Semiconductor Manufacturing Etch Process

Keywords: etch process, etching, dry etch, wet etch, plasma etch, RIE, reactive ion etch, etch selectivity, anisotropic etch

Semiconductor Manufacturing Etch Process

1. Overview

Etching is a critical pattern transfer process in semiconductor fabrication. After lithography defines a pattern using photoresist, etching selectively removes material to create transistors, interconnects, and other IC structures.

1.1 Fundamental Equation

The etch process can be characterized by the etch rate $R$:

$$ R = \frac{\Delta d}{\Delta t} \quad \text{[nm/min]} $$

where:

• $\Delta d$ = thickness removed (nm)
• $\Delta t$ = etch time (min)

2. Etch Categories

2.1 Wet Etching

Uses liquid chemicals to dissolve material isotropically.

• Characteristics:
• Isotropic (etches equally in all directions)
• High selectivity achievable
• Simple and low cost
• Batch processing capable

• Common Chemistries:
• $\text{SiO}_2$ etching: $\text{HF}$ (hydrofluoric acid)
• Si etching: $\text{HNO}_3 / \text{HF} / \text{CH}_3\text{COOH}$

• Etch Rate Model (for $\text{SiO}_2$ in HF):

$$ R_{\text{wet}} = A \cdot [\text{HF}]^n \cdot e^{-E_a / k_B T} $$

where:

• $A$ = pre-exponential factor
• $[\text{HF}]$ = HF concentration
• $n$ = reaction order
• $E_a$ = activation energy
• $k_B$ = Boltzmann constant ($1.38 \times 10^{-23}$ J/K)
• $T$ = temperature (K)

2.2 Dry Etching (Plasma Etching)

Uses plasma containing ions and reactive radicals for anisotropic etching.

• Sub-types:
• Physical Etching (Ion Milling)
• Chemical Plasma Etching
• Reactive Ion Etching (RIE)
• High-Density Plasma (ICP, ECR)
• Atomic Layer Etching (ALE)

3. Reactive Ion Etching (RIE)

3.1 Plasma Generation

RF power ionizes feed gas creating:

• Ions ($\text{Cl}^+$, $\text{F}^+$, $\text{Ar}^+$) → directional bombardment
• Radicals ($\text{Cl}^$, $\text{F}^$) → chemical reaction
• Electrons ($e^-$) → sustain plasma
• Neutrals → background species

3.2 Ion Energy

The ion energy at the wafer is determined by the plasma potential $V_p$ and DC bias $V_{dc}$:

$$ E_{\text{ion}} = e \cdot (V_p - V_{dc}) $$

where:

• $e$ = electron charge ($1.6 \times 10^{-19}$ C)
• $V_p$ = plasma potential (V)
• $V_{dc}$ = DC self-bias voltage (V)

3.3 Ion-Enhanced Etching Model

The synergistic etch rate combines physical and chemical components:

$$ R_{\text{total}} = R_{\text{chem}} + R_{\text{phys}} + R_{\text{synergy}} $$

where typically:

$$ R_{\text{synergy}} \gg R_{\text{chem}} + R_{\text{phys}} $$

This ion-radical synergy is the foundation of anisotropic plasma etching.

4. Key Performance Metrics

4.1 Selectivity

Definition: Ratio of etch rates between target material and mask/stop layer.

$$ S = \frac{R_{\text{target}}}{R_{\text{mask}}} $$

• Example Requirements:
• $\text{Si} : \text{SiO}_2$ selectivity $> 50:1$
• Photoresist selectivity $> 10:1$
• Etch stop selectivity $> 100:1$ (for thin films)

4.2 Anisotropy

Definition: Measure of directional etching preference.

$$ A = 1 - \frac{R_{\text{lateral}}}{R_{\text{vertical}}} $$

where:

• $A = 1$ → perfectly anisotropic (vertical only)
• $A = 0$ → perfectly isotropic
• $0 < A < 1$ → partially anisotropic

4.3 Uniformity

Within-Wafer Non-Uniformity (WIWNU):

$$ \text{WIWNU} = \frac{\sigma}{\bar{R}} \times 100\% $$

where:

• $\sigma$ = standard deviation of etch rate
• $\bar{R}$ = mean etch rate

Target: WIWNU $< 2\%$ for advanced nodes

4.4 Aspect Ratio

$$ AR = \frac{H}{W} $$

where:

• $H$ = feature depth/height
• $W$ = feature width

• Current Challenges:
• Logic contacts: AR $\approx 10:1$ to $20:1$
• 3D NAND channels: AR $> 60:1$ (trending toward $100:1$)
• DRAM capacitors: AR $> 50:1$

5. Etch Chemistry

5.1 Silicon Etching

• Primary Chemistries:
• $\text{Cl}_2 / \text{HBr}$ — high anisotropy
• $\text{SF}_6$ — high rate, more isotropic
• $\text{Cl}_2 / \text{HBr} / \text{O}_2$ — with sidewall passivation

• Reaction Mechanism (Chlorine-based):

$$ \text{Si}_{(s)} + 4\text{Cl}^* \rightarrow \text{SiCl}_{4(g)} \uparrow $$

5.2 Silicon Dioxide Etching

• Primary Chemistries:
• $\text{CF}_4$, $\text{C}_4\text{F}_8$, $\text{C}_4\text{F}_6$, $\text{CHF}_3$

• Reaction Mechanism:

$$ \text{SiO}_{2(s)} + \text{CF}_x^* \rightarrow \text{SiF}_{4(g)} + \text{CO}_{(g)} + \text{CO}_{2(g)} $$

• Selectivity Control: C/F ratio in plasma
• Higher C/F → more polymer → higher selectivity to Si
• Lower C/F → less polymer → faster oxide etch

5.3 Metal Etching

• Aluminum: $\text{Cl}_2 / \text{BCl}_3$ (BCl₃ scavenges H₂O and Al₂O₃)
• Tungsten: $\text{SF}_6$, $\text{NF}_3$
• Copper: Not plasma etchable (damascene process instead)

6. High-Density Plasma Sources

6.1 Inductively Coupled Plasma (ICP)

• Plasma Density: $n_e \approx 10^{11} - 10^{12}$ cm⁻³
• Advantages:
• Independent control of ion flux and ion energy
• Higher density than capacitive RIE
• Lower operating pressure (1-50 mTorr)

6.2 Power Relations

Ion Flux (proportional to plasma density):

$$ \Gamma_i = n_i \cdot v_{\text{Bohm}} = n_i \sqrt{\frac{k_B T_e}{m_i}} $$

where:

• $n_i$ = ion density
• $T_e$ = electron temperature
• $m_i$ = ion mass

Source Power controls plasma density:

$$ n_e \propto \sqrt{P_{\text{source}}} $$

Bias Power controls ion energy:

$$ E_{\text{ion}} \propto V_{\text{bias}} \propto \sqrt{P_{\text{bias}}} $$

7. Atomic Layer Etching (ALE)

7.1 Process Cycle

-
┌─────────────────────────────────────────────────────┐
│  Step 1: Surface Modification (Self-limiting)       │
│          Cl₂ adsorption → Si-Cl surface bonds       │
├─────────────────────────────────────────────────────┤
│  Step 2: Purge                                      │
│          Remove excess Cl₂                          │
├─────────────────────────────────────────────────────┤
│  Step 3: Removal (Self-limiting)                    │
│          Low-energy Ar⁺ bombardment                 │
│          E_ion < E_threshold(Si), > E_threshold(SiCl)│
├─────────────────────────────────────────────────────┤
│  Step 4: Purge                                      │
│          Remove SiClₓ products                      │
└─────────────────────────────────────────────────────┘
         ↓ Repeat ↓

7.2 Etch Per Cycle (EPC)

$$ \text{EPC} \approx 0.3 - 0.5 \text{ nm/cycle} \approx 1 \text{ monolayer} $$

7.3 Energy Window

For self-limiting removal, ion energy must satisfy:

$$ E_{\text{threshold}}^{\text{modified}} < E_{\text{ion}} < E_{\text{threshold}}^{\text{unmodified}} $$

• Example for Si ALE:
• $E_{\text{threshold}}(\text{Si-Cl}) \approx 12-15$ eV
• $E_{\text{threshold}}(\text{Si}) \approx 25-30$ eV
• Operating window: $15 < E_{\text{ion}} < 25$ eV

8. Etch Challenges at Advanced Nodes

8.1 High Aspect Ratio Etching (HARE)

• Ion Angular Distribution Broadening:

$$ \Delta\theta \propto \sqrt{\frac{T_i}{E_{\text{ion}}}} $$

where $T_i$ is ion temperature.

• Knudsen Transport Limitation:

$$ \Gamma_{\text{bottom}} = \Gamma_{\text{top}} \cdot \frac{W}{2H} = \frac{\Gamma_{\text{top}}}{2 \cdot AR} $$

8.2 Aspect Ratio Dependent Etching (ARDE)

Etch rate decreases with aspect ratio:

$$ R(AR) = R_0 \cdot f(AR) $$

where typically:

$$ f(AR) \approx \frac{1}{1 + \beta \cdot AR} $$

with $\beta$ being a process-dependent constant.

8.3 Line Edge Roughness (LER)

3σ LER Specification:

$$ \text{LER}_{3\sigma} < 0.1 \times \text{CD} $$

For 20 nm CD: LER $< 2$ nm (3σ)

9. Process Control

9.1 Endpoint Detection Methods

9.2 OES Endpoint Signal

For layer clearing:

$$ I_{\text{product}}(t) = I_0 \cdot e^{-t/\tau} \quad \text{(during clear)} $$

where $\tau$ is the decay time constant related to etch rate.

10. Key Equations Reference

Physical Constants

Common Etch Gases

• Silicon Etch: $\text{Cl}_2$, $\text{HBr}$, $\text{SF}_6$, $\text{NF}_3$
• Oxide Etch: $\text{CF}_4$, $\text{CHF}_3$, $\text{C}_4\text{F}_8$, $\text{C}_4\text{F}_6$
• Nitride Etch: $\text{CHF}_3$, $\text{CH}_2\text{F}_2$, $\text{CH}_3\text{F}$
• Metal Etch: $\text{Cl}_2$, $\text{BCl}_3$, $\text{SF}_6$
• Passivation: $\text{O}_2$, $\text{N}_2$, $\text{He}$
• Carrier/Dilution: $\text{Ar}$, $\text{He}$, $\text{N}_2$

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