What is the cleaning process of gallium nitride epitaxial wafers?

I. Main pollutant types and hazards

1. Particle pollution: Dust from the environment, operating tools or precursor residues can cause device short circuits or increased defect density;

2. organic pollutants：Such as photoresist residue, fingerprints, oil stains, etc., will hinder the uniform growth of the film;

3. Metal ions (such as Fe, Cu, Na, etc.): It will become a deep energy level impurity, reducing the carrier mobility;

4. natural oxide layer: A thin layer of Ga₂O₃ (about 1-5nm thick) is easily formed on the GaN surface, affecting the quality of the ohmic contact or heterojunction interface.

2. Typical cleaning process (step-by-step analysis)

1. Pretreatment: Removal of loose contaminants

• Ultrasonic cleaning (organic solvents)：
  useacetone(5-10 minutes) →Isopropyl alcohol (IPA)(5-10 minutes) Sonicate (power 300-500W) to remove surface organic matter and some particles.
  principle：Organic solvents dissolve organic matter, and ultrasonic vibrations peel off loosely attached particles.
  Notice: Avoid prolonged ultrasonication (>15 minutes) to prevent damage to the epitaxial layer or corrosion of the substrate (such as Si).
• Deionized water (DI water) rinse：
  Rinse with ultrapure water (resistance ≥ 18.2 MΩ・cm) for 3-5 minutes to remove residual organic solvent.

2. Deep decontamination: removal of metal ions and stubborn particles

• Alkaline cleaning (optional, for metal ions)：
  useSC1 solution(NH₄OH:H₂O₂:H₂O = 1:1:5-10, volume ratio), soak at 50-70℃ for 5-10 minutes.
  principle：Hydrogen peroxide oxidizes metal ions, and ammonia causes them to form soluble complexes, while also dissolving some organic matter.
  Notice: GaN has good stability in strong alkaline solutions, but the GaN epitaxial wafers on Si substrates need to be kept for a shorter time (≤5 minutes) to avoid Si corrosion.
• Acid cleaning (removal of oxide layers and metals)：

  • For natural oxide layer: useDilute HF solution(0.5%-5%) soak for 30 seconds - 2 minutes to remove Ga₂O₃ (reaction: Ga₂O₃ + 6HF → 2GaF₃ + 3H₂O).

  • For metal ions:SC2 solution(HCl:H₂O₂:H₂O = 1:1:5-10), soak at 50-70℃ for 5-10 minutes to dissolve alkali metal and heavy metal ions.
    Notice: HF concentration that is too high or exposure time that is too long will corrode the GaN body, so parameters need to be strictly controlled; GaN epitaxial wafers on SiC substrates can tolerate slightly higher concentrations of HF.

• Megasonic cleaning (removal of stubborn particles)：
  Megasonic cleaning (frequency 800kHz-2MHz) is performed in DI water for 3-5 minutes. The microjets generated by high-frequency sound waves are used to remove tightly attached particles. This is gentler than ultrasound and reduces material damage.

3. Final washing and drying

• DI water rinse: Rinse several times (at least 3 times, 1-2 minutes each time) to ensure that residual chemical agents are completely removed.

• dry：

  • Nitrogen drying: Use high-purity nitrogen (purity ≥99.999%) to blow at a 45° angle to avoid water marks.

  • Drying: If batch processing is required, a spin dryer (speed 1000-2000 rpm, 1-2 minutes) can be used with nitrogen to assist drying.

  
  

3. Key Considerations

1. Material compatibility：

   • When the substrate is Si, avoid prolonged immersion in strong alkaline solutions (to prevent Si from being corroded);

   • When the substrate is SiC, attention should be paid to the SiC surface oxide layer (which can be removed simultaneously by dilute HF);

   • Heterojunction epitaxial wafers (such as AlGaN/GaN) require controlled acid solution concentration to avoid excessive etching of the AlGaN layer.

2. Environment and reagent purity：

   • Cleaning needs to be doneClass 100 clean roomTo avoid secondary pollution;

   • Reagents must be electronic grade (such as UPSS grade) and DI water must be submicron filtered.

3. Parameter optimization：

   • The temperature, time, and solution ratio need to be fine-tuned according to the surface condition of the epitaxial wafer (such as the degree of contamination and the thickness of the oxide layer). It is recommended to determine the optimal process through small-batch tests.

4. Cleaning effect test

1. Particle detection: Check the surface particle count using an optical microscope or a particle counter (such as KLA-Tencor) (target: ≤10 particles/cm², particle size ≥0.5μm);

2. Surface compositionX-ray photoelectron spectroscopy (XPS) analysis confirmed that the metal ion concentration was ≤ 1×10¹⁰ atoms/cm² and no organic matter remained.

3. Surface morphology: Use atomic force microscopy (AFM) to ensure that the surface roughness (Ra) is ≤ 0.5nm and there is no etching damage.