MEMS 应用中不同厚光刻胶的比较研究

This work reports on recent advances in microfabrication process technology for mediumto high-aspect ratio structures realised by UV photolithography using different kinds ofphotoresists. The resulting structures were used as moulds and will be translated intometallic structures by electroplating. We used four types of photoresists: SPR 220-7 novalakbased (positive), SU8 epoxy based (negative), Ordyl P-50100 acrylate based (negative) dryfilm photoresist, and Diaplate 132 acrylate based wet photoresist (negative). The motivationfor this work was to find an alternative to SU-8 photoresist, which is difficult to process andremove after electroplating. Depending on the application, we found that Ordyl P-50100 dryfilm photoresist is the best alternative to SU8 for realization of approximately 100 um deepmoulds for electroplating in acidic electroplating solution. SPR 220-7 is a good alternativeto SU8 for fabrication of 50 um deep moulds and electroplating in alkaline solutions.The results presented in this paper will open up new possibilities for low-cost processesusing electroplating for MEMS applications.

Introduction

High-aspect-ratio structures are commonly used formany MEMS applications such as sensors, actuatorsandmicro-machines. The specific aim ofthis workis therealisation of 3D structures using micro moulds for/andelectroplating.There is a range of fabrication techniques for realising 3D structures: X-Ray and UV-LIGA techniquesDeep Reactive Ion Etching (DRIE), wet Si bulk micro.machining and excimer laser ablation [1].Originally the X-ray LIGA (Lithografie Galvanofor-mung Abformung) process was extensively used to realise very high aspect ratio microstructures. Howeverthis technique involves considerable process complexity, high-contrast X-ray masks, difficulties in structuring PMMA (polymethylmethacrylate) photoresistthicker than 200 um, and high cost, since a synchrotronradiation source is required. The DRIE (Deep Reactive[on Etching) technique has became very popular nowadays because of its high etching speed, good profilecontrol, and the capability to realise microstructureswith good uniformity and high-aspect-ratio. Howeverthis process causes a sidewall roughness (scalloping)with amplitudes of up to 1 um [2]. Moreover, for manyapplications micro structures with submicron resolu.tion are not required and medium to high aspect ratiosat micron resolution would be sufficient. This motivated the search for a cheaper and simpler method to realizemicro-moulds for electroplating.The most promising alternative technique is based onUV LIGA (thick photoresist photolithography). Verythick photoresist lithography requires photoresists withlayer thicknesses that are orders-of-magnitude thickerthan those used for the fabrication of ICs [3]. This paper reports on the development of very thick photoresist lithography techniques using four different kindsof photoresist with different formulations.Up till now SU8 negative tone photoresist has beenextensively used for many applications and is the mostwell known because of its outstanding characteristicsfor high aspect-ratio structures, its use as an etch maskits excellent chemical resistance, and its interesting mechanical and optical properties [4-7].SU8 is an epoxy based photopolymer. Its structure isdepicted in Fig. 1.

Other negative thick photoresists suitable for verythick structures are Diaplate and Ordyl P-50100. Theseare wet and dry film photoresists respectively, which areacrylate-based photopolymers. Fig. 2 shows their structures. The structure of the dry film resist has to have achemical resist formulation that is a very viscous, inorder to sandwich it between a polyolefin sheet and apolyester base, and to berolledup on a supportcore, andto enable it to be cut or finished to various widths and roll lengths, as illustrated in Fig. 3. There are differenttypes of dry film photoresists widely used and commer-cially available, such as Riston  [8], Ordyl BF 410 [9].c 5600 [10], DF 4615 [11], DFR-15 [12]. DryEtertecfilm resists are commonly used for the fabrication ofprinted circuit boards, photochemical machining, andsome formulations for electroplating applications [13].

Diaplate 133 was used for the electroforming of micro parts for MEMS by Niedermann etal.[14].The processing was developed for thicknesses up to 700 um. Ithas been applied to the fabrication of two demonstrators: nickel foils with precise holes and gear wheelsIn our work we investigated processing parameters forDiaplate 132.

However, there are positive thick photoresists, whichare suitable for 3D-structure fabrication. One ofthe photoresists is SPR 220-7, which is photopoly-mer, based on novalak resin, where novalak resin isCresol+Formaldehyde. Fig.4 shows the chemical for-mula of the polymerEach of these photoresists has its advantages and disadvantages depending on the application.