Strategies for Machining NIL Masters in Silicon

Micro and nano lenses have optical properties relevant in a large field of applications such as 3D optical elements, optical sensors, light guides, photovoltaic devices and complex integrated MEMS. According to the application, different materials can be used as substrate: silicon, glass, liquid crystals and others. Silicon is the most used material as it is well known and compatible with standard microfabrication techniques, enabling the integration with other silicon based devices, such as MEMS, FETs. In this work we present two approaches, based on isotropic deep reactive ion etching and gray scale lithography, to produce silicon based sub-10 um concave lenses masters for replicas. The main challenges in this process are the geometry and uniformity process control on a full wafer.

For the etching process, a photoresist mask is firstly defined. This mask is composed by several circles with a fixed diameter, and separated by a constant pitch, that was designed to obtain the required lenses dimensions. Then, the wafer is etched by DRIE with an isotropic etch to achieve the desired depth. At this point, the mask is removed and the full wafer is isotopically etched using the same process untill the lenses are fully patterned with the required dimensions.

The 3-dimensional structure of the micro lenses array may also be achieved using grayscale lithography. A grayscale lithography is defined on a photoresist mask with 128 gray levels, using a direct write laser system. These levels are distributed along the lenses’ surface, corresponding to different photoresist thicknesses. The structure is then transferred by DRIE to the silicon substrate. Since the thickness of the photoresist changes along the mask the silicon etch rate is proportional to the thickness of the photoresist, reproducing the 3D resist mask to the silicon wafer.