Today's predominant printing technique is a step-and-repeat projection system. A schematic of a typical modern projection system is shown in Figure 2.4. These systems are either all refractive or catadioptric, i.e., a configuration consisting of both lens and mirrors. Usually a reduction is built in. Early systems used 10:1 reduction lenses, 5:1 or 2:1 lenses have become more common due to limitations in mask sizes that can be fabricated with the needed precision. Reducing the image size has the great advantage that the features on the reticle do not have to be as small as the final image and are therefore much easier to fabricate. Another advantage is that mask defects and imperfections are also reduced in size and hence become less severe. Only a small region of the wafer, also called field, is exposed at a time (typically 0.5-3 cm2). Between exposures the wafer must be mechanically moved to the next field, hence the common name steppers.
State of the art I-line steppers including several resolution
enhancement techniques are used for the fabrication of 0.35 m
technology. Excimer-based steppers at a wavelength of 248 nm are already
in use for pilot production of 0.25
m devices, and optical printing
with a wavelength of 193 nm is seriously considered for the
0.18
m generation [20]. Hand in hand with the
trend towards smaller wavelengths, the numerical aperture of the lens is
increased.
The technical challenge is to produce a very large lens with little
aberration and high transparency in the DUV regime. Here, continual
progress has been achieved, too. State of the art lenses have evolved from
numerical apertures of 0.28 in 1978, to 0.38 in 1985, to roughly 0.65 today.
This trend will continue. Optical designers are vigorously
developing systems with numerical apertures as high as 0.8 [13].
To handle the resulting extremely shallow depth of focus,
e.g.,
DOF = 0.3
m for
= 193 nm and
NA = 0.8,
new systems for alignment and
focusing have been developed. The first steppers used global alignment and
focusing. Modern systems are capable of sight by sight control and
automatic adjustment of alignment and focus at every field of the wafer. This
also makes the use of large diameter wafers more feasible. The primary
disadvantage lies in a reduced throughput that typically lies at
20-50 wafers/hour.
A recently proposed method referred to as in-lens filtering or
apodization enhances the depth of focus by placing a special
amplitude and phase filter in the pupil plane of the stepper [21].
Such a configuration is shown in Figure 2.4.
For certain patterns like contact holes the depth of focus can be
greatly increased, but only at the expense of reduced peak intensity and
increased energy in the sidelobes of the aerial image [22].
However, up to now pupil plane filtering has primarily been of theoretical
interest since the pupil plane in microlithographic lenses is usually somewhere
inaccessible in the optical path and cannot be reached unless the lens
is disassembled. Furthermore different mask types require different filters
for optimum performance. Thus the in-lens filter cannot be a simple fixed
optical element, which makes this approach hardly practicable for factory
floor processes where a high number (up to 25)
of different masks are applied during
the fabrication process of an IC.