UV-LIGA工艺中SU-8胶内应力和热溶胀性研究
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摘要
随着MEMS(微电子机械系统)技术的迅速发展,基于SU-8胶的UV-HGA技术得到了广泛的应用。SU-8厚光刻胶已经成功地应用于高深宽比微结构的制作。然而SU-8胶在工艺过程中会产生很大的内应力和热溶胀变形,这些问题的存在对SU-8胶微结构的深宽比及电铸结构的尺寸精度造成了严重影响。本文研究工作主要集中在SU-8胶的胶层应力和热溶胀性两方面,研究结果有利于提高SU-8胶工艺的稳定性和电铸结构的尺寸精度。
本文以基片曲率法为基础,建立了SU-8厚光刻胶的内应力计算模型。利用ANSYS仿真软件揭示了基片直径,胶层厚度及温度变化对基片曲率的影响。并实验测量了不同后烘温度条件下的胶层应力。结果表明,基片直径和胶层厚度对胶层应力影响较小,后烘温度的影响最大,且降低后烘温度可以有效减小胶层应力。
以SU-8胶层应力为指标,对SU-8胶的光刻工艺参数进行了优化。通过三因素三水平的正交实验,测量了九组不同工艺条件下SU-8胶层内应力的大小,研究了前烘温度、曝光剂量以及后烘温度对胶层应力大小的影响。以正交实验数据为样本,引入模糊神经网络对影响SU-8胶内应力的工艺参数进行了优化仿真研究,建立了SU-8胶内应力的大小与前烘温度,曝光剂量和后烘温度三者之间的预测模型。同时对网络预测结果进行了实验验证。根据网络优化结果,得到了SU-8胶的最佳光刻工艺参数。
在UV-LIGA工艺制作微模具的基础上,利用ANSYS软件对SU-8胶的热溶胀变形量进行了仿真计算,得出了沟道侧壁的溶胀变形趋势。通过研究微通道线宽随时间的变化规律,结合一级动力学方程式建立了热溶胀变形的速率模型。综合仿真结果和速率模型,可以计算不同电铸时间下微模具的顶部线宽。SU-8胶在电铸过程中作为电铸微结构的胶模,起着保证金属结构尺寸精度的关键作用。然而,SU-8胶在热的电铸液中会产生热溶胀变形,因此使得电铸微结构的尺寸小于光刻后微通道的尺寸。
研究了不同后烘温度对SU-8胶的热溶胀性的影响。溶胀实验结果表明,随着后烘温度的降低,SU-8胶的溶胀速率及溶胀变形量增大。但后烘温度较低时对应的胶层应力较小。因此,应综合考虑胶层的内应力和热溶胀性的影响,在保证微结构稳定性的基础上再采取适当措施提高电铸微结构的尺寸精度。
With the rapid development of MEMS, UV-LIGA technology based on SU-8 photoresist has been applied extensively. Thick SU-8 photoresist has been widely used to fabricate high aspect ratio structure. However, the aspect ratio and dimension error of electroforming are largely limited by the well-know internal stress and thermal swelling of SU-8 photoresist induced by processing. The primary work of this study is mainly concentrated on the two aspects. The research result is beneficial for improving the stability of SU-8 processing and dimensional precision of electroforming.
Based on the theory of curvature method, a theoretical formula for SU-8 internal stress is established. And a finite element analysis (ANSYS) is performed to investigate the influence of curvature which involves substrate diameter, film thickness and post-exposure bake (PEB) temperature. Internal stresses of SU-8 under different temperatures are measured through experiments. The results show that the PEB temperature is the main factor in developing the resulted internal stress, and the effects of film and substrate diameter thickness are subordinate. Moreover, experimental results show that lower PEB temperature can effectively reduce the internal stress.
The processing parameters of negative SU-8 photoresist are optimized in terms of internal stress. A 3~3 factorial orthogonal array technique is designed to measure the internal stress of SU-8 photoresist under nine different process parameters. According to the results of orthogonal experiment, a neural network is employed to investigate the influence of process parameters on the internal stress. And the prediction model is built between the internal stress and three main process parameters: soft bake temperature, exposure energy and PEB temperature. The prediction results are in good agreement with the experimental results. And the lithographic process of SU-8 is optimized with the application of neural network.
SU-8 photoresist is the electroforming mould for metal structure, and it keeps the dimensional precision of micro structure. However, the feature width of microchannel will be seriously reduced after electroforming due to the thermal swelling of patterned SU-8 in electrolyte. Based on the UV-LIGA process of micro-mold insert, the deformation of thermal swelling of SU-8 photoresist is investigated by ANSYS simulation. And the swelling trend of micro channel's sidewall is represented. Combined with the deforming rate of feature width and the equation of first-order dynamics, a velocity model of thermal swelling is established. Top width of micro-mold under different electroforming time is able to calculate through simulative results and the velocity model.
The influence of different PEB temperature on thermal swelling is measured through swelling experiments. Experimental results show that, the swelling deformation of SU-8 is larger when at a lower PEB temperature. However, lower PEB temperature can effectively reduce the internal stress. So, the influence of the two effects above should be considered interactively, and proper step should be done under the insurance of stability of micro structure to improve the dimensional error of electroforming.
本文以基片曲率法为基础,建立了SU-8厚光刻胶的内应力计算模型。利用ANSYS仿真软件揭示了基片直径,胶层厚度及温度变化对基片曲率的影响。并实验测量了不同后烘温度条件下的胶层应力。结果表明,基片直径和胶层厚度对胶层应力影响较小,后烘温度的影响最大,且降低后烘温度可以有效减小胶层应力。
以SU-8胶层应力为指标,对SU-8胶的光刻工艺参数进行了优化。通过三因素三水平的正交实验,测量了九组不同工艺条件下SU-8胶层内应力的大小,研究了前烘温度、曝光剂量以及后烘温度对胶层应力大小的影响。以正交实验数据为样本,引入模糊神经网络对影响SU-8胶内应力的工艺参数进行了优化仿真研究,建立了SU-8胶内应力的大小与前烘温度,曝光剂量和后烘温度三者之间的预测模型。同时对网络预测结果进行了实验验证。根据网络优化结果,得到了SU-8胶的最佳光刻工艺参数。
在UV-LIGA工艺制作微模具的基础上,利用ANSYS软件对SU-8胶的热溶胀变形量进行了仿真计算,得出了沟道侧壁的溶胀变形趋势。通过研究微通道线宽随时间的变化规律,结合一级动力学方程式建立了热溶胀变形的速率模型。综合仿真结果和速率模型,可以计算不同电铸时间下微模具的顶部线宽。SU-8胶在电铸过程中作为电铸微结构的胶模,起着保证金属结构尺寸精度的关键作用。然而,SU-8胶在热的电铸液中会产生热溶胀变形,因此使得电铸微结构的尺寸小于光刻后微通道的尺寸。
研究了不同后烘温度对SU-8胶的热溶胀性的影响。溶胀实验结果表明,随着后烘温度的降低,SU-8胶的溶胀速率及溶胀变形量增大。但后烘温度较低时对应的胶层应力较小。因此,应综合考虑胶层的内应力和热溶胀性的影响,在保证微结构稳定性的基础上再采取适当措施提高电铸微结构的尺寸精度。
With the rapid development of MEMS, UV-LIGA technology based on SU-8 photoresist has been applied extensively. Thick SU-8 photoresist has been widely used to fabricate high aspect ratio structure. However, the aspect ratio and dimension error of electroforming are largely limited by the well-know internal stress and thermal swelling of SU-8 photoresist induced by processing. The primary work of this study is mainly concentrated on the two aspects. The research result is beneficial for improving the stability of SU-8 processing and dimensional precision of electroforming.
Based on the theory of curvature method, a theoretical formula for SU-8 internal stress is established. And a finite element analysis (ANSYS) is performed to investigate the influence of curvature which involves substrate diameter, film thickness and post-exposure bake (PEB) temperature. Internal stresses of SU-8 under different temperatures are measured through experiments. The results show that the PEB temperature is the main factor in developing the resulted internal stress, and the effects of film and substrate diameter thickness are subordinate. Moreover, experimental results show that lower PEB temperature can effectively reduce the internal stress.
The processing parameters of negative SU-8 photoresist are optimized in terms of internal stress. A 3~3 factorial orthogonal array technique is designed to measure the internal stress of SU-8 photoresist under nine different process parameters. According to the results of orthogonal experiment, a neural network is employed to investigate the influence of process parameters on the internal stress. And the prediction model is built between the internal stress and three main process parameters: soft bake temperature, exposure energy and PEB temperature. The prediction results are in good agreement with the experimental results. And the lithographic process of SU-8 is optimized with the application of neural network.
SU-8 photoresist is the electroforming mould for metal structure, and it keeps the dimensional precision of micro structure. However, the feature width of microchannel will be seriously reduced after electroforming due to the thermal swelling of patterned SU-8 in electrolyte. Based on the UV-LIGA process of micro-mold insert, the deformation of thermal swelling of SU-8 photoresist is investigated by ANSYS simulation. And the swelling trend of micro channel's sidewall is represented. Combined with the deforming rate of feature width and the equation of first-order dynamics, a velocity model of thermal swelling is established. Top width of micro-mold under different electroforming time is able to calculate through simulative results and the velocity model.
The influence of different PEB temperature on thermal swelling is measured through swelling experiments. Experimental results show that, the swelling deformation of SU-8 is larger when at a lower PEB temperature. However, lower PEB temperature can effectively reduce the internal stress. So, the influence of the two effects above should be considered interactively, and proper step should be done under the insurance of stability of micro structure to improve the dimensional error of electroforming.
引文
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[8]李雄.UV-LIGA技术光刻工艺的研究:(硕士学位论文).武汉:华中科技大学,2004.
[9]Yoshihisa Sensu,Atsushi Sekiguchi,Satoshi Mori et al.Profile simulation of SU-8 thick film resist Advances in Resist Technology and Processing ⅩⅫ,2005,5753(2):1170-1185.
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[12]刘景全,朱军,丁桂甫等.用SU-8胶制高深宽比微结构的试验研究.微细加工技术,2002,1:27-29.
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[16]Li Bo,CHEN Q F.Low-stress ultra-thick SU-8 UV photolithography process for MEMS.Microlith.,Microfab.,Microsyst.,2005,4(4),043008:1-8.
[17]张金娅,陈迪等.超厚负胶高深宽比结构及工艺研究.功能材料与器件学报,2005,11(6):251-254.
[18]Ruzzu A,Matthis B.Swelling of PMMA-structures in aqueous solutions and room temperature Ni-electroforming.Mcirosystem Technologies,2002,8:116-119.
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[20]邵淑英,范正修,范瑞瑛,邵建达.薄膜应力研究。激光与光电子学进展,2005,42(1):22-27
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[23]赵若飞,周晓东等.纤维增强聚合物复合材料界面残余应力研究进展.纤维复合材料,2000,20(2):21-24.
[24]Zhang X,Zhang T Y,Zohar Y.Measurements of residual stresses in thin films using micro-rotating structures.Thin Solid Films,1998,335:97-105.
[25]K(a|¨)mpfe B.Investigation of residual stresses in microsystems using X-ray diffraction.Mater.Sci.Eng.,2000,288A:119-125.
[26]李美栓,辛丽等。拉曼光谱方法测量金属表面氧化膜的残余应力.中国腐蚀与防护学报,1999,19:185-188.
[27]杨于兴,胡赓祥。薄膜X射线应力分析技术研究。上海交通大学学报,1994,26(6):52-58.
[28]G.G.Stoney.The tension of metallic films deposited by electrolysis.Proc.Roy.Soc.London,1909,9:172-175.
[29]王莎莎,陈兢,栗大超等.基于局部基底弯曲法的高灵敏度薄膜应力测试技术.半导体学报,2006,27(6):1129-1135.
[30]钱劲,刘成,张大成,赵亚溥.微电子机械系统中的残余应力问题.机械强度,2001,23(4):393-401.
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[2]李永海,丁桂甫,毛海平,张永华.LIGA/准LIGA技术微电铸工艺研究进展.电子工艺技术,2005,26(1):1-5.
[3]张杨林.MEMS技术的发展现状及应用.电子工艺技术,2005,4:66-68.
[4]张立国,陈迪,杨帆等.SU-8胶光刻工艺研究.光学精密工程,2002,10(3):266-269.
[5]肖日松.微电铸工艺参数对模具质量影响研究:(硕士学位论文).大连:大连理工大学,2005.
[6]Lorenze H,Despont M,Fahrni N,et al.High-aspect-ratio,ultra thick,negative-tone near-UV-LIGA photoresist and its applications for MEMS[J].Sensors and Actuators A 64(1998):33-39.
[7]J.Zhang,K.L.Tan,H.Q.Gong.Characterization of the polymerization of SU-8 photoresist and its applications in micro-electro-mechanical systems(MEMS).Polymer Testing.2001,20(6):693-701.
[8]李雄.UV-LIGA技术光刻工艺的研究:(硕士学位论文).武汉:华中科技大学,2004.
[9]Yoshihisa Sensu,Atsushi Sekiguchi,Satoshi Mori et al.Profile simulation of SU-8 thick film resist Advances in Resist Technology and Processing ⅩⅫ,2005,5753(2):1170-1185.
[10]朱军,赵小林,倪智萍.SU-8光刻胶的应用工艺研究.微细加工技术,2001,2:57-60.
[11]G.Engelmannetal.Fabrication of perfectly three-dimensional microstructures by UV depth lithography.SPIE,2045:306-313.
[12]刘景全,朱军,丁桂甫等.用SU-8胶制高深宽比微结构的试验研究.微细加工技术,2002,1:27-29.
[13]H.Lorenze,et.al,High-aspect-ratio,ultra thick,negative-tone near-UV-LIGA photoresist and its applications for MEMS,Sensors and Actuators A 64(1998):33-39.
[14]Hyun-Kee Chang,Young-Kweon Kim,UV-LIGA process for high aspect ratio structure using stress barrier and C-shape etch hole,Sensors and Actuators A 84(2000):342-350.
[15]John D.Williams and Wanjun Wang,Study on the post-baking process and the effects on UV lithography of high aspect ratio SU-8 microstructures.Microlith.,Microfab.,Microsyst.,2004,3(4):563-568.
[16]Li Bo,CHEN Q F.Low-stress ultra-thick SU-8 UV photolithography process for MEMS.Microlith.,Microfab.,Microsyst.,2005,4(4),043008:1-8.
[17]张金娅,陈迪等.超厚负胶高深宽比结构及工艺研究.功能材料与器件学报,2005,11(6):251-254.
[18]Ruzzu A,Matthis B.Swelling of PMMA-structures in aqueous solutions and room temperature Ni-electroforming.Mcirosystem Technologies,2002,8:116-119.
[19]S K Griffiths,J A W Crowell,B L Kistler and A S Dryden,Dimensional errors in LIGA-produced metal structures due to thermal expansion and swelling of PMMA.J.Micromech.Microeng,2004,14:1548-1557.
[20]邵淑英,范正修,范瑞瑛,邵建达.薄膜应力研究。激光与光电子学进展,2005,42(1):22-27
[21]范玉殿,周志峰.薄膜内应力的起源。材料科学与工程,1996,14(1):5-12.
[22]Hass G,Thun T E.Physics of Thin Films.Academic Press,New York,1966.
[23]赵若飞,周晓东等.纤维增强聚合物复合材料界面残余应力研究进展.纤维复合材料,2000,20(2):21-24.
[24]Zhang X,Zhang T Y,Zohar Y.Measurements of residual stresses in thin films using micro-rotating structures.Thin Solid Films,1998,335:97-105.
[25]K(a|¨)mpfe B.Investigation of residual stresses in microsystems using X-ray diffraction.Mater.Sci.Eng.,2000,288A:119-125.
[26]李美栓,辛丽等。拉曼光谱方法测量金属表面氧化膜的残余应力.中国腐蚀与防护学报,1999,19:185-188.
[27]杨于兴,胡赓祥。薄膜X射线应力分析技术研究。上海交通大学学报,1994,26(6):52-58.
[28]G.G.Stoney.The tension of metallic films deposited by electrolysis.Proc.Roy.Soc.London,1909,9:172-175.
[29]王莎莎,陈兢,栗大超等.基于局部基底弯曲法的高灵敏度薄膜应力测试技术.半导体学报,2006,27(6):1129-1135.
[30]钱劲,刘成,张大成,赵亚溥.微电子机械系统中的残余应力问题.机械强度,2001,23(4):393-401.
[31]A.Mezin.Coating internal stress measurement through the curvature method.Surface & Coatings Technology 2006,200:5259-5267.
[32]Claude A.Klein.How accurate are Stoney's equation and recent modifications.Journal of Applied Physics,2000,88(9):5487-5489.
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