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284 MEMS and Microstructures in Aerospace Applications

beryllium copper. This total of five layers of materials aligned and stacked in an
alternating fashion provided some unique assembly and packaging challenges. Each
silicon layer has five dies (one per pixel) with an array of aperture slits on each die.
The CuBe plates were precision machined to achieve the array of channels, each
with a fixed field-of-view, with placement accuracy between arrays sufficient to
allow for the integration of an array of five silicon die. The wafers were diced such
that each of the five dies could be individually aligned and bonded to the CuBe
plates using a flip-chip die-attach bonding technique. Low stress conductive and
nonconductive epoxies were selected for bonding the five layers to each other
because of high mismatches in coefficients of thermal expansion between the
silicon and CuBe. The bonded components of the sensor head were packaged within
the iridite-plated aluminum supporting structure via mounting brackets and alumi-
num rods used for maintaining a 1-mm offset to the MCP. The lower flange of
the MCP was adhesively attached to the insulating mounting plate, which is
attached to the housing with screws around the perimeter. The remaining
items were assembled and packaged into the spacecraft mechanical interface
housing using 2–56 and 4–40 screws. Spot welding a high voltage lead to
the upper and lower plates of the MCP provided electrical connection from
the HV power supply. A AuNi plated Kovar lead was welded to the CuBe electro-
static analyzer in order to provide an accessible site for soldering a scan
voltage supply and ground wire from the PCB. An Sn63Pb37 solder was used to
connect the power supply to the PCB, and from each preamplifier discriminator
circuit on the PCB to the plated through vias on each anode. In addition, ensuring a
conductive bleed path from every conductive surface to spacecraft ground mitigated
potential charging effects. The packaging scheme of the FlaPS instrument is
illustrated in Figure 12.9.

12.6.3 MICROMIRROR ARRAYS FOR THE JAMES WEBB SPACE TELESCOPE
In support of the James Webb Space Telescope (JWST), equipped with the multi-
object-spectrometer, individually addressable MEMS mirror arrays serving as a slit
mask for the spectrometer, will selectively direct light rays from different regions of
space into the spectrometer. An integrated micromirror array or CMOS driver chip
was designed at NASA GSFC. System requirements posed several challenges to
the packaging of the integrated MEMS chips. However, flip-chip technology
to bump-bond the large chips (9 9 cm) onto a silicon substrate in a 2 2 mosaic
pattern was used to eliminate the concern for global thermomechanical stresses
due to mismatched coefficients of thermal expansion between the chip and sub-
strate. Alignment of the chips forming the mosaic pattern was also a critical
system specification. The relative tilt angle between the chips was held within
0.058 by making use of the restoring force of the solder bumps to self-align the
chips during flip-chip solder reflow. The attached MMA or CMOS assembly was
placed inside a package and fixed via peripheral pressure contacts. And, finally,
input or output leads were made via tape-automated bonding from the package to
the chips. 14

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