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.

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.

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.