Protection of DRAM Spatial Light Modulators from Incident Light using a Multilevel Metallization Procedure

Liquid Crystal (LC) over silicon spatial light modulators (SLMs) have the potential to be used in miniature head mounted displays and in compact projection display systems. For this application the silicon backplane design is invariably based on a single transistor DRAM memory layout which offers the potential for very high resolution devices. However, a problem arises when these devices are incorporated into optical systems. Light incident onto the circuitry causes charge leakage to the substrate, this in tum reduces the voltage on the modulating electrode, which results in the LC switching from its intended state. This problem is compounded by the desire to use these devices with large light intensities which increases the rate of charge leakage. The light incident onto the circuitry can be reduced by employing a double metal process and using the second level to fabricate a large pixel electrode which then shields the underlying circuitry. However, the second metal layer is deposited onto a very uneven surface due to the topography of the circuitry. As the electrode is also used as the mirror to reflect the incident light the device has poor optical performance. To overcome this problem we have employed a post-processing procedure to provide the second level of metallization which was applied to commercially fabricated wafers. To ensure a high optical quality substrate onto which this second metal level is deposited the interlevel dielectric is planarised using chemical mechanical polishing.