CSEM Scientific and Technical Report 2008

Fabrication of Programmable Micro Diffraction Gratings
R. Lockhart, M. Tormen, R. P. Stanley
One-dimensional arrays of vertically actuated, optically flat micromirrors intended to function as programmable microdiffraction gratings (PMDG)
have been designed and fabricated.
Diffractive MEMS are interesting for optical devices as they
are compact, fast, efficient, and cost effective. These
advantages make them a viable solution for use in external
cavity lasers, miniaturized spectrometers and projection
displays. This activity seeks to build on the known advantages
of diffractive MEMS, examining the development of a fully
programmable MEMS diffractive device in which the vertical
displacement of n grating elements are individually and
accurately controlled (Figure 1). This approach allows for
increased flexibility in the generation of output spectra where
the intensity, phase and wavelength are manipulated through
appropriate vertical positioning of the grating elements.
Correlation spectroscopy and beam shaping are just two
examples of applications which stand to benefit from such a
device. This class of MEMS devices is known as
programmable micro-diffraction gratings (PMDG).
Figure 1: Schematic of a fully programmable micro diffraction grating
(PMDG)
While several research groups have developed working
examples of similar PMDG’s [1, 2] , there have been drawbacks
to these designs. The main MEMS challenges are keeping the
optical surface flat during actuation, avoiding a complex
multilayer architecture, and achieving a large vertical
displacement. Associated challenges are keeping the
actuation voltage to below breakdown and having a simplified
electrical fan-out required for the individual addressing of the
beams.
The solution developed at CSEM consists of an array of
beams which are individually deflected out-of-plane by way of
an electrostatic force applied between the substrate and a pair
of flexure beams located on either side of the optical element.
By decoupling the mechanical actuation from the optical beam,
distortions of the reflective surface are dramatically reduced.
Further improvements are achieved through a novel design of
the underlying electrodes limiting bending of the optical
elements to λ/20 at maximum deflection (λ=632.8 nm).
The electromechanical performance of this PMDG has been
simulated using 3D FEM modelling software Coventorware.
Results show that by applying a potential of