NIST and KLA improve accuracy of SEM measurements

Researchers at the National Institute of Standards and Technology (NIST) and KLA Corporation, a company specializing in inspection and measurement systems for the semiconductor industry, have made significant advancements in the accuracy of scanning electron microscope (SEM) measurements. SEMs are crucial for process control in semiconductor manufacturing, ensuring the production of high-performance chips with optimal yields.

SEM technology employs a focused electron beam to capture images of features as small as one nanometer, providing valuable insights into semiconductor device structures. These high-resolution SEMs are utilized in various inspection and metrology applications during chip manufacturing, such as defect detection, defect classification, critical dimension measurements, overlay measurements, and more. The information gathered through SEM analysis helps chip engineers refine and optimize their manufacturing processes.

One of the challenges in SEM imaging is the precise control of the electron beam. Even minor deviations from the desired beam path or slight misalignments in the beam angle can distort the resulting SEM image, potentially leading to inaccurate representations of the device structure. To address this issue, NIST and KLA collaborated to enhance the accuracy of SEMs by accounting for angular misalignments of the electron beam. This joint research project focused on accurately measuring beam tilt, achieving an impressive measurement accuracy of less than one milliradian (equivalent to five hundredths of a degree). This achievement required advancements in angular resolution and measurement validation techniques.

To measure the beam tilt, the researchers developed a prototype standard for electron microscopy and adopted a novel approach to analyze the resulting electron micrographs. The prototype standard consists of an array of silicon pillars called conical frusta, which taper gradually. These structures are highly sensitive to beam tilt, and any tilt manifests as a shift between the centers of the images obtained from the top and bottom edges of a frustum. Leveraging their expertise in simulating electron-matter interactions, the researchers used advanced computer simulations to demonstrate the potential for sub-milliradian accuracy. These simulations informed the ongoing design and fabrication of the standard artifacts used in the research.

Simulation of a scanning electron micrograph showing the effect of electron beam tilt as a shift between the centers of the top and bottom edges of a frustum. Credit: National Institute of Standards and Technology

The utilization of arrays comprising conical frusta positioned at known locations holds immense potential for measuring variations in beam tilt across the scanned and imaged region in SEMs. These measurements can effectively calibrate the magnification and distortion of the electron microscope. Moreover, this new standard has implications for other microscopy techniques employed in chip manufacturing, including atomic-force microscopy and super-resolution optical microscopy. By comparing results from different microscopy methods, the reliability and reproducibility of information transfer between these techniques can be enhanced, ultimately improving measurement accuracy.

Andrew C. Madison, a NIST researcher and the primary author of industry papers on this research, emphasized the impact of electron-beam tilt on SEM measurements, stating, “Electron-beam tilt shifts the apparent positions of device features, reducing the accuracy of SEM measurements.” He further explained that their novel standard and analysis method can detect and account for variations in electron-beam shift across the imaging field.

Samuel M. Stavis, another NIST researcher and principal investigator, highlighted the benefits of this research for SEM manufacturers, stating, “With this data, SEM manufacturers can implement calibrations and corrections that improve image quality and measurement accuracy.”

Yalin Xiong, Senior Vice President and General Manager at KLA Corporation, expressed the company’s commitment to exploring new technologies that push the boundaries of measurement capabilities in the semiconductor industry. He emphasized the significance of collaboration with research organizations, stating, “Collaboration with research organizations serves an important role in discovering innovations that can help advance process control for the chip industry.” The joint research project with NIST aims to enhance the accuracy of fundamental measurements used in characterizing chip manufacturing processes.

NIST intends to make the newly developed standard and analytical method widely accessible to the chip manufacturing industry and the scientific community. This will be accomplished through publications and eventual distribution of frustum arrays.

Source: National Institute of Standards and Technology

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