Image credit: PHX Welding
Electronic and semiconductor gases are process gases used in electronic manufacturing operations.
Gas Use in Electronics Manufacturing
The electronic gases family includes both pure gases and gas mixtures which are specially configured for specific manufacturing processes. Mass production of integrated circuits (IC) requires up to 30 different gases for the various processes involved. These operations, which are discussed in more detail below, include oxidation, deposition, etching, sputtering, doping, and inerting.
The image below shows the first half of the semiconductor diode fabrication process. Gases may be used in any of these steps, but are particularly important in deposition (c), crystal growth (b), etching (h), and doping (j).
Image credit: All About Circuits
As a rule, electronics gases must be high-purity gases. The precise nature of electronics manufacturing results in contaminants, even those with concentrations as small as trace impurities measured in parts-per-trillion, having a disastrous effect on product quality.
The variety of process gases used within the semiconductor manufacturing industry is relatively larger than the range used in almost every other application. Electronic gases are typically classified by their use within one or more of these processes, as described below.
Process gases may be produced and sold as cylinders (left) or distributed through bulk gas systems (right).
Image credit: USASafety | SEMI Gas Systems
Gases by Process
Silicon growth gases are sometimes known as "silicon precursors" and are used to deposit silicon layers on existing substrates. Depending on the type of gas used, these layers form when precursor gases react with heat or basic gases such as as hydrogen. Many halides—including silane, silicon tetrachloride, and silicon tetrafluoride—are employed as silicon precursors. Some specialty gases such as germane (GeH4) are also used to form and deposit silicon-germanium alloys.
Etching is a process used to strip unwanted layers from a wafer or substrate. Silicon, silicon dioxide, and silicon nitride wafers are typically etched using some type of fluorine-based (halide) gas. Halocarbons (gases involving carbon and fluorine) are commonly used for etching of silicon materials, while other halides such as chlorine and hydrogen fluoride are used for etching metallic (non-silicon) interconnects.
Choice of an etchant gas strongly depends on the ability of a gas mixture to selectively etch a film without adversely affecting a disparate film on the same wafer.
Dopants impart controlled impurities to a wafer in order to modify its electrical properties. These gases contribute an electron deficiency or "hole" (p type) or an additional electron (n type) to a substrate. Doping of semiconductor materials is the groundwork for the manufacturing of diodes and transistors.
Dopants typically derive from specialty gas groups including boron, arsine, and phosphine and are subclassified by the type of doping they impart. For example, n type doping is typically performed using arsine and phosphine, while p type doping is accomplished using diborane.
System Purging / Atmospherics
Purge gases (also known as "atmospherics") are used to purge process systems to prevent back-contamination. These are typically basic gases which may be sold in cylinder or bulk form, as purge gases may also be used for other processes within a manufacturing operation. For example, oxygen is commonly used as a purge gas and may also be used for etching polymer materials.
Common atmospherics include oxygen, hydrogen, nitrogen, argon, and helium.
Sputtering is a specialized deposition process through which a material from a source (or "sputtering target") is ejected onto a wafer. This process relies on reactive gases to increase the deposition rate of the material onto the substrate. Argon and other inert gases are widely used as sputtering gases.
The sputtering process, showing the use of argon (green particles) as a sputtering gas.
Image credit: READE Advanced Materials
Electronic process gases may be manufactured, used, and tested based on guidelines laid out in various standards. Depending upon the standard type, guidelines may refer to either the processes or the gases themselves. Specifications and standards from SEMI, an association serving the micro- and nano-electronics manufacturing industries, are especially relevant to particular process gas types.
SEMI C3 - Specifications for gases
SEMI C57 - Specifications and guidelines for argon
SEMI C3.55 - Specification for silane
National Institute of Standards and Technology (NIST) - Index of Semiconductor Process Gases