Rainer K. Fettig

Some Aspects on the Mechanical Analysis of Micro-Shutters

An array of individually addressable micro-shutters is being designed for spectroscopic applications. Details of the design are presented in a companion paper. The mechanical design of a single shutter element has been completed. This design consists of a shutter blade suspended on a torsion beam manufactured out of single crystal silicon membranes. During operation the shutter blade will be rotated by 90 degrees out of the array plane. Thus, the stability and durability of the beams are crucial for the reliability of the devices. Structures were fabricated using focused ion beam milling in a FEI 620 dual beam machine, and subsequent testing was completed using the same platform. This allowed for short turn around times. We performed torsion and bending experiments to determine key characteristics of the membrane material. Results of measurements on prototype shutters were compared with the predictions of the numerical models. The data from these focused studies were used in conjunction with experiments and numerical models of shutter prototypes to optimize the design. In this work, we present the results of the material studies, and assess the mechanical performance of the resulting design.

Magnetically actuated microshutter arrays

Two-dimensional microshutter arrays are being developed at NASA Goddard Space Flight Center (GSFC) for the Next Generation Space Telescope (NGST) for use in the near-infrared region. Functioning as focal plane object selection devices, the microshutter arrays are 2-D programmable masks with high efficiency and high contrast. The NGST environment requires cryogenic operation at 45 K. Arrays are close-packed silicon nitride membranes with a unit cell size of 100x100 micrometer. Individual shutters are patterned with a torsion flexure permitting shutters to open 90 degrees with minimized mechanical stress concentration. The mechanical shutter arrays are fabricated with MEMS technologies. The processing includes a RIE front-etch to form shutters out of the nitride membrane, an anisotropic back-etch for wafer thinning, and a deep RIE (DRIE) back-etch down to the nitride shutter membrane to form frames and to relieve the shutters from the silicon substrate. A layer of magnetic material is deposited onto each shutter. Onto the side-wall of the support structure a metal layer is deposited that acts as a vertical hold electrode. Shutters are rotated into the support structure by means of an external magnet that is swept across the shutter array for opening. Addressing is performed through a scheme using row and column address lines on each chip and external addressing electronics.

Nano-scale fatigue study of LPCVD silicon nitride thin films using a mechanical-amplifier actuator

This paper describes a nano-scale tensile test to study the fatigue properties of LPCVD silicon nitride thin films using a novel electrostatic actuator design. Mechanical-amplifier devices made in silicon nitride thin films can apply controllable tensile stress (2.0–7.8 GPa) to test structures with relatively low actuation voltages (5.7–35.4 VRMS) at the resonant frequencies of the devices. The test devices are fabricated using a surface micromachining technique in combination with deep reactive ion etching and ion milling. With the recently developed experimental techniques inside a focused-ion-beam system, in situ fatigue measurements are performed on silicon nitride test structures with beam widths of 200 nm. The silicon nitride test structures are found to exhibit time-delayed failures with continuous increases in their compliance. By reducing the applied tensile stress to 3.8 GPa, the test structures can survive cyclic loadings up to 108 cycles.

Fracture tests of etched components using a focused Ion beam machine

Many optical MEMS device designs involve large arrays of thin (0.5 to 1 (mu) m) components subjected to high stresses due to cyclic loading. These devices are fabricated from a variety of materials, and the properties strongly depend on size and processing. Our objective is to develop standard and convenient test methods that can be used to measure the properties of large numbers of witness samples, for every device we build. In this work we explore a variety of fracture tests configurations for 0.5 (mu) m thick silicon nitride membranes machined using the Reactive Ion Etching (RIE) process. Testing was completed using an FEI 620 dual focused ion beam milling machine. Static loads were applied using a probe, and dynamic loads were applied through a piezo-electric stack mounted at the base of the probe. Results from the tests are presented and compared, and application for predicting fracture probability of large arrays of devices are considered.

Fabrication of microshutter arrays for space application

Two-dimensional microshutter arrays are being developed at NASA Goddard Space Flight Center for the Next Generation Space Telescope (NGST) for use in the near-infrared region. Functioning as object selection devices, the microshutter arrays are designed for the transmission of light with high efficiency and high contrast. The NGST environment requires cryogenic operation at 45K. Arrays are close-packed silicon nitride membranes with a pixel size of 100 X 100 micrometers . Individual shutters are patterned with a torsion flexure permitting shutters to open 90 degrees with a minimized mechanical stress concentration. The mechanical shutter arrays are fabricated with MEMS technologies. The processing includes a RIE front-etch to form shutters out of the nitride membrane, an anisotropic back-etch for wafer thinning, and a deep RIE (DRIE) back-etch down to the nitride shutter membrane to form frames and to relieve shutters from the silicon substrate. Two approaches for shutter actuation have been developed. Shutters are actuated using either a combined mechanical and electrostatic force or a combined magnetic and electrostatic force. A CMOS circuit embedded in the frame between shutters allows programmable shutter selection for the first approach. A control of row and column electrodes fulfills shutter selection for the second approach.

Microshutter arrays for near-infrared applications on the James Webb Space Telescope

Magnetically actuated MEMS microshutter arrays are being developed at the NASA Goddard Space Flight Center for use in a multi-object spectrometer on the James Webb Space Telescope (JWST), formerly Next Generation Space Telescope (NGST). The microshutter arrays are designed for the selective transmission of light with high efficiency and high contrast. The JWST environment requires cryogenic operation at 45K. Microshutter arrays are fabricated out of silicon-on-insulator (SOI) wafers. Arrays consist of close-packed shutters made on silicon nitride (nitride) membranes with a pixel size of 100 × 100 m. Individual shutters are patterned with a torsion flexure permitting shutters to open 90°, with a minimized mechanical stress concentration. Shutters operated this way have survived fatigue life test. The mechanical shutter arrays are fabricated using MEMS technologies. The processing includes a multi-layer metal deposition, patterning of shutter electrodes and magnetic pads, reactive ion etching (RIE) of the front side to form shutters in a nitride film, an anisotropic back-etch for wafer thinning, and a deep RIE (DRIE) back-etch, down to the nitride shutter layer, to form support frames and relieve shutters from the silicon substrate. An additional metal deposition and patterning has recently been developed to form electrodes on the vertical walls of the frame. Shutters are actuated using a magnetic force, and latched electrostatically. One-dimensional addressing has been demonstrated.

Programmable two-dimensional microshutter arrays

We are developing a two-dimensional array of microshutters which can be used as a high efficiency, high contrast field selection device for a multi-object spectrometer for the Next Generation Space Telescope (NGST). The device is a close- packed array of shutters, with a typical size of 100 microns square and area filling factor of up to 80%. Each shutter, made of single crystal silicon with an appropriate optical coating, pivots on a torsion flexure along one edge. Each of the shutters is individually selectable. An original double- shutter mechanism is employed for actuation. Since the device works in transmission, there is no loss of contrast due to diffraction from the edges of unactuated pixels. When working in reflection, the device can also be used as a micromirror array.

Composite Infrared Spectrometer (CIRS) on Cassini

The Cassini spacecraft orbiting Saturn carries the composite infrared spectrometer (CIRS) designed to study thermal emission from Saturn and its rings and moons. CIRS, a Fourier transform spectrometer, is an indispensable part of the payload providing unique measurements and important synergies with the other instruments. It takes full advantage of Cassinis 13-year-long mission and surpasses the capabilities of previous spectrometers on Voyager 1 and 2. The instrument, consisting of two interferometers sharing a telescope and a scan mechanism, covers over a factor of 100 in wavelength in the mid and far infrared. It is used to study temperature, composition, structure, and dynamics of the atmospheres of Jupiter, Saturn, and Titan, the rings of Saturn, and surfaces of the icy moons. CIRS has returned a large volume of scientific results, the culmination of over 30 years of instrument development, operation, data calibration, and analysis. As Cassini and CIRS reach the end of their mission in 2017, we expect that archived spectra will be used by scientists for many years to come.

High-Tc superconducting bolometer on chemically-etched 7 μm thick sapphire

Abstract A transition-edge IR detector, using a YBa 2 Cu 3 O 7− x (YBCO) thin film deposited on a chemically etched 7 μm thick sapphire substrate has been built. To our knowledge, it is the first such high- T c superconducting (HTS) bolometer on chemically thinned sapphire. The peak optical detectivity obtained is 1.2×10 10 cm Hz 1/2 /W near 4 Hz. Results show that it is possible to obtain high detectivity with thin films on etched sapphire with no processing after the deposition of the YBCO film. We discuss the etching process and its potential for micro-machining sapphire and fabricating two-dimensional detector arrays with suspended sapphire membranes. A 30 μm thick layer of gold black provided IR absorption. Comparison is made with the current state of the art on silicon substrates.

High-resolution x-ray masks for high aspect ratio microelectromechanical systems applications

The advanced requirements of bio-MEMS and MOEMS, i.e., low sidewall surface roughness, submicron critical dimension, and high aspect ratio, necessitate the use of an intermediate mask and a soft x-ray lithography process to fabricate working x-ray masks that are suitable for deep x-ray lithography. Intermediate masks consist of 2 to 2.5-µm gold patterns on membranes/substrates that are highly transparent to x-ray radiation, whereas working masks possess greater than 5 µm of gold patterns. In this work, 1-µm silicon nitride membranes are produced by a low pressure chemical vapor deposition (LPCVD) process on both the front and backside of 100 prime grade wafers and anisotropic wet etch through silicon nitride etch masks. E-beam lithography is used to pattern 0.8- to 3-µm-thick resist layers with submicron resolution. In the case of the 3-µm resist layers, the features are electroplated with approximately 2 µm of gold to form an intermediate mask. The 0.8-µm-thick layers are electroplated with gold up to a thickness of 0.6 µm and form initial masks, which are in turn used in a soft x-ray lithographical process to make intermediate masks. The process of building a high-resolution intermediate x-ray mask, directly by e-beam patterning a 3 µm layer of e-beam resist, followed by gold electroplating, is found to be viable but requires the use of a high energy (>100 keV) e-beam writer. The stability of the resist pattern during soft x-ray lithography (SXRL) by use of an initial mask is found to be problematic. Double-side lithography and gold electroplating, can effectively reduce the aspect ratio of the mask pattern, eliminates the problems associated with the use of an initial mask to fabricate intermediate x-ray masks.