| Popis: |
Electrical interconnections are paramount components of microelectronic packages, being the link between the device and the external circuit. Accordingly, the performance of electronic systems is directly affected by the electrical, thermal and mechanical properties of the materials used in the interconnects. Conventional materials show limited electrical current handling and poor mechanical stability at high temperature, which constrains the applicable current density and requires thermal management with increased complexity. Pastes based on copper nanoparticles (Cu-NPs) can overcome these limitations, owing to Cu’s superior electromigration resistance and high-temperature stability compared to standard solder alloys. In the present work, we report on the development of processes to fabricate pure Cu interconnects by sintering Cu nanoparticle pastes with the aim of overcoming the limits of state-of-the-art-interconnects used in flip-chip and power device packaging. In particular, we investigate three major topics: (i) the robustness of Cu interconnects formed by oven-sintering Cu paste, (ii) the fabrication of Cu interconnects by laser-sintering of Cu paste, and (iii) the role played by the organics in sintering the Cu paste. In the oven-sintering route, flip-chip interconnects made of pure copper are formed following the “dip-based all-copper interconnect” approach, where the Cu pillars on the chip are first dip-coated with Cu paste, and then aligned and placed on the substrate’s Cu pads. After that, the assembly is heated to 200˚C in formic acid enriched nitrogen atmosphere in an oven. During this thermal step, the Cu paste is sintered, forming a rigid Cu joint that connects the Cu pillar and the pad. We demonstrate that the dip-transfer process is robust, independent of variations in the withdrawal velocity and copper pillar heights. Moreover, the scalability of the dip-based all-copper interconnect approach to fine pillar diameters and pitches is shown, leading to the potential packaging of devices with high interconnect density beyond typical solder-based interconnect limits. Moreover, reductions of the sintering temperature and residual porosity of the Cu joint are achieved, resulting in improved mechanical and electrical performance of the interconnect. Finally, the compatibility of this technology with standard pad finishing layers is demonstrated, thus enabling its use on standard printed circuit boards. The dip-based all-copper interconnect technology is further explored by developing a fast and formic-acid-free fabrication process, which exploits the laser-assisted sintering of Cu nanoparticle pastes. First, we study the sintering of thin films of Cu paste by irradiating a laser through the Si substrate. Herein, the effect of the input laser energy density on the processed Cu is investigated, observing that the sintering takes place only within a defined energy regime. Then, laser-assisted sintering is used for the first time to form dip-based all-copper interconnects. These interconnects are accomplished in air, reaching a maximal temperature below 400°C for only a few seconds. For these interconnects, a shear strength comparable to state-of-the-art dip-based all-copper interconnects is achieved. This novel approach allows a fast and localized heating of the assembly, potentially mitigating thermo-mechanical stresses caused by the sintering procedure. Finally, Cu pastes based on amine-passivated Cu nanoparticles are demonstrated to be oxide-free after production, and thus can be sintered in inert atmosphere without applying reducing agents. This renders them attractive for bonding of power devices to substrates, where the package topology challenges the application of formic acid-enriched nitrogen and laser-assisted sintering. The mechanisms of sintering of these Cu pastes are studied in order to develop tailored paste formulations for the die attachment process. On the one hand, the sintering onset temperature of these pastes is determined by measuring in situ the electrical characteristics during sintering and observed to be dependent only on the amine desorption temperature and nanoparticle size. On the other hand, the densification temperature is found to be proportional to the boiling temperature of the carrier solvent. For die areas larger than 25 mm2, high-boiling-point solvents cannot be completely evaporated, thus locally hindering the sintering process. Therefore, paste formulations based on solvents with a low boiling point and high vapor pressure are preferred for attaching large dies. In this regard, we report the successful attachment of dies with an area of 100 mm2 using a paste based on 1-nonanol. |
