Free-Space MEMS Tunable Optical Filter in (110) Silicon Ariel Lipson & Eric M. Yeatman Optical & Semiconductor Group Imperial College London Optical & Semiconductor Devices - E.E.E. Dept.

Outline Device - Optical Filter Optical analysis Fabrication Schematic Fabricated Device - Optical Filter Optical analysis Fabrication

Device Application  Optical Communication Network Lasers Fibre Broadcast System  Lasers Fibre 400 channels Tunable receiver Spectrometer Sensor

Device Configuration 1D Photonic band gap Two alternating materials Dimensions are in the order of ¼ of a wavelength (or odd multiples) Multiple reflections and phase matching create a wavelength selective mirror Two mirrors form a Fabry-Perot filter

Device Configuration 150mm Lensed Fiber Lensed Fiber Air 1D PBG structure 3 silicon bar mirrors and a cavity – Fabry Perot Dimensions ~2mm thick x 25mm deep Collimating Lensed fibers 9mm radius Gaussian beam Alignment grooves & springs created with an additional etch step Tuning by varying the centre cavity width Light beam Oxide Silicon 200mm Alignment Springs Filter

Optical Analysis The ideal case Transfer matrix formulation Mode overlap integral with the exit fiber Gaussian beam input Plane wave expansion of Gaussian beam using spatial Fourier transform f mair¼lc mgap½lc msi¼lc r1,t1(l,f) nH nL

Optical Analysis The ideal case Collimation decreases losses Layer thickness effects losses and pass band Optical communication systems need narrow pass bands ~100GHz

Optical Analysis The non-ideal case Due to imperfect deep etching, angles are introduced Fabry-Perot cavity analysis for a wedged cavity with a PBG mirror on either side mair¼lc mgap½lc msi¼lc r1,t1(l,f) r2,t2(l,f) J f L01 Lp1 Lp2

Optical Analysis The non-ideal case Simulation for an 9mm Gaussian beam with different etching angles Ripples appear at high frequencies Above >0.001 deg we get excess loss and pass band widening vertical etching! Optics Letters 1st Feb 2006

Optical Analysis The non-ideal case From “Principles of Optics” by Born and Wolf Fizeau fringes…

Fabrication DRIE followed by short KOH etching in (110) silicon: Vertical etching Smooth surfaces If kept short, does not effect too much other orientations Deep Reactive Ion Etching (DRIE): Suitable for all silicon orientations Scalloping and 1-2° deg sidewall angle. KOH etching in (110) silicon (Kendall 1979): Vertical etching, but limited by etch ratio between (111):(110) Smooth surfaces Not suitable for non (111) planes Depth limited by width and length

Fabrication DRIE KOH wet etching DRIE + KOH

Fabrication Process Flow 1. Dry Oxidation of a BSOI 5. DRIE – device layer 2. Back Etching 6. DRIE – handle layer Photo resist Oxide Silicon Gold 3. Device pattern 7. DRIE + KOH – device layer Fiber 4. Handle pattern 8. Release oxide

Fabrication Results – Static Filter 0.65nm pass band, -10.5dB loss and a 200Ghz channel spacing MEMS tuning mechanisms <0.01° deg verticality

Fabrication Results – Tunable Filter Back etching release works well. Moving cantilevers 10mm

Fabrication Results – Tunable Filter Undercuts Stiction because of wet processing KOH smoothing is far from ideal on BSOI. Under cut because of notching problem in DRIE. Additional loss due to lateral angles while tuning. Unsmooth surfaces Notching effect in DRIE

Summary 1D PBG Static filter was fabricated for DWDM networks with -10.5 dB fiber to fiber losses, 200 GHz channel spacing. Tunable filter but wide pass band. Combined DRIE + KOH wet etching on (110) silicon wafers for very vertical (<0.01°) and smooth surfaces.

Acknowledgments We would like to thank the UPC, Lambdax and Coventor for their support. Thanks to Dr. John Stagg, Dr. Munir Ahmad and Michael Larsson for their kind help. Further information: alipson@ic.ac.uk