For cleaning NH₄Cl, fully immerse the trap in a circulating water bath for about one hour. When the trap is clean, dry it in an oven. This removes all the water, which could contaminate the system when the trap is reinstalled.

Water leaks, during trap removal for cleaning, are essentially eliminated with the self-sealing, quick release coupling option. These couplings for the cooling water inlet and outlet ports ease installation and removal by eliminating the need to shut off the water source.

Vapor Sublimation Trap accomplishes collection of solidifiable vapor in two stages. The first stage, at the entrance of the trap, has a high capacity to trap more than 95 percent of all condensable gases. The figure is a cross section of the Vapor Sublimation Trap. It shows the high volume reserved for first stage trapping. This avoids rapid clogging frequently encountered with other traps. In just half the volume, the trap has the capacity to capture more than five times the vapor than the nearest competitor.

The second stage is a polishing scrubber designed for greater efficiency. This stage maximizes heat transfer between the gas and the cooling surface, while maintaining high flow conductance. The combination of cooling water coils and perforated stainless steel cones solidifies the residual vapors that have passed through the first stage.

Semiconductor CVD processes produce gaseous by-products that can readily be pumped out of the reaction chamber. However, they usually solidify in a vacuum pipe line since the line temperature is lower than the reaction chamber. A clogged line means longer down time and lower product yield, especially with the trend of larger wafers and smaller feature size.

One of the most common processes is LPCVD silicon nitride. Since sublimation is temperature driven, use heat to maintain the by-products in the vapor phase and use cooling to intentionally sublimate the vapors in the trap.

LPCVD Nitride

LPCVD silicon nitride process deposits solid silicon nitride on the wafer and creates an ammonium chloride (NH4 Cl) by-product. Ammonium chloride will solidify in the vacuum pump line if the line is not heated. To keep NH4 Cl in its vapor stage, the system temperature must be from 130°C to 150°C at 150 mTorr. This allows the system to transport it away from the furnace. Higher pressures will require a higher temperature (see the vapor pressure curve in the figure).

In a typical silicon nitride CVD process the NH3 /DCS ratio is around 3 to 1. A higher NH3 /DCS ratio leads to longer cumulative deposition time. There are several reasons for installing heated lines and a Vapor Sublimation Trap in a silicon nitride CVD process.

1. Collecting NH4 Cl in the trap for ease of maintenance;
2. Large trapping capacity leads to longer preventative maintenance cycles for increased uptime;
3. High trapping efficiency provides better protection of the pump, control valve and downstream instrumentation;
4. Lower NH4 Cl vapor pressure in the trap (due to lower temperature) results in higher process yield.

Other Semiconductor Processes

The Vapor Sublimation Trap can be used in other semiconductor processes where by-product vapors can be physically solidified and cleaned by dissolving them in a solution (like water). For example, ammonium hexaflurosilicate (NH₄)₂SiF₆ has been observed in a silicon nitride PECVD process due to the cross chemical reaction between the by-products formed in the deposition process and the etching process.