DRIE TECHNOLOGY: FROM MICRO TO NANOAPPLICATIONS

Transcription

1 DRIE TECHNOLOGY: FROM MICRO TO NANOAPPLICATIONS J-M. Thevenoud 1*, B. Mercier 2, T. Bourouina 2, F. Marty 2, M. Puech 1, N. Launay 1 1 Alcatel Micro Machining Systems 98 Avenue de Brogny, Annecy, France Tel Fax jean-marc.thevenoud@alcatelmicromachining.com 2 ESIEE, Ecole Supérieure d ingénieurs en Electronique et Electrotechnique 2 Boulevard Blaise Pascal Cité Descartes BP 99, Noisy le Grand, France Tel Fax mercierb@esiee.fr Abstract Over the past 20 years, the application of micro electronics technology to the fabrication of mechanical devices has completely changed the research in microsensors and microactuators to develop Micro Electrical Mechanical Systems (or MEMS or Microsystems). At the early stage, the micromachining process was mainly achieved by wet chemical etching such as Potassium Hydroxid (KOH) or Tetra Methyl Ammonium Hydroxid (TMAH). During the last 10 years, the Deep Reactive Ion Etching (DRIE) of Silicon has opened new fields of application in MEMS and device integration. Opposite to the wet anisotropic etching, the DRIE process prevents lateral etching of the Si resulting in highly anisotropic etch profiles at high etch rates and with high aspect ratio. Alcatel Micro Machining Systems rapidly grew to be one of the market leaders in deep plasma etching equipment. Over 200 tools are now in use at major industries and laboratories world-wide such as ESIEE, France. The goal is to be at the forefront of deep etching technology while bringing real benefits to customers. Today, fully integrated Microsystems on a single chip include biological or chemical sensors, movable parts and actuators, micro-fluidics, optics and electronics for the control. A review of the typical etched patterns has been carried out, the features sizes are ranging from large exposed areas for Si microphone to comb structures for 3D high precision inertial sensors to nanofeatures. In fact, an optimisation of the DRIE process by varying process parameters including reactive gas flow, pressure, and ion energy in order to minimize the rippling of the sidewalls usually called scalloping effect could even produce well defined nanostructures. The DRIE process coupled with adapted high resolution lithography tools to large surface allows the research community to explore the very innovating and exciting nano-world. 1

2 Introduction The use of DRIE (Deep Reactive Ion Etching) for the fabrication of mechanical devices has completely changed the research in microsensors and microactuators to develop Micro Electrical Mechanical Systems (or MEMS or Microsystems). This is a rapidly emerging technology combining electrical, electronic, mechanical, optical, material, chemical, and fluids engineering disciplines. Nowadays, the mature technology is widely used, the total sales in the MEMS market reached $5.4 billion last year (source: Yole Développement). For example, MEMS devices incorporated into cellphones include silicon microphones, 3D accelerometers, gyroscopes for image stabilization and GPS, microfuel cells, and also biochips for personal weather stations and health care monitoring. 1. DRIE technology The DRIE process prevents lateral etching of the Silicon resulting in highly anisotropic etch profiles at high etch rates and with high aspect ratio compare to wet chemical etching such as Potassium Hydroxid (KOH) or Tetra Methyl Ammonium Hydroxid (TMAH) Principle The most popular DRIE process is the well-known Bosch process [1] based on the use of alternative steps of SF 6 and C 4 F 8. The SF 6 is used to etch the Si, and the C 4 F 8 to passivate the surfaces and to achieve the anisotropic etch of the Silicon. This alternation of etching and passivation steps results in a waving side wall profile, also commonly called scalloping. With the control of the gas flows and pressures, this scalloping can be significantly reduced, to as low as 14 nm. Fig.1: Bosch Process, 250µm holes etched with the Bosch process at 32µm/min 1.2. Etch rate versus exposed area In 2002, the I-Speeder Project [2] developed by Alcatel with Bosch and PerkinElmer resulted in tools that had the fastest etch rate for Silicon etching in the market. The latest I- Productivity project has resulted in further improvement of the etch rate and is now exceeding 30 µm/mn, rate with an excellent etch depth uniformity (Fig. 2) 2

3 Fig. 2: Etch rate versus exposed area 1.3. High aspect ratio (SHARP process) Designers and manufacturers are continuously looking for improved device performances like higher sensitivity of the widespread capacitive detection or actuation of comb structures. Better sensitivity of MEMS devices requires higher aspect ratio features than can be achieved, using the Bosch process. The Aspect Ratio (A.R.) of a given feature, like a trench, is defined as the ratio between the depth of the trench and the width. The Alcatel patented SHARP (Super High Aspect Ratio Process) [3] allows to increase the aspect ratio in comparison with the basic Bosch process. When applying the innovative SHARP process to deep etching of very small features, it was demonstrated that an Aspect Ratio as high as 110 was achievable as shown in Fig. 3. For comparison, a Basic Bosch process doesn t allow to achieve aspect ratios higher than 30. Fig. 3: The SHARP Process for submicron features (Courtesy ESIEE) 1.4. Aspect ratio depending etching The ARDE (Aspect Ratio Depending Etching) is sometimes a limitation as the large features are naturally etched faster than the narrow ones. Alcatel has developed the capability to make disappear this phenomenon by optimizing the passivation step of the Bosch process [4]. The best results obtained presented less than 1% ARDE at 3 µm/mn etch rate, Fig 4. 3

4 200µm trenches Fig. 4: Example of 1% ARDE at 3.0 µm/min Etch Rate 2. The driver applications The DRIE allows to obtain fast etch rates, with a perfect control of the profile, high aspect ratio, smooth sidewalls. This is the perfect tool to be used for the MEMS manufacturing. 2.1 Physical sensors (Inertial, Pressure, etc ) Vibrations, motion and shock can be measured by sensing accelerations. Capacitive sensing provides the highest level of sensitivity combined with low power consumption and lower dependence on temperature variation. MEMS technology and especially DRIE micromachining is fully adapted and takes more and more importance in this field of application. As the Bosch process became available a large amount of new accelerometers, pressure sensors, gyro meters or microphones have been developed. Comb drives, cantilevers or bridges integrity has been satisfied by using LF Bosch process in order to avoid notching effects. Such structures found in a micro pressure sensor are presented Figure 5 [5] Fluidics and Bio applications Figure 5: SEM view of a pressure sensor Microfluidic devices are mainly used in the following applications: Ink jet printers, chemical analysis systems, biological sensing, drug delivery, molecular separation, amplification and sequencing or synthesis of nucleic acids. Fluidic MEMS includes mixers, CFD, pumping, filtration, flow control. 4

5 Today, the most interesting areas are in diagnostic devices for environmental monitoring, food safety, agriculture and human health. The use of microfluidic MEMS devices in place of larger-scale equipment will replace bulky and slow laboratory equipment with portable units that can be deployed in the field where the analysis is needed to provide fast detection of diseases, to improve the traceability in the food industry or to monitor liquid or gas environments. Integration of microfluidic elements with control circuits and detectors provides functions from sample to result. DRIE processes are used for silicon devices in order to create channels, holes, via holes,.etc. The surface quality as well as the roughness or biocompatibility is one of the most important issues for such devices. One example is shown in Figure 6 for illustrating a part of the wide range of DRIE needs in the field of Fluidics and Bio applications. Figure 6: SEM view of a silicon micro mixer [6] 2.3. Vibration based energy harvester [7] Various efforts have already been made to reduce the power consumption of sensors, and to make them autonomous i.e. independent of externally attached power sources. Hence energy should be harvested from the ambient environment to power these sensors. In addition, various chemical materials, currently being used in power sources, can be eliminated. One mechanism to harvest energy from environmental ambient vibrations is electrostatic. Electrostatic-based energy harvesters are more compatible with CMOS process and are considered good for miniaturization. Electrostatic converters need two sets of electrodes. First one is attached to a moving mass called as proof mass and second one is fixed to the substrate. A huge proof mass is normally used to target a low resonance frequency. External vibrations force the mobile part to move relatively to the substrate, which results in a mechanical-to-electrical energy transduction, if a constant charge or voltage is maintained on the electrodes while the capacitance decreases. The high aspect-ratio of the structures is obtained using the Alcatel DRIE pulsed process allowing smooth sidewalls, perfect anisotropy and no negative effect of over-etching time. Top view of the final device is shown in Figure 7. 5

6 SiO2 Mechanical stopper 50 µm Silicon beams Beam width 30 µm (a) (b) Figure 7: (a) & SEM top view after anodic bonding, with 2 close views (b) 2.4. From micro to nano: optical sensors and actuators [8] A nice illustration of DRIE applications for optical sensors is given in the study of a vertical Bragg mirrors on silicon to provide a tunable Fabry-Pérot cavity composed of two mirrors, where tuning is achieved by moving one of the mirrors using SOI electrostatic MEMS actuation. The realized structures are presented on figure 8. Air Si Light beam Fixed Bragg mirror Moving Bragg Silico Tunable Cavity n (a) (b) Figure 8: SEM photo of the realized Bragg mirror (a) and Sketch illustrating principle of operation (b) The DRIE technology can also be used for the fabrication of photonic crystal waveguides [9]. The etching of 205nm diameter rods 5µm height on a pitch of 57nm allows to realize such devices for 1.5 µm wavelength. Figure 9: 2D photonic waveguide bend in Silicon (Courtesy Delft Institute of Microeletronics and submicron Technology) 6

7 2.5. Others [10] Many other applications based on DRIE are very promising and may offer very simple solutions to problems such as protecting surfaces from rain, self-cleaning properties for windows or low friction devices in microfluidics. The purpose is to define surface texture which brings to the surface superhydrophobic properties. DRIE and especially Bosch process can be considered like a very simple way to fabricate such superhydrophobic surfaces. It allows high aspect ratios for micropillar arrays and hydrophobic Teflon-like polymer passivation of the pillar sidewalls. Also, the obtained surfaces did not need any further chemical treatment, and the structured surfaces were observed to be super-hydrophobic. Figure 10 displays the scanning electron micrograph (SEM) of 2µm diameter micropillar arrays of various periods and various heights, after DRIE and Figure 11 shows the result in term of hydrophobicity. Figure 10: SEM images of patterned micropillars after DRIE with Al etchmasks still visible. Etch depths and array periods are respectively: (a) 20µm/6µm, (b) 8µm/5µm, (c) 20µm/20µm Figure 11: Shapes of a drop of water on a flat surface (a) and on two micro-structured surfaces of a 26% density (b) Conclusion The Alcatel DRIE allows to obtain a wide variety of features, in terms of sizes, depths, shapes. This high versatility is used in a broad range of MEMS applications such as physical sensors, optical actuators, biochips and microfluidics. The MEMS market is now a mature market, and the MEMS devices are everywhere, from the telecommunication area to the entertainment world for motion monitoring. The size reduction and the new applications create the opportunity for the DRIE to be used for the realisation of micro and nanofeatures. 7

8 References [1] Laermer,F., Schilp,A., «Method of Anisotropically Etching Si» US patent 5,501,893. [2] European Commission (Framework V, IST , Semiconductor Equipment Assessement SEA : «I-SPEEDER». [3] M. Puech, N. Launay, N. Arnal, P. Godinat, JM. Gruffat, A novel plasma release process and super high aspect ratio process using ICP etching for MEMS, MEMS/NEMS seminar December 3 rd [4] M. Puech, N. Launay, N. Arnal, L. Rolland, JM. Gruffat, 6 th SEMI Microsystems/MEMS seminar, Dec. 4, 2002, pp [5] O. Gigan, H. Chen, O. Robert, G. Amendola, O. Francais, F. Marty, "Fabrication and characterization of resonant SOI micromechanical silicon sensors based on DRIE micromachining, freestanding release process, and process, and silicon direct bonding", Nano and Microtechonology: Materials, Processes, Packaging, and Systems, pp , du 16 au 18 Décembre 2002, Melbourne, Australia. [6] [7] Ayyaz Mahmood Paracha, P. Basset, F. Marty, A. Vaisman Chasin, P. Poulichet, T. Bourouina, "A high power density electrostatic vibration to electric energy converter based on an In- Plane Overlap Plate (IPOP) mechanism", Proceedings of DTIP of MOEMS & MEMS, Stresa, Italy, April 25-27, [8] B. Saadany, M. Malak, M. Kubota, F. Marty, Y. Mita, K. Diaa, T. Bourouina, "Free- Space Tunable and Drop Optical Filters Using Vertical Bragg Mirrors on Silicon", IEEE Journal on Selected Topics in Quantum Electronics, N 6, Vol. 12, pp , IEEE, du 1 Novembre au 31 Décembre [9] T. Zijlstra, E. van der Drift, M. J. A. de Dood, E. Snoeks, A. Polman, Fabrication of twodimensional photonic crystal waveguides for 1.5µm in silicon by deep anisotropic dry etching, J. Vac. Sci. Technol. B17 (6), pp , Nov/Dec 1999 [10] M. Callies, Y. Chen, F. Marty, A. Pepin, D. Quere, "Microfabricated textured surfaces for super-hydrophobicity investigations", MNE'2004, 2004, Rotterdam. 8