Atomic Force Spectroscopy using a force balance technique

这项研究的目的是制造原子力光谱仪，并采用力平衡方法进行表面表征。一些研究小组曾尝试使用力平衡法来准确测量力曲线，但没有成功。在本研究中，我们基于反馈控制理论对系统的稳定性进行了分析，明确了力曲线的测量困难是由于反馈控制器的频率响应不足造成的。当我们使用商用AFM微悬臂时，反馈控制器必须具有~100 MHz的频率响应。这样的频率响应太快，不能用数字计算机来实现。具有足够频率响应的模拟放大器也不能用于力曲线的测量，因为噪声水平的增加和力执行器响应的限制，反馈控制器所需的带宽可以通过具有高阻尼系数的微悬臂梁来减小。然后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，尤其是聚酰亚胺树脂，可以有效地提高该系数。采用微细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的力学性能进行数值模拟，发现1 MHz带宽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，适合在超高真空条件下进行测量。我们制作了一种带有探头尖端的聚酰亚胺微悬臂梁和1 MHz带宽的PID反馈控制器。然而，在本项目中，我们还没有测量到力曲线。

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.