Selective W for Coating and Releasing MEMS Devices
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is used by the silicon microelectronics industry as a gate electrode and local interconnect [1]. Parts fabricated from polysilicon, a material originally developed for its electronic properties, have been demonstrated to be mechanically robust [2]. However, wear has been identified as a significant failure mechanism for devices with load bearing surfaces [3-5]. Some approaches to the problem of wear include the introduction of a low friction polymeric coating, for example by PECVD (Teflon) or introduction of self assembled monolayer or through wet chemical routes after the release process [6, 7]. In these approaches the deposited layer itself is not hard and wear is diminished by the reduction in the coefficient of friction. The long-term behavior of these very thin layers of polymeric materials is unclear. A fundamentally different approach to the wear problem is to substitute the polysilicon with intrinsically hard materials such as diamond or silicon carbide. However, this runs counter to the great enabling strength of surface micromachining, leveraging of IC processing technology and tool sets. An even bigger drawback to this approach involves process integration. Most devices with contacting layers consist of a minimum of three mechanical levels fabricated using a complicated combination of deposition, photolithographic, etch, and planarization processes. The introduction of completely new materials and processing technologies into these complex process flows may be difficult. Therefore, development of better surface passivation and tribological coatings using standard IC processing tool set is of great importance for the successful widespread introduction of MEMS sensors and actuators with contacting surfaces. In this paper we demonstrate that coating with 135 Mat. Res. Soc. Symp. Proc. Vol. 605 © 2000 Materials Research Society
selectively deposited, self-limiting, tungsten coating can dramatically improving the wear characteristics of microengines. The selective deposition of tungsten through the silicon reduction of WF 6 was studied in detail in the late 1980's but never gained acceptance by the IC industry [8-13]. However, blanket tungsten CVD, using silane or hydrogen reduction is commonly used in the integrated-circuit industry, and this same tool set can be applied to the selective silicon reduction process outlined here. The selective deposition of tungsten is accomplished through silicon reduction of WF 6, which results in a self-limiting reaction [11, 12]. The self-limiting nature of this deposition process ensures the consistency necessary for process control and results in a very conformal coating. The ability to selectively deposit W after the removal of the sacrificial oxide and the low temperature of the deposition (
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