Abstract:
A process of providing semi-insulating thin film resistors with closer tolerance values by furnance-annealing the film to increase is resistance to less than the final intended value, and then focused heat source-annealing the film to within a close tolerance of the final intended value.

Description:
BACKGROUND 
     This invention relates to semiconductor devices, and more specifically to polycrystalline silicon resistors with high, and close tolerance, resistance values. 
     In an integrated circuit (IC) or other device formed on a semiconductor substrate, resistors may be formed from several hundred to several thousand angstroms of thin film deposited over insulation on the surface of the substrate, and connected between two conductors. In integrated circuit devices, such isolated resistors are useful for their range of sheet resistance values, low temperature coefficients, freedom of positioning on the surface of the IC, electrical isolation from other elements of the circuit, and trimmability by a focused heat source cutting or physically evaporating a pattern of the film to increase its resistance and to improve resistance tolerance precision. 
     Resistive thin films are often formed of amorphous silicon, doped with chromium and traces of carbon, boron or other elements for resistance and temperature coefficient optimization. Thin film resistors of this nature are usually deposited by the well-known technique of sputtering. In sputtering, a high intensity radio frequency field is applied, through electrodes or a coil, to a low pressure gas, from which a &#34;glow discharge&#34; of ions bombards or &#34;sputters&#34; a target composed of resistor constituents in appropriate proportions, dislodging atoms from the target to condense and form a thin film on a substrate positioned nearby. Even for use as a thin film resistor, silicon must have its conductivity increased by doping, with either P type (e.g., boron) or N type (e.g., arsenic phosphorous) impurities, for hole or electron flow, respectively. Silicon can be doped with impurities while or after being deposited as a film, but whenever doping is done, the need to control doping concentration precisely makes it difficult to obtain accurate resistance values. The sputtering equipment and process are difficult to control, leaving a need for an alternate technique of providing thin film resistors. 
     Thin film resistors can also be formed of polycrystalline silicon deposited by conventional CVD techniques. Polycrystalline silicon is an aggregation of small, randomly oriented grains of silicon crystals whose lattices mismatch, forming boundaries between grains. Solid State Electronics, Vol. 27, No. 11, pp. 995-1001, 1984, reports that resistive thin film dopant diffusion can be controlled by combining acceptor (e.g., boron) and donor (e.g., phosphorus) doping to form a high resistance or &#34;semi-insulating&#34; resistor. In this prior art process, a polycrystalline silicon film is deposited on insulating oxide over a silicon substrate, blanket implanted with phosphorous atoms, annealed a first time, masked and etched, forming a pattern of polycrystalline silicon resistor areas, which are then implanted with boron atoms to a concentration no greater than that of the phosphorus atoms. The doubleimplanted film is annealed a second time, at 1000° C. for at least 10 minutes, repairing implantation damage and increasing polycrystalline silicon grain size, but more importantly thermally activating dopants at an approximately predictable rate, so that practically all boron atoms become associated with, and are neutralized or deactivated by, phosphorus atoms, which offsets the conductivity, or increases the resistance, of the double-implanted resistor areas. After neutralization, resistor values are 10 3  to 10 4  times higher than values of conventional polycrystalline silicon thin film resistors doped with the same concentration of phosphorus. However, the resistance value tolerance remains wider than desired. 
     SUMMARY 
     This invention takes advantage of the effect of twice doped silicon resistance increasing with increasing anneal temperature, to trim high resistance films to closer resistance value tolerances, by using a laser or other focused heat source for a dynamic feedback controlled third anneal. 
     Polycrystalline silicon thin films are deposited, doped and annealed a first time, patterned, and doped a second time conventionally, but annealed a second time at a reduced temperature to limit the increase in resistance to less than the value ultimately intended. Then, the thin film is covered with passivation, through which contacts are formed with conductive leads, and connected to an external resistance measuring device. The resistance is monitored while the thin film is annealed a third time by a locally focused heat source, such as a laser beam, until the measuring circuit detects that the resistance has reached the intended final value, and turns off the focused heat source. The twice annealed resistor value tolerance is improved by the individually controlled third anneal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIGS. 1 through 6 are cross-sections illustrating steps in a sequence for forming resistors according to this invention. 
    
    
     DETAILED DESCRIPTION 
     The present invention provides high resistance thin film polycrystalline silicon resistors with improved (closer) tolerance resistance values. Referring to FIG. 1, the process preferably begins with a semiconductor wafer 10 having an upper surface 11 covered with a dielectric layer such as silicon dioxide 12, which is in turn covered with several hundred to several thousand angstroms of chemical vapor deposited polycrystalline silicon film 13. 
     Next, polycrystalline silicon film 13 is doped a first time by ion implantation or by thermal predeposition of N type atoms (e.g. phosphorous). N- doped film 13 and wafer 10 are annealed for example, in a suitable furnace at approximately 1000° C. for ten minutes, to activate N type dopant atoms. 
     Referring to FIG. 2, photoresist is applied and patterned to form mask M1 protecting (once doped and annealed) film 13 areas 14 designated for formation of conductors or resistors, while exposed areas are etched, leaving thin film pattern 14. Alternatively, film 13 can be left unpatterned and intact until after the second doping and annealing steps below. 
     If desired, selected areas of unpatterned film 13 or of patterned film 14 can be preserved in the once doped conductive state by employing a second photoresist mask M2, FIG. 3, to block a subsequent second doping. With or without optional mask M2, exposed film areas 15 are then doped with P type atoms (e.g. boron) by ion implantation or by thermal predeposition, to a concentration not exceeding the N type (phosphorus) dopant concentration. Alternatively, film 13 can be doped first with P type atoms, annealed, and doped second with N type atoms. After doping, mask M1 or M2 is removed. 
     Twice-doped film 15 and wafer 10 are annealed a second time to activate P type (boron) atoms to recombine with N type (phosphorus) atoms by the deactivation mechanism referred to in the prior art reference. However, the second anneal temperature is limited so that film 15 resistance increases to no closer than the maximum tolerance variation from the value ultimately intended. 
     Then, twice doped, twice annealed, and patterned film 15 is covered by transparent silicon oxide or semitransparent silicon nitride passivating layer 16, FIG. 4, formed, for example, by chemical vapor deposition. Next, photoresist is deposited and patterned to form mask M3 over passivating layer 16, exposed areas of which are then etched, opening contact holes 17 down to thin film resistor area 15. 
     Next, a conductive metallization layer is deposited and etched to form a metallization pattern including contacts 18 and 19, FIG. 5. 
     Contacts 18 and 19 are connected through conductive leads, not shown, to an external resistance measuring device such as an ohmmeter, curve tracer, test computer, or the like, not shown, which monitors the resistance of resistor 15 continuously and in a well known fashion to control and direct a focused heat source 25 such as an electron beam or laser through passivation 16 to scan and locally heat resistor 15 until the desired resistance value is attained. In contrast to prior art trimming techniques, this resistance adjustment or &#34;trimming&#34; by focused heat does not cut or eliminate the thin film. 
     Typically, an integrated circuited device includes many such thin film resistors 15, and the third anneal resistance &#34;trimming&#34; operation is repeated by switching the resistance monitoring device and focused heat source 25 to another resistor 15. 
     A preferred embodiment has been illustrated, of which modifications and adaptations within the scope of the invention will occur to those skilled in the art. The invention is limited only by the scope of the following claims.