Diborane gas regulator best practices

Thursday, January 28th, 2021

Of all the gases used in semiconductor manufacturing, Diborane (formula B2H6), is the most challenging for cylinder gas regulator reliability.   Diborane is normally supplied as a mixture with a balance of N2, He or H2.   Pure Diborane and its mixtures are not stable at room temperature.  At room temperature, Diborane decomposes slowly and at elevated temperatures it decomposes rapidly.

Figure 1 AP1710 Regulator: Diborane decomposition byproducts attached on high pressure and low pressure side of first stage poppet, causing seat leak

This poses a problem for pressure regulators.  When Diborane decomposes it creates heavier boron hydrides and hydrogen.  The heavier boron hydrides are volatile liquids and solids such as Decaborane  (formula B10H14). At room temperature and cylinder pressures, Decaborane can sublimate into vapor.   When Decaborane flows into the regulator, due to pressure and temperature drop, it can deposit on the poppet and seat causing a seat leak.

In addition to natural decomposition of Diborane, it is extremely reactive to oxygen and moisture.   The reaction of Diborane with oxygen will create Boron Trioxide (formula B2O3) and water. Diborane will react with moisture to create Boric acid (formula H3BO3) and Hydrogen. Boron Trioxide is a white amorphous solid that can also deposit on the regulator’s poppet and seat.

What can be done to maximize gas regulator performance and reliability?

From a gas management point of view, minimizing decomposition can be done by maintaining low storage temperatures throughout the supply chain of gas delivery, on-site storage, and cylinder cabinet use.   It has also been reported to us that mixtures with H2 rather than N2 improve stability of Diborane.

Another best practice is to change the cylinder at a minimum pressure such as 200 psi to ensure any possible B2H6 byproducts in the cylinder are not introduced into the gas system.

Figure 2 AP 1700 two-stage tied diaphragm regulator

From a design point of view, AP Tech recommends dual stage regulation with tied diaphragm regulators.  This can be done with two regulators in series or a single regulator such as the two-stage 1700 or 2700 models.   Dual stage regulation has the benefit of maintaining a steady supply pressure due to minimization of supply pressure effect and reduces the amount of temperature drop because pressure drop is spread across two regulators.  Tied diaphragm regulators feature a mechanical linkage between the diaphragm and the poppet. When there is an increase in outlet pressure, the tied diaphragm will pull the poppet up.  This proportionally increases the sealing pressure with outlet pressure increase and helps minimize seat leaks.  However, if an operator notices the outlet pressure is above set-point, care should be taken to not back off the regulator too much (reduce outlet pressure) during static flow conditions as this may damage the tied diaphragm regulator by pulling the poppet into the seat.

When specifying a dual stage regulator, it is important to monitor the outlet pressure of the first stage to monitor signs of a seat leak.   The MP option is a monitoring port available with the 1700 or 2700 models.  A continuous rise above set point during static flow periods indicates a seat leak.  Other design options include filtration before the regulator and purge glass bleed through the gas manifold’s cylinder connection. This bleed of purge gas through the cylinder connection minimizes air and moisture into the gas system during cylinder changes.  AP Tech offers the “CB” or constant bleed option for many of its valves for this purpose.

What should you do if you detect a continuous rise in outlet pressure during static flow conditions?  One option is to perform a regulator cycle purge by cycling vacuum on the outlet of the regulator and pressure through the inlet of the regulator.  This may improve the seat sealing performance, but vacuum purging the regulator should be done sparingly, more than several hundred cycles over the life of the regulator may affect performance.

If the installed regulator must be removed and exchanged with a new regulator, again it is very important to minimize air and moisture entering the exposed regulator inlets and outlets. As with the cylinder connection, positive purge gas pressure exiting the inlet and outlet will minimize air and moisture entering the gas system.  If too much air or moisture enters the system, no amount of purging can completely remove the moisture.  We are aware of situations where Diborane gas systems have chronic and repeated regulator failures due to the improper removal and replacement of the failed regulator.

It is also important to remember that the byproducts of B2H6 are toxic.  Generally, there are restrictions on shipping toxic materials if a failure analysis is required.   Based on our many years of experience with Diborane gas regulation, a seat leak is almost always caused by particles such as Decaborane and Boron Trioxide making the effort and risk of shipping a Diborane regulator suspected of seat leak for failure analysis unnecessary.

Want to learn more about our Diborane gas handling solutions?  Talk to one our experts, contact AP Tech here.