Biomedical Engineering Reference
In-Depth Information
(A)
(B)
LOR
(a)
(a)
Silicon
Thin photoresist
(b)
25 µm
(c)
(d)
30 µm
Thick photoresist
75 µm
(e)
Shallow exposure
Mask
(b)
(c)
(f )
Deep exposure
(d)
(e)
(g)
(h)
FIGURE 1.3
3-D.photoresist.structures..(A).Fabrication.process.for.3-D.positive.photoresist.structures..
(a).Spin.coating.of.LOR.(a.positive.photoresist).on.Si..(b).Photolithography.of.thin.photoresist.using.AZ.
340.developer..(c).Development.of.unmasked.LOR.in.AZ.400K.developer..(d) UV.exposure.and.dissolu-
tion.of.thin.photoresist.in.AZ.340..(e).Spin.coating.of.thick.photoresist..(f).Shallow.exposure..(g).Deep.
exposure..(h).Development.of.exposed.thick.photoresist.and.whole.LOR.in.AZ.400K..(B).SEM.pictures.
of.suspended.3-D.photoresist.structures..The.levitation.height.is.approximately.30
μ
m.and.recessed.
depth.by.shallow.exposure.is.approximately.25
μ
m..(From.Yun,.K.S..and.E. Yoon,.“Fabrication.of.com-
plex.multilevel.microchannels.in.PDMS.by.using.threedimensional.photoresist.masters,”.
Lab. Chip.
,.8,.
245,.2008..Reproduced.with.permission.from.The.Royal.Society.of.Chemistry.)
(For example, there is no point in using a very expensive projection system if the photomask
has very low resolution.) A high-resolution photomask is typically made of chrome (features)
on glass, which is placed in contact with the photoresist ilm during exposure. For applications
requiring lower resolution, a transparency ilm (e.g., containing features made with toner ink
on a plastic support) suices. Usually, the photomask is drawn in black and white using drawing
sotware and electronically sent to the printing facility. Some printing services, such as those
used by publishers and graphic designers, now ofer the printing of letter-size transparencies
at 65,000 dots per inch at very low cost. Unfortunately, 65,000 dpi does not correspond to the
expected 65,000 inch/dot = ~0.4 μm/dot because of mechanical inaccuracies of the printer head.
Nevertheless, ~5-μm-diameter circles and ~5-μm-wide lines or gaps with marginal “graininess”
are faithfully reproduced. In any case, care must be taken to ensure that the photomask is in
close contact with the photoresist layer. In microelectronics and other high-resolution applica-
tions in which contact between the photomask and the substrate is not desirable, the mask is
usually demagniied by optical projection during exposure; alternatively, exposure of the pho-
toresist may be done serially by scanning a focused electron beam, ion beam, or laser beam. A
requirement of high resolution is that the photoresist ilm should be thin; for example, less than
