Biennial Report 2016/2017

Reports
Figure 3: Relative grating depth measured at different
positions along the substrate radius. The radial offsets for
the two etching positions were R 0 = 107.9 mm and
R 1 = 260.6 mm, respectively.
Selected example for RIBE pattern
transfer developments
Besides the improvement of the technical
infrastructure and the development of algorithms
processing of large workpieces, considerable
efforts were made in the development and
optimization of the RIBE pattern transfer for the
fabrication of different high-end diffraction
gratings, in particular working closely with Carl
Zeiss Jena GmbH. One joint project was the
manufacturing of highly-dispersive, high-efficiency
transmission gratings by laser interference
lithography and dry etching [2] which will be
presented briefly below.
Diffraction efficiency for transmission gratings with
period-to-wavelength ratios smaller than 0.6 is
mainly limited due to reflection losses from
imperfect effective refractive index matching. To
address the mentioned losses and achieve high
efficiencies a process chain consisting of twobeam
laser interference lithography (LIL) and
RIBE has been developed to obtain subwavelength
gratings with optimized non-binary
profiles. The manufactured 1700 l/mm fused silica
gratings with a 1030 nm center wavelength
achieved diffraction efficiencies of above 96%,
significantly exceeding the theoretical limit for
widely used monolithic binary profiles in this
application.
In the experiment, the substrates were spin
coated with approx. 550 nm AZ1518 positive
resist after applying an adhesion promoter. The
resist thickness has to be sufficient for etching
about 1.5 µm into the fused silica substrate
without completely consuming the resist mask. If
the latter is too deep, the sidewall angles will
increase, which is not desirable with regard to
efficiency optimization. To create the gratings, the
resist-coated and softbaked substrate is exposed
to the stabilized interference pattern and
afterwards wet-chemically developed in basic
solution. The depth, duty cycle and sidewall-slope
of the resist profiles can widely be tuned by the
resist thickness, exposure dose, developer
concentration and development time. Since the
resist mask sidewalls erode during etching, a
sufficiently large duty cycle has to be provided,
which, together with a continuous profile, can be
attained by reducing exposure dose to a
comparatively low level and stopping the
immersion in the developer timely.
For the pattern transfer of the resist profile into the
fused silica substrate, reactive ion beam etching
has been used. As already pointed out above, ion
beam techniques, especially RIBE, offer the
advantage of separating the substrate from the
plasma. This allows for additional parameters for
proportional or non-proportional pattern transfer,
which are important for optimization of the etched
profile. For the RIBE process the SHAPION plant
was used. The diameter of the broad ion beam
was approximately 150 mm (FWHM). The plant
has a 4-axes motion system that, combined with
sophisticated motion algorithms described above,
allows a uniform pattern transfer for workpieces
with dimensions larger than the used ion beam
source. The ion source was operated with a beam
voltage of 700 V. During etching, the watercooled
substrate rotated with 4.8 rpm. A mixture of
CHF 3 /Ar/O 2 was used as etching gas with a total
mass flow of 4 sccm. Depending on the
composition of the etching gas, the selectivity, i.e.
etching rate of fused silica related to the value of
the resist, can be adjusted approx. from 0.1 to 15.
It must be noted that these selectivities were
determined for planar thin films of SiO 2 and resist,
respectively. For patterned samples, the
selectivity shrinks with decreasing feature size.
Nevertheless, for the used gas mixtures (3.0 sccm
CHF 3 /0.5 sccm Ar/0.5 sccm O 2 ), the selectivity is
sufficient to achieve etching depths of 1.5 µm with
the resist mask discussed above. After RIBE, the
remaining resist residuals were removed using
Caro's acid (sulfuric acid with hydrogen peroxide).
The final depth of the gratings was measured by
AFM using high aspect ratio Si tips. In addition to
the possibility to, within certain limits, control the
profile shape by means of the gas mixture via the
angular dependent etch rates, the beamlet
divergence of the broad ion beam, i.e. the angular
spread of the ion's straight trajectories, could play
an important role. For deeply etched grooves,
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