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 layer's 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”
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.
Semiconductor device shrink progresses steadily at a speed of one generation every two years and CD uniformity (CDU) requirement becomes severer as shown in ITRS. Higher level of CD control performance is the most important item for semiconductor exposure tools to meet 45nm node requirements. For this higher accuracy it is of course necessary to brush up the projection optics and the illuminator system in every detail. We need to reassess items which were ignored as error factor until now and include them into the CD budget and thereby control in high accuracy. These small factors include the effects of birefringence caused by glass materials and coating, transmission distribution at the pupil of projection optics and influence of spectrum stability of a laser used in Hyper NA lens, and so on. They will come into the budget in addition to the conventional aberration and illuminator uniformity as we start to use new exposure technology such as immersion or polarized illumination after 45nm node. In this paper, we list up items which influence CDU in the node after 45nm, and estimate sensitivity for CDU for each item. Then we set the target values of every item by breaking the CDU target value of ITRS in terms of projection optics, illumination system, and total performance of exposure equipment. We show data for some items, and describe a prospect for 45nm node era and beyond.