Biomedical Engineering Reference
In-Depth Information
containing the desired pattern in the form of opaque features on a transparent support. he
photoresist is usually spun on a lat substrate from solution to a thin ilm and dried before expo-
sure. he exposure chemically alters the photoresist by modifying its solubility in a certain
devel-
oper
solution. (he developer is speciic to the photoresist; in the case of DNQ-Novolac-based
photoresists, for example, the exposed areas can be dissolved in a basic solution of tetramethyl-
ammonium hydroxide whereas dissolution of the unexposed areas is inhibited by the presence
of unexposed DNQ.) By convention, a
positive photoresist
is one that becomes soluble when it
is exposed to light (as in
Figure 1.2
) and a
negative photoresist
is one that becomes insoluble.
Unfortunately, because the smallest dust particle distorts the spreading of the photoresist dra-
matically during spinning, photolithography must be carried out in clean room facilities, which
require an expertise uncommon in biological laboratories and are costly to build and maintain.
Although there are a number of photolithographic techniques that help in reducing the cost of
processing, particularly when the resolution requirements are less stringent, in general, clean
room-based photolithography remains inherently expensive. Despite its shortcomings, photoli-
thography is widely used beyond microelectronics processing because it is extremely reliable. It
has been optimized to produce high yields over large areas; the fabrication of a typical desktop
computer's microprocessor, for example, consists of more than twenty photolithographic steps,
so it had better be reliable! It should be noted that photolithography is a
subtractive
patterning
process; that is, patterning is carried out by selectively removing material (photoresist in this case).
1.3.2 Black or White versus Gray Scale
Take a look at
Figure 1.2
and you will notice that
all
three steps consist of a modiication of the
substrate that is homogeneous from edge to edge of the wafer: (1) the photoresist thickness is
uniform, (2) the light dose is the same on all the exposed areas (i.e., the photomask has only two
levels of opacity: fully transparent or fully opaque), and (3) the whole surface is homogeneously
exposed to the developer solution. As a result, the inal features are of a single height—it is a
“black-and-white” process.
In many cases, features of various (and variable) heights are needed. Ideally, one would like
a process in which feature height is not encoded by the spinning speed (which is homogeneous
across the wafer), but that could be varied across the wafer, in other words, that could be speci-
ied in the photomask in the form of grayscale variations. In practice, it has been hard to develop
such translucent materials. It is, however, possible to make three-dimensional (3-D) features
by patterning various photoresists (each with its own developer chemistry) in multiple steps
(
Figure 1.3
). Suspended bridges with various lengths and suspended plates with micropits and
microvilli are successfully formed.
here are a number of techniques that are capable of “
grayscale photolithography
,” that is,
of patterning multiheight features in one step, but they are generally more specialized, more
expensive, or restricted in terms of the patterns that can be produced. A recently developed digi-
tal projector formed from an array of micromirrors—used in some movie theaters—promises to
become the standard for photolithographic patterning because the fabrication of a photomask is
no longer necessary, and is inherently gray scale (see Section 1.3.6.1). As shown in Section 1.6.6.2,
a low-cost approach to grayscale photolithography uses luids as a feature of the photomask
(which is itself a microluidic device).
1.3.3 Resolution
In microfabrication, we refer to the “resolution” of any part of a process as the smallest linewidth
that can be achieved—it depends on the materials being patterned, on the substrate on which
they are being patterned, on the photomask, on the photoresist and equipment used, and on
environmental parameters such as temperature and humidity. Unless special tricks are used,
the resolution of a process is always limited by the lowest resolution of each of its subprocesses.
