A Batch Wafer Scale LIGA Assembly and Packaging Technique vai Diffusion Bonding Page: 4 of 8
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tests and is unique in the sense that all interfaces have
identical overlap area which allows it to be diffusion bonded
in one step while maintaining equal pressures at each
interface. Copper wires have been press fit into two locating
holes to provide interlayer alignment for the component. This
preliminary work identified successful bonding parameters of
temperatures near 500 C and pressures near 10 ksi with
periods of several hours.
The basic batch wafer-scale diffusion bonding scheme is
illustrated in Fig. 2. Two substrates are prepared, one with
and one without a sacrificial layer, and are subsequently
decorated with complementary DXRL formed and planarized
geometry. If fully released two-layer components are desired,
a sacrificial layer is applied to both substrates. For multiple
layer released components, a titanium layer may be used to
protect the second sacrificial layer until release. Typical
substrate materials are nickel, alumina, or silicon. Copper at 1
pm thick serves as the sacrificial material for nickel structures
and can be etched in wet chemistry that in no way attacks the
mechanical nickel material. The copper sacrificial layer also
must not interact with the mechanical material (nickel) in
order to retain its etchability and not disturb the planarity of
the mechanical material surface for subsequent diffusion
bonding steps. Copper has been found to be suitable for this
task and a diffusion barrier material has been found not to be
The substrates are also provided with DXRL patterned
alignment structures to allow the press fitting of alignment
pegs. The resulting alignment scheme uses commercially
available precision circular gauge pins which have a
diametrical tolerance of +1 m/-Opm. The pins are cut to
length and press fit into one of the substrates which will be
designated the foundation substrate without a sacrificial layer.
The other substrate (the sacrificial substrate) is then press fit
onto the foundation substrate. This step has been
demonstrated without the assistance of any optical instruments
and is aided by slightly beveling the end of the pins.
Prior to joining the substrates, the nickel is initially
cleaned with an oxygen plasma followed by an ammonium
hydroxide treatment to remove the nickel oxide . The
substrates are immediately placed in a hot press apparatus
which is evacuated to the 10~6 Ton range and backfilled with
argon prior to heating and pressing. The hot press
accommodates 6-inch diameter subtrates and is capable of
providing a 50 ton load at 1100 C. Since the bonding area
will likely never be greater than 50% of the substrate area, this
press is well suited for the 3 and 4 inch diameter substrates
that are used here at 10 ksi pressures.
Subsequent to bonding, the substrates are removed from
the hot press and placed in a liquid sacrificial layer etchant
that releases the entire sacrificial substrate. Additional layers
are processed in the same manner and it is evident that there is
no limit on the number of levels that may be joined.
The test mask set that was used includes a layout with a
variety of geometry which when mirrored and stacked
constitutes a variety of overlapping structures. Two level
nickel structures resulting from the batch alignment and
diffusion bonding sequence are shown in Fig. 3. Interlayer
alignment of less than 1 gm was achieved as revealed by the
two level x-y vernier measurement fiducials shown in Fig. 4.
These vernier patterns are placed near the outer edge of the 3
inch diameter pattern and co-located with the press fit pin
alignment structures thereby ensuring even better than 1 im
alignment between these areas.
- y_ sacrifidal layer (Cu)
+-LIGA layer#2 (Ni)
LIGA layer#1 (Ni)
Nickel substrate (structure)
U DIFFUSION BONDand RELEASE
FA 11 11-F
Fig. 2 Batch wafer-scale DXRL based diffusion bonding procedure.
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Christenson, T.R. & Schmale, D.T. A Batch Wafer Scale LIGA Assembly and Packaging Technique vai Diffusion Bonding, article, January 27, 1999; Albuquerque, New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc681650/m1/4/: accessed April 19, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.