Fabrication of high-aspect-ratio lightpipes

The authors report the development of two fabrication processes for creating high-aspect-ratio lightpipes in a 10 mu m thick SiO2 layer, with smooth, uniform, and straight vertical sidewalls. Both processes require only standard optical lithography, without the need for advanced electron beam or deep-UV lithography. One process employs a dielectric etch mask and the other uses a negative photoresist as the etch mask. The experiments show that the CF4-based reaction gases are best for deep etching with high selectivity and etch rate. Trenches with diameters or width of 1.5 mu m are demonstrated, with an aspect ratio of 7.2:1 and a sidewall angle of 87.4. The authors also achieve cylindrical lightpipes with an aspect ratio of 3.8:1 and a sidewall angle of 89.5. They anticipate that these high-aspect-ratio lightpipe structures would be useful for complementary metal-oxide semiconductor image sensors, where they would increase the efficiency of light collection, and reduce interpixel cross-talk. (C) 2011 American Vacuum Society. [DOI: 10.1116/1.3589809]