Integrated; ;: ;; Silicon Thermopile Infrared Detectors
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INTEGRATED; ;: ;; SILICON THERMOPILE INFRARED DETECTORS * , i *Fs - ^ ■^ ^ c^ INTEGRATED SILICON THERMOPILE INFRARED DETECTORS INTEGRATED SILICON THERMOPILE INFRARED DETECTORS Infrarooddetectoren op basis van geintegreerd silicium thermozuilen Proefschrift ter verkrijging van de graad van doctor in de technische wetenschappen aan de Technische Universiteit Delft op gezag van de Rector Magnificus, prof.dr. J.M. Dirken, in het openbaar te verdedigen ten overstaan van een commissie, door het College van Dekanen daartoe aangewezen, op donderdag 1 oktober 1987, te 16.00 uur door Pasqualina Maria Sarro geboren te Piedimonte Matese, Italië dottore in Fisica TR diss 1571 Dit proefschrift is goedgekeurd door de promotor Prof.dr.ir. S. Middeihoek ai miei genitori aan René en Marco ed alia mia nonna TABLE OF CONTENTS Page 1. INTRODUCTION 1 1.1 Aim of the work 1 1.2 Organization of the thesis 2 2. OVERVIEW OF INFRARED DETECTORS 3 2.1 Introduction 3 2.2 Detection of infrared radiation 3 2.2.1 Infrared radiation 3 2.2.2 The photon detection process 6 2.2.3 The thermal detection process 10 2.3 Thermal detectors 10 2.3.1 Thermopile detectors 11 2.3.2 Bolometer detectors 13 2.3.3 Pyroelectric detectors 15 2.2.4 Others 17 2.4 Optical detectors versus thermal detectors 18 3. THE SILICON THERMOPILE INFRARED DETECTOR 21 3.1 Introduction 21. 3.2 Thermoelectric effects 22 3.2.1 The Seebeck effect 22 3.2.2 The Peltier effect 25 3.2.3 The Thomson effect 27 3.2.4 The Seebeck coefficient 28 3.2.5 Figure of merit 33 3.3 Integrated silicon thermopiles 35 3.3.1 Thermopile performance 35 3.3.2 Use of thermopiles in thermal sensors 38 3.4 The silicon thermopile infrared detector 39 3.4.1 The working principle 39 3.4.2 Design criteria 40 vn 4. FABRICATION PROCESS 45 4.1 Introduction 45 4.2 The cantilever beam structure 46 4.2.1 The etch process 46 4.2.2 Influence of the oxide thickness 55 4.2.3 Influence of the aluminum interconnection pattern 57 4.2.4 Influence of other parameters 58 4.3 IR detector fabrication process 61 5. EXPERIMENTAL RESULTS 67 5.1 Introduction 67 5.2 The single detector 67 5.2.1 The detector layout 67 5.2.2 Responsivity to blackbody radiation 73 5.2.3 Relative detectivity, NEP and time constant 81 5.2.4 Spectral response 83 5.2.5 Spatial homogeneity 84 5.3 The infrared sensing array 86 5.3.1 The array layout 86 5.3.2 Responsivity to blackbody radiation 88 5.3.3 Relative detectivity, NEP and time constant 91 5.3.4 The array as part of a monochromatic radiation sensor 92 6. DISCUSSION AND CONCLUSIONS 99 REFERENCES 103 LIST OF SYMBOLS 108 SUMMARY 110 SAMENVATTING 112 ACKNOWLEDGMENTS 114 ABOUT THE AUTHOR 116 viii 1. INTRODUCTION 1.1 Aim of the work Electronic measurement and control systems in general consist of an input transducer, a signal processor and an output transducer [1.1]. In the input transducer, often called the sensor, a measurand such as temperature, pressure, radiation, chemical composition or magnetic field direction, is converted into an electrical signal. In the signal processor, the electronic signal is modified (amplified, filtered, etc.). In the output transducer, the electronic signal is converted into a signal which can be perceived by one of our senses (display) or which can cause some action (actuator). While an abundance of very sophisticated low-cost microelectronic components is available today, sensors with performance/price ratios comparable to that of microelectronic circuits are much in demand. One group of sensors of current interest are silicon sensors. Silicon is a very promising material for sensors not only because it shows many large physical effects which may be used for sensing purposes [1.2], but also because a dependable, diverse and sophisticated silicon planar technology is available nowadays. The application of silicon planar technology to sensors offers several advantages [1.3,1.4]: - The dimensions of the sensor can be very small, so that the measurand will not be significantly influenced by the sensor, the power consumption can be very small and the frequency response can be good. - The batch-fabrication technique allows large quantities of sensors to be produced, thereby reducing their price. - The sensor and the signal processing electronics (or a part of it) may be integrated on the same chip, to obtain a so-called smart sensor. Silicon also has very good mechanical properties and micromachining of three-dimensional structures is feasible. Further, it exhibits no hysteresis if subjected to repeated stress and in terms of their chemistry silicon and its oxide are inert in many hostile environments. Of course, the use of silicon also has some drawbacks such as a limited temperature range of operation (most sensors only work properly between - 50 and + 150°C). In addition, packaging often presents some difficulties (the sensor may have to operate in a hostile environment in which the usual integrated circuit (IC) encapsulation is inadequate). However, the advantages of integrated sensors greatly outweigh the disadvantages. 1 One of the physical effects that can be exploited for thermal sensing is the Seebeck effect. This self-generating effect, in which a temperature difference is converted into an electric voltage, is rather large in silicon. Thermocouples or thermopiles (a pile of thermocouples connected in series) based on this effect have been used to measure temperature differences or to convert thermal energy into electrical energy. Several thermal sensors, based on the Seebeck effect and able to measure mechanical, radiant and chemical signals, have been realized in silicon and some of them are fabricated by integrated circuit technology [1.5]. The aim of this work was to investigate the possibility of realizing one of these thermal sensors, namely a thermal infrared detector based on an integrated silicon thermopile. The use of infrared (IR) detectors, both thermal and photon, is not confined to research and development laboratories, but has many applications in industry, medicine, meteorology, astronomy and defence. In fact, without touching an object, IR technology can determine its existence, its shape, its temperature and its composition [1.6-1.7]. Thermal detectors, although generally slower and less sensitive than photon detectors, are still widely used, because they respond equally well to a broad range of infrared radiation, operate at room temperature and are inexpensive. These unique properties make them suitable for various tasks that cannot be fulfilled by photon type detectors, and as such are sufficient reasons to continue to develop them, particularly for applications where inexpensive, but reliable detectors are required [1.8]. The device presented in this thesis is a thermal type detector of infrared radiation, based on an integrated silicon thermopile. By using silicon not only as a supporting structure, but also as one of the two thermocouple materials, such a device benefits from both of the above-mentioned advantages offered by silicon IC technology and from the large value of the Seebeck coefficient in silicon. 1.2 Organization of the thesis In Chapter 2 the infrared radiation detection process will be briefly described and an overview of the thermal type infrared detectors will be presented. The Seebeck effect in silicon and the integrated silicon thermopile, the device exploiting this effect, will be investigated in Chapter 3. In that chapter the infrared detector based on the thermopile will be analyzed theoretically. The fabrication process used to fabricate both single detector and linear arrays will be described in Chapter 4, while the experimental results will be extensively presented in Chapter 5. Finally, a discussion of these results together with some conclusions will be the subject of Chapter 6. 2 2. OVERVIEW OF INFRARED DETECTORS 2.1 Introduction Although the infrared part of the spectrum was discovered in 1800 by Herschel, detection of infrared radiation goes back a long way. Man has always been aware of the heating effects of first the sun and then of fire. The only heat sensors available then were the skin sensors distributed over the body, with those in the hands and the face being the most convenient for use. The first experiment to sense heat emitted by a terrestrial object appears to be that made by della Porta at the end of the 16th century [2.1]. He noted (using his face) the concentration by a concave metal mirror of the heat of a distant candle and the cold from a block of ice. This experiment was repeated in Florence by the Accademia del Cimento in 1660 when, for the first time, a detector replaced the hand or face. The detector used was a thermometer, which was a prototype of the modern liquid-in-glass thermometer. Since then many detectors for infrared radiation have been discovered and are usually classified into two general classes: thermal detectors and photon detectors [2.2]. In photon detectors, the incident radiation excites electronic transitions which change the electronic state of the detector. In thermal detectors, the energy of its absorbed radiation raises the temperature of the detecting element. This increase in temperature will cause changes in the temperature dependent properties of the detector. Monitoring one of these changes enables the radiation to be detected. In the following section, after a short review of the infrared radiation characteristics, we will describe briefly these two types of detection processes. In the last section of this chapter we will review the most important types of thermal detectors, pointing out their characteristics and their limits. Finally, the advantages and disadvantages of both optical and thermal detectors will be briefly discussed.