Dettagli del Blog

From Material Fundamentals to Process-Driven Cleaning Strategies

Although both silicon and glass wafers share the common goal of being “cleaned,” the challenges and failure modes they face are fundamentally different. These differences arise from:

• The intrinsic material properties of silicon and glass

• Their distinct specification requirements

• The very different “philosophies” of cleaning driven by their end applications

Before comparing processes, we need to ask: What exactly are we cleaning, and what contaminants are involved?

What Are We Cleaning? Four Major Categories of Contaminants

Contaminants on wafer surfaces can be broadly divided into four categories:

1. Particle Contaminants

Examples: dust, metal particles, organic particulates, abrasive particles from CMP, etc.

Impact:

• Can cause pattern defects

• Lead to shorts or open circuits in semiconductor structures

2. Organic Contaminants

Examples: photoresist residues, resin additives, skin oils, solvent residues, etc.

Impact:

• Can act as “masks,” hindering etching or ion implantation

• Reduce adhesion of subsequent thin films

3. Metal Ion Contaminants

Examples: Fe, Cu, Na, K, Ca, etc., originating mainly from equipment, chemicals, and human contact.

Impact:

• In semiconductors: metal ions are “killer” contaminants. They introduce energy levels in the bandgap, increasing leakage current, shortening carrier lifetime, and severely degrading electrical performance.

• On glass: they can impair thin-film quality and adhesion.

4. Native Oxide or Surface-Modified Layer

• Silicon wafers:
  A thin silicon dioxide (SiO₂) layer (native oxide) naturally forms in air. Its thickness and uniformity are difficult to control, and it must be completely removed when fabricating critical structures such as gate oxides.

• Glass wafers:
  Glass is itself a silica network, so there is no separate “native oxide layer” to strip. However, the surface can be modified or contaminated, forming a layer that still needs to be removed or refreshed.

I. Core Goals: Electrical Performance vs. Physical Perfection

Silicon Wafers

The primary goal of cleaning is to ensure electrical performance.

Typical specifications include:

• Extremely low particle counts and sizes (e.g., effective removal of particles ≥ 0.1 μm)

• Ultra-low metal ion concentrations (e.g., Fe, Cu ≤ 10¹⁰ atoms/cm² or below)

• Very low organic residue levels

Even trace contamination can lead to:

• Circuit shorts or open circuits

• Increased leakage currents

• Gate oxide integrity failures

Glass Wafers

As substrates, glass wafers focus on physical integrity and chemical stability.

Key specifications emphasize:

• No scratches or non-removable stains

• Preservation of the original surface roughness and geometry

• Visual cleanliness and stable surfaces for subsequent processes (e.g., coating, thin-film deposition)

In other words, silicon cleaning is performance-driven, while glass cleaning is appearance- and integrity-driven—unless glass is pushed into semiconductor-grade use.

II. Material Nature: Crystalline vs. Amorphous

Silicon

• A crystalline material

• Naturally grows a non-uniform SiO₂ native oxide layer

• This oxide can threaten electrical performance and must often be uniformly and completely removed in critical process steps

Glass

• An amorphous silica network

• Bulk composition is similar to the silicon oxide layer on silicon

• Highly susceptible to:

  • Rapid etching in HF

  • Erosion by strong alkalis, which can increase surface roughness or distort geometry

Consequence:

• Silicon wafer cleaning can tolerate controlled, light etching to remove contaminants and native oxide.

• Glass wafer cleaning must be much gentler, minimizing attack on the substrate itself.

III. Process Philosophy: How Cleaning Strategies Diverge

High-Level Comparison

IV. Comparison of Typical Cleaning Solutions

Silicon Wafer Cleaning

Cleaning objective:
Thorough removal of:

• Organic contaminants

• Particles

• Metal ions

• Native oxide (where required by the process)

Typical process: Standard RCA Clean

• SPM (H₂SO₄/H₂O₂)
  Removes heavy organics and photoresist residues via strong oxidation.

• SC1 (NH₄OH/H₂O₂/H₂O)
  Alkaline solution that removes particles through a combination of lift-off, micro-etching, and electrostatic effects.

• DHF (dilute HF)
  Removes native oxide and certain metal contaminants.

• SC2 (HCl/H₂O₂/H₂O)
  Removes metal ions via complexation and oxidation.

Key chemicals:

• Strong acids (H₂SO₄, HCl)

• Strong oxidizers (H₂O₂, ozone)

• Alkaline solutions (NH₄OH, etc.)

Physical assistance and drying:

• Megasonic cleaning for efficient, gentle particle removal

• High-purity DI water rinsing

• Marangoni / IPA vapor drying to minimize watermark formation

Glass Wafer Cleaning

Cleaning objective:
Selective removal of contaminants while protecting the glass substrate and maintaining:

• Surface roughness

• Geometry and flatness

• Optical or functional surface quality

Characteristic cleaning flow:

1. Mild-alkaline cleaner with surfactants

   • Removes organics (oils, fingerprints) and particles by emulsification and dispersion.

2. Acidic or neutral cleaner (if required)

   • Targets metal ions and specific inorganic contaminants, using chelating agents and mild acids.

3. HF is strictly avoided throughout the process to prevent substrate damage.

Key chemicals:

• Weak-alkaline cleaning agents with:

  • Surfactants (e.g., alkyl polyoxyethylene ethers)

  • Metal chelating agents (e.g., HEDP)

  • Organic cleaning aids

Physical assistance and drying:

• Ultrasonic and/or megasonic cleaning

• Multiple pure-water rinses

• Gentle drying (slow lift-out, IPA vapor drying, etc.)

V. Glass Wafer Cleaning in Practice

In most glass processing plants today, cleaning processes are designed around the fragility and chemistry of glass and therefore rely heavily on specialized weak-alkaline cleaners.

Cleaning Agent Characteristics

• pH typically around 8–9

• Contain:

  • Surfactants to emulsify and detach oils and fingerprints

  • Chelating agents to bind metal ions

  • Organic additives to boost cleaning power

• Formulated to be minimally corrosive to the glass matrix

Process Flow

1. Clean in a weak-alkaline bath (controlled concentration)

2. Operate from room temperature up to ~60 °C

3. Use ultrasonic agitation to enhance contaminant removal

4. Perform multiple pure-water rinses

5. Apply gentle drying (e.g., slow lifting from the bath, IPA vapor drying)

This flow reliably meets the visual cleanliness and general surface cleanliness requirements for standard glass wafer applications.

VI. Silicon Wafer Cleaning in Semiconductor Processing

For semiconductor manufacturing, silicon wafers typically use standard RCA cleaning as the backbone process.

• Capable of addressing all four contaminant types systematically

• Delivers the ultra-low particle, organic, and metal ion levels required for advanced device performance

• Compatible with integration into complex process flows (gate stack formation, high-k/metal gate, etc.)

VII. When Glass Must Meet Semiconductor-Level Cleanliness

As glass wafers move into high-end applications—for example:

• As substrates in semiconductor processes

• As platforms for high-quality thin-film deposition

—the traditional weak-alkaline cleaning approach may no longer be sufficient. In such cases, semiconductor cleaning concepts are adapted to glass, leading to a modified RCA-type strategy.

Core Strategy: Diluted and Optimized RCA for Glass

• Organic removal
  Use SPM or milder oxidizing solutions such as ozone-containing water to decompose organic contaminants.

• Particle removal
  Employ highly diluted SC1 at lower temperatures and shorter treatment times, leveraging:

  • Electrostatic repulsion

  • Gentle micro-etching
    while minimizing attack on the glass substrate.

• Metal ion removal
  Use diluted SC2 or simpler dilute HCl/HNO₃ formulations to chelate and remove metal ions.

• Strict prohibition of HF/DHF
  HF-based steps must be absolutely avoided to prevent glass corrosion and surface roughening.

Throughout this modified process, the use of megasonic technology:

• Significantly enhances removal of nanoscale particles

• Remains gentle enough to protect the glass surface

Conclusion

The cleaning processes for silicon and glass wafers are essentially reverse-engineered from their end-use requirements, material properties, and physicochemical behavior.

• Silicon wafer cleaning pursues “atomic-level cleanliness” in support of electrical performance.

• Glass wafer cleaning prioritizes “perfect, undamaged surfaces” with stable physical and optical properties.

As glass wafers are increasingly incorporated into semiconductor and advanced packaging applications, their cleaning requirements will inevitably tighten. Traditional weak-alkaline glass cleaning will evolve toward more refined, customized solutions, such as modified RCA-based processes, to achieve higher levels of cleanliness without sacrificing the integrity of the glass substrate.