Workshop on Atomic Force Microscopy, Nanometrology and More

One of my colleagues from our global research center in India, K.G. V. Siva Kumar (Sivakumar), recently attended a workshop on Atomic Force Microscopy (AFM) based nanometrology and was kind enough to share his thoughts in a blog.  Before jumping into the blog, I would like to introduce him.

Sivakumar is a Lead Engineer in the Materials Characterization organization working on applications related to high temperature materials, polymeric membranes and composites that leverage his expertise in microscopy (SEM/EDS, AFM) & electrical characterization. He received his Bachelors in Metallurgical Engineering from Indian Institute of Technology Madras, followed by a PhD in Materials Science & Engineering from Georgia Institute of Technology for his work on `Electrical characterization of controlled Waspaloy microstructures’. Prior to joining GE Global Research, Sivakumar worked at the South Dakota School of Mines as a Research Scientist.

SivaAccurate metrology is just the opposite side of the coin for microscopists who are constantly faced with the challenge of quantifying attributes of the microstructure in question with enough statistical significance. With the maturation of nanoscience and nanotechnology, there are ever increasing opportunities of these transitioning into industrial applications. Therefore, ‘nanometrology’, defined as the science of measurement and related applications at the nanoscale, not only has scientific and industrial implications but also legal, under the purview of global trade. This was one of the opening themes of the AFM-based Nanometrology Workshop held at the National Physical Laboratory, New Delhi on Sep. 27th and 28th 2012. A parallel theme was that of understanding and evaluating experimental uncertainty associated with AFM-based measurements. The workshop was organized by NPL scientists Dr. Vijay Narain Ojha and Dr. Rina Sharma, who along with Dr. Vijaykumar Toutam were the instructors. Participants were from industry, government laboratories and academic institutions.

Dr. Ojha lectured on the standard methods for uncertainty estimation in measurements, put in place by experts comprising the Bureau International des Poids et Mesures (BIPM), International Electrotechnical Commission (IEC) and International Organization for Legal Metrology (OIML). Dr. Sharma subsequently gave a broad over view of the role of National Metrological Institutes (NMIs) (such as NPL India) in providing traceability to any measurement originating from industrial, academic or accredited laboratories of the respective countries. The NMIs themselves are participating members of the BIPM, the international governing body for providing a single, coherent measurement system throughout the world, traceable to the SI unit system. She underlined the fact that traceability is a pre-requisite to equivalence of measurements across any two entities in any two parts of the world who wish to compare measurements or engage in trade. Dr. Sharma also made a distinction between traceability and calibration as follows- traceability of a measurement is complete when the result can be related to a reference through a documented and unbroken chain of calibrations whereas calibration refers to determining and documenting the deviation of the measurement from the conventional, true value of the measurand.

Dr. Toutam presented in detail on atomic force microscopy, imaging artifacts and possible sources of error such as scanner non-linearity and bow, affecting topographic measurements. Dr. Sharma followed with a section on estimation of uncertainty in AFM measurements by accounting for factors such as Abbe’s error, tip and specimen mounting errors, drift, non-linearity, tip-wear and measurement noise. A transfer function for the measurand was written including these factors and the uncertainty in the measurand was then calculated by apriori knowledge of the factor uncertainties. Participants were given a tour of the AFM lab on the following day and were asked to calculate the experimental uncertainty in standard grating pitch measurements by accounting for some of the contributing factors discussed earlier.

The workshop was an eye-opener for me, particularly, when it came to systemic issues such as scanner non-linearity which can throw off AFM measurements if left unchecked. I believe that NMIs such as the National Physical Laboratory could be very resourceful to both industry and academic institutions by providing traceability and offering calibration services.

I would like to close with the following questions for those of you who use an AFM routinely and worked with one that has a closed-loop scanner.

  1. How often do you check calibration of the scanner? What is the pedigree of the standard that you use? Is it traceable?
  2. What is the best achievable accuracy using your system and how does this vary with scan size?
  3. Have you noted effects of tip-geometry affecting lateral measurements of features?
  4. Do you account for some of the aforementioned uncertainties involved in AFM-based metrology?

Below is an example of scanner non-linearity artifact where features that are of the same size are apparently compressed at the bottom of the image compared to the top.

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Below is an example of scanner non-linearity artifact where features that are of the same size are apparently compressed at the bottom of the image compared to the top.

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