Zone melted recrystallized silicon on diamond

In silicon-on-diamond (SOD) technology, diamond replaces the silicon-dioxide in the silicon-on-insulator structure. Diamond is good thermal conductor unlike silicon dioxide and a good electrical insulator like silicon dioxide. A high grade, high purity device-quality silicon wafer is chosen. An insulating diamond film of about 0.5 um or any suitable thickness is grown on the whole silicon wafer, including the rim area. A polycrystalline silicon of about 2 microns thick or so is then deposited on the whole wafer, including the rim area. Using the rim area silicon as the seed the polycrystalline silicon crystallizes into a single crystal by a zone melting recrystallization technique. The rim area is scribed off the wafer, leaving a recrystallized silicon-on-diamond (FIG. 1d). The structure top to bottom is recrystallized silicon-diamond and silicon-substrate. The structure is similar to ZMR SOI, but the insulator here is diamond instead of silicon dioxide and is therefore called ZMR SOD. The devices, such as MOSFETS, Bipolar transistors, JFETS and diodes, are fabricated in the recrystallized silicon that sits on top of the diamond film.

BACKGROUND OF THE INVENTION 
The present invention relates generally to devices fabricated on thin 
silicon films, and more specifically the invention pertains to 
radiation-resistant silicon substrate and for high temperature operation 
of devices grown on insulating diamond films. 
Recently, there has been increased interest in recrystallizing thin layers 
of semiconductor material, especially silicon, on a buried noncrystalline 
insulator layer as in, semiconductor-on-insulator technology. Typically, a 
layer of insulator material is formed on a semiconductor substrate and, 
patterned, a layer of semiconductor material, is deposited thereover, the 
semiconductor layer is melted in whole or in part, and one or more 
solidification fronts are caused to advance laterally across the 
semiconductor layer. See, for instance, U.S. Pat. No. 4,323,417, issued 
Apr. 6, 1982 to H. W. Lam, the disclosure of which is incorporated herein 
by reference. Electronic devices based on buried insulator structures 
offer promise of, increased dielectric isolation, useful in, high 
voltage-high power devices, reduced parasitic capacitance in integrated 
circuits, and of improved radiation hardness of devices. Common buried 
insulator is silicon dioxide. While these devices have proven excellent, 
the search continues for radiation resistant silicon substrate devices, 
which could operate at high temperatures. 
A promising buried insulator candidate is diamond. Diamond films have a 
resistivity of 10.sup.16 ohm-cm, and are excellent electrical insulators. 
The task of producing a recrystallized silicon layer on diamond device is 
alleviated, to some extent, by the systems disclosed in the following U.S. 
Patents, the disclosures of which are incorporated herein by reference: 
U.S. Pat. No. 4,444,620 issued to Kovacs et al; 
U.S. Pat. No. 4,885,052 issued to Fan et al; 
The use of silicon with diamond films is discussed in the article entitled 
"Silicon on Insulator using CVD Diamond Films" by K. V. Ravi and M. I. 
Landstrass. This paper was presented at the First International Symposium 
on Diamond and Diamond-Like Films by, in the Electrochemical Society 
Meeting, Los Angeles, Calif., May 1989, and is incorporated herein by 
reference. 
Silicon-on-diamond is a form of silicon-on-insulator technology. 
Silicon-on-diamond with a very high electrical resistivity and thermal 
conductivity diamond film is suitable for developing high temperatures, 
high speed ultra large scale integrated (ULSI) circuits. This structure is 
suitable for radiation hardened applications. In silicon-on-diamond (SOD) 
technology, diamond replaces the silicon dioxide in the 
silicon-on-insulator structure. Diamond is a good thermal conductor, 
unlike silicon dioxide, and is a good electrical insulator, like silicon 
dioxide. 
SUMMARY OF THE INVENTION 
The present invention includes a process for fabricating radiation 
resistant silicon substrate based devices of zone melted, recrystallized 
silicon on diamond insulator films. The process begins by growing a 0.5 
micron thick or a suitable thickness diamond insulating film on a silicon 
wafer such that the wafer has an exposed outer rim which circumscribes the 
diamond film. 
Next, a polysilicon coating is deposited over the diamond film so that the 
polysilicon coating contacts the rim of the silicon wafer. Exposure of the 
wafer to heat at temperatures of 1,400.degree. C. recrystallizes the 
polysilicon coating into a single recrystallized silicon crystal. 
After the zone melted recrystallization treatment is administered, the rim 
of the silicon wafer is removed off to complete the recrystallized silicon 
wafer. This finished product is believed to be a radiation resistant 
silicon substrate upon which high quality semiconductor devices may be 
fabricated. 
It is an object of the present invention to provide a zone melted 
recrystallized silicon on diamond deposited silicon wafer that is 
radiation resistant. 
It is another object of the present invention to make a semiconductor wafer 
that utilizes the thermal conductivity and high electrical insulation 
properties of thin diamond films. 
These objects together with other objects, features, and advantages of the 
invention will become more readily apparent from the following detailed 
description when taken in conjunction with the accompanying drawings 
wherein like elements are given like reference numerals throughout.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention includes a radiation - resistant silicon substrate 
device which is grown on an electrically insulating diamond film. 
The readers attention is now directed towards FIGS. 1a-1e which depict the 
sequence of steps in which the present invention is fabricated. As shown, 
a (3" to 8" diameter although other sizes could be used) high grade, high 
purity device-quality silicon wafer is used. An insulating diamond film of 
0.5 um thick (or any thickness technology permits) is grown on the whole 
silicon wafer, except for a rim about 1/4" wide (FIG. 1a). A 
polycrystalline silicon layer of 2 um thickness or more (or any thickness 
technology permits) is then deposited on this wafer covering the whole 
wafer, including the rim area (FIG. 1b). Using the rim area silicon as the 
seed the polycrystalline silicon is crystallized into a single crystal by 
a zone melting recrystallization technique. 
Zone-melting recrystallization (ZMR) is a process for producing thin 
crystalline films of silicon (Si) isolated from a silicon substrate by a 
buried insulating layer, i.e., SiO.sub.2. In the prior art ZMR process, a 
layer of SiO.sub.2 is deposited on a substrate, often a single-crystal 
wafer. Polycrystalline Si (polysilicon) is then deposited on the SiO.sub.2 
layer, followed by deposition of a capping layer or a wetting agent, such 
as, SiO.sub.2. This structure is then subjected to a heat treatment 
wherein the polysilicon film is melted. 
Typically, the heating is performed using a stationary bottom heater 
adjacent to the substrate surface. The stationary heater elevates the 
temperature of the polysilicon to about 1000.degree. C., near its melting 
point. A movable upper heating source is then translated past the 
structure adjacent to the capping film to supply sufficient heat to melt 
the polysilicon as the heating source moves along its path. Upon 
recrystallization the polysilicon film is transformed to a single, or 
nearly single crystalline film. Optionally, a single crystal seed material 
may be used to aid in epitaxial recrystallization. (See U.S. Pat. No. 
4,371,421 to Fan et al.) 
Silicon-on-Insulator (SOI) material prepared by the ZMR technique promises 
to satisfy the material needs for many important device applications, 
including radiation-hardened circuits, high temperature operation 
circuits, high voltage circuits, very high speed computating circuits, 
microsensors etc. In the present invention, the polysilicon layer is zone 
melted with the silicon layer to encapsulate the diamond insulator. Next, 
the rim area is scribed off the wafer (FIG. 1c), leaving a recrystallized 
silicon-on-diamond structure (FIG. 1d). The structure from top to bottom 
is recrystallized silicon, diamond, silicon substrate. The structure is 
similar to ZMR SOI, but the insulator here is diamond instead of silicon 
dioxide and is therefore called ZMR SOD. Devices can be fabricated in the 
recrystallized silicon layer that sits on top of the diamond film. FIG. 1e 
is a plain view of the wafer which shows the wafer diameter with a 1/4 
inch rim (any suitable rim area could also be used) that is to be trimmed 
after the ZMR step. 
It is also possible to grow a thick diamond film on silicon, with a 1/4" 
rim (any size rim suitable for recrystalllized system) as discussed above 
(FIG. 2a). A polysilicon film is deposited on the whole wafer and 
recrystallized by a ZMR technique (FIG. 2b). The silicon substrate is 
etched off to yield a recrystallized silicon and a (stand alone) diamond 
film (FIG. 2c). The structure is mounted on a heat sink (FIG. 2c) and the 
rim is scribed off (FIG. 2d). The final structure is recrystallized 
silicon on diamond mounted on a heat sink (FIG. 2e). 
In FIG. 2e, the heat sink is a mounting base, usually metallic, that 
dissipates, carries away, or radiates into the surrounding atmosphere the 
heat generated by a semiconductor device. The package of the device often 
serves as a heat sink, but, for devices of higher power, a separate heat 
sink on which one or more packages are mounted is required to prevent 
overheating and subsequent destruction of the semiconductor junction. 
In the present invention, the heat sink may be a mass of metal that is 
added to a device structure for the absorption or transfer of heat away 
from critical parts. It is generally made from aluminum to achieve high 
heat conductivity and minimum added weight. Most heat sinks are of 
one-piece construction. They may also be designed for mounting on 
printed-circuit boards. 
In one embodiment of the invention, the heat sink may be a metal part with 
a large surface area to facilitate the removal of heat. This heat sink may 
have fins mounted on or under a circuit component which produces heat, 
such as a power transistor, silicon rectifiers, etc. The heat sink absorbs 
and then radiates the heat to maintain a safe working temperature for the 
component. 
As mentioned in the above-cited Ravi et al reference, the use of diamond 
films with silicon has a number of advantages. Diamond has a dielectric 
constant of 5.5 and an electrical resistivity of 10.sup.16 ohm cm. The 
Ravi et al wafer is described as having: a silicon top layer, a diamond 
insulation layer, and a polysilicon substrate. The present invention has a 
recrystallized silicon top layer, a diamond insulation layer, and a 
silicon substrate which may be mounted on a heat sink. The objective of 
this effort is to apply the radiation hardness properties of insulating 
diamond films grown on silicon substrates and of devices fabricated in 
thin recrystallized silicon films forming a silicon-on-diamond (SOD) 
structure to fabricate silicon rad-hard, high temperature, high speed 
circuits. 
The following electronic properties are of interest in using diamond film 
as an insulating layer. Diamond is probably rad hard because it has high 
bond energy, low neutron cross section, and high band gap. Diamond is an 
electrical insulator and an excellent thermal conductor, and it is 
therefore, an ideal candidate to replace SiO.sub.2 in a SOI structure. The 
SOD structure instead of the SOI structure is thus ideal for fabricating 
rad hard circuits. 
There are a few small commercial companies growing diamond films on 
silicon, one of which is the crystalline company of Menlo Park, Calif. 
As described above, the present invention can be considered a process of 
fabricating radiation hardened, high speed, small feature size and ultra 
large scale integrated (ULSI) MOSFET and other integrated circuits. This 
process may be referred to as zone melted recrystallized silicon on 
diamond (ZMR SOD) and is described below. The first step of the process 
entails growing an 0.5 micron thick or any suitable thickness insulating 
diamond film on a silicon wafer. The above-cited Ravi et al reference 
describes the use of plasma enhanced chemical vapor deposition of diamond 
to fabricate a wafer silicon-on-diamond that includes a silicon layer on 
top and one polysilicon layer at bottom. Referring back to FIG. 1a, the 
silicon wafer of any diameter wafer with a 1/4 inch or suitable size rim 
circumscribing the diamond film. 
The second step of the process entails depositing a polysilicon layer of 
two microns or any suitable thickness over diamond and silicon wafer, as 
shown in FIG. 1b. Polysilicon deposition techniques are well known in the 
art, and need not be redescribed here. 
In the third step of the process, the polysilicon layer is converted to a 
recrystallized silicon single crystal using zone melting 
recrystallization. In the zone-melting method, the sample, e.g., a wafer, 
is typically coupled to a heat source that maintains the sample at a 
temperature slightly below the melting temperature of the semiconductor 
material, and a strip-like hot zone (in which the semiconductor material 
is molten) is scanned across the sample. A variety of means exists for 
producing the moving hot zone, e.g., a line-focused laser or other light 
source, or an electron beam. Graphite strip heaters are also used in the 
prior art. As described in the above-cited Kovacs patent, various heat 
sources are used to melt the thin semiconductor layers, including strip 
heaters, electron or laser irradiation, and irradiation with high 
intensity radiation from, e.g., tungsten halogen lamps. Various techniques 
for recrystallization are also known to the art. Among the techniques is a 
global melting approach, typically comprising simultaneous exposure of a 
whole wafer to high intensity visible and infrared radiation. In this 
step, the recommended temperature is 1400.degree. C. for the particular 
thickness of polysilicon being used. 
Once the zone melting recrystallation step is complete, the rim edge is 
trimmed off, and a complete wafer is ready for use for semiconductor 
device fabrications. 
The diamond films on silicon are radiation hard, good thermal conductors 
and good electrical insulators and are therefore ideal as substrates for 
developing radiation hard silicon VLSI circuits. Diamond is a good thermal 
conductor suitable for developing small geometry devices, thus increasing 
the frequency of operation. As a good electrical insulator it reduces the 
active volume of silicon on top, thereby making it a suitable to fabricate 
radiation hard circuit. 
While the invention has been described in its presently preferred 
embodiment it is understood that the words which have been used are words 
of description rather than words of limitation and that changes within the 
purview of the appended claims may be made without departing from the 
scope and spirit of the invention in its broader aspects.