DIMENSIONAL INSPECTION OF SAMPLES IN THE NANOMETER SCALE BY MEANS OF THE SUPERCONTINUUM LIGHT

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1 DIMENSIONAL INSPECTION OF SAMPLES IN THE NANOMETER SCALE BY MEANS OF THE SUPERCONTINUUM LIGHT Ondřej ČÍP, Radek ŠMÍD, Břetslav MIKEL, Martn ČÍŽEK, Bohdan RŮŽIČKA and Jose LAZAR Insttute o Scentc Instruments, Academy o Scences o the Czech Republc, Královopolská 147, Brno, Czech Republc, E-mal: ocp@sbrno.cz Abstract In a lot o cases o producton o semconductor mcrochps, nano-components lke MEMS, or optcal lthography, an even more precse length measurng devces are necessary or a relable abrcaton and qualty nspecton. Modern hgh-resoluton laser ntererometers are consdered n ths eld as these more precse measurng length sensors [1]. They use a wavelength o the laser lght lke a undamental length unt [2]. Presently, the progress n the nanometrology research s orented to emtosecond mode-locked lasers stablzed by technque o the optcal requency comb. The laser produces a supercontnuum lght, whch s composed o a cluster o coherent requency components n certan nterval o wavelengths. A value o the repetton rate o emtosecond pulses determnes spacng o these components n the requency doman. We put together a new method, whch uses a specal desgn o an optcal resonator workng lke a measurng devce. The measurng probe (one o two mrrors o the ty) montors changes o unknown dstance by pezoelectrc transducer (.e. cantlever o the atomc orce mcroscope). Then the repetton requency o the emtosecond laser s controlled by selected optcal modes o the ty. The down-converson o the optcal requency changes o certan mode o the ty nto the rado-requency doman s the man mpact o the method. There s excepton that the resoluton o the method wll be n the range o the sze o atoms. 1. INTRODUCTION Presently, the progress n the eld o optcal requency standards s orented to emtosecond mode-lock lasers stablzed by a technque o the optcal requency comb [3]. Such a laser produces a supercontnuum lght, whch s composed o a cluster o coherent requency components n certan nterval o wavelengths. A value o the repetton rate (requency) o emtosecond pulses determnes (n the requency doman) spacng o these coherent components. I we control the mode-lock laser by means o.e. atomc clocks we ensure requency o these components very stable. The comb spectrum plays a role as a requency rule whch s sutable or the metrology o length n varous branches: t can be used as a tool or generaton o precse dstance and at the same tme as a length measurng devce wth the atomc clock as the undamental reerence. 2. FREQUENCY COMB IN DIMENSIONAL METROLOGY The optcal requency comb s based on laser source perodcally generatng a tran o emtosecond pulses. The tran s characterzed by the central wavelength, perod o pulses, pulse shape and pulse to pulse phase sht. The tran produces a requency spectrum o comb lnes around the central optcal requency (wavelength). The requency o one selected spectral component o the comb s expressed as: = + (1) ceo rep

2 where s the number o a comb lne (typcally n the order o 10 6 ) and rep and ceo are requences typcally set n RF doman, called repetton and oset requency, respectvely. The repetton requency rep s ndrectly proportonal to the perod o the tran pulses and the oset requency ceo descrbes phase-sht between envelope o pulses and ther carrer wave. For ree runnng emtosecond laser both requences ( rep and ceo ) are typcally stable only up to Thereore ts better stablzaton s necessary to generatng precse and stable requency comb spectrum. On bass o Eq. 1. the stablty o the repetton requency rep aects stablty o -th component o the comb multplcatvely and the oset requency ceo addtvely. The repetton requency s very ntensvely present n RF spectrum and could be then easly retreved and phase locked to.e. a dvson o an atomc clock RF sgnal (typcally 10 MHz) by phase-lock servo-loop [4]. The oset requency s derved by sel-reerencng method called as the -to-2 technque. It s based on broadenng o 100 nm approx. wde spectrum o the emtosecond laser (wth central wavelength λ = 1541 nm) by means a photoncs crystal bre. In the bre a our-wave mxng technque produces very broad comb spectrum whch cover at least one octave o optcal requences (n our case an nterval rom 1000 nm to 2000 nm). Longer wavelengths (requency ) are requency doubled by non-lnear crystal to shorter wavelengths (requency 2 ). Doubled longer wavelengths are optcally mxed wth short wavelengths (requency 2 ) and the oset requency s obtaned: 2 = (2) 2 Ths beat-requency s also phase locked to the atomc clock RF sgnal by servo-loop. The result o the method s the comb requency spectrum wth stablty o all o requency components wth degree o relatve stablty o atomc clocks (thus more than ). ceo 3. FABRY-PEROT CAVITY AS A GENERATOR OF PRECISE LENGTH Fabry-Perot ty (FPC) conssts o two mrrors separated by the geometrcal dstance L. The FPC s an nstrument whch transers precse requences characterzed rom the optcal path to a dstance between mrrors [5]: ν = c c = 2 L opd (3) where ν s the ree spectral range (FSR), c s speed o the lght, n s the reractve ndex o the ty envronment, and L opd s the optcal path dstance between mrrors ( longtudnal mode transmtted through the FPC can be expressed: L opd = n L ). A requency o m-th m = (4) where m s optcal requency o m-th mode o the FPC. I the ty s nserted nto the vacuum chamber then the reractve ndex n equals 1.0 and L = L (geometrcal and optcal length o the ty s the same). opd Then or generaton o the precse length L opd s necessary to stablze only one o requency modes m o the ty to a stable optcal requency. For ths purposes the emtosecond laser deals such a stable requency spectrum. As we express n Chapt. 2, each -th component o the comb s perectly stable. I we hold the

3 ty length L at the rght poston the optcal requency o the -th component o the comb equals to the optcal requency o m-th mode o the FPC. Then: = m = ceo + rep = (5) L 2n ( ceo + rep) = (6) Because and m are nteger constant and c s speed o the lght then the stablty o the ty length L s determned only by stablty o the repetton and oset requency. 4. FABRY-PEROT CAVITY AS A MEASURING SENSOR FOR DIMENSIONAL INSPECTION The FPC can be used lke a dstance measurng sensor n cooperaton wth the emtosecond comb too. I we cancel the servo-loop lock o the repetton requency rep to the atomc clock RF sgnal the spectrum o the comb stays ree runnng. But we control the repetton requency or condton that the m-th requency lne o the FPC equals to -th component o the requency comb, t leads to expresson: rep c m ceo = (7) 2 nl I we montor the repetton requency by a counter whch s reerenced to RF sgnal derved rom atomc clocks then we are able to determne the L very precse on bass o Eq. 6. The optcal setup o the novel method or dmensonal nspecton s n Fg. 1. Because the character o the comb lght s broad optcal spectrum wth thousands narrow lghtwaves (supercontnuum) t s mpossble to use such a lght to drect llumnaton o the FPC. We have to rstly to separate -th component rom the emtosecond comb and to llumnate the FPC by only ths component wth respect to Eq. 5. It can be done.e. by supportng tunable laser dode (DFB) whch s requency locked to -th component by specal servo-loop. But due to poor sgnal-to-nose rato o the beat-note we proposed and vered other technque whch uses a passve lterng o only several components o the requency comb by means o an ultra narrow-band bre Bragg gratng. As s presented n Fg. 1, the output o the emtosecond laser s equpped by a bre crculator whch prevents back-relecton rom the long perod bre Bragg gratng and FPC nto the laser. The group o several components about -th lne o the comb s transmtted to the tap 3 o the crculator. Ths lght s sent to the FPC through the bre expander. The FPC s constructed wth a xed length L = 75 mm. The body o the FPC s made rom low-expanson materal (zerodur).

4 Fg. 1. Detaled scheme o the optcal setup or dmensonal nspecton by means o the requency comb technology. The poston o one o the ty mrrors (dstance measurng probe) s postoned by a pezoelectrc transducer. It s sutable or the rst harmonc detecton technque used n the servo-loop lock o the repetton requency o the emtosecond laser to the ty length o the FPC. The stablzaton o the oset requency ceo by means o -2 technque o the requency comb s remanng. 5. THE EXPERIMENTAL RESULTS The locked repetton requency rep s montored by a hgh-resoluton requency counter whch has the reerence sgnal rom atomc clocks. In ths setup the repetton requency express changes o the length o the ty, see Eq. 6. Thanks to perormed novel method we nspected the stablty o the dstance determned by the zerodur body o the FPC. Ths result was done wth the plot expermental setup presented n Fg. 1. The standard devaton o the dstance calculaton on bass o the repetton requency measurement s 0.02 nm or averagng tme τ = 1 s. The method wll be very promsng.e. or measurement o a cantlever movement at scannng probe mcroscope. ACKNOWLEDGEMENTS The authors wsh to express thanks or support to the grant projects rom Mnstry o Educaton, Youth and Sports o CR, projects No.: LC06007, 2C06012, the AS CR, projects No.: AV0 Z , Mnstry o Industry and Commerce, projects No: 2A-1TP1/127, FT-TA3/133, 2A-3TP1/113 and GA CR, projects: GA102/09/1276, GA102/07/1179.

5 REFERENCES [1] Msum, I. et al Sub-hundred nanometre ptch measurements usng an AFM wth derental laser ntererometers or desgnng usable lateral scales. Meas. Sc. Technol, 2005, 16: [2] Qunn, T.J. Practcal realzaton o the denton o the metre. Metrologa, 2001, 40: [3] Udem,T., Holzwarth, R., Zmmermann, M., et al Optcal requency-comb generaton and hghresoluton laser spectroscopy. Topcs n Appled Physcs, 2004, 95: [4] Bauch, A. Caesum atomc clocks: uncton, perormance and applcatons. Meas. Sc. Technol, 2003, 14: [5] Cp, O., Smd, R., Lazar, J. An ultra-stable generator o absolute length based on emtosecond modelock laser and optcal resonator. Proc. o the 2007 IEEE Int. Freq. Control Symposum, 2007, 1-4: