CHARIOT

CHARIOT software is used to simulate signal and images in scanning electron microscope (SEM) and energy and charge deposition in electron beam lithography (EBL).

The software allows simulation of a SEM signal from complex 3D patterns. Each layer can be specified and edited separately, including chemical composition, physical properties, and the geometry of the pattern. Layers can automatically be combined to imitate such processes as etch, film deposition, and lift off; in this way, a complex 3D pattern, like a microcircuit, can be specified.

CHARIOT simulation model considers:

elastic electron scattering: either Mott or Rutherford models
inelastic energy loss using either CSDA or DSDA, where processes of inner-shell ionization, outer shell ionization, and plasmon generation and decay are taken into account
semi-empirical model was used for low (E < 50 eV) energy electrons scattering
generation of fast and true secondary electrons, considering multiple generations of secondaries
electron propagation between layers
detector geometry, location and energy transfer function
charge and discharge of the sample

SEM

Accurate measurement of linewidth is a critical problem in sub-100 nm semiconductor manufacturing, where required accuracy is below 1 nm. CD-SEMs are usually used for such measurements. A cross-correlation of CD-SEMs, while demonstrating a good relative trend, is often subject to a significant absolute linewidth error. There is no proven algorithm for edge detection in CD-SEMs.

In calibration experiments, exact knowledge of a pattern is only possible for a standard-type pattern. Such traceable, characterized patterns are expensive and extremely limited in terms of materials and a type of a pattern. Using such a pattern, a SEM can be calibrated; however, for any other pattern or setup condition, the calibration should be repeated using a specific traceable pattern. In simulation, there is the luxury of knowing the pattern exactly – it is specified by the user. Simulated SEM signal and a pattern can be compared; an algorithm to extract an edge position can be established.

Scattering of an electron beam in a microstructure, generation of secondary electrons, and characteristics of the detector, as well as the material and shape of the features, determine a SEM signal. Because of this complexity, understanding the signal formation mechanism and accurate extraction of edge position are problematic. To respond to this problem, CHARIOT software was developed; this software employs advanced physical and computational models to comply with required accuracy, especially at low voltage.

It was demonstrated using CHARIOT that edge detection depends greatly on parameters of SEM settings, like beam diameter, and pattern properties, like the wall angle of a pattern. This data was also found from experiments. Both the signal and pattern are known when using CHARIOT. Then an offset for a specific SEM algorithm can be found. An algorithm for automatic edge detection in CD-SEMs can be tuned for beam parameters and the type of pattern.

EBL

An example of energy deposition in direct write EBL is shown in the Figure. Energy is shown in 200 nm resist on silicon. It is clear that the area of deposited energy is considerably smaller at 100 KV than at 50 KV.

Electron scattering in EBL (proximity effects) is non-desirable; it leads to CD-variation of fabricated pattern. The correction of the linewidth called proximity correction is based on the knowledge of so called proximity function, that describes absorbed energy as a function of a distance from the beam center. The CHARIOT software allows determining proximity function for any type of a multilayered substrate.