1. Field of the Invention
This invention relates to thin film electrical resistors used in integrated circuits, and specifically to a method for producing thin film resistors with selectable temperature coefficients and improved resistance value tolerances.
2. Description of the Prior Art
Electrical resistors are formed by high resistivity material connected between two electrodes. The resistor's resistance is equal to its length divided by its cross-sectional area, times a constant for the material. Because resistance is inversely proportional to cross-sectional area, small cross-section resistors are useful in high density, low power integrated circuits for providing high resistances in short lengths.
High resistivity resistors may be formed in a semiconductor substrate of a given conductivity type (N-type or P-type) and a given resistivity by forming a region in and on the surface of the semiconductor substrate with a predeposition step, this region being of a conductivity type opposite that of the substrate, and then diffusing the impurities introduced during the predeposition to a desired depth. By carefully controlling the amount of dopant introduced during the predeposition, a high sheet resistance resistor may be formed.
The introduction of impurities by thermal predeposition techniques does not allow the control in resistor value that is often required for circuit applications. The "doped" resistors also have a temperature coefficient of resistance (TCR) that is set by the relative dopant concentration in the resistor and cannot be set to provide desirable circuit operating characteristics. Diffused resistors are also not trimmable using laser techniques.
Resistors are also formed by implanting ions into an exposed region (i.e., not covered by an SiO.sub.2 layer) to form a high resistivity region in the substrate instead of using diffusion as described above, but while implantation allows better control of the resistance value, the TCR and trimmability of these resistors are the same as those of the resistors formed by thermal predeposition.
U.S. Pat. No. 4,196,228 to Priel et al. forms a resistor in a substrate by implanting boron ions through a 3,000-4,000 angstrom thick layer of SiO.sub.2 to a depth of "about 0.1 microns, which will diffuse into the semiconductor substrate to no more than 0.5 microns during the remaining heating steps in the fabrication process" (See Priel et al., U.S. Pat. No. 4,196,228, Col. 4, lines 58-61, and Col. 5, lines 37-42). Resistor values produced by this method are also fairly difficult to control due to the need for precise implantation depth and the inability to precisely control final depth after heating. In addition, the TCR and trimmability characteristics are close to those resistors formed by implanting into the bare semiconductor.
Besides having resistance values which are difficult to control, resistors formed in the substrate need reverse biasing to electrically isolate them from the substrate or from other circuit elements in the substrate.
Alternatively, resistors are made from thin films, such as doped polysilicon several hundred to several thousand angstroms thick, formed by chemical vapor deposition over a layer of insulation on the surface of a substrate. Thin film resistors can be made to have high sheet resistance values; may be located anywhere on the surface of an integrated circuit; are electrically isolated from other circuit elements without biasing; and may be laser-trimmed for closer resistance value tolerances. However, high yields of these doped polysilicon resistors using chemical vapor deposition are limited to resistance tolerances of approximately .+-.40% (See Miles et al., U.S. Pat. No. 4,225,877, Col. 1, lines 16-20). U.S. Pat. No. 4,225,877 to Miles et al. describes a polysilicon film resistor which is doped by ion bombardment to achieve high yield tolerances of .+-.15%.
A polysilicon layer is . . . formed over the silicon oxide covering the entire wafer surface by a standard low pressure chemical vapor deposition step . . . A polycrystalline layer is produced having a thickness of about 4000 angstroms. Boron ions are then implanted into the surface of the polysilicon, using a boron dopant such as boron or BF.sub.2 at 150 KeV . . . PA1 Then, by a standard photoresist and etching step, the polysilicon is selectively removed to define the lateral extent of the resistors, all of which are about 10 microns wide but of various particular lengths to determine the final resistance values (Miles et al., Col. 5, lines 32-56). PA1 The ion implanted polysilicon resistors can be made with high yields to resistance tolerance specifications of as little as .+-.25[%] (Miles et al., Col. 1, lines 63-65). [By annealing the polysilicon resistor in an inert atmosphere] the resistors can be made with high yield to an even smaller tolerance such as .+-.15% (Miles et al., Col. 2, lines 48-49).
It is important to note that neither polycrystalline nor single crystal silicon resistors can be made to have a temperature coefficient that is nominally zero by implantation of any amount of boron, phosphorus, arsenic, antimony or other common dopants. The temperature coefficient of such a 2 K.OMEGA./square resistor can be expected to be on the order of .+-.7000 ppm/.degree.C.
Thin film resistors are also made by the well-known technique of sputtering, described in U.S. Pat. No. 4,021,277 to Shirn et al. Sputtering thin film resistors entails using energetic particles, usually positive argon ions, to bombard a target composed of resistor constituents in the desired ratios, causing atoms to be energized and ejected from the target surface. A common source of these high energy ions for sputtering is provided by the well-known phenomenon of glow discharge resulting from an applied electric field between two electrodes in a gas, such as argon, at low pressures. Another common source of ions for sputtering is the ionization of a low pressure gas by exposure to a high intensity radio frequency (r.f.) electromagnetic field, which may be accomplished by the application of an r.f. voltage to electrodes, or r.f. current through a coil, in the vacuum chamber. Many practical variations of this method of ion production are employed. The rate of deposition of atoms on a substrate by sputtering is generally more readily controlled than by evaporation. (Evaporation is accomplished by heating the substance to be deposited to the point of evaporation and allowing the evaporated substance to condense into a thin film.) A sputtered film is also generally more uniform. Atoms sputtered from the target material condense to form a thin film on surfaces positioned near the target surface. To form thin film resistors, resistor material is sputtered onto an insulating layer over a substrate surface.
Common target constituents typically include a mixture of chromium silicide (CrSi.sub.2) and silicon carbide (SiC), which, after sputtering, constitute the bulk of the thin film resistor in approximately the same ratio as in the target. Other constituents in the target, such as carbon and boron, may be a small fraction of the target mass but have a significant effect on the temperature coefficient and sheet resistance of the resistor. These constituents, however, due to their low quantities in the target, may not get uniformly distributed in the thin film resistor and are usually not condensed in the same ratio as in the target. What is needed is a thin film resistor which can be sputter-deposited and which can be produced with high yields to small tolerances and with low temperature coefficients.