U.S. patent number 3,650,815 [Application Number 04/863,973] was granted by the patent office on 1972-03-21 for chemical vapor deposition of dielectric thin films of rutile.
This patent grant is currently assigned to Westinghouse Electric Corporation. Invention is credited to Rathindra N. Ghoshtagore, Robert F. Yut.
United States Patent |
3,650,815 |
Ghoshtagore , et
al. |
March 21, 1972 |
CHEMICAL VAPOR DEPOSITION OF DIELECTRIC THIN FILMS OF RUTILE
Abstract
Thin films of titanium dioxide in the rutile form are deposited
by chemical vapor deposition technique on a heated surface of a
substrate by reacting titanium tetrachloride with oxygen, in the
range of temperatures from 700.degree. to 900.degree. C.
Inventors: |
Ghoshtagore; Rathindra N.
(Monroeville, PA), Yut; Robert F. (Pittsburgh, PA) |
Assignee: |
Westinghouse Electric
Corporation (Pittsburgh, PA)
|
Family
ID: |
25342233 |
Appl.
No.: |
04/863,973 |
Filed: |
October 6, 1969 |
Current U.S.
Class: |
427/126.2;
427/126.3; 428/336; 438/785; 423/613; 428/328; 428/472;
427/255.36 |
Current CPC
Class: |
C23C
16/405 (20130101); Y10T 428/256 (20150115); Y10T
428/265 (20150115) |
Current International
Class: |
C23C
16/40 (20060101); C23c 011/00 (); C23c
013/00 () |
Field of
Search: |
;117/106,107,107.1,17.2R
;23/202 |
References Cited
[Referenced By]
U.S. Patent Documents
|
|
|
3373051 |
March 1968 |
Ting Li Chu et al. |
|
Other References
Powell, C. F. et al., Vapor Deposition, John Wiley & Sons,
N.Y., 1966, p. 426..
|
Primary Examiner: Kendall; Ralph S.
Assistant Examiner: Glynn; Kenneth P.
Claims
We claim as our invention:
1. A process for growing the rutile form of titanium dioxide on a
heated surface of a substrate comprising the steps of;
a. heating a surface of a substantially nonoxidizable substrate to
an elevated temperature of from 400.degree. to 1,100.degree. C.;
and
b. passing a gaseous mixture of oxygen and titanium tetrachloride
over the heated surface whereby the titanium tetrachloride reacts
with the oxygen at the heated surface to produce titanium dioxide
which is then deposited on the heated surface in the rutile form,
the minimum partial pressure of oxygen in the reactant gas mixture
having a relationship to the partial pressure of titanium
tetrachloride contained therein expressed by the formula
P.sub.O = 1.8 .times. 10.sup.3 (P.sub.TiCl ).sup.3
wherein the partial pressures are in mm. Hg, and the partial
pressure of titanium tetrachloride being from 0.028 to 0.90 mm.
Hg.
2. The process of claim 1 wherein
the reactant gas mixture includes a carrier gas selected from the
group consisting argon, neon, krypton, and helium.
3. The process of claim 1 wherein
the partial pressure of oxygen in the reactant gas mixture is one
atmosphere, and
the partial pressure of titanium tetrachloride ranges from 0.028
mm. Hg to 0.9 mm. Hg.
4. The process of claim 1 wherein
the partial pressure of titanium tetrachloride is from 0.058 mm. Hg
to 0.232 mm. Hg.
5. The process of claim 1 wherein
the surface of the substrate is heated to a temperature of from
about 700.degree. to about 900.degree. C.
6. The process of claim 1 wherein
the surface of the substrate is heated to a temperature of from
700.degree. to 850.degree. C., and
the ratio of the partial pressure of titanium tetrachloride to the
partial pressure of oxygen in the reactant gas mixture is less than
1.16 .times. 10.sup..sup.-1, the partial pressure of titanium
tetrachloride being from 0.028 to 0.90 mm. Hg.
7. The process of claim 6 wherein
the thin film of titanium dioxide grown on the heated surface of
the substrate is one grain in thickness.
8. The process of claim 1 wherein
the surface of the substrate is heated to an elevated temperature
no greater than 850.degree. C., and
the thin film of titanium dioxide grown on the heated surface of
the substrate is one grain in thickness.
9. The process of claim 1 wherein
the surface of the substrate is heated to a temperature of
approximately 800.degree. C.,
the thin film of titanium dioxide is grown to a thickness of at
least 100 A., and
the major preferred orientation of titanium dioxide crystallites
parallel to the surface of the substrate is (301) and the major
preferred orientation of the c-axes of the titanium dioxide crystal
with respect to the substrate is 78.5.degree..
10. The process of claim 9 wherein the substrate is made of
silicon.
11. The process of claim 1 wherein
the substrate is made of a material selected from the group
consisting of silicon, magnesium oxide, quartz, and aluminum
oxide.
12. The process of claim 1 wherein
the temperature of the substrate surface is 700.degree. C.;
the partial pressure of oxygen in the reactant gas mixture is one
atmosphere;
the partial pressure of titanium tetrachloride in the reactant gas
mixture is at least 0.058 and does not
the minimum average reactant gas mixture volume flow is the
equivalent of 12.5 liters per minute for a 54 mm. I.D. quartz
reactor tube.
13. The process of claim 1 wherein
the partial pressure of titanium tetrachloride is 0.116 mm. Hg;
and
the minimum average reactant gas mixture volume flow is the
equivalent of 6.5 liters per minute for a 54 mm. I.D. quartz
reactor tube.
14. The process of claim 1 wherein
the partial pressure of titanium tetrachloride is 0.232 mm. Hg;
and
the minimum average reactant gas mixture flow is the equivalent of
3.2 liters per minute for a 54 mm. I.D. quartz reactor.
15. The process of claim 1 wherein
the temperature of the substrate surface is 800.degree. C.;
the partial pressure of titanium tetrachloride in the reactant gas
mixture is at least 0.058 and does not exceed 0.90 mm. Hg, the
partial pressure of oxygen in the reactant gas mixture being one
atmosphere; and
the minimum average reactant gas mixture volume flow is the
equivalent of 51.0 liters per minute for a 54 mm. I.D. quartz
reactor.
16. The process of claim 1 wherein
the partial pressure of titanium tetrachloride is 0.116 mm. Hg;
and
the minimum average reactant gas mixture flow is the equivalent of
26.0 liters per minute for a 54 mm. I.D. quartz reactor.
17. The process of claim 1 wherein
the partial pressure of titanium tetrachloride is 0.232 mm. Hg;
and
the minimum average reactant gas mixture flow is the equivalent of
13.0 liters per minute for a 54 mm. I.D. quartz reactor.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to dielectric films of rutile titanium
dioxide suitable for use as active and passive components in
various electrical devices and particularly in solid state
devices.
2. Description of the Prior Art
Crystalline titanium dioxide in the rutile crystallographic texture
is known to have one of the highest dielectric constants of the
TiO.sub.2 textures. Both physical and chemical vapor deposition
techniques have been used for the preparation of titanium dioxide
films. The physical techniques employed have included the
evaporation of titanium followed by oxidation; reactive sputtering
of titanium in oxygen; and radio frequency sputtering of titanium
dioxide. The pyrolysis of organotitanium esters (e.g.,
tetraisopropyl titanate), the hydrolysis of titanium tetrachloride,
and the anodization of of titanium films are known chemical
deposition techniques for titanium dioxide. However, each of these
techniques has at least one undesirable feature in producing a
layer of device quality titanium dioxide. Some of these desirable
features are a lack of flexibility and capacity for device
production purposes, contamination particularly from residual
hydrogen or water vapor, high porosity of the film grown and lack
of control of the stoichiometry of the chemical reaction and the
consequent formation of the undesirable crystalline phases of
titanium dioxide (anatase and brookite).
An object of this invention is to provide a process for producing
titanium dioxide thin films which do not have the undesirable
features of prior art titanium dioxide thin films.
An object of this invention is to provide a process for producing a
thin film of rutile TiO.sub.2 for electrical devices.
Another object of this invention is to provide a process for
producing a thin film of rutile on a suitable substrate by the
chemical reaction of titanium tetrachloride with oxygen in a
predetermined temperature range.
Other objects of this invention will, in part, be obvious and will,
in part, appear hereinafter.
SUMMARY OF THE INVENTION
In accordance with the teachings of this invention there is
provided a process for growing a thin film of rutile on a heated
surface of a substrate. The thin film of rutile is produced by
heating a surface of the substrate to a temperature range of about
400.degree. to 1,100.degree. C. and passing a reactant gas mixture
of titanium tetrachloride and oxygen over the heated surface. The
titanium tetrachloride and the oxygen chemically react with each
other on the heated surface and deposit rutile on the heated
surface of the substrate.
DRAWING
For a better understanding of the nature and objects of this
invention reference should be had to the graphical representation
of the temperature dependence of the deposition rate of rutile at 1
atmosphere of oxygen partial pressure and different titanium
tetrachloride partial pressures.
DESCRIPTION OF THE INVENTION
Rutile is produced by the reaction of titanium tetrachloride and
oxygen and grown on a substrate surface in accordance with the
following chemical equation: TiCl.sub.4 (g) + O.sub.2 (g) .fwdarw.
TiO.sub.2 (rutile) (solid) + 2 Cl.sub.2 (gas) To provide source
material of the highest purity, multiple distillation of the
titanium tetrachloride is performed. The multiply distilled
titanium tetrachloride is placed in a bubbler apparatus where it is
maintained at a predetermined constant temperature. A temperature
of 25.degree. C. has been found to be suitable.
A sufficient volume of a carrier gas of oxygen or at least one
inert gas selected from the group consisting of argon, neon,
krypton, and helium or a gaseous mixture of oxygen and the inert
gas is caused to flow through the bubbler containing the titanium
tetrachloride to provide the required minimum partial pressure of
titanium tetrachloride for the chemical reaction of the process and
then passed into a reactor chamber. Before entering the reactor,
the carrier gas is joined with oxygen gas in an amount necessary
for the chemical reaction of the process and a reactant gas mixture
is produced which is caused to flow over and about a substrate
disposed on a suitable susceptor and heated within the reactor. The
reactor chamber may be made of quartz. Although silicide coated
graphite is suitable for making he susceptor, it is desirable that
the susceptor be quartz encapsulated graphite to minimize the
contamination of the substrate by outgasing of the graphite and
rapid consumption or erosion of the susceptor by oxidation.
The substrate may be made of any suitable material which is not
adversely affected by oxidation. Materials such, for example, as
silicon which will be slightly oxidized in the process, are
appropriate as their slightly oxidized surface does not adversely
affect the adherence of rutile to the substrate. Such substrates
are considered to be substantially non-oxidizable. Other suitable
substrate materials are magnesium oxide, quartz, and aluminum
oxide. The substrate is heated to a temperature of from about
400.degree. C. to about 1,100.degree. C. The preferred heating
range for the substrate is 700.degree. to 900.degree. C. to produce
the highest deposition rates of rutile. Rutile is formed below
400.degree. C. and above 1,100.degree. C. but either the rate of
growth or the nature of the thin film grown is undesirable.
Should the substrate be silicon and the surface upon which the
rutile is to be deposited is to be oxide free, prior to deposition
of rutile thereon the silicon substrate is baked at a temperature
of from 900.degree. to 1,000.degree. C. in hydrogen gas flowing at
a rate of 10 liters per minute for a reactor tube of an internal
diameter of 54 mm.
It is desirable that the thin film of rutile grown on a substrate
surface should be only one grain in thickness but the thickness of
the film should be closely controllable. Therefore, for practical
purposes the chemical reaction producing rutile should be carried
on at a temperature of no greater than 850.degree. C. At
temperatures of 850.degree. C. and lower, the deposit of the grown
rutile is single grain in thickness. Above 850.degree. C. the film
is multiple grain in thickness which is undesirable since the
electrical properties of the grown film are adversely affected
thereby. However, rutile deposits which have a physical structure
exhibiting a multiple grain thickness are suitable up to a
predetermined higher temperature. It has been found that the
temperature at which the reaction occurs to produce rutile alone
should not exceed 900.degree. C. by a significant amount. As the
temperature exceeds 900.degree. C., the proportion of the other
crystalline phases of TiO.sub.2 increases accordingly.
The rate of deposition of rutile increases, first order equation,
with an increase of partial pressure of titanium tetrachloride with
1 atmosphere of oxygen, at all temperatures. The peak deposition
rate for all partial pressures of TiCl.sub.4 with oxygen at 1
atmosphere, is about 850.degree. C. Above 850.degree. C., the rate
of deposition of rutile decreases linearly with temperature
increase, but the slope of the deposition rate curves decreases
more greatly with decreasing partial pressure of titanium
tetrachloride.
In the chemical reaction producing rutile in accordance with the
teachings of this invention, the relationship of the minimum
partial pressure of oxygen to the partial pressure of titanium
tetrachloride in the reactant gas mixture to provide for a
deposition rate independent of the oxygen present, is P.sub.O = 1.8
.times. 10.sup.3 (P.sub.T .sub.Cl cl ).sup.3. The partial pressures
are in mm. Hg. The reaction is dependent solely on the temperature
as shown in the graph.
In the reactant gas mixture the partial pressure of titanium
tetrachloride may vary from as low as 0.018 mm. Hg to as high as
0.9 mm. Hg for an oxygen partial pressure of one atmosphere.
However, controlled deposition thickness is best achieved when the
partial pressure of titanium tetrachloride in the reactant gas
mixture is from about 0.058 mm. Hg to 0.232 mm. Hg for a partial
pressure of oxygen of one atmosphere. The deposit thickness of
rutile may be controllably achieved at these partial pressures at
temperatures of from 700.degree. to 900.degree. C. at a deposition
rate of from 50 to 800 A. per minute.
The lateral grain size of the polycrystalline rutile film grown
varies with both the film thickness and the temperature of
deposition but attains a constant size above 900 A. of film
thickness. The final grain sizes are 1,500 A. at 800 C., 3,300 A.
at 700.degree. C. and 2,700 A. at 900.degree. C.
Reproducible and uniform rutile thin film growth rates on a heated
surface of a silicon substrate are achieved only above a minimum
gas flow rate at a given temperature and a given partial pressure
of the reactant gases. For example for a 54 mm. I.D. quartz reactor
tube at a specific temperature and titanium tetrachloride partial
pressure the minimum total gas flow rate is as shown in Table I.
---------------------------------------------------------------------------
TABLE I
Substrate TiCl.sub.4 Partial Minimum Surface Pressure* Volume Gas
Flow Temperature (mm. Hg) (liters/minutes)
__________________________________________________________________________
700.degree. C. 0.058 12.5 700.degree. C. 0.116 6.5 700.degree. C.
0.232 3.2 800.degree. C. 0.058 51.0 800.degree. C. 0.116 26.0
800.degree. C. 0.232 13.0
__________________________________________________________________________
Note: *Partial pressure of oxygen is one atmosphere
As indicated by the tabulated results, an increase in the partial
pressure of the titanium tetrachloride resulted in a decrease in
the minimum average volume gas flow necessary for reproducible and
uniform rutile thin film growth rates.
At any fixed titanium tetrachloride partial pressure and at all
temperatures below about 850.degree. C. for the heated surface of
the substrate, the growth rate of the thin film of rutile is
constant only above a minimum oxygen partial pressure. For a
partial pressure of 0.058 mm. Hg for titanium tetrachloride the
minimum partial pressure of oxygen required is 3.5 .+-. 0.3 .times.
10.sup..sup.-1 mm. Hg. At a partial pressure of 0.116 mm. Hg for
titanium tetrachloride, the minimum partial pressure of oxygen
required is 2.7 .+-. 0.2 mm. Hg. A minimum partial pressure of
oxygen of 20 .+-. 5 mm. Hg is required for a constant growth rate
of a thin film of rutile for a constant partial pressure of
titanium tetrachloride of 0.232 mm. Hg. Below these oxygen partial
pressure the rutile film deposition rate at any titanium
tetrachloride partial pressure decreases as the one half power of
the partial pressure of oxygen (P.sub.O .sup.1/2).
Stoichiometric rutile can only be deposited below a partial
pressure ratio of titanium tetrachloride to oxygen of 1.16 .times.
10.sup..sup.-1 at any temperature between 700.degree. and
850.degree. C.
The graphical representation in the specification shows the
temperature dependence of rutile deposition rate at one atmosphere
of oxygen partial pressure and different titanium tetrachloride
partial pressures.
To be effective as a dielectric film of material, the grown rutile
should be at least 100 A. in thickness to assure complete coverage
of the surface of the substrate upon which it is deposited.
Electron diffraction studies indicate that all the thin films of
rutile grown on the various substrates have a fiber texture and a
preferred orientation which is determined by the temperature of the
substrate upon which is deposited. Table II tabulates the results
of experiments in which thin films of rutile were grown in
accordance with the teachings of this invention on silicon
substrates whose surface temperature was varied. ##SPC1##
The most desirable material is produced at 800.degree. C. since the
c-axis of the grown rutile is 78.5.degree. with respect to the
silicon surface upon which it was grown. The relative dielectric
constant is the highest approaching density for pure rutile. The
refractive index is also the largest being 2.83.sup.+ . Other
desirable films are those grown at 457.degree. C., 515.degree. C.,
600.degree. C., and 700.degree. C. The two remaining growth
temperatures, namely 400.degree. and 900.degree. C., produce a
suitable rutile thin film but they have the lowest refractive index
and the lowest relative dielectric constant when compared to
air.
In all instances in the temperature range of from about 400.degree.
to about 1,100.degree. C., the adherence of the thin film of rutile
on a heated surface of the substrate was very good and uniform
polycrystalline rutile thin film resulted. In the preferred
temperature range of 700.degree. to 900.degree. C., the most
desirable rutile thin film properties were obtained. These were
such, for example:
Dielectric constant .about. 50-100
Dielectric loss .about. 0.1 at 50 kc.; 0.05 at 500 kc.
Thickness - 100 A. to greater than 1 micron
Grain size - 0.15 to 0.33 micron
Porosity - substantially zero percent
Refractive Index - 2.6 to 2.9
Although specific reference has been made to thin films of rutile
grown on silicon, the same properties are evident when rutile is
grown on silicon dioxide films and other substrate surfaces in
accordance with the teachings of this invention.
Metal-insulator-semiconductor devices of Al-SiO.sub.2 -TiO.sub.2
(rutile)-SiO.sub.2 -Si, Al-TiO.sub.2 (rutile)-SiO.sub.2 -Si,
Al-SiO.sub.2 -TiO.sub.2 (rutile)-Si, and Al-TiO.sub.2 -Si
structures have been successfully prepared and show acceptance and
full incorporation of rutile as a component layer in the
structures. The silicon wafers employed were of both n and P-type
material and both types employed had resistives as low as 0.01
ohm-cm. and as high as 20 to 40 ohm-cm.
Titanium dioxide produced in accordance with the teachings of this
invention is the rutile form and has no hydrogen entrapped within
the film as is the case in prior art processes using hydrogen as a
reducing gas. Consequently the thin films of rutile grown in
accordance with the teachings of this invention are much superior
to prior art thin films as far as electron charge stability is
concerned. The detrimental effect of protons in prior art rutile
thin films is therefore avoided.
* * * * *