Fabrication of a Single Crystal Silicon Substrate for AM-LCD Using Vertical Etching of (110) Silicon
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ABSTRACT In this paper, feasibility of crystalline silicon (c-Si) substrate for transmissive active matrix liquid crystal displays (AM-LCDs) has been investigated. The transparent pixel areas of AMLCD were formed by vertical etching of (110) silicon substrate using anisotropic etching property of aqueous KOH. Combining this vertical etching process with the conventional MOSFET fabrication process, the pixel switching devices, peripheral circuits and transparent apertures were successfully integrated on the same c-Si substrate. The pixel NMOS devices exhibit an electron mobility of about 600cm 2 /V.s, a subthreshold slope of 65mV/decade and ON/OFF current ratio of 9 decades at 5V drain voltage. And the gate delay time is 3.3ns at 10V power voltage, measured from a ring oscillator which has enhancement-load type NMOS inverters having the (W/L)load-(W/L)driver of 251.m/15prm - 501.tm/10rtm.
INTRODUCTION Glass has been exclusively used for the substrate material of AM-LCD. But, due to its low melting temperature, amorphous silicon thin film transistors (a-Si TFTs) or polycrystalline TFTs (poly-Si TFTs) fabricated at relatively low temperature have been widely used, although these devices cannot show high performance of bulk transistors formed on a c-Si wafer. In fact, c-Si has been considered little in the field of AM-LCD since its growing on the transparent glass or quarts is practically impossible. Recently however, the device characteristics up to bulk transistor's are frequently needed for advanced AM-LCDs pursuing high resolution, full integration and low power consumption, to name a few. Hereby, we have changed our view points and attempted to use a c-Si wafer as a substrate for AM-LCD. To open a light path, we completely etched the aperture region in the c-Si wafer vertically, while remaining the region where pixel devices and peripheral circuits were fabricated. In this light-path opening technique, the etchant and material pair of aqueous KOH and (110) cSi wafer, which has excellent vertical etching property was used. This pair is known to have more than 400:1 depth-to-undercut aspect ratio[l]. Thus, we propose the new substrate structure for AMLCDs as shown in Fig. 1, where the unit pixel area is defined by equally spaced strips of silicon and metal data lines. The switching devices are made on the front-side strips of the wafer and the aperture regions are formed by vertical etching started from the back side of the wafer. In this paper, we describe a fabrication process for the proposed structure, which easily combines the vertical etching process with the conventional MOSFET fabrication process.
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Mat. Res. Soc. Symp. Proc. Vol. 377 01995 Materials Research Society
Aperture Switching Metal (transparent silicon device data line nitride window) Front LPCVD Si3
Driv c ing
Metal data line
c-Si strip
LPCVDilicNn
Back side
c-Si stripiti (110) silicon substrate
device
(Overall View)
(Plane View)
Figure 1. Crystalline silicon substrate structure for AM-LCDs. The transparent areas are formed by ver
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