A micro-spectrometer has been realized based on an array of Fabry-Perot thin-film optical resonators. The 16 channel micro-spectrometer is IC fabrication compatible and
operates in the visible spectral range with an inter-channel shift of 6 nm. Each of the channels is sensitive in a single peak with FWHM of 16 nm. A FWHM < 2 nm and finesse
of 40 for narrowband operation is demonstrated.STW - Project DEL 55.3733.
TUDelft.
Fundação para a Ciência e Tecnologia - Praxis XXI-BD/5181/95

Bartek, M.

Correia, J. H.

Wolffenbuttel, R. F.

Universidade do Minho: RepositoriUM

High-selectivity single-chip spectrometer for operation at visible wavelengths

High-Selectivity Single-Chip Spectrometerfor Operation at Visible WavelengthsJ. H. Correia*,  M. Bartek and R. F. WolffenbuttelDelft University of Technology, ITS/Electrical Engineering,Lab. for Electronic Instrumentation/DIMES, 2628 CD Delft, The NetherlandsAbstractA micro-spectrometer has been realized based on an array ofFabry-Perot thin-film optical resonators. The 16 channelmicro-spectrometer is IC fabrication compatible andoperates in the visible spectral range with an inter-channelshift of  6 nm. Each of the channels is sensitive in a singlepeak  with FWHM of 16 nm. A FWHM < 2 nm and finesseof 40 for narrowband operation is demonstrated.IntroductionNumerous applications, e.g. systems for chemical analysisby optical absorption and emission line characterization,will benefit from the availability of low-cost single-chipspectrometers. Miniaturized spectrometers will offersignificant advantages over existing instruments, includingsize reduction, low cost, fast data-acquisition and highreliability. Previously developed micro-spectrometers [1-5],fabricated using bulk or surface micromachining, containmovable parts to perform wavelength tuning. As a result,these are less reliable and suitable only for operation in alimited spectral band (mostly near-IR) [5-6]. Moreover,high-voltage electrostatic actuation is necessary forresonance cavity tuning.Operation in the visible spectral range (with transmission ina single peak) requires cavity length below 500 nm.Fabrication and modulation of a such narrow airgapbetween the two mirrors is severally hindered by capillaryforces inside of the cavity. Also, electrostatic pull-in andsubsequent sticking of the two mirrors limits the operatingrange of the device to one third of the initial air gap [7].Micro-Spectrometer DesignWe have developed a new single-chip Fabry-Perot array-type micro-spectrometer suitable for the visible spectralrange which does not require electrostatic actuation. Therealized device, schematically shown in Fig. 1 (anindividual channel in cross-section) and Fig. 2 (the complete4x4 array micro-spectrometer), is fully fabricationcompatible with a standard IC processing.Fig.1: Schematic structure of the micro-spectrometer: individual channel incross-section.Fig. 2: A 4x4 array micro-spectrometer in perspective. Each of theFabry-Perot cavities is tuned to transmit in different spectral band. The impinging spectrum is filtered in the Fabry-Perot reso-nator and the intensity of the selected spectral component ismeasured in transmission using an underlying integratedphotodiode array. On top of each photodiode an Al/SiO2/Aglayer stack is deposited functioning as the Fabry-Perot opti-cal filter. The thickness of the PECVD silicon dioxide layer,which is enclosed inbetween two semi-transparent metallicmirrors, determines the wavelength tuned to. In N subse-quent plasma etching steps (using different photoresistmasks), the initially deposited PECVD oxide layer isthinned forming 2N channels, each with different resonancecavity length. An oxide layer is present between the cavityand the photodiode and introduces a wavelength-dependenttransmission of the incident radiation. Its thickness was de-signed for a flat transmittance over the visible spectra range[8-9]. Evaporated metallic mirrors were used instead ofhigh-performance dielectric mirrors, to maintain fabricationsimplicity (only one layer must be deposited).AlSiO2Silvern-epilayerp++ p++p+n++ n++Fabry-Perot cavity*On leave from University of Minho, Dept. of Industrial Electronics,Portugal, Grant #FCT-Program Praxis XXI-BD/5181/95.Another advantage of metallic mirrors is the suitability foruse over a wide spectral range [10]. Silver and aluminumhave been selected for high reflectivity at visiblewavelengths (see Fig. 3). Fabry-Perot filters using metallicmirrors cannot provide both high finesse (ratio between thefree spectral range and Full-Width-Half-Maximum,FWHM) and high transmittance simultaneously due to theoptical absorption in the metal layers (Fig. 4). Unlikemacroscopic applications of silver-based reflective coatings[11], the poor environmental resistance of silver is notcritical in a microsystem application. Sealing of a completesystem avoids any environmentally caused mirrordegradation.A thin-film optics software package was used to performoptimization of the FP filter. The transmittance for a 60 nm-Ag/1000 nm-SiO2/60 nm-Ag layer stack (Fig. 5) showsFWHM of 1.8 nm and finesse of 40. The Ag layer thicknessis a trade-off between achievable FWHM and absorptionloss. For fabrication compatibility reasons, Al layer wasused for the bottom mirror. Due to higher absorption in Al(when compared to Ag), the performance slightly decreasesas shown in Fig. 6 (a FWHM of 6 nm for a finesse of 12).Device FabricationPhotodiode array is fabricated using a standard IC proces-sing (bipolar or CMOS). Fabry-Perot etalons are added in apost-processing module. Firstly, a 20 nm Al layer is evapo-rated and patterned using lift-off. Then a PECVD oxide lay-er is deposited with thickness equal to the maximum cavitylength. Subsequently, N plasma etching steps are used toform 2N different thicknesses of the oxide layer. Thicknessaccuracy better than 2 nm was achieved. Silver layer is de-posited in the very end of the fabrication sequence and pat-terned using lift-off. Fig. 7 shows a photograph of the fabri-cated device with overall dimensions of 4.7 x 4.7 mm2.Fig. 3: Reflectance of Ag, Au and Al as a function of radiationwavelength [10].Fig. 4: Transmittance and finesse as a function of silver thickness.Fig. 5: Simulated transmittance vs. wavelength for 60 nm Ag  / 1000 nmSiO2 / 60 nm Ag layer stack.Fig. 6: Measured reflectance vs. wavelengths for a 45 nm Ag/ 1000 nmSiO2 / 20 nm Al layer stack.02040608010010 20 30 40 50 600102030405060TransmittanceFinesse FinesseTransmittance (%)Silver thickness (nm)051015202530400 450 500 550 600 650 700 750 800Wavelength (nm)Transmittance (%)0102030405060708090100400 450 500 550 600 650 700 750 800Wavelength (nm)Reflectance (%)Fig. 7: Photograph of the fabricated  micro-spectrometer with 4x4channels. The die area is 4.7x4.7 mm2.Experimental ResultsThe electrical characteristics and spectral responsivity weremeasured using an HP4142B DC Source/Monitor (full-scalerange from 10-15 A to 1 A and a resolution of 10-13 A). A100 W quartz tungsten halogen lamp with a monochromatorTRIAX-180 (1200 g/mm grating with a spectral dispersionof 3.6 nm/mm and a spectral resolution of 0.3 nm at546 nm), was used as the light source. A collimator lens wasused to image the light on the entrance slit. At the exit slit apinhole and a focusing lens were used to achieve a  beamwith a diameter of about 400 µm [12]. The measurementswere calibrated with a commercial photodiode HamamatsuS1336-5BQ.System operation is demonstrated on a 16 channel micro-spectrometer designed to be sensitive in the spectral rangebetween 400 to 500 nm with inter-channel shift of about6 nm. Each of the channels consists of a 20 nm-Al / SiO2 /45 nm-Ag layer stack, where oxide layer thickness changesbetween 220 nm and 300 nm with 5 nm steps. Opticalspectral measurements (using an external detector) show(see Fig. 8) that each of the channels is sensitive to only onenarrow spectral band with FWHM of 16 nm over the entirevisible spectral range (400 to 780 nm).Subsequently, measurements were performed on the devicewith integrated photodiodes shown in Fig. 7. A dark currentof 0.2 pA at 0 V and 1 pA at -4 V (see Table 1) wasmeasured for a photodiode active area of 1 mm2. The overalldevice performance is a trade-off between sensitivity,selectivity and the number of channels used. An increasedAg-layer thickness improves selectivity but causes higherabsorption losses and, therefore, sensitivity, is decreased.For 45nm-Ag and 20nm-Al layers used, a maximumtransmittance of about 15 % was measured.Fig. 9 presents the spectral responsivity (A/W) between400 nm to 800 nm for all 16 channels using on-chipphotodiodes. The ratio between the base line and the peakmaximum  ranges from 4 to 7. Large variations (comparingwith Fig. 8) in the height of the peaks is due to: the spectralsensitivity of the integrated photodiodes and the relativelyhigh stray light sensitivity (no optical shielding between theindividual Fabry-Perot channels was used).Fig. 8: Measured transmittance vs. wavelength for 45 nm Ag / SiO2/ 20 nmAl layer stack. The SiO2 layer thickness is used as a parameter and changesfrom 225 nm to 300 nm in 5 nm increments.Fig. 9: Spectral responsivity of the 16 channel micro-spectrometer for a45 nm Ag / SiO2/ 20 nm Al layer stack. The SiO2 layer thickness is used asa parameter and changes from 225 nm to 300 nm in 5 nm increments.0246810121416400 450 500 550 600 650 700 750 800Wavelength (nm)Transmittance (%)225 nm300 nm00.0020.0040.0060.0080.010.0120.014360 420 480 540 600 660 720 780Spectral Responsivity (A/W) 225 nm300 nmWavelength (nm) ConclusionsA single-chip spectrometer, combining Fabry-Perotresonance cavities on a distributed photodetectors array,was fabricated and characterized. The advantage of thedevice presented is that it can easily be tuned duringfabrication to cover a different spectral band, by adjustingthe etching time only, without affecting the device layout.Such a device is extremely suitable for applications inmicrosystems (e.g. µTAS), because of  its small size, highspectral selectivity and low cost. Also, micro-spectrometersfor the UV and IR are feasible using this technique.Current work aims on an entirely CMOS integratedmicrosystem that includes on-chip power management,signal conditioning circuits, A/D conversion and a digitalbus interface for external interface.AcknowledgmentsThe authors would like to thank the staff of Delft Institute ofMicroelectronics and Submicron Technology (DIMES),especially J. Groeneweg, for technical assistance infabrication of the devices. This work is supported in part bySTW (project DEL 55.3733), TUDelft and FCT-Portugal(Program Praxis XXI-BD/5181/95).References[1] J. D. Patterson and B. van Zeghbroeck,”Fabrication and analysis ofSi/SiO2 Micro-mechanical Modulators”, Digest IEEE/LEOS 1996Summer topical meeting on optical MEMS and their applications, pp.25-26, 1996.[2] T. A. Kwa, P.M.Sarro, R.F. Wolffenbuttel, “Backside-illuminatedsilicon photodiode array for an integrated spectrometer”, IEEETransactions on Electron Devices, Vol. 44, No. 5, pp.761-765, May1997.[3] N.F. Raley, D.R. Ciarlo, J.C. Koo, B. Beiriger, J. Trujillo, C. Yu, G.Lomis, R. Chow, “A Fabry-Perot Microinterferometer for VisibleWavelength”, Proc. 5th IEEE Solid State Sensor and ActuatorWorkshop., Hilton Head Island, SC, USA, 1992, pp. 170-173.[4] J. H. Correia, M. Bartek, R. F. Wolffenbuttel, “Bulk-micromachinedtunable Fabry-Perot microinterferometer for the visible spectralrange”, Proc. of Eurosensors XII, Southampton, UK, pp.287-290,September 1998.[5] A. T. T. D. Tran, Y. H. Lo, Z. H. Zhu, D. Haronian, E. Mozdy,“Surface micromachined Fabry-Perot tunable filter”, IEEE PhotonicsTechnology Letters, Vol. 8-3, pp. 393-395, 1996.[6] R. T. Carline, D. A. O. Hope, D. J. Robbins, M. B. Stanaway,“Vertical cavity longwave infrared SiGe/Si photodetector using aburied silicide mirror”, Proc. of  IEDM’97, Washington, USA, 1997. [7] S. D. Senturia, Narayan Aluru, J. White, “Simulating the Behavior ofMEMS Devices: Computacional Methods and Needs”, IEEEComputacional Science & Engineering, pp. 30-43, March 1997.[8] D.P. Poenar, R.F. Wolffenbuttel, “Optical properties of thin-filmsilicon-compatible materials, Applied Optics”, Vol. 36, No. 21, July1997, pp. 5122-5128.[9] R. F. Wolffenbuttel, “Photodiodes in silicon with an electrically-programmable UV response”, Sensors and Actuators A, A22, 1990,pp. 559-563.[10] H.A. Macleod,” Thin-film optical filters”, Adam Hilger Ltd, 2ndedition, 1986.[11] D.-Y. Song, R. W. Sprague, H. A. Macleod, M. Jacobson, “Progressin the development of a durable silver-based high-reflectance coatingfor astronomical telescopes”, Appl. Optics, Vol. 24, No. 8, 1985, pp.1164-1170.[12] G. M. Yee, N. I. Maluf, P. A. Hing, M. Albin, G. T. A. Kovacs,“Miniature spectrometers for biochemical analysis”, Sensors andActuators A, 58, pp.61-66, 1997.Table 1: Electrical and optical characteristicsFeature ResultTechnology Bipolar or CMOSDevice area 4.7 x 4.7 mm2FWHM@tSiO2=300 nm16 nmFinesse@tSiO2=300 nmSingle peakFWHM (simulated)@60 nm Ag mirrors @tSiO2=1 µm1.8 nmFinesse (simulated)@60 nm Ag mirrors @tSiO2=1 µm40Dark current@ 0 V 0.2 pADark current@ -4 V 1 pASpectral response maximum @-4V0.013 A/W@388 nmIncident angle range ±10 °

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High-selectivity single-chip spectrometer for operation at visible wavelengths

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Universidade do Minho: RepositoriUM

Universidade do Minho: RepositoriUM