Definition
Interferometric microscope is a precision optical instrument based on the principle of optical interference for non-contact measurement of the microscopic topography of the surface of an object. It obtains surface height information by measuring the interference fringes generated between the beam reflected by the measured surface and the reference beam, enabling surface profile measurement at nanoscale or even sub-nanometer resolution.
Principle
The core principle of interferometry microscopy is the phenomenon of interference in light. The light emitted by the light source inside the instrument is divided into two beams by the beam splitter: one beam of light shines on the surface of the sample to be measured, which is called object light; Another beam of light hits a highly flat reference mirror surface, called the reference light. The two beams of light are reflected and re-converge, and due to the difference in optical path, they will interfere, forming light and dark interference fringes. The morphology of these stripes is directly related to the high undulations of the sample surface. By recording the interference pattern by a photodetector (such as a CCD camera) and analyzing the stripe using a phase demodulation algorithm (such as phase shift interferometry), the height of each point on the surface relative to the reference plane can be accurately calculated, so as to reconstruct the three-dimensional surface topography.
The relationship between the basic optical path difference (OPD) and phase difference (φ) can be expressed as:
OPD = (λ / (2π)) * φ
where λ is the wavelength of the light source. The relationship between surface height h and optical path difference is:
h = OPD / 2
Measurement method
The main measurement methods of interferometry microscopy include phase-shift interferometry and vertical scanning interferometry. Phase-shift interferometry acquires multiple interferograms by introducing a known, step-changing phase shift by precisely moving the reference mirror through a piezoelectric ceramic driver during the measurement process. By mathematically processing these patterns, the phase distribution of the measured surface can be calculated point by point, and then the height information can be obtained. This method has fast measurement speed and high spatial resolution.
Vertical scanning interferometry, also known as white light scanning interferometry, uses a broadband white light source. Due to the short coherence length of white light, clear interference fringes are produced only when the optical paths of the object light and the reference light are nearly equal. By scanning the sample or interferometric lens in the vertical direction, the light intensity of each pixel with the scanning position is recorded, and the surface height corresponding to the point can be determined by analyzing the envelope peak or phase zero position of the curve. This method is suitable for measuring surfaces with large steps or high roughness.
Influencing factors
The measurement accuracy and reliability of interferometric microscopy are influenced by various factors. Ambient vibration and air turbulence can cause random fluctuations in the optical path, resulting in instability of interference fringes, which are often required for use on vibration isolation platforms or relatively stable environments. The coherence and stability of the light source directly affect the contrast and measurement range of the interference fringes. The optical properties of the surface being measured, such as reflectance, transparency, and color, can affect the strength of the returned light signal, and too low or too high reflectance may result in signal saturation or insufficient signal-to-noise ratio. In addition, the numerical aperture of the objective determines the lateral resolution and vertical measurement range of the instrument, with the larger the numerical aperture, the higher the lateral resolution, but the smaller the vertical measurement range. The calibration status of the instrument and the data processing algorithm used are also key to ensuring accurate measurement results.
Application
Interference microscopy is widely used in many fields that require high-precision surface topography analysis. In the semiconductor manufacturing industry, it is used to measure wafer surface film thickness, etch depth, and critical dimensions. In the field of precision optical processing, it is used to detect surface shape errors and surface roughness of optical components such as lenses and mirrors. In materials science, it can be used to study surface wear, corrosion, coating quality, and microstructure of metals, ceramics, polymers, and other materials. In the field of microelectromechanical systems, it is used to measure step heights and three-dimensional contours of microstructures. In biomedical engineering, it can also be used to observe the microscopic topography of the surface of certain biological tissues or artificial implants.
Selection
When choosing an interferometric microscope, it is necessary to consider the specific measurement needs comprehensively. First, key measurement parameters need to be identified, including vertical resolution (typically down to the sub-nanometer level), vertical measurement range (ranging from a few microns to several millimeters), lateral resolution (in relation to the objective lens and camera pixels), and measurement speed. Secondly, the appropriate technology type is selected according to the characteristics of the tested sample, for example, for smooth continuous surfaces, phase shift interferometers are more suitable. For surfaces with steep steps or roughness, vertical scanning white light interferometers are more advantageous. It is also necessary to consider whether the size, load-bearing capacity and freedom of movement of the sample stage meet the size and positioning requirements of the sample to be tested. The instrument's software capabilities, such as data analysis capabilities, the ability to generate reports in compliance with relevant standards (e.g., ISO, ASME), and the degree of automation, are also aspects that need to be evaluated during selection. Finally, the long-term stability of the instrument, maintenance complexity, and technical support services should also be factored into the decision-making process.