Note: Descriptions are shown in the official language in which they were submitted.
23~
1 46,767
FORMING IMPURITY REGIONS IN SEMICONDUCTOR
BODIES BY HIGH ENERGY ION IRRADIATION,
AND SEMICONDUCTOR DEVICES MADE THEREBY
BACKGROUND OF THE INVENTION
Field of the Invention:
The present invention relates to semiconductor
devices and particularly semiconductor devices having
buried impurity regions
Description of the Prior Art:
Semiconductor devices generally-require impurity
regions in them to impart electrical characteristics.
These impurity regions are formed typically by diffusion
of a selected impurity into the semiconductor body or the
inclusion of the impurity in the semiconductor body during
its formation, e.g., by epitaxial growth. Impurities
usually used for such purpose are phosphorus, antimony,
arsenic, boron, gallium, aluminum, and gold.
Formation of the impurity regions in semicon-
ductor devices by diffusion has inherent limitations. The
concentration of the impurity at the surface of the semi-
conductor body through which it is diffused is normally
fixed by the saturation solubility concentration of the
impurity in the semiconductor material. This results in
high impurity concentration at the surface of the semi-
conductor body which both chemically and electrically
degrade the surface portions of the body In addition,
the thickness of the impurity region in the semiconductor
body and the concentration gradient is fixed by th~ sur-
face concentration, diffusion rate, temperature and time
1 162326
2 46,767
of diffusion. The impurity region is thus restricted in
its thickness and profile gradient and oftentimes requires
considerable time to make. Additionally, the thickness of
the impurity regions are often difficult to control with
precision and may require simultaneous or successive
diffusion of more than one impurity, e.g., boron and
aluminum, to form the impurity region of the desired
electrical characteristics.
Forming the impurity region by epitaxial growth
similarly has inherent limitations. Epitaxial growths
require careful preparation of the substrate, whether it
be semiconductor or insulator, and careful control of the
deposition system during the growth. Even with careful
control, epitaxial semiconductor bodies are typically
limited in their electrical characteristics by the pres-
ence of fugitive impurities of low concentrations which
somehow find their way into the system. Also, epitaxial
growths are in general limited to rather narrow thickness-
es which makes the application of that technique unuseful
in making certain semiconductor devices, such as high-
power devices
Another technique used for forming shallow
impurity regions of high concentration near the surface of
semiconductor bodies is ion implantation. See, e.g., Ion
Beams by Wilson and Rewer (1973), and Ion Implantation _
Semiconductors by Mayer, Eriksson and Davies (1970). In
this technique, a low energy ion beam of generally about
20~ to 4Q0 Kev. was formed of ions of the impurity desired
in the impurity region, and a major surface of semicon-
ductor body bombarded with the ion beam. An impurityregion was thus formed adjacent the surface of the semi-
conductor body of a few microns in thickness of high, fix
concentration gradient.
Ion implantation has not been considered to be
useful in making thick impurity regions penetrating beyond
a few microns into the semiconductor body or impurity
regions of controllable concentration gradient. The
crucial limitation has been that the ion beam generators
1 16~326
3 46,767
available have produced monoenergetic ion beams which
produced very narrow impurity regions of use at the sur-
face of the semiconductor body. The thickness of the
impurity region in the semiconductor body had been extend-
ed by pivotally moving the body during ion implantation,but even with this technique, the thickness of the impur-
ity region was typically less than 5 microns and of uncon-
trolled impurity concentration gradient.
It has been known to interpose a scattering foil
of metal, e.g., aluminum, of about one-half a mil in
thickness between the ion beam source and semiconductor
body. But these foils were necessarily uniform thickness
to provide the desired scattering of the ion beam to
provide a substantially uniform dosage over a large area
of the semiconductor body. It was not conceived to shape
a material of low scattering properties to modify and
modulate the beam energy to tailor an impurity region of
large thickness and variable concentration gradient.
The problems are compounded when it is desired
to form a buried impurity region within a semiconductor
body. ~ A buried impurity region is one in the interior of
the body which has a high impurity concentration relative
to the adjacent impurity region or regions between it and
a working surface of the semiconductor device. Generally,
such buried impurity regions are formed by a series of
epitaxial growths or a combination of diffusion and epi-
taxial growth. See, e.g., U.S. Patent No. 3,237,042,
assigned to the same assignee as the present application.
These techniques are difficult and require considerable
time to perform, and even then the yields of devices are
relatively low. The formation of such buried impurity
regions is also compounded by the difficulty in controll^
ing autodoping of the lower concentration impurity regions
over the buried impurity regions and in turn the diffi-
culty in controlling the thickness and concentation grad-
ient of both the buried impurity region and the impurity
region adjacent to it.
The present invention overcomes all of these
1 ~62326
4 46,767
difficulties and disadvantages. It provides a way of
rapidly forming impurity regions generally, and buried
impurity regions in particular in a semiconductor body
with a high degree of precision. Manufacturing yields can
be greatly increased with accompanying marked decrease in
production costs. Further, it enables the making of
semiconductor devices heretofore not possible because of
the restrictions on the concentration gradient by the
inherent limitations of the formation techniques. The
present invention provides a flexibility in forming con-
centration gradients of impurity regions and the position-
ing of impurity regions in the semiconductor body previ-
ously unavailable.
SUMMARY OF THE INVENTION
The present invention is a method of forming an
impurity region or regions in a semiconductor body by high
energy ion irradiation. An ion beam is formed containing
ions of an impurity desired to form an impurity region or
regions in a selected semiconductor body. The ion beam is
Z0 of such enérgy that it can penetrate the semiconductor
body through a selected surface to a depth greater than
the maximum depth of a desired impurity region from the
selected surface.
A beam modifier is formed of a given material in
a non-uniform shape to modify the energy of the radiation
beam on transmission therethrough to form a transmitted
`energy beam capable of forming an impurity region of a
desired thickness and impurity gradient in the semicon-
ductor body a given distance from a selected surface
through which the semiconductor body is irradiated.
Preferably the beam modifier is made of a material such as
aluminum, beryllium or radiation-resistant silicone or
epoxy to reduce scattering ~nd to provide good resolution
for the transmitted ion beam.
The impurity region is formed by positioning the
selected surface of the semiconductor body to be exposed
to the ion beam through the beam modifier. The beam
transmitted through the beam modifier thus penetrates the
~ 18232~
46,767
semiconductor body through the selec~ed surface and forms
the desired impurity region within the body. The thick-
ness of the impurity region (i.e., its distance along the
transmission direction of the ion beam) and its distance
from the selected surface are accurately controlled by the
energ~ of the ion beam and thickness of the beam modifier.
The impurity concentration profile of the impurity region
is controlled with precision by the contour of the beam
modifier and the predetermined relative movement between
the beam ~odifier and the semiconductor body during dop-
ing.
The present invention is particularly useful in
preparing semicondùctor devices where buried regions and
regions of previously unusual doping concentration gradi-
ents are desired. In some embodiments the semiconductorbody may be preferably annealed after the doping operation
to reduce the damage to the crystalline lattice of the
semiconductor body.
Other details, objects and advantages of the
2~ invention become apparent as the following description of
the presently preferred embodiments and presently prefer-
red methods of practicing the same proceeds.
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings, the presently
preferred embodiments of the invention and the presently
preferred methods of practicing the invention are illu-
strated in which:
Figure 1 is an elevational view in cross-section
of a transistor with a buried collector region made in
accordance with the present invention;
Fig. 2 is an elevational view in cross-section
of a second transistor with a buried collector region made
in accordance with the present invention;
Fig. 3 is an elevational view in cross-section
of an integrated circuit made in accordance with the
present invention;
Fig. 4 is an elevational view in cross-section
of a semiconductor body wherein an impurity region is
t 162326
6 46,767
formed in accordance with the present invention;
Fig. 5 is an elevational view in cross-section
of a second semiconductor body in which an impurity region
is formed in accordance with the present invention; and
Fig. 6 is an elevational view in cross-section
of a third semiconductor body formed in accordance with
the present invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
Referring to Fig. 1, a transistor is made with a
buried collector region utilizing the present invention.
The transistor is formed in semiconductor body 10 having
opposed major surfaces 11 and 12, which has been attached
to substrate 13 at major surface 12 by alloying, electro-
- static technique, epitaxial growth or some other means.
The semiconductor body is doped during manufacture to the
desired impurity concentration for the collector region,
e.g., 1 x 1013 to 1 x 1014 per cubic centimeter. Sub-
strate 13 is preferably an insulator material appropriate
for the particular application.
Base region 15 and emitter region 14 are formed
by sequential diffusions of impurities of opposite conduc-
tivity type, e.g., gallium and phosphorus, through major
surface 11 using standard oxide masking and photoetch
techniques. The impurity utilized in forming base region
14 is also of opposite conductivity type from the impuri-
ties formed in the semiconductor body during its manufac-
ture. PN junctions 17 and 18 are thus formed in the
semiconductor body between emitter and base regions 14 and
15 and collector and base regions 15 and 16. The impurity
concentration of base region 15 typically ranges from
1 x 1015 to 1 x 1017 per cubic centimeter, and the impur-
ity concentration of emitter region typically ranges from
1 x 1017 to 1 x 1019 per cubic centimeter. Collector
region 12 is formed of the impurity concentration in the
remaining part of semiconductor body 10 using the irradia-
tion technique hereinafter described.
Buried collector region 19 is then formed in the
semiconductor body at a depth to provide the desired width
1 162326
7 46,767
of collector region 16. The buried collector region is a
conductor electrically connecting the transistor to anoth-
er electrical component preferably in the same semicon-
ductor body.
To forming collector region 19 an ion source is
provided which is capable of emitting particles with
molecular weight of at least one (1) to form ion beam 20.
Ion beam 20 is of an energy capable of penetrating semi-
conductor body 10 through selected surface 11 to a depth
greater than the desired depth of buried collector region
19 in the body.
The ion source may be any conveniently available
source which emits ions of an impurity desired to form
buried collector region with sufficient energy to pene-
trate body 10 to at least the desired depth of the buriedcollector region. Preferably the ion source is a Van de
Graaff accelerator emitting ions of boron because such
particles are relatively inexpensive to accelerate to
energy sufficient to penetrate the semi,conductor body to
the desired depth. Other ions suitable to form impurities
in semiconductor materials, such as phosphorus or alumi-
num, may be utilized; however, ions having a molecular
weight higher than 16 are presently impractical because
available ion sources, e.g., Van de Graaff accelerators,
do not generate high enough energy to cause penetration of
such higher molecular weight particles into material to a
significant depth. In any event, the ion selected must be
of the same conductivity type as that utilized for col-
lector region 16 to provide the desired electrical charac-
teristics to the buried collector region.
The ion source is also preferably a monoenerget-
ic source such as conventionally produced by Van de Graaff
accelerators to permit the buried collector region to be
more precisely controlled in thickness, width and dosage
gradient. For this reason, higher molecular particles
such as phosphorus or aluminum ions may be more useful in
certain applications where higher resolution is desired
for the impurity region because such ions have a narrower
1 ~62326
8 46,767
half-width to the defect generation distribution produced
in semiconductor materials. The narrower the half-width
the more precisely the impurity region desired for buried
collector region 19 can be positioned in semiconductor
body 10 and the more precisely the concentration and
concentration grad~ent can be controlled.
Additionally, it should be observed that it may
be appropriate in certain applications to use a non-mono-
energetic radiation source or to modify a monoenergetic
source so that it is not monoenergetic. For example, it
may be more desirable to have a more uniform particle
distribution over the area of semiconductor body 10 at a
sacrifice of resolution of the depth of the impurity
region. This can be done by utilization of a scattering
foil (not shown) in the path of the ion beam 20 between
the ion source and semiconductor body 10. Generally,
however, a monoenergetic radiation source is preferred to
provide the narrowest half-width for the defect generation
distribution of the beam in the material and in turn the
highest possible resolution for the impurity region formed
in the semiconductor body.
Positioned between the ion beam 20 and semicon-
ductor body 10 is beam modifier 21 Beam modifier 21 is
selected of a material to allow transmission of ion beam
20 through it normally without substantially scattering of
the beam, Although in some circumstances it may be ap-
propriate to incorporate a scattering foil with beam
modifier 21, typically it is desired that the transmitted
ion beam 22 not be dispersed due to variations in the
thickness of modifier 21. This improves the accuracy of
the placement and resolution of the impurity region. Beam
modifier 21 is of a material which modifies the energy of
the transmitted ion beam, but is preferably selected from
a material which does not modify the energy of the beam
greatly per unit thickness. This permits the accuracy of
the thickness and dosage gradient of the impurity region
to be more precisely controlled without critically con-
trolling dimensional tolerances of beam modifier 21
t 162326
9 46,767
Examples of such materials are those composed of low
atomic number elements such as aluminum or beryllium. If
the ion beam used not penetrated too deeply into semicon-
ductor body, radiation resistance silicone and epoxy may
be used. Also, the beam modifier is preferably made of a
material that is easily worked to the desired shape as
hereinafter explained.
Preferably semiconductor body 10 and beam modi-
fier 21 are moved relative to each other through a prede-
termined motion during the doping. This can be done byany suitable means. Beam modifier 21 is preferably oscil-
lated normal to the path of ion beam 20 parallel to major
surface 11 of semiconductor body 10 as indicated in Fig.
1. Alternatively, beam modifier 21 could be rotated or
otherwise moved relative to semiconductor body 10, or
semiconductor body 10 can be moved relative to beam modi-
fier 21 in a predetermined path.
Given the ion source and the composition of the
beam modifier 21, the thickness and impurity concentration
gradient of the impurity region to be formed in semicon-
ductor~body 10 becomes a function of the shape of the beam
modifier 21 and the relative motion between the semicon-
ductor body and the beam modifier doping. Typically the
relative movement between the beam modifier and the body
is fixed so that the thickness and impurity gradient of
the impurity region becomes a function solely of the shape
of the beam modifier.
One can therefore shape the beam modifier to
correspond to any desired width and impurity concentration
gradient desired for the impurity region and any desired
distance between major surface 11 and impurity region 19
in the semiconductor body. To understand the relation,
consider that the energy spectrum of ion beam 22 after
transmission through beam modifier 21 corresponding to the
desired positioning, width and impurity concentration
gradient desired for buried collector impurity region 19
is represented by the mathematical function dn(Et)/dEt =
h(Et). Further, consider that the beam modifier shape
,
l 162326
46,767
corresponding to this energy spectrum is represented by
the mathematical function F(~) = X, where X is the thick-
ness of the modifier and ~ is the distance along the
modifier from a coordinate. Also consider that ~ is a
mathematical function of X, F(X).
Now consider the energy of transmitted ion beam
22 at a point at major surface 11. The ion beam directed
at this point must pass through a thickness XQ of beam
modifier 21 which will reduce the energy of the beam from
E to E~. The flux density at the point on major surface
11 is 0 in ions per second, stated mathematically as
d = dn/dt. Since beam modifier 21 moves horizontally as
shown in Fig. 1 with a speed V = d ~ dt, the number of ions
which strike the selected point on major surface 11 in a
time dt is defined as dn = ~dt - 0/V~ d~. Since ~ = f~x)
and d~ = df(x)/dx dx, dn = ~/V~ df(x)/dx.dx.
The variable x is the thickness of beam modifier
21 through which the ion beam passes to the point on major
surface 11 at any moment in time and is functionally
related to the energy of the ions emerging from the beam
modifier in ion beam 22. Suppose that R (the range of the
ions) and E (the energy of the ions) are related by the
mathematical function R = g(E). The range of the incoming
ion in beam 20 to the beam modifier is Rp - g(Ep), and the
range of the exiting ions in beam 22 is R = g(E). The
thickness of beam modifier 21 can be expressed as a mathe-
matical function of the energy of the ion beam as follows:
x = Rp - R = g(Ep) - g(E).
Substituting this x in the preceding equation
the ion reaching the selected point on major surface 11
can be stated as:
dn = d, df (gdEp(E)- g(E)~ . ~ , dE
The thickness of beam modifier 21 can thus be
expressed mathematically in terms of the transmitted
energy spectrum of the ion beam as follows:
-~ -` 1 16232~
11 46,767
df (g(Ep)E) g(E)) = ~ a~ ( ~ )
For complex energy spectra, dn/dE, the shape of
the modifier 21 can be calculated by computer. For more
simple spectra the shape can be determined by simple hand
calculation.
For example, consider where buried collector
region 19 is to be rectangular with a constant impurity
concentration gradient across its thickness x and area.
dn/dE = K (a constant) and R = g(E) = E/m, i.e., the range
dependence on ion energy is linear with mass. The mathe-
matical function shown above reduces to:
~Ep E~
df ~ m m) V K dE
d (E) 0 (m)
Substituting x = Ep/m - E/m, the function fur-
ther reduces to:
15 ~ = VK
x ~
The general solution to this equation is:
f(x) = V~_ mx + b = 1
Stated another way for x = F(R):
_ d + b~
x - VK VK m
20The constant b can be obtained by letting x be
the thickness X (at ~ = 0) for the lowest energy in the
spectrum:
b = X~ V~_
The length of the beam modifier, L, depends on
X2, which is the thickness for E2. The beam modifier can
thus be shaped as shown in Fig. 1 with sawteeth surfaces
23 having slope ~/VKm, where Xl is the largest thickness
and X2 is the smallest thickness corresponding to the
desired width of buried collector region 19. The beam
modifier is preferably oscillated a large number of cycles
l 162326
12 46,767
of the energy spectrum of the transmitted ion beam 22 to
eliminate spec~rum distortions produced by small temporal
fluctuations in the energy and density of ion beam 20 as
it emitted from a typically ion source.
After beam modifier 21 is appropriately formed
and positioned as described, the transistor as formed with
emitter and base regions 14 and 15 is positioned with
major surface 11, which has been selected for reference
distance, to be exposed to the transmitted ion beam 22.
Semiconductor body 10 is then irradiated with ion beam 20
through oscillating beam modifier 21 to form buried col-
lector region 19 having a constant impurity ConCentratiOn
gradient. The ion dosage is selected to correspond to the
desired impurity concentration in buried collector region.
Emitter electrodes 24 and gate electrode 25 are
then formed on major surface 11 by standard metalizing and
photoetch techniques. For a power device, lateral edges
26 may be beveled and passivated by standard techniques
and electrode or other electrical components provided, to
which buried collector region 19 connects.
Referring to Fig. 2 a transistor similar to that
shown and described in connection with Fig. 1 is made
utilizing the present invention to simultaneously form
both buried collector region and the emitter region. The
same elements have been eorrespondingly numbered with the
prefix "1" before them. The transistor shown in Fig. 2 is
rim gated instead of center gated as shown in Fig. l to
provide for simultaneous formation of the emitter and
buried collector regions.
The two impurities region are simultaneously
formed by making beam modifier 121 in a step function
shape. The shape of surfaces 123 control the depth,
thickness and impurity concentration gradient of buried
collector region 119 and the shape of surfaces 123' con-
trol the thickness and impurity concentration gradient of
emitter region 114. As shown, it is assumèd that a con-
stant impwrity concentration gradient is desired in both
impurities regions. An ion shield 127 is provided to mask
~ 16232~
13 46,767
section areas of the transistor and confine the emitter
and buried collector regions 114 to certain parts of the
semiconductor body 10. This is to thereafter permit gate
electrode 125 to be formed on major surface 11 and made
ohmic contact with base region 115. The dosage of ion
beam is again controlled to provide the desired impurity
concentration in the impurity regions.
Referring to Fig. 3, an MOS field effect tran-
sistor is formed in an integrated circuit where the pres-
ent invention is used to form the isolation impurity
regions between the electrical compon~nts. The MOS field
effect transistor is formed in a semiconductor body 210
formed on substrate 211, for example, by electrostatic or
epitaxial techniques. Source and drain impurity region
212 and 213 are formed in the semiconductor body prefer-
ably simultaneously by standard photoetching and diffusion
techniques, and are spaced apart to form channel region
214 between them of the impurity formed in the semicon-
ductor body during its making.
The semiconductor body is then positioned for
irradiation through major surface 215 by ion beam 216
through beam modifier 217 which oscillates as indicated
during the irradiation. The ion beam 216 is of sufficient
energy to penetrate through semiconductor body 210 and the
beam modifier 217 is shaped with sawtooth surfaces 218 to
provide impurity regions 219 extending entirely through
the semiconductor body with a constant impurity concentra-
tion gradient. The scope of the sawteeth surfaces 218 and
their lengths are selected to this end given the energy
level of ion beam 216, the material of beam modifier 217,
and the width of semiconductor body 210.
The conductivity type of the ion of ion beam 216
is also selected to be opposite to the impurity in semi-
conductor body 210 to provide PN junctions in 220 the
semiconductor body isolating the transistor from the
remainder of the electrical components of the integered
circuit in the body. Also ion shield 221 is interposed in
the path of transmitted ion beam 222 to mask the area of
14 46,767
the semiconductor body containing the transistor from the
ion beam.
As shown, the isolating impurity regions 219 can
then be formed in semiconductor body 210 by irradiating
semiconductor body through major surface 215 with trans-
mitted ion beam 222. By this technique various electrical
components in semiconductor body 10 can be electrically
isolated rapidly and with high precision. Indeed, because
of the high resolution and precise positioning of the
isolating impurity regions 219, many more electrical
components can be formed for a given area of major surface
215 of semiconductor body 210, and quality and performance
of integered circuits can be increased.
Oxide layer 223 and passivating layer 224,
15source electrode 225, drains electrode 226 and gate elec-
trode 227 are then sequentially formed on major surface
215 of semiconductor body 210 by standard oxide growth,
photoetching and metalizing techniques to complete the
transistor and the integered circuit.
2~Referring to Fig. 4, a semiconductor 310 is
shown in which an impurity region with a variable concen-
tration gradient is formed using the present invention.
The impurity region as shown is buried in the semiconduct-
or body 310, or adjoining major surfaces 211 or 213 as
desired. The impurity region can thus be used in various
applications.
Fig. 4 shows the relation between the shape of
beam modifier 313 and impurity region 314 formed in semi-
conductor body 310. The surfaces 315, 316 and 317 are of
shapes and lengths corresponding to the thicknesses and
impurity concentration gradients of portions 318, 319 and
320, respectively. The maximum thickness of modifier 313
corresponds to the distance of impurity region 314 from
major surface 311 given the material of modifier 313 and
the energy of ion beam 320 to the beam modifier. The
transmitted ion beam 321 thus corresponds to the desired
overall positioning and impurity concentration profile of
impurity 314 to be formed in semiconductor body 310.
2326
15 46,767
The impurity region 314 is thus formed by posi-
tioning the semiconductor body 310 for irradiation through
major surface 311, selected for reference, by ion beam 320
transmitted through beam modifier 313; and irradiating
5semiconductor body 310 through major surface 311 with the
modified ion beam 321 while modifier 313 is oscillated as
indicated in Fig. 4. The relative shape of the dosage
gradient of the impurity region 314 is shown by the small
graph to the right of Fig. 4.
10Referring to Fig. 5, a second semiconductor body
1310 is shown in which an impurity region with an impurity
concentration gradient is formed using the present inven-
tion. The elements and their relation are the same as
that described in connection with Figure 4 and are identi-
fied with a prefix "1." The differences are the position-
ing of impurity region 1314 adjoining major surface 1311,
selected for reference, and the impurity concentration
gradient of impurity region 1314 and corresponding shape
of beam modified 1313. Shaped surfaces 1315 are parabolic
providi~g a parabolic impurity concentration profile to
impurity region 1314, as best shown by the small graph to
the left of Fig. 5.
Referring to Fig. 6, a third semiconductor body
2310 is shown in which two impurity regions with different
impurity concentration gradients are simultaneously formed
using the present invention. The elements and their
relation are the same as that described in connection with
Fig. 4 and are identified with a prefix "2." The differ-
ences are positioning and concentration gradients of the
impurity regions and the corresponding shape of the beam
modifier 2313. Shaped surfaces 2315 and 2316 are in step
function relation, with surfaces 2315 corresponding to
gaussion distribution concentration profile for buried
impurity region 2318 and surfaces 2316 corresponding to an
inverse gaussion distribution concentration profile for
buried impurity region 2319. The concentration gradients
of impurity regions 2318 and 2319 and their spatial rela
tion to each other is best seen by the small graph to the
l 16232~
16 46,767
left of Fig. 6.
As shown in Figs. 4, 5 and 6, the present inven-
tion provides a flexibility in locating impurity regions
and imparting concentrations profiles to such impurity
regions heretofore not known. Furthermore, the present
invention provides a speed and precision in forming impur-
ity heretofore not known.
While presently preferred embodiments have been
shown and described, it is distinctly understood that the
invention may otherwise be variously performed and embod-
ied within the scope of the following claims.
