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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:

2. Etch Categories

2.1 Wet Etching

Uses liquid chemicals to dissolve material isotropically.

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

where:

2.2 Dry Etching (Plasma Etching)

Uses plasma containing ions and reactive radicals for anisotropic etching.

3. Reactive Ion Etching (RIE)

3.1 Plasma Generation

RF power ionizes feed gas creating:

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:

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.

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

4.2 Anisotropy

Definition: Measure of directional etching preference.

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

where:

4.3 Uniformity

Within-Wafer Non-Uniformity (WIWNU):

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

where:

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

4.4 Aspect Ratio

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

where:

5. Etch Chemistry

5.1 Silicon Etching

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

5.2 Silicon Dioxide Etching

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

5.3 Metal Etching

6. High-Density Plasma Sources

6.1 Inductively Coupled Plasma (ICP)

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:

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:

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

8. Etch Challenges at Advanced Nodes

8.1 High Aspect Ratio Etching (HARE)

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

where $T_i$ is ion temperature.

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

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

MethodPrincipleApplication
OESOptical Emission SpectroscopyMonitor plasma species
InterferometryLaser reflection interferenceReal-time thickness
RGAResidual Gas AnalysisEtch product detection
Bias MonitoringDC bias changeLayer transition

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

ParameterEquationUnits
Etch Rate$R = \Delta d / \Delta t$nm/min
Selectivity$S = R_{\text{target}} / R_{\text{mask}}$ratio
Anisotropy$A = 1 - R_{\text{lat}} / R_{\text{vert}}$0-1
Aspect Ratio$AR = H / W$ratio
Ion Energy$E = e(V_p - V_{dc})$eV
Uniformity$\text{WIWNU} = \sigma / \bar{R} \times 100\%$%
Ion Flux$\Gamma_i = n_i \sqrt{k_B T_e / m_i}$cm⁻²s⁻¹

Physical Constants

ConstantSymbolValue
Electron charge$e$$1.602 \times 10^{-19}$ C
Boltzmann constant$k_B$$1.381 \times 10^{-23}$ J/K
Electron mass$m_e$$9.109 \times 10^{-31}$ kg
Avogadro's number$N_A$$6.022 \times 10^{23}$ mol⁻¹

Common Etch Gases

Source: ChipFoundryServices — Search this topic — Ask CFSGPT

etch processetchingdry etchwet etchplasma etchRIEreactive ion etchetch selectivityanisotropic etch

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