Micro optics is a vital technology for many applications today. The important role of micro-optical components is based on three different levels: miniaturization, high functionality and packaging. Miniaturized systems require micro optics manufacturing for light focusing, light shaping and imaging. In other industries, more precise applications of micro optical components are made.
Previously some alternative technologies including grinding, ultrasonic lapping, diamond drilling and cutting, ion-diffusion, laser ablation, casting, embossing, molding, ion-implanting, etc. have been developed for manufacturing of micro-optics. However, none of these technologies could match with the level of wafer-based manufacturing. Wafer-based manufacturing of micro optics is based on standard technologies from the semiconductor industry like resist coating, lithography, reactive ion etching, deposition, sputtering, and lift-off.
Wafer-optics manufacturing on research-level typically used 100mm wafer technology, whereas industrial manufacturing is typically based on 150mm and 200mm technology respectively. This choice is much related to the cost of owning and operating equipment. Today, the100mm equipment has been phased out by semiconductor industry and since 2004 the 300mm wafer-technology is the standard in semiconductor industry. For lens manufacturing, the best choice would be 200mm technology.
Wafer Cleaning and Photoresist Coating
Wafer-based technology for micro optics manufacturing starts with wet cleaning. Piranha etch, a mixture of sulfuric acid (H2SO4) and hydrogen peroxide (H2O2) is used to remove organic contamination. Additional ultra or mega-sonic cleaning, brush cleaning, high-pressure water jet or plasma cleaning might be used as well.
For manufacturing of diffractive optical elements (DOE), 20nm to several microns of a thin resist layer is spin-coated and patterned by photolithography. You might have to coat additional resist layers on top of the previous layers, if needed. For refractive optical elements (ROE), the required resist layer thickness may range from some microns to more than 100µm layers.
Today, micro optics industry uses immersion lithography steppers to print structures down to some 22 nanometers on 300mm wafers in mass-production. Compared to semiconductor industry, the lithography requirements for micro-optics are much more relaxed.
Due to a very limited depth-of-focus, projection lithography tools are not appropriate for thick resist exposure. So, mask aligners in contact or proximity mode are the preferred lithography tool.
Reactive Ion Etching
Here the resist structures are transferred into the bulk wafer material by plasma ion etching. Wafer processing reactive ion etching process removes atoms from the resist and wafer surface at different etch rates. Surface areas covered by resist structures are protected until the covering resist layer is removed. Typical etch rates range from less than 0.01µm/min to about 1µm/min depending on the ion energy and the reactive etch gases in the plasma chamber.
These well-established technologies allow the manufacturing of almost any shape of micro optical components. You will get the best quality of micro optics depending on your choice of manufacturing equipment and process control. To note, as all processes are standard semiconductor technology, the quality is merely a question of budget and the optimization effort.
However, for micro optics characterization and testing, neither the test equipment from semiconductor industry nor the classical optics manufacturing test equipment is suitable. This lack of suitable test equipment is a major problem for this industry today.