reflection interferometry
**Reflection interferometry** is an optical metrology technique that monitors **film thickness or etch depth in real-time** by analyzing the **interference pattern** of light reflected from the wafer surface. It is widely used for endpoint detection during etch and for thin-film thickness measurement.
**How It Works**
- A beam of light (monochromatic or broadband) is directed at the wafer surface.
- Light reflects from **both the top surface** and the **film-substrate interface** (and from any additional interfaces in multilayer stacks).
- The two reflected beams interfere — **constructively or destructively** — depending on the optical path difference, which is determined by the film thickness and refractive index.
- As the film thickness changes (during etch or deposition), the reflected intensity **oscillates** — producing a characteristic sinusoidal signal.
**Physics**
Constructive interference occurs when:
$$2 \cdot n \cdot d = m \cdot \lambda$$
Where $n$ is the refractive index, $d$ is the film thickness, $\lambda$ is the wavelength, and $m$ is an integer. Each complete oscillation in reflected intensity corresponds to a thickness change of $\lambda / (2n)$.
**Application: Etch Endpoint**
- During etch, the film gets thinner → reflected intensity oscillates.
- **Counting fringes**: Each fringe = a known thickness change. By counting fringes, the etch depth is tracked in real-time.
- **Endpoint Detection**: When the target film is completely removed, the oscillations stop (the film is gone), and the reflected signal stabilizes. This change indicates endpoint.
**Application: Film Thickness Measurement**
- For thickness measurement, **spectroscopic reflectometry** (broadband light) analyzes the entire reflection spectrum.
- The spectrum is fitted to a thin-film optical model to determine thickness with **sub-nanometer precision**.
- Non-contact, non-destructive measurement — ideal for in-line monitoring.
**Advantages**
- **Non-Contact**: No physical contact with the wafer — suitable for in-situ measurement during processing.
- **Real-Time**: Continuous monitoring enables real-time etch rate tracking and endpoint detection.
- **High Precision**: Sub-nanometer thickness resolution with spectroscopic reflectometry.
- **Simple Setup**: Requires only a light source, optical fiber, and detector/spectrometer.
**Limitations**
- **Transparent Films Only**: The film must be at least partially transparent at the measurement wavelength for interference to occur. Opaque metals cannot be measured this way.
- **Patterned Wafers**: On patterned wafers, the reflected signal is a complex average of multiple film stacks — interpretation requires modeling or calibration.
- **Minimum Thickness**: Very thin films (<10 nm) may not produce detectable interference fringes with monochromatic light (spectroscopic methods can extend the range).
Reflection interferometry is a **foundational metrology technique** in semiconductor manufacturing — its simplicity, real-time capability, and non-destructive nature make it indispensable for etch and deposition process control.