Application Note: Performing Step Metrology with Dektak Stylus Profilers
Learn about sub-nanometer repeatability in thickness and etch-depth measurements
Unmatched precision in surface characterization with Dektak® Stylus profilers
Numerous processes in semiconductor, optical coatings, and micro-electromechnical systems (MEMS), as well as research in flexible electronics and sensors, rely on precise thickness or etch-depth measurements. Among their many metrology capabilities, Dektak® stylus profilers deliver exceptional sub-nanometer repeatability to enable accurate step height measurements over the nanometer-to-millimeter vertical range. Through precise control of applied load, these systems also can provide accurate and repeatable metrology for soft matter, such as polymer thin films/coatings and indium-based devices. This application note discusses Dektak metrology capability over vertical dimensions (a wide range of certified step heights), and provides some specifications and case examples to benchmark its performance, showing why these profilers have been consistently helping manufacturers and materials engineers with their surface characterization needs for over half a century.
Definition of Metrology Terms
Metrology capability defines the performance of a measurement instrument by listing and quantifying sources of error so that a user can assess whether the system is applicable for a specific task. All errors are benchmarked versus standards that are certified by independent metrology reference laboratories, such as NIST in the USA, PTB in Germany, UKAS/NPL in the UK, or NMIJ in Japan.
Representation of six steps within the same diagram, default feature from Vision64 software
Sources of error are split into four categories:
1、Bias corresponds to the difference between the output result and its true value, also referred to as accuracy of a measurement.
2、Stability describes the amount of drift of the output result over a certain time period.
3、Repeatability represents the inherent random fluctuation of the output result while measuring the same location over a short period of time, without any user interaction. This indicates the precision of a measurement system, setting the hard boundaries below which no valid measurement can be made.
4、Reproducibility brings the influence of external parameters, such as operator, time, and repositioning into measurement consideration.
While repeatability is inherent to the performance of the measurement system and cannot be improved without hardware changes, reproducibility can be improved through appropriate analysis and automation routines so that the measurement becomes less sensitive to, or even completely independent from, users, time, or location. Likewise, bias or accuracy can be improved through a proper calibration process, while stability improvement would require environmental control and/or a redesign of the measurement system.
Graphic illustration of Accuracy and Repeatability versus target value.
Automatic Step Extraction
To diminish user influence, it is crucial to have step heights calculated using automated analysis routines. Dektak systems incorporate Vision64® software’s two standard automated analysis routines. The first one is based on the fixed position of cursors, and the second one is based on automatic positioning of cursors with respect to the step edge. For the latter, the average height difference between the cursor positions defines the film thickness or etch depth. In addition to the above methods, users have the freedom to tailor more advanced analysis by setting up preprocessing of data via an analysis tree.
Influence Factors for Accuracy
Dektak stylus profilers are equipped with the latest generation LIS-3 heads, which are based on Linear Variable Differential Transformer (LVDT) sensor technology.
This type of sensor is well reputed and widely adopted throughout industry as a robust and linear displacement sensor. Robustness comes from the fact that the magnet core is positioned away from the detecting coils, making it insensitive to large displacement or involuntary shocks. This sensor architecture also explains why Dektak stylus exchange is so fast and worry-free.
Additionally, the sensor head has been designed to keep metrology performance consistent through a very wide vertical range (up to 1mm) and load (from 0.03mg to 15mg) using a single head. This large range enables users to extend the use of Dektak stylus profilers across a wider range of applications.
Advanced Analysis setup for automatic step height determination. Multi-tree based Vision64 analysis allows different simultaneous calculations to deliver the best metrology performance and flexibility for a wide range of applications.
