A new atomic force microscopy (AFM) imaging method called the step-in mode for high aspect ratio structures has been proposed and developed. In the step-in mode operation, probe scanning is performed only while a probe is far from the surface of a sample. Therefore, this method can remove frictional and torsional force acting on the probe caused by probe scanning. In high aspect ratio measurement, it is demonstrated that the position error caused by torsional force is smaller than 1 nm using the probe with an aspect ratio of 5 and that the probe tip is not damaged in the step-in mode measurement. As an experimental result, using the probe with an aspect ratio of 5, the image of a shallow trench isolation (STI) structure with a top width of 315 nm and a depth of 580 nm is faithfully obtained.
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, AdvancedStep-in TM 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.
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 because 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 probe 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 1.2 nm 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 has a potential for application to critical dimension metrology for a LSI process monitor.
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