In this paper, we present an AFM based subsurface measurement technique that can be used for overlay and critical dimensions (CD) measurements through optically opaque layers. The proposed method uses the surface elasticity map to resolve the presence and geometry of subsurface structures. To improve the imaging performance of the AFM based subsurface measurements, we made use of photothermal excitation of the AFM cantilever together with a frequency modulation scheme. The experimental results show a significant improvement in the quality of the image, which leads to a more accurate and reliable CD and overlay measurement.
TNO is developing a novel Large Dynamic Range Atomic Force Microscope (LDR-AFM), primarily but not exclusively designed for sub-nm accurate overlay metrology. The LDR-AFM combines an AFM with a 6 degrees- of-freedom interferometric positioning stage, thereby enabling measurements of sub-nm features on a wafer over multiple millimeters marker-to-feature distances. The current work provides an overview of recent developments and presents the first results obtained after final integration of the complete system. This includes results on the AFM head development, the validated positioning stage performance, the first AFM images, and long-term stability measurements.
In this paper we present a new AFM based nano-patterning technique that can be used for fast defect repairing of high resolution photomasks and possibly other high-speed nano-patterning applications. The proposed method works based on hammering the sample with tapping mode AFM followed by wet cleaning of the residuals. On the area where a specific pattern should be written, the tip-sample interaction force is tuned in a controlled manner by changing the excitation frequency of the cantilever without interrupting the imaging process. Using this method several patterns where transferred to different samples with imaging speed. While the pattern was transferred to the sample in each tracing scan line, the patterned sample was imaged in retracing scan line, thus the outcome was immediately visible during the experiment.
The maximum amount of repulsive force applied to the surface plays a very important role in damage of tip or sample in Atomic Force Microscopy(AFM). So far, many investigations have focused on peak repulsive forces in tapping mode AFM in steady state conditions. However, it is known that AFM could be more damaging in transient conditions. In high-speed scanning, and in presence of 3D nano structures (such as FinFET), the changes in topography appear in time intervals shorter than the response time of the cantilever. In this case, the tip may crush into the sample by exerting much higher forces than for the same cantilever-sample distance in steady state situations. In this study the effects of steep upward steps in topography on the tip-sample interactions have been investigated, and it has been found that the order(s) of magnitude higher forces can be applied. The information on the worst case scenario obtained by this method can be used for selection of operation parameters and probe design to minimize damage in high-speed imaging. The numerically obtained results have been verified with the previous works in steady state regime. Based on this investigation the maximum safe scanning speed has been obtained for a case study.