A Novel Process of Fabrication of Ultrafine Microvias for Advanced Electronic Packaging

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9900238RailkarWe propose to study and develop a novel, femtosecond (fs) excimer laser based process for fabricating ultra-fine (20-50 pm) high aspect ratio ( 5) microvias for next generation electronic packaging applications. These microvias will be metallized using copper by damage-free fs-laser seeding followed by low current electroplating techniques. The vias will be fabricated in polyimide flex, silicon (Si) and alumina (A1203) substrates which are commonly used in packaging applications. Subsequent to the ir fabrication, the vias will be undergo mechanical testing and electrical characterization. The research will be performed at High Density Electronics Center (HiDEC), and Materials and Manufacturing Research Laboratory (MRL) at the University of Arkansas at Fayetteville. In this collaborative research, Irvine Sensors, CA, a leading company in advanced packaging, will implement the fabricated microvias in real application (letter of commitment attached).There is an ever growing need for ultra-fine microvias in packaging industry. As discussed in the National Technology Roadmap for Electronic Interconnections [issued by Institute for Interconnecting and Packaging Electronics Circuits (IPC)], the technological progress trend of via formation technology for packaging is significantly falling behind the progress in IC industry. Consequently, there is a extraordinary need for a technology that could fabricate such microvias on technologically important substrates. In this project, we identify polyimide, silicon, as well as A1203 substrates as those are widely accepted and used material for various high density multi-chip module (MCM) packaging assemblies, MCM-L, MCM-D and MCM-C, respectively.In the proposed project, a 380 femtosecond (fs), UV (Z=248 nm) excimer pumped dye laser will be used to investigate the through- and blind- microvia fabrication. Currently, various technologies such as wet etch, reactive ion etching, photosensitive polymer lithography and lasers - are used for fabricating vias for packaging applications. Dimensional uniformity, mechanical integrity and chemical cleanliness are critical for success of vias and thus the reliability the package. One of the leading candidates is pulsed nanosecond UV excimer laser. This laser is known [1,2] and typically used to etch vias of about 100 um diameter. The via diameter directly affects the ultimate density of vias possible within a unit area and volume, but the density indirectly depends also on the chemical degradation of the substrate surrounding the via. This, in turn completely relies on the process employed to fabricate vias. Recently, brittle fracture of high aspect ratio vias, drilled using nanosecond excimer laser has been observed [3]. The failure mechanism was identified as excessive polymer "burning" due to nanosecond laser induced heat affected zone [1]. This HAZ causes thermal run-away which further puts a natural limit on the ultimate size of the via fabricated using nano second UV laser. Femtosecond laser processing is known to be superior [3]. Consequently, use of ultrafast fs pulse lasers is inevitable. In view of the recent exciting results of fs laser microvia drilling in our laboratory and significant technological advances in fs lasers, we propose that fs excimer laser is a serious candidate for the advancement of science and technology of microvias fabrication for advanced packaging applications. Further, with embedded passives becoming an integral part of high density packaging, the thickness of substrates is on the increase. Use of fs lasers is the best choice for realization of high aspect ratio vias (5), and those could be drilled through integrated passive elements too, without thermally disturbing the surrounding geometry or chemistry, unlike ns lasers. Development of such technology will prove beneficial to companies such as Irvine Sensors, Motorola, Sheldahl, Cray, etc. and will provide us a cutting edge in the fierce global market. This program further benefits educational infrastructure, on and off campus, as students will be exposed to the state-of-the-art research experience and hands-on education. HiDEC and MRL are centers of excellence for packaging and surface engineering at the University of Arkansas (http://www.engr.uark.edu/~apm2). Our collaborations with industrial partners is unique and will benefit a wide range of applications in various fields such as electronics, bio-medical, automobile, telecommunication, and numerous other civilian and strategic applications.***