There are many factors to consider when monitoring the stability of CD-SEM tools in the semiconductor manufacturing environment. With decreasing feature size and high aspect ratio dimensions, metrology tool calibration, stability, monitoring and matching play a more significant role in obtaining consistent CD measurements. It is not easy to separate the cause of outlier CD measurements. Tool owners need to consider all possible factors when matching across toolsets. For example, the tool should demonstrate repeatable electrical beam alignments in order to minimize the contribution of CD-SEM drift to measurement error. In order to overcome error in CD measurement caused by CD-SEM tool drift, it is important to monitor critical tool parameters that can produce shifts in CD measurements.
Probe current is a critical CD-SEM parameter that affects CD measurement precision. Drifts in probe current can be the result of instabilities in the emission current, accumulation of contamination on the objective aperture, or misalignment of the SEM optics. Since measurement precision is impacted by drifts in probe current, Hitachi and HP began monitoring probe current on HP’s S9000 CD-SEMs in an effort to understand Ip drift effect on CD measurements.
HP and Hitachi utilized an Information Server system, which was developed by Hitachi High Technologies America, Inc., to facilitate data collection. Information server is a web-based program which will archive and monitor many parameters of Hitachi CD-SEM tools. Hitachi Applications Engineers worked with HP Metrology Engineering to put the capability in place.
In this paper, we will address probe current instability and its impact on CD measurements. We will explore the relationship between probe current, CD data, and errors in pattern recognition caused by probe current and alignment drift.
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.