Research On Electrothermal Mems Micromirror Towards Miniaturization Of Fourier Transform Spectrometer

As Internet of Things(Io T)technology booming,the MEMS based micro spectrometer is being developed towards the trend of smaller size,lower cost,higher integration and performance to meet the demanding of portable,on-site,real-time application in the field of food safety inspection,environmental monitoring,and medical care and etc.The MEMS based Fourier Transform Spectroscopy(FTS)combines the both advantages of MEMS and FTS,which can realize not only the miniaturization,portable and costless,but also the benefit of fine spectral resolution,high single –noise ratio(SNR)and accuracy.Thus,the MEMS based miniaturized FTS has been a hot research area in the worldwide.In this thesis,the research on electrothermal MEMS micromirror for the miniaturization of FTS in carried on,to solve the key challenge and bottleneck of the minimizing of MEMS based FTS,which will have a profound studying value and social significance.One of the key point for FTS miniaturization is minimizing the size of the system,movable mirror and its associated module without seriously sacrificing the scan range which determines the spectral resolution,optical throughput,SNR,the spectral band and others.In this thesis,a large-stroke electrothermal MEMS mirror device is developed,and the modeling and dual control for the issue of mirror plate tilting and uneven-velocity scanning is studied.An FTS demo system around the fabricated MEMS mirror device is fully established.Also a compact FTS module on a micro silicon optical bench(Si OB)is first reported and realized.More important,a novel monolithic FTS on chip(FTSOC)based on electrothermal actuation is firstly proposed.To be more specific,the main work and achievements are listed as below:(1)The state-of-art of the miniature FTS based on MEMS mirror,as well as the industrial overview in the world wide,are studied and summarized thoroughly.Some key technical challenges and bottleneck for MEMS-FTS minimization are concluded.Thus,the solution of electrothermal MEMS mirror towards miniaturization of FTS in proposed.(2)The principle and theory of FTS are introduced.Some of the main problem and issues during realization of FTS system are discussed.Especially the effects on FTS performance by the factors of the scan range,mirror plate titling,and non-uniform scanning speed and etc.of the movable mirror are analyzed,which provides the theoretical foundation and guideline for the MEMS movable mirror design,control and the MEMS based FTS system implement.(3)The theorem of electrothermal actuation is studied in depth.Based on the mechanism of stress-strain of the film material,the operation principle of electrothermal Bimorph is explained,as well as the relevant actuation equations are derived.The Bimorph actuator is designed and optimized based on the theoretical analysis.The mechanical characterization is discussed.Also,the steady-state temperature distribution and transient thermal response of the Bimorph actuator are studied by thermal modeling.(4)A large vertical displacement electrothermal MEMS mirror is developed.The parameter and structure of the MEMS mirror are designed and optimized based on both LSF and folded double-S shaped actuator presented.The mechanical and thermal simulation is carried out for device optimization and pre-characterize.Vary-designed MEMS mirror devices are fabricated by process combining the bulk silicon and surface micromachining.Both the static and dynamic characterization are finished.For a 3.4mm×3.4mm sized LSF mirror,a 780?m displacement is measured at 8Vdc.(5)The Voltages-based driving method is adopted due to its negative feedback feature,and MCU based drive & control circuit system is realized.To eliminate the mirror plate tilting,the open/close loop control are carried out.By recognizing the model of the electrothermal MEMS mirror,a dual close loop control system is established,which could improve the dynamic responsivity and anti-disturbance ability,and also the scanning stability in FTS system.Thanks to the close-loop control,the response time is improved to 6.4ms from140 ms,and the tilting angle is diminished to < ±0.0015°,and the uniform-speed scanning is achieved.(6)Around on the fabricated and well-controlled MEMS mirror,a complete FTS demo system is built.The data acquisition system based on Agilent I/O Card + Matlab is founded,and the whole data process algorithm is studied and realized by Matlab programming.The optical system based on open and close loop control schemes are configured respectively.The spectroscopy experiments are demonstrated and all the functions are well verified.Up to1016?m optical path difference(OPD)scan range and 0.55 nm spectral resolution are successfully achieved by our electrothermal MEMS mirror based FTS system.(7)A compact FTS system with a large-stroke electrothermal MEMS mirror and other optical components all integrated on a micro-machined silicon optical bench(Si OB)with the footprint of 2cm×2cm is firstly proposed.The linear OPDscan range generated by the MEMS mirror reaches up to 450?m and the tilting of the mirror plate is reduced down to <±0.002° by using a new open-loop control method.A spectral resolution of 1.1nm at 532 nm,has been achieved.The overall size of the system is reduced dramatically and the performance is improved significantly(8)A novel electrothermal MEMS based Monolithic FTS on chip(FTSOC)with both the movable mirror and fixed mirror vertically oriented and integrated and self-assembled for the first time.Both mirrors are single-crystal silicon based and their vertical orientation is attained by the initial bending results in the residual stress of proper designed thermal Bimorph array.According to the theoretical analysis,simulation and process calibration,the bending Bimorph array,a unique stopper mechanism and other parts are designed and optimized.The 9mm×9mm sized FTSCO is successfully fabricated and characterized,which has a capacity of ?2mm OPD scan range.A spectral experiment are demonstrated under the open-loop control,a spectral resolution of 0.56 nm is achieved.The promising result indicates that the large-stroke electrothermal MEMS based FTS can be fully integrated in chip-size with high spectral performance.