- Email: [email protected]
Soft micromolding and lamination of piezoceramic thick films
Sensors and Actuators A 97±98 (2002) 512±519
Soft micromolding and lamination of piezoceramic thick ®lms Tobias Rosqvist*, Stefan Johansson The AÊngstroÈm Laboratory, Department of Materials Science, Uppsala University, Box 534, SE-75121 Uppsala, Sweden Received 26 September 2001; received in revised form 4 December 2001; accepted 5 December 2001
Abstract Combining ceramic thick ®lm technology and lithographically based microstructuring technology we have performed micromolding of PZT thick ®lms with lateral features down to 10 mm and aspect ratios up to 7:1 using soft reusable polydimethylsiloxane (PDMS) molds replicated from SU-8 resist masters. For multilayer applications microstructured tapes were laminated using a green tape adhesive and low pressure to preserve dimensional stability. Single and multilayer bodies were ®red and the resulting microstructures were studied. The presented work proves it possible to integrate channel microstructuring with multilayer ceramic technology opening up for numerous applications in areas of actuators, ¯uid handling components and ultrasonic transducers. Related microstructuring techniques for powderbased ceramics are compared regarding reached resolution and aspect ratio. # 2002 Elsevier Science B.V. All rights reserved. Keywords: PZT; Micromolding; PDMS; Lamination; Microchannels
1. Introduction The issue of microstructuring piezoceramics is important in several areas of applications where one wish to, e.g. incorporate channels for ¯uid transportation [1] in a multilayer electroceramic component or to fabricate microcomposites [2]. Piezoceramic/polymeric microcomposites are used in ultrasonic transducers where composite features below 20 mm are favorable for high frequency operation above 10 MHz [3]. Previous examples of piezoceramic microstructuring techniques are based on serial machining, such as sawing [2] or laser cutting [4], injection molding using metal molds [5], removal of sacri®cial material such as lost polymer molds [2,6,7] or lost silicon molds combined with hot isostatic pressing (HIP) technology [8]. Further, piezoceramic and other ceramic materials has been microstructured using stamping of prefabricated green ®lms [9,10] and some recent work has been done on soft micromolding (elastic reusable molds) of alumina [11] and PZT [12]. Freeform microfabrication of piezoceramic structures, such as ink-jet dispensing of ceramic slurries [13] has also been investigated. The conventional building techniques for multilayer electroceramic components is tape casting and lamination [14] where a ceramic slurry is cast onto a ¯exible foil using a doctor blade, forming green tapes. The tapes are electroded, cut, stripped from the carrier foil, aligned and laminated * Corresponding author. Tel.: 46-18471-7236; fax: 46-18471-3572. E-mail address: [email protected] (T. Rosqvist).
forming a multilayer green body which in turn is cut into components. Firing the components in an oven removes the binder and sinters the ceramic particles to a monolithic body. The lamination of green tapes is most often performed using thermomechanical compression [15], which may not be suitable for lamination of microstructured tapes due to signi®cant shear strains present, inducing poor dimensional stability. To improve dimensional stability during lamination of structured or patterned tapes, the idea of using a green tape adhesive to join stacked green tapes before ®ring [16] has been introduced. Integration of channels in multilayer ceramic components has been performed in an alumina micro heat exchanger [10] by solvent assisted lamination of structured tapes and in a piezoceramic ink-jet head [1] by sacri®cial removal of integrated polymer structures. The aim of the presented work is to evaluate high aspect ratio PZT microstructuring using a reusable soft PDMS mold for single and multilayer applications. Some preliminary results on soft micromolding of PZT have been presented previously [17]. Electrode integration and low-pressure adhesive assisted lamination of microstructured green tapes is evaluated to allow integration of active channel microstructures in a multilayer piezoceramic component aiming at a process that can be combined with tape casting and lamination. 2. Experimental An overview of the replication process utilizing soft PDMS micromolds is shown in Fig. 1. Soft micromolds
0924-4247/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 2 ) 0 0 0 2 7 - 4
T. Rosqvist, S. Johansson / Sensors and Actuators A 97±98 (2002) 512±519
513
Fig. 1. Schematic view of replication process showing: (A) silicon/SU-8 master, (B) PDMS molding and (C) mold removal, (D) ceramic slurry molding and (E) mold removal, (F) green ceramic body and (G) sintered ceramic body.
were fabricated using molding and hardening of a polydimethylsiloxane (PDMS) [18] elastomer precursor (Sylgard 184, Dow Chemicals) onto silicon wafers with lithographically de®ned SU-8 (SM1060, Sotec Microsystems) master structures, with lateral features down towards 10 mm. The exposure of the SU-8 was mainly performed in a Karl SuÈss MA6 mask aligner using conventional high-resolution Cr masks, but for the structures shown in Fig. 9 a Karl SuÈss MJB21 mask aligner was used together with low-cost polymer transparency masks resolving 2540 dpi. The PDMS bulk thickness of the molds was de®ned using 1 mm spacers, and the hardening was performed in an oven at 80 8C for 2 h. The molds were peeled from the masters after cooling. A ceramic slurry suitable for tape casting, with a solids loading (PZT powder and polymer binder) of about 40 vol.%, was prepared according to Table 1 by ®rst dispersing the PZT powder for 3 h in solvent and dispersant using a ball mill, then adding binder and plasticizer followed by a second milling step. Ethanol was chosen as solvent due to swelling of PDMS by nonpolar solvents such as toluene [18]. The slurry was cast onto the PDMS mold using a manually operated doctor blade, dried for 2 h in room temperature and peeled off, giving a microstructured green tape. The PDMS molds were rinsed thoroughly in ethanol, blown dry using N2 gas and reused several times. Green tapes with base thickness of about 50 mm and areas up to 50 mm 50 mm have been fabricated. To evaluate mold ®lling and avoid gas bubble entrapment in complex geometries, such as the structures shown in Figs. 7±9, a lowpressure casting technique was utilized where the slurry was dispensed dropwise through a manually moveable glass pipette onto the PDMS mold at 200 mbar using an air ejector pump (Epovac, Struers). To evaluate electrode integration in microstructured tapes, platinum electrode paste (E1192, FERRO) was screenprinted on dry green tapes while still in the mold using a
meshed screen and a handheld squeegee. The electrode paste was dried in 60 8C for 5 min and then cooled. A second PZT layer was cast on top, followed by a second electrode layer and ®nally topped by another PZT layer. To integrate channel structures in a multilayer ceramic component, microstructured green tapes were cut to squares with 10 mm side and laminated. The lamination was performed using a green tape adhesive (70:30 wt.% ethanol/ PPG; PPG 1000, Sigma, Aldrich), which was applied to the ¯at backside of tapes by a wetted wipe followed by application of the structured tape with microstructures facing the adhesive. The laminates (10 10 mm2) were pressed using dead weights up to 2 kg at 80 8C for 1 h and then cut into smaller pieces. Single and multilayer bodies were ®red in air at a peak temperature of 1280 8C for 1 h with slow ramping (0.5 8C/min) up to 500 8C allowing binder burnout. The samples were investigated using SEM (LEO 440, Zeiss DSM 960 A) and optical interferometry (WYKO NT2000). Green material cross-sections were prepared by breaking after immersion in liquid N2 and all samples investigated using SEM were gold sputtered before microscopy. 3. Results and discussion Examples of resulting microstructures at different steps in the replication process are shown in Figs. 2±5. The structures designed for micromolding evaluation consisted of microridges separated with a period of four times the ridge width, covering an area of 1 cm2. The height, h, and period, p, of the microridges, presented in Table 2, were measured in SEM cross-sections and the measurements were con®rmed using optical interferometry.
Table 1 Ceramic slurry composition Ingredient
Weight %
Ceramic powder (PZT) Solvent (ethanol) Dispersant Binder (PVB) Plasticizer (butyl benzyl phthalate)
61 (TRS 600 FG, TRS Ceramics) 32 0.5 (Solsperse 20000, Zeneca) 4.5 (B-98, Tape casting warehouse) 2 (S-160, Tape Casting Warehouse)
Fig. 2. SEM image of SU-8 ridges on silicon, serving as master for PDMS mold fabrication.
514
T. Rosqvist, S. Johansson / Sensors and Actuators A 97±98 (2002) 512±519 Table 2 Measurements from SEM cross-sections State
h sa (mm)
h/h0b (%)
p sa (mm)
p/p0b (%)
SU-8 master PDMS mold Green PZT Sintered PZT
117.8 116.5 110.1 86.9
100 99.0 93.5 73.8
80.0 78.6 75.1 60.8
100 98.2 93.9 75.9
a
Fig. 3. Cross-sectional SEM image of microstructured PDMS mold.
Fig. 4. SEM image of replicated PZT microridges in green state. The powder size is about 1 mm.
Fig. 5. SEM image of replicated PZT microridges in sintered state. The grain size is about 3 mm.
Fig. 2 shows an SU-8 master structure with 20 mm wide and 118 mm high ridges with a period of 80 mm. At these dimensions we encountered some sticking and collapse of the SU-8 structures but by optimizing the process [19] it
b
0.5 0.6 0.6 0.7
1.2 1.6 0.6 0.6
s is the standard deviation. h0 and p0 refers to the height and period of the master.
should be possible to fabricate narrow ridges at least twice as high. The slight nonverticality that can be seen in Fig. 2 is a typical feature due to energy loss during exposure by absorption and scattering in the thick resist [19]. A cut cross-section of the PDMS mold is shown in Fig. 3. The shrinkage due to cross-linking during hardening of the PDMS is measured to about 1%. The molds were free from defects such as trapped gas bubbles or PDMS sticking or collapse. However, sticking between adjacent features may become limiting when increasing the aspect ratio of freestanding mold features above 2:1 together with close packing [18]. Features about 110 mm high and 15 mm wide in the green state were successfully molded and demolded as shown in Fig. 4, giving an aspect ratio of above 7:1. Due to solvent evaporation there is an overall shrinkage of the green body during drying which is thought to facilitate mold separation, but it has been observed that cracks can be induced in the tape when the tape thickness is 50 mm or less, originating from sharp corners in the mold. The drying shrinkage is believed to be relaxed mainly through viscous/plastic ¯ow of the slurry/green body, but also to some extent through PDMS mold deformation. This is observed as a slight bending of the ®lled and dried mold. The drying shrinkage also induces waviness on the backside of tapes with the same period as the microreplicated structure as shown in Fig. 6. For 50 mm thick tapes the peak-to-peak waviness is 20 mm for 80 mm wide ridges, 10 mm for 40 mm wide ridges and not observable for the 20 mm wide ridges shown in Fig. 4. The
Fig. 6. Interferometric line scan (WYKO NT2000) of back surface profile together with a sketched front surface profile (dotted) of a 50 mm thick micromolded green tape with 40 mm wide and 100 mm high ridges.
T. Rosqvist, S. Johansson / Sensors and Actuators A 97±98 (2002) 512±519
Fig. 7. SEM image of micromolded pillars in green state.
Fig. 8. SEM image of sintered micromolded pillars 50 mm high and 10 mm wide.
PDMS molds were cleaned and reused about 10 times with no apparent signs of degradation. With conventional doctor blade casting at atmospheric pressure there may be gas bubble entrapment forming defects in the order of some 100 mm within the replicated structure, preferably in the mold corners. The replicas fabricated using low-pressure casting, shown in Figs. 7±9 show excellent ®lling characteristics and no signs of trapped gas defects. A combination of low-pressure casting and doctor blade casting has the prospect of good mold ®lling results combined with tape thickness control also for complex mold designs. The shrinkage of features due to sintering as shown from green state, Fig. 4, to sintered body, Fig. 5, was measured to
515
Fig. 9. SEM image of replicated and sintered maze showing 50 mm high and 20 mm wide walls.
be about 20%, see Table 2. When the features were very small, such as the pillars shown in Figs. 7 and 8, they were bent stochastically after sintering due to the fact that they were only a few grains wide. The minimum feature size obtained by the soft micromolding technique appears to be close to the limit set by the ®nal grain size. The ®nal grain size of commercially available powder-based PZT is typically kept at some micrometers and a signi®cant reduction in grain size would in general severely degrade the piezoelectric properties of the material [20]. The surface roughness of sintered ceramics can in some cases [9] be better than the ®nal grain size. A comparison of related ceramic microstructuring techniques [1,4±12] is presented in Table 3. The reached aspect ratio of the soft micromolding technique is high if compared to other microstructuring techniques suitable for multilayer fabrication but surpassed by the lost mold techniques, which lack this multilayer possibility. As a demonstration of the freedom in microstructure design and the capabilities of soft micromolding, Fig. 9 shows a replicated and sintered piezoceramic maze with 50 mm high and 20 mm wide walls. For the evaluation of electrode and channel integration the mold used featured ridges about 100 mm wide with aspect ratio of about 1:1 separated with a period of four times the width. The resulting green state PZT replicas had channel
Table 3 Comparison of results obtained using presented (italics) and related ceramic microstructuring techniques Microstructuring technique
Material
Grain size (mm)
Resolutiona (mm)
Techniques with multilayer potential Soft micromolding Soft micromolding [11] Soft micromolding [12] Stamping [9] Stamping [10] Space forming [1]
PZT Al2O3 PZT ZrO2±CeO2 PZT PZT
2±5 0.5b 3 2 2±5b 2±5b
10 2 35 4 100 20
Other techniques Lost polymer mold [6] Lost Si mold/HIP [8] Slip pressing [7] Centrifugal casting [7] Injection molding [5] Laser cutting [4]
PZT PZT Al2O3 PZT PZT PZT
2±5b ± ± 2±5 2±5 2±5b
25 12 20 35 70 50
a b
The resolution is defined as the finest ceramic feature successfully fabricated. Estimated grain size from presented material. Sintered PZT usually has a grain size of 2±5 mm.
Aspect ratio 7:1 1.5:1 6:1 3:1 5:1 1:1 10:1 14:1 >10:1 4:1 10:1 20:1
516
T. Rosqvist, S. Johansson / Sensors and Actuators A 97±98 (2002) 512±519
Fig. 10. SEM cross-section of sintered PZT microstructure with two integrated Pt electrode layers (light gray) showing the effect of drying shrinkage on backside waviness.
structures with a depth of 102 3 mm and at a period of 375 6 mm. A slight curvature was observed of the end faces to be laminated, interpreted as a result of drying shrinkage in the green body, and the peak deviation from a ¯at surface was measured to 3 mm for the approximately 300 mm wide structure. A curvature of this order, which can be seen in the sintered sample shown in Fig. 10 does not prevent successful lamination but is rather a problem for channel de®nition since the lamination pressure has to be increased. The deviations must be small compared to the channel size to allow for multilayer lamination with limited channel deformations. If one intends to use integrated microchannel structures for, e.g. active micro¯uidic applications, electrodes need to be integrated above and below or between the channels in the piezoceramic component. A typical appearance of a sintered microstructured tape with integrated platinum electrodes can be seen in Fig. 10. The uppermost electrode layer in Fig. 10 had a thickness of approximately 2 1 mm. The average thickness of the electrode bottom layer (near microstructured surface) was 2 mm above channels but 10 mm halfway between channels. During screen printing the screen acts as a spacer de®ning the amount of electrode paste to be deposited when pressed towards the green surface by the squeegee, and since the screen does not follow the surface waviness this will yield thicker electrode layers in the valleys between the channel structures. The thicker electrode layers increase the volume of inactive material and render a stiffer composite structure due to the higher
Young's modulus of platinum (168 GPa), and are therefore undesirable. The screen printing technique is not easily modi®ed to allow for structured surfaces and planarization is therefore the most attractive route to get uniform electrode thickness. Alternative methods for electrode material deposition that could yield uniform electrode thickness on structured surfaces and at the same time having a potential for low cost fabrication are, e.g. spray or ink-jet-based techniques. Compared to the ridges examined in the previous section the backside waviness of the ®rst cast ceramic layer is much more pronounced due to the wide mold structures to be ®lled by slurry. The second electrode layer shows that the waviness is more or less eliminated after a second cast ceramic layer, in this case at a tape thickness of less than 200 mm indicating the tape thickness needed for planarization of the molded structure. However, it was found that multilayer slurry casting on a PDMS mold might introduce bubbles in the green tape/mold interface as if air was pressed through the previous cast tape. Probably this is caused by capillary action of solvent ®lling the porous green body during subsequent casting. This phenomenon will be further analyzed in future work. Integration of screen-printed electrodes above and below integrated channels should be possible if proper alignment is achieved during screen printing and lamination. The waviness allows for introduction of electrodes embedded between replicated channel structures if the thickness of deposited electrode layers can be controlled or the thickness variation accepted. For multilayer lamination and channel integration, ethanol/PPG adhesive was applied to the backside of green tapes. The application of adhesive gave a glazed appearance of the tape surface and it was seen as a 5±10 mm in®ltrated layer, from now on referred to as the adhesive layer, in green tape cross-sections as shown in Fig. 11. This layer is believed to consist of softened and easy ¯owing green material and the effect of this layer upon lamination can be studied in Fig. 12, where different local lamination pressures has given channel and adhesive layer deformations to different extensions. The difference in local lamination pressure is discussed below. The mechanism of material deformation and ¯ow in the lamination interface at too high lamination pressure is
Fig. 11. (a) SEM cross-section of green tape with applied green tape adhesive, which can be seen as a 5±10 mm infiltrated thick darker layer at the top of the tape indicated by an arrow. (b) Close-up on adhesive layer.
T. Rosqvist, S. Johansson / Sensors and Actuators A 97±98 (2002) 512±519
517
Fig. 12. SEM cross-sections of laminated channels in green state at different local lamination pressures (a), (b) and (c) showing the effect of the adhesive layer.
Fig. 13. Sketched flow of the adhesive layer during lamination as interpreted from cross-sections.
interpreted as illustrated in Fig. 13, where the adhesive layer is outlined and shadowed. The adhesive layer should be able to ¯ow more easily than the bulk material, resulting in contact between the surfaces to be laminated with a lower deformation of the molded microstructures than otherwise would be needed, therefore, allowing integration of channels. Nonuniform adhesive layer thickness due to the manual deposition does not appear to in¯uence the lamination results. A too low lamination pressure has also shown to yield defects or voids along the lamination surface between the channel structures, consistent with the observed curvature of the end faces to be laminated as shown in Fig. 10. The difference in local lamination pressure in the series shown in Fig. 12 is probably an effect of thickness variations in the
microstructured tape and nonparallel application of the lamination pressure. Thickness control of the microstructured tapes is of outmost importance when laminating with solid dead weights since the allowed plastic deformation of features during lamination must be negligible compared to the microstructure feature size. Due to edge effects during slurry casting the tape thickness is believed to vary over the tape for the small mold size used (<50 50 mm2). Using larger area molds the problem with thickness variations over the tape may be diminished. Cross-sections of laminated and sintered chips with integrated channels are shown in Fig. 14. The chips shown were successfully laminated at 80 8C for 1 h using green tape adhesive and a dead weight (1973 g over an area of 1 cm2) giving an effective lamination pressure of 0.3 MPa at surfaces between channels. By using the same parameters, but without the adhesive, no or very poor lamination was achieved. The height of integrated channels after sintering was measured to 77 3 mm and the period to 302 2 mm and the shrinkage of channel structures from green dimensions presented earlier was 20% in period and 25% in height. The difference is believed to be caused by deformations during lamination since the values of shrinkage are expected to be accurate within 1% when using the same slurry recipe and lamination procedure. Accordingly, both these factors must be accounted for when designing new microchannel structures.
Fig. 14. SEM cross-sections of laminated and sintered channel chips showing: (a) an overview of two chips; (b) a close-up on a single channel. Lighter areas on the polished surfaces are drying stains.
518
T. Rosqvist, S. Johansson / Sensors and Actuators A 97±98 (2002) 512±519
Fig. 15. SEM micrographs of (a) laminated and sintered channel chip with (b) close-up on laminated microridge with lamination surface indicated by an arrow.
A sintered PZT chip with integrated channels separated by narrow walls, suitable for, e.g. cooling channels is shown in Fig. 15. This structure was laminated at 80 8C for 1 h using the green tape adhesive and a dead weight (1259 g over an area of 0.7 cm2) giving an average lamination pressure over the surfaces to be laminated of 0.7 MPa. The thickness of the walls and roof is about 20 mm. No lamination defects were observed in the lamination border, i.e. the bottom side of the channels in the ®gure, indicated by an arrow in Fig. 15b. A slight bending of the walls is observed, probably due to stresses during lamination. 4. Conclusions The soft micromolding technique yields a high resolution and a high aspect ratio of piezoceramic microstructures combined with geometrical freedom. The obtained results using soft micromolding are comparable with the lost mold technique with additional features of reusable molds and of green material post-processing, such as cutting and lamination. Lamination of microstructured tapes has been successfully performed with good dimensional stability using low-pressure adhesive assisted lamination with an ethanol/PPG green tape adhesive yielding piezoceramic components with integrated microchannel structures. Future work will focus on applications utilizing the presented technology in the area of advanced microactuators and micro¯uidic components. Acknowledgements This work was supported by the Swedish National Board for Industrial and Technical Development (NUTEK), which is gratefully acknowledged. References [1] K. Utsumi, Development of multilayer ceramic components using green-sheet technology, Ceram. Bull. 70 (1991) 1050±1055.
[2] V.F. Janas, A. Safari, Overview of fine-scale piezoelectric ceramic/ polymer composite processing, J. Am. Ceram. Soc. 78 (1995) 2945± 2955. [3] K. Lubitz, A. Wolff, G. Preu, Microstructuring technology, IEEE Ultrasonics Symp. 1 (1993) 515±524. [4] R. Farlow, W. Galbraith, M. Knowles, G. Hayward, Micromachining of a piezocomposite transducer using a copper vapor laser, IEEE Trans. Ultrasonics, Ferroelectrics Frequency Control 48 (2001) 639± 640. [5] L.J. Bowen, R.L. Gentilman, H.T. Pham, D.F. Fiore, K.W. French, Injection molded fine-scale piezoelectric composite transducers, IEEE Ultrasonics Symp. (1993) 499±503. [6] Y. Hirata, H. Nakaishi, T. Numazawa, H. Takada, Piezocomposite of fine PZT rods realized with synchrotron radiation lithography, IEEE Ultrasonics Symp. (1997) 877±881. [7] W. Bauer, H.-J. Ritzhaupt Kleissl, J.H. Hausselt, Slip casting of ceramic microcomponents, Microsyst. Technol. 4 (1998) 125±127. [8] S. Wang, J.-F. Li, R. Watanabe, M. Esahi, Fabrication of lead zirconate titanate microrods for 1±3 piezocomposites using hot isostatic pressing with silicon molds, J. Am. Ceram. Soc. 82 (1999) 213±215. [9] J.A. Bride, S. Baskaran, N. Taylor, J.W. Halloran, W.H. Juan, S.W. Pang, M. O'Donnell, Photolithographic micromolding of ceramics using plasma etched polyimide patterns, Appl. Phys. Lett. 63 (1993) 3379±3381. [10] R. Knitter, E. GuÈnther, C. Odemer, U. Maciejewski, Ceramic microstructures and potential applications, Microsyst. Technol. 2 (1996) 135±138. [11] U.P. Schonholzer, L.J. Gauckler, Ceramic parts patterned in the micrometer range, Adv. Mater. 11 (1999) 630±632. [12] S. Gebhardt, A. Schonecker, R. Steinhausen, T. Hauke, W. Seifert, H. Beige, Fine scale 1±3 composites fabricated by the soft mold process: preparation and modeling, Ferroelectrics 241 (2000) 67±73. [13] G. Thornell, L. Klintberg, T. Laurell, J. Nilsson, S. Johansson, Desktop microfabrication-initial experiments with a piezoceramic, J. Micromech. Microeng. 9 (1999) 434±437. [14] A.J. Bell, Multilayer ceramic processing, in: N. Setter, E.L. Colla (Eds.), Ferroelectric ceramics. Tutorial Reviews, Theory, Processing and Applications, BirkhaÈuser Verlag, Basel, 1993, pp. 241±271. [15] J.S. Sung, K.D. Koo, J.H. Park, Lamination and sintering shrinkage behavior in multilayered ceramics, J. Am. Ceram. Soc. 82 (1999) 537±544. [16] N. Suppakarn, H. Ishida, J.D. Cawley, Roles of poly(propylene glycol) during solvent-based lamination of ceramic green tapes, J. Am. Ceram. Soc. 84 (2001) 289±294. [17] T. Rosqvist, S. Johansson, High aspect ratio soft micromolding of piezoceramic thick films, in: Proceedings of the Transducers'01, Vol. 2, Springer, Munich, Germany, 2001, pp. 1022±1025. [18] Y. Xia, G.M. Whitesides, Soft lithography, Angew. Chem. Int. Ed. 37 (1998) 550±575.
T. Rosqvist, S. Johansson / Sensors and Actuators A 97±98 (2002) 512±519 [19] H. Lorenz, M. Despont, N. Fahrni, J. Brugger, P. Vettiger, P. Renaud, High-aspect-ratio, ultrathick, negative-tone near-UV photoresist and its applications for MEMS, Sensors Actuat. A 64 (1998) 33±39. [20] W. Hackenberger, M.J. Pan, V. Vedula, P. Pertsch, W.W. Cao, C. Randall, T. Shrout, Effect of grain size on actuator properties of piezoelectric ceramics, in: Proceedings of the SPIE, Vol. 3324, 1998, pp. 28±36.
Biographies Tobias Rosqvist was born in 1973 in LinkoÈping, Sweden, and graduated from LinkoÈpings Universitet 1997 with a MSc in physics. He is since 1998,
519
a member of the Microstructure Technology (MST) Group at the Department of Materials Science, Uppsala University, as a PhD student. His research interest is within the area of microactuators, focusing on piezoceramic microactuator technology. Stefan Johansson, born in 1960, received his PhD degree in Materials Science at Uppsala University in 1988. He became associate professor in Materials Science in 1994 and professor in Materials Science in 2000. His research area includes topics such as microrobotics, micromanipulation and testing of micromechanical components. During the last years, his main focus has been microactuation for miniature components and, in particular, research related to the fabrication of miniature devices based on piezoceramic materials.
Soft micromolding and lamination of piezoceramic thick ®lms Tobias Rosqvist*, Stefan Johansson The AÊngstroÈm Laboratory, Department of Materials Science, Uppsala University, Box 534, SE-75121 Uppsala, Sweden Received 26 September 2001; received in revised form 4 December 2001; accepted 5 December 2001
Abstract Combining ceramic thick ®lm technology and lithographically based microstructuring technology we have performed micromolding of PZT thick ®lms with lateral features down to 10 mm and aspect ratios up to 7:1 using soft reusable polydimethylsiloxane (PDMS) molds replicated from SU-8 resist masters. For multilayer applications microstructured tapes were laminated using a green tape adhesive and low pressure to preserve dimensional stability. Single and multilayer bodies were ®red and the resulting microstructures were studied. The presented work proves it possible to integrate channel microstructuring with multilayer ceramic technology opening up for numerous applications in areas of actuators, ¯uid handling components and ultrasonic transducers. Related microstructuring techniques for powderbased ceramics are compared regarding reached resolution and aspect ratio. # 2002 Elsevier Science B.V. All rights reserved. Keywords: PZT; Micromolding; PDMS; Lamination; Microchannels
1. Introduction The issue of microstructuring piezoceramics is important in several areas of applications where one wish to, e.g. incorporate channels for ¯uid transportation [1] in a multilayer electroceramic component or to fabricate microcomposites [2]. Piezoceramic/polymeric microcomposites are used in ultrasonic transducers where composite features below 20 mm are favorable for high frequency operation above 10 MHz [3]. Previous examples of piezoceramic microstructuring techniques are based on serial machining, such as sawing [2] or laser cutting [4], injection molding using metal molds [5], removal of sacri®cial material such as lost polymer molds [2,6,7] or lost silicon molds combined with hot isostatic pressing (HIP) technology [8]. Further, piezoceramic and other ceramic materials has been microstructured using stamping of prefabricated green ®lms [9,10] and some recent work has been done on soft micromolding (elastic reusable molds) of alumina [11] and PZT [12]. Freeform microfabrication of piezoceramic structures, such as ink-jet dispensing of ceramic slurries [13] has also been investigated. The conventional building techniques for multilayer electroceramic components is tape casting and lamination [14] where a ceramic slurry is cast onto a ¯exible foil using a doctor blade, forming green tapes. The tapes are electroded, cut, stripped from the carrier foil, aligned and laminated * Corresponding author. Tel.: 46-18471-7236; fax: 46-18471-3572. E-mail address: [email protected] (T. Rosqvist).
forming a multilayer green body which in turn is cut into components. Firing the components in an oven removes the binder and sinters the ceramic particles to a monolithic body. The lamination of green tapes is most often performed using thermomechanical compression [15], which may not be suitable for lamination of microstructured tapes due to signi®cant shear strains present, inducing poor dimensional stability. To improve dimensional stability during lamination of structured or patterned tapes, the idea of using a green tape adhesive to join stacked green tapes before ®ring [16] has been introduced. Integration of channels in multilayer ceramic components has been performed in an alumina micro heat exchanger [10] by solvent assisted lamination of structured tapes and in a piezoceramic ink-jet head [1] by sacri®cial removal of integrated polymer structures. The aim of the presented work is to evaluate high aspect ratio PZT microstructuring using a reusable soft PDMS mold for single and multilayer applications. Some preliminary results on soft micromolding of PZT have been presented previously [17]. Electrode integration and low-pressure adhesive assisted lamination of microstructured green tapes is evaluated to allow integration of active channel microstructures in a multilayer piezoceramic component aiming at a process that can be combined with tape casting and lamination. 2. Experimental An overview of the replication process utilizing soft PDMS micromolds is shown in Fig. 1. Soft micromolds
0924-4247/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 2 ) 0 0 0 2 7 - 4
T. Rosqvist, S. Johansson / Sensors and Actuators A 97±98 (2002) 512±519
513
Fig. 1. Schematic view of replication process showing: (A) silicon/SU-8 master, (B) PDMS molding and (C) mold removal, (D) ceramic slurry molding and (E) mold removal, (F) green ceramic body and (G) sintered ceramic body.
were fabricated using molding and hardening of a polydimethylsiloxane (PDMS) [18] elastomer precursor (Sylgard 184, Dow Chemicals) onto silicon wafers with lithographically de®ned SU-8 (SM1060, Sotec Microsystems) master structures, with lateral features down towards 10 mm. The exposure of the SU-8 was mainly performed in a Karl SuÈss MA6 mask aligner using conventional high-resolution Cr masks, but for the structures shown in Fig. 9 a Karl SuÈss MJB21 mask aligner was used together with low-cost polymer transparency masks resolving 2540 dpi. The PDMS bulk thickness of the molds was de®ned using 1 mm spacers, and the hardening was performed in an oven at 80 8C for 2 h. The molds were peeled from the masters after cooling. A ceramic slurry suitable for tape casting, with a solids loading (PZT powder and polymer binder) of about 40 vol.%, was prepared according to Table 1 by ®rst dispersing the PZT powder for 3 h in solvent and dispersant using a ball mill, then adding binder and plasticizer followed by a second milling step. Ethanol was chosen as solvent due to swelling of PDMS by nonpolar solvents such as toluene [18]. The slurry was cast onto the PDMS mold using a manually operated doctor blade, dried for 2 h in room temperature and peeled off, giving a microstructured green tape. The PDMS molds were rinsed thoroughly in ethanol, blown dry using N2 gas and reused several times. Green tapes with base thickness of about 50 mm and areas up to 50 mm 50 mm have been fabricated. To evaluate mold ®lling and avoid gas bubble entrapment in complex geometries, such as the structures shown in Figs. 7±9, a lowpressure casting technique was utilized where the slurry was dispensed dropwise through a manually moveable glass pipette onto the PDMS mold at 200 mbar using an air ejector pump (Epovac, Struers). To evaluate electrode integration in microstructured tapes, platinum electrode paste (E1192, FERRO) was screenprinted on dry green tapes while still in the mold using a
meshed screen and a handheld squeegee. The electrode paste was dried in 60 8C for 5 min and then cooled. A second PZT layer was cast on top, followed by a second electrode layer and ®nally topped by another PZT layer. To integrate channel structures in a multilayer ceramic component, microstructured green tapes were cut to squares with 10 mm side and laminated. The lamination was performed using a green tape adhesive (70:30 wt.% ethanol/ PPG; PPG 1000, Sigma, Aldrich), which was applied to the ¯at backside of tapes by a wetted wipe followed by application of the structured tape with microstructures facing the adhesive. The laminates (10 10 mm2) were pressed using dead weights up to 2 kg at 80 8C for 1 h and then cut into smaller pieces. Single and multilayer bodies were ®red in air at a peak temperature of 1280 8C for 1 h with slow ramping (0.5 8C/min) up to 500 8C allowing binder burnout. The samples were investigated using SEM (LEO 440, Zeiss DSM 960 A) and optical interferometry (WYKO NT2000). Green material cross-sections were prepared by breaking after immersion in liquid N2 and all samples investigated using SEM were gold sputtered before microscopy. 3. Results and discussion Examples of resulting microstructures at different steps in the replication process are shown in Figs. 2±5. The structures designed for micromolding evaluation consisted of microridges separated with a period of four times the ridge width, covering an area of 1 cm2. The height, h, and period, p, of the microridges, presented in Table 2, were measured in SEM cross-sections and the measurements were con®rmed using optical interferometry.
Table 1 Ceramic slurry composition Ingredient
Weight %
Ceramic powder (PZT) Solvent (ethanol) Dispersant Binder (PVB) Plasticizer (butyl benzyl phthalate)
61 (TRS 600 FG, TRS Ceramics) 32 0.5 (Solsperse 20000, Zeneca) 4.5 (B-98, Tape casting warehouse) 2 (S-160, Tape Casting Warehouse)
Fig. 2. SEM image of SU-8 ridges on silicon, serving as master for PDMS mold fabrication.
514
T. Rosqvist, S. Johansson / Sensors and Actuators A 97±98 (2002) 512±519 Table 2 Measurements from SEM cross-sections State
h sa (mm)
h/h0b (%)
p sa (mm)
p/p0b (%)
SU-8 master PDMS mold Green PZT Sintered PZT
117.8 116.5 110.1 86.9
100 99.0 93.5 73.8
80.0 78.6 75.1 60.8
100 98.2 93.9 75.9
a
Fig. 3. Cross-sectional SEM image of microstructured PDMS mold.
Fig. 4. SEM image of replicated PZT microridges in green state. The powder size is about 1 mm.
Fig. 5. SEM image of replicated PZT microridges in sintered state. The grain size is about 3 mm.
Fig. 2 shows an SU-8 master structure with 20 mm wide and 118 mm high ridges with a period of 80 mm. At these dimensions we encountered some sticking and collapse of the SU-8 structures but by optimizing the process [19] it
b
0.5 0.6 0.6 0.7
1.2 1.6 0.6 0.6
s is the standard deviation. h0 and p0 refers to the height and period of the master.
should be possible to fabricate narrow ridges at least twice as high. The slight nonverticality that can be seen in Fig. 2 is a typical feature due to energy loss during exposure by absorption and scattering in the thick resist [19]. A cut cross-section of the PDMS mold is shown in Fig. 3. The shrinkage due to cross-linking during hardening of the PDMS is measured to about 1%. The molds were free from defects such as trapped gas bubbles or PDMS sticking or collapse. However, sticking between adjacent features may become limiting when increasing the aspect ratio of freestanding mold features above 2:1 together with close packing [18]. Features about 110 mm high and 15 mm wide in the green state were successfully molded and demolded as shown in Fig. 4, giving an aspect ratio of above 7:1. Due to solvent evaporation there is an overall shrinkage of the green body during drying which is thought to facilitate mold separation, but it has been observed that cracks can be induced in the tape when the tape thickness is 50 mm or less, originating from sharp corners in the mold. The drying shrinkage is believed to be relaxed mainly through viscous/plastic ¯ow of the slurry/green body, but also to some extent through PDMS mold deformation. This is observed as a slight bending of the ®lled and dried mold. The drying shrinkage also induces waviness on the backside of tapes with the same period as the microreplicated structure as shown in Fig. 6. For 50 mm thick tapes the peak-to-peak waviness is 20 mm for 80 mm wide ridges, 10 mm for 40 mm wide ridges and not observable for the 20 mm wide ridges shown in Fig. 4. The
Fig. 6. Interferometric line scan (WYKO NT2000) of back surface profile together with a sketched front surface profile (dotted) of a 50 mm thick micromolded green tape with 40 mm wide and 100 mm high ridges.
T. Rosqvist, S. Johansson / Sensors and Actuators A 97±98 (2002) 512±519
Fig. 7. SEM image of micromolded pillars in green state.
Fig. 8. SEM image of sintered micromolded pillars 50 mm high and 10 mm wide.
PDMS molds were cleaned and reused about 10 times with no apparent signs of degradation. With conventional doctor blade casting at atmospheric pressure there may be gas bubble entrapment forming defects in the order of some 100 mm within the replicated structure, preferably in the mold corners. The replicas fabricated using low-pressure casting, shown in Figs. 7±9 show excellent ®lling characteristics and no signs of trapped gas defects. A combination of low-pressure casting and doctor blade casting has the prospect of good mold ®lling results combined with tape thickness control also for complex mold designs. The shrinkage of features due to sintering as shown from green state, Fig. 4, to sintered body, Fig. 5, was measured to
515
Fig. 9. SEM image of replicated and sintered maze showing 50 mm high and 20 mm wide walls.
be about 20%, see Table 2. When the features were very small, such as the pillars shown in Figs. 7 and 8, they were bent stochastically after sintering due to the fact that they were only a few grains wide. The minimum feature size obtained by the soft micromolding technique appears to be close to the limit set by the ®nal grain size. The ®nal grain size of commercially available powder-based PZT is typically kept at some micrometers and a signi®cant reduction in grain size would in general severely degrade the piezoelectric properties of the material [20]. The surface roughness of sintered ceramics can in some cases [9] be better than the ®nal grain size. A comparison of related ceramic microstructuring techniques [1,4±12] is presented in Table 3. The reached aspect ratio of the soft micromolding technique is high if compared to other microstructuring techniques suitable for multilayer fabrication but surpassed by the lost mold techniques, which lack this multilayer possibility. As a demonstration of the freedom in microstructure design and the capabilities of soft micromolding, Fig. 9 shows a replicated and sintered piezoceramic maze with 50 mm high and 20 mm wide walls. For the evaluation of electrode and channel integration the mold used featured ridges about 100 mm wide with aspect ratio of about 1:1 separated with a period of four times the width. The resulting green state PZT replicas had channel
Table 3 Comparison of results obtained using presented (italics) and related ceramic microstructuring techniques Microstructuring technique
Material
Grain size (mm)
Resolutiona (mm)
Techniques with multilayer potential Soft micromolding Soft micromolding [11] Soft micromolding [12] Stamping [9] Stamping [10] Space forming [1]
PZT Al2O3 PZT ZrO2±CeO2 PZT PZT
2±5 0.5b 3 2 2±5b 2±5b
10 2 35 4 100 20
Other techniques Lost polymer mold [6] Lost Si mold/HIP [8] Slip pressing [7] Centrifugal casting [7] Injection molding [5] Laser cutting [4]
PZT PZT Al2O3 PZT PZT PZT
2±5b ± ± 2±5 2±5 2±5b
25 12 20 35 70 50
a b
The resolution is defined as the finest ceramic feature successfully fabricated. Estimated grain size from presented material. Sintered PZT usually has a grain size of 2±5 mm.
Aspect ratio 7:1 1.5:1 6:1 3:1 5:1 1:1 10:1 14:1 >10:1 4:1 10:1 20:1
516
T. Rosqvist, S. Johansson / Sensors and Actuators A 97±98 (2002) 512±519
Fig. 10. SEM cross-section of sintered PZT microstructure with two integrated Pt electrode layers (light gray) showing the effect of drying shrinkage on backside waviness.
structures with a depth of 102 3 mm and at a period of 375 6 mm. A slight curvature was observed of the end faces to be laminated, interpreted as a result of drying shrinkage in the green body, and the peak deviation from a ¯at surface was measured to 3 mm for the approximately 300 mm wide structure. A curvature of this order, which can be seen in the sintered sample shown in Fig. 10 does not prevent successful lamination but is rather a problem for channel de®nition since the lamination pressure has to be increased. The deviations must be small compared to the channel size to allow for multilayer lamination with limited channel deformations. If one intends to use integrated microchannel structures for, e.g. active micro¯uidic applications, electrodes need to be integrated above and below or between the channels in the piezoceramic component. A typical appearance of a sintered microstructured tape with integrated platinum electrodes can be seen in Fig. 10. The uppermost electrode layer in Fig. 10 had a thickness of approximately 2 1 mm. The average thickness of the electrode bottom layer (near microstructured surface) was 2 mm above channels but 10 mm halfway between channels. During screen printing the screen acts as a spacer de®ning the amount of electrode paste to be deposited when pressed towards the green surface by the squeegee, and since the screen does not follow the surface waviness this will yield thicker electrode layers in the valleys between the channel structures. The thicker electrode layers increase the volume of inactive material and render a stiffer composite structure due to the higher
Young's modulus of platinum (168 GPa), and are therefore undesirable. The screen printing technique is not easily modi®ed to allow for structured surfaces and planarization is therefore the most attractive route to get uniform electrode thickness. Alternative methods for electrode material deposition that could yield uniform electrode thickness on structured surfaces and at the same time having a potential for low cost fabrication are, e.g. spray or ink-jet-based techniques. Compared to the ridges examined in the previous section the backside waviness of the ®rst cast ceramic layer is much more pronounced due to the wide mold structures to be ®lled by slurry. The second electrode layer shows that the waviness is more or less eliminated after a second cast ceramic layer, in this case at a tape thickness of less than 200 mm indicating the tape thickness needed for planarization of the molded structure. However, it was found that multilayer slurry casting on a PDMS mold might introduce bubbles in the green tape/mold interface as if air was pressed through the previous cast tape. Probably this is caused by capillary action of solvent ®lling the porous green body during subsequent casting. This phenomenon will be further analyzed in future work. Integration of screen-printed electrodes above and below integrated channels should be possible if proper alignment is achieved during screen printing and lamination. The waviness allows for introduction of electrodes embedded between replicated channel structures if the thickness of deposited electrode layers can be controlled or the thickness variation accepted. For multilayer lamination and channel integration, ethanol/PPG adhesive was applied to the backside of green tapes. The application of adhesive gave a glazed appearance of the tape surface and it was seen as a 5±10 mm in®ltrated layer, from now on referred to as the adhesive layer, in green tape cross-sections as shown in Fig. 11. This layer is believed to consist of softened and easy ¯owing green material and the effect of this layer upon lamination can be studied in Fig. 12, where different local lamination pressures has given channel and adhesive layer deformations to different extensions. The difference in local lamination pressure is discussed below. The mechanism of material deformation and ¯ow in the lamination interface at too high lamination pressure is
Fig. 11. (a) SEM cross-section of green tape with applied green tape adhesive, which can be seen as a 5±10 mm infiltrated thick darker layer at the top of the tape indicated by an arrow. (b) Close-up on adhesive layer.
T. Rosqvist, S. Johansson / Sensors and Actuators A 97±98 (2002) 512±519
517
Fig. 12. SEM cross-sections of laminated channels in green state at different local lamination pressures (a), (b) and (c) showing the effect of the adhesive layer.
Fig. 13. Sketched flow of the adhesive layer during lamination as interpreted from cross-sections.
interpreted as illustrated in Fig. 13, where the adhesive layer is outlined and shadowed. The adhesive layer should be able to ¯ow more easily than the bulk material, resulting in contact between the surfaces to be laminated with a lower deformation of the molded microstructures than otherwise would be needed, therefore, allowing integration of channels. Nonuniform adhesive layer thickness due to the manual deposition does not appear to in¯uence the lamination results. A too low lamination pressure has also shown to yield defects or voids along the lamination surface between the channel structures, consistent with the observed curvature of the end faces to be laminated as shown in Fig. 10. The difference in local lamination pressure in the series shown in Fig. 12 is probably an effect of thickness variations in the
microstructured tape and nonparallel application of the lamination pressure. Thickness control of the microstructured tapes is of outmost importance when laminating with solid dead weights since the allowed plastic deformation of features during lamination must be negligible compared to the microstructure feature size. Due to edge effects during slurry casting the tape thickness is believed to vary over the tape for the small mold size used (<50 50 mm2). Using larger area molds the problem with thickness variations over the tape may be diminished. Cross-sections of laminated and sintered chips with integrated channels are shown in Fig. 14. The chips shown were successfully laminated at 80 8C for 1 h using green tape adhesive and a dead weight (1973 g over an area of 1 cm2) giving an effective lamination pressure of 0.3 MPa at surfaces between channels. By using the same parameters, but without the adhesive, no or very poor lamination was achieved. The height of integrated channels after sintering was measured to 77 3 mm and the period to 302 2 mm and the shrinkage of channel structures from green dimensions presented earlier was 20% in period and 25% in height. The difference is believed to be caused by deformations during lamination since the values of shrinkage are expected to be accurate within 1% when using the same slurry recipe and lamination procedure. Accordingly, both these factors must be accounted for when designing new microchannel structures.
Fig. 14. SEM cross-sections of laminated and sintered channel chips showing: (a) an overview of two chips; (b) a close-up on a single channel. Lighter areas on the polished surfaces are drying stains.
518
T. Rosqvist, S. Johansson / Sensors and Actuators A 97±98 (2002) 512±519
Fig. 15. SEM micrographs of (a) laminated and sintered channel chip with (b) close-up on laminated microridge with lamination surface indicated by an arrow.
A sintered PZT chip with integrated channels separated by narrow walls, suitable for, e.g. cooling channels is shown in Fig. 15. This structure was laminated at 80 8C for 1 h using the green tape adhesive and a dead weight (1259 g over an area of 0.7 cm2) giving an average lamination pressure over the surfaces to be laminated of 0.7 MPa. The thickness of the walls and roof is about 20 mm. No lamination defects were observed in the lamination border, i.e. the bottom side of the channels in the ®gure, indicated by an arrow in Fig. 15b. A slight bending of the walls is observed, probably due to stresses during lamination. 4. Conclusions The soft micromolding technique yields a high resolution and a high aspect ratio of piezoceramic microstructures combined with geometrical freedom. The obtained results using soft micromolding are comparable with the lost mold technique with additional features of reusable molds and of green material post-processing, such as cutting and lamination. Lamination of microstructured tapes has been successfully performed with good dimensional stability using low-pressure adhesive assisted lamination with an ethanol/PPG green tape adhesive yielding piezoceramic components with integrated microchannel structures. Future work will focus on applications utilizing the presented technology in the area of advanced microactuators and micro¯uidic components. Acknowledgements This work was supported by the Swedish National Board for Industrial and Technical Development (NUTEK), which is gratefully acknowledged. References [1] K. Utsumi, Development of multilayer ceramic components using green-sheet technology, Ceram. Bull. 70 (1991) 1050±1055.
[2] V.F. Janas, A. Safari, Overview of fine-scale piezoelectric ceramic/ polymer composite processing, J. Am. Ceram. Soc. 78 (1995) 2945± 2955. [3] K. Lubitz, A. Wolff, G. Preu, Microstructuring technology, IEEE Ultrasonics Symp. 1 (1993) 515±524. [4] R. Farlow, W. Galbraith, M. Knowles, G. Hayward, Micromachining of a piezocomposite transducer using a copper vapor laser, IEEE Trans. Ultrasonics, Ferroelectrics Frequency Control 48 (2001) 639± 640. [5] L.J. Bowen, R.L. Gentilman, H.T. Pham, D.F. Fiore, K.W. French, Injection molded fine-scale piezoelectric composite transducers, IEEE Ultrasonics Symp. (1993) 499±503. [6] Y. Hirata, H. Nakaishi, T. Numazawa, H. Takada, Piezocomposite of fine PZT rods realized with synchrotron radiation lithography, IEEE Ultrasonics Symp. (1997) 877±881. [7] W. Bauer, H.-J. Ritzhaupt Kleissl, J.H. Hausselt, Slip casting of ceramic microcomponents, Microsyst. Technol. 4 (1998) 125±127. [8] S. Wang, J.-F. Li, R. Watanabe, M. Esahi, Fabrication of lead zirconate titanate microrods for 1±3 piezocomposites using hot isostatic pressing with silicon molds, J. Am. Ceram. Soc. 82 (1999) 213±215. [9] J.A. Bride, S. Baskaran, N. Taylor, J.W. Halloran, W.H. Juan, S.W. Pang, M. O'Donnell, Photolithographic micromolding of ceramics using plasma etched polyimide patterns, Appl. Phys. Lett. 63 (1993) 3379±3381. [10] R. Knitter, E. GuÈnther, C. Odemer, U. Maciejewski, Ceramic microstructures and potential applications, Microsyst. Technol. 2 (1996) 135±138. [11] U.P. Schonholzer, L.J. Gauckler, Ceramic parts patterned in the micrometer range, Adv. Mater. 11 (1999) 630±632. [12] S. Gebhardt, A. Schonecker, R. Steinhausen, T. Hauke, W. Seifert, H. Beige, Fine scale 1±3 composites fabricated by the soft mold process: preparation and modeling, Ferroelectrics 241 (2000) 67±73. [13] G. Thornell, L. Klintberg, T. Laurell, J. Nilsson, S. Johansson, Desktop microfabrication-initial experiments with a piezoceramic, J. Micromech. Microeng. 9 (1999) 434±437. [14] A.J. Bell, Multilayer ceramic processing, in: N. Setter, E.L. Colla (Eds.), Ferroelectric ceramics. Tutorial Reviews, Theory, Processing and Applications, BirkhaÈuser Verlag, Basel, 1993, pp. 241±271. [15] J.S. Sung, K.D. Koo, J.H. Park, Lamination and sintering shrinkage behavior in multilayered ceramics, J. Am. Ceram. Soc. 82 (1999) 537±544. [16] N. Suppakarn, H. Ishida, J.D. Cawley, Roles of poly(propylene glycol) during solvent-based lamination of ceramic green tapes, J. Am. Ceram. Soc. 84 (2001) 289±294. [17] T. Rosqvist, S. Johansson, High aspect ratio soft micromolding of piezoceramic thick films, in: Proceedings of the Transducers'01, Vol. 2, Springer, Munich, Germany, 2001, pp. 1022±1025. [18] Y. Xia, G.M. Whitesides, Soft lithography, Angew. Chem. Int. Ed. 37 (1998) 550±575.
T. Rosqvist, S. Johansson / Sensors and Actuators A 97±98 (2002) 512±519 [19] H. Lorenz, M. Despont, N. Fahrni, J. Brugger, P. Vettiger, P. Renaud, High-aspect-ratio, ultrathick, negative-tone near-UV photoresist and its applications for MEMS, Sensors Actuat. A 64 (1998) 33±39. [20] W. Hackenberger, M.J. Pan, V. Vedula, P. Pertsch, W.W. Cao, C. Randall, T. Shrout, Effect of grain size on actuator properties of piezoelectric ceramics, in: Proceedings of the SPIE, Vol. 3324, 1998, pp. 28±36.
Biographies Tobias Rosqvist was born in 1973 in LinkoÈping, Sweden, and graduated from LinkoÈpings Universitet 1997 with a MSc in physics. He is since 1998,
519
a member of the Microstructure Technology (MST) Group at the Department of Materials Science, Uppsala University, as a PhD student. His research interest is within the area of microactuators, focusing on piezoceramic microactuator technology. Stefan Johansson, born in 1960, received his PhD degree in Materials Science at Uppsala University in 1988. He became associate professor in Materials Science in 1994 and professor in Materials Science in 2000. His research area includes topics such as microrobotics, micromanipulation and testing of micromechanical components. During the last years, his main focus has been microactuation for miniature components and, in particular, research related to the fabrication of miniature devices based on piezoceramic materials.