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Precision Metrology of thin plane-parallel optical surfaces

Now it is possible to measure the surface of thin parallel plates down to only 100 microns thick with the new AccuFlat from Zygo!

Key Features:

  • Polishing process characterisation of form, waviness & microwaviness


  • Enables metrology of transparent optics to 100 µm thickness


  • Provides low noise, high contrast interferogram for a wide range of surface reflectivities

Zygo AccuFlat

Laser Fizeau interferometric profilers based on phase-shifting interferometry routinely measure flat surfaces with high precision. However, because of interference from the other parallel surface, these tools often have difficulty measuring transparent plane-parallel parts like the glass disks used in high density hard drives and the pellicles used for semiconductor lithography applications.


Zygo AccuFlat
Zygo AccuFlat

Single acquisition measurement of the front surface of a 1 inch solid glass retroreflector. Note the low contrast fringes, yet the surface measurement shows neither artefacts nor fringe bleed through.

To satisfy the precision and resolution requirements for current and future hard disk and glass pellicle manufacturers, Zygo have developed the AccuFlat based on a novel low coherence equal-path interferometer. This new instrument operates at 455nm for enhanced vertical resolution with respect to traditional HeNe 633nm based interferometers, and uses a 4Mpix camera (2048x2048 pixel) camera with matching optics for high lateral resolution. In addition to solving the problem of separating parallel semi-transparent reflecting surfaces, the AccuFlat has extraordinarily low coherent noise and high overall performance with respect to alternative solutions, particularly for mid-spatial frequency measurements.

An industry standard method for assessing measurement precision is Gauge Repeatability and Reproducibility (Gauge R&R or GR&R). A GR&R uses an analysis of variance model to assess the measurement precision of the system through the precision/tolerance ratio (P/T). A P/T ratio of 10% usually indicates the system can reliably determine whether a part meets the tolerance.

GR&R tests were applied to the measurement of hard disk blanks for three parameters of importance to the manufacture and operation of these disks: Form PV and Waviness PV. The instrument measured the three parameters with 3% and 5% P/T ratio respectively, indicating that the instrument is an excellent tool for measuring the surface characteristics of these hard drive blanks.

The AccuFlat is of interest as a metrology tool for thin transparent parts as well as for high resolution, low-noise measurements of all flat parts. The novel in-line, normal-incidence equal path interferometer geometry and extended broad-band illumination suppresses multiple-surface interference and reduces coherent noise and optical artefacts. System noise is typically 3X to 5X lower than a comparable laser Fizeau interferometer. The system also incorporates new phase analysis software to reduce vibration sensitivity and passive shields for turbulence mitigation. Incorporating a 4Mpix camera and a matching low-distortion, high resolution imaging system, the instrument combines 50 micron sampling resolution with excellent system transfer function characteristics. Though the design is intended to satisfy the needs of current and future hard disk and pellicle metrology, the instrument can be used to measure any flat surface with high precision and without interference from other parallel surfaces, making it particularly useful for prisms, retroreflectors and waveplates.

Zygo AccuFlat Zygo AccuFlat

Measurement of the substrate (top) and top surface (bottom) of a mirror coated with a 100 micron thick fused silica layer.

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