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Barrier metals that prevent copper diffusion: How do they protect our circuits from damage?
With the advancement of technology, integrated circuits (ICs) have become the core of modern electronic devices, and their performance and efficiency are constantly improving. In this process, the use of copper interconnects demonstrates its advantages in reducing propagation delays and power consumption. Since copper interconnects were first introduced by IBM and Motorola in 1997, their introduction has led to significant improvements in IC performance.
Copper conducts electricity better than aluminum, which allows copper interconnects to operate at smaller sizes and consume less power.
However, the introduction of copper was not without its challenges. Since copper diffuses easily, this can cause damage to surrounding materials, especially silicon, which can lead to performance degradation. Therefore, researchers had to take steps to isolate the copper to protect the integrity of the circuit. This leads to a key solution: barrier metals.
The necessity of barrier metalThe barrier metal layer exists to completely surround all copper interconnects and prevent copper from diffusing into the surrounding material. The diffused copper not only affects the material's properties, but also causes the formation of deep energy traps in the silicon, which is very detrimental to the function of semiconductor devices.
The barrier metal needs to have sufficient conductivity to maintain good electrical contact, while also effectively limiting the diffusion of copper.
The thickness of the barrier metal layer is also important: if it is too thin, the copper will contaminate the device it connects to, while if it is too thick, the total resistance of two layers of barrier metal and a layer of copper will be higher than that of aluminum interconnects, which will undoubtedly offset the advantages of copper.
Evolution of process technology
With the evolution of process technology, the transition from aluminum to copper is inevitable. For the patterning of copper, scientists have developed a method called "Damascene" or "dual-Damascene". The method involves trenching the base silicon oxide layer and then depositing a thick layer of copper on top of the insulating layer. The excess copper is then removed through chemical mechanical planarization (CMP), leaving only the desired interconnect conductors.
Without the support of CMP technology, the realization of this technology would be impossible.
In addition, new alloy materials, such as copper-germanium alloys, have been proposed as potential interconnect materials to reduce copper diffusion issues. These studies suggest that more promising technologies may emerge in the future to further improve the interconnection issues of high-performance electronic devices.
Challenges of Electromigration
Electromigration is another challenge, a process in which a metal conductor changes shape when subjected to an electric current, eventually causing the conductor to break. Copper has significant advantages over aluminum in terms of resistance to electromigration, allowing copper interconnects to handle higher current densities.
The advantage of copper lies not only in its conductivity, but also in its stability in long-term use.
This improvement in electromigration resistance is one of the reasons that has attracted industry investment in copper-based technology. As circuit size decreases, the demand for higher performance grows, making copper interconnect technology more and more important.
The development of super conformal plating technology
In 2005, as processor frequencies increased, interconnect capacitive RC coupling became a speed bottleneck. To address this issue, the challenge is to reduce both resistance and capacitance.
Copper was introduced to reduce resistance, while silicon oxide was replaced by a low dielectric constant material to reduce capacitance.
Through the process of copper electroplating, the research team developed a "super-conformal" filling technology to solve the problem of uneven copper deposition inside tiny channels. This advancement in technology allows the coating to uniformly fill the vias, further improving the quality and performance of the interconnect.
Future Challenges and Thoughts
While copper interconnects continue to advance, challenges remain in the future. Researchers need to explore new materials and technologies to address new problems that may arise during the miniaturization process. Will new alternative materials emerge in the future to replace copper, or can new technologies solve existing diffusion and electromigration challenges? These are questions worth pondering.
