Material Selectivity & Process Optimization
The reactivity of halogen radicals follows the electronegativity trend:
Fluorine radicals are the most reactive, forming strong Si-F bonds (565 kJ/mol) compared to Si-Cl (381 kJ/mol) and Si-Br (310 kJ/mol). This high reactivity makes fluorine-based chemistries aggressive but less selective. The etch rate follows: R_Si = k × [X*]^a × √(E_ion), where X is the halogen radical, a is the reaction order (typically 0.5-1.5), and E_ion is ion energy. Fluorine chemistries exhibit spontaneous (purely chemical) etching, while Cl and Br require ion assistance.
CF₄ (tetrafluoromethane) is a workhorse chemistry for silicon and oxide etching. Upon plasma dissociation:
The F/C ratio is ~4, leading to fluorine-rich plasma. Silicon etching proceeds via:
SiF₄ is highly volatile (b.p. -86°C), enabling efficient removal. Si etch rate: 200-400 nm/min at moderate power. SiO₂ etch rate: 50-150 nm/min. Si:SiO₂ selectivity: 2-5:1. CF₄ is non-toxic but a potent greenhouse gas (GWP = 7,390), requiring abatement. Cost: $5-10 per wafer.
SF₆ (sulfur hexafluoride) provides extremely high fluorine density upon dissociation:
F/C ratio = ∞ (no carbon), yielding 6 F atoms per molecule. This creates very high Si etch rates (500-1000 nm/min), excellent for deep silicon etching (DRIE). However, SiO₂ etching is also fast (150-300 nm/min), giving poor selectivity (3-5:1). No polymer formation due to lack of carbon. SF₆ is non-flammable but highly electronegative. GWP = 23,500 (extreme). Cost: $8-15 per wafer.
Cl₂ (chlorine gas) is widely used for polysilicon and silicon etching:
Unlike fluorine, chlorine requires ion bombardment to overcome surface activation barrier. Spontaneous etching is negligible at room temperature. Si etch rate: 150-300 nm/min (ion-assisted). SiO₂ etch rate: < 5 nm/min (excellent selectivity > 50:1). Si₃N₄ etch rate: 10-30 nm/min. Cl₂ etching is highly anisotropic due to ion dependence. Safety concern: Cl₂ is toxic (TLV = 0.5 ppm). Cost: $3-6 per wafer.
HBr (hydrogen bromide) offers superior selectivity through polymer-assisted etching:
The key advantage is polymer formation on SiO₂ surfaces via H* and Br* recombination, passivating oxide while allowing silicon etching. Si etch rate: 100-250 nm/min. SiO₂ etch rate: 1-5 nm/min. Si:SiO₂ selectivity: 50-200:1 (excellent). Brominated polymers (HBr_x) protect oxide sidewalls. Lower reactivity than Cl₂ reduces damage. Anisotropy excellent. Safety: Corrosive, TLV = 3 ppm. Cost: $6-12 per wafer.
BCl₃ (boron trichloride) is specialized for metal etching, particularly aluminum:
AlCl₃ forms volatile products at elevated temperature (>150°C). Al etch rate: 200-400 nm/min at 250°C. Tungsten etch rate: 50-150 nm/min. BCl₃ also etches silicon (Si rate: 100-200 nm/min) with good anisotropy. Often mixed with Cl₂ for enhanced metal etch. BCl₃ provides boron doping of sidewalls, reducing corrosion. Safety: Highly reactive with moisture. Cost: $10-18 per wafer.
CHF₃ (fluoroform) is a polymer-forming chemistry with low F/C ratio:
F/C ratio = 3 (lower than CF₄), promoting CF₂ and CF polymer deposition. SiO₂ etch rate: 100-250 nm/min. Si etch rate: 20-50 nm/min. SiO₂:Si selectivity: 5-20:1 (reverse selectivity). Polymer thickness: 10-30 nm on sidewalls. Excellent for oxide contact etching where silicon must be protected. Anisotropy very high due to sidewall passivation. Lower GWP than CF₄ (GWP = 14,800). Cost: $7-13 per wafer.
Etch rates depend on chemistry, ion energy, and surface temperature:
where k values are chemistry-dependent rate constants, E_ion is ion energy (50-1000 eV), E_th is threshold energy for oxide etching (~20-50 eV), E_a is activation energy (0.1-0.5 eV), and T is temperature (K). Fluorine radicals dominate Si and SiO₂ etching, while chlorine/bromine provide selectivity through differential reactivity.
Selectivity S_ij is the ratio of etch rates:
Key selectivities for process design:
Selectivity is achieved through: (1) chemical reactivity differences, (2) polymer passivation, (3) ion energy threshold differences, and (4) temperature activation energy differences.
Fluorocarbon polymer deposition competes with etching:
Low F/C (< 2): Net deposition, polymerization dominates. Medium F/C (2-3): Balanced etch/dep, sidewall passivation. High F/C (> 3): Net etch, minimal polymer. CHF₃ (F/C=3) and C₄F₈ (F/C=2) are polymerizing. CF₄ (F/C=4) and SF₆ (F/C=∞) are etching. Polymer composition: (CF₂)_n with CF₃ termination. Helps anisotropy by passivating vertical sidewalls while horizontal surfaces are cleared by ion bombardment.
Gas cost per liter (approximate):
Total cost includes: gas consumption, abatement (30-50% added cost for PFCs), chamber cleaning (polymer removal), and maintenance (corrosive gas handling). Chlorine-based chemistries are most economical but require corrosion-resistant chambers.
Pressure affects mean free path and ion energy distribution:
At low pressure (1-10 mTorr): Long λ_mfp (>1 cm), high ion energy, low radical density, anisotropic. At medium pressure (10-50 mTorr): Balanced ion/radical ratio, moderate anisotropy. At high pressure (50-200 mTorr): Short λ_mfp (<1 mm), low ion energy, high radical density, more isotropic. Etch rate typically peaks at 20-50 mTorr where ion flux and radical density are both sufficient.
Etch rate temperature dependence follows Arrhenius equation:
Where A is pre-exponential factor, E_a is activation energy, k is Boltzmann constant, T is absolute temperature. Activation energies: Si with F* (0.05-0.15 eV, weak dependence), SiO₂ with CF_x (0.2-0.4 eV, moderate), Si with Cl* (0.3-0.6 eV, strong - ion-assisted). Temperature control within ±2°C is critical for uniformity. Higher temperature increases etch rate but may reduce selectivity and anisotropy.
Optimal chemistry selection depends on application:
Multi-gas mixtures allow tuning of F/C ratio, ion/radical balance, and selectivity. Real-time monitoring (OES, interferometry) enables endpoint detection and process control.