GaN ETCHING

1. GaN ETCHING With Cl2/CH4/Ar Process

2. SUMMARY • Ion bombardment control, • Thermal management, • No Defects (pits or pillars) formation, • Profile control • Plasma repeatability • Plasma uniformity • Loading effect • Optimum process conditions

3. Ion Bombardment Control Physical sputtering is due to cinetic energy of positive ions which impinge the etched areas. This leads to positive effects as: Anisotropic profiles, But this also may lead to significant damages as: Rough surface morphology (pits formation) Trenching Bad selectivity against mask and underlayers Non stoechiometric surfaces (impact on device performance). High density Plasma enhances chemical reaction and decreases RF bias. This leads to better etching efficiency while reducing the physical part of the etch mechanism. In Corial systems, a quartz plate (a few mm thick) covers the cathode. This insulating cover also decreases the ion energy and reduces excessive damages to the GaN surface.

4. Thermal Management Temperature control of substrate to etch leads to better process control. This leads to positive effects as: Better etch selectivity, Stable etch rates as surface reaction kinetic is well controlled. Negative effects are: High temperature of substrate during etching leads to excessive loss of N2. That leaves strong n-types conductive regions which affect multi layer epitaxy (Review of Dry Etching of GaN and Related Materials). In Corial systems, the temperature control of substrates during etching is performed using helium backside cooling and mechanical clamping.

5. Profile Control Device manufacturing may request vertical or tapered profiles according to applications and technologies. Process must enable low mask erosion to achieve vertical profiles. Hard masks as Si3N4, SiO2 and Ni are good candidates. Negative slopes can be obtained with a specific cathode design and particular process conditions. Controlled PR Mask erosion enables tapered to almost vertical profiles (55° to 85°). This process feature depends upon size of the substrate.

6. Plasma Repeatability Plasma ignition has dramatic influence on final GaN etching results. It is extremely important to ensure high reproducibility of plasma ignition. In Corial systems, the editable multi-step processes warrants the repeatability of plasma ignition and, thereby, stable etch performance.

7. Plasma Uniformity In many cases, as the wafer size is 50 mm diameter, batch loading is requested to achieve high throughput. Consequently, good uniformity accross the wafer and wafer to wafer is required. In Corial ICP source, the large size of the source (Ø270 mm) and a carefull design of the coil terminations enable excellent GaN etch uniformity.

8. Etch Rate (nm/mn) 800 400 1/4 of wafer 7 wafers Loading Effect In batch loading, the loading effect has to be considered. The etch rate decreases with the wafer size and the number of wafers.

9. RECAP of GaN PROCESS • Adequate adjustment of RF biasing versus ICP power to get smooth etched surfaces, • Control of wafer temperature by helium assisted heat exchange to maintain resist and device integrity, • Control of the etching profile by the flow rate of polymerizing gas (CH4) in the process recipe, • Repeatability of the etching results by a well controlled plasma ignition using a multi-step approach, • Uniform etching of a batch of seven 2” wafers thanks to the large size of the ICP source (Ø270 mm).