Micro-Scale Engineering I Microelectromechanical Systems (MEMS) Y. C. Lee

Micro-Scale Engineering I Microelectromechanical Systems (MEMS) Y. C. Lee Department of Mechanical Engineering University of Colorado Boulder, CO 80309-0427 leeyc@colorado.edu September 2, 2008 1

Three Lectures Tuesday, September 2 - MEMS Introduction Tuesday, September 9 - BioMEMS, Lab-on-a-Chip Tuesday, September 16 - BioMEMS, Cancer cell analysis, implantable MEMS 2

Contents Microelectromechanical systems (MEMS) Surface micromachining for a mirror - electrostatic actuation - pull-down voltage - stiction - thermal actuation Bulk micromachining for a pressure sensor - processing steps -piezoresistor Applications 3

Integrated Circuits Information Era 4

Spin Coating of Photoresist 6

Lithography Exposure to ultraviolet light through a quartz mask. S.M. Sze, Semiconductor Devices, Wiley, 1985. 7

Semiconductor Processing UV Light Photoresist Mask (Layout) Batch Processing 8

Foundry Processes for Microelectromechanical Systems (MEMS) Multi-User MEMS Processes (MUMPS) Example Design Anchor hole 2.0 0.5 0.6 poly2 1.5 Layers and Nominal Thickness in Microns Anchor gold 0.5 MCNC 0.75 2.0 poly0 nitride oxide 1 oxide 2 nitride poly0 poly2 (a) After Poly2 Deposition (b) Released Device 9

Add nitride (600 nm, LPCVD (low pressure chemical vapor deposition) Silicon Substrate 100 mm, n-type, 1-2 Ohm-cm, surface doped with phosphorus 10

Add Poly0 (500 nm, LPCVD) Silicon Substrate 11

Patterning through 1st level mask (Poly0) using Photolithography Design Poly0 Patterned Photoresist Photoresist Silicon Substrate 12

Removal of Unwanted Poly0 using Reactive Ion Etching Design Poly0 Patterned Photoresist Silicon Substrate 13

1st Oxide Deposition 2 um using LPCVD Design Poly0 Silicon Substrate 14

Patterning through 3rd level mask (Anchor1) using Photolithography Design Anchor1 and Deep RIE Photoresist Silicon Substrate 15

Blanket un-doped polysilicon deposition(poly1) 2 um using LPCVD... followed by 200 nm PSG deposition and annealing at 1050 C for 1 hr Silicon Substrate 16

Patterning through 4th level mask (Poly1) using Photolithography... and Deep RIE Silicon Substrate PSG layer etched first to form the RIE hard mask 17

Release of structures using HF (46% HF, room temperature, 1.5-2 minutes; followed by several minutes in DI water and then alcohol by at least 10 minutes in an oven at 110 C Silicon Substrate 18

Microelectromechanical Systems (MEMS) Multi-User MEMS Processes (MUMPS) Example Design Anchor hole 2.0 0.5 0.6 poly2 1.5 Layers and Nominal Thickness in Microns Anchor gold 0.5 MCNC 0.75 2.0 poly0 nitride oxide 1 oxide 2 nitride poly0 poly2 (a) After Poly2 Deposition (b) Released Device 19

Array of Piston Mirrors (W. D. Cowan and V. M. Bright) 20

MUMPs Chip 1cm 2 21

Electrostatic Force The energy stored at a given voltage is, U ( x, z) = 1 CV 2 2 1 ε0ε r A = V 2 z The force between the plates in the vertical direction is, electrostatic vertical force between plates F e 1 2 0 A F ( z) r z U ( x, z) = ε ε = = V z 2 1 z 1 2 = ε 1 0ε 2 2 r V A z 0 2 (negative z direction) 1 z z o force vs. distance is nonlinear metal dielectric air metal d The flexures can be modeled as cantilevers. For small deflections, a cantilever's restoring force is given by Hooke's law, F c = k d force vs. distance is linear where, k = flexure spring constant d = deflection distance 22

Restoring Force The flexure spring constant is given by, where, k = Ewt 3 L σ (1 2L 3 μ + cross-sectional spring constant E w t L σ = Young's Modulus = flexure width = flexure thickness = flexure length ) wt stress term (due to residual stress) = flexure material stress μ = flexure material Poisson's ratio The flexure spring constant is determined during device design and construction, it is not a function of voltage or deflection. The equation for k is only an approximation! For stable deflection the electrostatic force is balanced by the restorative flexure force, F = 4 e NF c where, N = number of flexures Anchor Flexure gold nitride poly0 poly2 23

Pull-Down Voltage assuming free space between the plates we get, 1 2 2 AV 2 = ε0 Nkd z solving for V as a function of d gives, V = z 2 ε0 Nkd A z z ( z d ) = 0 replacing by gives, z o metal dielectric air metal d V = ( z d ) 0 2Nkd ε A 0 This equation calculates the required voltage,, to deflect the top plate of the piston actuator a distance,, from the initial plate separation, z. V d 0 24

Maximum Deflection Note that we are trying to balance the nonlinear electrostatic force,, with a linear force, F c. An imbalance or snap-through occurs when the rate of change in voltage vs. deflection distance is zero: dv dd d = d s = 1 1 0 = 1 2Nk 2Nk 0 s s s 2 ε0a ε0a 1 1 2 2 0 = ( z0 ds ) ds ds 2 2 ( z d ) d d 1 2 1 1 1 1 2 2 2 2 0 = z0ds ds 2ds = z0ds 3ds d s = z 3 0 ds z0 where, = deflection distance from initial separation,, to snap-through. 1 2 z o metal F e dielectric air metal d deflection z 3 0 1 z 3 Note : The rule also applies 0 to cantilevers and microbridges. 0 voltage 25

An Example A plate: 250 μm square An air gap: 2 μm A flexure: 40 μm x 10 μm x 0.5 μm E = 169 GPa for the flexure k = 169,000 MPa x 10 x 0.5 3 /(4 x 40 3 ) = 0.8 μn/μm d = 1/3 x 2 μm = 0.7 μm V = (2/3 x 2μm) x SQRT [(2x1x0.8x0.7)/(8.85E-6 x 250 2 )] = 1.9 V ε o = 8.85E-6 pf/μm 1 Pa = 1 Newton/m 2 = 1 N/m 2 1 MPa = 1 μn/μm 1 V = SQRT (μn x μm/pf) Anchor Flexure gold nitride poly0 poly2 k = V Ewt 3 L 4 = ( z d ) 0 σ (1 2L 3 μ + 2Nkd ε A 26 0 ) wt

Piston Micromirrors Evolution of a Piston Micromirror Over Four MUMPs Fabrication Runs aluminum square mirror with four flexures 60 μm wide copper hexagonal mirror 50 μm wide gold hexagonal mirror AFIT 100 μm wide gold hexagonal mirror J.H. Comtois and V.M. Bright, AFIT 27

Simulation of Mirror Design 28

Multi-User MEMS Processes (MUMPS) Example Design Mirror Accelerometer? Anchor hole 2.0 0.5 0.6 poly2 1.5 Layers and Nominal Thickness in Microns Anchor gold 0.5 MCNC 0.75 2.0 poly0 nitride oxide 1 oxide 2 nitride poly0 poly2 (a) After Poly2 Deposition (b) Released Device 29

V. M. Bright and M. A. Michalicek, 1999 Piezoresistive Readout Sensors Static Pressure Sensor Single Axis Accelerometer Oxide Proof Mass Piezoresistor Piezoresistor Glass Support Glass Support After J. W. Gardner, Microsensors, Wiley, 1994. 30

Thermal Actuator 31

Thermal Actuator 32

Array of Thermal Actuators 33

V. M. Bright and M. A. Michalicek, 1999 Piezoresistive Readout Sensors Static Pressure Sensor Oxide Piezoresistor Glass Support 35

Processing Steps for Pressure Sensor Piezoresistor Glass Support 36

Low Pressure Chemical Vapor Deposition (LPCVD) Gas flow of dichloro silane (DCS) and ammonia 37

Reactive Ion Etching Chemical and Physical Etching Si Si 3 N 4 Si 3 N 4 Piezoresistor Glass Support 38

Etching Bulk Silicon V. M. Bright and M. A. Michalicek, 1999 Isotropic Etching Masking Material Anisotropic Etching (Two Wafer Orientations) {111} {110} Substrate No Agitation Substrate With Agitation Substrate XeF 2, HCl, HF Si 3 N 4 {111} Isotropic Si Substrate {100} Si 3 N 4 Piezoresistor Anisotropic KOH, EDP, Hydrazine After L. J. Ristic, ed., Sensor Technology and Devices, Artech House, 1994. Glass Support J. H. Comtois, Ph.D. Thesis, AFIT, 1995. 39

Piezoresistor Microcrystalline polysilicon deposited using LPCVD at ~610 C on silicon nitride. Boron-ion implantation at 30 KeV with the dose of 5X10 14 cm -2 or 5X10 14 cm -2. The doped boron was activated at 950 C for 10 min or 1050 C for 30 min. 40

Diffusion and Implantation of Dopants V. M. Bright and M. A. Michalicek, 1999 Dopant Gas Diffusion Dopant Profile High-Velocity Dopant Ions Implantation Dopant Profile Mask Ln( N ) Mask Ln( N ) After S. M. Sze, Semiconductor Devices, Wiley, 1985. x After S. M. Sze, Semiconductor Devices, Wiley, 1985. x Analyzer Magnet Vertical Scanner Horizontal Scanner Target Wafer Variable Slit (Beam Control) Acceleration Tube Ion Implantation System After S. M. Sze, Semiconductor Devices, Wiley, 1985. Example: Release Etch Ion Source Buried Etch Stop Region Formation of Rounded, Often Complex, Three-Dimensional Bulk Silicon Structures 41

Anodic Bonding Piezoresistor Glass Support Na + ions moved O molecules to form SiO2 42

V. M. Bright and K. Ishikawa, 2001 Examples of Accelerometers IMI Website; http://www.imi-mems.com/ High sensitivity Large dynamic range Capacitive accelerometers M. Lemkin et al., ISSCC Digest of Technical Papers, pp. 202-203, Feb. 1997. Noise Floor X-Axis 110 micro-g/rt. Hz Y-Axis 160 micro-g/rt. Hz Z-Axis 990 micro-g/rt. Hz Dynamic Range (100 Hz BW) X-Axis 84 db Y-Axis 81 db Z-Axis 70 db Proof Mass, Resonant Frequency X-Axis Y-Axis Z-Axis 0.375 micro-gram, 3.2kHz 0.259 micro-gram, 4.2kHz 0.389 micro-gram, 8.3kHz Resonant accelerometers Open view of an accelerometer assembly T. A. Roessig et al., IEEE Frequency Control Symposium, Orlando FL, May 1997. 44 The World of Microsystems, FSRM, 2000. FSRM website; http://www.fsrm.ch/

Applications and Markets of MEMS in Mobile Communications Brochure of the Report from Yole Développement, Feb. 2005 46

Applications and Markets of MEMS in Mobile Communications Silicon microphone: improves the manufacturability of microphone for similar performance compared to ECM 3D accelerometers: adds functions for man machine interface and silent mode activation RF MEMS passive and active devices: provides better integration of passive devices for RF module and faster frequency agility Gyroscope for camera stabilisation and GPS: enables real digital imaging, preserves the GPS signal Microfuel cell: provides longer lifetime for the batteries Chemical and Biochip: personal weather station and health care monitor Brochure of the Report from Yole Développement, Feb. 2005 48

Silicon Microwell Polymerase Chain Reaction (PCR) 49

Summary Microelectromechanical systems (MEMS) Surface micromachining for a mirror - electrostatic actuation - pull-down voltage - stiction - thermal actuation Bulk micromachining for a pressure sensor - processing steps -piezoresistor Applications 50