Method of improving silicon crystal perfection in silicon on sapphire devices

A process for forming a relatively defect free layer of silicon on an insulating substrate wherein as soon as growth islands are formed on the substrate, to a point just prior to the complete coverage of the substrate with silicon, the formation of the layer is temporarily terminated. The growth islands are maintained at a given temperature for a predetermined period, to allow any defects, which may have started during the initial formation of the growth island, to be self-cured or to annihilate themselves. Thereafter, the growth of silicon is continued until the desired layer thickness is achieved.

BACKGROUND OF THE INVENTION 
This invention relates to semiconductor processing and more particularly to 
a method of improving the crystalline perfection of a layer of 
monocrystalline silicon epitaxially deposited on an insulating substrate. 
It has long been known that the semiconducting properties of an epitaxially 
deposited layer of silicon in both MOS and bipolar device structures, 
fabricated on an insulative substrate, is closely related to both the 
chemical and crystallographic nature of the silicon-substrate interface. 
The nature of the interface is a function of both the condition of the 
single crystalline nature of the substrate exposed to the deposition 
atmosphere and the variables involved in the heteroepitaxial growth 
process of the silicon film. The properties specific to the thin silicon 
film grown on the insulative substrate are largely determined by the 
contamination of the silicon film as a result of reactions with the 
insulative substrate which take place during the period immediately prior 
to the complete coverage of the substrate surface. 
Detailed examinations of, for example, (100) silicon deposited on a (0112) 
sapphire will show the presence of defects such as stacking faults and 
microtwins all of which contribute to increased leakage currents and 
decreased mobilities in the silicon film in its final, fabricated form. 
One attempt to improve the deposition technique and to minimize any auto 
doping of the silicon film at the silicon-insulating substrate interface 
has been detailed in U.S. Pat. No. 3,885,061, which issued on May 20, 1975 
to J. F. Corboy et al. and entitled "Dual Growth Rate Method of Depositing 
Epitaxial Crystalline Lines." This reference teaches the deposition of a 
film in two stages, the first stage being the deposition of a very thin 
film (500-2000 Angstroms) using a "burst" technique followed by the 
deposition of the remainder of the film at a slower rate until the desired 
thickness is reached. While the process of this reference does, in fact, 
produce fewer defects and faults in the deposited silicon layer, it does 
not address itself to or attempt to cure those defects or faults that are 
generated in the growth islands when the initial burst produces the 
individual growth islands. 
SUMMARY OF THE INVENTION 
A novel process is described for forming a relatively defect free, 
epitaxially grown layer of silicon on a sapphire substrate which includes 
the initial step of forming growth islands on an insulative substrate to a 
point just prior to the complete coverage of the substrate with silicon, 
annealing the growth islands for a predetermined period to allow the 
defects to annihilate themselves and thereafter continuing the growth of 
the silicon until the desired thickness is reached.

DETAILED DESCRIPTION OF THE PROCESS 
While the foregoing exegesis will be presented in terms of using sapphire 
as the insulative substrate or carrier for the epitaxial growth of a 
silicon film, we do not wish to be so limited. Those skilled in the art 
will readily recognize that when the expression "sapphire" or 
"silicon-on-sapphire" (SOS) appears, it is also meant to include the use 
of such other materials as spinel or gallium phosphide as the substrate. 
The initial steps in the practice of our invention start with the proper 
preparation of the substrate in order to facilitate the most favorable 
conditions for the growth of sufficiently high quality epitaxial layer of 
silicon so as to be able to manufacture a commercially successful 
microelectronic device. The usual practice consists of growing a good 
quality sapphire either in a boule and thereafter slicing and polishing 
the surface or, in the alternative, the sapphire may be an edge defined, 
film-fed growth which involves pulling a continuous ribbon of sapphire 
from a shaping dye. In any event, these or any one of many other well 
known techniques may be used for the production of a sapphire substrate, 
all of which are well known to those skilled in the art. If necessary, the 
substrate is polished or otherwise prepared for the deposition of a layer 
of epitaxially grown silicon by annealing to relieve any stresses that may 
have been introduced therein during its formation. 
The next step is to place the sapphire substrate in an appropriate reaction 
chamber and raise the temperature therein to about 1000.degree. C. 
Thereafter, while maintaining the substrate at about 1000.degree. C., a 
mixture of silane (SiH.sub.4) and hydrogen (H.sub.2) is introduced into 
the chamber in such proportions as to achieve a silicon growth rate of 
about 0.4 micron/min. This silicon film growth is continued for a period 
of about 1-3 seconds. 
FIG. 1 depicts the epitaxial growth of silicon on sapphire substrate 10 
during the very beginnings of the formation of the silicon layer. Growth 
islands 12.1-12.5 are usually randomly formed on substrate 10 and have not 
grown sufficiently laterally to coalesce or grow together with the next 
adjacent growth island. 
FIG. 2 represents a period of time, of the order of about 1 second after 
the onset of processing, when there has been little coalescing of adjacent 
islands, such as illustrated by islands 12.2 and 12.3. What is preferred 
is that the islands be allowed to grow in the manner illustrated by 
islands 12.4 and 12.5, that is, to the point where they only just abut. 
However, it should be understood that any coalescing of islands up to 
about 90% of the islands 12.1-12.5 can be tolerated. At this stage, 
islands 12.1-12.5 will have a mean diameter of about 1400-1500 Angstroms 
and a minimum height or thickness of about 300-500 Angstroms. 
At this point in time, the next step is taken which step takes the form of 
terminating the exposure of the structure to the SiH.sub.4 /H.sub.2 
mixture by cutting off the flow thereof and maintaining the structure at 
the growth temperature of about 1000.degree. C. for a sufficient period of 
time to allow any crystallographic misorientation or defects (stacking 
faults and microtwinning) that may have started in any of the growth 
islands to be self-cured. In the example described above, using a growth 
temperature of about 1000.degree. C. and an initial growth rate of silicon 
of about 0.4 micron/min., we have found that a curing time of about two 
minutes is sufficient for the defects to become annihilated or cured. 
After the curing step, the flow of SiH.sub.4 and H.sub.2 is restarted and 
the epitaxial growth of silicon is continued at whatever rate is desired. 
In this example, a growth rate of about 2.0 microns/min. is utilized for 
the second growth and continued until the desired thickness of silicon is 
achieved. Typically, the finished thickness for an SOS device is about 0.6 
micrometer. 
Soon after restarting the SiH.sub.4 /H.sub.2 flow, all of the silicon 
growth islands 12.1-12.5 have coalesced and growth together as shown in 
FIG. 3. When the processing is continued, as shown in FIG. 4, silicon 
layer 12.6 forms over the now coalesced sublayer 12.1-12.5, the 
combination forming epitaxially grown silicon layer 12. 
Thus, we have presented a method of forming an epitaxially grown layer of 
monocrystalline silicon having fewer defects than heretofore possible by 
curing the defects almost as soon as they are developed. This is done by 
means of a heat treatment before the defects have had the chance to 
increase in size and before the defects have had a chance to affect any 
subsequently grown silicon.