Page 63 - An Introduction to Microelectromechanical Systems Engineering

42 Processes for Micromachining

routinely better than one micrometer. Depth of focus, however, is a more severe con-
straint on lithography, especially in light of the need to expose thick resist or accom-
modate geometrical height variations across the wafer. Depth of focus for contact
and proximity systems is poor, also limited by Fresnel diffraction. In projection sys-
tems, the image plane can be moved by adjusting the focus settings, but once it is
2
fixed, the depth of focus about that plane is limited to ±0.5 × λ/NA . Depth of focus
is typically limited to few microns.
Projection lithography is clearly a superior approach, but an optical projection
system can cost significantly more than a proximity or contact system. Long-term
cost of ownership plays a critical role in the decision to acquire a particular litho-
graphic tool.
While resolution of most lithographic systems is not a limitation for MEMS,
lithography can be challenging depending on the nature of the application; examples
include exposure of thick resist, topographical height variations, front to back side
pattern alignment, and large fields of view.

Thick Resist
Patterned thick resist is normally used as a protective masking layer for the etching
of deep structures and can also be used as a template for the electroplating of metal
microstructures. Coating substrates with thick resist is achieved either by multiple
spin-coating applications (up to a total of 20 µm) or by spinning special viscous
resist solutions at slower speeds (up to 100 µm). Maintaining thickness control and
uniformity across the wafer becomes difficult with increasing resist thickness.
Exposing resist thicker than 5 µm often degrades the minimum resolvable fea-
ture size due to the limited depth of focus of the exposure tool—different depths
within the resist will be imaged differently. The net result is a sloping of the resist
profile in the exposed region. As a general guideline, the maximum aspect ratio
(ratio of resist thickness to minimum feature dimension) is approximately three—in
other words, the minimum achievable feature size (e.g., line width or spacing
between lines) is larger than one third of the resist thickness. This limitation may be
overcome using special exposure methods, but their value in a manufacturing envi-
ronment remains questionable.

Topographical Height Variations
Changes in topography on the surface of the wafer, such as deep cavities and
trenches, are common in MEMS and pose challenges to both resist spinning and
imaging. For cavities deeper than about 10 µm, thinning of the resist at convex
corners and accumulation inside the cavity create problems with exposure and with
leaving insufficient resist thickness during etches (see Figure 3.3). Two recent devel-
opments targeting resist coating of severe topography are spray-on resist and
electroplated resist.
Exposing a pattern on a surface with height variations in excess of 10 µm is also
a difficult task because of the limited depth of focus. Contact and proximity tools
are not suitable for this task unless a significant loss of resolution is tolerable. Under
certain circumstances where the number of height levels is limited (say, less than
three), one may use a projection lithography tool to perform an exposure with a