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Design rule shrinkage and wider adoption of new device structures such as STI, copper damascene interconnects, and
deep trench structures have made the need for in-line process monitoring of step heights and profiles of device
structures more urgent. To monitor active device patterns, as opposed to test patterns as in OCD, AFM is the only non-destructive 3D monitoring tool. The barriers to using AFM in-line monitoring are its slow throughput and the accuracy degradation associated with probe tip wear and spike noise caused by unwanted oscillation on the steep slopes of high-aspect-ratio patterns. Our proprietary AFM scanning method, StepInTM mode, is the method best suited to measuring high-aspect-ratio pattern profiles. Because the probe is not dragged on the sample surface as in conventional AFM, the
profile trace fidelity across steep slopes is excellent. Because the probe does not oscillate and hit the sample at a high
frequency, as in AC scanning mode, this mode is free from unwanted spurious noises on steep sample slopes and incurs
extremely little probe tip wear. To take full advantage of the above properties, we have developed an AFM sensor that is
optimized for in-line use and produces accurate profile data at high speeds and incurs little probe tip wear. The control
scheme we have developed for the AFM sensor, which we call "Advanced StepInTM", elaborately analyses the contact
force signal, enabling efficient probe tip scanning and a low and stable contact force.
With a developed AFM sensor that realizes this concept, we conducted an intensive evaluation on the effect of low and
stable contact force scan. Probes with HDC (high density carbon) tips were used for the evaluation. The experiment
proves that low contact force enhances the measured profile fidelity by preventing probe tip slip on steep slopes.
Dynamics simulation of these phenomena was also conducted, and its results agreed well with the experimental results.
The low contact force scan also incurs extremely little probe tip wear, which is essential to assure high measurement
repeatability. An inherent property of StepInTM is that it causes little probe tip wear because of the minimal contact
between tip and sample. The effects of this property have been enhanced by adding low contact force scanning.
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