Q: What is the difference between a traceable calibration STANDARD and an ordinary calibration REFERENCE specimen?
A: The traceable calibration standard is a select grade of material that is individually measured and certified. The quality is higher, with fewer defects than would be found on the ordinary calibration specimen. Each piece is individually measured against a transfer standard previously measured by a national laboratory, which has certified that the measurement value is traceable to the international meter.
Our certificate of traceable calibration:
– describes the measurement procedure,
– identifies the calibration standard used and its uncertainty as stated by the national laboratory,
– summarizes the measurement results,
– and provides a statement of uncertainty for both the individual and average values. The uncertainty statement is given in a form that meets the international guidelines for expressing uncertainty of measurements.
Collaborative projects and joint publications with specialists at several national laboratories show the accuracy of our measurement procedures. For the 144-nm standard (model 150-2DUTC), ASM’s average pitch agreed with PTB’s value within 0.033 nm. For the 70-nm standard (model 70-1DUTC), ASM’s average pitch agreed with NMC’s and NIST’s values within 0.025 nm.
In contrast, the calibration reference specimen is supplied with a non-traceable certificate which states the average pitch based on a batch calibration. A rough statement of uncertainty is given, such as “accurate to +/- X nm” and “standard deviation of single values is less than Y nm.”
Customer comment: As a nano-metrologist, traceability to SI units and measurement uncertainty estimations are part of my daily life. After having carefully read your paper on the corresponding interlaboratory comparison (ILC) study, I am convinced that, in terms of SI traceability, your calibration standard meets our requirements. I do have a few remaining questions concerning the stated uncertainty assigned to the pitch size.
Q: In the 70-nm pitch study, the measurement uncertainties estimated by the three ILC participants (ASM, NIST and NMC) ranges from 0.017 nm to 0.028 nm. I guess that these uncertainties originate from repeatability data, meaning that the measurements performed in each laboratory have been performed on one day?
A: The stated uncertainties come from uncertainty models devised by each of the three labs and described in the paper. The uncertainty models include repeatability and many other uncertainty components. I cannot speak for NIST or NMC/A-STAR, but the two runs at ASM took place on different days.
Q: On the information sheet for the 70-nm pattern, it is mentioned that the pitch size is accurate within +/- 0.25 nm. However, further down, it is written that the single pitch value has a typical uncertainty of +/- 0.5 nm (95 % confidence interval). Which one is the certified uncertainty?
A: I apologize for some confusion between the rough statement of accuracy for the calibration reference (“accurate within +/- 0.25 nm”) with a single pitch uncertainty stated for the standard +/- 0.5 nm”). The following remarks may clarify the situation:
“Model 70-1D is a non-traceable calibration reference, not a traceable standard. No uncertainties are certified for this model. The calibration sheet we provide with the reference states: Pitch Period: 70.1 nm (+/- 0.25 nm)
The specimens are produced by a series of process steps including interferometric lithography. The period of the line-space pattern is uniform without stitching errors. Standard deviation of individual pitch values < 0.5 nm when measured in a 3 um image, using DiscTrack PlusTM. Point to point measurements will be less precise, due to surface and edge roughness.”
Q: The uncertainty values from the ILC study and shown on the information sheet differ by a factor of 10. So I guess, some other uncertainty contributions (e.g., sample homogeneity, stability, etc.) have been included in the overall uncertainty budget?
A: Model 70-1DUTC is a traceable standard. Its certificate of traceable calibration has certified uncertainties both for the mean pitch and for single pitch values. Typical uncertainties (K=2) are better than +/-0.05 nm (mean) and +/-0.50 nm (single values). The traceability path uses the NIST-measured specimen that was the subject of our paper. For this paper, ASM measured more than twice as many images as we do in our normal certification process, in order to produce a lower uncertainty for the average pitch.
Q: Do you have any information on the stability (shelf life) of the standard? How long can you guarantee the certified value and corresponding uncertainty?
A: We’ve been working with the 70-nm grating for more than 2 years and have seen no evidence of pitch change. We’ve been working with other gratings also on silicon substrates since 1996 and also have seen no evidence of pitch change. It is important to use and store the standard carefully and to avoid measuring in a dirty or damaged area. One SEM user reported apparent changes in line width when a small area was exposed to a very high doses of electrons by scanning at very high magnification for a very long time. It is known that silicon oxide can shrink by radiolysis and that cumulative dose is more important in this process than HV settings (D. Joy, personal communication 2011). ASM analyzed the user’s lower magnification image, which showed a “normal” region surrounding the “altered” region. The DiscTrack PlusTM high precision measurement software showed that the altered region had a large increase in contrast (>30%) between the lines and spaces and a small decrease in apparent line width (< 10%). However, there was no change in pitch (at the 0.1 nm level, which was limited by the quantity of data available for analysis), as long as the pitch measurements excluded features in the boundary between altered and normal regions. Therefore we recommend: For the most accurate results, always measure pitch in an area that has uniform contrast. Do not bridge across areas with noticeably different appearance.
Q: Will the sample come with a detailed cleaning protocol, if not, do you have any information on that and which you are willing to share with us?
A: The patterned area of the 70-nm grating is fragile. We cannot recommend any cleaning protocol and instead advise “do not clean”.