Positive PR Stripper (Water base) Electronic/EL Grade is an aqueous photoresist removal chemistry supplied for immersion and spray processing of positive-tone novolak/diazonaphthoquinone resists in semiconductor, flat-panel display, and microelectromechanical-system manufacturing. The product is specified as Electronic/EL Grade, which indicates that metallic, anionic, and particulate impurities are controlled for use in front-end device fabrication areas where residue-derived defects can alter threshold voltage, gate oxide integrity, and electrical leakage. The water-based formulation reduces organic solvent load relative to conventional stripper chemistries while retaining the alkaline activity required to lift and dissolve positive photoresist films. Typical positive-resist thicknesses processed with this class of material lie between 0.5 µm and 5.0 µm, although heavily implanted or plasma-hardened films may require longer contact time or higher bath temperature within the manufacturer’s qualified window.

The active chemical system of a water-based positive photoresist stripper in this class normally includes a high-boiling alkanolamine or amino alcohol, a water-soluble organic cosolvent, and a corrosion inhibitor package for aluminium, copper, titanium nitride, and tungsten. The water phase supplies polar solvent capacity for novolak resin fragments and diazonaphthoquinone photoactive compounds after alkaline hydrolysis or resin swelling. Because proprietary composition information is not released in publicly available literature, lot-specific pH, specific gravity, alkali reserve, and inhibitor concentration must be obtained from the certificate of analysis and safety data sheet. No uniform model designation is assigned in the current datasheet; the product is identified by the chemical description Positive PR Stripper (Water base) Electronic/EL Grade, and material identity is normally verified by FTIR spectral match against the supplier reference or by refractive index after dilution.

What Distinguishes EL-Grade Aqueous Stripping from Solvent-Based Resist Removal?

Conventional positive resist strippers based on organic solvents such as N-methyl-2-pyrrolidone, dimethyl sulfoxide, or glycol ether/hydroxylamine combinations offer rapid penetration of cross-linked resist crust but introduce volatile organic compound control, solvent recycling, and drying/rinse concerns. Positive PR Stripper (Water base) Electronic/EL Grade is formulated with water as a continuous phase, so rinsability is improved and volatile organic content is lower when measured by EPA Method 24. The trade-off is that aqueous alkaline systems can attack pH-sensitive metals if the corrosion inhibitor is depleted, and bath-life control must account for dissolved resist load, absorbed carbon dioxide, and water evaporation. A direct comparison is shown in Table 1.

Positive photoresist films are typically based on novolak resins and diazonaphthoquinone photoactive compounds. After patterning and etch, the film may contain a carbonized crust, halogenated residues, and cross-linked polymer chains. The alkaline aqueous stripper attacks the phenolic hydroxyl groups and ester linkages, causing the film to lift from the substrate. In water-based systems, swelling is often the primary removal mode; dissolution follows as the polymer fragments disperse into the aqueous phase. Mechanical agitation and elevated temperature accelerate penetration, but excessive temperature can decompose the corrosion inhibitor or initiate attack on exposed aluminium bond pads. The processing window therefore combines chemical activity, temperature, and bath age in a matrix that must be controlled for each device layer.

In production, the stripper is applied after plasma etch, ion implantation, or post-etch residue formation. The process sequence normally uses a rinse with ultrapure water meeting ASTM D5127 Type E-1 or better, followed by an optional isopropyl alcohol or intermediate solvent rinse only if the device structure traps water in high-aspect-ratio features. Bath temperature in recirculated batch immersion tools is commonly maintained between 50 °C and 80 °C; spray tools may use a lower set point because mechanical impingement assists removal. The exact set point for this product must be derived from process qualification on patterned substrates, because resist cross-link density, ion implant dose, and underlying film stack change the dissolution rate. Filtration through 0.1 µm to 0.45 µm cartridges is recommended to remove stripped resist agglomerates and particulate contamination. Bath agitation by recirculation pump, nitrogen bubbling, or ultrasonic/megasonic energy can improve clearing from dense via and trench arrays, but cavitation intensity should be controlled to avoid pattern damage in advanced node work.

Incoming inspection and bath monitoring generally use pH by ASTM E70, density by ASTM D4052, conductivity by ASTM D1125, and trace-metal analysis by ICP-MS according to EPA 6020B. Electronic/EL-grade control typically requires each cationic impurity to be below 100 µg/L in the as-supplied liquid, with more stringent limits for sodium, potassium, iron, and copper because these species influence device reliability. However, the exact acceptance limits are lot-specific and are stated on the certificate of analysis; the value 100 µg/L should not be interpreted as a universal specification. Bath life is commonly monitored by total organic carbon or UV-visible absorbance, with replacement triggered when the resist load causes clearing time to increase beyond the qualified limit or when particle counts exceed the process control limit.

Observed Production-Scale Behavior in Immersion and Spray Tools

Batch immersion tools with 40 L to 120 L working volume are typical for wafer cassettes, while larger flat-panel display lines use spray chambers or ultrasonic immersion tanks. Production-scale behaviour differs from laboratory beaker evaluation because dissolved resist accumulates in the bath, corrosion inhibitor components can be consumed on metal surfaces, and evaporation can increase the concentration of less volatile active species. In some tools, bath temperature overshoot at the heater surface can exceed 5 °C above set point, which may accelerate aluminium etching if the inhibitor package is already at the lower end of its working range. The processing window is therefore not defined by temperature alone; total contact time, metal passivation, and filtration efficiency must be validated together.

Common failure modes observed on manufacturing lines include redeposition of stripped photoresist on sidewalls and wafer bevels, via plugging caused by particle agglomeration, and galvanic attack at copper/aluminium interfaces. Redeposition is usually controlled by increasing bath turnover rate or by adding a post-strip intermediate rinse rather than by extending immersion time. If pH decreases by more than the supplier-defined range due to absorbed carbon dioxide or acidic carryover from prior cleaning operations, the clearing time for hardened resist can increase abruptly. For this reason, bath replenishment is normally performed on a bleed-and-feed schedule based on accumulated wafer area and not solely on elapsed time.

Point-of-use filtration may use polytetrafluoroethylene or polyvinylidene fluoride membranes rated at 0.05 µm to 0.2 µm, but the specific material must be compatible with the alkaline bath and the operating temperature. Filter change-out frequency is determined by differential pressure, typically not exceeding the manufacturer’s maximum. The recirculation loop should be designed to avoid dead legs where stripped resist can settle. For substrates with trenches or vias having aspect ratio above 10:1, capillary forces can retain aqueous stripper in features after rinsing. A low-surface-tension rinse or isopropyl alcohol vapor dry may be required. This product is water-based and therefore has higher surface tension than solvent-based formulations; pattern collapse during drying is not directly caused by the stripper but by subsequent rinse and dry. The use of a final solvent rinse should be evaluated against flammability and cleanliness requirements.

The product is used as supplied or diluted with ultrapure water according to the supplier’s bath preparation procedure. Typical bath preparation uses an automated dispense system to fill the recirculation tank, heats the bath to the target set point, and verifies pH and conductivity before wafer processing. Wafers are processed in cassette or single-wafer mode. In batch immersion, contact time may range from 5 min to 30 min depending on resist type, implant dose, and bath temperature. In spray processing, contact time may be shorter, often 2 min to 10 min, because mechanical force assists removal. After stripping, the wafers receive a quick dump rinse, an overflow rinse, and a spin rinse/dry. These ranges are not product-specific and must be confirmed by patterned-wafer qualification.

When Post-Etch Residue Load Exceeds Aqueous Bath Scavenging Capacity

The stripping rate of an aqueous positive photoresist stripper is strongly dependent on dissolved resist concentration. In low-load baths, the alkaline component and solvent are available to penetrate the resist surface and swell the film; removal occurs by sheet lift-off, fissure propagation, and dissolution. As the bath accumulates novolak fragments and diazonaphthoquinone by-products, the effective solvent strength of the continuous water phase is reduced. Production baths may show a nonlinear increase in clearing time once the total organic carbon exceeds the supplier-defined threshold. The bath then enters a regime where longer process time increases metal attack while not improving residue removal, because the spent resist film has already redeposited onto the wafer surface. Published quantitative kinetic data for this exact product is limited; therefore, the onset of this regime should be measured by patterned-wafer clearing tests using the actual post-etch residue condition, not inferred from generic stripper data.

Because water-based strippers evaporate more slowly than many solvent-based products, viscosity and surface tension can change through water loss or uptake. The product should be stored closed between uses at 15 °C to 25 °C or according to the supplier’s label. Exposure to strong acids should be avoided to prevent protonation of the alkaline active species and possible precipitation. Mixing with oxidizers such as hydrogen peroxide or sulfuric-peroxide mixtures is not recommended unless explicitly permitted by the supplier; uncontrolled mixing can generate heat and oxygen.

Corrosion inhibitor performance is pH-dependent. In the as-supplied alkaline range, the inhibitor forms a passivation film on copper and aluminium. If the bath pH falls below the supplier-defined lower control limit due to carbon dioxide absorption or acid drag-in, the passivation film can dissolve, and the bath can transition from selective resist removal to aggressive metal attack. This is a critical threshold; the bath must be checked at least once per shift or more frequently when the facility operates at high humidity or when acid-cleaning chemistry is used upstream without adequate rinse.

Relative to semi-aqueous strippers that contain an organic solvent phase and a water phase, the water-based EL-grade product reduces organic solvent content and simplifies waste treatment. Relative to solvent-amine strippers containing hydroxylamine or N-methylpyrrolidone, the water-based product lowers the vapor-phase hazard and can be used in closed recirculating baths with water-wet exhaust. Relative to aqueous inorganic strippers based on sulfuric acid and hydrogen peroxide, the water-based organic alkaline stripper is generally selective to positive novolak resist but may not remove silicon-heavy post-etch residue without additional fluoride-containing additives. Published data for this specific configuration is limited; the absence of a documented specification for a given application does not imply compatibility, and patterned-wafer testing remains mandatory before inserting the product into a production flow.

The material is not intended for negative-tone resists with epoxide or polyimide chemistries unless the manufacturer has qualified the specific resist. It should not be considered a final cleaning chemistry for contacts and vias where metallic residue must be removed; a separate post-strip residue cleaner may be required. At relative humidity above 60%, storage and dispense areas should be purged with dry nitrogen to prevent atmospheric absorption of carbon dioxide, which can reduce alkali reserve and shift the pH downward over time.

RoHS compliance for the as-supplied liquid is typically demonstrated by absence of the restricted heavy metals above threshold levels; however, RoHS Directive 2011/65/EU applies to articles, and the supplier declaration should be requested for any specific customer equipment. REACH registration obligations apply to imported substances; the mixture may contain registered or exempt components. The stripper is not intended for direct food contact or pharmaceutical use and is not an FDA-regulated material. Waste from water-based strippers is commonly treated by pH neutralization and collection of the organic fraction; the reduced solvent content permits connection to aqueous waste treatment systems that accept amine-containing solutions. The exact waste classification must be determined according to local regulations and, in the European Union, under the European Waste Catalogue, because the spent bath contains dissolved photoresist, trace metals, and possibly post-etch residue components such as fluorides or boron from implanted resists.