An unique approach to high-performance magnification calibration

Dimensional calibration standards are an important metrology tool for quality control, inspection, and fault analysis. Tools such as Atomic Force Microscopes (AFM), Scanning Probe Microscopes (SPM), or Scanning Electron Microscopes (SEMI) require regular calibration to meet the needs of current and projected production processes. Suitable calibration standards have been expensive, difficult to use, and of limited utility. These to a large degree, a result of the fabrication process and the accompanying measurement calibration paradigm. Any approach to microscope calibration should make calibration easier, less expensive, and more useful, An improved calibration standard would also be amenable to automation of the calibration process for use in production line instruments. The necessary features include: 1. The ability to calibrate the entire viewing field instead of discrete points. 2. The ability to easily locate and use the calibrated region. 3. The ability to calibrate on the nanometer scale where the most demanding applications push the state of the art. 4. Significantly reduced specimen costs. There is an alternative production method for calibration specimens which meets the above criteria, It is based on the concept of physically replicating a light interference pattern to provide the essence of an interferometer in a simple calibration specimen. Modern optics technology has reached the point where large area (many square centimeters), very accurate and regular interference patterns in 1 and 2 dimensions can be produced. The basic physics of the process enables the periodicity of. these patterns to be specified and controlled to fractions of a nanometer over these very large areas. This large-area interference pattern can be captured in a physical record suitable for viewing under the microscope. The issues affecting the accuracy and utility of this physical record and its preparation for use as a magnification standard will be discussed. Experience in use in AFM applications indicates that calibration samples produced by this method can deliver repeatable accuracy of 1.5 nm if properly employed and analyzed. This methodology can be extended to other imaging microscope technologies.