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:

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.