Eunji Shin (Industrial Product Management of Park Systems, Seoul, Korea)
Atomic force microscopy (AFM) is useful for investigating the surfaces and characteristics of different materials down to nanometer detail1-6. Accurate surface roughness measurement is not only a highly powerful technique for understanding the basic physics of materials, but also very useful in examining device structures and their failure mechanisms. In addition, roughness repeatability is one of the very important factors in industrial applications as it relates to the customer’s product reliability. Park AFM’s True Non-Contact Mode is especially essential for accurate and repeatable surface roughness measurement. In order to measure accurate and repeatable roughness, two key requirements need to be satisfied. First is maintaining the tip sharpness and the other is lowering the system noise. In this paper, we examine these two requirements for measuring surface roughness. Also, we demonstrate why True Non-Contact mode is powerful for accurate roughness measurement by testing a tip-check sample and ultra flat media samples using Park AFM.
Requirements for Accurate Roughness Measurement
To measure accurate and repeatable roughness, the first thing to consider is the tip effect. That is because the tip geometry directly correlates to the roughness value. As shown in Figure 1, when the tip is sharp, it obtains an accurate feature of the surface. But as the tip becomes blunt, it cannot approach the bottom of the sample, so it obtains a skewed feature of the surface. In short, as the tip radius increases, the roughness value decreases. That is why the tip sharpness needs to be preserved during the roughness measurements.
Figure 1. (a), (b) show the tip effect on roughness measurement.
The system noise is equally important. The reason is when the noise gets mixed into the AFM results, the AFM cannot accurately measure the ultra flat surface. To detect the smallest of sample features and to image the flattest of surfaces, low system noise is required. Therefore, Park Systems has engineered instruments that hold the industry’s lowest noise floor specification of < 0.5Å. As shown in Figure 2a, Park AFM systems routinely achieves about 0.2Å rms. Hence, it is good enough to measure the ultra flat sample, as shown in Figure 2b.