本實驗開發客製化2x2矩陣式四點導電AFM(Atomic force microscopy)探針，並且建立起四點探針和掃描式探針顯微術的整合技術，可以藉此控制電流流經的路徑，並進行量測微區表面結構的導電率，而且同時消除接觸電阻的影響。除此之外，探針設計為矩形排列，有別於常見的共線排列，根據Montgomery所提出的概念，將矩形排列的四點探針具備量測非等向性導電率的能力。四點AFM導電探針的設計，是根據Abaqus和Intellisuite軟體的機械性質和製程蝕刻的模擬結果，然後規劃微機電製程和微影技術中的光罩圖案，相鄰探針的間距決定了最小的試片結構的範圍，本實驗所設計的間距為60~80 µm，而探針高度約為7~8.5 µm。探針具備AFM形貌掃描的能力，並可進行微區表面定位電性分析。
在製程方面，本實驗為了避免良率的下降，使用了RCA clean中的Piranha溶液和SC-1溶液清洗晶圓表面，避免有機和顆粒汙染物導致製程上的誤差或失敗。在過程中，晶圓經歷過許多薄膜成長、微影曝光和蝕刻成型的步驟，而其中核心的探針成型的技術，是使用45 wt%的KOH溶液並對矽晶圓進行非等向性的濕式蝕刻，並獲得理想的探針針頭和懸臂樑；在蝕刻的步驟，溫度是容易影響成果的參數，因為不同晶面的蝕刻速率和速率比，會隨著溫度而有所改變，將導致蝕刻結果不如預期，而在此蝕刻步驟中30 ℃為最理想的溫度設定。除此之外，由於晶圓表面的形貌高低差(4~5 µm)，造成光阻塗佈不完整，導致微影製程的困難，以及進一步的電極絕緣失效的問題，應對的方法為修改微影技術的參數和使用有著更好附著性質的光阻：AZ P4620，取代原本的AZ 1500光阻，並大幅改善所碰到的問題。最後，由於探針上有四個電極，為了達到導電電極之間的彼此絕緣，吾人使用乾式氧化法，在晶圓上成長了具有高阻值的二氧化矽層，並使用Lift-off技術完成四個彼此絕緣的電極。
當製程結束，使用SEM拍攝並確認探針的功能和規格，並將會使用本實驗所設計的探針對HOPG(Highly Oriented Pyrolytic Graphite)標準試片進行量測和測試。並希望獲得初步的分析成果，並藉由獲得不同電阻率的表現來區分不同微結構，並證實四點導電AFM探針技術的功能。

Design, fabrication and characterization of 2x2 array four-point AFM conductive probe
Jing-Ting Liao
Bernard Haochih Liu, PH.D.
Department of Materials Science and Engineering, National Cheng Kung University

SUMMARY

The purpose of the research is to develop a technique that combines atomic force microscopy (AFM) and four-point probe. An innovative and customized 2x2 array four-point AFM conductive probe was invented. The probe with four tips and rectangle configuration is different from the common collinear four-point probe. The idea of a rectangle configuration is proposed by Montgomery that it is practical to measure anisotropic conductivity by redirecting current.
In the fabrication, I utilized RCA clean, photolithography, reactive ion etching, KOH wet etching, thermal oxidation, and sputtering for micromachining. However, the etched height difference on the wafer caused photoresist coating problems. The original photoresist, AZ 1500, was replaced by the other, AZ P4620. And it resulted to the better adhesion and coverage on the wafer. After fabrication, in order to assure the probe function, SEM was used to inspect probe geometry. The probe that was characterized has a probe spacing is about 60 to 80 µm and a tip height of 7 to 8.5 µm. The probe spacing determines the minimum range of the measurement structure and the tip height determines the maximum of the surface height difference.
With the topography scanning function of AFM, the four-point conductive AFM probe is able to measure the surface structure and the electrical property of materials. In the future, 2x2 array four-point AFM conductive probe will be used to scan the HOPG (Highly oriented pyrolytic graphite) standard sample from Bruker.

Keywords: Four-point probe, Atomic Force Microscopy, Scanning Probe Microscopy, Anisotropy, Microstructure

INTRODUCTION

The resistivity (ρ) is one of the basic physical properties of the materials. It can determine the type or the structure of materials by measuring the parameters. The value of resistivity is from 10-8 Ω•m to 106 Ω•m. Moreover, resistivity value influence the capacitance, threshold voltage, and the performance of the semiconductor element, for example: light-emitting diodes (LEDs)[1], and a transistor[2]. The common method to measure resistance is macro four-point probe method, which have higher accuracy compared to two-point probe measurement[3]. The method measures the homogeneous materials but cannot distinguish short-range varieties of conductivity of materials. In the recent researches, there are micro four-point probe study. For example, the research team from Denmark invented the micro four-point probe and the gap between tips is down to several hundred nanometers. They apply the probe to nano-manipulator, and proved the abilities of one-dimensional conductivity scanning and short-range varieties measurement of the probe[4]. Moreover, the current penetration depth is lower because of lower tip gaps, so the micro four-point probe measurement is able to distinguish the surface electrical property[5].
On the other hand, AFM was invented in 1986, which detects the interaction force between AFM probe and surface, and further interprets the topography of surface of sample[6]. The contact force of probe can be determined by AFM system. At the end, this study is combined AFM technique and micro-four point probe.

MATERIALS AND METHODS

Material:
Our materials are silicon(100), and silicon derivatives, for example: silicon nitride, and silicon oxide. The principles of micromachining are based on different etching rate of materials[7]. For example, the silicon nitride is not etched during KOH etching but the silicon(100) does. Therefore, the purposes of the silicon nitride and oxide is to restrict the etched shape during wet etching.
Design and Simulation:
In the design, the concept of Montgomery was applied. In the idea, it is practical that anisotropic conductivity can be measured by redirecting current in the rectangle configuration[8]. Moreover, Abaqus software and Intellisuite software were utilized separately to test the mechanical function and the fabrication flow of the 2x2 array four-point AFM conductive probe.
Fabrication and characterization:
The whole fabrication technique is included in the Microelectromechanical systems (MEMS) which is the technology of microscopic device.
During the fabrication, at first, the deposition of silicon oxide and silicon nitride was sequentially executed on the four-inch wafer to restrict the etched shape of silicon during KOH etching. Secondly, the patterning of silicon nitride and oxide was used by photolithography, reactive-ion etching (RIE), and buffered oxide etcher (BOE). Thirdly, the KOH etching and sputtering were conducted for micromachining and conductive layer separately. Finally SEM was utilized to inspect the 2x2 array four-point AFM conductive probe.

RESULT AND DISCUSSION

During simulation, Abaqus software was used to test the deflection and stress distribution of 2x2 array four-point AFM conductive probe. There are several findings in the Abaqus simulation. Because there are higher deflection of cantilevers, the spring constant of outer cantilevers are decreased in order to reducing the damage of the probe during probe scanning. Moreover, in simulation there were force concentrations which may destroy the probe on the bottom of the cantilever. Therefore, the bottom widths of cantilevers were increased then the phenomenon was minimized. In the Intellisuite simulation, fabrication flow and the tip height and the probe shape were confirmed, and the condition which the tips will always contact the surface of samples first was confirmed, too.
In the actual fabrication, AFM and Alpha-step profilometer were utilized to measurement the thickness of silicon dioxide and cantilevers after BOE and KOH etching to make sure the etching result was agreed to the design. Moreover the photoresist, AZ 1500, was found that these is coverage problem on the wafer because AZ 1500 was restricted by the etched structure after tip etching. As a result, AZ P4620 was utilized and there is better adhesion and coverage result. According to SEM images, the tips height are 7 ~8.5 micrometer, and tips gaps are 60 ~80 micrometer.

CONCLUSION

Several ideas were added in the design of design 2x2 array four-point AFM conductive probe. For example, the concept from Montgomery that four-point probe can measuring anisotropic conductivity by redirecting the applied and measured voltage with rectangle configuration of tips. Moreover, modify the specification of the probe to lower the damage on the cantilever, like: reduce the spring constant of outer cantilever and minimize the force concentration on the bottom of the cantilevers. Overall, these ideas make the probe more functional and durable.
The SEM result is consist with Intellisuite simulation result.
Furthermore, the geometry of the probe was examined by SEM and it is consistent with Intellisuite simulation result. Therefore, the probe fabrication is completed successfully.

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