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WSi2 interconnections for very-large-scale integrated circuits
Thin Solid Films, 83 (1981) 143-144
143
ELECTRONICS AND OPTICS
WSi 2 INTERCONNECTIONS FOR VERY-LARGE-SCALE INTEGRATED CIRCUITS* K. C. SARASWAT
Integrated Circuits Laboratory, Stanford University, Stanford, CA 94035 (U.S.A.)
With the advent of semiconductor technology device dimensions are continually decreasing and the number of devices per chip as well as the chip area are increasing. Therefore the length of interconnections is increasing, although their cross-sectional area is decreasing, resulting in a severe increase in the resistance. The RC delays associated with interconnections are beginning to reduce the performance of very-large-scale integrated (VLSI) circuits. Hence severe requirements are being imposed on the material used to form gates and interconnections in the MOS technology. Apart from low resistivity, in a self-aligned MOS technology other requirements are the ability to withstand various chemicals and various high temperature environments encountered during the fabrication process, the capability of being defined into fine patterns and the possession of good MOS and electromigration properties. Polycrystalline silicon is widely used for this application; however, its inadequate properties are beginning to limit the performance of the circuits. Its high resistivity degrades the speed of the circuits because of RC delay times. Although the grain size in as-deposited polycrystalline silicon can be very small, subsequent doping and high temperature processing increase it markedly. Because the grain boundaries provide low energy sites, chemical etchants attack them preferentially, and this causes a problem in defining very fine lines. Refractory metals such as molybdenum and tungsten can meet most of the above requirements but they cannot withstand high temperature oxidizing ambients because their oxides are generally volatile. They also cannot withstand chemical reagents commonly encountered during the fabrication of integrated circuits. Use of silicides of refractory metals such as WSi2, MoSi2, TaSi2 and TiSi 2 has been proposed for this application. In this work, first the formation and properties of WSi2 were investigated to determine its compatibility with the VLSI technology, and then MOS devices were fabricated to show the feasibility. Deposition of WSi 2 was investigated by the techniques of (1) sputtering from a hot-pressed target of WSi 2, (2) cosputtering from two individually controlled targets of tungsten and
* Abstract of a paper presented at the International Conference on Metallurgical Coatings, San Francisco, CA, U.S.A., April 6-10, 1981. 0040-6090/81/0000-0000/$02.50
© Elsevier Sequoia/Printed in The Netherlands
144
AUTHORS' ABSTRACTS
silicon, (3) chemical vapor deposition and (4) deposition of tungsten onto silicon with subsequent reaction by laser heating. The effect of annealing of the resistivity of the films was investigated. It was found that as-deposited amorphous films had a high resistivity and on annealing the films became polycrystalline with a reduction in resistivity. The reduction in resistivity could be attributed to the increase in grain size. The lowest obtainable resistivity was also a function of contaminants in the film. Thermal oxidation of these films in H20 and 0 2 was studied, and good quality SiO2 could be grown on top of them, although the process was markedly affected by the presence of contaminants in the films. The optical properties of WSi 2 were measured to allow ellipsometer measurements. The extinction coefficient and refractive index were found to be functions of the structure of the films, which changes as a function of annealing. Ohmic contacts to boron- and phosphorous-doped silicon were made and the contact resistance was measured as a function of the annealing conditions. Aluminum contacts to WSi 2 were stable up to 600 °C, above which aluminum reacted with WSi 2 to form WA112. MOS capacitors and transistors were fabricated with WSi 2 as the gate electrode. From the measurements on these devices the work function of WSi 2, and the mobile charge density, the fixed interface oxide charge density, the interface trap density, the flat-band voltage, the threshold voltage and the channel mobility for enhancement-mode and depletion-mode transistors were calculated. Details of these investigations will be presented.
143
ELECTRONICS AND OPTICS
WSi 2 INTERCONNECTIONS FOR VERY-LARGE-SCALE INTEGRATED CIRCUITS* K. C. SARASWAT
Integrated Circuits Laboratory, Stanford University, Stanford, CA 94035 (U.S.A.)
With the advent of semiconductor technology device dimensions are continually decreasing and the number of devices per chip as well as the chip area are increasing. Therefore the length of interconnections is increasing, although their cross-sectional area is decreasing, resulting in a severe increase in the resistance. The RC delays associated with interconnections are beginning to reduce the performance of very-large-scale integrated (VLSI) circuits. Hence severe requirements are being imposed on the material used to form gates and interconnections in the MOS technology. Apart from low resistivity, in a self-aligned MOS technology other requirements are the ability to withstand various chemicals and various high temperature environments encountered during the fabrication process, the capability of being defined into fine patterns and the possession of good MOS and electromigration properties. Polycrystalline silicon is widely used for this application; however, its inadequate properties are beginning to limit the performance of the circuits. Its high resistivity degrades the speed of the circuits because of RC delay times. Although the grain size in as-deposited polycrystalline silicon can be very small, subsequent doping and high temperature processing increase it markedly. Because the grain boundaries provide low energy sites, chemical etchants attack them preferentially, and this causes a problem in defining very fine lines. Refractory metals such as molybdenum and tungsten can meet most of the above requirements but they cannot withstand high temperature oxidizing ambients because their oxides are generally volatile. They also cannot withstand chemical reagents commonly encountered during the fabrication of integrated circuits. Use of silicides of refractory metals such as WSi2, MoSi2, TaSi2 and TiSi 2 has been proposed for this application. In this work, first the formation and properties of WSi2 were investigated to determine its compatibility with the VLSI technology, and then MOS devices were fabricated to show the feasibility. Deposition of WSi 2 was investigated by the techniques of (1) sputtering from a hot-pressed target of WSi 2, (2) cosputtering from two individually controlled targets of tungsten and
* Abstract of a paper presented at the International Conference on Metallurgical Coatings, San Francisco, CA, U.S.A., April 6-10, 1981. 0040-6090/81/0000-0000/$02.50
© Elsevier Sequoia/Printed in The Netherlands
144
AUTHORS' ABSTRACTS
silicon, (3) chemical vapor deposition and (4) deposition of tungsten onto silicon with subsequent reaction by laser heating. The effect of annealing of the resistivity of the films was investigated. It was found that as-deposited amorphous films had a high resistivity and on annealing the films became polycrystalline with a reduction in resistivity. The reduction in resistivity could be attributed to the increase in grain size. The lowest obtainable resistivity was also a function of contaminants in the film. Thermal oxidation of these films in H20 and 0 2 was studied, and good quality SiO2 could be grown on top of them, although the process was markedly affected by the presence of contaminants in the films. The optical properties of WSi 2 were measured to allow ellipsometer measurements. The extinction coefficient and refractive index were found to be functions of the structure of the films, which changes as a function of annealing. Ohmic contacts to boron- and phosphorous-doped silicon were made and the contact resistance was measured as a function of the annealing conditions. Aluminum contacts to WSi 2 were stable up to 600 °C, above which aluminum reacted with WSi 2 to form WA112. MOS capacitors and transistors were fabricated with WSi 2 as the gate electrode. From the measurements on these devices the work function of WSi 2, and the mobile charge density, the fixed interface oxide charge density, the interface trap density, the flat-band voltage, the threshold voltage and the channel mobility for enhancement-mode and depletion-mode transistors were calculated. Details of these investigations will be presented.