Plasma Stripping & Ashing – Principles, Gas Chemistry, and Equipment Guide

By NineScrolls Engineering · 2026-03-27 · 14 min read · Materials Science

Target Readers: Process engineers, equipment engineers, PIs/lab managers, R&D procurement teams, and technical decision-makers evaluating photoresist stripping and plasma ashing solutions. Newcomers will find the fundamentals and comparison tables helpful; experienced engineers can skip to the gas chemistry selection and process optimization sections.

TL;DR Summary

Plasma stripping (ashing) uses oxygen-based plasmas to convert organic films — photoresist, PMMA, polymer residues — into volatile CO₂ and H₂O, providing a dry, damage-controllable alternative to wet chemical stripping. By selecting the right gas chemistry (O₂, O₂/N₂, O₂/CF₄, or H₂/N₂), tuning RF power, pressure, temperature, and gas flow, engineers can strip photoresist at rates exceeding 1 µm/min while preserving sensitive substrates and metal interconnects. Beyond traditional PR removal, modern plasma strippers handle post-etch residue cleaning, descum, surface activation for bonding, organic contamination removal, and even selective etching of 2D materials (MoS₂, BN, graphene). This guide covers the underlying plasma chemistry, compares dry vs. wet stripping, provides gas-selection decision guidance, discusses special 2D material applications, and offers a full FAQ — everything needed to select, optimize, and run a plasma stripping process.

1) What Is Plasma Stripping / Ashing?

Plasma stripping — also called plasma ashing — is a dry-process technique that uses a low-pressure, RF-generated plasma to remove organic materials from a substrate surface. The most common application is photoresist removal after lithographic patterning and etching, but the technique extends to any scenario requiring controlled removal of organic films.

1.1 The O₂ Plasma Chemistry

In a typical oxygen plasma, the 13.56 MHz RF field dissociates O₂ molecules into a mixture of reactive species:

• Atomic oxygen radicals (O*): The primary etchant — these neutral, highly reactive species attack C–C and C–H bonds in organic materials
• Ozone (O₃): A secondary oxidant that contributes to organic decomposition
• O₂⁺ ions: Provide directional bombardment energy when accelerated across the plasma sheath
• UV photons: Break polymer chains through photo-dissociation, increasing surface reactivity
• Metastable O₂*: Excited-state molecules that transfer energy to surface species on contact

The fundamental stripping reactions for a generic hydrocarbon photoresist (CₓHᵧOᵤ) are:

• C + 2O* → CO₂↑ (primary carbon removal pathway)
• C + O* → CO↑ (secondary pathway, favored at low O* concentration)
• 2H + O* → H₂O↑ (hydrogen removal)
• CₓHᵧ + (2x + y/2) O* → xCO₂↑ + (y/2) H₂O↑ (net reaction)

All reaction products (CO₂, CO, H₂O) are gases at process conditions and are continuously pumped away, leaving a clean, residue-free surface. This self-cleaning characteristic is a major advantage over wet stripping, where dissolved resist can re-deposit.

1.2 Downstream vs. Direct Plasma Configurations

Plasma strippers operate in two main configurations:

• Direct plasma (in-situ): The wafer sits within or directly adjacent to the plasma discharge zone. Both radicals and ions reach the substrate, providing high strip rates (1–5 µm/min) but with more ion bombardment. Suitable for robust substrates and when speed is paramount.
• Downstream (remote) plasma: The plasma is generated upstream and only neutral radicals flow to the wafer. Ion bombardment is minimal, making this configuration ideal for damage-sensitive substrates (III-V compounds, MEMS devices, thin gate oxides). Strip rates are lower (0.3–2 µm/min) but controllability and gentleness are superior.

NineScrolls Striper systems use an adjustable plasma discharge gap that allows operators to tune the balance between direct and downstream exposure — effectively providing both configurations in a single platform.

1.3 Why "Ashing"?

The term "ashing" comes from the analogy to combustion: organic material is oxidized to gaseous products, leaving behind only inorganic residue (the "ash"). In semiconductor processing, this ash consists of metal oxides, ion-implant residues, or inorganic filler particles from the photoresist formulation. A complete strip process often includes an ashing step followed by a residue removal step to handle this inorganic remnant.

2) Plasma Stripping vs. Wet Chemical Stripping

Both plasma (dry) and wet chemical methods can remove photoresist, but they differ significantly in mechanism, performance, and environmental impact. The following table provides a detailed comparison:

When to choose plasma stripping: Post-etch residue removal, ion-implanted resist, surface activation, descum, or any process requiring precise uniformity control and minimal chemical waste. Plasma stripping is also the only viable option for many 2D material processing applications where wet chemistries would destroy the monolayer films.

When to choose wet stripping: Very thick resist layers (>20 µm), lift-off processes where partial resist dissolution is needed, or when the substrate cannot tolerate any ion bombardment or UV exposure.

3) Key Process Types

3.1 Standard Photoresist Stripping

The most common application of plasma ashing is removal of positive-tone photoresist (e.g., DNQ/novolac i-line resists, chemically amplified DUV resists) after pattern transfer. A typical process flow:

1. Pre-heat: Ramp wafer stage to target temperature (80–150 °C for standard PR; lower for temperature-sensitive substrates)
2. Pump-down: Evacuate chamber to base pressure (<50 mTorr)
3. Gas stabilization: Introduce O₂ at target flow rate (50–200 sccm); stabilize pressure (200–800 mTorr)
4. Plasma ignition: Apply RF power (300–600 W); confirm stable plasma via reflected power reading
5. Strip: Continue until endpoint detection signals resist clearance (typically 30–120 s for 1–2 µm thick resist)
6. Over-ash: Continue for 10–30% additional time to ensure complete removal, especially in via and trench features
7. Pump/purge: Evacuate reaction products; optional N₂ purge to remove residual O*

Process window for 1.5 µm i-line resist on Si: O₂ 100 sccm, 500 mTorr, 400 W, 120 °C stage temperature, strip rate ~2.5 µm/min, total process time ~60 s.

3.2 Post-Etch Residue Removal

After RIE or ICP-RIE etching, sidewall polymer deposits and redeposited etch by-products form residues that are not pure organic. These residues contain metal halides (from etched metal layers), silicon-containing polymers (from fluorocarbon passivation), and cross-linked photoresist (hardened by ion bombardment and UV exposure during etching).

Removing these residues requires more aggressive chemistry than standard PR stripping:

• O₂/CF₄ (95:5 to 90:10): The CF₄ additive generates fluorine radicals that attack Si-containing residues while O₂ handles the organic component. Etch selectivity to underlying SiO₂ must be monitored.
• O₂/N₂ (80:20): N₂ addition increases ion bombardment energy (heavier N₂⁺ ions) without introducing halogen species. Effective for moderate post-etch residues.
• Two-step process: (1) High-power O₂ to remove bulk organic, then (2) O₂/CF₄ at lower power to gently remove inorganic residue without attacking the underlying film stack.

3.3 Descum

Descum is a brief, low-power O₂ plasma treatment applied after photoresist development to remove thin organic residues (scum) from nominally cleared areas. These residues — typically 5–50 nm thick — result from incomplete development and can cause pattern transfer defects during subsequent etching.

Descum parameters are deliberately gentle:

• Power: 100–300 W (low to avoid resist erosion)
• Pressure: 200–500 mTorr
• Time: 10–30 seconds
• Temperature: Room temperature to 50 °C

The critical requirement is that descum removes scum without measurably thinning the patterned resist or altering its sidewall profile. For a 1 µm thick resist, a well-tuned descum process removes <20 nm of resist thickness while clearing scum completely.

3.4 Surface Activation

O₂ plasma exposure converts hydrophobic surfaces to hydrophilic by replacing surface C–H groups with polar C=O, C–OH, and COOH functionalities. This dramatically increases surface energy and is used for:

• Wafer bonding: Plasma-activated SiO₂ surfaces achieve bond energies >2 J/m² at room temperature, enabling direct bonding without adhesives
• Adhesion promotion: Improving metal or dielectric film adhesion to polymer substrates (e.g., PDMS, PI, PET)
• Wettability control: Ensuring uniform photoresist coating on hydrophobic surfaces
• Bio-functionalization: Creating reactive surface groups for biomolecule attachment in MEMS/microfluidics

Surface activation parameters are typically low power (100–300 W), low pressure (200–500 mTorr), and short duration (10–60 s). The activated surface has a limited lifetime — hydrophobic recovery begins within minutes to hours depending on the material — so subsequent processing should follow promptly.

3.5 2D Materials Processing

Plasma stripping finds a growing role in 2D materials research, where controlled removal of mask layers and selective etching of atomically thin films requires exceptional process control. This application is discussed in detail in Section 7.

4) Gas Chemistry Selection

The choice of process gas is the most impactful decision in plasma stripping. Each chemistry offers distinct advantages and limitations:

4.1 Pure O₂

The workhorse chemistry for standard photoresist stripping. O₂ plasma generates abundant atomic oxygen radicals that efficiently oxidize organic materials to volatile CO₂ and H₂O.

• Advantages: High strip rate, clean by-products, no halogen contamination, simple single-gas process
• Limitations: Oxidizes metal surfaces (Cu, Ti, TiN, W); cannot remove inorganic residues; may cause substrate oxidation damage
• Typical conditions: 100–200 sccm, 300–800 mTorr, 300–600 W

4.2 O₂/N₂ Mixtures

Adding 10–30% N₂ to O₂ increases the mean ion mass, enhancing physical sputtering of stubborn residues without introducing halogen contamination. N₂ also dilutes the O* radical density, providing a way to reduce strip rate for better endpoint control on thin resists.

• Advantages: Better post-etch residue removal than pure O₂; no fluorine contamination; improved uniformity at lower pressures
• Limitations: Still oxidizes metals; strip rate lower than pure O₂
• Typical conditions: O₂ 80 sccm + N₂ 20 sccm, 300–600 mTorr, 400–600 W

4.3 O₂/CF₄ Mixtures

The addition of 3–10% CF₄ to O₂ generates fluorine radicals that attack silicon-containing and metal-containing residues that pure O₂ cannot volatilize. This is the standard chemistry for post-etch residue removal after fluorocarbon-based RIE processes.

• Advantages: Removes both organic and inorganic (Si-containing) residues; higher strip rate than pure O₂ (fluorine catalyzes carbon oxidation)
• Limitations: Fluorine attacks SiO₂ (~1–5 nm/min depending on conditions); potential fluorine contamination of sensitive surfaces; CF₄ is a potent greenhouse gas
• Typical conditions: O₂ 95 sccm + CF₄ 5 sccm, 300–600 mTorr, 400–600 W
• Caution: Keep CF₄ fraction below 10% to avoid excessive SiO₂ etching. Monitor oxide loss with ellipsometry if process runs near sensitive gate or tunnel oxides.

4.4 H₂/N₂ (Forming Gas) for Metal-Sensitive Substrates

When the substrate contains exposed copper, titanium, or other easily oxidized metals, O₂-based chemistries are prohibited. H₂/N₂ plasmas provide an alternative stripping mechanism:

• H* radicals abstract hydrogen from the polymer chain, weakening C–C bonds
• N* radicals and NH* species attack the destabilized carbon backbone
• Volatile by-products: HCN, NH₃, small hydrocarbon fragments (CH₄, C₂H₂)

Key advantages: No oxidation of metals — H₂ plasma actually reduces metal oxides (CuO → Cu + H₂O); compatible with Cu BEOL processing; no halogen contamination.

Limitations: Strip rate is 3–5× lower than O₂ plasma (typically 0.3–1 µm/min); higher power and temperature needed to compensate; some concern about hydrogen embrittlement of certain metals at high doses.

• Typical conditions: H₂ 50 sccm + N₂ 150 sccm, 500–1000 mTorr, 500–800 W, 150–200 °C stage temperature

Gas Chemistry Selection Guide

5) Process Parameters & Optimization

Plasma stripping involves five primary process parameters, each with distinct effects on strip rate, uniformity, substrate damage, and residue removal completeness.

5.1 RF Power

RF power controls both plasma density (radical and ion generation) and DC self-bias (ion energy). In plasma stripping:

• Higher power (500–1000 W): Increases strip rate linearly up to a saturation point where radical recombination and gas depletion limit further improvement. Use for thick resists, hardened post-implant resists, and high-throughput requirements.
• Lower power (100–300 W): Reduces ion bombardment damage, minimizes substrate sputtering, and provides better rate control for thin films. Essential for descum, surface activation, and 2D materials work.
• Power density consideration: For a 200 mm (8") wafer, 400 W corresponds to ~1.3 W/cm². Scale proportionally for other wafer sizes.

5.2 Pressure Effects

Pressure is the second most influential parameter after RF power. In plasma stripping, higher pressures (500–1000 mTorr) are common — unlike RIE where low pressure is preferred for anisotropy. The rationale:

• Stripping is primarily a chemical (radical-driven) process — directionality is not needed for blanket resist removal
• Higher pressure increases O₂ dissociation rate and radical density
• More gas-phase collisions thermalize ions, reducing bombardment damage
• Higher pressure improves uniformity for large-area substrates

Exception: For descum and fine-feature residue removal, moderate pressures (200–500 mTorr) provide better penetration into high-aspect-ratio features.

5.3 Temperature Control

The NineScrolls Striper provides a water-cooled stage with temperature range of 5–200 °C, enabling precise thermal control:

• Low temperature (5–50 °C): Used for temperature-sensitive substrates (flexible polymers, biological samples, some III-V devices). Cooling also prevents thermal reflow of partially stripped resist.
• Moderate temperature (80–120 °C): Standard range for most PR stripping. Provides good balance of strip rate and process control.
• High temperature (150–200 °C): Required for hardened, cross-linked, or ion-implanted resists where increased thermal energy aids bond breaking and by-product desorption. Also beneficial for H₂/N₂ stripping where the lower radical reactivity must be compensated.

5.4 Endpoint Detection

Endpoint detection is critical for preventing over-ashing (which can damage underlying films) and under-ashing (which leaves residues). The NineScrolls Striper incorporates automated endpoint detection using:

• Optical emission spectroscopy (OES): Monitors the intensity of CO* emission at 483.5 nm and/or OH* emission at 309 nm. When photoresist is being stripped, these lines are strong. When the resist is cleared, CO* and OH* intensities drop sharply — the endpoint signal.
• Endpoint algorithm: First-derivative detection of the CO* signal drop, with configurable threshold and over-etch timer. The system automatically transitions from strip to over-etch mode upon endpoint detection.
• Benefits of endpoint detection:
  • Wafer-to-wafer consistency even with resist thickness variation
  • Minimizes over-ashing to protect underlying films
  • Compensates for chamber condition drift (aging, coating buildup)
  • Enables automated, unattended processing

5.5 Optimization Strategy

A systematic approach to plasma strip process development:

1. Define requirements: Target strip rate, acceptable substrate loss, uniformity specification, throughput goal
2. Select gas chemistry: Based on substrate and exposed materials (see Section 4)
3. Start with baseline: O₂ 100 sccm, 500 mTorr, 400 W, 100 °C — adjust from this starting point
4. Optimize power: Increase for faster stripping; decrease if substrate damage is observed (measure with contact angle, XPS, or electrical testing)
5. Tune pressure: Increase for higher rate and lower damage; decrease for better feature penetration
6. Adjust temperature: Increase if hardened resist is incompletely removed; decrease for sensitive substrates
7. Set endpoint: Calibrate OES endpoint on test wafers; set over-etch time to 10–30% of main strip time
8. Verify uniformity: Measure strip rate at center, mid-radius, and edge; adjust gas flow or pressure if non-uniformity >5%

6) Uniformity Considerations

Achieving uniform stripping across the entire wafer is essential — non-uniform stripping leads to either over-ashed regions (substrate damage) or under-ashed regions (resist residue). Key factors affecting uniformity:

• Gas distribution: The NineScrolls Striper uses a center pump-down design that creates symmetric radial gas flow across the wafer surface, ensuring uniform radical delivery from edge to center.
• Plasma uniformity: Electrode design, gap spacing, and chamber geometry determine plasma density distribution. The adjustable discharge gap allows fine-tuning of the plasma profile.
• Temperature uniformity: Non-uniform wafer heating causes local strip rate variation. The water-cooled stage maintains temperature uniformity across the wafer within ±2 °C.
• Loading effects: Dense resist patterns strip slower than isolated features due to local radical depletion. Higher gas flow and pressure help mitigate microloading.

The NineScrolls Striper achieves <5% non-uniformity (with standard edge exclusion) across wafer sizes from 4" to 12", verified by multi-point ellipsometry measurements on blanket oxide witness wafers.

7) Special Applications: 2D Materials Processing

The emergence of two-dimensional (2D) materials — molybdenum disulfide (MoS₂), hexagonal boron nitride (BN), and graphene — has created new demands for plasma stripping equipment. These atomically thin materials require process control at a fundamentally different scale than traditional semiconductor processing.

7.1 MoS₂ Processing

Monolayer MoS₂ (~0.65 nm thick) is a promising channel material for next-generation transistors. Plasma stripping is used in two contexts:

• Resist removal after patterning: PMMA or photoresist must be stripped from MoS₂ without damaging the underlying monolayer. Standard O₂ plasma at full power would etch the MoS₂ itself. The solution is ultra-low-power O₂ plasma (50–150 W) at elevated pressure (>500 mTorr) and reduced exposure time — conditions that maximize chemical (radical) stripping while minimizing ion bombardment.
• Selective etching: Controlled O₂ plasma exposure can selectively thin multi-layer MoS₂ to monolayer thickness. MoO₃ (the oxidation product) is volatile above ~700 °C but can also be removed by a subsequent water rinse at room temperature. Layer-by-layer thinning requires careful endpoint monitoring — typically via in-situ photoluminescence or Raman spectroscopy.

7.2 Hexagonal Boron Nitride (hBN) Processing

hBN serves as an ideal dielectric and encapsulation layer for 2D heterostructures. Plasma stripping applications include:

• Surface cleaning: Low-power O₂ plasma removes organic contaminants from hBN surfaces before device fabrication. hBN is more resistant to O₂ plasma than graphene or MoS₂, allowing slightly more aggressive conditions.
• Patterned etching: O₂/CF₄ plasma can etch hBN when selective patterning is required. The etch rate depends strongly on the number of layers and crystallographic orientation.

7.3 Graphene Processing

Graphene is extremely sensitive to O₂ plasma — even brief exposure can introduce defects (sp³ sites, vacancies) or completely remove monolayer graphene. This sensitivity is both a challenge and an opportunity:

• Challenge — resist removal: Stripping photoresist or PMMA from graphene without damaging it requires H₂/N₂ or Ar/H₂ plasma at very low power, or alternatively, thermal annealing (which avoids plasma damage entirely but is slower).
• Opportunity — patterned etching: O₂ plasma selectively etches graphene with high contrast against SiO₂ substrates, enabling device isolation and ribbon patterning. For this application, moderate power (200–400 W) and short, timed exposures provide clean, well-defined graphene edges.
• Controlled functionalization: Extremely brief O₂ plasma exposure (1–5 s at low power) introduces oxygen-containing groups that tune graphene's work function and wettability without complete removal. This is used for sensor and biosensor surface preparation.

7.4 Process Control Requirements for 2D Materials

Processing 2D materials with plasma stripping demands:

• Precise power control: Ability to operate stably at low RF power (100–300 W) without plasma flickering or mode instability
• Temperature control: Water-cooled stage (5–200 °C) to prevent thermal damage to 2D material/substrate interfaces
• Adjustable plasma exposure: Tunable discharge gap to control the ratio of radical vs. ion flux reaching the substrate
• Reliable endpoint: OES endpoint detection to prevent over-processing of atomically thin films
• Low base pressure: Clean vacuum environment to avoid contaminating pristine 2D surfaces

8) Equipment Selection Guide

When evaluating plasma stripping / ashing equipment, the following specifications and features should be prioritized:

8.1 Critical Specifications

8.2 Application-Specific Considerations

• High-volume manufacturing: Prioritize throughput (multi-wafer batch capability), automated endpoint, recipe storage, and SECS/GEM interface. Consider multi-wafer Striper configurations.
• R&D / University labs: Prioritize process flexibility (wide power/temperature/pressure ranges), multiple gas lines, manual override capability, and small footprint. The NineScrolls Striper's 0.8 × 0.8 m footprint fits even in space-constrained cleanrooms.
• 2D materials research: Prioritize low-power stability, precise temperature control (water-cooled stage), adjustable discharge gap, and clean vacuum base pressure.
• Failure analysis: Prioritize aggressive strip capability (high power, O₂/CF₄), large process window, and ease of use. Endpoint detection is less critical for deprocessing applications.

9) Frequently Asked Questions