Takahiro Yamamoto

Development of the point diffraction interferometer for extreme ultraviolet lithography: Design, fabrication, and evaluation

A point diffraction interferometer (PDI) for extreme ultraviolet lithography (EUVL) aspheric mirror measurement has been developed. In order to realize an accuracy of 0.1 nm rms, various optical error factors have been numerically analyzed and the maximum tolerable error has been determined. From the error estimation results, the optimal pinhole diameter has been determined as 0.5 μm. In a PDI, air turbulence reduces the precision and accuracy because of the long optical path. In order to avoid this problem, the apparatus is filled with helium gas, which has a smaller refractive index than that of air. By using this apparatus, precision of 0.03–0.04 nm rms and a system error of 0.10 (0.16) nm rms have been obtained for a spheric mirror with numerical aperture (NA) 0.08 (0.15). In aspheric mirror measurement, an accuracy of 0.74 (1.18) nm rms for NA 0.08 (0.15) has been obtained. The accuracy becomes 0.34 (0.97) nm rms for NA 0.08 (0.15) with 36-term Zernike polynomial fitting.

Accuracy evaluation of the point diffraction interferometer for extreme ultraviolet lithography aspheric mirror

We evaluated the accuracy of the point diffraction interferometer, which has been completed at the Atsugi Research Center’s Association of Super-Advanced Electronics Technologies for the precise measurement of extreme ultraviolet lithography optics. To evaluate the absolute accuracy, more precision is required. A pinhole with 0.5 μm diameter was used to generate a near completely spherical reference wave front, and helium gas filled the inside of the chamber to suppress air turbulence. With this apparatus, precision of 0.04 nm root-mean-square (rms) was achieved. Absolute accuracy was evaluated from the measurement of mirror rotation and displacement and an absolute accuracy of 0.17 nm rms was obtained for a spherical mirror with numerical aperture of 0.145. Absolute accuracy was improved to 0.11 nm rms by limiting the numerical aperture to 0.08.

Advanced point diffraction interferometer for EUV aspherical mirrors

An advanced point diffraction interferometer for measuring EUV aspherical mirrors with high accuracy has been developed. It is designed for measuring various EUV mirrors with high accuracy and high precision. It can measure the surface figure of all mirrors that will be used in high numerical aperture systems. Using this interferometer, 0.1nm rms precision and 0.2nm rms accuracy are expected.

Development of projection optics set-3 for high-numerical-aperture extreme ultraviolet exposure tool (HiNA)

We have developed a high-numerical-aperture extreme ultraviolet exposure tool (HiNA). HiNA is equipped with an illumination system, projection optics, a mask stage, and a wafer stage in the vacuum chamber. The projection optics consist of two aspherical mirrors (M1 and M2). The numerical aperture of the optics is 0.3. Thus far, we fabricated two sets of projection optics (set-1 and set-2). The wave-front errors of set-1 and set-2 were 7.5 and 1.9nm rms, respectively. We developed a third set of projection optics (set-3), the target wave-front error of which was less than 1nm rms. In set-3, we also attempted to reduce flare. We completed the mirror polishing, coating, and mirror adjustment of set-3. By using a recently developed polishing method, we reduced low-spatial-frequency roughness (LSFR), mid-spatial-frequency roughness (MSFR), and high-spatial-frequency roughness, simultaneously. The predicted wave-front error calculated from the LSFR number was 0.69nm rms. MSFR, which strongly affects the flare o...

Aspherical mirror measurement using a point diffraction interferometer

An point diffraction interferometry (PDI) system is used for measurement of EUV aspherical mirrors, because diffracted light by a small aperture has a nearly ideal spherical wavefront and EUV projection systems is designed with mild aspheres so that the mirrors can be tested at the center curvature without null optics. An advanced point diffraction interferometer has been developed and its precision and accuracy performance tested with a spherical mirror have been reported in last year1. After that, the diameter of the pinhole employed in the PDI system is switched from 1.0mm to 0.5mm in anticipation of measurement accuracy improvement. An aspherical mirror is measured, and the system error is estimated from the aspherical measurement data. In this system error estimation, an aspherical mirror designed for a four-mirrors EUV projection optics is used.

Subnanometer Fabrication of Optics by Plasma Chemical Vaporization Machining

We discuss the fabrication of optics by plasma chemical vaporization machining (CVM) to obtain them with a shape accuracy below the nanometer level, that is, subnanometer accuracy. We used 88-mm-diameter spheric mirrors made of fused silica as workpieces, which were roughly polished to 9.24 nm RMS before plasma CVM. To achieve subnanometer accuracy, the plasma CVM conditions were adjusted. Under these conditions, we successfully fabricated the mirrors with the desired shape accuracy of 0.63 nm RMS. This demonstrated that plasma CVM is capable of fabricating optics with subnanometer shape accuracy.

Absolute accuracy evaluation of aspherical null testing for EUVL mirrors

Interferometric null metrology can produce highly precise figure of aspherical surfaces. However, because the measurement is a direct comparison of the tested surface with the reference wavefront of the null optics, measurement accuracy is equivalent to the quality of the reference null wavefront. Although the asymmetric aberration of the reference null wavefront can be calibrated by rotating the tested surface, it is more difficult to calibrate the rotationally symmetric errors. Especially, the aspherical surface of EUVL mirrors must to be measured with higher accuracy, 0.2~0.3nmRMS. We have developed an aspherical null testing system using a null lens for EUVL mirrors. We analyzed the uncertainty of null lens in each process and estimated the measurement accuracy of aspherical null testing using null lens. If the compensator lens contains only one piece of lens, the measurement accuracy is estimated to be 0.20nmRMS. If the compensator contains two pieces, the measurement accuracy becomes 0.24nmRMS. To verify our estimation, we evaluate a sample lens with aplanatic surfaces that make no spherical aberrations. In this case, we can evaluate the quality of the transmitted wavefront absolutely. The difference between the calculated and the experimental wavefronts is much smaller than our estimation. To this extent, our aspherical testing technique using null lens has been verified to be able to meet the high demanding for EUVL mirror testing.

Fabrication of aspherical mirrors for HiNA (high numerical aperture EUV exposure tool) set-3 projection optics

Aspherical mirror fabrication of HiNA set-3 projection optics was completed. By using a new polishing method, we successfully reduced low spatial frequency roughness (LSFR), mid spatial frequency roughness (MSFR) and high spatial frequency roughness (HSFR) compared with HiNA set-1 and set-2 projection optics. MSFR, which strongly affects the flare of the optics, was remarkably reduced to less than 0.2nm rms. HiNA projection optical system with the numerical aperture of 0.3 consists of two aspheric mirrors (M1 and M2). We had already fabricated two sets of the HiNA projection optics. The wavefront error (WFE) of the set-1 optics was 7.5nm rms and that of the set-2 optics was 1.9nm rms. We tried to reduce the WFE and flare in the set-3 optics. The target number of WFE of the set-3 optics was less than 1nm rms. The LSFR, MSFR and HSFR of the M1 of the set-3 optics were 0.25nm rms, 0.17nm rms and 0.10nm rms, respectively. The LSFR and MSFR are almost half values compared with those of the M1 for the set-2 optics. The HSFR was also reduced from 0.13nm rms (set-2) to 0.10nm rms (set-3). The LSFR and MSFR of the M2 were 0.25nm rms and 0.20nm rms, respectively. The estimated wavefront error calculated from these LSFR numbers is 0.7nm rms.

Fabrication of aspherical mirrors for EUV projection optics set-3 of HiNA

We developed a high-numerical-aperture EUV exposure tool (HiNA). HiNA is equipped with an illumination system, projection optics, a mask stage and a wafer stage in the vacuum chamber. The projection optics consist of two aspherical mirrors (M1 and M2). The numerical aperture of the optics is 0.3. Thus far, we fabricated two sets of projection optics (set-1 and set-2). The wavefront errors of set-1 and set-2 were 7.5nm rms and 1.9nm rms, respectively. We developed the third set of projection optics (set-3), the target wavefront error of which was less than 1nm rms. In set-3, we also attempted to reduce flare. We completed the mirror polishing, coating and mirror adjustment of set-3. Using a new polishing method, we successfully reduced low-spatial-frequency roughness (LSFR), mid-spatial-frequency roughness (MSFR) and high-spatial-frequency roughness (HSFR) simultaneously. The predicted wavefront error calculated from the LSFR number was 0.69nm rms. MSFR, which strongly affects the flare of the optics, was significantly reduced to less than 0.2nm rms. The estimated flare was 7%, which is significantly reduced to one-fourth that of set-2. The wavefront error of set-3 was measured with the visible-light point diffraction interferometer (PDI) after coating and assembly. The wavefront error measured after adjustment and cramping of the adjustment system was 0.90nm rms, which is less than one-half the wavefront error of set-2.

testing at 1nm accuracy for sub-mm asphericity

This paper describes null interferometry at 1nm accuracy for testing aspherical surfaces of sub-mm deviation from the best fitting sphere. We have developed the two kinds of null compensators. The one is a “null lens” composed of almost perfect spherical surfaces and homogeneous glass. The other is a “zone plate” manufactured through the lithography process. The results of the two null testing were compared with the results by the ultra-precision CMM (Coordinate Measuring Machine). These totally different measurements differed only by an amount of 1.6nm rms. This result shows the accuracy of our null interferometry is almost 1nm rms.