Introduction to Microeletromechanical Systems (MEMS) Lecture 9 Topics MicroOptoElectroMechanical Systems (MOEMS) Grating Light Valves Corner Cube Reflector (CCR) MEMS Light Modulator Optical Switch Micromirrors Tunable IR Filter Interferometry Field Emission Display MEMS Overview Introduction & Background History & Market Methodology Devices & Structures Processes & Foundries Micromachining: lithography, deposition, etching,
Grating Light Valve MEMS display array fabricated in CMOS compatible process. Each pixel is made up of multiple ribbon-like structures, which can be moved up or down over a very small distance (only a fraction of the wavelength of light) by controlling electrostatic forces. The ribbons are arranged such that each pixel is capable of either reflecting or diffracting light. Invented by D. Bloom (Stanford) Developed since 1994 by Silicon Light Machines, Sunnyvale CA, www.siliconlight.com Acquired by Cypress Semiconductors 7/000 Exclusive license with Sony Grating Light Valve
Grating Light Valve Ribbons at same height: reflection Ribbons at height difference d: diffraction; no reflection when d=λ/4 (maximum diffraction) Intensity of 1st order diffraction lobes: I = I sin π d λ I 1 max max ( ) maximum1st order diffracted intensity Diffraction d Reflection d [D.T. Amm and R.W. Corrigan 1998] Movable Ribbons Grating Light Valve
Grating Light Valve Grating Light Valve Design features: Usually 6 ribbons per pixel: 150µm x 00µm Very high switching speeds Switching: thresholds, hysteresis CMOS fabrication compatible (Cypress Semiconductor) Allows a variety of system architectures, including very simple designs: D pixel array (passive array) 1D scan line Analog display (note: non-linear transfer function, snap-in point)
Grating Light Valve Grating Light Valve Different GLV Arrangements
Corner Cube Reflector (CCR) Corner cube retroreflector reflects light directly back down its incident path. Common in bicycle and road reflectors, and on the moon. [Comtois and Bright 1996] [Chu, Lo, Berg and Pister 1997] 1 kbps over 100 meters Parallel Beams CCR Design Specifications: 500 m distance 56 kbps Issues: Switching speed Accuracy Diffraction Size Parallel Beams
CCR Design Hinged Plate Membrane silicon nitrate poly0 poly1 poly Schematic Design: Reflective material: Au hinged plates (locked in vertical position) 1 membrane (horizontal, deformable) Use MCNC MUMPs (3 layer polysilicon process) CCR Design Determine minimum size of plates and membranes: Want ring diameter of 1st diffractive minimum < 1 m λ / d = sin θ tan θ = 0.5/500 = 10-3 d = λ 10 3 = 10-6 m = µm (KrCl excimer laser) Detector (Ø 1m) Laser CCR Distance 500 m
CCR Design Accuracy of mirror plates: Center of reflected beam should lie within 1 m radius θ tan θ = 0.5/500 = 10-3 = 1 mrad 0.057 Detector (Ø 1m) Laser CCR Distance 500 m CCR Design Mirror deflection: Gap, g Membrane Thickness, d Gap: oxide1 ( µm) + oxide (0.75 µm) =.75 µm Radius of curvature: r = ( r g) r = g + d + ( d ) 8g = 4mm silicon nitrate poly0 poly1 poly
CCR Design Membrane (bridge) Deflection: ρ y( x) = y( d E I = wt ) = ρd 3 y = ρ( 10 y 6 10 x /(4EI)( d x) Young's modulus, 4 7 m N 384EI 6 ρ m) 4 ρ force density (pressure) 1 bending moment of inertia (384 169 10 9 Pa 10 6 m (1.5 10 6 m) 3 1) Required force (conservative estimate): 7 m F = ρ d = y / 6 10 d N for y = g = µm weget F = 0. 74mN CCR Design Electrostatic actuation Capacitance: 1 ε C = F = 1 d g = 8.85 10 F m( 10 8 CV g =.88 10 V F m 6 m).75 10 6 m =.159pF Required voltage: V 0.74 10 N.88 10 3 = 8 F m = 160V This voltage is too high! solutions: More accurate analysis of required force (non-linear, pull-in point, ) Design modifications
Switching speed Estimate spring constant: K = F / y = 0.74mN µm = 370 Calculate mass: m = ρ Si V = ρ d Resonance frequency: Si CCR Design N m 3 t =.33 10 1.5 10 18 kg = 0.17µg ω = πf = K m f = ω / π = 1 π K m 33kHz 56 kbps is feasible! CCR Design Power Estimate work for switching membrane: E = g 0 Fds = g 0 Ksds = 1 Kg = 1 370 (.75 10 6 ) Nm 1.4nJ Power: P = Ef = 1.4nJ 56kHz = 78µW Compare with energy stored in capacitor 1 1 9 E = CV = 0159. 10 186 J. 75µJ P =.75µJ 56kHz = 154mW
CCR Design Other assumptions: Very smooth mirror surfaces No obstacles, no fog Is deflected membrane really curved, or mostly flat? Other problems: If we reduce the required voltage by reducing the membrane stiffness, then we also reduce the resonance frequency and the bit rate What else? MEMS Light Modulators Modulator Type Motion Side View Cantilever Bending Torsional Plate Rotation Membrane Drumhead Suspended Plate Vertical [Kovacs, 1998, p.46]
Optical Switches MEMS Optical Switches: modulation of light w/o conversion to/from electrical signal All optical network switch: 56 input and output channels Switch time ca. 10 ms 16 x 16 micromirror array MUMPs like polysilicon process Electrostatic DOF actuation with position feedback MicroStar Technology (Lucent Bell Labs 1999) Micromirror on Crystal Planes Beam Splitter MOEMS reflectors and beam splitters [Rosengren et al., 1994] V-groove for fiber positioning
Tunable IR Filter Parallel plate array polarizes light of wavelength λ > d Plates d F Two orthogonal arrays act as filter with cutoff frequency λ = d Flexures Pull on structure to increase cutoff wavelength [Ohnstein et al., 1995 and 1996] Interferometry Basic idea: two plates at distance d = n λ with n small (usually 1) Constructive interference for all wavelengths λ = d/n λ for visible light (or IR, UV) is within range of many micromachined thin film thicknesses d Refractive indices can be varied from 1.38 (MgF ) to.4 (TiO ) Semi-transparent Mirrors
Field Emission Display Strong electrostatic field pulls electrons off the sharp tips and accelerate them towards display [See for example, W. Hofmann, L.-Y. Chen, J. H. Das and N. C. MacDonald, 1996]