Multi-Stage Traps Clean Up Vacuum Systems

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Trapping Prevents Exhaust Line Clogging in Metal Etch Processes

In addition to deposition processes, vacuum systems are heavily used for etch processes. When using a dry pump with a metal etch process, pump failures are infrequent. However, the exhaust line tends to clog, causing process failures. Once the exhaust line clogs, it needs replacement, a costly and time-consuming task.

Users of etch systems from three popular manufacturers have installed three-stage traps in the exhaust line immediately following the pump (Figure 3). The traps consist of a knock-down stage, a water cooling stage and a long stainless steel gauze filtration stage. In all cases, they have replaced the time-consuming, costly exhaust line replacement with a simple filter change. In one case, the user replaced 20 feet of exhaust line every two months. Estimating the cost for new exhaust line at $100/linear foot, this operation cost the user at least $2,000 every two months. Installation of the multi-stage trap saved over $12,000 per year.

Figure 3. With metal etch processes, a multi-stage trap installed after the pump can prevent clogged gas lines.

Table 3 shows savings achieved on metal etch processes from two different etch manufacturers. Figure 4 shows the actual effluent trapped from an etcher from a third manufacturer. The trapping in these cases is different from trapping achieved in the foreline, upstream of the pump. With the trap after the pump, much less accumulation occurs in the knock-down stage. A fine powder collects in the water cooling stage, followed by heavy powder in the filter elements. This change in accumulation as the exhaust gas flows through the trap offers additional support for multi-stage trapping.

	Manufacturer E Aluminum Etch	Manufacturer F Metal Etch
Pump Type	Dry Pump	Dry Pump
Trapping Stages Installed	3 stage: Knock-down, cooling, long stainless steel gauze elements	3 stage: Knock-down, cooling, long stainless steel gauze elements
Months Between Exhaust Line Failures Before	5	2
Savings in Exhaust Line Replacement (wafer cost only)	$4,800 / year	$12,000 / year
Time Base	24 hours/day	24 hours/dayr
Maintenance Frequency Before	150 days	60 days
Maintenance Frequency After	unknown	30 days
Throughput Increase	--	1.1%
Typical Cost of Metal Etch Equipment	$2,100,00010	$2,100,00011
Value of Capacity Increase	--	$23,729

Table 3. Cost savings on metal etch processes based on exhaust line clogging

Source: Mass-Vac Inc.
*Cost for new exhaust line is estimated at $100/foot for 20 feet of line; downtime for replacement is estimated at 20 hours.

Figure 4. When placed after the vacuum pump in a metal etch process, the cooling, and filter stages are more effective than the knock-down stage, as shown in this cooling section from a low-capacity trap installed on an etcher.

Reduced Maintenance on Ion Implanter

An ion implanter using phosphine (PH³) and arsine (AsH³) required changing the exhaust line every two weeks due to build-up. Not only was this procedure costly in terms of downtime and parts, but it also exposed the equipment technicians to potentially hazardous waste material every time the exhaust line was handled. By installing a simple two-stage trap, the user reduced his maintenance frequency from once every two weeks to once every six months. This user estimates the trap collected 95% of the phosphorus coming out of the process. After six months, the exhaust line had only a slight coating. Assuming that it took four hours every two weeks to change the exhaust line before installation of the trap, and four hours to replace the filter every six months after installation of the trap, the throughput increase equates to $23,000 (Table 1), nearly 40 times the cost of this trap.

	Manufacturer G Ion Implanter
Trapping Stages Installed	2 stage: Knock-down, and 2µm polypropylene filter
Time Base	24 hours/day
Maintenance Frequency Before	14 days
Maintenance Frequency After	180 days
Throughput Increase	1.1%
Typical Cost of Metal Etch Equipment	$2,100,00012
Value of Capacity Increase	$23,333

Table 4. Capacity increase in an ion implanter

Newer high-capacity multi-stage traps (see "The MV Multi-Trap - Flexible configuration allows complete trapping customization") provide additional benefits for companies that are concerned about handling hazardous waste. Although slightly more expensive than conventional multi-stage traps, the high-capacity traps require less frequent maintenance, thus requiring less exposure to the hazardous contents inside. The reduced maintenance costs permit safety-conscious companies to dispose of the entire trap, avoiding the need for maintenance technicians to change the filters and expose themselves to the hazardous contents.

Multiple Stages are Required to Keep a Clean Vacuum System - and It Pays

The cost savings achieved by installation of a multi-stage trap on CVD, etch and ion implant systems far exceeds the cost of the trap and its consumables. Even when replacing a single stage trap, the multi-stage trap keeps the vacuum system cleaner, reducing maintenance downtime and eliminating catastrophic pump failures. The capacity increase afforded by these improvements is small, typically 1-2%. However, with the high cost of vacuum related processes, this small increase is worth many times the cost of the trap.

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Authors:

Herbert W. Gatti holds an associate's degree in chemical engineering and a BBA in management and engineering from Northeastern University. He is Director of Research and Development for Mass-Vac Inc., a rebuilder of mechanical pumps and distributor of vacuum equipment, which he founded in 1971.

Lise Laurin began her career as an LPCVD process engineer at Intel and has held a number of positions in semiconductor processing and marketing over the last 20 years. She founded Clear Tech in 1996 to provide technical marketing and consulting services to the semiconductor supplier community. She holds a Bachelor of Science degree in Physics from Yale University, and is an active member of the Semiconductor Safety Association and the SEMI New England Committee.

For further information contact info@massvac.com

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