Microelectronic Engineering 57–58 (2001) 825–831www.elsevier.com/ locate /mee

Evaluation and fabrication of AFM array for ESA-Midas /Rosettaspace mission

a a a a a aW. Barth , T. Debski , N. Abedinov , Tz. Ivanov , H. Heerlein , B. Volland ,a ,1 a , b b c*T. Gotszalk , I.W. Rangelow , K. Torkar , K. Fritzenwallner , P. Grabiec ,

c d d´K. Studzinska , I. Kostic , P. HudekaInstitute of Technological Physics, IMA, University of Kassel, FB-18, Heinrich-Plett Str. 40, D-34132 Kassel, Germany

bSpace Research Institute of the Austrian Academy of Sciences, Infeldgasse 12, A-80310 Graz, AustriacInstitute of Electron Technology, Al. Lotnikow 32/46, PL-02-668 Warsaw, Poland

dSlovak Academy of Sciences, Dubravska 9, SK-84237 Bratislava, Slovakia

Abstract

The MIDAS (Micro-Imaging Dust Analysis System) experiment is dedicated to the micro-textural and statistical analysisof cometary dust particles. The instrument is based on the technique of atomic force microscopy. The comparative simplicityand robustness of the technique lends itself to advanced space applications. The instrument is considered as essential for thismission since, for the first time, it has the capability of three-dimensional imaging of interplanetary and pristine cometaryparticles in the manometer to micrometer range. In this paper we describe our effort to evaluate and fabricate the AFMarrays for the ESA-Midas /Rosetta space mission. 2001 Elsevier Science B.V. All rights reserved.

Keywords: Atomic force microscopy; Atomic force microscopy space application; Piezoresistive cantilever sensors

1. Introduction

Fabrication of piezoresistive AFM sensors for ‘earth’ applications have been demonstrated and suchsensors are commercially available [1,2]. In the project presented here, we use our experience gainedin the last 10 years during the development and realization of the piezoresistive sensor family used indifferent scanning probe microscopy applications [3–8]. However the AFM sensor arrays for work inspace have to fulfil a lot of non-conventional requirements. Additionally a critical issue is thereliability of the piezoresistive AFM sensor in case of long-term space operation. Beginning with the

*Corresponding author.E-mail address: rangelow@hrz.uni-kassel.de (I.W. Rangelow).1Present address: Institute of Microsystems, Wroclaw University of Technology, ul. Janiszewskiego 11/17, 50-372 Wroclaw,Poland.

0167-9317/01/$ – see front matter 2001 Elsevier Science B.V. All rights reserved.PI I : S0167-9317( 01 )00558-5

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range of work temperature, acceleration on board the spacecraft, space radiation, etc., we develop andrealise from the technological point of view, a new sensor array specially designed for work in space.The most suitable location of the piezoresistors on the beam is the beam surface, because of the highermechanical stress, but the stability is not satisfactory due to the instability of the silicon/silicon-oxideinterface states.

2. Simulation

Every technological experiment must be preceded by detailed considerations of the assumptions,proposed solutions and expected results. Technical experience supported by advanced CAE techniquesand software is very relevant at this point. Such an approach is common practice in IET, where thesimulation package developed by SILVACO (USA) is used. Two simulation tools, ATHENA/SSUPREM4 and ATLAS/SPISCES [9], are the basic parts of our system.

ATHENA is a very complex program for numerical simulation of sequences of technologicalprocesses. It enables both one- and two-dimensional simulation of the following processes: diffusion,oxidation, deposition, chemical and plasma etching and ion implantation. The models of theseprocesses are controlled by a large number of parameters which determine the model’s complexityand characteristics. ATLAS is software for numerical two-dimensional simulation of electricalcharacteristics of semiconductor devices. It enables calculation of d.c., a.c. and transient characteris-tics. The ATHENA/SSUPREM4 input data file contains a detailed description of the main materialused in the device, its crystallographical orientation, type of conductivity and resistivity. Modeling ofthe subsequent processes is the next stage of simulation. The engineer determines the processparameters, such as operation duration, temperatures, process atmosphere, doping concentrations,type, stoichiometry and density of deposited layers, doses and energies of ion implantations. Afterthese calculations we obtain the doping concentration distributions, junction depths and the layerthicknesses in the final structure. Fig. 1a–c represents cross-sections of the piezoresistive AFMcantilever simulated by mean Virtual Wafer Fab (VWF).

The output data of process simulation establish the input data for the ATLAS/SPISCES program.At the moment all the electrodes in the devices must be defined. Next, the input signals must bedetermined. After the simulation we obtain a solution which comprises not only device electricalcharacteristics but spatial distributions of variables such as potential and current density.

3. Fabrication

The fabrication process is a modification of our basic double side silicon micromachining procedure[5] developed for the manufacturing of the piezoresistive AFM microprobes. First, 12-mm high sharptip was formed through anisotropic etching of h338j fast etching planes in alkali solution at 608C [6].Next, standard CMOS processing such as oxidation, phosphorus and boron diffusion, ion implanta-tion, dry and wet etching, insulator and aluminum film deposition and photolithography were

1sequentially applied to form piezoresistors, p diffusion connecting paths, contact windows andmetallic connections at the front side of the wafer [4–6]. We designed resistors by considering bulkscattering of electrical carrier and achieved deflection sensitivity of 0.8 mV per nm (for 1 V bridge

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Fig. 1. ATHENA simulations. A cross-sections of the piezoresistive cantilever: (a) tip etching, (b) piezoresistors diffusion,(c) final structure.

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Fig. 2. The SEM picture of the piezoresistive AFM cantilever array.

voltage). In the following back side processing sequence, a corner compensated membrane patternwas created by two-side photolithography process and back side anisotropic deep etching of silicon inalkali solution at 608C, to create a 15-mm thick silicon membrane in the future beam area [6,7].Finally, the cantilever was defined in the membrane by a last photolithography step at the top side andby silicon dry etching in Oxford Instruments ICP plasma system [7,8]. Figs. 2–4 represent the SEMpicture of the piezoresistive AFM cantilever array and tip details.

4. Measurements

4.1. Atomic force microscopy

The cantilever array (Fig. 5) will be working in tapping mode. The lateral resolution obtained intapping mode is comparable to that of contact mode. However, tapping mode images often representnot only the topography, but also to some extent the elastic properties of the sample underinvestigation.

4.2. Magnetic force microscopy (MFM)

Magnetic force microscopy is planned to cover some of the MIDAS tips with a thin layer of cobaltto map additionally the magnetic structures of the particles. Fig. 6 shows a characteristic of theresonance frequency of our cantilevers. The high resolution of MIDAS requires a mechanically stabledesign and environment. According to the current analysis of Rosetta’s microvibration sources, thepropagation of vibrations to MIDAS and their effect on its measurements, the dominant source ofmechanical noise will be movement of the high gain antenna and reaction wheels. The dust collectorwheel and the scanner system are mounted on a special antivibration plate (Fig. 7).

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Fig. 3. The SEM picture of the single cantilever.

5. Mission

The mission goal is a rendezvous with comet 46 P/Wirtanen. On its 8-year journey to the comet,the spacecraft will pass close to two asteroids (Otawara and Siwa are now the planned targets).Rosetta will study the nucleus of comet Wirtanen and its environment in great detail for a period ofnearly 2 years, the near-nucleus phase starting at a heliocentric distance of | 3.25 AU, with

Fig. 4. The SEM picture of the sensor’s tip.

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Fig. 5. The piezoresistive AFM 16-cantilever array module with holder.

far-observation activities leading ultimately to close observation (from | 1-km distance). Rosetta willbe launched in January 2003 by an Ariane-5 from Kourou, French Guiana. To gain enough orbitalenergy to reach its target, one Mars and two Earth gravity assists will be required. The long missionduration required the introduction of extended hibernation periods.

Fig. 6. Characteristic resonance frequency curve of the AFM sensor.

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Fig. 7. Micro-Imaging Dust Analysis System (MIDAS) AFM measurement system.

References

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