Yoshiyuki Sekine

Mask optimization for arbitrary patterns with 2D-TCC resolution enhancement technique

In this paper, a new resolution enhancement technique named 2D-TCC technique is proposed. This method can enhance resolution of line patterns as well as that of contact hole patterns by the use of an approximate aerial image. The aerial image, which is obtained by 2D-TCC calculation, expresses the degree of coherence at the image plane of a projection optic considering mask transmission at the object plane. OPC of desired patterns and placement of assist patterns can be simultaneously performed according to an approximate aerial image called a 2D-TCC map. Fast calculation due to truncation of a series in calculating an aerial image is another advantage. Results of mask optimization for various line patterns and the validity of the 2D-TCC technique by simulations and experiments are reported.

Extension of the 2D-TCC technique to optimize mask pattern layouts

The extendibility of 2D-TCC technique to an isolated line of 45 nm width is investigated in this paper. The 2D-TCC technique optimizes mask patterns placing assist pattern automatically. For 45 nm line patterns, the assist pattern width generally becomes much smaller than the exposure wavelength of 193 nm. Thus, the impact of the topography of a mask is examined using an electro-magnetic field (EMF) simulation. This simulation indicates that unwanted assist pattern printings are brought about by assist patterns with a smaller size than expected by the Kirchhoffs approximation. The difference, however, can be easily solved by giving a bias to the main pattern in the optimized mask. The main pattern bias decreases DOF very little. Furthermore, DOF simulated with a thick mask model is roughly the same as that simulated with a thin mask model. Therefore the topography of the optimized mask does not have an influence on the assist pattern position of the optimized mask. From these results, we have confirmed that the 2D-TCC technique can be extended to the optimization of 45 nm line patterns. As one of the notable features, the optimized aperiodic assist pattern greatly reduces MEEF compared with the conventional periodic assist pattern. To verify the feasibility of the 2D-TCC technique for 45 nm line, we performed experiment with an optimized mask. Experimental results showed that DOF increased with the number of assist pattern as simulation indicated. In addition, a defect whose length was twice that of the assist pattern did not have an influence on CD. From these results we have confirmed that the 2D-TCC technique can enhance the resolution of 45 nm line and has practical feasibility.

Shearing Interferometry for at Wavelength Wavefront Measurement of Extreme-Ultraviolet Lithography Projection Optics

We present a type of lateral shearing interferometer (LSI) for at-wavelength characterization of the projection lens for use in extreme-ultraviolet lithography (EUVL). LSI is one of the potential candidates for high Numerical Aperture (NA) optics testing at the EUV region. To address the problem of multiple-beam interference, we propose a general approach for derivation of a phase-shift algorithm that is able to eliminate the undesired 0th order effect. The main error source effects including shear ratio estimate, hyperbolic calibration, and charge coupled device (CCD) size, etc. are characterized and the measurement accuracy of the LSI is estimated to be within 7 mλ rms (0.1 nm rms at 13.5 nm wavelength) for testing the wavefront of EUVL projection optics.

Development of an experimental EUV interferometer for benchmarking several EUV wavefront metrology schemes

An experimental extreme UV (EUV) interferometer (EEI) using an undulator light source was designed and constructed for the purpose of developing wavefront measurement technology with the exposure wavelength of the projection optics of EUV lithography systems. EEI has the capability of performing five different EUV wavefront metrology methods.

Wave-front errors of reference spherical waves in high-numerical aperture point diffraction interferometers

In phase-shifting point diffraction interferometry (PS/PDI), a pinhole with a diameter of 34 nm is necessary to measure a wave-front aberration of extreme ultraviolet projection optics of numerical aperture (NA) 0.20. However, it is extremely difficult to process such a small pinhole, and light transmission through the pinhole becomes too low. Here, the diameter of a pinhole is optimized, together with the thickness of a Ta membrane, for a converging wave of NA 0.20 with no aberration so that the difference between a wave front produced by the pinhole and that of a spherical wave is minimized. On that condition, the optimum values are a diameter of 50 nm and a thickness of 200 nm. For these values, behaviors are examined in real cases, including focal point shifts and aberrations with incident light. Astigmatism in the aberrations has the most impact on a wave-front error, and a 0°–90° astigmatism (Z5) coefficient in the FRINGE Zernike polynomials of test optics is required to be less than 50 mλ to use th...

High precision binary optical element fabricated by novel self aligned process

Binary optical elements (BOE) tend to be used frequently in optical equipment because aspherical type BOE is easy to fabricate and the BOE can easily correct chromatic aberration. What makes this technology attractive to industry is that the BOE can be fabricated precisely by the semiconductor micro fabrication technique. However when it is used for more precise optics e.g. semiconductor exposure equipment, more precise BOE are necessary. In some manufacturing processes, we have confirmed that the alignment error between masks is the dominant factor to decrease diffraction efficiency by optical simulation. To study this problem, we have produced a BOE using a novel self-alignment method we have devised, then compared it to a BOE which has being made by a conventional method.

Feasibility of 37-nm half-pitch with ArF high-index immersion lithography

ArF water immersion exposure systems with a numerical aperture (NA) of over 1.3 are currently being developed and are expected to be used for the node up to 45-nm half-pitch. Although there are multiple candidates for the next generation node, we here focus on ArF immersion lithography using high-index materials. The refractive index of highindex fluids is typically about 1.64 and is larger than that of fused silica (~1.56). In this situation, the NA is limited by the refractive index of silica and is at most 1.45. An exposure system with 1.45 NA is not suitable for 32-nm hp node, but may be used for 37-nm hp node. In spite of this limitation, the system has the advantage of slight alterations from the current system using water as immersion fluid. On the other hand, high-index lens material is effective to increase the NA of projection optics further. At present, LuAG, whose refractive index is 2.14, is most promising as high-index lens material. The combination of high-index fluid and high-index lens material can enhance the NA up to about 1.55 and the exposure system would be available for the 32-nm half-pitch node. Although high-index immersion lithography is attractive since it is effective in raising resolution, such new materials should be examined if these materials can be used for high precision projection optics. Here, we have investigated optical characteristics of high-index materials in order to realize high-index immersion systems.

Application of large scale binary optical elements to high resolution projection optics used for microlithography

Binary optical elements (BOEs) are very attractive because of their compactness and high aberration correction abilities. However, in their application to high resolution optics, some critical issues such as the influence of fabrication errors and the behavior of spurious order diffraction beams must be carefully examined. An eight-level BOE, ideally giving 95 % diffraction efficiency, is fabricated by lithographic techniques using a stepper for pattern delineation. A large element exceeding the size of stepper exposure area can be obtained by pattern stitching in the radial and the tangential directions. The performance of these BOEs confirm that it is fundamentally possible to apply BOEs fabricated with currently available techniques to high resolution projection optics used for microlithography.

Fast computation of constructive and destructive interference areas in partially coherent imaging for resolution enhancement in optical microlithography

We present a method to determine constructive and destructive interference areas on the object plane in partially coherent imaging. This method is based on the interference pattern on the image plane. A function Gamma that shows constructive and destructive interference areas with respect to the origin on the object plane is derived as the product of mutual intensity on the object plane and the Fourier transform of the pupil function. The convolution integral of Gamma and object transmittance gives the constructive and destructive interference areas. Experimental results show that small clear openings placed at constructive interference areas enhance light intensity at desired positions. By applying this method to optical microlithography imaging, one can achieve resolution enhancement of fine features with a relatively small amount of computation.

Hyper-NA imaging in ArF immersion lithography

ArF immersion lithography (Lin, 2004) has emerged as the primary solution for the manufacturing of semiconductor device for 65-nm half-pitch node and beyond. The immersion technique allows the design of projection optics with a numerical aperture that exceeds unity. Pure water is the preferred immersion fluid for the first generation of the immersion exposure tool. The water has good transmissivity and relatively high refractive index of 1.44 at ArF wavelength of 193 nm (Burnett et al., 2004). An NA of up to about 1.3 seems feasible with water. Moreover, high-index materials have recently been proposed for the immersion fluid (Miyarnatsu et al., 2005; Peng et al., 2005) as well as for the lens material (Burnett et al., 2004). With the high-index materials, the immersion technique may ultimately allow an NA that is even larger than the refractive index of water. In this paper, we present analytical results on imaging properties of the ArF immersion lithography in hyper-NA region to provide insight on the potential and challenges of the immersion technique.