Processing of vertically aligned carbon nanotubes for heat transfer applications
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The development of wide band gap semiconductors for power and RF electronics as well as high power silicon microelectronics has pushed the need for advanced thermal management techniques to ensure device reliability. While many techniques to remove large heat fluxes from devices have been developed, fewer advancements have been made in the development of new materials which can be integrated into the packaging architecture. This is especially true in the development of thermal interface materials. Conventional solders are currently being used for interface materials in the most demanding applications, but have issues of high cost, long term reliability and inducing negative thermomechanical effects in active die. Carbon nanotubes have been suggested as a possible thermal interface material which can challenge solders because of their good thermal properties and 1-D structure which can enhance mechanical compliance between surfaces. In this work, we have developed a novel growth and transfer printing method to manufacture vertically aligned CNTs for thermal interface applications. This method follows the nanomaterial transfer printing methods pioneered at Georgia Tech over the past several years. This process is attractive as it separates the high growth synthesis temperatures from the lower temperatures needed during device integration. For this thesis, CNTs were grown on oxidized Si substrates which allowed us to produce high quality vertically aligned CNTs with specific lengths. Through the development of a water vapor assisted etch process, which takes place immediately after CNT synthesis, control over the adhesion of the nanotubes to the growth surface was achieved. By controlling the adhesion we demonstrated the capability to transfer arrays of vertically aligned CNTs to polyimide tape. The CNTs were then printed onto substrates like Si and Cu using a unique gold bonding process. The thermal resistances of the CNTs and the bonded interfaces were measured using the photoacoustic method, and the strength of the CNT interface was measured through tensile tests. Finally, the heat dissipation capabilities of the vertically aligned CNTs were demonstrated through incorporation with high brightness LEDs. A comparison of LED junction temperatures for devices using a CNT and lead free solder thermal interface was made.