Luận án: Thiết kế, chế tạo cảm biến áp suất SAW dùng Nitrure Nhôm (AlN)

UNIVERSITE JOSEPH FOURIER- GRENOBLE I SCIENCES.SANTE N° attribué par la bibliothèque |__|__|__|__|__|__|__|__|__|__| THESE pour obtenir le grade de DOCTEUR DE L’UNIVERSITE JOSEPH FOURIER Spécialité : Radio-Fréquence préparée au laboratoire CEA/LETI-Minatec - IMEP-LAHC dans le cadre de l’Ecole Doctorale "Electronique, Electrotechnique, Automatique, Télécommunication, Signal" présentée et soutenue publiquement par Trang HOANG Le 19/01/2009 TITRE Design and realization of SAW pressure sensor using Aluminum Nitride DIRECTEUR DE THESE Philippe BENECH JURY M.Daniel Barbier Président Mme. Hélène Tap-Beteille Rapporteur M. Gilles Despaux Rapporteur M.Patrice Rey Examinateur M.Philippe Robert Examinateur M.Philippe Benech Directeur de thèse Abstract Résumé L'objectif de cette thèse est la conception d'un capteur de pression à ondes de surface utilisant le nitrure d'aluminium (AlN). Les études théoriques, la réalisation et la caractérisation du capteur de pression sur différentes structures à ondes de surface sont présentées.

La modélisation du capteur est effectuée en utilisant un circuit équivalent basé sur le modèle de Mason et la méthode des modes couplés. Les paramètres des ondes de surfaces sont obtenus par calcul et analysés pour différentes structures telles que AlN/SiO2/Si, AlN/Si et AlN/Mo/Si. A partir de ces analyses, nous avons montré que la vitesse des ondes ainsi que le facteur de couplage peuvent dépendre du milieu de propagation. Pour chaque type de structure utilisant l'AlN, nous déterminons la plage d'épaisseur de la couche d'AlN pour laquelle la vitesse des ondes et le facteur de couplage présentent une faible dépendance au regard de l'épaisseur d'AlN.

Les dispositifs à ondes de surface doivent être conçus, en particulier pour le choix des épaisseurs de différentes couches, en tenant compte de la précision du procédé de fabrication, afin de réduire les dispersions de caractéristiques des capteurs. En outre, nous avons analysé le comportement mécanique de la membrane en présence d'une pression et nous en avons déduit la sensibilité du capteur. Les effets des variations de température sur une structure à ondes de surface (SAW) sont étudiés. Pour des applications dans le domaine de la mesure de pressions, nous proposons une méthode de réduction des effets des variations de température.

Pour le précédé de fabrication, nous proposons d'utiliser le micro-usinage de surface. Ce type de procédé de fabrication permet d'obtenir exactement les dimensions des membranes utilisées dans les capteurs de pression et il permet aussi de réaliser tout type de géométrie grâce au procédé d'arrêt de gravure du silicium. Les films de nitrure d'aluminium sont caractérisés au cours de la fabrication. Nous avons trouvé que pour améliorer le comportement piézoélectrique de l'AlN, trois voies sont possibles : utiliser une couche de molybdène sous l'AlN, réduire la rugosité de la couche se trouvant sous l'AlN jusqu'à 0,2 nm et augmenter l'épaisseur de l'AlN.

Les pertes acoustiques de propagation, le facteur de couplage, l'effet d'une couche de Mo et l'effet du film mince de polyimide sur la fréquence centrale sont analysés expérimentalement. En conclusion, la sensibilité de pression mesurée de notre dispositif est présentée. Ce dernier résultat est très prometteur. Mots clés : ondes acoustique de surface, Nitrure d'Aluminium (AlN), micro-usinage de surface, capteur de pression Abstract The goal of this thesis is the design of a surface acoustic wave (SAW) pressure sensor using Aluminium Nitride (AlN).

Theoretical studies, realization, and characterization of the pressure sensor on different SAW structures are presented. The modeling of the sensor was performed using an equivalent circuit based on Mason model and Coupling-Of-Mode. The theoretical study, SAW parameters in different structures of AlN/SiO2/Si, AlN/Si, and AlN/Mo/Si are calculated and analyzed. From these analysis, the wave velocity as well as coupling factor could depend on the wave propagation medium.

For each structure using AlN, we establish the range of thickness of AlN layer, in which there is a weak dependence of the wave velocity and coupling factor on the AlN layer thickness. The SAW devices should be designed, in particular for the choice of the thicknesses of the different layers, by taking into account the accuracy of the manufacturing process, to reduce dispersion effects on the sensors characteristics. Besides, we also performed the mechanical analysis of the membrane under pressure and we have deduced the pressure sensitivity. The effect of frequency variation due to temperature change in SAW device using AlN is given.

For pressure measurement applications, we propose a method to reduce temperature change effects. Concerning the fabrication process, we propose to use surface micro-machining. This kind of fabrication process allows to obtain exactly the dimensions of membranes used in pressure sensors and it also allows to have any kind of geometry due to the silicon etch stop wall. Characterizations of AlN film are done during fabrication process.

We found that to increase the piezoelectric behavior of AlN, there are three possible ways: using a bottom Mo layer, decreasing the roughness of the layer below the AlN layer up to 0.2nm and increasing the thickness of AlN. Acoustic propagation losses, coupling factor, effect of Mo layer and the effect of thin polyimide film on the center frequency are experimentally analyzed. Finally, the measured pressure sensitivity of our device is presented. This last result is promising.

Key words: surface acoustic wave (SAW), Aluminium Nitride (AlN), surface micro-machining; pressure sensor. Table of content Table of content Introduction. 1 UNIVERSITE JOSEPH FOURIER- GRENOBLE I.1 Acoustic wave devices.1 Thickness Shear Mode (TSM) resonator.2 Surface Acoustic Wave Device.1 SAW excitation and detection.2 SAW perturbation mechanisms.3 Acoustic Plate Mode (APM) devices .4 Flexural Plate Wave (FPW) or Lamb wave device .5 Comparation between four sensors .6 SAW sensor and the application pressure sensor in this research .1 SAW pressure sensor with one port IDT.2 SAW pressure sensor with two ports IDT. Aluminum Nitride (AlN) and its applications in SAW devices.1 Piezoelectric materials and the choice of AlN.2 General Information of AlN .3 Modelling SAW devices .1 Why Equivalent Circuit model is chosen? .2 The Finite Element Model (FEM).4 Micromachining process, the choice of surface micromachining.27 Chapter 2 SAW PARAMETERS ANALYSIS AND EQUIVALENT CIRCUIT OF SAW DELAY LINE.2 Calculation of SAW properties .1 Wave velocity, coupling factor in AlN/Si structure .2 Wave velocity, coupling factor in AlN/SiO2/Si structure.3 Wave velocity, coupling factor in AlN/Mo/Si structure.3 Equivalent circuit for SAW delay line based on Mason model .1 Equivalent circuit for IDT including N periodic sections .2 Equivalent circuit for propagation path.3 Equivalent circuit for SAW delay line .4 Equivalent Circuit for IDT Based On The Coupling-Of-Mode Theory.1 COM equation for particle velocities .2 Equivalent circuit for IDT based on com theory .3 Equivalent circuit for propagation path based on com theory .4 Equivalent circuit for SAW delay line based on COM theory .5 Comparison of Equivalent circuit of SAW device based on Mason model and COM thoery .52 Chapter 3 DESIGN OF SAW PRESSURE SENSOR DEVICES.2 Temperature compensated structure for SAW device .1 Temperature dependence of Si, SiO2, AlN properties .2 Temperature Coefficient of Frequency (TCF) and temperature compensated structure for SAW sensor 56 3.3 Pressure dependence of frequency and phase in saw delay line .1 Mechanical analysis of membrane under pressure .2 Pressure-dependence of frequency by pressure dependence of AlN elastic properties .3 Pressure-dependence of frequency by delay line .4 Pressure-dependence of phase shift.74 i Table of content Chapter 4 FABRICATION PROCESS.1 Trench, counter masque, hole, and PSG layers .2 Metal AlCu and polyimide layers .3 Creating the stop wall of etching SiO2- Trench .4 Non-selective epitaxy .5 COUNTER MASK lithography, etching Si and CMP process.7 HF Etching of the sacrificial layer .9 Depositing AlN as the piezoelectric layer and its properties .1 Influence of substrate roughness on crytal quality of AlN .2 Dependence of FWHM of AlN on AlN thickness.3 Dependence of FWHM of AlN on using bottom MO layer.4 AlN at high temperature.10 Metal layer AlCu for IDT and probes .11 Polyimide as absorber .101 Chapter 5 CHARACTERIZATION OF SAW DEVICE .1 The Square Resistance: Van Der Pauw.2 Isolation and continuity.3 Measuring under etching .4 Mask for parametric test.1 Propagation losses measurement.2 Piezoelectric coupling factor extraction .3 Comparison between experiment and simulation.4 Effect of Mo layer on performance of AlN/Si SAW device .5 Effect of thin Polyimide film.6 Device under pressure .128 CONCLUSION and PERSPECTIVE.

151 List of publication paper. 155 Appendix A Properties of Si, SiO2, AlN and Mo .137 Appendix B Development of calculation for equivalent circuit of SAW device.139 Appendix C Equipments used to control each fabrication step and to characterise device. 2 SCANNING ELECTRON MICROSCOPE (SEM). 3 VEECO PROFILING SYSTEM.

4 ATOMIC FORCE MICROGRAPH (AFM). 5 X-RAY DIFFRACTION (XRD) SYSTEM. 6 SURFACE PROFILER TENCOR P-11. 7 FLEXUS F2320 DUAL WAVELENGTH STRESS MEASUREMENT SYSTEM.

185 ii List of tables List of Tables Table 1.1 QUALITATIVE CHARACTERISTICS OF THE FOUR SENSOR FAMILIES DISCUSSED .2 ADVANTAGES AND DISADVANTAGES OF SOME PIEZOELECTRIC MATERIALS COMMON USED IN SAW DEVICES .3 COMPARISON BETWEEN BULK AND SURFACE MICROMACHINING TECHNOLOGIES.1 TEMPERATURE DEPENDENCE OF SI, SIO2 AND ALN PROPERTIES .2 TEMPERATURE DEPENDENCE OF C12 AND C13 OF ALN .3 VALUES OF TCFS IN SAW DEVICES AT DIFFERENT TEMPERATURE (0C) .4 THE PRESSURE DEPENDENCE OF ALN ELASTIC CONSTANTS .5 VALUES OF PCFS IN ALN/SIO2(KHSIO2= 0.1571)/SI SAW DEVICES AT DIFFERENT PRESSURE (BAR) .6 APE COEFFICIENT IN SAW DEVICE ALN/SIO2 (KHSIO2=0.7 THE VALUE OF COEFFICIENT APL .8 COMPARISON BETWEEN THE PRESSURE DEPENDENCE OF FREQUENCY SHIFT, PRESSURE DEPENDENCE OF ELASTIC CONSTANT AND EFFECT OF PRESSURE ON DELAY LINE IN SAW DEVICE (WAVELENGTH: 8μm).2 THE DIMENSION OF DEVICES.3 MEASURED VALUES OF TRAPEZOID GROOVE, A AND B .4 FABRICATION STEPS TO TEST SELECTIVE EPITAXY .5 SPEED OF ETCHING SACRIFICIAL LAYER SIO2 BY WET HF 49% AND VAPOR HF ON THE WAFER WITHOUT TRENCHES ETCHED .6 SPEED OF ETCHING SACRIFICIAL LAYER SIO2 BY WET HF 49% ON THE WAFER WITH TRENCHES ETCHED.7 PARAMETERS USED FOR DEPOSITION OF ALN.8 STEPS TO CHARACTERISE ALN IN HIGH TEMPERATURE, FOUR SAPMPLES .9 SOLUTION PROPERTIES OF PI-2610 .1 DESCRIPTION OF MASK 1 FOR PARAMETRIC TESTS: SQUARE RESISTANCE, ISOLATION, CONTINUITY, UNDER ETCHING .2 THE VALUE OF THE FINGER WIDTH IN MASK 1, MASK 2 (Figure 5.3 COMPARISON OF MEASUREMENTS OF SAW FILTERS D5.4 COMPARISON BETWEEN CENTER FREQUENCIES OF DEVICES WITH DIFFERENT LENGTHS OF POLYIMIDE .5 EFFECT OF DIFFERENT THICKNESSES OF POLYIMIDE ON CENTER FREQUENCY. 1 ELASTIC CONSTANT OF Si, SiO2, AlN AND Mo. 2 PIEZOELECTRIC CONSTANT OF AlN. 3 DIELECTRIC CONSTANTS OF Si, SiO2.

4 DIELECTRIC CONSTANTS OF AlN. 5 MASS DENSITY OF Si, SiO2, AlN AND Mo. 1 FWHM MEASUREMENT CONDITIONS .162 iii List of figures List of Figures Figure 1.1 Schematic sketches of the four types acoustic sensors (a) the Thickness Shear Mode (TSM) resonator, (b) the Surface Acoustic Wave (SAW) device, (c) the Acoustic Plate Mode (APM) device, (d) the Flexural Plate Wave (FPW) or Lamb wave device.2 The thickness shear mode resonator .3 Rayleigh waves move vertically in a direction normal to the surface plane of a surface acoustic wave (SAW) sensor. SAW waves are very sensitive to surface changes, but do not work well for most liquid sensing applications.4 The wave energy is confined to within one wavelength from the surface of a SAW sensor.

This characteristic yields a sensor that is very sensitive to interactions with the surface .5 SAW with IDT excitation and detection.6 In the shear-horizontal acoustic plate mode (SH-APM) sensor, the waves travel between the top and bottom surfaces of the plate, allowing sensing on either side.7 Schematic of a flexural plate wave device. The side view shows the different layers and membrane movement. Interdigital electrodes are used for actuation .8 Side views and cross sections of four devices .9 Environmental influences to the SAW sensors.10 SAW wireless pressure sensor with one broadband reflective delay line .11 SAW pressure sensor, one IDT and with external sensor circuit Z .12 General structure of a SAW pressure sensor with two IDTs .13 Hexagonal SAW device.2 Calculated values of wave velocity V0 and Vs in SAW device AlN/Si substrate depend on the normalized thickness khAlN of AlN layer .