Device For Indiffusing Dopants Into Semiconductor Wafers

Abstract

In a silicon tube whose diameter is two to three times larger than the diameter of the wafers to be diffused, a rod is situated centrally and in parallel to the tubular axis and has channels extending in parallel to the rod axis. These channels define, together with the tubular wall, cages for the stacks of wafers. In this manner, several parallel wafer stacks may be diffused in a single silicon tube. Favorable degrees of utilization of the silicon tube are obtained with five and six parallel stacks of wafers.

Parent Case Text

This is a (X) continuation, of application Ser. No. 108,723, filed Jan. 22, 1971, now abandoned.

Description

The present invention relates to a device for the indiffusion of dopants into semiconductor wafers, with a heatable tube of the same semiconductor material, adapted to accommodate wafers which are heated to diffusion temperature, in a vacuum or in a protective gas.

Such a device has previously been suggested. It has considerable advantage over devices wherein the semiconductor wafers are placed in a quartz tube. One advantage is, for example, the fact that the semiconductor material, i.e., silicon can withstand higher temperatures than quartz. As a result thereof, it is possible to effect diffusion in a tube of semiconductor material, at higher temperatures than in quartz tubes, thus speeding up the diffusion process. When diffusion is effected in a vacuum, quartz tubes soften at diffusion temperatures and are compressed by the outer air pressure. This may bend and tension the semiconductor wafers, situated in an evacuated quartz tube leading to disturbances and to dislocations in the crystal lattice of the semiconductor material. Recombination centers have a tendency to deposit at such dislocations, which has an adverse effect upon the qualities of the semiconductor components produced of this semiconductor material. Therefore, quartz discs having a diameter that is larger than the diameter of the semiconductor wafers, are provided in the evacuated quartz tubes which are also called quartz ampules. This prevents both a collapse of the quartz ampule and mechanical influences upon the semiconductor wafers. These support wafers must be relatively thick and require much space in the interior of the quartz ampule thereby losing much useful space. Support wafers are not necessary in evacuatable semiconductor ampules since, at diffusion temperatures, the latter are mechanically more stable than quartz ampules.

Diffusion devices of semiconductor material, for example silicon, have an additional advantage over quartz ampules insofar as the semiconductor wafers may come into contact with the tubular wall, without the disadvantage of causing undesired chemical reactions between the material of the ampule or tube and the semiconductor material, as is the case with quartz. Thus, the diameter of the semiconductor wafers may be almost as large as the inside diameter of the semiconductor tube.

This affords a good space utilization of the semiconductor tube. In addition, the semiconductor wafers are kept in their position by the inner wall of the tube so that mutual shifting of the wafers is prevented. When a semiconductor tube is completely filled with wafers, a twisting and bending of the wafers is reliably prevented.

Such semiconductor tubes can be produced by pyrolytic dissociation of a gaseous compound of said semiconductor material, in the presence of a reduction gas, for example hydrogen. Up to now, this method can be employed economically, only for larger tubular diameters, for example about 30 mm. If semiconductor wafers of smaller diameters are to be diffused in such tubes, the latter cannot be maintained in position thus the wafers may become mechanically stressed with resulting impairment of their electric properties.

The object of the present invention is to present a device of the afore-mentioned type, wherein it becomes possible to diffuse semiconductor wafers, whose diameters are smaller than the inner diameter of the semiconductor tube, without causing undesirable, mechanical stresses of the semiconductor wafers.

Our invention provides a holding device inside the tube which, together with the inner wall of the tube, forms a cage for the semiconductor wafers. THe holding device is of a material which does not react with the semiconductor wafers, at diffusion temperature. The holding device, preferably, has a rod parallel to the longitudinal axis of the tube. The rod is provided at its circumference with at least one recess parallel to its longitudinal axis. The recess, together with the wall of the tube, defines a cage for the semiconductor wafers by which the semiconductor wafers are kept in their position. The rod may, preferably, comprise over its circumference three, five or six recesses which are so formed that the rod has the cross section of a three, five or six-arm star. The tube has, preferably, a circular cross section.

The holding device may also be provided with recesses parallel to the tubular axis provided in the inner side of the tubular wall. These recesses, together with a rod situated in the tube in parallel to the tubular axis, define a cage for the semiconductor wafers. This cage helps to keep the semiconductor wafers in their position. The rod can then have a circular or star-shaped cross section.

A preferred feature of the invention is found in the fact that the rod is provided with two sealing discs where-between the semiconductor wafers are positioned. The thickness of the sealing discs is a multiple, at least double, of the thickness of the semiconductor wafers. The sealing discs also consist of a material which, under diffusion conditions, does not react with the semiconductor wafers. The rod and/or the sealing discs may be of the same semiconductor material as the semiconductor wafers, or may consist of sintered material, such as silicon carbide, for example. These sealing discs may have openings that may be somewhat smaller than the interior surface of the tube or they may be so designed that they seal the tube against the outer atmosphere.

The invention will be disclosed in greater detail with reference to the Drawings, which show several embodiment examples.

In the Drawings:

FIG. 1 is a device of the afore-mentioned type;

FIGS. 2, 3, 4, 5, 9 and 10 are a respective cross section through various embodiments of the invention;

FIGS. 6 and 7 are a longitudinal section through two embodiments of the invention, and

FIG. 8 is a suitable design of the device according to FIGS. 6 and 7.

FIG. 1 schematically illustrates a device of the afore-mentioned type, wherein semiconductor tube 1 is closed by a lid 2. The tube 1 accommodates semiconductor wafers with a diameter somewhat smaller than the inside diameter of the tube. The semiconductor wafers 3 are held in place by sealing discs 4 and 5 so that no matter which position is assumed by the tube 1, the semiconductor wafers 3 can become neither twisted nor bent. The tube 1 contains a dopant source 6 which, when the tube is heated by a heating coil 7, is heated to diffusion temperature.

FIG. 2 shows a cross section through a first embodiment of the invention. The same parts are provided here with the same reference numbers, as the device according to FIG. 1. It is obvious that the diameter of the wafers 3 is smaller than the inner diameter of tube 1. The semiconductor tube 1 is provided with a rod 8, positioned parallel to its longitudinal axis. The rod is provided with a recess 9 which lies parallel to its longitudinal axis. The recess 9 of the rod 8, together with the wall of the tube 1, defines a cage wherein the semiconductor wafters 3 are maintained in position.

The rod consists of a material which does not react with the semiconductor discs 3. Preferably, the rod may consist of the same semiconductor material as the wafers 3. When the discs are made, for example, of silicon, silicon is also selected for the rod. However, the rod may also consist, for example, of silicon carbide, SiC. If the rod 8 is of semiconductor material, one preferably starts with a full rod whose recess is obtained through an appropriate mechanical processing. When another work material is used, such as for example silicon carbide, the rod is preferably sintered.

FIG. 3 shows a device, wherein a rod 10 is arranged in the tube 1 and is provided with three recesses. This rod has a cross section shaped in form of a three-pointed star. The recesses are indicated as 11. The recesses 11, together with the wall of the tube 1, define cages wherein the semiconductor wafers 3 are kept in position. It is obvious that in this embodiment three parallel stacks of wafers are accommodated. The rod 10 may, in this case, also be comprised of the same semiconductor material as the wafers 3. The recesses may be produced, for example, by grinding out a rod of circular cross section. This is particularly simple when the areas which border the recesses 11, are made flat. They may also be of sintered material, such as for example, silicon carbide SiC. The recesses may be worked in, in this case, directly during the sintering processes through an appropriate sinter form.

FIG. 4 shows a device with a rod 12, which has divided over its periphery, five recesses 13, running parallel to the longitudinal axis. The rod 12 has a cross section like a five-pointed star. The recesses 13 are formed by faces of the rod which, preferably, are also flat. In this device, five parallel stacks of semiconductor wafers 3 may be accommodated. These stacks are fixed in their position relative to each oter, as well as relative to the tube 1. The utilization of the tube is relatively good as the cross section of the tubular interior, that is occupied by the semiconductor wafers, is relatively large compared to the entire interior area of the tube.

FIG. 5 shows another embodiment wherein a rod 14 is used and is provided with 6 recesses, distributed over its circumference. In this device, tube 1 is also relatively well utilized.

In the illustrated embodiments, the position of the semiconductor wafers is secured. Thus, individual discs can never slide out of the stack.

FIG. 6 shows, in longitudinal section, the embodiment according to FIG. 5. The same parts are provided here with the same reference numerals as in FIG. 5. It is obvious that the recesses 15 are shorter than the rod 14. Therefore, the latter has ends 16 and 17 above and below with greater dimensions and the original cross section form, for example, a circular cross section, if prior to the installation of the recesses 15 the rod 14 had a circular cross section. The ends 16 and 17 limit longitudinally the recesses 15 and keep together the stacks defined by the semiconductor wafers 3. This makes a twisting of the semiconductor wafers impossible. The cages formed by the recesses and the tubular wall therefore remove undesired mechanical stresses which may lead to a bending or twisting of the semiconductor discs. A further improvement in the adherence of the stacks is obtained so that the ends 16 and 17 are provided with sealing discs 18, having a central bore. The diameter of the bore of the sealing disc 19 is preferably such that it may be pressed upon the end 17. The outside diameter of the disc 19 is preferably somewhat smaller than the inner diameter of the tube 1, so that the dopant contained in the dopant source 6, may reach the semiconductor wafers 3. The other sealing disc 19 can simply be placed upon the stacks.

FIG. 7 shows a longitudinal section through another embodiment of the invention. An upper sealing disc 20 is presented. This disc is provided with a dense surface 21. Together with dense area 22, the sealing disc 20 is seated upon the upper rim 22 of the tube 1 and seals the same to the outside.

The semiconductor wafers 3 are stacked in a unit comprising the rod 14 and the end discs 18, 19 or 20, 19. The unit with the wafers is then accommodated in the tube 1 and the latter is gas-tightly sealed. The devices of FIGS. 6 and 7 may also be arranged horizontally rather than perpendicularly, as illustrated.

As shown in FIG. 8, the lower sealing disc 19 may have openings 23 and/or recesses 24, through which the dopant has access to the semiconductor wafers 3. It suffices, as a rule, that the openings and/or the recesses occupy an area which amounts to approximately 0.5 and 20 percent of the area of the sealing disc 19. The sealing discs are of a material which does not react with the semiconductor wafers, that means it is made of the same semiconductor material or, for example, of sintered material, such as silicon carbide SiC. The features of FIGS. 6 and 7 and 8 are applicable not only to the embodiment of FIG. 5, but also apply for all other embodiments.

The embodiments of FIGS. 6 and 7 are, accordingly, also suitable for diffusion in a flowing medium, wherein an inert gas which is charged with a dopant, is passed through the tube. To this end, the tube must be open at both sides. The sealing discs may then be constructed on both sides just as the sealing disc 19. It is also possible, however, to accommodate the tube 1 in a quartz ampule and to evacuate the same or to fill it with a protective gas. The tube 1 need not be sealed gas-tightly, in this case.

FIG. 9 illustrates another embodiment of the invention wherein recesses 34 are provided on the inside of a tube 30. These recesses form a cage together with a rod 31, positioned parallel to the axis of the tube. The cage secures the position of the semiconductor wafers 3. Tube 30 is of the same semiconductor material as the discs 3, while rod 31 may be of the same semiconductor material or even a sintered material, such as for example, SiC. This rod has a circular cross section.

In the embodiment shown in FIG. 10, in addition to the recesses 34, there is provided a rod 32 which also has recesses 36. The recesses 34 are positioned opposite the recesses 36. The tube which is indicated as 33, may comprise the same semiconductor material as the wafers 3. It is also possible to produce the tube of sintered material such as for example SiC, which does not react with the material of the semiconductor wafers. since sinter material is not gas tight, at the conditions which prevail during diffusion, it becomes necessary, in this instance, to place the tube 33 with the semiconductor wafers 3 and the rod 32, into a semiconductor tube 1, made of silicon, for example.

The embodiments of FIGS. 3 to 10 make it possible to place several charges of semiconductor wafers with variable characteristics, into a single tube and to keep them separated from each other. It should, of course, be realized that the space between the semiconductor wafers, stacked on one another, is adequate for diffusion of the dopant to all the wafers.

Claims

We claim:

1. A device for the indiffusion of dopants into semiconductor wafers comprising a heatable tube of a semiconductor material, an inner wall in said tube, said tube being adapted to accommodate wafers which are heated to diffusion temperature in vacuum or inert gas, means for providing a dopant source, and an elongated holding device for a stack of semiconductor wafers of a material which does not react with the semiconductor discs at diffusion temperature within said tube, said holding device together with the inner wall having, parallel to its longitudinal axis, at least one recess, which tube along the inner wall thereof and recess forms a channel for holding a stack of wafers.

2. The device of claim 1, wherein the holding device constitutes a rod parallel to the longitudinal axis of the heatable tube, at least one recess along the circumference of said holding device parallel to its longitudinal axis, said recess together with the inner wall of the tube forms a cage for the semiconductor wafers whereby the semiconductor wafers are kept in position perpindicular to the longitudinal axis.

3. The device of claim 2, wherein the said rod has three recesses giving the rod, along its cross-section, the shape of a three-pointed star.

4. The device of claim 2, wherein the said rod has five recesses giving the rod, along its cross-section, the shape of a five-pointed star.

5. The device of claim 2, wherein the said rod has six recesses giving the rod, along its cross-section, a six-pointed star.

6. The device of claim 2, wherein said rod has a crescent shaped cross-section.

7. The device of claim 1, wherein the inner wall of the tube has recesses parallel to the tube axis and a rod parallel to the tube axis together with the recesses form a cage for the semiconductor wafers whereby the semiconductor wafers are kept in position.

8. The device of claim 7, wherein the rod has a circular cross section.

9. The device of claim 7, wherein the rod has the same number of recesses as the tube wall and the recesses of the rod and the tube wall are in coincidence with each other.

10. The rod of claim 9, wherein the device has a star-shaped cross section.

11. The device of claim 2, wherein the rod is provided with sealing discs, between which the semiconductor wafers are arranged, the thickness of the sealing discs is at least double the thickness of the semiconductor wafers, and the sealing discs are of a material which, at diffusion temperature, does not react with the semiconductor wafers.

12. The device of claim 11, wherein at least one of the rod and the sealing discs is of the same material as the semiconductor wafers.

13. The device of claim 11, wherein at least one of the rod and the sealing discs is of sintered material.

14. The device of claim 11, wherein at least one of the rod and the sealing discs is of sintered silicon carbide.

15. The device of claim 11, wherein at least one sealing disc is provided with openings, the area of said openings constitute between 0.5 and 20 percent of the area of said sealing disc.

16. The device of claim 11, wherein the sealing disc is a thickening at the end of the rod.

17. The device of claim 2, wherein the rod is perpendicular to the longitudinal axis of the tube and the semiconductor wafers lie horizontally thereto.