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
PROGRAMMABLE DEVICES AND ASICs 113
Figure 4.47 Three main types of ball grid array packages: (a) plastic; (b) ceramic; and (c) tape-ball nonvolatile memory. For example, erasable programmable read-only memory (EPROM) in which the memory is erased by UV light and its easier-to-use successor, electronically erasable programmable read-only memory (EEPROM). In recent years, there has been a strong move toward software programmable array devices; these components do not have the mathematical capability of a microprocessor but are able to perform simple logical actions. As such, they can be high-speed stand-alone chips or glue chips - that is, chips that interface an analogue device with a microcontroller or communication chip. Examples of these are programmable logic devices (PLDs) and PGAs that may have several thousand
114 STANDARD MICROELECTRONIC TECHNOLOGIES
gates to define. The newest type of component is the programmable analogue array (PAA) device, and these may well become increasingly important in sensing applications in which the design of an operational amplifier circuit can be set and reset through I/O ports.
It should be noted that this classification of devices into hardware and software programming is not universally accepted. Sometimes, it may be more useful to distinguish
Standard devices
Standard components Application-specific ICs
Fixed Applications application by programming
1
Discrete Log fam
n
O
1 11
ic Hardware Software lies programming programming
(MASK) 1
"L PLA Microprocessor MOS ROM EPROM, EEPROM
PLD, PGA, AA
Semicustom Full Silicon custom compilation
i l l I
Gate Analogue Master Standard array array slice cell
Figure 4.48 Manufacturing methods for common microelectronic devices and ICs
Increasing risk
Increasing density
Increasing time to market
Increasing versatility
Increasing development
cost V
Decreasing cost/gate
Figure 4.49 Diagram showing the various trade-offs between the different technologies adopted to make an ASIC chip
PROGRAMMABLE DEVICES AND ASICs 115 between devices according to where they were programmed. In this case, devices may all be regarded as 'electrically' programmed and then they can be subdivided into those programmed by the manufacturer (as in mask-programmed parts) and those programmed by the user (as in field-programmable gate array).
The second class of components are those called application-specific integrated circuit ICs (ASICs) (see Figure 4.48). There are several types of ASIC and these are referred to as full-custom, semicustom, and silicon compilation. Full-custom ASICs are those that are defined down to the silicon level and, therefore, there is great scope for the optimisation of the device layout, reduction in silicon die area, and speed of operation. However, a full-custom design can be an expensive option and is only useful for large volumes.
Silicon compilation is the exact opposite; hence, it is rather wasteful of silicon and pushes up the process costs while minimising the design cost. The more common approach, and more relevant for the manufacturing of microtransducers, is that of a semicustom ASIC chip. This has four subdivisions. In gate arrays, the device has been partly processed and the designer simply defines the interconnection of the digital logic devices by one or two customised masks. Thus, most of the process is common to a number of end users, and hence the costs are greatly reduced. In analogue arrays, the same principle is applied, except that this time a range of analogue components are being connected and an analogue circuit is formed. In master slice, the wafer run can be split at a later stage into different subprocesses. The last type of semicustom approach is the standard cell in which the designer selects standard logic or analogue circuit functions from a software library and then connects them together on the silicon die. The design time is reduced by using standard cells with a standard process.
The various trade-offs of the ASIC technologies are illustrated in Figure 4.49 such as risk, cost, density, and flexibility. Strictly speaking, PLDs are not ASICs but they have been included here because they are often the main competitors to an ASIC chip.
Although the number of equivalent gates per chip in PLDs is only 500 to 3000, the cost advantage is often attractive.
When deciding upon which ASIC technology to use, it is important to weigh the relative costs involved, such as the development time and the nonrecurring engineering costs (mask making etc.), and design consideration such as the architecture required and the number of gates. In the final analysis, it is usually the volume that dictates the cost to manufacture the chips; the production charges per 1000 gates are shown against total volume in Table 4.14.
For example, modern microprocessor and memory chips are manufactured in enormous volume (millions of chips per year) and so the cost is dominated by the time to process and, hence, the size of wafer processed. Current microelectronic plants use wafers of a diameter of 8" or more, and companies have to build new plants that cost nearly one billion dollars as larger diameter wafers become available. This situation is usually not applicable to the manufacture of microsensors because of the much reduced volume and higher added value.
However, all of these production costs per kgate are low compared with the cost of fabricating a nonstandard component. For instance, when integrating a microtransducer or MEMS with a standard IC, it is nearly always necessary to develop nonstandard pre-or postprocessing steps, such as surface pre-or bulk micromachining (see next chapter). This cost issue is critical for the eventual success of a component on the market and therefore, we will return to it later on in Chapter 8, having first described the different fabrication methods and technologies associated with microtransducers and MEMS.
116 STANDARD MICROELECTRONIC TECHNOLOGIES
Table 4.14 Typical costs of different ASIC (and programmable device) technologies. Adapted from Ginsberg (1992)
aCosts shown for PC-based and workstation-based design, respectively
REFERENCES
Atkinson, J. (2001). University of Southampton, UK. Personal communication.
Colclaser, R. A. (1980). Microelectronics Processing and Device Design, Wiley & Sons, New York, p. 333.
Doane, D. A. and Franzon, eds. (1993). Multichip Module Technologies and Alternatives, Van Nostrand Reinhold, New York, p. 875.
Furakawa, S. (1985). Silicon-on-insulator: Its Technology and Applications, D. Reidel Publishing Company, Dordrecht, p. 294.
Ginsberg, G. L. (1992). Electronic Equipment Packaging Technology, Van Nostrand Reinhold, New York, p. 279.
Gise, P. and Blanchard, R. (1986). Modern Semiconductor Fabrication Technology, Prentice-Hall, New Jersey, p. 264.
Harper, C. A. (1997). Electronic Packaging and Interconnection Handbook, McGraw-Hill, USA.
Hart, P. A. H. (1994). Bipolar and Bipolar-MOS Integration, Elsevier Science, Amsterdam, p. 468.
Joly J., Kurzweil, K. and Lambert, D. (1995). "MCMs for computers and telecom in CHIPPAC programme," in W. K. Jones and G. Harsanyi, eds., Multichip Modules with Integrated Sensors, NATO ASI series, Kluwer Academic Publishers, Dordrecht, p. 324.
Jones, W. K. and Harsanyi, G., eds. (1995). Multichip Modules with Integrated Sensors, NATO ASI series, Kluwer Academic Publishers, Dordrecht, p. 324.
Sze, S. M. (1985). Semiconductor Devices, Physics and Technology, Wiley & Sons, New York, p. 523.
Udrea, F. and Gardner, J. W. (1998). UK Patent GB 2321336A,"Smart MOSFET gas sensor,"
Published 22.7.98, Date of filing 15.1.97. International Publication Number: WO 98/32009, 23 July 1998, Gas-sensing semiconductor devices.
5.1 INTRODUCTION
The emergence of silicon micromachining has enabled the rapid progress in the field of microelectromechanical systems (MEMS), as discussed previously in Chapter 1. Silicon micromachining is the process of fashioning microscopic mechanical parts out of a silicon substrate or, indeed, on top of a silicon substrate. It is used to fabricate a variety of mechanical microstructures including beams, diaphragms, grooves, orifices, springs, gears, suspensions, and a great diversity of other complex mechanical structures. These mechan-ical structures have been used successfully to realise a wide range of microsensors1
and microactuators. Silicon micromachining comprises two technologies: bulk micro-machining and surface micromicro-machining. The topic of surface micromicro-machining is covered in the next chapter. Further details can be found in the two-volume Handbook of Microlithog-raphy, Micromachining, and Microfabrication (Rai-Choudhury 1997).
Bulk micromachining is the most used of the two principal silicon micromachining technologies. It emerged in the early 1960s and has been used since then in the fabrication of many different microstructures. Bulk micromachining is utilised in the manufacture of the majority of commercial devices - almost all pressure sensors and silicon valves and 90 percent of silicon acceleration sensors. The term bulk micromachining expresses the fact that this type of micromachining is used to realise micromechanical structures within the bulk of a single-crystal silicon (SCS) wafer by selectively removing the wafer material.
The microstructures fabricated using bulk micromachining may cover the thickness range from submicrons to the thickness of the full wafer (200 to 500 um) and the lateral size ranges from microns to the full diameter of a wafer (75 to 200 mm).
Etching is the key technological step for bulk micromachining. The etch process employed in bulk micromachining comprises one or several of the following techniques:
1. Wet isotropic etching 2. Wet anisotropic etching 3. Plasma isotropic etching 4. Reactive ion etching (RIE) 5. Etch-stop techniques
1 Chapter 8 is devoted to this topic.