Multi-Stage Traps Clean Up Vacuum Systems
Throughput Increases in Silicon Dioxide Deposition Systems
Another common application for a multiple-stage trap is a CVD (chemical vapor deposition) process to deposit silicon dioxide onto silicon wafers. Silicon dioxide dust wears at a vacuum system, reducing pumping efficiency. Liquid silicon dioxide sources, such as tetraethylorthosilicate (TEOS), can recondense in the vacuum lines,6 causing valves to stick and product runs to be lost. With most silicon dioxide sources, multi-stage trapping reduces these problems, saving the user time and money.
SEMATECH, a semiconductor research consortium, installed a four-stage trap7 in a single-wafer TEOS reactor. The trap reduced the preventive maintenance frequency from once per shift to once every three months. SEMATECH estimates that the trap added 250 hours of operation per year to the system.
At another company, a batch reactor running TEOS averaged a pump failure every 6 months. To minimize these failures, the user performed preventive maintenance every 47 runs. To reduce the occurrence of pump failures, the user installed a three-stage trap, consisting of a knock-down stage, a stainless steel gauze filter and a 20-µm polypropylene filter. After installation of the trap, the user reported no pump failures for over one year and had increased the time between preventive maintenance sessions to every 88 runs.
In these examples, the cost savings include reduced line yield losses due to pump failure, reduced costs for pump rebuilding and a slight capacity increase. Table 1 summarizes the savings - in each case, the multi-stage trap paid for itself within months.
| Manufacturer A Single-wafer TEOS |
Manufacturer B Batch TEOS |
Manufacturer C Batch Silane-based Doped Silica |
|
| Pump Type | Dry Pump | Dry Pump | Dry Pump |
| Trapping Stages Installed | 4 stage: Knock-down, cooling, stainless steel gauze filter, 20µm filter | 3 stage: Knock-down, stainless steel gauze filter, 20µm filter | 3 stage: Knock-down, stainless steel gauze filter, 5µm filter |
| # of Pump Failures Before | N/A | 2 | N/A |
| Savings in Line Yield (wafer cost only) | $40,000 / year | ||
| Savings in Pump Rebuilds | $12,000/year | ||
| Time Base | 1 year | 4 hours/run | 24 hours/day |
| Maintenance Frequency Before | Every shift | 47 runs | 2 days |
| Maintenance Frequency After | 3 months | 88 runs | 10 days |
| Throughput (Capacity) Increase | 2.9% | 1.0% | 7.3% |
| Typical Cost of CVD Equipment | $2,200,0008 | $200,000 | $200,000 |
| Value of Capacity Increase | $62,785 | $2,000 | $14,600 |
Table 1. Significant cost savings in CVD systems from installation of a multi-stage trap to remove silicon dioxide.
Source: SEMATECH, Mass-Vac Inc. Data from Mass-Vac Inc. obtained from nonproprietary questionnaires answered before and after installation of the trap.
A silane-based doped silica deposition process in a batch reactor provides similar results. One user had frequent pump failures and exhaust line clogging, despite the presence of a single stage trap in the vacuum line. This trap used only a knock-down stage and required cleaning every 6,000 nm of deposition, or every two days. Once installed, the multi-stage trap requires cleaning every 35,000 nm, which occurs only every ten days. Inspection during cleaning shows heavy silicon dioxide powder accumulates in the knock-down stage, followed by a light powder coating of the filter elements. No accumulation appears in the vacuum line outside the trap, giving a good indication that clogged lines have been eliminated as a source of vacuum line failure. This multi-stage trap saved the user $14,600 - more than four times the cost of the trap (see Table 1).
Effluent Trapping Benefits Plasma Processes
Silicon nitride and silicon dioxide films may also be deposited using plasma-enhanced chemical vapor deposition (PECVD) processes. Plasma processes produce the same type of effluent as LPCVD systems, and plasma vacuum systems fail for the same reasons. In one process running silicon nitride, pump failures occurred every two to three months. In this case, a three-stage trap cleaned up the vacuum system. This trap consists of the knock-down stage, then a stainless steel gauze filtration stage followed by a 5-µm polypropylene filter stage. This trap allowed the user to reduce maintenance frequency from every 25,000 nm to every 50,000 nm of deposition, allowing up to 30 days between preventive maintenance.
Another user did not perform routine preventive maintenance on the vacuum system of a PECVD system, resulting in pump failures every two months. To eliminate these failures, the user installed a four-stage trap consisting of knock-down, water cooling, a stainless steel gauze filter and a 5-µm polypropylene filter. The trap requires cleaning after 60 days of service. The result is a scheduled down period of about four hours every two months, in place of a catastrophic failure at about the same frequency. Table 2 shows the minimum payback for these two plasma processes.
| Manufacturer C PECVD Silicon Nitride | Manufacturer D PECVD Silicon Nitride | |
| Pump Type | Oil-Based | Oil-Based |
| Trapping Stages Installed | 3 stage: Knock-down, stainless steel gauze, 5µm polypropylene filter | 4 stage: Knock-down, cooling, stainless steel gauze, 5µm polypropylene filter |
| Months Between Pump Failures Before | 2.5 | 2 |
| Savings in Line Yield (wafer cost only) | $72,000 / year | $12,000 / year |
| Savings in Pump Rebuilds9 | $9,600/year | $12,000/year |
| Time Base | 24 hours/day | 24 hours/day |
| Maintenance Frequency Before | 15 days | 60 days |
| Maintenance Frequency After | 30 days | 60 days |
| Throughput Increase | 0.6% | 0.28% |
Table 2. Trapping reduces costs in PECVD systems
Source: Mass-Vac Inc.
*Typical rebuild cost for an oil-based pump is $2000.
Downstream Traps also Protect Pumps
In the case of a different PECVD system, a multi-stage trap installed downstream of the pump solved clogging problems in the lower stage of the dry pump. Initial installation consisted of a knock-down stage and a stainless steel gauze filtration stage, followed by a 5-µm polypropylene filter stage. This setup required cleaning every 6-8 days. To reduce the cleaning frequency, the user replaced the 5-µm polypropylene filters with 20-µm filters. While there is too little data to calculate a payback for this user, they no longer find particles logging the lower stage of the pump, and they report a significant reduction in required maintenance.
NEXT: Trapping Prevents Exhaust Line Clogging in Metal Etch Processes
