Etching is a commonly adopted process in semiconductor micro-nano manufacturing, which refers to the selective removal of target materials from the wafer surface by chemical means. The general procedure of etching is to protect specific areas of the wafer with a masking material to prevent the underlying materials from being etched away. The masking material can be photoresist patterned by photolithography or more durable materials such as Si₃N₄. After the mask is formed, the sample is etched with an etchant. Once the etching is completed, the masking material is removed by a suitable method, thus finishing the entire etching process.

Etching is categorized into wet etching and dry etching. Wet etching uses liquid etchants, with the sample to be etched immersed in the etchant, and agitation is required during this process to achieve better etching process control. Wet etching is typically isotropic, leading to a significant lateral etching distance when etching thick films. Meanwhile, wet etching generates a large volume of toxic waste liquid. For the above reasons, wet etching is rarely used in modern semiconductor manufacturing processes.

Dry etching technology can achieve various etching effects from isotropic to anisotropic etching of the target materials, and features excellent etching stability and repeatability, making it an essential etching technology for fabricating high-precision electronic circuits. Dry etching technologies include plasma etching, reactive ion etching, inductively coupled plasma (ICP) etching, and electron cyclotron resonance plasma etching. Among them, ICP etching has attracted extensive attention due to its combination of high plasma density and low ion bombardment energy.

Principles of Inductively Coupled Plasma Etcher

Figure 1 shows a schematic diagram of the inductively coupled plasma (ICP) etcher. The etching system mainly consists of two radio frequency (RF) sources, namely the ICP RF source and the reactive ion etching (RIE) RF source. The ICP RF source ionizes the introduced process gas into plasma by means of a high-frequency alternating electromagnetic field, generating glow discharge. The RIE RF source is responsible for accelerating the plasma to bombard the surface of the sample to be etched, thereby achieving the etching effect. The plasma density can be controlled by adjusting the ICP power, and the ion bombardment energy on the sample can be altered by adjusting the RIE power. In addition, during the etching process, the sample is bonded to the etching stage with high-thermal-conductivity pump oil, and helium is continuously introduced to the back of the etching stage for cooling to ensure temperature stability throughout the etching process. The pressure and flow rate of helium are adjustable to control the cooling effect of the sample, and pump oil can be optionally used to further regulate the heat accumulation on the etched wafer. The ICP etching process generally involves both chemical reactions and physical bombardment simultaneously; therefore, the relative intensity of these two processes can be optimized by controlling the ratio of process gases to achieve the desired etching effect.

Advantages of Inductively Coupled Plasma Etching

The inductively coupled plasma etcher has the following advantages:

· Separate ICP RF generator and electrode RF generator allow independent control of ion density and energy.

· High-conductance pumping ports provide high gas throughput for the highest etching rate.

· Electrostatic shielding eliminates capacitive coupling, reduces electrical damage to devices, and decreases particle contamination in the reaction chamber.

· The system is equipped with a helium cooling device, providing excellent temperature control.

Compared with reactive ion etching, ICP etching has the following performance advantages:

· Higher etching rate due to higher ion density and radical density.

· Better control of selectivity and damage with lower ion energy.

· Better etching profile under high ion concentration and low chamber pressure.

· Higher process flexibility from independent control of ICP and electrode RF power