Computer Optics and Nanophotonics SU-8 BASED UV-LIGA FABRICATION PROCESS FOR REALIZATION OF NICKEL BASED MEMS INERTIAL SENSOR Payal Verma1, K. Zaman Khan2, S.A. Fomchenkov1, R. Gopal3 1 Samara National Research University, Samara, Russia 2 Semiconductor Technology and Applied Research Centre, Bangalore, India 3 CSIR – Central Electronics Engineering Research Institute, Pilani, India Abstract. This paper reports the complete fabrication process flow based on UV-LIGA technology for realization of metal based MEMS inertial sensor. In this process, nickel is used as the structural layer and copper as the sacrificial layer. The economical three mask process has been optimized and the detailed step by step procedure for carrying out the fabrication is presented. The opti- mized process parameters to achieve void free copper and nickel electroplated layers with extremely low roughness have been reported. Footing problem asso- ciated with lithography process has been analysed and its solution discussed. The fabrication results after each process step have been presented and dis- cussed. Scanning electron micrograph images of the released prototype inertial sensor devices have been presented to demonstrate the successful fabrication of the prototypes using the economical UV-LIGA process. Keywords: MEMS, UV-LIGA, SU-8 2010, Cu and Ni electroplating Citation: Verma Payal, Zaman Khan K, Fomchenkov SA, Gopal R. SU-8 based UV-LIGA fabrication process for realization of nickel based MEMS inertial sensor. CEUR Workshop Proceedings, 2016; 1638: 149-158. DOI: 10.18287/1613-0073-2016-1638-149-158 1 Introduction LIGA is a micro fabrication technique used to fabricate micro structures with high aspect ratio, from a variety of materials (plastics, metals and ceramics). LIGA is the German acronym for Lithographie, Galvanoformung (electro deposition), Abformung (molding). It was developed in the early 1980s at the Institute for Nuclear Process Engineering at the Karlsruhe Nuclear Research Center [1]. LIGA process is one of the few processes that offer lateral precision below one micrometer. LIGA finds application in the MEMS industry due its capability of forming molds from various materials with complex shapes and with high aspect ratio and reasonably good absolute tolerances, which is essential for the realization of high aspect ratio MEMS devices. The advantage of LIGA over other microfabrication techniques such Information Technology and Nanotechnology (ITNT-2016) 149 Computer Optics and Nanophotonics Verma Payal, Zaman Khan K et al… as bulk and surface micromachining is its capability of forming structures with com- parable dimensions not just in the lateral direction but also in the z-direction defining the thickness of the device. There are two major variations of LIGA namely, X-ray LIGA and UV-LIGA. X-ray LIGA is used for fabrication of microstructures with aspect ratio as high as 500:1 with lateral precision below one micrometer and parallel, smooth side walls. But the syn- chrotron source used to generate X-rays is expensive, hence rendering it out of reach for low cost production [2, 3]. However UV-LIGA has paved way for implementation of LIGA process in an economical manner [4]. Unlike the expensive X-ray absorbing mask plates used for X-ray lithography, UV lithography uses relatively cheaper coun- terparts made of chromium. In UV-LIGA, the thickness of the resist that can be used is limited to 150 to 200 µm, as the pattern suffers from distortion with increasing thickness of the resist, thereby limiting the aspect ratio of structures that can be real- ized. Hence there is a trade-off between the fabrication cost and process requirements. UV- LIGA process uses a polymer resist sensitive to UV-rays, which can be patterned using lithography techniques. After development of the resist, 3-dimensional structure with trenches is left behind on a conductive substrate into which the metallic struc- tures are electroplated. The resist can be coated to a thickness as per the required thickness of plating, considering practical limitations of the process. After electroplat- ing process, the resist is stripped leaving behind a metallic mold insert. This is fol- lowed by electroplating of the structural layer. This process can be used for realiza- tion of high aspect ratio metal based MEMS inertial sensor. Fig. 1. Solid model of the device Inertial sensor is used for measurement of angular rate of any moving object and finds application in diverse fields from consumer electronics to strategic applications such as Inertial Navigation systems. The device presented here is based on multi-DOF architecture to provide decoupled motion and increased robustness to fabrication and environmental variations. In this work, we present the process flow for fabrication of the inertial sensor using the economical fabrication technique, UV-LIGA. SU-8 mold formation and electroplating form the backbone of this in-expensive process, consist- ing of just three masking steps. Fabrication results after each unit process step have been presented and also the SEM images of the final released structure. Technology Information Technology and Nanotechnology (ITNT-2016) 150 Computer Optics and Nanophotonics Verma Payal, Zaman Khan K et al… of computer optics [5-12] is unable to provide the required parameters of MEMS- structures. 2 Design The schematic representation of the proposed inertial sensor structure is shown in Fig. 1. In this design, the structure is comprised of two masses, 𝑚1 and 𝑚2 , supported by flexures as shown in the figure. There is an intermediate frame mass, 𝑚𝑓 which acts as a decoupling mass. The spring, 𝑘𝑎 in the x-direction, connects the decoupling frame mass, 𝑚𝑓 , to an inner anchor.This structure is based on anchoring of the frame mass, which acts as a decoupling mass between the drive and sense masses. This con- figuration provides a reduced bandwidth and decoupled motion of the sense mass. The device has been designed considering fabrication compatibility with UV-LIGA process. The minimum feature size in the structure is 4µm gap between the comb fingers. The structure is designed with 8 µm x 8 µm perforations to aid in the sacrifi- cial release process. 3 UV-LIGA based fabrication process flow UV-LIGA process mainly consists of the well-established UV- Lithography process and Electroplating process. The process flow based on UV-LIGA implemented for fabrication of the MEMS inertial sensor is shown in Fig. 2. 3.1 Steps in UV-LIGA a) Silicon wafer is thermally oxidized in a furnace to form silicon-di-oxide layer of 1 µm thickness, for electrical isolation of the device from the substrate. b) 200 Å- 2000 Å Ti-Au is deposited, patterned and etched to form the metal inter- connects of the device. c) Again 200 Å- 2000 Å Ti-Au is deposited to form a seed layer for copper electro- plating (sacrificial layer). d) 8 µm SU-8 2010 photoresist mold is formed using optimized Lithography process for selective copper electroplating. e) Copper is electroplated to a thickness of 6 µm in the SU-8 resist mold using cop- per sulphamate bath with optimized process parameters to achieve non-porous copper layer. f) SU-8 resist is stripped using PG-remover and Plasma Asher. g) 11 µm SU-8 2010 photoresist mold is formed using Lithography for electroplating Nickel structural layer. h) 9 µm nickel electroplating is carried out in the SU-8 resist mold to form the struc- tural layer. i) SU-8 resist is stripped using PG-remover & Plasma Asher. This is followed by dicing operation to singulate the dies from the wafer. Sacrificial etching of copper Information Technology and Nanotechnology (ITNT-2016) 151 Computer Optics and Nanophotonics Verma Payal, Zaman Khan K et al… is carried out followed by etching of seed layer of Ti-Au (step c), leaving the nick- el structural layer freely suspended. Wiring metal Anchor Silicon Oxide layer pad metals Substrate (a) Thermally oxidized Silicon wafer (b) Pattern Ti/Au metallization pad on Silicon substrate (c) Sputter Ti/Au on the whole surface (d) Coat and pattern SU-8 photoresist as a seed layer as a mold for electroforming sacrificial copper (e) Electroplate copper as the sacrificial (f) Remove the photoresist mold used layer for copper electroplating (g) Coat and pattern thick photoresist (h) Electroplate nickel into the photoresist mold for nickel electroplating mold (i) Release the structures by removing structure mold and sacrificial Cu, Au and Ti Fig. 2. Fabrication steps of MEMS inertial sensor using SU-8 based UV-LIGA process 4 Fabrication results The prototype device has been fabricated using the process flow presented in this paper. Images of the fabricated device have been captured using Zeta optical profiler after each process step during fabrication. Fig. 3 (a) shows the image of the device after first lithography of the metal pattern and Fig. 3(b) shows the device after metal etch process. The formation of SU-8 2010 photoresist mold after UV exposure is shown in Fig. 3(c). Fig. 3(d) shows electroplated copper layer (captured after SU-8 2010 resist stripping). The bright portion is the Ti-Au layer while the rest of the area is plated with copper. The Ti-Au layer forms the anchors for the structural layer. The structural layer mold formation using SU-8 resist is shown in Fig. 3(e). The device Information Technology and Nanotechnology (ITNT-2016) 152 Computer Optics and Nanophotonics Verma Payal, Zaman Khan K et al… after Nickel electroplating and stripping of the SU-8 2010 resist is shown in Fig 3(f). The SEM image of the fabricated structure after sacrificial release is shown in Fig. 4. Fig. 3. Zeta optical profiler images of a MEMS inertial sensor captured after process Fig. 4. SEM image of the fabricated complete structure 5 Copper and Nickel electroplating results Copper electroplating is done at room temperature using copper sulphate solution to achieve ~6 µm thick sacrificial layer (Fig. 3(d)). Table 1 summarizes the operating conditions for copper electroplating process. Information Technology and Nanotechnology (ITNT-2016) 153 Computer Optics and Nanophotonics Verma Payal, Zaman Khan K et al… The above process parameters have been optimized after several electroplating trials by varying temperature, current density etc., to achieve a void free copper layer with fine grain size which is a very challenging art [13, 14]. Figure 5 shows the average surface roughness of the plated copper after (0.2 µm) process optimization measured using Zeta optical profiler. Similarly, the process parameters such as applied current and temperature have been optimized in order to achieve a shiny and uniform nickel layer using nickel sulfamate bath. The optimized parameters are summarized in Table 1. Figure 6 shows the surface roughness of the electroplated nickel layer during optimi- zation trials, profiled using Zeta optical profiler. Table 1. Operating conditions for copper and nickel electroplating process Parameters Condition for Cu Condition for Ni Plating current type DC DC Plating current density 10 A/ cm2 5 A/ cm2 Deposition rate 0.16 µm/min 0.1 µm/min Solution agitation Off 150 rpm Anode-cathode spacing 6.8 cm 6.4 cm Fig. 5. Improved surface of the deposited Cu after optimization. Fig. 6. Improved surface of the deposited Ni after optimization Information Technology and Nanotechnology (ITNT-2016) 154 Computer Optics and Nanophotonics Verma Payal, Zaman Khan K et al… 6 SU-8 2010 Removal After selective plating, SU-8 photoresist is removed. During stripping, a major con- cern is to ensure that the electroplated structure does not get damaged. This is argua- bly the most challenging step, because no solvent has been found that can simply dissolve SU-8 after hard bake. Several methods of striping SU-8 molds have been suggested and employed such as burning at 450-600 oC [15, 16], dissolving in molten salt bath at 350 oC [16], downstream chemical etching (DCE) at 225 oC [17, 18], plasma etching [Engelke et al. 2008] or solvent cracking at 75-80 oC. Therefore, diffi- culties associated with the removal of polymerized SU-8 remains a serious issue. In the fabrication of prototypes, N-methyl pyrrolidinone (NPM) based solvent Remover PG [http://www.microchem.com] at 75 oC has been used for removal of SU-8, which basically is peeled-off from the electroplated structures. To enhance the stripping, the implosion of ultrasonic agitation during the rinsing process is employed, which strikes the surface of the SU-8 mold and dislodges it from the electroplated structure. But, wet technique alone is found to be in-sufficient, as SU-8 residue still remained on the surface as shown in Fig. 7(a). For complete removal of resist, the wafer is ashed in a plasma stripper using CF4 and O2 gasses. This ensures the complete removal of SU-8 photoresist as shown in Fig. 7(b). (a) (b) Fig. 7. SU-8 photoresist stripping after Cu electroplating: (a) results after SU-8 removal in Remover PG; (b) results after plasma removal of SU-8 residue 7 SU-8 mold for realization of fine features in the structure The selectively Cu plated wafers are coated with primer (Omnicoat) in order to im- prove the adhesion between the electroplated copper and SU-8 photoresist. This is followed by drying the Omnicoat and coating of SU-8 photoresist at 1500 rpm and subsequent soft bake process which is done at 65 oC for a period of 7 min and then at a ramped up temperature of 95 oC for 14 min. The wafer and mask plate are held in- hard contact mode and exposed to UV light with wavelength of 365 nm. To ensure development at the bottom of the pattern, double exposure is given with each expo- sure lasting for 1.4 s with a brief interval of 10 s between the exposures. This is fol- lowed by a post exposure bake which is done in exactly the same conditions as soft bake explained above. After the post exposure bake, the resist is developed in SU-8 Information Technology and Nanotechnology (ITNT-2016) 155 Computer Optics and Nanophotonics Verma Payal, Zaman Khan K et al… Developer [http://www.microchem.com] for 30 s in stable condition and then stirring the solution for 30 s. The treated wafers are then immersed in Isopropanol for 30 s in stable condition and then stirred for 30 s. (a) (b) Fig. 8. Optimization for structure layer mold: (a) before optimization; (b) after optimization During this process, footing problem is encountered in comb fingers and beams (Fig. 8(a)). After some experiments, it is found that insufficient soft bake and post bake are the reason, which caused the resist to stick to copper. The baking processes have been Information Technology and Nanotechnology (ITNT-2016) 156 Computer Optics and Nanophotonics Verma Payal, Zaman Khan K et al… optimised by increasing the baking time up to 14 min for both 65 oC and 95 oC in soft bake and post bake respectively. This has resolved the issue (Fig. 8(b)). Conclusion Fabrication process based on UV-LIGA technique has been presented for realization of metal based MEMS inertial sensor. The process flow has been optimized after several trials and the same is presented in this paper. The structural nickel layer thick- ness implemented in the process is 9 µm and the minimum feature size is 4 µm, which is between the comb fingers. The process parameters optimized to achieve Copper and Nickel electroplated layers with surface roughness less than 0.2 µm and 0.02 µm respectively, have been presented. Footing problem associated with lithography pro- cess has been analysed and its solution discussed. The results of unit process steps and finally the SEM images of the fabricated MEMS inertial sensor are presented, demon- strating the successful fabrication of the prototype using UV-LIGA process. References 1. Madou MJ. Fundamentals of microfabrication. CRC Press, 2002. 2. Becker EW, Ehrfeld W, Hagmann P, Maner A, Munchmeyer D. Fabrication of microstruc- tures with high aspect ratios and great structural heights by synchrotron radition lithogra- phy, galvanoforming and plastic moulding (LIGA process). Microelectronics Engineering, 1986; 4: 35-56. 3. Kupka RK, Bouamrane, Cremers C, Megtert S. Microfabrication: LIGA-X and appli- cations. Applied Surface Science, 2000; 164: 97-110. 4. Verma P, Shekhar C, Arya SK, Gopal R. New design architecture of a 3 -DOF vibrato- ry gyroscope with robust drive operation mode and implementation. Microsystem Technologies, 2015; 21: 2175-2185. 5. Bruk MA, Zhikharev EN, Streltsov DR, Kalnov VA, Spirin AV, Rogozhin AE. Some pe- culiarities of a new method of microrelief creation by the direct electron-beam etching of resist. Computer Optics, 2015; 39(2): 204-210. DOI: 10.18287/0134-2452-2015-39-2-204- 210. 6. Kazanskiy NL, Moiseev OYu, Poletayev SD. Microprofile Formation by Thermal Oxida- tion of Molybdenum Films. Technical Physics Letters, 2016; 42(2): 164-166. DOI: 10.1134/S1063785016020085. 7. Kazanskiy NL, Khonina SN, Skidanov RV, Morozov AA, Kharitonov SI, Volotovskiy SG. Formation of images using multilevel diffractive lens. Computer Optics, 2014; 38(3): 425- 434. 8. Kazanskiy NL, Kolpakov VA, Podlipnov VV. Gas discharge devices generating the di- rected fluxes of off-electrode plasma. Vacuum, 2014; 101: 291-297. DOI: 10.1016/ j.vacuum.2013.09.014. 9. Kazanskiy NL. Research & Education Center of Diffractive Optics. Proceedings of SPIE, 2012; 8410: 84100R. DOI: 10.1117/12.923233. 10. Abul'khanov SR, Kazanskii NL, Doskolovich LL, Kazakova OY. Manufacture of diffrac- tive optical elements by cutting on numerically controlled machine tools. Russian Engi- neering Research, 2011; 31: 1268-1272. DOI: 10.3103/S1068798X11120033. Information Technology and Nanotechnology (ITNT-2016) 157 Computer Optics and Nanophotonics Verma Payal, Zaman Khan K et al… 11. Bezus EA, Doskolovich LL, Kazanskiy NL. Evanescent-wave interferometric nanoscale photolithography using guided-mode resonant gratings. Microelectronic Engineering, 2011; 88: 170-174. DOI: 10.1016/j.mee.2010.10.006. 12. Volkov AV, Kazanskiy NL, Moiseev OYu, Soifer VA. A Method for the Diffractive Mi- crorelief Formation Using the Layered Photoresist Growth. Optics and Lasers in Engineer- ing, 1998; 29: 281-288. 13. Takahashi KM. Electroplating copper onto resistive barrier films. Journal of Electrochemi- cal Society, 2000; 147: 1414-1417. 14. Flake J, Solomentsev Y, Cooper J, Cooper K. Wafer-scale profile evolution of electro- chemically deposited copper films. Journal of Electrochemical Society, 2003; 150: C195- C201. 15. Dentinger PM, Clift WM and Goods SH. Removal of SU-8 photoresist for thick film ap- plication, Microelectronic Engineering, 2002; 61-62: 93-100. 16. Joye CD, Calame JP, Garven M and Levush B. UV-LIGA microfabrication of 220 GHz sheet beam amplifier gratings with SU-8 photoresists, Journal Micromechanics and Micro- engineering, 2010; 20: 125016 (1-11). 17. Blain MG, Jarecki RL and Simonson RJ. Chemical downstream etching of tungsten, Jour- nal of Vacuum Science & Technology A, 1999; 16: 2115-2119. Information Technology and Nanotechnology (ITNT-2016) 158