1. Introduction
In precision manufacturing, semiconductor processing, and micro/nano-scale surface engineering, the demand for accurate vertical surface profile measurements continues to rise. Traditional contact-based or low-resolution methods are often insufficient in terms of accuracy and efficiency. Vertical profilometers, which specialize in non-contact surface height measurements, increasingly employ white light interferometry (WLI) as a core technique due to its high precision and robustness. This article introduces the working principles of WLI and explains how it enables nanometer-scale vertical resolution in vertical profilometry.

2. Fundamentals of White Light Interferometry
White light interferometry utilizes a broadband light source—such as an LED or halogen lamp—and is based on the Michelson interferometer principle. The light is split into a reference beam and a measurement beam. After reflection from the reference mirror and the sample surface, the beams are recombined to form interference fringes.

Unlike monochromatic light, white light has a very short coherence length. Therefore, interference fringes only appear when the optical path difference between the two beams is nearly zero. This property enables precise localization of the surface height along the vertical axis.

3. System Architecture of Vertical Profilometers Using WLI
A typical WLI-based vertical profilometer consists of the following key modules:

Broadband Light Source: Provides continuous spectrum light in the visible range;

Interferometric Objective (e.g., Mirau or Linnik type): Combines the reference and measurement beams;

Z-axis Scanning Mechanism: Moves the interferometric head or the sample vertically with nanometer-scale precision;

Imaging System: Captures interference image sequences using a CCD or CMOS camera;

Image Processing Algorithm: Analyzes fringe contrast variation at each pixel to determine surface height.

4. Achieving Nanometer-Scale Vertical Resolution
The core capability of WLI-based profilometry lies in its vertical resolution, which is realized through specific algorithms:

Envelope Peak Detection: Suitable for rough or stepped surfaces; determines surface height by locating the maximum contrast position in the interference envelope. Typical resolution: 1–10 nm.

Phase-Shifting Interferometry: Ideal for smooth surfaces; analyzes the phase changes of interference fringes to extract sub-nanometer height information.

These algorithms can be combined or automatically selected based on the surface type, enabling the system to adapt to various measurement scenarios.

5. Advantages and Limitations
Advantages:

Non-contact, non-destructive measurement;

High vertical resolution (down to 1 nm);

Applicable to reflective, transparent, or moderately rough surfaces.

Limitations:

Sensitive to environmental vibrations; isolation systems are recommended;

Limited ability to measure steep sidewalls or deep trenches;

Higher cost, more suited to mid- to high-end applications with strict accuracy requirements.

6. Conclusion
White light interferometry is a well-established optical technique that enables vertical profilometers to achieve high-resolution, non-contact surface measurements. With a solid physical foundation and proven reliability, WLI has found widespread application in semiconductor, optical, and precision materials industries. Looking ahead, its integration with faster scanning systems, enhanced algorithms, and automated platforms will further extend its capabilities in high-precision metrology.