Ryo Sasaki

The solution to enhance i-line stepper applications by improviing process overlay accuracy

Canon has developed the worlds first stepper with the shot shape compensator called “SSC”, which independently controls x and y magnification of shots on a wafer. This SSC function enables steppers to achieve overlay accuracy equivalent to scanners. The most suitable application of this function is back side illumination (BSI) image sensors, which has large distortion on wafers because of bonding and thinning processes. In this paper, we will provide the detailed descriptions along with the exposure results using our new solutions.

The solution to enhance i-line stepper applications by improving mix and match process overlay accuracy

In recent years, the demand for high sensitivity image sensors has become prominent, in correlation with the reduction of pixel size and higher pixel counts. Sensitivity is especially important for mobile applications and as a result, back side illumination (BSI) structure image sensors are emerging. The spread of BSI image sensors causes new technological challenges in the lithographic process. One of the challenges is related to the wafer distortion created during the bonding and thinning of the wafer. The challenge is to reduce the impact of the wafer distortion on the overlay accuracy, and we propose two unique solutions for this challenge: Extended Advanced Global Alignment (EAGA) and Shot Shape Compensator (SSC). EAGA is an alignment measurement function that can measure the position and shape of all shots on the wafer. SSC is an exposure function that adjusts the shape of exposure shots according to the shape of the underlying layers shot on the distorted wafer, by controlling both the XY magnification difference and skew component of the projection optical system. In order to realize the SSC system in i-line stepper, Canon has introduced a new compensation mechanism featuring “two-dimensional Alvarez” optical elements. One other challenge is to detect alignment marks located on the back surface of the silicon wafer and for this challenge, Canon has employed a new alignment system using infrared light. In this paper, we will provide detailed descriptions along with exposure results using these solutions. We will also delve into the possibility of additional process applications that can benefit from the enhanced overlay accuracy provided by Canon i-line lithography systems.