WO2003036737A2 - Stiffened surface micromachined structures and process for fabricating the same - Google Patents
Stiffened surface micromachined structures and process for fabricating the same Download PDFInfo
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- WO2003036737A2 WO2003036737A2 PCT/US2002/033351 US0233351W WO03036737A2 WO 2003036737 A2 WO2003036737 A2 WO 2003036737A2 US 0233351 W US0233351 W US 0233351W WO 03036737 A2 WO03036737 A2 WO 03036737A2
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B81—MICROSTRUCTURAL TECHNOLOGY
- B81B—MICROSTRUCTURAL DEVICES OR SYSTEMS, e.g. MICROMECHANICAL DEVICES
- B81B3/00—Devices comprising flexible or deformable elements, e.g. comprising elastic tongues or membranes
- B81B3/0064—Constitution or structural means for improving or controlling the physical properties of a device
- B81B3/0067—Mechanical properties
- B81B3/007—For controlling stiffness, e.g. ribs
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- the present invention generally relates to micromachined structures and their fabrication methods, and, more particularly, to a micromachined device with stiffening members to reduce stress-induced or inertial deformation and a method of fabricating the same.
- the Internet, cable television and teleconferencing has highlighted the increased requirement for communication bandwidth.
- the use of dense wavelength division multiplexing (DWDM) has increased the number of wavelengths carried on each optical fiber used to meet these high bandwidth requirements. These multiple wavelengths must be switched and rerouted to different fibers.
- the current method of converting the optical signal at each wavelength to slower electrical signals, switching, and then converting back to optical signals and transmitting back down an optical fiber has become the dominant power and space consumer of fiber communication systems. Therefore, it is desirable to develop an all-optical switching method to meet the demand for increased optical communication bandwidth.
- Micromechanical mirror systems are one method of obtaining all-optical switching.
- the small nature of an optical fiber makes the beam compatible with micromechanical mirrors.
- Movable micromechanical mirrors can be used to redirect the optical beam between fibers. This presents significant problems for the design of micromechanical mirrors.
- Surface micromachined devices are constructed from thin films containing internal stresses resulting from their fabrication process. As a result of these internal stresses, devices with high length-to-thickness ratios can deform considerably once released from the substrate. For example, the tip of a rectangular cantilever beam will tend to deflect out of the plane of the substrate when released.
- One class of devices particularly sensitive to surface deformation is micro mirrors used for optical cross-connects and in scanned-beam imaging systems. These mirrors may require diameters of several hundred micrometers, leading to very large length-to- thickness ratios. An ideal mirror would have an optically flat surface so that the reflected beam is not significantly deformed. This will aid in the coupling efficiency into the optical fiber.
- An ideal micromachined mirror should also have a large dynamic range.
- the mirror should be easily produced. Complex and exotic processes increase the production costs and reduce the yield, raising the ultimate cost of the cross connect.
- Residual stress during fabrication of a micromachined device can be partially reduced by controlling deposition conditions, by annealing deposited films, and by multilayer designs that attempt to balance stresses in a laminate structure.
- film stress remains a variable in most deposition systems, and solutions are needed that can increase the tolerance of a particular design to variation in film stresses.
- the surface micromachined structure can be stiffened by adding topology to the substrate that creates stiffening beams and ribs in the deposited material. These beams and ribs are used to add structural integrity to the mechanical members as discussed in (1) H. Y. Lin, W. Fang, "Rib-reinforced Micromachined Beam and its Applications," J. Micromech. Microeng., 10, pp. 93-99, 2000, and (2) J. Drake, H. Jerman, "A Micromachined Torsional Mirror for Track Following in Magneto-optical Disk Drives," 2000 Solid-State Sensor and Actuator Workshop, pp. 10-13, 2000.
- micromachined structures that can be made stiffer (i.e., with substantially reduced deformation due to internal stresses or excitation of unwanted vibration modes during dynamic operation) and have larger angular deflections (especially in the case of micromachined mirrors).
- the 3 -dimensional surface micromachined structures should preferably have increased stiffness to reduce the deformation resulting from stress gradients in materials and differential stresses in laminated films. It is further desirable to devise a fabrication method that produces such micromachined structures with ease and simplicity. It is also desirable to develop a silicon micromachining process that uses industry-standard processing steps to realize highly functional micromechanical devices, especially, micro mirrors for optical switching applications.
- the present invention contemplates a method of fabricating a thin- film micromachined device comprising etching a substrate to produce a mold therein; depositing a structural stiffening member on the substrate so as to backfill the mold with the structural stiffening member; patterning the stiffening member deposited on the substrate to form the thin- film micromachined device on the substrate; and etching the mold to release the micromachined device without removing the stiffening member that is backfilling the mold.
- the present invention contemplates a micromachined device comprising a structural stiffening member; and a thin-film micromachined structure formed from the stiffening member by patterning the stiffening member, wherein the stiffening member is initially deposited on a substrate backfilling a mold etched into the substrate, and wherein the mold is selectively etched after formation of the micromachined structure so as to release the micromachined structure without removing the stiffening member that is backfilling the mold.
- the mold may be produced in a number of lattice configurations including, for example, a ring configuration or a honeycomb configuration.
- the structural stiffening member includes one or more silicon nitride layers deposited on a silicon substrate.
- One or more layers of metal are also deposited and patterned on the stiffening member to form leads and capacitors for electrostatic actuation. Further, a portion of the mold is left incorporated into the released micromachined device for increased stiffness.
- the present invention contemplates a micromachined mirror comprising a structural stiffening member containing at least one layer of silicon nitride; one or more mechanical members formed from the stiffening member by patterning the stiffening member; and one or more layers of metal deposited and patterned on the stiffening member so as to form a reflective portion of the micromachined mirror and one or more electrostatic actuators for the mechanical members, wherein the stiffening member is initially deposited on a silicon substrate backfilling a mold etched into the substrate, and wherein the mold is selectively etched after patterning the one or more metal layers so as to release the micromachined mirror without removing the stiffening member that is backfilling the mold.
- micromachined devices built with vertical features or fins or ribs created by molding the substrate and backfilling the mold with silicon nitride exhibit increased out-of-plane bending stiffness.
- the increased bending stiffness resulting from stiffening fins or ribs substantially reduce stress-related deformations experienced by surface-micromachined devices with large length-to-thickness ratios.
- surface micromachining techniques to pattern stiffened micromachined devices out of silicon nitride and then releasing them by a sacrificial oxide etch and bulk etching of the silicon substrate, the out-of-plane deformation of the released micromachined structures can be significantly reduced.
- Fig. 1 illustrates an abbreviated fabrication process flow according to the present invention depicting a cantilever beam fabricated with trenches;
- Fig. 2 shows trenches forming a double ring configuration when etched into a silicon substrate;
- Fig. 3 illustrates an exemplary shape for one of the trenches depicted in Fig. 2;
- Fig. 4 depicts four exemplary configurations for trenches or stiffening lattice that can be produced to stiffen surface micromachined structures
- Fig. 5 illustrates a top view and a cross-sectional view of an exemplary micromachined device (a bi-axial micro mirror) formed upon release from the substrate;
- Fig. 6 illustrates an isometric view of an exemplary biaxial micro mirror released from the substrate after being formed according to the process depicted in Fig. 1;
- Fig. 7 illustrates some exemplary lattice (or trench) configurations for cantilever beams fabricated according to the process discussed with reference to Fig. 1 ;
- Fig. 8 shows experimental dimensions (in ⁇ m) for three types of cantilever beams — a beam having a flat cross section, a beam having a "T” cross section, and a beam having a "C” cross section;
- Fig. 9 shows optical interferometer images of exemplary micromachined cantilever beams with six different cross sections fabricated according to the process described with reference to Fig. 1;
- Fig. 10 depicts exemplary graphs showing displacement measured along the length of silicon nitride cantilevers for four different beam cross sections, corresponding to the top four cantilevers in Fig. 9;
- Fig. 11 shows the residual curvature or displacements measured for the same cantilevers as those shown in Fig. 10, but after sputtering 100 mn of gold on the surface of silicon nitride beams of Fig. 10;
- Fig. 12 illustrates an exemplary schematic of a silicon nitride cantilever beam used for finite element analysis of various beam configurations according to one embodiment of the present invention;
- Fig. 13 is a finite element model corresponding to the cantilever beam schematic shown in Fig. 12;
- Fig. 14 illustrates four finite element models for different cantilever beam configurations generated using the schematic illustrated in Fig. 12;
- Fig. 15 depicts simulated cantilever displacements for four different beam cross-sections as predicted by the finite element analysis
- Fig. 16 is a bottom-side view of a portion of a released lattice structure illustrating inclusion of silicon in the stiffening lattice for a micromachined structure;
- Fig. 17 illustrates an exemplary released bi-axial micro mirror with standard torsional hinges
- Fig. 18 illustrates an exemplary released bi-axial micro mirror with meander hinges
- Fig. 19 shows some details of an inner flexure for the released bi-axial mirror in Fig. 17;
- Fig. 20 shows the backside of another biaxial mirror fabricated using the process described with reference to Fig. 1.
- any reference in the specification to "one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the invention.
- the appearances of the phrase “in one embodiment” at various places in the specification do not necessarily all refer to the same embodiment.
- Fig. 1 illustrates an abbreviated fabrication process flow according to the present mvention depicting a cantilever beam 20 fabricated with trenches 14.
- the process in Fig. 1 is described hereiribelow in conjunction with Figs. 2-6.
- the process illustrated in Fig. 1 is a multi- step process, with the beginning step shown at the top and the concluding step shown at the bottom in Fig. 1.
- the fabrication flow shown in Fig. 1 is a modification of the process used to produce silicon nitride deformable membranes as discussed in D. L. Dickensheets, P. A. Himmer, R. A. Friholm, B. J.
- a silicon substrate or wafer 10 is masked with a DRIE (Deep Reactive Ion Etching) mask 12 and deep trenches 14 are etched in bulk silicon 10 using DRJE etching with short iteration times to minimize the typical scalloping of the sidewalls as discussed in A. A. Ayon, R. Braff, C. C. Lin, H. H. Sawin, M. A. Schmidt, "Characterization of a Time Multiplexed Inductively Coupled Plasma Etcher," J. Electrochem. Soc, 146, pp. 339- 349, 1999, the disclosure of which is incorporated herein in its entirety.
- the trenches 14 could be formed in many configurations. For example, Fig.
- FIG. 2 illustrates trenches 22 forming a double ring configuration when etched into a silicon substrate 24.
- Fig. 3 illustrates an exemplary shape for one of the trenches 22 depicted in Fig. 2.
- the configuration of the trenches produces a mold or stiffening lattice (e.g., the double ring configuration in Fig. 2) for the stiffening members of the final structure.
- Fig. 4 illustrates four exemplary configurations for trenches or stiffening lattice that can be produced to stiffen surface micromachined structures.
- the shapes in Fig. 4 are the backside of various configurations including the single ring configuration 26, the multiple ring configuration 28, the webbed rings configuration 30, and the honeycombed structure 32. Any other suitable lattice configuration may be produced independently or using one or more configurations shown in Fig. 4.
- oxide is thermally grown and/or deposited on the silicon substrate 10, which is followed by deposition of phosphosilicate glass (PSG) to form a sacrificial oxide layer 15 between the substrate 10 and the stiffening member (here, the silicon nitride layers 16, 18).
- PSG phosphosilicate glass
- the thermally grown oxide conformally coats the surface of the substrate 10.
- the trenches are then backfilled with a structural material or stiffening member to create vertical flanges that significantly increase the overall bending stiffness of the resulting micromachined device.
- the structural members for the micromachined device e.g., a micro mirror
- the structural members for the micromachined device are formed out of two layers of silicon nitride.
- the trenches 14 are backfilled with a first layer of silicon nitride 16.
- This first layer of stiffening member (here, silicon nitride) forms the mechanical members of the micromachined device when it is patterned and appropriately etched.
- a second nitride layer 18, usually a thinner layer, may be then deposited over the entire surface of the substrate and patterned to define the flexures in the micromachined device. This allows for an increased design space, with the flexure thickness being the additional design variable.
- metals may be deposited and patterned (not shown in Fig. 1, but illustrated in Figs. 5 and 6) as needed to form the leads and capacitors for the electrostatic actuation of the micromachined device.
- micromachined devices are then released from the surface of the substrate by wet etching the sacrificial PSG and thermal oxide with, for example, a concentrated hydrofluoric acid (HF) solution, followed by anisotropic etching of the silicon substrate 10 in a tetramethyl ammonium hydroxide (TMAH) solution ((CH 3 ) 4 NOH) to produce clearance for mechanical motion.
- HF concentrated hydrofluoric acid
- TMAH tetramethyl ammonium hydroxide
- CH 3 tetramethyl ammonium hydroxide
- FIG. 5 illustrates a top view 34 and a cross-sectional view 36 of an exemplary micromachined device (a bi-axial micro mirror) formed upon release from the substrate.
- the cross-sectional view 36 is a horizontal slice taken through the middle of the device (i.e., horizontally through the middle of the top view 34) and depicts a metal layer 40 deposited and patterned on top of two layers 38, 39 of structural stiffening member or material (here, silicon nitride layers).
- a substantially uniform or flat air gap 37 may be created at will under the released structure by appropriately timing the release etch process.
- the size of the air gap 37 is variable and it depends on the duration of the etching of the mold.
- TMAH TMAH for the release may result in very flat (to a few micrometers) air gaps, even though the initial etch patterns (for micromachined structures) are complex and dependent on the geometry of the structures to be released.
- an air gap e.g., the air gap 37 in Fig. 5 of controllable thickness may be generated, and that air gap may be uniformly flat beneath the released device.
- Fig. 6 illustrates an isometric view 42 of an exemplary biaxial micro mirror released from the substrate after being formed according to the process depicted in Fig. 1.
- chrome-gold is deposited and patterned to form the reflective surface, the electrodes, interconnects, the surface capacitor plates and bonding pads for the micro mirror device assembly.
- Actuation electrodes are positioned on the outer member to deflect the structure about the axis defined by the outer flexures and on the inner electrodes to deflect the structure about the axis defined by the inner flexures. Actuation is made by applying a potential between the substrate and the electrodes on the surface of the device.
- thermal oxide and phosphosilicate glass layers together serve as a sacrificial layer which later may be etched through access vias in the structural material to expose the top surface of the silicon mold when it is time to release the micromachined structures by etching the silicon mold.
- the presence of thermal oxide under the nitride layer(s) may dramatically increase the breakdown voltage for the fabricated micromachined structures. Breakdown occurs away from the released devices, between the metal layer and the silicon substrate where the films are all in contact with the silicon. The low-stress silicon breaks down at low voltages. Therefore, having a good dielectric (like thermal oxide) under the nitride layer(s) may allow application of several hundred volts of potential across the device films without breakdown.
- the thermal oxide layer is optional.
- low stress LPCVD silicon nitride is deposited directly on the substrate without thermal oxide or phosphosilicate glass first deposited.
- the silicon nitride layer is patterned to form the micromechanical device, and metal layers may be deposited and patterned as needed to form the leads and capacitors for the electrostatic actuation of the micromachined device.
- the device is released by etching the silicon through access vias in the silicon nitride, or from around and under the micromachined device using a selective etchant that removes the silicon in the substrate without removing the micromachined device.
- some of the silicon may remain integral to the finished micromachined device, as illustrated and discussed hereinbelow with reference to Figure 16.
- trenches measuring approximately 2.5 ⁇ m wide were etched into bulk silicon using deep reactive ion etching with the Bosch process. This was followed by a growth of thermal oxide ( ⁇ 1.0 ⁇ m) and then deposition of phosphosilicate glass (PSG) at 400°C ( ⁇ 0.5 ⁇ m). Thermal oxide grows conformally around the etched trench, but PSG typically does not coat the trench sidewalls and bottom. In this embodiment, the trench depth used was approximately 10 ⁇ m- 12 ⁇ m. However, if needed, trenches exceeding 30 ⁇ m deep (and upto 100 ⁇ m deep) and about 2 ⁇ m wide may be etched and filled.
- thermal oxide ⁇ 1.0 ⁇ m
- PSG phosphosilicate glass
- a 1.0 ⁇ m thick layer of low-stress LPCVD (low pressure chemical vapor deposited) silicon-nitride was then deposited and patterned using standard photolithography and reactive ion etching.
- a second layer of silicon nitride 0.5 ⁇ m thick was deposited and patterned similarly to complete the trench filling.
- a good conformal coating by the silicon nitride layer is obtained.
- the devices were released from the silicon substrate surface by wet etching the oxide in concentrated hydrofluoric acid, followed by silicon etching in TMAH to release the cantilevers (one such cantilever 20 is shown in Fig. 1) and achieve the desired clearance between released devices and the substrate.
- TMAH silicon etching in TMAH
- 100 ⁇ m gold was sputtered on the released devices.
- the devices were fabricated at the Stanford Nano fabrication Facility, USA.
- RIE etching may be used to pre-structure the substrate with trenches prior to deposition of thin films.
- the trenches can be back-filled with a structural material, such as low- stress silicon nitride, to create vertical flanges that significantly increase the overall bending stiffness of the resulting MEMS (microelectromechanical systems) device.
- a structural material such as low- stress silicon nitride
- Various flange configurations beneath a micromachined device may be formed to significantly increase the height to width aspect ratio of the device, thus increasing the overall bending stiffness.
- the back-filled trenches may serve as a lateral etch stop during the release of surface micromachined structures.
- the vertical silicon nitride members are part of the released silicon nitride structures. With the process described with reference to Fig. 1, surface micromachined structures of silicon nitride can be produced with nearly arbitrary control of the depth of the vertical members in the structures.
- Fig. 7 illustrates some exemplary lattice (or trench) configurations for cantilever beams fabricated according to the process discussed with reference to Fig. 1.
- the light lines 44, 46, 48 and 50 show the edges of cantilever beams and the dark lines 52, 54, 56 and 58 represent the trenches in different configurations.
- FEA Finite Element Analysis
- FIG. 8 shows experimental dimensions for three types of cantilever beams — a beam having a flat cross section 60, a beam having a "T" cross section 62 (T-beam), and a beam having a "C” cross section 64 (C-beam). All dimensions shown in Fig. 8 are in microns and are used to obtain the moment of inertia values of respective beams as discussed hereinbelow.
- Equation (1) k is the curvature, E is the modulus of elasticity and / is the moment of inertia.
- I ...(2)
- Equation (2) h is the thickness of the beam and b is the width of the beam. Substituting equation (2) into equation (1), one observes that the curvature k decreases proportional to h 3 . Thus, for a beam with a rectangular cross section and a fixed width, increasing the film thickness by h decreases the curvature by l/h 3 .
- I ⁇ n is the moment of inertia of the n piece
- a n is the area of the n piece
- d n is the perpendicular distance between the centroid of the n piece and the centroid of the entire composite cross-section.
- Fig. 8 shows dimensions for some typical beams. If the modulus of elasticity is assumed to be constant, then according to equation (1), stiffness can only be improved by increasing the moment of inertia.
- the composite moment of inertia can be calculated for the cross-sections shown in Fig. 8 to illustrate its effect on bending stiffness.
- the moments of inertia (measured in ⁇ m ) calculated from equation (3) by using the dimensions for the flat, T-beam and C-beam shown in Fig. 8 are 7.0, 414.1, and 633.4 respectively.
- both the T-beam and the C-beam are significantly stiffer in bending than the flat beam.
- the modified flexural rigidity can be determined analytically and with finite element analysis. By measuring the static deflection of the beams it is possible to determine a bulk or modified flexural rigidity. Useful lattice configurations that can be used for such measurements are shown in Fig. 7.
- Fig. 9 shows optical interferometer images of exemplary micromachined cantilever beams with six different cross sections fabricated according to the process described with reference to Fig. 1.
- the end view and top view of each cantilever beam is also shown in Fig. 9 along with its interferometer image.
- the light lines in the top views in Fig. 9 show the edges of the corresponding cantilever beam and the dark lines represent the trenches.
- the devices fabricated according to the process in Fig. 1 may be characterized by measuring surface deformation of released cantilevers. Both silicon nitride and gold-coated silicon nitride structures have been investigated.
- the primary experimental tool for making surface deformation measurements is an optical profilometer, consisting of a Mirau interferometer (imaging at 660 nm) and custom fringe analysis software for extracting surface profiles.
- Each fringe in the images shown in Fig. 9 represents a 330 nm change in surface height.
- the substrate is tilted from the optical axis, producing linear fringes across flat surfaces.
- All of the cantilevers in Fig. 9 measure 100 ⁇ m wide by 300 ⁇ m long, and are made of low stress LPCVD silicon nitride approximately 1 ⁇ m thick, with no other films.
- the vertical stiffening elements or trenches are 12 ⁇ m deep and approximately 2 ⁇ m wide.
- the first device i.e., the device at the top in Fig. 9) is a simple flat nitride cantilever.
- For the first device there is a small stress gradient in the silicon nitride that causes it to bend upward, with a tip deflection of 2.2 ⁇ m above the plane of the wafer. Cupping of the cantilever in both dimensions is evident by the curved fringes in the interferogram.
- the second (T-cross section) and third (C-cross section) cantilevers from the top in Fig.
- the bottom three cantilevers in Fig. 9 are examples of different lattices that are essentially a C-channel cross section with lateral elements to tie the two outer vertical elements together (as seen from the respective end views in Fig. 9).
- the fourth cantilever from the top is a simple triangular lattice
- the fifth is a diamond or X-lattice. Both of these exhibit very good flatness in both dimensions, with straight, evenly spaced fringes.
- the bottom cantilever is an example of an asymmetric triangular lattice. In this case, the bending moment due to the stress gradient in the silicon nitride leads to a twisting of the cantilever. Such structures could be useful for asymmetric torsion elements.
- FIG. 10 depicts exemplary graphs showing displacement measured along the length of silicon nitride cantilevers for four different beam cross sections, corresponding to the top four cantilevers in Fig. 9.
- the data in Fig. 10 were taken for cantilevers that were 300 ⁇ m long and 50 ⁇ m wide, with 12 ⁇ m deep vertical members or stiffening structures (or trenches).
- cantilever bending due to stress gradient in the nitride
- the flattest cantilever of the group is the cantilever with triangular lattice.
- FIG. 11 shows the residual curvature or displacements measured for the same cantilevers as those shown in Fig. 10, but after sputtering 100 nm of gold on the surface of silicon nitride beams of Fig. 10.
- This new laminate material exhibits greater bending moment, as evidenced by the increased deflection of the simple or flat nitride beam.
- the tip deflection has doubled, and the radius of curvature has been reduced by half with the introduction of the metal layer on top of the beam.
- the beams with vertical stiffening elements i.e., the T- beam, the C-beam, and the triangular lattice beam
- the increased bending stiffness of these elements has rendered them much less sensitive to variation in film stress.
- Fig. 12 illustrates an exemplary schematic of a silicon nitride cantilever beam used for finite element analysis of various beam configurations according to one embodiment of the present invention.
- Fig. 13 is a finite element model conesponding to the cantilever schematic shown in Fig. 12.
- the cantilever is released from the substrate 70 and consists of flanges 72, a silicon nitride layer 74, and a metal layer 76.
- the experimental beams were anchored to the substrate, the substrate is slightly undercut during the final etch step as is seen by the presence of the undercut 78 in the schematic of Fig. 12.
- Fig. 13 illustrates the boundary conditions used for the FEA model of the beam shown in Fig. 12.
- Figs. 12 and 13 flat beams, T-beams, C-beams and triangle-lattice beams were simulated using the finite element package ANSYS 5.6 (Houston, PA).
- Each of the beam models consisted of a 1.5 ⁇ m thick silicon nitride layer (i.e., layer 74 in Fig. 12) and 100 nm thick layer of metal (i.e., layer 76 in Fig. 12).
- the flange depth for the T-beam, C-beam and the triangle-lattice beam was set to 10 ⁇ m and the flange width was set to 1.5 ⁇ m.
- Fig. 14 illustrates four finite element models for different cantilever beam configurations generated using the schematic illustrated in Fig. 12.
- the drawing 80 in Fig. 14 represents a finite element model of the flat beam
- the drawing 82 is the finite element model of the T-beam
- the drawing 84 is the finite element model of the C-beam
- the drawing 86 is the finite element model of the triangle-lattice beam.
- Each model measures 50 ⁇ m wide by 300 ⁇ m long.
- the flat beam, T-beam and C-beam take advantage of a symmetry plane along the center of the respective beam.
- the elastic modulus and Poisson's ratio for the low-stress silicon-nitride and the metal layers have been obtained from Xin Zhang, Tong-Yi Zhang, Yitshak Zohar, "Measurements of residual stresses in thin films using micro-rotating structures," Thin Film Solids, 335, 97-105, 1998.
- a value of 220 GPa was used for the elastic modulus of the low- stress nitride and a value of 180 GPA was used for the metal.
- Poisson's ratio was set to 0.24 for the silicon-nitride and a value of 0.33 was used for the metal.
- residual stresses in both layers can vary considerably from one wafer to another.
- the Guckel rings and pointer devices offer a direct way to get an approximate residual stress value in the silicon nitride layer. However, these devices may not predict the stress level in the metal. In that situation, once the residual stress level in the silicon-nitride has been determined, the measured tip deflection of cantilever beams with metal can be compared to a finite element model with an assumed value for residual stress in the metal layer. The residual stress value in the model can then be adjusted so that the tip deflection of the model matches that of the experimental device. This one-point calibration scheme was used to set the stress value for the gold layer in the simulations depicted in Fig. 14.
- Fig. 15 depicts simulated cantilever displacements for four different beam cross- sections as predicted by the finite element analysis.
- the simulations shown in Fig. 15 include simulations for silicon nitride beams with 100 nm gold coating. It is noted that nonlinear geometry analyses were not necessary due to the small tip displacements relative to the length of the beam.
- the fabrication process according to the present invention produces three-dimensional structures with improved stiffness to resist out-of-plane bending.
- these structures may be made from a low-stress LPCVD silicon nitride, using a silicon substrate for the processing.
- the process of the present invention includes etching of the silicon substrate to form deep trenches, followed by deposition of a silicon dioxide layer and then deposition of the silicon nitride material that fills the trenches and coats the surface of the substrate. It is noted that the silicon dioxide layer may be omitted for some structures.
- Lithographic means may be used to pattern the deposited silicon nitride material to create useful thin-film micromachined structures.
- These structures may be made free-standing by chemical etches that attack the silicon dioxide and/or the underlying silicon, without damaging or removing the silicon nitride material.
- a substrate material other than silicon e.g., gallium arsenide, another semiconductor or dielectric material,etc.
- a structural material other than silicon nitride e.g., polysilicon or silicon carbide or a metal film (preferably molded) or silicon dioxide
- the mechanical members in the micromachined device may be formed of a ceramic material or a dielectric material or another suitable material that cannot be electroplated.
- etchants either liquid, gas or plasma
- etchants that ultimately dissolve the substrate material without attacking the structural material (here, silicon nitride) should preferably be selected.
- etchants may attack the substrate isotropically (like the common acid etch HNA (hydrofluoric acid, nitric acid and acetic acid) or anisotropically like the alkaline etch TMAH.
- HNA hydrofluoric acid, nitric acid and acetic acid
- anisotropically like the alkaline etch TMAH.
- the substrate itself may be considered as a sacrificial material in the fabrication process described hereinbefore.
- a deposited film material for the structural layer, followed by an etch of the substrate material in order to allow for motion of the resulting device, where the film material is either different from the substrate and not attacked by the etchant used to remove the substrate material, or else it is protected in some way such as encapsulation or by a galvanic process during the etch.
- a galvanic etch is the common use of a p-n junction as an etch stop during KOH (potassium hydroxide) etching.
- narrow trenches are used so that after deposition of the silicon nitride, the trenches were completely filled and closed off at the top surface. This may allow deposition of a second thin film of metal (chrome/gold in this embodiment) to make capacitive plates for actuation of the micromachined structures, with assurance that the metal film would be continuous and electrically conductive across the stiffening features.
- metal films include nickel, aluminum, or tungsten films. Closing-off the trenches after silicon nitride deposition may provide rigid micromachined structures that may resist tensile forces in the plane of the substrate but normal to the trench edges. Corrugated micromachined structures made by coating deep but wide trenches may be very compliant to such tensile stresses, pulling apart like an accordion.
- a polishing step is performed following the nitride deposition that closes off the trenches. Without this polishing step, surface features may exist where the vertical members or fins of micromachined structure have been formed. Therefore, it may be necessary to eliminate these features, which, preferably, may be done with a chemo-mechanical polish, resulting in a flat surface. In this way, micromachined structures such as optical mirrors may incorporate stiffening features across the entire surface, without degrading the optical properties of the surface.
- the stiffening members may be formed into lattices. These lattices may adopt properties that may be modeled as a bulk material, significantly simplifying the design process. On the other hand, very detailed and large finite element models to accurately represent the latticed structural detail may take a long time to simulate, hindering the design process. By extracting equivalent bulk material properties for the lattices, one can replace the latticed material with an equivalent solid material with appropriate properties, resulting in much simpler models that are useful for the design and analysis of structures that incorporate the stiffening lattices. Lattices may be designed that produce interesting or desirable bulk properties. Isotropic materials may result from symmetric lattices, such as hexagonal honeycomb structures.
- Anisotropic materials may result from asymmetric lattices.
- Further types of materials include, for example, materials with different Young's modulus along different axes, and different torsional rigidity for left-handed versus right-handed torsion.
- Out-of-plane bending may also be controlled by lattice engineering, allowing the construction of structures that, once released, may bend in a controlled manner. Applying a highly tensile film such as chromium film on the surface of the structures may generate a bending moment.
- Appropriate lattices may be engineered to achieve various curved surfaces, including cylinders, spheres and other higher order shapes.
- macroscopic mechanical parameters may be engineered by changing the microscopic patterns of the mold (e.g., building torsion springs that are stiffer when resisting a right-handed twist than when resisting a left-handed twist).
- the devices may be actuated with AC (alternating current) voltages rather than DC (direct current) voltages in order to prevent charge migration from causing mechanical drift of the micromachined structure.
- the frequency of the AC drive voltage may be kept much higher than important mechanical resonance frequency of the device being actuated with the AC voltage. This results in the device just responding to the average actuation force, without causing any mechanical device drift.
- Fig. 16 is a bottom-side view of a portion of a released lattice structure illustrating inclusion of silicon in the stiffening lattice for a micromachined structure.
- the term "top”, as used herein, refers to the top surface of a micromachined structure (e.g., the layer 76 in Fig. 12), whereas the term “bottom” refers to a view from a direction that is opposite from the top surface of the structure (e.g., the bottom of the substrate 70 in Fig. 12).
- the discussion of stiffened micromachined structures given hereinbefore focused on the structures made of only the deposited silicon nitride film. In those structures, care is taken to ensure that the silicon substrate from underneath the structures is preferably completely etched away.
- silicon substrate planes 87 and the exposed silicon nitride layer 88 are illustrated from a bottom- side view of a portion of a released latticed structure.
- smaller cells in the lattice may result in the silicon etch not reaching the silicon nitride film 88 from the bottom side, so that no silicon nitride gets exposed except for the lattice grid.
- This technique may be useful for improving the stiffness of the released structure significantly, and for adding to its mass for applications such as inertial sensing that require a massive movable element.
- Silicon dioxide may be a candidate material for the lattice grid in such an embodiment, since the structural properties of the resulting composite material may be determined more by the silicon than by the grid material.
- the fabrication methods of surface micromachined structures create stresses in the structural members upon release from the substrate and because such structures can be very compliant normal to the subsfrate, as discussed hereinbefore, the stresses in the members cause them to deform or bend out-of-plane.
- the thin film micromachined structure can be made more rigid to unwanted movements, both static and dynamic.
- a lattice of structural stiffening members may be first etched into the substrate material and then backfilled with the structural material. This can be a multi-step process, where the surface is planarized between layer depositions, or a single step process where the structural material fills the lattice mold while forming the surface structure.
- the surface deposited micromachined device can be stiffened upon release.
- the stiffening technique according to the present invention may be useful for inertial sensors where off-axis motions are critical to instrument performance, or in optical systems where abenations and unwanted vibrations can influence performance (e.g., to control deformations in uni-axial and bi-axial tilt mirrors), or to fabricate Gimbal structures with optimal properties, flexure width and length and outer ring radius to result in near zero axial tension on flexures and nearly spherical curvature of central plate due to film sfress gradients.
- the following discusses gold-coated silicon nitride micro mirrors designed for two orthogonal rotations.
- Fig. 6 illustrates an isometric view 42 of an exemplary biaxial micro mirror released from the substrate after being formed according to the process depicted in Fig. 1.
- Fig. 5 illustrates top and cross-sectional views of a bi-axial micro mirror.
- Micromachined silicon nitride mirrors are used to redirect light, in optical telecommunication systems, in endoscopic imaging devices, etc.
- two orthogonal flexures are used to support a reflective coated silicon nitride mirror. Applying voltages to surface electrodes can angularly deflect the mirror.
- a combination of two techniques can produce structures that can be made stiffer and have larger angular deflections, yet are easily produced.
- the mechanical members and mirror surface are made using standard surface micromachining techniques with structures to add rigidity to the members then released using a wet bulk etch of silicon substrate.
- Mirror diameters ranging from 100 ⁇ m to 500 ⁇ m were fabricated with electrostatic actuation used to achieve over four degrees of tilt for each axis.
- the micro mirror devices in Figs. 5 and 6 were released by etching the sacrificial PSG and thermal oxide with a hydrofluoric acid (HF) solution.
- HF hydrofluoric acid
- the clearance for mechanical motion was then produced by anisotropic etching the silicon substrate in a TMAH solution.
- TMAH a 5% solution of TMAH at 80 °C produced an etch rate of 25 ⁇ m/hr under the silicon nitride with the stiffening ribs.
- the silicon etch in TMAH was timed to produced the desired recess under the biaxial mirrors.
- thirty-two different biaxial mirror designs and numerous test structures were produced on a die with 34 dies on a four-inch wafer.
- the biaxial mirrors had a range of flexure geometries and dimensions, stiffening members, actuator sizes and reflective surface dimensions. Fig.
- FIG. 17 illustrates an exemplary released bi-axial micro mirror 90 with standard torsional hinges.
- Fig. 18 illustrates an exemplary released bi-axial micro mirror 92 with meander hinges.
- Other geometry for the flexures includes recessed hinges (not shown).
- the micro mirror 90 in Fig. 17 has a reflective surface with a 150 ⁇ m diameter with inner actuator 50 microns wide and outer electrode 100 microns wide.
- the flexures are all 50 ⁇ m long with inner flexture width being 6 ⁇ m and outer flexure width being 8 microns.
- the micro mirror 90 has two individual stiffening rings with webbing on the outer member and one webbed stiffening ring on the inner member.
- Fig. 19 shows some details of an inner flexure for the released bi-axial mirror 90 in Fig. 17.
- the step down between the two layers of nitride can be seen as arcs (for example, the arc 94 in Fig. 19) at the end of the flexures.
- the cusps over the backfilled trenches create a surface unsuitable for a reflective mirror, therefore no trenches were incorporated in the mirror area.
- the subsfrate under the mirror is flat to within a few microns.
- Fig. 20 shows the backside of another biaxial mirror fabricated using the process described with reference to Fig. 1.
- the backfilled trenches 96 that increase the structural strength of the device can be seen in Fig. 20.
- the height of the lattice- work may be determined by the etch depth into the silicon substrate, which may be typically 10-15 microns.
- Various geometries of lattice- work maybe produced ranging from simple concentric rings to interlaced webbings that completely fill the dimensions of the micromachined structure.
- the electrostatically actuated tilting mirror made of silicon nitride according to present invention standard surface micromachining techniques of lithography, wet and dry etching and thin films deposition are used.
- the mirror with stiffening members uses a substrate material (in this case silicon) that is different from the structural material (in this case silicon nitride), which allows a post-processing etch step to selectively etch the silicon substrate to an arbitrary depth underneath the released silicon nitride ⁇ mirror, without damaging the mirror. In this way, deep recesses under the device may be fabricated, allowing for large angular deflection of the mirror.
- Silicon nitride is used because of its good optical properties, low tensile stress, its ability to support multiple metal actuators on its surface, its dielectric properties, and excellent mechanical properties that make it not susceptible to fatigue.
- the mirror may be formed from a variety of metal films such as nickel, aluminum or tungsten, and other semiconducting materials such as polysilicon or dielectrics such as silicon carbide, hi the case of polysilicon, a non-silicon substrate material should preferably be used.
- the use of silicon nitride which is a dielectric, allowed the use of top-side electrodes for electrostatic actuation.
- chromium was deposited (for adhesion promotion) followed by gold deposition, and then electrodes were lithographically patterned for capacitive actuation.
- the counter electrode was the silicon subsfrate wafer.
- the counter electrode may be on some other surface placed adjacent to and substantially parallel to the silicon substrate.
- An example may be indium- tin-oxide coated glass.
- the mirror may be coated with a continuous metal film, with patterned actuation electrodes provided on an adjacent surface for the control of angular motion of the mirror.
- the substrate may be etched clear through, allowing optical access to the mirror from underneath.
- the use of a metal film material for the mirror structure may necessitate the use of an adjacent surface with patterned electrodes on it.
- actuation means may be devised, including electromagnetic, with either fixed magnets or electromagnets incorporated onto the mirror structure.
- Fixed magnets may be a film variety, deposited during the fabrication of the mirror, or they may be other types of magnets glued with an adhesive after the fabrication process was complete.
- An electromagnet may be formed with a deposited and/or plated coil incorporated onto the central plate of the mirror. Passing a current through this coil would generate a magnetic moment that may be acted upon by external magnetic fields. Combinations of electrostatic and electromagnetic actuation are also possible.
- Actuation provided by a mechanical coupling mechanism rather than by direct actuation on the mirror or gimbal ring are also possible.
- comb drive actuators may be used, with a mechanical coupling provided to the mirror or ring.
- Control of the surface curvature of an optical mirror may be achieved in two ways. The first is by taking advantage of the gimbal structure of a bi-axial tilt mirror. Changing the shape of the outer ring changes the way in which it curves, which in turn changes the tension applied to the torsion hinge connecting to the inner plate. Large tension on the hinges may lead to inner plate shapes that are more cylindrical, adding astigmatism to the optical beam reflecting from the plate. Compression of the hinges may lead to hinge buckling and unpredictable mechanical behavior. A small tensile force may allow the inner plate to curve in a more spherical shape, minimizing aberrations infroduced onto the optical beam. This tensile stress may be controlled by engineering the outer gimbal ring shape.
- the second approach to control mirror curvature is the incorporation of stiffening structures into the mirror.
- This approach is described hereinbefore where the substrate is first etched with a narrow trench pattern, prior to deposition of the silicon nitride structural material. These trenches are filled up and closed off during the nitride deposition, resulting in 3- dimensional film structures with significantly improved resistance to out-of-plane bending.
- Use of various lattice designs allows one to tailor the mechanical bending properties of these stiffened structures. In this way, the bending of both the central disk and the outer ring of the gimbal may be controlled.
- the tension felt by the inner torsion hinge may be controlled (achieving either tension or compression in that element), and thereby the residual curvature of the inner plate may also be controlled.
- the flatness of the mirror may be maintained to meet optical tolerances.
- the mirrors fabricated according to the method of the present invention have stiffening features incorporated around the perimeter of the central mirror plate, and onto the gimbal ring. No stiffening ribs are used in the area where the optical beam will strike the mirror, since such ribs may result in surface features that may cause scattering of the optical energy in the beam.
- a polishing step may be infroduced into the process after the nitride deposition so that stiffening structures could be used across the entire mirror plate, without compromising optical quality of the mirror surface.
- Micromirrors with honeycombed lattices across the entire mirror may also be fabricated using the method of the present invention, with an accompanying improvement in the flatness of the resulting mirror structures.
- bi-axial mirrors are described herein in detail, the fabrication process according to the present invention applies equally to uni-axial mirrors, which suffer much the same complications of the bi-axial mirrors. Such uniaxial mirrors don't have an outer gimbal ring around the mirror plate.
- Other useful mirror structures may also be fabricated using the method of the present invention. For example, translational mirrors designed for motion perpendicular to the plane of the substrate surface (often called piston mode motion) may be fabricated using the process of the present invention. Scanning interferometers may benefit from such minors.
- Optically flat mirrors, or minors with controlled curvature of the optical surface that are designed to operate with a large initial tilt angle, up to or exceeding 90 degrees may also be fabricated using the process of the present invention. Such "pop-up" minors may be useful for micro-optical systems that include an optical beam propagating parallel to the substrate surface.
- a deposited film material e.g., silicon nitride
- etch of the substrate mold in order to allow for motion of the device, where the film material is either different from the substrate and not attacked by the etchant used to remove the substrate material, or else it is protected in some way such as encapsulation or by a galvanic process during the etch.
- a galvanic etch is the common use of a p-n junction as an etch stop during KOH (potassium hydroxide) etching. It is observed that significantly stiffer minors may be fabricated from the intentional inclusion of some silicon into the silicon nitride stiffening lattice in the manner discussed hereinbefore with reference to Fig. 16.
- the minor with stiffening ribs still had some curvature (represented in the minor's electron microscope image by concentric fringes on the minor as compared to the straight fringes seen on the flat substrate of the minor), but substantially less curvature than that present in the minor without stiffening ribs.
- the reflective portion of the device (with stiffening ribs) was 150 microns in diameter and had less than one fringe across it.
- the source of the curvature in the device with stiffening ribs was the sfress induced on the loss stress nitride by the chrome- gold metal layers. Typically, the nitride has stress levels of 50-100 MPa, and the 50A of chrome and 1000 A of gold create additional stress in the layered film.
- a bi-axial minor fabricated with stiffening ribs using the methodology of the present invention was electrostatically actuated and its interferometric images were taken to profile the effect of actuation on the minor, hi the static case with no applied voltage, the interferometric image of the minor exhibited some fringes that were due to the combination of the surface deformation and substrate tilt, which was visible at the top of the image. It was apparent from the static case image that there was some curvature on the outer member of the minor as demonstrated by the nonlinear fringe pattern, but the center of the minor was relatively flat since its fringe pattern was nearly linear.
- a silicon substrate is first etched to produce a mold containing a plurality of trenches or grooves in a lattice configuration.
- Sacrificial oxide is then grown and/or deposited on the silicon subsfrate and then a stiffening member (silicon nitride) is deposited over the surface of the substrate, thereby backfilling the grooves with silicon nitride.
- the silicon nitride is patterned to form mechanical members and metals are then deposited and patterned to form the leads and capacitors for electrostatic actuation of mechanical members.
- the underlying silicon and sacrificial oxides are removed with a wet etch.
- the mold is etched from underneath the fabricated micromachined devices, leaving free-standing silicon nitride devices.
- the micromachined devices built with vertical features or fins or ribs created by molding the substrate and backfilling the mold with silicon nitride exhibit increased out-of-plane bending stiffness.
- the increased bending stiffness resulting from stiffening fins or ribs substantially reduce stress-related deformations experienced by surface-micromachined devices with large length-to-thickness ratios.
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Abstract
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Priority Applications (4)
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JP2003539116A JP2005506909A (en) | 2001-10-22 | 2002-10-21 | Reinforced surface micromachined structure and manufacturing method thereof |
EP02780484A EP1438256A2 (en) | 2001-10-22 | 2002-10-21 | Stiffened surface micromachined structures and process for fabricating the same |
AU2002343537A AU2002343537A1 (en) | 2001-10-22 | 2002-10-21 | Stiffened surface micromachined structures and process for fabricating the same |
US10/493,140 US20090065429A9 (en) | 2001-10-22 | 2002-10-21 | Stiffened surface micromachined structures and process for fabricating the same |
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US33043301P | 2001-10-22 | 2001-10-22 | |
US33043501P | 2001-10-22 | 2001-10-22 | |
US60/330,435 | 2001-10-22 | ||
US60/330,433 | 2001-10-22 |
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JP (1) | JP2005506909A (en) |
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Cited By (6)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
WO2008089786A1 (en) * | 2007-01-23 | 2008-07-31 | Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. | Micromechanical component having increased stiffness, and method for the production of the same |
DE102013205527A1 (en) * | 2012-04-04 | 2013-10-10 | Infineon Technologies Ag | MEMS COMPONENT AND METHOD FOR PRODUCING A MEMS COMPONENT |
US8888252B2 (en) | 2008-07-09 | 2014-11-18 | Hewlett-Packard Development Company, L.P. | Print head slot ribs |
CN105314587A (en) * | 2014-07-31 | 2016-02-10 | 英飞凌科技股份有限公司 | Micro mechanical structure and method for fabricating the same |
DE102009026629B4 (en) * | 2009-06-02 | 2017-09-21 | Robert Bosch Gmbh | Micromechanical system with a buried thin structured polysilicon layer and method for its production |
CN115140703A (en) * | 2022-07-08 | 2022-10-04 | 山东大学 | Pre-strain-assisted wrinkle-form micro-nano structure manufacturing device and method |
Families Citing this family (7)
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US6912082B1 (en) * | 2004-03-11 | 2005-06-28 | Palo Alto Research Center Incorporated | Integrated driver electronics for MEMS device using high voltage thin film transistors |
WO2011080883A1 (en) * | 2009-12-28 | 2011-07-07 | 株式会社ニコン | Electro-mechanical converter, spatial optical modulator, exposure device, and methods for manufacturing them |
JP5630015B2 (en) * | 2009-12-28 | 2014-11-26 | 株式会社ニコン | Spatial light modulator, exposure apparatus and manufacturing method thereof |
US20130027795A1 (en) | 2011-07-29 | 2013-01-31 | Gsi Group Corporation | Systems and methods for providing mirrors with high stiffness and low inertia involving chemical etching |
JP2013255975A (en) * | 2012-06-14 | 2013-12-26 | Fujitsu Ltd | Electronic device |
DE102019111634A1 (en) * | 2019-05-06 | 2020-11-12 | Lpkf Laser & Electronics Ag | Process for the production of microstructures in a glass substrate |
US11460688B2 (en) * | 2019-08-23 | 2022-10-04 | Tohoku University | Mirror device, scanning laser device and scanning laser display including same mirror device, and method for manufacturing mirror device |
Family Cites Families (3)
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US5660680A (en) * | 1994-03-07 | 1997-08-26 | The Regents Of The University Of California | Method for fabrication of high vertical aspect ratio thin film structures |
US5628917A (en) * | 1995-02-03 | 1997-05-13 | Cornell Research Foundation, Inc. | Masking process for fabricating ultra-high aspect ratio, wafer-free micro-opto-electromechanical structures |
US6411754B1 (en) * | 2000-08-25 | 2002-06-25 | Corning Incorporated | Micromechanical optical switch and method of manufacture |
-
2002
- 2002-10-21 WO PCT/US2002/033351 patent/WO2003036737A2/en active Application Filing
- 2002-10-21 AU AU2002343537A patent/AU2002343537A1/en not_active Abandoned
- 2002-10-21 JP JP2003539116A patent/JP2005506909A/en active Pending
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Cited By (9)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
WO2008089786A1 (en) * | 2007-01-23 | 2008-07-31 | Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. | Micromechanical component having increased stiffness, and method for the production of the same |
US8888252B2 (en) | 2008-07-09 | 2014-11-18 | Hewlett-Packard Development Company, L.P. | Print head slot ribs |
DE102009026629B4 (en) * | 2009-06-02 | 2017-09-21 | Robert Bosch Gmbh | Micromechanical system with a buried thin structured polysilicon layer and method for its production |
DE102013205527A1 (en) * | 2012-04-04 | 2013-10-10 | Infineon Technologies Ag | MEMS COMPONENT AND METHOD FOR PRODUCING A MEMS COMPONENT |
US9409763B2 (en) | 2012-04-04 | 2016-08-09 | Infineon Technologies Ag | MEMS device and method of making a MEMS device |
US9580299B2 (en) | 2012-04-04 | 2017-02-28 | Infineon Technologies Ag | MEMS device and method of making a MEMS device |
DE102013205527B4 (en) | 2012-04-04 | 2018-06-21 | Infineon Technologies Ag | Method of making an electrode of a MEMS device |
CN105314587A (en) * | 2014-07-31 | 2016-02-10 | 英飞凌科技股份有限公司 | Micro mechanical structure and method for fabricating the same |
CN115140703A (en) * | 2022-07-08 | 2022-10-04 | 山东大学 | Pre-strain-assisted wrinkle-form micro-nano structure manufacturing device and method |
Also Published As
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JP2005506909A (en) | 2005-03-10 |
EP1438256A2 (en) | 2004-07-21 |
WO2003036737A8 (en) | 2008-12-31 |
WO2003036737A3 (en) | 2003-11-20 |
AU2002343537A8 (en) | 2009-01-29 |
AU2002343537A1 (en) | 2003-05-06 |
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