Dong-Seok Nam

Fast hologram pattern generation by radial symmetric interpolation

In this paper, we present a fast hologram pattern generation method by radial symmetric interpolation. In spatial domain, concentric redundancy of each point hologram is removed by substituting the calculation of wave propagation with interpolation and duplication. Also the background mask which represents stationary point in temporal domain is used to remove temporal redundancy in hologram video. Frames are grouped in predefined time interval and each group shares the background information, and hologram pattern of each time is updated only for the foreground part. The effectiveness of proposed algorithm is proved by simulation and experiment.

Potential of solid immersion lithography using I-line and KrF light source

ArF technology is currently being used for 80nm resolution in the photo-lithography field, and ArF wet immersion technology is expected to be used for high resolution systems down to 50nm. Between ArF wet immersion technology and EUV technology, there is no proper technology that can cover the resolution range from 50nm to 30nm. In this paper, a new lithography technology using a Solid Immersion Lens (SIL) is introduced as an idea for very high resolution, and its resolution achievement is estimated through simulation. SIL technology is a near-field optics technology that achieves high resolution. A SIL is a hemispherical lens, and the incident beam is normal with respect to the surface of the lens. Because a high refractive index material is used for the SIL, very high numerical aperture provides high resolution. The resolution limit is estimated by calculating the vector irradiance inside the thin-film stack composed of the SIL, air gap, photoresist, anti-reflection layer and substrate. Feature size is estimated over reasonable exposure latitude at 20nm depth in the resist. Results show that, using a 365nm wavelength source, 70nm resolution is expected, and 50nm resolution is expected with a 248nm wavelength source. With a shorter wavelength light source and a proper SIL material of high refractive index for the wavelength, higher resolution can be achieved.

Experimental investigation of solid immersion lens lithography

There are several next generation technologies for high resolution lithography, such as ArF wet immersion, F2, EUV, etc. However, these technologies are very expensive because of projection lens and mask costs. Near-field optics using a solid immersion lens (SIL) can meet the requirement of high resolution in a cost-effective way. In this paper, a very compact and inexpensive high resolution system using a SIL is introduced and preliminary experimental results are presented using a 405nm laser diode system. The SIL is used with a modified conventional inverted microscope. The air gap between the SIL flat bottom surface and the wafer is kept less than 50nm. Optical reflected power from SIL bottom and wafer interface is used to control the gap. A high resolution experiment with 405nm wavelength is discussed.

Process latitude extension in low k1 DRAM lithography using specific layer-oriented illumination design

Improvement of process latitude is tested in typical DRAM patterns by using the optimized illumination for each layer pattern. The optimized illumination for a specific layer is generated by modifying the Fourier transformed image of the layer and by using in-house illumination optimization program, which can simulate the maximum process latitude. These illumination shapes are compared with each other, and it is confirmed that both illuminations are similar in shape. The typical DRAM patterns are exposed using the optimized illuminations, and the process latitude is compared with typical annular illumination cases. It is certain that the process latitude using the optimized illumination is greater than the high sigma annular illumination. By using the optimized illumination, the enlarged process latitude makes it possible to use lower grade tools for a critical layer. It is expected that the lifetime of low-grade exposure tools can be extended by this illumination optimization technique.

Effectiveness and confirmation of local area flare measurement method in various pattern layouts

It is well known that flare, which increases the background intensity and loses the image contrast, degrades the pattern fidelity and CD uniformity. Usually there is little mid and long-range flare at the initial exposure tool introduction except the short-range flare, so called, aberration. However, flare effect is observed in used exposure tools. To estimate the influence of flare, both lens quality of the exposure tool and mask pattern layout with various open ratios are important parameters to be considered. So it is very crucial to make a standard mask layout to measure the flare value as a tool specification. So far, CD variation of the long-range flare has been measured and reported. The long-range flare includes the average influence of the short and mid-range flare and affects more than several hundred- micron distances. Recently it is observed that lens contamination is a dominant component among sources of flare and induced by the pattern layout with its different open ratio. Being contaminated, the lens malfunctions with various types of scattering sources. These scattering sources make the mid and long range flare. This type of flare source has time dependence. If there are proper monitoring methods for the flare measurement, it is possible to maintain the lens quality within the limit of mid range flare. In addition, matching the flare value to CD distribution is not easy because there is no standard measurement method to distinguish the short and mid-range flare from the long-range one. In this paper a LOcal Area Flare Evaluation Reticle (LOAFER) method is suggested. The LOAFER is designed to measure the local area flare of the lens, that is, the short and mid-range flare and the local flare distribution of the exposure tool lens can be characterized. Then matching the result to the real device pattern will be introduced.

E-beam shot count estimation at 32 nm HP and beyond

Recent Low k1 era requires aggressive OPC technology with advanced lithography technology. The aggressive OPC contains the rounded pattern and a lot of assistant pattern which are the main source to increase the shot division. We have defined the shot complexity, which is defined by the ratio of number of shot between the interested pattern and the 1:1 L/S pattern. Based on shot complexity parameter, we have estimated the writing time as the device node decreases. We expect that the aggressive OPC and the high dose could generate severely the writing time issue in 32nm node era.

Image placement error of photomask due to pattern loading effect: analysis and correction technique for sub-45 nm node

As semiconductor features shrink in size and pitch, the image placement error at photomask has been interested as an important factor to be reduced. Especially, by the development of double exposure technique (DET) or double patterning technique (DPT) for sub-45 nm node the image placement error is required to be controlled tightly. Following ITRS roadmap, when DET or DPT is used the registration for sub-45 nm node is required to be less than 4 nm but this specification still corresponds to the challengeable goal. Among various sources of image placement errors, here, we focus on the error occurring at patterning process of photomask and discuss its effect on the photomask overlay. We name the image placement error occurred at patterning process due to e-beam charging effect, absorber etching effect, and so on as the pattern loading effect. We quantify the amount of pattern loading effect on registration error, analyze it with the help of simulation and experiment, and discuss the character of each error and correction method.

Improved scatterometry method of critical dimension measurements and its application for control of development process

Downscaling of microchip production technology continually increases requirements to precision of process control, and demands improvement of critical dimension (CD) measurement and control tools. In this paper we discuss the application of in situ method of critical dimension measurement for improvement of photomask development process. For this purpose scatterometry and fitting methods are applied to the CD end point detector system (CD EPD). The CD EPD system is different from the commonly used EPD system, which mainly detects the thickness of remaining resist. Measurement can be performed directly during development process, thus there is an advantage of measurement time decreasing in comparison with the ex situ method. In situ method allows one to control development precisely, and gives possibility to meet the requirements of process control. For the application of scatterometry to the CD measurement, diffraction analysis is carried out by using of rigorous coupled wave analysis (RCWA). We calculate the library of reflected spectra with various CD and heights of the pattern. These spectra are used for fitting with an experimentally measured one to get the CD and height. To increase precision and speed of measurements interpolation of spectra and various fitting methods are used.

Maskless lithography with the solid immersion lens nanoprobes

The International Technology Roadmap for Semiconductors (ITRS) shows that 45 nm and lower feature sizes are required in lithographic production before the year 2007. Both immersion lithography and EUV lithography can play major roles in realizing this goal. However, a maskless lithography system capable of producing 45 nm features is an attractive option for small-volume semiconductor fabrication, such as with ASIC manufactures. Compared with a conventional lithography system, the maskless feature of the system allows the chip designer to be free of the very expensive process of mask fabrication and to shortcut development time. In this paper, we discuss a new maskless lithography concept employing an array of solid immersion lens (SIL) nano-probes. The nano-probes are efficient near-field transducers. Each transducer is the combination a SIL, a dielectric probe tip and an antenna structure. The nano-probes are fabricated in arrays that dramatically improve throughput. By combining these technologies, it should be possible to fabricate an efficient array of near-field transducers with optical spot dimensions of around 20 nm when illuminated by a 405 nm laser diode source. This paper plans to address, for the first time, the efficient generation of an array of light spots with dimensions of λ/20 or less that couple efficiently into dielectric films, like photoresist.

Layer-specific illumination optimization by Monte Carlo method

Layer specific illumination has merits of enhancement of resolution, widening DOF and image fitness. For dense patterns like DRAM cell, layer specific illumination is a major candidate to drive low k1 lithography. To find out the best illumination for a specific pattern, diffracted image of the pattern and the ratio of captured first order to 0th order diffracted beam should be considered. By spectrum analysis, the best illumination is obtained for simple patterns like dense lines, brick wall, and dense contacts. In this paper, the procedure of obtaining the best illumination for specific patterns is presented. Comparing general illuminations such as annular, the resultant illumination is proved to have wider DOF and enhancement of resolution. The best illumination can also be found by Monte Carlo simulation. For simple one-dimensional case, its validity is proved. From the exposure results, wide DOF and enhancement of resolution is confirmed.