Monolithically integrated cantilevers with self-aligned tips for wavelength tuning in a photonic

crystal cavity-based channel-drop filter

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2011 J. Micromech. Microeng. 21 074004

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J. Micromech. Microeng. 21 (2011) 074004 (6pp) doi:10.1088/0960-1317/21/7/074004

Monolithically integrated cantilevers withself-aligned tips for wavelength tuning ina photonic crystal cavity-basedchannel-drop filterS M C Abdulla1, L J Kauppinen2, M Dijkstra2, M J de Boer1,E Berenschot1, R M de Ridder2 and G J M Krijnen1

1 Transducers Science and Technology Group, MESA+ Research Institute, University of Twente,PO Box 217, 7500 AE Enschede, The Netherlands2 Integrated Optical Microsystems Group, MESA+ Research Institute, University of Twente,PO Box 217, 7500 AE Enschede, The Netherlands

E-mail: shahinamumthaz@gmail.com

Received 24 December 2010, in final form 18 April 2011Published 22 June 2011Online at stacks.iop.org/JMM/21/074004

AbstractA technology to monolithically integrate micro-bimorph cantilevers equipped with tips that areself-aligned with respect to the holes of a 2D photonic crystal cavity-based channel-drop filteris presented. On electrostatic actuation, the tips move into the holes and provideelectromechano-optical modulation of light. The technology allows the fabrication of tips onspecific photonic crystal holes by controlling the hole diameter and the sacrificial layerthickness. The integrated device is both mechanically and optically characterized. A 180 pmwavelength shift at the first band edge of the photonic crystal cavity-based channel-drop filteris measured on the application of a 2 V dc voltage to the cantilever. This CMOS-compatibledevice is designed to operate in the C-band of the telecommunication wavelengths andconstitutes a promising candidate for future integrated all-optical devices.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

Photonic crystals (PhC) [1] are unique engineered structureswhich manipulate light passing through them by a ‘photonicband gap effect’. They consist of materials having a periodicvariation in the refractive index. In thin dielectric slabs with astrong two-dimensional periodic perturbation of the refractiveindex, known as photonic crystal slabs, light is confined tothe plane of the slab by total internal reflection and in-planeby the photonic band gap effect. Two-dimensional PhC slabwaveguides have attained ample attention and are proposedfor a plethora of applications such as lasers [2], modulators [3]and sensors [4].

Channel-drop filters constitute an essential componentfor future photonic integrated circuits. Among the variousproposed designs, a photonic crystal cavity-based channel-

drop filter (PhC-CDF) [5] is attractive owing to its compactsize. However to realize an active device out of it, it isimportant to be able to tune the dispersion properties of aPhC-CDF. Tuning in PhCs is realized by various methods suchas thermo-optically [6, 7], electro-optically [8], with liquidcrystal infiltration [9], by applying mechanical stress [10], etc.Apart from this, evanescent field perturbation using externalmechanical probing has been established as an effective wayto perturb the localized field inside a cavity. This has beendemonstrated using tips of an atomic force microscope (AFM)[11–13], fibre/glass [14, 15] and by a scanning near-fieldoptical microscope (SNOM) [16, 17]. However, due to thelack of monolithic integration and the difficulty in aligningmore than a single or a few tips in one device, this methodbecomes impractical.

0960-1317/11/074004+06$33.00 1 © 2011 IOP Publishing Ltd Printed in the UK & the USA

J. Micromech. Microeng. 21 (2011) 074004 S M C Abdulla et al

Figure 1. Schematic illustrating the top view of the micro-cantileverintegrated PhC-CDF.

Compared to the aforementioned methods, anelectrostatically actuated integrated micro-cantilever hasdistinct advantages such as higher switching speed,compactness of the device and near-zero energy dissipationin the stationary state. Recently, an integrated electrostaticallyactuated comb drive was used to mechanically tune a 1DPhC nano-cavity using the coupled cavity effect [18, 19]where the tuning is accomplished by varying the gap betweentwo nano-cavities. In this contribution, we demonstrate themonolithic integration of a micro-bimorph cantilever equippedwith tips that are self-aligned with respect to the holes ofa silicon on insulator (SOI)-based, 2D air-bridge-type, PhCcavity-based channel-drop filter. The electrostatically actuatedmicro-cantilever provides electromechano-optical modulationof light by inserting the tips into the PhC holes. We reportthe results of the mechanical characterization of the integrateddevice and demonstrate optical tuning.

The organization of this paper is as follows. Thefollowing section briefly describes the fabrication technologyfor integrating the bimorph cantilever with the PhC-CDF.Section 3 presents the mechano-optical characterization ofthe integrated device and section 4 provides a summary andconclusion of the paper.

2. Fabrication

2.1. Optical device fabrication

PhC-CDFs and their access waveguides are fabricated onSOITEC SOI wafers by the silicon photonics platform ePIXfab

(A)

(B)

Figure 2. SEM images showing the top views of (A) a PhC-CDF with its access waveguides and (B) a zoom-in of its cavity area.

(A)

(B)

Figure 3. Schematic showing (A) cross-sectional view through theslab holes in the X-direction and (B) cross-sectional view throughthe Y-direction.

[20], established at IMEC, Leuven. The PhC-CDF isdesigned to operate in the C-band (1530–1565 nm) of thetelecommunication wavelengths. Figure 1 shows a schematictop view of the micro-cantilever integrated PhC-CDF. Thethickness of the silicon device layer is 220 nm and that ofthe handle wafer is 700 μm. Before integration, the lowercladding is a 2 μm thick thermal oxide (refractive index n =1.45 measured at 633 nm) and the upper cladding is air (n =1.0). The final integrated device has lower and upper claddingsas air.

The PhC-CDF consists of two symmetrically arrangedmodified W1-type waveguides which are formed by a linedefect caused by removing a single row in the GK-direction[21]. These W1-type waveguides are separated by seven rowsof holes and a cavity is formed in between them by omitting arow of nine holes. The periodicity of the structure is 440 nm.The diameter of W1 waveguide boundary holes (WBH) is350 nm and that of the cavity boundary holes (CBH) is 200 nm.The rest of the PhC slab holes (SH) have a 270 nm diameter.Figure 2(A) shows a SEM image of the PhC-CDF with itsaccess waveguides whereas figure 2(B) shows a zoom-in ofthe cavity area indicating different hole diameters. The accesswaveguides of the PhC-CDF are designed to be single modefor TE-polarized light at a wavelength of 1550 nm. Figure 3shows the schematic cross-sectional view of the integrateddevice in the X- and Y-directions.

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J. Micromech. Microeng. 21 (2011) 074004 S M C Abdulla et al

2.2. Bimorph integration

Figure 4 shows the fabrication flow for the integration ofthe bimorph cantilevers with a PhC-CDF. (A) PhC-CDFs arefabricated [22] on a 200 mm SOI wafer by a 193 nm deep-UV lithography process at ePIXfab [20]. This wafer is firstdiced into four pieces to fit into the 100 mm wafer systems ofthe MESA+ clean room. (B) On top of these wafers, first a43 nm thick, low-pressure chemical vapour deposition(LPCVD) [23], silicon-rich-nitride (SiNx) layer is deposited.This layer acts as a protective layer (PL) for the accesswaveguides during the etching of a sacrificial layer (SL)in a later step. To have later access to the bottom siliconelectrode, the PL is removed from the backside of the waferby reactive ion etching (RIE). In order to fabricate tips, the PLhas to be removed from the PhC area of the wafer. For thisreason, (C) a 93 nm thick tetra-ethyl-ortho-silicate (TEOS)layer is deposited as an etch mask layer. Next (D) a darkmask (OiR 907–17, Fujifilm, thickness of resist = 1.7 μm) isused to define the etch windows in the PhC area. After thelithography, the wafer is first ozone treated to make the TEOSlayer hydrophilic and then chemically etched in BHF (bufferedhydrofluric acid, NH4F:HF = 7:1) to remove the TEOS fromthe etch windows. (E) After stripping the photoresist in 99%HNO3 for 20 min, the PL is removed from the etch windowby 85% H3PO4 at 180◦C. Following this, (F) the SOI wafersare first coated conformally by a 105 nm thick TEOS SL andthen by a 1.0 μm thick low stress LPCVD SiNx layer. Next a50 nm thick gold layer (Au) is sputtered as the upper electrodelayer for which a thin (8 nm) layer of chromium (Cr) is usedas an adhesive layer. The thickness of Cr is selected to be aslow as possible to reduce the stress in the electrode layer.

A second mask is used for (G) pattering the metal layersusing OiR 907–17 resist as etch mask. Subsequently, Auis etched by the gold etch solution at 30◦C (KI = 132 g,I2 = 18 g, DI = 1200 ml, in which glycerin (600 ml) isadded to reduce the excessive undercut). Followed by this, Cris wet etched in chromium etchant (MERCK 111547.2500).(H) A third mask is used for patterning the SiNx device layerof the bimorph cantilever, using a resist mask (OiR 908–35,Fujifilm, thickness of resist = 3.5 μm). Etching is performedby RIE (Elektrotech PF340 at 10◦C, 75 W, 10 mTorr, 25 sccmCHF3 and 5 sccm O2). Followed by this, SiNx is removedfrom the backside by RIE to have later access to the lowersilicon electrode. After this, the resist is first etched for1 min in an O2 plasma etcher (TEPLA 300E at 500 W,1.2 mbar, 200 sccm O2) for removing the possible fluorocarboncontamination produced by RIE etching. Subsequently, resiststripping is continued in 99% HNO3 for 20 min. Finally(I) sacrificial layer etching (SLE) is done by BHF (NH4F:HF =7:1) followed by a freeze drying release step [24]. In orderto avoid the photochemical etching of silicon in aqueous HF[25], which makes silicon porous, the SLE is carried out ina dark environment. Figure 5 shows the SEM images of theintegrated device and its access waveguides, taken at differentangles. Figure 6 shows the self-aligned tips on top of the SHand the WBH where the inset shows a zoom-in of the tip ontop of the two SHs.

(A)

(B)

(C)

(D)

(E)

(F)

(G)

(H)

(I)

Figure 4. Fabrication flow for the integrated device. Cross-sectionis taken in the X-direction on top of SHs.

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J. Micromech. Microeng. 21 (2011) 074004 S M C Abdulla et al

(A) (B)

Figure 5. SEM images of (A) top view and (B) oblique view of theintegrated device.

(A)(B)

Figure 6. SEM images showing the self-aligned tips (A) on top ofSH and WBH and (B) a zoom-in on top of the two SHs.

Timed etching of the SL defines the anchor region ofthe cantilever, which provides a convenient bond pad area of300 μm × 300 μm. The fabricated cantilever has a designedlength of 60 μm and a width of 10 μm. However, its actuallength is increased from the design length due to the undercut inbond pads caused by the SLE. The undercut, as measured withan optical microscope, is 5.5 μm. This increased effectivebimorph length results in a lower resonance frequency andpull-in voltage [26].

2.3. Self-aligned tip formation

Except for the metal, all other layers in this process aredeposited by the LPCVD method which has excellent stepcoverage. The height and width of the fabricated tips arestrongly related to the thickness of the SL (tSL) and the holediameter. Figure 7 shows the tip formation in three differentholes for an increased SL thickness.

Until tSL is 100 nm (half of the smallest PhC holediameter), all three different holes will have a self-alignedtip of height 220 nm on top of it. As soon as the thicknessincreases above 100 nm, the tip on top of the CBH will reduceits height and will convert to a sharp pin. Similarly untiltSL = 135 nm, all the SHs will have 220 nm high self-alignedtips on top and until tSL = 175 nm all the WBHs will have220 nm high tips on top. Since the yield of devices becomesincreasingly challengeable as tSL decreases, we have selectedSL to be 105 nm in this process. Hence, with a tSL = 105 nm,all the SH and WBH will have 220 nm high self-aligned tipswhereas CBH will not. Figure 8 shows the SEM images of

(A)

(B)

(C)

(D)

Figure 7. Effect of sacrificial layer thickness and hole diameter onthe tip formation.

(A) (B)

Figure 8. SEM images showing (A) self-aligned tips of differentwidths and (B) a zoom-in of two different types of tips where thePhC-CDF has been locally removed.

self-aligned tips on top of the SH and WBH, where the PhC-CDF has been removed locally from the wafer. The figureshows tips of ∼60 nm (270 − 2 × 105) on top of the SH and∼140 nm (350 − 2 × 105) on top of the WBH.

The dimensional uniformity of the self-aligned tips of acantilever mainly depends on the variation in the PhC holedimensions. For fabricating the PhC holes, an exposuredose sweep is performed and due to it the hole diameter (asmeasured) ranges between ±20 nm around the designed value[21]. Hence a maximum of 20 nm variation arises on theself-aligned tips due to the PhC hole diameter variation.

3. Mechano-optical characterization

In order to assess its mechanical performance, the integratedcantilever is characterized statically. Off-state tip deflectionwhich arises due to the stress in the top electrode layer [27]is measured by white light interferometry (WLIM) showing19 nm tip deflection for a 60 μm long cantilever. Due to thelocal etching of the protective nitride layer in the PhC area,the subsequently deposited conformal layers follow this stepwhich makes the cantilever have a stepped geometry on top ofthe PhC as shown in figure 9. The static displacement of thebimorph by applying a dc voltage of 1 V is measured as wellusing WLIM as seen in figure 10.

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J. Micromech. Microeng. 21 (2011) 074004 S M C Abdulla et al

(A)

(B)

(C )

Figure 9. (A) Optical microscopic image of the integrated device,(B) the off-state deflection measured by WLIM and (C) theinterferogram of the cantilever along its length. The shaded regionrepresents the anchor area of the cantilever.

Figure 10. Static displacement of a 60 μm long cantilever on theapplication of a dc voltage of 1 V.

For optical measurements, infrared light (500 μW) from atunable laser (Agilent 8164B and 81634B) is coupled througha polarization-maintaining fibre and an integrated gratingcoupler [21, 28] into the optical device. Output light isrecollected from an output grating coupler and is guided tothe photodetector through a single-mode fibre. The siliconhandle wafer of the device is silver-glued to a copper plateafter removing the native oxide from its backside, to have agood electrical connection. Figure 11 shows the 3D schematicof the integrated device with its electrical contact probes andvertical fibre couplers.

Figure 12 shows the measured transmission spectrum ofthe integrated device measured at the OUT 1 and OUT 2 ports(figure 1), when the cantilever is in its off-state. The first bandedge of the device starts around 1520 nm. It is observed thatthe light is coupled to the OUT 2 port near the band edge,possibly due to the presence of the cantilever. At the firstband edge, the OUT 1 port shows a −35 dB reduction intransmission whereas the OUT 2 port shows a transmissionreduction of −20 dB (in the cantilever off-state).

Figure 13 shows the static tuning of the OUT 1 firstband edge to a higher wavelength by inserting the tips intothe PhC holes. These preliminary measurements show a180 pm wavelength shift with a dc voltage as small as 2 V.

Figure 11. Schematic representation of the optical tuningmeasurement set-up with the vertical fibre couplers and electricalcontact probes.

Figure 12. Transmission spectra of the PhC-CDF in the OUT 1 andOUT 2 ports, when the cantilever is in its off-state.

Figure 13. Static tuning of the first band edge of the PhC-CDF inthe OUT 1 port by applying different voltages to the cantilever.

The measurement is performed at the band edge of the OUT 1port which extends from ∼ 1520 to 1522 nm. The slope of theband edge is steeper at a level close to −57 dB transmission,which allows for the best obtainable accuracy in measuring thewavelength shift of this edge. Since the mechanical and opticalmeasurements have to be performed on the same device,precaution has been taken not to pull in the cantilever during theinitial mechanical measurements. Hence, the voltage appliedfor mechanical measurements was limited to 1 V, which islower than the pull-in voltage of this cantilever (between 2 and3 V). However during optical measurements the voltage has

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J. Micromech. Microeng. 21 (2011) 074004 S M C Abdulla et al

been increased further to a voltage of 2 V in order to observethe maximum attainable wavelength shift.

4. Conclusion

A successful fabrication technology that enables the formationof electrostatically driven bimorph cantilevers with sharptips that are self-aligned with respect to the holes of aphotonic crystal cavity-based channel-drop filter is described.This CMOS-compatible technology allows for the fabricationof a new class of compact devices, exploiting modulationof the waveguiding properties of PhC-CDFs by changingthe proximity of the tips to (and into) the holes of thePhC. As an example, a micro-cantilever integrated PhC-CDFmodulator has been fabricated. The cantilever is mechanicallycharacterized and the first optical band edge is statically tunedto a higher wavelength by 180 pm with a dc voltage of 2 V.

Acknowledgments

The authors would like to acknowledge R Sanders and AHollink for their advice on the measurements. This projectis funded by the NanoNED programme of the Dutch Ministryof Economic Affairs.

References

[1] Yablonovitch E 1987 Inhibited spontaneous emission insolid-state physics and electronics Phys. Rev. Lett.58 2059–62

[2] Painter O J et al 1999 Room temperature photonic crystaldefect lasers at near-infrared wavelengths in InGaAsPJ. Light. Technol. 17 2082–8

[3] Teo S H G et al 2006 Induced free carrier modulation ofphotonic crystal optical intersection via localized opticalabsorption effect Appl. Phys. Lett. 89 091910

[4] Skivesen N et al 2007 Photonic-crystal waveguide biosensorOpt. Express 15 3169–76

[5] Fan S et al 1998 Channel drop filters in photonic crystals Opt.Express 3 4–11

[6] Chong H M H and DeLaRue R M 2004 Tuning of photoniccrystal waveguide microcavity by thermo-optic effect IEEEPhoton. Technol. Lett. 16 1528–30

[7] Cui Y et al 2010 Silicon-based thermo-optically tunablephotonic crystal lens IEEE Photon. Technol. Lett. 22 21–3

[8] Fushman I et al 2007 Ultrafast nonlinear optical tuning ofphotonic crystal cavities Appl. Phys. Lett. 90 091118

[9] Wang B et al 2010 Controlling mode degeneracy in a photoniccrystal nanocavity with infiltrated liquid crystal Opt. Lett.35 2603–5

[10] Cui Y et al 2010 Mechanically tunable negative-indexphotonic crystal lens IEEE Photon. J. 2 1003–12

[11] Marki I et al 2006 Tuning the resonance of a photoniccrystal microcavity with an AFM probe Opt. Express14 2969–78

[12] Takahata T et al 2006 Transmittance tuning of photonic crystalreflectors using an AFM cantilever Sensors Actuators A128 197–201

[13] Hopman W C L et al 2006 Nano-mechanical tuning andimaging of a photonic crystal micro-cavity resonance Opt.Express 14 8745–52

[14] Koenderink A F et al 2005 Controlling the resonance of aphotonic crystal microcavity by a near-field probe Phys.Rev. Lett. 95 153904

[15] Intonti F et al 2008 Spectral tuning and near-fieldimaging of photonic crystal microcavities Phys. Rev. B78 041401

[16] Mujumdar S 2010 Tunability parameters in SNOM tipmediated tuning of an ultra-high quality heterostructureresonator J. Opt. 12 035001

[17] Lalouat L et al 2007 Near-field interactions between asubwavelength tip and a small-volume photonic-crystalnanocavity Phys. Rev. B 76 041102

[18] Chew X et al 2010 Dynamic tuning of an optical resonatorthrough MEMS-driven coupled photonic crystalnanocavities Opt. Lett. 35 2517–9

[19] Chew X et al 2010 An in-plane nano-mechanics approach toachieve reversible resonance control of photonic crystalnanocavities Opt. Express 18 22232–44

[20] Dumon P et al 2009 Towards foundry approach for siliconphotonics: silicon photonics platform ePIXfab Electron.Lett. 45 581–2

[21] Kauppinen L J 2010 Compact integrated optical devices foroptical sensor and switching applications PhD ThesisUniversity of Twente

[22] Selvaraja S K et al 2009 Fabrication of photonic wire andcrystal circuits in silicon-on-insulator using 193 nm opticallithography J. Light. Technol. 27 4076–83

[23] Gardeniers J G E et al 1996 LPCVD silicon-rich silicon nitridefilms for applications in micromechanics, studied withstatistical experimental design J. Vac. Sci. Technol. A14 2879–92

[24] Legtenberg R and Tilmans H A C 1994 Electrostatically drivenvacuum-encapsulated polysilicon resonators: part I. Designand fabrication Sensors Actuators A 45 57–66

[25] Kolasinski K W 2003 The mechanism of Si etching in fluoridesolutions Phys. Chem. Chem. Phys. 5 1270–8

[26] Abdulla S M C et al 2009 Optimisation study of microcantilevers for switching of photonic band gap crystals Int.Conf. on Photonics in Switching (Pisa, Italy, 2009) pp 1–2

[27] Abdulla S M C et al Analysis of resonance frequency andpull-in voltages of curled micro-bimorph cantilevers TSTGroup, University of Twente, Enschede, The Netherlands(in preparation)

[28] Taillaert D et al 2006 Grating couplers for coupling betweenoptical fibers and nanophotonic waveguides Japan Soc.Appl. Phys. 45 6071–7

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