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Patterned sputter deposited SmCo-films for MEMS applications
Journal of Magnetism and Magnetic Materials 242–245 (2002) 1146–1148
Patterned sputter deposited SmCo-films for MEMS applications T. Budde*, H.H. Gatzen Institute for Microtechnology, Callinstr. 30 A, Hanover University, D-30167 Hanover, Germany
Abstract The hard magnetic material chosen for patterning was sputter deposited SmCo with a film thickness between 5 and 20 mm. As a mask material capable of being deposited in the thickness range required as well as withstanding the high process or annealing temperature, electroplated copper was selected. The copper mask was stripped using wet-chemical etching. r 2002 Elsevier Science B.V. All rights reserved. Keywords: MEMS; Structuring; Sacrificial layer
1. Introduction Building MEMS type magnetic microactuators poses a major challenge: typically, the energy such a system is capable of transmitting is proportional to its volume, which obviously decreases in a cube function when it is reduced. Therefore, any approach allowing an increase is of great interest. One way to extend the driving force is to use an actuator taking advantage of both soft and hard magnetic materials. While actuators, using only soft magnetic materials, allow forces pulling the magnetized motor part together, the use of hard magnetic materials also permits forces pulling them apart, thus increasing the total force difference available. For this approach, a move from a variable reluctance motor design [1] using only soft magnetic materials to a hybrid motor design by appropriately adding hard magnets was intended. In doing so, the major challenge was to come up with rather thick patterned layers of high-energy product hard magnetic material. The hard magnetic material chosen was SmCo. The intention was to create a layer thickness in the range of a couple of ten microns. Therefore, lift-off sputter deposition was chosen as the most promising combination of deposition and patterning technology. However, *Corresponding author. Tel.: +49-511-762-2767; fax: +49511-762-2867. E-mail address: [email protected] (T. Budde).
for achieving the desired hard magnetic properties of the SmCo-layer, either a deposition or a post-deposition annealing at elevated temperatures was required [2]. These temperatures are beyond the range of organic photoresists. Therefore, copper was selected as the mask material, which is capable of withstanding the process temperature, as well as being deposited in the thickness range required. This paper discusses the coating of the copper structure’s sidewall as a function of its angle, the influence of the high process temperature on the shape of the copper lift-off structures as well as challenges regarding the wet-chemical etching of the mask material.
2. Process description To optimize the process sequence, a series of experiments was performed. The wafer consisted of silicon. Creating the electroplated copper mask for the lift-off process started out with the sputter deposition of Cu-film required as a seed layer for the electroplating process. Next, a photoresist mask with a thickness range between 15 and 30 mm was created, serving as a micromold for the Cu mask structure. By appropriately varying the lithography process conditions, the sidewall angle was varied between 751 and 851. Next, the Cu mask material was electroplated, with thicknesses
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 1 3 0 4 - X
T. Budde, H.H. Gatzen / Journal of Magnetism and Magnetic Materials 242–245 (2002) 1146–1148
1147
ranging from 5 to 30 mm. After its completion, stripping the photoresist and ion milling the seed layer completed the Cu mask buildup. In a next step, the SmCo-film was deposited. As an underlayer as well as a capping layer for corrosion protection, Cr was used. Therefore, the masked wafer was coated with a Cr/SmCo/Cr trilayer using RF sputtering. The SmCo target has a nominal composition of SmCo5. The deposition rate was 0.2 nm/s for Cr and 2.8 nm/s for SmCo. The film thickness of the Cr underlayer and capping layer were 50 and 100 nm, respectively. The thickness for SmCo was varied between 5 and 20 mm. In order to develop its optimal magnetic properties, a SmCo-layer has to be crystalline. To achieve this crystalline structure, the SmCo-layer was deposited either on a cooled substrate, followed by a postannealing process, or at elevated temperature (400– 5501C). To finish the process, the Cu mask had to be stripped, thereby lifting off the SmCo-layer deposited on top of the Cu mask. This was achieved by wet-chemical etching. To protect the SmCo-layer from being damaged by the etchant, patterned photoresist was utilized, which was stripped after completing the wet-chemical etching process.
3. Experimental results The first process parameter investigated was the occurrence of coating the Cu mask’s sidewall as a function of its sidewall angle. During the investigation, the angle was varied by influencing the lithography process. Fig. 1 shows the cross-section of SmCo-coated copper lift-off structures with the sidewall angles varying between 751 and 851. It could be observed that the edges of the copper structures with an angle higher than 801 (Fig. 1a and b) were coated during the sputter process, while sidewalls with an angle of 751 remained uncoated (Fig. 1c). The second set of investigations was made regarding the effect of the high annealing or process temperature on the copper mask. It could be observed that the lift-off structure was not influenced by this temperature treatment (Fig. 2), which demonstrates the general applicability of this mask material to the process. To finally obtain the patterned sputter deposited SmCo-layer, the copper structure needed to be removed by using wet-chemical etching. The etchant chosen was iron(III)-chloride. Since the etchant shows only insufficient selectivity between etching the Cu mask and the SmCo hard magnet, the SmCo had to be protected by a photomask from the chemical attack. In doing so the copper structure could be etched. However, a little
Fig. 1. SmCo-coated (4 mm) copper lift-off structures (13 mm) with different sidewall angles: (a) 851, (b) 801 and (c) 751.
underetching of the hard magnet occurred at the edges, since a contact exists between the SmCo and the copper. After the complete removal of the copper structure, the photoresist needed to be stripped to obtain the patterned sputter deposited hard magnetic layer. Fig. 3 depicts a patterned SmCo-layer with a film thickness of 7 mm obtained by a lift-off copper structure of 30 mm and a sidewall angle of 751.
4. Conclusions and outlook A sidewall angle of 751 for the sacrificial copper structure was determined to be sufficient to prevent a coating of the edges during the SmCo sputter process. Furthermore, it was demonstrated that the process temperature as well as the temperature used during the annealing process did not affect the shape of the lift-off copper structure. It could be demonstrated that this process is capable of producing patterned hard magnetic
1148
T. Budde, H.H. Gatzen / Journal of Magnetism and Magnetic Materials 242–245 (2002) 1146–1148
Fig. 3. Topography of a patterned sputter deposited SmCofilm.
obtained. Furthermore, the suitability for other sputter deposited materials [5] will be investigated, to extend the application field of this structuring method.
Acknowledgements
Fig. 2. Cross-section of a SmCo-coated copper structure (a) as deposited and (b) after the annealing process with a maximum temperature of 6001C.
This research is supported in part by a grant of the German Research Foundation.
References SmCo-layers. A critical step of the patterning process turned out to be the etching of the lift-off structure. The etchant utilized for this process step exhibited only little selectivity between the copper and the hard magnetic layer. Therefore, a protection layer for SmCo was introduced to prevent a contact between the etchant and the hard magnetic film. However, at the edges of the patterned SmCo-layer minor underetching occurred. To prevent it, further improvement either of the Cu lift-off structure design [3] or of the utilized wet-chemical etchant is desirable. A digital etching process [4], where the copper is attacked by the etchant and the SmCo creates a protective coating layer, would be of great interest, since a simplification of the process will be
. . [1] H.H. Gatzen, H.-D. Stolting, S. Buttgenbach, H. Dimigen, A novel variable reluctance micromotor for linear actuation, Proceedings of Actuator 2000, Bremen, 2000, pp. 363– 366. [2] C. Prados, et al., J. Appl. Phys. 85 (8) (1999) 6148. [3] H. Hegde, J. Wang, A.J. Devasahayam, V. Kanarov, A. Hayes, R. Yevtukhov, J. Vac. Sci. Technol. B 17 (5) (1999) 2186. . [4] M. Kohler, Etching in Microsystem Technology, WILEYVCH Verlag GmbH, Weinheim, 1999. [5] Y. Konaka, G.A. Allen, Single- and multi-layer electroplated microaccelerometers, Proceedings of the Ninth Annual International Workshop on Micro Electro Mechanical Systems 1995, San Diego, CA, USA, February 11–15, 1996, pp. 168–173.
Patterned sputter deposited SmCo-films for MEMS applications T. Budde*, H.H. Gatzen Institute for Microtechnology, Callinstr. 30 A, Hanover University, D-30167 Hanover, Germany
Abstract The hard magnetic material chosen for patterning was sputter deposited SmCo with a film thickness between 5 and 20 mm. As a mask material capable of being deposited in the thickness range required as well as withstanding the high process or annealing temperature, electroplated copper was selected. The copper mask was stripped using wet-chemical etching. r 2002 Elsevier Science B.V. All rights reserved. Keywords: MEMS; Structuring; Sacrificial layer
1. Introduction Building MEMS type magnetic microactuators poses a major challenge: typically, the energy such a system is capable of transmitting is proportional to its volume, which obviously decreases in a cube function when it is reduced. Therefore, any approach allowing an increase is of great interest. One way to extend the driving force is to use an actuator taking advantage of both soft and hard magnetic materials. While actuators, using only soft magnetic materials, allow forces pulling the magnetized motor part together, the use of hard magnetic materials also permits forces pulling them apart, thus increasing the total force difference available. For this approach, a move from a variable reluctance motor design [1] using only soft magnetic materials to a hybrid motor design by appropriately adding hard magnets was intended. In doing so, the major challenge was to come up with rather thick patterned layers of high-energy product hard magnetic material. The hard magnetic material chosen was SmCo. The intention was to create a layer thickness in the range of a couple of ten microns. Therefore, lift-off sputter deposition was chosen as the most promising combination of deposition and patterning technology. However, *Corresponding author. Tel.: +49-511-762-2767; fax: +49511-762-2867. E-mail address: [email protected] (T. Budde).
for achieving the desired hard magnetic properties of the SmCo-layer, either a deposition or a post-deposition annealing at elevated temperatures was required [2]. These temperatures are beyond the range of organic photoresists. Therefore, copper was selected as the mask material, which is capable of withstanding the process temperature, as well as being deposited in the thickness range required. This paper discusses the coating of the copper structure’s sidewall as a function of its angle, the influence of the high process temperature on the shape of the copper lift-off structures as well as challenges regarding the wet-chemical etching of the mask material.
2. Process description To optimize the process sequence, a series of experiments was performed. The wafer consisted of silicon. Creating the electroplated copper mask for the lift-off process started out with the sputter deposition of Cu-film required as a seed layer for the electroplating process. Next, a photoresist mask with a thickness range between 15 and 30 mm was created, serving as a micromold for the Cu mask structure. By appropriately varying the lithography process conditions, the sidewall angle was varied between 751 and 851. Next, the Cu mask material was electroplated, with thicknesses
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 1 3 0 4 - X
T. Budde, H.H. Gatzen / Journal of Magnetism and Magnetic Materials 242–245 (2002) 1146–1148
1147
ranging from 5 to 30 mm. After its completion, stripping the photoresist and ion milling the seed layer completed the Cu mask buildup. In a next step, the SmCo-film was deposited. As an underlayer as well as a capping layer for corrosion protection, Cr was used. Therefore, the masked wafer was coated with a Cr/SmCo/Cr trilayer using RF sputtering. The SmCo target has a nominal composition of SmCo5. The deposition rate was 0.2 nm/s for Cr and 2.8 nm/s for SmCo. The film thickness of the Cr underlayer and capping layer were 50 and 100 nm, respectively. The thickness for SmCo was varied between 5 and 20 mm. In order to develop its optimal magnetic properties, a SmCo-layer has to be crystalline. To achieve this crystalline structure, the SmCo-layer was deposited either on a cooled substrate, followed by a postannealing process, or at elevated temperature (400– 5501C). To finish the process, the Cu mask had to be stripped, thereby lifting off the SmCo-layer deposited on top of the Cu mask. This was achieved by wet-chemical etching. To protect the SmCo-layer from being damaged by the etchant, patterned photoresist was utilized, which was stripped after completing the wet-chemical etching process.
3. Experimental results The first process parameter investigated was the occurrence of coating the Cu mask’s sidewall as a function of its sidewall angle. During the investigation, the angle was varied by influencing the lithography process. Fig. 1 shows the cross-section of SmCo-coated copper lift-off structures with the sidewall angles varying between 751 and 851. It could be observed that the edges of the copper structures with an angle higher than 801 (Fig. 1a and b) were coated during the sputter process, while sidewalls with an angle of 751 remained uncoated (Fig. 1c). The second set of investigations was made regarding the effect of the high annealing or process temperature on the copper mask. It could be observed that the lift-off structure was not influenced by this temperature treatment (Fig. 2), which demonstrates the general applicability of this mask material to the process. To finally obtain the patterned sputter deposited SmCo-layer, the copper structure needed to be removed by using wet-chemical etching. The etchant chosen was iron(III)-chloride. Since the etchant shows only insufficient selectivity between etching the Cu mask and the SmCo hard magnet, the SmCo had to be protected by a photomask from the chemical attack. In doing so the copper structure could be etched. However, a little
Fig. 1. SmCo-coated (4 mm) copper lift-off structures (13 mm) with different sidewall angles: (a) 851, (b) 801 and (c) 751.
underetching of the hard magnet occurred at the edges, since a contact exists between the SmCo and the copper. After the complete removal of the copper structure, the photoresist needed to be stripped to obtain the patterned sputter deposited hard magnetic layer. Fig. 3 depicts a patterned SmCo-layer with a film thickness of 7 mm obtained by a lift-off copper structure of 30 mm and a sidewall angle of 751.
4. Conclusions and outlook A sidewall angle of 751 for the sacrificial copper structure was determined to be sufficient to prevent a coating of the edges during the SmCo sputter process. Furthermore, it was demonstrated that the process temperature as well as the temperature used during the annealing process did not affect the shape of the lift-off copper structure. It could be demonstrated that this process is capable of producing patterned hard magnetic
1148
T. Budde, H.H. Gatzen / Journal of Magnetism and Magnetic Materials 242–245 (2002) 1146–1148
Fig. 3. Topography of a patterned sputter deposited SmCofilm.
obtained. Furthermore, the suitability for other sputter deposited materials [5] will be investigated, to extend the application field of this structuring method.
Acknowledgements
Fig. 2. Cross-section of a SmCo-coated copper structure (a) as deposited and (b) after the annealing process with a maximum temperature of 6001C.
This research is supported in part by a grant of the German Research Foundation.
References SmCo-layers. A critical step of the patterning process turned out to be the etching of the lift-off structure. The etchant utilized for this process step exhibited only little selectivity between the copper and the hard magnetic layer. Therefore, a protection layer for SmCo was introduced to prevent a contact between the etchant and the hard magnetic film. However, at the edges of the patterned SmCo-layer minor underetching occurred. To prevent it, further improvement either of the Cu lift-off structure design [3] or of the utilized wet-chemical etchant is desirable. A digital etching process [4], where the copper is attacked by the etchant and the SmCo creates a protective coating layer, would be of great interest, since a simplification of the process will be
. . [1] H.H. Gatzen, H.-D. Stolting, S. Buttgenbach, H. Dimigen, A novel variable reluctance micromotor for linear actuation, Proceedings of Actuator 2000, Bremen, 2000, pp. 363– 366. [2] C. Prados, et al., J. Appl. Phys. 85 (8) (1999) 6148. [3] H. Hegde, J. Wang, A.J. Devasahayam, V. Kanarov, A. Hayes, R. Yevtukhov, J. Vac. Sci. Technol. B 17 (5) (1999) 2186. . [4] M. Kohler, Etching in Microsystem Technology, WILEYVCH Verlag GmbH, Weinheim, 1999. [5] Y. Konaka, G.A. Allen, Single- and multi-layer electroplated microaccelerometers, Proceedings of the Ninth Annual International Workshop on Micro Electro Mechanical Systems 1995, San Diego, CA, USA, February 11–15, 1996, pp. 168–173.