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0066_Frame_C20.fm Page 111 Wednesday, January 9, 2002 1:46 PM

Mesh Seed Layer Mold Mold
Silicon Substrate Insulation Ni-Fe Ni-Fe Insulation
Layer Cu Layer
Mesh Seed Layer
Patterned Layers
Silicon Substrate
Insulation Layer
Cu
Mesh Seed Layer Patterned Layer
Silicon Substrate
Ni-Fe Ni-Fe
Ni-Fe Ni-Fe Cu
Insulation Layer Mesh Seed Layer
Cu
Mesh Seed Layer Silicon Substrate
Silicon Substrate Movable Member
Insulation Ni-Fe Ni-Fe Insulation Ni-Fe Ni-Fe
Layer Cu Layer Cu
Mesh Seed Layer Mesh Seed Layer
Silicon Substrate Silicon Substrate

FIGURE 20.132 Basic fabrication sequential steps for the microtransducer fabrication.
used as the photoresist, offers good planarization and pattern properties, stability at low temperatures,
and exhibits negligible hydrophilic properties.
The sketched fabrication process with sequential steps to make the electromagnetic microtransducer with
a movable member is illustrated in Fig. 20.132. On the silicon substrate, the chromium–copper–chromium
(Cr–Cu–Cr) mesh seed layer is deposited (through electron-beam evaporation) forming a seed layer for
electroplating. The insulation layer (polyimide Dupont PI-2611) is spun on the top of the mesh seed
layer to form the electroplating molds. Several coats can be done to obtain the desired thickness of the
polyimide molds (one coat results in 8–12 µm insulation layer thickness). After coating, the polyimide
is cured (at 280–310°C) in nitrogen for 1 h. A thin aluminum layer is deposited on top of the cured
polyimide to form a hard mask for dry etching. Molds for the lower conductors are patterned and plasma
etched until the seed layer is exposed. After etching the aluminum (hard mask) and chromium (top
chromium–copper–chromium seed layer), the molds are filled with the electroplated copper, applying
the described copper electroplating process. One coat of polyimide insulates the lower conductors and
the magnetic core (thus, the insulation is achieved). The seed layer is deposited, mesh-patterned, coated
with polyimide, and hard-cured. The aluminum thin layers (hard mask for dry etching) are deposited,
and the mold for the magnetic cores is patterned and etched until the seed layer is exposed. After etching
the aluminum (hard mask) and the chromium (top chromium–copper–chromium seed layer), the mold
is filled with the electroplated Ni x% Fe 100−x% thin films (electroplating process). One coat of the insulating
layer (polyimide) is spin-cast and cured to insulate the magnetic core and upper conductors. The via
holes are patterned in the sputtered aluminum layer (hard mask) and etched through the polyimide layer
using oxygen plasma. The vias are filled with the electroplated copper (electroplating process). A copper–
chromium seed layer is deposited and the molds for the upper conductors are formed using thick
photoresist. The molds are filled with the electroplated copper and removed. Then, the gap for the
movable member is made using the conventional processes. After removing the seed layer, the passivation
layer (polyimide) is coated and cured to protect the top conductors. The polyimide is masked and etched
to the silicon substrate. The bottom mesh seed layer is wet etched and the microtransducer (with the
ICs to control it) is diced and sealed.
Electroplated aluminum is the needed material to fabricate microstructures. In particular, aluminum
can be used as the conductor to fabricate microcoils as well as mechanical microstructures (gears, bearing,
pins, reflecting surfaces, etc.). Advanced techniques and processes for the electrodeposition of aluminum
are documented in [9].
As was reported, the magnetic core of microstructures and microtransducers must be fabricated. Two
major challenges in fabrication of high-performance microstructures are to make electroplated magnetic
thin films with good magnetic properties as well as planarize microstructures (stationary and movable

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