Envelope Method for Single-Layer Refractive-Index Fitting

Introduction

The envelope method is suitable for single-layer films with clear spectral extrema. It estimates refractive-index behavior from upper/lower envelope trends and is practical for transparent or weak-absorption layers.

Procedure

1. Measure transmittance/reflectance spectra of a single-layer sample.
2. Confirm the spectrum contains clear peak/valley envelopes in the target wavelength range.
3. Import spectrum data and initialize film thickness and substrate parameters.
4. Apply envelope-based fitting settings and execute fitting.
5. Evaluate fitted n/k curves and residual error; then save the accepted result.

Example

For a SiNx single layer, the imported spectrum shows stable oscillation envelopes. After envelope fitting, the simulated envelope overlaps measured data and produces a smooth n-curve in the working band.

Notes

• The method is sensitive to spectral noise and baseline drift; preprocess data when necessary.
• Ensure thickness initial value is reasonable, otherwise fitting may converge to local minima.
• Not recommended for multilayer stacks or spectra without clear envelope characteristics.