The semiconductor industry currently uses different types of semiconductor-based imagers, such as charge coupled devices (CCDs), complementary metal-oxide semiconductor (CMOS) active pixel sensors (APS), photodiode arrays, charge injection devices and hybrid focal plane arrays, among others, in which an array of microlenses causes incident light to converge toward each of an array of pixel elements. Semiconductor displays using microlenses have also been developed.
Microlenses are manufactured using subtractive processes and additive processes. In an additive process, lens material is formed on a substrate, patterned and subsequently formed into microlens shapes.
In conventional additive microlens fabrication, an intermediate material is patterned on a substrate to form a microlens array using a reflow process. Each microlens is separated by a minimum distance from adjacent microlenses, typically no less than 0.3 micrometers. Distances less than 0.3 micrometers may cause unwanted bridging of neighboring microlenses during reflow. In the known process, each microlens is patterned as a single shape, typically square, with gaps around it. Heat is applied during the subsequent step of reflowing, which causes the patterned microlens material to form a gel drop in a partially spherical shape, driven by the force equilibrium of surface tension and gravity. The microlenses then harden in this shape. If the gap between two adjacent gel drops is too narrow, they may touch and merge, or bridge, into one larger drop. The effect of bridging is that it changes the shape of the lenses, which leads to a change in focal length, or more precisely the energy distribution in the focal range. A change in the energy distribution in the focal range leads to a loss in quantum efficiency of, and enhanced cross-talk between, pixels. The gaps also allow unfocused photons through the empty spaces in the microlens array, leading to increased cross-talk between respective photosensors of adjacent pixel cells.
In addition, as the size of imager arrays and photosensitive regions of pixels decreases, it becomes increasingly difficult to provide a microlens capable of focusing incident light rays onto a photosensitive region. This problem is due in part to the increased difficulty in constructing a smaller microlens that has the optimal focal length for the imager device process and that optimally adjusts for optical aberrations introduced as the light passes through the various device layers. Also, it is difficult to correct the distortion created by multiple layered regions above the photosensitive area, for example, color filter regions, which results in increased crosstalk between adjacent pixels. Consequently, smaller imagers with untuned or nonoptimized microlenses do not achieve optimal color fidelity and signal/noise ratios.
It would be advantageous to have improved microlens structures and techniques for producing them.