Polishing process
With the continuous development of integrated circuit (IC) manufacturing technology, chip feature sizes are becoming smaller, interconnect layers are increasing, and wafer diameters are also constantly increasing. To achieve multi-layer wiring, the wafer surface must have extremely high flatness, smoothness, and cleanliness. Chemical mechanical polishing (CMP) is currently the most effective wafer planarization technology, which, together with photolithography, etching, ion implantation, and PVD/CVD, is known as the five core key technologies in IC manufacturing.
CMP equipment mainly consists of polishing head, polishing disc, conditioner, polishing solution delivery system, etc. The polishing head and its pressure control system are the most critical and complex components, which are the foundation and core of CMP technology to achieve nanoscale planarization. At present, the most advanced 300mm wafer polishing head abroad adopts pneumatic loading, which has functions such as partition pressure, vacuum adsorption, floating holding ring, and adaptive, making it very complex. With the continuous reduction of feature size and the continuous increase of wafer diameter, the requirements for CMP surface quality are also increasing, and traditional single zone pressure polishing heads can no longer meet the requirements. If the polishing head can divide the wafer into multiple areas for loading, the material removal rate in different areas can be controlled by changing the magnitude of the applied pressure. The polishing head of high-end 300mm wafer CMP equipment currently available internationally typically has three pressure zones. In addition, at the 45 nm technology node and below, current CMP equipment (polishing pressure>6.985 kPa) is prone to problems such as fracture, scratching, and interface delamination of Low-k materials. Ultra low pressure CMP (<3.448 kPa) will be the main development direction of CMP equipment and technology in the future.
The polishing head mainly plays the following roles during CMP process: ① Applying pressure to the wafer; ② Drive the wafer to rotate and transmit torque; ③ Ensure that the wafer and polishing pad are always in good contact without falling off or fragments. In addition, in high-end CMP equipment, it is best for the polishing head to be able to hold the wafer by its own structure without relying on external conditions to improve production efficiency.
Partition pressure polishing head is an important factor in measuring the level of CMP equipment technology. The core idea comes from the Preston model, according to which. According to the research of CHEN et al., the more partitions the polishing head has, the stronger its ability to adjust the material removal rate. But the more partitions there are, the more complex its structure and the greater the difficulty of research and development. There is no specific requirement for the size division of each area of the polishing head, which can be divided equally or based on the actual internal structure of the polishing head.
To prevent the wafer from being thrown out during rotation, the polishing head must have a retaining ring structure. There have been two types of retaining rings in the development of CMP technology: fixed retaining rings and floating retaining rings. Due to the inability of fixed retaining rings to avoid edge effects, current mainstream CMP equipment adopts floating retaining rings. By applying different pressures to the floating retaining rings, the contact state between the wafer and the polishing pad can be adjusted, effectively improving edge effects.
Due to the tight fit between the retaining ring and the polishing pad, a series of grooves must be designed at the bottom of the retaining ring to guide the polishing solution smoothly into the wafer/polishing pad interface. In addition, to improve the lifespan, the retaining ring should be made of high-strength, corrosion-resistant, and wear-resistant materials such as polyphenylene sulfide (PPS) or polyetheretherketone (PEEK).
As mentioned earlier, one important function of the polishing head is to clamp the wafer and achieve fast and reliable transfer between the loading and unloading station and the polishing station. In the development of CMP technology, there have been various clamping methods such as mechanical clamping, paraffin bonding, and vacuum suction cups, but these methods can no longer meet the requirements of high-end CMP equipment in terms of efficiency, reliability, and cleanliness. The multi zone polishing head uses vacuum adsorption method to clamp the wafer, and the basic principle is shown in Figure 2. Firstly, apply positive pressure to the multi zone airbag to squeeze out the air between the airbag and the wafer. Then, use the positive and negative pressure combination control of different zones of the airbag to form a negative pressure zone between the airbag and the wafer, firmly attaching the wafer to the polishing head. This method fully utilizes the multi zone airbag structure of the polishing head itself, and has the advantages of being fast, reliable, and pollution-free.
The pressure control system controls the pressure of the polishing head through air pressure, and its main functions include: ① pressure loading on the wafer and holding ring; ② Apply negative pressure to the polishing head to clamp the wafer; ③ Check if there is any air leakage in each chamber. The pressure control principle of the multi zone polishing head is shown in Figure 3. The main components of the polishing head air circuit include air source, pressure reducing valve, electrical proportional valve, vacuum generator, vacuum pressure regulating valve, two position three-way valve, two position two-way valve, and pressure sensor. The polishing head has five pressure chambers (Z1-Z5), each of which has functions such as applying positive pressure, extracting negative pressure, venting to the atmosphere, and leak detection. The positive pressure is controlled by a fully closed-loop system.
