Process Monitoring

Semiconductor Manufacturing Process Parameters Monitoring: Mathematical Modeling

1. The Fundamental Challenge

Modern semiconductor fabrication involves 500–1000+ sequential process steps, each with dozens of parameters requiring nanometer-scale precision.

Key Process Types and Parameters

Lithography: exposure dose, focus, overlay alignment, resist thickness
Etching (dry/wet): etch rate, selectivity, uniformity, plasma parameters (power, pressure, gas flows)
Deposition (CVD, PVD, ALD): deposition rate, film thickness, uniformity, stress, composition
CMP (Chemical Mechanical Polishing): removal rate, within-wafer non-uniformity, dishing, erosion
Implantation: dose, energy, angle, uniformity
Thermal processes: temperature uniformity, ramp rates, time

2. Statistical Process Control (SPC) — The Foundation

2.1 Univariate Control Charts

For a process parameter $$X$$ with samples $x_1, x_2, \ldots, x_n$ :

Sample Mean:

\bar{x} = \frac{1}{n}\sum_{i=1}^{n} x_i

Sample Standard Deviation:

\sigma = \sqrt{\frac{1}{n-1}\sum_{i=1}^{n}(x_i - \bar{x})^2}

Control Limits (3-sigma):

\text{UCL} = \bar{x} + 3\sigma

\text{LCL} = \bar{x} - 3\sigma

2.2 Process Capability Indices

These quantify how well a process meets specifications:

$$C_p$$ (Potential Capability):

C_p = \frac{USL - LSL}{6\sigma}

$C_{pk}$ (Actual Capability) — accounts for centering:

C_{pk} = \min\left[\frac{USL - \mu}{3\sigma}, \frac{\mu - LSL}{3\sigma}\right]

$C_{pm}$ (Taguchi Index) — penalizes deviation from target $$T$$ :

C_{pm} = \frac{C_p}{\sqrt{1 + \left(\frac{\mu - T}{\sigma}\right)^2}}

Semiconductor fabs typically require $C_{pk} \geq 1.67$ , corresponding to defect rates below ~1 ppm.

3. Multivariate Statistical Monitoring

Since process parameters are highly correlated, univariate methods miss interaction effects.

3.1 Principal Component Analysis (PCA)

Given data matrix $\mathbf{X}$ ( $$n$$ samples × $$p$$ variables), centered:

1. Compute covariance matrix:

\mathbf{S} = \frac{1}{n-1}\mathbf{X}^T\mathbf{X}

2. Eigendecomposition:

\mathbf{S} = \mathbf{V}\mathbf{\Lambda}\mathbf{V}^T

3. Project to principal components:

\mathbf{T} = \mathbf{X}\mathbf{V}

3.2 Monitoring Statistics

Hotelling's $$T^2$$ Statistic

Captures variation within the PCA model:

T^2 = \sum_{i=1}^{k} \frac{t_i^2}{\lambda_i}

where $$k$$ is the number of retained components. Under normal operation, $$T^2$$ follows a scaled F-distribution.

Q-Statistic (Squared Prediction Error)

Captures variation outside the model:

Q = \sum_{j=1}^{p}(x_j - \hat{x}_j)^2 = \|\mathbf{x} - \mathbf{x}\mathbf{V}_k\mathbf{V}_k^T\|^2

> Often more sensitive to novel faults than $$T^2$$ .

3.3 Partial Least Squares (PLS)

When relating process inputs $\mathbf{X}$ to quality outputs $\mathbf{Y}$ :

\mathbf{Y} = \mathbf{X}\mathbf{B} + \mathbf{E}

PLS finds latent variables that maximize covariance between $\mathbf{X}$ and $\mathbf{Y}$ , providing both monitoring capability and a predictive model.

4. Virtual Metrology (VM) Models

Virtual metrology predicts physical measurement outcomes from process sensor data, enabling 100% wafer coverage without costly measurements.

4.1 Linear Models

For process parameters $\mathbf{x} \in \mathbb{R}^p$ and metrology target $$y$$ :

Ordinary Least Squares (OLS):

\hat{\boldsymbol{\beta}} = (\mathbf{X}^T\mathbf{X})^{-1}\mathbf{X}^T\mathbf{y}

Ridge Regression ( $$L_2$$ regularization for collinearity):

\hat{\boldsymbol{\beta}} = (\mathbf{X}^T\mathbf{X} + \lambda\mathbf{I})^{-1}\mathbf{X}^T\mathbf{y}

LASSO ( $$L_1$$ regularization for sparsity/feature selection):

\min_{\boldsymbol{\beta}} \|\mathbf{y} - \mathbf{X}\boldsymbol{\beta}\|^2 + \lambda\|\boldsymbol{\beta}\|_1

4.2 Nonlinear Models

Gaussian Process Regression (GPR)

y \sim \mathcal{GP}(m(\mathbf{x}), k(\mathbf{x}, \mathbf{x}'))

Posterior predictive distribution:

Mean:

\mu_* = \mathbf{K}_*^T(\mathbf{K} + \sigma_n^2\mathbf{I})^{-1}\mathbf{y}

Variance:

\sigma_*^2 = K_{**} - \mathbf{K}_*^T(\mathbf{K} + \sigma_n^2\mathbf{I})^{-1}\mathbf{K}_*

GPs provide uncertainty quantification — critical for knowing when to trigger actual metrology.

Support Vector Regression (SVR)

\min \frac{1}{2}\|\mathbf{w}\|^2 + C\sum_i(\xi_i + \xi_i^*)

Subject to $\epsilon$ -insensitive tube constraints. Kernel trick enables nonlinear modeling.

Neural Networks

MLPs: Multi-layer perceptrons for general function approximation
CNNs: Convolutional neural networks for wafer map pattern recognition
LSTMs: Long Short-Term Memory networks for time-series FDC traces

5. Run-to-Run (R2R) Control

R2R control adjusts recipe setpoints between wafers/lots to compensate for drift and disturbances.

5.1 EWMA Controller

For a process with model $y = a_0 + a_1 u + \epsilon$ :

Prediction update:

\hat{y}_{k+1} = \lambda y_k + (1-\lambda)\hat{y}_k

Control action:

u_{k+1} = \frac{T - \hat{y}_{k+1} + a_0}{a_1}

where:
- $$T$$ is the target
- $\lambda \in (0,1)$ is the smoothing weight

5.2 Double EWMA (for Linear Drift)

When process drifts linearly:

\hat{y}_{k+1} = a_k + b_k

a_k = \lambda y_k + (1-\lambda)(a_{k-1} + b_{k-1})

b_k = \gamma(a_k - a_{k-1}) + (1-\gamma)b_{k-1}

5.3 State-Space Formulation

More general framework:

State equation:

\mathbf{x}_{k+1} = \mathbf{A}\mathbf{x}_k + \mathbf{B}\mathbf{u}_k + \mathbf{w}_k

Observation equation:

\mathbf{y}_k = \mathbf{C}\mathbf{x}_k + \mathbf{D}\mathbf{u}_k + \mathbf{v}_k

Use Kalman filtering for state estimation and LQR/MPC for optimal control.

5.4 Model Predictive Control (MPC)

Objective function:

\min \sum_{i=1}^{N} \|\mathbf{y}_{k+i} - \mathbf{r}_{k+i}\|_\mathbf{Q}^2 + \sum_{j=0}^{N-1}\|\Delta\mathbf{u}_{k+j}\|_\mathbf{R}^2

subject to process model and operational constraints.

> MPC handles multivariable systems with constraints naturally.

6. Fault Detection and Classification (FDC)

6.1 Detection Methods

Mahalanobis Distance

D^2 = (\mathbf{x} - \boldsymbol{\mu})^T\mathbf{S}^{-1}(\mathbf{x} - \boldsymbol{\mu})

Follows $\chi^2$ distribution under multivariate normality.

Other Detection Methods

One-Class SVM: Learn boundary of normal operation
Autoencoders: Detect anomalies via reconstruction error

6.2 Classification Features

For trace data (time-series from sensors), extract features:

Statistical moments: mean, variance, skewness, kurtosis
Frequency domain: FFT coefficients, spectral power
Wavelet coefficients: Multi-resolution analysis
DTW distances: Dynamic Time Warping to reference signatures

6.3 Classification Algorithms

Support Vector Machines (SVM)
Random Forest
CNNs for pattern recognition on wafer maps
Gradient Boosting (XGBoost, LightGBM)

7. Spatial Modeling (Within-Wafer Variation)

Systematic spatial patterns require explicit modeling.

7.1 Polynomial Basis Expansion

Zernike Polynomials (common in lithography)

z(\rho, \theta) = \sum_{n,m} Z_n^m(\rho, \theta)

These form an orthogonal basis on the unit disk, capturing radial and azimuthal variation.

7.2 Gaussian Process Spatial Models

y(\mathbf{s}) \sim \mathcal{GP}(\mu(\mathbf{s}), k(\mathbf{s}, \mathbf{s}'))

Common Covariance Kernels

Squared Exponential (RBF):

k(\mathbf{s}, \mathbf{s}') = \sigma^2 \exp\left(-\frac{\|\mathbf{s} - \mathbf{s}'\|^2}{2\ell^2}\right)

Matérn (more flexible smoothness):

k(r) = \sigma^2 \frac{2^{1- u}}{\Gamma( u)}\left(\frac{\sqrt{2 u}r}{\ell}\right)^ u K_ u\left(\frac{\sqrt{2 u}r}{\ell}\right)

where $K_
u$ is the modified Bessel function of the second kind.

8. Dynamic/Time-Series Modeling

For plasma processes, endpoint detection, and transient behavior.

8.1 Autoregressive Models

AR(p) model:

x_t = \sum_{i=1}^{p} \phi_i x_{t-i} + \epsilon_t

ARIMA extends this to non-stationary series.

8.2 Dynamic PCA

Augment data with time-lagged values:

\tilde{\mathbf{X}} = [\mathbf{X}(t), \mathbf{X}(t-1), \ldots, \mathbf{X}(t-l)]

Then apply standard PCA to capture temporal dynamics.

8.3 Deep Sequence Models

LSTM Networks

Gating mechanisms:

Forget gate: $f_t = \sigma(W_f \cdot [h_{t-1}, x_t] + b_f)$
Input gate: $i_t = \sigma(W_i \cdot [h_{t-1}, x_t] + b_i)$
Output gate: $o_t = \sigma(W_o \cdot [h_{t-1}, x_t] + b_o)$

Cell state update:

c_t = f_t \odot c_{t-1} + i_t \odot \tilde{c}_t

Hidden state:

h_t = o_t \odot \tanh(c_t)

9. Model Maintenance and Adaptation

Semiconductor processes drift — models must adapt.

9.1 Drift Detection Methods

CUSUM (Cumulative Sum)

S_k = \max(0, S_{k-1} + (x_k - \mu_0) - k)

Signal when $$S_k$$ exceeds threshold.

Page-Hinkley Test

m_k = \sum_{i=1}^{k}(x_i - \bar{x}_k - \delta)

M_k = \max_{i \leq k} m_i

Alarm when $M_k - m_k > \lambda$ .

ADWIN (Adaptive Windowing)

Automatically detects distribution changes and adjusts window size.

9.2 Online Model Updating

Recursive Least Squares (RLS)

\hat{\boldsymbol{\beta}}_k = \hat{\boldsymbol{\beta}}_{k-1} + \mathbf{K}_k(y_k - \mathbf{x}_k^T\hat{\boldsymbol{\beta}}_{k-1})

where $\mathbf{K}_k$ is the gain matrix updated via the Riccati equation:

\mathbf{K}_k = \frac{\mathbf{P}_{k-1}\mathbf{x}_k}{\lambda + \mathbf{x}_k^T\mathbf{P}_{k-1}\mathbf{x}_k}

\mathbf{P}_k = \frac{1}{\lambda}(\mathbf{P}_{k-1} - \mathbf{K}_k\mathbf{x}_k^T\mathbf{P}_{k-1})

Just-in-Time (JIT) Learning

Build local models around each new prediction point using nearest historical samples.

10. Integrated Framework

A complete monitoring system layers these methods:

Layer	Methods	Purpose
Preprocessing	Cleaning, synchronization, normalization	Data quality
Feature Engineering	Domain features, wavelets, PCA	Dimensionality management
Monitoring	$$T^2$$ , Q-statistic, control charts	Detect out-of-control states
Virtual Metrology	PLS, GPR, neural networks	Predict quality without measurement
FDC	Classification models	Diagnose fault root causes
Control	R2R, MPC	Compensate for drift/disturbances
Adaptation	Online learning, drift detection	Maintain model validity

11. Key Mathematical Challenges

1. High dimensionality — hundreds of sensors, requiring regularization and dimension reduction
2. Collinearity — process variables are physically coupled
3. Non-stationarity — drift, maintenance events, recipe changes
4. Small sample sizes — new recipes have limited historical data (transfer learning, Bayesian methods help)
5. Real-time constraints — decisions needed in seconds
6. Rare events — faults are infrequent, creating class imbalance

12. Key Equations

Process Capability

C_{pk} = \min\left[\frac{USL - \mu}{3\sigma}, \frac{\mu - LSL}{3\sigma}\right]

Multivariate Monitoring

T^2 = \sum_{i=1}^{k} \frac{t_i^2}{\lambda_i}, \quad Q = \|\mathbf{x} - \hat{\mathbf{x}}\|^2

Virtual Metrology (Ridge Regression)

\hat{\boldsymbol{\beta}} = (\mathbf{X}^T\mathbf{X} + \lambda\mathbf{I})^{-1}\mathbf{X}^T\mathbf{y}

EWMA Control

\hat{y}_{k+1} = \lambda y_k + (1-\lambda)\hat{y}_k

Mahalanobis Distance

D^2 = (\mathbf{x} - \boldsymbol{\mu})^T\mathbf{S}^{-1}(\mathbf{x} - \boldsymbol{\mu})

Process Monitoring

Take process monitoring further