High Density Interconnect Solutions for the HL-LHC

This proposal addresses a pressing need in the detector development community within experimental High Energy Physics (HEP), namely the ability to produce high density interconnect detectors that can survive the environments of the modern and future experiments. The HEP community has been involved in the development of highly segmented and miniaturized detection elements ever since silicon strip detectors were first invented in the late 1970s. The high luminosity LHC (HL-LHC) has raised a new challenge for the technology as these detectors will all need to be hardened against the unprecedented levels of radiation dosage expected at the HL-LHC. Continued expansion in scale, density, complexity, and radiation hardness of silicon-based detectors requires concurrent development of technologies that enable interconnections between detector elements, readout electronics and data acquisition systems. Moreover, these designs require layers of novel materials whose thermal, mechanical and radiation tolerance properties need to be studied independently and also within assemblies that include the interfaces. The issue present in the HL-LHC and other HEP experiments, is not the bump size and pitch, but rather the issue of Dielectric Breakdown. During operation, due to radiation damage, the ROIC chip needs to supply the sensor with upwards of 500-600V. The issue, is that at these potential differences, the ROIC will short to the sensor. In order to prevent shorting, the interconnects need to be surrounded by a dielectric with sufficient breakdown strength after irradiation. In the Phase I work effort, materials that have application to the processing methodology inherent to foundry wafer production and wafer back-end processing where researched. Of these materials, a set of candidate materials were selected for testing and evaluation. These materials were evaluated for their process applicability to back-end wafer processing and for radiation tolerances to doses equivalent to those expected by the Outer Tracker in the HL-LHC upgrade. This Phase I work effort resulted in a set of candidate materials that satisfy both the wafer processing requirements as well as the radiation tolerance requirements. This technology has commercial application and public benefits to research being done in the national labs, application to detectors used in nuclear reactors as well space physics applications. In the Phase II work effort, the Phase I work will be expanded upon. As we have developed testing and methodology techniques to make these measurements, we will extend the material selection and radiation testing to include material selections for the Inner Tracker of the experiments as well. These materials must survive more than a 10X increase in radiation damage, In addition, electro-mechanical analogue structures will be designed and tested through the entire process from wafer manufacture through electrical interconnect and bonding. These structures which will electrically, mechanical and thermally represent the projected real die will be irradiated with both proton and neutron sources and then evaluated for performance. This will enables us to bond up real test die and evaluate them with a very high degree of confidence.