Tungsten CMP using Fixed Abrasive
Publication Date: 2004-Jul-22
The IP.com Prior Art Database
This paper describes the polishing of tungsten metal using a fixed abrasive polishing pad along with a working fluid. Polishing down forces, polishing speeds of the platen and carrier were varied, and good tungsten removal rates were achieved. Removal rates varied depending on the process conditions. Dishing and erosion of various wafer features were also monitored and good results were achieved. Removal of the underlying Ti and TiN layers was estimated to require less than a minute of additional polishing. Blanket TEOS wafers were polished under the identical process conditions as that of tungsten. Low oxide removal rates under were measured, yielding a high tungsten/oxide removal rate ratio.
Fixed Abrasive Tungsten CMP
Introduction: This paper describes the polishing of tungsten pattern wafers using a fixed abrasive (FA) polishing pad and a substantially "abrasive free" polishing solution. A FA polishing pad typically comprises a three- dimensional, textured fixed abrasive article comprising a plurality of abrasive particles and a binder arranged in the form of a predetermined pattern of abrasive composites. An iron(III) nitrate solution was used as the polishing fluid and it is thought to be one of many that could be employed in conjunction with a FA pad for tungsten CMP. It is preferred that the polishing solution oxidizes tungsten at a reasonable rate. Although a preferred method for FA tungsten CMP is to have a substantially abrasive free polishing solution, it is believed that small quantities of an appropriate abrasive could be added to the abrasive free polishing fluid while obtaining useful results. Known benefits of the FA approach in other CMP systems are low dishing and erosion. Both results have been demonstrated using FA tungsten CMP as well. Tungsten removal rates of up to 5,000 Å/min can be obtained, depending on process conditions. The removal rates are well within the range of those required for acceptable wafer throughput. Additionally, when blanket TEOS wafer were polished under the identical process conditions using the FA pad and iron(III) nitrate solution, oxide removal rates were under 10 Å/min, yielding a very high tungsten/oxide ratio. Although Ti and TiN rates were not specifically measured, the FA approach rapidly removed the Ti/TiN layer as judged from the end-point time and total polish time. Experimental: Polishing Solutions: A 3% (wt) iron(III) nitrate solution in deionized water was used as the polishing fluid. It was prepared from iron(III) nitrate nonahydrate. Ex: 50.0 g iron(III) nitrate nonahydrate is added to 1950 g deionzed water. The solution was stirred with a Teflon coated magnetic stir bar until complete dissolution of the iron(III) nitrate nonahydrate was observed. Pads: A 20" diameter MWR66 fixed abrasive with a T63 geometry laminated to a 20/90 sub-pad, both commercially available from the 3M Company, was used for the polishing experiments. The pad was "broken-in" prior to tungsten polishing by polishing. This can be achieved by polishing blanket tungsten wafers at 4.0-5.0 psi down force at 100 rpm platen and carrier speeds with a solution flow rate of 120-150 ml/min for 4-6 minutes using the appropriate polishing solution.
Polishing Tool/Configuration: An Applied Materials Mirra polishing tool was used. All polishing was conducted with a slotted retaining ring and the slurry delivery arm out 2.5-3 inches from the pad center. Specific process conditions for each polishing solution are reported with the polishing results.
The solution flow rate was 150 ml/min. Polishing time for the pattern wafers was monitored by the end point software. Approximately 20 seconds of over-poli...