以負光阻SU-8開發雙反摺式疏水結構的部分曝光製程技術

Abstract

數位微流體晶片近年來在生醫應用上已展現許多優勢，其一是可操控微米尺度下的生物檢體，包含氣體、細胞、血清及樣本等等；其二是可與電腦整合，自動化操控，減少檢驗時間。目前數位微流體晶片使用的疏水層多以沉積疏水材料為主，而以沉積疏水材料製作疏水層有以下幾個缺點，如價格昂貴、容易脫落或不均勻，進而影響晶片品質，以及許多生物檢體易與疏水材料產生蛋白質吸附現象，使其失去疏水性能，因而不適用於微流體晶片。本研究在疏水層的文獻中，發現疏水結構有取代疏水材料的潛力；本研究總結了目前已知的疏水結構類型，發現雙反摺結構具備許多適用於微流體晶片的優點，一是無需沉積疏水材料，單純透過結構便將接觸角提升至150度以上；二是能使相當大範圍內表面張力的液體達超疏水門檻；三為疏水性能較不受蛋白質吸附的影響。目前雙反摺結構在製程上依然有步驟簡化、改善材料的物理及化學性質的目標；應用上則希望材料透光性能更好，可以更彈性地選擇基板。 本研究以負光阻開發了雙反摺的正向部分曝光製程，相較於現有的雙反摺結構製程，正向部分曝光製程同時具備製程精簡、基板可彈性選擇、透光度佳以及機械與化學性質穩定性高的優點，並成功以此製程做出雙反摺結構，使低表面張力的矽油接觸角自0度提升至146度，展現出雙反摺結構的疏油性能。 此外，目前尚無討論雙反摺結構高度的文獻，而本研究討論了液珠在雙反摺結構間凹陷的深度，推導出了結構高度與疏水穩定性的理論模型。由接觸角量測的方式得知，理論所預期的液珠狀態與測試結果相符；以負光阻雙反摺結構為例，在結構角接近垂直的狀況下，間距180um的雙反摺結構陣列，結構高度至少要大於90um，也就是間距的一半，方可成功撐起初始接觸角0度的矽油，而本研究作出高度分別為20um、60um、100um的負光阻雙反摺結構，實地滴油測試，確實只有100um的雙反摺結構方能成功疏油。相信本研究所推導的理論模型，對未來的疏水結構在設計上能有實質的幫助。

Digital microfluidic chips have demonstrated many advantages in biomedical applications in recent years. It can be used in operating biological specimen in microns, including gas, cells, serum samples and so on. And it can also be automatically manipulated by computer programs so as to reduce testing time. In these days, most of the hydrophobic layer of the digital microfluidic chips are fabricated by coating hydrophobic material. But there are several disadvantages. For example, the hydrophobic material is easy to fall off and uneven, the biological samples may cause protein adsorption and it is expensive. These demerits affect the quality of the chip, and even make the chip lose the hydrophobic properties. Therefore, coating hydrophobic material may not suitable for microfluidic chips. In our study of hydrophobic layer, we found the hydrophobic structure could substitute for hydrophobic material. Thus, we summarized the presently known hydrophobic structure types, we concluded that to apply double-reentrant structure in microfluidic chip has many advantages. One is it can rise contact angle to over 150 degrees through the structure. Another is it has good applicability for liquid with different surface tension. The other is that the hydrophobicity is less effected by protein adsorption. Up until now, there are still some goals for manufacturing procedure of double-reentrant structures like simplifying procedure, improving the physical and chemical properties of material. And on application, we expect the material has better transmittance and applicability for different substrates In this research, we developed the front-side partial exposure method on SU-8 for double-reentrant hydrophobic structures. Compared to the currently existing manufacturing processes of double-reentrant structure, front-side partial exposure method has the advantage of simple procedures, unrestricted substrates, great transmittance and high stability of physical and chemical properties simultaneously. We succeeded in making double-reentrant structures by this manufacturing procedure, and made the contact angle of silicon oil with low surface tension rise from 0 degree to 146 degrees, which showed the oleophobic ability. In addition, there is no paper discussing the height of double-reentrant structures so far. In our study, we discussed the sagging height between double-reentrant structures, and deduced the theory of the corelation between structure height and hydrophobic stability. By contact angle measuring, we found that the results of droplet state are in accordance with theoretical predictions. Take SU-8 double-reentrant structure as an example, when structure angle is about vertical, and the spacing of double-reentrant array is 180um, the structure height must be higher than 90um, which is half of the spacing, so as to make silicon oil, whose initial contact angle is close to 0 degree, stay oleophobic. In this research, we made three structures of different height, which are 20um, 60um and 100um separately and tested them with oil droplet. Consequently, only the 100um double-reentrant structure successfully made oil oleophobic,as we predicted. We believe that the model deduced in this research can provide practical assistance in hydrophobic structure design in the future.