Gold (Au) is, on one hand, a noble metal with excellent electrical properties and extremely stable chemical characteristics, widely used in chip manufacturing; on the other hand, it is also a contaminant strictly prohibited in the front-end processes of wafer fabrication, listed as a high-risk control substance.
01
Physical and Chemical Properties of Gold
| Property | Excellent Characteristics |
|---|---|
| Conductivity | Gold ranks third among all metals in electrical conductivity, after silver and copper |
| Oxidation Resistance | Not easily oxidized, suitable for long-term exposure |
| Chemical Stability | Does not react with most chemicals, suitable for use in various packaging environments |
| Ductility | Can be drawn into extremely fine wires, suitable for wire bonding |
| Surface Functionalization | Easily bonds with organic molecules, suitable for functional layers in MEMS and biochips |
02
Why are wafer fabs afraid of gold?
- Gold diffuses in silicon, especially at high temperatures, where gold atoms can rapidly migrate, causing cross-contamination.
- Deep-level traps introduced by gold in silicon act as recombination centers, significantly reducing carrier lifetime and increasing device leakage current.
03
Why do wafer/packaging fabs still use gold?
- Gold electrodes: In compound semiconductors like GaAs, InP, and GaN, gold’s low contact resistance and low loss at high frequencies make it ideal for RF modules such as PA and LNA.
- Gold plating in MEMS: Used to achieve functions like conductivity and corrosion resistance.
- Gold bumps: Used in flip-chip bonding, a “gold mushroom head” is formed using wire bonding equipment, then bonded via thermo-compression.
- Gold wire bonding: Gold wire is soft, conductive, oxidation-resistant, and has a wide bonding window, making it convenient to use.
- EUV mask metals: EUV masks are reflective and require metals with high reflectivity or absorption at 13.5 nm to create light intensity contrast between pattern and background areas. Gold has good absorption properties at 13.5 nm.
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