Semiconductor and New Compound Semiconductor Applications Information
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Trapping and filtration notes for specific wafer manufacturing processes
LPCVD – LOW PRESSURE CHEMICAL VAPOR DEPOSITION SILICON NITRIDE
The gases used in this process are Dichlorosilane (DCS, SiCl2H2), ammonia (NH3), and occasionally HCl. The byproducts generated from the process are ammonium chloride (NH4Cl), silicon nitride (Si3N4), and HCl.
Ammonium chloride is the most difficult to deal with. It leaves the process furnace in vapor form but readily condenses to a heavy powder that will accumulate in the cooler areas of the vacuum system. Powder build up in the foreline, valve, vacuum pump, and the exhaust line is a common occurrence with this process. Frequent cleaning and/or replacing of lines, valves, and vacuum pumps is an expensive proposition.
The proper use of a trap that includes a baffle, cooling, and filtration will remove most of this powder and contain it in one central location. Most of the ammonium chloride condenses on the baffle and cooling option. The filtration is needed to remove trace amounts of silicon nitride dust and small amounts of ammonium chloride that may pass through the first two stages.
The HCl presents a problem when using an oil mechanical pump. HCl will react and polymerize the oil in a relatively short period of time. Here the use of an external oil filtration system is important to remove the acid from the oil and prolong the life of the vacuum pump.
TEOS is the liquid precursor used in this process. The byproducts are silicon dioxide (SiO2) and occasional liquid TEOS depending upon the amount (cc) used for the process. Silicon dioxide will accumulate in the foreline and valve very readily and in some cases may extend down to the vacuum pump. It is a much finer powder than ammonium chloride.
The use of a trap that has a baffle and two stages of filtration will remove most of the powder from the gas stream. The silicon dioxide powder can be as small as 2.0 microns in size so it is important to use micron size filtration in the last stage.
The gases used in this process are silane (SiH4) and hydrogen (H2). The byproduct generated from this process is silica dust that is usually found in small quantities. This is usually a relatively clean process where a trap with a baffle and two stages of filtration can provide adequate protection of the vacuum system.
Doping of this process with arsine presents an added problem of arsenic particulate that will accumulate in the foreline, valve, and vacuum pump. Removing this material in a centrally located trap not only protects the system but makes dealing with this toxic compound much easier.
LOW TEMPERATURE OXIDE (LTO)
This process uses silane (SiH4) and oxygen (O2) to form un-doped silicon dioxide (SiO2). Dopants of diborane, boron trichloride, and phosphine (B.S.G., P.S.G., B.P.S.G.) are often used in this silicate glass process. The byproducts that can be generated from these processes are silicon dioxide, boron, and phosphorous. The use of a trap that has a baffle and two stages of filtration will remove most of this material and protect the vacuum system.
This process uses large amounts of HCl, Cl2, SF6, and BCl3 to etch particular layers off the surface of a wafer. The etching of aluminum is by far the most difficult process to deal with. The main byproduct generated from this process is aluminum chloride (AL2CL6). This material, like ammonium chloride, leaves the chamber in a vapor form but later condenses into a abrasive/corrosive powder that normally accumulates in the vacuum pump and exhaust line.
Here a trap with a baffle, cooling, and filtration will remove almost all of this particulate, protecting the vacuum pump and exhaust line. It is important to note that when using an oil lubricated vacuum pump an external oil filtration system must be used to remove the acid from the oil. When a dry vacuum pump is employed, most of the material will pass through the pump in vapor form and will condense in the exhaust line or scrubber. Here the trap will be most effective installed in the exhaust line, after the vacuum pump.
PECVD – PLASMA ENHANCED CHEMICAL VAPOR DEPOSITION SILICON NITRIDE
The gases used in this plasma enhanced process are ammonia (NH3) and silane (SiH4) along with some type of fluorine based chamber cleaning gas. The cleaning gases normally used are C2F6, CF4, and NF3. The mixture of the precursor gases with the fluorine based cleaning gas generates a byproduct of ammonium hexafluorosilicate ((NH4)2 SiF6). This material tends to condense in the vacuum pump, exhaust line, and scrubber. This material condenses into a fine powder that may be as small as 1.0 micron in size. A trap with a baffle, cooling, and two stages of filtration will remove up to 95% of this material.To optimize the effectiveness of the MV MULTI-TRAP®, proper installation and filtration media are essential. These depend on the system and process parameters. Please consult our sales department for recommendations or submit our on-line process questionnaire.
New Compound Semiconductor Applications
MULTI-TRAP FOR MOCVD PROCESSES
Metal organic chemical vapor deposition (MOCVD) is commonly used to deposit III-V metals on to substrates or wafers in the manufacture of compound semiconductors and LED’s. This is performed in high temperature vacuum reactors with the use of a variety of chemical precursors including arsine, phosphine, and trimethylgallium (TMG). Typical MOCVD processes include GaAs, InP, GaN, and mixtures thereof.
The byproducts generated from these processes pose a serious threat to the vacuum pump. Arsenic particulate, pyrophoric phosphorus powder, carbon soot, and metal organics can accumulate in vacuum lines, valves, and the vacuum pump. This leads to increased system maintenance, unscheduled system down time, and premature vacuum pump failure.
The MV MULTI-TRAP offers a variety of configurations that have been very successful in trapping these byproducts. The standard configuration, with a baffle section and two stages of custom filtration, has been very effective in trapping the particulate and metal organics generated from the GaAs and GaN processes.
In documented cases, the MULTI-TRAP® removed greater than 98% of the byproducts in question, and due to its large capacity, did not require service until 600 deposition runs had been completed. This capacity is twice that of the nearest trap competitor for the MOCVD GaAs and GaN process.
Another configuration proved very successful in condensing the phosphorus generated from the MOCVD InP process. This configuration includes a water cooled baffle section, a bottom cooling option, and one parallel bank of 9.5″ stainless steel gauze filters. Customers report that they experienced similar trap efficiency to the above processes and were able to achieve 400-600 deposition runs before the trap requires service.
MULTI-TRAP FOR THE HVPE GaN PROCESS
Hydride Vapor Phase Epitaxy (HVPE) is a commonly used process for the manufacture of free standing gallium nitride crystals or for high deposition coatings on substrates and wafers in the compound semiconductor industry. This process can be performed in either high temperature vacuum reactors or under sub-atmospheric or positive pressure reactors. The process is accomplished with a variety of III-V precursors and several process steps.
Liquid gallium, group III precursor, is reacted with HCl to form gallium chloride. Gallium chloride is then reacted with ammonia, group V precursor, to form gallium nitride, as follows.
Ga (l) + HCl (g) → GaCl (g) + H2 (g) NH3 (g) + GaCl (g) → GaN + HCl (g) + H2
A large amount of ammonium chloride is generated from this reaction. The ammonium chloride condenses in the cooler lines and vacuum pump. Large amounts of this powder clog lines and destroy vacuum pumps in a very short period of time.
Due to the amount of powder generated from this process, a stacked version MULTI-TRAP is used for increased capacity. Two stacked MULTI-TRAP modules, each set up to hold one parallel bank of 9.5″ filters and cooling option, offer enough capacity for this high deposition process.
The stacked version is currently used in several HVPE GaN production facilities and the standard trap is used in many research universities.
Please contact us for more details.