Stability of the reference flat used in Fizeau interferometer and its contribution to measurement uncertainty

The flatness of an optical surface can be evaluated using a Fizeau interferometer. There is strong demand for ensuring that the measurement uncertainty of flatness is of nanometer order over a measurement range of 300 mm or more; however, the measurement range and measurement uncertainty of flatness at the National Metrology Institute of Japan (NMIJ) are 300 mm and 10 nm, respectively. In a Fizeau flatness interferometer, the gap distance between the reference flat and the specimen is measured. To obtain the absolute profile of the specimen, the absolute profile of the reference flat should be measured in advance. The three-flat test is one of the methods used to measure the absolute profile of a reference flat. The reference flat, however, deforms under the force of gravity, and its absolute deformation value cannot be determined by the three-flat test. The deformation value of the reference flat can be corrected by the finite element method (FEM) analysis; however, it is difficult to ensure the validity of the analysis and there is a large uncertainty component of the Fizeau flatness interferometer. To verify the FEM analysis, we developed a scanning deflectometric profiler (SDP) that does not require a reference flat and can directly measure a profile. We calibrated an optical flat using a Fizeau flatness interferometer and the SDP. Finally, the deformation value of the reference flat under the force of gravity was evaluated by comparing the measurement results.

Section: Validation Of Deformation Analysismentioning

confidence: 95%

“…It is mounted in an aluminum ring with 54 supporting points as shown in figure 4. The deformation value of the reference flat under the force of gravity was calculated by FEM analysis [17]. The deformation value of the reference flat was 23 nm in the case of a 300 mm diameter.…”

Section: Deformation Of Reference Flat Under Gravitymentioning

confidence: 99%

Evaluation of the deformation value of an optical flat under gravity

Kondo¹,

Bitou²

2014

“…Next, the proposed method was compared with Fizeau interferometry. Sample (1) was measured with a Fizeau interferometer whose uncertainty is reported as λ/77 [4,5].…”

Section: Methodsmentioning

confidence: 99%

“…With the aim of measuring large specimens with high precision, an instrument has been developed that is equipped with a ROF whose diameter is 350 mm [4]. The measurement uncertainty was investigated and reported as λ/77, where λ is 633 nm [5].…”

Section: Introductionmentioning

confidence: 99%

Sub-nanometre double shearing heterodyne interferometry for profiling large scale planar surfaces

Yokoyama

Yoshimori

et al. 2004

A novel interferometric method, called a double shearing interferometry, is presented for profiling large-scale quasi-planar surfaces, such as semiconductor wafers, optical flats and x-ray mirrors. The surface profile is measured even under existence of the inclination of a scanning stage. By adopting the common-path optical configuration and the heterodyne detection, the proposed method realizes excellent resolution in sub-nanometer order and enables a robust measurement against unwanted disturbances due to the measurement environments. A height resolution of 0.1nm was experimentally achieved. A standard deviation of the analytical surface profiles was found to be 1.3nm, even when using a conventional screw-lead scanning stage. The measured results show that the surface profile and stage inclination are separately determined.We confirmed resultant profile was well consistent with the one measured with Fizeau interferometer.

“…Therefore, in order to obtain an accurate absolute profile of the measurement flat, the absolute profile of the reference flat must be known to high accuracy. In the National Metrology Institute of Japan, a Fizeau-type interferometer using a He-Ne laser light source has been adopted as the primary national-standard instrument for flatness calibration (named FUJI) [2]. In FUJI, the absolute profile of the reference flat was determined using a three-flat test, and an uncertainty of 10 nm in flatness measurements has been achieved over a 300 mm diameter [3].…”

Section: Introductionmentioning

confidence: 99%

High-precision, low-coherence Fizeau interferometer using a pulsed laser diode for measurement of transparent plates

Bitou¹,

Ueki²

2010

A high-precision, low-coherence Fizeau interferometer system using a pulsed laser diode has been developed for the measurement of the flatness of transparent plates. A pulsed laser diode with a wavelength of around 633 nm was used as the low-coherence light source. A Twyman-Green interferometer with the pulsed laser diode was connected to the Fizeau interferometer. By adjusting the optical path differences in both interferometers, the interference fringe pattern due only to the reference and measurement surfaces could be observed. The accuracy of the measurements was found to be similar to that of a conventional Fizeau interferometer.