Resistive Film

The invention relates to a resistive film comprising carbon and a metal, and to a discrete resistor which is provided with such a resistive film.

Resistive films of said type are already known. In DE-OS 2809623 a description is given of a method of manufacturing resistive films of Ta-C.sub.x, where 0.35>x>0.8, by means of cathode sputtering.

This method shows (see, for example, FIG. 3) that in the Ta-C system the low temperature coefficient (TC) of -25 ppm/K is associated with a resistivity of 200-300 .mu..OMEGA.cm. Consequently, these films are unsuitable for high-valued precision resistors, i.e. precision resistors having a resistivity in excess of 1000 .mu..OMEGA.cm.

EP 247.413-A1 also describes resistive films which are manufactured by sputtering zirconium/palladium, titanium/gold, zirconium/gold, hafnium/gold or titanium/palladium in a reactive gas atmosphere. According to the teachings of said document, as described in column 3, lines 16-19 of the description and in claim 3, only films consisting of nitrides, carbides or carbonitrides should be manufactured.

Consequently, the films manufactured in accordance with said document consist of metallically conductive inclusions (gold, palladium or platinum) in a metallically conductive matrix (carbide or nitride). Due to their high conductivity, such metal composite films are unsuitable for use as films having a high resistivity. The temperature dependence of the resistor is not further specified.

Present-day microelectronic applications, however, require resistance values in excess of 1 M.OMEGA. at the lowest possible temperature coefficients (TC) of the resistor. A prerequisite condition for the realization of such components are resistive-film materials having a high resistivity of at least 1000 .mu..OMEGA.cm at a very low temperature coefficient. The metal-metalcarbide-films in accordance with the state of the art cannot meet these requirements. For this reason, also CrSi systems for use in high-valued film resistors are utilized at present. Although these films are an improvement on the previously used deposited-carbon resistors, their properties, as regards high-impedance value, temperature coefficient and long-term stability do not meet the requirements to be satisfied by film systems for use as precision resistors in microelectronic applications.

The resistance of a discrete resistor can be increased by a microstructuring process (coiling for cylindrical resistor bodies and meandering for flat resistor bodies). However, the limited overall surface area of the resistor imposes an upper limit on the terminal value/basic value ratio to be attained in this process, because the conductor path must have a minimum width. However, the trends in the development of discrete resistors are toward miniaturization. At present, the surface area of the smallest components are only approximately 1.times.2 mm.sup.2. Consequently, the high-impedance requirement can only be met by increasing the resistivity of the film materials used.

For this purpose, it is an object of the invention to provide a film-resistor material which combines a resistivity in excess of 1000 .mu..OMEGA.cm with a temperature coefficient TC in the range between -100 and +100 ppm/K. It is a further object of the invention to provide a corresponding resistor which can suitably be used as a discrete component.

This object is achieved in that a resistive film is proposed which consists of 40-95 at. % of carbon, 4-60 at. % of one or more metal(s) and 1-30 at. % of hydrogen, whereby no carbide-formation has occurred, the percentages of the combined components of the film being equal to 100%. These films have preferably a resistivity in excess of 1000 .mu..OMEGA.cm and a temperature coefficient TC in the range between -100 and +100 ppm/K. Surprisingly it has been found that certain Me--C:H films have a resistivity in excess of 1000 .mu..OMEGA.cm and a temperature coefficient TC in the range between -50 and +50 ppm/K when no carbide formation has taken place between the metal(s) and the carbon. In a preferred embodiment, the metals are selected from the 1.sup.st and/or the 8.sup.th sub-group of the periodic table of the elements, in particular, the copper group and/or platinum group. In this case, Ag, Pt, Au and/or Cu proved to be very suitable. In a further preferred embodiment, the film contains preferably 60-75 at. % of carbon, 25-30 at. % of one or more metal(s) and 5-8% of hydrogen.

In a further preferred embodiment, carbon is partially replaced by silicon and/or boron and/or nitrogen. Advantageously, between 1 and 95%, preferably between 1 and 40% of the carbon and/or boron and/or nitrogen is replaced by silicon. This measure even leads to higher resistance values.

The films according to the invention consist of a highly cross-linked hydrocarbon matrix with, preferably, embedded nanocrystalline, metallically conductive particles. As regards the electrical properties of these particles, they behave like metal (positive temperature coefficient of resistance TC) if the metal content is high and if the metal content is sufficiently low they behave like semiconductors (TC<0). Consequently, for each Me--C:H system there is a composition at which TC=0. The resistivity associated with a film of TC=0, as estimated by interpolation, amounts to about 200-300 .mu..OMEGA.cm for the film-system titanium CH, tantalum CH and niobium CH, and to approximately 10,000 .mu..OMEGA.cm for platinum CH, gold CH and copper CH. Consequently, films comprising non-carbide-forming components, such as platinum, gold and copper deviate substantially from the well known empirical laws known as "Mooij's laws", according to which the vast majority of conductors combines a TC between -100 and +100 ppm/K with a resistivity between approximately 100 and 200 .mu..OMEGA.cm.

The Me--C:H films are manufactured by means of prior art methods, such as CVD or PVD. By means of a subsequent tempering process, preferably in air, the properties of the film are stabilized (pre-aging). The thereby induced changes in the film structure (increase in particle size, repair of crystal lattice, increase of matrix) as well as the changes in the chemical composition (incorporation of oxygen, removal of hydrogen and carbon) cause also a change of the electrical properties. Depending on the film system and metal content, it is possible to obtain a Tk near to 0 ppm/K by using appropriate aging conditions (temperature, time, surrounding medium). In order to protect the film during the aging against thermal decomposition, caused by oxygen from the air, it is possible to provide an additional passivation layer on the resistive film. A silicium-containing carbon/hydrogen layer (a-CSi:H) can suitably be used for this purpose.

Consequently, by means of the thin-film material in accordance with the invention, resistivities which are higher than in the case of CrSi (approximately 1000 .mu..OMEGA.cm) can be attained at an equal temperature coefficient. In addition, by virtue of the particular microstructure of the material (dense amorphous network), a considerably improved long-term stability is obtained.

The invention further relates to a resistor for use as a discrete component. In accordance with the invention, the above-described resistive film is subsequently provided on a substrate in a thickness of from 10 nanometer to 10 .mu.m, preferably from 50 nanometer to 5 .mu.m by means of the known methods. In a preferred embodiment a substrate of AlN, BN, Al.sub.2 O.sub.3, SiC or silicate is used.

The invention will be explained in greater detail by means of three exemplary embodiments and the drawing the sole FIGURE of which is a cross-sectional view of a resistor of the invention.

The sole FIGURE of the drawing is a cross-sectional view of a resistor of the invention.

A plasma is ignited in a parallel-plate-RF-sputtering device (13.56 MHz, 800 W, 1.5 kV DC-bias), comprising a gold target (15 cm), at a pressure of 0.03 mbar in a gas atmosphere of argon (46 sccm) and ethylene (3 "sccm"). (sccm means standard cubic centimeter per minute, and is equal to cm.sup.3 /min. under standard conditions.) An Au--C:H film 3 containing particles 5 of gold having a thickness of 1.5 .mu.m is deposited in 17 minutes on a quartz substrate 3 arranged at a distance of 6 cm from the target. Elementary analysis (electron beam microprobe) shows that the atomic gold content amounts to 0.55 and that the overall hydrogen content is less than 30 at. %. As regards the electrical characteristics of the film, the resistivity amounts to 2500 .mu..OMEGA.cm and TC amounts to 45 ppm/K at room temperature.

Pt--C:H films were manufactured by RF sputtering. The distance between the target and the substrate was 5.5 cm, the overall pressure was 0.020 mbar. The acetylene content of the gas phase was 2% (remainder: argon). Target voltage 1.5 kV, substrate bias +20 V. In this manner, 0.5 .mu.m thick films were formed on ceramic substrates in 30 minutes. Elementary analysis demonstrated that the atomic platinum content amounted to 0.09 and that the overall water content was less than 30 at. %. As regards the electrical characteristics of the film after a tempering process (1 hour, air, 300.degree. C.), the resistivity amounted to 19,000 .mu..OMEGA.cm and TC amounted to 40 ppm/K at room temperature.

Pt--Si--C:H films have been manufactured by means of reactive RF-sputtering with tetramethylsilane (TMS). The distance between the target and the substrate was 5.5 cm, the target voltage was 2.0 kV. At a pressure of 0.01 mbar the TMS-partial pressure was 0.001 mbar (remainder: argon). At a coating process duration of 1 hour, films having a thickness of 2 .mu.m were manufactured. By elementary analysis it was found that the atomic platinum content amounted to 0.33, the atomic silicon content to 0.12 and the atomic hydrocarbon content to 0.55. The overall hydrogen content was less than 30 at. %. As regards the electrical characteristics of the film after a tempering process (8 h, air, 300.degree. C.), the resistivity amounted to 63,000 .mu..OMEGA.cm and TC amounted to -46 ppm/K at room temperature.