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followed by titanium (Ti) which showed a slight increase in
the bond strength with SU-8. The bond of SU-8 to Al was
even stronger, with a value of 12.1 (±2.8) MPa. The
measured bond strength between SU-8 and Si was 18.5(±
4.6) MPa. However, after less than five replicas the SU-8
was delaminated from the aluminum.
11-13
May 2011, Aix-en-Provence, France
The PMMA replica shown in Fig.3 was produced in six
passes at a temperature of 166 °C for both upper and lower
rollers while applying a pressure of 6 bars and a feed rate
0.1 m/min. In this particular case, only the number of passes
varies in the optimization process as the other parameters
were physically or software limited.
Fig. 1. SU-8/Si mould with micro-features of 100 µm wide and 80 µm height.
Inset: Magnification view of the ridge section 250X
This low number of replication is not acceptable for a roll
embossing process and SU-8/Al is not durable to be used as
stamp. Delamination is explained by reaching a relatively
high PEB temperature (95 °C, 2 min) without ramping on
two different materials with thermal expansion coefficient
mismatch.
On the another side the SU-8/Si stamp has been used in
roll-to-flat configuration and microfeatures of 100 µm width
and 80 µm depth were successfully replicated in Topas
8007 COC and PMMA. The COC replica shown in Fig.2
was produced in a two-pass process at a temperature of 110
and 145 °C for upper and lower roll respectively, using a
pressure of 6 bars and a feed rate of 0.3 m/min. These
processing parameters show an increase in embossing
temperature and reduction in speed compared with the
parameters used with the silicon moulds [7]. This increase
was expected as the thermal conductivity of SU-8 is less
than that of silicon (on the order of 0.3 vs. 149 W/m.°K,
respectively).
Fig. 3. PMMA replica embossed from SU-8/Si mould with micro-features
of 100 µm wide and 80 µm deep. Inset: Magnification view of the channel
section 250X
B. SU-8 e-beam dose evaluation and patterning of submicronic
features
The e-beam exposure dose variation was used to measure
the resist height versus dose characteristic curve. The
exposure dose was obtained by measuring the current using
a Faraday cage. The remaining SU-8 resist thickness was
measured by a profilometry at the 5µm features after
development and PEB. Fig. 4 shows the remaining SU-8
resist thickness versus exposure.
Normally, SU-8 contrast and sensitivity are determined
for a resist thickness which is half the initial film thickness
(D H=0.5 , which also depends on PEB temperature and
development conditions).
The optimum dose depends on pattern size and period so
that doses were selected for each feature size after SEM
observation to avoid proximity effect and to maintain a
given profile.
1400
Remaining resist thickness vs dose
1200
Remaining resist thickness (nm)
1000
800
600
400
200
Fig. 2. Topas COC 8007 replica embossed from SU-8/Si mould with microfeatures
of 100 µm wide and 80 µm deep. Inset: Magnification view of the
channel section 250X
0
0 0.5 1 1.5 2 2.5 3 3.5 4
Dose (µC/cm2)
Fig. 4. Remaining SU-8 2002 resist thickness (after a 2 min PEB at 95 °C &
10 min development in SU-8 developer) versus e-beam exposure dose at 30
keV, measured by profilometry, for pads of nominal width 5 µm.
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