以pH-ISFET元件為基礎之尿素感測器之製造與研究

Abstract

離子感測場效電晶體(Ion-sensitive Field Effect Transistor)是由Bergveld在1970年首先提出，由於它的尺寸小、反應速度快、可承受外部應力，且與現今的CMOS製程相容，所以在現在的感測元件開發中具有相當大的潛力。基於以上所提之優點，製造一個以pH-ISFET元件為基礎之尿素感測器並研究其特性為本論文主要目標。 首先，為了將酵素薄膜有效固定在ISFET之ZrO2感測層上，我們首創以Nafion 混合尿素酶之結構將Nafion 包埋住尿素酶以達到酵素固定化之目的。由實驗結果得知當酵素電極中尿素酶與Nafion 的混合比例為5:1時，該EnFET尿素感測器具有較廣之感測範圍(8 mg/dl ~ 240 mg/dl)、高感測度[0.64mV/(mg/dl)]、低偵測極限(8 mg/dl)、線性反應、反應時間短(25 ~ 60 sec)、生命期長(> 7 days)以及儲存穩定度佳等特性。 但是由於缺乏一個穩定且微小化的固態參考電極使得EnFET的應用受到很大的限制。這個問題在藉由一個EnFET/REFET差動對的輸出後,固態參考電極因為金屬/溶液接面產生的不穩定電壓會以共模訊號的形式而被消除掉。 在本篇論文中，我們以Nafion 混合光阻(FH-6400)或Polyimide的結構來修飾ISFET的感測層，成功地製造出一系列對尿素靈敏度極低的參考電晶體(REFET)。並從EnFET/REFET差動對的輸出結果以及EnFET/REFET轉移電導特性比較中選擇Nafion 混合光阻(FH-6400)該結構作為REFET感測層之材料。由實驗結果顯示對尿素的感測度達到0.63 mV/(mg/dl)，感測範圍也達到8 mg/dl到240 mg/dl之程度，而此輸出特性非常接近由玻璃參考電極所量測之結果。 為了要實現一個最簡單且小型結構的EnFET，在微小化的技術上，必須要整合一個固態參考電極在單一EnFET晶片上，不需要額外再使用到REFET或玻璃電極。因此，我們也把Nafion 混合光阻(FH-6400)的結構應用到固態參考電極的表面修飾上，使得固態參考電極因為金屬/溶液接面產生的不穩定電壓被消除掉。因為從前面的實驗結果可知道，Nafion 混合光阻的結構具有使REFET的感測層維持在一個固定的電位且保護它不受離子的干擾的效果。由實驗結果可看出，令人困擾的電壓不穩問題，大幅地獲得改善。一個單一的EnFET整合固態參考電極在不需搭配REFET或玻璃參考電極的情況下，對尿素的感測度仍然可達到0.47 mV/(mg/dl)而且感測範圍也達到10 mg/dl到240mg/dl之程度。

ISFET( Ion-sensitive Field Effect Transistor ) was first developed by Bergveld in 1970s, and because of its small size, fast response, rigidity and compatibility with standard CMOS process, ISFET is an attractive candidate of modern sensor device. Based on the advantages described previously, it is our main objective to fabricate a pH-ISFET based urea sensor and study the characteristics of it in the research. First of all, for the purpose of enzyme immobilization, we first apply urease-mix- Nafion structure as enzymatic membrane coated onto ZrO2 sensing layer by Nafion entrapment method. From the experimental results, it is obviously that when the ratio of urease to Nafion is 5:1, the urea-EnFET exhibits wide sensing range of between 8 and 240 mg/dl, high sensitivity of 0.64 mV/(mg/dl), low detection limit of 8 mg/dl, linear response, short response time of from 25 to 60 seconds, long lifetime, and good storage stability. Due to the lack of a stable and miniaturized solid-state reference electrode, the application of EnFET will be restricted seriously. One approach to solve this problem is to use a differential measurement consisting of an ion-sensitive structure (EnFET) and an ion-insensitive structure (REFET). With this arrangement, the common mode unstable voltage generated from the thermodynamically undefined metal/electrolyte interface can be eliminated. In this thesis, we apply the Nafion -mix-PR or Nafion -mix-Polyimide structures to modify the ISFET sensing layer and successfully obtain a series of REFETs with low urea sensitivity. According to the EnFET/REFET differential measurement results with different REFET’s sensing layers and the transconductance characteristics of EnFET and REFETs, we finally choose the Nafion -mix-PR structure as REFET’s sensing layer. From the experimental results, the urea sensitivity can achieve 0.63 mV/(mg/dl), and the urea sensing range is from 8 mg/dl to 240 mg/dl because of the ultra low sensitivity REFET treated by PR-mix-Nafion structure. The performance is extremely close to those of glass reference electrode. In order to realize the single EnFET integrated with the simple and compact structure of solid-state reference electrode by miniaturized technology, and the EnFET sensor is fabricated without the additional REFET or glass reference electrode. We also apply the Nafion -mix-PR structure to modify the surface of the solid-state reference electrode, so the unstable voltage generated from the thermodynamically undefined metal/electrolyte interface can be eliminated. From the experimental results presented previously, we can know the Nafion -mix-PR structure can maintain a constant voltage for the sensing layer of REFET and prevent it from disturbing by the ions. The experimental results show the troublesome and unstable problem can be greatly improved. Without EnFET/REFET arrangement in differential measurement or glass reference electrode, the urea sensitivity of single EnFET integrated with Ti/Pd/Nafion -mix-PR solid-state reference electrode still can reach to 0.47 mV/(mg/dl) and the sensing range is from 10mg/dl to 240 mg/dl.