Page 273 - An Introduction to Microelectromechanical Systems Engineering

252 Packaging and Reliability Considerations for MEMS

solutions remain proprietary to the manufacturers. For instance, a dilute silicon
etchant will round the corner mentioned earlier, but the duration of the etch, the
type of etchant used, and concentration are all considered trade secrets.
Shocks can also cause fracture by exciting undamped mechanical resonant
modes. By virtue of the instantaneous energy they impart on an object, shocks have a
very broad spectral signature (up to hundreds of kilohertz) that overlaps with the
resonant frequencies of many micromachined elements. Proper mechanical design
should address the appropriate damping of any resonant mode that may be excited
by shock. This is of particular significance to suspended structures made of single-
crystal silicon packaged in vacuum because they make excellent low-frequency reso-
nators with high quality factors. While single-crystal silicon is generally less suscep-
tible to fracture than polysilicon or amorphous silicon, its crystalline nature offers
little internal damping. External damping (e.g., air viscous damping) is the only
means to reduce the quality factor of single-crystal-silicon beams and the risk of
fracture under shock.
Corner rounding, travel limiters, and damping are examples of design modifica-
tions intended to improve immunity to shock. The cause-and-effect relationship
tends to be well understood either through modeling or extensive testing. Yet, there
are other factors related to fabrication and process control that can have an equally
dramatic effect on immunity to shock but whose details can be quite arduous to
unravel. For instance, a poorly controlled electrochemical etch in the fabrication of a
pressure sensor can yield a membrane lacking thickness uniformity that can jeopard-
ize its mechanical properties, including sensitivity to mechanical shock. Similarly,
increased scalloping and undercut in a poorly controlled deep reactive ion etch
(DRIE) can seriously degrade the mechanical properties of a micromachined struc-
ture. Investigating a failure that may be connected to poor process control is a costly
and time-consuming proposition. Instead, it is more important and economical to
implement preventive process controls over the critical fabrication and packaging
steps.
Packaging plays a very important role in shielding a micromechanical structure
from the effects of externally applied shocks. For example, the elastic nature of most
room-temperature vulcanizing (RTV) rubbers used in die attach is useful to mini-
mize the transmission of stress from the package to the sensitive micromechanical
elements in a shock [37]. Intermediate substrates and supporting material between
the die and the outer casing of the package can also be useful in protection and
lessening the impact of a shock.

Delamination
Though not a frequent failure mechanism, delamination is a concern in all multilay-
ered stacks. The fabrication methods of such stacks are detailed in Chapter 3 and
include silicon fusion bonding, LIGA, as well as surface micromachining.
Delamination is often the result of poor process control. In silicon fusion bonding,
an unreliable bond can usually be traced to minute surface defects, chemical con-
tamination, excessive bowing of the substrates under stress, or poor hydration of the
surfaces prior to bringing the two wafers into contact. A submicron defect is suffi-
cient to create an imperceptible void between the two surfaces that can later propa-
gate under the effect of aging and other environmental factors, such as shock. Silicon