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.
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.
Atomic Force Microscope (AFM) is a powerful metrology tool for process monitoring of semiconductor manufacturing
because of its non-destructive, high resolution, three-dimensional measurement ability. In order to utilize AFM for
process monitoring, long-term measurement accuracy and repeatability are required even under the condition that probe
is replaced. For the measurement of the semiconductor's minute structure at the 45-nm node and beyond, AFM must be
equipped with a special probe tip with smaller diameter, higher aspect ratio, sufficient stiffness and durability. Carbon
nanotube (CNT) has come to be used as AFM probe tip because of its cylindrical shape with small diameter, extremely
high stiffness and flexibility.
It is said that measured profiles by an AFM is the convolutions of sample geometry and probe tip dimension. However,
in the measurement of fine high-aspect-ratio LSI samples using CNT probe tip, horizontal measurement error caused by
attractive force from the steep sidewall is quite serious. Fine and long CNT tip can be easily bent by these forces even
with its high stiffness. The horizontal measurement error is caused by observable cantilever torsion and unobservable tip
bending. It is extremely difficult to estimate the error caused by tip bending because the stiffness of CNT tips greatly
varies only by the difference of a few nanometers in diameter.
Consequently, in order to obtain actual sample geometry by deconvolution, it is essential to control the dimension of
CNT tips. Tip-end shape also has to be controlled for precise profile measurement.
We examined the method for the measurement of CNT probe tip-diameter with high accuracy and developed the
screening technique to obtain probes with symmetric tip-ends. By using well-controlled CNT probe and our original
AFM scanning method called as Advanced StepInTM mode, reproducible AFM profiles and deconvolution results were
obtained.
Advanced StepInTM mode with the dimension- and shape-controlled CNT probe can be the solution for process
monitoring of semiconductor manufacturing at the 45-nm node and beyond.
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 have developed a method of deconvolving this probe deflection effect that is well suited to our AFM scanning mode,
Advanced Step-inTM mode. In this scanning mode, the probe is not dragged on the sample surface but approaches the
sample surface vertically at each measurement point. The CNT probe deformation is stable because we do not use
cantilever oscillation that can cause instability, but we detect static flexure of the cantilever. Consequently, it is possible
to estimate the amount of CNT probe deflection by detecting the degree of cantilever torsion. 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 makes vertical sidewall profile measurement possible.
A new inline metrology tool utilizing atomic force microscope (AFM) suited for LSI manufacturing at the 45-nm node
and beyond has been developed. The developed AFM is featuring both of high-speed wafer processing (throughput: 30
WPH) and high-precision measurement (static repeatability: 0.5nm in 3σ). Several types of carbon nanotube (CNT)
probes specially designed for the AFM have also been developed. The combination of Advanced StepInTM mode and
CNT probes realizes high precision measurement for high-aspect-ratio samples such as photoresist patterns. In
Advanced StepInTM mode, a probe tip approaches and contacts a sample surface, and then moves away from the surface
and toward a new measurement position. A series of these actions is performed in a short time (3.8 ms for single
measurement point) full-automatically. Advanced StepInTM mode not only ensures gentle probe tip contact and precise
measurement of high aspect ratio samples, but also minimum tip wear. CNT probes can provide long term performance,
while eliminating the need for probe exchange. The developed AFM also realizes flatness measurement of 10-nm level
in a wide area of 40x40-mm maximum. This performance is sufficient for the evaluation of CMP processes at the 45-nm
node.
We have developed a new atomic force microscope method that we call Step-in mode. The Step-in mode can realize a high aspect ratio structure observation without tip damage becasue of its unique probing method. Three types of high aspect ratio probe, a silicon probe sharpened by focused ion beam, a high density carbon probe and a carbon nanotube probe, are analyzed to make clear which probe is appropriate for high aspect ratio structure metrology. It is demonstrated that fine measurement can be carried out with all types of probes and we conclude that the high density carbon probe is the best at the present time. Experimental results show that the pitch repeatability for a standard grating sample is 1.2 nm at 3σ, height repeatability is 1.2nm at 3σ and width repeatability of a poly-silicon gate with side-wall is less than 3 nm at 3σ. It is also demonstrated that there is no tip damage after taking 1000 profiles of 512 data points in the Step-in mode. The experimental results show that the Step-in mode. The experimental results show that the Step-in mode has a potential for application to critical dimension metrology for a LSI process monitor.
New imaging techniques in atomic force microscopy have been developed to suppress bending of the sharpened probe during scanning. After analyzing the bending of the probe, it is clear that the bending is caused by response of servo control and slip of the probe on the slope. It is essential that we do not scan the probe under contact on a sample surface and we approach the surface at a contact force of <10 nN for slip-free operation. The technique controls the probe such that approaching and gap-controlling are done without scanning after the probe has been stepped from pixel to pixel, and the step movement is done after lifting the probe up from the sample surface without servo controlling. This technique permits us to use a very sharpened and slim probe so that we can observe a steep structure, such as a dry-etched groove and hole, and a photoresist pattern with a high aspect ratio faithfully. We clarify that it is possible to apply this technique to monitoring the steep structures with an inline process monitor in the large scale integration process without cracking the wafer.
New imaging technique in AFM has been developed to suppress bending of the sharpened probe during scanning. After analyzing the bending of probe, it is clear that the bending is caused by response of servo control and slipping of the probe on the slope. It is needed that we do not scan the probe under contact of it on a sample surface for friction- free and we approach it to the surface at a contact force of < 10nN for slip-free. The technique controls the probe such that approaching and gap-controlling are done without scanning after only one step xy-scanning is competed, and the xy-scanning is done without servo controlling after lifting the probe up from the sample surface. This technique permits to use very sharped and slim probe and to observe a steep structure such as a dry-etched groove and hole, and a photoresist pattern with a high aspect ratio faithfully. We can clarify that it is possible to apply this technique to monitoring the steep structures as an in-line process monitor in the LSI process without cracking the wafer.
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