Why Does a Semi-Insulating SiC Substrate Cleanroom Require Additional Electromagnetic Shielding?

Semi-insulating SiC substrates typically have a resistivity higher than 1×10⁷ Ω·cm and are widely used in 5G RF power amplifiers, high-frequency devices, and base station components. Unlike conductive SiC substrates, semi-insulating substrates exhibit different responses to electromagnetic interference (EMI) and electrostatic accumulation due to their high electrical resistance.

Therefore, a cleanroom designed for semi-insulating SiC substrate manufacturing requires not only conventional contamination control but also additional electromagnetic shielding measures to ensure process stability and device yield.

1. Sources of Electromagnetic Sensitivity in Semi-Insulating SiC Substrates

Semi-insulating SiC substrates achieve high resistivity through vanadium compensation doping or intrinsic defect compensation mechanisms. The concentration and distribution of deep-level impurities and crystal defects directly influence resistivity uniformity across the substrate.

During processing and inspection, substrates are exposed to electromagnetic fields generated by the surrounding environment. These external electromagnetic fields can induce charge redistribution within the semi-insulating SiC substrate.

Potential electromagnetic sources inside a cleanroom include:

• Variable-frequency drives (VFDs)
• Motors and motion-control systems
• Fan filter units (FFUs)
• Ionizers
• Lighting ballasts
• Wireless communication devices

These sources generate electromagnetic fields ranging from 50 Hz power-frequency fields to GHz-level RF signals.

When exposed to alternating electromagnetic fields, induced charges and localized current effects can modify the surface electrical potential of the substrate.

Changes in substrate surface potential may affect downstream semiconductor processes:

• Photolithography:
  Semi-insulating SiC substrates serve as carriers for photoresist coating. Non-uniform surface potential can influence photoresist spreading behavior, resulting in coating thickness variation.
• Chemical Mechanical Planarization (CMP):
  Surface potential variations may affect abrasive particle adsorption and slurry interaction with the substrate surface, influencing polishing uniformity.
• Inspection Processes:
  Electron-beam and ion-beam inspection systems are sensitive to electromagnetic disturbances. Surface potential fluctuations can degrade imaging accuracy and measurement stability.

2. Selection of Electromagnetic Shielding Areas

A semi-insulating SiC substrate cleanroom does not require electromagnetic shielding throughout the entire facility. Shielding requirements should be determined according to:

• Substrate exposure conditions
• Process sensitivity
• Equipment electromagnetic susceptibility

When substrates remain inside sealed wafer carriers, the carriers themselves provide a certain level of electromagnetic protection.

Wafer carriers may use:

• Conductive polymer materials
• Metal-coated polymer structures

The carrier housing should also be electrically grounded.

However, once substrates are removed from carriers and exposed directly to the cleanroom environment, electromagnetic protection must be provided by the cleanroom shielding structure.

The following areas require priority electromagnetic shielding:

Photolithography Area

After photoresist coating, the substrate surface becomes highly sensitive to electrical potential variations. Electromagnetic shielding helps maintain coating uniformity and lithography accuracy.

CMP Area

Surface potential control is important for maintaining stable slurry behavior and polishing consistency.

Inspection Area

Electron-beam and ion-beam inspection equipment are highly sensitive to electromagnetic interference. Independent shielding protection is recommended.

By comparison, substrate storage areas and transfer corridors generally have lower shielding requirements because substrates remain inside electromagnetic-shielded carriers during transportation and storage.

3. Electromagnetic Shielding Layer Design

Cleanroom electromagnetic shielding structures typically use:

• Metal plates
• Metal mesh layers
• Embedded conductive shielding structures

installed within walls, ceilings, and floors.

Common shielding materials include:

• Copper foil
• Galvanized steel sheets
• Stainless steel plates

The shielding material thickness and structure are determined according to the required shielding effectiveness.

Target Shielding Effectiveness

Shielding performance should be specified according to different frequency ranges:

The target values are determined based on whether electromagnetic exposure at specific frequencies can generate sufficient induced charge to affect process yield.

Electrical Continuity of Shielding Layer

The shielding structure must maintain continuous electrical conductivity.

Requirements include:

• Shielding panel joints should use conductive gaskets or conductive adhesive tapes.
• Overlapping width should be ≥50 mm.
• Openings in shielding structures should use waveguide ventilation windows.
• Doors should be equipped with conductive sealing strips.
• Door frames and sealing strips must maintain full-perimeter electrical contact when closed.

Grounding Design

The shielding layer should adopt single-point grounding.

Recommended grounding conditions:

• Low-impedance grounding system
• Ground resistance ≤1 Ω

Multiple grounding points may create ground loops. Induced currents within ground loops can generate secondary magnetic fields, reducing overall shielding effectiveness.

4. Management of Electromagnetic Pollution Sources Inside the Cleanroom

Electrical equipment inside cleanrooms is a major source of electromagnetic interference.

Potential EMI sources include:

• FFU motors
• Ionizers
• Wafer transfer motors
• Lighting ballasts

Equipment installed inside shielded areas should undergo electromagnetic emission evaluation.

Recommended equipment selection:

FFU Systems

Use brushless DC motors instead of AC motors to reduce electromagnetic emissions.

Ionizers

Use stable DC ionizers rather than pulsed AC ionizers, which may generate high-frequency electromagnetic noise.

Lighting Systems

Use LED lighting with DC drivers to minimize power-frequency magnetic fields generated by fluorescent lamp ballasts.

Equipment Grounding and Cable Shielding

Equipment metal enclosures must be grounded.

Proper grounding:

• Reduces electromagnetic radiation from equipment surfaces
• Prevents electrostatic charge accumulation

Recommended cable management:

• Power cables: shielded cables with both ends grounded
• Signal cables: twisted-pair shielded cables with single-end grounding

5. Coordination Between Electrostatic Control and Electromagnetic Shielding

Electrostatic accumulation on semi-insulating SiC substrates is closely coupled with electromagnetic shielding.

Surface electrostatic charges can generate local electric fields. When combined with external electromagnetic fields, these effects may increase surface potential non-uniformity.

Ionizer and Shielding Coordination

Ionizers inside shielded areas must be designed together with the shielding grounding system.

Ionizers generate ion pairs that neutralize surface charges. During this process, ion movement toward the shielding structure may create small currents.

Although these currents generally do not produce measurable voltage drops in the grounding system, ionizer placement should ensure:

• Full ion coverage of substrate handling areas
• No direct ion airflow toward shielding surfaces

Unified Grounding System

The following grounding systems should share a common grounding network:

• Electromagnetic shielding grounding
• Equipment grounding
• ESD protection grounding
• Cleanroom equipotential grounding

Potential differences between independent grounding systems can generate ground currents. These currents may create magnetic fields inside the shielding structure and reduce shielding performance.

6. Verification of Electromagnetic Shielding Effectiveness

After installation, the shielding system must undergo electromagnetic shielding effectiveness testing.

The test frequency range should include:

• Power-frequency magnetic fields
• RF electric fields
• Microwave frequencies

Measurement points should include:

• Shielding panel joints
• Door gaps
• Opening areas
• Critical process locations

Testing Method

According to shielding effectiveness testing standards:

1. A transmitting antenna is placed outside the shielded area.
2. A receiving antenna measures electromagnetic field strength at multiple points inside.
3. Locations failing to meet target shielding levels are identified and reinforced.
4. Verification testing is performed after improvement.

Periodic Re-Testing

Shielding effectiveness decreases over time due to:

• Aging of conductive gaskets
• Loss of elasticity in door seals
• Mechanical degradation of joints

The re-test cycle should be determined according to:

• Substrate electromagnetic sensitivity
• Shielding material aging characteristics

Typical frequency:

Once every 1–2 years

7. Cleanroom Zoning and Shielding Requirements

8. Conclusion

The high resistivity of semi-insulating SiC substrates makes them more sensitive to electromagnetic interference and electrostatic accumulation in cleanroom environments.

Variations in substrate surface potential can affect:

• Photoresist coating uniformity
• CMP polishing stability
• Electron-beam inspection accuracy

Therefore, electromagnetic shielding requirements should be defined according to process sensitivity.

Critical areas such as:

• Photolithography zones
• CMP areas
• Inspection areas

should incorporate building-level electromagnetic shielding structures.

Shielding effectiveness targets should be separately specified for power-frequency, RF, and microwave ranges. The shielding structure must maintain electrical continuity and adopt single-point grounding.

Internal electromagnetic pollution sources should be controlled through equipment selection, grounding design, and cable shielding management.

Electromagnetic shielding and electrostatic protection must be designed as an integrated system with a unified grounding network. After construction, shielding effectiveness must be verified and periodically re-tested to ensure long-term process stability and semiconductor yield performance.