AFM atomic force microscope
AFM: Atomic Force Microscope (AFM), an analytical instrument that can be used to study the surface structural planes of solid materials, including insulators. It studies the surface structure and properties of substances by detecting extremely weak atomic interactions between the surface of the sample to be tested and a micro force sensing element.
According to the changes in the interaction force between the needle tip and the sample surface, AFM mainly has three operating modes: contact mode, non-contact mode, and tapping mode.
Contact Mode: The most direct imaging mode for AFM. Throughout the entire scanning imaging process, the probe tip always maintains contact with the sample surface, and the interaction force is repulsive. During scanning, the force exerted by the cantilever on the needle tip may damage the surface structure of the sample, so the magnitude of the force ranges from 10 to 10 to 10-6N. If the surface of the sample is soft and cannot withstand such force, it is not advisable to use contact mode for imaging the sample surface.
Non contact mode: When detecting the surface of the sample in non-contact mode, the cantilever oscillates at a distance of 5-10nm above the sample surface. The interaction between the sample and the needle tip is controlled by van der Waals forces, usually 10-12N. The sample is not damaged and the needle tip is not contaminated, making it particularly suitable for studying the surface of soft objects. The drawback is that it is very difficult to achieve this mode in a room temperature atmospheric environment. Because water in the air will inevitably accumulate on the surface of the sample, it will build a small capillary bridge between the sample and the needle tip, sucking the needle tip and the surface together, thereby increasing the pressure on the surface from the tip.
Tapping Mode: Tapping mode is a hybrid concept that lies between contact mode and non-contact mode. The cantilever oscillates above the sample surface at its resonant frequency, and the needle tip only briefly contacts/strikes the sample surface periodically. This means that the lateral force generated when the needle tip contacts the sample is significantly reduced, so the AFM tapping mode is one of the best choices when detecting soft samples. Once AFM starts imaging and scanning the sample, the device immediately inputs relevant data into the system, such as surface roughness, average height, maximum distance between peaks, valleys, and peaks, for object surface analysis. At the same time, AFM can also complete force measurement work, measuring the bending degree of the cantilever to determine the magnitude of the force between the needle tip and the sample.
Figure 1 Comparison of three operating modes of AFM (a) Contact mode; (b) Non contact mode; (c) Tap mode.
As for the application of atomic force microscopy in the integrated circuit manufacturing industry, it can be mainly divided into three categories:
1. SCM: Measure the change rate of sample capacitance with alternating voltage dC/dV. SCM is a qualitative analysis of doping morphology, and its signal is affected by external environment. The change in color only represents the strength of the sample signal and cannot directly determine the concentration. The detectable concentration range of SCM is 1E15~1E20atom/cm3. The use of SCM for electrical characterization of semiconductor devices and the use of high spatial resolution for electrical feature characterization are key to understanding the functionality of semiconductor devices. Scanning capacitance microscopy is a powerful method for characterizing semiconductor devices due to its non-destructive scanning ability and high-precision nanoscale feature measurement. In semiconductor manufacturing, SCM provides doping distribution that facilitates fault analysis and design improvement.
2. C-AFM: Measure the current between the needle tip and the sample.
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3. AFM: Scan the surface of the sample to measure its roughness.
