Biomedical Engineering Reference
In-Depth Information
WHAT DOES IT TAKE?
Despite the ubiquitous statement
found in BioMEMS papers—“our technique
is straightforward and inexpensive”—making small things is not easy, and it is costly.
One can become an expert at using the techniques, but that does not make them cheaper.
he user fee in a fully stafed clean room of a major research university in the United
States is on the order of $100/h per student, although many university clean rooms have
caps around $500 to $1000/month per student. he typical time for training a starting
graduate student in standard photolithography is several weeks. It involves taking safety
training and scheduling a couple of sessions per piece of equipment (one for learning basic
operation procedures, the other for the student to demonstrate self-suiciency) with pro-
fessional trainers. Minimal training includes the spin coater, the mask aligner, and the
developing station. Counting indirect costs, tuition, and beneits, a graduate student costs
at least $3000/month in the U.S. So, the total bill for training can average several thou-
sands of dollars. Ater training, the student is ready to start his or her own microfabrica-
tion process, which requires adjusting spin coating parameters, photoresist type, length
of exposure, and developing or baking times, depending on the size of the features. his
process may easily take another month or two of full dedication to the project. Clearly,
microfabrication is not something to pursue unless you
absolutely
have to.
Incidentally, the early printed circuits were etched on copper foil that was masked by photo-
patterned gelatin. Photoengraving methods have been further sophisticated by the development
of dry photoresist ilms, which can be rolled onto thin metal plates for exposure and chemi-
cal etching. Referred to today by the industry as “photochemical machining” or “photoetch-
ing” (misleading names because the etching reaction is not a photochemical one), it is presently
used for making stencil masks (i.e., plates containing through-holes), molds (e.g., for injection-
molding), and calibration grids (e.g., for microscopy), among other utensils, at low cost.
1.3 Photolithography
1.3.1 Basics: Photoresist and Photomask
Photolithography is the starting point of most microfabrication technologies. In general, as sche-
matized in
Figure 1.2
, it involves the selective exposure to light of a thin coating of
photoresist
.
One of the most commonly used types of photoresists contains diazonaphthoquinone (DNQ)
as the photosensitive component (absorbing light between 300 and 450 nm), which is mixed
with Novolac resin (a phenolic polymer). Ultraviolet (UV) light is shone through a
photomask
Photoresist
coating
Selective illumination
through photomask
Dissolution
of photoresist
FIGURE 1.2
Photolithography.
