Etching removes layers of SiO2, metals, and polysilicon, according to the desired patterns delineated by the resist. The two major methods of etching are wet chemical etching or dry chemical etching.
Wet Chemical Etching: Wet etching is accomplished by submersion of the wafer in an acid bath. Common wet etchant chemical solutions are shown in Table 2. In general, etching solutions are housed in polypropylene, temperature-controlled baths. The baths are usually equipped with either a ring-type plenum exhaust ventilation or a slotted exhaust at the rear of the etch station. Vertical laminar-flow hoods are used to supply uniformly-filtered, particulate-free air to the top surface of the etch baths.
Dry Chemical Etching: Dry etching is commonly used due to its ability to better control the etching process and reduce contamination levels. Dry processing effectively etches desired layers through the use of gases, using either, a chemically reactive gas, or through physical bombardment of argon atoms.
Chemical - Plasma Etching: Plasma etching systems have been developed that can effectively etch silicon, silicon dioxide, silicon nitride, aluminum, tantalum, tantalum compounds, chromium, tungsten, gold, and glass. Two kinds of plasma etching reactor systems are in use -- the barrel (cylindrical), and the parallel plate (planar). Both reactor types operate on the same principles and vary primarily in configuration only. The typical reactor consists of a vacuum reactor chamber made usually of aluminum, glass, or quartz. A radiofrequency (RF) energy source is used to activate fluorine-based or chlorine-based gases which act as etchants. Wafers are loaded into the chamber, a pump evacuates the chamber, and the reagent gas is introduced. The RF energy ionizes the gas and forms the etching plasma, which reacts with the wafers to form volatile products which are pumped away. Table 3 identifies the materials and plasma gases in use for etching various layers.
Physical Bombardment: Physical etching processes are similar to sandblasting; argon gas atoms are used to physically bombard the layer to be etched, and a vacuum pump system is used to remove dislocated material. Sputter etching is one physical technique involving ion impact and energy transfer. The wafer to be etched is attached to a negative electrode, or "target," in a glow-discharge circuit. Positive argon ions bombard the wafer surface, resulting in the dislocation of the surface atoms. Power is provided by an RF energy source. Ion beam etching and milling are similar physical etching processes which use a beam of low-energy ions to dislodge material. The ion beam is extracted from an ionized gas (argon or argon/oxygen) or plasma, created by an electrical discharge.
Reactive ion etching (RIE) is a combination of chemical and physical etching. During RIE, a wafer is placed in a chamber with an atmosphere of chemically reactive gas (CF4 or CCl4) at a low pressure. An electrical discharge creates an ion plasma with an energy of a few hundred electron volts. The ions strike the wafer surface vertically, where they react to form volatile species that are removed by the low pressure in-line vacuum system.
The following are the potential hazards of etching.
- Possible employee exposure to acids used for wet chemical etching. Typical acids may include mixtures of HF, HCl, H2SO4, etc. (see Table 2).
- Occupational Health Guidelines for Chemical Hazards. US Department of Health and Human Services (DHHS), National Institute for Occupational Safety and Health (NIOSH) Publication No. 81-123, (1981, January). Provides a table of contents of guidelines for many hazardous chemicals. The files provide technical chemical information, including chemical and physical properties, health effects, exposure limits, and recommendations for medical monitoring, personal protective equipment (PPE), and control procedures.
- Possible employee exposure to fluorinated, chlorinated, and other reactive gases used for dry etching (see Table 3).
- Identify gas hazards and perform appropriate exposure evaluations.
- Provide appropriate ventilation to reduce gas concentration levels in the air.
- Provide PPE as appropriate to prevent contact with gases. [29 CFR 1910 Subpart I]
- Use respiratory protection when necessary to further reduce exposure and protect employees. [29 CFR 1910.134]
- Use gas monitoring systems with automatic shut-offs and alarm systems, as appropriate.
- Design and use specialized processing, material handling, and storage equipment for gases. Consider both normal use and emergency scenarios. Process Safety Management (PSM) requirements may also apply. [29 CFR 1910.119]
OSHA Safety and Health Topics Pages:
- Compressed Gas and Equipment
- Personal Protective Equipment (PPE)
- Process Safety Management (PSM)
- Respiratory Protection
Radiofrequency (RF) Radiation
- Possible employee exposure to radiofrequency (RF) radiation used as an ionizing source for dry etching.
- See Possible Solutions: Radiofrequency (RF) and Infrared (IR) Radiation.
- Install interlocks and emergency shut-offs on etching equipment.