dry etching

Dry etching is the Coburn–Winters synergy: ion bombardment perpendicular to the wafer surface enhances the chemical etch rate of silicon in XeF$_2$ from 1 nm/min to 55 nm/min — a 9.2$\times$ amplification over the sum of chemical and physical rates alone — because the ion breaks Si–Si back-bonds that fluorine radicals cannot attack thermally, and this single mechanism is what every plasma etch reactor from 1974 to 2024 exploits to cut vertical features into horizontal films. ```flowchart Reactive gas (Cl2, CF4, SF6, HBr…) → RF/ICP plasma dissociates molecules into radicals + ions → radicals adsorb on wafer surface (chemical, isotropic) → ions accelerated through sheath strike surface at normal incidence (directional) → ion impact breaks bonds, desorbs etch products (synergy) → volatile products pumped away → net result: vertical etch with 10× rate enhancement over either mechanism alone ``` **The synergy ratio is not a fixed constant — it depends on ion energy, flux ratio, surface temperature, and chemistry — and exploiting this dependence is how dry etching achieves selectivity.** In a C$_4$F$_8$/O$_2$/Ar plasma etching SiO$_2$ over Si, the fluorocarbon polymer deposited on both materials is thicker on Si (lower sputter yield) than on SiO$_2$ (higher bond polarity assists removal). At 200 eV bias the synergy ratio for SiO$_2$ reaches 15$\times$ while Si stays below 2$\times$, yielding a selectivity of 10:1 to 20:1. Lowering ion energy to 50 eV suppresses the Si synergy entirely while SiO$_2$ retains a ratio of 5$\times$ — the basis for infinite selectivity in production etch-stop schemes. Every selectivity recipe in semiconductor manufacturing is a deliberate manipulation of the synergy ratio difference between two materials. **Anisotropy emerges because ions arrive perpendicular to the surface while radicals arrive from all angles.** The vertical etch rate on horizontal surfaces is $R_v = R_\text{chem} + R_\text{synergy}$ where the synergy term requires ion bombardment. Sidewalls receive negligible ion flux (the IADF is typically less than 2$^\circ$ in ICP at 5 mTorr), so the lateral rate is $R_l \approx R_\text{chem}$ only. Anisotropy $A = 1 - R_l/R_v = 1 - 1/(1 + S)$ where $S$ is the synergy ratio. For $S = 10$, anisotropy is 0.91; for $S = 20$, it reaches 0.95. Adding sidewall passivation (fluorocarbon polymer, SiO$_x$Cl$_y$ from HBr/O$_2$) suppresses lateral attack entirely, pushing $A$ above 0.99 — the requirement for sub-10 nm features where 1% undercut equals 0.05 nm lateral loss. **The reactor architecture evolution from barrel to ALE is a progression toward independent control of the synergy's two inputs: radical flux and ion energy.** Barrel reactors (1970s) provided only radicals — isotropic etching, no pattern transfer capability. RIE (Hosokawa at NTT, 1974) introduced directional ions via the self-bias mechanism but coupled density to energy through a single RF source, limiting the accessible synergy space. ICP (1990s) decoupled density from energy with separate coil and bias generators, expanding the ion flux by 50$\times$ (from $3 \times 10^{15}$ to $1.3 \times 10^{17}$ cm$^{-2}$ s$^{-1}$) while independently controlling energy from 20 to 500 eV. ALE (2015) takes control to the atomic limit: a saturation dose of radicals adsorbs one monolayer, then a calibrated ion pulse removes exactly that monolayer and stops — synergy confined to a single atomic layer per cycle. **The etch equipment market reached 18 billion USD in 2023, driven by the 3D stacking revolution that multiplies the number of etch steps per wafer.** A 2D planar DRAM at the 20 nm node required approximately 30 etch steps; a 200-layer 3D NAND die requires over 120 etch steps including the single most challenging etch in semiconductor manufacturing — the 100:1 aspect-ratio channel hole through alternating oxide/nitride. Lam Research commands roughly 45% of the market (Kiyo, Flex, Versys platforms), Tokyo Electron holds 27% (Tactras, Vigus), Applied Materials 15% (Sym3, Producer Selectra), and Hitachi High-Tech 8% (M-7000 series). The installed base exceeds 40,000 etch chambers worldwide. **Gas chemistry determines which bonds break and which survive — the chemical half of the synergy equation.** Silicon etches in Cl$_2$/HBr because Cl radicals form volatile SiCl$_4$ (boiling point $-$58$^\circ$C) while the Si surface remains Cl-terminated between ion impacts; SiO$_2$ resists this chemistry because Si–O bonds (bond energy 799 kJ/mol) are stronger than Si–Cl (bond energy 416 kJ/mol). Fluorocarbon gases (C$_4$F$_8$, C$_4$F$_6$, CHF$_3$) etch SiO$_2$ by forming volatile SiF$_4$ and CO$_2$ while depositing a CF$_x$ polymer that passivates Si. Silicon nitride etches selectively in CH$_2$F$_2$/O$_2$ because the N–H bond offers a hydrogen abstraction pathway unavailable to SiO$_2$. Metal etches demand their own volatile products: Cl$_2$/BCl$_3$ for aluminum (AlCl$_3$, bp 183$^\circ$C), SF$_6$ for tungsten (WF$_6$, bp 17$^\circ$C), and Cl$_2$/O$_2$ for ruthenium (RuO$_4$, bp 40$^\circ$C) — each product volatile enough to desorb at the 60–80$^\circ$C wafer temperatures used in production. Oxygen additions control polymer thickness; argon dilution provides physical sputtering momentum. The gas mixture is the selectivity knob; ion energy is the rate and damage knob. **Critical-dimension tolerance has shrunk from $\pm$25 nm at the 500 nm node to $\pm$0.3 nm at the 2 nm GAA nanosheet node, making etch the single largest contributor to edge-placement error.** At the 14 nm FinFET node the fin width is 8 nm and etch contributes $\pm$1 nm — already 12% of the feature. At the 2 nm node a gate-all-around nanosheet is 5 nm thick; the inner spacer etch that defines channel length must hold $\pm$0.3 nm across a 300 mm wafer, demanding better than 1% uniformity in ion flux, ion energy, and radical-to-ion ratio simultaneously. This is why atomic-layer etching is not optional at advanced nodes — conventional continuous-wave etching cannot guarantee sub-angstrom reproducibility. | Architecture | Era | Density (cm$^{-3}$) | Pressure | Synergy control | |---|---|---|---|---| | Barrel / downstream | 1970s | $10^{9}$ | 300–1000 mTorr | Radicals only (isotropic) | | RIE (parallel plate) | 1974–1995 | $10^{9}$–$10^{10}$ | 50–200 mTorr | Coupled (one knob) | | ICP / high-density | 1995–present | $10^{11}$–$10^{12}$ | 2–20 mTorr | Decoupled (two knobs) | | CCP dual-frequency | 2000–present | $10^{10}$–$10^{11}$ | 10–100 mTorr | Partially decoupled | | ALE (pulsed ICP) | 2015–present | $10^{11}$ | 5–50 mTorr | Self-limiting (per layer) | **Dry etching's unsolved frontier is the atomic-scale etch stop — detecting and halting at a single monolayer interface between two materials of similar composition.** In a SiGe/Si superlattice for gate-all-around FETs, the etch must remove SiGe selectively without attacking the 5 nm Si channel underneath — a composition difference of only 30 atomic percent germanium. Optical emission spectroscopy (OES) cannot resolve a monolayer endpoint; mass spectrometry adds 2–5 s latency. The current solution is ALE with radical selectivity tuning (Cl$_2$ adsorbs preferentially on SiGe over Si by 4$\times$ at 300$^\circ$C), but reproducibility across 300 mm wafers remains the rate-limiting step for 2 nm node yield. Read dry etching through a *synergy ratio* lens rather than a *plasma physics* lens: every decision in the etch process — gas chemistry, pressure, bias voltage, reactor architecture, pulsing scheme — acts by changing the magnitude or material-selectivity of the ion-enhanced chemical rate amplification that Coburn and Winters measured at 9.2$\times$ in 1979, and the entire 50-year evolution of etch technology is a search for independent control over that single ratio. --- **Etch Chamber Cross-Section Diagram.** The following schematic shows the major hardware subsystems of a generic ICP etch chamber in cross-section — the architecture responsible for 80% of production dry etching at advanced nodes. Gas enters through a top showerhead, the ICP coil generates high-density plasma through a dielectric window, and a separate RF bias on the wafer electrode controls ion energy independently. The vacuum system maintains 2–20 mTorr while exhausting volatile etch products. **Etch Chamber Schematic — Signal and Control Flow.** The process chamber does not operate in isolation; it is embedded in a control hierarchy that maps recipe setpoints to hardware actuators and closes feedback loops through in-situ sensors. The schematic below traces the signal path from recipe parameters through the RF delivery chain, gas delivery, and vacuum subsystem, showing where each sensor provides real-time feedback to the chamber controller. **Etch Chamber Plasma Schematic — Species, Fields, and Transport.** Inside the chamber volume the plasma is a self-organized system of electrons, ions, radicals, and electric fields. The schematic below maps the spatial structure from the ICP coil through the bulk plasma, presheath, sheath, and wafer surface — showing where each species dominates and how the two RF sources (coil and bias) partition their energy into density generation versus ion acceleration.