In advanced optical lithography the requirements of focus control continues to tighten. Usable depth of focus (DoF) is already quite low due to typical sources of focus errors, such as topography, wafer warpage and the thickness of photoresist. And now the usable DoF is further decreased by hotspots (design and imaging hotspots). All these have put extra challenges to improve focus metrology, scanner focus stability calibrations and on-product correction mechanisms.
Asymmetric focus targets are developed to address robustness in focus measurements using diffraction-based focus (DBF and μDBF) metrology. A new layout specific calibration methodology is introduced for baseline focus setup and control in order to improve scanner focus uniformity and stability using the measurements of the above mentioned asymmetric targets. A similar metrology is also used for on product focus measurements. Moreover, a few novel alternative methods are also investigated for on-product focus measurements.
Data shows good correlation between DBF and process on record (POR) method using traditional FEM. The new focus calibration demonstrated robustness, stability and speed. This technical publication will report the data from all the above activities including results from various product layers.
A prototype of a digital video storage system (CD-watcher) has been developed and attached to a Hitachi S-9380 CD-SEM. The storage system has several modes that are selectable depending on the phenomenon of interest. The system can store video images of duration from a few seconds to a few weeks depending on resolution, sampling rate, and hard disc drive capacity.
The system was used to analyze apparent focusing problems that occurred during the execution of automated recipes. Intermittent focusing problems had been an issue on a particular tool for a period of approximately three months. By reviewing saved images, the original diagnosis of the problem appeared to be auto focus. Two days after installation, the CD-watcher system was able to record the errors making it possible to determine the root cause by checking the stored video files. After analysis of the stored video files, it was apparent that the problem consisted of three types of errors. The ability to record and store video files reduced the time to isolate the problem and prevented incorrect diagnosis.
The system was also used to explain a complex phenomenon that occurred during the observation a particular layer. Because it is sometimes difficult to accurately describe, and to have others easily understand, certain phenomena in a written report, the video storage system can be used in place of manual annotation.
In this report, we describe the CD-watcher system, test results after installing the system on a Hitachi S9380 CD-SEM, and potential applications of the system.
ArF-resist-shrinkage and line-edge roughness-induced CD errors are the two main challenges for CD SEM. The requirement of measurement precision for the 65-nm node is less than 0.5 nm. The current CD SEM ADI precision is between 0.7 to 0.9 nm after shrinkage curve correction. Optical CD (OCD) has provided three major advantages. That is more sampling (> 2500:1), insensitivity to line edge roughness, and less resist damage. These advantages facilitate much better measurement precision (< 0.3 nm) than CD SEM and make OCD a potential APC metrology candidate. However, the recipe and library generation of OCD is more complicated and time consuming than CD SEM. Any thin-film variation will disturb the CD accuracy and recipe coverage range of OCD. For different pitches and film combinations, new OCD libraries need to be generated. Matching through all pitches between CD SEM and OCD is also very difficult. We propose a new concept on optical-CD-like CD SEM measurement, i.e. average line width (ALW) and contact hole diameter (ACD) measurement at high resolution and low magnification (HRLM) CD SEM. The resolution chosen is below 2nm and the magnification is 50KX. The low magnification CD measurement can average the e-beam dosage and reduce the ArF shrinkage. Several repeated patterns such as line/space and hole arrays are measured to get an averaged CD under lower magnification condition. These low magnification average CDs increase the sampling size and they are insensitive to the line edge roughness. The CD linearity of ALW/ACD and the CD matching to current CD SEM methodology will be presented. Small step FEM CD by low magnification and high magnification CD measurement will be studied. The difference between low/high magnification SEM and optical CD will be also studied.
The concept of system invariance is the principle of scaling law in optical lithography. Both the conservation of the intensity threshold of the aerial image and the invariant pupil filling of the diffracted light with the normalized numerical aperture (NA) have to be satisfactory in order to ensure the invariance for a system in a variety of optical settings. Two well-known scaling equations with k1 and k2 factors characterize the capability of the manufacturing process in microlithography. In theory, the validity of these two equations has to be based on the principle of invariance. Therefore, any optical parameters in exposure tool could be scaling validly and properly, once they obey the principle of invariance.