Continuous Epitaxial Deposition System

Abstract

An epitaxial deposition system includes slice transporting boats which index individual slices between work stations. At the work stations, the temperature of the slices is controlled and the boats are sequentially filled with various gases, including an etching gas and a deposition gas. The work stations are surrounded by a sealed enclosure which receives gases discharged from the boats.

Description

In the electronic component manufacturing industry, certain products are produced by processes in which epitaxial layers are deposited on substrate slices. Heretofore, most epitaxial deposition processes have been designed to form layers on a large number of slices at the same time. This practice is known as batch processing.

Batch processing has several inherent disadvantages. First, in order to reduce the cost of each layer, batch processing is usually carried out in relatively large chambers. Deposition chambers must be purged and heated before the start of each deposition process. When large chambers are employed, the purging and heating procedure requires a relatively long time.

Second, it is very difficult to heat all portions of a large chamber uniformly. Also, it is difficult to supply deposition gas to a large number of slices on a uniform basis. Irregularities in slice heating and exposure to deposition gas result in differences between the epitaxial layers that are formed on various slices.

This invention relates to an epitaxial deposition system that operates on a continuous basis. The system operates on each slice in exactly the same way. Use of the system results in the fabrication of extremely uniform epitaxial layers.

In the preferred embodiment of the invention, epitaxial layers are formed on substrate slices by discharging a plurality of gases, at least one of which is a deposition gas, and by sequentially moving each slice into engagement with each gas. Preferably, the slices are positioned in boats and are engaged with the gases by sequentially filling the boats with each of the gases.

A more complete understanding of the invention may be had by referring to the following detailed description when taken in conjunction with the drawing, wherein:

FIG. 1 is a side view of a deposition system employing the invention in which certain parts have been broken away and certain parts have been shown schematically more clearly to illustrate certain features of the invention, and

FIG. 2 is an illustration of a slice transporting boat useful in the practice of the invention.

Referring now to the drawing and particularly to FIG. 1 thereof, there is shown a deposition system 10 employing the invention. The system 10 includes a plurality of work stations A, B, C, D, and E and a plurality of boats 12 which are employed to transport substrate slices between the work stations. As the slices are transported between the work stations, the system 10 operates to form epitaxial layers on the substrates. The system 10 operates on a continuous basis in that an epitaxial layer is formed on each substrate slice automatically as the slice is transported between the work stations by one of the boats 12.

Referring now to FIG. 2, the structural details of the boats 12 are shown. Each boat 12 is formed from quartz and includes a flat floor 14 and an upper portion 16 which may be of any convenient shape. One end of the upper portion 16 is closed. The other end is open to permit slices to be positioned within and removed from the boat. An induction susceptor 18 comprised of a silicon carbide coated graphite cylinder is positioned on the floor 14. In use, a slice is positioned on the susceptor 18 of each boat 12 for transportation through the system 10.

The boats 12 each include a gas inlet hole 20. The hole 20 is formed in the floor 14 and extends to a mixing chamber which is separated from the remainder of the boat 12 by a wall 22. The wall 22 has a slot 24 formed through it which directs gas from the mixing chamber over the susceptor 18. A gas outlet hole 26 is formed through the upper portion 16 of each boat 12.

Referring now to FIG. 1, the boats 12 are moved between the stations A, B, C, D, and E in trainlike fashion. The boats 12 are positioned in direct contact with each other in the train so that the closed end of one boat 12 operates to seal the open end of the next adjacent boat. The train of boats is preferably moved through the system 10 by an indexing mechanism (not shown) which advances the train one boat length each time it is actuated. By this means, each boat 12 is sequentially indexed into engagement with each work station of the system 10.

The deposition system 10 includes an enclosure 28 which extends over all of the work stations of the system. The enclosure 28 is formed from quartz and includes a flat floor 30 over which the boats 12 travel as they are indexed between the work stations. The enclosure 28 also includes an upper member 32 comprising an enlarged central portion 34 having a vent 36 formed in it and reduced end portions 38.

The end portions 38 of the enclosure 28 have interior dimensions substantially identical to the exterior dimensions of the upper portions 16 of the boats 12. Nitrogen (N.sub.2) is continually forced out of the ends of the enclosure 28 between the end portions 38 and the boats. The flow of nitrogen between the end portions 38 and the boats 12 seals the interior of the enclosure 28 against the entry of air.

Work station A includes a plurality of passageways 40 and a delivery tube 42. A coolant such as water is continually circulated through the passageways 40 to maintain station A at a relatively low temperature. The delivery tube 42 extends through the floor 30 of the enclosure 28 and is connected to a source of nitrogen (N.sub.2).

As each boat 12 is indexed to station A, the gas inlet hole 20 formed in the floor 14 of the boat is brought into alignment with the upper end of the delivery tube 42. Nitrogen flows into the boat 12 from the delivery tube 42 to purge the interior of the boat of air. Air from the boat, and subsequently nitrogen from the tube 42 flows out of the gas outlet hole 26 formed in the upper portion 16 of the boat 12 into the central portion 34 of the enclosure 28. From the enclosure 28 the air and nitrogen flow out of the system 10 through the vent 36. While the boat 12 is at work station A it is maintained at a reduced temperature by the flow of coolant through the passageways 40.

Work station B includes a heating device 44 and a delivery tube 46. The heating device 44 comprises a pancake type induction heating coil that is connected to a suitable source of induction heating power. The delivery tube 46 extends through the floor 30 of the enclosure 28 and is connected to a source of hydrogen (H.sub.2) and to a source of hydrogen chloride (HCl).

As each boat is indexed to work station B, the heating device 44 immediately begins to increase the temperature of the interior of the boat. As the temperature of the boat is raised, hydrogen is fed into the interior of the boat 12 through the delivery tube 46 to further purge the boat of air. Subsequently, either pure hydrogen chloride, hydrogen, or a mixture of the two is fed into the interior of the boat 12. The slot 24 formed in the wall 22 of the boat 12 directs the hydrogen chloride over the upper surface of a slice positioned on the induction susceptor 18. The hydrogen chloride etches the surface of the slice to render the surface absolutely clean.

Work station C is the epitaxial deposition station of the system 10. Station C includes a heating device 48 comprised of a pancake type induction heating coil and a suitable source of induction heating power. Station C also includes a delivery tube 50 which extends through the floor 30 of the enclosure 28 and which is connected to a source of deposition gas.

Before a boat 12 is indexed to Station C, the temperature of the slice positioned on the susceptor 18 of the boat is raised to a temperature suitable for epitaxial deposition by the heating device 44 of the Station B. At station C, the slice is maintained at the deposition temperature by the heating device 48. The delivery tube 50 directs a deposition gas into the interior of the boat 12 through the gas inlet hole 20. In the boat 12, the slot 24 directs the deposition gas over the slice positioned on the susceptor 18. As the gas engages the heated slice, an epitaxial layer is formed on the slice.

The nature of the deposition gas that is supplied to the boats 12 through the tube 50 depends upon the nature of the epitaxial layer to be formed. Ordinarily, the deposition gas will be comprised of a mixture of various gases. For example, if a silicon epitaxial layer is to be formed, the deposition gas may include silicon tetrachloride (SiCl.sub.4), hydrogen (H.sub.2), and an appropriate donor gas such as dibrane, arsine, phosphine, etc. depending upon the type of doping desired.

When the deposition of an epitaxial layer on the slice contained in a boat 12 has been completed, the boat is indexed to work station D. Station D includes a plurality of coolant passageways 52 and a delivery tube 54. The tube 54 extends through the floor 30 of the enclosure 28 and is connected to a source of hydrogen (H.sub.2).

At work station D, each boat 12 is cooled by the flow of a coolant such as water through the passageways 52. Simultaneously, hydrogen is fed into the interior of the boat through the tube 54 and the hole 20. The hydrogen forces the deposition gas out of the boat 12 and thereby stops the deposition process.

Work station E includes a delivery tube 56 which is connected to a source of nitrogen (N.sub.2). At work station E, nitrogen is introduced into the interior of the boat 12 through the tube 56. The nitrogen purges the boat 12 of the hydrogen that was introduced into the boat at the station D. The nitrogen also further reduces the temperature of the interior of the boat.

It should be understood that while the boats 12 are positioned at each of the work stations A, B, C, D, and E, the delivery tubes at the work station cause the various gases employed in the system 10 to flow through the boats 12 on a continuous basis. That is, while a boat is at each work station, a gas continuously flows through the inlet hole 20, through the slot 24, and through the outlet hole 26 of the boat. The gases flowing from the outlet holes 26 of the boats 12 merge together in the enlarged central portion 34 of the enclosure 28 and flow through the vent 36 of the enclosure 28 combined state. Thus, the boats 12 not only transport slices between the various work stations of the system 10, but also prevent unintentional contact between the slices and the various gases employed in the system.

It should be understood that the deposition system illustrated in the drawing is a basic system and that many modifications to the system are possible. For example, in many systems, additional stations identical to the work station B will be provided to assure proper slice preheating before the beginning of epitaxial deposition. In such a case, hydrogen chloride will ordinarily only be supplied to the boats at the last type B work station.

Similarly, in many systems more than one type C work station will be provided. In such systems, a portion of the epitaxial deposition process is carried on at each such work station.

Finally, in many systems it will be desirable to provide additional work stations similar to the station C at which different deposition gases are directed over the heated slices. In such systems multiple epitaxial layers of different types are formed on each slice as it passes through the system.

Of course, the structural details of the work stations comprising the system 10 are illustrated by way of example only and may be freely substituted. For example, heat pipes can be employed in the work stations B and C instead of induction heating coils. In such a case, either induction type or resistance type heating elements can be utilized in heating the heat pipes. Similarly, the work stations A and B can be cooled by heat pipes connected between the work stations and suitable heat sink.

Like the structural details of the work stations, the structural details of the boat 12 of the system 10 can be varied to provide specific performance characteristics in the system. For example, it has been found that many deposition gases do not react properly with slice surfaces when the surfaces are positioned exactly horizontally. Accordingly, in many systems it is desirable to position the work stations along an upwardly-extending plane so that the boats 12 travel angularly upwardly through the system 10. In such a case, the boats 12 may advantageously be equipped with suitable baffles and outlet holes to guide the various gases employed in system 10 with respect to the slices contained in the boats.

In some systems, induction susceptors will not provide adequate heat transfer to the slices. In such a case, heat pipes can be substituted for the susceptors in the boats. When heat pipes are employed they can often be formed integrally with the boat structure.

In many systems, radiation losses through the upper portions of the boats will be considerable. To this end, it will often be desirable to provide a radiation shield in each boat. This can be accomplished by forming the upper portion of each boat from two layers of quartz and positioning a layer of metal between the quartz layers. When such a layer of metal is enclosed in an inert gas atmosphere, it assumes a high and stable temperature and thereby minimizes radiation losses from the slice contained in the boat.

The epitaxial deposition system illustrated in the drawing differs from prior systems principally in that it is a continuous process. Thus, each work station of the system operates on a steady state basis. And, the boats are indexed at the end of equal time intervals to transport slices between the stations.

The continuous nature of the system results in distinct advantages over prior systems. For example, because the heating devices of the system are operated continuously and because the boats locate each slice on exactly the same position with respect to the heating devices, the system provides extremely uniform slice heating. That is, every slice is heated at the same rate and to the same temperature as every other slice.

The continuous nature of the system also results in more uniform gas flow than has been possible heretofore. The various gases employed in the system are continuously supplied to their respective delivery tubes at the same temperature and pressure. Insofar as possible, the boats of the system are constructed exactly alike. Therefore, the gas supplied at each work station of the system flows through each boat in exactly the same manner.

The advantages of the continuous nature of the system may be summarized simply; every slice is treated alike. That is, because of the uniform slice heating and gas flow characteristics of the system, the epitaxial layer that is deposited on one slice is the same as the layer deposited on any other slice. Thus, the use of the system results in an extremely uniform product.

In addition to being continuous, the epitaxial deposition system illustrated in the drawings is superior to prior systems because it is sequential. That is, slices emerge from the system in exactly the same order in which they are introduced. Sequential operation permits the results of changes in the operational parameters of the systems to be more easily traced. Also, in the event that the system fails to produce a satisfactory product, the cause of the failure is more easily determined.

Although only one embodiment of the invention is illustrated in the drawing and described herein, it will be understood that the invention is not limited to the embodiment disclosed but is capable of modification, rearrangement and substitution of parts and elements without departing from the spirit of the invention.

Claims

I claim:

1. A deposition system comprising:

a. means for discharging a plurality of selected gases into a plurality of work stations;

b. a plurality of boats and means for sequentially moving said boats into said work stations, each boat having at least first and second compartments, such that said selected gases sequentially flow through said first and second compartments; and

c. means for maintaining said boats and work stations at predetermined temperatures.

2. A deposition system in accordance with claim 1 in which each of said boats has a substantially flat bottom portion, a curved top portion joining said bottom portion along two edges, one substantially flat end portion joining said bottom and curved top portion thereby closing one end of said boat and a partition having an opening therein joining said bottom and top portions thereby dividing said boat into two compartments.

3. A deposition system in accordance with claim 1 which includes at least one work station having tubes through which a coolant may be circulated to reduce the temperature by said work station and a boat positioned therein.

4. A deposition system in accordance with claim 1 which includes at least one work station having heaters which may be used to increase the temperature of said work station and a boat positioned therein.

5. A deposition system in accordance with claim 1 in which each of said boats includes a susceptor on which a semiconductor slice may be placed and heated by induction heating.

6. A deposition system in accordance with claim 1 wherein said boats move in trainlike fashion into and through said work stations, such that the closed end of each of said boats is positioned adjacent to the open end by a following boat thereby closing said open end by said following boat.