Wet Etch Processes in CMOS Fabrication encompass the liquid-phase chemical reactions used to selectively remove specific thin films from wafer surfaces — including oxide etching with HF-based solutions, silicon etching with TMAH/KOH, metal etching with acid mixtures, and nitride removal with hot phosphoric acid — providing high selectivity, low damage, and batch processing capability that complement dry etch techniques throughout the process flow.
Key wet etch chemistries include: Buffered Oxide Etch (BOE) — HF (49%) buffered with NH4F in ratios of 6:1 to 100:1, providing controlled SiO2 etch rates (300-1000 Å/min for thermal oxide) with stable pH. The NH4F buffer maintains consistent HF concentration as HF is consumed, ensuring etch rate stability. BOE is used for contact hole cleaning, pad oxide removal, and pre-gate clean. Dilute HF (dHF) — HF diluted 50:1 to 1000:1 in DI water, provides lower etch rates for precise thickness removal. Critically, dHF etches different oxide types at very different rates: thermal SiO2 ~10 Å/min, PECVD oxide ~50 Å/min, and flowable oxide/SOD hundreds of Å/min — this selectivity variation is both a tool (for selective layer removal) and a challenge (STI recess non-uniformity).
Hot phosphoric acid (H3PO4) at 155-165°C etches silicon nitride at 40-60 Å/min with >30:1 selectivity to thermal oxide — used for nitride hard mask strip, spacer pull-back, and LPCVD nitride removal. Temperature control is critical: ±1°C affects etch rate by ~5%, and water content must be maintained by DI water injection to compensate for evaporation. TMAH (tetramethylammonium hydroxide) at 2-25% concentration and 60-90°C etches silicon anisotropically along crystal planes — (100) planes etch 10-50× faster than (111) planes — used for dummy gate removal in the RMG process and MEMS etching. SC1 (APM: NH4OH/H2O2/H2O at 1:1:5) at 60-80°C removes particles and organic contamination with slight oxide etch; SC2 (HPM: HCl/H2O2/H2O at 1:1:5) at 60-80°C removes metallic contamination; SPM (piranha: H2SO4/H2O2 at 4:1) at 120-150°C strips photoresist and heavy organics.
Wet etch challenges at advanced nodes include: etch uniformity across 300mm wafers (controlled by solution flow dynamics, temperature uniformity, and wafer rotation); pattern density effects (micro-loading where dense features etch faster due to local reagent depletion); surface tension effects in high-AR features (liquid may not penetrate sub-20nm trenches, causing incomplete etching); and environmental/cost concerns (HF waste treatment, acid consumption, and water usage are significant compared to dry processing).
Modern wet etch tools include single-wafer spin processors (SEZ, Lam) that spray chemistry on a spinning wafer for precise etch control, and batch immersion tanks that process 25-50 wafers simultaneously for cost-effective cleaning steps. Single-wafer tools dominate for critical etch steps requiring tight CD and uniformity control.
Wet etch processing provides irreplaceable selectivity and gentleness in CMOS manufacturing — while dry etch dominates pattern transfer, wet chemistry remains essential for cleaning, selective removal, and surface preparation steps where plasma damage is unacceptable.