Multi-channel Laser Interferometer for Parallel Characterization of MEMS Microstructures

K. Liżewski1, A. Styk1, M. Józwik1, M. Kujawińska1, R. Paris2, and S. Beer3

1 Institute of Micromechanics and Photonics, Warsaw University of Technology, 8 Sw. A. Boboli St., Pl-02-525 Warsaw, Poland 2 Institut für Mikroelektronik- und Mechatronik-Systeme Gemeinnützige GmbH, Ehrenbergstr. 27, D - 98693 Ilmenau, Germany 3 CSEM, Technoparkstrasse 1, 8005 Zürich, Switzerland

Abstract. In the paper the new approach towards microsystems characterization at wafer level, developed under EU project “SMART InspEction system for High Speed and multifunctional testing of MEMS and MOEMS” (SMARTIEHS) is presented. The goal of the project is to provide fast, cost effective and flexible optical inspection system. Two different interferometer approaches are pursued in the project’s development process: a refractive optics based Mirau type low coherence interferometer and a diffractive optics based Twyman-Green laser interferometer. In the paper the design and the tests of the laser interferometer are presented. The multifunctional approach of the interferometer measurement concept allows inspecting MEMS shape and deformation as well as characterizing spatial distribution of vibration amplitude at a resonance frequency within one instrument.

1 Introduction

Development of micro technology and increasing number of systems utilizing M(O)EMS devices in many field imposes strict conditions on manufacturers [1]. In general during different steps of production process contact and non-contact meas-urement methods applied to investigate substrates, coatings or single M(O)EMS devices are used [2]. Such approach makes production expensive as well as time consuming. To increase the efficiency of measurement the parallel measurement scheme has been proposed. The main aim of this scheme is to measure several units at once. The realization of the proposed scheme came with SMARTIEHS system (created under EU project SMARTIEHS “SMART InspEction system for High Speed and multifunctional testing of MEMS and MOEMS”) [3]. The goal of the project is to provide fast, cost effective and flexible optical inspection system utiliz-ing two types of interferometer arrays and hence allowing inspection of several objects at the time.

This paper focuses on the laser interferometer array designed and manufactured for SMARTIEHS system as one of two interferometer types. In the system laser

760 K. Liżewski et al.

interferometer is mainly responsible for objects’ dynamic properties characteriza-tion (resonant frequency and mode shape evaluation) but it also allows for static properties measurement.

2 Inspection System Mechanical Design

The design of the SAMRTIEHS instrument platform is motivated by industrial requirements on one hand and by a modular design and flexibility on the other hand. The mechanical design of the platform is determined by the three following substructures:

• A commercial prober system is used as system platform. The prober sys-tem performs a chuck movement in x, y, z and rotation around z as well as a scope movement in x, y and z. These features are used to realize a semiautomatic test system.

• The optical system consists of interferometer wafers, light sources with the corresponding lenses, beam splitters and a camera unit. The mechanical mounting of these components has to realize the required optical path lengths including various adjustment possibilities to handle tolerances.

• A high precision z-drive is needed for focusing the measurement inter-ferometer towards the MEMS devices. Furthermore the z-drive has to realize a highly uniform movement and stable positioning of the optical system in order to perform measurement tasks.

The mechanical unit was manufactured and is assembled at SUSS probe station (Suss prober PA 200 [4]). The main aim of the unit is to assure mechanical stabil-ity and protection for optical measurement head. The optical head consists of the two interferometer types. For each interferometer special modules holding an array of interferometers and light sources as well as beamsplitters were designed. CAD model of system is presented on Figure 1.

SMARTIEHS test system

vacuum chuck

scanning unit

optical unit

Fig. 1. Cross section of SMARTIEHS system attached to the prober

Multi-channel Laser Interferometer for Parallel Characterization 761

In order to perform measurements each interferometer type requires scanning or step-like changing of the optical path difference. For this purpose the high pre-cision drive consisting of three voice coil drives is realized (see Figure 2). The selection of the voice coils is motivated by the large required travel range of 1 mm with a linearity of better than 1%.

camera module

weight compen-sation

guiding spring

voice coil

scanning platform

Fig. 2. Concept of the z-scan platform

The position signals in z-axis (scanning axis) are measured by three

commercial interferometers by SIOS and their measuring heads are fixed at the carrier frame. Based on the SIOS interferometer signal resolution of 3 nm and after an interpolation the controller deliver the required positioning accuracy of 10 nm.

Besides the z-scan the platform enables pitch and roll motions (rx- and ry- di-rection) for the parallel alignment of the interferometer array to the MEMS-wafer. To realize a straight-lined and uniform z-motion the platform is weight compensated by pull-springs and guided with star-shaped leaf springs which provide a ratio of horizontal to vertical stiffness of over 10000. The nonlinearity of the leaf springs stiffness is compensated by feed forward control. Figure 3 presents the photography of the assembled SMARTIEHS system mounted on probe station.

762 K. Liżewski et al.

Fig. 3. The picture of SMARTIEHS system

3 Laser Interferometer Unit

Compact designed of optical wafer and large scale of integration allows extending single channel to multi-channel concept. Figure 4 shows the scheme of optical wafer with array of laser interferometers. A pitch between individual channels is adapted to distance between measurement structures on MEMS wafer under test. In SMARTIEHS project pitch is defined as 13.46 mm and final optical wafer consists an array of 5x5 independent interferometer channels.

The presented interferometer configuration is an adaptation of Twyman- Green interferometer. This type of interferometer is often used to non- contact measure-ments of reflecting MOEMS devices featuring low gradient of shape and continu-ity surface. In the project classical optical elements were replaced by special de-signed and optimized diffractive optical elements etched on glass substrates. The picture of the manufactured interferometer array is presented on Figure 5.

Fig. 4. Scheme of multi-channel interferometer

Multi-channel Laser Interferometer for Parallel Characterization 763

Fig. 5. Manufactured array of laser interferometers

The beam radiated by the laser diode placed in cartridges (light module in the system, see Figure 1.) is collimated by lens (f = 4,51 mm) in order to obtain plane wavefront. Illumination is then directed down to optical wafer by the beam split-ter. Optical wafer is placed under cube beam splitter in distance 2 mm. Optical axis is deflected 90° and the beam illuminates perpendicularly first diffractive structure DOE 1. Diffractive structures (DOE) used in the system are gratings with precisely defined parameters and different profiles etched on two optical glass substrates. The substrates are bonded together by 35µm thick elastomer layer deposited on the backside of the upper grating substrate. Diffraction structures geometrical parameters are shown on Figure 6.

DOE 1 grating diffracts the incident collimated beam in order to illuminate the beam splitting diffraction structure – DOE 2. A key task is to optimize power efficiency of first order grating up to 70 % of entire incident beam. Second optical wafer consists of two DOEs etched on top surface and one mirror coated on bot-tom side.

a) b) c)

Fig. 6. Designs of the diffraction grating: a) DOE 1, b) DOE 2, and c) DOE 3

DOE 2 works as a beam splitter in the interferometer unit and divides beam into reference and object beam and recombine them after reflections. Transmitted

764 K. Liżewski et al.

light of DOE 2 creates reference beam that after reflections on mirror and DOE 3 goes back to DOE 2. DOE 3 is a reference element in this configuration. Dif-fracted light of DOE 2 is directed towards object surface and is called object beam. Both beams are joined by DOE 2 and give interference pattern. Diffractive structures were manufactured by electron lithography and reactive ion etching process. A period of first two structures was designed to be 1 µm while period of the third grating structure is 500 nm. After reflections from reference and object surface both beams are imaged on camera along the same optical axis by mi-crolens placed on a top of first glass wafer. The diameter of lens is 3.2 mm while radius of curvature is 9.55 mm. Information about the investigated parameters of the object are obtained by analysis of acquired interferograms.

4 Signal Processing

In the project the specially designed Smart-Pixel Camera, consisting of the array of 5x5 imagers, is used. Every imager characterizes with the 140x140 smart-pixels that allows for the direct demodulation of the time dependent interferometric sig-nal at the detection level with demodulation frequencies (ωD) up to 100 kHz [5]. The working principle lies in direct demodulation implemented within every pixel creating two main signals: I (in-phase) and Q (quadrature-phase), see Figure 7. The interferometer employing the camera requires linear scanning (linear change of optical path difference) combined with synchronous camera demodulation frequency. The scan length in one demodulation (lock-in) period is λ/2. The opti-cal signal collected and converted into the electrical signal is integrated and sam-pled in every pixel. Each pixel has background suppression circuit to avoid satura-tion and small input signal contrast.

Fig. 7. Smart-pixel camera working principle

The signal samples are further multiplied by a signal of the same frequency and averaged on two paths, phase shifted by π/2 with respect to each other. A sample and hold stage in both circuit branches allows simultaneous demodulation and read-out, which means that while the stored values are read out of the imager pixel field, the input signal is demodulated and generate next values. These two branches create the I and Q channels of the imager.

Multi-channel Laser Interferometer for Parallel Characterization 765

This unique camera is especially useful for the vibration measurement tech-nique that was developed within described project [6]. Since the project assumes that measured vibration amplitude of the objects will not exceed 80 nm, and the measurements have to be full-field, standard measurement methods for resonant vibration characterization, such as time averaging interferometry can not be used. The proposed method id based on classical two beam interferometry, with addi-tional sinusoidal modulation of the light source. Due to the light modulation and additional small changes of the intensity caused by the measured vibration, the time dependent signal in each pixel of the camera is obtained. This signal can be further processed by the camera in the demodulation mode since it oscillates with the frequency specially matched to the camera demodulation frequency. The fre-quency match is realized when the light modulation frequency (ωL) and the object excitation frequency (ωV) differs of the value equal to the demodulation frequency

i.e. DVL ωωω −= . The interferometric signal is then described by:

−+++

++=

)cos()sin(2

)cos(

1

)(

0

00

LVDV t

A

ItI

ϕϕωδϕλ

γχπδϕγ , (1)

where I0 is the average of the interferogram, γ is the contrast of the interferogram, χ is the laser source modulation depth, ϕ0 is the phase of the interferogram (de-pends on the shape of the object), ϕL and ϕV are the initial phases of light source and excitation control signals respectively and δ is an additional phase shift that may be introduced intentionally.

The first two terms of the above equation are constant. The third term, how-ever, oscillates directly with camera lock-in frequency. The observed signal is a classical two beam interferogram with the vibration amplitude encoded in its con-trast/modulation distribution. Two Smart Pixel camera output signals (I and Q, respectively) are:

)cos()sin()2

( 00 LVV I

AI ϕϕδϕ

λπγχ −+= (2)

)sin()sin()2

( 00 LVV I

AQ ϕϕδϕ

λπγχ −+= (3)

From the above signals one may calculate expressions containing information on amplitude vibration:

( ) ( ) )sin()2

( 0022 δϕ

λπγχ +=+= I

AQIMD V . (4)

In order to find the vibration characteristic of measured objects, first resonant frequencies need to be found and then distribution of the amplitude (vibration mode shape) at resonant frequency needs to be measured.

766 K. Liżewski et al.

5 Experimental Results

In the SMARTIEHS system laser interferometer array is used to investigate the static and dynamic properties of measured MEMS devices. The static properties such as topography and deformations are measured using conventional Temporal Phase shifting technique for fringe pattern processing. For static measurements the IR sensor of the lateral size of 600 μm was used as an object. Figure 8 presents the evaluated wrapped phase (mod 2π) proportional to the topography of the investi-gated device in 10 channels measured parallel (at single measurement cycle).

Fig. 8. Evaluated mod 2π phase distribution measured parallel at 10 different channels

Fig. 9. Frequency spectrum for vibrating object measured in (1,3) channel

Multi-channel Laser Interferometer for Parallel Characterization 767

The dynamic properties of the objects were measured with proposed quasi het-erodyned method. As an object the special calibrated piezoelectric transducer with the silicon mirror glued at the top was used. The object was excited by controlled voltage signal with changeable frequency and amplitude. That approach allows for selective measurements process. An exact vibration frequency could be set and investigated. The measurements were performed for selected frequencies from the 10 – 70 kHz range. The smart-pixel camera demodulation frequency was set to 1 kHz. Figure 9 shows the results of resonant frequency search within the range of 8,5 – 11 kHz in (1,3) channel. It is clearly visible that the measured object vibrated at 10 kHz – the exact frequency of applied excitation signal.

6 Conclusions

The system concept introduced in this paper is a novel solution in industry meas-urements. It is based on a parallel inspection approach. An array of micro optical sensors integrated on a probing wafer is adapted to the M(O)EMS wafer under test and is thus able to inspect more than 100 structures within one measurement cycle on an 8 inch wafer. The developed inspection system can be improved with re-gards to the spatial resolution, the numbers of channels, the range of measured parameters and the measurement speed. The main limitation for a smaller footprint of the channels is the size of the smart pixel imager. The footprint of the micro optical interferometers is only a few mm2. The next generation smart pixel cam-eras will be smaller and have increased spatial resolution and measurement speed.

Acknowledgements

SMARTIEHS is a collaborative project funded under the Grant Agreement 223935 to the 7th Framework Program Objective 2007-3.6.

This work was co-supported by statutory founds of IMiF PW. The authors want to thank all involved people for their valuable contribution in

SMARHIES project realization.

References

[1] NEXUS MST/MEMS Market Analysis III 2005-2009 report (2009) [2] Osten, W. (ed.): Optical inspection of Microsystems. Taylor and Francis, New York

(2006) [3] SMARTIEHS - SMART InspEction system for High Speed and multifunctional testing

of MEMS and MOEMS, http://www.ict-smartiehs.eu [4] Cascade Microtech, Inc., http://www.cmicro.com/ [5] Beer, S.: Real-time photon-noise limited optical coherence tomography based on pixel-

level analog signal processing, PhD dissertation, University of Neuchâtel, Neuchâtel (2006)

[6] Styk, A., Lambelet, P., Røyset, A., et al.: SPIE, vol. 7387, p. 73870M (2010)