Summary 500 words
The International Linear Collider is a proposed e+e- linear collider designed for precision physics in the post-LHC era. The SiD concept is one of the three designs for particle detectors being designed and developed. SiD includes a Si-W electromagnetic tracking calorimeter, whose thin onion-skin construction of detection (silicon) and interaction (tungsten) layers allows for implementation of particle flow algorithms. Achieving both an adequate tracking performance and a sufficient number of radiation lengths, while maintaining a reasonable size for the detector, thin layers are needed in the ECAL's structure. Consequently, detection and readout hardware is required to be ~1 mm thick. We accomplish this by employing ~5" hexagonal silicon sensors, which are read out by the KPiX chip being developed at SLAC. The KPiX and data-bus flex cable are bump-bonded to the sensors. In this talk, the technologies needed to electrically bond these components with strict thickness and alignment parameters are presented. Test assemblies using the following technologies and the above hardware are to be tested at a beamline at SLAC this summer; results are shared.
Under-bump metallization, including metal types and thicknesses in the metal stack, plays a crucial role in the bond-ability of the parts. We explain sputtering and zincating techniques as they provide us with procedures to add metal layers to bonding pads for high solderability and report on effective stacks. We have developed the techniques and experience to successfully tack solder balls to bonding pads. Two eutectic solder compositions with differing melting temperatures are used for a novel two stage bonding process. Utilizing specialized flip chip bonding equipment, we can easily achieve the alignment tolerances of ~5 um. Additional bonding parameters include heating profiles (temperatures and durations) and coplanarity. During the solder heating, the chemical oxidation of metal layers destroys bond quality unless reduction agents are used; the results from both liquidus reducing flux and forming gas trials are presented. Our experiences with bond compression control and thermal expansion mediation of substrate materials have created a robust bonding procedure. Most of this process is conducted in-house at University of California Davis physics labs.
Finally, other techniques are investigated for bonding the readout and detector components, e.g. gold ball bonding. One alternative is presented in detail: "anisotropic conducting film" is a general term for an interconnection method utilizing conducting and insulating material to achieve unidirectional electrical conductance through the bulk of a medium. BTech brand ACF uses tightly packed nickel rods (2-8 micron diameter) suspended in a polymer matrix to conduct along the length of the rod while insulating in the transverse plane. Since the uniformity of the nickel rods allow X-Y translation of the BTech without affecting Z-axis conduction, this technology removes the burden of precision placement of bonding media (e.g. solder balls) on bonding pads. Resistance versus temperature during the bonding of ACF film is a complex process; changes in resistance as the polymer matrix liquifies and the nickel rods shift are explained. This technology and its potential use in high energy physics detectors is described.