Enhancement of AlN Slender Piezoelectric Cantilevers Actuation by PECVD Silicon Nitride Coating

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Procedia Engineering 47 ( 2012 ) 104 – 107

doi: 10.1016/j.proeng.2012.09.095

Proc. Eurosensors XXVI, September 9-12, 2012, Kraków, Poland

Enhancement of AlN slender piezoelectric cantilevers

actuation by PECVD Silicon Nitride coating

A.T.Tran

a,*

, G. Pandraud

a

, H.Schellevis

a

and P. M. Sarro

a

a_{Laboratory of Electronic Components, Technology & Materials, DIMES, Delft University of Technology, Delft, The Netherlands}

Abstract

Actuation enhancement for AlN piezoelectric cantilevers is achieved by coating slender AlN beams with a thin PECVD silicon nitride (SiN) layer. Very good linearity and high deflection, up to 19 nm/V of actuation deflection for 200 μm long cantilevers, at quasi-static mode, is obtained for a 500 nm SiN top layer. This value is three times larger than our previously reported value for cantilevers without the SiN layer. The achieved results make these cantilevers, fabricated in a CMOS compatible process, very promising micro/nano actuators.

Keywords: AlN; slender piezoelectric cantilevers; silicon nitride;

1. Introduction

Thin films of piezoelectric layers are very interesting for actuation devices, in particular for NanoElectroMechanical Systems (NEMS) [1]. Piezoelectric cantilevers based on thick silicon beams (5-30 μm) have been widely investigated [2-4]. However, controlling the final thickness of these beams is difficult unless expensive SOI substrates are employed. Furthermore, the miniaturization of these cantilevers as required by nanoscale applications is rather challenging.

Recently we have realized slender cantilevers based on Ti/AlN/Ti (figure 1a) [5] suitable for nanoscale device fabrication. In this paper, we present a new configuration (figure 1b), consisting of AlN slender piezoelectric cantilevers coated by a thin PECVD layer of SiN. This additional high stiffness, but very thin, layer results in high performance piezoelectric devices as predicted by the analytical model of the quasi-static mechanical behavior of piezoelectric cantilever reported in [6]. Due to its availability in CMOS technology and well-known properties, SiN is actually preferred to other candidates, such as diamond. The improvement of the cantilevers performance was investigated by quasi-static vibrometer

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A.T.Tran et al. / Procedia Engineering 47 ( 2012 ) 104 – 107

measurements. The results demonstrate the potential of the thin structural cantilevers for actuation applications.

Fig 1: Schematic drawing of (a) the AlN slender cantilever and (b) the AlN slender cantilever with the thin SiN layer. 2. Device Fabrication

AlN slender cantilevers with Ti electrodes have been deposited using a Sigma 204 DC magnetron PVD system, with the conditions reported in [7]. The process to fabricate Ti/AlN/Ti slender cantilevers with a 500 nm PECVD SiN is schematically depicted in figure 2. After sputtering of the AlN/Ti/AlN/Ti stack and deposition of a backside SiO2 mask, a contact to the bottom electrode is opened by dry etch of

the Ti top layer, followed by removal of the AlN using a wet etchant (MF322 developer). A backside SiO2 mask layer is defined by C2F6/CHF3 dry etch. Next, a 500 nm thick PECVD SiN layer is deposited

and patterned on top of the AlN/Ti stack. Then a thin Al protection layer is deposited to protect the SiN layer. The cantilever is defined using a SiO2 hard mask on top of the Al layer. Finally, the device is

released by Bosch DRIE process, followed by removal of SiO2 mask layer, Al and AlN stop layers.

Cross-session and top view SEM images of the SiN coated Ti/AlN/Ti stack of a 50 μm wide cantilever are shown in figure 3.

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a) b)

Fig 3: Cross section SEM images of 500nm SiN layer on Ti/AlN/Ti stack (a) and top view of 50 μm wide cantilevers with a length of 50 μm (left), 100 μm (center) and 200 μm (right).

3. Device Characterization

We measured the actuation of the cantilevers with a laser doppler vibrometer (Polytec OFV 5000). For quasi-static mode, the cantilevers were actuated at a very low frequency (1kHz) well below the resonance frequency. The deflections were determined at the tip of the cantilevers. The dependence of quasi-static deflection of the cantilevers with and without the SiN layer for varying input amplitude is displayed in figure 4. High linearity and large influence of the presence of the 500 nm nitride SiN layer are observed. It can be clearly noticed that the slender cantilever with the SiN structural layer leads to a greater tip deflection. For the 200 μm long cantilevers, a value of 19 nm/V (tip deflection per input amplitude unit) is measured. This is a threefold increase with respect to the slender AlN cantilever without the top SiN layer presented in [5]. The higher actuation obtained is due to the asymmetry of the piezoelectric structure [6].

Furthermore, the deflection actuation of AlN/SiN cantilevers of different lengths was investigated (figure 5). The highly linear behavior in figure 5a is preserved also for the shorter (100μm and 50μm long) cantilevers. The bending of the cantilevers depends quadratically on the cantilever length (fig 6). This indicates that the AlN piezoelectric layer is uniform throughout the length of the cantilever in agreement with [8].

Fig 4: The deflection measured using a Polytec OFV 5000 vibrometer for a 200 μm long cantilever without (1) and with a 500nm silicon nitride top layer (2).

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a)

b)

Fig 5: The deflection of cantilevers of different lengths coated with 500 nm thick SiN layer (a) and dependence of the deflection on the length of the AlN/SiN cantilevers (b).

4. Conclusions

In this paper, we report on the enhancement of the slender AlN cantilevers actuation, obtained by addition of a SiN thin film. The device is still very thin (~ 1.4 ȝm) as desirable for nanoscale device fabrication. The high static actuation deflection (19 nm/V for 200 μm long cantilevers) demonstrates the great performance enhancement achieved, while CMOS compatibility is preserved.

Acknowledgements

The authors would like to thank the staff of the DIMES-ICP group for technical help and Dr. O. Wunnicke from NXP Semiconductors for assistance in vibrometer measurements. This work is partially funded by MicroNed (SMACT 2-D), as well as the Vietnam International Education Development (VIED).

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