Page 259 - An Introduction to Microelectromechanical Systems Engineering

238 Packaging and Reliability Considerations for MEMS

correspondingly. Today, TO-type packages remain in use for few applications, in
particular high-power discrete devices and high-voltage linear circuits (e.g., opera-
tional amplifiers).
Metal packages are attractive to MEMS for the same reasons the integrated cir-
cuit industry adopted the technology over 30 years ago. They satisfy the pin count
requirements of most MEMS applications; they can be prototyped in small volumes
with rather short turnaround periods; and they are hermetic when sealed. But a
major drawback is the relatively large expense of metal headers and caps; they cost a
few dollars per assembled unit, at least ten times higher than an equivalent plastic
package. Early prototypes of the ADXL family of accelerometers from Analog
Devices (see Chapter 4) were available in TO-type hermetic metal packages. How-
ever, pressure to reduce manufacturing costs has led the company to adopt a stan-
dard plastic dual-in-line (DIP) solution and establish first-level packaging (at the die
level) using proprietary chip-encapsulation methods.
A metal hermetic package, including the familiar TO-8-type and the tub-like but-
terfly package, is frequently made of ASTM F-15 alloy (Kovar), though steel is also
possible. Because Kovar has a low coefficient of thermal expansion that is matched to
fused silica (a common optical material), it is a metal of choice for butterfly packages
used in optical and photonic applications. A sheet of metal is first formed into a
header or a tub-like housing. Holes are then punched, either through the bottom for
plug-in packages or the sides for flat or tub-like packages. An oxide is then grown
over the package housing. Metal leads are placed through the holes and beads of
borosilicate glass, such as Corning 7052 glass, are placed over the leads. Fusing of the
glass to metal at a temperature above the melting temperature of glass (~ 500ºC) pro-
duces a hermetic metal-to-glass seal. Etching the metal oxides reveals a fresh alloy
surface that is then plated with either nickel or gold—both of which allow wire bond-
ing and soldering. Standard headers, butterfly packages, and lids are commercially
available and can be readily modified in conventional machine shops. For instance,
metal tubes can be brazed to drilled ports in the header and a companion coverlid
to provide access to fluids in pressure and flow sensors and microvalves (see
Figure 8.11). Similarly, a feed-through tube may be brazed to the sidewalls of a
butterfly package for eventual optical interconnecting using a fiber [see Figure
8.7(a)]. In the final packaging assembly, the micromachined structures as well as
other components (e.g., optical elements) are mounted directly on the header or
within the tub of the package. Wire bonds to the plated package leads establish elec-
trical connectivity. If necessary, optical or fluidic connections are also made, as dis-
cussed earlier. Finally, the soldering or seam welding of the header or butterfly
package to a coverlid (or cap), most often made of the same alloy, hermetically seals
the assembly.
One example of metal packaging applies to the tunable laser from Santur Corp.
discussed in Chapter 5. The packaging includes positioning the array of distributed
feedback lasers (DFBs), the tilting micromirror, and a host of optical elements
within the butterfly package, as well as making the appropriate electrical and fiber
interconnections [see Figure 8.12(a)]. The die that holds the micromirror is first
attached and wire bonded to a ceramic chip with electrical pads. This micromirror
subassembly is mounted on its side over an underlying ceramic plate that also holds
the DFB array, two beam splitters, and a InGaAs quadrant detector. The ceramic