1. 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.

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

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

4. 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

5. 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

6. 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

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

8. 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

9. 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

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

11. 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

12. Fabrication
DRIE
KOH wet etching
DRIE +
KOH

13. 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

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

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

16. 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

17. Summary
1D PBG Static filter was fabricated for DWDM networks with 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.

18. 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: