Critical underfocus in low-magnification electron microscopy: rationale and practice.

Area of Science:

• Materials Science
• Physics
• Microscopy

Background:

• Accurate focusing is critical for high-resolution imaging in transmission electron microscopy (TEM).
• Current methods for achieving optimal focus are often empirical and lack precision, relying on subjective interpretations of diffraction patterns.

Purpose of the Study:

• To introduce a precise, empirical method for achieving optimal focus in TEM.
• To provide a systematic approach for calibrating focus based on image transfer characteristics.

Main Methods:

• Utilizing a beam deflector (wobbler) to establish a reference point for perfect focus.
• Applying a calibrated 'critical underfocus' based on the modulation transfer function (MTF).
• Calibrating underfocus values for specific microscope and imaging conditions.

Main Results:

• The proposed method offers a more objective and reproducible way to set focus compared to traditional techniques.
• Demonstrated that 'critical underfocus' values are dependent on multiple imaging parameters.

Conclusions:

• This precise focusing method enhances image quality and reliability in TEM.
• The calibration procedure accounts for variables affecting contrast, leading to more accurate results.