The dopant sources used in the predeposit step are in three distinct chemical states: gas, liquid and solid. Table 5 identifies the various types of diffusion source dopants and their physical states. Gases are generally supplied from compressed gas cylinders with pressure controls or regulators, shut-off valves and various purging attachments and are dispensed through small-diameter metal tubing.
Liquids are dispensed normally from bubblers, which saturate a carrier gas stream, usually nitrogen, with the liquid dopant vapours, as is described in the section on wet oxidation. Another form of liquid dispensing is through the use of the spin-on dopant apparatus. This entails putting a solid dopant in solution with a liquid solvent carrier, then dripping the solution on the wafer and spinning, in a manner similar to the application of photoresists.
Solid sources may be in the shape of a boron nitride wafer, which is sandwiched between two silicon wafers to be doped and then placed in a diffusion furnace. Also, the solid dopants, in powder or bead form, may be placed in a quartz bomb enclosure arsenic trioxide , manually dumped in the source end of a diffusion tube or loaded in a separate source furnace in line with the main diffusion furnace. In the absence of proper controls, arsenic exposures above 0.
These exposures occurred when no precautions were taken to limit the amount of dust in the air. However, when residues were kept wet during cleaning, exposures were reduced to far below the airborne exposure limit. In the older diffusion technologies safety hazards exist during the removal, cleaning and installation of furnace tubes. The hazards include potential cuts from broken quartz ware and acid burns during the manual cleaning. In newer technologies these hazards are lessened by in situ tube cleaning that eliminates much of the manual handling.
Diffusion furnace operators experience the highest routine cleanroom exposure to extremely low-frequency electromagnetic fields e. Average exposures greater than 0.
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This study also noted that cleanroom personnel working in the vicinity of diffusion furnaces had average measured exposures that were noticeably higher than those of other cleanroom workers. This finding was consistent with point measurements reported by Rosenthal and Abdollahzadeh , who found that diffusion furnaces produced proximity readings 5 cm or 2 inches away as high as 10 to 15 microteslas, with the surrounding fields falling off more gradually with distance than other cleanroom equipment studied; even at 6 feet away from diffusion furnaces, the reported flux densities were 1.
These emission levels are well below current health-based exposure limits set by the World Health Organization and those set by individual countries. Ion implantation is the newer method of introducing impurities elements at room temperature into silicon wafers for junction formation. Ionized dopant atoms i. At the end of their path, they strike the wafer and are embedded at various depths, depending on their mass and energy.
As in conventional diffusion, a patterned oxide layer or a photoresist pattern selectively masks the wafer from the ions.
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A typical ion implantation system consists of an ion source gaseous dopant source, usually in small lecture bottles , analysis equipment, accelerator, focusing lens, neutral beam trap, scanner process chamber and a vacuum system normally three separate sets of in-line roughing and oil-diffusion pumps. The stream of electrons is generated from a hot filament by resistance, an arc discharge or cold cathode electron beam.
The annealing process helps repair the damage to the exterior surface of the implanted wafer caused by the bombardment of dopant ions. With the advent of a safe delivery system for arsine, phosphine and boron trifluoride gas cylinders used in ion implanters, the potential for catastrophic release of these gases has been greatly reduced. These small gas cylinders are filled with a compound to which the arsine, phosphine and boron trifluoride are adsorbed.
The gases are pulled out of the cylinders by use of a vacuum. Ion implanters are one of the most significant electrical hazards in the semiconductor industry. Even after power is off, a significant shock potential exists within the tool and must be dissipated prior to working inside the implanter.
A careful review of maintenance operations and the electrical hazards is warranted for all newly installed equipment, but especially for ion implanters. Exposures to hydrides probably a mixture of arsine and phosphine as high as 60 ppb have been found during ion implanter cryo-pump maintenance Baldwin, Rubin and Horowitz Also, high concentrations of both arsine and phosphine can off-gas from contaminated implanter parts that are removed during preventive maintenance Flipp, Hunsaker and Herring Portable vacuum cleaners with high-efficiency particulate attenuator HEPA filters are used to clean arsenic-contaminated work surfaces in ion implantation areas.
HEPA vacuums, when discharging to the workspace, can also efficiently distribute the distinctive, hydride-like odour associated with ion implanter beam line cleaning Baldwin, Rubin and Horowitz While a concern, there have been no published reports of significant dopant gas exposures during oil changes of vacuum pumps used with dopants—possibly because this is usually done as a closed system. The lack of reported exposure may also be a result of low levels of off-gassing of hydrides from the used oil.
The result of a field study where ml of used roughing pump oil from an ion implanter which used both arsine and phosphine was heated only showed detectable concentrations of airborne hydrides in the pump head space when the pump oil exceeded 70 o C Baldwin, King and Scarpace Since normal operating temperatures for mechanical roughing pumps are 60 to 80 o C, this study did not indicate the potential for a significant exposure. During ion implantation, x rays are formed incidental to the operation. Most implanters are designed with sufficient cabinet shielding which includes lead sheeting strategically placed around the ion source housing and adjacent access doors to maintain employee exposure below 2.
These levels were reduced to less than 2. Additional lead shielding was added to attenuate possible exposures Baldwin, Rubin and Horowitz In addition to x-ray exposures from ion implanters, the possibility of neutron formation has been postulated if the implanter is operated above 8 million electron volts MeV or deuterium gas is used as an ion source Rogers However, typically implanters are designed to operate at well below 8 MeV, and deuterium is not commonly used in the industry Baldwin and Williams Chemical vapour deposition CVD involves the layering of additional material on the silicon wafer surface.
CVD units normally operate as a closed system resulting in little or no chemical exposure to the operators. However, brief hydrogen chloride exposure above 5 ppm can occur when certain CVD prescrubbers are cleaned Baldwin and Stewart Two broad categories of deposition are in common use—epitaxial and the more general category of non-epitaxial CVD. Epitaxial growth is rigidly controlled deposition of a thin single crystal film of a material which maintains the same crystal structure as the existing substrate wafer layer.
It serves as a matrix for fabricating semiconductor components in subsequent diffusion processes. Most epitaxial films are grown on substrates of the same material, such as silicon on silicon, in a process referred to as homoepitaxy. Growing layers of different materials on a substrate, such as silicon on sapphire, is called heteroepitaxy IC device processing. Three primary techniques are used to grow epitaxial layers: vapour phase, liquid phase and molecular beam. Liquid-phase and molecular-beam epitaxy are primarily used in the processing of III-V e. Vapour-phase epitaxy is used to grow a film by the CVD of molecules at a temperature of to 1, o C.
Vapours containing the silicon and controlled amounts of p- or n-type dopants in a carrier gas usually hydrogen are passed over heated wafers to deposit doped layers of silicon. The process is generally performed at atmospheric pressure. Table 6 identifies the four major types of vapour-phase epitaxy, parameters and the chemical reactions taking place.
Whereas epitaxial growth is a highly specific form of CVD where the deposited layer has the same crystalline structure orientation as the substrate layer, non-epitaxial CVD is the formation of a stable compound on a heated substrate by the thermal reaction or decomposition of gaseous compounds. CVD can be used to deposit many materials, but in silicon semiconductor processing the materials generally encountered, in addition to epitaxial silicon, are:. Table 7 identifies the three major categories of CVD using operating temperature as a mechanism of differentiation.
Heat is applied to provide the necessary energy for the chemical reaction in addition to controlling the surface temperatures of the reactor and wafers. After the reaction is complete, the unreacted source gas plus the carrier gas are exhausted through the effluent handling system and vented to the atmosphere. Passivation is a functional type of CVD. It involves the growth of a protective oxide layer on the surface of the silicon wafer, generally as the last fabrication step prior to non-fabrication processing.
After the devices have been fabricated in the silicon substrate, they must be connected together to perform circuit functions. This process is known as metallization. Metallization provides a means of wiring or interconnecting the uppermost layers of integrated circuits by depositing complex patterns of conductive materials, which route electrical energy within the circuits. The broad process of metallization is differentiated according to the size and thickness of the layers of metals and other materials being deposited.
These are:. The most common metals used for silicon semiconductor metallization are: aluminium, nickel, chromium or an alloy called nichrome, gold, germanium, copper, silver, titanium, tungsten, platinum and tantalum. Thin or thick films may also be evaporated or deposited on various ceramic or glass substrates. Thin film metallization is often applied through the use of a high-vacuum or partial-vacuum deposition or evaporation technique.
The major types of high-vacuum evaporation are electron beam, flash and resistive, while partial-vacuum deposition is primarily done by sputtering. To perform any type of thin film vacuum metallization, a system usually consists of the following basic components:. Electron-beam evaporation , frequently called E beam , uses a focused beamof electrons to heat the metallization material. A high-intensity beam of electrons is generated in a manner similar to that used in a television picture tube.
A stream of electrons is accelerated through an electrical field of typically 5 to 10 kV and focused on the material to be evaporated. The focused beam of electrons melts the material contained in a water-cooled block with a large depression called a hearth.
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The melted material then vaporizes within the vacuum chamber and condenses on the cool wafers as well as on the entire chamber surface. Then standard photoresist, exposure, development and wet or dry etch operations are performed to delineate the intricate metallized circuitry. Flash evaporation is another technique for the deposition of thin metallized films. This method is primarily used when a mixture of two materials alloys are to be simultaneously evaporated.
In flash evaporation, a ceramic bar is heated by thermal resistance and a continuously fed spool of wire, stream of pellets or vibrationally dispensed powder is brought in contact with the hot filament or bar. The vaporized metals then coat the interior chamber and wafer surfaces. Resistive evaporation also known as filament evaporation is the simplest and least expensive form of deposition. The evaporation is accomplished by gradually increasing the current flowing through the filament to first melt the loops of material to be evaporated, thereby wetting the filament.
Once the filament is wetted, the current through the filament is increased until evaporation occurs. The primary advantage of resistive evaporation is the wide variety of materials that can be evaporated. Maintenance work is sometimes done on the inside surface of E-beam evaporator deposition chambers called bell jars.
When the maintenance technicians have their heads inside the bell jars, significant exposures can occur. Removing the metal residues that deposit on the inside surface of bell jars may result in such exposures. For example, technician exposures far above the airborne exposure limit for silver were measured during residue removal from an evaporator used to deposit silver Baldwin and Stewart Cleaning bell jar residues with organic cleaning solvents can also result in high solvent exposure.