Microactuated Self-assembling Of 3d Polysilicon Structures With

MICROACTUATED SELF-ASSEMBLING OF 3D POLY SILICON STRUCTURES WITH RESHAPING TECHNOLOGY Y. Fukuta, D. Collard, T. Akiyama*, E.H. Yang, H. Fujita LIMMS / CNRS-IIS ,Institute of Industrial Science, The University of Tokyo Roppongi 7-22-1, Minatoku, Tokyo 106,Japan (*) Present address: IMT, University of Neuchatel, rue Jacquet Droz 1, Ch-2007 Neuchatel, Switzerland SELF ASSEMBLING BY (SCRATCH DRIVE ACTUATION AND RESHAPING TECHNOLOGY

ABSTRACT

The basic concept for the self assembling of 3D microstructures is depicted in Fig. 1for the case of a simple stage.

This abstract proposes and experimentally confirms the automatic self-assemblingof 3D polysilicon structures, compatible with both mass production and IC based surface machining. This technique combines an integrated actuation based on the Scratch Drive Actuator (SDA), for the structure raising up, and the reshaping technology to obtain the permanent 3D shapes. Complex 3D structures have been successfully realised and their electrostatic actuation have been obtained. This self-building capability of 3D devices from silicon surface micromachining opens new integration capabilities and new application field for

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SDA \

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(a) Micro structure with actuator

MEMS.

INTRODUCTION

(b) Elastic deformation by actuator

The batch fabrication of integrated three-dimensional (3D) structures is a cornerstone for the production of new kinds of sensors and actuators that will enlarge the application field of MEMS. Even if 3D polysilicon structure have already been successfully demonstrated for optical [l] or biomedical applications [2], the raising up of the surface micro machined parts requires external manipulations that cannot be extended for mass production. Self assembling of 3D structures with active polymers [3] have also been demonstrated, but the assembling needs a liquid environment not compatible with silicon processing. To overcome these limitations, this paper proposes, the automatic self assembling of 3D polysilicon structures, compatible with both mass production and silicon micro machining. This self assembly involves both internal actuation with Scratch Drive Actuator (SDA) [4] to rise up the structures by beam buckling, and reshaping technology [5] to obtain the permanent 3D shapes.

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Reshaped part

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(c) Reshaping

(d) 3D permanent micro structure

F i p r e 1 : Principle of the self-assembling by integrated SDA actuation and reshaping technology.

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This structure is made from a polysilicon layer pattemed by surface micro machining, that is plane initially, Fig. l(a).

As friction effects are involved, SDA functional yield depends on its dimensions. Yield above 60 % have been obtained for SDA lengths ranging from 35 pm to 60 pm [7], the other dimensions being given in the legend of Fig. 3. During the self assembling of the 3D polysilicon structures, the SDA bias is applied on the buried shield layer. The moving parts remains grounded during the motion while programmable current source is provided during the Joule heating.

The SDA is actuated and its forward stepping motion produces a pushing force that makes the beam buckling. The stage is lifted up, out of the substrate plane, as shown in Fig. l(b). Then, in order to obtain a permanent 3D shape, plastic deformation of the beam is produced. A current resulting in Joule heating is applied through the beam, while the SDA position is maintained by DC bias, Fig. l(c). This Joule heating primary occurs at the lifted part of the beam, as the heat sink produced by the substrate is reduced. As a result of the annealing effect, the plastic deformation of the beam occurs. After removing all the bias, the permanent 3D structure is obtained, Fig l(d). In this self-assembling process, no external manipulation is required and a l l is carried out in the micro world. This sequence is successfully applied to a 1 pm thick polysilicon plate and the final and permanent 3D shape is displayed by the SEM picture of Fig. 2.

Insulator

Substrate

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Bushing /

Insulator Fiz. 2 . SEM picture of the self-assembled 3D pol ysilicon plate.

Fig. 3: (a) Schematic view of the SDA with electric bias. SDA dimensions for the self-assembling are: k 5 0 pm, W=75 pm, t = l pm and h=1.5 pm. (b and c) Model for the step motion of the SDA showing the evolution of the plate deformation according to an applied pulse.

SDA ACTUATION AND PROCESS The SDA [3] basic structure and its operation mode are given in Fig. 3 . The motion is produced by successive bending and relaxation of the polysilicon plate. The bending is induced by electrostatic actuation and during the relaxation, non symmetrical friction effects produce a reptile like motion. With square pulse voltage of +/150 V, t h s actuator can develop a forward force in the 60 p N range The incremental step, AX, as small as 40 nm have also been measured. These characteristics allow the SDA to buckle 200pm long, 10 pm wide and 1 pm thick polysilicon beams . [5].

The SDA, stages and mechanical links are fabricated with silicon based surface machining in which polysilicon structures are released using the sacrificial oxide technique. The process uses 4 mask levels: (i) a "shield layer" that defines the buried polysilicon plates that are used to locally biased the SDA, (ii) the "bushing" mask for the definition of the SDA bushmg area, (iii), the "contact mask" to seal the movable

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The various stages of the fabrication sequence are presented in Fig. 5 (a-f) along the cross section line of the layout. Starting from low doped 3 inches (100) silicon wafers, a 0.35 pm thick silicon oxide layer was grown by dry oxidation at 1100 "C. A 0.3 pm-thick polysilicon layer was then deposited by low pressure chemical vapour deposition (LF'CVD) and phosphorus doped using coated doping film (OCD) and dnve-in. With the "shield" mask photolithography, the polysilicon layer was patterned to form the buried electrode in a sF6 plasma (fig Sa). The polysilicon was then oxidised in dry ambient to obtain a 0.2 pm thick Si02. A 0.3 pm-thck silicon-rich silicon nitride film followed by a 2.0 pm-thick silicon oxide (as sacrificial layer) were deposited by LPCVD. With the "bushing" mask, moulds were patterned by reactive ion etching (RIE) with CHF3. The depth of the bushing mould determines the SDA bushing height (here 1.5 pm-depth), Fig. 5(c). Using tlhe "poly" mask, both LPCVD-silicon oxide, silicon nitride and thermal oxide was removed by RIE with CHF3, Fig. 5(d), so that subsequent deposited polysilicon can locally contact the buried shield layer. A 1.0 pm-thick LPCVD polysilicon layer was deposited on the wafer surface. This polysilicon layer defines the main components: SDA, mechanical link and 3D structures. After OCD doping and drive-in, this structural polysilicon layer was pattemed and delineated by RIE with SF6 + Sic14 gas using the "poly" mask, Fig. 5(e). Finally, the polysilicon structures were released with wafer dipping into 50 96HF to fully dissolve the sacrificial silicon oxide, Fig. 30.

surface polysilicon layer to the buried one, and, finally, (iv) the "poly" mask that defines the SDA plate and mechanical parts. A typical layout and corresponding mask levels are given in Fig. 4-5, showing a SDA, located on a bias shield layer and mechanically linked to a polysilicon beam.

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Shield

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Bushing

Silicon

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Nitride

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3D SELF-ASSEMBLING OF COMPLEX STRUCTURES.

This self-assembling technique, combining SDA actuation and reshaping technology has been applied to polysilicon stages obtained wirh the above mentionned process. Two examples are here demonstrated in order to validate the extention of the 3D self assembling to more complex geometrical shapes. The first example concerns a rotational stage. As polysilicon plate can be lift up, rotational stage with large rotation angle can be realised from surface micro machined flat parts. Fig 6(a) gives the principle of the self-assembling of such stage. 2 SDA are simultaneously actuated to lift up the structure (squared pulse +/- 180 V, 250 Hz). Once the beam deformation allows a sufficient lift up of the stage, current was applied through the buckling beams. A 4mA current applied during 5s was sufficient to reshape each supporting beam that are 400 pm long and 10 pm wide. This current value is coherent with previous experiments [5] in which a minimum

Fii. 4 and 5: SDA masks definition and evolution of the structure at various stages of the process.

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threshold current was deduced to be 3.5 m A to obtain a complete reshaping for the same beam dimensions.

reshaped parts

Under the plate; additional electrodes are buried to allow rotational motion. SEM pictures of the reshaped structures are given in Fig. 6(b,c), as shown in these pictures, the stage is rotated and in contact with the substrate. The closed view of the reshaped area confirms that no macroscopic degradation is observable on the narrow polysilicon beams, due to the Joule heating.

rotational plates

The self-assembling of a suspended z-plate actuator IS also investigated. The plate is attached to supporting pillars by suspension springs, as shown in Fig. 7(a). The structure is raised up by the bending of the 4 supported pillars produced by the SDA pulling force. Once the deformation is completed, the pillars are reshaped by applying current through their attachment pads.

actuators

reshaped parts

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m:3D structure with suspended stage. (a) Principle

(c) 3D rotational stage. (a) Principle of the self assembling. (b) SEM of the 3D permanent structure. (c) Closed view of the reshaped area

w:

of the self assembling. (b) SEM of the 3D permanent structure. Each pillar has a length of 150 pm with a width of 5 pm. The reshaping was performed by 5 mA over 5s for each twin pillar bridge.

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[4] T. Aluyama and K. Shono, "Controlled stepwise J. motion in polysilicon microstructures', Microelectromechanical Syst., Vol. 2 , p. 106-110, 1993.

Fig. 7(b) shows a 3D reshaped structures obtained with thls principle. In this case, the force produced by 2 SDA is high enough to lift up the complete structure containing in fact 2 suspended plates (plate size: 100*100 pm2, height: 100pm). All electrodes under the plates have to be properly grounded to avoid sticking by parasitic electrostatic coupling during the SDA motion. After the self-assembling, a small z plate motion have been obtained by applying voltage on the buried undemeath electrodes.

[5]Y. Fukuta, T. Akiyama and 13. Fujita, "A Reshaping technology with Joule Heat for ThreeDimension polysilicon Structures", Proc. 8th Int. Con. on Solid State Sensor and Actuator, Stockholm, Sweden, p. 174,1995. [6] T. Akiyama and H. Fujita, "A quantitative analysis of scratch drive actuator using buckling motion", Proc. IEEE Micro Electro Mechanical Systems, Amsterdam, The Netherlands, pp.3 10-315, 1995.

CONCLUSION

[7] T. Akiyama, D. Collard and H. Fujita, "Scratch Drive Actuator with mechanical links for selfassembling of Three-Dimensional MEMS', J . Microelectromechanical Syst., tot be published.

The self assembly of 3D polysilicon structures have been demonstrated for the first time. The developed concept consists in combining integrated motion by SDA and reshaping technology to acbeve permanent 3D deformations. Complex 3D structures have been successfully realised and their electrostatic actuation have been obtained. This self-building capability of complex 3D M E M S from batch processed surface micro machined parts opens new integration capabilities and new application field for MEMS.

ACKNOWLEDGEMENT The authors would like to thank Prof. H. Toshiyoshi, Prof. S. Konishi, Dr D. Chauvel and Dr. F. Chollet and Mr. Y. Mita for their support in process experiments. One of the author is supported by the Japanese Society for the Promotion of Science (JSPS).

REFERENCES [l] 0. Solgaard, N. C. Tien, M. Daneman, M.H. Kiang, A. Friedberger, R.S. Muller, K.Y. Lau, "Precision and performance of polysilicon micromirrors for hybrid integrated optics, SPIE Proceed., Vol. 2383, p. 89, San Jose, ca. USA,7-9 Feb. 1995.

121 G. Lin, K.S.J Pister and K.P. Roos, "Standard CMOS piemresistive sensor to quantify heart cell contractile forces, Proc. IEEE Micro Electro Mechanical Systems, San Diego, Ca. USA, pp. 150155,1996. [3] E. Smela O.L. Inganas and I. Lundstrom, "Controlled Folding of Mmometer Size Microstructures", Science, Vol. 268, p. 1735-1738, 1995.

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