SU-8金属应变式微力传感器的设计与制作
|
摘要
微力传感器是微机电系统(MEMS)领域中不可缺少的一种传感器,在现有力传感器的研究中,能够对微小力测量的传感器相对较少。此外,在微装配和微操作过程中,为了避免破坏操作对象以及快速的完成操作,很多时候也都需要对力进行感知。本文设计了一款聚合物材料的微力传感器,并且采用了新颖的加工工艺将其制作出来,随后对其进行了相关的测试。
本文选用SU-8胶作为微力传感器的基底材料,金属铜作为电极和压阻,利用电阻应变计的应变原理,从理论上分析了它与常用半导体式微力传感器的性能差别。SU-8胶微力传感器为典型的双层悬臂梁结构,根据被测对象的大小,本文确定了传感器的大体尺寸,又根据摸索出的工艺经验,确定了传感器的具体结构尺寸,最终设计了五种不同金属线宽的微力传感器。SU-8胶作为一种负性光刻胶,良好的耐腐蚀性以及生物兼容性使得此款微力传感器的应用范围更加广泛。
为了制作SU-8微力传感器,本文探索研究了两种加工工艺。一种为剥离工艺,先加工出SU-8胶膜作为基底层,然后在上面涂胶,光刻出与金属层相反的结构,溅射金属铜后进行剥离操作。另一种为湿法腐蚀工艺,同样先加工出SU-8胶膜,然后在其上溅射金属铜,再涂胶,光刻出与金属层相同的结构,以这层结构为掩蔽层进行湿法腐蚀操作。最后再进行释放操作,就能得到此款微力传感器。
为了便于进行微力传感器的测试,本文设计制作了传感器的外围电路,其中包括惠斯顿电桥电路、三级放大滤波电路和电源电路。电桥电路将传感器的电阻变化量转换为电压变化量。三级放大滤波电路将电桥的输出信号由μV级别放大到mV级别,并进行一定的滤波处理。电源电路主要是为放大电路提供±12V的工作电压。
根据微力传感器的结构特点及测量需要,本文搭建了两套装置。一套为传感器的标定装置,其作用是完成对微力传感器偏转灵敏度的测量和力灵敏度的标定。标定结果显示10μm线宽的微力传感器综合性能最好,其力灵敏度为0.55mV/μN。另一套装置为微夹钳夹持力的测量装置,测量对象为本课题组设计制作的561钳口常闭型微夹钳。本文利用10μm线宽的微力传感器成功测量出了微夹钳不同钳口距离下的夹持力,并且将测量值与仿真值进行了比较。
Micro-force sensor is a kind of indispensable sensor in the field of microelectro mechanical systems (MEMS). However, there are relatively few sensors for micro force detecting among the existing sensors. Furthermore, in order to prevent operation from damaging the object and complete the operation quickly, force needs to be measured in the process of microassembry and micromanipulation. This paper presents the design of a polymer micro-force sensor and novel processing technologies are adopted to make it. At last some related tests are done.
SU-8is chosen as the substrate material for the micro-force sensor, copper as the electrode and the piezoresistance. Based on the strain theory of the resistance strain gauge, performance of the SU-8micro-force sensor is in comparison with that of the semiconductor piezoresistive micro-force sensor. SU-8micro-force sensor has a typical structure of double-layer cantilever. The specific structure size of the sensor is identified according to the size of measured object and explored process experience. And five kinds of micro-force sensors with different metal width are designed. As a negative photoresist, SU-8has good corrosion resistance and biological compatibility that makes the SU-8micro-force sensor more extensive in the scope of application.
In order to make the SU-8micro-force sensor, two kinds of fabrication process are investigated in this paper. One is the lift-off process, which begins with the processing of a SU-8film as the basal layer of the micro-force sensor, and then makes the structure opposite to metal layer by spin-coating and lithography, at last sputters copper and processes the lift-off technology. The other is the wet etching process, including processing a SU-8film, sputtering metal on it and patterning the masking layer structure which is same as metal layer for wet etching using spin-coating and lithography. Finally SU-8micro-force sensor is obtained after releasing process.
The sensor circuits are designed to facilitate the use of the SU-8micro-force sensor, including the Wheatstone bridge circuit, three-stage amplifying filter circuit and the power supply circuit. Resistance variation is converted into voltage variation by Wheatstone bridge circuit. And the μV-leveled signal is amplified with filter processing to mV-leveled one using the three-stage amplifying filter circuit. Power supply circuit mainly provides±12V voltage for the three-stage amplifying filter circuit.
For the structure features of the SU-8micro-force sensor and the measurement needs, this paper puts up two sets of device. One set is the calibration system of the micro-force sensor, which is used for the deflection sensitivity measurement and the force sensitivity calibration. The results show that the combination property of micro-force sensor with10μm width is the best, and the force sensitivity is0.55mV/μN. The other set is clamping force measuring device, and the measuring object is jaw normally close561microgripper which is designed by this research group. Using the micro-force sensor of10μm width, we successfully measure the clamping forces of different jaw and compare the measured values with the simulation values.
本文选用SU-8胶作为微力传感器的基底材料,金属铜作为电极和压阻,利用电阻应变计的应变原理,从理论上分析了它与常用半导体式微力传感器的性能差别。SU-8胶微力传感器为典型的双层悬臂梁结构,根据被测对象的大小,本文确定了传感器的大体尺寸,又根据摸索出的工艺经验,确定了传感器的具体结构尺寸,最终设计了五种不同金属线宽的微力传感器。SU-8胶作为一种负性光刻胶,良好的耐腐蚀性以及生物兼容性使得此款微力传感器的应用范围更加广泛。
为了制作SU-8微力传感器,本文探索研究了两种加工工艺。一种为剥离工艺,先加工出SU-8胶膜作为基底层,然后在上面涂胶,光刻出与金属层相反的结构,溅射金属铜后进行剥离操作。另一种为湿法腐蚀工艺,同样先加工出SU-8胶膜,然后在其上溅射金属铜,再涂胶,光刻出与金属层相同的结构,以这层结构为掩蔽层进行湿法腐蚀操作。最后再进行释放操作,就能得到此款微力传感器。
为了便于进行微力传感器的测试,本文设计制作了传感器的外围电路,其中包括惠斯顿电桥电路、三级放大滤波电路和电源电路。电桥电路将传感器的电阻变化量转换为电压变化量。三级放大滤波电路将电桥的输出信号由μV级别放大到mV级别,并进行一定的滤波处理。电源电路主要是为放大电路提供±12V的工作电压。
根据微力传感器的结构特点及测量需要,本文搭建了两套装置。一套为传感器的标定装置,其作用是完成对微力传感器偏转灵敏度的测量和力灵敏度的标定。标定结果显示10μm线宽的微力传感器综合性能最好,其力灵敏度为0.55mV/μN。另一套装置为微夹钳夹持力的测量装置,测量对象为本课题组设计制作的561钳口常闭型微夹钳。本文利用10μm线宽的微力传感器成功测量出了微夹钳不同钳口距离下的夹持力,并且将测量值与仿真值进行了比较。
Micro-force sensor is a kind of indispensable sensor in the field of microelectro mechanical systems (MEMS). However, there are relatively few sensors for micro force detecting among the existing sensors. Furthermore, in order to prevent operation from damaging the object and complete the operation quickly, force needs to be measured in the process of microassembry and micromanipulation. This paper presents the design of a polymer micro-force sensor and novel processing technologies are adopted to make it. At last some related tests are done.
SU-8is chosen as the substrate material for the micro-force sensor, copper as the electrode and the piezoresistance. Based on the strain theory of the resistance strain gauge, performance of the SU-8micro-force sensor is in comparison with that of the semiconductor piezoresistive micro-force sensor. SU-8micro-force sensor has a typical structure of double-layer cantilever. The specific structure size of the sensor is identified according to the size of measured object and explored process experience. And five kinds of micro-force sensors with different metal width are designed. As a negative photoresist, SU-8has good corrosion resistance and biological compatibility that makes the SU-8micro-force sensor more extensive in the scope of application.
In order to make the SU-8micro-force sensor, two kinds of fabrication process are investigated in this paper. One is the lift-off process, which begins with the processing of a SU-8film as the basal layer of the micro-force sensor, and then makes the structure opposite to metal layer by spin-coating and lithography, at last sputters copper and processes the lift-off technology. The other is the wet etching process, including processing a SU-8film, sputtering metal on it and patterning the masking layer structure which is same as metal layer for wet etching using spin-coating and lithography. Finally SU-8micro-force sensor is obtained after releasing process.
The sensor circuits are designed to facilitate the use of the SU-8micro-force sensor, including the Wheatstone bridge circuit, three-stage amplifying filter circuit and the power supply circuit. Resistance variation is converted into voltage variation by Wheatstone bridge circuit. And the μV-leveled signal is amplified with filter processing to mV-leveled one using the three-stage amplifying filter circuit. Power supply circuit mainly provides±12V voltage for the three-stage amplifying filter circuit.
For the structure features of the SU-8micro-force sensor and the measurement needs, this paper puts up two sets of device. One set is the calibration system of the micro-force sensor, which is used for the deflection sensitivity measurement and the force sensitivity calibration. The results show that the combination property of micro-force sensor with10μm width is the best, and the force sensitivity is0.55mV/μN. The other set is clamping force measuring device, and the measuring object is jaw normally close561microgripper which is designed by this research group. Using the micro-force sensor of10μm width, we successfully measure the clamping forces of different jaw and compare the measured values with the simulation values.
引文
[1]王立鼎,刘冲.微机电系统科学与技术发展趋势[J].大连理工大学学报,2000,40(5)：505-508.
[2]徐敬波,赵玉龙,蒋庄德,等.一种集成三轴加速度、压力、温度的硅微传感器[J].仪器仪表学报,2007,28(8)：1393-1398.
[3]杨静,高建忠,赵玉龙,等.一种MEMS热微执行器的设计与制作[J].MEMS器件与技术,2005,(4)：175-178.
[4]Duc T C, Lau G K, Creemer J F, et al. Electrothermal Microgripper With Large Jaw Displacement and Integrated Force Sensors[J]. JOURNAL OF MICROELECTROMECHANICAL SYSTEMS,2008,17(6):1546-1555.
[5]张彦国.聚合物微流控芯片超声波熔融键合技术研究[D].大连：大连理工大学,2009.
[6]王俊波,吴旻宪,崔铮,等.一种用于乳酸检测的新型微流控芯片[J].仪器装置与实验技术,2008,36(5)：710-714.
[7]关荣锋,王晓雪MEMS机油压力传感器设计[J].传感器与微系统,2009,28(6)：86-88.
[8]褚丹雷.非接触式压力微传感器的设计[J].仪表技术与传感器,2001,(12)：4-6.
[9]郑志霞,薛建国,陈军.基于MEMS加速度微传感器制作工艺及测试[J].莆田学院学报,2007,14(5)：59-61.
[10]程未,曾晓鹭,卞剑涛.基于MEMS技术的微电容式加速度传感器的设计[J].传感器技术,2003,22(8)：75-77.
[11]刘帅.柔性电热微夹钳的关键技术研究[D].大连：大连理工大学,2009.
[12]张成.SU-8柔性微夹钳的拓扑优化设计与制作工艺[D].大连：大连理工大学,2010.
[13]Chu H K, Mills J K, Cleghorn W L. Design of a High Sensitivity Capacitive Force Sensor[C]. International Conference on Nanotechonlogy, Hong Kong,2007:29-33.
[14]Muntwyler S, Beyeler F, Nelson B J. Three-axis micro-force sensor with sub-micro-Newton measurement uncertainty and tunable force range [J]. J. Micromech. Microeng,2010, (20):1-8.
[15]刘红梅.用于MEMS的PZT压电厚膜及硅基压电悬臂梁的制备研究[D].哈尔滨：哈尔滨工程大学,2008.
[16]卢晓光.压电薄膜微力传感器特性研究[D].大连：大连理工大学,2006.
[17]Yamashita K, Yang Y, Nishimoto T, et al. Piezoelectric Vibratory-Cantilever Force Sensors and Axial Sensitivity Analysis for Individual Triaxial Tactile Sensing[C]. IEEE,2011.
[18]Villanueva G, Plaza J A, Montserrat J, et al. Crystalline silicon cantilevers for piezoresistive detection of biomolecular forces [J]. ScienceDirect Microelectronic Engineering, 2008,(85):1120-1123.
[19]Duc T C, Creemer J F, Sarro P M. Piezoresistive Cantilever Beam for Force Sensing in Two Dimensions[J]. IEEE SENSORS JOURNAL,2007,7(1):96-104.
[20]Calleja M, Rasmussen P, Johansson A, et al. Polymeric mechanical sensors with integrated readout in a microfluidic system[C]. Proceedings of SPIE Smart Sensors, 2003,5116:314-321.
[21]Klejwa N, Harjee N, Kwon R, et al. TRANSPARENT SU-8 THREE-AXIS MICRO STRAIN GAUGE FORCE SENSING PILLAR ARRAYS FOR BIOLOGICAL APPLICATIONS[C]. IEEE,2007:2259-2262.
[22]Liu X Y,0'Brien M, Mwangi M, et al. PAPER-BASED PIEZORESISTIVE MEMS FORCE SENSORS[C]. MEMS 2011, Cancun, MEXICO,2011:133-136.
[23]Koyama A, Kan T, Iwase E, et al. FORCE SENSOR BASED ON METAL NANOPARTICLE[C]. IEEE, 2010:428-431.
[24]Lorenz H, Laudon M, Renaud P. Mechanical Characterization of a new high-aspect-ratio near UV-photoresist[J]. Microelectronic Engineering,1998,41-42:371-374.
[25]李雄.UV-LIGA技术光刻工艺的研究[D].武汉：华中科技大学,2004.
[26]Kouba J, Engelke R, Bednarzik M, et al. SU-8:promising resist for advanced direct LIGA applications for high aspect ratio mechanical microparts[J]. Microsystem Technologies,2007,13(3-4):311-317.
[27]Chronis N, Lee L P. Electrothermally Activated SU-8 Microgripper for Single Cell Manipulation in Solution[J]. Journal of Microelectromechanical Systems, 2005,14(4):857-863.
[28]Colinjivadi K S, Lee J B, Draper R. Viable cell handling with high aspect ratio polymer chopstick gripper mounted on a nano precision manipulator [J]. Microsystem Technologies,2008,17(9-11):1627-1633.
[29]RASMUSSEN P A, THAYSEN J, BOUWSTRA S, et al. Modular design of AFM probe with sputtered silicon tip[J]. Sensors and Actuators A,2001, (92):96-101.
[30]WOUTERS K, PUERS R. Determining the Young's modulus and creep effects in three different photo definable epoxies for MEMS applications[J]. Sensors and Actuators A,2009, (156):196-200.
[31]THAYSEN J, YALCINKAYA A D, VESTERGAARD R K, et al. SU-8 based piezoresistive mechanical sensor[C]. The Fifteenth IEEE International Conference,2002:320-323.
[32]THAYSEN J, YALCINKAYA A D, VETTIGER P, et al. Polymer-based stress sensor with integrated readout[J]. JOURNAL OF PHYSICS D, 2002, (35):2698-2703.
[33]Ibbotson R H, Dunn R J, Djakov V, et al. Polyimide microcantilever surface stress sensor using low-cost, rapidly-interchangeable, spring-loaded microprobe connections[J]. ScienceDirect Microelectronic Engineering, 2008, (85):1314-1317.
[34]Gammelgaard L, Rasmussen P A, Calleja M, et al. Microfabricated photoplastic cantilever with integrated photoplastic/carbon based piezoresistive strain sensor[J]. APPLIED PHYSICS LETTERS,2006,88(113508):1-3.
[35]Yang G Y, Wen Y H, Foldy C, et al. Sensor for Stiffness Measurements Within the Adult Rat Hippocampus[J]. IEEE SENSORS JOURNAL,2008,8(11):1894-1899.
[36]Doll J C, Nahid H, Klejwa N, et al. SU-8 force sensing pillar arrays for biological measurements[J]. Lab Chip,2009,9(10):1449-1462.
[37]黄长艺,严普强.机械工程测试技术基础[M].北京：机械工业出版社,2001.
[38]王树振,单威,宋玲玲.AD620仪用放大器原理与应用[J].微处理机,2008,(4)：38-40.
[39]周娟,袁良豪,曹德森.压力传感器信号调理电路设计[J].北京生物医学工程,2007,26(4)：395-398.
[40]康华光.电子技术基础[M].北京：高等教育出版社,2005.
[41]方凯.高温区高精度集成运放OP-07[J].微处理机,1990,(1)：1-10.
[42]段立颖,王荣,徐伟,等.LM118J-8和OP07运算放大器电路抗γ辐射实时模拟研究[J].核技术,2007,30(1)：34-36.
[43]刘德强.数字式低频函数信号发生器的设计[D].吉林：吉林大学,2010.
[44]张昱,潘武MEMS封装技术[J].纳米技术与精密工程,2005,3(3)：194-198.
[45]沈广平,秦明MEMS传感器的封装[J].电子工业专用设备,2006,(136)：28-35.
[46]Chu J k, Zhang R, Chen Z p. A novel SU-8 electrothermal microgripper based on the type synthesis of the kinematic chain method and the stiffness matrix method [J]. J. Micromech. Microeng,2011,21 (5):054030.
[2]徐敬波,赵玉龙,蒋庄德,等.一种集成三轴加速度、压力、温度的硅微传感器[J].仪器仪表学报,2007,28(8)：1393-1398.
[3]杨静,高建忠,赵玉龙,等.一种MEMS热微执行器的设计与制作[J].MEMS器件与技术,2005,(4)：175-178.
[4]Duc T C, Lau G K, Creemer J F, et al. Electrothermal Microgripper With Large Jaw Displacement and Integrated Force Sensors[J]. JOURNAL OF MICROELECTROMECHANICAL SYSTEMS,2008,17(6):1546-1555.
[5]张彦国.聚合物微流控芯片超声波熔融键合技术研究[D].大连：大连理工大学,2009.
[6]王俊波,吴旻宪,崔铮,等.一种用于乳酸检测的新型微流控芯片[J].仪器装置与实验技术,2008,36(5)：710-714.
[7]关荣锋,王晓雪MEMS机油压力传感器设计[J].传感器与微系统,2009,28(6)：86-88.
[8]褚丹雷.非接触式压力微传感器的设计[J].仪表技术与传感器,2001,(12)：4-6.
[9]郑志霞,薛建国,陈军.基于MEMS加速度微传感器制作工艺及测试[J].莆田学院学报,2007,14(5)：59-61.
[10]程未,曾晓鹭,卞剑涛.基于MEMS技术的微电容式加速度传感器的设计[J].传感器技术,2003,22(8)：75-77.
[11]刘帅.柔性电热微夹钳的关键技术研究[D].大连：大连理工大学,2009.
[12]张成.SU-8柔性微夹钳的拓扑优化设计与制作工艺[D].大连：大连理工大学,2010.
[13]Chu H K, Mills J K, Cleghorn W L. Design of a High Sensitivity Capacitive Force Sensor[C]. International Conference on Nanotechonlogy, Hong Kong,2007:29-33.
[14]Muntwyler S, Beyeler F, Nelson B J. Three-axis micro-force sensor with sub-micro-Newton measurement uncertainty and tunable force range [J]. J. Micromech. Microeng,2010, (20):1-8.
[15]刘红梅.用于MEMS的PZT压电厚膜及硅基压电悬臂梁的制备研究[D].哈尔滨：哈尔滨工程大学,2008.
[16]卢晓光.压电薄膜微力传感器特性研究[D].大连：大连理工大学,2006.
[17]Yamashita K, Yang Y, Nishimoto T, et al. Piezoelectric Vibratory-Cantilever Force Sensors and Axial Sensitivity Analysis for Individual Triaxial Tactile Sensing[C]. IEEE,2011.
[18]Villanueva G, Plaza J A, Montserrat J, et al. Crystalline silicon cantilevers for piezoresistive detection of biomolecular forces [J]. ScienceDirect Microelectronic Engineering, 2008,(85):1120-1123.
[19]Duc T C, Creemer J F, Sarro P M. Piezoresistive Cantilever Beam for Force Sensing in Two Dimensions[J]. IEEE SENSORS JOURNAL,2007,7(1):96-104.
[20]Calleja M, Rasmussen P, Johansson A, et al. Polymeric mechanical sensors with integrated readout in a microfluidic system[C]. Proceedings of SPIE Smart Sensors, 2003,5116:314-321.
[21]Klejwa N, Harjee N, Kwon R, et al. TRANSPARENT SU-8 THREE-AXIS MICRO STRAIN GAUGE FORCE SENSING PILLAR ARRAYS FOR BIOLOGICAL APPLICATIONS[C]. IEEE,2007:2259-2262.
[22]Liu X Y,0'Brien M, Mwangi M, et al. PAPER-BASED PIEZORESISTIVE MEMS FORCE SENSORS[C]. MEMS 2011, Cancun, MEXICO,2011:133-136.
[23]Koyama A, Kan T, Iwase E, et al. FORCE SENSOR BASED ON METAL NANOPARTICLE[C]. IEEE, 2010:428-431.
[24]Lorenz H, Laudon M, Renaud P. Mechanical Characterization of a new high-aspect-ratio near UV-photoresist[J]. Microelectronic Engineering,1998,41-42:371-374.
[25]李雄.UV-LIGA技术光刻工艺的研究[D].武汉：华中科技大学,2004.
[26]Kouba J, Engelke R, Bednarzik M, et al. SU-8:promising resist for advanced direct LIGA applications for high aspect ratio mechanical microparts[J]. Microsystem Technologies,2007,13(3-4):311-317.
[27]Chronis N, Lee L P. Electrothermally Activated SU-8 Microgripper for Single Cell Manipulation in Solution[J]. Journal of Microelectromechanical Systems, 2005,14(4):857-863.
[28]Colinjivadi K S, Lee J B, Draper R. Viable cell handling with high aspect ratio polymer chopstick gripper mounted on a nano precision manipulator [J]. Microsystem Technologies,2008,17(9-11):1627-1633.
[29]RASMUSSEN P A, THAYSEN J, BOUWSTRA S, et al. Modular design of AFM probe with sputtered silicon tip[J]. Sensors and Actuators A,2001, (92):96-101.
[30]WOUTERS K, PUERS R. Determining the Young's modulus and creep effects in three different photo definable epoxies for MEMS applications[J]. Sensors and Actuators A,2009, (156):196-200.
[31]THAYSEN J, YALCINKAYA A D, VESTERGAARD R K, et al. SU-8 based piezoresistive mechanical sensor[C]. The Fifteenth IEEE International Conference,2002:320-323.
[32]THAYSEN J, YALCINKAYA A D, VETTIGER P, et al. Polymer-based stress sensor with integrated readout[J]. JOURNAL OF PHYSICS D, 2002, (35):2698-2703.
[33]Ibbotson R H, Dunn R J, Djakov V, et al. Polyimide microcantilever surface stress sensor using low-cost, rapidly-interchangeable, spring-loaded microprobe connections[J]. ScienceDirect Microelectronic Engineering, 2008, (85):1314-1317.
[34]Gammelgaard L, Rasmussen P A, Calleja M, et al. Microfabricated photoplastic cantilever with integrated photoplastic/carbon based piezoresistive strain sensor[J]. APPLIED PHYSICS LETTERS,2006,88(113508):1-3.
[35]Yang G Y, Wen Y H, Foldy C, et al. Sensor for Stiffness Measurements Within the Adult Rat Hippocampus[J]. IEEE SENSORS JOURNAL,2008,8(11):1894-1899.
[36]Doll J C, Nahid H, Klejwa N, et al. SU-8 force sensing pillar arrays for biological measurements[J]. Lab Chip,2009,9(10):1449-1462.
[37]黄长艺,严普强.机械工程测试技术基础[M].北京：机械工业出版社,2001.
[38]王树振,单威,宋玲玲.AD620仪用放大器原理与应用[J].微处理机,2008,(4)：38-40.
[39]周娟,袁良豪,曹德森.压力传感器信号调理电路设计[J].北京生物医学工程,2007,26(4)：395-398.
[40]康华光.电子技术基础[M].北京：高等教育出版社,2005.
[41]方凯.高温区高精度集成运放OP-07[J].微处理机,1990,(1)：1-10.
[42]段立颖,王荣,徐伟,等.LM118J-8和OP07运算放大器电路抗γ辐射实时模拟研究[J].核技术,2007,30(1)：34-36.
[43]刘德强.数字式低频函数信号发生器的设计[D].吉林：吉林大学,2010.
[44]张昱,潘武MEMS封装技术[J].纳米技术与精密工程,2005,3(3)：194-198.
[45]沈广平,秦明MEMS传感器的封装[J].电子工业专用设备,2006,(136)：28-35.
[46]Chu J k, Zhang R, Chen Z p. A novel SU-8 electrothermal microgripper based on the type synthesis of the kinematic chain method and the stiffness matrix method [J]. J. Micromech. Microeng,2011,21 (5):054030.