Nobutaka Magome

Double-patterning requirements for optical lithography and prospects for optical extension without double patterning

Double patterning (DP) has now become a fixture on the development roadmaps of many device manufacturers for half pitches of 32 nm and beyond. Depending on the device feature, different types of DP and double exposure (DE) are being considered. This paper focuses on the requirements of the most complex forms of DP, pitch-splitting (where line density is doubled through two exposures) and spacer processes (where a deposition process is used to achieve the final pattern). Budgets for critical dimension uniformity and overlay are presented along with tool and process requirements to achieve these budgets. Experimental results showing 45-nm lines and spaces using dry ArF lithography with a k1 factor of 0.20 are presented to highlight some of the challenges. Finally, alternatives to DP are presented.

Development of aerial image based aberration measurement technique

To maintain the best imaging performance of current high NA DUV scanners, in-situ aberration measurement is becoming more important than ever. In this paper, we present an aerial image based aberration measurement technique that can measure the aberrations up to 37th Zernike polynomial term. Our aberration measurement technique uses aerial image sensor (AIS) on DUV scanners. AIS is a slit scanning type aerial image sensor that can capture the one-dimensional intensity distribution of aerial images. Unlike previous photo resist image based aberration measurement technique, presented technique does not require the three-beam interference condition or the two-beam interference condition because it utilizes the image intensity information. This can eliminates the geometrical restriction in determination of the pupil sampling points. Thus, we made optimization of pupil sampling so that it can minimize the random error propagation in each Zernike coefficients. This optimization was done on a trial and error basis and we observed that the random error propagation significantly depended on pupil sampling plan. The measured aberration was correlated to the programmed aberration induced by lens element displacement. Also the measurement repeatability was evaluated and confirmed. The overall performance of this aberration measurement technique is found to be appropriate for in-situ aberration monitor of current high NA scanners.

Double patterning requirements for optical lithography and prospects for optical extension without double patterning

Double patterning (DP) has now become a fixture on the development roadmaps of many device manufacturers for half pitches of 32 nm and beyond. Depending on the device feature, different types of DP and double exposure (DE) are being considered. This paper focuses on the requirements of the most complex forms of DP, pitch splitting, where line density is doubled through two exposures, and sidewall processes, where a deposition process is used to achieve the final pattern. Budgets for CD uniformity and overlay are presented along with tool and process requirements to achieve these budgets. Experimental results showing 45 nm lines and spaces using dry ArF lithography with a k1 factor of 0.20 are presented to highlight some of the challenges. Finally, alternatives to double patterning are presented.

New alignment sensors for wafer stepper

Two new alignment sensors for wafer steppers are developed to attain high alignment accuracy on all layers by targeting layers that are difficult to align using the existing alignment sensor, which is based on a laser beam scanning system. The Field Image Alignment (FIA) is a bright- field TV image processing alignment system using broadband illumination. The major advantage of FIA is that due to the broadband light source used for illumination, the edges of the alignment mark can be detected without being influenced by the interference fringes formed by the photoresist. Additionally, even if the cross section of the alignment mark is asymmetrical, the asymmetry can be accurately captured and alignment at the proper position can be achieved. The Laser Interferometric Alignment (LIA) is a grating alignment system based on an optical heterodyne interferometry technique. The advantage of this sensor is that is not affected by surface irregularities such as grains because it will process only specific spatial frequency components diffracted from the alignment mark. Therefore, the spatial frequency components which are diffracted from the metallic grains will be disregarded and will not influence alignment. This allows the alignment to be successful even for low step height or deformed marks. With the development of FIA and LIA, the authors have successfully complemented the existing sensor, so that a high alignment accuracy for the mass production of VLSI with 0.5-0.35 micron rules can be achieved on most layers.

Recent performance results of Nikon immersion lithography tools

Nikons production immersion scanners, including the NSR-S609B and the NSR-S610C, have now been in the field for over 2 years. With these tools, 55 nm NAND Flash processes became the first immersion production chips in the world, and 45 nm NAND Flash process development and early production has begun. Several logic processes have also been developed on these tools. This paper discusses the technical features of Nikons immersion tools, and their results in production.

Impact of CD and overlay errors on double-patterning processes

Double patterning (DP) is today the main solution to extend immersion lithography to the 32 nm node and beyond. Pitch splitting process with hardmask transfer and spacer process have been developed at CEA-LETI-Minatec. This paper focuses on experimental data using dry ArF lithography with a k1 factor of 0.20 ; the relative impact of each DP step on overlay and CD uniformity budgets is analyzed. In addition, topography issues related to the presence of the patterned hard mask layer during the second imaging step is also investigated. Tool-to-itself overlay, image placement on the reticle and wafer deformation induced by this DP process are evaluated experimentally and resulting errors on CD budget have been determined. CD uniformity error model developed by Nikon describing the relationship between CD and overlay in different DP processes is validated experimentally.

Overlay improvement by using new framework of grid compensation for matching

Overlay accuracy is a key issue in the semiconductor manufacturing process. To achieve overlay requirements, we developed compensation functions, i.e. Enhanced Global Alignment (EGA), Super Distortion Matching (SDM), and Grid Compensation for Matching (GCM). These functions are capable to reduce all the components except local linear components caused by a wafer global deformation. In this paper we introduce a novel correction framework which includes new compensation function called Shot Correction by Grid Parameter; thereby enabling further enhancements to overlay. Using this novel framework, we show both simulation and experimental data demonstrating improved overlay accuracy.

Laser Alignment Signal Simulation for Analysis of Al Layers

Laser alignment systems are widely used in optical lithography for LSI production. They, especially the dark field type, have high signal-to-noise sensitivity to step height or grating line alignment marks, because of a sharp and high intensity spot owing to coherency of laser beam. The coherency often distorts alignment signals according to a little change of interference condition caused by variation of alignment marks. This phenomenon causes overlay accuracy to Al layers to deteriorate. This is because that the Al layers sometimes have a cracked or granulated rough surface and asymmetrical alignment marks which produce noise and deformed signals. In order to analyze and optimize these effects, a computer program has been developed. Laser Step Alignment (LSA) is used as a model system. With this program, it is possible to calculate the alignment signal from 3-dimensional alignment mark and resist profiles. Then, the overlay errors are analyzed from the simulated signals at various detection levels. In one optimized case, when a concave mark is used, the lower detection level should be selected to minimize the influence of Al asymmetrical coverage. This case corresponds to many experimental results.

Outlier rejection with mixture models in alignment

Outliers in measurement often interfere with alignment. They are caused by sudden damages in the alignment mark, and existence of particles, resist damages and so on. In a conventional way to identify outliers, the observations that have larger residual than previously determined threshold are identified as outlier. It works well only with the operator’s labor of adjusting the threshold according to the deviation of ordinaries (non-outliers). However, labor is a problem especially in Small-Quantity Large-Variation fabrication such as for ASIC, System-LSI and so on. A novel method for elimination of the labor has been developed. It utilizes normal mixture models whose number of components is determined based on the Maximum Penalized Likelihood (MPL) method. It can be regarded as an identification method that determines threshold adaptively using ordinaries’ deviation. Simulation results show that the penalty coefficient, the only parameter of the method, can be a constant in the variation of ordinaries deviation. It also shows that in the absence of outliers, the accuracy of the method is comparable with the maximum likelihood estimation that is commonly considered to be the best method when the observations follow the normal distribution. The method performs better than conventional ones when there are a sufficient number of observations (no less than ten) in the standard Enhanced Global Alignment (EGA). Superiority of the adaptive method is dependent upon the probability of outlier occurrence, variation of the situation, the number of observations and the complexity of the model fitted to the observations.

Management of pattern generation system based on i-line stepper

A Device mask of 180nm generation was fabricated by Photomask Repeater system and the performance of it proved to be high by the results of fabricated mask. Great margins between the results of the fabricated mask and specifications suggest that lower graded masks can be used as master masks. From this point of view, error budgets were estimated about CD uniformity and pattern placement. The required specifications for master mask were estimated for 180nm and 130nm lithography. In CD uniformity the specification is 50nm(3?) for 180nm and 30nm(3?) for 130nm lithography. In pattern placement the specification is 75nm(3?) for 180nm and 50nm(3?) for 130nm lithography. In defect size the specification is lOOOnm for 180nm and 900nm for 130nm lithography. The requirements of master mask are rather rough even for 130nm lithography and enough realistic.