A Novel Latching Relay Fabricated Using an Oxide Molded Tungsten Process
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J11.6.1
A Novel Latching Relay Fabricated Using an Oxide Molded Tungsten Process J.G. Fleming, Michael Baker and David Luck Org. 1749, MS 1080, Sandia National Laboratories, P.O. Box 5800, Albuquerque, NM 87185. ABSTRACT In this paper we describe an oxide molded tungsten process applied to the fabrication of a novel latching relay. The steps in the process are: deposition of a sacrificial oxide, patterning of the oxide, filling of the resulting mold with a blanket film of tungsten using chemical vapor deposition (CVD), and then the removal and planarization of excess tungsten through chemical mechanical polishing (CMP). The process for the incorporation of dielectric isolation has also been developed. The resulting tungsten structures are under high tensile stress, which appears to be compensated in process by the compressive stress of the oxide mold. All the steps are low temperature and the entire process is backend CMOS compatible. This process has been used to fabricate a latching relay which relies on the internal stress of the tungsten and always generates force in a pulling mode. Parts have been successfully fabricated and tested, the devices generate very high forces for a MEMS device and give reasonable contact resistances even without noble metal contacts. INTRODUCTION In this paper, we describe a new processing technology and material system for MEMS. The process uses modifications of tungsten (W) plug interconnect process developed for and supported by CMOS technology. There has been work in the past that describes the use of selective W deposition for MEMS fabrication [1]. However, since this time the blanket W CVD and CMP process used here has become the CMOS industry standard. A major drawback to the use of blanket CVD W for MEMS is the high built-in stress of the material. In fact, the stress of blanket deposited CVD W is sufficiently high to prevent its subsequent use of many typical fabrication tools. This stress arises from two components. The first is a combination of the compressive stress associated with the sputtered TiN adhesion layer required for CVD W deposition (~-1GPa under our deposition conditions) and a high level of built-in stress rising from the CVD W deposition process [2]. The second component of stress arises from the difference in thermal expansion coefficients between W and the Si substrate. This can probably be compensated for by changing the deposition process [2]. However, we have found that we can successfully address and even take advantage of this property in the design of latching relays. PROPERTIES OF W AND THE FABRICATION PROCESS Tungsten is a very dense refractory metal with a relatively high Young’s modulus ~420GPa. While the resistivity of our as-deposited material, is roughly twice that of deposited Al, it is still two orders of magnitude less resistive than polysilicon. Tungsten has a relatively low thermal expansion coefficient (TCE) for a metal, but its TCE is still slightly greater than that of silicon.
J11.6.2
In cooling to room temperature the tungsten is
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