Application of Thin-Film Micromachining on Glass
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In this case, air or vacuum replaces the dielectric. EXPERIMENT Development of a bridge structure The general process sequence for the fabrication of the bridge structures is summarized in Figure 1. A metal is sputtered onto a glass substrate, followed by photolithography and a plasma etch to pattern the bottom electrode (Fig. la). Next, a low density silicon nitride layer is deposited and a second lithographic and etching step is done to define the sacrificial layer (Fig. 1 b). A third layer is deposited and patterned in such a way that it crosses the bottom metal electrode over the sacrificial layer (Fig. I c). Finally, the structure is immersed for 15-20 minutes, depending on the width of the bridge, in 1:10 buffered HF (BHF) to remove the sacrificial layer. BHF is an isotropic etch for the silicon nitride. The silicon nitride developed for this application has a high etch rate (100 A/s) in BHF. It was deposited by rf glow discharge decomposition of a silane/ammonia gas mixture (1:10) at a process pressure of 100 mTorr, a substrate temperature of 250 'C, and an rf power of 300mW/cm . The dimensions of the bridges to be presented in this paper have widths in the range of 10 pgm and spans ranging from 13 to 28 ptm. The typical thickness of the bridge is 2500 A. The airgap defined when all the sacrificial silicon nitride has been removed is approximately 5000 A 85 Mat. Res. Soc. Symp. Proc. Vol. 507 © 1998 Materials Research Society
7059 glens (a) Bottom electrode deposition and patterning
(b) Sacrificial layer deposition and patterning
(c)Bridge deposition and patterning
Fig. 1 Summary of the basic processing steps for fabricating a bridge structure.
E (d) Sacrificial layer removal
corresponding to the thickness of the sacrificial layer. These dimensions were chosen because they correspond to those commonly used in standard TFTs. Specially designed test structures enabled visual monitoring during the sacrificial etch process to ensure the complete removal of the silicon nitride under the bridge. These test structures are composed of square stacks of the constituent layers of the bridge structure, including the sacrificial layer. Several test structures of different dimensions are used and as the sacrificial layer is completely etched away during the silicon nitride etch, the top layer is released. Smaller test structures will release their top layers before the larger test structures, thus allowing an estimation of the progress of the etching of the sacrificial layer. An example of a micromachined device based on the bridge structure: air-gap TFTs The development of a bridge structure enables the fabrication of air-gap TFTs with top or bottom gate electrode. The bottom gate TFT has a metal gate composed of a bilayer of 2500 A AlSiCu/loo0 A TiW(N), followed by a 5000 A air gap, a 2500 A a-Si:H bridge which acts as the active layer, and finally 5000 A of AISiCu as source and drain contacts (Fig. 2). The a-Si:H was deposited by rf glow discharge of silane at a process pressure of 100 mTorr, a substrate tempera
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