Research On The Deep Sub-Nanometer Accuracy Reference Wavefront In Phase Shifting Point Diffraction Interferometer

Extreme Ultraviolet Lithography (also known as EUVL) is a lithographyprocess adapt to the mass production of ultra-large-scale integrated circuits with27nm~11nm features. The image of a desired circuit pattern is formed on asemiconductor wafer with an optical imaging system that operates at a wavelength of13.5nm. At the extreme ultraviolet wavelengths, the complex refractive index of anymedium is very close to one, and extreme ultraviolet projection lithography utilizesall-reflective multilayer-coated optics due to the strong absorption of EUV radiationby all materials. In order to achieve diffraction-limited performance, according toMarachel’s criterion, the wave front aberration of the projection objective lensshould be less than1.0nm RMS. Wave front errors of the aspheric reflective opticalelements over the entire range of spatial frequencies as well as the multilayer coatingdefects will affect the performance of the projection objective lens, the figure errorof the aspheric elements in the six-mirrors system should be less than0.2nm RMS.The high-performance requirements of the projection lens in EUVL, poseunprecedented challenges to optical fabrication, optical testing and optical alignment.The measurement accuracy of the traditional interferometers, such as Fizeauinterferometer or a Twyman-Green interferometer, is limited by the referenceelement, far able to meet the requirements during EUVL development. A pointdiffraction interferometer (PDI) with sub-nanometer wave front-measuring accuracy can be realized by using the near-perfect spherical wave diffracted by a quasi-pointsource. The measurement accuracy of a PDI is limited by many factors, such as thequality of the diffracted reference spherical wave, the image acquisition system, themeasurement environment, and so on. The quality of the diffracted referencespherical wave, which determined by the quality of the tiny pinhole in a pinholeplate, is the most subtle one that limited the measurement accuracy of a PDI.Due to the figure error of the optical elements and the wave front aberration ofthe imaging system in EUVL should be characterized with high accuracy, it isnecessary to study the quality of the diffracted reference wave front in a PDI both bytheoretically and experimentally. The main contents in this paper are as follows:1Based on Richard-Wolf vector diffraction integral formula and the incidentlight is linearly polarized light or circularly polarized light, the effect of theaberrations presented in the focusing lens on the intensity and phase distribution nearthe focal plane is in depth-study this time. The wave in the focal plane is no longer aperfect plane wave due to focusing lens aberrations and the miss-alignment of thepinhole mirror, and the results will be different if use the plane wave as the incidentwave during pinhole diffraction calculation.2The field distribution within a single-mode fiber in fiber point diffractioninterferometer is calculated base on the theory of optical waveguide. Single-modefiber can only transfer the LP01mode. The aberrations, introduced by previouselements in a system, coupled into the fiber, will be effectively filtered. The far fielddistribution of a single mode optical fiber with tilted end face is calculated analyticalby using the Scalar Rayleigh-Sommerfeld diffraction integral formula, the initialfield used in calculated is in the form of Bessel function. The deviation of the farfield diffracted by the single mode optical fiber used in this experiment is thencalculated. The fiber end face inclination does not affect the sphericity of itsdiffracted wave front.3The distribution of the far field diffracted by a tiny pinhole on a conductivescreen with zero thickness and infinite conductivity is calculated base on vector Rayleigh-Sommerfeld integral formula in the case of oblique illumination. Thedifference of the diffracted far field for linearly polarized light and circularlypolarized light illumination is then clearly. The oblique angle of the incident wavedoes not affect the sphericity of the diffracted wave front. Only the intensitydistribution in the diffracted wave deviates from the center axis of the pinhole. Thesphericity of the diffracted wave front is almost the same for linearly polarizedillumination and circularly polarized illumination. The main aberration present inthe diffracted wave is astigmatism when the incident light is linearly polarized andthe rotationally symmetric aberration when the incident light is circularly polarized.4The effect of the pinhole roundness on the quality of the diffracted wave frontin the far field is analyzed by using the vector Rayleigh-Sommerfeld integral. Edgeroughness of the tiny pinhole has a significant impact on the deviation of the wavefront, and has little effect on the intensity, while the ovality of the tiny pinhole has alittle impact on wave front deviations, but has an important impact on the intensitydistribution.5The FDTD simulation is carried out to calculate the diffraction of a tinypinhole on the screen with finite conductivity and finite thickness. The factorsaffecting the quality of the diffracted wave front, including the material, thethickness, the diameter, the size and the shape of a pinhole, the aberrations present inthe focusing lens, the miss-alignment of the pinhole, and so on. The candidate forpinhole material is Cr and Al, and their thickness should not be less than200nm.When a system with NA0.3is under testing, then the pinhole with a diameter of800nm is a good choice, while for testing an element of NA0.3, the diameter of thepinhole should be about500nm. In the alignment process of the pinhole, the amountof defocus must be greater than-175nm, and the amount of shift should be no morethan125nm.6The high precision wave front fitting is realized by using the algorithm ofGram-Schmidt. The algorithm can also be used in the annular Zernike polynomialfitting, and has the same accuracy. 7The deviation of the wave front diffracted by a single mode optical fiber ismeasured analogous to Young’s two-hole experiment. The systematic errors in theexperiment are precision analyzed. The separation of the two point sources willintroduce the comatic aberration to the testing result, and the incline of the detectorduring the measurement will introduce the astigmatism to the testing result. Toreduce the approximation error in wave front with large NA, the OPD is expanded tobinomial and the high power terms are considered. The deviation of the wave frontdiffracted by each fiber is measured by experimentally. The deviation of the wavefront diffracted by two pairs of fiber are0.1416±0.0084nm RMS and0.1560±0.0211nm RMS, respectively; therefore, the measured deviation of the wave frontdiffracted by the single mode optical fiber is about λ/3500RMS.In a word, the deep sub-nanometer accuracy diffracted reference wave front in aPDI is studied both by theoretically and experimentally in this paper, which is meetthe measuring accuracy requirement in testing the figure error of the optical elementand wave front aberration of the imaging system in EUVL. This work Reservetechnical conditions for testing EUVL projection optics with high accuracy, and laysfoundation for assessing the sphericity of the reference wave front with moredivergence angle and measurement accuracy of a PDI with large NA.