U.S. patent application number 13/887571 was filed with the patent office on 2013-09-19 for boron-comprising inks for forming boron-doped regions in semiconductor substrates using non-contact printing processes and methods for fabricating such boron-comprising inks.
This patent application is currently assigned to Honeywell International Inc.. The applicant listed for this patent is HONEYWELL INTERNATIONAL INC.. Invention is credited to Wenya Fan, Roger Yu-Kwan Leung, De-Ling Zhou.
Application Number | 20130240794 13/887571 |
Document ID | / |
Family ID | 42283363 |
Filed Date | 2013-09-19 |
United States Patent
Application |
20130240794 |
Kind Code |
A1 |
Leung; Roger Yu-Kwan ; et
al. |
September 19, 2013 |
BORON-COMPRISING INKS FOR FORMING BORON-DOPED REGIONS IN
SEMICONDUCTOR SUBSTRATES USING NON-CONTACT PRINTING PROCESSES AND
METHODS FOR FABRICATING SUCH BORON-COMPRISING INKS
Abstract
A method for fabricating a boron-comprising ink is provided. The
method includes providing an inorganic boron-comprising material,
combining the inorganic boron-comprising material with a polar
solvent having a boiling point in a range of from about 50.degree.
C. to about 250.degree. C., and combining the inorganic
boron-comprising material with a spread-minimizing additive that
results in a spreading factor of the boron-comprising ink in a
range of from about 1.5 to about 6.
Inventors: |
Leung; Roger Yu-Kwan; (San
Jose, CA) ; Zhou; De-Ling; (Sunnyvale, CA) ;
Fan; Wenya; (Campbell, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
HONEYWELL INTERNATIONAL INC. |
Morristown |
NJ |
US |
|
|
Assignee: |
Honeywell International
Inc.
Morristown
NJ
|
Family ID: |
42283363 |
Appl. No.: |
13/887571 |
Filed: |
May 6, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12344745 |
Dec 29, 2008 |
|
|
|
13887571 |
|
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Current U.S.
Class: |
252/500 |
Current CPC
Class: |
C09D 11/52 20130101;
C09D 11/30 20130101; C08G 77/46 20130101; C08G 77/20 20130101; C09D
11/38 20130101; H01L 21/2225 20130101 |
Class at
Publication: |
252/500 |
International
Class: |
C09D 11/00 20060101
C09D011/00 |
Claims
1. A method for fabricating a boron-comprising ink, the method
comprising the steps of: providing an inorganic boron-comprising
material; combining the inorganic boron-comprising material with a
polar solvent having a boiling point in a range of from about
50.degree. C. to about 250.degree. C.; and combining the inorganic
boron-comprising material with a spread-minimizing additive that
results in a spreading factor of the boron-comprising ink in a
range of from about 1.5 to about 6.
2. The method of claim 1, wherein the step of providing the
inorganic boron-comprising material comprises providing a material
selected from the group consisting of boron oxide, boric acid,
borates having a formula B(OR).sub.3, where R is an alkyl group,
and combinations thereof.
3. The method of claim 1, wherein the step of combining the
inorganic boron-comprising material with the polar solvent
comprises combining the inorganic boron-comprising material with a
material selected from the group consisting of iso-stearic acid,
ethanol, propylene glycol butyl ether, ethylene glycol, triethylene
glycol, and mixtures thereof.
4. The method of claim 1, wherein the step of combining the
inorganic boron-comprising material with the spread-minimizing
additive comprises combining the inorganic boron-comprising
material with a material selected from the group consisting of
iso-stearic acid, polypropylene oxide (PPO),
vinylmethylsiloxane-dimethylsiloxane copolymer, polyether-modified
polysiloxanes, organo-modified polysiloxanes, and combinations
thereof.
5. The method of claim 1, further comprising the step of adding a
functional additive to the inorganic boron-comprising material,
wherein the functional additive comprises a material selected from
the group consisting of viscosity modifiers, dispersants,
surfactants, polymerization inhibitors, wetting agents, antifoaming
agents, detergents and surface-tension modifiers, flame retardants,
pigments, plasticizers, thickeners, rheology modifiers, and
mixtures thereof.
6. The method of claim 1, further comprising adding a functional
additive chosen from viscosity modifiers, dispersants, surfactants,
polymerization inhibitors, wetting agents, antifoaming agents,
detergents and other surface-tension modifiers, flame retardants,
pigments, plasticizers, thickeners, rheology modifiers, or mixtures
thereof.
7. The method of claim 1, wherein providing the inorganic
boron-comprising material comprises providing a polymeric borazole
resin.
Description
PRIORITY CLAIMS
[0001] This is a divisional application of U.S. application Ser.
No. 12/344,745, filed Dec. 29, 2008.
FIELD OF THE INVENTION
[0002] The present invention generally relates to dopants and
methods for doping regions of semiconductor-comprising substrates,
and more particularly relates to boron-comprising inks for forming
boron-doped regions in semiconductor substrates using non-contact
printing processes and methods for fabricating such
boron-comprising inks.
BACKGROUND OF THE INVENTION
[0003] Doping of semiconductor substrates with
conductivity-determining type impurities, such as n-type and p-type
ions, is used in a variety of applications that require
modification of the electrical characteristics of the semiconductor
substrates. Well-known methods for performing such doping of
semiconductor substrates include photolithography and screen
printing. Photolithography requires the use of a mask that is
formed and patterned on the semiconductor substrate. Ion
implantation then is performed to implant conductivity-determining
type ions into the semiconductor substrate in a manner
corresponding to the mask. Similarly, screen printing utilizes a
patterned screen that is placed on the semiconductor substrate. A
screen printing paste containing the conductivity-determining type
ions is applied to the semiconductor substrate over the screen so
that the paste is deposited on the semiconductor substrate in a
pattern that corresponds inversely to the screen pattern. After
both methods, a high-temperature anneal is performed to cause the
impurity dopants to diffuse into the semiconductor substrate.
[0004] In some applications such as, for example, solar cells, it
is desirable to dope the semiconductor substrate in a pattern
having very fine lines or features. The most common type of solar
cell is configured as a large-area p-n junction made from silicon.
In one type of such solar cell 10, illustrated in FIG. 1, a silicon
wafer 12 having a light-receiving front side 14 and a back side 16
is provided with a basic doping, wherein the basic doping can be of
the n-type or of the p-type. The silicon wafer is further doped at
one side (in FIG. 1, front side 14) with a dopant of opposite
charge of the basic doping, thus forming a p-n junction 18 within
the silicon wafer. Photons from light are absorbed by the
light-receiving side 14 of the silicon to the p-n junction where
charge carriers, i.e., electrons and holes, are separated and
conducted to a conductive contact, thus generating electricity. The
solar cell is usually provided with metallic contacts 20, 22 on the
light-receiving front side as well as on the back side,
respectively, to carry away the electric current produced by the
solar cell. The metal contacts on the light-receiving front side
pose a problem in regard to the degree of efficiency of the solar
cell because the metal covering of the front side surface causes
shading of the effective area of the solar cell. Although it may be
desirable to reduce the metal contacts as much as possible so as to
reduce the shading, a metal covering of approximately 5% remains
unavoidable since the metallization has to occur in a manner that
keeps the electrical losses small. In addition, contact resistance
within the silicon adjacent to the electrical contact increases
significantly as the size of the metal contact decreases. However,
a reduction of the contact resistance is possible by doping the
silicon in narrow areas 24 directly adjacent to the metal contacts
on the light-receiving front side 14.
[0005] FIG. 2 illustrates another common type of solar cell 30.
Solar cell 30 also has a silicon wafer 12 having a light-receiving
front side 14 and a back side 16 and is provided with a basic
doping, wherein the basic doping can be of the n-type or of the
p-type. The light-receiving front side 14 has a rough or textured
surface that serves as a light trap, preventing absorbed light from
being reflected back out of the solar cell. The metal contacts 32
of the solar cell are formed on the back side 16 of the wafer. The
silicon wafer is doped at the backside relative to the metal
contacts, thus forming p-n junctions 18 within the silicon wafer.
Solar cell 30 has an advantage over solar cell 10 in that all of
the metal contacts of the cell are on the back side 16. In this
regard, there is no shading of the effective area of the solar
cell. However, for all contacts to be formed on the back side 16,
the doped regions adjacent to the contacts have to be quite
narrow.
[0006] As noted above, both solar cell 10 and solar cell 30 benefit
from the use of very fine, narrow doped regions formed within a
semiconductor substrate. However, the present-day methods of doping
described above, that is, photolithography and screen printing,
present significant drawbacks. For example, it is prohibitively
difficult, if not impossible, to obtain very fine and/or narrow
doped regions in a semiconductor substrate using screen printing.
In addition, while doping of substrates in fine-lined patterns is
possible with photolithography, photolithography is an expensive
and time consuming process. In addition, both photolithography and
screen printing involve contact with the semiconductor substrate.
However, in applications such as solar cells, the semiconductor
substrates are becoming very thin. Contact with thin substrates
often results in breaking of the substrates. Further, screen
printing cannot be used to dope rough or textured surfaces, which
are commonly used in solar cell design to trap light within the
semiconductor substrate. Moreover, because photolithography and
screen printings use custom designed masks and screens,
respectively, to dope the semiconductor substrate in a pattern,
reconfiguration of the doping pattern is expensive because new
masks or screens have to be developed.
[0007] Accordingly, it is desirable to provide boron-comprising
inks for forming boron-doped regions in semiconductor substrates
using non-contact printing processes. It also is desirable to
provide methods for fabricating boron-comprising inks for forming
such boron-doped regions using non-contact printing. Furthermore,
other desirable features and characteristics of the present
invention will become apparent from the subsequent detailed
description of the invention and the appended claims, taken in
conjunction with the accompanying drawings and this background of
the invention.
BRIEF SUMMARY OF THE INVENTION
[0008] A boron-comprising ink is provided in accordance with an
exemplary embodiment of the present invention. The boron-comprising
ink comprises boron from or of a boron-comprising material and a
spread-minimizing additive that results in a spreading factor of
the boron-comprising ink in a range of from about 1.5 to about 6.
The boron-comprising ink has a viscosity in a range of from about
1.5 to about 50 centipoise and, when deposited on a semiconductor
substrate, provides a post-anneal sheet resistance in a range of
from about 10 to about 100 ohms/square, a post-anneal doping depth
in a range of from about 0.1 to about 1 .mu.m, and a boron
concentration in a range of from about 1.times.10.sup.19 to
1.times.10.sup.20 atoms/cm.sup.3.
[0009] A method for fabricating a boron-comprising ink is provided
in accordance with an exemplary embodiment of the present
invention. The method comprises the steps of providing an inorganic
boron-comprising material, combining the inorganic boron-comprising
material with a polar solvent having a boiling point in a range of
about 50.degree. C. to about 250.degree. C., and combining the
inorganic boron-comprising material with a spread-minimizing
additive that results in a spreading factor of the boron-comprising
ink in a range of from about 1.5 to about 6.
[0010] A method for formulating a boron-comprising ink is provided
in accordance with another exemplary embodiment of the present
invention. The method comprises the steps of combining an amine and
a boron donor, heating the amine and the boron donor combination to
form a polymeric borazole resin, adding a solvent having a boiling
point in a range of about 50.degree. C. to about 250.degree. C. to
the polymeric borazole resin, adding a spread-minimizing additive
that results in a spreading factor of the boron-comprising ink in a
range of from about 1.5 to about 6, and adding a viscosity modifier
to the polymeric borazole resin. The viscosity modifier results in
the boron-comprising ink having a viscosity in a range of from
about 1.5 to about 50 centipoise.
[0011] A method for fabricating a boron-comprising ink is provided
in accordance with a further exemplary embodiment of the present
invention. The method comprises the steps of providing
boron-comprising nanoparticles having an average dimension of no
greater than 100 nm and combining the boron-comprising
nanoparticles with a dispersant that forms a uniform and stable
suspension with the boron-comprising nanoparticles. A
spread-minimizing additive that results in a spreading factor of
the boron-comprising ink in a range of from about 1.5 to about 6 is
added.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The present invention will hereinafter be described in
conjunction with the following drawing figures, wherein like
numerals denote like elements, and wherein:
[0013] FIG. 1 is a schematic illustration of a conventional solar
cell with a light-side contact and a back side contact;
[0014] FIG. 2 is a schematic illustration of another conventional
solar cell with back side contacts;
[0015] FIG. 3 is a cross-sectional view of an inkjet printer
mechanism distributing ink on a substrate;
[0016] FIG. 4 is a cross-sectional view of an aerosol jet printer
mechanism distributing ink on a substrate;
[0017] FIG. 5 is a flowchart of a method for forming boron-doped
regions in a semiconductor substrate using an non-contact printing
process in accordance with an exemplary embodiment of the present
invention;
[0018] FIG. 6 is a flowchart of a method for fabricating a
boron-comprising ink for use in the method of FIG. 5 in accordance
with an exemplary embodiment of the present invention;
[0019] FIG. 7 is a flowchart of a method for fabricating a
boron-comprising ink for use in the method of FIG. 5 in accordance
with another exemplary embodiment of the present invention;
[0020] FIG. 8 is an illustration of the molecular structure of a
polymer borazole resin formed in accordance with the method of FIG.
7; and
[0021] FIG. 9 is a flowchart of a method for fabricating a
boron-comprising ink for use in the method of FIG. 5 in accordance
with yet another exemplary embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0022] The following detailed description of the invention is
merely exemplary in nature and is not intended to limit the
invention or the application and uses of the invention.
Furthermore, there is no intention to be bound by any theory
presented in the preceding background of the invention or the
following detailed description of the invention.
[0023] Boron-comprising inks for forming boron-doped regions in
semiconductor substrates using non-contact printing processes and
methods for fabricating such boron-comprising inks are provided
herein. As used herein, the term "non-contact printing process"
means a process for depositing a liquid conductivity-determining
type dopant selectively on a semiconductor material in a
predetermined patterned without the use of a mask, screen, or other
such device. Examples of non-contact printing processes include but
are not limited to "inkjet printing" and "aerosol jet printing."
Typically, the terms "inkjet printing," an "inkjet printing
process," "aerosol jet printing," and an "aerosol jet printing
process" refer to a non-contact printing process whereby a liquid
is projected from a nozzle directly onto a substrate to form a
desired pattern. In an inkjet printing mechanism 50 of an inkjet
printer, as illustrated in FIG. 3, a print head 52 has several tiny
nozzles 54, also called jets. As a substrate 58 moves past the
print head 52, or as the print head 52 moves past the substrate,
the nozzles spray or "jet" ink 56 onto the substrate in tiny drops,
forming images of a desired pattern. In an aerosol jet printing
mechanism 60, illustrated in FIG. 4, a mist generator or nebulizer
62 atomizes a liquid 64. The atomized fluid 66 is aerodynamically
focused using a flow guidance deposition head 68, which creates an
annular flow of sheath gas, indicated by arrow 72, to collimate the
atomized fluid 66. The co-axial flow exits the flow guidance head
68 through a nozzle 70 directed at the substrate 74 and focuses a
stream 76 of the atomized material to as small as a tenth of the
size of the nozzle orifice (typically 100 .mu.m). Patterning is
accomplished by attaching the substrate to a computer-controlled
platen, or by translating the flow guidance head while the
substrate position remains fixed.
[0024] Such non-contact printing processes are particularly
attractive processes for fabricating doped regions in semiconductor
substrates for a variety of reasons. First, unlike screen printing
or photolithography, only an ink used to form the doped regions
touches or contacts the surface of the substrate upon which the ink
is applied. Thus, because the breaking of semiconductor substrates
could be minimized compared to other known processes, non-contact
printing processes are suitable for a variety of substrates,
including rigid and flexible substrates. In addition, non-contact
printing processes are additive processes, meaning that the ink is
applied to the substrate in the desired pattern. Thus, steps for
removing material after the printing process, such as are required
in photolithography, are eliminated. Further, because non-contact
printing processes are additive processes, they are suitable for
substrates having smooth, rough, or textured surfaces. Non-contact
printing processes also permit the formation of very fine features
on semiconductor substrates. In one embodiment, features, such as,
for example, lines, dots, rectangles, circles, or other geometric
shapes, having at least one dimension of less than about 200 .mu.m
can be formed. In another exemplary embodiment, features having at
least one dimension of less than about 100 .mu.m can be formed. In
a preferred embodiment, features having at least one dimension of
less than about 20 .mu.m can be formed. In addition, because
non-contact printing processes involve digital computer printers
that can be programmed with a selected pattern to be formed on a
substrate or that can be provided the pattern from a host computer,
no new masks or screens need to be produced when a change in the
pattern is desired. All of the above reasons make non-contact
printing processes cost-efficient processes for fabricating doped
regions in semiconductor substrates, allowing for increased
throughput compared to screen printing and photolithography.
[0025] Referring to FIG. 5, a method 100 for forming a boron-doped
region in a semiconductor substrate includes the step of providing
a semiconductor substrate (step 102). As used herein, the term
"semiconductor substrate" will be used to encompass semiconductor
materials conventionally used in the semiconductor industry from
which to make electrical devices. Semiconductor materials include
monocrystalline silicon materials, such as the relatively pure or
lightly impurity-doped monocrystalline silicon materials typically
used in the semiconductor industry, as well as polycrystalline
silicon materials, and silicon admixed with other elements such as
germanium, carbon, and the like. In addition, "semiconductor
substrate" encompasses other semiconductor materials such as
relatively pure and impurity-doped germanium, gallium arsenide, and
the like. In this regard, the method 100 can be used to fabricate a
variety semiconductor devices including, but not limited to,
microelectronics, solar cells, displays, RFID components,
microelectromechanical systems (MEMS) devices, optical devices such
as microlenses, medical devices, and the like.
[0026] The method 100 further includes the step of providing an ink
formed of or from a boron-comprising material (hereinafter, a
"boron-comprising ink") (step 104), which step may be performed
before, during or after the step of providing the semiconductor
substrate. Methods for fabricating a boron-comprising ink are
described in more detail in reference to FIGS. 6-9. The
boron-comprising ink should meet at least one of several
performance criteria for non-contact printing. First, the ink is
formulated so that it can be printed to form fine or small
features, such as lines, dots, circles, squares, or other geometric
shapes. In one exemplary embodiment of the invention, the ink is
formulated so that features having at least one dimension of less
than about 200 .mu.m can be printed. In another exemplary
embodiment of the invention, the ink is formulated so that features
having at least one dimension less than about 100 .mu.m can be
printed. In a preferred embodiment of the present invention, the
ink is formulated so that features having a dimension of less than
about 20 .mu.m can be printed.
[0027] Second, during the printing process and during pausing of
the printing process, the ink results in minimal, if any, clogging
of the non-contact printer nozzles. Clogging of the nozzles results
in down-time of the printer, thus reducing throughput. In one
exemplary embodiment, the boron-comprising ink has a viscosity in
the range of about 1.5 to about 50 centipoise (cp). Further, the
ink is formulated so that, after it is deposited on the substrate
and high-temperature annealing (discussed in more detail below) is
performed, the resulting doped region has a sheet resistance in the
range of about 10 to about 100 ohms/square (.OMEGA./).
[0028] Moreover, the ink is formulated so that the boron and/or the
boron-comprising ink do not significantly diffuse from the penned
area, that is, the area upon which the ink is deposited, into
unpenned areas before the high temperature anneal is performed.
Significant diffusion of the boron and/or the boron-comprising ink
from the penned area, either by vapor transport or by diffusion
through the substrate, before annealing at the proper annealing
temperature may significantly adversely affect the electrical
properties of devices comprising the resulting doped regions. The
boron-comprising ink also is formulated so that significant
diffusion of the boron from the penned area into unpenned areas
during the annealing process is minimized or prevented altogether.
In other words, localized doping, in contrast to blanket doping, is
desirably effected. Significant diffusion of the boron from the
penned area into unpenned areas, either by vapor transport or by
diffusion through the substrate during the annealing process,
should be minimized or eliminated so as to achieve localized doping
without significantly changing the boron distribution outside of
the penned area. In addition, the ink provides for shallow but
highly concentrated doping, with a post-anneal doping depth in the
range of from about 0.1 to about 1 micrometers (.mu.m) and a boron
concentration in the range of about 1.times.10.sup.19 to
1.times.10.sup.20 atoms/cm.sup.3.
[0029] The boron-comprising ink is applied to the substrate using a
non-contact printer (step 106). The boron-comprising ink is applied
to the substrate in a pattern that is stored in or otherwise
supplied to the printer. Examples of inkjet printers suitable for
use include, but are not limited to, Dimatix Inkjet Printer Model
DMP 2811 available from Fujifilm Dimatix, Inc. of Santa Clara,
Calif. An example of an aerosol jet printer suitable for use
includes, but is not limited, to, an M3D Aerosol Jet Deposition
System available from Optomec, Inc. of Albuquerque, N. Mex.
Preferably, the ink is applied to the substrate at a temperature in
the range of about 15.degree. C. to about 80.degree. C. in a
humidity of about 20 to about 80%. Once the pattern of
boron-comprising ink is formed on the substrate, the substrate is
subjected to a high-temperature thermal treatment or "anneal" to
cause the boron of the boron-comprising ink to diffuse into the
substrate, thus forming boron-doped regions within the substrate
(step 108). The time duration and the temperature of the anneal is
determined by such factors as the initial boron concentration of
the boron-comprising ink, the thickness of the ink deposit, the
desired concentration of the resulting boron-doped region, and the
depth to which the boron is to diffuse. In one exemplary embodiment
of the present invention, the substrate is placed inside an oven
wherein the temperature is ramped up to a temperature in the range
of about 800.degree. C. to about 1200.degree. C. and the substrate
is baked at this temperature for about 2 to about 90 minutes.
Annealing also may be carried out in an in-line furnace to increase
throughput. The annealing atmosphere may contain 0-100% oxygen in
an oxygen/nitrogen or oxygen/argon mixture. In a preferred
embodiment, the substrate is subjected to an anneal temperature of
about 1050.degree. C. for about ten (10) minutes in an oxygen
ambient.
[0030] Boron-comprising inks used in the method of FIG. 5 may be
manufactured using a variety of boron-contributing materials. In
accordance with one exemplary embodiment of the present invention,
the boron-comprising ink may be formed from an inorganic
boron-comprising material. Referring to FIG. 6, in accordance with
an exemplary embodiment of the present invention, a method 150 for
fabricating a boron-comprising ink includes the step of providing
an inorganic boron-comprising material (step 152). Inorganic
boron-comprising materials for use in method 150 include, but are
not limited to, boric acid (B(OH).sub.3), boron oxide
(B.sub.2O.sub.3), and other borates having the formula B(OR).sub.3,
where R is an alkyl group, such as, for example, a methyl, ethyl,
or propyl group, or a combination thereof.
[0031] The method further includes combining the inorganic
boron-comprising material with a polar solvent. Polar solvents
suitable for use comprise any suitable polar pure fluid or mixture
of fluids that is capable of forming a solution with the
boron-comprising material and that causes the boron-comprising ink
to have a viscosity in the range of about 1.5 to about 50 cp. In
some contemplated embodiments, the solvent or solvent mixture may
comprise those solvents that are not considered part of the
hydrocarbon solvent family of compounds, such as alcohols, ketones
(such as acetone, diethylketone, methylethylketone, and the like),
esters, ethers, amides and amines. Examples of solvents suitable
for use in formulating the boron-comprising ink include alcohols,
such as methanol, ethanol, propanol, butanol, and pentanol,
anhydrides, such as acetic anhydride, and other solvents such as
propylene glycol monoether acetate and ethyl lactate, and mixtures
thereof. The inorganic boron-comprising material may be combined
with the polar solvent using any suitable mixing or stirring
process that forms a homogeneous mixture. For example, a reflux
condenser, a low speed sonicator or a high shear mixing apparatus,
such as a homogenizer, a microfluidizer, a cowls blade high shear
mixer, an automated media mill, or a ball mill, may be used for
several seconds to an hour or more to combine the components.
[0032] In preferred embodiment of the invention, the
boron-comprising material is combined with at least one polar
solvent having a high boiling point in the range of about
50.degree. C. to about 250.degree. C. In this regard, the boiling
point of the resulting dopant-comprising ink is modified to
minimize the drying rate of the ink and, thus, minimize clogging of
the printer nozzles. Examples of solvents with high boiling points
suitable for use include ethanol, iso-stearic acid, propylene
glycol butyl ether, ethylene glycol, triethylene glycol, and the
like, and combinations thereof.
[0033] In another exemplary embodiment, a spread-minimizing
additive is added (step 156). The spread-minimizing additive is an
additive that modifies the surface tension, and/or wettability of
the boron-comprising ink so that spreading of the ink when penned
onto the substrate is minimized In a preferred embodiment of the
invention, the boron-comprising ink has a spreading factor in the
range of from about 1.5 to about 6. The term "spreading factor" of
a non-contact printing process ink is defined in terms of an inkjet
printing process and is the ratio of the average diameter of a dot
of the ink deposited by a nozzle of an inkjet printer to the
diameter of the nozzle when the semiconductor substrate is at a
temperature in a range of from 50.degree. C. to about 60.degree.
C., the temperature of the ink at the nozzle is in a range of about
20.degree. C. to about 22.degree. C., the distance between the tip
of the nozzle proximate to the substrate and the substrate is about
1.5 millimeters (mm) and the jetting frequency, that is, the number
of ink drops jetted from the nozzle per second, is 2 kilohertz
(kHz). By minimizing the spreading of the ink on the substrate,
fine features, such as those described above having at least one
feature that is less than about 200 .mu.m or smaller, can be
achieved. Examples of spread-minimizing additives include, but are
not limited to, iso-stearic acid, polypropylene oxide (PPO), such
as polypropylene oxide having a molecular weight of 4000 (PPO4000),
vinylmethylsiloxane-dimethylsiloxane copolymer, such as VDT131
available form Gelest, Inc. of Tullytown, Pa., polyether-modified
polysiloxanes, such as Tegophren 5863 available from Evonik Degussa
GmbH of Essen, Germany, other organo-modified polysiloxanes, such
as Tegoglide 420 also available from Evonik Degussa GmbH, and the
like, and combinations thereof.
[0034] In an optional exemplary embodiment of the invention, a
functional additive is added to the inorganic boron-comprising
material before, during, or after combination with the solvent
(step 158). For example, it may be desirable to minimize the amount
of the resulting boron and/or boron-comprising ink that diffuses
beyond the penned area into unpenned areas of the substrate before
the predetermined annealing temperature of the annealing process is
reached. As noted above, diffusion of the boron and/or
boron-comprising ink beyond the penned area into unpenned areas
before annealing can significantly affect the electrical
characteristics of the resulting semiconductor device that utilizes
the subsequently-formed doped region. Thus, in a further exemplary
embodiment, a viscosity modifier that results in the
boron-comprising ink having a viscosity in the range of about 1.5
to about 50 cp is added. Preferably, the resulting boron-comprising
ink is soluble in the viscosity modifier. Examples of such
viscosity-modifiers include glycerol, polyethylene glycol,
polypropylene glycol, ethylene glycol/propylene glycol copolymer,
organo-modified siloxanes, ethylene glycol/siloxane copolymers,
polyelectrolyte, oleic acid and the like, and combinations thereof.
Examples of other suitable additives that may be added to the
inorganic boron-comprising material include dispersants,
surfactants, polymerization inhibitors, wetting agents, antifoaming
agents, detergents and other surface-tension modifiers, flame
retardants, pigments, plasticizers, thickeners, rheology modifiers,
and mixtures thereof.
[0035] In accordance with another exemplary embodiment of the
present invention, the boron-comprising ink may be formed so that
it comprises a polymeric borazole (PBZ) resin. Referring to FIG. 7,
in accordance with an exemplary embodiment of the present
invention, a method 200 for fabricating a boron-comprising ink
comprising a PBZ resin includes the step of combining a boron donor
and an amine to form a PBZ resin (step 202). The boron donor may
comprise boron halides such as boron trichloride (BCl.sub.3), boron
tribromide (BBr.sub.3), and boron trifluoride (BF.sub.3), and
alkylboron compounds such as boron trifluoride etherate
(CH.sub.3CH.sub.2)OBF.sub.3, methyldicloroboron
((CH.sub.3)BCl.sub.2), and the like, and combinations thereof. The
amine may comprise an alkylamine such as, for example,
cyclohexylamine, butylamine, hexylamine, dipropylamine,
tripropylamine, and combinations thereto. In one exemplary
embodiment, the boron donor and the amine are combined at
temperatures in the range of about -60.degree. C. and about
-5.degree. C. to form an intermediate triaminoborane and amine
hydrochloride salt. In another exemplary embodiment, the boron
donor and the amine are combined in the presence of an inert,
non-polar solvent or solvent mixture with a relatively low boiling
point. Examples of suitable inert solvents include low boiling
point hydrocarbon solvents such as pentane, hexane, heptane, and
octane, which have a boiling point of less than about 100.degree.
C. The solvent may be added first to the boron donor, first to the
amine, or may be added when the boron donor and the amine are
combined.
[0036] The reaction mixture is filtered to remove the amine
hydrochloride salt to obtain a solution containing the
triaminoborane intermediate. The low boiling point solvent in the
solution then is evaporated to produce a neat triaminoborane. In
one exemplary embodiment, the solvent is heated under nitrogen
atmosphere to about 300.degree. C. for about one to about two hours
and then further heated to about 380.degree. C. to about
420.degree. C. for about two to about four hours. Upon completion
of the polymerization reaction, a PBZ resin having the molecular
structure illustrated in FIG. 8 is formed, where X and Y can be
hydrogen, a halogen such as chlorine, a hydroxyl group, an alkyl
group, an aryl group, or a cycloalkyl group and n is a number in
the range of from about 5 to about 100.
[0037] Referring back to FIG. 7, in accordance with one exemplary
embodiment of the invention, once formed the PBZ resin can be
isolated, such as by filtering the PBZ resin from solution, and a
spread-minimizing additive is added thereto (step 204). Any of the
above-described spread-minimizing additives may be used. The
polymeric borazole resin also is combined with at least one solvent
having a high boiling point in the range of about 50.degree. C. to
about 250.degree. C. (step 206). In this regard, the boiling point
of the resulting boron-comprising ink is adjusted to minimize the
drying rate of the ink. Examples of solvents with high boiling
points suitable for use include any of the high boiling point
non-polar solvents. In some contemplated embodiments, the solvent
or solvent mixture comprises aliphatic, cyclic, and aromatic
hydrocarbons. Aliphatic hydrocarbon solvents may comprise both
straight-chain compound and compounds that are branched. Cyclic
hydrocarbon solvents are those solvents that comprise at least
three carbon atoms oriented in a ring structure with properties
similar to aliphatic hydrocarbon solvents. Aromatic hydrocarbon
solvents are those solvents that comprise generally benzene or
naphthalene structures. Contemplated hydrocarbon solvents include
toluene, xylene, p-xylene, m-xylene, mesitylene, solvent naphtha H,
solvent naphtha A, alkanes, such as pentane, hexane, isohexane,
heptane, nonane, octane, dodecane, 2-methylbutane, hexadecane,
tridecane, pentadecane, cyclopentane, 2,2,4-trimethylpentane,
petroleum ethers, halogenated hydrocarbons, such as chlorinated
hydrocarbons, nitrated hydrocarbons, benzene, 1,2-dimethylbenzene,
1,2,4-trimethylbenzene, mineral spirits, kerosene, isobutylbenzene,
methylnaphthalene, ethyltoluene, and ligroine.
[0038] In an optional embodiment of the present invention, other
functional additives also may be added to the PBZ resin (step 208).
For example, a viscosity modifier can be added to cause the
resulting boron-comprising ink to have a viscosity in the range of
about 1.5 to about 50 cp. An example of a viscosity modifier
suitable for use in preparing the boron-comprising ink includes,
but is not limited to, polypropylene glycol. Any of the other
above-described functional additives also may be added. While FIG.
7 illustrates that the step of adding a functional additive (step
208) is performed after the step of adding a high boiling-point
solvent (step 206) and after the step of adding a spread-minimizing
additive (step 204), it will be appreciated that the functional
additive can be added before, during, or after the step of adding
the spread-minimizing additive (step 204) and/or before, during, or
after the step of adding the high boiling-point solvent (step
206).
[0039] In accordance with a further exemplary embodiment of the
present invention, the boron-comprising ink may be formed from
boron-comprising nanoparticles. Referring to FIG. 9, in accordance
with an exemplary embodiment of the present invention, a method 250
for fabricating a boron-comprising ink includes the step of
providing boron-comprising nanoparticles (step 252). Examples of
boron-comprising nanoparticles suitable for fabricating a
boron-comprising ink include, but are not limited to, boron oxide
nanoparticles, boron nitride nanoparticles, boron carbide
nanoparticles, and boron (metal) nanoparticles. In one exemplary
embodiment, the boron-comprising nanoparticles have an average
dimension, such as an average diameter, length, or width, that is
no greater than 100 nm. In a preferred embodiment, the
boron-comprising nanoparticles have an average dimension of no
larger than about 10 nm. A smaller size of nanoparticle facilitates
less tendency for clogging and more uniform distribution of the
boron-comprising ink.
[0040] The method 250 further includes combining the
boron-comprising nanoparticles with at least one dispersant that
forms a uniform and stable suspension with the nanoparticles and
does not dissolve the nanoparticles (step 254). In one exemplary
embodiment, a dispersant that stabilizes the nanoparticles by
adjusting the pH of the nanoparticles so that they are alkaline,
that is, with a pH greater than about 7, is combined with the
nanoparticles. An example of such a dispersant includes, but is not
limited to, ammonium hydroxide, sodium hydroxide, and
tetramethylammonium hydroxide. In this regard, at least a portion
of the boron on the surface of the nanoparticles forms
BO.sup.-NH4.sup.+, which prevents agglomeration of the
nanoparticles by electrostatic repulsion. In another exemplary
embodiment, a dispersant that stabilizes the nanoparticles with
organic groups is combined with the nanoparticles. Examples of such
dispersants include, but are not limited to, alkylchlorosilanes,
trialkylchlorosilanes, acetyl chloride, acetyl anhydride, and
alkylalkoxysilanes. In this regard, at least a portion of the boron
on the surface of the nanoparticles forms stable B--O--SiR.sub.3,
B--O--R, or B--O--COR, where R is an alkyl or alkoxy group. In a
further exemplary embodiment, a dispersant that stabilizes the
nanoparticles by charging the nanoparticles is combined therewith.
Examples of such dispersants include, but are not limited to,
aminoalkylalkoxy silanes. In this regard, at least a portion of the
boron on the surface of the nanoparticles forms
B--O--SiR.sub.2NH.sub.2, where R is an alkyl or alkoxy group. The
nanoparticles then can be further stabilized by protonation.
Protonation can be achieved by adding an acid, such as, for
example, nitric acid, to form B--O--SiR.sub.2NH.sub.3.sup.+. The
dispersant also may comprise a combination of the stabilizing
dispersants described above. The nanoparticles and the dispersant
are mixed using any suitable mixing or agitation process that
facilitates formation of a homogeneous and stable suspension, such
as any of the suitable methods described above. Heat also may be
used to facilitate formation of the suspension.
[0041] A spread-minimizing additive also is added to the
boron-comprising nanoparticles (step 256). Any of the
above-described spread-minimizing additives may be used. While FIG.
9 illustrates that the step of adding the spread-minimizing
additive (step 256) is performed after the step of combining the
nanoparticles with the dispersant (step 254), it will be
appreciated that the spread-minimizing additive also may be added
to the nanoparticles before or during the step of combining the
nanoparticles with the dispersant. The nanoparticles, with or
without reaction with a dispersant, and the spread-minimizing
additive are mixed using any suitable mixing or agitation process
that facilitates formation of a homogeneous and stable suspension,
such as any of the suitable methods described above. Heat also may
be used to facilitate formation of the suspension.
[0042] In an optional exemplary embodiment of the invention, one or
more other functional additives may be added to the nanoparticles
before, during, or after combination with the dispersant (step
258). Examples of other suitable additives that may be added
include dispersants, surfactants, polymerization inhibitors,
wetting agents, antifoaming agents, detergents and other
surface-tension modifiers, flame retardants, pigments,
plasticizers, thickeners, viscosity modifiers, rheology modifiers,
and mixtures thereof. While FIG. 9 illustrates that the step of
adding one or more other functional additives (step 258) is
performed after the step of adding the spread-minimizing additive
(step 256), it will be appreciated that the other functional
additive(s) may be added to the nanoparticles before, during, or
after the step of combining the nanoparticles with the dispersant
(step 254).
[0043] The following are examples of methods for fabricating
boron-comprising inks for use in forming boron-doped regions of
semiconductor substrates using non-contact printing processes. The
examples are provided for illustration purposes only and are not
meant to limit the various embodiments of the present invention in
any way.
EXAMPLE 1
[0044] In an exemplary embodiment of the present invention, a
boron-comprising ink was prepared by dissolving about 3.5 grams
(gm) boric acid in about 46.5 gm ethanol. The solution was spun
onto a four-inch n-type wafer at 500 revolutions per minute (rpm)
with no baking. The coated wafer was heated at 1050.degree. C. for
10 minutes in 2.5% oxygen. The wafer was deglazed using 20:1
diluted hydrofluoric acid (DHF). Sheet resistance after deglazing,
measured using a four-point probe test, was 75 ohms/sq.
EXAMPLE 2
[0045] In an exemplary embodiment of the present invention, a
boron-comprising ink was prepared by dissolving about 3.51 gm boron
oxide in about 46.5 gm ethanol. The solution was spun onto a
four-inch n-type wafer at 500 rpm with no baking. The coated wafer
was heated at 1050.degree. C. for 10 minutes in 2.5% oxygen. The
wafer was deglazed using 20:1 DHF. Sheet resistance after
deglazing, measured using a four-point probe test, was 82
ohms/sq.
EXAMPLE 3
[0046] A polymeric borazole (PBZ) resin was prepared from a high
temperature polymerization of boron trichloride and cyclohexylamine
A PBZ solution A ink then was prepared by dissolving 450 gm PBZ
resin in 1060 gm toluene. About 160 gm cyclohexylamine was added
and mixed thoroughly. The final PBZ solution A ink had a solid
content of about 37%.
[0047] A PBZ solution B ink was prepared by mixing 450 gm PBZ
solution A ink with 400 gm toluene and 40 gm cyclohexylamine
[0048] The PBZ solution B ink was spun onto a four-inch n-type
wafer at a spin speed of 1000 rpm with no baking. The coated wafer
was then heated to 1050.degree. C. for 30 minutes in 2.5% oxygen.
The film thickness was about 204 nm. The sheet resistance after
deglazing in 20:1 DHF was 14.8 ohms/sq.
[0049] The PBZ solution B ink also was spun onto a four-inch n-type
wafer at a spin speed of 1000 rpm with no baking. The coated wafer
then was heated to 950.degree. C. for 30 minutes in air. The sheet
resistance after deglazing in 20:1 DHF was 47 ohms/sq.
EXAMPLE 4
[0050] An ink comprising 20.6 weight percent (wt. %) PBZ resin
formed as described in Example 3, 55.9 wt. % xylene, 14.7 wt. %
cyclohexylamine and 8.8 wt% polypropylene glycol (molecular weight
of 4000) was prepared. The viscosity of the ink was 11.6 cp. The
solution was spun onto a four-inch n-type wafer at 1000 rpm with no
baking. The coated wafer was heated to about 1050.degree. C. in air
and held at 1050.degree. C. for about 15 minutes. The wafer was
deglazed in 20:1 DHF. The sheet resistance after deglazing as
measured using a four-point probe test was 10.6 ohm/sq. An area of
2 centimeters (cm) by 2 cm was printed using a Dimatix Inkjet
Printer Model DMP 2811 with a nozzle having a 21 micrometer .mu.m
diameter.
EXAMPLE 5
[0051] An ink comprising 63.5 wt. % PBZ resin formed as described
in Example 3 and 36.5 wt. % xylene was prepared. The viscosity of
the solution was about 7.0 cp. A film was print-coated onto a
four-inch n-type wafer using a Dimatix Inkjet Printer Model DMP
2811. The printed wafer was heated to about 1050.degree. C. in
about 15% oxygen and held at that temperature for about 30 minutes.
The wafer was deglazed in 20:1 DHF. Sheet resistance of the printed
areas was 18.9 ohms/sq.
EXAMPLE 6
[0052] An ink comprising 93.3 wt. % PBZ solution B ink formed as
described in Example 3 and 16.7 wt. % oleic acid was prepared.
Viscosity of the solution was about 3.4 cp. The solution was spun
onto a four-inch n-type wafer at 1000 rpm with baking at 80.degree.
C. for one minute, 170.degree. C. for one minute, and 250.degree.
C. for one minute. The coated wafer then was heated to 1050.degree.
C. in 15% oxygen and held at that temperature for 30 minutes. Film
thickness before deglazing was about 32.3 nm. The wafer was
deglazed in 20:1 DHF. Sheet resistance as measured by a four-point
probe test was about 26.6 ohms/sq.
EXAMPLE 7
[0053] An ink comprising about 90 wt. % PBZ solution B ink formed
as described in Example 3 and about 10 wt. % oleic acid was
prepared. The viscosity of the resulting ink was 2.2 cp. The
solution was spun onto a four-inch n-type wafer at 1000 rpm with
baking at 80.degree. C. for one minute, 170.degree. C. for one
minute, and 250.degree. C. for one minute. The coated wafer was
heated to 1050.degree. C. in 15% oxygen and held at that
temperature for about 30 minutes. Film thickness before deglazing
was about 275.7 nm. The wafer was deglazed in 20:1 DHF. Sheet
resistance as measured by a four-point probe test was 33.8
ohms/sq.
EXAMPLE 8
[0054] An ink comprising 36.6 wt. % PBZ resin formed as described
in Example 3, 10.9 wt. % cyclohexylamine and 52.8 wt. % xylene was
prepared. The solution was spun onto a four-inch wafer at a spin
speed of 1000 rpm with no baking. The coated wafer was heated to
950.degree. C. for 30 minutes in air. The wafer was deglazed in
20:1 DHF. Sheet resistance after deglazing was 51 ohms/sq.
[0055] Accordingly, boron-comprising inks for forming boron-doped
regions in semiconductor substrates using non-contact printing and
methods for fabricating such boron-comprising inks have been
provided. While at least one exemplary embodiment has been
presented in the foregoing detailed description of the invention,
it should be appreciated that a vast number of variations exist. It
should also be appreciated that the exemplary embodiment or
exemplary embodiments are only examples, and are not intended to
limit the scope, applicability, or configuration of the invention
in any way. Rather, the foregoing detailed description will provide
those skilled in the art with a convenient road map for
implementing an exemplary embodiment of the invention, it being
understood that various changes may be made in the function and
arrangement of elements described in an exemplary embodiment
without departing from the scope of the invention as set forth in
the appended claims and their legal equivalents.
* * * * *