We describe a scanning heterodyneinterferometer for imaging surface vibrations with a wide frequency range, with current electronics, up to 6GHz. The heterodyne operation facilitates measurement of absolute amplitude and phase of the surface vibration without calibration. Currently, the setup allows detection of vibration amplitudes down to ∼1pm with a lateral resolution of <1μm. The interferometer is designed to accommodate the different sample types, e.g., surface and bulk acoustic wave devices and micromechanical resonators. The absolute-amplitude and phase information allows for a thorough characterization of surface vibrations in such components and provides direct information of the vibration fields not obtainable via electrical measurements.inPeer reviewe

Kimmo Kokkonen

Matti Kaivola

Rooth S.

Yatsuda H.

Kokkonen, Kimmo

Kaivola, Matti

Aaltodoc Publication Archive

This is an electronic reprint of the original article.This reprint may differ from the original in pagination and typographic detail.Author(s): Kokkonen, Kimmo & Kaivola, MattiTitle: Scanning heterodyne laser interferometer for phase-sensitiveabsolute-amplitude measurements of surface vibrationsYear: 2008Version: Final published versionPlease cite the original version:Kokkonen, Kimmo & Kaivola, Matti. 2008. Scanning heterodyne laser interferometer forphase-sensitive absolute-amplitude measurements of surface vibrations. Applied PhysicsLetters. Volume 92, Issue 6. 063502. ISSN 0003-6951 (printed). DOI:10.1063/1.2840183.Rights: © 2008 American Intitute of Physics (AIP). This article may be downloaded for personal use only. Any otheruse requires prior permission of the author and the American Institute of Physics.http://scitation.aip.org/content/aip/journal/aplAll material supplied via Aaltodoc is protected by copyright and other intellectual property rights, andduplication or sale of all or part of any of the repository collections is not permitted, except that material maybe duplicated by you for your research use or educational purposes in electronic or print form. You mustobtain permission for any other use. Electronic or print copies may not be offered, whether for sale orotherwise to anyone who is not an authorised user.Powered by TCPDF (www.tcpdf.org)Scanning heterodyne laser interferometer for phase-sensitive absolute-amplitudemeasurements of surface vibrationsKimmo Kokkonen and Matti Kaivola  Citation: Applied Physics Letters 92, 063502 (2008); doi: 10.1063/1.2840183 View online: http://dx.doi.org/10.1063/1.2840183 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/92/6?ver=pdfcov Published by the AIP Publishing  Articles you may be interested in Phase sensitive absolute amplitude detection of surface vibrations using homodyne interferometry without activestabilization J. Appl. Phys. 108, 114510 (2010); 10.1063/1.3504636  Damping of surface acoustic vibration induced by electrons trapped on SnO 2 nanocrystal surface Appl. Phys. Lett. 95, 211903 (2009); 10.1063/1.3266870  Optical generation of long-lived surface vibrations in a periodic microstructure J. Appl. Phys. 105, 123530 (2009); 10.1063/1.3153956  Novel phase measurement technique of the heterodyne laser interferometer Rev. Sci. Instrum. 76, 093105 (2005); 10.1063/1.2038527  A multiplexed two-wave mixing interferometer for laser ultrasonic measurements of material anisotropy AIP Conf. Proc. 615, 984 (2002); 10.1063/1.1472903    This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:130.233.216.240 On: Tue, 19 May 2015 09:49:21Scanning heterodyne laser interferometer for phase-sensitiveabsolute-amplitude measurements of surface vibrationsKimmo Kokkonena and Matti KaivolaOptics and Molecular Materials Laboratory, Helsinki University of Technology, P.O. Box 3500,FI-02015 TKK, FinlandReceived 5 December 2007; accepted 15 January 2008; published online 12 February 2008We describe a scanning heterodyne interferometer for imaging surface vibrations with a widefrequency range, with current electronics, up to 6 GHz. The heterodyne operation facilitatesmeasurement of absolute amplitude and phase of the surface vibration without calibration.Currently, the setup allows detection of vibration amplitudes down to 1 pm with a lateralresolution of 1 m. The interferometer is designed to accommodate the different sample types,e.g., surface and bulk acoustic wave devices and micromechanical resonators. Theabsolute-amplitude and phase information allows for a thorough characterization of surfacevibrations in such components and provides direct information of the vibration fields not obtainablevia electrical measurements. © 2008 American Institute of Physics. DOI: 10.1063/1.2840183Recent advances in wireless communication systemshave led to extensive research and development of high-frequency, low-loss filters based on surface acoustic waveSAW and bulk acoustic wave BAW technologies. Fur-thermore, microelectromechanical system MEMS resona-tors are under intense research, e.g., due to the potential theyoffer as replacements for conventional reference oscillators,with improved performance and additional benefits of inte-gration into silicon circuits. To develop better componentsthat have their operation based on mechanical vibrations, it isimportant to be able to directly study the actual vibrationfields. Consequently, optical setups based on various tech-niques1–8 have been developed in order to facilitate noncon-tact vibration measurements that do not disturb the deviceoperation.Ideally, an optical probe should provide both amplitudeand phase information of the studied surface vibration. Itshould be able to scan both large and small areas with highmeasurement speed and with a high lateral resolution. Fur-thermore, the probe should be applicable to measuring mod-ern high-frequency devices. In this letter, we describe thedesign and operation of a heterodyne interferometer that al-lows the mapping and analyzing of vibration fields havingvibration amplitudes as small as 1 pm, with a lateral reso-lution better than 1 m. The heterodyne concept enables ac-quisition of both absolute amplitude of the surface vibrationand its phase, irrespective of variations in the local opticalsurface reflectance.The ability to map the amplitude and phase data of agiven device opens up a wealth of possibilities for furtherdata analysis. The absolute-amplitude measurements enable,e.g., exact quantitative analysis of SAW attenuation along thepropagation direction. The phase information combined withthe amplitude data allow powerful data analysis when two-dimensional Fourier methods are applied. For instance, thecontributions of standing and propagating waves within adevice can be separated, the acoustic reflections analyzed,and dispersion curves extracted. Furthermore, in the case of,e.g., BAW devices, the ability to measure dispersion proper-ties facilitates extraction of the material parameters of thedeposited thin films. In addition, by stepping the phase, thedynamics of the vibration fields can be visualized in order tobring further insight into the device operation.Consider the interference of two laser beams with inten-sities I1 and I2 originating from the same laser source. If oneof the beams is shifted in frequency from the original opticalfrequency f to f + fmfm f, the interference signal averagedover the optical cycle isIt = I1 + I2 + 2I1I2 cos2fmt + t , 1where t is the optical phase difference between thebeams. When one of the beams is reflected back from asinusoidally vibrating sample surface, the phase differencecan be written ast = 0 −4A cos2fvibt +  , 2where A is the amplitude of the normal component of thesurface vibration,  is the wavelength of the laser light, andfvib and  are the frequency and phase of the vibration, re-spectively. The term 0 is a slowly varying compared tofvib phase term that represents any arbitrary optical phasevariations between the two arms of the interferometer due to,e.g., variations in the ambient conditions. When the surface-vibration amplitude A is small compared to , the interfer-ence term of Eq. 1 may be expanded as9I12t = 2I1I2cos2fmt + 0+2Acos2fm + fvibt +  + 0− cos2fm − fvibt −  + 0	 . 3The error in calculated amplitudes due to this approximationis less than 1%, when A10 nm and =632.8 nm.When observed in the frequency domain, the signalconsists of the modulation peak fm and two satellitepeaks fm	 fvib. When measuring actual samples, only theaElectronic mail: kimmo.kokkonen@tkk.fi.APPLIED PHYSICS LETTERS 92, 063502 20080003-6951/2008/926/063502/3/$23.00 © 2008 American Institute of Physics92, 063502-1 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:130.233.216.240 On: Tue, 19 May 2015 09:49:21modulation peak and the upper fm+ fvib satellite peakhereafter called the signal peak are detected see Fig. 1.The modulation and signal peaks two frequencies aredetected simultaneously, and the absolute amplitude ofthe surface vibration can be acquired from their amplituderatio. Also, by comparing the phases of the two peaks, thephase of the surface vibration is acquired, and any slowvariations in the optical path lengths cancel out. The abilityto measure the absolute amplitude of the surface vibrationprovides immunity to, e.g., variations in the local opticalsurface reflectivity of the sample. Furthermore, the hetero-dyne detection diminishes radio frequency rf leakage prob-lems as there is a frequency offset between the detected fre-quency fm+ fvib and the frequency at which the sample isdriven fvib.The optical layout of the heterodyne interferometer, de-picted in Fig. 2, is that of a modified Mach-Zehnder interfer-ometer. A linearly polarized and collimated beam from asingle-mode HeNe laser Spectra-Physics model 117A issplit into two in an acousto-optical modulator AOM In-traAction model AOM-405A1. The frequency-shifted beamI in Fig. 2 acts as a reference beam while the zeroth-orderbeam 0 is the probe beam that propagates to the sample andback. The probe beam is focused with a microscope objec-tive Nikon 50 /0.55 ELWD to an 820 nm sized spotfull width at half maximum on the sample surface. The  /4plate rotates the polarization plane of the probe beam to fa-cilitate correct beam steering at the polarizing beam splitterPBS, where the two, now orthogonally polarized, beamsare combined. The beams then propagate through a linearpolarizer to a fast 0–12 GHz photodetector PD, featuringa flat frequency response better than 	1 dB up to 6 GHzNew Focus model 1554-A.The heterodyne laser interferometer features a sampleholder with six degrees of freedom. This allows thesample to be correctly oriented with respect to the probebeam tilt and rotation and translation of the sample alongthree axes. The scanning of the sample is accomplishedby computer-controlled linear motor stages NewportM-MFN25cc providing a 25 mm range of motion alongeach axis, with the smallest incremental step size of 55 nm.The interferometer setup is computer controlled and usespoint-to-point scanning with a frequency sweep at eachspatial point. Using frequency sweep enables fast scan-ning, since a single areal scan serves to measure multiplefrequencies.With the current setup and a detection bandwidthof 30 Hz, the smallest detectable surface vibration is 1 pm,as measured with an actual sample. This corresponds toa measurement sensitivity of 210−4 nm /Hz. Thetheoretical detection limit for a heterodyne interferometerperfect interference, shot noise limited detection is givenby10,11Amin =2eBP0, 4where e is the elementary charge, B the detection bandwidth,P0 the total optical power,  the responsivity A/W of thephotodetector, and  the wavelength of the laser. Using thevalues of 0.5 mW for the optical power, 0.2 A /W for theresponsivity, and 632.8 nm for the laser wavelength resultsin a theoretical detection limit of 110−5 nm /Hz, whichwith 30 Hz detection bandwidth corresponds to Amin0.05 pm. The difference between the obtained and theoret-ical sensitivity is mainly due to the limited optical poweravailable at the photodetector, which results in reduction ofthe available dynamic range. The measurement speed of thesystem depends on the chosen detection bandwidth. Usingthe 30 Hz detection bandwidth results in scanning speed of94 000 points /h, which can be increased by choosing alarger detection bandwidth.As an example of measurement data, we present selectedscanning results from a solidly mounted BAW resonator op-erating at 932 MHz. The sample geometry and structurealong with electrical characteristics are presented in Fig. 3.The sample is fabricated on a glass substrate, which allowsfor focusing the laser beam through the glass to the bottomof the acoustic mirror stack. The absolute-amplitude detec-tion enables direct comparison of the vibration fields mea-sured, under different optical conditions, from both sides ofthe sample. Therefore, using absolute-amplitude and phaseinformation, the performance of the acoustical mirror stackcan be studied in detail.12 The measurement is demanding,since the highest vibration amplitudes on top of the resonatorFIG. 1. Typical frequency spectrum of the photodetector signal consisting ofa modulation peak at fm and of a signal peak at fm+ fvib.FIG. 2. Optical setup of the heterodyne interferometer: PBS—polarizingbeamsplitter, M—mirror, AOM—acoustooptic modulator, and PD—highspeed photodetector.063502-2 K. Kokkonen and M. Kaivola Appl. Phys. Lett. 92, 063502 2008 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:130.233.216.240 On: Tue, 19 May 2015 09:49:21are on the order of 300 pm, and the acoustical mirror pro-vides an isolation of 35 dB, resulting in maximum ampli-tudes of the order of 5 pm on the bottom of the mirror stack.Examples of measured vibration and phase fields on top ofthe resonator are presented in Figs. 4a and 4b. The corre-sponding vibration fields at the bottom of the mirror stackare presented in Fig. 4c along with Fig. 4d correspondingFourier-filtered amplitude images.This letter reports a scanning heterodyne laser interfer-ometer for studying high-frequency surface-vibration fieldson, e.g., SAW, BAW, and MEMS components. The interfer-ometer setup enables accurate characterization, in terms ofboth absolute amplitude and phase, of the surface vibrationand, hence, is a versatile instrument for research on the phys-ics of the vibrations and for applied research of electroacous-tic devices. So far, the setup has been used to characterizeSAW, BAW, and MEMS devices, with operating frequenciesranging from 3 MHz to over 2.5 GHz. For recent results onSAW phononic crystals, see Ref. 13. The interferometer pro-vides absolute amplitude, high sensitivity, phase, spatialscanning, high lateral resolution, and high speed. Further-more, the inertness to rf leakage is essential when measuringmodern high-frequency devices.K. Kokkonen thanks Finnish Cultural Foundation andNokia Foundation for scholarships.1H. E. Engan, IEEE Trans. Sonics Ultrason. SU-25, 372 1978.2J. V. Knuuttila, P. T. Tikka, and M. M. Salomaa, Opt. Lett. 25, 613 2000.3J. E. Graebner, B. P. Barber, P. L. Gammel, D. S. Greywall, and S. Gopani,Appl. Phys. Lett. 78, 159 2001.4S. Rooth, S. Bardal, T. Viken, O. Johansen, and E. Halvorsen, Proc.-IEEEUltrason. Symp. 1, 201 2001.5H. Yatsuda, S. Kamiseki, and T. Chiba, Proc.-IEEE Ultrason. Symp. 1, 132001.6G. G. Fattinger and P. T. Tikka, Appl. Phys. Lett. 79, 290 2001.7K. L. Telschow, V. A. Deason, D. L. Cottle, and J. D. Larson, IEEE Trans.Ultrason. Ferroelectr. Freq. Control 50, 1279 2003.8H. Martinussen, A. Aksnes, and H. E. Engan, Opt. Express 15, 113702007.9J. P. Monchalin, Rev. Sci. Instrum. 56, 543 1985.10R. L. Whitman and A. Korpel, Appl. Opt. 8, 1567 1969.11J. W. Wagner and J. B. Spicer, J. Opt. Soc. Am. B 4, 1316 1987.12K. Kokkonen and T. Pensala, Proc.-IEEE Ultrason. Symp., 460 2006.13K. Kokkonen, S. Benchabane, A. Khelif, V. Laude, and M. Kaivola, Appl.Phys. Lett. 91, 083517 2007.FIG. 3. Color online a Photograph of a BAW resonator, b example ofmeasured amplitude distribution at 938 MHz, c schematic representationof the layer stack, and d measured impedance response.FIG. 4. Color a Amplitude and b phase images of the vibration fieldson top of the resonator, and c amplitude at the bottom of the acousticmirror stack. d Fourier-filtered amplitude images of c with suppressednoise reveal similar vibration fields as on top of the resonator. All the scanscover the active area of the resonator 297297 m2 with a lateral stepsize of 0.99 m, resulting in 90 000 data points in each image.063502-3 K. Kokkonen and M. Kaivola Appl. Phys. Lett. 92, 063502 2008 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:130.233.216.240 On: Tue, 19 May 2015 09:49:21

Scanning heterodyne laser interferometer for phase-sensitive absolute-amplitude measurements of surface vibrations

Crossref

English

https://aaltodoc.aalto.fi/bitstream/handle/123456789/16110/A1_kokkonen_kimmo_2008.pdf?sequence=1&isAllowed=y

Scanning heterodyne laser interferometer for phase-sensitive absolute-amplitude measurements of surface vibrations

Abstract

Similar works

Full text

Available Versions

Aaltodoc Publication Archive

Crossref