Epitaxy

Introduction

The single-crystal GaAs wafers are used as substrates for the growth of very thin layers of the same or other III-V compounds having the desired electronic or optical properties. The crystal structure of the grown layer matches that of the substrate. Epitaxy is the process of growing thin films of crystals, in which the substrate determines the crystallinity and orientation of the grown layer. A variety of epitaxial growth techniques are used in III-IV display and device production. The two most common techniques are Vapor Phase Epitaxy (VPE) and Liquid Phase Epitaxy (LPE).

VPE uses a heated stream of gaseous elements or compounds that interact at the surface of the substrate to form the crystalline layer. VPE is primarily used in LED epitaxy. In LPE, the crystalline layer is formed by exposing the substrate to a heated metallic solution saturated with the desired layer components. This method is primarily used in microwave IC epitaxy.

In addition to VPE and LPE, vacuum epitaxy in the form of molecular beam epitaxy (MBE) has developed as an extraordinarily versatile technique. MBE of GaAs consists of an ultra-high vacuum system containing sources for atomic or molecular beams of Ga and As and a heated substrate wafer. The molecular beam sources are usually containers for liquid Ga or solid As. The sources have an orifice that faces the substrate wafer. When the container is heated, atoms of Ga or molecules of As effuse from the orifice. For GaAs, growth usually takes place with a substrate temperature above 450 ºC.

VPE is the primary method in use and is discussed in this section. There are two major techniques of VPE, based on two different chemistries:

• The III-halogens (GaCl₃) and V-halogens (AsCl₃) or V-hydrogen (AsH₃ and PH₃).
• The III metal-organics and V-hydrogen, such as Ga₃(CH₃) and AsH₃.

The thermochemistries of these techniques are very different. The halogen reactions are usually "hot" or "cold", in which the III-halogen is generated in a hot zone by reaction of the III element with HCl, and then diffuses to the cold zone, where it reacts with the V species to form III-V material. The metal-organic chemistry is a hot wall process in which the III metal-organic compound cracks or pyrolyzes away the organic group, and the remaining III and hydride V react to form III-V.

Reactor Load and Unload

To begin, the degreased and polished wafers initially receive a pre-epitaxy etch and cleaning step. This involves a sequential wet chemical dipping operation using sulfuric acid, hydrogen peroxide, and water (5:1:1); a de-ionized water rinse; and finally, an isopropyl alcohol clean/dry.

The primary technique in use for VPE in LED processing is the III-halogen and V-hydrogen system. It involves a two-cycle process; first, growing the epitaxial layer of GaAsP on the GaAs substrate, next, an etch cycle to clean the quartz reactor chamber of impurities. During the epitaxial growth cycle, the pre-cleansed GaAs wafers are loaded into a vertical quartz reactor chamber containing an upper reservoir of elemental liquid gallium over which anhydrous HCl gas is metered, forming GaCl₃. A V-hydrogen-hydride gas mixture of 10%-AsH₃ and 5%-PH₃/H₃ carrier is also metered into the reaction chamber with the addition of 50 ppm dimethyl telluride and 25 ppm diethyl telluride gaseous dopants. The chemical species in the "hot zone" of the quartz reactor react, and in the "cold zone" form the desired layer of GaAsP on the wafer substrate as well as on the interior of the reactor chamber. Effluents from the reactor are routed to a hydrogen torch system for pyrolysis and vented to a wet scrubber system or other exhaust conditioning system.

The etch cycle is performed at the end of the grow cycle and on new quartz reactors to clean the interior surface of impurities. Undiluted hydrogen chloride gas is metered into the chamber for periods of 5-15 minutes. The effluents are vented to the wet scrubber system for neutralization. At the end of both the growth and etch cycles, an extended nitrogen purge is used to flush the reactor chamber of toxic and corrosive gases.

The following are potential hazards of reactor load and unload.

• Chemicals
• Flammable Gases, Fire
• Toxic, Irritative, and Corrosive Gases
• Arsenic

Chemicals

Potential Hazard

• Possible employee exposure to chemicals used for pre-cleaning. Common chemicals include acids (H₂SO₄), caustics (H₂O₂), and isopropyl alcohol.

Possible Solutions

• See Possible Solutions: Chemicals.

Additional Information

• 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.

Flammable Gases, Fire

Potential Hazard

• Possible ignition of flammable gases, resulting in fire and/or explosion. Possible exposure to gases above permissible limits.

Possible Solutions

• See Possible Solutions: Flammable Gases, Fire.
• Provide PPE as appropriate to prevent contact with gases. [29 CFR 1910 Subpart I]
• 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.

Additional Information

OSHA Safety and Health Topics Pages:

• Compressed Gas and Equipment
• Personal Protective Equipment (PPE)

Toxic, Irritative, and Corrosive Gases

Potential Hazard

• Possible employee exposure to toxic, irritative, and corrosive gases, including HCl, AsH₃, and PH₃.

Possible Solutions

• See Possible Solutions: Toxic, Irritative, and Corrosive Gases and Liquids.

Additional Information

• 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.

Reactor Cleaning

After each growth cycle, the VPE reactors must be opened, the wafers removed, and the lower portion of the reactor physically cleaned. The EPI operator performs the cleaning process by scraping the lower quartz reactor and the base plate using a metal tool. The operator collects the particulate material (mixture of GaAs, GaAsP, arsenic oxides, phosphorous oxides, and entrapped hydride gases) in a metal container positioned below the vertical reactor, and uses a high-efficiency vacuum for the final cleanup.

The following are potential hazards of reactor cleaning.

• Arsenic
• Toxic, Irritative, and Corrosive Gases and Liquids