chosen geometric shape in a grinding machine, which process includes:

a. locating said object so as partially to interrupt a beam of light of predetermined width provided by a light source, said beam having a centerline,

b. selectively varying said predetermined width by variably masking said beam between an observation area and the source of said light beam, and

c. shifting said object with respect to said light beam while rotating said object within said beam so as substantially to equalize the amounts of light passing by opposite sides of said object and reaching said observation area, whereby after said equalizing the centerline of said object is aligned with the centerline of said light beam and thereby identified, whereby said object may be rotated about the so-identified centerline in a grinding machine or the like to grind said object to a chosen geometrical shape.

2. The process defined in claim 1 wherein said shifting is carried out while simultaneously varying the width of said beam, so that the centerline established for s;:.id object is also the centerline of a cylinder within said irregularly shaped object.

3. A process for obtaining the centerline ofa cylinder ofa specified diameter from an unground semiconductor rod consisting of many different diameters, which process includes:

a. locating the semiconductor rod so as partially to interrupt a beam of light of predetermined width provided by a light source;

b. varying said predetermined width by masking said beam between the source of said light beam and said rod, and

c. shifting said rod with respect to said light beam while rotating said rod within said beam so as to equalize the amounts of light passing by opposite sides of said rod, whereby an optimum centerline in said unground rod for minimum undersize grinding losses is thereby established.

4. The process defined in claim 3 wherein said shifting said rod is effected by lateral positioning of its opposite ends while varying the width of said beam.

5. A process for determining the central axis of a cylinder of a specified diameter within an unground semiconductor rod of varying diameters comprising:

a. providing by means of a light source a collimated light beam of predetermined initial width,

b. locating said unground rod in the path of said collimated light beam and near the centerline thereof, said rod having a diameter less than the initial width of said beam and partially interrupting said beam and dividing it into two separate beams of collimated light,

c. masking said beam of said collimated light between said rod and the source of collimated light to thereby vary the amounts of light passing each side of said unground rod,

d. continuously repositioning said rod while continuously diminishing the width of said light beam to maintain the widths of said two beams approximately equal at given points along the length of said rod,

e. thereafter diminishing the width of said two beams to or toward zero at given locations along the length of said rod, whereby the centerline of said cylinder may be brought into the same plane as that containing the centerline of said light beam, and

f. affixing index means such as rotational bearings on each end of said unground rod and on said centerline of said cylinder, whereby said rod may then be ground down to a cylinder having said specified diameter and said undersize losses reduced or completely eliminated.

6. The process defined in claim 5 wherein the continuous repositioning of said rod includes rotating said rod while varying the width of said beam.

7. Rod centering apparatus for establishing a centerline in an unground rod for a cylinder therein of a prescribed diameter, including in combination:

a. means for focusing a beam of collimated light towards said unground rod so that said rod partially interrupts said beam of light and allows separate beams of collimated light to pass said rod on each side thereof,

b. means for supporting said rod and having a centerline in the same plane as the centerline ofsaid collimated light beam,

0. means for diminishing toward zero the light passing each side of said rod at one or more points along the length thereof to enable the centerline of said cylinder to be moved into the same plane with the centerline of said collimated light beam, and

d. rod positioning means for moving said rod in the X and Y directions on said supporting means, so that the amounts of light passing on each side of said rod may be varied.

8. Apparatus defined in claim 7 which further includes light imaging means mounted adacent said supporting means and forming images of the two separate collimated light beams passing by each side of said rod, whereby said diminishing small amounts of light passing by each side of said rod may be visually observed and maintained substantially equal as said amounts are reduced to or toward zero as said unground rod is manipulated and positioned on said supporting means.

9. Apparatus defined in claim 7 wherein said supporting means includes a pair of tables rotatably mounted to receive said unground rod on the opposite ends thereof and each having a centerline axis of rotation in the same plane as the centerline axis of said collimated light beam, whereby when the small amounts of light passing said rod are diminished to zero, said cylinder has its centerline moved into the same plane and coincident with the rotational axis of said tables and can be marked thereon in preparation for a subsequent grinding operation. 1

10. Apparatus defined in claim 7 wherein said diminishing means includes masking means located between said focusing means and said supporting means, said masking means being symmetrically mounted with respect to said plane and including means for increasing or decreasing the amounts of light passing each side of said rod.

11. Apparatus defined in claim 9 wherein said tables each includes means therein for thrusting an adhesive carrying bearing toward the ends of said rod and on said centerline of said cylinder thereby to facilitate the mounting of said rod in a lathe for grinding said rod down to said cylinder of said prescribed diameter.

12. Apparatus defined in claim 8 wherein:

a. said supporting means includes a pair of tables rotatably mounted to receive said unground rod on the opposite ends of the rod and each table having a centerline axis of rotation in the same plane as the centerline axis of said collimated light beam, whereby when the small amounts of light passing said rod are diminished to zero, said cylinder has a desired centerline thereof moved into the same plane and coincident with the rotational axis of said tables and can be marked thereon in preparation for a subsequent grinding operation,

b. said diminishing means includes masking means located between said focusing means and supporting means, said masking means being symmetrically mounted with respect to said plane and including means for increasing or decreasing the amounts of light passing each side of said rod, and

c. said tables each includes means therein for thrusting an adhesive carrying bearing toward the ends of said rod and on the said centerline established therein to facilitate the easy mounting of said rod in a grinding machine or the like for grinding said rod down to a prescribed diameter or other chosen geometrical shape symmetrical about said centerline.

Claims

1. A process for locating a centerline for a selected geometrical shape within an irregularly shaped object to be ground about said centerline to a chosen geometric shape in a grinding machine, which process includes: a. locating said object so as partially to interrupt a beam of light of predetermined width provided by a light source, said beam having a centerline, b. selectively varying said predetermined width by variably masking said beam between an observation area and the source of sAid light beam, and c. shifting said object with respect to said light beam while rotating said object within said beam so as substantially to equalize the amounts of light passing by opposite sides of said object and reaching said observation area, whereby after said equalizing the centerline of said object is aligned with the centerline of said light beam and thereby identified, whereby said object may be rotated about the so-identified centerline in a grinding machine or the like to grind said object to a chosen geometrical shape.

2. The process defined in claim 1 wherein said shifting is carried out while simultaneously varying the width of said beam, so that the centerline established for said object is also the centerline of a cylinder within said irregularly shaped object.

3. A process for obtaining the centerline of a cylinder of a specified diameter from an unground semiconductor rod consisting of many different diameters, which process includes: a. locating the semiconductor rod so as partially to interrupt a beam of light of predetermined width provided by a light source; b. varying said predetermined width by masking said beam between the source of said light beam and said rod, and c. shifting said rod with respect to said light beam while rotating said rod within said beam so as to equalize the amounts of light passing by opposite sides of said rod, whereby an optimum centerline in said unground rod for minimum undersize grinding losses is thereby established.

5. A process for determining the central axis of a cylinder of a specified diameter within an unground semiconductor rod of varying diameters comprising: a. providing by means of a light source a collimated light beam of predetermined initial width, b. locating said unground rod in the path of said collimated light beam and near the centerline thereof, said rod having a diameter less than the initial width of said beam and partially interrupting said beam and dividing it into two separate beams of collimated light, c. masking said beam of said collimated light between said rod and the source of collimated light to thereby vary the amounts of light passing each side of said unground rod, d. continuously repositioning said rod while continuously diminishing the width of said light beam to maintain the widths of said two beams approximately equal at given points along the length of said rod, e. thereafter diminishing the width of said two beams to or toward zero at given locations along the length of said rod, whereby the centerline of said cylinder may be brought into the same plane as that containing the centerline of said light beam, and f. affixing index means such as rotational bearings on each end of said unground rod and on said centerline of said cylinder, whereby said rod may then be ground down to a cylinder having said specified diameter and said undersize losses reduced or completely eliminated.

7. Rod centering apparatus for establishing a centerline in an unground rod for a cylinder therein of a prescribed diameter, including in combination: a. means for focusing a beam of collimated light towards said unground rod so that said rod partially interrupts said beam of light and allows separate beams of collimated light to pass said rod on each side thereof, b. means for supporting said rod and having a centerline in the same plane as the centerline of said collimated light beam, c. means for diminishing toward zero the light passing each side of said rod at one or more points along the length thereof to enable the centerline of said cylinder to be moved into the same plane with the centerline of said collimated light beam, and d. rod positioning means for moving said rod in the X and Y directions on said supporting means, so that the amounts of light passing on each side of said rod may be varied.

9. Apparatus defined in claim 7 wherein said supporting means includes a pair of tables rotatably mounted to receive said unground rod on the opposite ends thereof and each having a centerline axis of rotation in the same plane as the centerline axis of said collimated light beam, whereby when the small amounts of light passing said rod are diminished to zero, said cylinder has its centerline moved into the same plane and coincident with the rotational axis of said tables and can be marked thereon in preparation for a subsequent grinding operation.

12. Apparatus defined in claim 8 wherein: a. said supporting means includes a pair of tables rotatably mounted to receive said unground rod on the opposite ends of the rod and each table having a centerline axis of rotation in the same plane as the centerline axis of said collimated light beam, whereby when the small amounts of light passing said rod are diminished to zero, said cylinder has a desired centerline thereof moved into the same plane and coincident with the rotational axis of said tables and can be marked thereon in preparation for a subsequent grinding operation, b. said diminishing means includes masking means located between said focusing means and supporting means, said masking means being symmetrically mounted with respect to said plane and including means for increasing or decreasing the amounts of light passing each side of said rod, and c. said tables each includes means therein for thrusting an adhesive carrying bearing toward the ends of said rod and on the said centerline established therein to facilitate the easy mounting of said rod in a grinding machine or the like for grinding said rod down to a prescribed diameter or other chosen geometrical shape symmetrical about said centerline.