Wafer Fabrication Equipment - Page 1 of 2: Epitaxy; Oxidation; Diffusion; Ion Implant Equipment

Semiconductor Wafer Fabrication Equipment

Epitaxial Deposition Equipment

The equipment used in depositing epitaxial layers on the wafer is often referred to as ‘epitaxial reactor’. Epitaxial reactors are high-temperature chemical vapor deposition systems. There are two major types of epitaxial deposition equipment, namely, the ‘pancake reactor’ and the ‘barrel reactor’. These two types got their names from the shapes of their susceptor, which is the part that holds the wafers during the epitaxial deposition process.

A basic epitaxial reactor should consist of at least the following: a) a reactor tube or chamber to isolate the epitaxial deposition environment; b) a system that distributes the various chemical species for epitaxial deposition in a very controlled manner; c) a system for heating the wafers; and d) a system for scrubbing the effluent gases. Applied Materials is an example of a manufacturer of Epi deposition equipment.

Oxidation Systems

The formation of silicon dioxide (SiO2) on a silicon substrate is known as oxidation. Generally this is accomplished by thermal oxidation, wherein the wafer is exposed to an oxidizing environment at elevated temperature. Thus, an oxidation system has to have a heat source to elevate the temperature of the oxidizing environment and a system for delivering the oxidizing gases to the wafers.

A basic oxidation system would consist of the following: a) a cabinet that houses the various parts of the furnace; b) a heating system; c) a temperature measurement and control system; d) process tubes where the wafers undergo oxidation; e) a system for moving the oxidizing gases into and out of the process tubes and f) a loading station where the wafer boats are loaded into and unloaded from the process tubes.

Note that thermal oxidation is basically a diffusion process, so it can also be accomplished by diffusion systems designed not only for depositing dopants into wafers, but for oxidation purposes as well. Tempress Systems, Bruce Technologies, and Tystar are examples of manufacturers of oxidation furnaces.

Diffusion Systems

Diffusion is the transfer of a species resulting from concentration gradients. Diffusion can pertain to either oxidation or dopant deposition, but it is generally used to refer to the latter.

In the early days of semiconductor manufacturing, diffusion was extensively used in the controlled deposition of impurity atoms or dopants into the silicon substrate, which is the foundation of p-n junction formation. Ion implantation has become the primary means for dopant deposition in recent years, but diffusion is still necessary in certain applications.

A typical diffusion system, also known as a diffusion furnace, is very similar to (and in some cases, the same as) an oxidation furnace. It is an equipment designed to provide an environment of high temperature and controlled gas flow to wafers. It consists of a heating element, a diffusion tube, a diffusion boat, and a dopant delivery system. The wafer is exposed to the dopant gases at high temperature inside the furnace, a mechanism that is very similar to oxidation for growing SiO2 films. ASM, Tempress Systems, and Tystar are examples of manufacturers of diffusion furnaces.

Ion Implantation Equipment

Ion implantation is used in wafer fabrication to selectively deposit dopant ions into the surface of the wafers. This process involves the direct introduction of highly energetic, charged atomic species onto the target substrate. Its application to semiconductor manufacturing requires a great deal of control to ensure that the dopants are introduced in precise quantities at the correct location and depth without inducing any damage to the silicon lattice structure of the substrate. Needless to say, an ion implantation equipment is very complex, needing to accurately implant and monitor the species being introduced.

A typical ion implantation equipment consists of a feed source, an ion source, a device for extracting and analyzing ions, an acceleration tube, a scanning system, a system end station, a high vacuum system, and a computerized control system.

The feed source contains the material where the species for implantation will come from. The acceleration tube determines the energy content of the ions while the scanning system ensures uniform distribution of the ions over the target. The system end station measures the implant dose and minimizes dose errors. Applied Materials, Eaton, and Varian are examples of manufacturers of ion implantation systems.

Physical Vapor Deposition Systems

Physical vapor deposition, or PVD, is the process of depositing a material over a substrate by first converting the material to gaseous state, transporting it across the substrate through pressure control, and allowing the vapor to condense over the target area. This is widely used in the deposition of thin-film aluminum layers on wafers. The source material may be converted into vapor either by evaporation or sputtering.

A typical sputter-type PVD system consists of a sputter chamber, a pre-processing chamber, vacuum pumps, power supplies, sputtering targets, sputtering gas supply, flow control systems, monitors, wafer handling mechanisms and a computerized controller. Varian, Novellus, and KDF are examples of manufacturers of PVD equipment. See also: PVD by Sputtering; PVD by Evaporation.

Chemical Vapor Deposition Systems

Chemical Vapor Deposition, or CVD, is the process of transforming gas molecules known as the precursor into solid thin-film or powder material on the surface of a substrate. CVD comes in various methods. In the fabrication of semiconductor devices, however, the most popular method is known as Plasma-Enhanced CVD, or PECVD. PECVD uses plasma to decompose a reactant gas, such as silane (SiH4), to produce reaction products that precipitate on the surface of the substrate as a new layer.

Inside a PECVD reactor, a strong electric field ignites a plasma between two electrodes, one of which holds the substrate. This plasma ignition cracks the molecular bonds of the process gas, which in turn are able to crack more process gas molecules before reaching the surface of the substrate. Eventually a new layer is deposited on the substrate. For example, when silane is used as precursor, plasma ignition frees up Si and SiH radicals, which also crack more silane molecules on their way to the surface of the substrate, where silicon is deposited. Novellus and Applied Materials are examples of suppliers of CVD systems. See also: Chemical Vapor Deposition.

Photolithography Equipment

Photolithography is an optical process used to create circuit patterns on the silicon wafer. This consists of using photoresist materials and masks to selectively expose or cover areas on the die to which new materials may be added or from which existing materials may be removed. Photolithography consists of a series of steps and, consequently, requires several individual equipment to accomplish these steps.

The equipment used in photolithography include: 1) resist coating equipment to deposit the photoresist on the wafer; 2) ovens for soft-baking the photoresist; 3) exposure systems to subject the resist to some form of radiation; 4) development systems to remove or retain (depending on photoresist type) the exposed areas of the resist, leaving behind a mask pattern that may be used for other wafer fabrication processes. Nikon, Canon, and Karl Suss are examples of manufacturers of photolithography equipment. See also: Optical Lithography.

Etching Equipment

Etching is the process of removing materials or layers from the wafer, of which there are two types: wet and dry etching. As these names imply, wet etching involves the use of liquid chemicals while dry etching involves the use of reactant gases to remove materials.

Wet etching equipment consists of systems that: 1) allow diffusion of reactants to the surface of the wafer; 2) provide the proper conditions for chemical reaction of these reactants with the material being removed; and 3) extract the reaction products from the surface.

Plasma etching is one type of dry etching, using plasma to produce chemically reactive species from inert gases. The reactive gases are then made to react with the material to be etched. Plasma etchers come in many configurations, but a typical plasma etching system consists of: 1) an etching chamber; 2) a pumping and pressure control system; 3) an RF power supply; 4) gas handling systems; and 5) electrodes. Technics and Tegal are examples of manufacturers of etching systems.

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