Micro Electro Mechanical System Design - James J. Allen Part 7 pptx

Design Realization Tools for MEMS 161 FIGURE 5.7 Layout of the seismic mass level of an accelerometer utilizing only one drawing layer. FIGURE 5.8 Layout of the seismic mass level of an accelerometer utilizing two layers. © 2005 by Taylor & Francis Group, LLC 162 Micro Electro Mechanical System Design FIGURE 5.9 Use of a bit by bit XOR logical function to combine layers (layer and layer_CUT) to form a mask definition. FIGURE 5.10 Alternative approaches to layout of an annular MMPOLY2 feature. © 2005 by Taylor & Francis Group, LLC Design Realization Tools for MEMS 163 SUMMiT. Because surface micromachining is an alternating stack of two types of materials (i.e., structural and sacrificial), mechanical layers are attached by etching a hole or via in the sacrificial material; this enables the next mechanical layer material deposited to attach to the mechanical material below at the via. Figure 5.13 shows SEM images of various layers of SUMMiT anchored to each other. The attachment between the layers at the via produced by the SACOX_CUT can be seen between the adjacent layers. The term “SACOX_CUT” refers to a generic operation of opening a via in a sacrificial oxide layer to allow attachment of adjacent structural layers. SACOX#_CUT is the use of a SACOX_CUT on a specific SACOX# layer. A particular layer cannot be directly anchored to ground in the SUMMiT technology because deep SACOX_CUTs are not allowed. The SACOX_CUT FIGURE 5.11 Alternative approaches to the layout of an MMPOLY2 island inside an MMPOLY2 annular feature. a. MMPOLY2 island within a MMPOLY2 annular feature. MMPOLY2 b. Layout with two MMPOLY2_CUT polygons within a MMPOLY2 polygon. MMPOLY2 MMPOLY2_CUT c. Layout with three MMPOLY2_CUT polygons. MMPOLY2 © 2005 by Taylor & Francis Group, LLC 164 Micro Electro Mechanical System Design enables the mechanical layers immediately above and below to be attached. For example, a SACOX3_CUT will enable MMPOLY3 and MMPOLY2 to be attached. Figure 5.14 shows a demonstration of the five layers of the SUMMiT process. Figure 5.15 show a cross-section visualization and the masks for a post that extends from MMPOLY0 to MMPOLY4. Notice that the SACOX_CUTs are all on top of each other and the size of the SACOX_CUT becomes bigger at each higher level. The size increase enables the mechanical material at the higher level to attach to the shoulder of the via to the mechanical material on the level immediately below. This is denoted as a nested anchor. The nested anchor method can produce the smallest size post possible. How- ever, a nested anchor will have encased silicon dioxide trapped inside the post as shown in the cross-section of Figure 5.15. The encased silicon dioxide is due to the inability of the etching processes to remove material completely at locations that have significant vertical topography. This artifact is known as a stringer and is discussed in Section 3.3 and Section 5.3.1.6. The residual stress of the entrapped silicon oxide in the post can cause slight deflections [7]; this may be a design consideration, depending upon the application. A staggered anchor is a method of reducing the amount of encased silicon dioxide in an anchor; however, the required size of the post will increase. Figure 5.16 shows a cross-section visualization of a post utilizing a staggered anchor approach. 5.2.2 ROTATIONAL HUBS Rotational hubs are structures that enable 360° rotation similar to a wheel and axle. The ability to implement this structure at the microscale with no assembly FIGURE 5.12 Cross-hatch patterns for the SUMMiT V masks. NITRIDE_CUT SACOX1_CUT DIMPLE1_CUT MMPOLY0 MMPOLY1_CUT PIN JOINT_CUT MMPOLY2 SACOX3_CUT SACOX2 DIMPLE3_CUT MMPOLY3 DIMPLE4_CUT MMPOLY4 SACOX4_CUT © 2005 by Taylor & Francis Group, LLC Design Realization Tools for MEMS 165 FIGURE 5.13 Scanning electron microscope images of nested SACOX_CUTs to anchor layers to each other and to ground. (Courtesy of Sandia National Laboratories.) MMPOLY0 (a) One layer anchored to ground (b) Two layers anchored to ground (c) Three layers anchored to ground MMPOLY0 MMPOLY2 MMPOLY3 MMPOLY4 MMPOLY1 & MMPOLY2 MMPOLY3 MMPOLY1 & MMPOLY2 MMPOLY1 & © 2005 by Taylor & Francis Group, LLC 166 Micro Electro Mechanical System Design FIGURE 5.14 Example of the five mechanical levels of SUMMiT anchored to each other and to ground. The anchors utilized nested SACOX_CUTs except as noted in the figure where staggered SACOX_CUTs are used. (Courtesy of Sandia National Laboratories.) FIGURE 5.15 Masks and cross-section of a post composed of anchored layers utilizing nested SACOX_CUTs. FIGURE 5.16 Cross-section of an anchored layer stack using staggered SACOX_CUTs. trapped oxide MMPOLY4 MMPOLY3 MMPOLY2 MMPOLY0 SACOX4_CUT SACOX3_CUT SACOX2_CUT © 2005 by Taylor & Francis Group, LLC Design Realization Tools for MEMS 167 necessary is an enabling feature for MEMS devices that require mechanisms. Two methods can be used to produce a rotational hub in the SUMMiT technology: a cap and post hub and a low-clearance hub, which are discussed next. A cap and post hub can be implemented in any three-level surface microma- chine technology, and it is the simplest hub design that can be utilized. Figure 5.17 shows the masks and a cross-section of a cap and post hub implemented in the SUMMiT technology. The central feature of this type of hub is a central post with a cap of sufficient diameter so that a rotating wheel will be constrained vertically. The figure’s cross-section shows the rotating wheel composed of MMPOLY1 and MMPOLY2, which are laminated together. Functionally, the rotating wheel could be only one layer instead of two. An MMPOLY3 cap is supported by a post of MMPOLY1 and MMPOLY2. The implementation of the cap and post structure is similar to the anchors discussed in the previous section. The clearance for the rotating wheel is defined by the ability of the lithography process to etch layers MMPOLY1 and MMPOLY2 at the rotating interface. The vertical clearance is defined by the thickness of the sacrificial oxide layer or the ability to produce structures such as dimples to constrain the vertical motion. Dimples are small “bumps” under- neath surface micromachined layers that prevent broad area surface contact when the layers contact the substrate or each other. Dimples can also be used to minimize clearances. The low-clearance hub is a feature that SUMMiT was especially designed to implement ( Figure 5.18). This hub utilizes the ability to deposit and etch thin films of sacrificial material accurately (i.e., silicon dioxide) to control the clear- ance in the hub. Figure 5.18 shows the layout and cross-section of the low- clearance hub, and Figure 5.19 shows an FIB cross-section of a low-clearance hub and pin joint fabrication in SUMMiT. A pin joint is very similar to a hub, but is not attached to ground. A pin joint enables linkages between rotating members, as shown in Figure 5.19. Figure 5.20 shows a cross-section of the SUMMiT fabrication sequence for the low-clearance hub at several key points in the process: • Figure 5.20a shows the fabrication at the point at which SACOX1 has been deposited and patterned to produce dimples and anchor MMPOLY1. MMPOLY1 has been deposited and patterned with the PIN_JOINT_CUT mask. A combination of anisotropic and wet etching has been performed to form the features beneath MMPOLY1. • Figure 5.20b shows the process after the SACOX2 layer has been deposited, patterned, and etched and the MMPOLY2 layer deposited. At this stage, SACOX2 can be seen to define the clearances in the internals of the low-clearance hub. The low-clearance hub lateral and vertical clearances in SUMMiT are 0.3 µm. • Figure 5.20c shows the cross-section after the MMPOLY2 etch has been performed. This etch can etch the laminated MMPOLY1 and MMPOLY2 layers, thus providing an even outside surface for the © 2005 by Taylor & Francis Group, LLC 168 Micro Electro Mechanical System Design FIGURE 5.17 Cap and post hub layout and cross-section visualization. © 2005 by Taylor & Francis Group, LLC Design Realization Tools for MEMS 169 FIGURE 5.18 Low-clearance hub and pin joint layout and cross-section visualization. © 2005 by Taylor & Francis Group, LLC 170 Micro Electro Mechanical System Design rotating wheel and etch release holes through the rotating wheel disc. Note that the MMPOLY2 etch stops on the SACOX2 layer in the internal hub features. • Figure 5.20d shows the cross-section of the released low-clearance hub and pin joint structure. 5.2.3 POLY1 BEAM WITH SUBSTRATE CONNECTION The MMPOLY1 beam with a substrate connection is a simple structure illustrating the application of two features useful in design of a number of devices in the SUMMiT technology. The MMPOLY1 layer can be patterned in either of two ways in SUMMiT: • The MMPOLY1 layer can be patterned directly using the MMPOLY1_cut mask and etch. • The MMPOLY1 layer can also be patterned indirectly by using SACOX2 as a “hard” mask and etching with the MMPOLY2 etch. In the previous section, Figure 5.20c showed that the MMPOLY2 etch would etch the MMPOLY1 and MMPOLY2 layers except when the MMPOLY1 layer is protected by SACOX2. In this case, the SACOX2 layer was used as a “hard” mask to stop the MMPOLY2 etch. For the MMPOLY1 beam shown in Figure 5.21, the SACOX2 mask is used to define the MMPOLY1 beam via the MMPOLY2 etch. The MMPOLY1 beam is attached to the substrate using a SACOX1_CUT as discussed in Section 5.2.1. If a connection is to be established to the substrate for electrical grounding purposes, the NITRIDE_CUT mask is used to define the etch of the NITRIDE layer. 5.2.4 DISCRETE HINGES The concept of discrete hinges for MEMS applications was initially proposed by Pister [8]. Since that time, a number of different variations and types of hinges FIGURE 5.19 A focused ion beam (FIB) cross-section of a rotational hub and pin joint. (Courtesy of Sandia National Laboratories.) © 2005 by Taylor & Francis Group, LLC [...]... for plate-to-plate hinge B A floor pin (e) cross-section B-B after release etch floor pin staple (c) cross-section A-A after release etch plate 1 staple plate 2 (b) cross-section A-A after MMPOLY2 etch Design Realization Tools for MEMS 175 176 Micro Electro Mechanical System Design 5.3 DESIGN RULES The term design rules originally comes from the microelectronics industry These rules are a formal communication... plate-to-plate hinge B B A A (e) MMPOLY3 mask A A (b) SACOX2 mask A B B A A (c) MMPOLY2 mask B B A (f) Aligned composite masks for the plate-to-plate hinge B B A 174 Micro Electro Mechanical System Design B floor © 2005 by Taylor & Francis Group, LLC FIGURE 5.25 SUMMiT masks and cross-sections for the plate-to-plate hinge (d) cross-section B-B after MMPOLY2 etch pin A (a) composite masks for plate-to-plate... trade-offs between design variables, is paramount Chapter 7 will discuss development of design synthesis models utilizing Lagrange’s equations to formulate the governing equations of the MEMS device © 2005 by Taylor & Francis Group, LLC 188 Micro Electro Mechanical System Design Design Synthesis Models System Level Models Lumped parameter Limited number of degrees of freedom Phenomenological or Macro-Models... http://www.artwork.com/gdsii/asm3500/index.htm 7 M.S Baker, M.P de Boer, N.F Smith, L.K Warne, M.B Sinclair, Integrated measurement-modeling approaches for evaluating residual stress using micromachined fixed-fixed beams, J Microelectromechanical Syst., 11(6), 74 3 75 3, December 2002 8 K.S.J Pister, M.W Judy, S.R Burgett, Fearing, Microfabricated hinges, Sensors Actuators A, 33, 249–256, 1992 9 R Yeh, E.J.J Kruglick, K.S.J Pister, Surface micromachined... FIGURE 5.20 Low-clearance hub and pin joint cross-section visualization at various stages in the SUMMiT fabrication etch laminated MMPOLY1 and MMPOLY2 (a) Hub and pin joint after pin joint etch Design Realization Tools for MEMS 171 172 Micro Electro Mechanical System Design SACOX1_CUT NITRIDE_CUT MMPOLY0 SACOX2 substrate connection (a) Layout nitride silicon dioxide substrate (b) Cross-section visualization... model of the part) Recent research [45– 47] has developed the basic algorithms for the part- to-art problem (i.e., solid model of the part to the layout art) With further development and the use of the partto-art algorithms, a new method of design realization for MEMS may be possible Instead of the MEMS designer producing two-dimensional layouts to produce the three-dimensional MEMS device, he or she may... of the MEMS device; the part- to-art algorithm could produce the two-dimensional layouts of the masks to make the part in a particular technology This would enable a MEMS designer to work in the same manner as a macroworld design engineer, who develops a three-dimensional solid model of a device to be designed MEMS analysis capability is essential to the ability of the MEMS designer to engineer a device... providing the MEMS designer with meaningful information Figure 5.39 illustrates how MEMS twoand three-dimensional visualization tools assist the MEMS designer from layout to the fabricated device The three-dimensional visualiza- © 2005 by Taylor & Francis Group, LLC 186 Micro Electro Mechanical System Design Conformal Deposition Planar Deposition Wet Etch Dry Etch FIGURE 5.38 MEMS three-dimensional visualization... are designed, fabricated, tested, and placed into a library for use in further design Models of the components will have already been developed and available Ideally, a designer would be able to implement a significant portion of any design by placing and coupling the components together and simulating the total system response uti- © 2005 by Taylor & Francis Group, LLC 184 Micro Electro Mechanical System. .. Automatic design rule checking operation (SUMMiT™ MEMS design tools — courtesy of Sandia National Laboratories.) • • Design rule errors: a design rule violation that requires mandatory attention Design rule advisory: a design rule violation that requires the designer to evaluate the necessity for correction Figure 5.35 shows the operation of the automatic design rule-checking capability within the SUMMiT™ design . 2005 by Taylor & Francis Group, LLC 176 Micro Electro Mechanical System Design 5.3 DESIGN RULES The term design rules originally comes from the microelectronics industry. These rules are a. and cross-section. (a) SUMMiT TM layout (b) A-A cross-section moveable plate pin staple encased oxide A A © 2005 by Taylor & Francis Group, LLC 174 Micro Electro Mechanical System Design FIGURE. LLC Design Realization Tools for MEMS 175 FIGURE 5.25 SUMMiT masks and cross-sections for the plate-to-plate hinge. A A B B (a) composite masks for plate-to-plate hinge (b) cross-section A-A after