Hybrid Surface-/Bulk-Micromachining Processes for Scanning Micro-Optical Components


This dissertation discusses the design and fabrication of micro-optical scanners that have  high-quality  optical-surface  properties  and  are  capable  of  previously  unattainable high  scanning  rates.   Scanners  having  high-quality  optical  surfaces  are  fabricated  and characterized in both diffractive and reflective applications.

A  first  project  investigates  methods  of  creating  scanning  rectangular  diffraction gratings using well-established fabrication methods of silicon surface-micromachining in a foundry process.  We then introduce new methods to form an actuated blazed grating, a diffraction grating having a triangular surface profile that provides improved diffraction efficiency and wavelength resolution when compared to the rectangular grating.  We use KOH  anisotropic  etching  of  single-crystal  silicon  to  create  a  mold  tailored  by  crystal planes in the substrate.   The triangular shape of the  mold  shapes an  LPCVD-deposited thin-film  polysilicon  plate.   The polysilicon  plate  can  be  lifted  out  of  the  mold  in  the silicon substrate after an interposed thin film of silicon dioxide has been selectively etched by  HF  to  release  it.  Hence,  by  integrating  bulk-silicon  micromachining  steps  into  the surface-micromachining process, we create the high-quality diffractive surface properties of a blazed grating on a scanner structure.   Improvements over the scanned rectangular grating in terms of diffraction efficiency and wavelength resolution are demonstrated to be significant.  The measured diffraction efficiency of 60% in the blazed grating is four times that  of  the  rectangular  grating  at  an  incident  wavelength  of  632.8 nm.    Wavelength resolution  for  the  high-order  blazed  grating  is  not  limited  by  the  linewidth  of  the fabrication process in contrast to the case  of first-order rectangular gratings.   Thus, the wavelength  resolution  of  the  blazed  grating  holds  a fivefold  increase  from  that  of  a comparably sized rectangular grating made in a minimum 2μm-linewidth process.

Some  limitations  of  the  surface-micromachined  scanner,  such  as  non-rigid  optical plates that deform dynamically during scanning and weak actuators that are only useful for resonant scanning, prevent the use of these scanners in high-performance applications. Examples of these  demanding applications are: in external-cavity tunable lasers (whichrequirestatic-positioningcapability),andindensewavelengthmultiplexing/demultiplexing (which requires minimum resolvable wavelengths of 1 nm or less).   For our design  of the surface-micromachined blazed diffraction  grating, actuation  was only satisfactory at resonance, and deformation of the optical plate during resonant scanning degraded the minimum resolvable wavelength of the grating from 1 nm to 2.7 nm.   This performance  limits  the  surface-micromachined  grating  to  less  demanding  applications such as portable scanning spectrometers.

The limitations apply both to scanning gratings and mirrors.  In the case of scanning mirrors  for high-resolution  display  applications, much  improvement  is  demonstrated  in scanning mirrors that we have developed that use thick single-crystal silicon as structural material and which are built using high-aspect-ratio micromachining. With a new process, we have produced torsional scanners driven by high-force actuators.   The mirrors are built on a  thin  film  of polysilicon  stretched  across a  thick, single-crystal silicon support  rib. They  are  rigid  and  remain flat  even  under  high-speed  scanning  stresses.   We  call  the mirrors  STEC/TOS  (Staggered  Torsional  Electrostatic  Combdrive  /  Tensile  Optical-Surface).  They have low inertia and are capable of high-speed resonant scanning without appreciable surface deformations (too small to compromise the optical resolution of the mirror).We  demonstrate  STEC/TOS  micromirrors  of  differing  designs,  some  have resonant frequencies as high as 81 kHz and others are capable of dc mechanical-deflection angles up to 8.3°.  We have controlled the tensile stress in the polysilicon optical surface sufficiently  to  limit  static-  and  dynamic-deformations  to  less  than  70 nm  across  the polysilicon optical surface.

Scanning imaging applications such as a high-resolution scanning display require two scanning micromirrors having diverse requirements.  One microscanner must be capable of large-angle, high-speed  scanning  and  the other must be  capable  of large-angle, low-frequency mirror positioning.  We have addressed the design constraints and tradeoffs for the STEC/TOS micromirrors in these two applications.

To achieve large-angle, high-speed scanning without significant static- and dynamic-surface deformations, the micromirror must be lightweight and have relatively large-force actuators and stiff torsional hinges.  The polysilicon mirror surface should be moderately tensile-stressed (200-300 MPa) and supported by a stiff single-crystal silicon support rib. A tradeoff exists between stiffness of the silicon support rib, primarily determined by its width, and  the  overall  mass  of the  mirror.   Another  tradeoff  exists  between  achievable static- and dynamic-surface deformations.  Higher tensile stress in the polysilicon surface produces lower dynamic-surface deformation but also applies more stress to the silicon support rib and thus contributes to increased static-surface deformation.

In the second case (that of the large-angle  low-frequency scanner), the  micromirror should have relatively large-force actuators and soft torsional hinges.   The softer hinges provide lower restoring forces, thus allowing larger-angle scanning for a fixed amount of actuator force.   As dynamic-surface deformations and overall mass are lesser issues for this scanner, its design is easier with only static-surface deformations to consider.  A low tensile stress (under 100 MPa tensile) polysilicon-mirror surface supported by a very wide(and thus very stiff) single-crystal silicon support rib is sufficient to ensure minimal static-surface deformations.

The  design  of  MEMS  micromirrors  has  typically  focused  the   use  of   existing fabrication technologies without giving equal attention to the optical performance of these devices, especially their performance under dynamic conditions.  The research described here targets designs and fabrication technologies that can create optical MEMS scanners with high-quality optical surfaces that do not deform nor degrade during operation.   We have  pushed  the  limits  by  creating  optical  scanners to  scan  at  previously  unattainable speeds (as high as 81 kHz).   In this work, we have developed a fabrication process that produces robust  scanners  and  brought  the  technology  to  a  level  suitable  for  transfer  to industry.

Kam Y. Lau
Paul Wright
Publication date: 
December 31, 2001
Publication type: 
Ph.D. Dissertation
Nee, J. T. (2001). Hybrid Surface-/bulk-micromachining Processes for Scanning Micro-optical Components. United States: University of California, Berkeley.

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