U.S. patent application number 11/868927 was filed with the patent office on 2008-01-31 for methods of forming a doped semiconductor thin film, doped semiconductor thin film structures, doped silane compositions, and methods of making such compositions.
Invention is credited to James Montague Cleeves, Vladimir K. Dioumaev, Wenzhuo Guo, Klaus Kunze, Brent Ridley, Joerg Rockenberger, Fabio Zurcher.
Application Number | 20080022897 11/868927 |
Document ID | / |
Family ID | 38870457 |
Filed Date | 2008-01-31 |
United States Patent
Application |
20080022897 |
Kind Code |
A1 |
Zurcher; Fabio ; et
al. |
January 31, 2008 |
METHODS OF FORMING A DOPED SEMICONDUCTOR THIN FILM, DOPED
SEMICONDUCTOR THIN FILM STRUCTURES, DOPED SILANE COMPOSITIONS, AND
METHODS OF MAKING SUCH COMPOSITIONS
Abstract
Methods for forming doped silane and/or semiconductor thin
films, doped liquid phase silane compositions useful in such
methods, and doped semiconductor thin films and structures. The
composition is generally liquid at ambient temperatures and
includes a Group IVA atom source and a dopant source. By
irradiating a doped liquid silane during at least part of its
deposition, a thin, substantially uniform doped
oligomerized/polymerized silane film may be formed on a substrate.
Such irradiation is believed to convert the doped silane film into
a relatively high-molecular weight species with relatively high
viscosity and relatively low volatility, typically by
cross-linking, isomerization, oligomerization and/or
polymerization. A film formed by the irradiation of doped liquid
silanes can later be converted (generally by heating and
annealing/recrystallization) into a doped, hydrogenated, amorphous
silicon film or a doped, at least partially polycrystalline silicon
film suitable for electronic devices. Thus, the present invention
enables use of high throughput, low cost equipment and techniques
for making doped semiconductor films of commercial quality and
quantity from doped "liquid silicon."
Inventors: |
Zurcher; Fabio; (Brisbane,
CA) ; Guo; Wenzhuo; (Cupertino, CA) ;
Rockenberger; Joerg; (Redwood City, CA) ; Dioumaev;
Vladimir K.; (Mountain View, CA) ; Ridley; Brent;
(San Carlos, CA) ; Kunze; Klaus; (Albuquerque,
NM) ; Cleeves; James Montague; (Redwood City,
CA) |
Correspondence
Address: |
THE LAW OFFICES OF ANDREW D. FORTNEY, PH.D., P.C.
401 W FALLBROOK AVE STE 204
FRESNO
CA
93711-5835
US
|
Family ID: |
38870457 |
Appl. No.: |
11/868927 |
Filed: |
October 8, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10949013 |
Sep 24, 2004 |
|
|
|
11868927 |
Oct 8, 2007 |
|
|
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Current U.S.
Class: |
106/287.14 |
Current CPC
Class: |
H01L 21/02532 20130101;
H01L 21/02576 20130101; H01L 21/02579 20130101; H01L 21/0262
20130101; H01L 21/02628 20130101 |
Class at
Publication: |
106/287.14 |
International
Class: |
C09D 1/00 20060101
C09D001/00 |
Claims
1. A composition, comprising: a) a compound of the formula
A.sub.xH.sub.y, where each A is independently Si or Ge, x is from 3
to 20, and y is from x to (2x+2); and b) at least one dopant of the
formula B.sub.aR.sup.1.sub.b and/or
(R.sup.2.sub.3A).sub.rA.sub.c(BR.sup.1.sub.2).sub.s, where a is
from 1 to 20; b is an integer corresponding to the number of
binding sites available on the a instances of B, each R.sup.1 is
independently H, alkyl, aralkyl or AR.sup.2.sub.3, and at least one
of the b instances of R.sup.1 is alkyl, aralkyl or AR.sup.2.sub.3;
R.sup.2 is hydrogen, alkyl, aryl, aralkyl or A.sub.yH.sub.2y+1; c
is 1 to 4, r+s=2c+2, and s.gtoreq.1.
2. The composition of claim 1, wherein said compound has the
formula (AH.sub.x).sub.k, where k is from 3 to 12, and each of the
k instances of x is 1 or 2.
3. The composition of claim 2, wherein said compound is monocyclic,
k is from 4 to 8, and x is 2.
4. The composition of claim 1, wherein A is Si.
5. The composition of claim 1, wherein said dopant has the formula
B.sub.a'R.sup.1.sub.b', where a' is 1 or 2; b' is 3a', at least a'
instances of R' are C.sub.1-C.sub.6 alkyl, and the remainder of the
b' instances of R.sup.1 are independently H, C.sub.1-C.sub.6 alkyl,
C.sub.6-C.sub.10 aryl, C.sub.7-C.sub.10 aralkyl or AR.sup.2.sub.3,
where R.sup.2 is hydrogen or A.sub.yH.sub.2y+1
(1.ltoreq.y.ltoreq.4).
6. The composition of claim 5, wherein R.sup.1 is C.sub.1-C.sub.6
alkyl.
7. The composition of claim 6, wherein R.sup.1 is methyl, ethyl,
propyl or butyl.
8. The composition of claim 6, wherein A is Si.
9. The composition of claim 1, wherein c is 1 and R.sup.1 is H,
C.sub.1-C.sub.6 alkyl, C.sub.6-C.sub.10 aryl, or AR.sup.2.sub.3,
where R.sup.2 is hydrogen or A.sub.yH.sub.2y+1
(1.ltoreq.y.ltoreq.4).
10. The composition of claim 9, wherein R.sup.1 is H, t-butyl,
phenyl, or AH.sub.3.
11. The composition of claim 1, wherein from 0.00001 to 50 vol % of
said composition consists essentially of said dopant and from 0.5
to 99.999 vol % of said composition consists essentially of said
compound.
12. The composition of claim 11, further comprising a solvent in
which said compound and said dopant are soluble.
13. The composition of claim 12, wherein said solvent is selected
from the group consisting of alkanes, substituted alkanes,
cycloalkanes, substituted cycloalkanes, arenes, substituted arenes,
and (cyclic) siloxanes.
14. The composition of claim 13, wherein said solvent is selected
from the group consisting of C.sub.5-C.sub.10 monocycloalkanes;
C.sub.3-C.sub.8 monocycloalkanes substituted with from 1 to 2n
C.sub.1-C.sub.4 alkyl or halogen substituents or from 1 to n
C.sub.1-C.sub.4 alkoxy substituents; C.sub.10-C.sub.14
polycycloalkanes; siloxanes of the formula
(R.sup.3.sub.3Si)(OSiR.sup.3.sub.2).sub.p(OSiR.sup.3.sub.3), where
p is from 0 to 4, and each R.sup.3 is independently H,
C.sub.1-C.sub.6 alkyl, benzyl or phenyl substituted with from 0 to
3 C.sub.1-C.sub.4 alkyl groups; cyclosiloxanes of the formula
(SiR.sup.4.sub.2O).sub.q, where q is from 2 to 6, and each R.sup.4
is independently H, C.sub.1-C.sub.6 alkyl, benzyl or phenyl
substituted with from 0 to 3 C.sub.1-C.sub.4 alkyl groups; and
C.sub.3-C.sub.8 fluoroalkanes substituted with from 1 to (2n+2)
fluorine atoms, where n is the number of carbon atoms in the
selected solvent.
15. The composition of claim 14, wherein said solvent is a
C.sub.6-C.sub.10 monocycloalkane or a C.sub.10-C.sub.14
polycycloalkane.
16. The composition of claim 11, wherein A is Si.
17. The composition of claim 12, wherein from 0.5 to 50 vol % of
said composition consists essentially of said compound.
18. The composition of claim 17, wherein from 1 to 35 vol % of said
composition consists essentially of said compound.
19. The composition of claim 18, wherein from about 5 to 25 vol %
of said composition consists essentially of said compound.
20. The composition of claim 12, wherein from about 1 to about 25
wt. % of said composition consists essentially of said compound,
and dopant is present in an amount providing from about 0.0001 to
about 10 at. % of B atoms with respect to A atoms in said compound.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of U.S. application Ser.
No. 10/949,013 (Attorney Docket No. IDR0302), filed Sep. 24, 2004,
pending, which is incorporated herein by reference in its
entirety.
FIELD OF THE INVENTION
[0002] The present invention generally relates to the fields of
doped silane and semiconductor thin films and thin film structures,
methods of forming doped silane and semiconductor thin films, and
doped liquid phase silane compositions useful in such methods.
DISCUSSION OF THE BACKGROUND
[0003] There are a number of silanes that are liquid at ambient
temperatures (e.g., from about 15.degree. C. to about 30.degree.
C.) or that can be formulated into an ink composition that is
liquid at ambient temperatures. Liquid silanes, such as
cyclopentasilane or cyclohexasilane, have been investigated as
candidate "liquid silicon" precursors. However, to date, it has
been challenging to make semiconducting thin films of commercial
qualities and quantities from "liquid silicon" precursors. One such
challenge has related to doping such "liquid silicon" precursors
and/or the films formed therefrom.
[0004] Methods have been proposed for covalently binding dopant
atoms such as phosphorous and boron to silicon atoms in certain
liquid (cyclo)silanes. For example, photochemical reactions between
(cyclo)silanes and certain phosphines and/or boranes are disclosed
in U.S. Pat. No. 4,683,145 and U.S. Patent Publication No.
2003/0229190. Heterocyclic doped silanes are disclosed in U.S. Pat.
No. 6,527,847 and U.S. Patent Publication No. 2003/0045632, and a
method for synthesizing such doped silanes is disclosed in U.S.
Pat. No. 6,527,847. The properties of thin films formed from such
compounds are somewhat disappointing, given the relative proportion
of dopant atoms in the film-forming mixture. Also, the results are
not quite as reproducible as would be generally desired for
commercial applications.
[0005] The mechanisms behind the disappointing results are not well
understood. However, there may be a number of critical steps
involved in forming doped semiconducting films from doped liquid
silanes, such as forming the covalent bonds between dopant atoms
and silicon, preserving these covalent bonds during subsequent
synthesis steps and in initial processing steps to form a thin film
or thin film structure, and activating the dopant atoms once the
thin film or thin film structure is formed.
[0006] Thus, there has been a long-felt need in the art for a
"liquid silicon" compound and/or composition, particularly a doped
"liquid silicon." Such a composition would primarily comprise
silicon atoms (other than solvent, to the extent any solvent is
present as a main component), would include a dopant or dopant
precursor, would be liquid at ambient temperatures (to facilitate
handling, deposition and further processing), and would yield
commercial quality doped semiconducting films upon subsequent
processing (e.g., annealing or curing). However, to date, methods
of making a thin doped semiconducting film or film structure from
liquid silanes have not been sufficiently reliable for high-volume
commercial use.
SUMMARY OF THE INVENTION
[0007] Embodiments of the present invention relate to doped silane
and/or semiconductor thin film structures, methods of forming doped
silane and/or semiconductor thin films, and doped liquid phase
silane compositions useful in such methods. In one aspect, the
present invention concerns a method of coating a substrate,
comprising the steps of (a) coating the substrate with a liquid
phase composition comprising a doped silane; and (b) irradiating
the liquid phase composition sufficiently to (i) cross-link,
isomerize, oligomerize and/or polymerize the doped silane, (ii)
form a substantially uniform layer on the substrate, the layer
comprising a doped oligo- and/or polysilane, and/or (iii) increase
an average molecular weight, increase a viscosity and/or reduce a
volatility of the composition. At least part of the irradiating
step is performed during at least part of the coating step.
Optionally, the method may form a thin film pattern, and may
comprise printing a liquid phase composition comprising a doped
silane in a pattern on a substrate and irradiating the composition
and/or pattern.
[0008] In another aspect, the present invention further relates to
a method of making a doped semiconductor film, comprising the steps
of (1) curing an at least partially crosslinked, oligomerized
and/or polymerized doped silane film to form a cured doped
semiconductor layer (which may be hydrogenated and/or amorphous);
and (2) annealing the cured doped semiconductor layer sufficiently
to at least partially activate the dopant and form the doped
semiconductor film. It is believed that annealing (i) activates at
least a portion of the dopant and (ii) may (re)crystallize the
semiconductor film. This aspect of the invention is useful for
making device structures (such as transistor terminals, diodes,
resistors and/or capacitor plates). Thus, in a further aspect, the
present invention relates to doped semiconductor thin film
structures and device structures that may be made using one or more
of the present methods.
[0009] The present invention also relates to doped liquid phase
silane, germane and silagermane compositions, generally comprising
a Group IVA atom source and a dopant source. The Group IVA atom
source and the dopant source may be separate compounds and/or
groups or parts in the same compound. In certain embodiments, the
doped liquid phase compositions may further comprise a solvent.
[0010] The present inventors have discovered that a thin,
substantially uniform doped oligomerized/polymerized silane film
may be deposited onto a substrate by conversion of a doped liquid
silane composition into a relatively high-molecular weight species
with relatively high viscosity and relatively low volatility. Such
conversion (typically by cross-linking, isomerization,
oligomerization and/or polymerization) may be achieved by
irradiating the silane film during the coating thereof (preferably
by spin-coating) onto the substrate. The film may be irradiated
with ultraviolet (UV) light or other form of radiation suitable for
inducing cross-linking, isomerization, oligomerization and/or
polymerization. Such irradiation generally yields a film of
oligomeric and/or polymeric hydrogenated silanes, which can later
be converted (generally by heating and [optionally] subsequent
laser irradiation) into a doped, hydrogenated, amorphous silicon
film or a doped polycrystalline semiconductor film suitable for
electronic devices. Thus, the present invention advantageously
provides commercial qualities and quantities of doped semiconductor
films from a doped "liquid silicon" composition.
[0011] These and other advantages of the present invention will
become readily apparent from the detailed description of preferred
embodiments below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a graph showing the dopant
concentration/distribution as a function of film depth/thickness in
a first exemplary thin film made in accordance with the present
invention.
[0013] FIG. 2 is a graph showing the dopant
concentration/distribution as a function of film depth/thickness in
a series of second exemplary thin films made in accordance with the
present invention.
[0014] FIG. 3 is a graph showing a conventional current as a
function of applied voltage (I-V) curve for a thin film made in
accordance with the present invention (see the "Ohmic Contact"
example described in paragraph [0097] below).
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0015] Reference will now be made in detail to the preferred
embodiments of the invention, evidence and/or an example of which
is shown in the accompanying Figures and in the Table(s) and
Example(s) herein below. While the invention will be described in
conjunction with the preferred embodiments, it will be understood
that they are not intended to limit the invention to these
embodiments. On the contrary, the invention is intended to cover
alternatives, modifications and equivalents that may be included
within the spirit and scope of the invention as defined by the
appended claims. Furthermore, in the following detailed description
of the present invention, numerous specific details are set forth
in order to provide a thorough understanding of the present
invention. However, it will be readily apparent to one skilled in
the art that the present invention may be practiced without these
specific details. In other instances, well-known methods,
procedures, components, and circuits have not been described in
detail so as not to unnecessarily obscure aspects of the present
invention.
[0016] For the sake of convenience and simplicity, the terms
"C.sub.a-C.sub.b alkyl," "C.sub.a-C.sub.b alkoxy," etc., shall
refer to both branched and unbranched moieties, to the extent the
range from a to b covers 3 or more carbon atoms. Unless otherwise
indicated, the terms "arene" and "aryl" refer to both mono- and
polycyclic aromatic species. The terms "silane" and "(cyclo)silane"
may be used interchangeably herein, and unless expressly indicated
otherwise, these terms refer to compounds or mixtures of compounds
that consist essentially of (1) silicon and/or germanium and (2)
hydrogen. The terms "heterosilane" and "hetero(cyclo)silane" may be
used interchangeably herein, and unless expressly indicated
otherwise, these terms refer to compounds or mixtures of compounds
that consist essentially of (1) silicon and/or germanium, (2)
hydrogen, and (3) dopant atoms such as B, P, As or Sb that may be
substituted by a conventional hydrocarbon, silane or germane
substituent. The term "doped silane" refers to a composition
comprising (1) a Group IVA atom source (generally consisting of one
or more Group IVA elements [such as Si and/or Ge] and hydrogen) and
(2) a dopant source (generally consisting essentially of one or
more conventional semiconductor dopant atoms such as B, P, As, or
Sb, which may have one or more covalently bound mono- or divalent
hydrocarbon or silane substituents), and which may include a single
species such as a hetero(cyclo)silane or plural species such as a
(cyclo)silane and an organo- or silylphosphine or -borane. The term
"semiconductor" refers to a compound or material having
semiconducting properties or that contains atoms conventionally
associated with semiconducting material (e.g., Si and/or Ge, which
may be doped with or bound to conventional dopant atoms such as B,
P, As or Sb). The prefix "(cyclo)-" generally refers to a compound
or mixture of compounds that may contain a cyclic ring, and the
prefix "cyclo-" generally refers to a compound or mixture of
compounds that contain a cyclic ring. Somewhat similarly, the term
"(re)crystallize" (and grammatical variations thereof) means to
crystallize or recrystallize, depending on the morphology of the
film immediately prior to a process step effecting such
crystallizing or recrystallizing. Also, the term "(spin) coating"
refers to the present coating step, which in a preferred
embodiment, comprises spin coating.
[0017] The phrase "amorphous or polycrystalline" refers to films or
thin film structures that contain an amorphous material, a
polycrystalline material, or a material that is partially amorphous
and partially crystalline or polycrystalline. The term "transistor
terminal" may refer to any terminal type of any transistor type,
such as a gate, source and/or drain of a conventional thin film
transistor (TFT), field effect transistor (e.g., a MOSFET, IGFET,
etc.), or a base, collector or emitter of a bipolar transistor
(e.g., a BJT, a microwave transistor, a power transistor, etc.).
Also, for convenience and simplicity, the terms "part," "portion"
and "region" may be used interchangeably, as may the terms
"connected to," "coupled with," "coupled to," and "in communication
with," but these terms are also generally given their
art-recognized meanings.
[0018] The present invention concerns a method of coating a
substrate with a doped, oligomerized/polymerized silane film,
comprising the steps of (a) coating the substrate with a liquid
phase composition comprising a doped silane; and (b) irradiating
the liquid phase composition sufficiently to (i) cross-link,
isomerize, oligomerize and/or polymerize the doped silane, (ii)
form a substantially uniform layer on the substrate, the layer
comprising a doped oligo- and/or polysilane, and/or (iii) increase
an average molecular weight, increase a viscosity and/or reduce a
volatility of the composition. At least part of the irradiating
step is performed during at least part of the coating step.
Optionally, the method may form a thin oligomerized/polymerized
silane film pattern, and may comprise printing a liquid phase
composition comprising a doped silane in a pattern on a substrate
and irradiating the composition.
[0019] A further aspect of the invention concerns a method of
making a doped semiconductor film, comprising the steps of (1)
curing an at least partially crosslinked, oligomerized and/or
polymerized doped silane film to form a cured doped semiconductor
layer (which may be hydrogenated and/or amorphous); and (2)
annealing the cured doped semiconductor layer sufficiently to at
least partially activate the dopant and form the doped
semiconductor film. It is believed that annealing activates at
least a portion of the dopant in the doped film(s) and may
(re)crystallize the semiconductor film.
[0020] An even further aspect of the present invention relates to
doped semiconductor thin film structures and device structures
(such as thin film transistor terminals and/or MOS capacitor
terminals or plates) that may be made using one or more of the
present methods. The thin film transistor generally comprises the
doped, amorphous or polycrystalline semiconductor film, a
transistor terminal layer above or below the film, and one or more
metallization structures in contact with the film. The method of
making a transistor or MOS capacitor generally comprises at least
one of the present methods of (1) one of (i) coating a substrate
with a doped oligomerized/polymerized silane film or (ii) printing
a doped oligomerized/polymerized silane film in a pattern on a
substrate, and/or (2) making a doped amorphous or polycrystalline
semiconductor film therefrom, and (3) forming a metallization
structure in electrical communication with the film.
[0021] The present invention also relates to doped liquid phase
silane compositions and method(s) of making such compositions,
which generally comprise combining one or more Group IVA atom and
dopant source compounds with a solvent (or with another Group IVA
atom and/or dopant source compound), and mixing the compound(s) and
the solvent/other compound sufficiently to form a solution.
[0022] The invention, in its various aspects, will be explained in
greater detail below with regard to exemplary embodiments.
[0023] Exemplary Methods of Depositing Doped Liquid Silane
Compositions and Making Doped, Oligomerized/Polymerized Silane
Films
[0024] In one aspect, the present invention relates to a method of
coating a substrate with a doped oligomerized/polymerized silane
film, comprising the steps of: (a) coating the substrate with a
liquid phase composition comprising a doped silane; and (b)
irradiating the liquid phase composition at least partly during the
coating step. Typically, the irradiating step is performed
sufficiently to (i) cross-link, isomerize, oligomerize and/or
polymerize the doped silane, (ii) form a substantially uniform
layer on the substrate, the layer comprising a doped oligo- and/or
polysilane, and/or (iii) increase an average molecular weight,
increase a viscosity and/or reduce a volatility of the composition.
Optionally, the method may form a thin doped
oligomerized/polymerized silane film pattern, and may comprise
printing a liquid phase composition comprising a doped silane in a
pattern on the substrate and irradiating the composition. Thus, in
this aspect of the present invention, the term "depositing" (and
grammatical variations thereof) encompasses both "coating" and
"printing," as well as other forms of deposition, unless the
context of such use clearly and unambiguously limits its
meaning.
[0025] As disclosed in copending U.S. application Ser. No.
10/789,274, filed Feb. 27, 2004 (Atty. Docket No. IDR0080),
irradiating a liquid-phase silane composition while depositing the
composition on a substrate leads to uniform silane film formation,
in terms of both film thickness and film coverage over the
substrate. An exemplary apparatus for simultaneously coating a
substrate with a liquid phase silane composition and irradiating
the composition is also disclosed in U.S. application Ser. No.
10/789,274, filed Feb. 27, 2004 (Atty. Docket No. IDR0080), the
relevant portions of which are incorporated herein by
reference.
[0026] Without intending to be bound by any particular theory, it
is believed that irradiating the composition during coating causes
the silane and dopant therein to oligomerize, polymerize and/or
crosslink, thereby reducing (i) the volatility of the composition
and/or (ii) any tendency of the composition to bead up (a
phenomenon believed to be related to the surface tension of the
composition), and increasing (A) the viscosity of the composition
and/or (B) the average molecular weight of the (doped) silane
compound(s) therein. At the same time, irradiating the composition
during coating is believed to enable the composition to form a
uniform, thin layer using otherwise standard conventional coating
techniques (e.g., spin coating). Thus, in various embodiments of
the present method, the coating step and the irradiating step may
be conducted simultaneously for a length of time of from 1 to 300
seconds, from 3 to 180 seconds, or from 5 to 120 seconds. In one
implementation, the coating step and the irradiating step are
conducted simultaneously for about 30-35 seconds. When performed as
described herein, the present method generally yields a film of
doped oligomerized/polymerized silane, covering>80%, >90% or
.gtoreq.95% of the surface area of the substrate being coated.
[0027] However, the coating step in the present invention may
comprise spin coating, inkjetting, dip-coating, spray-coating, slit
coating, extrusion coating, or meniscus coating the composition
onto the substrate. Preferably, the coating step comprises spin
coating. Alternatively, in various embodiments of the
pattern-forming method, printing may comprise inkjetting, gravure-,
offset- or flexo-printing, spray-coating, slit coating, extrusion
coating, meniscus coating, microspotting and/or pen-coating the
doped silane composition in a pattern or through a stencil onto the
substrate (preferably inkjetting). In this case, only predetermined
portions of the substrate (generally corresponding to the pattern)
are coated with the composition.
[0028] In the pattern-forming method, it is not necessary to print
the doped silane composition and simultaneously irradiate it. For
example, in certain embodiments using inkjet printing technology,
the source of radiation may be placed in close proximity to the
inkjet head (e.g., within a few mm), but not physically close
enough to provide a dose of radiation sufficient to cross-link,
isomerize, oligomerize and/or polymerize the doped silane as the
composition is being printed onto the substrate (although the
substrate and/or inkjet head may move a sufficient distance within
less than one to a few seconds for the printed pattern to be
irradiated). As a result, the pattern-forming method does not
require simultaneous irradiation, although simultaneous irradiation
while printing may provide some of the same benefits to
pattern-forming methods as it does to coating methods. Also, in a
preferred embodiment of the present printing method, the
irradiating step may comprise irradiating the printed/patterned
composition with ultraviolet (UV) light for less than 1 second. In
a high-throughput printing system, the residence (irradiation) time
of the doped silane film under a UV lamp is preferably very small.
One may compensate for such a short irradiation time by increasing
the UV power sufficiently to provide an effective dose of
radiation.
[0029] With regard to a preferred coating method, spin coating may
be conducted at a speed of from 100 to 20,000 revolutions per
minute (RPM). In a preferred embodiment, the spin coating is
conducted in two stages. Thus, spin coating may comprise (i) a
first spin coating stage conducted at a first speed, and (ii) a
second spin coating stage conducted at a second speed greater than
the first speed. In various implementations, the first speed may be
from 100 to 2,000 RPM, from 200 to 1,500 RPM, or from 300 to 1,000
RPM, and the second speed may be from 1,000 to 20,000 RPM, from
1,500 to 15,000 RPM, or from about 2,000 to 10,000 RPM. Of course,
one may select any endpoint for the first and/or second speeds from
any of these ranges or from within any of these ranges, as long as
the second speed is greater than the first speed. For example, the
first speed may be from 100 to 1,000 RPM, and the second speed may
be from 1,500 to 10,000 RPM. In one such implementation, the first
speed is about 500 RPM and the second speed is about 2,000 RPM. In
this embodiment, the second speed is usually greater than the first
speed by an amount of 2.times. or more, 3.times. or more, or
4.times. or more.
[0030] The spin coating step, as well as each stage of the spin
coating step (if applicable), may be conducted for a predetermined
length of time. For example, the first spin coating stage may be
conducted for a first length of time of from 1 to 60 seconds, from
1 to 30 seconds, or from 3 to 10 seconds, and the second spin
coating stage may be conducted for a second length of time of from
5 to 300 seconds, from 10 to 120 seconds, or from 15 to 60 seconds.
Similar to the spin coating speed(s) described above, one may
select any endpoint for any applicable length of spin coating time
from any of these ranges or from within any of these ranges.
Generally, however, the second length of time is greater than the
first length of time.
[0031] Similarly, the irradiating step may be conducted for a
length of time, for example, of from 3 to 600 seconds, from 5 to
300 seconds, or from 10 to 150 seconds. Alternatively, as for the
high-throughput printing embodiment discussed above, the
irradiating step may comprise irradiating the spin coated
composition with UV light for less than 1 second, optionally
compensating for such a short irradiation time by increasing the UV
power sufficiently to provide an effective dose of radiation. As
for spin coating, one may select any endpoint for any applicable
length of irradiating time from any of these ranges, or from within
any of these ranges. Generally, however, (spin) coating is
performed for a greater length of time than irradiating. In one
implementation, the (spin) coating and irradiating steps are
stopped at about the same time (approximately simultaneously).
While any form of radiation (e.g., an electron beam or light, and
more particularly, any wavelength of light) may be used that
accomplishes one or more of results (i)-(iii) in paragraph [0022],
the irradiating step preferably comprises irradiating with
ultraviolet light.
[0032] In addition to the irradiating step described above, the
present method may further comprise irradiating the doped silane
composition prior to the coating step. Such pre-coating irradiation
may be at a dose and/or for a length of time sufficient to increase
the average molecular weight, increase the viscosity and/or reduce
the volatility of the composition containing the doped silane,
presumably by oligomerizing, polymerizing and/or crosslinking the
silane and dopant compound(s) therein.
[0033] The method may further comprise the step of depositing the
liquid phase composition onto the substrate prior to the coating
step, and the depositing step may further comprise rotating the
substrate while depositing the liquid phase composition. While
these steps are largely conventional, in one embodiment, the
depositing step comprises depositing the liquid phase composition
along a radius of the substrate. While the phrase "along a radius
of the substrate" is easily understood in the case where the
substrate has a substantially circular surface to be coated, this
phrase is also applicable in the cases where the surface of the
substrate to be coated is substantially square, rectangular, oval,
etc. (such as in the case of a flat panel display; see, e.g., U.S.
application Ser. No. 10/789,274, filed Feb. 27, 2004 [Atty. Docket
No. IDR0080], the relevant portions of which are incorporated
herein by reference).
[0034] When the depositing step includes rotating the substrate,
rotating may be conducted at a speed of 500 RPM or less, 300 RPM or
less, or 100 RPM or less. In fact, such substrate rotating may be
performed manually. If one rotates the substrate while depositing
the liquid phase composition thereon, one may effectively coat the
entire substrate surface by depositing the composition along a
radius of the substrate. Thus, the substrate preferably comprises a
securable substrate having a substantially smooth, flat surface
upon which the liquid phase composition can be spin coated. In such
a case, the substrate may comprise a wafer, plate, disc, sheet
and/or foil of a semiconductor (e.g. silicon), a glass, a ceramic,
a dielectric, plastic and/or a metal, preferably a member selected
from the group consisting of a silicon wafer, a glass plate, a
ceramic plate or disc, a plastic sheet or disc, metal foil, a metal
sheet or disc, and laminated or layered combinations thereof.
[0035] In one embodiment, the liquid phase composition consists
essentially of components in the liquid phase at ambient
temperatures. Use of all liquid phase components avoids a number of
potential problems associated with use of solid-phase components,
such as non-uniformities in distribution of the components in the
composition (e.g., when the composition is in the form of a colloid
or suspension) and/or in the thin film formed on the substrate
(e.g., when the solid-phase component[s] move along the substrate
surface at a lower rate than the liquid-phase components in the
composition).
[0036] As described above, the radiation used in the irradiating
step preferably comprises ultraviolet light. Typically, such
radiation includes light within the range of 200 nm to 440 nm,
preferably 220 nm to 400 nm, more preferably from 250 to 380 nm. A
suitable source of such radiation may comprise a mercury vapor
and/or arc lamp. In the coating embodiments, the power output of
the preferred UV lamp may be adjusted to about 0.1-20, 0.25-10 or
0.5-5 milliwatt/cm.sup.2. In the printing embodiments, the power
output may be higher, since the irradiation time will undoubtedly
be in the shorter ends of the irradiation time ranges described in
paragraph [0029] above. In either case, the power output of the
lamp may be focused at the location of the substrate and/or silane.
To adjust the UV intensity at the surface of the substrate, the
radiation from the UV lamp may be passed through a mask (e.g., a
quartz plate having a chrome pattern of varying density thereon).
Alternatively, the UV intensity may be adjusted by regulating the
voltage applied to the UV lamp power supply.
[0037] In an alternative embodiment, the method may further
comprise, after the irradiating step, heating the doped silane film
to a temperature of from about 100.degree. C. to about 200.degree.
C. to (i) remove some, most or all of the remaining solvent and/or
(ii) further oligomerize and/or polymerize the doped silane, prior
to subsequent processing. In various sub-embodiments, such heating
is conducted for a length of time of from 3 to 60 minutes, 5 to 30
minutes, 10 to 20 minutes, or any time range having endpoints
therein. In one implementation, such heating (to remove solvent
and/or to oligomerize and/or polymerize the silane) is conducted by
placing the substrate on a hot plate under an inert atmosphere,
although one could also use a conventional oven or furnace in which
the atmosphere can be controlled for such heating.
[0038] To help reduce any incidence of inadvertent and/or undesired
oxide formation, the coating, printing, irradiating and/or heating
step(s) may be conducted under an inert and/or reducing gas
atmosphere. Thus, the method may further comprise, prior to any of
these steps, the steps of (i) purging an atmosphere in which the
substrate is placed, then (ii) introducing an inert and/or reducing
gas into the atmosphere. In a preferred embodiment, the composition
is coated or printed, irradiated and heated under an inert
atmosphere. However, as will be explained below with regard to the
method for forming a doped semiconductor film, it may be desirable
in some cases to cure the oligomerized/polymerized silane film
under a reducing gas atmosphere. In such an embodiment, it may also
be convenient to irradiate the oligomerized/polymerized silane film
under the reducing gas atmosphere to be used in the curing
step.
[0039] Particular parameters of the UV spin coating process and
recipe can have a strong influence on the properties of the doped
amorphous or polycrystalline film resulting from the present
coating method, followed by the present curing/annealing method
described below. For example, increasing (1) the time of
irradiation during spin coating, (2) the percentage (by weight or
volume) of silane and/or dopant in the composition, (3) the
relative amount of higher molecular weight (hetero)silanes in the
composition, (4) the viscosity of the liquid phase composition, (5)
the UV irradiation time and/or (6) the UV intensity generally
yields a doped, hydrogenated, amorphous semiconductor film and/or
doped, at least partially polycrystalline semiconductor film with
an increased thickness. Conversely, increasing the spin speed
(e.g., during the final stage, the highest-speed stage and/or a
stage during which irradiation is simultaneously conducted)
generally yields a hydrogenated, amorphous and/or (partially)
polycrystalline semiconductor film with a decreased thickness.
However, the films may have a less uniform thickness and/or may
become more likely to form cracks upon curing if (a) the UV power
intensity is too high (generally larger than 0.3 mW/cm.sup.2 at a
typical silane/dopant loading of .about.20 vol % of the spin
coating composition), (b) the UV irradiation time is too long, (c)
the mass/volume loading of the silane/dopant in the liquid phase
composition is too high, and/or (d) the viscosity of the liquid
phase composition is too high. Film uniformity and tendency to form
cracks appear to be less significant problems in cured patterns
formed by inkjetting, presumably because the stress(es) in the
inkjetted pattern films/"islands" is less than that of a more
uniformly deposited or "blanket" coating film.
[0040] Exemplary Methods for Making Doped Semiconductor Thin
Films
[0041] In another aspect, the present invention concerns a method
of making a doped semiconductor film, comprising the steps of: (1)
curing an at least partially crosslinked, oligomerized and/or
polymerized doped silane (hereinafter, the "doped,
oligomerized/polymerized silane") to form a cured doped
semiconductor layer (which may also be hydrogenated and/or
amorphous); and (2) annealing the cured doped semiconductor layer
sufficiently to activate at least part of the dopant and form the
doped semiconductor film. Without wishing to be bound by a
particular theory, it is believed that annealing activates at least
part of the dopant and may (re)crystallize the semiconductor film.
In certain embodiments, the doped, oligomerized/polymerized silane
is formed by one of the present methods of making a doped,
oligomerized/polymerized silane film described above.
[0042] The curing step generally comprises heating the doped
oligomerized/polymerized silane (which is typically in the form of
a film, layer, pattern, or islands) to a temperature of at least
about 300.degree. C. (preferably at least about 350.degree. C., and
more preferably about 400.degree. C. or higher), generally for a
length of time sufficient to cure (e.g., form a doped, and
optionally, hydrogenated and/or amorphous semiconductor layer) the
doped oligomerized/polymerized silane. Such heating may be
conducted for a length of time of at least 1 minute, 3 minutes or 5
minutes. While the maximum heating time may be typically about 30
minutes, 45 minutes, 1 hour or 15 hours, TFT quality silicon films
can be obtained after heating at about 300.degree. C. (or more) for
several hours (e.g., from 2, 3 or 4 hours to 12, 8 or 6 hours). In
one embodiment, curing comprises heating the substrate and the
doped oligomerized/polymerized silane layer to a temperature of
from about 400.degree. C. to about 500.degree. C., generally for a
length of time of about 20 minutes. Alternatively, curing may
comprise conventional electron beam curing of the polysilane
film.
[0043] It is believed that annealing the semiconductor film by
irradiating or heating sufficiently (1) electrically activates at
least a portion of the dopant in the cured, doped (and optionally,
hydrogenated and/or amorphous) semiconductor layer and (2) may
crystallize the cured, doped semiconductor layer when it is
amorphous; however, under certain conditions, it may be possible to
activate at least some of the dopant during the curing step without
(re)crystallizing the semiconductor layer to any significant extent
to form an activated, doped, amorphous semiconductor layer, or to
crystallize the doped amorphous semiconductor layer (e.g., during
curing) without activating some or all of the dopant. Without
wishing to be bound by any particular theory, it is believed that
dopant activation by irradiation (or heating at a sufficiently high
temperature) causes dopant atoms in the composition to react with
silicon or other Group IVA atoms in or from the silane (thereby
chemically "embedding" dopant atoms in the polymerized silane
and/or amorphous/polycrystalline semiconductor film), and may
enable dopant atoms in the composition to migrate along a lattice
of silicon and/or other Group IVA atoms to a position in the
lattice where the dopant atoms may have an improved or optimal
electrical effect.
[0044] Annealing may comprise irradiating (e.g., with light or a
conventional electron beam) or heating (e.g., by furnace annealing
or rapid thermal annealing) the doped (and optionally, hydrogenated
and/or amorphous) semiconductor layer. In one embodiment, annealing
comprises irradiating using a laser, which may be rastered in one,
two or more dimensions, or which may be selectively focused (e.g.,
through a mask) on predetermined regions of the doped hydrogenated,
amorphous semiconductor film corresponding to a pattern. It may not
be necessary to remove non-irradiated portions of a cured film that
is subject to laser irradiation through a mask to activate the
dopant and, optionally, crystallize the exposed semiconductor film
where such non-irradiated portions are not sufficiently
electrically active to adversely affect the electrical properties
of a device containing such a film. (Re)crystallizing by
irradiating generally employs a dose of radiation from a laser
sufficient to change the crystalline structure of the cured doped,
hydrogenated, amorphous semiconductor layer. Typically, the
radiation in the present annealing step comprises light
(preferably, ultraviolet [UV] light). The dose of radiation may
comprise from 100 to 10,000, from 500 to 5000, or from 1000 to 3000
mJ/cm.sup.2 of ultraviolet light. In one embodiment, dopant
activation may comprise irradiating with UV light from a
conventional UV flash lamp, where the substrate having a doped
semiconductor layer thereon is preheated, then flash-irradiated
with a dose of UV light sufficient to activate at least part of the
dopant.
[0045] The present cured, doped semiconductor layer may still
contain about 5-15 at % hydrogen after curing at 400-500.degree. C.
for less than 1 hour. Laser irradiation of such a film, even at
relatively low power densities, may damage the film with minimal or
no crystallization. Thus, in embodiments employing laser
irradiation (particularly high-power laser irradiation), the
hydrogen content of the semiconductor layer (which may be amorphous
prior to laser irradiation) should be reduced to below 5 at %,
preferably below 3 at %. Thus, the method may further comprise
heating the (amorphous) semiconductor layer at a temperature of
400-550.degree. C. (e.g., about 500 C.) for at least 1 hour,
sufficient to reduce the hydrogen content of the doped (amorphous
or at least partially polycrystalline) semiconductor layer or film
to below 5 at %, preferably below 3 at %. Alternatively, hydrogen
reduction and annealing may comprise irradiating with UV light from
a conventional laser capable of delivering a number of low-power
pulses (e.g., on the order of 1-100 ns) of radiation sufficient to
reduce the hydrogen content to below 5 at % (preferably below 3 at
%), then irradiating with a number of higher power UV light pulses
(still generally on the order of 1-100 ns) sufficient to activate
at least part of the dopant and (optionally) crystallize the doped
semiconductor layer/film. In one implementation, the doped,
oligomerized/polymerized silane film was cured at 500.degree. C.
for 15 hours to reduce the hydrogen content of the doped amorphous
semiconductor film to below 3 at %.
[0046] Annealing by heating may be conducted at a temperature
sufficiently low (e.g., 450-550.degree. C.) and for a sufficient
length of time (e.g., from 10 minutes to 24 hours) to activate the
dopant, but not crystallize the amorphous film. This embodiment
yields an electrically activated, doped, hydrogenated, amorphous
semiconductor (e.g., Si) film.
[0047] Annealing by heating (e.g., rapid thermal annealing [RTA] or
furnace annealing) may also be conducted at a temperature, for a
length of time and/or at a temperature ramp (e.g., rate of change
[increase and/or decrease]) sufficient to change the crystalline
structure of the cured doped (and optionally, hydrogenated and/or
amorphous) semiconductor layer (e.g., [re]crystallize it), in
addition to activating some or all of the dopant. For silicon
films, the annealing and/or crystallization temperature is
typically at least 600.degree. C., at least 650.degree. C., at
least 700.degree. C., at least 750.degree. C. or at least
800.degree. C., for a minimum time of from 3, 6, 12 or 24 hours
and/or up to a maximum time of 24, 36 or 48 hours. In one
implementation, the RTA temperature is about 900.degree. C.
However, embodiments including Ge will crystallize at a
significantly lower temperature. For example, over the course of a
24 hour period, a doped amorphous SiGe film having 30 at % Ge is
expected to crystallize at a temperature of around 500-525.degree.
C., while a corresponding LPCVD Si film crystallizes at a
temperature of around 600.degree. C. Films containing mostly (or
consisting essentially of) Ge may crystallize within temperature
and time period ranges suitable for curing (e.g., 500-550.degree.
C. for 1-16 hours). Thus, the term "amorphous" in the phrase
"doped, hydrogenated, amorphous semiconductor" is not intended to
exclude doped, hydrogenated semiconductor films that may exhibit
some or a substantial degree of (poly)crystallinity following the
present curing step, prior to annealing (which may still be desired
for dopant activation).
[0048] One may also induce crystallization (in addition to
activating some or all of the dopant) using conventional
metal-promoted (re)crystallization. Suitable metal-based
crystallization promoters and processes for their use in
crystallizing an amorphous semiconductor film (e.g., as formed from
semiconductor nanoparticles containing Si and/or Ge) may be
disclosed in copending application Ser. No. 10/______, filed
______, 2003 and entitled "Nanoparticles and Method for Making the
Same" (Atty. Docket No. NANO-2400), the relevant portions of which
are incorporated herein by reference.
[0049] The present method of making a doped semiconducting film may
further comprise, prior to curing, the step of irradiating a liquid
phase composition comprising a doped silane on a substrate to form
the crosslinked, oligomerized and/or polymerized doped silane. The
method may also further comprise, prior to such irradiating, the
step of depositing the liquid phase composition on the substrate
(e.g., by coating or printing). Exemplary depositing techniques may
include spin-coating (as discussed above), dip-coating,
ink-jetting, spray-coating, etc., which may be adapted for
simultaneous or near-simultaneous irradiation with UV light.
However, in a preferred embodiment, the depositing step comprises
spin coating.
[0050] As for the exemplary method(s) of making doped
oligomerized/polymerized silane films described above, at least
part of the irradiating step may be performed during at least part
of the depositing step (e.g., the coating step and the irradiating
step may be conducted simultaneously for some length of time).
Thus, the irradiating step may be conducted for a length of time
sufficient to (i) cross-link, isomerize, oligomerize and/or
polymerize the doped silane, (ii) form a substantially uniform
layer on the substrate, the substantially uniform layer comprising
a doped oligo- and/or polysilane, and/or (iii) increase an average
molecular weight, increase a viscosity and/or reduce a volatility
of the liquid phase composition. In various embodiments, this
length of time may be less than 1 second, at least 1 second, at
least 3 seconds, at least 5 seconds, at least 10 seconds or at
least 30 seconds. Generally, simultaneous deposition and
irradiation causes the Group IVA atom and/or dopant source in the
liquid phase composition to oligomerize and/or polymerize,
typically by radiation-induced cross-linking, oligomerization
and/or polymerization.
[0051] In this aspect of the present invention, the substrate may
be as described above for the exemplary method(s) of making doped
oligomerized/polymerized silane films. Also as for the exemplary
method(s) described above, the liquid phase composition may consist
essentially of components in the liquid phase at ambient
temperatures.
[0052] As for the exemplary method(s) of coating a substrate with a
doped oligomerized/polymerized silane film, the coating or
printing, irradiating, heating, curing and annealing steps may be
conducted under an inert and/or reducing gas, although the present
doped films tend to be oxidation stable after curing at
400-500.degree. C. Thus, the method may further comprise the steps
of (i) purging an atmosphere in which the substrate is placed, then
(ii) introducing the inert and/or reducing gas into the atmosphere,
prior to the (1) coating or printing and (2) irradiating steps.
Similarly, the irradiating step may be conducted for a length of
time sufficient to (i) cross-link, isomerize, oligomerize and/or
polymerize the doped silane, (ii) form a substantially uniform
layer on the substrate, the layer comprising a doped oligo- and/or
polysilane, and/or (iii) increase an average molecular weight,
increase a viscosity and/or reduce a volatility of the
composition.
[0053] In a typical implementation, the substrate having a doped
oligomerized/polymerized silane film thereon (prepared as described
above for the method of depositing a doped oligomerized/polymerized
silane film onto a substrate) is placed onto a hotplate and heated
for about 5-20 minutes at a temperature of from 80.degree. C. to
about 120.degree. C. Thereafter, the substrate is placed into an
oven for a time of from about 5-20 minutes up to about 10-15 hours
at a temperature of about 350-550.degree. C. under an inert gas
flow (e.g., argon having <1 ppm O.sub.2). This procedure yields
a doped, hydrogenated, amorphous silicon film of .about.10-120 nm
thickness using the coating method and up to .about.400 nm
thickness using the patterning method, depending on the formulation
of the doped silane composition.
[0054] Exemplary Doped Silane Compositions and Methods of Making
the Same
[0055] The present invention further relates to a composition
comprising (1) a Group IVA atom source, (2) a dopant source, and
(optionally) a solvent in which the Group IVA atom source and
dopant source are soluble. The Group IVA atom source generally
consists of a Group IVA element (e.g., Si and/or Ge) and hydrogen
(and/or isotopes thereof, such as deuterium). The dopant source
generally (but not exclusively) consists essentially of one or more
conventional semiconductor dopant atoms (e.g., B, P, As or Sb) that
may have at least one substituent covalently bound thereto
consisting essentially of a hydrocarbyl, silyl, germyl, or
silagermyl group. Surprisingly, the presence of a carbon-containing
substituent on the dopant atom does not result in a significant
increase in the amount of carbon in doped semiconductor films
formed from the present compositions, or in significant adverse
effects on the electrical, physical and mechanical properties of
such films, relative to undoped films formed from structurally
analogous (cyclo)silane compounds.
[0056] Generally, the present composition is in the liquid phase at
ambient temperatures (e.g., from about 15.degree. C. to about
30.degree. C.). The present composition may be characterized as
being at least one of the following types: [0057] A novel
hetero(cyclo)silane and a solvent; [0058] A novel mixture of a
silane compound and a dopant compound; [0059] A novel
hetero(cyclo)silane/solvent mixture; or [0060] A novel silane
compound/dopant compound/solvent mixture.
[0061] Each of these types of compositions is useful for making
doped semiconductor films (e.g., by one or more of the exemplary
methods described above), and thus, can provide a source of doped
"liquid silicon." However, any liquid-phase composition (novel or
not) that includes a Group IVA atom source and a dopant source may
be useful in the exemplary methods described above. Each of the
various types of the present doped "liquid silicon" compositions
will be described in greater detail below, and suitable examples of
each type of composition will be provided herein.
[0062] Exemplary Novel Hetero(cyclo)silane-Based Compositions
[0063] A variety of novel hetero(cyclo)silanes are contemplated by
the present inventors for use in the present invention. For
example, the hetero(cyclo)silane may comprise a heterocyclosilane
compound of the formula (AH.sub.z).sub.n(DR.sup.1).sub.m, where n
is from 2 to 12, m is 1 or 2, each of the n instances of A is
independently Si or Ge, each of the n instances of z is
independently 1 or 2, each of the m instances of D is Sb, As, P or
B, and each of the m instances of R.sup.1 is alkyl, aryl, aralkyl,
or AR.sup.2.sub.3, where R.sup.2 is hydrogen, alkyl, aryl, aralkyl,
or A.sub.yH.sub.2y+1 (e.g., 1.ltoreq.y.ltoreq.4). In certain
preferred embodiments of this compound, A is Si, z is 2, n is
(predominantly) 4, m is 1, and/or R.sup.1 is C.sub.1-C.sub.6 alkyl
(e.g., t-butyl), C.sub.6-C.sub.12 aryl (e.g., phenyl),
C.sub.7-C.sub.12 aralkyl, SiH.sub.3, or Si(SiH.sub.3).sub.3,
although other embodiments may be preferred under certain
circumstances. Such heterocyclosilanes are described in copending
application Ser. No. ______, filed ______, 2004 and entitled
"Heterocyclic Semiconductor Precursor Compounds, Compositions
Containing the Same, and Methods of Making Such Compounds and
Compositions" (Atty. Docket No. IDR0301), the relevant portions of
which are incorporated herein by reference to the extent
possible.
[0064] Alternatively, the hetero(cyclo)silane may comprise a
hetero-substituted (cyclo)silane compound of the formula
(A.sub.nH.sub.z').sub.m(DR.sup.1.sub.3-m).sub.q, where n is from 3
to 12, z' is from (n-q) to (2n+2-q), m is from 1 to 3, each of the
n instances of A is independently Si or Ge, D is Sb, As, P or B, q
is from 1 to n (preferably 1 or 2), and each of the (3-m) instances
of R.sup.1 is independently H, alkyl, aryl, aralkyl, or
AR.sup.2.sub.3, where R.sup.2 is hydrogen, alkyl, aryl, aralkyl, or
A.sub.yH.sub.2y+1 (e.g., 1.ltoreq.y.ltoreq.4). In certain preferred
embodiments of this compound, A is Si, n is (predominantly) 5,
(A.sub.nH.sub.z') is monocyclic (i.e., z' is 2n-1), m and q are
each 1, and/or R.sup.1 is C.sub.1-C.sub.6 alkyl (e.g., t-butyl),
C.sub.6-C.sub.12 aryl (e.g., phenyl), C.sub.7-C.sub.12 aralkyl,
SiH.sub.3, or Si(SiH.sub.3).sub.3, although other embodiments may
be preferred under certain circumstances. For example, in one
embodiment, m is 1, and for each of the q instances of
DR.sup.1.sub.2, one R.sup.1 may be H and the remaining instances
selected from C.sub.1-C.sub.6 alkyl, C.sub.6-C.sub.12 aryl,
SiH.sub.3, and Si(SiH.sub.3).sub.3.
[0065] Such hetero-substituted cyclosilanes in which m is 1 and q
is from 1 to n are described in copending application Ser. No.
______, filed ______, 2004 and entitled "Dopant Group-Substituted
Semiconductor Precursor Compounds, Compositions Containing the
Same, and Methods of Making Such Compounds and Compositions" (Atty.
Docket No. IDR0303), the relevant portions of which are
incorporated herein by reference to the extent possible. Such
hetero-substituted (cyclo)silanes in which m is 1, 2 or 3 may be
prepared by alkali metal (e.g., Na or K)-induced cleavage of a
Si--H bond in a (cyclo)silane of the formula A.sub.nH.sub.z'+1,
followed by quenching with 1/x mole-equivalents of a heteroatom
compound of the formula DR.sup.1.sub.3-xX.sub.x, where X is a
halogen (e.g., Cl) and x is 1, 2 or 3. Such hetero(cyclo)silanes
and their preparation are also described in copending application
Ser. No. ______, filed ______, 2004 and entitled "Dopant
Group-Substituted Semiconductor Precursor Compounds, Compositions
Containing the Same, and Methods of Making Such Compounds and
Compositions" (Atty. Docket No. IDR0303), the relevant portions of
which are incorporated herein by reference to the extent
possible.
[0066] In an alternative embodiment, the present composition
comprises a hetero-substituted silane compound of the formula
(AH.sub.p).sub.n(DR.sup.1.sub.2).sub.2, where n is from 3 to 12,
each of the n instances of A is independently Si or Ge, each of the
n instances of p is independently 1 or 2, D is Sb, As, P or B, and
each R.sup.1 is independently H, alkyl, aryl, aralkyl, or
AR.sup.2.sub.3, where R.sup.2 is hydrogen, alkyl, aryl, aralkyl, or
A.sub.yH.sub.2y+1 (e.g., 1.ltoreq.y.ltoreq.4). In certain preferred
embodiments of this compound, A is Si, n is (predominantly) 5, p is
2, and R.sup.1 is H, C.sub.1-C.sub.6 alkyl (e.g., t-butyl),
C.sub.6-C.sub.12 aryl (e.g., phenyl), C.sub.7-C.sub.12 aralkyl,
SiH.sub.3, or Si(SiH.sub.3).sub.3, although other embodiments may
be preferred under certain circumstances. Such heterosilanes may be
prepared by alkali metal (e.g., Li)-induced cleavage of a Si--Si
bond in a cyclosilane of the formula (AH.sub.p).sub.n, followed by
quenching with 2 mole-equivalents of a heteroatom compound of the
formula DR.sup.1.sub.2X, where X is a halogen (e.g., Cl). Such
heterosilanes and their preparation are also described in copending
application Ser. No. ______, filed ______, 2004 and entitled
"Dopant Group-Substituted Semiconductor Precursor Compounds,
Compositions Containing the Same, and Methods of Making Such
Compounds and Compositions" (Atty. Docket No. IDR0303), the
relevant portions of which are incorporated herein by reference to
the extent possible.
[0067] Any of the compositions useful in the present invention
(including these exemplary compositions based on novel
hetero(cyclo)silanes) may further comprise a (cyclo)silane of the
formula A.sub.xH.sub.y, where x is from 3 to 20, each A is
independently Si or Ge, and y is from n to (2n+2). In general, the
(cyclo)silane may comprise any compound consisting essentially of
Group IVA atoms and hydrogen (and isotopes thereof, such as
deuterium) that (i) is liquid or solid at ambient temperatures and
(ii) can be cross-linked, isomerized, oligomerized and/or
polymerized upon irradiating with an appropriate dose of radiation.
However, in preferred embodiments, the (cyclo)silane comprises (or
consists essentially of) a compound of the formula
(AH.sub.z).sub.k, where k is from 3 to 12 (more preferably from 4
to 6), each A is independently Si or Ge (more preferably Si), and
each of the k instances of z is independently 1 or 2 (more
preferably 2, in which case the cyclosilane is monocyclic).
Representative (cyclo)silane compounds of the formula
A.sub.xH.sub.y and an exemplary method for their preparation are
described in greater detail in copending application Ser. No.
10/789,317, filed Feb. 27, 2004 (Atty. Docket No. IDR0020), the
relevant portions of which are incorporated herein by
reference.
[0068] Alternatively, suitable (cyclo)silane compounds may be
prepared in accordance with known methods (see, e.g., U.S. Pat.
Nos. 4,554,180, 4,683,145, 4,820,788, 5,942,637 and 6,503,570;
Kumada, J. Organomet. Chem., 100 (1975) 127-138; Ishikawa et al.,
Chem. Commun., (1969) 567; Hengge et al., J. Organomet. Chem., 212
(1981) 155-161; Hengge et al., Z. Anorg. Allg. Chem., 459 (1979)
123-130; and Hengge et al., Monatshefte fur Chem., 106 (1975)
503-512, the relevant portions of which are incorporated herein by
reference).
[0069] In various embodiments of the present composition, from
about 1 to 99.999 vol %, from 0.5 to 90 vol %, or from 10 to 50 vol
% of the composition consists essentially of the (cyclo)silane
compound(s). Such compositions may contain proportions of
(cyclo)silane compound and hetero(cyclo)silane sufficient to
provide a desired doping level in the doped semiconductor film. For
example, from 0.00001, 0.0001 or 0.001 to about 10, 20 or 50 vol %
of the composition may consist essentially of the
hetero(cyclo)silane, and from about 0.5, 1 or 10 to about 20, 50 or
99.9999 vol % of the composition may consist essentially of the
(cyclo)silane compound. Alternatively, the hetero(cyclo)silane may
be present in an amount providing from about 0.0001, 0.001 or 0.005
to about 1, 5 or 10 at. % of D atoms with respect to A atoms in the
(cyclo)silane compound.
[0070] Any of the compositions useful in the present invention
(including these exemplary compositions based on novel
hetero(cyclo)silanes) may further comprise a solvent in which the
silane and/or dopant are soluble. The solvent in the present
composition is one that is generally easily and/or thoroughly
removable from the composition. Thus, apolar and/or non-polar
solvents (e.g., saturated hydrocarbons such as C.sub.5-C.sub.12
alkanes, aliphatic ethers such as di-C.sub.2-C.sub.6 alkyl ethers,
methyl C.sub.4-C.sub.6 alkyl ethers and di-C.sub.1-C.sub.4 alkyl
C.sub.2-C.sub.6 alkylene diethers [e.g., glyme], cyclic ethers such
as tetrahydrofuran and dioxane, (cyclo)siloxanes, arenes such as
benzene, toluene and xylenes, etc.) may be included in compositions
suitable for use in the present methods of forming a doped,
oligomerized/polymerized silane film (e.g., coating a substrate
with a doped, oligomerized/polymerized silane film and/or printing
a doped, oligomerized/polymerized silane film in a pattern on a
substrate). Preferably, the solvent is a C.sub.5-C.sub.12 mono- or
bicycloalkane (e.g., cyclohexane, cycloheptane, cyclooctane,
decalin, etc.).
[0071] The higher the volume percentage (or mass loading) of silane
compounds in an ink composition that includes a solvent, the
thicker the film. For example, a silane ink composition containing
about 20 vol % of silanes generally forms a film having a thickness
of about 100 nm using the exemplary film-forming process described
in copending U.S. application Ser. No. 10/789,274, filed Feb. 27,
2004 [Atty. Docket No. IDR0080]). Similarly, an ink composition
containing about 5 vol % of silanes generally forms a film having a
thickness of about 20 nm using the same exemplary film-forming
process.
[0072] The composition may further comprise one or more
conventional additives, such as a surface tension reducing agent, a
surfactant, a binder and/or a thickening agent, in conventional
amount(s). However, such additives are not at all necessary. As a
result, the present composition may consist essentially of one or
more hetero(cyclo)silane compounds, one or more (cyclo)silane
compounds, and a solvent. Alternatively, as discussed above, the
present composition may consist essentially of the
hetero(cyclo)silane and (cyclo)silane compounds, without the
addition of a solvent.
[0073] Exemplary Compositions Based on Novel Mixtures of
(Cyclo)silanes and Dopant Compounds
[0074] In a further embodiment, the present doped "liquid silicon"
composition may comprise (1) a compound of the formula
A.sub.xH.sub.y, where each A is independently Si or Ge, x is from 3
to 20, and y is from x to (2x+2); and (2) a dopant of the formula
D.sub.aR.sup.1.sub.b, where D is Sb, As, P or B; a is from 1 to 20;
each of the b instances of R.sup.1 is independently H, alkyl, aryl,
aralkyl or AR.sup.2.sub.3, where R.sup.2 is hydrogen, alkyl, aryl,
aralkyl or A.sub.yH.sub.2y+1 (e.g., where 1.ltoreq.y.ltoreq.4, such
as SiH.sub.3 and Si(SiH.sub.3).sub.3), and at least one of the b
instances of R.sup.1 is alkyl, aryl, aralkyl or AR.sup.2.sub.3; and
b is an integer corresponding to the number of binding sites
available on the a instances of D. Alternatively, the present doped
"liquid silicon" composition may comprise (I) the compound of the
formula A.sub.xH.sub.y, and (II) a dopant of the formula
(R.sup.2.sub.3A).sub.rA.sub.c(DR.sup.1.sub.2).sub.s, where c is 1
to 4, r+s=2c+2, s.gtoreq.1 (preferably s.gtoreq.3), and R.sup.1 and
R.sup.2 are as described for D.sub.aR.sup.1.sub.b. In either case,
the composition may further comprise (i) a solvent in which the
compound and the dopant are soluble, and/or (ii) one or more
hetero(cyclo)silane compounds as described above.
[0075] In one embodiment, the dopant has the formula
D.sub.a'R.sup.1.sub.b', where a' is 1 or 2; b' is 3a', at least a'
instances of R.sup.1 are C.sub.1-C.sub.6 alkyl, C.sub.6-C.sub.10
aryl, C.sub.7-C.sub.10 aralkyl or AR.sup.2.sub.3 (preferably
AR.sup.2.sub.3), where R.sup.2 is hydrogen or A.sub.yH.sub.2y+1
(1.ltoreq.y.ltoreq.4; preferably y=1), and the remainder of the b'
instances of R.sup.1 are independently H, C.sub.1-C.sub.6 alkyl,
C.sub.6-C.sub.10 aryl, C.sub.7-C.sub.10 aralkyl or AR.sup.2.sub.3.
In various implementations, the dopant has the formula
D(AH.sub.3).sub.3, wherein D is P or B, and/or A is Si.
[0076] In another embodiment, the dopant has the formula
(R.sup.2.sub.3A).sub.rA.sub.c(DR.sup.1.sub.2).sub.s, where D is Sb,
As, P or B; c is from 1 to 4 (preferably 1); r+s=2c+2; s.gtoreq.1
(preferably s.gtoreq.3); and each of R.sup.1 and R.sup.2 are as
described above. Alternatively, the dopant may have the formula
A.sub.cR.sup.3.sub.r(DR.sup.1.sub.2).sub.s, where each A is
independently Si or Ge; D is Sb, As, P or B; c is 1 to 4 (e.g., 1
or 2); r+s=2c+2; s.gtoreq.1 (preferably s.gtoreq.3); each of the r
instances of R.sup.3 is independently H, DR.sup.1.sub.2 or
AR.sup.2.sub.3; and each of the 2s instances of R.sup.1 is
independently H, alkyl, aryl, aralkyl or AR.sup.2.sub.3, where
R.sup.2 is hydrogen, alkyl, aryl, aralkyl or A.sub.yH.sub.2y+1
(e.g., where 1.ltoreq.y.ltoreq.4, such as SiH.sub.3 and
Si(SiH.sub.3).sub.3)). For example, the dopant may have the formula
(Me.sub.3Si)Si(PH.sub.2).sub.3 or (Me.sub.3Si).sub.3SiPH.sub.2,
where Me is a methyl group. Preferably, R.sup.1 is H,
C.sub.1-C.sub.6 alkyl, C.sub.6-C.sub.10 aryl, or AR.sup.2.sub.3,
and R.sup.2 is hydrogen or A.sub.yH.sub.2y+1 (1.ltoreq.y.ltoreq.4).
In certain embodiments, the dopant has the formula
A.sub.c(DR.sup.1.sub.2).sub.2c+2, R.sup.1 is H, t-butyl, phenyl, or
AH.sub.3 (preferably H or AH.sub.3), D is P or B, and/or A is Si
(e.g., Si(PH.sub.2).sub.4, Si(P[t-Bu].sub.2).sub.4, or
Si(P[SiH.sub.3].sub.2).sub.4).
[0077] Such compositions may contain suitable proportions of
(cyclo)silane compound and dopant to provide a desired doping level
in the doped semiconductor film. For example, from 0.00001, 0.0001
or 0.001 to about 10, 20 or 50 vol % of the composition may consist
essentially of the dopant and from about 0.5, 1 or 10 to about 20,
50 or 99.9999 vol % of the composition may consist essentially of
the compound. Alternatively, the dopant may be present in an amount
providing from about 0.0001, 0.001 or 0.005 to about 1, 5 or 10 at.
% of D atoms with respect to A atoms in the (cyclo)silane
compound.
[0078] Furthermore, the composition may further comprise a solvent
(e.g., as discussed above). In certain embodiments, the solvent is
selected from the group consisting of alkanes, substituted alkanes
(e.g., with from 1 to 2n+2 [preferably 1 or 2] of the substituents
described above), cycloalkanes, substituted cycloalkanes (e.g.,
with 1 or 2 of the substituents described above), arenes,
substituted arenes (e.g., with from 1 to 6 [preferably 1 or 2] of
the substituents described above), and (cyclic) siloxanes. As for
the other exemplary compositions described herein, "cycloalkanes"
refers to both mono- and polycycloalkanes, and examples of suitable
cycloalkane solvents for the present composition include
C.sub.5-C.sub.12 mono- or bicycloalkanes (e.g., cyclohexane,
cycloheptane, cyclooctane, decalin, etc.).
[0079] Exemplary Compositions Based on Novel Mixtures of
Hetero(cyclo)silane Compounds and Solvents
[0080] A still further aspect of the present invention relates to a
doped hetero(cyclo)silane composition comprising (1) a compound of
the formula A.sub.x'H.sub.y'D.sub.z, where each A is independently
Si or Ge, x' is from 3 to 20, y' is from x to (2x+z+2), D is Sb,
As, P or B, and z is from 1 to 4; and (2) a solvent selected from
the group consisting of cycloalkanes, substituted cycloalkanes,
fluoroalkanes, and (cyclic) siloxanes. The (cyclic) siloxane
solvents are generally those that are liquid at ambient
temperatures (e.g., 15-30.degree. C.). Preferably, the compound of
the formula A.sub.x'H.sub.y'D.sub.z is cyclic, x' is from 3 to 12,
y' is from x to (2x+z), and/or z is 1 or 2. In certain embodiments,
the compound of the formula A.sub.x'H.sub.y'D.sub.z is monocyclic,
x' is from 4 to 6, y' is (2x+z), and z is 1. As for other preferred
embodiments described above, D may be P or B, and A may be Si. Such
compounds are known, and exemplary processes for their preparation
are described in U.S. Pat. Nos. 4,683,145 and 6,527,847, and U.S.
Patent Publication No. 2003/0229190.
[0081] The composition may contain from 0.00001 to 50 vol %, from
0.001 to 35 vol %, or from about 0.01 to 25 vol % of the compound.
Viewed a little differently, from 0.00001 to 50 vol %, from 0.001
to 35 vol %, or from about 0.01 to 25 vol % of the composition may
consist essentially of the compound.
[0082] The exemplary hetero(cyclo)silane/solvent composition herein
has novelty and/or particular advantage in the present invention as
a result of the selection of solvent. For example, when the solvent
is a mono- or bicycloalkane, the solvent is relatively easily
and/or completely removed from the composition in the process of
film formation. Perhaps more importantly, the solubility and
stability of (doped) silanes tends to be higher in mono- or
bicycloalkane than in arene-based solvents. Thus, cycloalkanes,
(cyclic) siloxanes and fluoroalkanes are preferred in the novel
mixtures for their removability from the coated and/or printed
composition.
[0083] In various embodiments, the solvent is selected from the
group consisting of C.sub.6-C.sub.12 monocycloalkanes;
C.sub.3-C.sub.8 monocycloalkanes substituted with from 1 to 2n
C.sub.1-C.sub.4 alkyl or halogen substituents or from 1 to n
C.sub.1-C.sub.4 alkoxy substituents, where n is the number of
carbon atoms in the monocycloalkane ring; C.sub.10-C.sub.14
polycycloalkanes; siloxanes of the formula
(R.sup.3.sub.3Si)(OSiR.sup.3.sub.2).sub.p(OSiR.sup.3.sub.3), where
p is from 0 to 4, and each R.sup.3 is independently H,
C.sub.1-C.sub.6 alkyl, benzyl or phenyl substituted with from 0 to
3 C.sub.1-C.sub.4 alkyl groups; cyclosiloxanes of the formula
(SiR.sup.4.sub.2O).sub.q, where q is from 2 to 6, and each R.sup.4
is independently H, C.sub.1-C.sub.6 alkyl, benzyl or phenyl
substituted with from 0 to 3 C.sub.1-C.sub.4 alkyl groups; and
C.sub.3-C.sub.8 fluoroalkanes substituted with from 1 to (2n+2)
fluorine atoms, where n is the number of carbon atoms in the
selected solvent. Preferably, the solvent is a C.sub.5-C.sub.10
monocycloalkane (e.g., cyclooctane) or a C.sub.10-C.sub.14
polycycloalkane (e.g., decalin).
[0084] Exemplary Compositions Based on Novel Mixtures of
(Cyclo)silane Compounds, Dopants and Solvents
[0085] A still further aspect of the present invention relates to a
doped liquid silane composition comprising (A) a compound of the
formula A.sub.xH.sub.y, as described above; (B) a dopant of the
formula D.sub.aH.sub.b, where D is Sb, As, P or B, a is from 1 to
20, and b is from 0 to 26; and (C) a solvent selected from the
group consisting of cycloalkanes, substituted cycloalkanes,
fluoroalkanes, and (cyclic) siloxanes. The solvent is generally one
of those described for the exemplary hetero(cyclo)silane/solvent
composition described above. The dopant may be an elemental
substance (e.g., red or yellow phosphorous), a known borane, or
phosphine or arsine. Examples of suitable boranes include
B.sub.2H.sub.6, B.sub.5H.sub.9, B.sub.6H.sub.10, B.sub.6H.sub.12,
B.sub.9H.sub.15, B.sub.10H.sub.14, B.sub.10H.sub.16,
B.sub.13H.sub.19, B.sub.16H.sub.20, and B.sub.20H.sub.26.
[0086] Exemplary Methods of Making Doped Silane Compositions
[0087] The invention further relates to a method of making the
present composition. This method generally comprises the steps of:
combining the silane compound with the dopant and/or solvent, and
mixing the silane compound and the dopant and/or solvent to form
the composition. Somewhat surprisingly, mono- and polycycloalkanes
(e.g., monocyclooctane, decalin) provide ink formulations with
improved stability relative to aromatic hydrocarbons.
[0088] In one implementation, the composition may be prepared by
mixing about 10-25 vol. % of one or more (doped) silane compounds
(which may have >90% purity, .gtoreq.95% purity, or .gtoreq.98%
purity in combination with other cyclic, linear or branched
silanes) with a dopant and/or a solvent under an inert (e.g.,
argon) atmosphere. The mixture is stored in amber vials (to prevent
UV or other radiation exposure) at ambient temperatures or lower.
This composition consisting of liquid-phase components may be used
directly in the present methods of coating or printing a thin
doped, oligomerized/polymerized silane film on a substrate and/or
of forming a doped amorphous or polycrystalline semiconductor thin
film, as described above.
[0089] Exemplary Semiconducting Thin Films and Devices
[0090] The present invention further relates to a semiconducting
thin film structure comprising a substantially uniform layer of
doped semiconducting material on a substrate, the doped
semiconducting material comprising (a) a hydrogenated, amorphous or
at least partially polycrystalline Group IVA element, the Group IVA
element comprising at least one of silicon and germanium, and (b) a
dopant. In certain embodiments, the Group IVA element in the thin
film structure comprises or consists essentially of silicon, and
the dopant (which may be B, P, As or Sb, but which is preferably B
or P) may have a concentration of from about 10.sub.16 to about
10.sub.21 atoms/cm.sup.3. Notably, the concentration profile of the
dopant in the thin film does not vary significantly throughout the
substantial thickness or depth of the film (see, for example, FIG.
1 and the corresponding description thereof below).
[0091] The doped semiconducting thin film structure may have a
thickness of from 0.005 .mu.m to 1000 .mu.m (preferably from 0.01
.mu.m to 200 .mu.m, more preferably from 0.01 .mu.m to 100 .mu.m).
The films prepared by the present method(s) and/or from the present
composition(s) generally show (i) greater adhesion, uniformity
(e.g., in thickness or morphology) and/or (ii) carrier mobility,
and/or (iii) reduced carbon content, relative to films made by an
otherwise identical process (a) in which coating and irradiation
were not performed simultaneously (i.e., in discrete steps), (b) in
which irradiation of the composition comprising a doped silane was
performed only prior to deposition, and/or (c) from an otherwise
identical composition including an arene solvent (when the present
composition includes only a cycloalkane solvent). Furthermore, such
films do not contain significantly greater amounts of carbon
relative to undoped films formed from structurally analogous
cyclosilanes (e.g., cyclopentasilane).
[0092] The present invention also concerns a device such as a thin
film capacitor, diode (e.g., a Schottky diode, Zener diode,
photodiode, etc.), resistor or thin film transistor, comprising the
present doped semiconductor thin film, a device terminal layer
above or below the doped semiconductor thin film, and one or more
metallization structures in contact with the doped semiconductor
thin film. In one embodiment, the thin film transistor comprises a
bottom-gate transistor that includes a transistor terminal layer
below the doped semiconductor thin film. In such an embodiment, the
substrate contains the transistor gate, and the doped semiconductor
thin film contains the transistor sources and drains.
Alternatively, the transistor terminal layer may be above the doped
semiconductor thin film, in which case the transistor terminal
layer comprises a gate layer, and the doped semiconductor thin film
comprises a source/drain terminal layer. The transistor terminal
layer may comprise a conventional semiconducting material, a
conventional conducting material, or a laminate of two or more
conventional semiconducting and/or conducting materials (e.g.,
heavily doped silicon with a transition metal silicide, such as
nickel silicide, titanium silicide or tungsten silicide, thereon).
In either case, the metallization structures in the thin film
transistor may comprise a contact structure in physical and/or
electrical contact with the source and drain structures of the
source/drain terminal layer (and optionally, in physical and/or
electrical contact with the gate of the transistor terminal
layer).
[0093] In another embodiment, the device comprises a thin film
capacitor, such as a MOS capacitor. In one implementation, the
capacitor comprises a lower metal layer, such as Al, under an oxide
layer, such as Al.sub.2O.sub.3, on which a semiconductor (e.g.,
doped amorphous Si or polysilicon) layer may be formed (e.g., in
accordance with the present invention). Generally, an upper metal
layer (e.g., of Al, an Al alloy, Ni or Ag) is then formed on the
doped semiconductor layer. In another embodiment, the semiconductor
layer comprises (i) a lower undoped or slightly doped amorphous
silicon or polysilicon layer and (ii) an upper heavily doped
amorphous silicon or polysilicon layer. Generally the upper, more
heavily doped silicon layer is thinner than the bottom, less doped
or undoped, silicon layer. Alternatively, the capacitor layers may
be reversed (e.g., upper metal on oxide on doped silicon on lower
metal). Zener diodes may be made by a similar process, in which a
plurality of (doped) semiconductor layers having different dopant
types (e.g., p, n or i) and/or concentration levels may be formed
sequentially, one on another, as is known in the art. Schottky
diodes may also be made by a similar process, in which one or more
(doped) semiconductor layers and a metal layer are formed in
contact with one another (e.g., in a stacked or laminate-type
structure) so to form a metal-semiconductor junction, as is known
in the art. Photodiodes comprising the present doped semiconductor
film may also be formed in accordance with the description(s)
herein and with techniques known in the art, such that a
photoconductive or photosensitive material (e.g., the present doped
semiconductor film) may be configured to receive light and provide
variable (but predictable and/or predetermined) electrical
properties and/or functions in response to such light.
[0094] The invention further relates to a method of making a device
such as a capacitor and/or transistor, comprising at least one of
the present methods of (i) coating or printing a doped
oligomerized/polymerized silane film on a substrate and (ii) making
a doped semiconductor thin film, and forming a metallization
structure in electrical communication with the doped film. The
metallization structure may be formed by conventional metal
deposition (e.g., by conventional sputtering or evaporation) and
photolithography, by printing and/or deposition techniques using
metallic inks, by conventionally dispensing commercial metal
pastes, by conventional electro- or electroless plating, or
alternatively, by laser patterning techniques to yield metal
source/drain (and optionally, gate) contacts. The method may
further comprise conventionally growing or depositing an oxide
and/or nitride (e.g., silicon oxide, silicon nitride) on the
substrate (e.g., a conventional single-crystal silicon wafer).
[0095] Conventional isolation processes, such as "island isolation"
techniques that involve masking of the silicon layer, optional
etching, and either thermal growth or deposition of silicon
dioxide, may be employed to isolate TFTs from each other. However,
island isolation may not be necessary if dopant activation was
achieved by laser irradiation through a mask or in a pattern,
leaving non-irradiated, electrically inactive cured semiconductor
layer portions between the laser irradiated, activated
semiconductor film portions. Typical channel lengths may be from 3
to 50 microns, and typical channel widths may be from 10 to 1000
microns. The metal contacts may be annealed for a length of time
and at a temperature sufficient to enhance bonding and/or improve
the electrical contact of the metal to the thin film, and/or
improve device performance (e.g., from 1 to 120 minutes, 10 to 90
minutes, or 15 to 60 minutes, at 200-500.degree. C.,
250-400.degree. C. or 300-350.degree. C.) under an inert atmosphere
(e.g., argon, nitrogen, nitrogen/hydrogen or argon/hydrogen).
Conventional metallization ("back-end"), assembly and packaging
processing may then be employed to complete the finished
devices/products.
[0096] An exemplary thin film transistor (TFT) may be made as
follows. A silicon wafer (which may be, and preferably is,
[heavily] doped with a conventional n- or p-type dopant, more
preferably heavily doped) has an oxide layer grown thereon by a
conventional wet or dry method. In various embodiments, the oxide
layer has a thickness of from 20 to 200 nm, from 30 to 150 nm, or
from 50 to 125 nm. A doped, oligomerized/polymerized silane film is
deposited thereon as described above, and the doped,
oligomerized/polymerized silane film may be cured and (optionally)
annealed as described above with regard to the exemplary methods of
forming thin doped semiconductor films to form a substantially
uniform doped silicon film. The resulting film stack can be further
processed after curing and (optionally) annealing the doped
semiconductor film by conventional metal
deposition/photolithography to yield a bottom-gate TFT structure
with metal source/drain contacts, where the silicon wafer may act
as the gate. The metal for the source/drain contacts (as well as
for any subsequent metallization layers) can be aluminum,
conventional alloys of aluminum with copper and/or silicon, copper
(which can be formed by conventional techniques for making copper
metallization in an integrated circuit or a printed circuit board),
nickel, silver, gold, etc. A TFT formed in accordance with the
present method may have mobilities of up to 100 or up to 10
cm.sup.2/Vs if it includes polysilicon, but otherwise, generally up
to 1.0, 0.5, 0.25, 0.20, 0.15 or 0.12 cm.sup.2/Vs and on/off ratios
greater than about 10.sup.3, about 10.sup.4, about 10.sup.5, or
about 10.sup.6.
EXAMPLES
Synthesis of cyclo(phenylphospha)tetrasilane
[0097] In a 1 L 4-neck flask equipped with addition funnels, a
thermometer and a gas dispersion tube, 10 g of
nonaphenylcyclophosphatetrasilane obtained as described in
copending U.S. application Ser. No. ______, (Attorney Docket No.
IDR0301, entitled "Heterocyclic Semiconductor Precursor Compounds,
Compositions Containing the Same, and Methods of Making Such
Compounds and Compositions" and filed concurrently herewith, the
relevant portions of which are incorporated herein by reference)
and 0.3 g freshly sublimed AlCl.sub.3 are suspended in 200 ml of
dry toluene. Under vigorous stirring, dry HCl gas is bubbled
through this suspension at ambient temperature until an almost
colorless to yellow solution is obtained. Under continuous HCl
bubbling, the solution is stirred for 5-8 hrs.
[0098] 50 mL of a 1M ethereal solution of LiAlH.sub.4 (Aldrich) is
added under vigorous stirring to the toluene solution at 0.degree.
C. The resulting suspension is further stirred at room temperature
for another 15 hrs after the addition. Two phases are formed upon
removing 150 ml solvent under reduced pressure. The lower phase
containing precipitated byproduct is removed with a separatory
funnel to yield about 125 ml of a clear solution. The solvents are
further removed under reduced pressure. The product is purified by
re-condensation to afford 0.5 ml clear colorless liquid (yield:
40%).
[0099] 1H-NMR, .sup.29Si-NMR, .sup.31P-NMR and GC/MS analysis of
the liquid confirm that a mixture of heterocyclosilanes has been
formed with cyclo(phenylphospha)tetrasilane as the main component.
Cyclopentasilane can be identified as a second component. Other
silane species are formed as well as aromatic and aliphatic
byproducts.
[0100] Procedure for Forming a P-Doped Silane Film
[0101] A spin coater coupled with a 6 inch square low-pressure
mercury UV grid lamp (designed and built in accordance with the
disclosure of copending U.S. application Ser. No. 10/789,274, filed
Feb. 27, 2004 [Atty. Docket No. IDR0080]) was placed into an inert
atmosphere glove box, largely to prevent silane oxidation during
deposition and curing.
[0102] A P-doped silane ink was prepared by mixing
cyclo(phenylphospha)tetrasilane (c-[SiH.sub.2].sub.4P-Ph) with a
volume of cyclooctane sufficient to provide an ink containing
.about.20 vol % of cyclo(phenylphospha)tetrasilane. The resulting
P-doped silane ink was stored in a refrigerator in silanized amber
vials to prevent inadvertent decomposition due to UV exposure or
hydroxylation (e.g., from latent --OH groups on the surface of the
glass vial).
[0103] A substrate (e.g., a 100 mm silicon wafer, n.sup.+-type,
0.01 Ohm cm resistivity, with 100 nm dry oxide thermally grown
thereon) is placed onto the vacuum chuck of the spin-coater. About
0.3 ml of the P-doped silane ink is dispensed onto the substrate
from a syringe equipped with a 0.2 micron PTFE syringe filter while
slowly rotating the substrate and moving the syringe tip from the
center to the edge of the substrate. After dispensing, UV
spin-coating is started by accelerating the substrate with the
liquid silane thereon to 500 rpm for 5 seconds, then to 3000 rpm
for an additional 30 sec. Three seconds after the acceleration is
initiated, the UV lamp is turned on for a total of about 32
seconds. The power output of the UV lamp is about 0.1-0.3
mW/cm.sup.2 at the location of the substrate and silane film. This
procedure generally yields a polymerized, hydrogenated doped silane
film covering more than 90% of the substrate.
[0104] Procedure for Forming a Thin Doped Amorphous Silicon
Film
[0105] The substrate having a silane film formed thereon from a
cyclo(phenylphospha)-tetrasilane ink as described above is placed
onto a hotplate and heated for about 10 minutes to 100.degree. C.
in an inert atmosphere (argon or nitrogen), then it is placed for
about 20 minutes into an oven at a temperature of about 400.degree.
C. under inert gas flow (e.g., argon having <1 ppm O.sub.2).
This procedure yielded a hydrogenated, n-doped amorphous silicon
film of .about.100 nm thickness.
[0106] Recrystallization of a Doped Thin Silicon Film
[0107] The n-doped amorphous silicon film formed from a
cyclo(phenylphospha)-tetrasilane ink as described above is
recrystallized to form a doped polycrystalline silicon film using a
KrF (248 nm) excimer laser with a pulse width of 38 ns (10 pulses
at 180 mJ/cm.sup.2 fluence in an N.sub.2 purged environment). As an
alternative to laser treatment, a film formed by the same
UV-spincoating process described above is annealed by RTA or
furnace annealing at 900.degree. C. under 200 sccm N.sub.2 flow to
obtain essentially the same polycrystalline film. Both laser
treatment and annealing (RTA) of the doped, hydrogenated silicon
film are believed to electrically activate the P dopant in the
film.
[0108] P-Doped Silicon Film Characterization
[0109] The dopant concentration in the recrystallized film formed
from a cyclo(phenylphospha)tetrasilane ink is measured using
Secondary Ion Mass Spectrometry (SIMS). As shown in FIG. 1, the
phosphorus concentration in the polycrystalline film is typically
from about 10.sup.20 to about 5.times.10.sup.20 atoms/cm.sup.3
(0.2-1 at. %), and phosphorus is uniformly distributed throughout
the entire film.
[0110] A standard four-point Kelvin Resistor measurement is taken
to measure the electrical effect of the dopant on the film. The
median sheet resistance of the film is in the range of from 1750 to
5000 .OMEGA.cm.sup.-2.
[0111] Doped Silane Inks Using Organophosphines as Dopant
Precursors
[0112] Doped silicon films were made from an ink containing a
cyclosilane (20 vol % of a cyclosilane mixture containing primarily
C--Si.sub.5H.sub.10 in cyclooctane) with an organophosphine
compound as dopant. The amount of the phosphine compound was chosen
such that the ratio of Si-- to P-atoms in the ink was 10:1. The ink
was UV-spincoated as described above on Si wafers having .about.100
nm thermal oxide thereon to form doped oligomerized/polymerized
silane films. The doped oligomerized/polymerized silane films were
cured using the curing conditions described above for forming a
thin doped amorphous silicon film. After curing at 400-500.degree.
C. in inert atmosphere, however, the films are still highly
resistive, indicating that the dopant is not activated.
[0113] Dopant activation is achieved by heating the films in a
rapid thermal annealing (RTA) furnace for 1 min. at 900.degree. C.
in a nitrogen atmosphere. The results for films obtained in this
manner are shown in Table 1 below for representative
organophosphines such as tert-butyl phosphine,
di-(tert-butyl)phosphine and tri-(tert-butyl)phosphine. Other
organophosphines may be used, and have demonstrated similar
results. TABLE-US-00001 TABLE 1 Properties of doped silicon films
formed from cyclosilane-organophosphine inks. Film Thickness Doping
Level Resistivity Dopant [nm] [10.sup.20 at/cm.sup.3] [.quadrature.
cm] tert-butylphosphine 60 1.2 0.015 t-BuPH.sub.2
di-(t-butyl)phosphine 44 1 0.027 (t-Bu).sub.2PH
tri-(t-butyl)phosphine 87 1.4 0.014 (t-Bu).sub.3P
[0114] Characterization of P-Doped Silicon Films from
Organophosphine-Doped Silane Inks
[0115] A uniform dopant distribution in these films is evident from
the SIMS profiles for phosphorous atoms, as shown in FIG. 2. Dopant
concentrations around 1*10.sup.20 at/cm.sup.3 and film
resistivities around 0.01-0.03 {tilde over (.quadrature.)}cm have
been realized. Higher and/or lower dopant concentrations and
conductivities can be accomplished by changing the ratio of silicon
to dopant atoms in the ink.
[0116] Similar results are obtained by crystallizing and activating
the doped amorphous Si films by laser-induced crystallization,
furnace thermal annealing and metal-induced crystallization.
[0117] Ohmic Contact of Doped Silicon Films to Metallization
[0118] The contact resistance of doped silicon layers to Al metal
was determined by spincoating, curing and RTA activating a doped
silicon film formed from a silicon ink doped with
di-(tert-butyl)phosphine (Si:P ratio=10:1) on a N.sup.+ Si wafer as
described above. After activation, 200 square microns of Aluminum
were shadow evaporated onto the film, and the silane was RIE etched
in the exposed areas (i.e., those areas of the doped silane film
not having Al evaporated onto it). I-V curves of this vertical
stack were measured with the wafer at a potential of 0 V. The
resulting I-V curve demonstrating an Ohmic contact between the
doped silicon and the Al metal is shown in FIG. 3. A contact
resistance of <0.002 Ohm/cm.sup.2 can be determined from the
data in FIG. 3.
CONCLUSION/SUMMARY
[0119] Thus, the present invention provides doped semiconductor
thin film structures, methods of forming doped
oligomerized/polymerized silane and/or semiconductor thin films,
and doped liquid phase silane compositions useful in such methods.
The present semiconductor thin films may have a dopant
concentration of from about 10.sup.16 to about 10.sup.21
atoms/cm.sup.3 that does not vary significantly throughout the
substantial thickness or depth of the film. The present composition
is generally liquid at ambient temperatures and includes a Group
IVA atom source and a dopant source, thus providing a doped "liquid
silicon" composition.
[0120] By irradiating a doped liquid silane while coating, a thin,
substantially uniform doped silane film may be formed onto a
substrate. Such irradiation is believed to convert the doped silane
film into a relatively high-molecular weight species with
relatively high viscosity and relatively low volatility, typically
by cross-linking, isomerization, oligomerization and/or
polymerization. A film formed by the irradiation of doped liquid
silanes can later be converted (generally by heating and,
optionally, irradiating) into an at least partially amorphous,
hydrogenated doped silicon film or a doped polysilicon film
suitable for electronic devices. Thus, the present invention
enables use of high throughput, low cost equipment and techniques
for making semiconductor films of commercial quality and quantity
from doped "liquid silicon."
[0121] The foregoing descriptions of specific embodiments of the
present invention have been presented for purposes of illustration
and description. They are not intended to be exhaustive or to limit
the invention to the precise forms disclosed, and obviously many
modifications and variations are possible in light of the above
teaching. The embodiments were chosen and described in order to
best explain the principles of the invention and its practical
application, to thereby enable others skilled in the art to best
utilize the invention and various embodiments with various
modifications as are suited to the particular use contemplated. It
is intended that the scope of the invention be defined by the
Claims appended hereto and their equivalents.
* * * * *