Page 53 - Chip Scale Review_May June_2021-digital

Figure 4: Refractive index vs. intensity of UV exposure: a) For a standard UV cure material, there is a clear tendency of lower refractive index with higher UV intensity to
be visible; and b) DELO KATIOBOND OM614 shows a much reduced sensitivity to intensity variation.
Shrinkage is key in any imprint process, Such high temperatures are, however, Results and discussion
as the level of shrinkage directly relates to a challenge for all organic materials— As discussed in the above sections,
the level of deviation of the replicated lens especially concerning yellowing and manufacturing of fully-functional and
from the master. In the case of monolithic form stability. While requirements on reliable monolithic lens arrays requires a
imprint, shrinkage becomes even more yellowing might be more relaxed for very good control of RLT uniformity, high
important: compared to “standard” POG imprint applications that require thin layers transfer fidelity and ultra-high precision of
imprint, the overall thickness of the imprint only, monolithic lenses need the highest lens-to-lens alignment. The combination
material is much thicker, so shrinkage is requirements as even low levels of yellowing of all these factors define how well the
more evident. Fortunately, compensation might introduce visible effects due to outgoing beam deviation and coupling losses
for shrinkage can (at least to are controlled in an ultimate
some extent) be achieved by microoptics stack. All required
considering it in the design measurements to qualify the
phase. Therefore, a reproducible monolithic lens arrays are done
and uniform shrinkage is using a laser confocal microscope
probably more important than at eight different locations over
the level of shrinkage itself. the 8-inch area. The investigation
In order to reduce shrinkage- of RLT uniformity is carried
induced variations (like TTV out on five monolithic wafers
or lens height deviation as that are treated under the same
discussed in the following process conditions. The desired
paragraph), it is necessary to monolithic thickness is 200µm.
consider both the process (i.e., The within-wafer and wafer-to-
machine) part as well as the wafer RLT variations are plotted
material part. The machine in Figure 6a. The imprints
needs to eliminate as much as achieve a TTV of <20µm and the
possible all variations from the Figure 5: Transmission spectrum of a 100µm-thick layer of optical imprint RLT uniformity of <10%, within-
process (e.g., excellent wedge material on glass. While a typical polymer material would show severe yellowing wafer, and are therefore within
compensation, homogeneous after exposure to reflow soldering (260°C peak temperature), major efforts were typical customer specification
intensity distribution, etc.). undertaken to formulate a material that shows only very minor yellowing. limits of TTV<25µm. The
T he mater ial, however, average RLT varied <8µm from
needs to provide as much robustness and the rather high layer thickness. Figure 5 wafer-to-wafer. The transfer fidelity of the
predictability as possible. “Robustness” shows the transmission spectrum of DELO imprint process is determined by evaluating
in this sense refers to an insensitivity KATIOBOND OM614 before and after the the lens height and diameter. The lens
to inevitable variations (e.g., intensity reflow soldering process. diameter measurements showed a deviation
variation from top to bottom of the Monolithic lenses require a hard, but not of ±2µm from the 485µm lens diameter on
imprinted structure, see Figure 4) while brittle, material. Hardness is important to the master. An overall diameter uniformity
“predictability” refers, for example, to a maintain good form stability and scratch of the imprinted lenses of <1% is reached,
negligible batch-to-batch variation of the resistance. If, however, the material was as shown in Figure 6b. The lens height
level of shrinkage. too brittle, effects like chipping might deviation of ±3µm from the master’s value of
One of the biggest advantages of UV- happen during the dicing process or 90µm, with a uniformity of <3%, is achieved,
replicated monolithic lenses over standard handling. DELO KATIOBOND OM614’s as shown in Figure 6c. Measurements of
injection molded lenses is the ability to Young’s modulus of 3GPa (at room a set of three wafers resulted in a front to
withstand a standard reflow soldering temperature) is a near-to-perfect balance of back alignment accuracy of <1.3µm for
process with peak temperatures of 260°C. these requirements.

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