A comparison of new thick photoresists for solder bumping

Abstract

The performance requirements for ultra-thick photoresists are rapidly increasing with the dramatic growth in lithographic applications that require electroplating processes. Two of the main applications for ultra-thick photoresists are advanced packaging and nanotechnology (MEMS). Flipchip packaging has become widely adopted to address electrical device performance and chip form factor considerations. The growth in the nanotechnology market is driven by a wide range of products, which include accelerometers, ink jet print heads, biomedical sensors and optical switches. The requirements of thick photoresists for solder electroplating are significantly different from typical thin photoresists used in front end of line applications. As the photoresist becomes thicker, processing times increase for many process steps. Photospeed gets slower due to the requirements for more chemical reactions per area of coating. Coating uniformity and edge bead control also become more difficult as photoresist films get thicker and time delay issues between process steps can arise. This result has led to the requirement for special photoresist formulations for thick photoresist films. These are traditionally positive tone DNQ-Novolak materials such as AZ 50XT. Such materials can be designed to work for a particular range of thicknesses, but as the desired thicknesses increases the processing times can become very long for high volume manufacturing. Many new bumping schemes require photoresists in a 60 to 70 μm thickness range. While DNQ-Novolak chemistry can work, there is a desire for faster alternatives to improve total cost of ownership (COO) of the lithography cell. In order to have fast photospeeds and reasonable processing times a chemistry that is very photo efficient is needed. Negative tone cross linking chemistries, which can give tens of thousands of chemical events for one photochemical event, provide excellent photospeed and process times. Positive tone chemically amplified photoresist provide hundreds or thousands of chemical events per photochemical event. They are somewhat slower in photospeed than free radical materials, but still provide reasonable photospeeds. This paper compares the lithography and processing performance of these two newer types of thick film chemistries with the performance of a state of the art DNQ-Novolak thick film photoresist. The lithographic performance of these three ultra-thick positive photoresists were optimized to control critical dimensions (CD), sidewall angles and aspect ratios. The experimental results includes process latitude studies, electroplating performance and stripping performance. The general result is that negative free radical chemistry has the edge in photo-speed and processing times, but positive photoresist is better for stripping and perhaps for process integration.