Page 92 - An Introduction to Microelectromechanical Systems Engineering

Combining the Tools—Examples of Commercial Processes 71

structural element and an organic polymer as a sacrificial layer. Chapter 5 describes
this particular device in greater detail.

Combining Silicon Fusion Bonding with Reactive Ion Etching
The silicon fusion bonding with reactive ion etching (SFB-DRIE) process involves
the formation of tall structures in crystalline silicon to overcome the thinness
limitation of surface micromachining [36]. Instead of depositing thin polysilicon
layers, crystalline silicon substrates are fusion bonded to each other in a stack. Each
substrate is polished down to a desired thickness, then patterned and etched before
the next one is bonded. An optional intermediate silicon dioxide between the silicon
substrates is not a sacrificial layer but is rather for electrical and thermal insulation.
The process allows the building of complex three-dimensional structures one thick
layer at a time.
The basic process flow begins by etching a cavity in a first wafer, referred to as
the handle wafer (see Figure 3.30). A second wafer is silicon fusion bonded on. An
optional grind and polish step reduces the thickness of the bonded wafer to any
desired value. CMOS electronic circuits can then be integrated on the top surface of
the bonded stack without affecting any of its mechanical properties. Finally, a DRIE
step determines the shape of the microstructures and mechanically releases them as
soon as the etch reaches the embedded cavity. This cavity takes the role of the sacri-
ficial layer in surface micromachining and ensures that the micromechanical struc-
tures are free to move except at well-defined anchor points.
The high aspect ratio and depth available using the SFB-DRIE process add new
dimensions to the design and fabrication of complex three-dimensional structures
(see Figure 3.31). A range of new applications, including those integrating fluid flow
functions such as valving and pumping, can be addressed with this process. Robust
thermal actuators made of crystalline silicon are also feasible with an available out-
put force approaching one newton. This process is now a manufacturing platform
at GE NovaSensor of Fremont, California.

DRIE of SOI Wafers
The availability of double-sided aligners, DRIE tools, and SOI wafers led to a rela-
tively simple process for fabricating three-dimensional microstructures that became
popular in the late 1990s. The process begins with DRIE of the thinner top layer of
an SOI wafer to form the desired structure (see Figure 3.32). The etch stops with
high selectivity on the buried oxide layer. If undercut of the silicon at the oxide inter-
face control is not desired, the specialized stop-on-oxide recipe discussed earlier can
be used. A large area of the back side, corresponding to the structure on the front
side, is etched to the buried oxide layer. Finally, the now-freestanding buried oxide
is etched away, typically with hydrogen fluoride [hydrofluoric acid (HF)] vapor or a
liquid HF solution, both of which selectively etch the oxide. If liquid HF is used and
the structure is fragile, it must be handled carefully to avoid breakage during etch-
ing, rinsing, and drying. A variation on the process is to etch the device structure
from the top, then release it by etching the underlying oxide, which may be as thick
as 2 µm, in liquid HF. If the structure is sufficiently stiff, it can be dried without spe-
cial handling. If it is too compliant, critical-point drying can be used. Similar
processes are in development or commercial use by companies including the