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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 4 - SMARTHERM (Pilot line production of functionalized CNTs as thermal interface material for heat dissipation in electronics applications)

Teaser

Summary

The global economy is benefiting from the fast evolution of the semiconductor industry. The primary driving force of the semiconductor industry is the increasing integration density, which has lead not only to higher transistor density but also to
increasing power density. Power density is expected to increase up to 135 W/cm2 in 2024 for single chip packages. This puts significant stress on thermal management technology. Obviously, there is a need for improved thermal management
within the field, and innovative TIMs constitutes a key component in reaching this goal. TIMs with dispersion of CNTs in polymer matrices for improved thermal conductivity have been reported. However, the thermal conductivity in these
composites is insufficient and fundamental limitations stem from the huge interfacial contact resistance between the CNTs and the contact resistance between the CNT ends and target surfaces. Therefore, SMARTHERM project is initiated aiming
to build up a pilot production line for high-performance TIMs based on functionalized CNTs. The utilization of vertically aligned CNT structure eliminated the CNT-CNT contacts along the heat transfer path and the functionalization at the CNT
ends dramatically decreased the contact resistance. The main outcomes of the SMARTHERM project are two types of CNT based TIMs manufactured in a roll-to-roll manner which allows large scale production at industrial level. The TIMs will be
demonstrated by two demonstrators proposed by TRT. The consortium consists of 5 partners from 3 European countries and integrates competence from 2 big companies, 2 SMEs and 1 university. TRT and SHT have been funded by the FP7
framework programme NANOPACK and SMARTPOWER, in which SHT and TRT have obtained great amount of knowledge on CNT based TIMs. Therefore, SMARTHERM project will make use of the previous research results and push the material
a big leap forward to the market.

Work performed

In order to use the VACNT as an efficient TIM, one issue is to ensure low contact resistance between the CNT ends and target surfaces. This resistance is indeed identified as a major contribution to the total resistance of such an interface. Several methods have been proposed to deal with this issue, such as the application of pressure on the interface. or the thermocompression bonding, which uses a metal as an interface material between the CNT and the superstrate. However, the industrial approach that is yielding the best results is the use of a heated polymer thin film as an adhesive layer at the CNT/superstrate interface. An azide functionalized polymer has been designed that has the ability to form covalent bonds with CNTs, thus reducing the overall thermal contact resistance. Moreover the polymer was also designed to sustain the required processing temperatures of microelectronics packaging. In the frame of the SMARTHERM project, the polymer will be synthesized at TRT and the processing conditions of the functional polymer will be adjusted; the industrial method is a localized spray deposition method, with a targeted thickness of polymer below one micron. The most appropriate spray deposition conditionswill be determined to achieve the required thermal performances at the level of a production line.

Final results

In order to develop CNT based TIMs with high thermal performance, vertically aligned CNTs are applied in SMARTHERM project. Therefore CVD method is selected to synthesize the raw CNT material.
In order to break the limitation of the heater in the CNT growth process, a CVD chamber allows roll-to-roll synthesis of vertically aligned CNTs for large scale TIM production is designed. The manufacturing process of the
CNT based TIMs . Figure 2 shows the first type of CNT TIM manufactured in this project, i.e. the CNT TIM on native growth substrate. The fabrication process starts from a Si wafer with
deposited Al2O3/Fe as catalyst, as seen in Figure 2(a). The wafer is annealed at high temperature with carbon precursor to grow CNT forests, followed by the spray coating of polymeric material for
functionalization the CNT ends. the second type of CNT TIM manufactured in the free-standing manner. The fabrication process starts from a stainless steel substrate with deposited Al2O3/Fe as catalyst. The growth of the CNT forests is performed in the same chamber as the growth on Si wafer.Afterwards, polymeric material is infiltrated into the CNT forests to make a composite material . In Oxygen plasma is used to reveal the CNT ends. The CNT-polymer composite is then peeled off from the substrate and functionalization is applied on both sides of the film to obtain a free-standing CNT TIM.