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Metal Etch Application Notes
OVERVIEW
To accurately understand the abatement issues for any given process, it is necessary to understand the process in the tool, the chemicals that are put in, and how the process modifies them. The following overview represents our understanding of this process and is the basis of this recommendation.
The removal of thin films from the surface of a wafer is an important step in the manufacture of today's semiconductor devices. One of the most important removal steps is the patterning of metal layers which serve as connections between layers on a microchip, or other semiconductor device. The wafer is first patterned by photolithography, and then the etching process removes exposed metal areas, transferring the pattern to the metal layer.
There are many different metals which are used in today's semiconductor devices. The most common layers removed via a metal etch step are aluminum, tungsten, titanium, and various alloys of these metals. Although the exact layer being removed may differ, the process recipes and flow rates are very similar for the different types of layers.
Most metal etch tools operate using reactive ion etching, RIE. In this process, the etch gases are introduced into an electrical field. This field ionizes the gases, creating a plasma full of energetic ions. This plasma etches the wafer through several mechanisms. The first involves actual physical etching. This occurs when the highly excited ions smash into the wafer, breaking off a part of the surface. The second mechanism utilizes the high reactivity of the ionic species present in the plasma. These species diffuse to the wafer surface where they react with the exposed layer to form a volatile by-product that is swept away in the effluent gas stream.
The actual chemistry of the plasma involves many complex reactions. The exact reactions are difficult to quantify, and many of the by-products may only last a few fleeting moments. However, EcoSys expects that a majority of the effluent will be mineral acids like HCl and HF, halogen gases, primarily Cl2 and possibly F2, and etch by-products, like AlCl3, or WF6. The amount of fluorine compounds created will depend on the efficiency of the plasma. There could also be some CO2 formed in the plasma from the decomposition of the fluorinated organics. Simplified chemical reactions for etching various metals are shown below:
Aluminum: Al(s) + 3 Cl- à AlCl3 (g)
Tungsten: W(s) + 6 F- à WF6 (g) OR W(s) + 6 Cl- à WCl6 (g)
Titanium: Ti(s) + 4 Cl- à TiCl4 (g)
The halogen ions are created by the plasma field. Different ratios of process gases are used to affect the exact etch rate and selectivity
TYPICAL RECIPE
Although there are many different layers which can be etched using a metal etch process, the basic recipe features Cl2 and BCl3. SF6 is often added when etching tungsten layers. Some newer applications feature recipes which include even more fluorinated hydrocarbons. However, these recipes are still in the minority to the gases shown below.
ABATEMENT CONSIDERATIONS
As with most semiconductor processes, there are many abatement considerations for a metal etch process. The two primary concerns are worker safety and facilities protection. To illustrate the dangerous nature of metal etch 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 which is an Immediate Danger to Life and Health. This is the concentration which represents a danger after 15 minutes of exposure. This table includes all the gases which might be present in this process, including process by-products.
It is easy to see the dangers that metal etch gases present to workers' health. Point of use scrubbing protects workers by removing dangerous acid gases at the earliest possible point. This greatly reduces the dangers associated with a leak in the exhaust system.
The second abatement consideration is facilities protection. Metal etch is a notoriously dirty process that demands point of use abatement. The etch by-product aluminum chloride has a low vapor pressure, and it will condense out in exhaust lines. Boron trichloride will react with water vapor or oxygen to form boric oxide and boric acid. Both compounds are sticky white powders that accumulate quickly, and will clog pump exhaust lines, or house duct work. Point of use abatement is used to control these substances, and prevent their formation in downstream duct work. Corrosion of duct work is also greatly reduced by removal of acid gases at an early point of the exhaust system.
BAZM SOLUTION: Vector
The Vector series of water scrubbers are a proven abatement device for all metal etch applications. The Vector water scrubber has evolved over many years, and now showcases many features which make it an excellent choice for the abatement of metal etch processes.
In metal etch applications, the high efficiency of the Vector can be coupled with low water usage. For a typical metal etch process, the water consumption rate can be set as low as 0.5 gallons per minute. The new Vector Ultra can be equipped with up to four independent entries. The entry lay out is specially designed to prevent clogging, by introducing a co-annular flow of nitrogen with the process gas. This flow is designed to shield the process gas from back-streaming water vapors. This keeps solid forming reactions within the main body of the scrubber, and out of the entry pipe. Furthermore, the inner walls of the lower entry area are continually washed with water, which sweeps away any solids which might try to form in that area. In a typical metal etch application, the entry should require simple, routine cleaning about once every month.
After leaving the entry, the process gas contacts the water in the main scrubber barrel. The water is introduced at the top of the packed column through a rotating spray bar, which helps provide a uniform flow of water through the packed bed of the column. The main barrel is a fused column construction. This eliminates corners where gas can escape scrubbing, and the fused construction eliminates glued joints which can leak. The packing material used inside the column provides a high surface area, allowing for excellent mass transfer from the gas phase to the liquid phase. This helps the Vector scrubbers to achieve such high efficiency.
After absorbing process gases, the liquid exits the packing and flows into a sump at the bottom of the unit. A pump at the bottom of the unit removes water from the sump. Most of this flow from the sump is circulated through the system via the spray bar. The remainder of the flow is pumped to the drain. The pressurized drain system is an integral design of the Vector unit and not an externally attached option. This enables the unit to be placed in the sub-fab below the waste drain level. Level switches in the sump of the scrubber regulate the sump level. Fresh water is metered in to replenish the water in the sump of the scrubber.
BAZM recommends installing the Vector as close to the pump as possible and heating the line from the pump to the Vector inlet to 150 degrees centigrade. The heating should be performed using Hot N2, or a heat blanket, which should extend all the way to the Vector entry. Heating the pump exhaust line keeps condensable solids, like AlCl3, in the vapor phase, preventing it from condensing into a yellowish-brown solid. Proper installation of the heat blanket is essential to the proper operation of the non-clogging entry. If not, BCl3 will react with water vapor from the air to form boric acid via the following reaction:
BCl3 + 3 H2O ® H3BO3 (s) + 3 HCl(g)
Boric acid is a white powder that will clog lines very quickly. Properly heated pump exhaust lines, and leak tight piping, prevent a significant amount of solid material from depositing out before the Vector scrubber. The Vector scrubber itself removes a high percentage of these gases from the vapor stream, thereby reducing clogging and corrosion downstream of the scrubber. This greatly reduces downtime and allows for a great saving on maintenance costs. The Vector water scrubber also offers protection to workers by reducing the concentration of acid gases in the exhaust stream.
The Vector water scrubber allows multiple process streams to be abated by one point of use scrubber. To achieve the best maintenance cycles, BAZM recommends that each process chamber be plumbed to its own inlet. Most of the metal etch gases are easy to scrub in a water scrubber. However, when multiple chambers are combined to a single Vector, the flow rate of chlorine may become too high for the Vector to provide acceptable scrubbing efficiency. The Vector Ultra has two new features which can greatly improve chlorine scrubbing efficiency.
Special Feature: Polishing Scrubber
The first feature is a standard part of the Vector Ultra. It is a polishing scrubber bed which fits directly into the exhaust pipe of the unit. With this feature, some of the incoming fresh water for the Vector is introduced at the top of the polishing scrubber. From there, it flows countercurrently to the exiting gas, providing additional scrubbing capacity. This polishing scrubber can provide excellent increases in chlorine scrubbing efficiency.
ABATEMENT CHEMISTRY
The Vector water scrubber will abate most of the gases involved in metal etch processing. The following reactions summarize the chemistry taking place in a Vector scrubber.
Vector
HCl (g) ® HCl (aq)
HF (g) ® HF (aq)
Cl2 (g) + H2O ® HCl (aq)
F2 (g) + H2O à HF(aq)
BCl3 (g) + 3 H2O ® H3BO3 (aq) + 3 HCl (aq)
2 AlCl3 (g) + 3 H2O ® Al2O3 (s) + 6 HCl (aq)
With chemical injection:
HCl (aq) + NaOH(aq) à NaCl(aq) + H2O
HF (aq) + NaOH(aq) à NaF(aq) + H2O
Any CF4, CHF3, or SF6 which survives the plasma will not be abated by a Vector scrubber. Any CO2 which might form in the plasma will pass through the water scrubber as well.
EFFICIENCY
The Vector is a high efficiency scrubber. EcoSys applications laboratory testing has shown the following efficiencies for metal etch gases:
For any addition questions, please contact BAZM Solution - your gas abatement expert.
To accurately understand the abatement issues for any given process, it is necessary to understand the process in the tool, the chemicals that are put in, and how the process modifies them. The following overview represents our understanding of this process and is the basis of this recommendation.
The removal of thin films from the surface of a wafer is an important step in the manufacture of today's semiconductor devices. One of the most important removal steps is the patterning of metal layers which serve as connections between layers on a microchip, or other semiconductor device. The wafer is first patterned by photolithography, and then the etching process removes exposed metal areas, transferring the pattern to the metal layer.
There are many different metals which are used in today's semiconductor devices. The most common layers removed via a metal etch step are aluminum, tungsten, titanium, and various alloys of these metals. Although the exact layer being removed may differ, the process recipes and flow rates are very similar for the different types of layers.
Most metal etch tools operate using reactive ion etching, RIE. In this process, the etch gases are introduced into an electrical field. This field ionizes the gases, creating a plasma full of energetic ions. This plasma etches the wafer through several mechanisms. The first involves actual physical etching. This occurs when the highly excited ions smash into the wafer, breaking off a part of the surface. The second mechanism utilizes the high reactivity of the ionic species present in the plasma. These species diffuse to the wafer surface where they react with the exposed layer to form a volatile by-product that is swept away in the effluent gas stream.
The actual chemistry of the plasma involves many complex reactions. The exact reactions are difficult to quantify, and many of the by-products may only last a few fleeting moments. However, EcoSys expects that a majority of the effluent will be mineral acids like HCl and HF, halogen gases, primarily Cl2 and possibly F2, and etch by-products, like AlCl3, or WF6. The amount of fluorine compounds created will depend on the efficiency of the plasma. There could also be some CO2 formed in the plasma from the decomposition of the fluorinated organics. Simplified chemical reactions for etching various metals are shown below:
Aluminum: Al(s) + 3 Cl- à AlCl3 (g)
Tungsten: W(s) + 6 F- à WF6 (g) OR W(s) + 6 Cl- à WCl6 (g)
Titanium: Ti(s) + 4 Cl- à TiCl4 (g)
The halogen ions are created by the plasma field. Different ratios of process gases are used to affect the exact etch rate and selectivity
TYPICAL RECIPE
Although there are many different layers which can be etched using a metal etch process, the basic recipe features Cl2 and BCl3. SF6 is often added when etching tungsten layers. Some newer applications feature recipes which include even more fluorinated hydrocarbons. However, these recipes are still in the minority to the gases shown below.
ABATEMENT CONSIDERATIONS
As with most semiconductor processes, there are many abatement considerations for a metal etch process. The two primary concerns are worker safety and facilities protection. To illustrate the dangerous nature of metal etch 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 which is an Immediate Danger to Life and Health. This is the concentration which represents a danger after 15 minutes of exposure. This table includes all the gases which might be present in this process, including process by-products.
It is easy to see the dangers that metal etch gases present to workers' health. Point of use scrubbing protects workers by removing dangerous acid gases at the earliest possible point. This greatly reduces the dangers associated with a leak in the exhaust system.
The second abatement consideration is facilities protection. Metal etch is a notoriously dirty process that demands point of use abatement. The etch by-product aluminum chloride has a low vapor pressure, and it will condense out in exhaust lines. Boron trichloride will react with water vapor or oxygen to form boric oxide and boric acid. Both compounds are sticky white powders that accumulate quickly, and will clog pump exhaust lines, or house duct work. Point of use abatement is used to control these substances, and prevent their formation in downstream duct work. Corrosion of duct work is also greatly reduced by removal of acid gases at an early point of the exhaust system.
BAZM SOLUTION: Vector
The Vector series of water scrubbers are a proven abatement device for all metal etch applications. The Vector water scrubber has evolved over many years, and now showcases many features which make it an excellent choice for the abatement of metal etch processes.
In metal etch applications, the high efficiency of the Vector can be coupled with low water usage. For a typical metal etch process, the water consumption rate can be set as low as 0.5 gallons per minute. The new Vector Ultra can be equipped with up to four independent entries. The entry lay out is specially designed to prevent clogging, by introducing a co-annular flow of nitrogen with the process gas. This flow is designed to shield the process gas from back-streaming water vapors. This keeps solid forming reactions within the main body of the scrubber, and out of the entry pipe. Furthermore, the inner walls of the lower entry area are continually washed with water, which sweeps away any solids which might try to form in that area. In a typical metal etch application, the entry should require simple, routine cleaning about once every month.
After leaving the entry, the process gas contacts the water in the main scrubber barrel. The water is introduced at the top of the packed column through a rotating spray bar, which helps provide a uniform flow of water through the packed bed of the column. The main barrel is a fused column construction. This eliminates corners where gas can escape scrubbing, and the fused construction eliminates glued joints which can leak. The packing material used inside the column provides a high surface area, allowing for excellent mass transfer from the gas phase to the liquid phase. This helps the Vector scrubbers to achieve such high efficiency.
After absorbing process gases, the liquid exits the packing and flows into a sump at the bottom of the unit. A pump at the bottom of the unit removes water from the sump. Most of this flow from the sump is circulated through the system via the spray bar. The remainder of the flow is pumped to the drain. The pressurized drain system is an integral design of the Vector unit and not an externally attached option. This enables the unit to be placed in the sub-fab below the waste drain level. Level switches in the sump of the scrubber regulate the sump level. Fresh water is metered in to replenish the water in the sump of the scrubber.
BAZM recommends installing the Vector as close to the pump as possible and heating the line from the pump to the Vector inlet to 150 degrees centigrade. The heating should be performed using Hot N2, or a heat blanket, which should extend all the way to the Vector entry. Heating the pump exhaust line keeps condensable solids, like AlCl3, in the vapor phase, preventing it from condensing into a yellowish-brown solid. Proper installation of the heat blanket is essential to the proper operation of the non-clogging entry. If not, BCl3 will react with water vapor from the air to form boric acid via the following reaction:
BCl3 + 3 H2O ® H3BO3 (s) + 3 HCl(g)
Boric acid is a white powder that will clog lines very quickly. Properly heated pump exhaust lines, and leak tight piping, prevent a significant amount of solid material from depositing out before the Vector scrubber. The Vector scrubber itself removes a high percentage of these gases from the vapor stream, thereby reducing clogging and corrosion downstream of the scrubber. This greatly reduces downtime and allows for a great saving on maintenance costs. The Vector water scrubber also offers protection to workers by reducing the concentration of acid gases in the exhaust stream.
The Vector water scrubber allows multiple process streams to be abated by one point of use scrubber. To achieve the best maintenance cycles, BAZM recommends that each process chamber be plumbed to its own inlet. Most of the metal etch gases are easy to scrub in a water scrubber. However, when multiple chambers are combined to a single Vector, the flow rate of chlorine may become too high for the Vector to provide acceptable scrubbing efficiency. The Vector Ultra has two new features which can greatly improve chlorine scrubbing efficiency.
Special Feature: Polishing Scrubber
The first feature is a standard part of the Vector Ultra. It is a polishing scrubber bed which fits directly into the exhaust pipe of the unit. With this feature, some of the incoming fresh water for the Vector is introduced at the top of the polishing scrubber. From there, it flows countercurrently to the exiting gas, providing additional scrubbing capacity. This polishing scrubber can provide excellent increases in chlorine scrubbing efficiency.
ABATEMENT CHEMISTRY
The Vector water scrubber will abate most of the gases involved in metal etch processing. The following reactions summarize the chemistry taking place in a Vector scrubber.
Vector
HCl (g) ® HCl (aq)
HF (g) ® HF (aq)
Cl2 (g) + H2O ® HCl (aq)
F2 (g) + H2O à HF(aq)
BCl3 (g) + 3 H2O ® H3BO3 (aq) + 3 HCl (aq)
2 AlCl3 (g) + 3 H2O ® Al2O3 (s) + 6 HCl (aq)
With chemical injection:
HCl (aq) + NaOH(aq) à NaCl(aq) + H2O
HF (aq) + NaOH(aq) à NaF(aq) + H2O
Any CF4, CHF3, or SF6 which survives the plasma will not be abated by a Vector scrubber. Any CO2 which might form in the plasma will pass through the water scrubber as well.
EFFICIENCY
The Vector is a high efficiency scrubber. EcoSys applications laboratory testing has shown the following efficiencies for metal etch gases:
| Gas | DRE |
|---|---|
| Cl2 | >95% |
| HCl | >99% |
| BCl3 | 99% |
| AlCl3 | 99% |
| HF | >99% |
For any addition questions, please contact BAZM Solution - your gas abatement expert.
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