Molding of Polymeric Microstructures - American Chemical Society

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Molding of Polymeric Microstructures T. Hanemann, V. Piotter, R. Ruprecht, and J. H. Hausselt Forschungszentrum Karlsruhe GmbH, Institut für Materialforschung III, Postfach 3640, 76021 Karlsruhe, Germany

Improved molding techniques allow for an economic mass production of micro components with lateral dimensions in the micrometer, structural details in the submicrometer range and high aspect ratios of up to 500. The capabilities of micro injection molding as well as examples of microstructures made from various polymers and related applications like pump housings, cell containers etc., will be demonstrated. Photomolding of UV curable resins as a new reaction injection molding process and related results dealing with optimized curing times and molding properties of different resin compositions will be discussed.

The market success o f microcomponents made from various metals, ceramics, polymers or composite materials strongly depends on the development o f an economic process technology. For low cost fabrication o f macroscopic technical plastic products, different molding techniques, especially injection molding, have been established. Hence, the attempt to adapt this reliable technology for the replication o f microcomponents was obvious. Actually the L I G A process developed at the Forschungszentrum Karlsruhe (FZK), Germany, ( L I G A is the german abbreviation o f lithography with synchrotron radiation, electroplating and molding) is one o f the most important technologies for the fabrication o f microstructures with lateral dimensions in the micrometer, structural details i n the submicrometer range, high aspect ratios (height to width ratio) o f up to several hundreds and a final average surface roughness o f less than 50 n m (7,2). The L I G A technique allows the fabrication o f m o l d inserts which contain the inverted surface topology o f the desired microcomponents. Alternatively, the mold inserts can be manufactured by various microstructuring techniques like mechanical ultraprecision milling, laser ablation (Laser L I G A ) , reactive ion etching, and lithography by means o f light or particle beams. They are suitable i n all relevant micromolding procedures like Thermoplastic Injection M o l d i n g (TIM), hot embossing as well as i n Reaction Injection M o l d i n g (RIM). In the following discussion, different injection molding techniques and some molded microstructures w i l l be presented.

© 1 9 9 8 American Chemical Society Ito et al.; Micro- and Nanopatterning Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

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Thermoplastic M i c r o Injection M o l d i n g Suitable and reliable molding techniques are basic preconditions for the economic production o f microsystem components in large scale series. In contrast to standard molding, micromolding requires modifications o f the molding machinery, the molding tool's construction as well as the adaptation o f the molding parameters to the small geometrical dimensions and the large flow length to wall thickness ratio (= aspect ratio). Injection molding o f L I G A microstructures has been developed at FZK since 1986. Basic differences to conventional injection molding machines are the higher precision and the integration o f peripheral equipment specific to microtechnology. The microstructures i n the L I G A or micromechanically fabricated mold inserts correspond to blind holes. A s a consequence the whole mold cavity has to be evacuated prior to the material's injection. Hence the machine periphery comprises a vacuum unit for evacuating the mold's cavity and temperature control units for each half o f the molding tool. A t FZK a full automatically driven two component Ferromatik Milacron K 5 0 S 2 F injection molding machine allows small and middle scale plastic microstructure fabrication (Figure 1). In cooperation with a machine vendor an a l l electric injection molding machine has been adapted for the requirements o f micromolding. In actuality, small and micro components made from various polymer materials with the following specifications can be molded: • plate-shaped microparts with microstructures o f any lateral form • volume o f the standard substrate base plate: 20 x 60 x 2 m m (width x length x height) • microstructure height up to 1.6 m m • smallest wall thickness down to 20 urn • smallest structural detail 0.2 um • aspect ratio up to 30 : 1 • suitable materials: P M M A , P C , P E , P S U , P O M , P A 1 2 , P E E K , etc. A typical micromolding cycle starts with closing o f the molding tool, evacuation o f the cavities and heating up to the injection temperature. A t injection temperature, the plastic melt is being injected into the mold. After complete filling the molding tool has to be cooled down to the demolding temperature followed by demolding, while at the same time new plastic material is conveyed and melted. The most critical steps are the molding and the demolding procedures. Thermal shrinkage during the cooling process has to be compensated by a corresponding holding pressure. A reliable demolding without any cracks or flaws is supported by plane parallel structural walls and the excellent surface quality o f L I G A microstructures with an average roughness o f less than 50 nm. Due to the time consuming heating and cooling steps the cycle times i n all micromolding techniques are considerably longer than those for conventional macroscopic molding techniques. A s a direct consequence, the reduction o f the cycle time is one o f the key issues for economic success o f microstructured products. In case o f micro injection molding, the shortest cycle times reported are 70s using P O M and a L I G A mold insert containing microstructures with an aspect ratio o f 2.5 (J). Using materials with higher viscosities and microstructures with larger aspect ratios the cycle times increase consequently. A s an example, injection molding o f high performance thermoplastics such as P E E K results i n a cycle time o f about 2 minutes using micromechanically made mold inserts carrying microstructures with an aspect ratio o f up to 5. In case o f higher aspect ratios, cycle times often amount up to ten minutes. Figure 2 shows the reduction o f the cycle time achieved at FZK since October 1994 for L I G A microstructures made from P M M A with aspect ratios o f up to 20. The installation o f a modified dual cycle temperature equalization followed by the optimization o f the molding parameters has resulted i n a total cycle time reduction o f more than 60 % (4). Latest experiments yield cycle times for automated manufacturing 3

Ito et al.; Micro- and Nanopatterning Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

Figure 1. F u l l automatical driven two component injection molding machine.

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Figure 2. Reduction of the injection molding cycle time since 1994.


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using P M M A o f less than 5 minutes i f the molding tool's geometry, the temperization system, and the molding parameters are being further optimized. Reaction Injection Molding (RIM). Thermally initiated reaction injection molding is another advanced technique for the fabrication o f microstructures. In addition to the described technique, basically only a mixing and metering unit for the cast resins as well as a temperature control for an accurate polymerization temperature has to be included. Due to the l o w viscosity o f the cast resins, a relatively simple and reliable mold filling and excellent molding accuracies are obtained. Thermally curable resins based on acrylates, methacrylates, amides and silicones are practical (5). To support demolding, internal mold release agents have to be added which influence the thermomechanical properties of the polymers. Applications The molding process represents the link between product development and a middle or large scale production. A s mentioned earlier the injection molding o f polymer microstructures is determined by the microstructure's aspect ratio as well as by the desired field o f application and the resulting material requirements which determine the selection o f the polymer. In the following some examples for successfully molded microcomponents are presented: • M i c r o injection molding o f microoptical benches and a L I G A microspectrometer for the NIR-range (structure height 145 um, grating step height approx. 1 urn, resulting aspect ratio « 160) using P M M A is under development towards an established molding technique (Figure 3) (6). The first commercial breakthrough was the fabrication o f a L I G A microspectrometer (spectral range from 380 - 750 nm, structure height 100 urn, grating step height 0.2 um, resulting aspect ratio o f 500 (7)) on a middle scale series o f approximately 4000 units by hot embossing o f thin P M M A foils (8). • M a m m a l tissue cells can be cultivated i f cubic shaped microcontainers (300 x 300 x 310 um ) with a wall thickness o f 50 um are used (Figure 4, left side). The injection molded P M M A structures possess small pyramidal rectangular holes (40 x 40 um ) with smallest size ( 6 x 8 um ) at the bottom side for the nutritious supply of the cells (P). • M i c r o membrane pumps with thermo-pneumatic actuators for the conveyance o f gases or liquids are mounted i n housings made from chemically and thermally resistant polymers ( P S U or P E E K ) for an extended lifetime (Figure 4, right side). • Electrostatically driven actuators like microvalves are integrated i n housings made from carbon black containing P A 12. They are used i n process technology and i n micro analytical systems. The diameters of the valves are 280 and 300 um with a smallest wall thickness o f 50 um (Figure 5, left side) (10). • A subsequent electroforming on conducting polymers (here: P A 12 with carbon black) allows the fabrication o f metal microstructures for e.g. spin nozzles test structures (Figure 5, right side) (4). For special applications micro components with high temperature stability or small thermal expansion coefficient are necessary. Hence, further process develop ments like micro casting as well as metal and ceramic micro injection molding targeted to the fabrication o f metal and ceramic microstructures are under investigation. (11,12) 3

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New Process Development: Photomolding of U V Curable Systems In thermally initiated reaction injection molding the relative slow polymerization starts at elevated temperatures (up to 150 °C) after filling o f the microstructured mold inserts with the l o w viscosity resin. Typical polymerization times necessary for a


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Figure 3. Mold insert and injection molded NIR-microspectrometer (PMMA).

Figure 4. Micro cell container (PMMA) and micro membrane pump housing (PSU).

Figure 5. Micro valve plates, spin nozzles (PA12/carbon black) and electroformed nickel raw structure.


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complete hardening o f the bulk material can take up to 30 m i n for a thickness o f 3 m m at a curing temperature o f 115 °C. After curing the solidified polymer can be demolded at temperatures between 40 and 80 °C i n case o f P M M A . Our process development now tries to combine the features o f reaction injection molding with the advantages o f the photopolymerization o f methylmethacrylate resins using suitable photoinitiators (75). A s a direct consequence the whole molding process can be realized at ambient temperature without any heating or cooling steps. Hence the cycling time should be reduced to lower values. The most important process characteristics are described in the following: • degassing of the resin prior to injection • variation o f the injection pressure from 0.1 up to 5 M P a • evacuation of the mold insert prior to the resin injection • use o f U V - A and - B transparent glass molding tool • holding pressure compensates the reaction shrinkage during curing • demolding by integrated ejector pins • integration o f a small sized powerful UV-source with homogeneous intensity distribution over a large illuminated area (at least 50 x 100 mm ) i n the experimental setup. The first design o f the laboratory injection molding equipment has been integrated i n a press with a maximum force o f 60 k N . In principle the setup consists o f two parts: the lower part with the light source and the upper part containing the molding tools. The lower glass molding tool faces the lamp, the upper molding tool carries the mold insert, ejector pins, connectors for vacuum pump and external temperature control as well as the polymer resin reservoir (14). 2

Photopolymerization Times. Commercially available liquid methylmethacrylate resins like Plexit (Roehm G m b H ) contain normally approximately 30 - 35 weight% polymer dissolved i n the parent monomer. Further increase i n the polymer concen tration results i n a corresponding increase o f the viscosity and cannot be used for reaction injection molding. In our experiments we decided to investigate either pure commercial Plexit, dilutions with the monomer ( M M A or Diluent30 (stabilized M M A ) ) or with Ethyltriglycolmethacrylate ( E T G M ) as a bulky comonomer with respect to the polymerization times and the brittleness o f the solidified material ( P M M A ) . Additionally, mixtures containing internal release agents like Butylstearate ( S B E ) or plastizers like Dibutylphthalate ( P S B E , Aldrich) were prepared (14). The photopolymerization time o f a polymer resin depends directly on the choice o f a suitable photoinitiator. The absorption o f a photoinitiator from Ciga Additive G m b H fits with the emission range o f an U V A/B-source (F-type). Commercial resins for inks or wood impregnation contain 0.5 - 5.0 weight% photoinitiator, further increase elevates normally the curing times due to a pronounced absorption o f the initiator close to the surface. The resulting reduction o f the effective light intensity i n the bulk material is known as self-quenching process (13). A s a consequence, first experiments have to evaluate the proper photoinitiator concentrations. With respect to the relative thick molded substrate base plate ( 1 - 5 mm) and the desired short polymerization times, a large irradiation density o f up to 600 m W / c m for the photopolymerization o f various resin compositions at different photoinitiator contents was applied. A l l given data were normalized to a thickness o f 5 m m (Figure 6). A more detailed investigation on pure Plexit yielded i n the fastest polymerization time o f 2.5 m i n @ 3.0 weight% initiator and 600 m W / c m irradiation density. In comparison to previous experiments with irradiation densities o f 250 m W / c m and 350 m W / c m a mean reduction o f the curing time o f 35% could be observed (Figure 7). Stronger dilutions with monomer or other additives extends the hardening process especially at small initiator concentrations. But i n all cases the polymerization times are strongly reduced i n comparison to thermally initiated reaction injection molding. 2

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Figure 6. Change o f the photopolymerization times with resin composition.

Figure 7. Reduction o f the curing times.


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F i r s t M o l d i n g Experiments. In case o f micro injection molding o f P M M A , the injection and holding pressures are between 50 - 70 M P a at an injection temperature around 230 °C. The photomolding approach reduces the injection and holding pressures to values between 0.5 and 3 M P a at ambient temperature. In micro injection molding, normally, thermal shrinkage o f the polymer material during cooling down supports demolding. Here, due to the isothermal process, successful demolding depends strongly on the static friction o f the mold insert and the polymer's surface, respectively, as well as on the polymer's elastic behaviour. A s a consequence, the surface roughness o f the mold insert has to be as low as possible, which is fulfilled i f the mold inserts are fabricated either micromechanically or by the L I G A technique. Surprisingly resin compositions containing the above mentioned intrinsic release agents or plastizers ( E T G M , P S B E , S B E ) show worse demolding properties due to a pronounced brittleness. The best results have been achieved using pure Plexit with 3 weight% photoinitiator. A series o f structureless parts with base plate thicknesses between 1 and 5 m m and a rough polymerization time o f 1 min/mm thickness could be molded i n good quality. The whole process development o f photoinduced reaction injection molding is targeted towards the molding o f microstructures. Successful experiments using pure Plexit containing 3 weight% photoinitiator and 1 weight% intrinsic release agent (Wuertz G m b H ) allowed the first molding o f polymer slabs (width: 300 um, depth: 600 um) on a substrate base plate (thickness 2 mm) using a micromechanically fabricated mold insert (Figure 8). It is expected that by the end o f the year molding o f slab structures with dimensions < 100 um w i l l be possible i n reliable quality. So, a quite elegant method would be available for the fabrication o f plastic microstructures e.g. waveguiding elements for future applications i n computing or telecommunication.

Figure 8. M o l d insert and R E M image o f photomolded part.

Acknowledgment W e gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft. Additionally we thank all who contributed successfully to this work.

Literature Cited 1. Becker, E.W.; Ehrfeld, W.; Hagmann, P.; Maner, A.; Münchmeyer, D. Microelectronic Engineering 1986, 4, 35. 2. Bacher, W.; Menz, W.; Mohr, J. IEEE Trans. Ind. Electr. 1995, 42(5), 431. 3. Michaeli, W.; Rogalla, A. F&M Feinwerktechnik 1994, 104, 641. 4. Piotter, V.; Hanemann, T.; Ruprecht, R.; Hauβelt, J. H. Microsystem Technologies 1997, 3(3), 129. 5. Hagmann, P.; Ehrfeld, W. Makromol. Chem. Macromol. Symp. 1989, 24, 241.


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6. Heckele, M.; Bacher, W.; Hanemann, T.; Ulrich, H. Precision Plastic Optics for Optical Storage, Displays, Imaging, and Communications; SPIE-Proc. 3135; SPIE: San Diego, CA, 1997; 06-12. 7. Heckele, M.; Bacher, W. Micromachine Devices 1997, 2(2), 1. 8. Müller, C.; Mohr, J. Interdisciplinary Science Reviews 1993, 18(3), 273. 9. Weibezahn, K.; Knedlitscheck, G.; Dertinger, H.; Schaller, T.; Schubert, K.; Bier, W., 1993, DP 4132379.3-41. 10. Fahrenberg, J.; Bier, W.; Maas, D.; Menz, W.; Ruprecht, R.; Schomburg, W.K.; Proc. Micro Mechanics Europe, Pisa, Italy, 1994; 17. 11. Piotter, V.; Hanemann, T.; Ruprecht, R.; Thies, A.; Hausselt, J. H. Micromachining and Microfabrication Technologies III; SPIE-Proc. 3223; SPIE: Austin, TX, 1997; 13-22. 12. Ruprecht, R.; Hanemann, T.; Piotter, V.; Haußelt, J.H. Proc. Micro Materials (Micro Mat '97); Berlin, Germany, 1997; 238. 13. Chang, C.-H.; Mar, A.; Tiefenthaler, A.; Wostratzky, D. in Handbook of Coatings Additives; Calbo, L. J., Ed.; Marcel Dekker, Inc.: New York, NY, 1992, Vol. 2; 1-43. 14. Hanemann, T.; Ruprecht, R.; Hauβelt, J. H. Polymeric Materials Science & Engineering 1997, 77, 418.


Molding of Polymeric Microstructures - American Chemical Society

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