Sarah J. Lukes

SU-8 2002 Surface Micromachined Deformable Membrane Mirrors

This paper describes two surface micromachining processes, i.e., one using a wet-etch release and the other using a dry-etch release, to create deformable membrane mirrors made from a thin film of low-stress SU-8 2002. The mirrors are designed for electronic focus and aberration control in imaging systems, and exhibit a large range of motion and high optical quality. The processes result in free-standing membrane mirrors with in-plane film stress as low as 12.5 MPa while attaining well-defined lithographic features as small as 3 μm in a 2.5-μm-thick film. We achieved a maximum deflection of 14.8 μm for a 3-mm × 4.24-mm elliptical boundary mirror, limited by electrostatic pull-in. Using a 3-mm × 4.24-mm mirror, which is designed for operation with a circular beam at 45° incidence angle, we demonstrate focus control while compensating spherical aberration in an optical microscope over a depth of 137 μm using a 50× 0.4-NA objective lens.

SU-8 focus control mirrors released by XeF2 dry etch

SU8-2002 deformable membrane mirrors for primary focus control and compensation of focus-induced spherical aberration have been fabricated using a surface micromachining process with dry etching of silicon in XeF2. This process has a higher yield and realizes larger mirrors with a twofold improvement in stroke, relative to a wet release etch process previously described. The use of 3 mm x 4.24 mm elliptical mirrors for 45° incidence focus control in microscopy is described.

Agile scanning using a MEMS focus control mirror in a commercial confocal microscope

We present the use of a large stroke deformable membrane mirror for focus control over a range of 123 μm in a commercial confocal microscope at 0.5-0.64 NA. The MEMS mirror is much faster than lens or stage translation allowing synchronization to x-y scanning for agile control over the 3D shape of the surface that is scanned. We describe a novel, low-actuator-count MEMS mirror designed specifically for focus control in scanning laser microscopes and present images taken with the mirror deployed on an Olympus Fluoview FV300.

Surface micro-machined SU-8 2002 membrane mirrors for focus control

We have built 1 mm circular and 1 mm × 1.4 mm elliptical surface micro-machined SU-8 2002 deformable mirrors for focus control. The SU-8 2002 membrane layers are 2.25 μm thick with gold as the reflective coating on their surfaces. Three annular electrodes were used for electrostatic actuation. Stable deflection was observed up to 6.8 μm for a 1 mm × 1.4 mm elliptical mirror with an air gap of 14 μm. The frequency response of the membrane exhibited a 3 dB frequency larger than 10 kHz.

Feedback-stabilized deformable membrane mirrors for focus control

This paper describes a method to extend the range of motion of a deformable, continuous membrane mirror beyond the limit of open-loop electrostatic instability through feedback control. The feedback scheme employs capacitive sensing directly at the mirror actuation electrodes and is based on frequency modulation of a coupled ring oscillator using a differential measurement technique. Analysis of the system shows that the range of stable deflection depends on the relative dynamics of the device and the feedback control circuitry. Experimental results demonstrate stable closed-loop deflection of our silicon nitride membrane test device to 69% of the air gap and confirm the dependence of the maximum stable displacement on overall loop dynamics.

MEMS mirror for flexible z-axis control in a commercial confocal microscope

We describe the use of a deformable membrane mirror for fast and flexible focus control in a commercial confocal microscope. En-face and oblique imaging over a 55 μm focal range has been demonstrated.

Stress engineering for free-standing SU-8 2002 thin film devices

In this paper we describe a process for creating thin SU-8 2002 films between 1.5 μm and 3.0 μm thick that are hardbaked and can withstand a release etch in either aqueous or plasma silicon etchants. Resulting films are characterized using both wafer bow and membrane bulge tests to monitor in-plane stress and Youngs modulus. We explore the influence on final film stress of several process variables including hard bake temperature, exposure dose, film thickness, and various temperature profiles. We observe resultant film stress in the range of 13.8 to 32 MPa, and Youngs modulus in the range of 2.1 to 5.2 GPa for free-standing membranes. Illustrative process recipes are described for both patterned and un-patterned SU-8 2002 membrane devices.

MEMS focus control and spherical aberration correction for multilayer optical discs

We present the use of a large-stroke deformable membrane mirror at 45° incidence to achieve a very compact optical system capable of fast multi-layer focusing in an optical disc unit. The MEMS mirror replaces a lens translation mechanism and liquid crystal compensator, resulting in a single optical element to control both focus depth and compensation of attendant focus-dependent spherical aberration. We outline the membrane optical requirements in terms of stroke and aberration compensation required for multi-layer focusing for current DVD and BD standards. We demonstrate an adjustable range of at least 1.6 μm peak wavefront spherical aberration correction at a membrane displacement of 7 μm, which should be sufficient capability for quadruple layer BDXL™ discs.

MEMS deformable mirrors for focus control in vital microscopy

This paper describes deformable membrane mirrors designed for focus control and aberration compensation in vital microscopy and shows microscope images obtained using these mirrors. The mirrors are metalized polymer membranes ranging from 1-3 mm in diameter using the photo-cured epoxy SU-8 2002, constructed using a die-bonding process. They are electrostatically actuated using three concentric electrodes to provide large displacement while minimizing spherical aberration.

Feedback stabilized deformable membrane mirrors for focus control

This paper describes a method to extend the range of motion of a deformable, continuous membrane mirror beyond the limit of open-loop electrostatic instability through the use of a feedback control scheme. The feedback scheme is based on capacitive sensing of the mirror. The sensing is achieved by coupling the actuation electrodes to a ring oscillator. We use a differential technique, where the frequency of the coupled oscillator is compared to a reference ring oscillator. Analysis of the system using a simplified parallel-plate model shows that the range of stable deflection depends on the dynamics of the device and control circuitry, and suggests that stable full-gap displacement can be achieved under certain conditions. Experimental results are provided, showing stable closed-loop deflection of our silicon nitride test device to 61% of the air gap, consistent with the predictions of our model.