A novel surface micromachining process to fabricate AlN unimorph suspensions and its application for RF resonators

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Abstract

A novel surface micromachining process is reported for aluminum nitride (AlN) thin films to fabricate piezoelectric unimorph suspension devices for micro actuator applications. Wet anisotropic etching of AlN thin film is used with a Cr metal mask layer in the microfabrication process. Tetra methyl ammonium hydroxide (TMAH) of 25 wt.% solution is used as an etching solution for the AlN thin films. Polysilicon is used as a structural layer. Highly c-axis oriented AlN thin films are deposited by RF reactive sputtering. Thin layers of chromium on either side of the AlN are used as top and bottom electrodes and also as a mask to etch the AlN and polysilicon layers. The fabricated suspended unimorph structures are tested for scattering parameters using a vector network analyzer. Results show resonant frequencies of devices above 1.7 GHz with an effective electromechanical coupling factor, Keff21.7%.

Introduction

Aluminum nitride (AlN) thin films are interesting in MEMS applications because of their piezoelectricity, high acoustic velocity and chemical stability at high temperatures. Surface micromachined AlN thin film piezoelectric microstructures find many applications in modern telecommunication devices in the form of resonators and filters. A sandwich of AlN thin film between two electrode layers forms a basic configuration of piezoelectric unimorph structures as shown in Fig. 1. Applying a cyclic electric field across this piezoelectric AlN capacitor results in the AlN thin film to expand and contract alternatively causing excitation of acoustic waves. It is also known that thin structural layers can be beneficial for certain applications such as thin film bulk acoustic resonators (TFBAR). The quality of AlN thin films is decisive for electromechanical coupling. AlN grown with (0 0 2) orientation perpendicular to the substrate is favourable for such piezoelectric device applications [1]. Methods such as MOCVD, MOVPE, PLD, RF sputtering, sublimation etc. are used in the literature [2], [3], [4] to grow AlN thin films on several substrates. RF reactive sputtering is one of the common methods used to deposit polycrystalline AlN thin films with preferentially (0 0 2) orientation perpendicular to the substrate in many kinds of substrates [5].

AlN thin film patterning, etch selectivity to the mask layer, compatibility with surface micromachining processes and minimum feature size of free-standing piezoelectric microstructure are key factors in the fabrication process. Wet patterning of AlN thin film using 0.6 wt.% tetra methyl ammonium hydroxide (TMAH) solution at room temperature and a Cr mask layer was already reported [6]. Fabrication of SOI-based AlN thin film suspended devices involving a very thick (15 μm) silicon structural layer was reported earlier [7]. Also, micromachined AlN piezoelectric resonating structures on SiO2 structural layers with germanium as a sacrificial layer have been reported recently [8].

This paper reports on a new silicon-based surface micromachining process for integrating AlN thin films into a surface micromachining process and the fabrication of AlN unimorph suspension devices. These devices use SiO2 as a sacrificial layer. Anisotropic wet etching of AlN using TMAH (25%) solution is used to pattern AlN microstructures using a Cr layer that simultaneously serves as top electrode. The dimensions of the active part of the device are (480 μm × 25 μm). The characteristics of these devices are measured using HP 8510C vector network analyzer (VNA) and an RF probe station.

Section snippets

Deposition of AlN thin films

Highly (0 0 2) textured AlN thin films are necessary for piezoelectric device applications. A Nordiko-2000 RF reactive sputtering machine has been used for the deposition process. The used deposition parameters for AlN thin films are shown in Table 1. For piezoelectric actuating structures, a stack of Cr/AlN/Cr layers was deposited in a single run without breaking the vacuum to ensure better adhesion of the Cr layers with AlN. The deposition was done at substrate temperatures <400 °C which is

Anisotropic wet etching of AlN

AlN thin films on silicon substrates were used to study the etching behaviour of AlN thin films with Cr mask layers. The thickness of the mask layer is about 40–50 nm and it was found to be dense and uniform over the AlN thin films. The Cr layer was patterned first to make etch openings for AlN. It was done by a UV photolithography process with a mask having regular beam structures with a minimum width of 2 μm. A TMAH (25 wt.%) solution was used to etch AlN at room temperature without stirring the

Surface micromachining process

A schematic process description to fabricate AlN piezoelectric free standing microstructures is shown in Fig. 5. A stack of Cr/AlN/Cr layers forms an active area for piezoelectric actuation on the polysilicon structural layer. Three masks were used in the fabrication process. The first mask was used to define the area to be freed by sacrificial etching and the second and third masks were used to pattern the electrode area for top and bottom electrode, respectively.

An ordinary p-type (1 0 0)

Results

The fabricated structures were characterized using a vector network analyzer (VNA) for a frequency range of 1–2 GHz in a one-port resonator configuration. An RF probe station along with a HP 8510C VNA was calibrated using a standard calibration sample for a characteristic impedance of 50 Ω in open, short and load modes. Ground–signal–ground (G–S–G) probes were used with a spacing of 125 μm between them. The calibration procedure was repeated for every selected frequency range of measurements.

Conclusions

AlN thin film piezoelectric microstructures on polysilicon structural layers were fabricated by a surface micromachining process for actuator applications. Highly textured AlN thin films were sputtered on Cr metal layers by RF reactive sputtering in a single run process to form unimorph piezoelectric structures. Top Cr layers are used as electrode and mask to etch the AlN thin films and polysilicon layers. Anisotropic wet etching of AlN thin film is unique in this process with negligible

Acknowledgements

The authors would like to thank Mr. Henk de Vries, Mesa+Test Facility Centre for providing facilities to characterize our devices. This research is supported by the Mesa+Research Institute, University of Twente, The Netherlands.

S. Saravanan received his MS by research degree in precision engineering and instrumentation from the Indian Institute of Technology, Madras, India, in 2000. Since 2001, he has been doing his PhD in the MESA+Institute for Nanotechnology, University of Twente, The Netherlands. His research interests are microactuators and microfabrication processes. His thesis work involves the integration of AlN thin films in surface micromachining process technology and realization of its microstructures as

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S. Saravanan received his MS by research degree in precision engineering and instrumentation from the Indian Institute of Technology, Madras, India, in 2000. Since 2001, he has been doing his PhD in the MESA+Institute for Nanotechnology, University of Twente, The Netherlands. His research interests are microactuators and microfabrication processes. His thesis work involves the integration of AlN thin films in surface micromachining process technology and realization of its microstructures as piezoelectric actuators and thin film acoustic resonators.

J.W. Erwin Berenschot received the BSc degree in applied physics from the Technische Hogeschool in Enschede, The Netherlands, in 1990. Since 1992, he has been employed as a micromachining engineer at the transducer science and technology group of the MESA+Research Institute. His main research area is development and characterization of etching and deposition techniques for the fabrication of micro systems. He has published over 30 reviewed journal papers on micromachining and related topics, and four patent applications.

Dr. Gijs Krijnen received his MSc degree in electrical engineering with honours from the University of Twente following a study on magnetic recording carried out at the Philips Research Laboratories, Eindhoven. In 1992, he received the doctorate degree with honours from the same university and was rewarded the 1993 Veder price of the Dutch Electronics and Radio Engineering Society (NERG) for his PhD thesis on nonlinear integrated optics devices. From 1992 to 1995, he was a fellow of the Royal Netherlands Academy of Arts and Sciences and studied second- and third-order non-linear integrated optics devices. In this period, he was a visiting scientist at the Center for Research and Education in optics and lasers in Orlando, Florida, USA. In 1995–1997, he worked on integrated optic devices for optical telecommunication simultaneously at the University of Twente and the Delft University of Technology. Since1998, he is associate professor in the transducers science and technology (TST) group of the MESA+Research Institute and responsible for the micro-actuator research. His current interests include micro-electro-mechanical systems (MEMS) including bio-mimetic flow-sensors and micro-actuators and nano opto-electro mechanical systems (NOEMS).

Miko Elwenspoek (born 9 December 1948 in Eutin, Germany) studied physics at the Free University of Berlin (West). His masterthesis dealt with Raleigh scattering from liquid glycerol using light coming from a Mössbauer source. From 1977–1979, he worked with Prof. Helfrich on lipid double layers. He conducted experiments on osmotic shrinkage of giant lipid vesicles, and wile preparing light scattering experiments from those giant vesicles worked out the theory of light scattering from large aspheric particles and spherical bubbles. In 1979, he started his PhD work with Prof. Quitmann. This work dealt with relaxation measurements on liquid metals and alloys, in particular alkali metal alloys. The experimental technique used in these experiments is related to nuclear magnetic resonance, but the alignment of the nuclei is done by a nuclear reaction using high energetic α-radiation. This work resulted in a thesis (Freie Universität Berlin, 1983). In 1983, he moved to Nijmegen, The Netherlands, to study crystal growth in the group of Prof. Bennema of the University of Nijmegen. The emphasis lay on growth of organic crystals (in particular naphthalene) from melt and solution. In 1987, he went to the University of Twente, to take charge of the micromechanics group that was part of the sensors and actuators lab, now MESA+Research Institute. Since then his research focused on microelectromechanical systems. In the first years the work was concentrated to coaching design and modelling of micropumps and resonant sensors by PhD students, but since the beginning of the 90ths more and more attendance was given to electrostatic microactuators, including electrostatic motors, and thermal and electromechanical actuators with the aim of building microrobots. Further, research in fabrication technology got much attention, with emphasis on physical chemistry of wet chemical anisotropic etching, materials science of thin films (such as ZnO, TiNi, PZT and fluorocarbon), reactive ion etching of silicon, polymers and metals, wafer bonding and chemical-mechanical polishing. In 1996, he became full professor in transducers science and technology at the Faculty of Electrical engineering, University of Twente. He is fellow of the Institute of Physics and of IEEE. He is enthusiastic on teaching on academic level. In 2001, students elected him as the the best lecturer in the electrical engineering program. He was a co-founder of the education “advanced technology” at the UT and of the master nanotechnology. He enjoys classical music, painting and drawing, and hiking. He is co-author of two books (Silicon Micromachining, by M. Elwenspoek and Henri Jansen, Cambridge University Press, Cambridge, 1998; Mechanical Microsensors, by M. Elwenspoek and Remco Wiegerink, Springer, Heidelberg, 2001), and (co)author of ca. 300 scientific papers.

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