Yutaka Hojyo

A non-uniform SEM contour sampling technique for OPC model calibration

OPC model calibration techniques that use SEM contours are a major reason for the modern day improved fitting efficiency in complex mask design compared to conventional CD-based calibration. However, contour-based calibration has a high computational cost and requires a lot of memory. To overcome this problem, in conventional contour-based calibration, the SEM contour is sampled uniformly at intervals of several nanometers. However, such sparse uniform sampling significantly increases deviations from real CD values, which are measured by CD-SEM. We also have to consider the shape errors of 2D patterns. In general, the calibration of 2D patterns requires higher frequency sampling of the SEM contour than 1D patterns do. To achieve accurate calibration results, and while considering the varied shapes of calibration patterns, it is necessary to set precise sampling intervals of the SEM contour. In response to these problems, we have developed a SEM contour sampling technique in which contours are sampled at a non-uniform rate with arbitrary mask shapes within the allowable sampling error. Experimental results showed that the sampling error rate was decreased to sub-nm when we reduced the number of contour points.

Generation of large field SEM image by panorama composition technology for nano-order measurement

Semiconductor manufacturing has a pressing need for a method to accurately evaluate the global shape deformation of a photomask pattern. We thus propose a novel composition technique for a large field panorama image of scanning electron microscopy (SEM). The proposed method optimises the arrangement of segmented imaging regions (SIRs), which are components of a panorama image, on the basis of the design data of the photomask pattern layout. The quantity of the line pattern segment, which is a clue to the connection in an overlapping region between adjoining SIRs and the connectability of any two SIRs, is evaluated. As a result of the optimisation, it is guaranteed that all SIR images can be connected theoretically. For 30 evaluation points, the maximum connection error of the SIR images was 1.5 nm in a simulation using pseudo-SEM images. The maximum total measurement error, which includes the connection error and CD measurement error from the panorama image, is estimated at 2.5 nm. This error was equivalent to about 1.4% of the photomask line width (target: 3%). The experiments using real SEM images demonstrate the effectiveness of the proposed method. It was visually confirmed that a large field, high-resolution and seamless panorama image can be generated.

High-accuracy optical proximity correction modeling using advanced critical dimension scanning electron microscope–based contours in next-generation lithography

Optical proximity correction (OPC) modeling is traditionally based on critical dimension (CD) measurements. As design rules shrink and process windows become smaller, there is an unavoidable increase in the complexity of OPC resolution enhancement technique (RET) schemes required to enable design printability. The number of measurement points for OPC modeling has increased to several hundred points per layer, and metrology requirements are no longer limited to simple 1-D measurements. Contour-based OPC modeling has recently arisen as an alternative to the conventional CD-based method. In this work, the technology of contour alignment and averaging is extended to arbitrary 2-D structures. Furthermore, the quality of scanning electron microscope (SEM) contours is significantly improved in cases where the image has both horizontal and vertical edges (as is the case for most 2-D structures) by a new SEM image method, which we call fine SEM edge (FSE). OPC model calibration is done using SEM contours from 2-D structures. Then, the effectiveness of contour-based calibration is examined by doing model verification. The experimental results of the model quality with innovative SEM contours that was developed by Hitachi High-Technologies Corporation (Ibaraki-ken, Japan) are reported. This combination of advanced alignment and averaging, and FSE technologies, makes the best use of the advantage of contour-based OPC-modeling, and should be of use for next-generation lithography.

Weighting evaluation for improving OPC model quality by usingadvanced SEM-contours from wafer and mask

OPC modeling has been complex procedure in 28nm node, and it becomes difficult to obtain enough OPC modeling accuracy if calibration is done by using only CD value. Therefore it becomes essential to take pattern shape variation into consideration especially in 2D pattern calibration. Thus utilizing SEM-contour has become important technology. In SPIE advanced lithograpy 2010 [3], Contour-based OPC-modeling by using Advanced SEM-contour which is combined with Fine SEM Edge, alignment and averaging technologies was examined, and model quality was significantly improved. Also, in SPIE advanced lithography 2011, an advanced hybrid OPC modeling which uses 1D CD measurements by CD-SEM and 2D contours created by the advanced SEM-contouring technology and panoramic Mask SEM-contour showed high predictability for both 1D and 2D, even though the relationship between 1D and 2D calibration has trade-off. In this study, weighing function of Calibre ContourCal, a product of Mentor Graphics, was evaluated using the OPC data set same as that used in SPIE2011. The weighting can be set for 1D structure and 2D structure separately. In this paper, the quality of OPC model by applying different weighting is discussed.

The assessment of the impact of mask pattern shape variation on the OPC-modeling by using SEM-Contours from wafer and mask

As design rules shrink, Optical Proximity Correction (OPC) becomes complicated. As a result, measurement points have increased, and improving the OPC model quality has become more difficult. From the viewpoint of decreasing OPC calibration runtime and improving OPC model quality concurrently, Contour-based OPC-modeling is superior to CD-based OPC-modeling, because Contour-based OPC-modeling uses shape based rich information. Hence, Contour-based OPC-modeling is imperative in the next generation lithography, as reported in SPIE2010. In this study, Mask SEM-contours were input into OPC model calibration in order to verify the impact of mask pattern shape on the quality of the OPC model. Advanced SEM contouring technology was applied to both of Wafer CD-SEM and Mask CD-SEM in examining the effectiveness of OPC model calibration. The evaluation results of the model quality will be reported. The advantage of Contour based OPC modeling using Wafer SEM-Contour and Mask SEM-Contour in the next generation computational lithography will be discussed.

Improved Alignment Accuracy Using Lens-Distortion Correction for Electron-Beam Lithography in Mix-and-Match with an Optical Stepper

The mix-and-match use of an electron-beam (EB) lithography system and an optical stepper is an effective approach to achieving higher resolution while maintaining high throughput in the fabrication of Gbit DRAMs and advanced ASIC devices. In this approach, the highly accurate alignment of patterns exposed by different lithography methods is essential. In this paper, the lens distortion of the optical stepper is examined and distorted patterns are aligned with an EB lithography system. The overlay accuracy in this mix-and-match application was 51.4 nm (3σ) which corresponds to Gbit DRAMs fabrication.

The unified mask data format based on OASIS for VSB EB writers

We have developed a unified mask data format named “OASIS.VSB” for Variable-Shaped-Beam (VSB) EB writers. OASIS.VSB is the mask data format based on OASIS released as a successive format to GDSII by SEMI. We have defined restrictions on OASIS for VSB EB writers to input OASIS.VSB data directly to VSB EB writers just like the native EB data. We confirmed there was no large problem in OASIS.VSB as the unified mask data format through the evaluation results. The latest version of OASIS.VSB specification has been disclosed to the public in 2005.

Enhancement of unified mask data formats for EB writers

We have developed a unified mask data format named “OASIS.NEO1” for Variable-Shaped-Beam (VSB) EB writers as enhancement of unified mask data format named “NEO2”. OASIS.NEO is a pattern data format based on OASISTM3 released as GDSII replacement by SEMI. We have developed OASIS.NEO for practical use of unified mask data formats in mask data preparation (MDP) flow. For practical use, it is necessary to input OASIS.NEO data directly to VSB EB writers just like the native EB data. So we have defined restrictions on OASIS for VSB EB writers referring the restrictions in NEO based on GDSII named “GDSII.NEO4”. In this paper we proposed the specification of OASIS.NEO.