SiC Surface Metal Detection & Cleaning

For SiC device manufacturers, surface metal control is the difference between high yield and field failure. As SiC fabrication lines increasingly integrate with silicon-based processes, a critical issue—surface metal contamination—has become equally relevant. Even trace amounts of metallic elements (Na, K, Fe, Cu, Ni, Cr) with areal densities as low as 10⁸ cm⁻² can introduce deep-level defects, increasing reverse leakage current, reducing breakdown voltage, and causing threshold voltage shifts.

This article reviews the sources of contamination, presents standardized ICP-MS detection protocols, compares conventional RCA with a fluoride-free catalytic cleaning approach, and provides full specifications for 6-inch 4H-SiC wafers that meet the industry’s stringent <5×10¹⁰ cm⁻² control threshold.

1. Sources of Surface Metal Contamination and Device Impact

Understanding contamination origins is the first step. Metallic impurities arise from:

Crystal growth: Volatilized residual metals (Fe, Ni, Cr, Al) from graphite crucibles and insulation.
Wafering & polishing: Residues from CMP slurries and abrasive particles.
Environmental exposure: Airborne particles and handling tool contacts.

From a semiconductor physics perspective, transition metals (Fe, Cu, Ni, Cr) introduce deep-level recombination centers, shortening minority carrier lifetime and degrading breakdown robustness. Alkali metals (Na, K) accumulate at interfaces, causing threshold voltage instability. The industry-accepted control threshold is ≤5×10¹⁰ atoms/cm²—a standard that PAM-XIAMEN rigorously validates for every production lot. Take the typical specifications – 6-Inch n-Type 4H-SiC Polished Wafers for example:

Parameter	6‑inch SiC Polished SiC Wafer
Grade	Production Grade	Research Grade	Dummy Grade
Diameter	150.0 ± 0.2mm
Thickness	350 ± 25μm
Polytype	4H
Off‑orientation	4°±0.15° toward <11-20>
Primary flat orientation	[1-100] ± 5°
Conductivity type	n‑type (nitrogen‑doped)
Resistivity	0.015 – 0.025Ω·cm
Primary flat length	47.5 ± 1.5mm	47.5 ± 1.5mm	47.5 ± 1.0mm
Secondary flat	None	None	None
LTV(5×5mm²)	≤3μm	≤5μm	≤10μm
TTV	≤5μm	≤10μm	≤15μm
Bow	−25 ~25μm	−35~35μm	−45 ~45μm
Warp	≤35μm	≤45μm	≤55μm
Si‑CMP Roughness (5×5µm²)	Ra≤0.2nm	Ra≤0.2nm	Not specified
Micropipe Density	<0.2ea/cm²	<10ea/cm²	<15ea/cm²
Metal Impurity Content	≤5×10¹⁰ atoms/cm²	≤5×10¹⁰ atoms/cm²	Not required
Basal Plane Dislocation (BPD)	≤1500ea/cm²	≤2000ea/cm²	Not required
Threading Screw Dislocation (TSD)	≤300ea/cm²	≤1000ea/cm²	Not required
Particles (size ≤ 2.0 μm)	≤60ea/wafer	Not required	Not required
Scratches	≤5ea/wafer, total length ≤ diameter	Total length ≤ 2×diameter	Not required

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2. Detection Standard: ICP-MS Protocol for SiC Surfaces

Precise measurement demands ultra-high sensitivity. Inductively coupled plasma mass spectrometry (ICP-MS) offers ng/L-level detection limits (≈10⁷–10⁸cm⁻² surface density), multi-element analysis, and a dynamic range spanning 10⁸ to 10¹²cm⁻².

Standardized Sample Prep – Direct Acid Droplet Extraction (DADE):

(a) A mixed solution (HNO₃, HF, H₂O₂) is dispensed onto the wafer surface.

(b) The droplet is swept across the entire surface for ≥15 seconds to extract metal ions.

(d) Mass concentration is converted to surface areal density (atoms/cm²).

This method applies to 100–200mm SiC substrates, polished wafers, and epi-wafers, quantifying nearly 20 elements (Na, Al, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ag, W, Au, Hg).

At PAM-XIAMEN, 4H-SiC polished wafers are tested for surface metal element content according to standards before shipment, ensuring full traceability and compliance.

3. Evolution of Cleaning Processes: From RCA to Fluoride‑Free Catalytic Cleaning

Detection guides process improvement. For the removal of surface metal contamination, the long‑established RCA cleaning method (comprising SC‑1 (NH₄OH/H₂O₂/H₂O) and SC‑2 (HCl/H₂O₂/H₂O), typically supplemented by a dilute HF dip to strip chemical oxides) has proven effective for silicon. However, its direct transfer to SiC reveals new challenges.

3.1 Potential Damage from the RCA‑HF Process on SiC Surfaces

Recent surface analyses indicate that HF treatment induces undesirable chemical modifications on SiC. X‑ray photoelectron spectroscopy (XPS) detects the formation of C–F covalent bonds between fluorine and surface carbon atoms, and first‑principles density functional theory (DFT) calculations further confirm that such surface bonding narrows the bandgap in the near‑surface region, altering the intrinsic electronic properties. Thus, the conventional RCA process is not damage‑free, and its potential impact on SiC surface electronic structure warrants attention from device researchers.

3.2 Fluoride‑Free Three‑Step Catalytic Cleaning

To overcome these limitations, Kubo et al. developed a three‑step cleaning process that completely omits hydrofluoric acid. Its core innovation lies in the deliberate use of a transition‑metal complex (copper complex) as a catalytic medium, generating reactive radicals that efficiently remove particles. The procedure is as follows:

Step 1: Immersion in an alkaline solution containing copper complex and hydrogen peroxide to remove particles and organic wax residues.
Step 2: Immersion in a strong alkaline solution (pH 14) to dissolve metallic impurities.
Step 3: Immersion in a strong acid solution (pH 1) to eliminate any remaining metal ions.

This process is entirely HF‑free and requires only three steps (Fig. 1).

Fig. 1 Schematic comparison of the conventional RCA cleaning sequence and the fluoride‑free three‑step method

3.3 Cleaning Efficacy and Surface Integrity Assessment

Atomic force microscopy (AFM) topographs (Fig. 2) show that after the fluoride‑free three‑step cleaning, the SiC surface is free of residual particles, whereas the RCA‑cleaned surface still exhibits some particles. The deionized‑water contact angle decreases from 70° (before cleaning) to 42° after the new process, confirming effective removal of hydrophobic organic contaminants.

Fig. 2 AFM images of SiC wafer surfaces: (a) as‑received; (b) after RCA cleaning; (c) after fluoride‑free three‑step cleaning

Candela surface particle inspection on 3‑inch SiC wafers further confirms a significant reduction in particle counts. Importantly, despite the use of copper complex in the cleaning bath, total reflection X‑ray fluorescence (TXRF) analysis (Fig. 3) detects no copper or other metal residues on the cleaned surface (Fe below 1.0×10¹⁰ atoms/cm²), demonstrating that the subsequent strong acid/base steps effectively remove any introduced metal species, avoiding secondary contamination.

Fig. 3 TXRF spectrum of a SiC wafer surface after the fluoride‑free three‑step cleaning

The success of this method highlights the key role of catalytically generated reactive oxygen species in particle oxidation and removal, while also eliminating the safety hazards and surface‑chemical damage associated with HF. It offers a combination of process safety, cleaning efficiency, and surface integrity preservation.

From precise detection of surface metals to non‑damaging cleaning processes, SiC wafer surface quality control is evolving toward higher sensitivity, lower damage, and greener practices. The establishment of the ICP‑MS standard provides a reliable tool for material R&D and production monitoring, while the fluoride‑free catalytic cleaning approach mitigates the potential impact on SiC’s electrical properties from the outset, promising improved device yield and long‑term reliability.

Whether you need SiC wafers for research or for industrial applications, please contact us email at [email protected] and [email protected].

References:

T/CASAS 032-2023. Test method for the content of metal elements on the surface of silicon carbide wafer—Inductively coupled plasma mass spectrometry
Kubo, M., Hidaka, M., Kageyama, M., Okano, T., & Kobayashi, H. (2012, June). Novel Cleaning Method of SiC Wafer with Transition Metal Complex. In Materials Science Forum (Vol. 717, pp. 877-880). Trans Tech Publications Ltd.