Objective Wavefront aberration describes the properties of a small-aberration imaging optical system. In a high-quality microscope objective lens and space telescope, the wavefront errors should be within λ/14 RMS (where λ is the operational wavelength and RMS is the root mean square value). To meet required wavefront quality of the optical systems for extreme ultraviolet lithography, the error must be less than 0.45 nm RMS. Therefore, wavefront measurements are highly demanded. At present, wavefronts are typically measured by Hartmann sensors, Fizeau interferometers, Twyman-Green interferometers, shearing interferometry, or point-diffraction interferometry. The Shark-Hartmann sensor covers a large measurement range and can quickly measure the wavefront, but with lower resolution than interferometry. The Fizeau and Twyman-Green interferometers cannot measure to higher accuracy than their standard lenses, and cannot be installed in systems with limited space.

In the present study, we report a phase-shifting point-diffraction interferometer with several advantages: high optical field uniformity, high measurable numerical aperture, and a quasi-common optical path. The optical signals are transmitted through single-mode fibers that improve the flexibility of the interferometer system. Our results are anticipated to assist wavefront-aberration detection in high-precision photolithographic projection lenses.

Methods We developed a dual-hole point diffraction interferometer (DHPDI) based on a dual-fiber optical path. First, we designed the measuring principle of the interferometer. The interferometer uses a diode-pumped solid-state laser with multi-longitudinal modes. The laser operating wavelength is 532 nm and the coherence length is several centimeters. The two laser beams form a quasi-common optical path interferometer structure. The intensities of the beams are controlled by interference arms connected with adjustable attenuators, one of which is connected to a phase shifter. The two single-mode optical fibers of the object surface output two beams of coherent light. The end faces are imaged by the lens at two pinholes of the object surface mask, and are filtered by the pinholes to form two standard spherical-wavefront illumination imaging systems. One wavefront becomes the measurement wavefront and the other becomes the reference wavefront through the imaging system to be measured. The two beams overlap and produce an interference pattern at a charge-coupled device camera. The wavefront phase map is measured using a phase-shifting method. In the experiments, a DHPDI and a dual-fiber point-diffraction interferometer (DFPDI) were set up to detect the same projection objective lens. The experimental results were analyzed and the measurement results of both interferometers were compared to verify the effectiveness of the DHPDI.

Results and Discussions This paper proposes our DHPDI for measuring wavefront aberrations of imaging systems. Its advantages are high optical field uniformity, a high measurable numerical aperture, a quasi-common optical path, and a phase-shift element besides the imaging optical path of the system (Fig. 1). The DHPDI is designed with two measurement modes: point-diffraction measurement mode and system-errors measurement mode (Fig. 2). In point-diffraction mode, the DHPDI measures the geometric optical path error and detector tilt error of the test light and point-diffraction light, which mainly appear as coma aberration and astigmatism, respectively. These geometric optical path differences can be quickly and conveniently calibrated in system-errors mode. Both measurement modes can be used together for high-precision detection of wave aberrations in the imaging system. We constructed a DHPDI system that measures the wavefront aberration of a 5× demagnification projection objective lens with a numerical aperture of 0.3, and supplied it with a 532 nm laser (see Methods for laser details). The DHPDI was verified in experiments (Figs. 3 and 4), and its results were compared with those of the DFPDI (Figs. 5--7). The experimental results confirmed the theoretical deviation. When detecting the wavefront aberrations of the same projection objective lens, both measurement methods gave nearly consistent wavefront distributions, with a relative error of 0.07 nm RMS.

Conclusions We have demonstrated an advanced DHPDI. With a pinhole diameter of 700 nm, the deviation of the diffracted wavefront from spherical meets the requirements of wave aberration detection in high-precision imaging systems. The optical signals are transmitted through single-mode fibers, enabling a flexible interferometer system. The DHPDI also allows convenient adjustment of the interference contrast and phase-shifting outside the imaging optical path.

We then constructed DHPDI and DFPDI systems for measuring the wavefront aberration of a 5× projection objective lens with a numerical aperture of 0.3. In both modes, the contrast in the interferogram exceeded 65%. Moreover, the intensity uniformity of the interferogram in DHPDI was approximately twice that in DFPDI. Such uniform intensity can improve the accuracy of pupil-edge detection. The relative error of the wavefront distribution of the two detection results is less than 0.1 nm RMS , and the theoretical deviation was verified in the experiments.