Method of automatically focusing a charged particle beam on a surface region of a sample, method of calculating a converging set of sharpness values of images of a charged particle beam device and charged particle beam device for imaging a sample

A method of calculating a converging set of sharpness values of images of a charged particle beam device, comprising:

• providing a first focusing strength range and a focusing strength accuracy as input parameters to a convergence algorithm;
• calculating a first focusing strength value based on a golden ratio or Fibonacci numbers and depending on the first focusing strength range;
• calculating a second focusing strength value based on a golden ratio or Fibonacci numbers;
• imaging a sample with the first focusing strength value and the second focusing strength value to obtain images;
• calculating sharpness values from the images with a sharpness function, wherein the sharpness function is based on at least one of a Fourier transform of an image or a sum in a frequency space;
• determining a second focusing strength range narrower than the first focusing strength range based on the sharpness values;
• and
• iteratively replacing the first focusing strength range with the second focusing strength range and imaging the sample to obtain further images and calculating further sharpness values from the further images.

A charged particle beam device for imaging a sample, comprising:

• a charged particle source configured to emit a charged particle beam;
• an objective lens configured to focus the charged particle beam propagating along an optical axis on the sample with a focusing strength value;
• and
• a controller configured to adjust the focusing strength value according to a method of focusing the charged particle beam on a surface region of the sample, comprising:
• calculating a first sharpness value of a first image, wherein the first sharpness value is calculated with a sharpness function including a sum in a frequency space;
• determining a second focusing strength value based on the calculated first sharpness value and based on a golden ratio or Fibonacci numbers;
• and
• acquiring a second image for the second focusing strength value.

A charged particle beam device for imaging a sample, comprising:

• a charged particle source configured to emit a charged particle beam;
• an objective lens configured to focus the charged particle beam propagating along an optical axis on the sample with a focusing strength value;
• and
• a controller configured to adjust the focusing strength value according to a method of calculating a converging set of sharpness values of images of a charged particle beam device, comprising:
• providing a first focusing strength range and a focusing strength accuracy as input parameters to a convergence algorithm;
• calculating a first focusing strength value based on a golden ratio or Fibonacci numbers and depending on the first focusing strength range;
• calculating a second focusing strength value based on a golden ratio or Fibonacci numbers;
• imaging a sample with the first focusing strength value and the second focusing strength value to obtain images;
• calculating sharpness values from the images with a sharpness function, wherein the sharpness function includes a Fourier transform of an image and/or wherein the sharpness function is provided as a sum in a frequency space;
• determining a second focusing strength range narrower than the first focusing strength range based on the sharpness values;
• and
• iteratively replacing the first focusing strength range with the second focusing strength range and imaging the sample to obtain further images and calculating further sharpness values from the further images.