The present invention relates to a method for etching tungsten containing material on a substrate.
In integrated circuit fabrication, refractory metals, such as for example tungsten, tantalum, titanium, molybdenum, and their silicides, are used to form high density, high speed, electrically conductive interconnect lines and/or contact plugs for electrically connecting active semiconducting devices formed on silicon or gallium arsenide substrates. In a typical process, a blanket tungsten layer is deposited over an underlying titanium, titanium-nitride, titanium-tungsten, which serves as an barrier or adhesion layer. This is followed by a masking step in which patterned photoresist features are formed over the tungsten layer. To form interconnect lines on the substrate, an etch step is used to remove the tungsten material from the regions not covered by the photoresist features. Tungsten layers can also be used for filling openings to form electrically conducting contact plugs below interconnect wiring layers, by chemical vapor deposition of a blanket tungsten layer that fills the openings, followed by etching of the blanket tungsten layer to form the desired configuration of interconnect lines and/or plugs. Tungsten is desirable for both interconnect lines and contact plugs because (i) tungsten has a higher melting temperature of about 3410.degree. C. compared to aluminum which melts at 660.degree. C., which allows high temperature processing of the substrate; (ii) tungsten exhibits reduced electromigration or diffusion when applied over a silicon layer, (iii) tungsten reacts with silicon at temperatures exceeding 600.degree.-700.degree. C., whereas aluminum reacts with silicon at lower temperatures of 250.degree. C., (iv) tungsten deposition generally provides more uniform filling of high aspect ratio contact plugs, better step coverage, and reduced formation of electrical discontinuities that cause shorts or breaks.
Conventional processes for etching tungsten containing layers use a capacitive RIE plasma of halogen-containing gases, and more typically fluorinated gases, such as CF.sub.4, SF.sub.6, CBrF.sub.3, and NF. One problem with conventional etching processes lies in their inability to anisotropically etch tungsten containing layers to provide features having substantially vertical sidewalls which are not inwardly or outwardly tapered. Excessive etching at the sidewalls of the etched features results in the undesirable inwardly or outwardly sloped sidewalls.
Conventional tungsten etch methods that provide increased anisotropic etching use gases such as CBr, NF.sub.3, or CHF.sub.3 to form thick passivating deposits on the sidewalls of the etched features to reduce horizontal etching, as described in J. Vac. Sci. Technol. B, page 3272 (1985). However, these methods often result in etched features which have wider dimensions at the base due to the increased thickness of the passivating film that forms on the freshly etched sidewalls of the features as the etch process proceeds to completion. In yet another method, gas mixtures of SF.sub.6 /CBrF.sub.3 or SF.sub.6 /CHF.sub.3 are used to reduce undercutting, but these mixtures nonetheless yield isotropic profiles with substantial undercutting at the base of the etched feature, as for example, disclosed in Tennant, et al., J. Vac. Sci. Technol. B, page 71836 (1989). In yet another etching method, as described in U.S. Pat. No. 4,992,136 to Tachi, et al., the tungsten film is etched using (I) an etchant gas such Cl.sub.2 or SF.sub.6 ; and (ii) a passivating film forming halogen gas containing C or Si to form deposits on the surface of the substrate during etching, such as CCl.sub.4, CF.sub.4, CHF.sub.3, CHCl.sub.3, SiF.sub.4, or SiCl.sub.4. However, this method generally provides tapered angles for the sidewalls of the etched features because it is difficult to precisely control the ratio of the etchant gas to the film forming gas during the etching process, particularly towards the completion of the etching process when a high concentration of volatile passivating polymeric species is formed in the etchant chamber.
Conventional etching processes also often fail to maintain the critical dimensions of the etched features, which are predefined and desirable dimensions of the etched features, used to determine the electrical properties of the etched features, in the design of integrated circuits. The critical dimensions are those dimensions that have a significant effect on the electrical properties of the etched features. In modern integrated circuits, the line widths of interconnect lines and diameters of contact plugs are becoming increasingly smaller to levels below 0.25 microns, to accommodate higher circuit densities. Because the electrical resistance of these features is proportional to the cross-sectional area of the etched features, it is important to maintain consistent and uniform dimensions without variations of across an etched feature or between different etched features. Tapering cross-sections, cross-sectional profiles that vary as a function of the spacing between the features, or other variations in the profile of the features is no longer acceptable in modern integrated circuits. The critical dimensions are typically measured as a ratio or difference between the width W.sub.r of the resist features and the width W.sub.e of the resultant etched features. The closer the width of the etched feature to that of the resist feature the more predictable and reliable are the electrical properties of the etched feature.
Thus, it is desirable to have a process for etching tungsten-containing layers which exhibits anisotropic etching and provides etched features having substantially straight sidewalls. It is further desirable for the etching process to provide etched features having consistent and reproducible critical dimensions. It is also desirable for the etching process to exhibit reduced and more controllable formation of passivating film deposits on the sidewalls of the etched features. It is further desirable for the etching process to have a large processing window tolerant of process fluctuations and provide a high process throughput.