Filter Selection for Hot Phosphoric Acid and SPM: Why All-Fluoropolymer Construction Matters

Hot phosphoric acid and SPM are the two most demanding chemistries in semiconductor wet processing: temperatures are often 125–160 °C, and one is a concentrated acid while the other is a strong oxidizer. When choosing filters for these two chemistries, many people's first thought is “just use PTFE” — that is the right direction, but it is only half right.

What do these two chemistries do in the process?

What the two have in common is “high temperature + high corrosivity,” and this combination tests every material in the filter at the same time.

Why is the membrane alone not enough?

The wetted components of a cartridge filter include at least the membrane, the support and drainage layers, the cage, the core, the end caps, and the O-rings; once the filter is installed in a housing, the housing body itself has to be added as well. A hot, strongly oxidizing chemistry will find the weakest one among them.

The membrane ratings are taken from the SPM (125–160 °C) row of the Process Chemical Compatibility Chart on JIUNYUAN's “Semiconductor Process Filtration” category page (◎ long-term compatible / ○ generally compatible / △ conditional, soak test first / ✕ incompatible). That chart lists phosphoric acid only at 60 °C or below; for hot phosphoric acid, read the “Strong / oxidizing acid” and “> 100 °C” columns of the Membrane / Chemical Selection Chart on the “Products” page. Hardware temperature limits come from the individual product specifications.

This is why “all-fluoropolymer filters” exist: the membrane is PTFE, and the support layers, cage, core, and end caps are PFA, from the same fluoropolymer family, so the flow path has no dissimilar-material interface, and you do not end up with “the membrane is still fine, but the hardware has already failed.” For a comparison of O-ring and seal materials, see “Filter O-ring Material Selection: FFKM, EPDM, Viton Compared”; for housing materials, see “Why All Teflon Filter Housings Are Essential in Strong Acid and Alkali Environments: A Materials Science Analysis.”

Temperature and differential pressure: use the ratings at operating temperature

Even with an all-fluoropolymer construction, the ratings change with temperature. Take the specifications of JIUNYUAN's two all-fluoropolymer filters as an example:

Fluoropolymers soften at high temperature and become more prone to creep, so the higher the temperature, the lower the differential pressure they can withstand. This has three practical consequences:

1. Match the filter type to the operating temperature. For a loop operating at 150–160 °C, a cartridge filter rated to 100 °C is out of range; choose a type whose temperature rating covers the operating temperature — among JIUNYUAN products, that is the disposable capsule rated to 180 °C.
2. Set the change-out differential pressure from the high-temperature rating. If you keep using the 25 °C value of 0.58 MPa as the baseline, that is already nearly 6 times the 0.1 MPa allowed at 180 °C.
3. When you need a lower differential pressure, increase the membrane area or run several filters in parallel. Ø68 mm filters have 1.2 m² per 10 inches; Ø83 mm filters are available with 1.8, 2.2, or 2.8 m².

Cold start: viscosity raises differential pressure

Concentrated phosphoric acid and concentrated sulfuric acid are both more viscous than water, and their viscosity changes substantially with temperature. Filter flow curves are usually measured with ultrapure water (UPW) at 25 °C; for the actual chemistry, the differential pressure has to be multiplied by that chemistry's viscosity ratio at the operating temperature:

In other words, at the same flow rate, the differential pressure right after a cold start, before the chemistry has heated up, will be noticeably higher than at hot steady state. During heat-up, we recommend circulating at a low flow rate first and ramping up to the set flow rate only after the temperature has stabilized; differential pressure alarm values should also take temperature into account, rather than using the same number throughout. For the full flow rate and differential pressure calculation, see “Liquid Filter Flow Rate Sizing: Meeting Process Demand.”

A problem specific to SPM: bubbles and dewetting

In hot acid, H₂O₂ continuously decomposes and releases oxygen. Once bubbles enter the housing, they gather on the upstream side of the filter and even get into the pores of the hydrophobic PTFE membrane, pushing the liquid out — this is called dewetting. Membrane area occupied by gas no longer passes liquid. Outwardly, this shows up as rising differential pressure and falling flow, which is easily misjudged as a clogged filter and leads to premature replacement.

A problem specific to hot phosphoric acid: silica precipitation

When silicon nitride is etched, silicon dissolves into the phosphoric acid. Keeping dissolved silicon within an appropriate range helps raise the etch selectivity over silicon oxide; but if it builds up too far, it precipitates as silica particles or colloids and even redeposits onto the wafer surface. For the filter, this means the load gradually increases the longer the bath is in use:

• Plan the filter change-out interval together with bath management, rather than going only by a fixed number of days.
• Watch the differential pressure trend, not just single readings. The same differential pressure means different things right after a bath change and near the end of the bath's use.
• Balance capture against loading when choosing the rating. The finer the rating, the more it captures, and the faster the load builds up. For how to choose a rating, see “Liquid Filter Rating Selection: Matching Process Needs.”

High temperature speeds up the release of extractables

As temperature rises, the rate at which filter materials release metals and organics usually increases as well. So for hot chemical loops, beyond material compatibility, you also need to look at the metal cleanliness grade: JIUNYUAN's all-fluoropolymer disposable capsule filters are available at < 0.5 / < 3 / < 10 µg/device, and its all-fluoropolymer cartridge filters at < 3 / < 10 / < 25 µg/device. The all-fluoropolymer construction of the disposable capsule withstands 180 °C and can tolerate flushing with hot chemistry and hot UPW. For how metal extractables are measured, see “Filter Metal Extractables: How ICP-MS Evaluates Filter Cleanliness.”

Selection checklist

1. Operating temperature: What is the maximum steady-state temperature? How do temperature and flow rate change during start-up and heat-up?
2. Chemical composition: concentration, mixing ratio, and whether it contains gas-generating components such as H₂O₂.
3. All wetted materials: check the membrane, support layer, cage, core, end caps, bonding method, O-rings, and housing against the compatibility chart one by one.
4. Temperature rating of the type: whether the operating temperature is within that type's maximum temperature.
5. Max. differential pressure at operating temperature: base the change-out differential pressure and alarm values on this.
6. Viscosity conversion: convert to the actual differential pressure using the chemistry's viscosity ratio at operating temperature, and check the differential pressure at cold start.
7. Pre-wet method: for SPM loops, prefer filters shipped UPW pre-wet to avoid IPA residue.
8. Venting design: housing vent ports; core vent configuration for disposable capsules.
9. Cleanliness grade: choose the metal cleanliness grade according to the filter's position in the process, and plan the flush before start-up.
10. Qualification before deployment: run a soak test at the actual chemical concentration, temperature, and run time, and re-check flow and integrity.

FAQ

Sources

• JIUNYUAN, “Semiconductor Process Filtration”: Process Chemical Compatibility Chart
• JIUNYUAN, “Products”: Membrane / Chemical Selection Chart
• JIUNYUAN, “Semi-grade All-PTFE Pleated Filter,” “Semi-grade All-PTFE Disposable Capsule Filter,” and “All-PTFE High Chemical Compatibility Filter” product specifications
• JIUNYUAN, “PFA Filter Housing with Fool-proof Lock”