Diffusion involves introducing controllable dopant atoms (e.g., boron, phosphorus, arsenic) into a silicon wafer to form PN junctions/resistive regions. During this process, metallic impurities irreversibly damage the electrical properties and device structures of the silicon wafer, posing a far greater hazard than particle or organic contamination. The core reasons are as follows:

1.Rapid Diffusion, Deep Penetration, and Irremovability

Metallic impurities (e.g., iron, copper, nickel, sodium, aluminum) exhibit a much higher diffusion rate in the silicon lattice than process dopant atoms. Under high-temperature diffusion conditions, they rapidly penetrate the bulk region of the silicon wafer—even reaching the substrate bottom. Once diffused, no physical or chemical means can effectively remove them, resulting in permanent damage.

2.Formation of Deep-Level Recombination Centers, Degrading Minority Carrier Lifetime

Metallic impurities introduce deep-level defects in the silicon band gap, acting as strong recombination centers for non-equilibrium carriers. This drastically reduces the minority carrier lifetime of the silicon wafer, directly leading to deterioration of device switching characteristics, a surge in leakage current, and a reduction in reverse breakdown voltage—ultimately scrapping the entire wafer.

3.Disruption of PN Junction Characteristics, Inducing Electrical Failure

Metallic impurities distort the impurity distribution of the diffused junction, causing PN junction leakage, soft breakdown, and abnormal on-resistance. They are the primary cause of batch failure in devices such as diodes, transistors, and MOSFETs.

4.Interference with Doping Precision, Causing All Process Parameters to Deviate

The core of diffusion is the precise control of dopant concentration and junction depth. Metallic impurities interfere with the solubility and diffusion coefficient of dopant atoms, leading to deviations in sheet resistance and junction depth from the process window. This results in complete loss of parameter consistency across the entire batch of devices.

5.Special Sensitivity of Pre-Diffusion Cleaning

The purpose of pre-diffusion cleaning is to remove surface metal residues. If cleaning is substandard and trace metal residues remain, their hazard will be "amplified" during subsequent high-temperature diffusion. In contrast to particles and organics (which can be partially eliminated via thermal decomposition/volatilization), there is no room for remediation of metal contamination.