U.S. patent number 3,664,943 [Application Number 04/874,013] was granted by the patent office on 1972-05-23 for method of producing tantalum nitride film resistors.
This patent grant is currently assigned to Oki Electric Industry Company Limited. Invention is credited to Toshiaki Koikeda, Shun Kumagai.
United States Patent |
3,664,943 |
Kumagai , et al. |
May 23, 1972 |
METHOD OF PRODUCING TANTALUM NITRIDE FILM RESISTORS
Abstract
Hexagonal type tantalum nitride films are made by reactive
sputtering with a sputtering current of more than 0.25
m.A./cm.sup.2 at a sputtering voltage of more than 4.5 K.V. in a
nitrogen gas atmosphere in which the mixed gas pressure of argon
and nitrogen is 0.8 .times. 10.sup.-.sup.2 torr to 3 .times.
10.sup.-.sup.2 torr and the partial pressure of nitrogen gas is 5
.times. 10.sup.-.sup.5 torr to 10.sup.-.sup.3 torr, the substrate
to receive the film being presputtered for a time of not more than
one hour while being heated to a temperature of about
550.degree.-770.degree. C., which temperature is maintained during
the main sputtering.
Inventors: |
Kumagai; Shun (Tokyo,
JA), Koikeda; Toshiaki (Tokyo, JA) |
Assignee: |
Oki Electric Industry Company
Limited (Tokyo, JA)
|
Family
ID: |
12838441 |
Appl.
No.: |
04/874,013 |
Filed: |
November 4, 1969 |
Foreign Application Priority Data
|
|
|
|
|
Jun 25, 1969 [JA] |
|
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44/49700 |
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Current U.S.
Class: |
204/192.15 |
Current CPC
Class: |
C23C
14/0036 (20130101) |
Current International
Class: |
C23C
14/00 (20060101); C23c 015/00 () |
Field of
Search: |
;204/192 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Mack; John H.
Assistant Examiner: Kanter; Sidney S.
Claims
What is claimed is:
1. A method of making by reactive sputtering films of tantalum
nitride having a predominantly hexagonal crystalline structure and
exhibiting a temperature coefficient of substantially zero, which
comprises the steps of providing within a chamber a tantalum
cathode in spaced relation to an anode supporting the substrate to
be coated with said film, introducing into said chamber an inert
atmosphere under a pressure of 0.8 .times. 10.sup..sup.-2 to 3
.times. 10.sup..sup.-2 torr, composed of argon and nitrogen with
sufficient nitrogen to give a partial pressure of nitrogen of 5
.times. 10.sup..sup.-4 to 10.sup..sup.-3 torr, presputtering said
substrate while shielded for a time not exceeding 1 hour, while
heating said substrate to a temperature of about
550.degree.-700.degree. C., and while maintaining the substrate
temperature within said range of about 550.degree.-700.degree. C.,
applying across said anode and cathode an electrical potential of
at least about 4.5 K.V. with a current of at least about 0.25
M.A./cm.sup.2 to initiate sputtering to said cathode with
consequential deposition of tantalum nitride upon said
substrate.
2. The method of claim 1 wherein said sputtering voltage is at
least 5 K.V. and said sputtering current is at least 0.3
M.A../cm.sup.2.
3. The method of claim 1 wherein said presputtering is carried out
for at least about 0.5 hour.
Description
This invention relates to an improved method of making tantalum
nitride film resistors by a cathodic sputtering and more
particularly to a producing method improved by noting the degree or
temperature coefficient at which the resistance value of a tantalum
nitride film resistor fitted in a device circuit or the like varies
reversible in response to the variation of its using
temperature.
An object of the present invention is to provide a method of
obtaining high reproducability a resistor having a zero or very low
temperature coefficient as often required with a view to cancelling
it with a temperature corfficient with another element in a
communication device or the like.
Another object of the present invention is to provide a method
wherein the relation between the presputtering time and temperature
coefficient is solved and the presputtering time can be reduced and
further substantially omitted.
In the accompanying drawings:
FIG. 1 is a partly sectional elevation of a cathodic sputtering
apparatus used in a conventional producing method;
FIGS. 2 and 3 are temperature coefficient characteristic diagrams
of a tantalum nitride film resistor by a conventional producing
method;
FIG. 4 is a schematic view of a cathodic sputtering apparatus to be
used in the present invention;
FIG. 5 if a temperature coefficient-presputtering time
characteristic diagram for explaining the method of the present
invention;
FIGS. 6 to 9 are curve diagrams showing various characteristics of
a tantalum nitride film resistor by the producing method of the
present invention.
A tantalum nitride film resistor is generally made by using such
sputtering apparatus as is shown in FIG. 1.
It shall be explained with reference to the drawing. An argon gas
of about 10.sup..sup.-2 torr containing a small amount of nitrogen
gas is introduced into a vacuum chamber 1. A voltage is impressed
between a cathode 2 formed of tantalum which is to be a mother
material and an anode 3 from a high voltage electric source 4 so
that a glow discharge may be caused. The tantalum sputtered in case
plus ions in the atmosphere produced thereby are accelerated to
collide with the anode 2 is deposited on an opposed substrate 5 and
is made to react with nitrogen in an active state in such case to
form a tantalum nitride film. A resistor pattern is formed by using
a mask at the time of sputtering or by photoetching after
sputtering. 6 in FIG. 1 is a substrate supporter.
Thereafter, it is heat-treated in air in order to inhibit the
variation with the lapse of time and the resistance value is
adjusted by an anodizing technique in order to eliminate
fluctuation of the initial resistance value.
The thus produced tantalum nitride film resistor has substantially
satisfied the requirements of communication devices in the
precision of its initial resistance value and the absence of
substantial variation with the lapse of time but the third factor,
that is, the temperature coefficient of the resistor was -70 to
-100 p.p.m. in the plateau wherein reproducibility was guaranteed
in daily production.
As its characteristics are shown in FIG. 2 by taking the partial
pressure of the nitrogen gas on the abscissa, the present
applicants have clarified it by a plasma sputtering method in which
not only the partial pressure of nitrogen shown in FIG. 2 but also
the substrate temperature in the producing process contributes to
the temperature coefficient of the tantalum nitride film resistor
and have further given such suggestion even in cathodic sputtering
but no method high in the reproductivity has yet come to be
established.
Particularly, in a method of making a tantalum nitride film
resistor by cathodic sputtering (or plasma sputtering alike), as
different from ordinary evaporation deposition, in order to secure
the reproductivity of the temperature coefficient and to
secondarily reduce the temperature coefficient, a shielding plate
to shield the substrate prior to sputtering is provided and a
pretreating process to coat this shielding plate is provided which
is called presputtering.
Such presputtering has been carried out for several minutes in an
initial method of making tantalum nitride film resistors. However,
as the requirements for the temperature coefficient from
communication devices or the like have become strict in recent
years, the relation between the temperature coefficient and
presputtering time has been made clear. According to a report, as
shown in FIG. 3, in order to secure the reproductivity of the
temperature coefficient, a presputtering time of 8 to 10 hours in
required. Generally various different reports have been made.
However, at least 4 to 5 hours have been required. In some case,
there has been heard on opinion that 10 or more hours are required.
By the way, in FIG. 3, an area resistance value is also
mentioned.
The present invention shall be explained in detail in the
following.
FIG. 4 shows a cathodic sputtering apparatus used in the present
invention. 11 is a vacuum chamber called a bell jar. 12 is a
cathode formed of tantalum and is a target. 13 is an anode which is
also a substrate supporting platform. 14 is a high voltage electric
source for impressing a high voltage on the above mentioned cathode
12 anode 13. 15 is a substrate. 16 is a gas introducing pipe for
introducing a gaseous mixture of nitrogen and argon. 17 is a
thermocouple or other temperature sensing means for detecting the
temperature of the substrate 15. 18 is a heater for heating the
substrate 15 through the anode 13. 19 is a meter for indicating the
temperature detected by the thermocouple 17 and may be also a
controlling system for the heater 18 as required. 20 is an electric
source terminal for the heater 18. 21 is an exhaust port. Further,
as it is difficult to detect the accurate temperature of the
substrate 15 itself and the anode 13 and substrate 15 are fitted to
each other, they can be considered to be substantially at the same
temperature. Therefore, in fact, the temperature of the anode 13
was detected and was made the temperature of the substrate 15.
In the apparatus in FIG. 4, the vacuum chamber 11 is exhausted with
a vacuum exhaust system (not illustrated) connected to the exhaust
port 21. The vacuum exhaust system to be used here consists of a
rotary pump and oil diffusing pump having ordinary performances.
The finally reached vacuum degree was 10.sup..sup.-6 torr.
Then a gaseous mixture of about 10.sup..sup.-2 torr or preferably
0.8 .times. 10.sup..sup.-2 to 3 .times. 10.sup..sup.-2 torr was
introduced into the vacuum chamber 11 and the substrate 15 was
shielded with a shielding plate (not illustrated) and was
presputtered for 30 minutes to 1 hour. Simultaneously with these
operations, the substrate 15 was heated with the heater 18 to
elevate the temperature of the substrate 15 to a desired value.
Such reduction of the presputtering time in the present invention
was made on the basis of the following actual results. Further,
according to the present invention, presputtering for more than 1
hour is not detrimental but has no special advantage.
That is to say, prior to the completion of the present invention,
the present inventors investigated the relation between the
temperature coefficient and substrate temperature and obtained such
results as are exemplified in FIG. 5.
In FIG. 5 wherein no means of consciously heating the substrate was
provided and only the presputtering time was varied under the fixed
conditions of a mixed gas pressure of 1.5 .times. 10.sup..sup.-2
torr, partial pressure or nitrogen of 1.4 .times. 10.sup..sup.-4
torr, sputtering voltage of 5 K.V. and sputter-current of 0.5
m.A./cm.sup.2., there is shown a relation between the substrate
temperature just after the presputtering, that is, just before the
main sputtering and the temperature coefficient of a tantalum
nitride film resistor of a film thickness of 800 A. formed by the
main sputtering and treated properly.
The curve of the temperature coefficient in FIG. 5 and the relation
between the presputtering time and temperature coefficient when the
substrate temperature was elevated to 400.degree. C. were
investigated. An example of the results was as shown in FIG. 6 and,
with a presputtering time of 30 minutes, the reproductivity of the
temperature coefficient could be well secured. Further, in FIG. 6,
the characteristics of the specific resistance are also shown.
Such fact was recognized by the present inventors to be as
follows.
That is to say, in the conventional presputtering, in the initial
period, the cathode surface, that is, target surface is cleaned to
remove the oxide and deposited gases and mostly to activate the
surface so that the sputtering from the surface may be stable. When
such initial atmosphere stabilizing time has passed, the time will
be spent for the substrate temperature to rise and balance and will
be a substrate temperature stabilizing time during which the
reproductivity of the temperature coefficient is secured.
Therefore, if the substrate temperature is kept at a determined
value in advance, the presputtering time will be reduced. Further,
in a continuous apparatus wherein the substrate and therefore also
the anode accompanying it can be continuously replaced as different
from the batch process apparatus shown in FIG. 4, if a means of
heating the substrate is provided separately, the presputtering
time will become substantially unnecessary.
Now, in FIG. 4, after such presputtering as is described above, the
shielding plate is removed and the main sputtering is started to
form a tantalum nitride film on the substrate 15.
During this main sputtering, the substrate temperature usually
tends to rise. Therefore, it is preferable to provide a means of
keeping the substrate temperature constant, but as a usually
required film thickness of about 600 to 100 A. is formed in several
minutes to 10 and several minutes, the temperature rise will be so
small that no necessity of providing a special means is seen in
practice.
In the stage of the main sputtering, the temperature coefficient is
so closely relates with such other conditions as the partial
pressure of nitrogen and the main sputtering starting temperature
that the main sputtering in the present invention is carried out
under the conditions of a partial pressure of nitrogen of 5 .times.
10.sup..sup.-5 torr to 10.sup..sup.-3 torr and main sputtering
starting temperature of 550.degree. - 700.degree. C.
FIGS. 7 and 8 show these relations. These characteristics were
measured under the conditions of a mixed gas pressure of 1.5
.times. 10.sup..sup.-2 torr, sputtering voltage of 5 K.V. and
sputtering current of 0.5 m.A./cm.sup.2.
FIG. 7 shows the partial pressure of nitrogen during sputtering and
the electric characteristics of the film in the case of the
producing method according to the present invention, A representing
the temperature coefficient and B representing the specific
resistance. According to it, over a wide range of the partial
pressures of nitrogen of 5 .times. 10.sup..sup.-5 torr to
10.sup..sup.-3 torr, a so-called plateau is formed and a
temperature coefficient of zero is realized.
FIG. 8 is to explain the present invention more particularly. The
temperature coefficients in the plateau are represented by means of
the substrate temperatures (A is for 300.degree. C., B is for
400.degree. C., C is for 500.degree. C., D is for 550.degree. C., E
is for 650.degree. C. and F is for 700.degree. C.) during
sputtering as parameters. According to it, when the substrate
temperature was elevated, the value of the temperature coefficient
reduced as in -150 ppm./.degree. C. at 300.degree. C., about -30
ppm./.degree. C. at 500.degree. C. and 0 ppm./.degree. C. at
550.degree. C. until zero ppm./.degree. C. was obtained. As the
other sputtering conditions than the substrate temperature are the
same in this experiment, it is very effective as a method of freely
controlling the temperature coefficient to thus freely control the
substrate temperature during sputtering. Further, it is found to be
very excellent as a method of making them stably at a high
reproductivity that a wide plateau is covered.
Further, the temperature coefficient depends on the sputtering
voltage and sputtering current during sputtering. They are
correlated with each other and with the mixed gas pressure within
the vacuum chamber. It is difficult to definitely indicate only one
of them. However, they were required to be considerably larger than
were anticipated.
FIG. 9 shows the relation between the sputtering current and
temperature coefficient when the sputtering voltage was made 5
K.V., the partial pressure of nitrogen was made 1.4 .times.
10.sup..sup.-4 torr and the substrate temperature at the time of
starting sputtering was made a parameter.
As illustrated, though the temperature coefficient does not become
remarkably small with only the increase of the sputtering current,
the variation of the temperature coefficient reduces with the
variation of the current value and the reproductivity is
guaranteed. Though the saturation region (the region of a current
value of more than 0.3 m.A./cm.sup.2. in this example) guaranteeing
the reproductivity varies more or less with the setting of the
mixed gas pressure and sputtering voltage, a sputtering current of
at least 0.25 m.A./cm.sup.2. is required for the saturation
region.
The relation between the sputtering voltage and temperature
coefficient is quite similar to the above mentioned relation
between the sputtering current and temperature coefficient. Thus,
is order to secure the reproductivity in the temperature
coefficient, a sputtering voltage of at least 4.5 K.V. or
preferably 5 K.V. is required.
Further, as both sputtering current (See FIG. 9) and sputtering
voltage are saturated with their increase irrespective of the
setting of the substrate temperature, in the region under such
sputtering conditions, as seen in the diagram, it is very effective
to elevate the substrate temperature (so that the temperature
coefficient may vary to be -150 p.p.m./.degree. C. at 300.degree.
C., -30 p.p.m./.degree. C. at 500.degree. C. and 0
p.p.m./.degree.C. at 550.degree. C.). Thus a tantalum nitride film
having a temperature coefficient which is zero or small is realized
by elevating and controlling the substrate temperature.
Further, by the observation with an X-ray diffraction and electron
beam diffraction, it is found that a film showing a zero
temperature coefficient in the plateau is mostely of a hexagonal
type and that a film made at a high substrate temperature is also a
stable tantalum nitride film.
Further, as tantalum nitride film resistor producing conditions,
there are a heat-treatment in air after sputtering and a surface
oxide film formation by anodization. However, their influences on
the tantalum nitride film made by the producing method of the
present invention are not different from those on any conventional
one. The stability of the tantalum nitride film sputtered by
keeping the substrate temperature high is found to be nothing
inferior to that of any conventional product.
As example shall be explained in the following. In the present
invention, 30 resistors were formed by the producing steps of a
sputtering, electrode evaporation deposition and photoetching on a
substrate made of glazed ceramics of 50 .times. 25. The temperature
coefficient was measured from the temperature difference between
25.degree. and 125.degree. C. The film thickness of the resistor
used for the measurement was about 800 A. Further, the reliability
testing sample to be left at a high temperature was heat-treated in
air at 300.degree. C. for 5 hours after the resistors were formed.
Table 1 shows the results of high temperature aging tests of
tantalum nitride films made by controlling the substrate
temperature to control the temperature coefficient. ##SPC1##
The sample used in this high temperature aging test was a tantalum
nitride film in which the temperature coefficient was controlled to
be of any value by varying only the substrate temperature to be of
any value at a partial pressure of nitrogen 1.4 .times.
10.sup..sup.-4 torr, sputtering current of 200 m.A. and sputtering
voltage of 5 K.V. in the middle of the plateau. It is also found
from this table that, in the range of the temperature coefficient
of this resistance, substantially no difference is seen and that,
by making the temperature coefficient of the resistance zero by
such producing method, the excellent stability of the tantalum
nitride film is not impaired.
With respect to the crystal structure, the film in which the
temperature coefficient was reduced by elevating the substrate
temperature in the plateau was a tantalum nitride film mostly of a
hexagonal type structure.
As evident from the above explanation, in the conventional tantalum
nitride resistor, the temperature coefficient was of such large
value as -100 p.p.m./.degree. C. to -70 p.p.m./.degree. C. in the
plateau, whereas, in the present invention, the temperature
coefficient is freely controlled in the plateau, a tantalum nitride
film having a characteristic of a zero temperature coefficient
which has been impossible heretofore can be attained stably at a
high reproductivity and its effect is high.
* * * * *