Methodology and mechanism study on high aspect ratio (HAR) contact bottom image in scanning electron microscopy

Abstract

Contact patterns that have high aspect ratio (HAR) are inevitable as the design rule has been shrunk in semi-conductor fabrication processes. HAR contacts have serious troubles to monitor the Critical Dimension (CD) of the contact bottom images with Scanning Electron Microscope (SEM). Because we can not see the bottom images anymore with general methods as the contact is getting deep. We must be able to extract secondary electrons from the contact bottom to monitor the bottom images in the contact patterns. One possible solution that we may suggest is using positive charges on the wafer surface as a driving force for secondary electrons from the contact bottom. If the positive charges are generated on the wafer surface, an electric field will be created between the contact bottom and the wafer surface. The electric field will drive the secondary electrons from the contact bottom to the wafer surface, which makes the contact bottom images. High surface voltage can be acquired when the electron energy and the magnification in pre-charge are smaller, but it requires longer charging time. High probe current can help the charging time in this case, though it may cause some damages on the wafer. After all, optimized determination is required considering the charging time and the surface voltage at various aspect ratios. In addition, there is one thing that we must consider. When the charged contact pattern is exposed to electrons at high magnification, the surface voltage on the wafer surface tends to be stabilized at lower voltage which causes fading away of the contact bottom images. Therefore, electron exposure must be minimized at high magnification by setting the focus a little away from the observation point and so on.