White Light Interferometer Scanning Modes Compared: Vertical Scanning Interferometry (VSI) vs. Phase-Shifting Interferometry (PSI)

The white light interferometer is the mainstream optical instrument for surface topography measurement. Its measurement modes fall mainly into two categories: Phase-Shifting Interferometry (PSI) and Vertical Scanning Interferometry (VSI). Each mode suits surfaces with different characteristics, and understanding their differences is the prerequisite for correctly selecting and using a white light interferometer.

1. Why Two Measurement Modes Are Needed

A white light interferometer obtains surface height information by analyzing interference fringes, yet the surface characteristics of different samples vary enormously: optical mirror surfaces have extremely small height differences but demand extremely high precision, while machined parts can have surface undulations of tens of micrometers. A single measurement mode can hardly satisfy the precision and range requirements of all scenarios at once. Therefore, mainstream white light interferometers offer multiple measurement modes to accommodate different surface features and measurement objectives.

2. Phase-Shifting Interferometry (PSI)

2.1 Principle

PSI introduces a precise phase shift into the reference beam (usually driven by a piezoelectric ceramic transducer, PZT) and captures multiple interferograms at equal step intervals. A phase-shifting algorithm then solves the phase value of each pixel, reconstructing the surface topography with higher precision.

2.2 Characteristics

PSI is the highest-resolution measurement mode of the white light interferometer, with a vertical resolution down to the sub-nanometer level or better. It is suitable for ultra-smooth reflective surfaces such as optical flats, mirrors, and semiconductor wafers. Its limitation lies in a limited effective measurement range—typically less than one wavelength (a few hundred nanometers)—and greater sensitivity to vibration and environmental disturbances. Therefore, PSI is suited to scenarios with small surface undulations and extremely high precision requirements, and is not suitable for stepped or rough surfaces.

3. Vertical Scanning Interferometry (VSI)

3.1 Principle

VSI performs continuous or stepwise vertical scanning along the Z axis, capturing a series of interferograms during the scan. At each pixel, the light-intensity signal varies with scan position and forms an interference curve whose envelope peak corresponds to the zero optical path difference (OPD) position. By extracting the peak position algorithmically, the height of that point is obtained, thereby reconstructing the 3D topography of the entire surface.

3.2 Characteristics

VSI offers a large measurement range, determined by the scanning stroke, and can cover surfaces with large undulations. It applies to a broad range of surfaces, including medium-to-high-roughness surfaces and surfaces with significant variations in reflectivity. Compared with PSI, VSI is relatively insensitive to vibration and has a relatively wide vertical measurement range—from a few nanometers to several micrometers—striking a good balance between vertical resolution and measurement range. As a result, it has become a highly versatile mode, widely used in semiconductor packaging, MEMS, precision machining, and other fields.

4. VSI vs. PSI Comparison

5. Logic for Choosing a Measurement Mode

The choice of measurement mode mainly depends on the sample's surface characteristics and the measurement objective:

• If the sample is an ultra-smooth surface (e.g., optical lenses, wafers, reflectors) with small form undulations and high precision requirements, choose PSI mode to obtain the highest resolution;
• If the sample is a rough surface or contains large undulating structures such as big steps or deep grooves, choose VSI mode to ensure range coverage;
• When a surface contains both smooth and rough regions, a hybrid scanning mode can be used, with the algorithm adaptively selecting the optimal processing method pixel by pixel.

6. Summary

PSI and VSI address the problems of "measuring more precisely" and "measuring more," respectively: PSI excels in resolution and is suited to mirror-level surfaces, while VSI excels in range and adaptability and is suited to engineering surfaces. Understanding the differences between the two modes is the foundation for correctly selecting a white light interferometer and fully exploiting the instrument's capabilities.