Wafer Level SEM Metrology for Critical Dimension Measurements

In the semiconductor manufacturing industry, microscopes are used to measure feature linewidths of a few tens of nanometers for process monitoring and control At this scale, the largest component of measurement uncertainty is usually associated with the interaction of the probe (e.g., mechanical stylus, photons, or a beam of charged particles) with the specimen. The principal challenge in linewidth metrology is to accurately define the position of the physical edge of a feature within the metrology instrument response profile. Each instrument exhibits its own characteristic response profile due to the bandwidth of the electronics, signal collected or probe used. The measurand also contributes to the response as well. The result is an increased linewidth measurement uncertainty.  This uncertainty can easily reach undesirable levels when dealing with nanometer structures, and NIST is continually striving to decrease this measurement limitation.

NIST will develop a reference SEM (scanning electron microscope) for calibration of transfer artifacts that are measured using the same imaging physics.  The metrology for this reference instrument will be based on a laser interferometer stage with sub-nanometer resolution.  In order to reduce linewidth measurement uncertainty, new measurement techniques producing sharper edge profiles will be developed and adopted. Ultimately, however, no method has been able to fully achieve the required resolution, and it becomes necessary to assign the physical edge to a definite position within the broadened signal. This in turn requires understanding and modeling the physical process that produce the broadening so that it can be compensated.

Therefore, this work will be supported by accurate Monte Carlo simulation modeling (JMONSEL) of the incident measurement beam-specimen interactions to determine the instrument response profile from various line-edge shapes. JMONSEL supports secondary electron generation for samples in vacuum (standard SEM mode) or a gaseous environment (Variable Pressure SEM Mode) and utilizes a “model-based library (MBL)” method pioneered by this project to provide edge detection. NIST will use the MBL to quickly determine the line-edge shape by comparing the measured image intensity profile to a library of pre-computed images that can be quickly scanned and interpolated to determine the best match.

Immediate goals of the project include:

1. Demonstrate traceable linewidth measurement on amorphous silicon patterns on 300 mm wafers using the new reference metrology SEM with standard uncertainties at or better than 2 nm for nominal linewidths less than 100 nm.
2. Improve the NIST model of secondary electron generation to include many-body effects (e.g., screening) and exhibit better agreement with NIST’s database of inelastic mean free paths (standard reference database 71).
3. Test the sensitivity of measurement results to varying model assumptions.