Fabrication of pixelated wiregrid polarizers for real-timelaser beam profile analysis
Florian Schlachter1, Michael Hornung1, Stephan Suckow1, Max Christian Lemme1,2
1

AMO GmbH, Otto-Blumenthal-Str. 25, 52074 Aachen, Germany
Chair of Electronic Devices, RWTH Aachen University, Otto-Blumenthal Str. 2, 52074, Germany

2E-mail: schlachter@amo.deThe exact knowledge of the laser beam profile, wave front and polarization state is very often thecriteria for an effective and long-term stable usage in industrial applications. Especially thecontrol of the laser beam during operation of high-power lasers, e.g. in material processing, isessential. Here the thermal lens for beam guidance and focusing optics can result in aconsiderable deviation of the beam parameters from the targeted process values [1]. Due to thecomplex and transient nature of thermally induced effects, established measurement methods
only cover partial aspects of the problem, which is relevant e.g. for the time-resolved analysis ofstress birefringence and associated wave front deformations in thermally loaded optics [2]. A
real-time measurement method for comprehensive, quantitative light field analysis could
significantly reduce obstacles in the industrial use of high-power lasers.
For the high-resolution real-time acquisition of all relevant beam parameters a measuring
principle based on the wave front curvature sensor was further developed and extended by thespatially resolved determination of the polarization state. In this work we report on thefabrication of a pixelated wire grid polarizer to enable this analysis technique by using
nanoimprint lithography. The elementary cell of this nanogrid array consists of four gratings withdifferent orientations covering exactly four pixels of the corresponding CCD detector (figures 1and 2). With this new sensor, the wave front can be reconstructed either separately for eachpolarization direction or by averaging the four partial gratings. It is also possible to characterize
the polarization state locally by determining the Stokes parameters.
The pixelated nanogratings of the polarizing element consist of 1:1 lines & spaces patterns with apitch of 180 nm, arranged in four different orientations per elementary cell (figure 2). This
polarizing element is defined in a 180 nm thick aluminum (Al) layer located on a 2 mm thickquartz substrate. The respective etch mask is fabricated by substrate conformal imprint
lithography (SCIL) which allows the distortion-free replication of nanopatterns with criticaldimensions well below 100 nm [3]. In order to enhance the selectivity between the aluminumand the resist etch mask, an additional silicon dioxide (SiO2)-layer of 50 nm is deposited onto thealuminum layer acting as a hard mask during the reactive ion etching (RIE) process. The grating
structures are finally transferred into the Al by a two-stage RIE process. In the first step theresidual layer of the imprint resist and the SiO2 hard mask are opened in a fluoroform (CHF3)based plasma. The underlying Al layer is then etched in a combined etch plasma consisting ofboron trichloride (BCl3) and chlorine (Cl2) as main etch gases and fluoroform (CHF3), whichguarantees highest anisotropy resulting in Al lines with aspect ratios up to 3:1 and perpendicularsidewalls (see figures 3 and 4). Corresponding optical measurements of these polarizing
elements resulted in polarization extinction ratios up to 389:1 at 1064 nm wavelength.
In conclusion pixelated wire grid polarizers based on Al gratings with aspect ratios up to 3:1 andextinction ratios up to 389:1 based on a fabrication process chain consisting of a distortion freeSCIL feature replication and specifically tailored RIE etching processes were demonstrated.
This work has been financially supported by the German Federal Ministry of Education andResearch (BMBF) under "Photonik Plus – Neue optische Basistechnologien" program within theresearch project “NANOPOL” (Grant No. 13N14231).

Reference:
[1]
[2]
[3]

D. Shealy et al., Appl. Opt. 45 (2006), 5118-5131.
R. Weber et al., Optical Materials, Vol. 11 (1999), 245-254.
R. Ji et al., Microelectronic Engineering, Vol. 87 (2010), 963-967.

Figure 1 Schematic view of the pixelated Figure 2 Schematic view of the pixelated
polarizer. From left to right: CCD cell, elementary cell showing the four different
polarizing element, polarization direction and grating orientations.
beam profile.

Figure 3 SEM image of the centre of thepixelated elementary cell showing the single
grating blocks.

Figure 4 cross section SEM image of thenanograting defined in aluminum with a heightof 200 nm and a width of 90 nm.