Petr Cizmar

Optimization of accurate SEM imaging by use of artificial images

Today the vast majority of the scanning electron microscopes (SEMs) are incapable of taking repeatable and accurate images at high magnifications. Geometric distortions are common, so are drift, vibration, and problems related to disturbing electro-magnetic fields, contamination. These issues tend to degrade the quality of the image. Hence, in many cases it is not the focusing ability of the electron optical column, but these factors that limit the achievable resolution, repeatability and accuracy. This is a significant issue for nanometer-scale measurements, because the errors are many times greater than the measured distances. However, there are new image acquisition techniques that could improve the accuracy and repeatability of such measurements. One of these techniques is being developed at National Institute of Standards and Technology (NIST). This technique is based on cross-correlation combined with frequency filtering. Because the power this technique strongly depends on many conditions, like shape and periodicity of the sample features, noise, frequencies of the vibrations, etc., it needs to be properly evaluated and limits of usability of the technique should be specified. For this, a statistically significant number of SEM images varying in all of these parameters is necessary and the parameters must be known. Unfortunately, this is impossible to achieve using an SEM, because most of these parameters are random and the instrument parameters are unknown. A possible solution is to use the artificial SEM images, which can simulate these effects repeatably and deterministically. It is not an issue to generate a large number of such images in a reasonable time. The artificial image generator was developed at NIST originally for the evaluation of resolution-calculation techniques, but it is also very usable for assessment of new imaging techniques too. The new version of the generator is being developed at NIST. It allows for modeling different types of samples and several new effects, which also allow for taking into account the different characteristics of different SEMs. This paper describes, how the artificial images are built and how they may be used to improve the new SEM imaging techniques.

Can we get 3D-CD metrology right?

Our world is three-dimensional, and so are the integrated circuits (ICs), they have always been. In the past, for a long time, we have been very fortunate, because it was enough to measure a simple critical dimension (CD), the width of the resist line, to keep IC production under acceptable control. This requirement has changed in the last few years to contour and now to three-dimensional measurements. Optical lithography is printing photoresist features that are significantly smaller than the wavelength of the light used, and therefore it is indispensable to use optical proximity correction (OPC) methods. This includes modeling and compensation for various errors in the lithography process down to sub-nanometer, essentially atomic levels. The process has to rely on sophisticated and complex simulations and on accurate and highly repeatable dimensional metrology. The necessary dimensional metrology is beyond the conventional one-dimensional line width measurements, and must include two - and three-dimensional measurements of the contours and shapes of structures. Contour metrology needs accurate and highly repeatable measurements on sets and individual OPC structures, for which the critical dimension measurement scanning electron microscope (CD-SEM) is the key metrology tool. Three-dimensional (3D) metrology is now indispensable for IC technology, but current metrology tools and methods cannot fulfill the requirements. We believe that with the implementation of new methods it is feasible to develop 3D metrology that will well serve IC production, even on structures in the few nanometer-size range.

Advances in modeling of scanning charged-particle-microscopy images

Modeling artificial scanning electron microscope (SEM) and scanning ion microscope images has recently become important. This is because of the need to provide repeatable images with a priori determined parameters. Modeled artificial images are highly useful in the evaluation of new imaging and metrological techniques, like image-sharpness calculation, or drift-corrected image composition (DCIC). Originally, the NIST-developed artificial image generator was designed only to produce the SEM images of gold-on-carbon resolution sample for image-sharpness evaluation. Since then, the new improved version of the software was written in C++ programming language and is in the Public Domain. The current version of the software can generate arbitrary samples, any drift function, and many other features. This work describes scanning in charged-particle microscopes, which is applied both in the artificial image generator and the DCIC technique. As an example, the performance of the DCIC technique is demonstrated.

Advanced Image Composition with Intra-Frame Drift Correction

Drift Corrected Image Composition (DCIC) is a real-time technique that allows for acquiring more accurate images applicable in nanometer-scale imaging and metrology. This technique may be improved by adding intraframe drift correction as a further step in image-frame processing before the final composition. This new technique allows for further correction of the drift-related distortions in the individual frames. This is important especially for the metrological applications in nano-scale. This intra-frame drift correction requires knowledge of all dead times. This work proposes two possible approaches, how these values may be found.

Accurate and Traceable Dimensional Metrology with a Reference CD-SEM

NIST is currently developing two Reference scanning electron microscopes (SEMs), which are based on FEI Nova 600* variable vacuum, and on FEI Helios* dual-beam instruments. These were installed in the new Advanced Metrology Laboratory at NIST where the temperature variation is under 0.1 C° and the humidity variation is under 1%. Both SEMs are equipped with field emission electron guns and are capable of better than 1 nm spatial resolution. The ESEM has large sample capability, allowing for measurements on 200 mm wafers, 300 mm wafers and 150 mm photolithography masks, with a 100 mm by 100 mm measurement area in the center. The dual-beam instruments laser stage will work on smaller samples and has a 50 mm by 50 mm measurement area. The variable vacuum instrument is especially suitable for measurements on a large and diverse set of samples without the use of conductive coating. These will be among the most scrutinized of SEMs. A detailed, thorough work of combined measurements and optimization of the SEMs themselves is underway, which includes the assessment of resolution, signal transfer characteristics, distortion and noise characteristics in various working modes. Accurate three-dimensional modeling, including all aspects of beam formation, signal generation, detection and processing is under development. Establishment of modeling and measurement methods to ascertain the threedimensional shape and size of the electron beam is also underway. All these are needed to properly interpret the obtained data in accurate, physics-based measurements and will permit three-dimensional size and shape determination on a scale ranging from a few nanometers up to a few centimeters. Accuracy and traceability will be ensured through calibrated laser interferometry.