Patent Grant
H546
The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon. The present invention relates broadly to thin-film resistors and in particular to the formation of thin-film resistors on silicon substrates by ion implantation. Direct ion implantation is frequently used in the fabrication process of integrated circuits as a useful alternative to high-temperature diffusion. In this process a beam of impurity ions is accelerated to kinetic energies ranging from several keV to several MeV and is directed onto the surface of the semiconductor. As the impurity atoms enter the crystal, they give up their energy to the lattice in collisions and finally come to rest at some average penetration depth. Depending on the impurity and its implantation energy, the penetration depth in a given semiconductor may vary from a few hundred angstroms to about 1 .mu.m. The distribution of implanted impurities in the semiconductor material is approximately a gaussian distribution. A uniformly doped region may be achieved by several implantations at different energies. The obvious advantage of implantation is that it can be done at relatively low temperatures. The ions can be blocked by metal or photo-resist layers; therefore, the photo-lithographic techniques may be used to define ion implanted doping patterns. Very shallow (tenths of a micron) and well-defined doping layers can be achieved. One of the major advantages of ion implantation is the precise control of doping concentration it provides. Since the ion beam current can be measured accurately during implantation, a precise quantity of impurity can be introduced. This control over doping level, along with the uniformity of the implant over the wafer surface, make ion implantation particularly attractive for the fabrication of Si integrated circuits. One problem with this doping method is the lattice damage which results from collisions between the ions and the lattice atoms. However, most of this damage can be removed in Si by heating the crystal after the implantation. This process is called annealing. Although Si can be heated to temperatures in excess of 1000.degree. C. without difficulty, some other compounds tend to dissociate at high temperatures. By using annealing methods it is possible to dope Si or compound semiconductors with good control over doping concentration and with the geometrical tolerances that are required for electronic device fabrication. The state of the art of thin-film resistors is well represented and alleviated to some degree by the prior art apparatus and approaches which are contained in the following U.S. patents: Ang et al patent describes thin-film resistors which are made of thin-film materials that include tantalum. The substrate materials recited in this patent are silicon, ceramic, quartz and glass. Plough, Jr. et al patent is concerned with a high resistance thin-film resistor with military specifcation stability. It is formed by depositing a thin metal film on a substrate such as glass. Vugts patent discusses a thin-film resistor that is formed on chromium silicon of an insulating substrate. The resistance range of this material is 100 kilo-ohms to 10 meg-ohms per square. Moksvold patent discloses a method of forming an integrated resistor element by ion implantation. The implantation results in a resistor bar on a semiconductor wafer. Tsang patent improves film adhesion between metallic silicide and polysilicon in thin-film integrated circuit structures by ion implantation. The patented method includes the steps of depositing a metallic silicide on a substrate and then implanting selected ions at predetermined doses and energies into the silicide layer whereby tensile stress generated during fabrication processes is reduced. This invention pertains to a thin-film resistor structure and the method of making same for radiation-hardened integrated circuits. The resistor is formed using a thin-film metallic conductor layer such as tantalum or chromium silicide which is deposited by ion implantati,on on the surface of fused phosphosilicate glass (PSG) or borophosphosilicate glass (BPSG) substrate. Implantation is at an energy level which provides sufficient penetration to insure good adhesion and the resistor is annealed at temperatures up to 700.degree. C. The resistor is formed and annealed prior to deposition of metal, e.g. aluminum, on the substrate. Advantages of ion implantation include very accurate control of dosage, and good adhesion of deposited films. It is one object of the present invention, therefore, to provide an improved thin-film resistor on a silicon substrate. It is another object of the invention to provide an improved thin-film resistor wherein a thin-film metallic conductive layer is deposited by ion implantation. It is still another object of the invention to provide an improved thin-film resistor wherein ion implantation of a silicon substrate is achieved at an energy level of 20-180 kilovolts. It is an even further object of the invention to provide an improved thin-film resistor wherein ion implantation achieves sufficient penetration of and good adhesion to the substrate material. It is yet another object of the invention to provide an improved thin-film resistor wherein the resistor is annealed after implantation at temperatures between 400.degree. to 700.degree. C. It is still a further object of the invention to provide an improved thin-film resistor wherein very accurate control of resistor material depth and geometry is achieved by ion implantation. These and other advantages, objects and features of the invention will become more apparent after considering the following description taken in conjunction with the illustrative embodiment in the accompanying drawing.
