Osiander / MEMS and microstructures in Aerospace applications DK3181_c002 Final Proof page 18 1.9.2005 11:49am 18 MEMS and Microstructures in Aerospace Applications FIGURE 2.2 The NMP ST5 MEMS thermal louver actuator block with shutter array (Source: JHU/APL.) balance One potential solution to this design problem is to employ the MEMS micromachined shutters to create, in essence, a variable emittance coating (VEC) Such a VEC yields changes in the emissivity of a thermal control surface to allow the radiative heat transfer rate to be modulated as needed for various spacecraft operational scenarios In the case of the ST5 flight experiment, the JHU/APL MEMS thermal shutters will be exercised to perform adaptive thermal control of the spacecraft by varying the effective emissivity of the radiator surface 2.2.2 JWST MICROSHUTTER ARRAY NASA’s James Webb Space Telescope (JWST) is a large (6.5-m primary mirror diameter) infrared-optimized space telescope scheduled for launch in 2011 JWST is designed to study the earliest galaxies and some of the first stars formed after the Big Bang When operational, this infrared observatory will take the place of the Hubble Space Telescope and will be used to study the universe at the important but previously unobserved epoch of galaxy formation Over the past several years, scientists and technologists at NASA GSFC have developed a large format MEMS-based microshutter array that is ultimately intended for use in the JWST near infrared spectrometer (NIRSpec) instrument It will serve as a programmable field selector for the spectrometer and the complete microshutter system will be © 2006 by Taylor & Francis Group, LLC Osiander / MEMS and microstructures in Aerospace applications DK3181_c002 Final Proof page 19 1.9.2005 11:49am Vision for Microtechnology Space Missions 19 composed of four 175 by 384 pixel modules This device significantly enhances the capability of the JWST since the microshutters can be selectively configured to make highly efficient use of nearly the entire NIRSpec detector, obtaining hundreds of object spectra simultaneously Micromachined out of a silicon nitride membrane, this device, as shown in Figure 2.3 and Figure 2.4, consists of a 2-D array of closely packed and independently selectable shutter elements This array functions as an adaptive input mask for the multiobject NIRSpec, providing very high contrast between its open and closed states It provides high-transmission efficiency in regions where shutters are commanded open and where there is sufficient photon blocking in closed areas Operationally, the desired configuration of the array will be established via ground command, then simultaneous observations of multiple celestial targets can be obtained Some of the key design challenges for the microshutter array include obtaining the required optical (contrast) performance, individual shutter addressing, actuation, latching, mechanical interfaces, electronics, reliability, and environment requirements For this particular NIRSpec application, the MEMS microshutter developers also had to ensure the device would function at the 37 K operating temperature of the spectrometer as well as meet the demanding low-power dissipation requirement Figure 2.5 shows the ability to address or actuate and provide the required contrast demonstrated on a fully functional 128 by 64 pixel module in 2003 and the development proceeding the 175 by 384 pixel flight-ready microshutter module that will be used in the JWST NIRSpec application This is an outstanding example of applying MEMS technology to significantly enhance the science return from a space-based observatory FIGURE 2.3 JWST microshutters for the NIRSpec detector (Source: NASA.) © 2006 by Taylor & Francis Group, LLC Osiander / MEMS and microstructures in Aerospace applications DK3181_c002 Final Proof page 21 1.9.2005 11:49am 21 Vision for Microtechnology Space Missions 2.2.4 NMP ST6 INERTIAL STELLAR CAMERA NASA’s NMP is sponsoring the development of the inertial stellar compass (ISC) space avionics technology that combines MEMS inertial sensors (gyroscopes) with a wide field-of-view active pixel sensor (APS) star camera in a compact, multifunctional package.6 This technology development and maturation activity is being performed by the Charles Stark Draper Laboratory (CSDL) for a Space Technology-6 (ST6) flight validation experiment now scheduled to fly in 2005 The ISC technology is one of several MEMS technology development activities being pursued at CSDL7 and, in particular, is an outgrowth of earlier CSDL research focused in the areas of MEMS-based guidance, navigation, and control (GN&C) sensors or actuators8 and low-power MEMS-based space avionic systems for space.9 The ISC, shown in Figure 2.6, is a miniature, low-power, stellar inertial attitude determination system that provides an accuracy of better than 0.18 (1-Sigma) in three axes while consuming only 3.5 W and is packaged in a 2.5-kg housing.10 The ISC MEMS gyro assembly, as shown in Figure 2.7, incorporates CSDL’s tuning fork gyro (TFG) sensors and mixed signal application specific integrated DPA Housing DC - DC Converter Processor PWA CGA Housing DPA PSE PWA Lens and Camera Support Assembly Alignment Reference Cube Lens Assembly Baffle DC - DC Converter Controller and PSE PWA Gyro PWA Camera PWA FIGURE 2.6 The NMP ST6 inertial stellar camera (Source: NASA JPL/CALTECH ST6.) © 2006 by Taylor & Francis Group, LLC Osiander / MEMS and microstructures in Aerospace applications DK3181_c002 Final Proof page 23 1.9.2005 11:49am Vision for Microtechnology Space Missions 23 that serves to free up precious spacecraft resources For example, the mass savings afforded by using the MEMS-based ISC could be allocated for additional propellant or, likewise, the power savings could potentially be directly applied to the mission payload These are some of the advantages afforded by using MEMS technology 2.2.5 MICROTHRUSTERS Over the past several years MEMS catalytic monopropellant microthruster research and development has been conducted at NASA’s GSFC.11 MEMS-based propulsion systems have the potential to enable missions that require micropropulsive maneuvers for formation flying and precision pointing of micro-, nano-, or pico-sized satellites Current propulsion technology cannot meet the minimum thrust requirements (10–1000 mN) or impulse-bit requirements (1–1000 mNÁsec), or satisfy the severely limited system mass (10 m >10À3 Extremely large material suite Assembled as fabricated Yes Yes Assembly required Yes No Two-dimensional high aspect ratio Assembly required Limited Yes for SOI bulk processes Two-dimensional high aspect ratio Multi-layer Two-dimensional Very flexible Threedimensional Parallel processing at the wafer level Parallel processing at the wafer level Parallel processing at the wafer level Serial processing LIGA ~3 to 5 mm >1 mm >2 mm ~10À2 Electroplated metals or injection molded plastics Assembly required Limited No Conventional Machining The evaluation of a fabrication process for an application requires the assessment of a number of factors: The process-critical dimension (i.e., the smallest dimension that can be fabricated) The process precision (i.e., dimensional accuracy or nominal device dimension) Materials available for fabrication Assembly requirements to produce a functioning device Process scalability (i.e., can large quantities of devices be produced?) Integrability with other fabrication processes (e.g., microelectronics) A large assortment of MEMS fabrication processes have been developed, but they may be grouped into three broad categories, which are discussed in further detail in subsequent sections © 2006 by Taylor & Francis Group, LLC 3D structures formed by mold fabrication, followed by injection molding or electroplating 3D structures formed by wet or dry etching of silicon substrate Structures formed by deposition and etching of sacrificial and structural thin films Groove Nozzle Membrane Silicon Substrate p++ (B) [100] Wet Etch Patterns [111] 54.7Њ Poly Si Channels Dry Etch Patterns Mold 1.9.2005 8:59pm Silicon Substrate Holes Silicon Substrate FIGURE 3.1 MEMS fabrication technology categories (Courtesy: Sandia National Laboratories.) 39 © 2006 by Taylor & Francis Group, LLC Osiander / MEMS and microstructures in Aerospace applications DK3181_c003 Final Proof page 39 LIGA MEMS Fabrication Bulk Micromachining Surface Micromachining Osiander / MEMS and microstructures in Aerospace applications DK3181_c003 Final Proof page 42 42 1.9.2005 8:59pm MEMS and Microstructures in Aerospace Applications NH4 F $ NH3 þ HF (3:2) Wet-etching methods can also be used on crystalline materials to achieve anisotropic directional etches For example, a common directional wet etchant for crystalline silicon is potassium hydroxide (KOH) KOH etches 100 times faster in the (1 0 0) direction than the (1 1 1) direction Patterned silicon dioxide can be used as an etch mask for these types of etches Very directional etches can be achieved with these techniques as illustrated in Figure 3.4 Note the angular features (54.7 8) that can be etched in silicon Table 3.2 lists some of the common etchants for crystalline silicon and their selectivity If there are no etch stops in a wet-etching process the two options available to the process engineer are a timed etch or a complete etch through the material A timed etch is difficult to control accurately due to the many other variables in the process such as temperature, chemical agitation, purity, and concentration If this is not satisfactory, etch stops can be used to define a boundary for the etch to stop on There are several etch-stop methods that can be utilized in wet etching: pþ (boron diffusion or implant) etch stop Material-selective etch stop Electrochemical etch stop Boron-doped silicon has a greatly reduced etch rate in KOH The use of born-doped regions, which are either diffused or implanted, has been used either to form features or as an etch stop as seen in Figure 3.5 Also, a thin layer of a material such as silicon nitride, which has a greatly reduced etch rate, can be deposited on a material to form a membrane on which etching will stop An electrochemical etch stop can also be used as shown in Figure 3.6 Silicon is a material that readily forms a silicon oxide layer, which will impede etching of the bulk material The formation of the oxide layer is a reduction–oxidation reaction that can be impeded by a reverse-biased p–n junction, which prevents the current SiO2 Mask 1 FIGURE 3.4 Directional etching of crystalline silicon © 2006 by Taylor & Francis Group, LLC φ = 54.7Њ φ h ... Micromachining Surface Micromachining Osiander / MEMS and microstructures in Aerospace applications DK3181_c003 Final Proof page 42 42 1.9 .20 05 8:59pm MEMS and Microstructures in Aerospace Applications. .. beneficial © 20 06 by Taylor & Francis Group, LLC Osiander / MEMS and microstructures in Aerospace applications DK3181_c0 02 Final Proof page 26 1.9 .20 05 11:50am 26 2. 3.1 INVENTORY MEMS and Microstructures. .. LLC Osiander / MEMS and microstructures in Aerospace applications DK3181_c0 02 Final Proof page 32 1.9 .20 05 11:50am 32 MEMS and Microstructures in Aerospace Applications engineering approaches