Automated SEM Metrology

Linewidth, placement, distance and statistics

Nanolithography and nanofabrication are one thing, dimension check another. Given that nanostructures are still shrinking, nanometrology is becoming increasingly important for verifying a nanofabrication process.

Automated process control is increasingly finding its way into NON semiconductor fab-like environments such as Nanofabrication Centers or Quality Control, with the goal of stabilizing processes.

By precisely measuring nm-sized features reliably and – based on these results – by feedbacking to, and thus correcting, the process, yield and device performance can effectively be improved.

Removing measurement subjectiveness is an important factor with respect to nanofabrication process reproducibility. A fully automated measurement procedure with automated statistical evaluation is therefore of the essence.

Raith provides this in all its turnkey systems, which can be operated in a “CD-SEM like” manner. Efficient and fully automated nanometrology operations and statistical evaluations thereof are available, as well as measurement algorithms such as linewidth, placement, pitch and distance. All these nanometrology operations can be seamlessly integrated in the GDSII design, overlaid with real SEM images and executed from the position list. Data acquisition and processing takes place fully automatically in the background. (Statistical) results can be displayed in the GUI, specifically recalled individually, and exported later in standard formats if required.

Line edge roughness and coupling gap dimension as indicators for device performance

With regard to the SEM image of a ring resonator coupling device: Both the minimum line edge roughness (LER) along the entire waveguide as well as the accuracy and stability of the “coupling gap” (here 0.4 μm) are of crucial importance with respect to the device performance. Corresponding GDSII design: First the waveguide is written using the unique FBMS exposure mode; then a marker is used for highly precise alignment relative to the waveguide before the ring is written. A conventional manual measurement for gap and LER can be applied, but is tedious and time consuming for a large number of devices (here 135 devices).

The solution: Automated Nanometrology – fully automated over full wafer

Line edge roughness and coupling gap are measured automatically. First, a GDSII design is setup for automated nanometrology for both LER (10 linewidth measurements at different waveguide locations) and a single distance measurement for gap determination. Then it is applied to 135 devices exposed at different places. Finally, 1,200 fully automated measurements and automated statistical evaluation have taken place.

All images:
Linjie Zhou, Katsunari Okamoto, and S. J. Ben Yoo, et al., University of California Davis, and James Conway, et al., Stanford Nanofabrication Facility