Process For Patterning Features In Poly(Acrylic Acid) For Microelectronic Applications
Abstract
Development of patterning micro-scale feature in poly(acrylic acid) film or also known as PAA is one technique for engineering application. PAA is a water soluble polymer which make it very sensitive to water contact so a very careful attention is needed during the photolithographic process. The time process of photolithography is reduced by half to obtain well-defined pattern. Plasma ash process is used to remove the remaining photoresist and PAA under it. For patterning micro-scale feature in PAA by plasma ash process holds potential for fabrication of polymer microstructure, sacrificial layer micromolding, and patterned substrate micromolding.
Background
3D devices and its structure is important in modern micromechanic and microengineering. Laser micromachining, stereolithography, ion beam milling and self assembly processes was include in fabrication method. Traditionally, fabrication technology focus more on silicon based technique, however by 3D structure technique may require release from sacrificial layers such as SiO2 and Si. Hydrofluoric acid (HF) is used to removed the unwanted part through the process of etching. Usually to produce pattern and additional treatment provided before releasing the structure during utilizing the sacrificial layers. Photolithography used for micro-scale pattern on flat and smooth surface. Conventional photolithography is impossible for patterning of thin water soluble polymeric film due to the solubility of the polymer to the solution of the developer. A conventional photolithography exposure with the photoresist development time for 50% during the patterning process. Plasma ashing process is used to removed the remaining photoresist and PAA film. This technique was used for patterning PAA in a solder based self assembly (SBSA) process to show the process which include physical vapor deposition of metal, electroplating, photolithography and solder immersion can be perform on defined polymeric shape. Self assembly is useful in creating 3D structure. Heating solder and removing sacrificial layeris needed by self assembly process to start the folding of metal pattern without external force.
Result and Discussion
The pink colour inside the square indicate the silicon substrate according to Figure2(a) and phtoresist is the yellow part. The faint ring indicate by dash line is cause by etching of PAA area by water. In Figure2(b) the ring area etched PAA which leaves the ring of the expose substrate around the shape once resist is strip. The dash line at the edge indicate the over etch of PAA after development. MF319 developer contain water which make a problem to PAA since it is soluble in water. A process is develop to combine a partially complete development process and uses oxygen plasma ashing to remove both photoresist and PAA. PAA is heated with 115oC to remove the excess of water and strengthen the film as Figure1(a). A thin layer of unexposed photoresist will be removed while the expose photoresist and PAA are also remove to open up the PAA windows after ashing since the oxygen plasma process has poor selective of polymer. The benefit of oxygen plasma ashing process is PAA removal process can be control by adjusting the plasma treatment time and it also a residue free area within the open window. PAA assisted SBSA ProcessPlasma ash treatment is use in fabrication process flow to produce number of square pattern. The sample are covered with electroplate Cu after metal deposition, photolithography and electroplating. Cu electroplating involve the immersion of the entire wafer for 20 min in a sulphuric based acid bath is the most critical but the liquid does not penetrate PAA which could cause dissolution of PAA.
