Microelectronics

If renamed today, microelectronics would be called nanoelectronics since sizes have been pushed well below 1 p.m. The fabrication of modern electronic devices, such as large-scale integrated circuits, involves an elaborate sequence of physical and chemical operations. As the size scale becomes smaller, it is increasingly necessary to recognize the molecular nature of matter. This and the next sections describe technology applicable to the microscale that is somewhat less than state-of-the art. 

A chip fabrication process starts with a wafer of high-purity silicon that has been cut from a single crystal. Electronic functionality is achieved by creating a multilayer structure in and on the surface of the wafer in a precise geometric pattern. The pattern is laid down by a process known as photolithography using the following sequence of steps  

The surface is coated with a polymer, typically by spin coating 

An image is formed on the surface using hard ultraviolet or soft x rays. If the polymer is a photoresist, it crosslinks in those areas exposed to radiation. If the polymer is a negative photoresist, it degrades in those areas exposed to radiation. 

For the case of a photoresist, a solvent removes the polymer that is not crosslinked and thus exposes the underlying surface. 

The freshly exposed surface is treated with a chemical agent, or dopant, to modify its electrical properties (e.g., to create transistors). 

FIGURE 11.11 Etching of silicon wafer to create interconnections (a) desired profile achievable by ion bombardment (b) profile obtained by chemical etching. 

A single 0 or 1 is called a bit. The number of bits required to characterize an equivalent analog signal is eight and this is called a For example, the sequence of 8 bits corresponding to the byte representing the letter a is  

Other bytes correspond to other letters, munbers, pimctuation, and mathematical symbols. 

In 1940 G. Stibitz ofthe Bell Laboratories used relays (Fig. 15.10) to demonstrate the possibility of using binary numbers to perform calculations. It soon became clear that relays were far too slow and energy intensive and they were replaced by diodes (Fig. 10.19). The first NC machine (MIT 1952) used diodes. However, these were replaced by transistors as soon as they became available (late 1950s). 

Chip production is carried out in clean rooms where filtered air is maintained slightly above atmospheric pressure, since it is very important that dust particles not create problems in microprocessor chip production. Where possible, robots are employed, but where human intervention is required, workers are completely shrouded from head to foot in special lint-fi ee material. 

Integrated circuit chips are mainly produced at a few locations in 

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A lively and optimistic survey of a new breed of businessmen who are breaking away from huge companies to form dynamic enterprises in microelectronics, biotechnology and other developing areas. 

Metal to ceramic (oxide) adhesion is very important to the microelectronics industry. An electron transfer model by Burlitch and co-workers [75] shows the importance of electron donating capability in enhancing adhesion. Their calculations are able to explain the enhancement in adhesion when a NiPt layer is added to a Pt-NiO interface. 

One of the more recent advances in XPS is the development of photoelectron microscopy [ ]. By either focusing the incident x-ray beam, or by using electrostatic lenses to image a small spot on the sample, spatially-resolved XPS has become feasible. The limits to the spatial resolution are currently of the order of 1 pm, but are expected to improve. This teclmique has many teclmological applications. For example, the chemical makeup of micromechanical and microelectronic devices can be monitored on the scale of the device dimensions. 

Undeniably, one of the most important teclmological achievements in the last half of this century is the microelectronics industry, the computer being one of its outstanding products. Essential to current and fiiture advances is the quality of the semiconductor materials used to construct vital electronic components. For example, ultra-clean silicon wafers are needed. Raman spectroscopy contributes to this task as a monitor, in real time, of the composition of the standard SC-1 cleaning solution (a mixture of water, H2O2 and NH OH) [175] that is essential to preparing the ultra-clean wafers. 

Light microscopy is of great importance for basic research, analysis in materials science and for the practical control of fabrication steps. Wlien used conventionally it serves to reveal structures of objects which are otherwise mvisible to the eye or magnifying glass, such as micrometre-sized structures of microelectronic devices on silicon wafers. The lateral resolution of the teclmique is detennined by the wavelength of tire light... 

Campbell S A 1996 The Science and Engineering of Microelectronic Fabrication (New York Oxford University Press)... 

CVD gaseous reactants (precursors) delivered to a heated substrate in a flow reactor undergo tliennal reaction to deposit solid films at atmospheric or reduced pressure, and volatile side products are pumped away. CVD is used for conductors, insulators and dielectrics, elemental semiconductors and compound semiconductors and is a workliorse in tire silicon microelectronics industry. 

William T, Coffey, Department of Microelectronics and Electrical Engineering, Trinity College, University of Dublin, Dublin, Ireland... 

The advantages of miniaturization are now being exploited in areas beyond microelectronics. Adaptation of materials and processes originally devised for semiconductor manufacture has allowed fabrication of sensors (for example, pressure meters and accelerometers used in the automotive industry) (6,7), complex optical (8) and micromechanical (6,7,9) assembHes, and devices for medical diagnostics (6,7,10) using Hthographic resists. 

L First manufacturing use of chemically amplified resists Plasma-developed resist first described X-ray proximity lithography demonstrated Bis-azide rubber resists introduced DNO-novolac resist for microelectronics introduced Photoresist technology first applied to transistor fabrication DNO-novolac resist patented by Kalle... 

R. Aden, G. Wadraff, W. Hinsberg, L. Simpson, and R. Kunz, in Polymers for Microelectronics, MCS Symposium Series 537, American Chemical Society, Washington, D.C., 1994, p. 165. 

Equally important as tape casting in the fabrication of multilayer ceramics is thick film processing. Thick film technology is widely used in microelectronics for resistor networks, hybrid integrated circuitry, and discrete components, such as capacitors and inductors along with metallization of MLC capacitors and packages as mentioned above. 

R. R. Tummala and E. J. Rymas2ewski, Microelectronics Packaging Handbook, Van Nostrand Reinhold, New York, 1989. 

H. T. Sa whill and co-workers, "Low Temperature Co-Firable Ceramics with Co-Fired Resistors," International Society of Hybrid Microelectronics Proceedings, 1986, pp. 473—480. 

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