Used Plasma Etch vs Wet Etch: Choosing the Right Process for Your Budget

Etching is a fundamental semiconductor process that removes material to create patterns defined by photolithography. Two dominant approaches exist: plasma (dry) etching and wet (chemical) etching. Each has distinct advantages, costs, and applications. For fabs building capacity on a budget, the used equipment market offers compelling options for both approaches—if you understand the trade-offs.

Process Fundamentals

Plasma Etch (Dry Etching)

Plasma etching uses ionized gas (plasma) to remove material through physical sputtering, chemical reaction, or a combination of both.

Key mechanisms:

• Physical sputtering: Ion bombardment physically ejects material
• Chemical etching: Reactive species form volatile byproducts
• Reactive Ion Etching (RIE): Combines both mechanisms for anisotropic profiles

Common chemistries:

• Silicon/SiO₂: CF₄/O₂, SF₆, C₄F₈
• Aluminum: Cl₂/BCl₃
• Photoresist: O₂ (ashing)

Wet Etch (Chemical Etching)

Wet etching uses liquid chemicals to dissolve exposed material isotropically or anisotropically depending on the chemistry and substrate.

Common chemistries:

• Silicon dioxide: Buffered HF (BHF), diluted HF
• Silicon: HNA (HF:HNO₃:CH₃COOH), TMAH (anisotropic)
• Aluminum: H₃PO₄:HNO₃:CH₃COOH:H₂O
• Photoresist: NMP, DMSO, sulfuric peroxide

Technical Comparison

Anisotropy

Plasma Etch: Highly Anisotropic

• Vertical sidewalls achievable
• Critical for sub-micron features
• Aspect ratios >20:1 possible with advanced ICP systems

Wet Etch: Isotropic (mostly)

• Undercuts mask by approximately etch depth
• Limits minimum feature size
• Exception: TMAH etching of silicon (anisotropic along crystal planes)

| Feature Size | Plasma Etch | Wet Etch | |--------------|-------------|----------| | >10 µm | Capable | Suitable | | 3-10 µm | Capable | Marginal | | 1-3 µm | Required | Not suitable | | <1 µm | Required | Not suitable |

Selectivity

Plasma Etch: Moderate Selectivity

• Selectivity to mask and stop layers: 5:1 to 20:1 typical
• Advanced processes achieve 50:1+ with optimized chemistries
• Endpoint detection enables precise stopping

Wet Etch: High Selectivity

• Selectivity to mask: 100:1+ common
• Crystal-plane selectivity in anisotropic etching
• Limited by isotropic undercut

Surface Damage

Plasma Etch: Potential Damage

• Ion bombardment can cause lattice damage
• Charging effects in sensitive devices
• UV exposure from plasma can damage sensitive structures

Wet Etch: Minimal Damage

• No ion bombardment
• No charging effects
• Gentler on delicate structures

Throughput

Plasma Etch: Batch or Single-Wafer

• Batch systems: 50-100 wafers/hour
• Single-wafer systems: 20-60 wafers/hour (depending on process)
• Process times: 30 seconds to 10+ minutes

Wet Etch: Batch Processing

• 25-50 wafers per batch (cassette)
• Batch time: 5-30 minutes
• Effective throughput: 50-200 wafers/hour

Equipment Options and Used Market Pricing

Plasma Etch Systems

Lam Research 2300 Series

The Lam 2300 platform dominates the plasma etch market with multiple configurations:

Versys (Dielectric Etch)

• Applications: Oxide, nitride, low-k dielectrics
• Technology: Dual-frequency capacitive coupling
• Used price (2026): $350,000 - $700,000

Kiyo (Conductor Etch)

• Applications: Polysilicon, aluminum, tungsten
• Technology: ICP (Inductively Coupled Plasma)
• Used price (2026): $400,000 - $800,000

Flex (MEMS/Deep Silicon)

• Applications: Deep silicon etching (Bosch process)
• Technology: High-density ICP
• Used price (2026): $300,000 - $600,000

Applied Materials Centura MXP+ / DPS

• Applications: Metal etch, polysilicon etch
• Technology: Transformer-coupled plasma (TCP)
• Used price (2026): $250,000 - $550,000

TEL Tactras / Unity

• Applications: Dielectric and conductor etch
• Technology: Dual plasma sources
• Used price (2026): $300,000 - $650,000

Wet Etch Systems

Batch Wet Benches

Metrologic / CFM Technologies

• Configuration: 4-8 tank manual or semi-automated
• Applications: RCA clean, oxide etch, photoresist strip
• Used price (2026): $25,000 - $120,000

Submicron Systems (SCS)

• Configuration: Semi-automated to fully automated
• Applications: High-purity processing
• Used price (2026): $40,000 - $150,000

Single-Wafer Wet Processors

Mattson Aspen

• Technology: Spray processing
• Applications: Precision cleaning, selective etching
• Used price (2026): $80,000 - $250,000

SEZ (now Lam Research) Spin Processors

• Technology: Single-wafer spin etch
• Applications: Backside cleaning, film removal
• Used price (2026): $60,000 - $200,000

Cost of Ownership Analysis

Capital Cost Comparison

| Equipment Type | Used Price Range | Relative Cost | |----------------|------------------|---------------| | Plasma Etch (Lam 2300) | $300,000 - $800,000 | 1.0x (baseline) | | Wet Bench (automated) | $80,000 - $250,000 | 0.3x - 0.4x | | Wet Bench (manual) | $15,000 - $50,000 | 0.05x - 0.1x |

Operating Costs

Plasma Etch:

• Consumables: Process gases ($20,000-$80,000/year), chamber parts ($30,000-$100,000/year)
• Power: 50-150 kW depending on configuration
• Maintenance: $50,000-$150,000/year for service contracts
• Facilities: Exhaust, cooling water, process gas delivery

Wet Etch:

• Consumables: Chemicals ($15,000-$50,000/year), DI water, filters
• Power: 10-30 kW (mostly for heating and pumps)
• Maintenance: $10,000-$40,000/year
• Facilities: Chemical delivery, waste treatment, exhaust

Total Cost of Ownership (5-year, 200mm fab)

Plasma Etch System:

• Acquisition: $500,000
• Operating costs: $600,000
• Total: $1,100,000

Automated Wet Bench:

• Acquisition: $150,000
• Operating costs: $200,000
• Total: $350,000

When to Use Each Process

Use Plasma Etch When:

Feature size < 3 µm

• Sub-micron features require anisotropic etching
• Isotropic wet etch would destroy pattern fidelity

High aspect ratios required

• Deep trenches, through-silicon vias (TSVs)
• Aspect ratios >5:1 demand directional etching

Precise depth control needed

• Endpoint detection in plasma systems
• Critical for gate etch, contact etch

Vertical sidewalls critical

• Gate structures, capacitor plates
• Wet etch undercut unacceptable

Sensitive to contamination

• Some devices can't tolerate liquid exposure
• Plasma etch offers cleaner process environment

Use Wet Etch When:

Feature size > 10 µm

• Large geometry patterns tolerate isotropic etching
• Cost savings significant at relaxed design rules

High selectivity required

• Etching one layer while stopping on another
• Wet chemistries often more selective than plasma

Surface damage must be minimized

• No ion bombardment damage
• Preferred for some MEMS and sensitive devices

High throughput needed for simple processes

• Batch processing of 25-50 wafers
• Effective for blanket etches, cleaning

Budget constraints exist

• 60-80% lower capital cost
• Lower operating costs for simple processes

Hybrid Strategies

Many fabs use both technologies strategically:

Example: Power Device Manufacturing

• Plasma etch: Gate definition (critical dimension)
• Wet etch: Field oxide removal (non-critical, high throughput)
• Plasma etch: Contact etch (anisotropic, precise)
• Wet etch: Backside grinding damage removal

Example: MEMS Manufacturing

• Plasma etch: Device layer patterning
• Wet etch (TMAH): Release etch (anisotropic, selective)
• Wet etch: Sacrificial layer removal

Used Equipment Selection Guidelines

Evaluating Used Plasma Etch Systems

Critical inspection points:

• Chamber condition (coating degradation, particle generation)
• RF generator functionality and matching
• Pump system performance (turbo and roughing)
• Endpoint detection system calibration
• Software licenses and documentation

Red flags:

• Excessive chamber coating buildup
• RF matching network issues
• Obsolete control systems without support
• Missing process recipes

Evaluating Used Wet Etch Systems

Critical inspection points:

• Tank material compatibility with intended chemistries
• Exhaust system integrity and flow rates
• Robot alignment and repeatability (automated systems)
• Temperature controller accuracy
• Chemical delivery system condition

Red flags:

• Unknown process history (chemical incompatibility risk)
• Cracked or crazed tank materials
• Inadequate exhaust capacity
• Missing safety interlocks

FAQ

Q: Can I replace a plasma etch system with a wet bench to save money?

A: Only if your process geometries allow. For features >10 µm with tolerance for isotropic profiles, wet etching can substitute. For sub-micron features, high aspect ratios, or precise depth control, plasma etching remains necessary. Attempting to substitute wet etch for critical plasma etch processes will result in yield loss exceeding any equipment cost savings.

Q: What's the minimum feature size achievable with wet etching?

A: Practical limit is approximately 3 µm for isotropic wet etches, assuming 1:1 aspect ratio (etch depth equals minimum feature size). For anisotropic wet etching (TMAH on silicon), features down to 1-2 µm are possible along specific crystal orientations. Below these limits, plasma etching is required.

Q: How do I decide between batch wet benches and single-wafer wet processors?

A: Batch wet benches offer higher throughput and lower cost per wafer for standard processes (cleaning, blanket etches). Single-wafer processors provide better process control, reduced chemical consumption, and compatibility with fragile substrates (III-V materials, MEMS). For high-volume production of standard devices, batch systems are more economical. For development, low-volume, or precision applications, single-wafer processors are preferred.

Q: Can plasma etch systems handle the same chemistries as wet benches?

A: No—plasma etching uses gaseous chemistries (CF₄, SF₆, Cl₂, etc.) while wet etching uses liquid chemistries (HF, HNA, TMAH). The chemistries are fundamentally different, though both may etch the same materials (silicon, oxide, aluminum). Process transfer from wet to dry (or vice versa) requires complete recipe redevelopment.

Conclusion

Plasma etch and wet etch serve complementary roles in semiconductor manufacturing. Plasma etching dominates critical applications requiring anisotropic profiles and sub-micron resolution, while wet etching remains valuable for cost-effective processing of larger features and high-throughput cleaning operations. The used equipment market offers significant savings for both approaches—50-70% below new equipment costs—with wet benches offering particularly attractive economics for budget-constrained operations.

When building or expanding etch capacity, consider a hybrid approach: plasma etch for critical layers and wet etch for non-critical processes. This strategy optimizes both technical performance and capital efficiency.

Need help selecting etch equipment for your application? Contact Caladan Semi for guidance on plasma etch systems, wet benches, and hybrid process solutions. We provide tested and certified equipment with process support and warranty options.