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Ion Implantation
OVERVIEW
An important step in semiconductor manufacturing involves manipulating the properties of semiconductor films by implanting dopant atoms at the near surface of poly or single crystal silicon films. The dopants alter the physical properties of the semiconductor film by changing its resistivity and conductivity. During ion implantation, dopant atoms are vaporized, accelerated, and directed at a substrate. The individual implanted ion enters the crystal lattice, collides with the substrate atoms and electrons, gradually loses its energy and finally comes to rest at some depth within the lattice. The wafers are typically batch processed, on a spinning platen. The ion beam is directed back and forth across the spinning platen, until all areas have received a certain dosage. Ion implant has become the preferred method for doping thin film regions because it allows for better control of the location and doping density in a specific region.
The most common dopants are atoms of arsenic, phosphorous, and boron. Some less common dopants are germanium, indium, and silicon. The source of the dopant atoms can be any number of compounds, with the most common sources being gases. Solids and liquids can also be used as dopant sources. Solid and liquid source ion implanters generally do not generate toxic gas emissions; therefore abatement issues tend to surround only the ion implanters with gas sources. The most common gaseous sources are boron trifluoride (BF3), arsine (AsH3), and phosphine (PH3). Less common gas sources include germanium tetrafluoride (GeF4), and silicon tetrafluoride (SiF4).
Ion implant tools come in a variety of configurations. The three most common configurations are medium current, high current, and high energy. Each kind of ion implanter incorporates a number of roughing pumps, the effluent of which may contain toxic source gases. Medium current ion implanters will use one or two roughing pumps at the source area and/or target chamber and two cryogenic pumps at the target chamber. High current machines can incorporate two cryo pumps at the target chamber and as many as three roughing pumps at the source area, at the accelerator or beamline, and/or at the target chamber.
TYPICAL RECIPE
Gas Flow Rate
AsH3 - 5 sccm
PH3 - 5 sccm
BF3 - 5 sccm
GeF4 - 2 sccm
SiF4 - 2 sccm
He - 15 sccm
Ar - 10 sccm
N2 - 2 slpm
ABATEMENT CONSIDERATIONS
The primary reason for abatement of ion implant processes is worker safety. To illustrate the dangerous nature of implant processes, the following chart of exposure limits has been prepared. TLV is the Threshold Limit Value for a gas. This is the maximum time weighted average exposure for an 8-hour period. IDLH is the level that is an Immediate Danger to Life and Health. This is the concentration that represents a danger after 15 minutes of exposure. Although some of these gases do have explosive limits, the flow rates used in ion implant applications guarantee that the tool effluent will not approach flammability limits. This table includes all the gases that might be present in this process, including process by-products.
It is easy to see the health dangers that ion implant gases present to fab workers. Point-of-use gas abatement protects workers as well as the environment surrounding the fab by removing the highly toxic gases at the earliest possible point. This greatly reduces the dangers associated with a leak in the exhaust system.
Due to the extremely toxic nature of arsenic containing compounds, abatement options for ion implantation are limited. Arsine is not water soluble, making water scrubbers ineffective without the addition of permanganate, hypobromite, or hypochlorite solutions, which are other dangerous chemicals. Even with chemical injection, the effluent water will become contaminated with arsenic, which is a very closely regulated water pollutant.
Thermal combustion and oxidation of arsine generates arsenic trioxide, As2O3, an equally toxic dust that will coat all downstream duct work. Any workers required to work with this toxin would need to wear proper protective equipment
More and more semiconductor manufacturers are realizing the cost benefits of point-of-use abatement over traditional dilution. When the cost of a continuous nitrogen dilution is compared with the purchase and operation of a point of use scrubber that is capable of handling multiple implanters, the scrubber is shown to be more attractive.
BAZM SOLUTION: GST SDS Dry Scrubber
Dry scrubbing is the best approach for the low levels of effluent encountered in ion implant processes. BAZM offers the SDS, a completely passive system designed for hydride abatement from ion implant applications. Effluent gases flow directly from the pump exhaust into the SDS with its 37gallon can of dry media.
Target gases are chemisorbed onto the resin and are contained within the canister. After a resin canister has become consumed, it is essential that it be decommissioned. This simple process ensures that all trapped hydride gases are fully reacted to their most stable form. This decommissioning process also ensures that any solid phosphorus materials, which are pyrophoric, that may have accumulated in the canister as a by-product of the implantation process are converted to the non-pyrophoric phosphorus oxides. On typical ion implant processes, a single 37Gallon can will last over 1 year.
The dry media technology is designed to chemisorb target gases. As the acid and hydride gases flow through the resin bed they are first adsorbed by the resin material, and then chemically reacted to form inert solid material. This means that the gases are not simply bonded onto solid resin particles, but are converted to a different chemical by reaction with the resin material. This bonding eliminates the possibility of out-gassing, which can occur when gases only form weak physical bonds with resin material, as in the case of carbon beds.
ABATEMENT CHEMISTRY
All targeted gases are removed from the effluent stream to concentrations below TLV. If a toxic gas monitor is employed, it will ensure that effluent gases are below TLV. Fluorinated species react with the first layer of resin in the canister a proprietary metal hydroxide.
2BF3 + 3M(OH)2à B2O3 (s) + 3MF2 (s) + 3H2O
SiF4 + 2M(OH)2à SiO2 (s) + 2MF2 (s) + H2O
GeF4 + 2M(OH)2 à GeO2 (s) + 2MF2 (s) + H2O
The fluorinated species will also react with the proprietary metal oxides in the second resin:
2BF3 + 3MO à B2O3 (s) + 3MF2 (s)
SiF4 + 2MO à SiO2 (s) + 2MF2 (s)
GeF4 + 2MO à GeO2 (s) + 2MF2 (s)
Although the fluorinated species can react with both layers, the hydride gases will pass through the first layer and will only react with the metal oxides of the second resin layer via the following chemical reactions:
2AsH3 + 3MO à M3As (s) + As (s) + 3H2O
2PH3 + 3MO à M3P(s) + P (s) + 3H2O
Both of these reactions are taken to completion via reaction with air introduced during chamber pump downs and air oxidation procedures:
4M3As + 11O2à 2M3(AsO4)2 (s) + 6MO (s)
4As + 3O2à 2As2O3 (s)
4M3P + 11O2à 2M3(PO4)2 (s) + 6MO (s)
4P + 5O2à 2P2O5 (s)
EFFICIENCY
In a SDS dry scrubber, effluent gases will be reduced to below TLV levels. The actual process efficiency is determined by the gas concentrations at the inlet of the canister. However, under typical ion implant operating conditions, the efficiency is >99% for all gases.
For any addition questions, please contact BAZM Solution - your gas abatement expert.
An important step in semiconductor manufacturing involves manipulating the properties of semiconductor films by implanting dopant atoms at the near surface of poly or single crystal silicon films. The dopants alter the physical properties of the semiconductor film by changing its resistivity and conductivity. During ion implantation, dopant atoms are vaporized, accelerated, and directed at a substrate. The individual implanted ion enters the crystal lattice, collides with the substrate atoms and electrons, gradually loses its energy and finally comes to rest at some depth within the lattice. The wafers are typically batch processed, on a spinning platen. The ion beam is directed back and forth across the spinning platen, until all areas have received a certain dosage. Ion implant has become the preferred method for doping thin film regions because it allows for better control of the location and doping density in a specific region.
The most common dopants are atoms of arsenic, phosphorous, and boron. Some less common dopants are germanium, indium, and silicon. The source of the dopant atoms can be any number of compounds, with the most common sources being gases. Solids and liquids can also be used as dopant sources. Solid and liquid source ion implanters generally do not generate toxic gas emissions; therefore abatement issues tend to surround only the ion implanters with gas sources. The most common gaseous sources are boron trifluoride (BF3), arsine (AsH3), and phosphine (PH3). Less common gas sources include germanium tetrafluoride (GeF4), and silicon tetrafluoride (SiF4).
Ion implant tools come in a variety of configurations. The three most common configurations are medium current, high current, and high energy. Each kind of ion implanter incorporates a number of roughing pumps, the effluent of which may contain toxic source gases. Medium current ion implanters will use one or two roughing pumps at the source area and/or target chamber and two cryogenic pumps at the target chamber. High current machines can incorporate two cryo pumps at the target chamber and as many as three roughing pumps at the source area, at the accelerator or beamline, and/or at the target chamber.
TYPICAL RECIPE
Gas Flow Rate
AsH3 - 5 sccm
PH3 - 5 sccm
BF3 - 5 sccm
GeF4 - 2 sccm
SiF4 - 2 sccm
He - 15 sccm
Ar - 10 sccm
N2 - 2 slpm
ABATEMENT CONSIDERATIONS
The primary reason for abatement of ion implant processes is worker safety. To illustrate the dangerous nature of implant processes, the following chart of exposure limits has been prepared. TLV is the Threshold Limit Value for a gas. This is the maximum time weighted average exposure for an 8-hour period. IDLH is the level that is an Immediate Danger to Life and Health. This is the concentration that represents a danger after 15 minutes of exposure. Although some of these gases do have explosive limits, the flow rates used in ion implant applications guarantee that the tool effluent will not approach flammability limits. This table includes all the gases that might be present in this process, including process by-products.
| Gas | TLV | IDLH |
|---|---|---|
| AsH3 | 0.05 ppm | 3 ppm |
| PH3 | 0.3 ppm | 50 ppm |
| BF3 | 1 ppm | 25 ppm |
| GeF4 | 1 ppm | 25 ppm |
| SiF4 | 25 ppm | 100 ppm |
It is easy to see the health dangers that ion implant gases present to fab workers. Point-of-use gas abatement protects workers as well as the environment surrounding the fab by removing the highly toxic gases at the earliest possible point. This greatly reduces the dangers associated with a leak in the exhaust system.
Due to the extremely toxic nature of arsenic containing compounds, abatement options for ion implantation are limited. Arsine is not water soluble, making water scrubbers ineffective without the addition of permanganate, hypobromite, or hypochlorite solutions, which are other dangerous chemicals. Even with chemical injection, the effluent water will become contaminated with arsenic, which is a very closely regulated water pollutant.
Thermal combustion and oxidation of arsine generates arsenic trioxide, As2O3, an equally toxic dust that will coat all downstream duct work. Any workers required to work with this toxin would need to wear proper protective equipment
More and more semiconductor manufacturers are realizing the cost benefits of point-of-use abatement over traditional dilution. When the cost of a continuous nitrogen dilution is compared with the purchase and operation of a point of use scrubber that is capable of handling multiple implanters, the scrubber is shown to be more attractive.
BAZM SOLUTION: GST SDS Dry Scrubber
Dry scrubbing is the best approach for the low levels of effluent encountered in ion implant processes. BAZM offers the SDS, a completely passive system designed for hydride abatement from ion implant applications. Effluent gases flow directly from the pump exhaust into the SDS with its 37gallon can of dry media.
Target gases are chemisorbed onto the resin and are contained within the canister. After a resin canister has become consumed, it is essential that it be decommissioned. This simple process ensures that all trapped hydride gases are fully reacted to their most stable form. This decommissioning process also ensures that any solid phosphorus materials, which are pyrophoric, that may have accumulated in the canister as a by-product of the implantation process are converted to the non-pyrophoric phosphorus oxides. On typical ion implant processes, a single 37Gallon can will last over 1 year.
The dry media technology is designed to chemisorb target gases. As the acid and hydride gases flow through the resin bed they are first adsorbed by the resin material, and then chemically reacted to form inert solid material. This means that the gases are not simply bonded onto solid resin particles, but are converted to a different chemical by reaction with the resin material. This bonding eliminates the possibility of out-gassing, which can occur when gases only form weak physical bonds with resin material, as in the case of carbon beds.
ABATEMENT CHEMISTRY
All targeted gases are removed from the effluent stream to concentrations below TLV. If a toxic gas monitor is employed, it will ensure that effluent gases are below TLV. Fluorinated species react with the first layer of resin in the canister a proprietary metal hydroxide.
2BF3 + 3M(OH)2à B2O3 (s) + 3MF2 (s) + 3H2O
SiF4 + 2M(OH)2à SiO2 (s) + 2MF2 (s) + H2O
GeF4 + 2M(OH)2 à GeO2 (s) + 2MF2 (s) + H2O
The fluorinated species will also react with the proprietary metal oxides in the second resin:
2BF3 + 3MO à B2O3 (s) + 3MF2 (s)
SiF4 + 2MO à SiO2 (s) + 2MF2 (s)
GeF4 + 2MO à GeO2 (s) + 2MF2 (s)
Although the fluorinated species can react with both layers, the hydride gases will pass through the first layer and will only react with the metal oxides of the second resin layer via the following chemical reactions:
2AsH3 + 3MO à M3As (s) + As (s) + 3H2O
2PH3 + 3MO à M3P(s) + P (s) + 3H2O
Both of these reactions are taken to completion via reaction with air introduced during chamber pump downs and air oxidation procedures:
4M3As + 11O2à 2M3(AsO4)2 (s) + 6MO (s)
4As + 3O2à 2As2O3 (s)
4M3P + 11O2à 2M3(PO4)2 (s) + 6MO (s)
4P + 5O2à 2P2O5 (s)
EFFICIENCY
In a SDS dry scrubber, effluent gases will be reduced to below TLV levels. The actual process efficiency is determined by the gas concentrations at the inlet of the canister. However, under typical ion implant operating conditions, the efficiency is >99% for all gases.
For any addition questions, please contact BAZM Solution - your gas abatement expert.
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