In lithography, as photoresist (PR) and thinner are delivered all the way to the coating tool, one of the most common membrane materials for point-of-use (POU) filtration in front of the tool is UPE. When lithography engineers select a filter, these are the three questions they ask most often:
- Why is UPE chosen for most photoresist filtration?
- Is a 2 nm rating necessarily better than 10 nm?
- Can developer and photoresist use the same filter?
This article answers them in order and includes the actual specification figures for JIUNYUAN's semi-grade UPE filter.
- What is UPE?
- Four things photoresist filtration must deal with
- Why is UPE suited to lithography?
- UPE vs Nylon 66: sieving and adsorption
- Is a finer rating better? Look at the pressure-drop cost
- Start-up and usage notes
- FAQ
What is UPE?
UPE is ultra-high molecular weight polyethylene. Like ordinary PE, it consists only of carbon and hydrogen. The difference is the length of the molecular chains: UPE's molecular weight reaches several million. The longer the chains, the tougher and more wear-resistant the material, and this is what allows it to be made into a thin yet strong microporous membrane.
A molecular structure of only carbon and hydrogen, with no polar functional groups, gives it two properties. First, it is chemically stable and has good intrinsic cleanliness. Second, it is naturally hydrophobic: water cannot wet its membrane pores on its own. These two points are exactly what define UPE's strengths and limitations in lithography.
Four things photoresist filtration must deal with
All four become harder to handle the smaller they are, and two of them (gels and microbubbles) are directly related to pressure drop. This is also why filter selection for lithography cannot be based on rating alone. For how defects carry through to yield, see “How Wrong Filter Selection Hurts Wafer Yield.”
Why is UPE suited to lithography?
1. Low extractables
JIUNYUAN's Semi-grade UPE Pleated Filter has low metal and organic extractables, with metal cleanliness selectable at < 3 or < 25 µg/device. For a chemical like photoresist, which carries impurities straight onto the wafer, the cleanliness of the filter itself is the first threshold. For how extractables are measured, see “Filter Metal Extractables: How ICP-MS Evaluates Filter Cleanliness” and “Filter Organic Extractables: NVR vs TOC Test Methods Compared.”
2. Ratings from 2 nm to 0.2 µm
A single membrane material covers 2, 5, 10, 20, 30, and 50 nm as well as 0.1 and 0.2 µm. From pre-filtration at the chemical supply end to POU filtration in front of the tool, everything can be planned within one material system.
3. Photoresist solvents can wet it directly
Although UPE is hydrophobic, the organic solvents commonly used in photoresist have low surface tension and can wet the membrane pores. Conversely, water and aqueous developers cannot spontaneously wet UPE and require pre-wet treatment (see the usage notes below).
4. Compatibility with common lithography chemicals
| Chemical / property | UPE | Nylon 66 | PP | Hydrophobic PTFE |
|---|---|---|---|---|
| IPA (20 °C) | ◎ | ◎ | ◎ | ◎ |
| PGMEA / PGME (20 °C) | ◎ | ○ | ○ | ◎ |
| TMAH 5% (20 °C) | ◎ | ○ | ◎ | ◎ |
| Hydrophilic / hydrophobic | Hydrophobic | Hydrophilic | Hydrophobic | Hydrophobic |
Compatibility ratings are taken from the process chemical compatibility chart on JIUNYUAN's “Semiconductor Process Filtration” category page: ◎ long-term compatible / ○ generally compatible, watch concentration and temperature limits / △ conditional, run a soak test first / ✕ incompatible.
JIUNYUAN UPE filter specifications
| Item | Specification |
|---|---|
| Membrane | Hydrophobic UPE microporous membrane |
| Rating | 2 / 5 / 10 / 20 / 30 / 50 nm, 0.1 / 0.2 µm |
| Support layers and hardware | HDPE support layer, drainage layer, cage, core, and end caps |
| O-ring | EPDM or TEV (modified EPDM), 222 style |
| Membrane area (10-inch) | Ø68 mm: 1.2 m²; Ø83 mm: 1.8 m² |
| Max temperature | 40 °C |
| Max forward differential pressure | 0.34 MPa (49 psi) @ 25 °C |
| Max reverse differential pressure | 0.24 MPa (34 psi) @ 25 °C |
| Metal cleanliness | Selectable: < 3 / < 25 µg/device |
| Supplied as | Dry, or UPW pre-wet |
UPE vs Nylon 66: sieving and adsorption
Another JIUNYUAN filter for lithography is the Semi-grade Nylon66 Pleated Filter. This is not a case of one replacing the other; the two differ in how they capture contaminants:
| Item | UPE | Nylon 66 |
|---|---|---|
| Hydrophilic / hydrophobic | Hydrophobic | Naturally hydrophilic |
| Capture mechanism | Mainly pore-size sieving | In addition to sieving, polar amide groups adsorb polar impurities |
| Wetting | Wetted by photoresist solvents; aqueous chemicals require pre-wet | Wets with UPW alone; no alcohol pre-wet required |
| Rating | 2 nm – 0.2 µm | 2 nm – 0.1 µm |
| Membrane area (Ø68 mm, 10-inch) | 1.2 m² | 0.7 or 1.3 m² |
| Max temperature | 40 °C | 40 °C |
| PGMEA / PGME | ◎ | ○ |
The two can divide the work by defect type, or be used together in series: use UPE at positions where the main job is sieving out hard particles and gels; consider Nylon 66 when additional adsorption of polar impurities is needed. The actual combination should be verified with wafer defect data. For a full comparison of the four main materials, see “Liquid Filter Materials Compared: PES, PTFE, PP, UPE.”
Is a finer rating better? Look at the pressure-drop cost
The table below lists the clean initial pressure drop of JIUNYUAN UPE filters at each rating (UPW at 25 °C, single Ø68 mm 10-inch cartridge, 1.2 m² membrane area), using 0.5 GPM (about 1.9 LPM) as the example:
| Rating | Pressure-drop slope (kPa/GPM) | Pressure drop at 0.5 GPM | Relative to 50 nm |
|---|---|---|---|
| 2 nm | 30.29 | ≈ 15.1 kPa | ≈ 9.0× |
| 5 nm | 23.76 | ≈ 11.9 kPa | ≈ 7.1× |
| 10 nm | 14.09 | ≈ 7.0 kPa | ≈ 4.2× |
| 20 nm | 7.928 | ≈ 4.0 kPa | ≈ 2.4× |
| 30 nm | 6.054 | ≈ 3.0 kPa | ≈ 1.8× |
| 50 nm | 3.362 | ≈ 1.7 kPa | 1× |
Slopes are taken from the flow curves of JIUNYUAN UPE filters (UPW, clean initial pressure drop); the pressure drops in the table are reference values calculated from the slopes.
At the same flow rate, the pressure drop at 2 nm is about 9 times that at 50 nm. This creates two practical problems:
- Photoresist and solvents are more viscous than water. The actual pressure drop must be further multiplied by that liquid's viscosity ratio. For example, for a liquid 3 times as viscous as water, the initial pressure drop of a 2 nm filter at 0.5 GPM is about 45 kPa.
- The higher the pressure drop, the more easily gels are squeezed through the membrane pores, and the more easily dissolved gas comes out of solution as microbubbles. Finer ratings therefore have an even greater need for low flow, larger membrane area, or multiple cartridges in parallel to bring the pressure drop down.
Start-up and usage notes
- Choose the right supply format: UPE filters can be supplied dry or UPW pre-wet.
- Displace stepwise for photoresist and solvents: in photoresist and solvent service, displace the wetting fluid stepwise with the process solvent so that no interface residue is left to cause defects. Water has limited miscibility with some photoresist solvents, and switching directly may leave a two-phase interface inside the membrane pores.
- Complete wetting first for aqueous chemicals: developer and other aqueous chemicals cannot spontaneously wet hydrophobic UPE, so choose a UPW pre-wet model or complete wetting first; naturally hydrophilic Nylon 66 is also an option to consider.
- Stay at or below 40 °C: this is the maximum temperature of JIUNYUAN UPE filters.
- Venting: at start-up, vent all air from the housing and the filter so that bubbles are not carried downstream.
- Monitor pressure drop: the maximum forward differential pressure is 0.34 MPa and the maximum reverse differential pressure is 0.24 MPa (25 °C); in routine operation, judge loading from the trend of the pressure drop relative to its initial value.
- Cleanliness grade: the closer a position is to the wafer, the more worthwhile it is to choose the higher metal cleanliness grade (< 3 µg/device).
- Verify with defect data: introduce the filter with the actual photoresist or solvent, and track results with wafer defect data.
FAQ
Can UPE filters be used for developer (TMAH)?
In terms of chemical compatibility, yes: UPE is rated ◎ for TMAH 5%. But developer is aqueous, so hydrophobic UPE must be wetted first; you can choose a model supplied UPW pre-wet. If you want to wet the filter directly with water, consider naturally hydrophilic Nylon 66 (rated ○ for TMAH 5%).
Can UPE be used with high-temperature or strongly oxidizing chemicals?
Not recommended. The maximum temperature of JIUNYUAN UPE filters is 40 °C. In the compatibility chart, UPE is rated ✕ for SPM at 125–160 °C and △ for 98% sulfuric acid and 70% nitric acid. For these conditions, use a PTFE membrane with an all-fluoropolymer construction instead; see “Chemical Compatibility Guide for Filter Material Selection.”
How do I choose between 2 nm and 5 nm?
At the same flow rate, the initial pressure drop at 2 nm is about 1.3 times that at 5 nm. We recommend first using defect data to confirm whether 2 nm is really needed, then deciding the membrane area and the number of cartridges in parallel based on the pressure-drop budget. To confirm a filter's actual retention performance at nanometer particle sizes, you can commission Filter Retention Testing.
Can used UPE filters be reclaimed?
JIUNYUAN's PTFE Filter Reclaim & Recycle Service uses SPM, aqua regia vapor, and HF/HNO₃, and its accepted scope is filters made of PTFE membrane / PFA material. UPE does not withstand strongly oxidizing acids and is outside the applicable scope of this reclaim process.
Can PP membrane filters be used to filter photoresist?
They can be evaluated. The applications of JIUNYUAN's Semiconductor-grade PP Membrane Filter Cartridge include photoresist, PGMEA, PGME, and others. PP is rated ○ for PGMEA / PGME, while UPE is rated ◎. When choosing, compare chemical compatibility, cleanliness grade, and cost, and verify with actual defect data.
Sources
- JIUNYUAN, “Semi-grade UPE Pleated Filter” product specifications and flow curves
- JIUNYUAN, “Semi-grade Nylon66 Pleated Filter” product specifications
- JIUNYUAN, “Semiconductor Process Filtration” process chemical compatibility chart