掃描微波阻抗顯微鏡：介電常數與電導率的奈米級成像

Scanning Microwave Impedance Microscope: Nanoscale Mapping of Permittivity and Conductivity

掃描微波阻抗顯微術（scanning microwave impedance microscopy，sMIM）是以原子力顯微鏡（atomic forcemicroscope，AFM）為平台的分析技術，應用於分析材料和元件特性。從針尖與樣品界面反射的微波掌握了針尖下方樣品表面的電性訊息，即時偵測並分析處理反射的微波訊號，使得掃描微波阻抗顯微鏡能直接取得材料的的介電常數和電導率，從而取得針尖底下奈米微區上的樣品電性，伴隨著探針在樣品表面的掃描，掃描微波阻抗顯微鏡可以直接對樣品表面的介電常數和導電率進行二維甚至三維成像。當原子力顯微鏡型式的掃描微波阻抗顯微鏡探針在樣品表面掃描時，掃描微波阻抗顯微鏡能將電阻（sMIM-R）與電容（sMIM-C）特性的變化成像，由於掃描微波阻抗顯微術是基於探針尖與樣品之間的電容耦合，所以這種偵測方法不需要與樣品有電性導通的接觸；而在導通的情況下，於樣品或待測元件上施加交流偏壓，掃描微波阻抗顯微鏡就類似傳統的掃描電容顯微鏡，能提供載子分布輪廓，以相同的方式，掃描微波阻抗顯微鏡也能提供獨特的非線性電阻特性成像。兼具一般性訊號與樣品交流偏壓調制訊號，掃描微波阻抗顯微鏡可適用於研究具有複雜組成的材料表面或者待測元件，例如良導電性、半導電性以及絕緣性的區域。做為一種近場方法，掃描微波阻抗顯微鏡的解析度僅受限於探針的針尖半徑，其電性成像可輕易達到20 nm以下的橫向解析度。使用具有同軸遮罩結構的波導探針，高訊雜比以及aF以下的靈敏度得以在掃描微波阻抗顯微鏡中實現。有這些獨特的能力，掃描微波阻抗顯微鏡在應用範圍的廣度上優於其他以原子力顯微鏡為基礎的量測模式。本文將介紹掃描微波阻抗顯微術與多功能原子力顯微鏡平臺的整合，當其與峰值力輕敲模式結合時，有機會在脆弱的樣品上（如奈米碳管）得到掃描微波阻抗顯微術的量測結果。 Scanning microwave impedance microscopy(sMIM) is an AFM-based technique for materials and device characterization. The reflected microwave signal from the tip-sample interface holds information of the electrodynamic properties of the sample surface underneath the tip apex. Detecting and processing in real time of the refl ectance allows sMIM to directly access the permittivity and conductivity of the material. When an AFM-type sMIM probe is scanning across the sample surface, sMIM is capable of imaging variations in resistive(sMIM-R) and capacitive(sMIM-C) properties. This detection approach does not require adding electrical contact to the sample as sMIM is based on the capacitive coupling between the tip and the sample. By AC-biasing the sample or device under test(DUT) , sMIM also provides carrier profi ling(dC/dV) capability similar to traditional scanning capacitance microscopy(SCM) . In the same way, it also uniquely offers mapping of nonlinear resistive properties(dR/dV) . With both regular and AC-sample-bias modulated sMIM signals, sMIM is suitable for studying surfaces with complex composition or DUTs with a broad dynamic range, e.g., metallic, semiconducting and insulating domains. As a near fi eld method, the resolution of sMIM is only limited by the tip radius of the probe and it can easily achieve a lateral resolution of < 20 nm for electrical mapping. Sub-aF sensitivity and high S/N ratios are realized by using waveguide tips with coaxial shielding. Having these unique capabilities, sMIM is superior to other AFM-based electrical modes for a broad range of applications. This article provides an introduction of sMIM and its integration with versatile AFM platforms. When combined with PeakForce Tapping, it is possible to obtain sMIM results on delicate samples such as carbon nanotubes.