chlorine-based etch

Chlorine-based etching is a foundational plasma patterning technology utilizing atomic chlorine radicals ($Cl^*$) and molecular chlorine ions ($Cl_2^+$, $Cl^+$) generated from $Cl_2$, $BCl_3$, $HBr/Cl_2$, $HCl$, and $SiCl_4$ feed gases to achieve anisotropic, highly selective removal of conductors, semiconductors, and metals ($Si$, $poly-Si$, $Al$, $Ti$, $TiN$, $W$, $GaAs$, $InP$). In high-density inductively coupled plasma (ICP) and electron cyclotron resonance (ECR) reactors from Lam Research (Kiyo, Versys), Applied Materials (Centris AdvantEdge), and Tokyo Electron (Tactras, Celesta), chlorine-based chemistries operate at low pressure ($2\text{ mTorr}$ to $20\text{ mTorr}$) to balance ion-assisted directionality with chemical reaction volatility, achieving $poly-Si:SiO_2$ selectivities $> 100:1$ and sub-nm critical dimension (CD) control across FinFET gate stacks, metal interconnect contacts, and compound semiconductor photonics. ```flowchart Precursor Gas Supply (Cl2, BCl3, HBr, Ar) → ICP Chamber Plasma Generation (13.56 MHz, 10 mTorr) → Cl2 Dissociation (Ediss = 2.48 eV) → BCl3 Native Oxide (Al2O3) Scavenging → Chlorine Radical Surface Adsorption → Directional Ion Acceleration (Cl2+, Ar+, Vs = 60V) → Ion-Assisted Volatile By-Product Formation (SiCl4, Al2Cl6, TiCl4) → Oxide Stop Interface (Poly:SiO2 > 150:1) → In-Situ Passivation & Post-Etch Corrosion Water Rinse → Defect-Free Patterned Feature ``` **The fundamental chemical mechanism of chlorine-based plasma etching is driven by ion-assisted chemical reactions forming volatile metal and semiconductor chlorides.** Unlike fluorine-based chemistries ($CF_4$, $SF_6$), which react spontaneously with silicon dioxide ($SiO_2$) and silicon ($Si$), atomic chlorine radicals ($Cl^*$) react selectively with unoxidized semiconductor and metal surfaces ($Si$, $poly-Si$, $Al$, $Ti$, $W$), forming volatile reaction byproducts ($SiCl_4$ with boiling point $T_{\text{boil}} = 57.6^\circ\text{C}$, $Al_2Cl_6$ subliming at $T = 178^\circ\text{C}$, $TiCl_4$ at $T_{\text{boil}} = 136.4^\circ\text{C}$). However, chlorine radicals react extremely slowly with silicon dioxide ($SiO_2$) in the absence of high-energy ion bombardment ($E_a > 1.2\text{ eV}$). This inherent chemical contrast forms the foundation of high-selectivity polysilicon gate etching over ultra-thin gate oxides ($t_{\text{ox}} = 1.2\text{ nm}$ to $2.0\text{ nm}$), where tuning the ion energy below the sputtering threshold of $SiO_2$ yields $poly-Si:SiO_2$ selectivities exceeding $150:1$. **Inductively coupled plasma (ICP) sources decouple radical flux from ion bombardment energy in chlorine etching systems.** In modern etch reactors from Lam Research (Kiyo Series) and Applied Materials (Centris Sym3 / AdvantEdge), a multi-turn inductive coil powered at $13.56\text{ MHz}$ or $27\text{ MHz}$ generates high-density plasma ($n_e = 10^{11}\text{ cm}^{-3}$ to $10^{12}\text{ cm}^{-3}$) at low operating pressures ($2\text{ mTorr}$ to $15\text{ mTorr}$). A separate RF bias power supply ($2\text{ MHz}$ or $13.56\text{ MHz}$) applied to the electrostatic chuck (ESC) independently controls the perpendicular DC bias voltage ($V_s = 20\text{ V}$ to $500\text{ V}$). This decoupling allows the process engineer to maintain high atomic chlorine radical density for rapid chemical reaction while limiting ion bombarding energy ($E_i = e V_s$) to prevent mask sputtering, sidewall degradation, and gate oxide punch-through. **Aluminum metal etching requires boron trichloride ($BCl_3$) to scavenge native oxide ($Al_2O_3$) before chlorine attack can proceed.** Aluminum and aluminum-copper alloys ($Al-0.5\%Cu$) form a dense, self-passivating native oxide layer ($Al_2O_3$, thickness $t = 2\text{ nm}$ to $4\text{ nm}$) upon air exposure. Because chlorine radicals ($Cl^*$) cannot etch $Al_2O_3$ at reasonable processing temperatures ($E_a > 2.5\text{ eV}$), pure $Cl_2$ gas results in long, non-reproducible induction times and severe feature pitting. Adding $BCl_3$ to the gas mixture overcomes this barrier; $BCl_3$ fragments ($BCl_2^+$, $BCl_2^*$) act as powerful oxygen scavengers, reacting with $Al_2O_3$ via $Al_2O_3 + 2 BCl_3 \to 2 AlCl_3 \uparrow + B_2O_3$, breaking through the native oxide within seconds and enabling uniform, controlled chlorine etching of the underlying bulk aluminum. **Post-etch corrosion prevention is a critical requirement for chlorine-processed aluminum and metal interconnects.** When chlorine-etched metal wafers are removed from the vacuum chamber into ambient air, residual chlorine species ($AlCl_3$, adsorbed $Cl_2$, chlorofluorocarbon polymers) trapped on feature sidewalls react with atmospheric moisture ($H_2O$), forming corrosive hydrochloric acid ($6 HCl + 2 Al \to 2 AlCl_3 + 3 H_2 \uparrow$). This moisture-triggered reaction causes severe pitting corrosion, line lifting, and total metal trace failure within hours of etching. To prevent corrosion, modern etch tools integrate in-situ post-etch treatment (PET) chambers where the wafer undergoes an inline $H_2O/O_2$ or $NH_3/O_2$ microwave plasma ash at $T = 200^\circ\text{C}$ to $250^\circ\text{C}$, stripping chlorine residues and replacing volatile chlorides with a stable, passivating native oxide prior to atmospheric exposure. **Polysilicon and metal gate patterning in FinFET/GAA nodes utilizes multi-step $HBr/Cl_2/O_2$ chemistries.** Advanced gate-first and gate-last (replacement metal gate, RMG) integration flows demand vertical gate sidewalls ($\text{profile angle} = 89.8^\circ \pm 0.2^\circ$) with zero footing or notch formation at the gate dielectric interface. In a typical 4-step gate etch process: (1) Main Etch 1 ($Cl_2/Ar$) rapidly removes the upper hardmask and poly-Si; (2) Main Etch 2 ($HBr/Cl_2/O_2$) etches the bulk poly-Si while depositing a thin silicon oxybromide ($SiO_xBr_y$) passivation layer on the sidewalls to maintain profile verticality; (3) Soft Landing ($HBr/O_2$ at low bias $V_s < 30\text{ V}$) approaches the gate dielectric with zero sputtering; and (4) Over-Etch ($HBr/O_2/He$) clears poly residues in trench corners without eroding the underlying $1.2\text{ nm}$ $SiO_2$ or $HfO_2$ high-k gate dielectric. **Compound semiconductor etching of $GaAs$, $GaN$, and $InP$ leverages temperature-tuned chlorine volatility kinetics.** In optoelectronic and RF device fabrication (VCSELs, micro-LEDs, power HMETs), chlorine plasma etching patterns III-V compound semiconductors. While gallium ($Ga$) forms highly volatile $GaCl_3$ ($T_{\text{boil}} = 201^\circ\text{C}$), indium ($In$) forms $InCl_3$, which exhibits very low volatility below $150^\circ\text{C}$ ($T_{\text{sublimation}} = 600^\circ\text{C}$). As a result, etching $InP$ in $Cl_2/Ar$ plasmas at room temperature causes indium enrichment and severe surface roughness ($R_a > 5\text{ nm}$). By heating the wafer chuck to $T = 150^\circ\text{C}$ to $200^\circ\text{C}$, $InCl_3$ desorption rate increases by $> 40\times$, restoring smooth stoichiometric etching ($R_a < 0.3\text{ nm}$) and high etch rates ($> 1.5\ \mu\text{m/min}$). | Metric / Parameter | Poly-Si Gate Etch | Al-Cu Metal Line Etch | TiN / TaN Barrier Etch | GaAs Photonic Etch | InP Mesa Etch (Elevated T) | |---|---|---|---|---|---| | Primary Gas Mixture | HBr / Cl2 / O2 / He | Cl2 / BCl3 / Ar | Cl2 / Ar / CH4 | Cl2 / Ar / BCl3 | Cl2 / Ar (T = 180°C) | | Operating Pressure | 4 mTorr to 12 mTorr | 8 mTorr to 15 mTorr | 3 mTorr to 8 mTorr | 2 mTorr to 5 mTorr | 3 mTorr to 6 mTorr | | Substrate DC Bias (Vs) | 25 V to 80 V | 120 V to 250 V | 60 V to 150 V | 50 V to 120 V | 80 V to 160 V | | Etch Rate (Vertical) | 320 nm/min | 650 nm/min | 180 nm/min | 1200 nm/min | 850 nm/min | | Selectivity to Stop | > 150:1 (vs SiO2) | > 25:1 (vs SiO2) | > 40:1 (vs SiO2) | > 30:1 (vs AlGaAs) | > 20:1 (vs InGaAsP) | | Primary By-Product | SiCl4 / SiBr4 | Al2Cl6 / CuCl2 | TiCl4 / TaCl5 | GaCl3 / AsCl3 | InCl3 / PCl3 | | Sidewall Passivation | SiOxBry / SiOxCly | BClx Polymer | C-H-Cl Polymer | GaOx / BClx | InClx Passivation | Read Chlorine-Based Etch through a *chemical volatility and surface passivity* lens rather than a *generic ion sputtering* lens. In advanced semiconductor manufacturing, chlorine chemistry is selected precisely because it combines high volatility for metals and semiconductors with near-zero spontaneous reaction rates on oxides and nitrides. Every critical performance metric in chlorine processing — from $BCl_3$ native oxide breakthrough and $HBr/Cl_2/O_2$ gate oxide selectivity to $180^\circ\text{C}$ $InP$ stage heating and in-situ anti-corrosion ash — represents the deliberate optimization of surface chemical kinetics over physical erosion. Master these volatile chloride formation rules and post-etch passivation requirements, and your process modeling will accurately predict CD bias, oxide selectivity, and corrosion-free yield across advanced logic, memory, and compound semiconductor devices. --- ## Plasma Gas Chemistry and Atomic Chlorine Dissociation Kinetics High-density ICP plasmas dissociate molecular $Cl_2$ into atomic radicals ($Cl^*$) and reactive ions ($Cl_2^+$, $Cl^+$), balancing gas phase dissociation with surface reaction rates. In high-density $Cl_2$ ICP discharges, low electron impact dissociation energy ($E_{\text{diss}} = 2.48\text{ eV}$) drives high atomic chlorine radical concentrations ($n_{\text{Cl}} \approx 2.5 \times 10^{13}\text{ cm}^{-3}$ at $10\text{ mTorr}$), providing abundant chemical reactants for isotropic and anisotropic metal/semiconductor etching. The steady-state atomic chlorine radical density $n_{\text{Cl}}$ in an ICP discharge is balanced between electron-impact dissociation of $Cl_2$ and wall recombination kinetics: $$k_{\text{diss}} n_e n_{\text{Cl}_2} = \frac{1}{4} n_{\text{Cl}} \bar{v}_{\text{Cl}} \left( \frac{A_{\text{wall}}}{V_{\text{chamber}}} \right) \gamma_{\text{rec}}$$ where $k_{\text{diss}} = 3.2 \times 10^{-9}\text{ cm}^3/\text{s}$ at $T_e = 3.5\text{ eV}$, $n_e = 4.0 \times 10^{11}\text{ cm}^{-3}$, $\bar{v}_{\text{Cl}} = 420\text{ m/s}$ at $400\text{ K}$, chamber volume-to-area ratio $V/A = 6.5\text{ cm}$, and wall recombination coefficient $\gamma_{\text{rec}} = 0.15$ on anodized aluminum. Solving for dissociation fraction $\alpha_{\text{diss}} = n_{\text{Cl}} / (2 n_{\text{Cl}_2,0})$ at $10\text{ mTorr}$ gas density ($n_0 = 2.4 \times 10^{14}\text{ cm}^{-3}$) yields $\alpha_{\text{diss}} \approx 35.4\%$, supplying an atomic chlorine flux $\Gamma_{\text{Cl}} = 2.6 \times 10^{17}\text{ radicals/(cm}^2\cdot\text{s)}$ to the wafer. --- ## Polysilicon and Metal Gate Selectivity Mechanics over Gate Oxides Chlorine-based $HBr/Cl_2/O_2$ chemistries achieve extreme selectivity ($> 150:1$) over ultra-thin gate oxides by forming protective silicon oxybromide passivants while maintaining low bias energies. Chlorine radicals etch polysilicon rapidly while leaving silicon dioxide unreacted. By capping ion bias energy at $V_s = 25\text{ V}$ ($E_i = 35\text{ eV} < E_{\text{sputter,SiO}_2} = 50\text{ eV}$), $poly-Si:SiO_2$ selectivity reaches $152:1$. The overall etch selectivity $S_{\text{poly/ox}}$ of polysilicon relative to $SiO_2$ under ion-assisted $HBr/Cl_2/O_2$ etching is formulated as: $$S_{\text{poly/ox}} = \frac{ER_{\text{poly}}}{ER_{\text{SiO}_2}} = \frac{Y_{\text{Si,Cl}} \Gamma_i + k_{\text{chem,Si}} \Gamma_{\text{Cl}}}{Y_{\text{SiO}_2,\text{Cl}} \Gamma_i + k_{\text{chem,SiO}_2} \Gamma_{\text{Cl}}}$$ Because spontaneous chemical reaction of $Cl$ with $SiO_2$ is negligible ($k_{\text{chem,SiO}_2} \approx 0$), $ER_{\text{SiO}_2}$ is dictated entirely by ion physical sputtering $Y_{\text{SiO}_2,\text{Cl}} \Gamma_i$. At low RF bias ($V_s = 25\text{ V}$, ion flux $\Gamma_i = 4.5 \times 10^{15}\text{ cm}^{-2}\text{s}^{-1}$), $Y_{\text{SiO}_2,\text{Cl}} \approx 0.008\text{ SiO}_2/\text{ion}$, yielding $ER_{\text{SiO}_2} = 2.1\text{ nm/min}$. With $ER_{\text{poly}} = 320\text{ nm/min}$, selectivity evaluates to $S_{\text{poly/ox}} = 320 / 2.1 = 152.4:1$, preventing gate oxide punch-through across a $100\%$ over-etch cycle. --- ## Aluminum Etching, Native Oxide Breakthrough, and BCl3 Scavenging Aluminum etching requires $BCl_3$ to scavenge native $Al_2O_3$ oxide before $Cl_2$ can react with bulk aluminum to form volatile $Al_2Cl_6$. Native $Al_2O_3$ ($3.0\text{ nm}$) resists $Cl_2$ attack. $BCl_3$ scavenging breaks through $Al_2O_3$ in $4.2\text{ s}$, enabling rapid bulk aluminum etching ($650\text{ nm/min}$) to form volatile $Al_2Cl_6$ ($P_{\text{vap}} = 1.2\text{ Torr}$ at $60^\circ\text{C}$). The thermodynamic driving force for $BCl_3$ scavenging of native $Al_2O_3$ is governed by the negative Gibbs free energy change of reaction ($\Delta G_{\text{rxn}}^\circ = -248.5\text{ kJ/mol}$): $$\frac{1}{3} Al_2O_3\text{ (s)} + \frac{2}{3} BCl_3\text{ (g)} \to \frac{2}{3} AlCl_3\text{ (g)} + \frac{1}{3} B_2O_3\text{ (s)}$$ The breakthrough time $t_{\text{breakthrough}}$ for a native oxide of thickness $t_{\text{ox}} = 3.0\text{ nm}$ under $BCl_3^+$ ion flux $\Gamma_{\text{BCl}_3^+} = 1.2 \times 10^{15}\text{ cm}^{-2}\text{s}^{-1}$ at bias voltage $V_s = 150\text{ V}$ is: $$t_{\text{breakthrough}} = \frac{\rho_{\text{Al}_2\text{O}_3} \cdot t_{\text{ox}}}{Y_{\text{scavenge}} \cdot \Gamma_{\text{BCl}_3^+}} = \frac{(2.35 \times 10^{22}\text{ molecules/cm}^3) \cdot (3.0 \times 10^{-7}\text{ cm})}{(1.4\text{ molecules/ion}) \cdot (1.2 \times 10^{15}\text{ cm}^{-2}\text{s}^{-1})} = 4.20\text{ seconds}$$ After $4.2\text{ s}$, the native oxide is completely breached, initiating steady-state bulk $Al$ etching. --- ## Post-Etch Chlorine Corrosion Mechanisms and In-Situ Passivation Atmospheric exposure of chlorine-etched metal lines causes $HCl$ acid formation and severe pitting corrosion, requiring integrated in-situ $H_2O/O_2$ plasma stripping. Residual $AlCl_3$ trapped on sidewalls reacts with moisture to form $HCl$, creating catalytic pitting loops. Integrated in-situ $H_2O/O_2$ plasma stripping at $250^\circ\text{C}$ extracts chlorine ($< 0.5\text{ at}\%$) and encapsulates metal lines in a protective $Al_2O_3$ shell. The catalytic cyclic reaction for atmospheric aluminum corrosion is driven by moisture hydrolysis of residual chlorine: $$\text{Step 1: } AlCl_3\text{ (residual)} + 3 H_2O\text{ (air)} \to Al(OH)_3\text{ (s)} + 3 HCl\text{ (aq)}$$ $$\text{Step 2: } 6 HCl\text{ (aq)} + 2 Al\text{ (metal)} \to 2 AlCl_3\text{ (aq)} + 3 H_2\text{ (g)} \uparrow$$ Because $AlCl_3$ is regenerated in Step 2, a single residual chlorine atom can catalyze the dissolution of over $10^4$ aluminum atoms. In-situ microwave $H_2O/O_2$ plasma ash at $250^\circ\text{C}$ extracts chlorine via $AlCl_3 + \text{O}^* / \text{OH}^* \to Al_2O_3 + 3 HCl \uparrow$, reducing surface chlorine concentration below the corrosion threshold ($[Cl] < 0.5\text{ atomic}\%$ by XPS) and extending corrosion-free ambient queue time beyond 168 hours. --- ## Compound Semiconductor (GaAs, InP) and Metal Interconnect Etching Etching III-V semiconductors and metal barriers requires temperature-tuned chlorine plasma kinetics to ensure stoichiometric byproduct volatility. $GaCl_3$ is volatile at room temperature ($T_{\text{boil}} = 201^\circ\text{C}$), permitting room-temperature $GaAs$ etching. $InCl_3$ vapor pressure is negligible below $150^\circ\text{C}$; heating the wafer chuck to $180^\circ\text{C}$ increases $InCl_3$ desorption rate by $> 40\times$, enabling smooth $InP$ mesa etching. The desorption rate $R_{\text{des}}$ of reaction products $InCl_3$ and $GaCl_3$ follows the Clausius-Clapeyron activation relationship: $$R_{\text{des}}(T) = v_0 \cdot \exp\left( -\frac{\Delta H_{\text{sub}}}{k_B T} \right)$$ where $\Delta H_{\text{sub}}(InCl_3) = 1.62\text{ eV}$ ($156.3\text{ kJ/mol}$) and $\Delta H_{\text{sub}}(GaCl_3) = 0.64\text{ eV}$ ($61.7\text{ kJ/mol}$). Heating an $InP$ wafer from $T_1 = 293\text{ K}$ ($20^\circ\text{C}$) to $T_2 = 453\text{ K}$ ($180^\circ\text{C}$) accelerates $InCl_3$ desorption by a factor of: $$\frac{R_{\text{des}}(453\text{ K})}{R_{\text{des}}(293\text{ K})} = \exp\left[ \frac{1.62\text{ eV}}{8.617 \times 10^{-5}\text{ eV/K}} \left( \frac{1}{293} - \frac{1}{453} \right) \right] = \exp(22.68) = 7.07 \times 10^9$$ This exponential increase in product volatility enables stoichiometric $InP$ etching without indium droplet accumulation or surface roughening. --- ## Metrology, Residual Gas Analysis (RGA), and Defect Qualification Chlorine etch process qualification combines inline XPS surface analysis, Residual Gas Analysis (RGA) mass spectrometry, and automated optical corrosion defect inspection. In residual gas analysis (RGA), optical emission spectroscopy (OES), X-ray photoelectron spectroscopy (XPS), and automated darkfield defect inspection at TSMC, Intel, Samsung, SK hynix, Micron, and IBM, chlorine processes modeled in Synopsys Sentaurus and Coventor SEMulator3D are qualified on KLA 29xx tools by verifying endpoint detection within $< 0.5\text{ s}$, surface chlorine residue $< 0.5\text{ atomic}\%$, and gate oxide breakdown field $E_{\text{BD}} > 14\text{ MV/cm}$. RGA mass spectrometry tracks volatile byproduct evolution during etching. For silicon etching in $Cl_2/Ar$, the dominant cracking fragment ion is $SiCl_3^+$ ($m/z = 133$). The partial pressure $P_{133}$ drops sharply at the polysilicon/oxide interface: $$\Delta P_{133}(t) = P_{133,0} \cdot \left[ 1 - \text{erf}\left( \frac{t - t_{\text{endpoint}}}{\tau_{\text{clearance}}} \right) \right]$$ Triggering the RF power shutoff or switching to the over-etch step when $P_{133}$ falls below $10\%$ of its main-etch baseline limits gate oxide exposure to energetic ions for $< 1.5\text{ s}$, preserving dielectric breakdown fields $E_{\text{BD}} > 14\text{ MV/cm}$.