ELECTRIC RESISTOR EQUIPPED WITH THIN FILM CONDUCTOR AND POWER DETECTOR

Abstract: The electric resistor of this invention is comprised of a Si-Ge alloy thin film containing amorphous and microcrystal phases which serve as an electric resistance, thereby keeping the resistance value ratio substantially constant and uninfluenced by frequency changes which range from d.c. to 32 GHz. In addition, the power detector of this invention uses a thermocouple which is made by connecting the conductor film with the above-mentioned alloy thin film having great thermoelectric power. The thermocouple is provided with beam lead electrodes at cold junction areas to thereby produce large temperature differences between the hot and cold junctions, so that the thermocouple is provided with a sufficient thermal gradient to detect very low power with high accuracy.

Description: This invention relates to an electric resistor having the features of the generic clause of claim 1. Such an electric resistor is known from the prior art as described in the following paragraph. To make such electric resistors in the microelectric field, a nichrome vaporized film and a tantalum nitride film are used. These thin films feature the capability of photolithography. However, the surface of a substrate used to form a resistor must be fine-grained since the thickness of the films, with good high frequency characteristics, is about 200 Å. These films have another drawback, in that the film quality deterioration temperature is as low as about 200°C. The low deterioration temperature means that the burnout temperature is low. Therefore, the working environment for electric resistors, using conventional thin film resistance materials, has been limited considerably. In regards to the power detector, in Japanese Patent Disclosure No. 84-23263 one of the inventors of this invention has disclosed a thermocouple device made by connecting a mixed crystal film of the amorphous and microcrystal phases with a metal resistance film. A power detector using this thermocouple device is quite capable of stable measurement during high power detection. However, this power detector comprising the thermocouple is too low in sensitivity to perform stable measurement and has poor linearity in the theremoelectro-motive force of the thermocouple in detecting low power, such as 1 µW for example. U.S. Patents Nos. 4835059 and 4766008, that one of the inventors of this invention have filed, discloses a Si-Ge alloy thin film conductor in which the amorphous and microcrystal phases coexist. This thin film conductor, though made of a semiconductor material, has a relatively large conductivity but the temperature coefficient is small like that of metals and also offers a feature that the thermoelectric power (the Seebeck coefficient magnitude) is large like that of semiconductors; therefore, the thermo-electromotive force is large. In the above patent applications, the applicability to electric resistors and power detectors is indicated. Document EP-A-0161556 and document GB-A-2179790, which is not prepublished, disclose a SiGe alloy film conductor in which the amorphous and microcrystal phases coexist. In view of this prior art it is the object of the invention to provide an electric resistor comprised of a thin film resistor that is superior in its photolithography ability and can be used at high temperatures, which is capable of utilizing an insulating substrate, but not necessarily of a fine-grained texture but can consist of almost any optional type material, which displays the characteristics of being applicable to a variety of frequencies, ranging from direct current to a frequency range of 32 GHz and which is easy to manufacture. This object is solved by an electric resistor having the features of Claim 1. Preferred embodiments are disclosed in the dependent claims. By using an alloy thin film conductor chiefly consisting of silicon and germanium, in which the amorphous and microcrystal phases coexist, the temperature coefficient of the conductivity is small and the conductivity is substantially constant from direct current to frequencies up to 32 GHz. The above-mentioned alloy thin film conductor is manufactured using microelectronics processes such as the plasma CVD process and the photo assisted CVD process as indicated in U.S. Patent No. 47 66 008. The Si-Ge alloy thin film conductor thus produced has a crystallinity (the proportion of microcrystals to the total) of 10% to 90%. The microcrystals consist of silicon and germanium. The average grain size is about 50 to 500 Å. The conductivity is at least 0.1 S·cm-¹. The ratio of conductivity change with the measuring frequency is less than 20% (less than 1 dB) from direct current to 32 GHz. This electric resistor has the above-mentioned thin film conductor, two first electrodes and a second electrode, all arranged on an insulating substrate. The thin film conductor has a trapezoidal portion having two mutually parallel edges and an edge at right angles with the two parallel edges. The first electrodes have a rectangular shape longer than the two parallel edges of the thin film conductor. Each of the first electrodes has a portion in contact with the two parallel edges of the thin film conductor and a portion extending beyond the thin film conductor. The second electrode is in contact with the side at right angles with the two parallel edges and arranged between the extended portions of the first electrodes and certain distances from the first electrodes. These electrodes, constituting the input and output terminals of current, are structured to enable the electric resistor to utilize the current transfer phenomenon of the thin film conductor. The electric resistor in this invention makes it possible to (1) make an electric resistor having a substantially constant resistance value ratio with frequency changes from direct current to 32 GHz; (2) make an electric resistor which is stable at above 600°C by using a chrome/platinum multi-layer vaporized film disclosed in Japanese Patent Application No. 85-274390 as the electrodes; and (3) make on an integrated circuit substrate an electric resistor displaying the characteristics applicable from direct current to a frequency range up to 32 GHz. Fig. 1A is a top view example of the electric resistor according to this invention; Fig. 1B is another top view example of the electric resistor according to this invention; Fig. 2 shows the frequency characteristics of the dark conductivity of the Si-Ge alloy thin film conductor in this invention; Fig. 3 shows the frequency characteristics of the resistance value ratio of the electric resistor according to this invention; Fig. 4 compares the laser Raman spectrum of the thin film conductor according to this invention with that of a thin film resistor of a comparative example; Fig. 5 compares the temperature dependency of the dark conductivity of the thin film conductor according to this invention with that of a thin film resistor of a comparative example; An embodiment of the electric resistor according to this invention will now be described. The process of depositing Si-Ge alloy thin film conductor by CVD (Chemical Vapour Deposition) is one technique of microelectronics. In this embodiment, a plasma CVD furnace, which is available on the consumer market, is used. Similarly, a thermal CVD furnace or a photo assisted CVD furnace can be used. It is only necessary to use a furnace which is capable of depositing an alloy thin film consisting of a amorphous phase and a microcrystal phase on an insulating substrate. In this embodiment, a p-type film was formed, but an n-type film can be formed by the same process. Insulating substrates were placed in the furnace, into which a source gas had been introduced. The substrates that can be used for this purpose have an insulating property, at least on the surface, such as a glass substrate, mica, polyimide film, various types of semiconductor substrates and metal plates covered with an insulating thin film. The insulating substrates used in this embodiment do not need to necessarily have a fine-grained surface texture. A mixed gas of saline (SiH₄) and germane (GeH₄) was used as the source gas introduced into the furnace. Diborane (B₂H₆) diluted with hydrogen was used as a p-type dopant. An example of depositing conditions is shown in Table 1. Sample No. 1 2 3 Gas flow ratio (SiH₄/GeH₄) 1 1 1 Electric discharge pressure (Torr) 2 2 2 Discharge power (W) 15 80 300 Discharge power density (W/cm²) 0.02 0.11 0.40 Substrate temperature (°C) 380 380 380 The characteristics of the thin film conductor thus obtained are indicated in U.S. Patent No. 48 35 059 described previously. Fig. 4 shows an example of a laser Raman spectrum of Si-Ge alloy thin film consisting of an amorphous phase and a microcrystal phase, deposited on a glass substrate under the conditions specified in Table 1. The light-angle scattering method was used as a measuring technique to obtain data. In Fig. 4, the axis of abscissa represents Raman shift (cm⁻¹), and the axis of ordinates represents a Raman strength (in optimal units). The waveforms A, B, and C indicate the Raman spectra of the deposited films obtained at a discharge power of 15W, 80W and 300W. Waveform A, which has a broad pattern, indicates that the Si-Ge alloy thin film consists of a completely amorphous phase. Waveforms B and C, having sharp peaks, indicate that a microcrystal phase and an amorphous phase coexist in the alloy thin films. Since the Raman shifts in waveforms B and C have peak values which are in the neighborhoods of 290 cm-¹, 400 cm-¹ and 490 cm-¹, it is suggested that the Si-Ge alloy thin films be chiefly composed of Ge-Ge, Si-Ge and Si-Si bonds. An X-ray analysis pattern (not shown) and the Raman spectra in Fig. 4 indicates that the Si-Ge alloy thin films rarely have clusters consisting of only silicon microcrystals or germanium microcrystals, but are mainly composed of microcrystals consisting only of Si-Ge alloy. Fig. 5 shows an example of the temperature characteristics of the dark-conductivity of the Si-Ge alloy thin film conductor. In Fig. 5, the axis of abscissa represents the reciprocal of the absolute temperature (l/T), and the axis of ordinates represents the dark-conductivity (δ D ). Waveform A shows the temperature characteristics of the dark-conductivity of the Si-Ge alloy thin film consisting only of the amorphous phase deposited at a discharge power of 15W. This figure shows that the dark-conductivity of the alloy thin film of waveform A is proportional to the -1/4th power of (1/T) in the low temperature range and to (1/T) in the high temperature range. From this, it is understood that the electric conduction mechanism is governed by the hopping conduction in the low temperature range and by the band conduction in the high temperature range. These temperature characteristics are the same as those of the well-known amorphous silicon film. An excellent feature of the Si-Ge alloy thin film conductor of this invention is that the measured absolute value of the dark-conductivity is three digits higher than that of the amorphous Si film. Waveforms B and C show the temperature characteristics of the dark-conductivity of two Si-Ge alloy thin films, containing microcrystals with a crystallinity of 30% and 90%, deposited at a discharge power of 80W and 300W, respectively. Of note are the features that the dark-conductivity is measured to be more than 100 S·cm-¹, so high as to be equivalent to that of semimetals and that the temperature change in the dark-conductivity is so small as to be less than 1 %/K. Such a stable dark-conductivity, which is only minutely affected by temperature changes, is extremely advantageous in forming thin film resistors and IC chips with high accuracy. Among Si-Ge alloy thin films having a dark-conductivity of more than 1 S·cm-¹, not shown diagrammatically, which were formed under the depositing conditions in Table 1, it has been confirmed that the temperature coefficient is extremely small; less than 1 %/K. Fig. 2 shows the changes in frequency of the dark-conductivity of a Si-Ge alloy thin film formed under the depositing conditions in Table 1. In this figure, the axis of ordinates represents the dark-conductivity ratio (%), and the axis of abscissa represents the measuring frequency (Hz). The black dots indicate the frequency characteristics of a sample deposited under depositing condition A. It can be seen from the black dots that the dark conductivity begins to decrease around 200 kHz and has decreases by more than 10 percent at 4 MHz. The white dots indicate the frequency characteristics of samples deposited under depositing conditions B and C and show that the dark-conductivity is constant up to 4 MHz and begins to increase slightly at 10 MHz. This corresponds to the changes in the Si-Ge alloy thin film from the amorphous phase to the microcrystal phase as shown in Fig. 4. Hence, if the Si-Ge alloy thin film contains a microcrystal phase, the dark-conductivity of the film will remain constant within a wide range of frequencies; from direct current to high frequencies. Fig. 1A shows an electric resistor which is made of the Si-Ge alloy thin film conductor of this invention. The figure shows an application of the electric resistor as a terminal resistor used in a wide range of frequencies, from direct current to high frequencies in coplanar microstrip lines thereon. In this electric resistor 11, provided on insulating substrate 12, is an Si-Ge alloy thin crystal film conductor 13 in which the amorphous phase and the microcrystal phase coexist as described earlier. This film conductor is in a rectangular shape indicated by width D and length L. The two first electrodes, 14A and 14B, are arranged in parallel on the insulating substrate. The first electrodes are rectangular in shape and their length is longer than the length of the Si-Ge alloy thin film conductor. First electrodes 14A and 14B are partly in contact with the two, parallel, lateral sides of the Si-Ge alloy thin film conductor. The remaining parts of the first electrodes extend beyond the Si-Ge alloy thin film conductor. Second electrode 15 is also provided on the insulating substrate and has a rectangular form and width 2d which is narrower than space D (the width of the Si-Ge alloy thin film conductor) between the first electrodes. The second electrode, being in contact with that edge of the Si-Ge alloy thin film conductor which is at right angles with the edges with which the first electrodes are in contact, is located between the opposing extended parts of first electrodes 14A and 14B with clearances provided from the first electrodes. As is clear from the figure, the head ends of first electrodes 14A and 14B and second electrode 15 are on the same line. The first electrodes correspond to the external conductor of a coaxial cable and the second electrode to its internal conductor. The space D between the first electrodes and the width 2d of the second electrode determine the characteristic impedance of the cable as well as the relative dielectric constant of the substrate. The length L of the Si-Ge alloy thin film conductor, that is, the film resistor, is determined taking the attenuation factor into account. A design example is shown in the following. The sheet resistivity of a thin film conductor generally does not affect the frequency characteristics of the characteristic impedance and the attenuation factor. However, when a design example having D = 2d and L ≧ 0.7D was analyzed using a simple model, this model did not hold at ultra high frequencies. The above frequency characteristics were determined by the properties of the resistor and the attenuation factor was more than 20 dB at ultra high frequencies. The actual dimensions of a thin film resistor are determined by the configuration of the coaxial line used. Generally, thin film resistors of D = 3.5 mm or so are used. Fig. 3 shows the ratio of the change in the resistance value (resistance value ratio) of the electric resistor of Fig. 1A to the change in frequency. The axis of ordinates represents the resistance value ratio (percent) and the axis of abscissa the measuring frequency (Hz). It is confirmed from this figure that the electric resistor of this invention shows an almost constant resistance value up to 32 GHz. The above-mentioned electric resistor of this invention can be easily made of the deposition of an amorphous Si-Ge alloy thin film conductor using the plasma CVD process, a micro processing technique using photolithography and a metal film deposition technique using vapour deposition. Metal films that can be used to constitute the first and second electrodes include an aluminium evaporated film, nichrome/gold multi-layer evaporated film and chrome/platinum multi-layer evaporated film. The chrome/platinum multi-layer evaporated film, above all else, is stable and effective at temperatures above 600°C. This is indicated in Japanese Patent Application No. 85-274390 (the Title of the Invention: Ohmic Junction Device) by the inventor of this invention. The deposition conditions of the aforementioned amorphous Si-Ge alloy thin film conductor by the plasma CVD process are governed by the flow ratio of SiH₄/GeH₄ and carrier gas (H₂) discharge pressure, discharge power and substrate temperature. This film can be formed under the conditions other than given in Table 1, namely, SiH₄/GeH₄ = 0.1 to 100, discharge pressure : 0,1 to 10 Torr, discharge power : 0.1 to 10 W/cm², substrate temperature : 300° to 500°C. By the photo-assisted CVD process, this alloy thin film conductor can be formed at a low temperature, for example, when the substrate temperature is about 200°C. This invention is not limited to the embodiment of Fig. 1A. As an example, the thin film conductor may be formed in a hexagon, as shown in Fig. 1B, such that inclined sides are formed between the two mutually parallel sides and the side at right angles with these two sides and the latter single side is shorter than the distance between the two parallel sides. If a device is made by making the Si-Ge alloy thin film conductor shown in Fig. 1A synmetrical about the right end of the rectangular Si-Ge alloy thin film conductor, the device produced will be a resistance attenuator. The attenuated amount of resistance is for the most part proportional to the length L but even if length L is varied, the characteristic impedance stays unchanged.

Claims: An electric resistor (11) having a thin film conductor (13) comprising: an insulating substrate (12), a thin film conductor (13) mounted on said insulating substrate (12), and a couple of first electrodes (14A,14B) characterized in that said thin film conductor (13) comprises a silicon-germanium alloy film conductor in which the amorphous and microcrystal phases coexist, said thin film conductor (13), as seen from the top view, has two mutually parallel edges and one edge which is positioned at right angles to the two parallel edges, that said couple of first electrodes (14A,14B) are provided in parallel on said insulating substrate, said first electrodes (14A,14B) having a rectangular shape of which the length of the rectangle is longer than the two parallel edges of said thin film conductor (13) and each having a portion in contact with a respective one of the two parallel edges of said thin film conductor (13) and a portion extending from said thin film conductor, and that a second electrode (15) is disposed on said insulating substrate (12) between the extended portions of said first electrodes (14A,14B) and in contact with said one edge of said thin film conductor (13). The electric resistor (11) of Claim 1 characterized in that said thin film conductor (13) has a rectangular shape. The electric resistor (11) of Claim 1 characterized in that said thin film conductor (13) has a hexagonal shape, and inclining edges located between said two mutually parallel edges and said one edge at right angles to both of said two parallel edges, said one edge, which is positioned at right angles to both of said two parallel edges and with which said second electrode (15) is in contact, being shorter than the distance between said two parallel edges. The electric resistor (11) of Claim 1 characterized in that said thin film conductor (13) has the dark-conductivity δ D which is not smaller than 1 . S cm⁻¹ and the temperature coefficient δ of the dark-conductivity which is not larger than 1 %/K. The electric resistor (11) of Claim 1 characterized in that the resistance value ratio of the thin film conductor (13) changes within a range of 1 dB up to 32 GHz. The electric resistance (11) of Claim 1 characterized in that said electrodes (14A,14B,15) are formed of chrome/platinum evaporated film.

Inventors: KODATO, SETSUO, NAKAMURA, MASAO
Applicants: ANRITSU CORPORATION
Application year: 1987
Main classification: H01C7
IPC codes: H01C1, H01C7, G01R1, G01R19, H01L35