Laser Processing of Polyimide and Molybdenum Substrates for Extreme Environment Electronics

Abstract

A desire in today’s electronic packaging technologies is to create more dense electronic packaging schemes. One method to achieve more dense electronic package integration is to use three-dimensional interconnects to minimize interconnect physical dimensions. Three-dimensional interconnect technologies, such as interposers, require a method of connecting electronics to electronic packages vertically. Thru silicon vias are a widely studied and explored vertical interconnect technology used in a variety of electronic packaging applications. Though silicon has high material hardness, the brittle nature of silicon substrates is a concern for robustness and reliability. These concerns only increase when the desired interconnect application is in extreme environments, such as cryogenic temperatures. A potential alternative to silicon in thru-via applications in extreme environments is molybdenum. Molybdenum is a commonly used material in many applications, such as automotive and aerospace, and has high material strength. Molybdenum is also a low coefficient of thermal expansion material, comparable to silicon, which in combination with high material strength lends to high reliability at extreme temperatures. A challenge of working with refractory metals such as molybdenum is that conventional CNC machining techniques require the use of machining coolants that may not be compatible with fabricated electronics and/or processes can become costly due to tool wear and the need for specialized equipment. Additionally, using molybdenum as a thru-via substrate requires the need for an electrically insulated layer, such as polyimide, within drilled molybdenum thru-holes to prevent the molybdenum substrate from shorting via connections. One potential solution to the above challenges is developing laser processing techniques to cut and drill molybdenum and polyimide. Laser processing is becoming a preferred method for inexpensive prototyping with high resolution and design versatility. This work seeks to meet the challenges described by developing laser cutting and drilling techniques for both molybdenum and polyimide for the use in cryogenic applications. This work will describe two laser processing configurations utilizing two different lasers to laser machine molybdenum and polyimide. This work will first describe the process strategies of laser cutting and drilling molybdenum as a stand-alone substrate. Aspects of laser processing on molybdenum, such as pattern repetitions and scanning speeds, will be described. Process parameter variations will be compared directly and evaluated. Additionally, post-laser processing steps, such as abrasive blasting, will be described and discussed. Next, the fabrication steps to achieve an insulating layer of polyimide in laser drilled holes as well as on each surface of the molybdenum will be detailed. Once polyimide has been cured, laser processing parameters to realize thru-holes in the polyimide-filled molybdenum drills will be presented. Adhesion of polyimide on the molybdenum substrate will be verified and demonstrated through the use of confocal scanning acoustic microscopy before and after polyimide laser processing. Cryogenic performance will be verified by completing confocal scanning acoustic microscopy after submersion in liquid nitrogen (~77 K) and compared with the pre-submersion microscopy images. Processes detailed in this work will open opportunities for future development of molybdenum-based interconnect structures for use in cryogenic applications. Acknowledgement We thank the Alabama Micro/Nano Science and Technology Center (AMNSTC) for providing access to fabrication and characterization facilities used in this work.