CHARIOT

Screenshot of simulation layout showing secondary electrons

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CHARIOT software is used to simulate the signal and images in scanning electron microscopy (SEM), including charging, complex electromagnetic fields and detectors. The software also simulates energy and charge deposition in electron beam lithography (EBL). The main emphasis of the software is on the accurate simulation of slow secondary electrons and charging.

The software is used by major semiconductor companies, including equipment makers, chip makers, and maskmakers. Equipment makers use Chariot to optimize SEM systems at the design stage for better image quality by optimizing the design of subsystems such as detectors, additional electrodes, etc. Chip makers use it to improve the contrast and signal to noise ratio for their specific layers or applications by optimizing the setup of systems that they use for those specific applications.

Typical applications include:

  • calibration of CD-SEM for accurate dimensions;
  • finding the best setup parameters for specific layers or materials, such as the resist pattern
  • optimizing the parameters of pre-charge in metrology and defect inspection, especially if the bottom of contact holes is of interest
  • optimizing the detector setup used in SEM

In EBL, the software is used to accurately predict the point spread function used in proximity correction, as well as to simulate the distribution of absorbed energy in 3D patterns, and to simulate the shot noise.

CHARIOT software uses an advanced physical model of electron scattering, called the discrete loss approximation (DLA) model, which makes the software incomparably more accurate than the typical slowing down approximation (SDA) model. SDA models based on the Bethe formula lump all the scattering events into one number, depending on the electron energy and distance traveled. They work well for electron energies over about 8 kV, but electrons with lower energies require a different model. The DLA model considers all scattering events separately, simulating electron interactions with inner and outer shells of specific atoms, generation of plasmons, Auger electrons, modeling of all generations of secondary electrons until they stop. The use of advanced physics results in much higher accuracy, which is necessary for modern microelectronics and other areas.

The simulation model considers:

  • elastic electron scattering: either Mott or Rutherford models
  • inelastic energy loss using an accurate discrete loss approximation (DLA) model, where such processes as inner-shell ionization, outer shell ionization, plasmon generation and decay are taken into account
  • a semi-empirical model used for low (E < 50 eV) energy electron scattering
  • the generation of fast and true secondary electrons, considering multiple generations of secondaries
  • electron propagation between layers
  • detector geometry, location and energy transfer functions
  • charge and discharge of the sample; local electrical fields and potentials due to charging
  • additional electrodes with potentials over the sample
  • electron trajectories in local and global electromagnetic fields

See more details, examples or references.