SU-8 based deep x-ray lithography/liga

Transcription

1 SU-8 based deep x-ray lithography/liga Linke Jian* a,b, Yohannes M. Desta a, Jost Goettert a, Martin Bednarzik b, Bernd Loechel b, Jin Yoonyoung a, Georg Aigeldinger a, Varshni Singh a, Gisela Ahrens c, Gabi Gruetzner c, Ralf Ruhmann c,reinhard Degen d a Center for Advanced Microstructures and Devices, Louisiana State University 6980 Jefferson Highway, Baton Rouge, LA 70806, USA b BESSY GmbH, Albert-Einstein-Strasse 15, Berlin, Germany c micro resist technology (MRT) GmbH, Köpenicker Str. 325, Berlin, Germany d Micromotion GmbH, An der Fahrt 13, Mainz, Germany ABSTRACT Poly-methylmethacrylate (PMMA), a positive resist, is the most commonly used resist for deep X-ray lithography (DXRL)/LIGA technology. Although PMMA offers superior pattern quality but it is also extremely insensitive. In this paper, we present our research results on SU-8 as negative resist for deep X-ray lithography. The results show that SU-8 is over two order of magnitude more sensitive to X-ray radiation than PMMA and the accuracy of the SU-8 microstructures fabricated by deep X-ray lithography is superior to UV-lithography and comparable to PMMA structures. The good pattern quality together with the high sensitivity offers rapid prototyping and direct LIGA capability. Moreover, the combinational use of UV and X-ray lithography as well as the use of positive and negative resists made it possible to fabricate complex multi-level 3D microstructures. The new process can be used to fabricate complex multi-level 3D structures for MEMS, MOEMS, Bio-MEMS and other micro-devices. Keywords: SU-8, X-ray lithography, 3D-lithography, MEMS, LIGA applications 1. INTRODUCTION LIGA, an acronym for the German words for lithography, electroplating, and molding, is a technique used to produce high aspect ratio microelectromechanical systems (MEMS) out of metals, ceramics, or plastics. The LIGA process utilizes X-ray radiation produced by synchrotrons as a lithographic light source 1. X-ray lithography is superior to optical lithography when it comes to the fabrication of tall, high aspect ratio microstructures due to the shorter wavelength, high penetration depth and larger depth of focus of the X-ray photons. High molecular weight poly-methylmethacrylate (PMMA), a positive resist, is the most commonly used resist for deep X-ray lithography (DXRL). Although PMMA offers superior pattern quality it is also extremely insensitive 2. A typical exposure time at the Center for Advanced Microstructures and Devices (CAMD) can take several hours for a 500 µm thick PMMA resist layer with 2 scanning length. In order to make the LIGA process more attractive to commercial users and provide an avenue for rapid prototyping, the exposure time should be significantly reduced while maintaining accuracy in typical LIGA microstructure features such as resolution, verticality and roughness of the sidewalls. One way of achieving this goal is by finding an alternative resist that is more sensitive to X-ray radiation. SU-8 is an epoxy based, chemically amplified, solvent developed negative resist; it is typically patterned with nm UV aligners. SU-8 is especially suitable for thick resist applications where high aspect ratio and resistance to harsh etching and plating conditions are required. SU-8 also possesses physical and photo-plastic properties that permit microstructures to be made of the resist itself 3,4,5,6,7. Furthermore, some previous investigations have shown the potential of SU-8 as resist in X-ray lithography 8,9,10,11. *Linke.jian@bessy.de; phone ; fax ; SPIE USE (p.1 of 12) / Color: No / Format: A4/ AF: A4 / Date: :22:36

2 For X-ray lithography in PMMA, three characteristic doses describe the exposure process: the damage dose, the development dose and the threshold dose. The definitions and their values of these characteristic doses are available for the positive resist PMMA 2. In this paper, we present our research results on SU-8 as negative resist for deep X-ray lithography. The new definition of characteristic dose values for SU-8 is given and their values are determined experimentally. High aspect ratio SU-8 microstructures, up to 1500 µm tall, have been prepared by deep X-ray lithography and the patterning accuracy of SU-8 microstructures has been measured. The results show that SU-8 is over two orders of magnitude more sensitive to X-ray radiation than PMMA and the accuracy of the SU-8 microstructures fabricated by deep X-ray lithography is superior to UV-lithography and comparable to PMMA structures. The good pattern quality together with the high sensitivity offers rapid prototyping and direct LIGA capability. Moreover, the combinational use of UV and X-ray lithography as well as the use of positive and negative resists made it possible to fabricate complex multi-level 3D microstructures. The new process can be used to fabricate complex multi-level 3D structures for MEMS, MOEMS, Bio-MEMS and other microdevices. 2. DEEP X-RAY LITHOGRAPHY OF SU-8 NEGATIVE RESIST 2.1 Pre- and post exposure processes SU-8 photo resist consists of EPON SU-8 resin, GBL solvent and a triaryl sulfonium salt as initiator. The resist used in this investigation is available commercially from MicroChem Corp. (MCC) of Massachusetts and manufactured in five standard grades with increasing viscosity, SU-8(5), SU-8(10), SU-8(25), SU-8(50), SU-8(100). In Germany, SU-8 resist is available from micro resist technology (MRT) GmbH. SU-8 can be spun in thickness ranges as determined by viscosity 6. Samples with thickness of SU-8 layer from 100 µm to 1500 µm have been prepared. The100 µm to 800 µm layers were single or multiple spin-coated while the 1000 µm and 1500 µm layers were cast on the substrates. Pre-baking is necessary for removing solvent from the resist layer after spin coating. The pre-bake was performed at 96 C for a time depending upon the thickness of the SU-8 resist. To keep internal stress to a minimum the sample was put into an oven at C and ramped to 96 C over a period of 5 min. Prior to taking the samples out of the oven the temperature is cooled down to below the SU-8 glass transition temperature (Tg=55 C). For the negative SU-8 resist, post exposure baking (PEB) is necessary to complete cross-linking reaction. This is critical to the final quality of the microstructures. The temperature and time of PEB should be optimized to get best result. A period of relax time before development is necessary to minimize internal stress. NANO SU-8 Developer, available commercially from MCC and MRT, was used in immersion development mode. SU-8 resist, cross-linked by X-ray exposure and post exposure bake, is virtually insoluble in the developer while the unexposed SU-8 resist will be fully developed. The development time is dependent on the thickness of the resist. For high aspect ratio structure, ultrasonic is used. Rinse is done with fresh developer, I.P.A and D.I. water. 2.2 Characteristic doses in deep X-ray lithography In deep X-ray lithography, there are three characteristic doses which are critical parameters, as shown in Figure 1 schematically. The definitions and values of these three characteristic doses for positive resist PMMA are as follows, Threshold dose (Dth): maximum dose underneath absorber to avoid development. For given light source, this dose determines the minimum Au absorber thickness. For PMMA, Dth =100 J/ccm. Development dose (Ddv): minimum dose needed at resist-substrate interface to completely dissolve the resist. For PMMA, Ddv =3,500 J/ccm Damage dose (Ddm): maximum dose allowed at top of resist before bubble formation occurs. For PMMA, Ddm =20,000 J/ccm For SU-8 negative resist the characteristic doses have to be redefined as follows, SPIE USE (p.2 of 12) / Color: No / Format: A4/ AF: A4 / Date: :22:36

3 Threshold dose (Dth): minimum dose required underneath the absorber to initiate a cross-linking reaction during postexposure bake (PEB). Typically if the dose is higher than Dth a skin-layer is built during the developing process. Development dose (Ddv): minimum dose needed at resist-substrate interface to complete cross-linking reaction so that the resist withstands the development. Damage dose (Ddm): maximum dose allowed at top of resist before any visible damage of the resist occurs. For SU-8 resist in DXRL, the values of the characteristic doses have been studied 10 and more detail results will be given in next section. X-ray Membrane Absorber D dm D th D dv Resist Figure 1: Schematic diagram of three characteristic doses used for X-ray lithography 2.3 Deep X-ray lithography experiments Initially samples with 100 µm thick SU-8 resist were used in experiments to determine the characteristic doses. In addition, samples with ranging from 100 µm to 1500 µm were used to confirm the values and further optimize the process parameters. Two of the X-ray masks which were used in the experiments to investigate the X-ray exposure parameters and the patterning accuracy of SU-8 microstructures are shown in Figure 2. Mask-1 was made from graphite membrane with gold absorber and there is an array of 4x4 similar patterns in this mask. Use of adjustable beam apertures allows exposing each row with different doses. It was used to investigate the effects of different exposure doses. Mask-2 consists of triangle pattern with 5µm smallest grating structures and rectangle pattern of different width with down to 5µm smallest feature size. This mask was made from titanium membrane with gold absorber and provided by the Institute of Mikrostrukturtechnik, Forschungszentrum Karlsruhe. It was used for pattern quality analysis 12. (a) Mask-1 (b) Mask-2 Figure 2: X-ray masks used for X-ray lithography of SU-8 negative resist SPIE USE (p.3 of 12) / Color: No / Format: A4/ AF: A4 / Date: :22:36

4 3. EXPERIMENTAL RESULTS AND DISCUSSION 3.1 Characteristic doses of X-ray lithography for SU-8 resist The values of the characteristic doses of deep X-ray lithography for SU-8 resist were determined by a series of deep X- ray exposure experiments. These values are listed in Table 1 and the values for PMMA are also included in the table for comparison. Table 1 Characteristic dose values of SU-8 resist Resist Characteristic SU-8 PMMA dose Damage dose (Ddm) / 20,000 J/ccm Development dose (Ddv) 10 J/ccm 3,500 J/ccm Threshold dose (Dth) 0.05 J/ccm 100 J/ccm Different from X-ray exposure to PMMA in which the energy of X-ray is used to scissoring the chain of the resist, the absorbed X-ray energy is initiating a cross-linking reaction making the SU-8 resist insoluble after PEB in the developing step. So the damage dose is not critical to SU-8 resist, any radiation damage such as bubbles or foaming has not been observed even with very high exposure doses up to 10,000 J/ccm. The development dose for the negative resist SU-8 was determined from our deep X-ray lithography experiments as 10 J/ccm. The X-ray dose deposited on the bottom of the SU-8 resist should be higher than this development dose. Compared to the PMMA, SU-8 is over two orders of magnitude more sensitive to X-ray radiation than PMMA. As a result, the typical exposure time of a layer of SU-8 is about 0.5-1% of that of PMMA. Exposed at threshold dose the SU-8 resist starts cross-linking during PEB. Therefore the dose absorbed in the surface of the SU-8 resist underneath the gold absorber of the X-ray mask must be less than this threshold-dose. Figure 4 illustrates the effect of different exposure doses at the resist surface. In case the absorbed dose exceeds the threshold dose, a skin-like layer of SU-8 was left after development, resulting in poor feature definition (Figure 4a, b, c and Figure 5a). Good pattern quality is achieved when keeping the absorbed dose low enough (Figure 4d and Figure 5b). The experimentally determined threshold dose for the negative resist SU-8 is about 0.05 J/cm 3. The threshold-dose is an important characteristic dose because it determines the minimum thickness of Au absorber on the X-ray mask for a given resist height. (a)bottom dose 400 J/cm 3 (b)bottom dose 300 J/cm 3 (c)bottom dose 200 J/cm 3 (d)bottom dose 100 J/cm 3 Figure 3: Microscope pictures of the test structures patterned into 100 µm SU-8 resist with different exposure doses SPIE USE (p.4 of 12) / Color: No / Format: A4/ AF: A4 / Date: :22:36

5 Figure 4: SEM pictures of the SU-8 structures patterned into 100 µm SU-8 resist with different exposure doses 3.2 The quality and accuracy of the SU-8 structures In order to analyze the patterning accuracy, mask-2 with 5µm smallest feature size, shown in Figure 2b, was used. WYKO NT3300 Optical Profiler has been used to evaluate the quality and accuracy of the SU-8 structures, the sidewall profile, in particular dimensional accuracy as a function of height and sidewall roughness, was measured with 400 µm tall SU-8 microstructures. Figure 5 shows a 400 µm high SU-8 grating structure with 20 µm steps used for measurement of roughness and profile of the sidewall. Figure 5: 400 µm high SU-8 grating Figure 6: Measurement of the sidewall surface roughness Position µm 300 µm 200 µm 100 µm Figure 7: Measurement of the profile of a 400 µm high SU-8 grating with 20 µm steps SPIE USE (p.5 of 12) / Color: No / Format: A4/ AF: A4 / Date: :22:36

6 The average roughness (R a ) of the sidewall was determined by the WYKO optical profiler is 26 nm (Figure 6). In comparison, the average R a of PMMA patterned under similar conditions is 30 nm 13. Figure 7 illustrates the profile of the grating and the widths of the steps at four locations along the height of the structure. The measurement result shows that the step-widths are reproduced with variations of no more than 0.1 µm. The overall quality and accuracy of the SU-8 microstructures produced by x-ray lithography is much better than those produced by UV lithography 5 and comparable with similar PMMA structures. 3.3 SU-8 microstructures fabricated by deep X-ray lithography Comparing with those characteristic doses of the PMMA, SU-8 is a much more sensitive X-ray resist. So the X-ray exposure time for SU-8 is dramatically shorter. Figs.9,10,11 show the examples of the 400 µm, 1000 µm and 1500 µm thick SU-8 microstructures fabricated by deep X-ray lithography. The exposure times of these structures are in the order of a few minutes. Figure 8: 400 µm tall grating with 5µm steps Figure 9:1000 µm tall microstructure Figure 10: 1500 µm tall microstructure 3.4 Fabrication of complex multi-level 3D structures The negative tone of the SU-8 resist is advantageous in fabricating complex multilevel and 3D-like structures. Multiple layers of SU-8 can be applied, baked and exposed one layer at a time with the necessary overlay registration being done prior to each exposure. After the final layer has been exposed and baked, all the layers can be developed at the same time. The result is a substrate with multi-level microstructures made of SU-8 resist as shown in Figure 11. The Figure 12 shows a micro-rotary engine housing which has a size of 1.6 mm by 1.6 mm and three multi-levels of 0.3 mm, 0.6 mm and 0.9 mm structure with smallest feature size of 15µm. The process is a combination of UV lithography and x-ray lithography with negative SU-8 resist. This micro-engine housing was fabricated by three aligned exposures; the high accuracy of the alignment exposure is made possible by the optical overlay system integrated in the DEX 02 Scanner (JENOPTIK Mikrotechnik GmbH, Jena, Germany), which allows multiple exposures and has an internal overlay alignment accuracy of ± 0.3 µm 10,11. A new combinational lithography process has been developed for fabricating multi-level complex 3D structures. This new process takes advantage of the different tones of resists and their different sensitivity to UV and X-ray radiation. It involves the use of UV lithography and X-ray lithography as well as the use of positive and negative resists. Figure 13 shows some structures (mirror array, cantilever array, multilevel channel, etc.) which were made by this new combinational process. The concept of this novel lithography process is shown in Figure 14. First, the alignment marks are patterned on a substrate. PMMA and SU-8 layers are then applied on the substrate. After the application of PMMA and SU-8 layers, the top SU-8 layer is patterned by use of UV or X-ray lithography. Since the radiation dose from the UV or X-ray radiation used for exposing the SU-8 layer is too small to sufficiently expose the underlying PMMA layer, there is almost no damage to the PMMA layer during this process. Finally, the underlying PMMA layer is patterned without damaging the upper cross-linked SU-8 layer. SPIE USE (p.6 of 12) / Color: No / Format: A4/ AF: A4 / Date: :22:36

7 Alignment marks UV photolithography Pattern alignment marks X-ray lithography Aligning pattern 1 st level to make bottom plate 1 st layer of SU-8 X-ray lithography Aligning pattern 2 nd level to make gear 2 nd layer of SU-8 1 st layer of SU-8 X-ray lithography Aligning pattern 3 rd level to make frame 3 rd layer of SU-8 2 nd layer of SU-8 1 st layer of SU-8 Figure 11: Fabrication process of three-levels structure of micro-engine housing (a) (b) Figure 12: A micro engine housing (a) made in a multi-level SU-8 process using UV and x-ray lithography and cross-section of the engine housing (b). SPIE USE (p.7 of 12) / Color: No / Format: A4/ AF: A4 / Date: :22:36

8 Figure: 13 Multi-level 3D structures made by the combinational lithography process Alignment marks UV photolithography Pattern alignment marks Apply PMMA and SU-8 layers Layer of SU-8 Layer of PMMA UV or X-ray lithography Pattern SU-8 layer with alignment X-ray lithography Pattern PMMA layer with alignment Figure 14: The concept of the combinational lithography process SPIE USE (p.8 of 12) / Color: No / Format: A4/ AF: A4 / Date: :22:36

9 4. LIGA PROCESS WITH SU-8 RESIST AND APPLICATION 4.1 Direct LIGA process In LIGA process, metallic micro-structures are made by replicating structures from photo-resists using electroplating. These metallic structures usually serve as mould insert for further mass replication steps. Direct mass fabrication of LIGA metal structures is not economical commercially due to the fact that low sensitivity of PMMA to X-ray and long irradiation time. Reduction in the irradiation time from hours to minutes through the use of SU-8 resist and hence the reduction in the cost of X-ray lithography make the direct mass fabrication of LIGA metal structures more attractive to commercial users. The joint, focused effort including Micromotion, micro resist technology, BESSY and CAMD has resulted the development of the fabrication concept based upon deep x-ray lithography in SU-8 resist, electroplating of NiFe alloy, polishing, release of metal structure from the SU-8 mold, and assembly of the Micro Harmonic Drive gear train 13,14. Low-cost and large pattern area of graphite membrane mask (Figure 15) increases the cost-effectiveness. Figure 16 shows SU-8 gear-structures of 500µm and 1mm height fabricated by deep x-ray lithography. These SU-8 gearstructures will be used for followed NiFe alloy electroplating. Figure show the results of a test Ni electroplating. Figure 15: X-ray mask, gold on graphite Figure 16: 500µm and 1mm thick SU-8 structures Figure 17: SU-8 micro gears Figure 18: Ni deposited into SU-8 mold Figure 19: Ni micro gears WYKO NT3300 Optical Profiler was used to evaluate the quality and accuracy of the SU-8 microstructures and its counterpart metallic microstructures replicated by electroplating process. SPIE USE (p.9 of 12) / Color: No / Format: A4/ AF: A4 / Date: :22:36

10 Fig.20 shows the profile, measured along the height of greater than 800 µm. It can be clearly seen from the Figure 20a that the Ni microstructure is curved, this is due to the bowing present in SU-8 microstructures (Figure 20b). The peak to valley depth varies from 4 to 8 µm from one structure to other. The bowing of SU-8 microstructures is likely due to the residual stresses coming from the cross-linking of SU-8 resist. To realize the straight walls for thickness of the order of 1000 µm, further studies are needed to optimize the processing parameters. Figure 20a: Sidewall profile of Ni microstructure Figure 20b Sidewall profile of SU-8 microstructure 4.2 LIGA process with PDMS intermediate molding In the standard LIGA technique, the patterned PMMA resist is used as mold for electroplating to make a metallic mold inserts. The patterned SU-8 microstructures can also be electroplated to make metallic mold inserts. However, the cross-linked SU-8 resist is resistant to most organic and inorganic resist strippers. In order to avoid the difficulties of SU-8 stripping, an intermediate process is introduced to replicate the SU-8 microstructures in an easy to remove elastomer material, Poly-dimethylsiloxane (PDMS). Figure are the SEM images of a three-level SU-8 microstructure fabricated by deep x-ray lithography, a PDMS mold replicated from the three-level SU-8 microstructure, SPIE USE (p.10 of 12) / Color: No / Format: A4/ AF: A4 / Date: :22:36

11 and the nickel mold insert fabricated by electroplating through the PDMS mold. The detail process and parameters for this multi-level mold insert were presented in SPIE Proceeding of Micromachining and Microfabrication Figure 21: Three-level SU-8 microstructure fabricated by a combination of UV and x-ray Figure 22: PDMS molds replicated from SU-8 microstructures Figure 23: Nickel mold insert 5. SUMMARY AND CONCLUSIONS Negative tone photo resist SU-8 has been used for deep X-ray lithography and the fabrication of high-aspect-ratio microstructure. The process conditions were investigated and optimized. The characteristic doses for SU-8 were defined and a series of X-ray exposure experiments were performed on samples of SU-8 resist ranging from µm. The values of these characteristic doses were determined from these experiments, that is, the threshold dose of 50 mj/ ccm, the development dose of 10 J/ ccm and the damage dose is not critical to negative SU-8 resist. Comparing with those characteristic doses of the PMMA, SU-8 is a much more sensitive X-ray resist. So the X-ray exposure time for SU-8 is dramatically shorter. The typical exposure time of a layer of SU-8 is less than 1% of that of PMMA. This time advantage offers rapid prototyping capability in deep X-ray lithography. The quality of the SU-8 microstructures fabricated by deep X-ray lithography is much better than UV-lithography and comparable with PMMA X-ray lithography. The smallest structure details patterned so far are 5 µm. They can be transferred into 400 µm thick resist with high dimensional accuracy, vertical and smooth sidewall. The negative tone of the SU-8 resist and its flexibility in resist application (multiple spin and cast) are the advantages in fabricating some 3D-like structures. SU-8 can be an alternative resist to the predominated positive resist PMMA in X-ray lithography. The new combinational lithography process, which includes the combinational use of positive PMMA and negative SU- 8 resists as well as the use of UV lithography and X-ray lithography, has been developed for fabricating multi-level complex 3D structures. SU-8 based LIGA process can be used in direct and cost-effective mass fabrication of LIGA metal structures and in the case of removing SU-8 is difficult, an intermediate PDMS molding process is introduced. ACKNOWLEDGEMENTS The authors acknowledge the financial support from the State of Louisiana and the DARPA8968. The authors appreciate the UC Berkeley's Combustion Processes Laboratories for the cooperation in fabricating micro engine and would also like to thank the Institute of Mikrostrukturtechnik, Forschungszentrum Karlsruhe for their generosity in providing the X-ray mask. SPIE USE (p.11 of 12) / Color: No / Format: A4/ AF: A4 / Date: :22:36

12 REFERENCE 1. J. Hruby, LIGA technologies and applications, MRS Bulletin, April M. Madou, Fundamentals of Microfabrication, CRC, K.Y. Lee, N. LaBianca, et al, "Micromachining applications of a high resolution ultra-thick photoresist, J. Vac. Sci. Technol. B13(6), H.Lorenz, M.Despont, N.Fahrni, J.Brugger, P.Renaud, and P.Vettiger, "High-aspect-ratio, ultra-thick, negative-tone near-uv photoresist and its applications for MEMS," Sensors and Actuators A 64, Z. G. Ling, K. Lian, and L. K. Jian, Improved Patterning Quality of SU-8 Microstructures by Optimizing the Exposure Parameters, Proc. SPIE 3999, 1019, C.K. Malek, Mask prototyping for ultra-deep X-ray lithography: preliminary studies for mask blanks and high-aspect-ratio absorber pattern, Proc. SPIE 3512, 227, A. Bogdanov, S. Peredkov, Use of SU-8 photoresist for very high aspect ratio X-ray lithography, Micro-, Nanoengineering 1999, Rome. 10. C. Cremers, F. Bouamrane, L.Singleton, R. Schenk, SU-8 as Resist Material for Deep X-ray Lithography, Microsystem Technologies, v.7,n.1, L.K. Jian, G. Aigeldinger, Y. Desta, and J.Goettert, SU-8 as Negative Resist in Deep X-ray Lithography, Fourth International Workshop on High-Aspect-Ratio Micro-Structure Technology Book of Abstracts, June 17-19, 2001, Baden-Baden, Germany. 12. S. Achenbach, Diplomarbeit, Karlsruhe University, L.K. Jian, et al, SU-8 Based Lithography Processes for Fabricating Complex Multi-level 3D Structures, Pacific Rim Workshop on Transducers and Micro/Nano Technologies, July 22-24, 2002, Xiamen, China 14. J. Goettert, et al, Cost Effective Fabrication of High Precision Microstructures Using a Direct LIGA Approach, COMS 2002, Ann Arbor, Michigan, USA 15. R. Degen, R. Slatter, Hollow shaft micro servo actuators realized with the Micro Harmonic Drive, ACTUATOR 2002, Bremen, Germany 16. L.K. Jian, et al, Multilayer Microstructures and Mold Inserts Fabricated with X-ray Lithography of SU-8 Negative Photoresist, SPIE 2001 Intel. Symposium on Micromachining and Microfabrication, San Francisco, USA SPIE USE (p.12 of 12) / Color: No / Format: A4/ AF: A4 / Date: :22:36