Ingot Growing

In the Gallium Arsenide ingot and wafer growth process, elemental forms of gallium (Ga) and arsenic (As), plus small quantities of dopant material (silicon, tellurium, or chromium) react at elevated temperatures to form ingots of doped single crystal GaAs. Three generalized methods of ingot production are used:

  1. Horizontal Bridgeman (HB)
  2. Gradient Freeze (GF)
  3. Liquid Encapsulated Czochralski (LEC Cz)

The reaction of As vapor with Ga metal at elevated temperatures, in sealed quartz ampoules, form the bulk polycrystalline GaAs compound. Typically, an As reservoir contained at one end of the ampoule is heated to approximately 600ºC. This generates approximately 1 atm of As vapor pressure in the system, a prerequisite for obtaining stoichiometric GaAs. The As vapor reacts with the Ga metal which is heated to approximately 1260ºC and located at the other end of the ampoule in a quartz board. After the reaction is complete, single-crystal growth begins by programmed cooling (Gradient Freeze) or by physically moving either the ampoule or furnace to provide proper temperature gradients for growth (Horizontal Bridgeman). This indirect approach (Arsenic transport) for compounding and growth of GaAs is used because of the high vapor pressure of As at its melting point and at the melting point of GaAs, about 20 atm at 812ºC, and 60 atm at 1238ºC, respectively.

Another approach to the commercial production of bulk single-crystal GaAs that is gaining favor is the liquid encapsulated Czochralski technique (LEC Cz). A Cz crystal puller is loaded with chunk GaAs in a quartz ampoule with an outer graphite receptor. The bulk GaAs melts at temperatures close to 1238ºC, and the crystal is pulled in a pressurized atmosphere of approximately 100 atm. A viscous glass, B2O3, completely encapsulates the melt, which prevents melt dislocation when the dissociation vapor (As) is less than or equal to the pressure of an inert gas (argon, typically) applied in the puller chamber.

The following description delineates processing steps in use in LED production facilities. The Horizontal Bridgeman (HB) and Gradient Freeze (GF) methods of single-crystal gallium arsenide ingot growth are the dominant techniques in use in LED production.

Charge Preparation

A long cylindrical quartz ampoule is loaded with elemental arsenic in chunk form in a locally exhausted laminar flow hood. A smaller quartz boat is charged with liquid gallium, dopant, and a small GaAs single-crystal seed. The quartz boat is then loaded into the other section of the quartz ampoule already containing the elemental arsenic.

The following are potential hazards for charge preparation.

Ampoule Load and Seal

First, the quartz ampoules are attached to an oil-diffusion vacuum pump assembly. The system is evacuated and the ampoule is heated with a hydrogen/oxygen torch. Finally the end of the ampoule is sealed, which creates a charged and sealed quartz ampoule ready for furnace growth.

The following are potential hazards for ampoule load and seal.

Flammable Gases, Fire

Potential Hazard

  • Possible ignition of flammable gases, resulting in fire and/or explosion. Employees may also be exposed to gases above permissible limits.

Possible Solutions

Furnace Growth

The two dominant methods of single-crystal ingot growth (HB and GF) use charged and sealed quartz ampoules in a high temperature furnace enclosure which is vented to a wet scrubber system.

The HB system consists of a two zone furnace in which the sealed quartz ampoule has separate temperature zones: the arsenic "cold" finger end at 610ºC, and the quartz gallium/dopant/seed-crystal boat containing the melt at 1240ºC. The basic principle in HB involves traversing two heated zones (one above the melting point of GaAs, and one below the melting point), over a boat of GaAs to provide the precisely controlled freezing of molten GaAs. The seed crystal, maintained at all times in the freeze zone, provides the initial crystal starting structure defining direction and orientation of the crystalline structure within the boat. Silicon-carbide liners called support tubes, which allow the resistance heater assembly to mechanically move the full distance of the ampoule, suspend the quartz boat and ampoule of gallium and arsenic within the heater chamber. Additionally, the ampoule rests on a table which must be raised and lowered during growth to provide the proper interface of the GaAs melt with the seed crystal.

GF is the second ingot growth method in use. A high-temperature furnace utilizing resistance heating is kept at 1200-1300ºC, with 1237ºC being the melt/freeze point of GaAs. The total ingot growth process duration is three days, comprised of the following steps:

  1. Furnace firing to temperature
  2. GaAs synthesis
  3. Seed melt
  4. Cool down

The quartz ampoule is also raised and lowered during the growth process by the use of a scissor-type manual jack.

The following are potential hazards for furnace growth.

Ampoule Explosion/Implosion

Potential Hazard

  • Possible explosion or implosion of ampoules during ingot growth, resulting in personnel injury and/or damage to equipment and facilities.

Possible Solutions

  • Identify and eliminate possible causes of explosion/implosion. Use a Process Hazard Analysis (PHA) for identifying and controlling such hazards.
  • Implement appropriate emergency procedures, including ventilation and PPE controls in the event of an explosion/implosion.

Additional Information

OSHA Safety and Health Topics Page:

Ampoule Breakout

After the single-crystal GaAs ingot is grown within the sealed quartz ampoule, the quartz boat containing the ingot, plus the seed crystal, is removed from the ampoule, either by cutting off the sealed end of the ampoule with a wet circular saw, or heating and cracking the ampoule with a hydrogen/oxygen torch. The quartz ampoules are recycled by wet etching the condensed arsenic on the interior surface with HCl/HNO3 or H2SO4/H2O2.

The following are potential hazards for ampoule breakout.

Flammable Gases, Fire

Potential Hazard

  • Possible ignition of flammable gases, resulting in fire and/or explosion. Employees may also be exposed to gases above permissible limits.

Possible Solutions

Acid and Caustic Solutions

Potential Hazard

  • Possible employee exposure to acid and caustic solutions used for etching ampoules for recycling. Solutions of HCl, H2SO4, HNO3, and H2O2 are commonly used.

Possible Solutions

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.
Ingot and Remelt Sandblasting/Cleaning

The single-crystal GaAs ingots and polycrystalline defects must be sandblasted and cleaned to remove exterior oxides and contaminants. The sandblasting is done in a glove-box type bead blaster utilizing either silicon carbide or calcined alumina blasting media. Wet cleaning is done in wet chemical baths provided with local exhaust ventilation and utilizing HCl/HNO3 or alcohol rinses (isopropyl and/or methyl).

The following are potential hazards for ingot and remelt sandblastin/cleaning.

Chemicals

Potential Hazard

  • Possible employee exposure to chemicals used for cleaning. Acid solutions of HCl/HNO3 and isopropyl/methyl alcohol solutions are commonly used.

Possible Solutions

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.