Atomic Force Spectroscopy using a force balance technique

本研究的目的是利用力平衡法制作一台原子力光谱仪，以进行表面表征。一些研究小组曾试图用力平衡法来精确测量力曲线，但没有成功，在本研究中，我们基于反馈控制理论分析了系统的稳定性，明确了测量力曲线的困难是由于反馈控制器的频率响应不足造成的。当我们使用市售的AFM微悬臂梁时，反馈控制器必须具有± 100MHz的频率响应。这样的频率响应太快而不能由数字计算机执行。具有足够频率响应的模拟放大器也由于噪声水平的增加和力致动器响应的限制而无法用于力曲线测量，具有高阻尼系数的微悬臂梁可以减小反馈控制器所需的带宽。讨论了通过改变材料和气氛来提高微悬臂梁阻尼系数的方法。因此，塑料微悬臂梁，特别是聚酰亚胺树脂，是有效的，以提高系数。采用微光刻技术制备了聚酰亚胺微悬臂梁，并对其力学性能进行了实验研究。通过对聚酰亚胺树脂力学性能的数值模拟，我们发现带宽为1MHz的放大器可用于力平衡系统。此外，聚酰亚胺具有良好的耐热性，适合在超高真空下进行测量。我们制作了一个带探针和PID反馈控制器的聚酰亚胺微悬臂梁，带宽为1MHz。然而，我们在本项目期间尚未测量力曲线。

The aim of this study was fabrication of an atomic force spectroscope with a force balancing method for surface characterizations. Some research groups had tried to measure force curves accurately by using the force balancing method, but they did not succeeded in it. In this study, we analyzed a system stability based on the feedback control theory, and it was cleared that the difficulty of measuring the force curves was caused by an insufficient frequency response of the feedback controller. When we use a AFM micro-cantilever commercially sold, the feedback controller must have frequency response of 〜100MHz. Such a frequency response is too fast to be performed by digital computer. An analog amplifier which has an enough frequency response also is of no use in the force curve measurement because of an increasing in noise level and the limit of force actuator's response.The required bandwidth of the feedback controller can be reduced by a micro-cantilever with a high damping coefficient. Then we discussed some ways to increase the damping coefficient of the micro-cantilever by changing material and an atmosphere. Consequently a plastic micro-cantilever, especially polyimide resin, is valid to increase the coefficient. A polyimide micro-cantilever was fabricated with a micro-lithography technology and its mechanical properties were investigated experimentally. From a numerical simulation based on the mechanical properties of the polyimide resin, we found that an amplifier with 1MHz bandwidth was usable to the force-balancing system. Moreover polyimide has a good heat proof, which is suitable for measurement in a ultra-high vacuum.We fabricated a polyimide micro-cantilever with a probe tip and a PID feedback controller with 1MHz bandwidth. However we have not measured the force curves in the term of this project.