Online proceedings - EDA Publishing Association

(2mm " 120!m " 120!m), k z and k y are calculated as 0.78
N/mm, for the outer beams (3.7mm " 120!m " 120!m)
0.12 N/mm. The springs are parallel in z direction providing
a combined spring constant k z for each buckled beam flexure
of 0.10N/mm. For lateral deflections, the direction of
the applied force is relevant. The evaluation of the force
generation model in that direction exceeds the scope of this
article.
D. Manufacturing considerations and feasibility tests
To manufacture the prototype of the integrated actuator,
the Heriot Watt University supports facilities, which include
an automated screen printer with camera alignment (model
DEK Horizon 265), an isostatic press (model KEKO ILS-4),
a cofiring oven (model Nabertherm 30º-3000º), a CO 2 laser
cutter (EPILOG Mini 18"12, 10.6!m wavelength) and a
self-built powder blaster. The design has to comply with
these capabilities and support rapid prototyping methods.
To focus the manufacturing process on the functional layers,
the layers containing the magnets are reusable acrylic
frames, as depicted in figure 6. Via filling and sacrificial
material filling is processed as manual stencil coating step
using laser cut Mylar TM , which is delivered with the unblanked
green sheet as support. The emulsion screen is the
only externally manufactured mask.
The device concept contains two critical aspects that have
to be carefully taken into account, firstly, the structural integrity
and position of the comparatively large actuated area,
including the beam flexures, during the whole manufacturing
process. To prevent deformations and displacements,
the gaps have to be filled with sacrificial material. The
green sheet is in contact with the stencil during automated
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May 2011, Aix-en-Provence, France
printing process (and manual stencil filling step for the prototype),
so that the raising stencil causes a contact vacuum
after the print is finished. Thus, mechanical structuring all
features at once would damage the structure. To prevent
this, structuring and printing has to be split in two process
steps, so that the mobile parts remain fixed.
A second manufacturing issue is the accurate top and bottom
metallization of the flexures. The standard LTCC process
printing accuracy depends on the thixotropic viscosity
of the metallization, the metal particle size, the mesh density
and thread diameter of the screen, as well as the alignment
accuracy and configuration of the screen printer. The
used screen printer is specified to guarantee 40!m alignment
accuracy for appropriate fiducial qualities. The internal
c p /c pk statistics show that the machine repeatability is
25!m. The metallization paste is specified to support line
widths of 100!m. Hence, the dimensions of the coils and
counter electrodes are adapted to the size of the magnet
pairs in order to prevent fringing effects in the actuator elements.
This tolerance is configured to be twice the maximum
assumed alignment error (40!m).
Alignment and minimum printing feature size is especially
crucial for the metallization of the flexures if the screen
printing process step follows after the mechanical structuring
and gap filling. The beam widths of 120!m have to be
completely covered to prevent an increased electrical resistance
whilst preventing overprinting to exclude the risk
of short circuits between top and bottom side. To solve this
problem, the mechanical structuring is performed after the
metallization printing process, where the track widths are
increased by twice the maximum alignment error. Thus, the
structuring process removes the LTCC material including
the metallization, and the process becomes self-aligning.
Fig. 7a: Test beam structures manufactured using powder blasting. The masks
were manufactured of 500!m PMMA sheets and clamped on the substrate
during the process. The lamination stacking pins are used for the alignment of
the mask.
Fig. 7b: Powder blasting alignment test. An expired paste was printed for
lower material consumption. In spite of explicitly laser cutting inaccurate
alignment marks causing 70!m misalignment, the metallization is still in
range to process completely covered flexures.
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