使用自激振荡法提高原子力光子扫描隧道组合显微镜的扫描速度

介绍了采用频率偏移控制样品/探针间距的原子力光子扫描隧道组合显微镜(AF/PSTM)。制备了尖端直径100nm,锥角为60～90°的锐利大锥角探针用于轻敲自激振荡模式的AF/PSTM,该探针固定在压电陶瓷片上置于一个正反馈回路中激励探针振荡。使用锁相器解调自激振荡探针的频率,调整Z方向压电陶瓷的运动使得锁相器检测到的值维持恒定来跟踪样品的起伏。对外加激励模式和自激振荡模式进行了对比。理论分析表明,自激振荡方法减小了探针响应时间;测试试验显示,采用自激振荡模式AF/PSTM的带宽为50Hz,比外加振荡模式快一个数量级。采用改进后的仪器对光栅样品以1Hz的速度进行了扫描,扫描结果显示,采用自激振荡的方式得到的形貌和光学图像比外加激励模式更清晰,不仅响应时间更快,通过提高Q值还可以提高分辨率而不会增加系统进入稳态的时间。

Self-oscillating electronics for improving scanning speed in AF/PSTM

An atomic Force Photon Scanning Tunneling Microscope (AF/PSTM) is reported here,which uses a shift in resonance frequency to control the distance between a sample and a fiber probe. For self-oscillating tapping mode AF/PSTM applications, a sharp end and large conical angle optical fiber tip with an apex diameter smaller than 100 nm and a cone angle of 60～90° is fabricated. The tip is mounted on a piezo,then the piezo is placed in a positive feedback circuit to excite the probe to oscillate. The oscillation frequency of the tip is demodulated by a PLL, and its value is maintained a constant by controlling the negative feedback loop of the Z piezo to track the sample surface. Both theory and experiment are carried out to compare the externally excitation mode and self-oscillating mode. Results show that the self-oscillation mode reduces the response time of the tip in theory,and the bandwidth of self-oscillating mode in proposed AF/PSTM is 50 Hz in experiments, which is higher an order of magnitude than that of externally excitation mode response. A sample of grating is scanned by proposed instrument with a scanning rate of 1 Hz,and results demonstrate that the topography and optical images obtained by self-excitation mode have clearer contrast than those obtained by externally excitation mode. Besides faster response, the self-oscillating mode can improve the system resolution by increasing Q without effecting on the system response speed.