Electrografted seed layers for metallization of deep TSV structures

This paper describes electrografted (eG) copper seed layers deposited on a wide range of Through Silicon Via (TSV) dimensions. Deposition is achieved on patterned substrates insulated with different materials (SiO₂ for example) and covered by titanium or tantalum based diffusion barriers deposited by PVD, CVD or ALD. This eG technique is fully compatible with standard electroplating tools, either at 200 mm or 300 mm scale. Characterization of electrografted copper layers in vias with half pitch between 5 mum and 100 mum and aspect ratio from 1:1 to 10:1 is presented, with step coverage (ratio bottom/top thickness) going up to 100%. From these conformal eG seed layers, several examples of successful copper gap filling are shown, using conventional plating baths. Copper bulk properties have been studied by several techniques: sheet resistance measurement, mechanical stress analysis, X-Ray diffraction (XRD) and Electron backscattered diffraction (EBSD). Electrical properties of both electrografted copper and conventional plated copper (on top of PVD seed) are equivalent, which was predicted from the copper microstructure, similar for both copper deposition techniques. Contaminant levels in the copper bulk, observed by TOF- SIMS (time-of-flight secondary ion mass spectrometry), is lower in the case of eG copper compared to conventional plated copper, and especially the level of carbon, sulphur and chlorine. Some reliability tests have been performed with 50 mum*100 mum TSVs, looking to copper gap filling evolution after various thermal cycles, going up to 400degC. Work is ongoing to collect electrical data, using specific test vehicles integrating electrografted seed layers with design rules typical of TSV technology. AquiViatrade stack, under development at Alchimer, is presented at the end of the paper. AquiVia stack includes the insulator layer, the diffusion barrier and the copper seed, all these layers being deposited in a wet path using grafting technol- ogy.