We are living in an unpreceded time with life style never expected by our ancestors. Especially, the high-speed internet and mobile computation have changed every fundamentals of our human society. All of these revolutions were made possible by powerful computation capability provided by small silicon (Si) chips. The number of transistors, the basic unit of Si chips, has amazingly maintained high rate of advancement following the Moore’s Law for the last 60 years. However, the further minimization of transistors has come to an end due to physical and chemical limitations. To keep up with the rapid technology development pace, new technology breakthrough is desperately needed in the present time. Among all promising technologies, advanced packaging appears to be one of the most likely paths going forward. Numerous advanced packaging methods have been developed in the decade that is coming to the end. New system integration and chip stacking into third dimension from the current cutting-edge 2.5D/3D packaging is leading our way into the next chapter in technology world. From these new packaging methods, the number of transistors can be further increased and thus provides a feasible way to make smaller and smaller Si chips, which is crucial for the mobile computation age.
A key foundation for every advanced packaging technology is new cutting-edge materials, which can endure tough fabrication processes and also provide new functionalities, which were never able to be achieved from traditional materials. Key materials for advanced packaging include polymeric dielectrics, copper layers (metal tracks through electroplating), epoxy molding compounds (EMC), thermal conductive layers, underfill materials, etc.. Among them, high quality dielectric materials, broadly used in redistribution layers (RDL), is undoubtedly the most important material. These dielectrics are used in form of thin films which embed tiny copper tracks and therefore requires an unique set of material properties in the aspects of resolution, electrical and mechanical properties, reliability, etc..
Meanwhile, photosensitive dielectrics could save a number of fabrication steps compared to traditional polymeric dielectric (non-photosensitive) and inorganic dielectric materials. This would greatly reduce cleanroom usage time and fabrication cost. Therefore, photosensitive dielectrics have been broadly accepted as a foundation material in most advanced IC packaging processes. However, the requirements for photosensitivity coupled with outstanding material properties made it extremely challenging to identify an ideal material from the existed materials in early days. A number of prestigious material companies including Siemens, Dow Chemical, and Toray Industries took the odyssey and their R&D work led to photosensitive negative-tone polyimide (PI from Siemens and Toray) and benzocyclobutene (BCB from Dow). Later, positive-tone polybenzoxazole (PBO) and other polyimide with new photochemistries were added to the material family.
Among the PI, PBO, and BCB materials, BCB is exclusively manufactured and marketed by former Dow Electrical Materials (now DuPont Electronics & Imaging) and it will be discussed in a future blog. In this series of blogs, we will go over photosensitive PI /PBO materials as shown the following figure. The two materials are very similar in aspects of structures, properties, etc.. Unless there are distinctions between the two, I would discuss the two materials as a pair in the upcoming blogs. Excellent material properties associated with PI/PBO materials are generally believed to be related to ring-closure reaction as demonstrated in the figure. In my next few blogs, I will introduce these critical PI/PBO material perspectives including material properties, marketing, key manufacturers, various material types, underline photochemistries, etc.
Figure 1-1. Formation of PI/PBO