Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim Part 5 pdf

MONOLITHIC MOUNTING 103 Al or Au wire Die or chip Die-bond material Figure 4.38 Die- and wire-bonding technique -TAB lead — Chip • Die-bond material Substrate \ Figure 4.39 Tape-automated bonding technique hot thermode produces a faster throughput than wire bonding. Moreover, the reduced inductance of a probe means that the devices can be AC-tested. The disadvantages of TAB include the relatively high cost of the process and the need for a large device footprint. This problem is overcome in flip-chip mounting. 4.4.3 Flip TAB Bonding In flip TAB bonding, the die is mounted upside down on the substrate, as shown in Figure 4.40. The major advantage of flip TAB over regular TAB mounting is that the die can be subsequently attached to a metal lid for better thermal management. 4.4.4 Flip-Chip Mounting Finally, flip-chip mounting of the die has a number of key advantages. It provides an excellent contact between the die and substrate by eliminating the wire or beam lead Chip Flip TAB lead Support material Substrate Figure 4.40 Flip TAB technique 104 STANDARD MICROELECTRONIC TECHNOLOGIES Chip — Solder bumps "" Substrate Figure 4.41 Flip-chip mounting technique entirely (see Figure 4.41). Solder bumps are placed on the substrate and then the die is mounted facedown, and the solder is melted to make the connection. The small footprint and pitch, coupled with short interconnect of about 50 urn, and hence low inductance, make this a very attractive technology at a relatively low cost. Full details of these bonding methods may be found in textbooks such as Doane and Franzon (1993). 4.5 PRINTED CIRCUIT BOARD TECHNOLOGIES Once electronic components have been made and packaged, such as the monolithic ICs described in Sections 4.3 and 4.4, they need to be connected with other components to form a circuit board. The most common way to do this is to make a PCB, which is also known as a printed wiring board (PWB). There are a number of different PCB technologies based on different dielectric materials and their fabrication process. Here, we consider the three main kinds of organic PCBs - solid, flexible, and moulded; the ceramic PCB is known as a thick film hybrid circuit board and is discussed in Section 4.6.1. 4.5.1 Solid Board Solid (and flexible) PCBs generally consist of an organic dielectric material on top of which is a thin metal layer - predominantly copper. The copper layer is patterned using a photoresist material and an acid etch to define the tracks between the electronic compo- nents. In the case of surface-mount devices, a single-sided organic PCB can be used as illustrated in Figure 4.42(a). Single-sided PCBs are simpler to make and are increasingly used with the greater availability of surface-mount components. However, the majority of organic PCBs are double-sided with multilayer boards used in special cases, such as the need to introduce ground planes and thereby reduce the electrical interference between high-speed switching logic and analogue circuitry (Figure 4.42(b) and (c)). A double- sided PCB has copper tracks patterned on both sides of the dielectric material. Electrical connections between the layers are formed by drilling holes through the board, and this is followed by the plating of the sides of the holes. Clearly, the metal will be thinner here, and passing large currents down through holes can be a problem. Finally, a solder mask is prepared and, if required, a protective layer is patterned, leaving just the solder areas exposed. In a solid organic PCB, the dielectric material consists of an organic resin reinforced with fibres. The fibres are either chopped or woven into the fabric, and the liquid resin is added and processed using heat and pressure to form a solid sheet. The most PRINTED CIRCUIT BOARD TECHNOLOGIES 105 - Copper interconnect Dielectric -Plated through hole (b) L —Dielectric Copper interconnect Blind via —\ /-Buried via Dielectric-/ Copper interconnect- (c) Plated through hole Figure 4.42 Schematic cross section of three types of organic PCBs: (a) single-sided; (b) double-sided; and (c) multilayered Table 4.8 Material properties of some common fibres used in organic PCBs Thermal expansion Dielectric constant at 1 MHz Dissipation factor at 1 MHz Maximum elongation Softening temperature Specific gravity Specific heat capacity Tensile strength Thermal conductivity Young's modulus Units ppm/°C - 10 –3 % °c g/cm 3 J/g.°c kg/mm W/m.°C kg/mm e-glass 5.0 5.8 1.1 4.8 840 2.54 0.827 350 0.89 7400 s-glass 2.8 4.52 2.6 5.5 975 2.49 0.735 475 0.9 8600 Quartz 0.54 3.5 0.2 5.0 1420 2.20 0.966 200 1.1 7450 Aramid –5.0 a 4.0 1.0 4.5 300 1.40 1.092 400 0.5 13000 a Along axis of fibre; radial is 60 ppm/°C commonly used fibres are paper, e-glass, s-glass, quartz, and aramid. The precise choice of the dielectric material depends on the technical demands presented by the device and application proposed, and the properties, such as the permittivity and loss factor, are frequently the most important. Table 4.8 gives some of the properties of the fibres that are commonly used in organic PCBs. 4.5.2 Flexible Board In flexible PCBs, the resin used to make a solid dielectric material is replaced by a thin flexible dielectric material and the metal is replaced by a ductile copper foil. Again, a 106 STANDARD MICROELECTRONIC TECHNOLOGIES Etch Laminate Cover film Adhesive | Etch Copper Adhesive Base film Cover film Adhesive Copper Adhesive Base film Adhesive Copper Adhesive Cover film Cover film" Adhesive Copper Adhesive Base film Adhesive Copper Adhesive Cover film Adhesive Copper Adhesive Base film firm firm mm nrm Single-sided flex-printed wiring :> nrm mm mm mm Y///////////////////////////A mm mm mm mm Y///////////////////////////A Double-sided flex-printed wiring Etch Laminate mm rmn mm nrm nrm mrn rmn mm mm mm mm mm y/////////////////////////zm Multilayer flex-printed wiring Figure 4.43 Schematic cross section of three types of flexible PCBs: (a) single-sided; (b) double-sided; and (c) multilayered Table 4.9 Material properties of some resins used in organic PCBs CTE Dielectric constant at 1 MHz Poisson's ratio Temperature Thermal conductivity a Young's modulus Units ppm/°C - - °C W/m.°K GPa Epoxy 58 4.5 0.35 130 0.3 3.4 Polyimide 49 4.3 0.33 260 0.3 4.1 Cyanate ester 55 3.8 0.35 260 0.3 3.4 PTFE 99 2.6 0.46 - 0.3 0.03 a Approximate values number of different organic materials can be used to make a flexible wiring board such as polyimide (Kapton), polyester terephthalate (Mylar), random fibre aramid (Nomex), Teflon, and polyvinyl chloride (PVC). The copper foil is processed as before by optical lithography, and layers can be joined together to form multilayer laminates. The layers are usually bonded together using an adhesive such as acrylic, epoxy, polyester, and PRINTED CIRCUIT BOARD TECHNOLOGIES 107 Table 4.10 Material properties of some dielectric films used in flexible organic PCBs Density Dielectric constant at 1 MHz Dielectric strength, min. Dimensional stability, max. Dissipation factor at 1 MHz Elongation, min. Initial tear strength Tensile strength, min. Volume resistivity (damp heat) Units Polyimide g/cm 3 1.40 4.00 kV/mm 79 % 0.15 10 –3 12 % 40 g 500 MPa 165 Q-cm 10 6 FEP 2.15 2.30 79 0.3 0.7 200 200 17 10 7 Polyester 1.38 3.40 79 0.25 7.0 90 800 138 - Epoxy polyester 1.53 - 5.9 0.20 0 15 1700 34 10 5 Aramid paper 0.65 3.00 15.4 0.30 10 4 - 28 10 6 polytetrafluroethylene (PTFE). Figure 4.43 shows the way in which single-sided, double- sided, and multilayer flexible PWBs are constructed. Table 4.9 gives some typical properties of the resins used in flexible organic PCBs. Care is needed to match these properties with those of the copper layer and the nature of the circuit, for example, high frequency or high power. Flexible PCB dielectric and adhesive films are now manufactured to a standard, and Table 4.10 shows the Class 3 properties of some dielectric films according to the standard IPC-FC-231. Accordingly, organic PCB laminates can now be constructed with increased confidence in their performance. 4.5.3 Plastic Moulded The most common forms of PCB - the organic PCB and the ceramic PCB (see next section) - are planar, that is, the metal interconnects are formed in two dimensions with plated through holes joining one layer to another. However, it is possible to make a three-dimensional PCB by the moulding of a suitable plastic. A three-dimensional PCB can be made from extruded or injection-moulded thermoplastic resins with a conductive layer that is selectively applied on its surface. However, high-temperature thermoplastics are required to withstand the soldering process, and commonly used materials are polyethersulfone, polyetherimide, and polysulfone. Plastic moulded PCBs have several advantages over organic PCBs, such as superior electrical and thermal properties and the ability to include in the design, noncircular holes, connectors, spacers, bosses, and so on. More often than not, a moulded PCB is in essence an IC chip carrier package. Plastic moulded PCBs may prove to be advantageous in microtransducers and MEMS applications, in particular, when the assembled microstructure has an irregular structure or needs special clips or connectors. The plastic moulded IC package may also be used as part of a hybrid MEMS before full integration is realised. Future Micro-moulds may be fabricated using microstereolithography (see Chapter 7). 108 STANDARD MICROELECTRONIC TECHNOLOGIES 4.6 HYBRID AND MCM TECHNOLOGIES 4.6.1 Thick Film PCBs can also be formed on a ceramic board, and these may be referred to as ceramic PCBs. A ceramic board, such as alumina, offers a number of advantages over organic PCBs, because a ceramic board is much more rigid, tends to be flatter, has a lower dielectric loss, and can withstand higher process temperatures. In addition, alumina is a very inert material and hence is less prone to chemical attack than an organic PCB. Ceramic PCBs can be processed in a number of different ways, such as thick-film, thin- film, co-fired, and direct-bond copper. The most important technology is probably the thick film. Circuit boards have been made for more than twenty years using this technology and are usually referred to as hybrid circuits. In thick-film technology, a number of different pastes have been developed (known as inks), and these pastes can be screen-printed onto a ceramic base to produce interconnects, resistors, inductors, and capacitors. Example: 1. Artwork is generated to define the screens or stencils for the wiring layers, vias, resistive layers, and dielectric layers. 2. Ceramic substrate is cut to size using laser drilling, and perforations that act as snapping lines are included after the process is complete. 3. Substrate is cleaned using a sandblaster, rinsed in hot isopropyl alcohols, and heated to 800 to 925 °C to drive off organic contaminants. 4. Each layer is then in turn screen-printed to form the multilayer structure. Each paste is first dried at 85 to 150 °C to remove volatiles and then fired at 400 to 1000 °C. 5. The last high-temperature process performed is the resistive layer (800 to 1000 °C). 6. A low-temperature glass can be printed and fired at 425 to 525 °C to form a protective overlayer or solder mask. Thick-film technology has some useful advantages over other types of PCB manufacture. The process is relatively simple - it does not require expensive vacuum equipment (like thin-film deposition) - and hence is an inexpensive method of making circuit boards. Figure 4.44 shows a photograph of a thick-film PCB used to mount an ion-selective sensor and the associated discrete electronic circuitry (Atkinson 2001). The thick-film process is useful here not only because it is inexpensive but also because it forms a robust and chemically inert substrate for the chemical sensor. The principal disadvantage of thick- film technology is that the packing density is limited by the masking accuracy - some hundreds of microns. Photolithographically patterned thin-film layers can overcome this problem but require more sophisticated equipment. 4.6.2 Multichip Modules Increasingly, PCB technologies are being used to make multichip modules (MCMs). A multichip module is a series of monolithic chips (often silicon) that are connected and HYBRID AND MCM TECHNOLOGIES 109 Figure 4.44 ISFET sensor and associated circuitry mounted on a ceramic (hybrid) PCB. From Atkinson (2001) 250 200 100 50 0 PCB SMT brids 50 microns HDMI 25 microns HDMI 10 microns HDMI 0 WSI 50 Silicon efficiency (%) Figure 4.45 Silicon efficiency rating and line width of different interconnection and substrate technologies. After Ginsberg (1992) packaged to make a self-contained unit. This module can then be either connected directly to peripheral ports for communication or plugged into another PCB. One important reason for using MCM instead of a conventional die-packaging approach is that the active silicon efficiency rating is improved (see Figure 4.45). In other words, the total area of the semiconductor die is comparable to the MCM substrate area. As can be seen from the figure, conventional PCB technologies and even SMT and hybrid are much poorer than the high-density MCM methods. The ceramic-based technology is referred to as an MCM-C structure; other MCM-C technologies include high-temperature co-fired ceramic (HTCC) and low-temperature co- fired ceramic (LTCC). Table 4.11 lists the relative merits of different MCM-C technologies. 110 STANDARD MICROELECTRONIC TECHNOLOGIES Table 4.11 Relative merits of MCM-C technologies, with one being the best Adapted from Doane and Franzon (1993), Property Top-layer dimensional stability Low K values High-conductivity metallisation High mechanical strength High thermal conductivity CTE matched to alumina or silicon Hermeticity Excellent dielectric control Surface roughness Thick-film 1 1 1 2 2 2 2 3 3 HTCC 3 3 3 1 1 3 1 1 2 LTCC 2 1 1 3 3 1 1 1 1 Benefit Improved wire-bond, assembly yield stability Improved high-frequency performance Smaller line and space designs More rugged package Good thermal characteristics Capability of assembly Development of packages More consistent electrical performance Better high-frequency performance Table 4.12 Properties of some commonly used MCM-C materials. Adapted from Doane and Franzon (1993) Property Purity Colour CTE at 25 to 400°C Density Dielectric constant at 1 MHz Dielectric loss tangent at 1 MHz Dielectric strength Flexural strength Resistivity Specific heat capacity Thermal conductivity Units % - 10- 6 /°C g/cm 3 - 10 –3 kV/mm GPa ft-cm J/g.°K W/m.°K AI 2 O 3 99.5 White 7.6 3.87 9.9 0.1 24 400 10 14 - 20-35 AI 2 O 3 96 White 7.1 3.7 9.5 0.4 26 250 10 14 - 20-35 BeO 99.5 White 9.0 3.01 6.5 0.4 9.5 170-240 10 15 - 250-260 A1N 98-99.8 Dark grey 4.4 3.255 8.8-8.9 0.7-2.0 10-14 280-320 >10 13 0.74 80-260 The choice of ceramic substrate is important and the >99 percent alumina has a low microwave loss, good strength and thermal conductivity, and good flatness. However, it is expensive and 96 percent alumina can be used in most applications. In cases in which a high thermal conductivity is required (e.g. power devices), beryllia (BeO) or aluminum nitride (A1N) can be used, although these involve a higher cost. Table 4.12 summarises the key properties of the ceramic substrates. In addition, modules wherein interconnections are made by thin films are classified as MCM-D and those made by plastic (organic) laminate-based technologies are classified as MCM-L. Table 4.13 shows a comparison of the typical properties of the three main types of MCM interconnection technologies. HYBRID AND MCM TECHNOLOGIES 111 Table 4.13 Comparison of MCM interconnection technologies. (1993) Property MCM class Dielectric material: Dielectric constant Thickness/layer (um) Min. via diameter (urn) Conductive materials: Thickness (am) Line width (um) Line pitch (um) Bond pad pitch (um) Maximum number of layers Electrical properties: Line resistance (£2 -cm) Sheet resistance (mfi/sq) Propagation delay (ps/cm) Stripline capacitance of 50 Q line (pF/cm) Thick film MCM-C Glass-ceramic 6-9 35-65 200 Cu (Au) 15 100-150 250-350 250-350 5 to 10+ 0.2-0.3 3.0 90 4.3 HTCC MCM-C Alumina 9.5 100-750 100-200 W(Co) 15 100-125 250-625 200-300 50+ 0.8-1 10.0 102 2.1 Adapted from Doane and Franzon Thin film MCM-D Polyimide 3.5 25 25 Cu (Al, Au) 5 10-25 50-125 100 4-to 10 1.3-3.4 3.4 62 1.25 Laminate MCM-L Epoxy-glass 4.8 120 300 Cu 25 75-125 150-250 200 40+ 0.06-0.09 0.7 72 1.46 MCM technology has several advantages for integrating arrays of microtransducers and even MEMS (Jones and Harsanyi 1995). First, the semiconductor dies can be fabricated by a different process, with some dies being precision analogue (bipolar) components and others being digital (CMOS) logic components. Second, the cost of fabricating the MCM substrate is often less expensive than using a silicon process, and the lower die complexity improves the yield. Finally, the design and fabrication of a custom ASIC chip is a time- consuming and expensive business. For most sensing technologies, there is a need for new silicon microstructures, precision analogue circuitry, and digital readout. Therefore, fabri- cating a BiCMOS ASIC chip that includes bulk- or surface-micromachining techniques is an expensive option and prohibitive for many applications. Figure 4.46 shows the layout of a multichip module (MCM-L) with the TAB patterns shown to make the interconnections (Joly et al. 1995). This MCM-L has been designed for a high-speed telecommunications automatic teller machine (ATM) switching module, which, with a power budget of 150 W, is a demanding application. 4.6.3 Ball Grid Array There are a number of other specialised packaging technologies that can be used as an alternative to the conventional PCB or MCM. The main drive for these technologies is to reduce the size of the device and maximise the number of I/Os. For example, there are three types of ball grid array (BGA) packages. Figure 4.47 shows these three types: the plastic BGA, ceramic BGA, and tape BGA. The general advantages of BGA are the smaller package size, low system cost, and ease of assembly. The relative merits of 112 STANDARD MICROELECTRONIC TECHNOLOGIES Figure 4.46 Example of a high-density MCM-L substrate with TAB patterns. From Joly et al. (1995) plastic and ceramic PGA packages are similar to those already discussed for PCBs and MCMs. The tape EGA uses a TAB-like frame that connects the die with the next layer board. 4.7 PROGRAMMABLE DEVICES AND ASICs The microtechnologies described in this chapter are used to make a variety of different microelectronic components. Figure 4.48 shows the sort of devices that can be made today. These are subdivided into two classes - standard components, which are designed for a fixed application or those that can be programmed, and application-specific ICs (ASICs), which are further subdivided. The standard components that may be regarded as having fixed application are discrete devices (e.g. n-p-n transistors), linear devices (e.g. operational amplifiers), and IC logic families of TTL and CMOS (e.g. logic gates and binary counters, random access memory). The other types of standard component may be classified as having the application defined by hardware or software programming. In hardware programming, the application is defined by masks in the process, and examples of these devices include programmable logic arrays (PLAs) and read-only memory (ROM) chips. There has been a move in recent years to make software programmable components. The most familiar ones are the microprocessors (such as the Motorola 68 000 series or Intel Pentium) that form the heart of a microcomputer and its [...]... kgates 50 10 10 10 10 10 N/A 1 2-3 1 5- 1 0 3-4 2-3 N/A 3-4 2-3 8 7 6 1 0-2 0 7-1 5 5- 1 2 Device Capability Density (kgates) Full custom RAM, ROM, Analogue RAM, ROM, Analogue Logic only 1-1 00 Long 1-1 0a 1 -5 0 Moderate 15a -5 0 1 -5 0 Moderate 1 5- 1 00 Standard and compiled cells Gate arrays Development time PLDs Fixed logic 0 .5 FPGAs Fixed logic 1-3 ... Figure 5. 11, that only the p-type sample, and not the n-type sample, would be etched This is the doping-selective effect that is used as an etch-stop 128 SILICON MICROMACHINING: BULK PP(n) PP(p) ::::•:•:• •:• !&*( •^ - j Oxide gr . cells Gate arrays PLDs FPGAs Capability RAM, ROM, Analogue RAM, ROM, Analogue Logic only Fixed logic Fixed logic Density (kgates) 1-1 00 1-1 0 a 1 -5 0 1 -5 0 0 .5 1-3 Development time Long Moderate Moderate Short Moderate NRE costs (k€) 50 15 a -5 0 1 5- 1 00 < ;5 5- 2 0 Production volume (1000s) <2 .5 2. 5- 1 0 >10 <2 .5 2. 5- 1 0 >10 <2 .5 2. 5- 1 0 >10 <2 .5 2. 5- 1 0 >10 <2 .5 2. 5- 1 0 >10 Production cost . cells Gate arrays PLDs FPGAs Capability RAM, ROM, Analogue RAM, ROM, Analogue Logic only Fixed logic Fixed logic Density (kgates) 1-1 00 1-1 0 a 1 -5 0 1 -5 0 0 .5 1-3 Development time Long Moderate Moderate Short Moderate NRE costs (k€) 50 15 a -5 0 1 5- 1 00 < ;5 5- 2 0 Production volume (1000s) <2 .5 2. 5- 1 0 >10 <2 .5 2. 5- 1 0 >10 <2 .5 2. 5- 1 0 >10 <2 .5 2. 5- 1 0 >10 <2 .5 2. 5- 1 0 >10 Production cost in k€ per kgates N/A 1 2-3 1 5- 1 0 3-4 2-3 N/A 3-4 2-3 8 7 6 1 0-2 0 7-1 5 5- 1 2 a Costs shown for PC-based and . 10+ 0. 2-0 .3 3.0 90 4.3 HTCC MCM-C Alumina 9 .5 10 0-7 50 10 0-2 00 W(Co) 15 10 0-1 25 25 0-6 25 20 0-3 00 50 + 0. 8-1 10.0 102 2.1 Adapted from Doane and Franzon Thin film MCM-D Polyimide 3 .5 25 25 Cu