Proc. SPIE. 9173, Instrumentation, Metrology, and Standards for Nanomanufacturing, Optics, and Semiconductors VIII
KEYWORDS: Semiconductors, Metrology, Calibration, Silicon, Atomic force microscopy, Transmission electron microscopy, Atomic force microscope, Critical dimension metrology, Photomicroscopy, System on a chip
In atomic force microscopy (AFM) metrology, the scanning tip is a major source of uncertainty. Images taken with an
AFM show an apparent broadening of feature dimensions due to the finite size of the tip. An AFM image is a
combination of the feature shape, the tip geometry and details of the tip-sample interaction. Here we describe the use of
a new multi-feature characterizer for CD-AFM tip, and report initial measurement results. The results are compared with
those obtained from the current tip characterizer.
To use atomic force microscope to measure narrow vertical features is challenging. Using carbon nanotube (CNT) probes is a possible remedy. 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 27-nm-diameter and 265-nm-long CNT probe, the probe deflection at the bottom is estimated as large as 5.8 nm. This phenomenon is inevitable when using long and thin probes. We proposed a method to correct this probe deflection effect. Detecting torsional motion of the base cantilever of the CNT probe makes it possible to estimate the CNT probe deflection. Using this information, we have developed a technique for correcting the probe deformation effect from measured profiles. This technique, in combination with correction of the probe shape effect, enables vertical sidewall profile measurement with AFM.
Image of the atomic force microscopy (AFM) is the convolution of probe shape and specimen geometry. However, probe
shape for AFM imaging is not equivalent to the actual probe shape. Gap distance was controlled with the interaction
between probe and specimen. Imaging parameters for controlling gap distance between probe and specimen surface is
one of the origins of image artifacts. Artifacts of the AFM image were analyzed as a function of set-point in dynamic
mode, using well defined reference specimen. Two kinds of typical objects, such as single protrusion and narrow gap
were used for the analysis of artifacts in the AFM image.
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.