Breakthrough in Advanced Semiconductor Packaging: Hydrogen Plasma Cleaning of Copper and Tin for Flux-Free Flip Chip and Hybrid Bonding
For advanced 3D IC integration, two technologies are being developed to bond chips together: flip-chip micro-bumps and hybrid bonding (J. H. Lau, Semiconductor Advanced Packaging, Springer Nature, Singapore, 2021). In the former case, fine-pitch solder micro-bumps, consisting of copper pillars with tin caps, are fused together by thermocompression bonding. Hybrid bonding, on the other hand, involves direct copper-to-copper bonding at pitches down to 10 microns. Here, copper vias in each chip are fused together without solder by SiO2-SiO2 chip-to-wafer or wafer-to-wafer bonding.
Metal oxide on the surface of copper, tin and indium is a concern with these advanced integration technologies since the oxide can prevent the metal from each chip fusing together and making a reliable interconnect. For solder micro-bumps with a pitch of 50 microns or less, flux cannot be used to remove the oxide since it is impossible to wash away the flux residue. Formic acid vapor may be used instead; however, this requires a bond
temperature above 200°C, which can cause heat damage to the semiconductor package. For hybrid bonding, the SiO2 layers must be activated for fusion bonding, but not in such a way that the copper vias become oxidized or the surface roughness of the chip and wafers are changed in an undesirable way.
Surfx Technologies is proud to announce a new method of removing metal oxidation from copper and tin: atmospheric pressure hydrogen and argon plasma cleaning (J. Lee, T. S. Williams, and R. F. Hicks, “Atmospheric pressure plasma reduction of copper oxide to copper metal,” J. Vac. Sci. Technol. A 39, 023001, 2021). Hydrogen atoms produced by electron impact dissociation in the Surfx plasma head flow down onto the substrate and react with the metal oxide, converting it back to metal and water vapor. For copper, the reaction is CuOs + 2H = Cus + H2O. One may ask what is so special about Surfx’s plasma that would make one want to select it over traditional vacuum plasma treatment. There are several reasons:
1. Due to the higher electron density and very much higher hydrogen concentration in the Surfx plasma, the amount of active H atoms bathing the semiconductor surface is about 19,000 times higher than in a vacuum plasma. 2. Surfx’s plasma heads are small and compact and can be easily integrated into thermocompression bonders (TCBs) for oxide removal immediately before the bonding step. 3. The high concentration of H atoms generated by the atmospheric pressure argon and hydrogen plasma enables a high throughput to be achieved. Dies with micro-bumps can be stripped of oxidation in as little as one second. 4. For hybrid bonding, the hydrogen plasma quickly activates the silica (SiO2) surface, converting it to a hydrophilic state with a water contact angle below 5°. At the same time, the hydrogen plasma removes any oxidation from the surface of the copper vias. 5. The Surfx plasma system can process 300 mm wafers and die on tape frame in the same machine without contaminating the substrates with particles. Preliminary tests have proven that five particles or less, greater than 0.1 micron, are added by the process.
A picture of the Surfx wafer processing system, model STW-10, is shown below. It is designed for plasma activation of 300 mm wafers supplied in FOUPs and dies on tape frame supplied in cassettes. The machine is qualified for use in class 10 cleanrooms and meets all SEMI standards for environmental, health and safety. Surfx offers the STA-10 plasma system for processing IC packages in high volume using a variety of carriers. Moreover, the integrator’s version of the plasma machine may be easily incorporated into other equipment that is integral to semiconductor packaging. This gives the customer total control over bonding, molding, and printing operations with just-in-time surface cleaning. For more information contact us or visit www.surfxtechnologies.com.
