Endpoint-controlled etch uses an in-situ signal to decide when a target film has cleared or reached a defined remaining thickness, then applies a controlled transition, overetch, or stop. It replaces a purely fixed-time assumption with measurement-informed control, but it does not make etch rate, selectivity, profile, or within-wafer clearing uniform. A valid endpoint system connects a physical signal to wafer state, declares detection latency and failure handling, and proves the resulting structure with independent metrology.

Endpoint is a process event, not merely a timestamp. Clearing begins at the fastest location and ends at the slowest. If a 500 nm film etches at 100 nm/min on average, nominal clear time is 300 s. With a 5% radial rate range, the first and last regions do not clear together. A detector may respond when enough exposed underlayer changes the chamber-average signal, followed by a qualified overetch such as 20% or 60 s. The overetch budget must clear the slow region without unacceptable mask or underlayer loss.

The control sequence needs explicit states: stabilization, eligible detection window, signal processing, endpoint latch, recipe transition, overetch, and abnormal fallback. Detection should be inhibited during ignition, gas switching, pressure settling, or known emission transients. A signal jump at 5 s cannot be accepted when the fastest physically possible clear is 180 s. Bounds derived from incoming thickness and qualified etch-rate range protect against false triggers.

Endpoint time is useful as a process monitor but not a complete rate measurement. $R=t_f/t_{ep}$ estimates average rate only when starting thickness $t_f$, detection state, patterned loading, and overlying layers are comparable. A shift from 300 s to 330 s can reflect 10% slower etching, 10% thicker film, changed open area, optical-window coating, or algorithm drift. Confirm the load-bearing cause before adjusting RF power or gas.

Optical emission spectroscopy tracks plasma species through time. Excited reactants and volatile products emit at characteristic wavelengths; an optical window, fiber, spectrometer, detector, and acquisition system measure intensity. Endpoint may appear as product emission falls, reactant emission rises, a ratio changes, or a multivariate spectral score crosses a boundary. The chosen line must respond to the material transition and remain distinguishable from continuum, overlapping species, chamber-wall emission, and source drift.

Single-line OES is interpretable but sensitive to common-mode changes. Dividing a product line by a stable reference line can suppress plasma-intensity drift, provided the reference is actually stable. A trace sampled at 10 Hz produces one point every 100 ms; averaging 20 points improves noise at the cost of roughly 2 s temporal smoothing. At 5 nm/s etch rate, 2 s corresponds to 10 nm of additional removal before controller and recipe latency are included.

Low exposed area reduces endpoint contrast. If only 0.5% of wafer area is open, changing surface chemistry may contribute little to the chamber-integrated spectrum. Longer integration improves signal-to-noise but delays response. Pattern-density changes between products can move signal amplitude and shape without changing local clear physics. Build product-family models or normalization rather than applying a high-open-area threshold blindly to a low-open-area mask.

Window state is part of the measurement system. Deposits attenuate wavelengths nonuniformly, etch cleans can change transmission, fibers can move, and viewport temperature can drift. A reference lamp or broadband baseline can detect sensitivity loss. A line decreasing 30% over 200 wafers may be window coating, chamber chemistry, or both. Monitor dark level, saturation, spectral calibration, and reference response; PM should restore measurement capability as well as chamber surfaces.

Interferometry measures optical change at the wafer surface. An incident beam reflected from the film surface and interfaces produces intensity oscillations as optical thickness changes. For near-normal incidence, one fringe corresponds approximately to $Δd=λ/(2n)$ when refractive index $n$ is adequately known. At 633 nm and $n=1.46$, one fringe represents about 217 nm. Counting fringes can estimate rate; fitting phase can predict remaining thickness or detect transition to an underlayer.

Interferometry samples the illuminated spot, unlike chamber-integrated OES. Spot placement must represent the critical pattern region and remain stable through wafer rotation or stage motion. Roughness, topography, multilayers, plasma glow, changing refractive index, and low reflectance complicate traces. A center spot can endpoint while the edge retains 30 nm. Multi-site interferometry or a qualified overetch is needed when spatial variation matters.

Reflectometry can monitor broad spectral change, while laser interferometry emphasizes phase at selected wavelengths. Transparent films support fringes; opaque metal transitions may be better served by OES, reflectance change, or electrical/plasma parameters. No endpoint modality is universally superior. Choose according to film optical properties, pattern fraction, selectivity, chamber geometry, expected signal, and acceptable latency.

Nonoptical signatures provide independent or fallback evidence. Plasma impedance, match-network position, reflected power, DC bias, chamber pressure, throttle position, residual-gas signal, and motor current can shift when exposed material changes plasma chemistry. These signals are already available at high rate on many tools but may respond weakly or ambiguously. A reflected-power transition from 15 W to 35 W at 1 kW forward power is evidence only when RF delivery is stable and arcing is excluded.

Mass spectrometry can follow reactants or products with chemical sensitivity, but sampling-line residence time and wall reactions add delay. At a 500 ms transport delay and 200 ms filter delay, a true transition appears 700 ms late before controller latency. Chamber pressure and gas flow change residence time, so delay calibration should cover the recipe range. Residual-gas instruments also require maintenance and fragmentation-aware interpretation.

Machine-learning or principal-component methods can combine wavelengths and equipment traces for weak endpoints. The model must be trained on representative product, chamber, PM, seasoning, and fault states. A high validation accuracy does not protect against spectral drift outside training space. Preserve raw signals, model revision, preprocessing, feature bounds, confidence, and deterministic fallback. A model should not hide an impossible endpoint at 40 s when physics requires at least 180 s.

Decision logic must be deterministic, bounded, and testable. Threshold, slope, change-point, ratio, or model output needs minimum duration, hysteresis, eligible window, timeout, and quality flag. A rule might require normalized slope below −0.02/s for 2 s after 180 s, then latch once. Requiring 3 consecutive samples at 10 Hz adds at least 200 ms from first to third sample. Controller scan, network transfer, PLC logic, and recipe transition add further latency; measure the complete chain.

False early endpoint risks residue, micro-masking, opens, and incomplete contact. Missed endpoint risks excess underlayer loss, mask erosion, CD change, charging, and profile damage. Cost is asymmetric, so thresholds should reflect device risk rather than maximize generic classification accuracy. Test injected spikes, flat lines, saturation, dropped samples, wrong recipe step, window attenuation, and sensor disconnection. A safe fallback may complete a bounded timed etch and hold the wafer, not silently run indefinitely.

Overetch is controlled margin, not compensation for an unstable main etch. Define it as time, percentage of measured endpoint, or a separate selective chemistry. If endpoint occurs at 300 s and overetch is 20%, total time is 360 s. If rate to the underlayer is 1 nm/s during overetch, the potential loss budget is 60 nm at already-cleared sites before loading and selectivity are considered. A chemistry switch can improve selectivity but introduces its own settling and endpoint-transient behavior.

Define film transition and last-clear requirement → Select OES, interferometry, reflectance, mass, RF, or fused signals → Establish calibrated baseline and physical earliest/latest bounds → Acquire representative wafers across chambers, patterns, and PM age → Design filter, normalization, threshold, persistence, timeout, and fallback → Measure sensor-to-recipe latency → Execute endpoint transition and capped overetch → Map residual, underlayer loss, CD, profile, and defects → Challenge weak signal, coated window, spikes, and disconnects → Release model and monitor endpoint-time and signal-shape drift

Independent wafer metrology closes the endpoint loop. Cross-sectional SEM, profilometry, AFM, ellipsometry, reflectometry, XPS, and SIMS answer different questions about residual film, loss, roughness, composition, and depth. Four-point probe or Hall effect can show electrical change in conductive films; corona-Kelvin or Semilab techniques may reveal surface or junction consequences; DLTS can test trap-related damage. Keithley and Keysight instruments can quantify leakage or contact resistance. NIST-traceable standards support measurement chains but do not validate recipe physics.

Qualification spans thickness, pattern density, wafer position, chamber, kit age, window state, and upstream variation. Report endpoint-time distribution, signal-to-noise, detection latency, false-trigger rate, timeout rate, residual map, underlayer loss, CD/profile, and defectivity. A chamber matching time while its residual map differs is not matched. A clean endpoint trace with unacceptable profile is not a successful etch.

Through the signal-to-clear-state and bounded-overetch lens, endpoint-controlled etch is a measurement-and-control system embedded inside plasma processing. Its strength comes from a physically justified signal, explicit temporal logic, measured latency, safe fallback, and independent proof that the slowest relevant feature cleared without spending more mask, underlayer, profile, or reliability margin than the process allows.