Porous silicon nitride semiconductor dopant carriers

New porous semiconductor dopant carriers are disclosed together with a method for the diffusion doping of semiconductors by the vapor phase transport of an n or p type dopants, such as phosphorus, arsenic, antimony, boron, gallium, aluminum, zinc, silicon, tellurium, tin and cadmium to the semiconductor host substrate; wherein the dopant source comprises a dopant containing porous, inert, rigid dimensionally stable and thermal shock resistant reaction sintered Si.sub.3 N.sub.4 carrier material.

FIELD OF INVENTION 
This invention relates to novel porous, inert, rigid, dimensionally stable 
and thermal shock resistant, vapor diffusion Si.sub.3 N.sub.4 containing 
dopant carriers, to methods of providing such carriers, to diffusion 
sources containing said carriers and to a method for diffusion doping 
semiconductors utilizing Si.sub.3 N.sub.4 containing carriers. 
BACKGROUND OF THE INVENTION 
Semiconductor elements have multiple applications and utility in the 
electronics industry and are used in rectifiers, transistors, photodiodes, 
solar batteries, radiation detectors, charged particle detectors, 
integrated circuits and various other applications. They have been known 
in the industry for many years and the term semi-conductor element is 
generally accepted throughout the industry and intended in this 
application to generically include semiconductor devices and parts thereof 
formed of host substrates comprising elements, alloys and intermetallic 
compounds of silicon, germanium, silicon/germanium, gallium arsenide, 
indium phosphide and the like. Such semiconductor elements can be of any 
convenient or suitable shape or form but are typically commercially used 
in the form of circular, rectangular or triangular wafers or disks. 
In order to achieve the various electrical rectification characteristics so 
important to their multiple applications and utilities, semiconductor 
elements typically have an active impurity incorporated within the host 
substrate, during manufacture or later by diffusion, which impurity 
affects the electrical rectification characteristics of the semiconductor 
element. These active impurities are usually classified as donor 
impurities or acceptor impurities; the donor impurities including 
phosphorus, arsenic, antimony, silicon, tellurium, tin, and the like; and, 
the acceptor impurities including boron, gallium, aluminum, zinc, cadmium 
and the like. 
The semiconductor element may have a region thereof containing an excess of 
donor impurities thus yielding an excess of free electrons. Such region is 
termed an impurity doped ntype region. Similarly, the semiconductor 
element may have a region thereof containing an excess of acceptor 
impurities, which results in a deficit of electrons, such region being 
termed an impurity doped p-type region. The boundary between such p-type 
and n-type regions is termed the n-p or p-n junction. In many applications 
the uniformity of the impurity distribution within the p or n type region, 
as well as the sharpness of the p-n or n-p junction, is critical to the 
efficiency of the semiconductor element. 
Multiple means have been proposed for incorporating various active 
impurities in the semiconductor element. Typically, the active impurity 
may be intimately incorporated during preparation of the host substrate or 
may be incorporated by deposition on the host substrate during 
manufacture. 
DESCRIPTION OF THE PRIOR ART 
The deposition of active impurities at the surface of the semiconductor 
host substrate during manufacture typically comprises the high temperature 
diffusion of vaporized dopant atoms into the body of the host substrate. 
Typically the diffusion of the doping substance into the host substrate is 
achieved by heating a predetermined quantity of dopant, together with the 
host substrate in a closed receptacle in such manner that dopant atoms 
will permeate the semiconductor body from all or select sides. One method 
involving deposition of dopants on a limited surface of a semiconductor 
element is described in U.S. Pat. No. 3,287,187 wherein an oxide of the 
host substrate material is deposited on the host substrate followed by 
diffusion of the doping substance into the substrate surface area by 
heating the host substrate. 
U.S. Pat. No. 3,923,563 depicts a typical method of deposition and 
diffusion wherein porous, rigid dimensionally stable wafers are formed by 
compacting and sintering refractory oxide powders. The thus formed wafers 
are then impregnated with aluminum metaphosphate, arsenic oxide or 
antimony oxide by treatment with solutions thereof in suitable organic or 
aqueous solvents. These wafers function as the source of dopant vapors and 
are positioned in a suitable furnace in the vicinity of the host 
substrate. The dopant wafer and host substrate are heated to temperatures 
between about 850.degree. C. to about 1250.degree. C. wherein the dopant 
wafer liberates phosphorus, arsenic or antimony oxide vapors which pass 
through the furnace and contact the host substrate. The vapors appear to 
react with the hot silicon surface and the elemental phosphorus, arsenic 
and/or antimony diffuse into the host substrate with continued heating to 
create the semiconductor element. 
U.S. Pat. No. 3,920,882 discloses a solid dopant source comprising a 
porous, inert, rigid, dimensionally stable, refractory support impregnated 
with a dopant component. The porous supports are formed by compacting and 
sintering refractory oxide powders such as stabilized zirconia powder, 
alumina powders, silica powders, thoria and the like; they are compacted, 
sintered and thereafter impregnated with a solution of the dopant 
component. 
U.S. Pat. No. 3,849,344 discloses a solid dopant source comprising a hot 
pressed composition containing preferably about 70 wt percent silicon 
nitride and about 30 wt percent of a phosphorus/silicon compound. The 
patent describes the hot pressing technique as resulting in uniform 
composites composed of discrete particles of its components, held together 
by the plastic deformation of the particles. Suitable phosphorus/silicon 
compounds are described as the reaction products of phosphorus and silicon 
oxides. The patent does disclose that up to about 95 weight percent of the 
composition can be silicon nitride. 
OBJECTS OF THE INVENTION 
One object of the invention is to provide novel solid dopant carriers 
comprised of reaction sintered Si.sub.3 N.sub.4. 
Another object of the invention is to provide novel solid dopant sources 
comprising a dopant and a carrier containing reaction sintered Si.sub.3 
N.sub.4. 
A further object of the invention is to provide methods for the preparation 
of dopant sources and dopant carriers containing reaction sintered 
Si.sub.3 N.sub.4. 
A still further object is to provide a method for the diffusion doping of 
semiconductor host substances by a dopant source comprising a dopant and a 
porous, inert, rigid non-oxide containing reaction sintered Si.sub.3 
N.sub.4 carrier material. 
These and other objects will be apparent from the following description of 
the invention. 
DESCRIPTION OF THE INVENTION 
It has been discovered that solid dopant sources can be provided, which are 
capable of liberating active impurities at elevated temperatures and which 
are so dimensionally stable as to have substantially no deformation or 
slump while maintaining high thermal shock resistance, such sources being 
comprised of reaction sintered Si.sub.3 N.sub.4. By reaction sintered 
Si.sub.3 N.sub.4 is meant that elemental silicon particulate material is 
first formed to an appropriate solid dopant source configuration and is 
then, nitrided, at elevated temperatures, to cause at least some formation 
of Si.sub.3 N.sub.4 together with bonding of either silicon nitride to 
silicon nitride or silicon nitride to elemental silicon. 
The solid dopant carrier of the invention can be prepared by various means. 
One preferred means is to compact crushed particulated elemental silicon 
within an appropriate die to form an appropriate "green compact" of the 
desired configuration. The green compact is then fired for a time and at a 
temperature sufficient for sintering, in the presence of nitrogen, to 
yield the porous, dimensionally stable, reaction sintered Si.sub.3 N.sub.4 
dopant carrier of the invention. 
Initially, particulate elemental silicon is selected having a particle size 
sufficient to yield a final sintered product having an appropriate 
porosity and pore size which is varied dependent upon the dopant which is 
sought to be carried. Generally, it is desirable to obtain a sintered 
carrier having a volume porosity of at least about 20% and preferably in 
the range of 40% to 80%. The pore size of the carrier is also critical in 
that they should not be so small as to significantly restrict the flow of 
dopant into the carrier. Typically, average pore size in the range of from 
about 5 microns to about 250 microns has been found appropriate for most 
dopants. Appropriate pore size can typically be achieved by utilizing 
particle sizes from about 1 micron to about 150 microns. Mixtures of 
elemental silicon with silicon nitride have been found effective for 
producing the desired reaction sintered product. In such instance mixtures 
containing up to about 75% by weight Si.sub.3 N.sub.4 can be effectively 
reaction sintered to produce the desired product. 
Compacting of the particulate compounds of this invention can act to 
achieve two purposes, firstly to form the "green compact" for sintering 
and secondly to achieve a convenient and suitable size and shape of the 
carrier. It should be understood that compacting is not a necessary 
element of this invention. 
In many instances, it is desirable to hold the crushed particulate together 
by a binding means to expedite compacting and to assure appropriate 
porosity during sintering. Typical binders which have been found useful 
for molding the particulate compounds of the invention into suitable form 
include organic binders such as starches, dextrines, gums, flours, casein, 
gelatins, albumins, proteins, lignins, cellulosics, bitumens, rubbers, 
alginates, waxes and the like; synthetic resins such as vinyls, acrylics, 
wax emulsions, paraffin, cellulosics, glycols, epoxies, phenolics and the 
like; and inorganic binders such as silicates, colloidal silica, colloidal 
alumina, colloidal aluminates, colloidal silicates and the like. 
In certain instances, various additive compounds may be included with the 
particulate compounds of the invention for purposes such as accelerating 
sintering or improving the mechanical or thermal strength of the moldings. 
In such instance, it is important that the amount and type of such 
additive compounds be controlled so that they do not adversely effect the 
dopant vaporization or contribute non-desirable diffusable impurities 
which adversely effect or otherwise undesirably influence the electrical 
properties of the semiconductor elements. 
The additive compounds can be granular or fibrous in shape. Though not 
generally necessary, fibrous additives have been found effective in 
enhancing the thermal shock resistance of molded wafers. Granular 
decomposable additives have been found effective in increasing the 
porosity of the sintered wafers. It should be understood, however, that 
though various additives can be utilized in the practice of this 
invention, it has generally been found that the compounds themselves are 
so superior that further additives are unnecessary. 
In the formation of solid dopant carriers the particulate elemental silicon 
is mixed with a binder as before described, with or without an appropriate 
additive, then molded or compacted into a suitable die. Compacting the 
compound/binder mixture is not necessary but in some instances may help 
form the particulate mixture to a desirable green density for sintering. 
The formed mixture can then be reaction sintered by heating in the 
presence of nitrogen to between about 1,000.degree. and 1,800.degree. C. 
until a porous, inert, rigid, Si.sub.3 N.sub.4 containing structure is 
created. 
During the sintering process, the surface of the carrier is subjected to a 
nitrogen atmosphere. The nitrogen can be in the form of a gas or nitrogen 
containing compound such as forming gas, ammonia, etc. Typically, the 
carrier is treated by a "static" system wherein the nitrogen is charged to 
the reactor furnace, however, it is preferred to use a flow system wherein 
nitrogen gas is caused to continually flow past the surface of the 
carrier. The nitrogen source present during the heating process causes the 
elemental silicon to be converted to Si.sub.3 N.sub.4 which in turn 
effects the sintering process. The thus formed reaction sintered product 
was found to be porous, inert, rigid, dimensionally stable and thermal 
shock resistant. 
The solid carrier can be formed in any convenient size and shape, but 
usually it is formed in substantially the same size and shape as the 
semiconductor element it will be doping. One advantage of the instant 
invention is that the starting compound may be molded, compacted and 
sintered into rods, billets, etc., which thereafter can be cut into 
wafers, disks, etc., rather than pressing each wafer, disk, etc., 
individually. It has been found that wafers produced using the compounds 
of this invention retain their form when subjected to heat treatment with 
the semiconductor element and exhibit superior thermal shock resistance. 
After formation of the solid dopant carrier, it must be impregnated with 
one or more appropriate dopants and/or other additives for utilization 
therewith. Any suitable means of impregnation can be utilized with the 
carrier of this invention including applying molten dopant, powdered 
dopant, solutions, suspensions, sputtering, molecular beam, vapors and the 
like. 
A preferred means involves the heating of the carrier with a solution or 
suspension of the dopant material in organic or aqueous solvent. 
Generally, the concentration of the solution or suspension is selected to 
yield a concentration of dopant on the carrier of at least about 10% by 
weight. After treatment of the carrier with a dopant solution or 
suspension, the carrier is typically dried by heating. 
Multiple dopants can be utilized with the solid carrier of the invention. 
Typical dopants include compounds containing the elements phosphorus, 
arsenic, antimony, boron, gallium, aluminum, zinc, indium, and the like. 
The thus formed dopant sources are typically ready for use in the vapor 
deposition process without any further processing steps being required. 
Typically, wafers of the dopant source are arranged in trays together with 
wafers of the semiconductor host substrate to be doped and heated to 
temperatures from about 500.degree. C. to about 1400.degree. C. until 
appropriate quantities of the active dopant impurities have been deposited 
on the semiconductor host substrate surface. 
The following examples are provided to illustrate the invention and are not 
meant as a limitation thereof. All temperatures are in degrees centigrade 
unless otherwise indicated.

EXAMPLE 1 
48 grams of metallic silicon, screened through a 50 mesh screen, was dry 
blended with 32 grams of methyl cellulose (4,000 cp) for 1 hour in a 
rotary mill at room temperature. The resulting particulate composition was 
formed into average 2.01 inch diameter, 0.04 inch thick wafers, by 
pressing into an appropriate die at 4,000 psi. The thus formed wafers were 
placed on an alumina plate and nitrided by heating to a temperature of 
1,400.degree. C. in a 4 inch mullite tube furnace for 39 hours in the 
presence of a flowing (2 L/min) gaseous nitrogen atmosphere. The resulting 
wafers were found to be comprised of Si.sub.3 N.sub.4 and had a diameter 
of 1.84 inches and a thickness of 0.07 inches. The thus formed wafers, 
upon visual inspection, appeared to have maintained their structural 
integrity, did not show deformation such as bending or warping and had a 
smooth, porous surface. 
EXAMPLE 2 
60 grams of metallic silicon, screened through a 50 mesh screen, was dry 
blended with 30 grams of methyl cellulose (4,000 cp) for 1 hour on a 
rotary mill at room temperature. The resulting particulate composition was 
formed into average 2.01 inch diameter, 0.08 inch thick wafers by pressing 
the material into an appropriate die at 4,000 psi. The formed wafers were 
placed on an alumina plate and nitrided by heating to a temperature of 
1,400.degree. C. in a 4 inch mullite furnace for 39 hours in the presence 
of a flowing (2L/min) nitrogen gas atmosphere. The resulting wafers were 
found to be comprised of Si.sub.3 N.sub.4, had a diameter of 1.95 inches 
and a thickness of 0.08 inches. The thus formed wafers upon visual 
inspection appeared to have maintained their structural integrity, did not 
show deformation such as bending or swelling and had a smooth, porous 
surface. 
EXAMPLE 3 
A foamed polyurethane wafer, 3.0 inches in diameter and 0.1 inch in 
thickness was impregnated using the process of copending application 
81010A/203D by immersion in a silicon slip containing 60 grams metallic 
silicon, 39 grams deionized water, 0.5 grams of ammonium alginate, 0.3 
grams of styrene malaic anhydride copolymer and 0.2 grams of ammonium 
carboxylate. The impregnated wafer was hand squeezed to remove excess 
fluids and was measured, showing a diameter of 3.2 inches and a thickness 
of 0.106 inches. The thus treated wafer was placed on an alumina plate and 
nitrided, by heating, to a temperature of 1,400.degree. C. in a 4 inch 
mullite furnace for 1.5 hours, then at 1450.degree. C. for 12 hours in the 
presence of flowing (2L/min) gaseous nitrogen atmosphere. The thus formed 
wafers were found to be comprised of Si.sub.3 N.sub.4 and had 
substantially retained the structural porosity of the foamed polyurethane, 
though the polyurethane had essentially decomposed. The dimension of the 
wafers were taken and were found to be an average 3.2 inches in diameter 
and 0.123 inches thick. The wafers did not show deformation such as 
bending or warping and had a porous surface. 
EXAMPLE 4 
187.5 grams of metallic silicon, screened through a 50 mesh screen, was 
blended for one hour on a rotary mill at room temperature, with 62.5 grams 
of Cere-Amic (gelatinized corn flour), 9.0 grams of dextrine and 15.0 
grams of deionized water. The thus blended particulate material was formed 
into a wafer by pressing into a 3.1 inch diameter billet mold, at 3.0 psi. 
The thus formed billet was heated slowly to 900.degree. C. and held at 
that temperature for 12 hours in the presence of nitrogen gas atmosphere. 
The treated billet was then placed in a 6 inch graphite tube furnace on a 
graphite setter and nitrided at 1400.degree. C. for 37 hours in the 
presence of a flowing (30 Std Ft.sup.3 /hn) gaseous nitrogen atmosphere. 
The thus nitrided billet was then sliced with a diamond cutting shear to 
form wafers 60 mil in thickness. The wafers displayed good mechanical 
integrity. 
EXAMPLE 5 
170 grams of metallic silicon, screened through a 100 mesh screen, was 
blended for one hour on a rotary mill at room temperature, with 30 grams 
of Cere-Amic, 7.5 grams of dextrine and 15.0 grams of deionized water. The 
thus blended particulate material was formed into a wafer by pressing into 
a 3.1 inch diameter billet mold, at 3.0 psi. The thus formed billet was 
heated slowly to 900.degree. C. and held at that temperature for 12 hours 
in the presence of nitrogen gas atmosphere. The treated billet was then 
placed in a muffle tube furnace on a graphite setter and heat treated in 
an Argon atmosphere to 1200.degree. C. for 3 hours, then heated at 
1450.degree. C. for 24 hours in the presence of a gaseous nitrogen 
atmosphere. The billet increased in weight 44% and was found to have a 
bulk density of 1.35 gm/cc. The porosity of the billet was determined to 
be 60%. The thus nitrided billet was then sliced with a diamond cutting 
shear to form wafers 60 mil in thickness. 
EXAMPLE 6 
Si.sub.3 N.sub.4 wafers, produced by the method of Example 5 are sprayed, 
at room temperature, with an aqueous dopant suspension comprising 100 
parts SiP.sub.2 0.sub.7, 122 parts de-ionized water, 1.5 parts ammonium 
alginate, 1.0 parts of styrene malaic anhydride copolymer and 0.7 parts of 
ammonium carboxylate, the amount of dopant sprayed on is an amount 
sufficient to effect a 50% weight add-on calculated after drying for 1 
hour at 100.degree. C. The dried dopant containing wafer is thereafter 
fired at 1,000.degree. C., in air, for 30 minutes to sinter the dopant 
containing source wafer. 
The aforesaid prepared dopant source wafer is heated in a diffusion furnace 
with a single crystal silicon host substrate semi-conductor element for 60 
minutes at 1,000.degree. C. in a nitrogen atmosphere. The resultant 
phosphorus doped semiconductor element is etched with a 10% aqueous 
hydrofluoric acid solution and tested in accord with ASTM F43-78 to 
determine sheet resistivity. ASTM F43-78 defines a four point probe 
technique for ascertaining the ratio of potential gradient parallel to the 
current in the material to the current density. The element is confirmed 
to have a uniform n-type region and found to have a sheet resistivity of 
3.68.+-.10% ohms/square. The used phosphorus containing dopant wafer does 
not show deformation such as bending or swelling and retains its porous 
surface. 
EXAMPLE 7 
In a similar manner to Example 6, dopant source wafers are prepared by 
spraying with an aqueous suspension comprising 100 parts A1As0.sub.4, 122 
parts de-ionized water, 1.5 parts ammonium alginate, 1.0 parts of styrene 
malaic anhydride copolymer and 0.7 parts ammonium carboxylate, to a dry 
weight add-on of 50% calculated after drying for 1 hour at 1,000.degree. 
C. The dried dopant containing wafer is fired at 1,000.degree. C., in air, 
for 30 minutes. 
A single crystal silicon host substrate semiconductor element, which has 
been heated at 1,000.degree. C. for 120 minutes with the dopant wafer, 
etched with 10% hydrofluoric acid and tested in accord with ASTM F43-78 is 
found to have a sheet resistivity of 44.+-.10% ohms/square and have a 
uniform n-type region. The used arsenic containing dopant wafer does not 
show deformation such as bending or swelling and retains its porous 
surface.