Measurement of Structure and Strain by Transversal Ultra-High Resolution

Polarization-Sensitive Optical Coherence Tomography

Karin WIESAUER, David STIFTER, Upper Austrian Research GmbH, Linz, Austria Michael PIRCHER, Erich GÖTZINGER, Christoph K. HITZENBERGER, Centre of

Biomedical Engineering and Physics, Medical University Vienna, Austria Rainer ENGELKE, Gabi GRÜTZNER, Gisela AHRENS, micro resist technology GmbH,

Berlin, Germany Reinhold OSTER Eurocopter Deutschland GmbH, München, Germany

Abstract. Optical coherence tomography (OCT) is a technique for contactless and non-destructive imaging of internal structures within semi-transparent materials. When OCT is performed polarization-sensitively (PS-OCT), additionally information about internal anisotropies and strains is obtained. Originally developed for the biomedical sector, OCT increasingly attracts interest for material characterization. In this work, we apply PS-OCT imaging for non-biological samples using a novel transversal ultra-high resolution PS-OCT setup. We demonstrate the advantages of this technique for structural analysis and strain mapping for different types of samples: we evaluate photoresist moulds for the fabrication of micro-electromechanical parts (MEMS), and we investigate the glass-fibre composite outer shell of helicopter rotor blades where defects have formed.

1. Introduction

Optical coherence tomography (OCT) is a contactless and non-destructive technique for imaging of internal structures within semi-transparent materials. Originally developed for ophthalmologic examinations of the human retina [1], it has become an established method on the bio-medical sector, where its applications comprise the investigation of a variety of biological tissues [2-9]. Only recently, the benefits of OCT for non-destructive testing of non-metallic materials have been realized: OCT has been used to detect subsurface cracks in ceramics, Teflon or SiC [10,11], to image polymer matrix or glass-fibre composites [12-14], injection moulded plastic parts [14], thin multi-layer foils and coatings [15], but also to investigate the properties of paper [16] or the subsurface morphology of archaic jades [17].

An extension of OCT is polarization-sensitive (PS-)OCT. In addition to the reflectivity image obtained by classical OCT, PS-OCT provides depth resolved information about the birefringence and the orientation of the optical axis within a sample [18]. PS-OCT adds valuable information for the characterization of materials: in unloaded samples, birefringence may occur due to internal anisotropies or residual stresses (strains, respectively), and by externally loading of samples, stress-birefringence calibration measurements allow even a quantitative measurement of stresses [19]. However, to date only few examples can be found where PS-OCT has been applied for materials research,

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despite the applications presented e.g. in refs. [14], [19] and [20] that clearly demonstrate the high potential of PS-OCT for strain-mapping and stress analysis in materials.

In this paper, we present non-biological applications of PS-OCT for structural analysis and strain-mapping. For this purpose, we have extended existing OCT techniques beyond the state-of-the art. We have combined ultra-high resolution (UHR-)OCT with the concept of en-face (transversal) scanning developed by Hitzenberger et al. [21] and refined by Pircher et al. for PS-OCT [22,23]. By transversal OCT, images parallel to the surface at a defined adjustable depth are obtained, in contrast to conventional cross-sectional OCT imaging. Additionally, we have extended this setup for polarization–sensitive measurements. Our system provides, to the best of our knowledge, transversal UHR-PS-OCT with the highest axial resolution obtained so far. We show the potential of this setup for problems posed in material research: we evaluate high aspect-ratio moulds in thick photoresist layers on gold-coated wafers for the production of micro electromechanical parts (MEMS) where highly strained areas are revealed. Furthermore, the glass-fibre reinforced outer shell of helicopter rotor blades is investigated, where delaminated areas and cracks due to externally loading of the samples are present.

2. Concepts of Classical OCT, UHR- and PS-OCT

OCT is based on the principles of white-light interferometry (see, e.g. ref. [24]). Low-coherent light is reflected and backscattered from interfaces or scattering structures located at different depths within the sample. Information about the optical depth is obtained within an interferometer setup, e.g. a Michelson interferometer, where a depth scan (A-scan) of the reference mirror provides a reflectivity depth profile of the sample. Commonly, cross-sectional scans (B-scans) are obtained by combining a series of subsequent A-scans. Using light sources in the near infrared, the penetration depth of the probing beam can reach some millimetres in materials such as plastics, polymers, resins and compounds.

In the past few years, a number of novel concepts have lead to OCT systems with enhanced performance. These new techniques include e.g. Fourier-domain OCT [25], Doppler OCT [26], PS-OCT [27], transversal OCT [21] and full-field OCT [28], giving rise to an improved sensitivity and imaging speed, exploiting new contrast mechanisms, and employing new imaging concepts. Additionally, the development of new light sources has driven the depth resolution of OCT down to the 1 µm range [29] (UHR-OCT). Because the depth resolution of OCT is mainly determined by the coherence length of the light source [30], which depends on the square of the centre wavelength and on the reciprocal bandwidth, UHR-OCT requires high-broadband light sources such as e.g. femtosecond (fs-)lasers. It has been demonstrated in our previous work [15], that UHR imaging is essential for many problems posed in materials research, where often structures with typical sizes of only a few microns are of interest like, e.g. the diameter of embedded fibres in reinforced composite materials, the thickness of thin layers and coatings, or the size of particles and inclusions.

Information about additional physical parameters can be obtained by PS-OCT imaging. In contrast to conventional OCT, where the intensity of the light returned from the sample is measured, PS-OCT maps the polarization state of the backscattered light, providing simultaneous information on reflectivity, birefringence and the orientation of the optical axis. This eventually results in an enhanced structural contrast, and optical anisotropies within the material can be visualized. In most of the set-ups, the sample is illuminated by circularly polarized light. Depending on the birefringence within the sample, the polarization is changed into a generally elliptical one. The polarization state of the interferometric signals is recorded by a polarization-sensitive detection system. From the

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signals of the two horizontal detection channels, the reflectivity, the retardation between the two polarization components and the orientation of the optical axis are determined [18]. From the retardation data, the birefringence can be calculated.

3. Experimental Setup

Our transversal UHR-PS-OCT system shown in Fig. 1 follows the principles of time-domain OCT, using a Mach-Zehnder interferometer geometry with separate paths of the reference- and the sample beam. A detailed description of the basic setup can be found in ref. [15]. Specific characteristics are the acousto-optic modulators (AOMs) in the reference arm, which introduce a carrier frequency of 2 MHz for heterodyne signal detection, and the xy-galvano scanner unit for scanning the laser beam over the sample with a frequency up to 500 Hz. For en-face imaging, the laser beam raster-scans in the x and y directions while the reference mirror is kept at a fixed position. Transversal scan areas up to 3 x 3 mm2 can be measured with a pixel rate of 500 kHz. Alternatively, conventional cross-sectional images (i.e. B-scans) can be obtained by performing one depth scan of the reference mirror while scanning the laser beam repeatedly along the x direction. For matching the focal position of the sample lens with the coherence gate, dynamic focussing has been implemented for our OCT measurements by automatically shifting the sample accordingly along the z direction during a depth scan.

Figure 1. Schematic setup of the transversal UHR-PS-OCT. The bold arrows indicate the polarization states of the laser beam at the respective positions. Abbreviations: SMF – single mode fibre, (N)PBS – (Non)-

polarizing beamsplitter, AOM – acousto-optic modulator, QWP – quarter wave plate, DAQ – data acquisition.

For polarization-sensitive measurements, a polarizer and two quarter wave plates

(QWP in Fig. 1) are added. The horizontally polarized light becomes polarized by 45° with respect to the horizontal after double-passing the QWP (fast optical axis at 22.5°) in the reference arm. The QWP in the sample arm (fast optical axis at 45°) produces circularly polarized light illuminating the sample. The polarization state of the interferograms is

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measured by using polarization-sensitive beamsplitters (PBS in Fig. 1) and a second set of balanced photoreceiver and lock-in amplifier.

As light source for UHR-imaging, we use a fs-Ti:sapphire mode-locked laser (Integral OCT from Femtolasers) with attached single mode fibre and an average output power of 50 mW ex-fibre at a pulse rate of 85 MHz, resulting in 4 mW power at the sample position. The nominal spectral full-width-at-half-maximum (FWHM) is 150 nm at a centre wavelength of 800 nm. We have measured a FWHM of the coherence peak of 2.5 µm in air, corresponding to a depth resolution of 1.8 µm for typical plastic materials. In combination with a lateral resolution better than 4 µm measured experimentally, our setup is capable of ultra-high resolution OCT measurements. A sensitivity of 97 dB of the UHR-PS-OCT system has been determined for the pixel rate given above.

4. Cracks and Delaminations in Aerospace Parts

We have used the transversal UHR-PS-OCT setup described above for the investigation of defects in the outer shell of a helicopter rotor blade. The rigid outer shell consists of a glass-fibre reinforced composite (GFC) material, as it is commonly used for the fabrication of aerospace parts. Prior to the OCT measurements, the blade was subjected to loading tests, resulting in the formation of cracks and delaminated areas in the GFC material.

Figure 2. UHR-PS-OCT scans across a crack in the GFC-shell of a helicopter rotor blade. Cross-sectional (a) reflectivity and (c) retardation image (colour-coded form 0° to 90°), and (d) orientation of the optical axis

(colour-coded form 0° to 180°). (b) En-face reflectivity image. The vertical arrows indicate the position of the crack, in (a) the two arrows mark a delaminated area beneath the crack. In (c) and (d), the areas below the

crack show enhanced birefringence and an orientation of the optical axis approximately perpendicular to the direction of the crack.

Fig. 2(a) shows a cross-sectional scan across a crack, with the crack perpendicular to the image plane. The crack (with its position indicated by the vertical arrow) is only slightly visible because it is oriented parallel to the incident light, thus providing nearly no backscattering or reflecting surfaces. Beneath the crack, a delaminated area (marked by the arrows) is observed, where structural damage of the glass-fibre epoxy composite has

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occurred. When a transversal scan at an optical depth indicated by the dashed line in Fig. 2(a) is performed, the crack running vertically across the fibre bundles becomes entirely visible (Fig. 2(b)). It should be noted that the single fibres visible within the fibre bundles in (a) and (b) can only be resolved due to UHR-imaging [15].

Analysis of the polarization information of the cross-sectional scan reveals an increased birefringence beneath the crack, as it is shown in Fig. 2(c), where the retardation signal is colour-coded from 0° to 90°. Additionally, the optical axis corresponding to the direction of anisotropies or strains within the sample is found to be oriented more or less perpendicularly with respect to the direction of the crack (Fig. 2(d), orientation of the optical axis colour-coded from 0° to 180°). Although the crack itself is hardly visible in the PS-images, the region where a crack is present becomes clearly distinguishable from defect-free areas due to the enhanced birefringence and the defined orientation of the optical axis, demonstrating that PS-OCT provides a promising contrast mechanism for crack detection.

5. Structural Analysis and Strain Mapping of MEMS Moulds

A field of current research are high aspect-ratio moulds for the fabrication of micro electromechanical parts (MEMS). These moulds are defined in thick photoresist layers by a lithographic process, with e.g. smallest lateral dimensions of 30 µm at a photoresist layer thickness of 1 mm. A non-destructive control of the resist moulds prior to the MEMS production by electroplating is required, in order to separate faulty moulds for a reproducible and high quality of the MEMS. OCT, and in particular our transversal scanning UHR-PS-OCT system is promising tool for characterization of the moulds, as is shown in the following.

We have investigated photoresist moulds on a gold-coated silicon wafer for the fabrication of micro-gear wheels (Fig. 3(a)). Using cross-sectional UHR-OCT, information about the layer thickness and the refractive index of the resist are obtained [15]. The cross-sectional image shown in Fig. 3(b) reveals the different optical levels of the structure, i.e. the surface, the bare wafer where the resist layer has been removed by etching, and the resist-wafer interface. A resist layer thickness of 1.2 mm was measured for this sample, and from the offset between the bare wafer and the resist-wafer interface due to the larger optical path length of the light within the material, a refractive index of 1.60 was obtained.

Figure 3. (a) Photo of a high-aspect ratio photoresist mould on a wafer for the production of micro-gear wheels. (b) Cross-sectional OCT image and schematic drawing of the sample illustrating the different optical

levels. (Wheels designed by Micromotion GmbH)

By en-face imaging, the different optical levels can be investigated selectively. Figures 4(a) and (b) show en-face images recorded at the optical level of the bare wafer and the resist-wafer interface, respectively ((a) and (b) belong to different wheel structures on the same wafer). The shape and the geometrical properties of the structure and possible defects are revealed immediately. In particular at the bare wafer, particles or resist residues

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were detected (some of them indicated by the arrows in Fig. 4(a)), and at the interfacial level, cracks in between the wheels become visible (arrows in Fig. 4(b)).

Figure 4. Transversal UHR-PS-OCT reflectivity images of (a) the bare wafer and (b) the resist-wafer interface of different micro-wheel photoresist moulds. The arrows indicate (a) contaminations at the bare

wafer and (b) cracks in the photoresist layer. (c) Corresponding retardation image (colour-coded from 0° to 90°) and (d) orientation of the optical axis (colour-coded from 0° to 180°, and displayed as vector-field)

recorded at the interface.

Transversal UHR-PS-OCT measurements reveal the in-plane strain distribution and

the orientation of the optical axis within the sample. In Figs. 4(c) and (d), the PS-information obtained simultaneously with the reflectivity image in (b) at an optical depth of the resist-wafer interface is displayed. As can be seen from the retardation image shown in Fig. 4(c) (retardation colour-coded from 0° to 90°), the major part of the interface is nearly unstrained (blue), but especially in between the wheels an increased birefringence (red) is present. The optical axis, related to the directions of the strain fields, indicates radial strains around the wheels as is shown in Fig. 4(d), where the orientation of the optical axis is displayed colour-coded from 0° to 180° and as vector field. Images like these demonstrate the high potential of transversal UHR-PS-OCT for direct mapping of the in-plane strain distribution of MEMS moulds. It can be used for an optimization of the moulds by avoiding highly strained areas where preferentially defects occur.

6. Conclusions

In this work, we have presented transversal ultra-high resolution OCT for polarization sensitive measurements with the highest axial resolution ever reported. We have demonstrated the high potential of this method for contactless and non-destructive testing of materials by performing structural analysis and strain mapping for different samples:

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defects such as cracks and delaminated areas were imaged within the GFC outer shell of a helicopter rotor blade, where PS-imaging has been shown to provide a contrast mechanism for crack detection.

The advantages of transversal scanning were demonstrated for photoresist moulds for MEMS production. The structures were investigated selectively at the optical depths of the different surfaces and interfaces, revealing contaminations at the bare wafer and cracks in the resist layer. Transversal PS-measurements additionally revealed the in-plane strain distribution and the orientation of the optical axis within the moulds. These example demonstrate that transversal scanning directly provides in-plane information about internal structures and strains which is of interest for many applications and which can hardly be obtained by cross-sectional imaging.

In conclusion, transversal UHR-PS-OCT is promising tool for strain-mapping in semitransparent materials. UHR-PS-OCT is capable of simultaneous imaging of internal strains and structures with sizes in the micrometer range, or for strain in micro-parts themselves. So far, OCT for material investigations has been demonstrated on polymers and plastics, ceramics and compound materials. This opens a wide field for the application of PS-OCT for non-destructive structural analysis and strain-imaging.

Acknowledgements

This work has been supported by the Austrian Science Fund FWF (Projects: P16585-N08 and L126-N08) and the European Commission (FP6 CRAFT Project: COOP-CT-2003-507825).

References

[1] D. Huang, E. A. Swanson, C. P. Lin, J. S. Schuman, W. G. Stinson, W. Chang, M. R. Hee, T. Flotte, K. Gregory, C. A. Puliafito, and J. G. Fujimoto, "Optical Coherence Tomography", Science 254, 1178-1181 (1991). [2] A. Puliafito, M. R. Hee, C. P. Lin, E. Reichel, J. S. Schuman, J. S. Duker, J. A. Izatt, E. A. Swanson, and J. G. Fujimoto, "Imaging of macular diseases with optical coherence tomography", Ophthalmology 102, 217-229 (1995). [3] C. Wirbelauer, J. Winkler, G. O. Bastian, H. Häberle, and D. T. Pham, "Histopathological correlation of corneal disease with optical coherence tomography", Graefe's Arch. Clin. Exp. Ophthalmol. 240, 727-734 (2002). [4] E. Goetzinger, M. Pircher, M. Sticker, A. F. Fercher, and C. K. Hitzenberger, "Measurement and imaging of birefringent properties of the human cornea with phase-resolved polarization-sensitive optical coherence tomography", J. Biomed. Opt. 9, 94-102 (2004). [5] C. E. Saxer, J. F. de Boer, B. H. Park, Y. Zhao, Z. Chen, and J. S. Nelson, "High-speed fiber-based polarization-sensitive optical coherence tomography of in vivo human skin", Opt. Lett. 25, 1355-1357 (2000). [6] M Pircher, E. Götzinger, R. Leitgeb, and C. K. Hitzenberger, "Three dimensional polarization sensitive OCT of human skin in vivo", Opt. Expr. 12, 3236-3244 (2004). [7] A. Baumgartner, S. Dichtl, C. K. Hitzenberger, H. Sattmann, B. Robl, A. Moritz, F. Fercher, and W. Sperr, "Polarization-sensitive optical coherence tomography of dental structures", Caries Res. 34, 59-69 (2000). [8] D. Fried, J. Xie, S. Shafi, J. D. B. Featherstone, T. M. Breunig, and C. Le, "Imaging caries lesions and lesion progression with polarization sensitive optical coherence tomography", J. Biomed. Opt. 7, 618-627 (2002). [9] G. J. Tearney, H. Yabushita, S. L. Houser, H. T. Aretz, I. K. Jang, K. H. Schlendorf, C. R. Kauffman, M. Shishkov, E. F. Halpern, and B. E. Bouma, "Quantification of macrophage content in atherosclerotic plaques by optical coherence tomography", Circulation 107, 113-119 (2003). [10] M. Baskansky, M. D. Duncan, M. Kahn, D. Lewis III, and J. Reintjes, "Subsurface defect detection in ceramics by high-speed high-resolution optical coherent tomography", Opt. Lett. 22, 61 (1997). [11] M. D. Duncan, M. Bashkansky, and J. Reintjes, "Subsurface defect detection in materials using optical coherence tomography", Opt. Express 13, 540 (1998).

7

[12] J. P. Dunkers, R. S. Parnas, C. G. Zimba, R. C. Peterson, K. M. Flynn, J. G. Fujimoto, and B. E. Bouma, "Optical coherence tomography of glass reinforced polymer composites", Composites A 30, 139–145 (1999). [13] J. P. Dunkers, F. R. Phela, D.P. Sanders, M. J. Everett, W. H. Green, D. L. Hunston, and R.S. Parnas, "The application of optical coherence tomography to problems in polymer matrix composites", Opt. Laser Eng. 35, 135 (2001). [14] D. Stifter, P. Burgholzer, O. Höglinger, E. Götzinger, and C. K. Hitzenberger, "Polarisation-sensitive optical coherence tomography for material characterisation and strain-field mapping", Appl. Phys. A 76, 947 (2003). [15] K. Wiesauer, M. Pircher, E. Götzinger, S. Bauer, R. Engelke, G. Ahrens, G. Grützner, C. K. Hitzenberger, and D. Stifter, "En-face scanning optical coherence tomography with ultra-high resolution for material investigation", Optics Express 13, 1015 (2005). [16] E. Alarousu, L. Krehut, T. Prykari, and R. Myllyla, "Study on the use of optical coherence tomography in measurements of paper properties", Meas. Sci. Technol. 16, 1131-7 (2005). [17] M.-L. Yang, C.-W. Lu, I.-J. Hsu, and C.C. Yang, "The use of Optical Coherence Tomography for Monitoring the Subsurface Morphologies of Archaic Jades", Archaeometry 46, 171 (2004). [18] C. Hitzenberger, E. Götzinger, M. Sticker, M. Pircher, and A. F. Fercher, "Measurement and imaging of birefringence and optic axis orientation by phase resolved polarization sensitive optical coherence tomography," Opt. Express 9, 780-790 (2001). [19] K. Wiesauer, A. D. Sanchis Dufau, E. Götzinger, M. Pircher, C. K. Hitzenberger, and D. Stifter, "Non-destructive quantification of internal stress in polymer materials by polarisation sensitive optical coherence tomography", Acta Materialia 53/9, 2785-2791 (2005). [20] J.-T. Oh and S.-W. Kim, "Polarization-sensitive optical coherence tomography for photoelasticity testing of glass/epoxy composites", Opt. Express 11, 1669-1676 (2003). [21] C. K. Hitzenberger, P. Trost, P.-W. Lo, and Q. Zhou, "Three dimensional imaging of the human retina by high speed optical coherence tomography", Opt. Express 11, 2753 (2003). [22] M. Pircher, E. Goetzinger, R. Leitgeb, and C. K. Hitzenberger, "Transversal phase resolved polarization sensitive optical coherence tomography", J. Phys. Med. Biol. 49, 1257 (2004). [23] M. Pircher, E. Götzinger, R. Leitgeb, H. Sattmann, and C. K. Hitzenberger, "Ultrahigh resolution polarization sensitive optical coherence tomography", Proc. of SPIE 5690, 257-262 (2005). [24] B. E. Bouma and G.J. Tearney, eds.: "Handbook of Optical Coherence Tomography" (Marcel Dekker, New York, 2002). [25] A. F. Fercher, C. K. Hitzenberger, G. Kamp, and S. Y. El-Zaiat, "Measurement of intraocular distances by backscattering spectral interferometry", Opt. Commun. 117, 43-48 (1995). [26] Z. Chen, T. E. Milner, S. Srinivas, X. Wang, A. Malekafzali, M. J. C. van Gemert, and J. S. Nelson, "Noninvasive imaging of in vivo blood flow velocity using optical Doppler tomography", Opt. Lett. 22, 1119-1121 (1997). [27] M. R. Hee, D. Huang, E. A. Swanson, and J. G. Fujimoto, "Polarization-sensitive low-coherence reflectometer for birefringence characterization and ranging", J. Opt. Soc. Am B 9, 903-908 (1992). [28] A. Dubois, L. Vabre, A.-C. Boccara, and E. Beaurepaire, "High-resolution full-field optical coherence tomography with a Linnik microscope", Appl. Optics 41, 805-812 (2002). [29] Drexler, U. Morgner, F. X. Kärntner, C. Pitris, S. A. Boppart, X. D. Li, E. P. Ippen, and J. G. Fujimoto, "In vivo ultrahigh-resolution optical coherence tomography", Opt. Lett. 24, 2112 (1999). [30] E. A. Swanson, D. Huang, M. R. Hee, J. G.Fujimoto, C. P. Lin, and C. A. Puliafito, "High-speed optical coherence domain reflectometry", Opt. Lett. 17, 152 (1992).

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