endpoint-controlled etch

**Endpoint-controlled etch** uses **real-time monitoring** of the etch process to detect exactly when the target material has been completely removed (or a specific etch depth reached), and then transitions to the next step or stops. It provides **active feedback** rather than relying on a predetermined time. **Why Endpoint Detection Matters** - Incoming film thickness varies from wafer to wafer and across the wafer. A fixed etch time may result in **under-etch** (residual material remaining) or **over-etch** (damage to underlying layers). - Endpoint detection adapts automatically — it stops (or transitions) at the right time regardless of incoming variation. - Critical for etch steps where the **stop layer is thin or sensitive** (e.g., gate oxide, barrier metal). **Endpoint Detection Methods** - **Optical Emission Spectroscopy (OES)**: The most common method. Monitors **plasma emission light** — each material produces characteristic spectral lines when etched. When the target material is consumed, its emission lines **decrease** while stop-layer-related lines **increase**. - Example: During SiO₂ etch, monitor the CO emission line (from the reaction SiO₂ + fluorocarbon → SiF₄ + CO). When the oxide is gone, CO emission drops. - **Laser Interferometry (Reflectometry)**: Shines a laser on the wafer and monitors reflected intensity. As the film gets thinner, the reflected light **oscillates** due to thin-film interference. Each oscillation corresponds to a known thickness change, allowing precise depth tracking. - Particularly useful for **transparent films** (oxides, nitrides) where interference fringes are strong. - **Mass Spectrometry (RGA)**: Analyzes the **etch byproducts** in the exhaust gas using a residual gas analyzer. When the target material is consumed, its characteristic etch products disappear. - High sensitivity but slower response time than OES. - **Broadband Optical Emission**: Uses a spectrometer to capture the full emission spectrum and applies multivariate analysis or machine learning to detect endpoint — more robust than single-wavelength OES. **Endpoint + Overetch** - In practice, the endpoint signal indicates the material is "almost gone" (typically when ~70–90% of the target is cleared from the densest area). - After endpoint, a **timed overetch** (10–50% of the main etch time) ensures complete clearing of residual material from sparse areas. - The soft landing recipe is often used during this overetch phase. Endpoint-controlled etch is **essential for critical etch steps** at advanced nodes — it directly reduces CD variation, prevents stop-layer damage, and adapts to incoming process variability.