Micromachined IR spectrometers for chemical sensing

MICROMACHINED IR SPECTROMETERS FOR CHEMICAL SENSING LEE FEIWEN NATIONAL UNIVERSITY OF SINGAPORE 2008 MICROMACHINED IR SPECTROMETERS FOR CHEMICAL SENSING LEE FEIWEN (B Eng (Hons.), NUS) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF ENGINEERING DEPARTMENT OF MECHANICAL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2008 Acknowledgements ACKNOWLEDGEMENTS I wish to express my most sincere gratitude to my project supervisors, Dr Zhou Guangya and A/P Chau Fook Siong for their invaluable advice and assistance during the entire course of this project I would also like to express my appreciation to staff at the Institute of Materials Research & Engineering This includes Mr Tang Xiaosong, Eric, Ms Teo Siew Lang, Mr Wang Weide, Mr Neo Kiam Peng and Mr Cheong Khee Leong from SERC Nanofabrication & Characterisation (SNFC) Department for their tremendous help and assistance during the fabrication phase of the project Special thanks also go out to Mr Tan Yee Yuan for his superb guidance and advice in designing and building the various electronic devices and also to Mr Wong Chee Leong for his prompt and superb assistance in helping to take SEM pictures I would also like to take this opportunity to thank all the research fellows and fellow graduate students whom I met over the course of the project They include Dr Yu Hong Bin, Mr Wang Shou Hua, Mr DuYu, Mr Chew Xiong Yeu, Jason, Mr Cheo Koon Lin, Kelvin, Ms Leung Hui Min, Mr Koh Tian Yi, Alvin, Ms Zheng Rongyan, Mr Seah Weeter, Mr Tiang Junhui, Aska and Mr Wang Sirui They were great companions and certainly provided great support in the completion of the project Lastly, thanks to all friends who have helped me in one way or another -i- Summary SUMMARY In this thesis, the development of Fourier Transform micro-spectrometer based on lamellar grating principle is reported This micro-spectrometer has the potential to be used as chemical sensor for environmental monitoring Most commercial spectrometers have been characterized as being too delicate, too expensive and too bulky; limiting their usage to be only within laboratory premises The ability to miniaturize current spectrometer will certainly be welcomed as it increases the portability of the equipment; allowing spectrometers to perform diagnostic tests in the field at real time The miniaturization of the spectrometer is achieved using Microelectrical Mechanical Systems (MEMS) technology The first prototype of a lamellar grating based micro-spectrometer has proven the feasibility of the idea A grating displacement of ~28μm has been achieved using electrostatic actuation, yielding spectroscopic resolution of 14.3nm and 10.1nm at 632.8nm and 532nm respectively The resolution of the spectra is the minimum separation for two spectra lines in the spectrum so as to enable clear identification Most importantly, the resolution of an interferometer varies inversely as the maximum optical path difference and it is true for both the visible and IR range of the light spectrum The second lamellar grating based micro-spectrometer, which is actuated using electromagnetic actuation, has several advantages over electrostatic actuation such as bidirectional actuation, larger actuation force and linear relationship with input current The maximum optical path difference (OPD) achieved by the device is increased significantly using the electromagnetic actuator A spectra resolution of 3.8nm at a wavelength of 632.8nm -ii- Summary and 3.44nm at 532nm is achieved Finally, the idea of achieving the same performance of the spectrometer at a lower voltage requirement is explored This is done by actuating the micro-spectrometer at the resonant frequency of the device While a decent displacement is achieved at a relatively low voltage input, a new electronic data acquisition system is designed to capture the IR interferogram when the device is in motion The spectrometer is used to measure the output of a tunable laser source from wavelength at 1520nm to 1590nm at 10nm intervals The peak of the calculated spectra is very close to the actual wavelength of the input IR, with a maximum difference of less than 5nm -iii- Table of Contents Table of Contents ACKNOWLEDGEMENTS ……………………………………………………… I SUMMARY ………………………………… …………………………………… II TABLE OF CONTENTS ……………………………………………………….… IV LIST OF TABLES …………………………………………………………………VII LIST OF FIGURES ……………………………………………………………….VIII LIST OF SYMBOLS …………………………………………………….……… XII CHAPTER INTRODUCTION ……………………………………………………1 1.1 Introduction to Optical Spectroscopic Chemical Sensors ……………………… 1.2 Overview of Spectroscopy ……………………………………………………… 1.3 Types of Spectrometers ………………………………………………………… 1.4 Fourier Transform Infrared Spectroscopy (FTIR) …………………………………8 1.4.1 History and Development of FTIR ……………………………………… 1.4.2 Advantages of FTIR ……………………………………………………….9 1.5 Integration of FTIR Spectroscopy and MEMS ………………………………… 12 1.6 Introduction to Micro-Electro Mechanical Systems (MEMS) ………………… 14 1.6.1 Overview of Microfabrication Technology ………………………………15 1.7 Literature Review of MEMS-based spectroscopy ……………………………… 19 1.8 Project Objective …… ………………………………………………………….21 CHAPTER WORKING PRINCIPLES OF FTIR …………………………… 25 2.1 Review on Interference and Diffraction ………………………………………….25 2.2 Working Principle of the Michelson Interferometer …………………………… 28 -iv- Table of Contents 2.3 Working Principle of the Lamellar grating Interferometer ………………………30 CHAPTER AN ELECTROSTATIC DRIVEN LAMELLAR GRATING MICRO-SPECTROMETER FABRICATED WITH POLY MUMPs PROCESS ……………………………………………………………………………35 3.1 Design and Fabrication ………………………………………………………… 35 3.2 Characterization of the micro-spectrometer …………………………………… 37 3.2.1 Electro-mechanical Characterization …………………………………… 38 3.2.2 Optical Characterization ………………………………………………….42 3.3 Discussion of results …………………………………………………………… 45 CHAPTER AN ELECTROMAGNETIC DRIVEN LAMELLAR GRATING MICRO-SPECTROMETER FABRICATED WITH SOI MUMPs PROCESS ……………………………………………………………………………48 4.1 Electromagnetic Actuation ……………………………………………………….49 4.2 Design and Fabrication ………………………………………………………… 50 4.3 Characterization of the micro-spectrometer …………………………………… 54 4.4 Discussion of results …………………………………………………………… 57 CHAPTER RESONANT-SCANNING LAMELLAR GRATING FOURIER TRANSFORM MICRO-SPECTROMETER WITH A LASER REFERENCE SYSTEM …………………………………………………………………………… 59 5.1 Experimental setup ……………………………………………………………….61 5.2 Architecture of electronic data acquisition system ……………………………….63 5.2.1 Modulation and conditioning of IR interferogram …………………… 64 5.2.2 Modulation and conditioning of driving signal ……………………… 65 5.2.3 Modulation and conditioning of DPSS laser signal …………………….66 5.2.4 Data Acquisition software …………………………………………… 72 -v- Table of Contents 5.3 Analytical study and discussion of the DPSS laser signal ……………………… 73 5.4 Characterization of the micro-spectrometer …………………………………… 76 5.5 Discussions of results …………………………………………………………….78 CHAPTER CONCLUSION AND FUTURE WORK ……………………… 82 6.1 Conclusion………… ……………………………………………………………82 6.2 Future Works …………………………………………………………………… 83 REFERENCES …………………………………………………………………… 85 LIST OF PUBLICATIONS ARISING FROM PROJECT……………… 88 APPENDIX A ……………………………………………………………………… 90 APPENDIX B ……………………………………………………………………… 91 APPENDIX C ……………………………………………………………………… 95 -vi- List of Tables LIST OF TABLES Table 4.1: Dimensions of structure ………………………………………………… 51 Table 5.1: Peak Recorded and FWHM at various wavelength of IR radiation ………78 Table 5.2: Maximum frequency of the interferogram for the various light source … 80 -vii- List of figures LIST OF FIGURES Figure 1.1: Simple configuration of a prism spectrometer …………………………….4 Figure 1.2: (a) Configuration of a plane transmission grating spectrometer, (b) A Fastie-Ebert mount for a blazed reflection grating …………….……… Figure 1.3: (a) A Michelson interferometer, (b) A Mach-Zehnder interferometer Figure 1.4: A Fabry Perot spectrometer ……………………………………… …… Figure 1.5: Schematic of a simple microfabrication process …………………………16 Figure 1.6: SEM of the device showing details of the fixed and movable micromirrors which are obtained by KOH etching … ……………… ……………….19 Figure 2.1: Young’s double slit experiment Light get diffracted when it falls on S0 which is narrow and so it illuminates both S1 and S2 Diffraction also takes place at S1 and S2 and interference occurs in the region where the light from S1 overlaps that from S2 ……………………………………… ……… 26 Figure 2.2: Geometric construction for describing Young’s double slit experiment 27 Figure 2.3: Schematic of the Michelson interferometer M1 is the movable mirror and M2 is the fixed mirror …………………… ……….…………………… 29 Figure 2.4: Optical diagram of the lamellar grating with light of wavelength, λ , incident normally on the grating surface and diffracted at an angle α The width of the slit is a and the depth of the grating is d ………… ……… 31 Figure 2.5: Graph of Intensity of the light beam, I(d) against displacement of the moving finger, d…………………………… ……………………… 34 -viii- Chapter proportional such that a detector of high sensitivity will have a small bandwidth and a detector of larger bandwidth will have lower sensitivity Other novel methods to improve the spectral resolution of the micro-spectrometer should be continued to be explored This will be discussed in the next chapter under future work 81 Chapter Chapter Conclusion and Future Work 6.1 Conclusion The integration of Micro-electrical Mechanical Systems (MEMS) technology and FTIR spectroscopy has proven to be the solution for the realization of a truly small and portable micro-spectrometer Various lamellar grating based Fourier transform Infrared electromagnet (FTIR) micro-spectrometers have been presented in this report and the performance of the spectrometers in term of spectral resolution and accuracy have been reasonably good Though the performance of the micro-spectrometer is not superior to present commercial bench-top spectrometers, the proposed microspectrometers can still achieve reasonably good results in chemical identification purposes for chemical sensors It is believed that the benefits of being a portable spectrometer system will simply outweigh this slight disadvantage Through the various modification in designs and actuation methods, a lamellar grating based FTIR spectrometer which possesses good resolution (~20nm) with relatively low power voltage (~2.2V) has been developed and presented The presented design is not without limitations, but it has the potential of developing into a feasible portable gas sensing system On a final note, the work on developing miniaturized spectrometer system should be further explored due to its immense potential benefits The next section describes the future suggestions for future work that can be carried out to enhance the resolution of the micro-spectrometers 82 Chapter 6.2 Future Work Figure 6.1: Schematic of a MEMS synthetic FTIR spectrometer Other novel methods of improving the spectral resolution of the micro-spectrometer can be explored A plausible idea is MEMS synthetic FTIR spectrometer that utilizes multiple FTIR micro-spectrometers to significantly boost the spectral resolution The basic idea behind the MEMS synthetic FTIR spectrometers is inspired by synthetic aperture radar (SAR) technology As shown in Figure 6.1, an array of MEMS lamellar grating spectrometers will be employed, each scanning a limited range with different initial optical retardations As shown schematically in Figure 6.2, the light spectrum is obtained by Fourier transformation of the “synthetic” interferogram that results from combining the interferograms obtained from the individual lamellar grating interferometers Consequently, the resolution of MEMS synthetic FTIR spectrometer can be greatly improved as the maximum optical path difference in the “synthetic” interferogram is N-fold In this way, the difficulties of developing high-resolution 83 Chapter miniaturized FTIR spectrometers imposed by the lack of sufficient maximum stroke of microactuators can be largely avoided To achieve this objective, the fundamental theories behind MEMS synthetic FTIR spectrometers need to be investigated and methods to synchronize the multiple micro-spectrometers as well as the algorithms to construct “synthetic” interferograms need to be developed Figure 6.2: Synthetic interferogram constructed using multiple interferometers to achieve enhanced resolution 84 References REFERENCES [1] Chung, Raymond Basics Principles of Spectroscopy pp 34-70, New York McGraw Hill.1970 [2] Daniel Malacara and Brian J Thompson Handbook of optical engineering chapter 9, Marcel Dekker, Inc 2001 [3] Karl Dieter Moller and Walter G Rothschild Far-Infrared Spectroscopy pp 34-38, John Wiley & Sons, Inc 1971 [4] M Francon Optical Inteferometry pp 74-81, Academic Press 1966 [5] J.M Vaughan The Fabry Perot interferometer : history, theory, practice, and applications pp3-10, Bristol, Eng ; Philadelphia, A Hilger 1989 [6] Bell, R J Introductory Fourier Transform Spectroscopy pp 72-81, New York: Academic Press 1972 [7] Vaugham, J.M The Fabry Perot Inteferometer: history, theory, practice and applications pp 28-32, A Hilger 1989 [8] Connes, J and Connes, P near-Infrared Planetary Spectra by Fourier Spectroscopy, J Optical Soc Am., 56, pp 896-910, 1966 [9] Marc J Madou Fundamentals of Microfabrication The Science of Miniaturization (2nd edition) CRC Press 1997 [10] Kyoungik Yu, Daesung Lee, Uma Krishnamoorthy, Namkyoo Park and Oalv Solgaard Micromachined Fourier Transform Spectrometer on Silicon Optical Bench Platform, Sensors and Actuators A, 130-131, pp 523-530, 2006 85 References [11] Omar Manzardo, Hans Peter Herzig, Cornel R Marxer and Nico F de Rooij Miniaturized Time-scanning Fourier Transform Spectrometer based on Silicon Technology, Optics Letters, Vol 24 No 23, pp 1705-1707, 1999 [12] Andreas Kenda, Christian Drabe, herald Schenk, Albert Frank, Martin Lenzhofer, Werner Scherf Application of a Micromachined Translatory Actuator to an optical FTIR Spectrometer, Proc Of SPIE, Vol 6186 618609, pp 1-7, 2006 [13] Ulrike Wallrabe, Christian Solf, Jurgen and Jan G Korvink Miniaturized Fourier Transform Spectrometer for the near infrared wavelength regime incorporating an electromagnetic linear actuator, Sensors and Actuators A, 123-124, pp.459-467, 2005 [14] Collin S.D., A.P Wallace, R.L Smith, J.M Sirota and C Gonzalez MicroPhotonics Systems Implementation, Proc Of Tranducers ’99, pp 124-127, 1999 [15] Omar Manzardo, Roland Michaely, Felix Schadelin, Wilfried, Thomas Overstolz, Nico De Rooij and Hans Peter Herzig Miniature Lamellar Grating interferometer based on Silicon Technology, Optics Letters, Vol 29 No 13, pp 14371439, 2004 [16] Caglar Ataman, Hakan Urey and Alexandra Wolter A Fourier Transform Spectrometer using Resonant Vertical Comb Actuators, J Micromech Microeng., 16, pp 2517-2523, 2006 [17] Caglar Ataman, Hakan Urey and Alexandra Wolter A MEMS Based Visible- NIR Fourier Transform Microspectrometer, Proc SPIE, 6186 61860C, 2006 [18] O Manzardo Micro-sized Fourier Spectrometers, Neuchatel, University, Thesis (doctoral), pp 105-107, 2002 [19] MUMPs MEMSCAP The Power of a Small WorldTM, 20th Feb 2008 http://www.memscap.com/en_mumps.html 86 References [20] Yu Hingbin, Lee Feiwen, Zhou Guangya and Chau Fook Siong An electromagnetically driven lamellar grating based Fourier transform microspectrometer, J Micromech Microeng., 18, 055016 (6pp.), 2008 [21] Zhou Guangya and Philip Dowd Tilted folded beam suspension for extending the stable travel range of comb drive actuators, J Micromech Microeng., 13, pp 178183, 2003 [22] Hou M T K, Huang G K W and Huang J Y et al Extending displacements of comb drive actuators by adding secondary comb electrodes, J Micromech Microeng., 16, pp 684-691, 2006 [23] L Palchetti and D Lastrucci Spectral noise due to sampling errors in Fourier- transform spectroscopy, Applied Optics, Vol 40 No 19, pp 3235-3243, 2001 [24] D L Cohen, Performance degradation of a Michelson interferometer due to random sampling errors, Applied Optics, Vol 38 No 1, pp 139-151, 1999 87 List of Publications List of Publications Journal papers Lee Feiwen, Zhou Guangya, Yu Hongbin and Chau Fook Siong A MEMS based resonant-scanning lamellar grating Fourier Transform micro-spectrometer with a laser reference system Sensors and Actuators A (2008) Paper submitted Yu Hongbin, Zhou Guangya, Chau Fook Siong and Lee Feiwen Optofluidic variable aperture OPTICS LETTERS, vol.33, No 6, (2008) , pp 548-550 Yu Hongbin, Zhou Guangya, Chau Fook Siong and Lee Feiwen.Yield improvement for anodic bonding with suspending structure Sensors and Actuators A 143 (2008) pp 462–468 Yu Hongbin, Zhou Guangya, Chau Fook Siong , Lee Feiwen, Wang Shouhua and Zhang Mingsheng An electromagnetically driven lamellar grating based Fourier transform microspectrometer J Micromech Microeng 18 (2008), 055016-1-6 Yu Hongbin, Zhou Guangya, Chau Fook Siong and Lee Feiwen.Simple method for fabricating solid microlenses with different focal lengths IEEE Photonics Technology Letters (2008) Paper Accepted Yu Hongbin, Zhou Guangya, Chau Fook Siong and Lee Feiwen.Tunable ShackHartmann wavefront sensor based on liquid-filled microlens array J Micromech Microeng (2008) Paper submitted Yu Hongbin, Zhou Guangya, Chau Fook Siong and Lee Feiwen.A variable optical attenuator based on optofluidic technology J Micromech Microeng (2008) Paper submitted 88 List of Publications Conference Paper Lee Feiwen, Zhou Guangya, Chau Fook Soing Surface Micro-machined Fourier Transform Infra-red (FTIR) micro-spectrometer In Proc International Conference on Materials for Advanced Technologies (ICMAT) MEMS Technology and Devices, July 2007, Singapore, pp 130-133 89 Appendices Appendix A The derivation process of Wiener-Kintchine theorem is as follows [18]: Assume one of the interfering light waves is expressed as u (t ) and the other one is u (t + δ ) , where δ is the OPD difference between these two waves The interferogram is defined as the autocorrelation function of two interference waves I (δ ) = u (t )u (t + δ ) = ∫ u (t )u (t + δ )dδ While the spectrum is define as B (σ ) = u (σ ) , where u (σ ) is the Fourier transform of u (t ) B (σ ) = = ∫ u (t ) exp( i 2πσ t )dt ∫ rec (T , t )u (t ) exp( i 2πσ t ) dt ∫ rec (T , t ' )u (t ' ) exp( − i 2πσ t ' ) dt ' Replacing t ' by t + δ , ∫ u (t ) exp( i 2πσ t ) dt ∫ u (t + δ ) exp[ − i 2π ( t + δ )σ ]d δ = ∫ u ( t ) u ( t + δ ) exp( − i 2πσδ ) d δ ∫ rec (T , t ) rec (T , t + δ ) dt 4 442 4 4 B (σ ) = equals to = ∫ exp( − i 2πσδ ) I (δ ) d δ 90 (b) Appendix B.1: Electronic circuit design for IR signal Appendices Appendix B.1 (a) 91 Appendix B.2: Electronic circuit design for driving signal Appendices Appendix B.2 92 Appendix B.3: Electronic circuit design for DPSS laser Appendices Appendix B.3 93 Appendix B.4: Electronic circuit design for ADC Appendices Appendix B.4 94 Appendices Appendix C (a) (b) (c) (d) (e) (f) (g) (h) Appendix C: Interferogram of IR source at wavelength of (a)1520nm, (b)1530nm, (c)1540nm, (d)1550nm, (e)1560nm, (f)1570nm, (g)1580nm, (h)1590nm 95 .. .MICROMACHINED IR SPECTROMETERS FOR CHEMICAL SENSING LEE FEIWEN (B Eng (Hons.), NUS) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF ENGINEERING DEPARTMENT... transform infrared (FTIR) spectrometers based on MEMS technology, which has the potential to form a new generation of multi-component infrared chemical sensing system in the field of environmental... (2.16) The first term in Eqs (2.16) is the single facet or mirror term; the second term accounts for the accounts for the array of N identical mirrors in either the front or back plane The first and