Silicon Wafer Production

The silicon employed for microelectronic and photovoltaic applications must first go through extensive processing to ensure that the material is of utmost purity. This section will describe these steps, with a discussion of perhaps the most intriguing conversion in the realm of materials science the synthesis of high-purity polished silicon wafers from a naturally occurring form of silicon - sand. 

Sea sand is primarily comprised of silicon dioxide (silica), which may be converted to elemental silicon (96-99% purity) through reaction with carbon sources such as charcoal and coal (Eq. 2). Use of a slight excess of Si02 prevents silicon carbide (SiC) from forming, which is a stable product at such a high reaction temperature. Scrap iron is often present during this transformation in order to yield silicon-doped steel as a useful by-product. 

The purity of silicon in this first step is only ca. 98%, and is referred to as metallurgical grade silicon (MG-Si). In order for the silicon to be used for electronics applications, additional steps are necessary to decrease the number of impurities. Reaction of MG-Si with hydrogen chloride gas at a moderate temperature converts the silicon to trichlorosilane gas (Eq. 3). When SiHCl3 is heated to a temperature of ca. 1,150°C, it decomposes into high-purity silicon and gaseous by-products (Eq.4). This reaction is typically performed in a bell-shaped Siemens-type reactor, where Si is deposited onto heated electrodes. 

Recently, afluidized-bed approach has been developed wherein SiHa and H2 gases are fed into the bottom of a vertical reactor held at a temperature 600°C. Silicon seed crystals are suspended in the chamber due to the gas flow, and decomposition of the gaseous precursor causes the nucleation/growth of silicon on the surface of the seeds. When grown to large sizes, the particles no longer remain suspended and are collected at the bottom of the reactor. 

These chemical processes result in electronic-grade silicon (EG-Si), with a purity of 99.999999999% that is, only one out of every billion atoms in the solid is something other than silicon To put this into perspective, imagine stacking yellow tennis balls from the earth s surface to the moon replacing only one of these with a blue ball would represent the level of impurities in EG-Si. Every year, between 15,000 and 20,000 tones of EG-Si is manufactured throughout the world for an ever-increasing number of applications. 

In order to ensure high purity of the ingot, this process is normally performed in vacuo ca. 10 Torr), or under an inert atmosphere e.g., 99.999% Ar) using an inert chamber such as quartz. Even under these reaction conditions, the CZ method technique suffers from O and C impurities that arise predominantly from the crucible walls. It should be noted that small quantities of O impurities are actually desirable, as they may trap unwanted transition metal impurities, a process referred to as gettering. The CZ technique is especially useful to yield doped semiconductors. For example, to yield p-doped Si, the desired concentration of pure Ga metal is added to molten Si within the crucible. 

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For epitaxial silicon wafers, product design focuses on optimizing the geometry of the plasma-enhanced, chemical-vapor-deposition (PECVD) reactor. To increase productivity, and maintain acceptable thickness uniformity, on the order of 5%, a simple optimization strategy locates a design that completes the deposition in 62 s. Then, for a standard manufacturing process, the economics are driven by the wafer costs, which are provided by a vendor at 206/wafer. At a sales price of 260/epitaxial wafer, the investor s rate of return is 18.3% and the return on investment is 25.3%. 

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. 

In this chapter we describe the basic principles involved in the controlled production and modification of two-dimensional protein crystals. These are synthesized in nature as the outermost cell surface layer (S-layer) of prokaryotic organisms and have been successfully applied as basic building blocks in a biomolecular construction kit. Most importantly, the constituent subunits of the S-layer lattices have the capability to recrystallize into iso-porous closed monolayers in suspension, at liquid-surface interfaces, on lipid films, on liposomes, and on solid supports (e.g., silicon wafers, metals, and polymers). The self-assembled monomolecular lattices have been utilized for the immobilization of functional biomolecules in an ordered fashion and for their controlled confinement in defined areas of nanometer dimension. Thus, S-layers fulfill key requirements for the development of new supramolecular materials and enable the design of a broad spectrum of nanoscale devices, as required in molecular nanotechnology, nanobiotechnology, and biomimetics [1-3]. 

When classifying chemical products, Seider et al. [3] identify three categories (1) basic chemicals (commodity and specialty chemicals, bio-materials, and polymeric materials) (2) industrial chemicals (films, fibers, paper,. ..) and (3) configured consumer products (dialysis devices, post-it notes, transparencies, drug delivery patches,. ..). In the manufacture of epitaxial silicon wafers, a thin film of crystalline silicon is often deposited on a polished crystalline silicon... 

D. A. Brass and A. G. Lee, The Production of Epitaxial Silicon Wafers via Plasma Enhanced Chemical Vaposition, Univ. Pennsylvania, Towne Library, 2003. 

These advantages are commercially used in the so-called photolithography, a technique that allows the production of very tiny and accurate nanometer scale structures on the surface of semiconductors (e.g., silicon wafers). 

A coating composition was prepared by dissolving the step 1 product (3.00 g), and 1,3,5,7-adamantanetetracarboxylic acid (0.552 g) in /V,/V-dimcthylacctamidc (20.13 g) and then filtering through 0.2 pm membrane. The coating solution was then spin coated onto an 8-inch silicon wafer and heated to 300°C for 30 minutes and then further heated to 400°C for an additional 30 minutes. The film that formed had a thickness of 298 nm, a density of 1.05 g/cm3, and a dielectric constant of 2.3. 

Diborane(6), B2H6. This spontaneously flammable gas is consumed primarily by the dectronics industry as a dopant in the production of silicon wafers for use in semiconductors. It is also used to produce amine boranes and the higher boron hydrides. Callery Chemical Co., a division of Mine Safety Appliances Co., and Voltaix, Inc., are the main U.S. producers of this substance. Several hundred thousand pounds were manufactured worldwide in 1990. 

It is well established (elucidated in several articles in this encyclopedia) that the production of integrated circuits (ICs) requires manufacturing techniques of extreme precision and sophistication, The purity of materials used is also far higher than experienced by most other materials-processing industries. It has been observed by Howard, Jackel, Mankiewich. and Skocpol (AT T Bell Laboratories), m a 1986 paper, that a singlecrystal silicon wafer 15 cm or more in diameter can be obtained with concentrations of undesired dopants at less than 1 part in 10 billion and with only about one defect per square centimeter. Accuracy m recent years is in terms of a few nanometers, and feature sizes in commercial circuits are down to 1 micrometer (micron) and getting smaller, Thus, it is no surprise... 

Little is known about the interactions between the transport properties in the melt and the production of defects at the melt-crystal interface. An exception is the swirl microdefect seen during processing of dislocation-free silicon wafers (118). The origins of this defect (119) are related to temperature oscillations and remelting of the interface. Kuroda and Kozuka (120) have studied the dependence of temperature oscillations on operating parameters in a CZ system but have not linked the oscillations to convective instabilities in the melt. 

MIP features down to 1.5 pm on 4-in silicon wafers was reached (Fig. 6). A wide range of micrometric patterns with different geometries can be obtained, such as lines, spirals, circle matrices, and circular, squared, or hexagonal patterns (Fig. 6 bottom). Multiplexed chips containing three different polymers were also fabricated, paving the road to mass production of biomimetic chips. Fluorescence microscopy was used to test for the binding of fluorescent model analyte to the micropattems. 

Sample integrations similar to pharmaceutical approaches were already examined in 1997 [39]. Here, a chip-like microsystem was integrated into a laboratory automaton that was equipped with a miniaturized micro-titer plate. Microstructures were introduced later [40] for catalytic gas-phase reactions. The authors also demonstrated [41] the rapid screening of reaction conditions on a chip-like reactor for two immiscible liquids on a silicon wafer (Fig. 4.8). Process conditions, like residence time and temperature profile, were adjustable. A third reactant could be added to enable a two-step reaction as well as a heat transfer fluid which was used as a mean to quench the products. 

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