應變式原子力探測計之設計與製作

Abstract

原子力探測計在試片表面輪廓的描繪上是廣泛被使用的一種儀器，它在工程上應用 非常廣泛，例如微力量的讀取，量測表面輪廓及快速資料存取等應用。本篇提出的應變 式原子力探測計不同與其它以光學、壓電及電容為主的原子力探測計。除了懸臂樑上的 探針外，加上一個惠司同電橋的應變規。此設計的探針是在懸臂樑的下端，而應變規設 計在上端。 首先我們分析應變與懸臂樑撓度的關係，且使用了有限元素法加以驗證。並且在電 阻設計上考慮了靈敏度的分析並且加入惠司同電橋的電路以補償溫度效應。最後獲得懸 臂樑的撓度與輸出電壓的數值關係，這個關係將做為量測與修改結構尺寸的基本根據。 在製程部份，已經完成用氫氣化鉀從背後把矽完全蝕刻掉以產生主體，結構 主體以2μm厚的Si-rich的氮化矽為主，包括了200μm×40μm的懸臂樑及5μm長的探針 ，並以複晶矽作為電橋的應變規。使用氮化矽做為背後蝕刻時保護正面應變規的材料， 已獲得很好的隔離效果。在電阻變化及電壓輸出的測試結果方面，顯示出與模擬分析相 吻合。

Atomic force microscopy (AFM) is a popular device for inspecting topography of the sample surface. Here we propose a novel process to fabricate a strain-type atomic force microscopy, which is different from other AFM, like the optical, tunneling, and capacitive type. It incorporates strain gauge and Wheatstone bridge circuit into the cantilever with a sensing tip. In this design, the tip is fabricated on the bottom of the cantilever and the gauge circuit is fabricated on the top of the cantilever. First the analytical expressions of the strain and the deflection of the cantilever beam are derived and verified by the finite element method. We also perform sensitivity analysis in the resistance design. Then the gauge circuit using Wheatstone bridge is analyzed to compensate temperature effect. Finally the relation of the deflection of the cantilever beam and output voltage is obtained. In the fabrication process, the polysilicon acts as the resistor, and a 5μm tip is made. The 2μm thick Si-rich nitride forms the main structure including the 200μm×40μm cantilever beam, which is fabricated by etching the silicon from the back side of the wafer. In the back etching process, the thinner Si3N4 is shown to be able to protect the strain gauge on the front side. The testing results on resistance change and output voltage show good agreement with simulation results.