What Is an Etch Spacer Process Oes Endpoint Algorithm

An etch spacer process Optical Emission Spectroscopy endpoint algorithm (etch spacer process OES endpoint algorithm) is a high-speed multi-spectral plasma diagnostic and statistical pattern recognition engine designed to capture the clearing of conformal gate sidewall films ($Si_3N_4, SiBCN, SiOCN$) across 3D FinFET and nanosheet architectures. In advanced logic transistors, an anisotropic dry etch in hydrofluorocarbon chemistry ($CH_3F/O_2/Ar$ or $CH_2F_2/CF_4$) strips the horizontal spacer layer from the silicon source/drain landing pads while preserving the vertical dielectric film protecting the gate. Because the spacer layer is only 5 nm to 12 nm thick and pattern open areas are low, volatile nitrogen byproducts ($CN^*$ and $N_2^*$) generate transient, low-amplitude optical emission pulses that can clear in under 15 seconds. A spacer OES endpoint algorithm operates at high sampling rates (100 Hz to 200 Hz), tracking cyanogen radical emission ($CN^*$ at 388.3 nm) and atomic silicon ($Si^*$ at 288.2 nm) while ratioing them against unreacted fluorine radicals ($F^*$ at 703.7 nm) or argon actinometers ($Ar^*$ at 750.4 nm). By applying multi-line ratioing, noise-adaptive Savitzky-Golay differentiation, and dynamic inflection zero-crossing, the algorithm identifies the exact millisecond horizontal clearance finishes, commanding a low-bias over-etch before energetic plasma ions recess the silicon channel. At a high level, how a spacer OES endpoint algorithm tracks byproduct decay and commands soft landing looks like this:

Conformal spacer main etch
CH3F / O2 / Ar anisotropic CCP/ICP discharge
High-speed CCD acquisition
100-200 Hz multi-channel spectrometer collection
Spacer OES multi-spectral tracking engineTransient byproduct tracking, reactant ratioing, and zero-crossing
Nitride byproduct CN* (388.3nm) decay
Violet system bandhead collapses as SiN exhausts
Fluorine reactant F* (703.7nm) surge
Unconsumed halogen etchant spikes at clearance
Dual-line ratio I(CN) / I(F)
Normalizes fluoropolymer chamber seasoning
d²(Ratio)/dt² zero-crossing lock
Sub-millisecond horizontal clearing trigger
↻ per-sample 100Hz OES
High-selectivity soft landing
Shift to ultra-low bias CH3F/O2/He over-etch
Silicon source/drain preservation
Clears footing residues with <0.3nm silicon loss

A few principles are essential for understanding how spacer OES endpoint algorithms operate, why transient ultra-thin film etching demands high-frequency optical sampling, and how multi-line stoichiometry ratios reject polymer-induced spectral drifting:

Spectral emission lines in hydrofluorocarbon spacer etch chemistries. In FinFET and GAA gate sidewall formation, silicon nitride ($Si_3N_4$) or carbon-doped boron nitride ($SiBCN$) is etched using carbon- and hydrogen-containing fluorocarbons mixed with oxygen:

$$Si_3N_4 + 12F \rightarrow 3SiF_4 \uparrow + 2N_2 \uparrow \quad ; \quad Si_3N_4 + CF_x \rightarrow SiF_4 \uparrow + CN \uparrow + N_2 \uparrow$$

Under high-density electron collision in the plasma, these molecular fragments emit distinct optical wavelengths:
- *Nitride Byproduct Signatures (Collapse at Endpoint):* Cyanogen radical ($CN^*$) violet system bandhead ($B\,^2\Sigma^+ \rightarrow X\,^2\Sigma^+$) emits with extraordinary intensity at 388.34 nm. Molecular nitrogen first-positive and second-positive systems ($N_2^*$) emit around 337.1 nm and 357.7 nm.
- *Silicon Volatilization Lines (Collapse at Endpoint):* Atomic silicon ($Si^*$) emits in the ultraviolet at 288.16 nm ($3p^2\,^1D_2 \rightarrow 3p4s\,^1P_1^\circ$).
- *Halogen Reactants and Actinometers (Surge at Endpoint):* Atomic fluorine ($F^*$) emits in the near-infrared at 685.6 nm and 703.7 nm ($3p\,^2P^\circ \rightarrow 3s\,^2P$). Argon gas ($Ar^*$) line at 750.4 nm or 811.5 nm serves as an invariant actinometric baseline. As the horizontal nitride film clears from horizontal surfaces, fluorine consumption ceases abruptly, causing the 703.7 nm line to surge upwards.

The dual-line ratio metric cancels chamber polymer drift. Hydrofluorocarbon plasmas ($CH_3F/O_2$) continuously deposit a thin fluoropolymer film ($CF_x$) on the chamber walls and quartz optical viewport. Single-wavelength intensity monitoring ($I(388.3\text{ nm}) < \text{Threshold}$) suffers from false tripping as window transparency degrades across wafer lots. To eliminate drift, the algorithm computes a cross-spectral ratio between decaying byproduct and surging halogen reactant:

$$R_{\text{OES}}(t) = \frac{I_{CN^*}(388.3\text{ nm})}{I_{F^*}(703.7\text{ nm})} \quad \text{or} \quad \frac{I_{CN^*}(388.3\text{ nm})}{I_{Ar^*}(750.4\text{ nm})}$$

Because the numerator plummets while the denominator surges, the endpoint transition exhibits a steep, orders-of-magnitude mathematical cliff with amplified signal-to-noise ratio ($SNR > 26\text{ dB}$).

Spacer Etch OES Endpoint Algorithm Dynamics CH3F/O2 emission lines, CN/F ratio cliff, and low-bias soft-landing handoff Key Spacer Spectral Lines Nitride Byproducts (Fall ↓) • CN* Violet: 388.3 nm (Primary) • N2* Second Pos: 337.1 nm • Si* Atomic: 288.2 nm (UV) Directly tracks SiN removal Reactants / Ref (Surge ↑) • F* Halogen: 703.7 nm (NIR) • F* Radical: 685.6 nm • Ar* Actinometer: 750.4 nm Spikes as SiN clears Real-Time Spectral Dynamics & Ratio Cliff HORIZONTAL CLEAR TRIGGER (t_EP) CN* (388.3nm) Nitride Byproduct ↓ F* (703.7nm) Reactant ↑ Ratio Cliff (CN / F) Process Time (s) → Intensity / Ratio CN*(388nm) / F*(703nm) Dual-line ratio cliff detection 100-200 Hz Sampling Rate Sub-50ms clearance resolution Zero Recess Soft Landing Preserves active silicon S/D pads spacer OES algorithms ratio falling CN* byproducts against surging F* halogens at 100-200 Hz sampling rates second-derivative zero-crossing locks horizontal clearance instantly to preserve silicon source/drain junctions

High-speed sampling (100–200 Hz) and Savitzky-Golay filtering. Because spacer films are only 8 nm to 12 nm thick, the entire main etch step lasts only 10 to 20 seconds. Standard 1 Hz or 5 Hz spectrometers capture too few data points during the critical clearing transition, leading to substantial over-etch latency. Modern spacer OES systems deploy high-speed multi-channel CCD spectrometer arrays reading at 100 Hz to 200 Hz (one spectrum every 5 to 10 ms). To filter high-frequency electron plasma noise without introducing phase delay, the algorithm applies real-time Savitzky-Golay convolution:

$$y_i = \sum_{j=-m}^{m} c_j \, x_{i+j}$$

This computes smoothed real-time first ($dR/dt$) and second ($d^2R/dt^2$) derivatives with zero phase lag, enabling the system to detect the inflection zero-crossing point in under 30 milliseconds.

Over-etch selectivity modulation and 3D profile freeze. The moment the zero-crossing trigger fires:
1. *RF Bias Attenuation:* Substrate RF bias is reduced from 120 W to <25 W, lowering ion bombardment energy below the silicon physical sputtering threshold ($E_{\text{ion}} < 25\text{ eV}$).
2. *Feedgas Optimization:* The gas flow shifts to a polymer-rich $CH_3F/O_2/He$ ratio, achieving $>50:1$ selectivity of $Si_3N_4$ over underlying crystalline silicon. Residual nitride footings along the base of the gate are chemically scavenged while leaving the source/drain junction recess under 0.3 nm.

Read a spacer etch process OES endpoint algorithm through a *high-speed-chemical-tripwire* lens: sampling hundreds of spectra per second, it tracks the sudden vanishing of cyanogen radicals to trigger a soft-landing downshift before energetic plasma ions can bite into delicate source and drain junctions.

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