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

Transcription

1 Micro-Scale Engineering I Microelectromechanical Systems (MEMS) Y. C. Lee Department of Mechanical Engineering University of Colorado Boulder, CO leeyc@colorado.edu January 15,

2 Contents Microelectromechanical systems (MEMS) Surface micromachining for a mirror - electrostatic actuation - pull-down voltage - stiction - thermal actuation Accelerometers and Gyroscopes Market and Trend 2

3 Integrated Circuits Information Era 3 Transistor counts: Intel 80386, 1985, 275,000 Intel Pentium 4, 2006, 184,000,000 Intel i7, 2011, 2,270,000,000

4 4

5 Spin Coating of Photoresist 5

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

7 Semiconductor Processing UV Light Photoresist Mask (Layout) Batch Processing 7

8 Foundry Processes for Microelectromechanical Systems (MEMS) Multi-User MEMS Processes (MUMPS) Example Design Anchor hole poly2 1.5 Layers and Nominal Thickness in Microns Anchor gold 0.5 MCNC poly0 nitride oxide 1 oxide 2 nitride poly0 poly2 (a) After Poly2 Deposition (b) Released Device 8

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 9

10 Add Poly0 (500 nm, LPCVD) Silicon Substrate 10

11 Patterning through 1st level mask (Poly0) using Photolithography Design Poly0 Patterned Photoresist Photoresist Silicon Substrate 11

12 Removal of Unwanted Poly0 using Reactive Ion Etching Design Poly0 Patterned Photoresist Silicon Substrate 12

13 1st Oxide Deposition 2 um using LPCVD Design Poly0 Silicon Substrate 13

14 Patterning through 3rd level mask (Anchor1) using Photolithography Design Anchor1 and Deep RIE Photoresist Silicon Substrate 14

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 15

16 Patterning through 4th level mask (Poly1) using Photolithography... and Deep RIE Silicon Substrate PSG layer etched first to form the RIE hard mask 16

17 Release of structures using HF (46% HF, room temperature, minutes; followed by several minutes in DI water and then alcohol by at least 10 minutes in an oven at 110 C Silicon Substrate 17

18 Microelectromechanical Systems (MEMS) Multi-User MEMS Processes (MUMPS) Example Design Anchor hole poly2 1.5 Layers and Nominal Thickness in Microns Anchor gold 0.5 MCNC poly0 nitride oxide 1 oxide 2 nitride poly0 poly2 (a) After Poly2 Deposition (b) Released Device 18

19 Electrostatic Actuator Pull In Release Voltage Applied (Volts) 19

20 MUMPs Chip 1cm 2 20

21 Electrostatic Force The energy stored at a given voltage is, U ( x, z) = 1 CV ε0ε r A = V 2 z The force between the plates in the vertical direction is, electrostatic vertical force between plates F e 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 d = flexure spring constant = deflection distance 21

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 µ = Young's Modulus = flexure width = flexure thickness = flexure length ) wt stress term (due to residual stress) = flexure material stress = 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 22

23 Pull-Down Voltage assuming free space between the plates we get, 2 1 ε0av Nkd 2 = 2 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 23

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 2Nk 2Nk 0 s s s 2 ε0a ε0a = ( z0 ds ) ds ds 2 2 ( z d ) d d = 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 Note : The rule also applies z 3 0 to cantilevers and microbridges. 0 voltage 24

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 /(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 )] = 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 25 0 ) wt

26 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 26

27 Digital Mirrors 27

28 Contents Microelectromechanical systems (MEMS) Surface micromachining for a mirror - electrostatic actuation - pull-down voltage - stiction - thermal actuation Accelerometers and Gyroscopes Market and Trend 28

29 29

30 Multi-User MEMS Processes (MUMPS) Example Design Mirror Accelerometer? Anchor hole poly2 1.5 Layers and Nominal Thickness in Microns Anchor gold 0.5 MCNC poly0 nitride oxide 1 oxide 2 nitride poly0 poly2 (a) After Poly2 Deposition (b) Released Device 30

31 STMicroelectronics LIS331DLH 3-axis accelerometer for iphone 4 Processing Chip 31

32 Three-Axis Accelerometer for iphone 4 32

33 Applications of Accelerometers 33

34 MEMS Gyroscope 34

35 MEMS Gyroscope 35

36 Why was it a major breakthrough? 36

37 Applications of Gyroscope 37

38 Contents Microelectromechanical systems (MEMS) Surface micromachining for a mirror - electrostatic actuation - pull-down voltage - stiction - thermal actuation Accelerometers and Gyroscopes Market and Trend 38

39 Why MicroNanoBio? 39

40 MEMS Market 40

41 MEMS for Smartphones 41

42 MEMS for Smartphones 42

43 Low-Cost MEMS 43

44 Trend 44