Home Knowledge Base Semiconductor Manufacturing Process: Etch Modeling

Semiconductor Manufacturing Process: Etch Modeling

1. Introduction

Etch modeling is one of the most complex and critical areas in semiconductor fabrication simulation. As device geometries shrink below $10\ \text{nm}$ and structures become increasingly three-dimensional, accurate prediction of etch behavior becomes essential for:

2. Fundamentals of Etching

2.1 What is Etching?

Etching selectively removes material from a wafer to transfer lithographically defined patterns into underlying layers—silicon, oxides, nitrides, metals, or complex stacks.

2.2 Types of Etching

$$ R = A \exp\left(-\frac{E_a}{k_B T}\right)$$

where:

2.3 Plasma Etching Mechanisms

1. Physical Sputtering

$$ Y(E_i) = A \left( \sqrt{E_i} - \sqrt{E_{th}} \right)$$

where $E_{th}$ is the threshold energy

2. Chemical Etching

$$ \text{Si} + 4\text{F} \rightarrow \text{SiF}_4 \uparrow$$

3. Ion-Enhanced Etching

$$ \eta = \frac{Y_{ion+chem}}{Y_{ion} + Y_{chem}}$$

3. Hierarchy of Etch Models

3.1 Empirical Models

Data-driven, fast, used in production:

$$ CD_{final} = CD_{litho} + \Delta_{etch}$$
$$ \Delta_{etch} = f(\text{pitch}, \text{density}, \text{orientation})$$
$$ \Delta(x,y) = \iint K(x-x', y-y') \cdot I(x', y') \, dx' dy'$$

3.2 Feature-Scale Models

Semi-empirical, balance speed and physics:

$$ \frac{d\vec{r}_i}{dt} = R(\theta_i, \Gamma_{ion}, \Gamma_{n}) \cdot \hat{n}_i$$
$$ \frac{\partial \phi}{\partial t} + R(\vec{x}) |\nabla \phi| = 0$$
$$ P_{remove} = 1 - \exp\left( -\sum_j \sigma_j \Gamma_j \Delta t \right)$$

3.3 Physics-Based Plasma Models

Capture reactor-scale phenomena:

$$ \frac{\partial f}{\partial t} + \vec{v} \cdot \nabla f + \frac{q\vec{E}}{m} \cdot \nabla_v f = \left( \frac{\partial f}{\partial t} \right)_{coll}$$
$$ J_{ion} = \frac{4\epsilon_0}{9} \sqrt{\frac{2e}{M}} \frac{V^{3/2}}{d^2}$$
$$ \frac{\partial n_i}{\partial t} + \nabla \cdot (n_i \vec{v}_i) = S_i - L_i$$

3.4 Atomistic Models

Fundamental understanding, computationally expensive:

$$ m_i \frac{d^2 \vec{r}_i}{dt^2} = -\nabla_i U(\{\vec{r}\})$$
$$ P = \min\left(1, \exp\left(-\frac{\Delta E}{k_B T}\right)\right)$$
$$ k_i = u_0 \exp\left(-\frac{E_{a,i}}{k_B T}\right)$$

4. Key Physical Phenomena

4.1 Anisotropy

Ratio of vertical to lateral etch rate:

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

Mechanisms for achieving anisotropy:

$$ f(\theta) \propto \cos^n(\theta)$$

where higher $n$ indicates more directional flux

4.2 Selectivity

Ratio of etch rates between materials:

$$S_{A/B} = \frac{R_A}{R_B}$$

Example selectivities required:

ProcessSelectivity Required
Oxide/Nitride$> 20:1$
Poly-Si/Oxide$> 50:1$
Si/SiGe (channel release)$> 100:1$

4.3 Loading Effects

Microloading

Local depletion of reactive species in dense pattern regions:

$$R_{dense} = R_0 \cdot \frac{1}{1 + \beta \cdot \rho_{local}}$$

where:

Macroloading

Wafer-scale depletion:

$$R = R_0 \cdot \left(1 - \alpha \cdot A_{exposed}\right)$$

where $A_{exposed}$ is total exposed area fraction

4.4 Aspect Ratio Dependent Etching (ARDE)

Deep, narrow features etch slower due to transport limitations:

$$R(AR) = R_0 \cdot \exp\left(-\frac{AR}{AR_0}\right)$$

where $AR = \text{depth}/\text{width}$

Physical mechanisms:

1. Ion Shadowing

$$ \theta_{shadow} = \arctan\left(\frac{1}{AR}\right)$$

2. Neutral Transport

$$ D_K = \frac{d}{3} \sqrt{\frac{8 k_B T}{\pi m}}$$

3. Byproduct Redeposition

4.5 Profile Anomalies

PhenomenonDescriptionCause
BowingLateral bulge in sidewallIon scattering off sidewalls
NotchingLateral etching at interfaceCharge buildup on insulators
MicrotrenchingDeep spots at cornersIon reflection at feature bottom
FootingUndercut at bottomIsotropic chemical component
TaperingNon-vertical sidewallsInsufficient passivation

5. Mathematical Foundations

5.1 Surface Evolution Equation

General form for surface height $h(x,y,t)$:

$$\frac{\partial h}{\partial t} = -R_0 \cdot V(\theta) \cdot \sqrt{1 + |\nabla h|^2}$$

where:

5.2 Ion Angular Distribution

At wafer surface, ion flux angular distribution:

$$\Gamma(\theta, \phi) = \Gamma_0 \cdot f(\theta) \cdot g(E)$$

Common models:

$$ f(\theta) = \frac{1}{\sqrt{2\pi}\sigma_\theta} \exp\left(-\frac{\theta^2}{2\sigma_\theta^2}\right)$$
$$ f(E) \propto \frac{E}{(E + E_b)^3}$$

5.3 Visibility Calculation

For a point on the surface, visibility to incoming flux:

$$V(\vec{r}) = \frac{1}{2\pi} \int_0^{2\pi} \int_0^{\theta_{max}(\phi)} f(\theta) \sin\theta \cos\theta \, d\theta \, d\phi$$

where $\theta_{max}(\phi)$ is determined by local geometry (shadowing)

5.4 Surface Reaction Kinetics

Langmuir-Hinshelwood mechanism:

$$R = k \cdot \theta_A \cdot \theta_B$$

where surface coverages follow:

$$\frac{d\theta_i}{dt} = s_i \Gamma_i (1 - \theta_{total}) - k_d \theta_i - k_r \theta_i$$

5.5 Plasma-Surface Interaction Yield

Ion-enhanced etch yield:

$$Y_{etch} = Y_0 + Y_1 \cdot \sqrt{E_{ion} - E_{th}} + Y_{chem} \cdot \frac{\Gamma_n}{\Gamma_{ion}}$$

where:

6. Modern Modeling Approaches

6.1 Hybrid Multi-Scale Frameworks

Coupling different scales:

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  <text x="380" y="48" fill="#8b98a5" font-size="12" text-anchor="middle">reactive ions + radicals selectively remove material through a patterned mask — the subtractive half of lithography</text>

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  <text x="360" y="168" fill="#f87171" font-size="7">ions ↓</text>

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  <text x="55" y="152" fill="#6b7684" font-size="7">source →</text>
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  <!-- Key metrics -->
  <text x="490" y="185" fill="#8b98a5" font-size="8">CD control: ±1 nm (EUV node)</text>
  <text x="490" y="199" fill="#8b98a5" font-size="8">Selectivity: &gt; 10:1 (etch:mask)</text>
  <text x="490" y="213" fill="#8b98a5" font-size="8">Aspect ratio: &gt; 60:1 (3D NAND)</text>

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  <text x="45" y="263" fill="#60a5fa" font-size="8.5" font-weight="600">CCP RIE:</text>
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  <text x="45" y="295" fill="#fbbf24" font-size="8.5" font-weight="600">ALE:</text>
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  <text x="45" y="325" fill="#6b7684" font-size="8">Wet etch: isotropic, used for cleans and sacrificial release</text>

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  <text x="410" y="325" fill="#6b7684" font-size="8">Equipment: Lam Research, TEL, Applied Materials</text>

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  <text x="100" y="380" fill="#60a5fa" font-size="9" text-anchor="middle" font-weight="600">Fin/NS etch</text>
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  <!-- Key insight -->
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  <text x="380" y="427" fill="#fbbf24" font-size="9.5" text-anchor="middle">Etch is the pattern transfer engine: lithography defines the pattern, etch carves it into silicon — at atomic precision.</text>

  <text x="380" y="460" fill="#6b7684" font-size="11" text-anchor="middle">Modern chips have 60+ etch steps — each one must hold sub-nanometer uniformity across a 300mm wafer.</text>
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6.2 Machine Learning Integration

$$ \hat{y} = f_{NN}(\vec{x}; \vec{w})$$
$$ \mathcal{L} = \frac{1}{N} \sum_{i=1}^{N} \|y_i - \hat{y}_i\|^2 + \lambda \|\vec{w}\|^2$$
$$ \mathcal{L}_{total} = \mathcal{L}_{data} + \alpha \mathcal{L}_{physics}$$
$$ CD_{predicted} = g(P, T, V_{bias}, \text{OES}, ...)$$

6.3 Computational Lithography Integration

Major EDA tools couple lithography + etch:

1. Litho simulation → Resist profile $h_R(x,y)$ 2. Etch simulation → Final pattern $h_F(x,y)$ 3. Combined model: $$ CD_{final} = CD_{design} + \Delta_{OPC} + \Delta_{litho} + \Delta_{etch} $$

7. Challenges at Advanced Nodes

7.1 FinFET / Gate-All-Around (GAA)

$$ R_{SiGe} \gg R_{Si}$$

7.2 3D NAND

Extreme aspect ratio challenges:

GenerationLayersAspect Ratio
96L96~60:1
128L128~80:1
176L176~100:1
232L+232+~150:1

Critical issues:

7.3 EUV Patterning

$$ LER_{final} = \sqrt{LER_{litho}^2 + LER_{etch}^2}$$

7.4 Stochastic Effects

At small dimensions, statistical fluctuations dominate:

$$\sigma_{CD} \propto \frac{1}{\sqrt{N_{events}}}$$

where $N_{events}$ = number of etching events per feature

8. Industry Tools

8.1 Commercial Software

CategoryTools
TCAD/ProcessSynopsys Sentaurus Process, Silvaco Victory Process
Virtual FabCoventor SEMulator3D
Equipment VendorLam Research, Applied Materials (proprietary)
Computational LithoSynopsys S-Litho, Siemens Calibre

8.2 Research Tools

9. Future Directions

9.1 Digital Twins

Real-time chamber models for closed-loop process control:

$$\vec{u}_{control}(t) = \mathcal{K} \left[ y_{target} - y_{model}(t) \right]$$

9.2 Atomistic-Continuum Coupling

Seamless multi-scale simulation using:

9.3 New Materials

Modeling requirements for:

9.4 Uncertainty Quantification

Predicting distributions, not just means:

$$P(CD) = \int P(CD | \vec{\theta}) P(\vec{\theta}) d\vec{\theta}$$

Key metrics:

Summary

Etch modeling spans from atomic-scale surface reactions to reactor-scale plasma physics to fab-level empirical correlations. The art lies in choosing the right abstraction level:

ApplicationModel TypeSpeedAccuracy
Production OPC/EPCEmpirical/ML★★★★★★★☆☆☆
Process DevelopmentFeature-scale★★★☆☆★★★★☆
Mechanism ResearchAtomistic MD/MC★☆☆☆☆★★★★★
Equipment DesignPlasma + Feature★★☆☆☆★★★★☆

As geometries shrink and structures become more 3D, accurate etch modeling becomes essential for first-time-right process development and continued yield improvement.

etch modelingplasma etchRIEreactive ion etchingetch simulationDRIE

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