Highest Resolution Electrical Measurements

ScanWaveTM Sensitivity on a NIST Capacitive Reference Standard

Validating ScanWave Sensitivity

Defining a systems sensitivity and noise floor are essential parameters for the research community. One measure of our sensitivity to capacitance utilizes a NIST developed capacitance standard manufactured by MC2 Technologies1. The measurements and analysis follow a a method published in the literature2, and as such, validate the ScanWave sensitivity on a known commercially available Standard using an objective procedure. The results are an order of magnitude more sensitive than those presented in the publication.

 

 

The Standard Sample

The capacitance standard contains 4 SiO2 steps, each 50nm thick, and patterned Au dots having 4 different diameters: 1 um, 2um, 3um and 4um (Figure 1). The 4 different dot sizes deposited on each of the 4 steps results in 16 different capacitor values. Figure 1 a) shows the side view of the sample structure. When the AFM probe is in contact with the sample, the electrical equivalent model is deLined by two capacitors in series: the Lirst is deLined by the Au pad and oxide step and the second is the contribution from the Si substrate. The sensitivity of the system is deLined by the ability to measure (sense) the variation from one step to another and one Au dot size to another. The smallest measurable change in capacitance on the standard sample is from the smallest Au dot on the thickest step.

 

 

 

 

 

 

Figure 2 a) shows a sMIM-C image of the sample imaged simultaneously with the topography; b) shows a line proLile through the smallest Au dot structure on the thickest oxide step. This measurement is the smallest incremental capacitance of the Standard sample to measure. By measuring at this location we can determine ScanWave’s sensitivity and noise Lloor on the smallest capacitor of the reference standard. Figure 2 b) shows a red circle where we measure the noise Lloor of the system in mV: 2mV.

 

The sensitivity of the measurement is calculated by converting the sMIM signal in mV to Farads following a calibration procedure described in the paper that uses measurements from the 16 gold dots. The sensitivity is calculated in this case to be 0.15aF/mV. Once we have determined the sensitivity, we can then calculate the noise Lloor in Farads. We use the 2mV value measured from the proLile in Figure 2 b) and apply our sensitivity conversion to calculate the Linal noise Lloor in capacitive units, 0.3aF. This value is almost 10x less than the value reported in the journal article.

 

Figure 2 a) shows an sMIM-C image of MC2 capacitance standard; b) shows the line profile extracted across the two smallest Au dots on the thickest oxide step. The profile has been converted to units of femto-Farads using the procedure from reference 2. The red circle shows the extremely small noise floor relative to the Au dot capacitance measurement.

Figure 1. A top view and side view schematic of the MC2 Technologies Capacitance Standard showing the 4 different size Au dots patterned on each of the 4 step thicknesses.

Conclusion

ScanWave is shown to have the highest sensitivity on a NIST traceable capacitance standard. The standard is available in the public market and using methods developed by third parties we have demonstrated an order of magnitude improved sensitivity and noise floor over the published results. ScanWave utilizes shielded probes and optimized electronics to maximize the sensitivity to minute changes in impedance, such as capacitance, encountered in Atomic Force Microscopy experiments.

 

References

  1. MC2-Technologies, SMM calibration kit, URL: http://www.mc2-technologies.com/index.php/productspage/smm-calibration-kit.html
  2. H.P. Hubert, Rev. Sci. Instrum. 81(11), 113701. (2010)

 

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