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.