AFM method for sidewall measurement through CNT probe deformation correction and its accuracy evaluation

Abstract

To use atomic force microscope (AFM) to measure dense patterns of 32-nm node structures, there is a difficulty in providing flared probes that go into narrow vertical features. Using carbon nanotube (CNT) probes is a possible alternative. However, even with its extremely high stiffness, van der Waals attractive force from steep sidewalls bends CNT probes. This probe deflection effect causes deformation (or "swelling") of the measured profile. When measuring 100-nm-high vertical sidewalls with a 24-nm-diameter and 220-nm-long CNT probe, the probe deflection can cause a bottom CD bias of 13.5 nm. This phenomenon is inevitable when using long, thin probes whichever scanning method is used. We proposed a method to deconvolve this probe deflection effect. By detecting torsional motion of the base cantilever for the CNT probe, it is possible to estimate the amount of CNT probe deflection. Using this information, we have developed a technique for deconvolving the probe deformation effect from measured profiles. This technique, in combination with deconvolution of the probe shape effect, enables vertical sidewall profile measurement.We have quantitatively evaluated the performance of the proposed method using an improved version of a "tip characterizers" developed at the National Institute of Advanced Industrial Science and Technology (AIST), which has a well-defined high-aspect-ratio line and space structure with a variety of widths ranging from 10 to 60 nm. The critical dimension (CD) values of the line features measured with the proposed AFM method showed good matches to TEMcalibrated CD values. The biases were within a range of ±1.7 nm for combinations of three different probes, five different patterns, and two different threshold heights, which is a remarkable improvement from the bias range of ±4.7 nm with the conventional probe tip shape deconvolution method. The static repeatability was 0.54 nm (3σ), compared to 1.1 nm with the conventional method. Using a 330-nm-deep tip characterizer, we also proved that a 36-nm-narrow groove could be clearly imaged.

Accurate measurement of very small line patterns in critical dimension scanning electron microscopy using model-based library matching technique

“…These profiles were deconvolved with the carbon-nanotube (CNT) tip shape by accounting for tip bending due to van der Waals force. 16,17 The SWAs are almost equal at ∼2 deg, although the pattern height decreases a little as the line width decreases.…”

Advances in the atomic force microscopy for critical dimension metrology

“…Profile dist ortion due to bending of the CNT tip [151] is believed to be an important issue with the CNT probes, even the probe bending of several nanometers was measured experimentally [154]. Therefore, deconvolution of the profile distortion (due to the CNT probe deflection) by the detection of cantilever's base torsional motion is indispensable to recover the true surface profile [155,156].…”

Analysis for system errors in measuring the sidewall angle of a silica waveguide with confocal laser scanning microscope (CLSM)

“…Around 2008-2009, the AFM technique was improved to be an acceptable tool of sidewall profile measurements [12]. In particular, after a carbon nanotube (CNT) probe was invented where an attractive force between the sidewall surface and CNT probe tip is detected, the AFM based measurement skill became the most mature and has been proliferated, in which a theoretical model of deconvolution of deflection effect of probe tip and the detected data was invented to create the error correction effect and finally the angle error range was reduced from the conventional 3°-6° to the current 4°-5° [13]. In the past decade, the development and applications of AFM based measurements have shown the advanced accuracy of the surface roughness of the planar object.…”

“…In the past decade, the development and applications of AFM based measurements have shown the advanced accuracy of the surface roughness of the planar object. However, the AFM-based measurement skill has apparently shown the intrinsic drawbacks: (1) the easy-broken expensive probe determined highcost and (2) the difficulty in measuring the steep sidewalls [12,13].…”