Transfer printing: an emerging technology for massively parallel assembly
9 shows a placement accuracy vector process (210 °C for 40 minutes under flowing anticipated that transfer printing could be
plot. The consistent displacement vector N
2
). Following this thermal process, the applied to the manufacture of advanced
patterns indicate that a slight rotational printed chips pass scotch tape tests, and backplanes for flat panel x-ray detectors
(θ) misalignment occurred during each standard wafer processing can be per- where each pixel requires an amplifier
transfer print. Figure 10 displays the same formed without disturbing the devices. circuit to lower noise
15
.
data in a color-coded plot. In this case the Measurements revealed that there was no In the concentrated photovoltaics
magnitude of the displacement vectors is change in the chip placement following the (CPV) industry, standard assembly equip-
converted to a color scale. The slight rota- BCB soft cure process. ment is being used to pick-and-place small
tional error is again apparent in this graph. high efficiency compound semiconductor
Finally, the combined X and Y placement applications of transfer printing solar cells
16
. There is a general trend toward
accuracy data are shown on a histogram One of the advantages of the transfer print- handling ever-smaller solar cells, but current
plot in Figure 11. A Gaussian curve fit ing approach is that all of the demanding assembly technology places constraints on
of this data set indicate that placement fabrication steps for high performance the numbers and sizes of cells that can be
accuracy better than ± 4 µm @ 3σ can be semiconductor devices can take place on economically handled. Transfer printing
achieved using transfer printing with an the source wafer instead of the target sub- opens a path to large assemblies of pre-
elastomeric stamp. The keys to achieving strate. As a result, the inherent mechanical cisely positioned microscale high efficiency
this placement accuracy include the ability and chemical instabilities of the target sub- solar cells.
to manufacture a mechanically stable high- strate material (i.e. glass, plastic, paper, etc.) Electronic packages, such as Freescale’s
fidelity transfer stamp and the ability to do not limit the choice of the semiconduc- Redistributed Chip Package
17
, require
align the chips to the substrate by looking tor process or device type. This implies that transfer of many die onto a reconfigured
through the transparent stamp. This allows transfer printing is an attractive method wafer. With transfer printing, the package
for the alignment to be performed with for the manufacture of display or sensor designer could consider assembling smaller
the chips in close proximity to the target backplanes on glass or plastic. For display devices and the improved placement accuracy
substrate. applications, transfer printing could be would allow for interconnections with
Following the transfer printing, the used to print high performance driver tighter tolerance and higher yield.
wafers went through a BCB soft cure circuitry for advanced pixel concepts. It is
Figure 7. Transfer printing yield map. One chip was
missing from this 1024 chip array, which translates
to 99.9% yield.
Figure 8. Methodology to determine the placement accuracy of the transfer printed chips using image analysis.
Figure 9. Placement accuracy vector plot illustrating Figure 10. Placement accuracy color plot. The color Figure 11. Placement accuracy histogram plot.
the slight rotational misalignment that occurred on scale bar units are microns.
this sample.
22 – Global Solar Technology – November/December 2008 www.globalsolartechnology.com
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