U.S. patent application number 10/384186 was filed with the patent office on 2004-09-09 for ultrathin oxide films on semiconductors.
Invention is credited to Klemperer, Walter G., Lee, Jason, Mikalsen, Erik A., Payne, David A..
Application Number | 20040175960 10/384186 |
Document ID | / |
Family ID | 32927206 |
Filed Date | 2004-09-09 |
United States Patent
Application |
20040175960 |
Kind Code |
A1 |
Klemperer, Walter G. ; et
al. |
September 9, 2004 |
ULTRATHIN OXIDE FILMS ON SEMICONDUCTORS
Abstract
A method of making a semiconductor structure includes contacting
a surface of a semiconductor with a liquid including
Zr.sub.4(OPr.sup.n).sub.16 to form a modified surface, activating
the modified surface, and repeating the contacting and activating
to form a layer of zirconia on the semiconductor surface.
Inventors: |
Klemperer, Walter G.;
(Champaign, IL) ; Lee, Jason; (Liverpool, GB)
; Mikalsen, Erik A.; (Urbana, IL) ; Payne, David
A.; (Champaign, IL) |
Correspondence
Address: |
BRINKS HOFER GILSON & LIONE
P.O. BOX 10395
CHICAGO
IL
60610
US
|
Family ID: |
32927206 |
Appl. No.: |
10/384186 |
Filed: |
March 6, 2003 |
Current U.S.
Class: |
438/785 ;
257/E21.271 |
Current CPC
Class: |
C23C 18/1275 20130101;
C23C 18/1216 20130101; H01L 21/02189 20130101; C23C 18/04 20130101;
C23C 18/143 20190501; C23C 18/1225 20130101; H01L 21/02282
20130101; H01L 21/316 20130101 |
Class at
Publication: |
438/785 |
International
Class: |
H01L 021/469 |
Goverment Interests
[0001] The subject matter of this application was in part funded by
the Department of Energy (Grant nos. DEFG02-91 ER45439), through
the Frederick Seitz Materials Research Laboratory at the University
of Illinois at Urbana-Champaign. The government may have certain
rights in this invention. Any opinions, findings, and conclusions
or recommendations expressed in this publication do not necessarily
reflect the views of the U.S. Department of Energy.
Claims
1. A method of making a semiconductor structure, comprising:
contacting a surface of a semiconductor with a liquid comprising
Zr.sub.4(OPr.sup.n).sub.16 to form a modified surface; activating
the modified surface; and repeating the contacting and activating
to form a layer of zirconia on the semiconductor surface.
2. The method of claim 1, wherein the liquid comprising
Zr.sub.4(OPr.sup.n).sub.16 is anhydrous.
3. The method of claim 2, wherein the liquid comprising
Zr.sub.4(OPr.sup.n).sub.16 further comprises methylcyclohexane.
4. The method of claim 3, further comprising rinsing the modified
surface with methylcyclohexane after the contacting and before the
activating.
5. The method of claim 1, wherein the Zr.sub.4(OPr.sup.n).sub.16 is
analytically pure.
6. The method of claim 1, wherein the contacting is performed in an
inert atmosphere.
7. The method of claim 1, wherein the activating comprises
irradiating the modified surface.
8. The method of claim 1, wherein the activating comprises heating
the modified surface.
9. The method of claim 1, wherein the activating comprises vacuum
treating the modified surface.
10. The method of claim 1, wherein the activating comprises
contacting the modified surface with an oxidizing agent.
11. The method of claim 1, wherein the activating comprises
contacting the modified surface with an aqueous liquid to form a
hydrolyzed surface.
12. The method of claim 11, wherein the aqueous liquid further
comprises n-propanol.
13. The method of claim 11, further comprising drying the
hydrolyzed surface after the contacting and after the
activating.
14. The method of claim 1, wherein the contacting and activating
are repeated at least two times.
15. The method of claim 1, wherein the contacting and activating
are repeated at least ten times.
16. The method of claim 1, wherein the contacting and activating
are repeated until the zirconia has an equivalent oxide thickness
of not more than 2 nanometers.
17. The method of claim 1, further comprising heat treating the
structure after the contacting and activating.
18. The method of claim 17, wherein the heat treating comprises
heating the structure to at least 100.degree. C. for at least 10
minutes.
19. The method of claim 17, wherein the heat treating comprises
heating the structure to at least 300.degree. C. for at least 20
minutes.
20. The method of claim 17, wherein the heat treating comprises
heating the structure to at least 600.degree. C. for at least 30
minutes.
21. The method of claim 1, wherein the semiconductor comprises
silicon.
22. A method of making a semiconductor structure, comprising:
obtaining a liquid comprising analytically pure
Zr.sub.4(OPr.sup.n).sub.16; contacting a surface of a semiconductor
with the liquid in an inert atmosphere to form a modified surface;
rinsing the modified surface; hydrolyzing the modified surface with
an aqueous liquid comprising n-propanol to form an activated
surface; drying the activated surface; repeating the contacting,
rinsing, hydrolyzing, and drying to form a layer of zirconia on the
semiconductor surface; and heat treating the semiconductor
comprising the layer of zirconia.
23. The method of claim 22, wherein the obtaining comprises
distilling Zr(OPr.sup.n).sub.4 and collecting analytically pure
Zr.sub.4(OPr.sup.n).sub.16.
24. The method of claim 23, wherein the obtaining further comprises
dissolving the analytically pure Zr.sub.4(OPr.sup.n).sub.16 in
methylcyclohexane.
25. The method of claim 22, wherein the semiconductor comprises
silicon.
26. The method of claim 25, wherein the semiconductor is
Si(111).
27. The method of claim 26, wherein the surface of Si(111) has been
treated with an aqueous solution of n-propanol and dried prior to
contacting with the liquid comprising
Zr.sub.4(OPr.sup.n).sub.16.
28. The method of claim 22, wherein the aqueous liquid comprises
water and n-propanol in a weight ratio of 1:4.
29. The method of claim 22, wherein the repeating is performed at
least two times.
30. The method of claim 22, wherein the repeating is performed at
least ten times.
31. The method of claim 22, wherein the repeating is performed
until the zirconia has an equivalent oxide thickness of not more
than 2 nanometers.
32. The method of claim 22, wherein the heat treating comprises
heating the semiconductor to at least 100.degree. C. for at least
10 minutes.
33. The method of claim 22, wherein the heat treating comprises
heating the semiconductor to at least 300.degree. C. for at least
20 minutes.
34. The method of claim 22, wherein the heat treating comprises
heating the semiconductor in an inert atmosphere to at least
600.degree. C. for at least 30 minutes.
35. A semiconductor structure comprising: a semiconductor
substrate; and a layer comprising zirconia on the substrate; the
layer having an equivalent oxide thickness of not more than 2
nanometers; wherein the semiconductor structure has a leakage
current less than 0.002 A/cm.sup.2 when subjected to a potential of
1 volt.
36. The semiconductor structure of claim 35, wherein the
semiconductor substrate comprises silicon.
37. The semiconductor structure of claim 36, wherein the
semiconductor substrate is Si(111).
38. The semiconductor structure of claim 35, wherein the
semiconductor structure has a leakage current less than 0.001
A/cm.sup.2 when subjected to a potential of 1 volt.
39. The semiconductor structure of claim 35, wherein the layer is
formed by contacting the substrate with a liquid comprising
Zr.sub.4(OPr.sup.n).sub.16 to form a modified surface, activating
the modified surface, and repeating the contacting and
activating.
40. The semiconductor structure of claim 39, wherein the
Zr.sub.4(OPr.sup.n).sub.16 is analytically pure and the liquid is
anhydrous.
41. The semiconductor structure of claim 35, wherein the layer is
formed by contacting the substrate with an anhydrous liquid
comprising analytically pure Zr.sub.4(OPr.sup.n).sub.16 to form a
modified surface, hydrolyzing the modified surface, repeating the
contacting and hydrolyzing, and heat treating the structure after
the contacting and hydrolyzing.
42. A metal oxide semiconductor capacitor comprising: a
semiconductor substrate comprising a first surface and a second
surface; a layer comprising zirconia on the first surface; a first
layer of a conductor on at least a portion of the zirconia layer;
and a second layer of a conductor on at least a portion of the
second surface; wherein the capacitor has a leakage current less
than 0.002 A/cm.sup.2 when subjected to a potential of 1 volt in
accumulation.
43. The metal oxide semiconductor capacitor of claim 42, wherein
the capacitor has a stretchout of less than 1.5 volts.
44. The metal oxide semiconductor capacitor of claim 42, wherein
the capacitor has a stretchout of less than 1 volt.
45. The metal oxide semiconductor capacitor of claim 42, wherein
the capacitor has a stretchout of less than 0.7 volt.
46. The metal oxide semiconductor capacitor of claim 42, wherein
the capacitor has a leakage current less than 0.001 A/cm.sup.2 when
subjected to a potential of 1 volt in accumulation.
47. The metal oxide semiconductor capacitor of claim 42, wherein
the layer comprising zirconia has an equivalent oxide thickness of
not more than 2 nanometers.
48. The metal oxide semiconductor capacitor of claim 42, wherein
the semiconductor comprises silicon.
49. The metal oxide semiconductor capacitor of claim 42, wherein
the first and second layers of conductor comprise a conducting
metal.
50. The metal oxide semiconductor capacitor of claim 49, wherein
the first and second layers of conductor independently comprise a
member selected from the group consisting of aluminum, copper and
gold.
51. A method of making a semiconductor device, comprising: making a
semiconductor structure by the method of claim 1; and forming a
semiconductor device from said structure.
52. A method of making a semiconductor device, comprising: making a
semiconductor structure by the method of claim 22; and forming a
semiconductor device from said structure.
53. A method of making an electronic device, comprising: making a
semiconductor device by the method of claim 51; and forming an
electronic device, comprising said semiconductor device.
54. A method of making an electronic device, comprising: making a
semiconductor device by the method of claim 52; and forming an
electronic device, comprising said semiconductor device.
55. A semiconductor device, comprising the semiconductor structure
of claim 35.
56. A semiconductor device, comprising the metal oxide
semiconductor capacitor of claim 42.
57. An electronic device, comprising the semiconductor device of
claim 55.
58. An electronic device, comprising the semiconductor device of
claim 56.
Description
BACKGROUND
[0002] In the development of microelectronics, there is an ongoing
effort to reduce the size of microelectronic devices and the
elements that make up the devices. As these dimensions continue to
shrink, the need for alternative gate dielectric materials will
become more important. Silica, having an empirical formula of
SiO.sub.2 and commonly referred to as silicon dioxide, has
conventionally been the material of choice for gate oxides because
it readily forms on a silicon substrate by oxidation of the
silicon. At thicknesses below about 2 nm, however, leakage currents
through silica films become unacceptably high during normal
operating conditions. Replacement of silica with materials having a
higher dielectric constant ("high-K materials") has been
investigated. Films of high-K materials, however, have typically
been plagued by poor interfaces and high cost of production.
[0003] One class of high-K films is the metal oxide family of
general empirical formula MO.sub.x (where "M" is a metal and x is
from 0.01 to 4). Metal oxide films can be prepared through a
variety of techniques. For example, vapor deposition of a metal
oxide can be accomplished by treatment of the surface with a
vaporized metal (i.e. physical vapor deposition). Vapor deposition
of a metal oxide can also involve treatment of the surface with a
vaporized metal alkoxide of the general formula M(OR).sub.y, where
y is from 1 to 8, and R is an alkyl group. This process is referred
to as chemical vapor deposition (CVD). "Alkyl" refers to a
substituted or unsubstituted, straight, branched or cyclic
hydrocarbon chain containing from 1 to 20 carbon atoms. The
chemisorbed layer formed is then treated with an activating agent
such as an oxidizing agent or water, or by exposure to heat or
light to form the MO.sub.x film. See, for example, Tada, H.
Langmuir, 11, 3281 (1995); and Zechmann, C. A. et al. Chem. Mater.,
10, 2348 (1998).
[0004] One particularly interesting high-K material is zirconia,
commonly referred to as zirconium oxide. The term "zirconia" is
defined herein as a substance having an empirical formula of
ZrO.sub.2, and which may include trace amounts of impurities such
as hafnium, water, or hydrocarbons. Zirconia has good performance
characteristics as a dielectric gate material due to its stability
on silicon.
[0005] In addition to vapor deposition processes, a conventional
method of forming films of zirconia or other metal oxides on a
semiconductor such as silicon is atomic layer deposition (ALD). The
ALD process involves a high temperature condensation of evaporated
metal-containing precursors on the semiconductor surface, followed
by a hydrolysis reaction with water, and then repeating the
condensation and hydrolysis one or more times. Anhydrous zirconium
alkoxides are stable at ambient temperature and form zirconia
through a series of hydrolysis (1) and condensation (2)
reactions:
Zr--O-alkyl+H.sub.2O.fwdarw.Zr--OH+alcohol (1)
Zr--OH+Zr--O-alkyl.fwdarw.Zr--O--Zr.sup.+ alcohol (2).
[0006] This approach of using alternating surface reactions can be
employed in a CVD chamber to grow zirconia films on a substrate
using Zr[--OC(CH.sub.3).sub.3].sub.4 at temperatures ranging from
150.degree. C. to 300.degree. C. Hydroxyl (--OH) groups on the
surface of the substrate are believed to provide initial sites for
condensation reactions. The reactions between zirconium alkoxide
groups and the hydroxyl groups yield a single layer of chemisorbed
zirconium alkoxide according to reaction (3):
Surf-OH+Zr(O-alkyl).sub.4.fwdarw.Surf-O--Zr--(O-alkyl).sub.3+alcohol
(3).
[0007] The adsorbed layer [--O--Zr--(O-alkyl).sub.3] is "protected"
from multilayer formation by the remaining unreacted --O-alkyl
groups. The zirconium alkoxide adsorbed on the surface is then
"deprotected" through alkyl group elimination by hydrolysis
according to reaction (4):
Surf-O--Zr--(O-alkyl).sub.3+3H.sub.2O.fwdarw.Surf-O--Zr--(OH).sub.3+3
alcohol (4).
[0008] A second exposure to zirconium alkoxide results in further
surface condensation as in (3). Through repeated
condensation-elimination cycling, a robust zirconia film is formed
layer by layer. See, for example, Kukli, K. et al. Chem. Vap.
Deposition, 6 (2000), p. 297.
[0009] The conventional methods of forming metal oxide films,
including zirconia and HfO.sub.2 films, on semiconductors have met
with mixed success. Disadvantages of these methods include the high
cost of using elevated temperatures and/or reduced pressures for
depositing the metal oxide precursors on the semiconductor surface.
Also, due to the elevated temperatures used, a thick oxide
interface, containing silicon and the metal from the metal oxide
being formed, can be present between the silicon substrate and the
desired high-K metal oxide film. Irregularities in the surface of
the metal oxide films and/or in the interface between the film and
the semiconductor can also be problematic.
[0010] It is thus desirable to provide thin metal oxide films on
semiconductors using lower temperature processes. Preferably, these
metal oxide films do not contain significant surface irregularities
and can be formed reproducibly. High-quality, ultrathin metal oxide
films would likely be useful as gate dielectrics in semiconductor
structures, as dielectrics in metal oxide semiconductor capacitors,
and as barrier layers in semiconductor processing.
BRIEF SUMMARY
[0011] In a first embodiment of the invention, there is provided a
method of making a semiconductor structure, comprising contacting a
surface of a semiconductor with a liquid comprising
Zr.sub.4(OPr.sup.n).sub.16 to form a modified surface; activating
the modified surface; and repeating the contacting and activating
to form a layer of zirconia on the semiconductor surface.
[0012] In a second embodiment of the invention, there is provided a
method of making a semiconductor structure, comprising obtaining a
liquid comprising analytically pure Zr.sub.4(OPr.sup.n).sub.16;
contacting a surface of a semiconductor with the liquid in an inert
atmosphere to form a modified surface; rinsing the modified
surface; hydrolyzing the modified surface with an aqueous liquid
comprising n-propanol to form an activated surface; drying the
activated surface; repeating the contacting, rinsing, hydrolyzing,
and drying to form a layer of zirconia on the semiconductor
surface; and heat treating the semiconductor comprising the layer
of zirconia.
[0013] In a third embodiment of the invention, there is provided a
semiconductor structure comprising a semiconductor substrate and a
layer comprising zirconia on the substrate, the layer having an
equivalent oxide thickness of not more than 2 nanometers. The
semiconductor structure has a leakage current less than 0.002
A/cm.sup.2 when subjected to a potential of 1 volt.
[0014] In a fourth embodiment of the invention, there is provided a
semiconductor substrate comprising a first surface and a second
surface; a layer comprising zirconia on the first surface; a first
layer of a conductor on at least a portion of the zirconia layer;
and a second layer of a conductor on at least a portion of the
second surface. The capacitor has a leakage current less than 0.002
A/cm.sup.2 when subjected to a potential of 1 volt in
accumulation.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a diagram of a representative structure for
Zr.sub.4(OPr.sup.n).sub.16.
[0016] FIG. 2 is a diagram of a process of forming a layer of
zirconia on a semiconductor substrate.
[0017] FIG. 3 is a graph of areal zirconium concentration versus
condensation-activation cycles for four samples as measured by
Rutherford Backscattering Spectrometry (RBS).
[0018] FIGS. 4a-d are graphs of peak area ratios versus
condensation-activation cycles as measured by high-resolution X-ray
photoelectron spectroscopy (XPS).
[0019] FIG. 5 is a graph of current density versus voltage for a
metal oxide semiconductor (MOS) capacitor having the structure:
Au/zirconia/p-Si(111)/Au.
[0020] FIG. 6 is a graph of current density versus voltage for the
structure: Au/silica/p-Si(111)/Au.
[0021] FIG. 7 is a graph of normalized capacitance density versus
voltage, performed at 1 MHz, for a metal oxide semiconductor (MOS)
capacitor having the structure: Au/zirconia/p-Si(111)/Au.
[0022] FIG. 8 is a graph of capacitance density versus voltage,
performed at 100 kHz, for a metal oxide semiconductor (MOS)
capacitor having the structure: Au/zirconia/p-Si(111)/Au.
[0023] FIG. 9 is a diagram of a MOS capacitor.
DETAILED DESCRIPTION
[0024] In a first embodiment of the invention, a method of making a
semiconductor structure includes contacting a surface of a
semiconductor with a liquid comprising Zr.sub.4(OPr.sup.n).sub.16
to form a modified surface, activating the modified surface, and
repeating the contacting and activating to form a layer of zirconia
on the semiconductor surface.
[0025] In a second embodiment of the invention, a method of making
a semiconductor structure includes obtaining a liquid containing
analytically pure Zr.sub.4(OPr.sup.n).sub.6, contacting a surface
of a semiconductor with the liquid in an inert atmosphere to form a
modified surface, rinsing the modified surface, hydrolyzing the
modified surface with an aqueous liquid containing n-propanol to
form an activated surface, and drying the activated surface. The
method also includes repeating the contacting, rinsing,
hydrolyzing, and drying to form a layer of zirconia on the
semiconductor surface, and heat treating the semiconductor
comprising the layer of zirconia.
[0026] In a third embodiment of the invention, a semiconductor
structure includes a semiconductor substrate and a layer containing
zirconia on the substrate. The zirconia containing layer may have
an equivalent oxide thickness of 2 nanometers or less, and the
semiconductor structure may have a leakage current less than 0.002
A/cm.sup.2 when subjected to a potential of 1 volt.
[0027] In a fourth embodiment of the invention, a semiconductor
substrate includes a first surface and a second surface, a layer
containing zirconia on the first surface, a first layer of a
conductor on at least a portion of the zirconia layer, and a second
layer of a conductor on at least a portion of the second surface.
The capacitor may have a leakage current less than 0.002 A/cm.sup.2
when subjected to a potential of 1 volt in accumulation.
[0028] The term "semiconductor structure," as used herein, is
defined as any structure containing a semiconducting material
(a.k.a. "semiconductor") and another material that is not
semiconducting. Examples of semiconducting materials include
silicon, germanium, and mixtures thereof; doped titanium dioxide;
2-6 semiconductors, which are compounds of at least one divalent
metal (zinc, cadmium, mercury and lead) and at least one divalent
non-metal (oxygen, sulfur, selenium, and telurium) such as zinc
oxide, cadmium selenide, cadmium sulfide, mercury selenide, and
mixtures thereof; and 3-5 semiconductors, which are compounds of at
least one trivalent metal (aluminum, gallium, indium, and thalium)
with at least one trivalent non-metal (nitrogen, phosphorous,
arsenic, and antimony) such as gallium arsenide, indium phosphide,
and mixtures thereof. Preferred semiconducting materials include
silicon, germanium, gallium arsenide, and cadmium sulfide.
[0029] The term "inert," as used herein, is defined as chemically
non-reactive in the context of the substance or environment. For
example, an inert atmosphere does not contain ingredients that can
chemically react with the substances used in the inert atmosphere.
Likewise, an inert solvent in a liquid mixture does not react with
the other ingredients present in the mixture or with other
substances with which the solvent is brought into contact.
[0030] The term "anhydrous," as used herein, is defined as having
only undetectable amounts of water, if any.
[0031] The metal alkoxides used to prepare metal oxide films on
semiconductors according to the present invention are substances
having the general formula M.sub.4(OR.sup.n).sub.16. The notation
"Rn" denotes an unbranched alkyl group bonded to the oxygen through
a terminal carbon, also referred to as an "n-alkyl" group.
Specifically, metal alkoxides useful for forming metal oxide films
on semiconductors include Zr.sub.4(OPr.sup.n).sub.16 and
Hf.sub.4(OPr.sup.n).sub.16. Preferably, the metal alkoxide is
Zr.sub.4(OPr.sup.n).sub.16, which can be further reacted and
processed to form thin, high-quality films of zirconia.
[0032] The species having formula M.sub.4(OR.sup.n).sub.16 can also
be represented by the empirical formula M(OR.sup.n).sub.4.
Typically, metal alkoxides having this empirical formula are not,
in fact, M.sub.4(OR.sup.n).sub.16, but actually include a variety
of compounds and molecular formulas, such that the overall molar
ratio of metal to alkoxide in a sample is approximately 1:4. The
reactivity of these conventional metal alkoxides contributes to
their utility in the formation of metal oxide (MO.sub.x) films, as
illustrated in general reaction scheme (5):
M(O--CH.sub.2--CH.sub.2--R).sub.4.fwdarw.MO.sub.2+2
CH.sub.2.dbd.CHR+2 HO--CH--CH.sub.2--R (5).
[0033] This reaction is believed to be a chain reaction including
steps (6) and (7):
2 M-O--CH.sub.2--CH.sub.2--R+H.sub.2O.fwdarw.M-O-M+2
HO--CH.sub.2--CH.sub.2--R (6)
HO--CH.sub.2--CH.sub.2--R.fwdarw.CH.sub.2.dbd.CHR+H.sub.2O (7).
[0034] Metal n-alkoxide decomposition can thus be induced by trace
amounts of an initiator, usually water or alcohol. (Bradley, D. C.
et al. Trans. Faraday Soc., vol. 55, 2117-2123 (1959); Bradley, D.
C. et al. J. Appl. Chem., vol. 9, 435-439 (1959)) In fact, zirconia
is known to be a catalyst for alcohol dehydration. (Zechmann et
al., 1998)
[0035] Although metal alkoxides readily react to form MO.sub.x, a
disadvantage to this reactivity is the difficulty of preparing
absolutely pure M(OR.sup.n).sub.4. Rather, conventional samples of
M(OR.sup.n).sub.4 actually contain a certain amount of M-O-M and/or
M-OH moieties. For example, Zr(IV) n-alkoxides and other Zr(IV)
alkoxides containing .beta.-hydrogen atoms are extremely difficult
to purify. (Turevskaya, E. P. et al. Russ. Chem. Bull., vol. 44,
734-742 (1995); Turova, N. Y. et al. Polyhedron, vol. 17, 899-915
(1998)) Crystallization does not generally improve the purity of
Zr(OPr.sup.n).sub.4, and repeated recrystallizations typically
yield increasingly impure material, possibly because of the extreme
moisture-sensitivity of the compound.
[0036] One example of a metal alkoxide is
Zr.sub.4(OPr.sup.n).sub.16, which can be prepared from
tetra-n-propyl zirconate, Zr(OPr.sup.n).sub.4. The starting
material Zr(OPr.sup.n).sub.4 may be prepared by treating ZrCl.sub.4
with n-propanol in the presence of ammonia. (Bradley, D. C. et al.
J. Chem. Soc., 280-285 (1951)) Another method of preparing
Zr(OPr.sup.n).sub.4, which typically yields product with higher
purity, involves alkoxide exchange of tetra-isopropyl zirconate
(Zr(OPr.sup.n).sub.4) with n-propanol. (Bradley, D. C. et al. J.
Chem. Soc., 2025-2030 (1953)) The tetra-n-propyl zirconate prepared
by the conventional alkoxide exchange method is characterized as a
highly viscous liquid with a boiling point of 208.degree. C. at 0.1
mm Hg. Tetra-n-propyl zirconate is typically provided as a 70 wt %
solution in n-propanol, and the alcohol solvent can be removed by
distillation at 85-98.degree. C. under nitrogen (N.sub.2) at
ambient pressure to provide a waxy solid. The solid may then be
fractionally distilled under N.sub.2 at a pressure of about
10.sup.-2 mm Hg. The fraction that distills at 225-245.degree. C.
is a clear, colorless liquid that solidifies upon contact with the
receiving flask. This fraction is pure Zr(OPr.sup.n).sub.4 and is
believed to have the molecular formula
Zr.sub.4(OPr.sup.n).sub.16.
[0037] In the method of the present invention,
Zr.sub.4(OPr.sup.n).sub.16 is purified by rapid fractional
distillation of Zr(OPr.sup.n).sub.4 at low pressure, for example
10.sup.-2 mm Hg. The first distillation fraction, a highly viscous
liquid at ambient temperature and pressure, is collected between
185-220.degree. C. The amount of material collected in the first
fraction is dependent upon the purity of the crude material, with
relatively pure starting material yielding a relatively small
amount of the first fraction. The first fraction, containing both
Zr.sub.4(OPr.sup.n).sub.16 and its hydrolysis product
Zr.sub.3O(OPr.sup.n).sub.10, is similar to the liquid produced by
the. Zr(OPr.sup.n).sub.4/n-propanol exchange. The second
distillation fraction, collected between 225 and 245.degree. C., is
analytically pure. Zr.sub.4(OPr.sup.n).sub.16, a white solid at
ambient temperature and pressure. The method of purification by
distillation at low pressure and high temperature over a short time
period can likely be extended to other metal alkoxide systems,
including alkoxide compounds of hafnium, titanium, scandium,
yttrium, indium, and ytterbium.
[0038] Analytically pure tetra-n-propyl zirconate is identified by
proton nuclear magnetic resonance spectroscopy (.sup.1H NMR) in
cyclohexane-d.sub.12 solution at ambient temperature. In the
spectrum for. Zr.sub.4(OPr.sup.n).sub.16, four triplets are
observed in the methyl proton region at .delta. 0.94, 0.90, 0.86,
and 0.83 (at 500 megahertz (MHz)), with relative intensities
3:2:2:1. If the hydrolysis product. Zr.sub.3O(OPr.sup.n).sub.10 is
present in the sample, however, low-intensity triplets can be
observed at .delta. 0.93 and 0.92 (at 500 MHz). (Turova et al.,
1998). Unless special precautions are taken, NMR samples of.
Zr.sub.4(OPr.sup.n).sub.16 are generally contaminated with 1-3% of
this impurity due to the extreme moisture-sensitivity of.
Zr.sub.4(OPr.sup.n).sub.16. Also, elemental analyses generally
report slightly higher values for zirconium and slightly lower
values for carbon, relative to the amounts calculated for each
based on the ratios in the empirical formula. Analytically pure.
Zr.sub.4(OPr.sup.n).sub.16 is defined as having a purity of at
least 97% as measured by NMR spectroscopy. Preferably the.
Zr.sub.4(OPr.sup.n).sub.16 used in the present invention has a
purity of at least 97% as measured by NMR spectroscopy, and more
preferably has a purity of at least 99% as measured by NMR
spectroscopy. The crystal structure of analytically pure.
Zr.sub.4(OPr.sup.n).sub.16 has been reported in. Day, V. W. et al.,
Inorg. Chem., vol. 40, 5738-5746 (2001). Without wishing to be
bound by any theory of operation, it is believed that.
Zr.sub.4(OPr.sup.n).sub.16 has the molecular structure illustrated
in FIG. 1.
[0039] The analytically pure. Zr.sub.4(OPr.sup.n).sub.16 can be
used to form thin zirconia films which are substantially free of
defects. Referring to the diagram of FIG. 2, the process for
producing these films preferably follows a sequence of deposition
100 of the metal alkoxide, activation 200 of the modified surface
12 containing the metal alkoxide, and repetition 300 of the
deposition and activation until a film 14 of the desired thickness
is formed. An optional heat treatment 400 can be performed to
provide a semiconductor structure 20 having the zirconia film 16 on
the semiconductor substrate 10. The semiconductor substrate 10 to
be coated is contacted 100 with a solution of.
Zr.sub.4(OPr.sup.n).sub- .16 in an inert solvent that does not
react with the zirconium alkoxide or the substrate during the time
necessary to form films. Useful inert solvents include
diethylether, methylene chloride, 1,2-dichloroethane, and
hydrocarbons such as methyl cyclohexane, toluene, benzene, heptane,
and pentane. The substrate is thus coated with a layer 12 of the
metal alkoxide. Preferably, the deposition is carried out in an
inert atmosphere, such as argon or nitrogen, to avoid premature
hydrolysis of the zirconium compound.
[0040] The. Zr.sub.4(OPr.sup.n).sub.16 may be adsorbed onto the
surface or, preferably, chemically reacts with the surface to bond
at least one of the zirconium atoms to the surface either directly
or through a. Zr--O-- bond. This treatment with.
Zr.sub.4(OPr.sup.n).sub.16 is referred to herein as "condensation,"
since it is believed that the alkoxy substituent is eliminated as
an alcohol (analogous to reaction (3)). Referring again to FIG. 2,
this modified surface 12 is then activated by treatment 200 with an
activating agent. Without wishing to be bound by any theory of
operation, it is believed that the activation can form one or more.
Zr--OH bonds, possibly by a process analogous to reaction (4). The
activated surface 14 can then be treated 300 with additional.
Zr.sub.4(OPr.sup.n).sub.16, allowing for condensation of more.
Zr.sub.4(OPr.sup.n).sub.16 moieties with the surface, and these
moieties can also be activated.
[0041] Repetition of the deposition of. Zr.sub.4(OPr.sup.n).sub.16
and the activation of the modified surface containing the
condensed. Zr.sub.4(OPr.sup.n).sub.16 can allow for a gradual
buildup of a zirconia film 14. After the treatments with.
Zr.sub.4(OPr.sup.n).sub.16 and the activation are complete, it may
be desirable to subject the film to a heat treatment (arrow 400 in
FIG. 2). Without wishing to be bound by any theory of operation, it
is believed that the interaction of. Zr.sub.4(OPr.sup.n).sub.16
with an activated surface results in the condensation of the
zirconium compound with the surface and elimination of at least one
equivalent of propanol, thus binding the metal alkoxide to the
surface. The alkoxide is highly reactive, and this reactivity
allows the metal alkoxide to be condensed and activated even at
room temperature.
[0042] The condensation and activation of.
Zr.sub.4(OPr.sup.n).sub.16 onto semiconductor surfaces at
temperatures lower than those conventionally used for metal oxide
film formation helps to reduce the development of a second oxide
film at the interface between the semiconductor and the metal
oxide. For example, conventional high temperature processes
involving silicon substrates can result in the formation of silica
layers between the silicon and the metal oxide. Preferably, the
condensation and activation of the metal alkoxide is performed at
temperatures well below those used in vapor deposition or ALD
processes. Preferably the condensation and activation are performed
at temperatures below 150.degree. C., more preferably at
temperatures below 100.degree. C., even more preferably at
temperatures below 50.degree. C., and even more preferably at
temperatures of about 25.degree. C.
[0043] The activation of a the modified surface of a semiconductor
(i.e. having one or more layers of condensed.
Zr.sub.4(OPr.sup.n).sub.16) serves to eliminate alkoxy groups from
the surface. The activated surface can then be treated with
additional. Zr.sub.4(OPr.sup.n).sub.16, followed by another
activation treatment. Continued repetition of the condensation and
activation steps provides for a gradual buildup of zirconia on the
semiconductor.
[0044] Activation of a modified surface layer of condensed.
Zr.sub.4(OPr.sup.n).sub.16 can be performed by a variety of
methods, including for example irradiation, heating, vacuum
treatment, hydrolysis, and treatment with an oxidizing agent. These
methods can be used individually, or two or more of the methods can
be used simultaneously or sequentially. For example, the modified
surface can be subjected to UV irradiation, and this irradiation
may be performed on the entire surface, or it may be performed in a
pattern, such as a mask pattern as used in conventional
semiconductor processing. In another example, the semiconductor
containing the modified surface can be heated to thermally
eliminate alkoxy groups. If the activation is performed by heating,
it is preferred that the temperature be maintained below
150.degree. C. In another example, the semiconductor containing the
modified surface can be vacuum treated by subjecting the
semiconductor to a reduced pressure environment. For example, the
semiconductor can be vacuum treated by holding it under a pressure
of 10 mm. Hg or lower, preferably of 1 mm. Hg or lower, and more
preferably of 0.1 mm. Hg or lower.
[0045] Activation of a modified surface layer of condensed
Zr.sub.4(OPr.sup.n).sub.16 can also be performed by contacting the
surface with a reagent. In one example, the modified surface can be
contacted with water, or with a liquid mixture containing water, to
hydrolyze the surface. In another example, the modified surface can
be contacted with one or more oxidizing agents. Oxidizing agents
include, for example, salts of sodium, potassium, ammonium, or
phosphonium with chlorates, perchlorates, perbromates, periodates,
sulfates, persulfates (S.sub.2O.sub.8.sup.-2), or monopersulfates
(HSO.sub.5.sup.-1). Specific examples of oxidizing agents include
KIO.sub.4, NaIO.sub.4, KHSO.sub.5, NaHSO.sub.5,
(NH.sub.4)HSO.sub.5, (NH.sub.4).sub.2S.sub.2O.sub.8,
K.sub.2S.sub.2O.sub.8, Na.sub.2S.sub.2O.sub.8, KClO.sub.4,
NaClO.sub.4, and NH.sub.4ClO.sub.4, H.sub.2O.sub.2, benzoyl
peroxide, di-t-butyl peroxide, and sodium peroxide. Preferably the
oxidizing agent does not contain metals that could be difficult to
remove from the surface, contaminating the zirconia layer.
Oxidizing agents can be dissolved or dispersed in a liquid, such as
an aqueous liquid or an anhydrous inert solvent. Examples of inert
solvents for oxidizing agents include methyl cyclohexane, toluene,
benzene, heptane, and pentane.
[0046] The activation step is preferably a hydrolysis of the
modified surface through contact with an aqueous liquid. An aqueous
liquid refers to a liquid mixture containing water, and can contain
other solvents and/or reagents that can facilitate the controlled
activation of the surface. More preferably, the hydrolysis liquid
is an aqueous liquid containing n-propanol, and even more
preferably has a water to propanol ratio of about 1:4. Without
wishing to be bound by any theory of operation, it is believed that
introduction of water to the system results in hydrolysis of other
Zr--O--C bonds, with the elimination of additional propanol and the
formation of Zr--OH bonds. These hydroxyl groups (--OH) are thus
believed to be available for condensation with additional
Zr.sub.4(OPr.sup.n).sub.16 to form Zr--O--Zr bonds.
[0047] The surface of the substrate can be rinsed after both the
condensation and the activation procedures. For example, the
surface may be rinsed with an inert solvent after a deposition of
the zirconium alkoxide to ensure that only zirconium alkoxide which
is bound to the surface remains. In another example, the surface
may be rinsed with propanol after an activation step involving
hydrolysis to assist in removing any residual water. After the
hydrolysis and rinsing, the structure may be dried, optionally
under vacuum conditions, to reduce the amount of reactive water or
alcohol that could prematurely react with the metal alkoxide.
[0048] The thickness of the final metal oxide layer formed is
dependent on the number of cycles of condensation and activation
preformed. For example, a single cycle of condensation and
activation, optionally followed by a heat treatment step, could be
used to form an extremely thin layer of zirconia. To form metal
oxide films that are useful as insulating or dielectric layers in
semiconductor structures or semiconductor devices, it is preferred
to use two or more cycles. In other preferred embodiments, the
number of condensation and hydrolysis cycles performed may be four
or more, ten or more, fifteen or more, or twenty or more.
[0049] Once a sufficient amount of zirconium has been deposited
through condensation and activation cycles, the semiconductor
structure can be heat treated. Preferably the semiconductor
structure is heat treated at a temperature of at least 100.degree.
C., more preferably at a temperature of at least 300.degree. C.,
more preferably still at a temperature of at least 600.degree. C.
Preferably the heat treatment is performed for at least 10 minutes,
more preferably is performed for at least 20 minutes, and more
preferably still is performed for at least 30 minutes. The heat
treatment may be performed in an inert atmosphere, such as an argon
atmosphere, and may also be performed under vacuum conditions (i.e.
10 mm Hg or less).
[0050] Metal oxide films on semiconductors may be characterized by
parameters including equivalent oxide thickness and leakage
current. The equivalent oxide thickness is the thickness of a film
of silica that would be required to provide the same capacitance as
provided by the metal oxide film. A metal oxide film with a
dielectric constant (K) higher than silica can provide the same
capacitance as a silica film having a smaller thickness. Because
the high-K dielectric film can have an increased thickness, the
leakage current at a given voltage is smaller compared to a silica
film having the same capacitance. When forming zirconia films on
silicon according to the present invention, dielectric films can be
formed having equivalent oxide thicknesses of 2 nanometers (nm) or
smaller, but with leakage currents of 0.002 amperes per square
centimeter (A/cm.sup.2) or lower when subjected to 1.0 volt. More
preferably, dielectric films can be formed having equivalent oxide
thicknesses of 2 nanometers (nm) or smaller, with leakage currents
of 0.001 A/cm.sup.2 or lower when subjected to 1.0 volt. The
leakage current as a function of applied voltage for a
semiconductor structure of the present invention is illustrated in
FIG. 5. In contrast, the leakage current as a function of voltage
for a semiconductor with a silica film rather than a metal oxide
film of the present invention is illustrated in FIG. 6.
[0051] Another parameter for characterizing the quality of metal
oxide films is the amount of stretchout in a graph of capacitance
versus voltage. The measurement of capacitance versus voltage (C-V)
is possible when the semiconductor coated with the metal oxide film
on one surface is sandwiched between conductive contacts, a
structure referred to as a metal oxide semiconductor (MOS)
capacitor. Referring to FIG. 9, an example of a MOS capacitor 30
has a semiconductor substrate 32 with a zirconia layer 34. A
conductive material is present on the semiconductor as ohmic
contact 38 and on the zirconia layer as contact 36. Conductive
materials that can be used as the contacts in a MOS capacitor
include conductive metals and conductive non-metals. Preferably the
conductive contacts are formed of conductive metals such as
aluminum, copper and gold. The measured capacitance is reported in
capacitance per unit area (i.e. per cm.sup.2), where the area of
contact between the zirconia layer and contact 36.
[0052] The stretchout for a MOS capacitor is defined as the
difference in applied voltage required to reduce the capacitance
from 95% of its maximum value to 5% of its maximum value. The
stretchout region is illustrated in FIG. 7 as the drastic decrease
in capacitance from -0.6 volts to 0 volts. Between these voltages,
the capacitance decreases from 95% of its maximum value at -1 volt
to 5% of the maximum value. Conventional high-K metal oxide films
exhibit a stretchout of 1.5 volts or greater. Referring to FIG. 7,
zirconia films of the present invention exhibit a stretchout of 0.6
volts. Preferably, the stretchout of the metal oxide films of the
present invention is less than 1.5 volts, more preferably less than
1 volt, and even more preferably less than 0.7 volt. It is believed
that the stretchout of a metal oxide film in a MOS capacitor is due
to surface irregularities. (Nicollian, E. H. et al. MOS (Metal
Oxide Semiconductor) Physics And Technology, Wiley-Interscience,
(2003) chapter 6) The semiconductor structures of the present
invention may be incorporated into a semiconductor device such as
an integrated circuit, for example a memory cell such as an SRAM, a
DRAM, an EPROM, an EEPROM etc.; a programmable logic device; a data
communications device; a clock generation device; etc. Furthermore,
any of these semiconductor devices may be incorporated in an
electronic device, for example a computer, an airplane or an
automobile. The related processing steps, polishing, cleaning, and
deposition steps, for making semiconductor devices are well known
to those of ordinary skill in the art, and are also described in
Encyclopedia of Chemical Technology, Kirk-Othmer, Volume 14, pp.
677-709 (1995); Semiconductor Device Fundamentals, Robert F.
Pierret, Addison-Wesley, 1996; Wolf, Silicon Processing for the
VLSI Era, Lattice Press, 1986, 1990, 1995 (vols 1-3, respectively),
and Microchip Fabrication 4th. edition, Peter Van Zant,
McGraw-Hill, 2000.
EXPERIMENTAL
[0053] Materials and Methods
[0054] Tetra-n-propyl zirconate was purchased from ALDRICH CHEMICAL
(Milwaukee, Wis.) as a 70% solution by weight in n-propanol.
Molecular sieves (3 .ANG. Linde type A, GRACE DAVISON, Columbia,
Md.) were activated by heating at 250.degree. C. for at least 24
hours and cooling under vacuum. Solvents such as HPLC grade
n-heptane, HPLC grade n-pentane, methylcyclohexane, and toluene
were obtained from FISHER SCIENTIFIC (Suwanee, Ga.) or ALDRICH, and
were dried over activated molecular sieves, refluxed over Na, and
freshly distilled prior to use. Anhydrous methylcyclohexane
(ALDRICH) was refluxed over molten sodium, freshly distilled, and
degassed using three freeze-pump-thaw cycles or sparging with Ar
gas (S. J. SMITH) for at least 30 min prior to use. Anhydrous
n-propanol (ALDRICH) was sparged with Ar gas for at least 30 min
prior to use. Diethylether and benzene (FISHER SCIENTIFIC) were
dried over activated molecular sieves, refluxed over
Na/benzophenone, and freshly distilled prior to use.
Cyclohexane-d.sub.12 (CAMBRIDGE ISOTOPE LABORATORIES, Andover,
Mass.) was dried over activated molecular sieves for at least 24
hours prior to use and subsequently distilled.
Methylcyclohexane-d.sub.14 was dried over Na/K alloy for 24 hours,
degassed using three freeze-pump-thaw cycles, and distilled from
Na/K alloy. All other solvents were dried over activated molecular
sieves for at least 24 hours prior to use.
[0055] For measuring the electrical properties of the thin films,
silicon(111) wafer strips were cut from single-side polished
p-type, CZ grown wafers with a resistivity of 3 to 6
ohm-centimeters (MEMC ELECTRONIC MATERIALS). For other analytical
measurements, n-type silicon wafer strips were also used. All water
(H.sub.2O) used was ultrapure water (18 Mohm-cm), which was
obtained directly from a BARNSTEAD NANOPURE II filtration system
with a 4-module cartridge configuration and 0.2 .mu.m pore size
final filter. Acetone, 2-propanol (FISHER SCIENTIFIC), 30% ammonium
hydroxide, 30% hydrogen peroxide (J. T. BAKER), 38% hydrochloric
acid (CORCO), and 40% ammonium fluoride were electronic grade, and
1,1,1-trichloroethane (ALDRICH) was reagent grade.
[0056] Tetra-n-propyl zirconate is an extremely moisture-sensitive
material, and all manipulations were carried out under an argon or
nitrogen atmosphere using standard Schlenk and dry box
techniques.
[0057] All glass and TEFLON materials used in the course of
cleaning, etching, and storing silicon(111) wafer strips strips
were cleaned in an 80.degree. C. bath of 5:1:1H.sub.2O:30%
NH.sub.4OH:30% H.sub.2O.sub.2 by volume for 1 h and rinsed for 30
sec in a flowing stream of H.sub.2O. These materials included fused
quartz and conventional glassware as well as TEFLON utensils,
including containers, tubing, and tweezers. The glassware used
during the film deposition process was immersed in a saturated
ethanolic KOH solution, rinsed with dilute HCl, rinsed with
deionized water, and oven-dried for 12 hours at 120.degree. C. The
glassware used for synthesis of the tetra-n-propyl zirconate was
then thoroughly flame-dried before use by passing the flame from a
Bunsen burner over the entire surface of the flask under vacuum
(ca. 10.sup.-2 mm Hg). Water vapor was observed upon contact of the
flame with the glass, and glassware was heated for approximately
three minutes until no further water vapor was visible. The flask
was then allowed to cool under vacuum. TEFLON stopcocks were
employed instead of glass stopcocks, and TEFLON stoppers were
employed instead of glass or rubber stoppers.
[0058] Preparation of Tetra-n-Propyl Zirconate
(Zr.sub.4(OPr.sup.n).sub.16- )
[0059] A 250 mL, two-neck round bottom flask with ground glass
joints was charged with 100 mL of a 70 wt % solution of
partially-hydrolyzed tetra-n-propyl zirconate in n-propanol. The
flask was then joined to a nitrogen inlet and a distillation
apparatus constructed from a 24 mm i.d. Vigreux reflux column 20.3
cm in length, a distillation head with a thermometer, and a Liebig
condenser with a jacket length of 20.0 cm. Components of the
distillation apparatus were not connected by ground-glass joints
but were instead integrated into a single piece of glassware. A 100
mL single-neck receiving flask was joined to the still body by an
elbow fitted with a nitrogen/vacuum inlet. All ground glass joints
were sealed with silicone grease and secured with copper wire.
[0060] n-Propanol was removed from the partially-hydrolyzed
tetra-n-propyl zirconate solution under nitrogen by heating the
distillation flask in a silicone oil heating bath and collecting
all material that distilled at temperatures less than 100.degree.
C. at ambient pressure. The waxy yellow solid remaining in the
distillation flask was allowed to cool to room temperature.
[0061] Tetra-n-propyl zirconate was distilled under vacuum from the
same distillation flask used for the removal of propanol, but using
a different distillation apparatus suited for higher temperatures
and lower pressures. A heating bath containing 40 wt % NaNO.sub.2,
7 wt % NaNO.sub.3, and 53 wt % KNO.sub.3 at 37.degree. C. was
employed. The distillation flask was fitted with a still body
identical to the one described above except that the Liebig
condenser was replaced with a simple, 1.25 cm i.d. glass condenser.
A cow receiver equipped with a nitrogen/vacuum inlet and fitted
with one 50 mL and two 100 mL Schlenk flask receivers spaced 450
apart was attached to the still body. All ground-glass joints were
sealed with KRYTOX.RTM. LVP fluorinated grease (70% perfluoroalkyl
ether, 30% polytetrafluoroethylene, DUPONT, Wilmington, Del.) and
secured with copper wiring. A thermocouple probe was attached to
the surface of the condenser, which was subsequently wrapped with
heating tape insulated with braided fibrous glass, and the still
body and condenser were heavily insulated with glass wool and
aluminum foil. The system was evacuated to ca. 10.sup.-2 mm Hg
pressure, the condenser was heated to ca. 175.degree. C., and
finally, the temperature of the heating bath was raised to
290.degree. C.
[0062] Three distinct distillation fractions were observed as the
temperature at the distillation head was allowed to rise to
270.degree. C., and these three fractions were collected as
follows. Less than 3 mL of a yellow oil distilled between
185.degree. C. and 220.degree. C., the precise amount obtained
depending upon the purity of the crude material. About 40 g of
analytically pure tetra-n-propyl zirconate was collected between
225.degree. C. and 245.degree. C. as a clear, colorless liquid that
solidified immediately upon contact with the collection flask.
Finally, about 10 g of a third fraction was collected between 250
and 270.degree. C. as a waxy white or slightly yellow solid. About
20 g of the crude material remained in the distillation pot.
Extreme caution was exercised to maintain the condenser at an
elevated temperature throughout the distillation, since
solidification of the distillate in the condenser at lower
temperatures would generate a closed and hence extremely hazardous
system.
[0063] Material in the distillation pot could not be raised to
temperatures above about 200.degree. C. for longer than about 40
minutes. If the distillation was carried out more slowly, a
distinct second distillation fraction was not observed, and the
tetra-n-propyl zirconate collected at elevated distillation
temperatures was seriously contaminated.
[0064] Tetra-n-propyl zirconate is highly soluble in diethylether,
n-propanol, toluene, benzene, methylene chloride,
1,2-dichloroethane, and hydrocarbons such as n-heptane, n-pentane,
and methylcyclohexane. It can be crystallized from n-heptane,
n-pentane, toluene, methylene chloride, and 1,2-dichloroethane.
[0065] Analysis of Tetra-n-Propyl Zirconate
(Zr.sub.4(OPr.sup.n).sub.16)
[0066] Both 500 MHz .sup.1H and 125.6 MHz .sup.13C{.sup.1H} NMR
spectra were measured on a UNITY 500 spectrometer (VARIAN, Palo
Alto, Calif.), and those recorded at 750 and 188.6 MHz,
respectively, were measured on a UNITY INOVA 750 spectromete
(VARIAN). Gradient-enhanced .sup.1H-.sup.1H COSY experiments,
gradient phase-sensitive .sup.1H--.sup.13C heteronuclear
multiple-quantum coherence (HMQC) experiments, and .sup.13C
inversion-recovery experiments were performed using standard pulse
programs. Chemical shifts were internally referenced to
tetramethylsilane (.delta.=0.00). NMR samples were typically
prepared by distilling 0.75 mL of deuterated solvent into a 5 mm
o.d. NMR sample tube containing ca. 45 mg tetra-n-propyl zirconate.
The tube was then flame-sealed under vacuum. Elemental analysis was
performed by the University of Illinois Microanalytical Service
Laboratory.
[0067] Analytical calculation for Zr.sub.4O.sub.16C.sub.48H.sub.112
in weight percent is as follows: C, 44.00; H, 8.62; Zr, 27.85.
Weight percentages measured were as follows: C, 43.68; H, 8.87; Zr,
28.33.
[0068] .sup.1H NMR (500 MHz, cyclohexane-d.sub.12,22.degree. C.):
.delta. 4.20-3.90 (16H, m, --OCH.sub.2CH.sub.2CH.sub.3), 2.13 (2H,
br sext, J=7.7 Hz, --OCH.sub.2CH.sub.2CH.sub.3), 1.90 (2H, br m,
--OCH.sub.2CH.sub.2CH.s- ub.3), 1.80 (2H, br m,
--OCH.sub.2CH.sub.2CH.sub.3), 1.67 (4H, sext, J=7.4 Hz,
--OCH.sub.2CH.sub.2CH.sub.3), 1.60 (6H, br m,
--OCH.sub.2CH.sub.2CH.s- ub.3), 0.94 (9H, t, J=7.4 Hz,
--OCH.sub.2CH.sub.2CH.sub.3), 0.90 (6H, t, J=7.5 Hz,
--OCH.sub.2CH.sub.2CH.sub.3), 0.86 (6H, t, J=7.6 Hz,
--OCH.sub.2CH.sub.2CH.sub.3), 0.83 (3H, t, J=7.5 Hz,
--OCH.sub.2CH.sub.2CH.sub.3);
[0069] .sup.1H NMR (750 MHz, methylcyclohexane-d.sub.14,
-20.degree. C.): .delta. 3.91-4.15 (16H, m,
--OCH.sub.2CH.sub.2CH.sub.3), 2.12 (2H, br sext,
--OCH.sub.2CH.sub.2CH.sub.3), 1.89 (2H, br m,
--OCH.sub.2CH.sub.2CH.sub.3), 1.78 (2H, br m,
--OCH.sub.2CH.sub.2CH.sub.3- ), 1.66 (4H, sext, J=7.3 Hz,
--OCH.sub.2CH.sub.2CH.sub.3), 1.59 (4H, sext, J=7.3 Hz,
(--OCH.sub.2CH.sub.2CH.sub.3), 1.58 (2H, sext, J=7.3 Hz,
--OCH.sub.2CH.sub.2CH.sub.3), 0.95 (6H, t, J=7.3 Hz,
--OCH.sub.2CH.sub.2CH.sub.3), 0.94 (3H, t, J=7.3 Hz,
--OCH.sub.2CH.sub.2CH.sub.3), 0.90 (6H, t, J=7.3 Hz,
--OCH.sub.2CH.sub.2CH.sub.3), 0.87 (6H, t, J=7.3 Hz,
--OCH.sub.2CH.sub.2CH.sub.3), 0.83 (3H, t, J=7.3 Hz,
--OCH.sub.2CH.sub.2CH.sub.3),
[0070] .sup.13C{.sup.1H} NMR (125.6 MHz,
cyclohexane-d.sub.12,22.degree. C.): .delta. 73.55 (1C,
--OCH.sub.2CH.sub.2CH.sub.3), 73.49 (2C,
--OCH.sub.2CH.sub.2CH.sub.3), 73.25 (2C,
--OCH.sub.2CH.sub.2CH.sub.3), 72.41 (3C,
--OCH.sub.2CH.sub.2CH.sub.3), 28.86 (2C,
--OCH.sub.2CH.sub.2CH.sub.3), 28.47 (1C,
--OCH.sub.2CH.sub.2CH.sub.3), 27.92 (2C,
--OCH.sub.2CH.sub.2CH.sub.3), 26.85 (2C,
--OCH.sub.2CH.sub.2CH.sub.3), 24.30 (1C,
--OCH.sub.2CH.sub.2CH.sub.3), 10.96 (3C,
--OCH.sub.2CH.sub.2CH.sub.3), 10.60 (2C,
--OCH.sub.2CH.sub.2CH.sub.3), 10.19 (1C,
--OCH.sub.2CH.sub.2CH.sub.3), 10.00 (2C,
--OCH.sub.2CH.sub.2CH.sub.3),
[0071] .sup.13C{.sup.1H} NMR (188.6 MHz, methylcyclohexane-d.sub.4,
-20.degree. C.): .delta.73.28 (--OCH.sub.2CH.sub.2CH.sub.3), 73.13
(--OCH.sub.2CH.sub.2CH.sub.3), 73.06 (--OCH.sub.2CH.sub.2CH.sub.3),
72.07 (--OCH.sub.2CH.sub.2CH.sub.3), 72.03
(--OCH.sub.2CH.sub.2CH.sub.3), 28.68 (--OCH.sub.2CH.sub.2CH.sub.3),
28.27 (--OCH.sub.2CH.sub.2CH.sub.3), 27.74
(--OCH.sub.2CH.sub.2CH.sub.3), 26.70 (--OCH.sub.2CH.sub.2CH.sub.3),
23.96 (--OCH.sub.2CH.sub.2CH.sub.3), 11.03
(--OCH.sub.2CH.sub.2CH.sub.3), 11.01 (--OCH.sub.2CH.sub.2CH.sub.3),
10.64 (--OCH.sub.2CH.sub.2CH.sub.3), 10.39
(--OCH.sub.2CH.sub.2CH.sub.3), 10.14
(--OCH.sub.2CH.sub.2CH.sub.3).
[0072] Preparation of H--Si(111)
[0073] Samples of Si(111) were cut from a single-side polished
wafer into 1.times.1 cm.sup.2 strips. The sized Si(111) strips were
sonicated for 5 minutes in 1,1,1-trichloroethane heated to
60.degree. C., rinsed with cascading water for 5 minutes, sonicated
for 5 minutes in acetone heated to 50.degree. C., rinsed with
cascading water for 5 minutes, sonicated for 5 minutes in
isopropanol heated to 80.degree. C., and rinsed with cascading
water for 5 minutes. Next, the strips were immersed for 15 minutes
in a solution of 5:1:1H.sub.2O:30% NH.sub.4OH:30% H.sub.2O.sub.2 by
volume heated to 80.degree. C., rinsed with cascading water for 5
minutes, and then immersed for 15 minutes in a solution of
5:1:1H.sub.2O:12 M HCl:30% H.sub.2O.sub.2 by volume heated to
80.degree. C. After rinsing with cascading water for 5 minutes, the
Si(111) samples were stored in water until needed. To obtain
hydrogen-terminated Si(111) surfaces, these Si(111) samples were
removed from the water, immersed in a 40% NH.sub.4F solution for 5
minutes, and rinsed for 30 seconds in running water. The wafer
strips were blown dried of any remaining water with the use of a
brisk flow of Ar gas.
Example 1
Deposition of Metal Oxide Films on a Semiconductor
[0074] Zirconia films were grown on either freshly prepared
H--Si(111) or pretreated Si(111). The pretreated wafers were
prepared by immersing freshly prepared H--Si(111) wafers in a
1:4H.sub.2O:n-propanol solution for 90 minutes. Zirconia films were
grown by immersing the Si(111) wafer into a solution containing
Zr.sub.4(OPr.sup.n).sub.16 and H.sub.2O. Solutions containing
Zr.sub.4(OPr.sup.n).sub.16 were prepared in an inert atmosphere
environment by dissolving Zr.sub.4(OPr.sup.n).sub.16 (150 mg, 0.115
mmol) into methylcyclohexane (15 mL) in a 50 mL glass beaker with a
screw top. Solutions containing H.sub.2O were prepared by mixing
H.sub.2O and n-propanol in a 1:4 ratio by volume in a 50 mL glass
beaker.
[0075] Film deposition was achieved as follows. The H--Si(111) or
pretreated Si(111) sample was first brought into an inert
atmosphere environment and then immersed in the methylcyclohexane
solution containing Zr.sub.4(OPr.sup.n).sub.16 for at least 10
minutes with the glass beaker screw top secured to minimize
evaporation and contamination. After the allotted time, the Si(111)
sample was removed and immediately rinsed 3 times with
methylcyclohexane, still inside an inert atmosphere environment.
When the sample had completely dried after the last rinse, the
sample was removed from inside the inert atmosphere environment and
was immediately immersed into the 1:4H.sub.2O:n-propanol solution
for 30 seconds. After the Si(111) sample was removed from this
solution, the sample was dried for 30 seconds using a brisk Ar
flow. Film growth continued when the Si(111) sample was brought
back into the inert atmosphere environment for an additional cycle.
The amount of zirconia deposited was adjusted by repeated
application of the deposition procedure just described. Each such
application is referred to herein as a deposition cycle.
[0076] The zirconia films on silicon were heat treated in a 1-inch
diameter fused quartz tube using a single zone tube furnace
(LINDBERG/BLUE M). The samples were heat treated at 600.degree. C.
for 30 minutes under a flow of Ar gas and allowed to cool to room
temperature under Ar.
[0077] Rutherford Backscattering Spectrometry Analysis of Metal
Oxide Film on a Semiconductor
[0078] Silicon samples with zirconia films were analyzed using a
Van der Graaf accelerator with 2.0 meV .sup.4He+ions and spot
diameter of 2 mm. The RBS chamber was maintained at a pressure of
.about.10.sup.-6 Torr during the experiment. The angle from the
beam to the detector was 30.degree. with both the beam and detector
positioned 15.degree. from the sample surface normal. The stage was
constantly rotated around the surface normal throughout the
experiment to minimize the likelihood of channeling, which may
inadvertently occur with crystalline materials. Acquisition times
of about 30 minutes were typically used.
[0079] Because the zirconia films were not excessively thick, the
density of Zr atoms on Si(111) was calculated from the ratio of the
expressions for the Si height of the step edge and area of the Zr
peak:
(Nt).sub.Zr=(A.sub.Zr/H.sub.Si,0)((.sigma..sub.Si/(.sigma..sub.Zr)(E/[.eps-
ilon..sub.0])
[0080] where (Nt).sub.zr is the areal concentration of Zr in atoms
per cm.sup.2 at the surface, A.sub.zr is the area in counts-channel
of the Zr peak, H.sub.Si,0 is the height in counts of the Si
substrate step edge due to the backscattered He ion from the
topmost layer of the surface, .sigma..sub.Si and .sigma..sub.Zr are
the average differential scattering cross sections between the He
ions and Si or Zr evaluated at the incident energy and
backscattering angle from the incoming beam to the detector, E is
the energy width of a channel, and [.epsilon..sub.0] is the
stopping cross section factor evaluated at the surface for a given
scattering geometry. The area of the Zr peak was calculated using a
Gaussian fit for the Zr peak and a polynomial equation fit for the
background due to post pile-up. Both .sigma..sub.Si and
.sigma..sub.Zr were calculated to have values of
2.816.times.10.sup.-25 cm.sup.-2 and 2.375.times.10.sup.-24
cm.sup.2, respectively, from the Rutherford scattering equation
where the incident energy was 2.0 MeV and the backscattering angle
was 150.degree.. The energy width of a channel, E, was derived from
the channel positions of the Si edge and Zr peak in the spectrum
against the known energy values according to their kinematic
factors for a 2.0 MeV beam. Finally, [.epsilon..sub.0] has a value
of 9.0999.times.10.sup.-14 eV/(atoms/cm.sup.2) and was calculated
using the .sup.14Si electronic stopping power curve equation and
the surface energy approximation. See, for example, Chu, W.-K.,
Mayer, J. W., Nicolet, M.-A. Backscattering Spectrometry; Academic
Press: New York, 1978; and Zeigler, J. F. Helium: Stopping Powers
and Ranges in All Elemental Matter, Pergamon: New York, 1977; Vol.
4.
[0081] Graphs of areal Zr concentration versus number of deposition
cycles is shown in FIG. 3 for four different series of samples,
where each set of points was measured from films deposited on the
same substrate. These samples were obtained by breaking off samples
of the silicon wafer in the course of the deposition process after
a given number of deposition cycles were completed. These results
show how the deposition rate varied from 9.7.times.10.sup.13 to
1.7.times.10.sup.14 Zr atoms/cm.sup.2 per deposition cycle.
[0082] XPS Analysis of Metal Oxide Film on a Semiconductor
[0083] X-ray photoelectron spectra were measured using a
XPS(PHYSICAL ELECTRONICS PHI 5400) spectrometer equipped with a
dual Mg/Al K.alpha. X-ray source as well as a monochromatic Al
K.alpha. X-ray source consisting of a quartz crystal monochromator,
a concentric hemispherical analyzer, and a multichannel detector.
The angle between the dual Mg/Al K.alpha. X-ray source and the
detector was locked at 54.70, while the angle between the
monochromatic Al K.alpha. X-ray source and the detector was locked
at 900. The samples were exposed to air for minimum periods of time
during transport to the XPS facility. The pressure inside of the
XPS analytical chamber remained below 2.times.10-8 Torr during data
collection. The Si(111) wafer was mounted with either the
[2{overscore (1)}{overscore (1)}] or [0{overscore (1)}{overscore
(1)}] directions parallel to the plane of incidence and oriented
toward the detector. The angle from the detector to the surface
normal of the wafer piece was adjusted by rotating the sample
stage.
[0084] X-ray photoelectron spectra were collected either in a
survey mode, using a pass energy of 178.95 eV (1.0 eV/step) with a
binding energy range of 1100 to 0 eV, or in a high-resolution mode,
using a pass energy of 35.75 eV (0.1 eV/step, 100 msec/step) with a
binding energy range focusing on the regions of the Zr 3d,
C.sub.1s,O 1 s, and Si 2p photoemission peaks. The spectra obtained
in the high-resolution mode were standardized to the hydrocarbon
peak at 285 eV in the C 1 s region.
[0085] Four different elements were identified from the survey
scans from their characteristic binding energies: zirconium,
silicon, carbon and oxygen. In high resolution spectra, distinct
peaks could be resolved at 182.3, 102, and 99 eV and were assigned
to Zr 3d electrons from Zr.sub.4+ in zirconia, to Si 2p electrons
from Si.sup.4+ in silica, and to Si 2p electrons from Si.sup.0 in
elemental (bulk) silicon, respectively, by comparison with
published reference values. See, for example, Moulder, J. F.;
Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray
Photoelectron Spectroscopy; Perkin-Elmer: Eden Prairie, Minn.,
1992. The intensity of the peak assigned to Si.sup.4+ increased
relative to the intensity of the peak assigned to elemental (bulk)
silicon after heat treating.
[0086] The relative intensities of selected XPS peaks are plotted
in FIG. 4 as a function of the number of deposition cycles, where
each set of data was measured from films deposited on the same
substrate as described above for FIG. 3. The plot of Zr 3d/Si 2p
(bulk) ratios versus number of cycles reflects the increasing
amounts of zirconia deposited as the number of deposition cycles
was increased. The amount of silica formed appeared to increase
during film growth according to the observed increase in Si 2p
(Si.sup.4+)/Si 2p (bulk) ratios with increasing numbers of
deposition cycles (FIG. 4b). Decreases in the Si 2p (Si.sup.4+)/Zr
3d ratio (FIG. 4c) and the Si 2p (Si.sup.4+)/O 1 s ratio (FIG. 4d)
with increasing number of deposition cycles suggests that silica
was not uniformly distributed in the zirconia film but was instead
concentrated near the silicon/zirconia interface.
Example 2
Electrical Property Measurements of Metal Oxide Film on a
Semiconductor
[0087] Metal oxide semiconductor (MOS) capacitors were fabricated
by sputter deposition of 1000 .ANG. thick Au contacts on the
deposited zirconia film with a top electrode area of 58.times.58
.mu.m.sup.2. The backside ohmic contact was deposited after etching
the oxide with dilute HF and was also 1000 .ANG. thick. A magnetron
sputtering system (AJA INTERNATIONAL) was employed to deposit Au
films at a growth rate of 1-2 .ANG. per second. The current-voltage
(I-V) curves were measured using a HP 4140B pA meter. The
capacitance-voltage (C-V) curves were measured using a HP 4284A
impedence LCR meter at frequencies between 100 Hz and 1 MHz,
usually reported at 100 kHz and 1 MHz for 0.05 V .alpha.-oscillator
strength.
[0088] A comparable metal-insulator-semiconductor (MIS) structure
for gate oxide capacitor is usually used during testing of new
materials for its convenience. The electrical response is similar
to that of the polysilicon gate devices and the ease of fabrication
facilitates obtaining data on the gate oxide material alone. The
basic evaluation of an insulating layer consists of leakage current
and capacitance measurements. See, for example, Degraeve, R.;
Cartier, E.; Kauerauf, T.; Carter, R.; Pantisano, L.; Kerber, A.;
Groeseneken, G. MRS Bulletin 2002, 27, 222. The capacitance-voltage
(C-V) behavior of an MIS structure provides information on the
dielectric properties of the insulating gate oxide as well as its
interface with the metal and underlying Si substrate.
[0089] The leakage current density in accumulation at -1 V was in
the range of 10.sup.-3 to 10.sup.-4 Amps/cm.sup.2 for the zirconia
films on Si(111) grown after 12 to 20 deposition cycles. A typical
current density-voltage is shown in FIG. 5. FIG. 6 shows the
current density-voltage plot of the control sample where a
H--Si(111) was immersed in alternating methylcyclohexane and
H.sub.2O:n-propanol solutions, rinsed with methylcyclohexane, blown
dry with Ar for a total 16 cycles, and then heat treated for 30
minutes at 600.degree. C. in Ar. An experimental procedure was,
used for a control sample processed by following the procedure
described above for deposition of zirconia on H--Si(111) using 16
deposition cycles but replacing the solution of
Zr.sub.4(OPr.sup.n).sub.16 in methylcyclohexane with neat
methylcyclohexane. The current density observed at 1 V was at least
105 times greater than the current density observed for zirconia
films prepared under the same conditions using 16 deposition
cycles.
[0090] FIG. 7 shows a C-V curve of a Si(111) with a zirconia film
grown after 16 cycles and heat treated in Ar. At an applied voltage
of 1.0 V and a frequency of 1 MHz, a capacitance density of 1.7
.mu.F/cm.sup.2 was achieved. The equivalent oxide thickness for the
zirconia film was 2.0 nm. The capacitance density was slightly
higher at lower frequencies, as seen in FIG. 8, but the distortion
at the midway point of the C-V curve was more pronounced. Films
with equivalent oxide thicknesses below 2 nm using the
Zr.sub.4(OPr.sup.n).sub.16 precursor in methylcyclohexane were
typically achieved, but the distortion was almost always present in
the C-V curves and was most likely due to interface traps. See, for
example, Schroder, D. K. Semiconductor Material and Device
Characterization; Wiley: New York, 1990.
[0091] Although the invention has been described and illustrated
with reference to specific illustrative embodiments thereof, it is
not intended that the invention be limited to those illustrative
embodiments. Those skilled in the art will recognize that
variations and modifications can be made without departing from the
true scope and spirit of the invention as defined by the claims
that follow. It is therefore intended to include within the
invention all such variations and modifications as fall within the
scope of the appended claims and equivalents thereof.
* * * * *