Micromechanical Sensors

Transcription

1 Preface This report gives a very brief overview of micromechanical sensors, a field which is relatively new but has already filled bookshelves with meters of journal and conference papers and which has attracted great activity in the past years. Due to the broad nature of the subject it can only form a first sampling and it is impossible to go into any substantial detail on the many subjects that could fill chapters or entire books on there own. Micromechanical sensors have become possible due to the development of micromachining technologies and many of the concepts and principles on which the sensors rely can only be used courtesy of this technology. That is why a substantial part of this report is dedicated to this technology. Just not to leave the reader completely in the dark, at the end a extensive list of references is included for further reading. Introduction Micromechanical sensors form a class of sensors which we here loosely define as sensors which are sensistive in one way or another to mechanical properties and which are made by micromachining technology (disregarding the actual size of the sensors). In practice, many of the sensors complying to this definition will indeed be rather small as compared to their "macro" counterparts although the latter ones can be rather small as well (especially when fabricated using precision engineering technology). In this part of the report we will not go in detail into all the specifics of micromechanical sensors, simply since we neither have the room nor the time to cover an area of research being so wide and so multidisciplinary in nature. What we will do, though, is give a short overview of some important aspects of micromechanical sensors and compile a list of references which will be useful for further reading 1. Miniaturisation and scaling laws One aspect of micromechanical sensors is that they are made by micromachining technology which basically has been adopted from microelectronics. As such, the field of micromechanical sensor research and fabrication forms a part of a wider field called Micro System Technology (MST) (mostly used in Europe) or Micro Electro-Mechanical Systems (MEMS). Although in the field of MEMS there is no driving force to constantly reduce feature sizes, the efforts made in this direction in the field of microelectronics are used beneficially. It offers MEMS researchers and engineers a toolbox increasingly meeting their requirements in terms of accuracy and feature size. If one had to give an indication of the feature sizes of MEMS devices one could say that these roughly have dimensions on the order of one micrometer to one millimetre, albeit that this is no restricting definition. Although these dimensions may look modestly small in respect of today s advent of nanotechnology, there is still a huge transition going from "macro-sensors" to micro-sensors which is related to various scaling laws. This may be easily appreciated looking at a unit-cell in the form of a cube with (linear) unit dimension l. Since the volume, and hence the mass, are proportional to l 3, and the surface is proportional to l 2, the ratio of body to surface forces will be proportional to l. In other words, scaling down a "macro-sensor" from 1 cm to 10 micrometer, means that the surface forces will play a 1000-fold more important role. This is for example reflected in the fact that gravity normally does not play an important role in micromechanical sensors 2 (nor do inertial forces) except when sensors are especially aimed at measuring these effects 3. Looking at diffusion, be it particles or fluxes (e.g. heat), diffusion times scale with l 2. Hence, thermal processes will be 100 times faster when one reduces a system by a factor of 10. More examples of scaling laws can be found in [Elwenspoek00] In this respect there is one publication which has been very useful for this report and which covers quite some material used in this report [Elwenspoek00] E.g.: the deflection of a beam under its own weight scales as l 2. So a ten times smaller beam, bends 100 times less. In fact in these situations the small mass associated with micromechanical sensors is a disadvantage. GK 1

2 These examples show that we cannot (always, simply) transfer our (intuitive) understanding of mechanical systems from the macro-world to the micro-domain. In fact, we always have to look carefully at the systems, especially at the numbers that describe specific properties of the physics that we study (e.g. the Reynolds number). By doing so, we will find limitations as well as new possibilities of micro-sensors relative to their macro-world counterparts. The systems aspect of micromechanical sensors There is an important aspect of microsensors: being small, and generally producing small signals, they do not trivially interface with the macro-world. Therefore, micromechanical sensors cannot be viewed as separate entities but they should actually be considered as microsystems involving not only the sensing microstructure but the interfacing and packaging as well. Hence, the design and development-process of microsensors should simultaneously include the interfacing and packaging aspects in order to successfully come to a working micro-sensor-system. In some cases, where one cannot rely on standard technology (i.e. in the form of foundry-processes 4 ), it may be even necessary to design the microfabrication processes hand in hand with the microsystems. The foregoing also reveals the multi-disciplinary nature of microsensorsystems; not only are the mechanical properties of structures important, but generally all the physical domains that take part in the transduction of the measurand to the eventual signal. This may involve such things as mechanics, for the actual response of the sensor (i.e. a bending membrane), optics (for read-out of a displacement), opto-electronic conversion and electronics for further signal processing. Moreover, the sensorsystem needs to be packaged. Although packaging of electronic integrated circuits is not a trivial thing, packaging a microsensor may even be more challenging by the very sole reason that a sensor should somehow "be open" to the environment in order to measure the measurand. This means, that apart from an electrical interconnection, it needs at least one other physical connection. Also from this point of view, the design and development of microsensorsystems requires multidisciplinary teams and system approaches, rather than sole monodisciplinary depth. MEMS technology In this section some very basic remarks will be made on MEMS technology. Rather than going into any detail of technological matters, the purpose of this section is to give some background of the technology that has enabled the development of microsensor structures, which are the prime subject of this report and which will be discussed in the next section. Silicon Although the term MEMS is not restricted to silicon micromachining, most of today s MEMS technology is based on silicon. This if, off course, for good reasons. Silicon mono-crystalline wafers offer a good combination of qualities, ranging from ideally elastic (no creep 5 or hysteresis) to a good heat conductor, from low to intermediate electrical conductivity (depending on type and doping), from having a small thermal expansion coefficient to being stable up to high temperatures [Petersen82]. More importantly, silicon wafers are produced and used on a large scale for integrated microelectronics resulting in low prices and compatible equipment. Additionally (with certain restrictions on technology) silicon wafers allow for monolithic integration of mechanical and electronic functions in one and the same chip offering a high potential for both cheap and advanced sen- 4 5 An example of a foundry service is the MUMPS process offered by Microelectronics Center of North Carolina (MCNC). This process currently contains a surface micromachining technology consisting of 3 polysilicon structural layers in combination with the required sacrificial layers and a siliconnitride layer (i.e. for electrical isolation). Creep is the phenomena that a material continues to deform under a constant load. Hysteresis on the other hand is the effect that the deformation of a material is dependent on its history, being for example different for in-or decreasing loads. GK 2

3 sorsystems 6. Therefore, the majority of MEMS devices is made on silicon wafers as the starting substrate. Photolithography Probably the most decisive characteristic for calling something "a MEMS device" is the use of photolithography in its fabrication. Basically, photolithography comes down to applying a layer of a light-sensitive material (photoresist) on a flat substrate, illuminating it by some source using some pattern (a mask) thereby making the illuminated parts either soluble 7 or insoluble and then removing the soluble parts. Silicon Oxidise Spin photoresist A B C Light Illuminate Mask Develop Etch oxide & remove photoresist D E F Figure 1: Various steps in a photolithography process (after [Elwenspoek00]). The way the photoresist is illuminated can vary. One way is to have a mask which is more or less pressed against the photoresist-layer (contact). In another set-up a mask is close to the substrate (proximity) whereas in the third method a mask is put in a projection system to have a projection of the mask on the photoresist-layer. The photoresist-layer is usually the first layer used to further shape the underlying structure. This is normally done by material-selective etching processes (i.e. in which the photoresist layer is etched much slower than the underlying material). On their turn, the underlying layers may be well used as a mask layer for further processing 8. Since photolithography only allows to define two-dimensional patterns, the structures that can be fabricated are only moderately shaped in the direction perpendicular to the plane of the photoresist layer. However, special etching processes as well as repetitive use of deposition-photolithography-etching-cycles can reduce this limitation (see e.g. figure 2). Growth and deposition To make structures in or on silicon, one almost always needs additional materials. Either to obtain specific properties related to these materials or because one needs a layer to shield the underlying material during a specific process (i.e. etching). To this end various methods exist. E.g. layers can be grown by using part of the underlying layer in an oxidation process (i.e. the growth of siliconoxide on top of silicon, thereby using the top part of the silicon wafer). Alternatively all required material for growth is transported to the substrate. Here again there are choices; either the material gets to the substrate in the form it needs to be grown in or in the form of intermediate products which react to the eventual compound at the substrate. In the first case, depending on the material there may be a choice of deposition methods. For atomic species (metals) evaporation and/or sputtering may be possible. Evaporation is only possible for materials with a high vapour pressure and can only be carried out under vacuum conditions. Sputtering on the other hand relieves the high vapour pressure condition by using charged particles (i.e. argon ions) to remove material from a target prior to its deposition on a substrate. For compound materials Chemical Vapour Deposition A good example of a highly developed, advanced as well as economic sensor system, showing both mechanical and electronic functionality, is the impact sensor often found in today s cars for triggering of airbags. Photo resists are said to be of the positive type if they become soluble when exposed to light. Negative photoresists become insoluble when exposed to light. This is for example the case when a thin silicon-oxide layer is etched to make openings to the underlying silicon. The silicon, then, is etched using another etching process which does not etch the silicon-oxide layer. GK 3

4 (CVD) may be more suitable. In the latter case one is almost solely left to CVD. Important materials that can be deposited by CVD are silicon-nitride (chemically inert, hard material, electrical insulator with moderate thermal conductivity), poly-silicon (having properties which may resemble those of single crystalline silicon and often used as structural layer), silicon-oxide (often used as mask material or sacrificial layer, see below) and phosphor silicate glass (PSG, mainly used as sacrificial layer). Doping Doping of silicon, mostly with either boron or phosphor, is not only a means for changing the electrical properties of silicon. It also changes mechanical properties as well as the etching behaviour in certain etching solutions. For example etch rates of anisotropically wet chemical etched silicon drop significantly when silicon is doped with boron. This fact can be used to have well-defined stopping positions for etching, rather than timed etching. Doping can be obtained various ways. One of them is ion implantation where high energy ions from an accelerator hit the silicon and penetrate to a certain depth, precisely controlled by the energy of the ions, into substrate. Implantation energies of kev s are required to reach depths of a few 100 nm. Depending on the usage of the doping an annealing step may follow to spread the doping more homogeneously over a thicker layer. Another method of doping is by indiffusion at elevated temperatures. This can for example be accomplished by putting the silicon wafers, side by side with boron or phosphorous wafers, in a tube at high temperature. Bulk micromachining Roughly, bulk micromachining is the technology by which structures are made in the silicon substrate rather than on top of it by selectively removing material. The purpose of this technology is to make structures that are released or undercut such that elements come into existence that can somehow move with respect to the frame of the wafer [Kovacs98]. The technology has many aspects [Elwenspoek99] but probably one of the most important is the etching of silicon, be it a small layer, or entirely through the silicon wafer. There exist various methods for etching. The most important flavours are wet-chemical etching and dry etching (e.g. Reactive Ion Etching, RIE). Etching methods can also be distinguished on the basis of the etch-profiles they produce: isotropic (etch-speed equal in all directions leading to rounded structures), anisotropic (etching speed highly dependent on the crystallographic directions leading to sharp edges and corners) and directional (mostly due to geometrical effects of dry etching. Examples of anisotropic wet-chemical etching are etching in KOH, EDP solutions (see figure 2 left). Isotropic wet-chemical etching is obtained using solutions containing e.g. fluoride and can be tailored using doping or voltage or light assisted etching [Tjerkstra99]. Dry etching has a great variety of implementations which share some similarities: they all take place in a vessel which can be brought at "certain vacuum" ( Torr) and they all use accelerated (reactive) particles (ions, atoms) for etching. Additionally chemical species may be added to assist in etching or passivation. Etching mechanisms can vary from physical removal due to bombardment (directional) to chemical reactions followed by removal of (volatile) reaction products. By combination of both effects, tailoring their mutual influence, it is possible to obtain etching process that vary between isotropic and almost perfectly directional. GK 4

5 Figure 2: Left: example of a structure fabricated in silicon by anisotropic etching in a KOH solution [Oosterbroek99]. Right: structures etched out of silicon by means of powderblasting. The etching methods mentioned above share the fact that they generally result in moderate etching speeds (microns/minute). Sometimes one may want to etch over large depths in the shortest times possible with limited requirements on accuracy. In this case a technique well-known from precision engineering may come in handy: sand-blasting. The technology is relative new in micromaching but seems to be able to "fill the gap" in etching needs between micromachining and precision engineering ( micron, see figure 2 right). Surface micromachining Although bulk-micromachining offers many possibilities to fabricate interesting structures, entirely new structures have become possible due to surface micromachining [Howe83, Howe86, Bustillo98]. One of the goals of this technology is to make small features just above the (silicon) substrate which can move (partly) free from the substrate. This is accomplished by using one layer as a sacrificial layer. Etching away this layer, a structural layer becomes free. The technology is drawn schematically in figure 3. In a first step (A) a sacrificial layer is deposited on the silicon substrate. Subsequently this layer is shaped using photolithographic and etching techniques (B). Next a layer of the structural material is deposited over the sacrificial layer (C). Again using photolithography and etching the structural layer is shaped and holes are made to allow etching fluid to reach the sacrificial layer in selected areas (D). Finally the sacrificial layer is completely removed by wet-chemical etching thereby releasing the structural layer partly. GK 5

6 A B C D E Figure 3: Surface micromachining. Left: processing sequence for micro-machining (after [Elwenspoek99]). Right: examples of structures made by surface micro-machining: a ball bearing like groove for a wobble motor [Legtenberg96a] and a shuffle-motor [Tas98] Various combinations of structural and sacrificial materials exist. E.g. silicon nitride and polysilicon, gold and titanium, nickel and titanium, polyimide and aluminium, tungsten and silicon-dioxide and aluminum and polymer, However, the most widely used surface micromachining technology uses polysilicon as the structural material and either silicon-dioxide or phosphor silicate glass (PSG) as the sacrificial layer [Elwenspoek99]. Bonding and Chemical Mechanical Polishing To even further enhance the possibilities and complexity of micromachined devices (partly structured) wafers (of various materials) can be bonded together. Again various methods exist. Anodic bonding can be applied to a pair of a good conducting layer (metal, silicon) and a slightly conducting layer (glass). Using a voltage over the waferpair induces a small depletion layer in the glass wafer resulting in a large electrical field over a short distance near the interface of the wafers. The resulting electrostatic pressure brings the wafers in such close contact that they may join together. Executing the process at high temperatures ( C) will allow the formation of covalent bonds at the glass-conductor interface. Anodic bonding is possible for wafers with a roughness of less than 1 micrometer. Another method of bonding is silicon fusion bonding. This type of bonding can occur between layers that have a roughness of 1 nanometer or less. For wafers having such smooth surfaces, they do not contact at 3 points but at large areas. The bonding energy of wafers covered with native oxide are thought to originate from hydrogen bridging [Stengl89] which can be promoted by annealing in a wet-oxidation oven at temperatures of about 700 C. Since the requirements for silicon fusion bonding, especially the extreme smoothness of the wafers, are generally hard to meet, it is a difficult process. However, there is a technology which can help to make the process more applicable: Chemical Mechanical Polishing (CMP). GK 6

7 Figure 4:Left: Schematic view of a CMP machine. Right: LPCVD Si 3 N 4 surface before (top) and after (bottom) CMP (the RMS roughness value is reduced from 3.6. nm to 0.4 nm [Gui98]) In CMP the wafer is placed on a rotating polishing head and moved under a certain pressure over a polishing pad. A slightly alkalic slurry containing nanometer sized silica particles is added. The combined action of wear and etching yields the smooth surfaces. This process can also be applied to other materials among others silicon nitride, silicon oxide and poly silicon [Gui98]. It appears that all materials that can be polished down to a roughness better than 1 nm are bondable [Berenschot94]. Micromechanical sensors principles Mechanical principles Micromechanical sensors, as the name suggest, contain some mechanical structure of which certain properties depend on specific (physical) environmental conditions. In general one can say that the mechanical structure is deformed in some way. How this deformation is sensed and turned into a (electrical) signal is a second question. w a) x b) c) d) Figure 5: Various ways of connecting a beam to its surroundings: a) clamped-free (cantilever beam) b) clamped-clamped (bridge) c) simply supported-simply supported d) clamped-simply supported. The way mechanical structures deform, not only depends on their shapes but also on the mechanical properties (Youngs modulus E, Poisson ratio υ, mechanical load (stress-)distribution σ(x,y,z), the way they are connected to their surroundings (see figure 5 and 6) and, hopefully, some environmental (physical) parameter(s), e.g. pressure, temperature, humidity, acceleration, rotation etc. Examples of mechanical members are beams (one/two side clamped, free-hanging, etc.), membranes, diaphragms, mass-suspension systems (e.g. for acceleration sensing). GK 7

8 a w(r) r b x Figure 6: Two examples of (circular) membranes. Left: simple diaphragm. Right: centrally strengthened diaphragm (boss). Apart from static deformations mechanical changes can be well observed in dynamic behaviour. As an example we mention the influence of stress on the resonance frequency of micro-bridges (see figure 7). On increasing the stress the resonance frequency will increase as well, an effect wellknown from e.g. macroscopic string-instruments. Resonant sensors can achieve very high resolutions of 100 ppm or better. Figure 7: Example of a micro-bridge resonator for flow-measurments. Sensing principles Once a mechanical structure shows a static or dynamic change, there are several ways of sensing these changes. Depending on the technologies used and the scale of the mechanical effects one or the other method may be favourable. One of the sensing principles best known from macroscopic mechanical structures is the strain gauge. The electrical resistance of a piece of metal depends on its size and shape. For a rod with cross-section S and length l the resistance R is given by R l = λs with λ the specific conductivity. If as a consequence of an external load the resistor changes its dimensions the resistance will change due to both a change in length and in cross-section. The relative change of the resistance is given by [Elwenspoek00]: dr R = ( 1+ 2υε ) where ν is the Poisson ratio. The ratio of the relative change of the resistance per unit strain is called the gauge factor G. Typical values of υ are , so for most materials the gage factor is between 1 and 2. The yield strain of metals mostly is well below 1% (this is about the yield strain of high quality steel) so changes in resistance of metal strain-gauges is at maximum 1 to 2%. Metal strain gauges are very well developed. There are commercial strain-gauges of all possible kinds, including temperature compensated strain-gauges, and including strain-gauges matching a great number of materials (with respect to thermal expansion coefficient). As an indication there are loadcells based on metal strain-gauges with a precision of 1:100,000, guaranteed in a temperature range between -40 and +80 C (outperforming current sensors made by micro-machining). The dominant effect in metal strain-gauges is the change of the geometry, the materials property λ is independent of the strain. This is different in semiconductors. Here the effect is called piezo-resistivity. Conductivity, and therefore all effects connected to conductivity such as piezo-resistivity in crystals are anisotropic [Kloek94]. The piezoresistive coefficient π is defined as the relative change of the resistivity per unit of stress. There is a longitudinal and a transversal effect: the resistivity changes in a direction parallel to the strain and normal to the strain. The effect is observed in single crystalline GK 8

9 silicon as well as in polysilicon thin films. In single crystalline silicon the effect is exploited by doping p-regions in n-type silicon microstructures. Polysilicon thin films can be deposited on top of microstructures, doped and patterned. The gauge-factor for p-doped <110> Si is 133, much larger than the geometrical effects. A second method for deformation (displacement-) sensing is using electrostatics. Basically when two electrically conducting bodies are put in each others neighbourhood they form a capacitor. Changing the geometry this capacitance will change as well. This change can either be detected by a variation in voltage (e.g. when using "frozen electrical charges" as in the case of electret microphones) or by the change in capacitance (e.g. by incorporating the structure in an oscillator circuit such that the oscillation frequency becomes a measure for the position of the two bodies relative to each other [Wiegerink99]. Yet other sensing is based on temperature and heat conduction effects. Various principles are possible. E.g. when two plates are positioned relatively close to each other, the heat transfer through the medium filling the gap in between the plates is largely influenced by the pressure of the medium only saturating at pressures for which the mean free-path becomes smaller than the gap [Burger98]. Other quantities may be sensed indirectly by determining the temperatures. This is for example the case for flow-sensors where the cooling of a wire(-like) structure by a passing flow is a measure for the magnitude of the flow (anemometry). Temperature (-differences) can be well measured by the temperature resistance effect, for example by using platinum wires. Many sensing principles are based on geometrical changes. However, when using piezo-electric materials, the transduction takes place inside the material. Applying a stress over a film of piezoelectric material directly results in a voltage-drop over the thickness (and width) of the layer. Although piezo-electric transduction is often used in macroscopic sensors and actuators, it is less often found in micro-mechanical transducers the reason being a lack of materials which can be deposited with sufficient quality and durability (e.g. zinc-oxide suffers from charge injection whereas materials like PZT are more difficult to apply in thick films). In many cases it may be advantageous to have no electrical in- and output signals to a sensor, e.g. when operating in environments with a large level of electromagnetic radiation or in explosive environments. In such circumstances optical read-out may be the method of choice. Optical signals can be derived on the basis of tunnelling effects (optical couplers), interference effects (Mach- Zehnder interferometer), (frequency dependent) absorption and polarisation-rotation. When using integrated optics (optical waveguides), a good sensitivity requires the mechanical structures to be within less than a wavelength ( micrometer) away from the light guides. As a last sensing principle we mention here the use of tunnelling currents. In this method charges (electrons) do tunnel through a non-conducting gap (e.g. air) from one conductor to another, one of them generally having the shape of a sharp tip. Tunnelling only occurs at small gaps (nanometer sizes) and tunnelling currents are exponentially dependent on gap distance: I V e t b α φ i x g with V b the bias-current, α i is ( -1 ev -0.5 ), φ the effective height of the tunnelling barrier and x g the gap distance in [Liu98]. To offer a convenient way of detection, tunnelling is often combined with an actuator to form a feedback control keeping the gap distance constant [McCord98]. Force sensors There are several techniques to measure forces and pressures. Very often, the force to be measured is converted into a change in length or height of a piece of material, the spring element. The geometrical change is subsequently measured by one of the sensing mechanisms mentioned in the previous section. Sometimes the sensor element and the spring element can not be distinguished, i.e. the sensor element itself is also the spring element. For example, in piezoelectric force transducers, the deformed crystal both supports the load and supplies the output signal. More sophisticated systems incorporate an electronic feedback to balance the external force or pressure by an equal but oppositely directed counter-force or pressure. The obvious advantage of such a GK 9

10 system is that the spring element can be omitted, thus eliminating problems like linearity, creep and hysteresis related to the spring element. However, application of such systems is limited to relatively small forces and pressures because of the limited size of the counterforce or pressure that can be exerted. A simple sensor structure to measure forces consists of a silicon beam with integrated strain gauge. Figure 8: Examples of force-sensors using strain-gauges (left) or resonator strain gauges (middle) and a schematic of how the structures are electronically interfaced [vanmullem91]. Pressure sensors Closely related to force sensors are pressure sensors. In pressure sensors the spring element is always a membrane. Conventional pressure sensors used metal membranes. A breakthrough was achieved in the early 1980 s when micromechanics was introduced and the metal membranes were replaced by (monocrystalline) silicon membranes, which suffer much less from creep, fatigue and hysteresis. Furthermore, the combination of small size, high elastic modulus and low density of silicon results in sensors with a very high resonance frequency [Elwenspoek00]. The first silicon pressure sensors were based on a piezoresistive read-out mechanism. At the moment, piezoresistive pressure sensors are still the most widely used. Piezoresistors may be diffused in the membrane or deposited on top of the membrane. Usually, the resistors are connected in a Wheatstone bridge configuration for temperature compensation. The main advantages of a piezoresistive read-out mechanism are the simple fabrication process, the high linearity and the fact that the output signal is conveniently available as a voltage. The main problems are the large temperature sensitivity and drift. Furthermore, because of the low sensitivity of piezoresistors, piezoresistive devices are not suitable for accurate measurement of very low pressure differences. Capacitive read-out mechanisms are inherently less sensitive to temperature variations and an extremely low power consumption can be obtained. However, the capacitance to be measured is usually very small and an electronic interface circuit is required, which either has to be integrated on the sensor die or at least has to be positioned very close to the sensor chip. Compared to piezoresistive sensors the obtained sensitivity is significantly higher. Usually a capacitance change of 30 to 50 % can easily be obtained while the change in resistance of piezoresistive devices is limited to 2 to 5 %. Capacitive structures also offer the possibility of force-feedback as the electrostatic force between the capacitor plates can be used to compensate the external pressure [Esashi98]. The highest accuracy is obtained using resonant sensors. These sensors have an output signal in the form of a change in resonance frequency of a vibrating element. A problem with resonant sensors is the complexity of the fabrication process since they normally require a vacuum sealing. Furthermore, in the common case that the vibrating element is integrated on a deflecting membrane problems may arise from the mechanical coupling between the resonantor and the membrane. GK 10

11 Figure 9: Left: Schematic cross section of a typical bulk micromachined piezoresistive pressure sensor. Combination of a piezoresistive pressure sensor with (middle) an NMOS process [Tanigawa85], and (right) a CMOS process [Kress91]. Figure 9 (left) shows a typical example of a bulk micro-machined piezo-resistive pressure sensor. The resistors may be diffused in the membrane or deposited on the membrane with an intermediate isolation layer (usually SiO 2 ). The membrane is etched from the backside of the wafer and is usually several tens of micrometers thick. A timed etch stop is simple and has the advantage that it does not require doping of the membrane with boron. However, the reproducibility of the membrane thickness is rather poor. A boron etch stop gives good control over the membrane thickness, however the high doping level prohibits the use of diffused strain gauges. Therefore, often an electrochemical etch stop is used with a more lightly doped membrane. Because the membrane is etched from the backside of the wafer it is very well possible to combine this with a standard IC fabrication process. Examples of sensors with on-chip electronics were developed at Toyota [Sugiyama91] (they combined a piezoresistive pressure sensor with a bipolar electronic circuit to provide temperature compensation and to convert the output voltage into a frequency (which can be easily interfaced with digital electronics)), Hitachi [Yamada83] and NEC [Tanigawa 1985]. Figure 9 (middle) shows a schematic cross section of the latter, where an NMOS operational amplifier is integrated with the sensor for amplification and temperature compensation of the strain gauge signal. A combination with a standard CMOS process was proposed [Kress91]. In this case the CMOS circuit was realized in an n-type epitaxial layer as shown in Figure 9 (right). Load-cells Load-cells are in fact force-sensors optimised to measure loads. Classical load-cells are made by high-quality steel beams with attached strain-gauges which measure the compression under the load. Recently a load-cell made in silicon was reported [Wiegerink99]. It uses compression of a large number of silicon poles intertwined by capacitors which form part of a modified Martin Oscillator. The structure allows to accurately measure loads, even in the case of inhomogeneous loading. Acceleration sensors Acceleration sensors come in a variety of sorts both regarding their performance as well as the underlying principles used for sensing. On the rough end of the spectrum one finds the sensors meant to be used e.g. in car as airbag deployment sensors whereas on the other end the very sensitive (micro-g) sensors are found which are intended for use e.g. for seismic applications. Parameter Airbag Vehicle stability Navigation Range 50 g 2g 1g Frequency range Hz Hz Hz Resolution <100 mg <10 mg <4 µg Max. shock in 1 ms >2000 g >2000 g >20 g Off axis sensitivity <5% <5% <0.1% Temperature range C C C Temp. Co. of sensitivity <900 ppm/ C <900 ppm/ C <50 ppm/ C Table 1: Specifications of accelerometers for three applications GK 11

12 (taken from [Yazdi98]). Micromechanical accelerometers are currently produced in such large quantities that they form the second largest sales volume (following pressure-sensors), a figure mainly due to their large deployment in automotive applications [Yazdi98] next to applications in e.g. camera stabilization, active monitoring in biomedical applications, vibration monitoring, three-dimensional mice, headmounted displays for virtual reality, etcetera. Figure 10: Simplified lumped element representation of an accelerometer. Basically an accelerometer consists of a mass connected by some kind of suspension to a frame as well as (some degree of) mechanical damping. When the frame is accelerated, the mass exerts an inertial force on the suspension leading to a deformation of the suspension and a displacement of the mass relative to the frame of the sensor. Both the deformation of the suspension and the displacement of the proof-mass can be used to obtain a signal, e.g. deformation by strain-gauges and displacement by capacitive effects. The performance of accelerometers is limited by the thermal motion of the proof mass. According to the laws of thermodynamics, the thermal energy of a system in equilibrium is k B T/2 for each energy storage mode, where k B is the Boltzmann constant and T is the temperature. The small proof mass of micromachined accelerometers results in rather large thermal movements. An equivalent acceleration spectral density, the so-called total noise equivalent acceleration (TNEA) can be calculated and is given by [Gabrielson93]: 2 an 4kBTD 4kBTωr TNEA = = = f M QM where Q is the quality-factor and ω r the resonance-frequency of the mass-spring-damper system. From this expression it follows that in order to measure low acceleration levels a large proof mass and high quality factor are required. Piezoresistive accelerometers Piezeoresistive accelerometers make use of the bending of the suspension structures by measuring the deformation by piezoresistive strain-gauges. The gauges are placed at the edge of the support rim and proof-mass where the stress variation is maximum. This type of device is relatively simple and can be used easily in a bridge type configuration allowing for simple read-out electronics as well. Disadvantages are the large thermal sensitivity and smaller overall sensitivity requiring larger proof-masses (asking for bulk- rather than surface-micromachining). GK 12

13 spring Figure 11: Left: Example of a piezoresistive accelerometer fabricated using bulk-micromachining [Roylance1979]. Right: 3-dimensional capacitive accelerometer with integrated electronics [Lemkin97]. Capacitive accelerometers Using the proof-mass as a moving part of a capacitor, capacitive read-out of the position of the proof-mass becomes possible. Capacitive accelerometers have been made having large ranges in performances depending on application and design (automotive to micro-gravity devices). Advantages are low-cost production, high sensitivity, good dc-response and noise-performance, lowpower dissipation, and a simple structure. The largest disadvantage is their high impedance making them susceptible to electromagnetic interference (requiring special interfacing and/or shielding and packaging) [Yazdi98]. A special version of a capacitive accelerometer is described in [Park98] where electrostatic actuation is used to reduce the stiffness of the suspension (in one direction) during operation thereby reducing the resonance frequency and thus the noise equivalent acceleration (see above). Tunnelling accelerometers Tunnelling accelerometers employ electrostatic actuation to keep a conductive tip at fixed distance (sub-nanometer) from a second electrode in a closed loop system [Liu98]. This type of accelerometer can achieve very high sensitivity [Strobelt99]. However, low-frequency noise levels are less favourable and the requirement of a stiff feedback loop reduces the useful bandwidth [Bernstein99]. Resonant accelerometers These devices work in a way similar to the piezoresistive devices. However, instead of measuring deformations of the suspension by strain-gauges, the deformation is measured by changes of the resonance frequency of resonating beams connected or integrated in the suspension(s). An important benefit is that the resonance frequency can be directly converted into a digital signal. Thermal accelerometers Thermal accelerometers consist of heater-sensor bridges, made of metal having a high temperature resistance coefficient, within an encapsulation. The heater produces a "heat bubble" which becomes unsymmetrically positioned over the sensors by acceleration. This causes a temperature difference of the sensor bridges (thus a voltage difference) on both sides of the heater which is converted by the temperature resistance effect into a voltage[leung98]. Advantages are the lack of moving parts as well as simple fabrication technology. Apart from the sensing schemes described above a full optical implementation has been proposed in [Chollet99]. An implementation using piezo-electric effects (rather than piezo-resistive) was recently described in [dereus99]. The piezo material used was zinc-oxide. Accelerometers with three-axis sensitivity are presented in [Lemkin97] and [Mineta96]. GK 13

14 Vibration sensors Vibration sensors have much in common with accelerometers. In fact, every accelerometer can off course be used as a vibration sensor. However, vibration sensors do not have to be sensitive for DC accelerations which means that vibration sensors can be optimised differently especially to high sensitivities at specific frequencies [Bernstein99]. Applications of vibration sensors are found in geophysical sensing, machinery vibration, failure prediction etc. A device with optical interconnects (using opto-electronic conversion) is described in [Peiner98]. Figure 12: Two phases of the fabrication of a vibration sensor made by bulk-micromachining including deep reactive ion etching [Bernstein99]. Angular rate sensors Almost all micromachined angular rate sensors use vibrating elements to sense the rotation. The velocity of the vibrating element together with a rotation results in a Coriolis acceleration in a direction perpendicular to the direction of the vibration. As a result the vibrating element starts to vibrate in a second mode, perpendicular to the first mode. The amplitude of this secondary vibration is directly proportional to the rate of rotation since the Coriolis acceleration is given by: a Coriolis = 2Ω v where Ω is the rotational frequency and v the velocity of the object in radial direction. Depending on application various requirements can be specified. Table 2 gives an overview of the requirements. Rate grade gyroscopes have merely automotive application and have been developed mostly in recent years. Parameter Rate grade Tactical grade Inertial grade Angle Random Walk >0.5 /H /H <0.001 /H Bias drift /H /H <0.01 /H Scale factor accuracy 0.1-1% % <0.001% Full scale range /sec >500 /sec >400 /sec Max. Schock in 1 msec 10 3 g g 10 3 g Bandwidth >70 Hz 100 Hz 100 Hz Table 2: Specifications of gyroscopes for three applications (taken from [Yazdi98]). Currently the most accurate gyroscopes are optical. Especially the ring laser gyroscopes have demonstrated inertial grade performance whereas fibre-optic gyroscopes have found tactical grade application. These levels of performance have not yet been obtained by MEMS devices [Kubena99]. GK 14

15 Figure 13:Left: Illustration of the Coriolis effect: a ball moving from the centre to the edge of a rotating disk moves along a curved trajectory on the disk. Right: a tuning-fork angular rate sensor based on the Coriolis effec [Elwenspoek00]t One well-known structure used in angular rate sensors is the tuning-fork structure depicted in figure 13 right. The tines are driven differentially and at a constant amplitude as indicated. A rotation of the stem of the tuning-fork results in a Coriolis acceleration of the tines in a direction perpendicular to the drive direction. The Coriolis force can be detected from the bending of the tines or from the torsional vibration of the tuning-fork stem. Actuation of the oscillation of the tines can be electrostatic, piezo-electric or electromagnetic whereas the sensing of the second mode can be capacitive, piezo-resistive or piezo-electric [Yazdi98]. Figure 14 shows a micromachined angular rate sensor developed by Putty and Najafi [Putty94]. It consists of a ring which is supported by eight semi-circular support springs. A number of drive and detection electrodes are located around the structure. The ring is electrostatically driven in an inplane elliptically shaped flexural mode with a fixed amplitude. When it is subjected to a rotation around the z-axis, Coriolis forces cause energy to be transferred from the primary mode to the secondary flexural mode, which is located 45 o apart from the primary mode. The amplitude of the secondary vibration is proportional to the angular rate and is detected capacitively. The inherent symmetry of the structure makes it less sensitive to spurious vibrations. Furthermore, since the drive and sense modes are in principle degenerate, a good match between the resonant frequencies is obtained resulting in a large sensitivity. Frequency mismatch due to mass or stiffness asymmetries occuring during the fabrication process can be compensated electronically by applying suitable bias voltages to the electrodes around the structure. Figure 14: Structure of a vibrating ring gyroscope. The ring is attached to an anchor in it s center by semicircular springs. The ring vibrates in a mode in which it s shape changes into an ellipse and the direction of the main axis of the ellipse changes as a result of angular rotation [Putty94]. The first version of the vibrating ring sensor was fabricated by electroforming nickel into a thick polyimide or photoresist mold on a silicon substrate containing the electronic control circuitry [Putty94, Sparks97]. Recently, an improved version was made using a polysilicon ring [Ayazi98] which obtained tactical grade with a walk-off as small as 0.05 deg/ Hz. Other designs have been proposed with a ring of mono-crystalline silicon [Hopkin97]. Although good results have been GK 15

16 obtained, a problem is the asymmetry introduced by the anisotropy of the mechanical parameters of monocrystalline silicon. This asymmetry causes significant coupling between the drive and detection mode and compensation is requred, either electronically or mechanically by selectively adjusting the thickness of the ring. Tunneling gyroscopes Much of the effort of gyroscopes is put into a precise tuning of the resonance frequencies of the drive and sense modes of the resonators. This is required to get sense signals large enough to be reliably determined from the mechanical sense modes at rather small Ω. It has been proven to be difficult to achieve a well-defined match due to fabrication limitations. Therefore in [Kubena99] a new approach is proposed in which a tunneling sensing mechanism is used to determine the sense vibration. Using surface micromachining a 2 µm thick, 300 µm long nickel cantilever is made which is put electrostatically into vibration parallel to the substrate. Since the tunneling transduction can have sensitivities as large as Å Hz at 1 khz, no mechanical gain is necessary in order to sense displacements near the Brownian noise floor. Noise figures of 27º/h/ Hz are obtained while improvements to a thermal noise-floor of 3º/h/ Hz are expected. Fluid and Flow sensors Flow sensing is very complex. There are two distinct reasons for the complexity: hydrodynamics is a science for itself and there are many phenomena that can be exploited for flow sensing. Micro flow sensors published so far are based on thermal principles or on the measurement of pressure distributions. Optical and acoustical possibilities have not yet been explored [Elwenspoek00]. Thermal flow sensors Flow sensors based on thermal principles ad some heat locally to a streaming fluid, and measure the resulting temperature distribution close and far away from the heater, and/or the heat loss of the heater. The key to understanding flow sensors is a hydrodynamic phenomenon known as the boundary layer. Close to walls there is a region with large gradients in the flow velocity. This region is confined within a layer of a certain thickness, which is called the boundary layer. If there is a heater involved then in most fluids (liquid metals are the exception) also a thermal boundary layer builds up, which is submerged within the hydrodynamic boundary layer. Extensions of sensors and obstructions must be compared with the extend of the boundary layer, and sensors operate completely different whether the boundary layer is thick or thin compared to dimensions important in the sensor design. We can classify thermal flow sensors in three basic categories: Anemometers, Calorimetric flow sensors and Time of flight sensors. Generally, anemometers consist of a single element which is heated and the temperature of which is measured. Since the electrical resistivity of most materials depends strongly on the temperature, and the element is heated by an electrical current, the measurement of the resistivity of the element is a natural choice. Any flow will enhance the rate at which heat is transported away from the element. The anemometer can be operated in two modes: constant power and constant temperature. The simplest mode is the mode where the anemometer is fed with constant power, and the temperature of the hot element is measured. The response time in this mode is given by the heat capacity of the hot element and the rate at which heat is transferred to the medium (RC-time). When the temperature is held constant - this requires a feed back loop - the power needed to keep the temperature constant is measured. In this mode the anemometer is considerably faster than in the constant power mode. The signal of the anemometer is proportional to the square root of the flow velocity as a consequence of the boundary layer. Calorimetric flow sensors and time of flight sensors require two or more elements. A standard configuration consists of a heater surrounded by temperature sensitive elements arranged symmetrically downstream and upstream. Any flow will carry away heat in the direction of the flow and accordingly will cool down the heater and change the temperature distribution around it. The temperature difference upstream and downstream is measured. The signal is proportional to the flow velocity for small velocities, but saturates and even decreases at higher flow velocity. Calorimetric flow sensors operate at very low flow velocities. Examples of sensors made in microtechnology are GK 16