Method of restoring Si crystal lattice order after neutron irradiation

Si crystal lattice damage caused by neutron irradiation in homogeneously doping Si crystals with p.sup.31 is removed by an annealing process wherein the minimum temperature is adjusted in accordance with the make-up of the irradiation flux utilized during neutron irradiation and in accordance with the carbon concentration within the irradiated Si crystal.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to homogeneously doped Si crystals and somewhat more 
particularly to a method of restoring Si crystal lattice order after 
neutron irradiation. 
2. Prior Art 
Homogeneously doped Si crystals may be produced via neutron irradiation so 
that the nuclear reaction: 
##STR1## 
occurs within the irradiated Si crystal. The silicon crystal utilized as 
the stock or starting material for the irradiation generally is produced 
by thermal decomposition of silicon-containing compounds and generally 
contains carbon as an impurity. This is known, for example, from N. 
Schink, "Determination Of Carbon In Trichlorosilane", Semiconductor 
Silicon, (The Electrochemical Society, 1969) pages 85-88. However, such 
neutron irradiation causes lattice disorder or damage detrimental to the 
electrical properties of the doped crystal. Such neutron-induced lattice 
damage may be removed by annealing. For example, German Letters Patent No. 
1,214,789 suggests a method of producing homogeneously n-doped Si crystals 
by irradiating such crystals with thermal neutrons and then heat-treating 
the so-irradiated crystals at an elevated temperature for a sufficient 
period of time to remove the lattice damage cause by the neutron 
irradiation. In accordance with the prior art, the time period of the heat 
treatment is dependent upon the intensity of the neutron flux in the 
nuclear reactor during the irradiation process. Accordingly, the 
respective temperature and time is determined by the degree of crystal 
lattice damage or distortion produced by the irradiation process. The 
above-referenced prior art patent suggests that neutron-induced crystal 
lattice damage may be removed by annealing an irradiated Si crystal for 24 
hours in a furnace at 1000.degree. C. Other prior art, for example, German 
Offenlegungsschrift No. 25 16 514 (owned by the instant assignee and 
substantially corresponding to Burtscher et al U.S. Ser. No. 676,646, 
filed Apr. 14, 1976, now abandoned) suggests that such annealing be 
carried out for a time period at least equal to the time period of a 
subsequent diffusion process and at a temperature at least as high as that 
utilized during such subsequent diffusion process. 
However, it has been determined that when semiconductor components are 
produced from such prior art annealed crystals, the electrical properties, 
particularly the specific electrical resistance, sometimes vary during 
subsequent diffusion and the like processes. 
SUMMARY OF THE INVENTION 
The invention provides a method of restoring Si crystal lattice order in a 
Si monocrystal homogeneously doped via neutron irradiation comprised of an 
improved annealing process so that the semiconductor components produced 
from the so-annealed crystals exhibit reproducible electrical properties, 
which are congruent with at least the specific electrical resistance 
property of the annealed crystals. 
In accordance with the principles of the invention, neutron-irradiated 
carbon containing Si crystals are subjected to an annealing or 
heat-treatment process for at least 30 minutes at a minimum temperature 
adjusted in accordance with the make-up of the irradiation flux utilized 
during neutron irradiation (i.e., in accordance with the ratio of thermal 
neutrons to fast neutrons) and in accordance with the carbon concentration 
within the irradiated crystals. Of course, this carbon concentration is 
first determined in any known manner before the annealing process. 
In embodiments of the invention where the neutron flux utilized to 
irradiate Si crystals contains at least 99% thermal neutrons (i.e., the 
neutron flux contains a ratio of thermal neutrons to fast neutrons of 
100:1), the annealing temperature is set at a value of at least 
700.degree. C., independently of the carbon concentration in the 
irradiated crystals. 
In embodiments where the neutron flux utilized to irradiate Si crystals 
contains a ratio of thermal neutrons to fast neutrons in the range of 1:1 
to less than 10:1, the annealing temperature is set at a value greater 
than 1100.degree. C. if the irradiated crystals have a carbon 
concentration greater than 3.multidot.10.sup.16 atoms/cm.sup.3 and the 
annealing temperature is set in the range of 750.degree. to 1000.degree. 
C. if the irradiated crystals have a carbon concentration less than 
3.multidot.10.sup.16 atoms/cm.sup.3. 
In embodiments where the neutron flux utilized to irradiate Si crystals 
contains a ratio of thermal neutrons to fast neutrons in the range of 10:1 
to less than 100:1, the annealing temperature is set at a value greater 
than 1000.degree. C. if the irradiated crystals have a carbon content 
greater than 3.multidot.10.sup.16 atoms/cm.sup.3 and the annealing 
temperature is set to be at least equal to 750.degree. C. if the 
irradiated crystals have a carbon concentration less than 
3.multidot.10.sup.16 atoms/cm.sup.3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The invention provides a method of restoring Si crystal lattice order after 
such crystal has been subjected to neutron irradiation whereby stable and 
reproducible electrical properties are attained in the so-treated 
crystals. 
In accordance with the principles of the invention, a neutron-irradiated 
carbon containing Si crystal is annealed over a time period of at least 30 
minutes at a minimum temperature adjusted in accordance with the make-up 
of the irradiation flux (i.e., in accordance with the ratio of thermal to 
fast neutrons within such flux) and in accordance with the carbon 
concentration within the irradiated Si crystal. 
In an exemplary embodiment of the invention, a carbon containing Si crystal 
is irradiated with a neutron flux comprised of at least 99% thermal 
neutrons (i.e., the ratio of thermal to fast neutrons within such a flux 
is 100:1) and the so-irradiated crystal is then annealed for at least 30 
minutes at a temperature at least equal to 700.degree. C., independently 
of the carbon concentration within the irradiated crystal. 
In another embodiment of the invention, a carbon containing Si crystal is 
irradiated with a neutron flux comprised of less than 99% thermal neutrons 
(i.e., the ratio of thermal to fast neutrons within such a flux is smaller 
than 100:1) and the so-irradiated crystal is then annealed for at least 30 
minutes at a temperature adjusted in accordance with the actual make-up of 
the neutron flux and in accordance with the carbon concentration within 
the irradiated crystal. 
In instances in the foregoing embodiment wherein the proportion of thermal 
to fast neutrons within the irradiation flux is in the range of 1:1 to 
less than 10:1 and the carbon concentration within the irradiated crystal 
is greater than 3.multidot.10.sup.16 atoms/cm.sup.3, the annealing 
temperature is adjusted to be greater than 1100.degree. C. However, with 
the foregoing neutron flux, when the carbon concentration within an 
irradiated crystal is less than 3.multidot.10.sup.16 atoms/cm.sup.3, the 
annealing temperature is adjusted to be in the range of 750.degree. to 
1000.degree. C. 
In instances in the foregoing embodiment wherein the proportion of thermal 
to fast neutrons within the irradiation flux is in the range of 10:1 to 
less than 100:1 and the carbon concentration within the irradiated crystal 
is greater than 3.multidot.10.sup.16 atoms/cm.sup.3, the annealing 
temperature is adjusted to be greater than 1000.degree. C. However, with 
the foregoing neutron flux, when the carbon concentration within an 
irradiated crystal is less than 3.multidot.10.sup.16 atoms/cm.sup.3, the 
annealing temperature is adjusted to be at least 750.degree. C. 
In other words, the foregoing embodiments of the invention comprise 
annealing neutron-irradiated carbon containing Si crystals at temperature 
conditions which are determined by the ratio of thermal to fast neutrons 
within the neutron flux produced by a nuclear reactor utilized to 
irradiate the crystals and by the carbon concentration within the 
so-irradiated crystals so that the annealing temperature is greater than 
1000.degree. C. when the carbon concentration within the irradiated 
crystal is greater than 3.multidot.10.sup.16 atoms/cm.sup.3 and the 
annealing temperature is at least equal to 750.degree. when the carbon 
concentration within the irradiated crystal is less than 
3.multidot.10.sup.16 atoms/cm.sup.3. In this manner, the annealing effect 
is independent from the furnace atmosphere existing during an annealing 
process. 
In the development of the invention, it was also determined that the 
minimum temperature for annealing Si crystal discs having a thickness of 
less than 2 mm and with the same carbon concentration as in relatively 
large diameter silicon rods, may be even further lowered relative to the 
annealing temperature required for such rods. 
As is apparent from the foregoing specification, the present invention is 
susceptible of being embodied with various alterations and modifications 
which may differ particularly from those that have been described in the 
preceding specification and description. For this reason, it is to be 
fully understood that all of the foregoing is intended to be merely 
illustrative and is not to be construed or interpreted as being 
restrictive or otherwise limiting of the present invention, excepting as 
it is set forth and defined in the hereto-appended claims.