120 Microengineering, MEMS, and Interfacing: A Practical Guide Note: Do not break design rules unless you are absolutely sure you know what you are doing. They are there for a purpose. It is usually desirable to exceed the design rules as far as possible. If a 4-µm overlap is specified, an 8-µm overlap would be preferable. Furthermore, it is helpful, although not essential, to work in multiples of the minimum feature size. 4.5.1 DEVELOPING DESIGN RULES The first principle is to comply with the minimum feature size. No structures or gaps should have dimensions below the minimum feature size. This will set the absolute minimum width and separation rules (Figure 4.19a). Process characteristics contribute to separation rules. Diffusion processes proceed laterally under the mask as well as into the silicon. The minimum separation between diffused impurities (P-well or N-well) is likely to be larger (Figure 4.19b). Taking the process characteristics into account, overlap requirements are set principally by the accuracy of alignment during photolithography. If it is possible to align one layer with another to ±1 µm, then an overlap of at least 2 µm would FIGURE 4.18 (Continued) Mosis Scmos layout rules–9–metal 2 Rule Description Lambda 9.1 Minimum width 3 9.2 Minimum spacing 3 9.3 Minimum overlap of via 1 1 9.4 Minimum spacing when either metal line is wider than 10 lambda 6 Metal 1 (c) Metal 2 Via Metal 2 9.1 9.2a 9.2b 9.3 DK3182_C004.fm Page 120 Friday, January 13, 2006 10:59 AM Copyright © 2006 Taylor & Francis Group, LLC 122 Microengineering, MEMS, and Interfacing: A Practical Guide 4.6 GETTING THE MASKS PRODUCED Assuming that the masks are to be produced by an independent supplier or brokerage, it will be necessary to provide the design files, obviously, and also additional information regarding the physical format of the mask. 4.6.1 MASK PLATE DETAILS The specification of the blanks to be used to create the set of masks will either be relatively simple for most silicon-related micromachining processes, where the demands are not great, or quite complex for odd requirements such as x-ray lithography masks. Normally, it would be necessary to obtain the latter from a specialist provider; for UV lithography, the masks will normally be chrome on low-expansion glass or quartz. Mask plates should, clearly, be larger than the wafer being processed: a 2-in. (50-mm)-diameter wafer would require 2.5-in. (63.5-mm) square blank, and a 4-in. (100-mm)-diameter wafer would require a 5-in. (125-mm) square blank. Table 4.1 lists some common plate dimensions. The chrome may either be “bright chrome” or have a low-reflection coating (LRC) applied. This can help to some extent with optical aberrations during the lithography process. Quartz (fused silica) glass is the preferred material for stability, but low-expansion glass is a cheaper substrate that can be used; usually, MEMS dimensions are not so critical as to require quartz. In the last instance, FIGURE 4.20 In (a) both M1 and V1 are aligned to a mark on the substrate, with an error of ±1 µm. This means that Xvia will be the nominal distance on the mask design, X, ±1 µm. The same is true of Xmetal, which means that Xvia can be as much as X+1 µm and Xmetal as little as X − 1 µm. The difference between the two structures on the final device will then be as great as 2 µm. This is addressed in (b), where M1 is aligned to A1 and V1 to M1. While Xmetal is still X±1 µm, and the alignment error between V1 and M1 is now ±1 µm, Xvia is now X ± 2 µm. Xvia Xmetal Substrate (A1) Via (V1) Metal (M1) Xvia Xmetal Substrate (A1) Via (V1) Metal (M1) ( b )( a ) DK3182_C004.fm Page 122 Friday, January 13, 2006 10:59 AM Copyright © 2006 Taylor & Francis Group, LLC 124 Microengineering, MEMS, and Interfacing: A Practical Guide 4.6.4 STEP AND REPEAT If the design is of a single chip, to be repeated across the entire mask (see Figure 1.2 in Chapter 1), then details need to be supplied. Care should be taken to ensure that the information given matches the desired pitch (spacing) of the chip designs and that the entire design can fit onto the blank selected. 4.6.5 PLACEMENT REQUIREMENTS It is normal practice to place all designs symmetrically about the center of the plate. However, it is possible to specify that the design be mirrored about the vertical axis. There are several reasons why this may be required: • If the design is double sided, then masks for the back side of the wafer will need to be mirrored (with respect to the other layers). • As seen in Chapter 1, the design is written onto the chromium side of the plate, but when performing lithography this is the side that is in contact with the substrate. This may result in the design appearing as a mirror image following photolithography. Some mask manufactures will normally mirror layers so that the final design comes out as it appears in the design file, whereas others do not do so. It is advisable to check beforehand and to place features (e.g., text) in an obvious place on the design so that the orientation can be verified. This will not normally have functional implications but can mean that bonding pads, etc., appear on the opposite side of the design to that expected. Note that by specifying which layers are to be mirrored at this stage, rather than using the layout software, it is possible to verify the alignment of features in double-sided designs before going on to mask manufacture. 4.7 GENERATING GERBER FILES Chapter 1 introduced the reader to some cheaper alternatives to chrome masks. These options included the use of printed circuit board (PCB) artwork and the use of laser-cut stencils. The common design file current in the PCB industry is the Gerber file format. Modern PCB design software can readily cope with multiple layers and can perform design rule checks similar to those performed by layout software. There is no confusion between internal units, lambda units, and user units. The design is created in either millimeters or mils (thousandths of an inch). The only thing to be aware of is that when generating the final Gerber files, it is normally possible to set the precision of the dimensions — the number of places before and after the decimal point; the software will automatically round any dimensions to the specified precision. In the author’s experience, PCB layout tools are slightly more difficult to come to grips with than simple polygon pushers. At the time of writing, there are a number of PCB design packages that are available in one form or another for downloading from the Internet; see Table 4.2. DK3182_C004.fm Page 124 Friday, January 13, 2006 10:59 AM Copyright © 2006 Taylor & Francis Group, LLC 126 Microengineering, MEMS, and Interfacing: A Practical Guide 4.8 MASK DESIGN — KEY POINTS 1. The design will be created using a layout editor in two dimensions: a. The output will be CIF or GDSII. b. Each layer will be numbered (GDSII) or named (CIF). c. Each individual layer of the design will be transferred to a single mask plate. 2. The layout editor will make use of a technology file that outlines the design (layer names, numbers, colors for rendering, etc.): a. Units will be design units (used by the software), microns, or lambda units (half the minimum feature size). b. Plan the design beforehand. c. Use the grid for help when drawing the design. d. Text may not appear on the design layer unless drawn as a polygon or track. 3. The design will be hierarchical. 4. The design will normally include a number of standard features specified by the foundry: a. Frame b. Alignment marks c. Scribe lane d. Bonding pads e. Test structures 5. Each layer of the design should include a mask set identifier and layer identifier. 6. Always work to the design rules given by the foundry, and use the layout editor’s DRC where possible to check: a. Minimum feature size b. Minimum overlap c. Minimum separation d. Exact dimensions where required 7. Masks are normally chrome on quartz or chrome on low-expansion glass: a. The mask plate should be about 1 in. (2.5 mm) larger than the design. 8. When the mask set is produced, it is possible to individually specify each layer: a. As being light or dark field b. As being mirrored Film Mask Parameters • Resolution: 8000, 16000, or 32000 dpi (dots per inch) • Minimum feature size: 6 µm (These were available at the time of writing from JD Photo-Tools Ltd., Unit 4a, Meridian Centre, King St., Oldham, OL8 1EZ, U.K., www .jdphoto.co.uk.) DK3182_C004.fm Page 126 Friday, January 13, 2006 10:59 AM Copyright © 2006 Taylor & Francis Group, LLC 127 Part II Microsystems II.1 INTRODUCTION A microsystem can be considered to be a device or unit made up of a number of microengineered components. A convenient model of a microsystem is that of a control system (Figure II.1); many proposed microsystems take this form. Microsensors detect changes in the parameter to be controlled; electronic control logic then operates microactuators based on information from the sensors to bring the parameter to be controlled within the desired limits. An example of such a system would be the refreshing of a medium in a small cell culture dish. Sensors could detect changes in pH, pO 2 , or pCO 2 , and a micropump could deliver new culture medium from a reservoir as required. Not all devices need follow this control-system scheme. For instance, an accelerom- eter designed to inflate an air bag in the event of a car crash may not only incorporate a micromachined acceleration sensor but also electronics to condition the signal and detect a rapid deceleration, and microactuators that put a force on the sensor, allowing the device to be tested before the driver moves off. Microsystems may be constructed from component parts produced using different technologies on different substrates, which are then assembled together, i.e., a hybrid system. For example, a silicon chip would be used to implement control circuitry, whereas the actuators it controlled could be micromolded in plastic or electroplated metal (using the LIGA technique, perhaps). Alternatively, all components of a system could be constructed on a single substrate using one technology (a monolithic system). Hybrid systems have the advantage that the most appropriate technology for each component can be selected to optimize system performance. This will often lead to a shorter development time because microfabrication techniques for each component may already exist, and compro- mises will not have to be made to ensure that each component can be fabricated without damaging components already existing on the substrate. Monolithic devices will typically be more compact and more reliable than hybrid devices (fewer interconnections that can go wrong, for example). Further, once the fabrication process has been developed, they can be manufactured more cheaply because less assembly is required. Various microsensors and microactuators are discussed in Chapter 5 and Chapter 6. DK3182_S002.fm Page 127 Monday, January 16, 2006 12:47 PM Copyright © 2006 Taylor & Francis Group, LLC Microsystems 129 The intelligent component of the system would be implemented using micro- electronic components or a computer. Although microelectronics itself is not addressed, Part III gives a good introduction to techniques that may be employed to interface sensors and actuators to a computer or microcontroller. Some of the microsensors described in Chapter 5 are micromechanical devices that have to be driven into resonance. This is an additional role for the microactuators described in Chapter 6. DK3182_S002.fm Page 129 Monday, January 16, 2006 12:47 PM Copyright © 2006 Taylor & Francis Group, LLC 131 5 Microsensors 5.1 INTRODUCTION A transducer is a device that converts one physical quantity into another. The change in refractive index of some crystals under an applied magnetic field is one example of how this occurs (magnetooptic effect). Deformation of a piezo- electric crystal under an applied electric field is another. Sensors and actuators are special types of transducers. In the present context, a sensor is a device that converts one physical or chemical quantity to an electrical one for processing by the microsystem. Similarly, an actuator is a device that converts an electrical quantity into a physical or chemical one. Many of the sensors described in this chapter have been developed within the microelectronics industry and do not involve any special micromachining techniques. However, some of these sensors can be enhanced by the use of micromachining techniques (e.g., for thermal isolation). 5.2 THERMAL SENSORS There are a number of different types of thermal (temperature) sensors. Two of the most common types are thermocouples and thermoresistors (thermistors). 5.2.1 T HERMOCOUPLES When two dissimilar metals (e.g., copper and iron) are brought together in a circuit and the junctions are held at different temperatures, a small voltage is generated and an electrical current flows between them. A working thermocouple is shown in Figure 5.1. It consists of a sensing junction at temperature Ta and a reference junction at temperature Tb . The voltage developed by the thermocouple is measured with a high-resistance voltmeter. The open circuit voltage (i.e., as measured by an ideal voltmeter with infinite input impedance) is related to the temperature difference ( Ta − Tb ), and the difference in the Seebeck coefficients of the two materials ( Pa − Pb ) is (5.1) V will typically be on the order of millivolts or tens of millivolts for metal thermocouples with temperature differences on the order of 200 ° C. Semiconductor materials often exhibit a better thermoelectric effect than metals. It is also possible to integrate many semiconductor thermocouples in VPaPbTaTb=− −()() DK3182_C005.fm Page 131 Friday, January 13, 2006 10:59 AM Copyright © 2006 Taylor & Francis Group, LLC Microsensors 133 to couple thermistors directly to amplifier circuits without the requirement for a bridge configuration. The nonlinearity would typically be dealt with by calibrating the device. Microengineering techniques can be used in a variety of ways to enhance thermal sensors. As mentioned earlier, they can be used to thermally isolate the sensing element from the rest of the device. Also, arrays of sensors can be produced to give signals that are larger than what one sensor on its own would produce. If the device is small and thermally isolated, then its response time (the time the sensor takes to heat up or cool down in response to changes in the temperature of the environment) can be quite fast. With silicon-based devices, there is, of course, all the potential benefits that could come if electronics were integrated onto the chip (e.g., calibration done on-chip, self-testing). 5.2.3 T HERMAL F LOW -R ATE S ENSORS There are a number of ways by which the flow rate of gases (and liquids, although clogging of the sensor may be more of a problem) can be monitored by the use of thermal sensors. One can measure the temperature of a fluid as it enters and then leaves the sensor having been passed over a heating resistor; the temperature difference will be proportional to the mass flow rate. Another possibility is to maintain the sensor at a constant temperature (using heating resistors with thermal sensors for feedback control) and measure the amount of power required to maintain the temperature. Again, this will be proportional to the mass flow rate of material over the sensor. This type of sensor is typified by the use of a platinum resistor both as the sensing and heating element. Equation 5.2 gave the relationship between temperature and resistivity for a metal-sensing element (for platinum, a is approximately 3.9 × 10 − 3 and b is approximately 0.58 × 10 − 6 , so the response is roughly linear). If a heating current, I , is flowing through the element, then this can be related to the mass flow rate by (5.3) where p is a constant related to the heat loss under zero-flow conditions, and q is a constant dependent on the geometry of the system and the fluid. Note that these constants will also be dependent on the temperature difference between the fluid and the heating element. In the case of a MEMS sensor, the element will be situated on a thermally isolating structure such as a bridge, otherwise heat will be dissipated through the silicon (or another) substrate. In order to make a measurement, the element can be incorporated into a Wheatstone bridge (see Chapter 11, Section 11.4) as shown in Figure 5.2. In Figure 5.2, a bridge with two sensing arms is shown. When the bridge is in balance ( V dif = 0 ), one arm has a resistance of approximately 11000 Ω, whereas the other has a resistance of approximately 278 Ω . This implies that the 100- Ω platinum element (100 Ω referring to its nominal resistance at 0 ° C) has been IpqM 2 =+ DK3182_C005.fm Page 133 Friday, January 13, 2006 10:59 AM Copyright © 2006 Taylor & Francis Group, LLC Microsensors 135 It is worth remembering that, although photodiodes are carefully designed to have specific response characteristics, light falling on any pn junction can poten- tially induce charge carriers. This will appear as unwanted noise in a signal, and it is, therefore, desirable to ensure that sensitive electronics are packaged so as to prevent light from falling on them. 5.3.2 P HOTOTRANSISTORS The phototransistor has a much higher current output than a photodiode for comparable illumination levels. However, it does not operate as fast as photo- diodes (about 100 kHz being the top limit) and also has higher dark current. The phototransistor is essentially a transistor with the base current supplied by the illumination of the base–collector junction; it can be considered to be similar to a photodiode supplying the base current to a transistor (Figure 5.3). Normal transistor action amplifies the small base current. 5.3.3 C HARGE -C OUPLED D EVICES CCDs can be built as large linear and two-dimensional arrays; the latter are often used in small video cameras. They consist of a large number of electrodes (gates) on a semiconductor substrate. A thin insulating layer is situated between the metal gates and the semiconducting substrate. The operation of a CCD is shown schematically in Figure 5.4. The substrate has been doped so that the main current carriers are positive (i.e., “holes” — the term originates from semiconductor physics). When a positive voltage is applied to every third gate (V1), the majority carriers are repelled from the region beneath (Figure 5.4a), leaving “wells.” When light falls on the device, additional carriers are generated (as with photodiodes). The positive carriers are repelled from the gate, but the negative charge carriers (electrons) are attracted to the gate and fill the wells (Figure 5.4b). After the carriers accumulate, the entire array may be read out by shifting the carriers from one well to the next, the number of carriers being proportional to the amount of light that fell on each well. The electrical potential on the gates to one side of those already biased (V2) is increased; thus the charge is now shared between wells under two gates (Figure 5.4c). Then the first potential (V1) is then switched off. The charge is transferred to the adjacent well (Figure 5.4d), and so on. FIGURE 5.3 Phototransistor. B E C Light DK3182_C005.fm Page 135 Friday, January 13, 2006 10:59 AM Copyright © 2006 Taylor & Francis Group, LLC Microsensors 137 Zinc oxide for piezoelectric or pyroelectric applications is usually applied by spinning on a solgel and then baking. This is discussed further in the section on piezoelectric actuators in Chapter 6. One common application of these devices is in motion detectors for intruder alarms. A lens cuts the sensor’s field of view into discrete sections. As someone moves across the field of view, thermal radiation from their body falls on the sensor, resulting in discrete pulses as the person moves from one part of the field of view to the next. It is thus possible to build relatively cheap motion detectors that can be tuned to respond to a particular rate of motion. 5.4 MAGNETIC SENSORS There are many ways of sensing magnetic fields. Optical sensors can be based on crystals that exhibit a magnetooptic effect or on specially doped optical fibers. Coils can be used, although microfabricated coils are generally two dimensional, which are often not useful for many applications. The continuing development of high-temperature superconductors is also broadening the possibilities for sen- sors based on superconducting quantum interference devices (SQUIDs), which are capable of detecting the magnetic fields in the heart or brain. There are also a variety of other devices. Many measurements can be made, however, using Hall effect sensors. These are very common and are outlined in the following text. A Hall effect sensor is shown diagrammatically in Figure 5.5. The sensor consists of a conducting material, usually a semiconductor, and a current is passed between two contacts on opposite sides of the device. Two sensing contacts are placed on two other sides of the device opposite each other and perpendicular to the current flow. A magnetic field perpendicular to the plane of the contacts causes a deviation in the current flow across the device. This in turn is detected as a potential difference between the two sensing contacts. Hall effect sensors operate typically in the range 0.1 mT to 1 T (the Earth’s magnetic field is about 0.05 mT). Hall effect IC packages that typically give an output of about 10 mV per mT are available. FIGURE 5.5 Hall effect sensor. Sense (V) Current Magnetic field DK3182_C005.fm Page 137 Friday, January 13, 2006 10:59 AM Copyright © 2006 Taylor & Francis Group, LLC [...]...DK3 182 _C005.fm Page 1 38 Friday, January 13, 2006 10:59 AM 1 38 Microengineering, MEMS, and Interfacing: A Practical Guide 5.5 CHEMICAL SENSORS AND BIOSENSORS There is a wide variety of different chemical sensors, especially if one includes biosensors as a subclass of chemical sensors, as is done here A large proportion of chemical sensors are based on metal-oxide-semiconductor field effect... charged electrons are the main carriers in these n-type silicon regions: the source and the drain Both n-type and p-type silicon are used to form diodes; current will flow from p-type to n-type, but not vice versa Thus, to keep the bulk of the silicon substrate from interfering with the transistor (gate, drain, and source), this is connected to the most negative part of the circuit (often connected inside... at a reference potential by an electrode placed in it Generally, the reference potential is Reference electrode + Solution Vref Source Ion selective membrane Oxide drain FIGURE 5.7 Ion-sensitive field effect transistor (ISFET) structure (not to scale) Copyright © 2006 Taylor & Francis Group, LLC DK3 182 _C005.fm Page 140 Friday, January 13, 2006 10:59 AM 140 Microengineering, MEMS, and Interfacing: A... near the gate and attracts electrons, forming, between the drain and source, a small channel in which the majority charge carriers are electrons Current can flow through this channel, and the amount of current that can flow depends on how large the channel is and, thus, on the voltage applied to the gate In the ISFET, the gate metal is replaced with an ion-selective membrane (Figure 5.7), and the device... many potential uses for chemical sensors, their use is often complicated by calibration requirements 5.5.1 ISFET SENSORS ISFETs sense the concentration (activity level) of a particular ion in a solution These devices are generally based on the enhancement-mode MOSFET structure, shown in Figure 5.6 Source Gate Drain Metal Oxide n n p-type silicon Substrate connection FIGURE 5.6 n-channel enhancement-type... has been investigated considerably because glucose is important in diabetes and in many industrial fermentation processes The operation of a glucose-oxidase-based sensor is shown schematically in Figure 5 .8 The enzyme is immobilized on a platinum electrode and covered with a thin polyurethane membrane to protect the enzyme layer and reduce the dependence of the sensor on blood oxygen levels Glucose oxidase,... flowing from the drain to the source, so the ionic concentration will be directly related to the solution reference potential with respect to the substrate potential (in the circuit shown in Figure 5.7) One significant problem in the design and fabrication of ISFETs is ensuring that the ion-selective membrane adheres to the device If the integrity of the membrane is compromised, then the device is useless;... device is a pH sensor, which employs a glass (oxide) as the “membrane.” 5.5.2 ENZYME-BASED BIOSENSORS Enzymes are highly specific in the reactions they catalyze If an enzyme can be immobilized on a sensing substrate and the reaction products detected, then one has the basis of a highly selective biosensor The enzyme-based biosensor described in the following text is for monitoring glucose levels; this... scale) Copyright © 2006 Taylor & Francis Group, LLC DK3 182 _C005.fm Page 139 Friday, January 13, 2006 10:59 AM Microsensors 139 A MOSFET has a metal gate electrode, insulated from the semiconductor (silicon) wafer by a thin layer of silicon dioxide (oxide) The bulk of the semiconductor (i.e., the substrate) is doped with impurities to make it p-type silicon; in this material current is carried by positive... sensors have received much attention One of these, based on the glucose oxidase enzyme, will be outlined One thing to note is that a lot of research has gone into these sensors — biosensors and blood glucose sensors in particular Although progress has been made, there are still a lot of problems to be solved One big problem in this area is that the sensor performance drifts or degrades over time, often . on low-expansion glass or quartz. Mask plates should, clearly, be larger than the wafer being processed: a 2-in. (50-mm)-diameter wafer would require 2.5-in. (63.5-mm) square blank, and a 4-in main carriers in these n-type silicon regions: the source and the drain . Both n-type and p-type silicon are used to form diodes; current will flow from p-type to n-type, but not vice versa (V) Current Magnetic field DK3 182 _C005.fm Page 137 Friday, January 13, 2006 10:59 AM Copyright © 2006 Taylor & Francis Group, LLC 1 38 Microengineering, MEMS, and Interfacing: A Practical Guide