Hideki Komatsuda

Development status of projection optics and illumination optics for EUV1

Final adjustment of EUV1 projection optics was completed and its performance was evaluated. Wavefront error of 0.6nmRMS in average through the exposure field was achieved. The maximum and minimum wavefront errors in the whole field were 0.8nmRMS and 0.3nmRMS, respectively. Flare of the projection optics was estimated from the measured power spectrum density (PSD) of each aspheric mirror of the projection optics. The flare value for Kirk pattern with the radius of 1μm was estimated to be about 10%. Completed projection optics was installed into the main body of EUV1. Optimization of polishing process was further pursued. Consequently, LSFR of 38pmRMS, MSFR of 80pmRMS and HSFR of 68pmRMS were achieved. Assemble of the illumination-optics unit for EUV1 was completed and its performance was evaluated using an illumination-optics test stand. Irradiation uniformity on the mask plane, pupil fill and so on were measured with the test stand using a visible light and EUV radiation. Completed illumination-optics unit was installed into the main body of EUV1. Reflection-type spectral purity filter (SPF) and high-NA projection-optics design were investigated as new R&D items for the future optics of EUV exposure tools.

Feasibility study of EUV scanners

EUV lithography is a successor to DUV/VUV lithography, and is the final photon base lithography technology. The concept of EUV scanners for 50nm node and below is considered by clarifying the similarities and differences between EUV scanners and DUV scanners. Illumination optics, projection optics, wafer alignment sensors and wafer focus sensors are examined. And the throughput model, overlay budget and focus budget are introduced. The concrete design of illumination optics and the requirements for sources are described. Numerical aperture, magnification and field size are discussed. EUV scanners for 50nm node and below are realized.

Development progress of optics for extreme ultraviolet lithography at Nikon

The full-field extreme ultraviolet (EUV) exposure tool named EUV1 is integrated and exposure experiments are started with a numerical aperture of the projection optics of 0.25, and conventional partial coherent illumination with a coherence factor of 0.8. 32-nm elbow patterns are resolved in a full arc field in static exposure. In a central area, 25-nm line-and-space patterns are resolved. In scanning exposure, 32-nm line-and-space patterns are successfully exposed on a full wafer. Wavefront error of the projection optics is improved to 0.4-nm rms. Flare impact on imaging is clarified, dependent on flare evaluation using the Kirk test. Resolution enhancement technology (RET) fly-eye mirrors and reflection-type spectral purity filters (SPFs) are investigated to increase throughput. High-NA projection optics design is also reviewed.

Fabrication of a complex-shaped mirror for an extreme ultraviolet lithography illumination system

We propose and discuss several fabrication processes for a complex-shaped mirror, which is a fly-eye mirror, used in an extreme ultraviolet lithography (EUVL) illumination system. The mirror has a complex reflective surface consisting of many concave mirror elements that are sections of a sphere; the top of each element is arc shaped. In the present study, we focus on one process in which all elements are fabricated individually and then arranged side-by-side to form the mirror. Thus, as the first step in this process, we fabricate the arc-shaped elements made of invar with electroless nickel plating. The resultant reflective surfaces have a peak-to-valley (PV) surface accuracy of 0.31 μm. The surfaces have the rms roughnesses of about 0.23 and 0.35 nm in areas of 110×140 μm and 1×1 μm, respectively. The slope accuracies of the surfaces relative to the bottom surfaces are –166 and 43 arc-sec in the y and x directions, respectively. Thus, the mirror elements for the fly-eye mirror can be fabricated very accurately with smooth surfaces, although the mirror elements have a special shape compared to that of general optics.

Development of optics for EUV lithography tools

Nikon is now conducting a development of the full-field EUV exposure tools for EUVL process development named EUV1, which will be delivered in 2007. Polishing and coating of six different kinds of mirrors for the projection optics of EUV1 were finished and adjustment of the projection optics has been started. Sophisticated polishing process for aspheric mirrors, which can reduce LSFR, MSFR and HSFR down to less than 0.1nmRMS simultaneously, were developed. Process conditions of Mo/Si multilayer coatings have been optimized to obtain high reflectivity, low internal stress and graded coating simultaneously. Wavefront error of the projection optics under adjustment process is now 3nmRMS. We will try to achieve a wavefront error of less than 1nmRMS by further precise adjustment. Fabrication process of flys eye mirrors, which is a key device of illumination optics of EUV1, was developed. All the mirrors of the illumination optics for EUV1 were finished and evaluation of its performance using an illumination-optics test stand has been started. Development and fabrication of both the projection optics and the illumination optics for EUV1 are satisfactorily in progress.

Development progress of optics for EUVL at Nikon

Full-field EUV exposure tool named EUV1 integrated and exposure experiments were started with the numerical aperture of the projection optics of 0.25 and conventional partial coherent illumination with the coherence factor of 0.8. 32nm elbow patterns were resolved in full arc field in static exposure. In the central area 25nm line-and-space patterns were resolved. In scanning exposure, 32nm line-and-space patterns were successfully exposed on a full wafer. Wavefront error of the projection optics was improved to 0.4nmRMS. Flare impact on imaging was clarified depend on the flare evaluation using Kirk test. RET flys eye mirrors and reflection-type SPF are investigated to increase throughput. High-NA projection optics design is also reviewed.

Fabrication and testing of a complex-shaped mirror constructed with silicon mirror elements

We have fabricated a complex-shaped mirror array and evaluated its fabrication accuracy. The mirror is constructed with 16 mirror elements with an arc-shaped contour and a spherical reflective surface, and is part of an arc-shaped fly-eye mirror for use in an extreme ultraviolet lithography system. All the mirror elements are manufactured individually, using single-crystal silicon as the mirror material, and are then arranged side by side on a base plate by magnetic attraction. The spherical mirror elements are manufactured to have highly accurate and smooth surfaces, although they have a different shape from those of general optics, and are assembled to form the fly-eye mirror. To evaluate the positioning errors of the mirror elements of the assembled fly-eye mirror, an optical testing system using visible rays are developed. From the measured results using this system, the average tilt error of the mirror elements is 69 arcsec, assuming that the system has a collimation error, which we corrected through calculation. Thus, the fly-eye mirror is fabricated with a high accuracy, demonstrating that the fabrication process is useful in realizing complex-shaped mirrors.

Fabrication of a complex-shaped mirror by arranging silicon mirror elements

We propose a method for the fabrication of the mirror elements that compose the complex-shaped fly-eye mirrors for use in an extreme ultraviolet lithography (EUVL) system. In this method, silicon blocks are polished to have a spherical shape and smoothness, from which the mirror elements are cut out by wire electrical discharge machining. Thus, this method enables more effective fabrication of the mirror elements than the method proposed previously. To evaluate the effectiveness of the present method, the mirror elements having a flat reflective surface are fabricated as a preliminary experiment. The resultant mirror elements have highly accurate and smooth surfaces, although they have a unique shape compared to that of general optics. Moreover, to construct the fly-eye mirror, we propose a method for arranging the mirror elements, in which mirror elements are fixed to a base plate containing magnets. This allows the accurate arrangement of all the reflective surfaces relative to the base plate. To enable their attraction to the magnets, the bottom of the silicon mirror elements is plated with metal. Experiments confirm the mirror elements are accurately arranged. Thus, we demonstrate that the proposed methods are useful in realizing the fly-eye mirror.

Fabrication of a fly-eye mirror for an extreme ultraviolet lithography illumination system

We proposed and discussed several fabrication processes for an arc-shaped and a rectangular fly-eye mirror, used in an EUVL illumination system proposed by Komatsuda. In particular, in the present study, we focused on one process in which all elements are fabricated individually and then arranged side-by-side to from the mirror. Thus, as the first step in this process, we fabricated arc-shaped and rectangular elements; the former and the latter were made of invar and fused silica, respectively. The slope accuracies of the surfaces for the arc-shaped and the rectangular mirror elements were +/- 15 and +/- 22 seconds, respectively. These values indicate that the mirror elements were fabricated with sufficient accuracy to satisfy the fly-eye mirror specifications. Moreover, both mirrors had the midfrequency roughness of about 0.25 nm RMS which satisfies the desired specifications. However, the high- frequency roughnesses of the former and the latter were 1.08 nm RMS and 0.59 nm RMS respectively, which must be improved.

Fabrication of a fly-eye mirror for an extreme-ultraviolet lithography illumination system by arranging silicon mirror elements

A novel EUVL illumination system including two fly-eye mirrors was proposed by Komatsuda. However, these mirrors are difficult to realize because they have a complex-shaped reflective surface constructed from many concave mirror elements. In the present study, we discuss a fabrication process for the fly-eye mirrors, in which all elements are fabricate individually and are then arranged side-by-side to form the fly-eye mirrors. We propose a fabrication method for the mirror elements in which silicon blocks are ground and polished into a spherical surface, and are then cut into the shape of the mirror element using a wire electric- discharge machine. Using the proposed method, we successfully fabricated mirror elements having a flat reflective surface in the preliminary experiment. Moreover, we propose a method of arranging the mirror elements to construct the fly-eye mirrors. In this method, to arrange the elements accurately without any contamination, the elements are fixed to a base plate containing magnets by attraction of their bottom surfaces. The bottom surfaces are plated with metal to enable their attraction to the magnets. The mirror elements were accurately arranged to satisfy the fly-eye mirror specifications by this magnetic method.