USING PLANAR FRESNEL ZONE PLATE LENSLETS TO REPLACE PROTRUDING TYPES OF MICRO-LENSLET AS FOCUSING ELEMENTS FOR IMAGING CMOS DEVICES
Original Publication Date: 1999-Jan-01
Included in the Prior Art Database: 2002-Aug-01
In a typical imaging CMOS cell, the light sensi- tive area occupies less than 30% of the total cell area. Special kinds of micro-lenslets are placed on the top of the cell to increase the light collection efficiency. Different kinds of micro-lenslet struc- tures have been proposed and implemented. The typical microlenslet now in use is made by pattem- ing resist and flowing it to a rounded shape by heat- ing. Prior art micro-lenslet structures have a com- mon drawback from the application point view: the micro-lenslets protrude above the top surface of the device. As a result, the device surface readily attracts particles during the final stages of manufac- ture, testing and packaging, and during its lifetime in use. Optical devices are rendered non-functional by the presence of particles in the photosensitive array area. A contaminated device is very difficult to clean because of the protrusion and mechanical softness of the micro-lenslets. There are additional process steps and special handling costs associated with lenslet protection during testing, die separation, and packaging of image CMOS devices with protru- sion-type micro-lenslets.