Resistor

1. Field of the Invention

The invention relates to a resistor comprising an insulating substrate on which a thin film of chromium silicon is present.

2. Description of the Prior Art

The material CrSi is particularly suitable for resistance layers having a surface resistance of 1-20 k.OMEGA. per square centimeter. Herewith resistors can be made having resistances in the high-ohmic range from 100 k.OMEGA. to 10 M.OMEGA.. The resistivity of CrSi.sub.x varies with the composition and is approximately 8.times.10.sup.-3 .OMEGA.cm in a composition having approximately 30 at.% Cr.

Such a resistor is known inter alia from an article by R. K. Waits in J. Vac. Sci. Techn. 6, 308-315 (1969). The most usual method of manufacturing said resistor is by sputtering the Cr-Si resistance material on the substrate which usually consists of ceramic material.

For the practical application of the compound in a resistance layer, the value of x may vary from 1-5.

A disadvantage of these resistors is that the resistance varies considerably at a temperature of 150.degree. C., for example between +3.5 and +8% after 1,000 hours.

It is therefore an object of the invention to produce an improved stability of these chromium silicon resistors.

The resistor according to the invention is characterized in that the CrSi.sub.x layer comprises nitrogen as a dopant.

When the dopant is present throughout the layer thickness, this is in a quantity of at least 1 at.% and at most 10 at.%.

As a result of said doping the variation of the resistance value has been reduced to less than 1% after 1,000 hours at 150.degree. C.

A disadvantage of this doping is that the temperature coefficient of the resistor in the temperature range of -55.degree. to +150.degree. C. becomes from weakly positive for the undoped CrSi.sub.x to rather strongly negative (up to approximately -200.times.10.sup.-6 /.degree.C.) for the nitrogen-doped material. This high temperature coefficient can be increased to above -100.times.10.sup.-6 by ageing at a temperature of approximately 450.degree. C.

According to a further elaboration of the invention the CrSi-layer has a nitrogen doping in at least one thickness zone, on the outside and/or the side adjoining the substrate, in combination with a non-doped zone.

The advantage of this layer construction is that with a suitable mutual ratio of the layer thicknesses the temperature coefficient of the resistor (TCR) of the layer combination can be adjusted between 0 and -100.times.10.sup.-6 /.degree.C., while the stability in the case of two nitrogen-doped layers is equally good as that of a layer doped with nitrogen throughout its thickness and, in case only one layer is present, said stability is reasonably approached.

The nitrogen-doped layers on each side of the non-doped layer have a thickness of, for example, 30 nm, while the overall thickness of the layer may be, for example, 70-1,000 n.m. The nitrogen content of these doped layers is approximately 50 at.%. An insulating layer is formed so that it is assumed that Cr-Si-nitrides are formed.

For the manufacture of the resistors according to the invention, a layer is provided from a target of chromium silicon on the substrate by means of sputtering in an atmosphere of an inert carrier gas (for example, argon) with such a nitrogen pressure, dependent on the sputtering current and the filling of the sputtering device, that 1-10 at.% nitrogen is incorporated in the deposited material.

The addition of nitrogen to the sputtering atmosphere results in an increase of the resistance and a decrease of the variation after ageing at 350.degree. C. At the nitrogen pressure at which the resistance value starts increasing noticeably, the temperature coefficient of resistance decreases and the resistance value becomes more stable. Too large an increase of the nitrogen pressure causes a non-reproducible resistance value to be obtained in this method. At a sputtering current of 0.5 A the maximum usable nitrogen pressure is approximately 3.3.times.10.sup.-2 Pa (2.5.times.10.sup.-4 Torr). At a nitrogen pressure of approximately 2.times.10.sup.-2 Pa (1.5.times.10.sup.-4 Torr) it is possible to manufacture a resistor having a TCR beneath 100.times.10.sup.-6 /.degree.C. and a variation of at most 0.1% after being kept at 150.degree. C. for 80 hours.

In order to manufacture the resistors according to the preferred embodiment, the substrates are first subjected to a sputtering process with a Cr-Si-plate in an atmosphere of the inert carrier gas to which nitrogen has been added, the nitrogen addition is then discontinued while the sputtering in the undoped carrier gas proceeds and finally nitrogen is again added to the carrier gas.

For illustrating the invention, the manufacture of a series of resistors will now be described.

Resistors having a uniform Cr-Si-N resistance layer.

A quantity of approximately 35,000 ceramic rods having a diameter of 1.7 mm and a length of 6.5 mm were provided in a sputtering device with a sputtering plate of Cr-Si of a composition 28 at.% Cr and 72 at.% Si.

The device was first evacuated and then a mixture of argon gas and nitrogen was introduced at a pressures of 0.2 Pa (1.5.times.10.sup.-3 Torr) and 0.02 Pa (1.5.times.10.sup.-4 Torr), respectively.

The sputtering was carried out for 15 minutes with a current of 0.5 A and a voltage of -400 Volts on the sputtering plate with respect to the substrates.

The resulting resistors of 3.8 kOhm with a standard deviation of .+-.20% and which were doped with 6 at.% nitrogen were heated at 450.degree. C. for 4 hours. The TCR of the resistors was approximately -90.times.10.sup.-6 /.degree.C.

The resistors were subjected to a test consisting of being kept at 150.degree. C. for 80 hours in air. The variation in the resistance value resulting from this test was less than 0.1%.

A quantity of approximately 35,000 ceramic rods of the same dimensions as in Example 1 were provided in the same sputtering device.

After evacuating the device a mixture of argon and nitrogen was introduced at pressures of 0.2 Pa (1.5.times.10.sup.-3 Torr) and 1.06.times.10.sup.-3 Pa (8.times.10.sup.-4 Torr), respectively. The sputtering was carried out at a current strength of 1A and a voltage of -400 V on the sputtering plate with respect to the substrates for 71/2 minutes. The nitrogen was then omitted from the gas current and sputtered in an atmosphere of only argon at a pressure of 0.2 Pa (1.5.times.10.sup.-3 Torr). The sputtering in said atmosphere with a current strength of 0.4A was continued for 10 minutes. Finally nitrogen was again introduced into the gas flow to the same pressure and sputtered with the same current strength and for the same period of time as stated for the first layer. Resistors were obtained with a resistance value of 9.4 kOhm.+-.20%. The TCR of said resistors was -30.times.10.sup.-6 /.degree.C. after ageing at 350.degree. C. for 3 hours. The nitrogen doping in the inner layer and in the outer layer was 50 at.%.

The resistors were subjected to a test by heating them at 150.degree. C. for 160 hours. The variation in the resistance value at a result of said test was 0.1%.

A part of the resistors according to Examples 1 and 2 was completed by providing them with connection caps and wires, trimming them with a laser to values 3 and 7 MOhm respectively and finally painting them. When said resistors were heated at 150.degree. C. for 1000 hours, they showed a variation of 0.85% for resistors of example 1 and 0.75% for resistors of Example 2, respectively.