光刻微光学工艺

Photolithography is the engine that  empowered semiconductor industry  to reduce the minimum feature size of  the components of a microchip from  some 50 microns in the 1960s to below  14 nanometers today. Diffractive and  refractive micro-optical components  play a decisive role in modern photolithography systems, e.g. for laser line  width narrowing, laser beam shaping  (customized illumination), as phaseshift masks, for optical proximity  correction, and for diffraction-based  overlay. Wafer-based manufacturing  of high-quality micro-optics and their  importance for photolithography will  be explained.

Photolithography Nowadays, we are used to have tens  of Gigabytes of memory in our smart  phones. Retail prices for flash memory  are around one dollar per Gigabyte. In  1980, the first Gigabyte hard drive ever,  the IBM 3380, weighed 250 kg and cost  more than $ 80,000. Some ten years ago,  a first Gigabyte SD flash memory card  was introduced for a retail price of $ 500.  The $ 500 will buy today’s leading edge  SD cards with 512 GB, 500 times more  for the same price. Semiconductor technology is moving forward with an incredible pace since more than fifty years.  The driving force behind is “shrinkage”,  also referred to as “die shrink”, i.e. the  ability of semiconductor industry to  reduce the minimum feature size of the  components of a microchip from some  50 microns in the early 1960s to below  14 nanometers today. Die shrink allows  manufacturing more chips on a wafer,  reducing manufacturing costs, minimizing the power consumption and  improving the performance in terms of  speed, storage capacity and customer

      Fig. 1 8 inch wafer populated with diffractive and refractive micro-optical elements (gold  mirror coating) (a); double-sided microlens arrays for beam twisting (b); and SEM images  of 8-level diffractive optical elements (CGH) designed for laser beam shaping at 193 nm (c)  and at 248 nm wavelength (d).

convenience. The key enabling technology behind shrinkage is photolithography. In a photolithography process, the  layout of a microchip is copied onto a  photosensitive layer on the wafer.  In 1960s and 1970s, mask aligners  in contact or proximity mode were the  dominating photolithographic technology. They were replaced by scanners and  projection steppers in the 1980s. In 1995,  state-of-the-art projection lithography  systems were operating at 248 nm wavelength providing a resolution of 250 nm  (half-pitch). Nowadays, modern 193 nm  steppers are able to print features below  40 nm (half-pitch, single exposure) – a  fifth of the wavelength – and far below  Abbe’s diffraction limit. Double patterning, multiple patterning, directed selfassembly (DSA) and other lithography  enhancement techniques are used to  achieve below 14 nm half-pitch today. A resolution enhancement from  250 nm resolution (single exposure) in  1995 to below 40 nm today was achieved  by further improving the projection  optics and by the introduction of immersion lithography, allowing a high  numerical aperture of NA = 1.35. When  lens optimization reached its very limits with surface qualities on the atomic  scale, significant improvements could  only be achieved by optimizing the mask  illumination. Shaping the illumination  light, also referred to as pupil shaping,  allows the optical path from reticle to  wafer to be optimized and has a major  impact on aberrations and diffraction  effects. Highly-efficient micro-optical  components are perfectly suited for

Fig. 2 Scheme of the classical Köhler illumination as proposed by August Köhler in 1893  for microscope illumination (a), lens-array-based Köhler integrator for flat-top illumination  (b), scheme of two double-sided cylindrical microlens arrays as used for Köhler integrators  (c); and photography of a mounted microlens-based Köhler integrator as used for mask  aligner illumination systems (d).