Solder-based fine pitch microbumps along with through silicon vias (TSV) enable high density interconnects for die-to-die and die-to-wafer stacking for different applications. With reduction in bump dimensions, several challenges arise that need to be addressed. For example, selection of under bump metallization (UBM), solder and barrier between them can determine the consumption of the UBM by solder and transformation to intermetallic compound (IMC), during thermocompression bonding. IMC are known to be brittle and can result in fracture/crack formation and therefore it is important to select the right metallurgy to ensure there is sufficient Sn and UBM left to ensure the reliability of the device for the time it is in operation. Characterizing the thickness of each layer of the microbump at every stage during thermal process prior to bonding becomes very critical. Current options for thickness measurements rely on off-line cross-section SEM measurements or in-situ electrical tests. Picosecond Ultrasonic (PULSE™) technology is a proven work horse for metal film metrology in several wafer fabs. The wide scale adoption of the technology has been primarily driven by its unique capability to provide rapid, first principles in-line thickness measurements.
In this study, we systematically investigate the effectiveness of PULSETM metrology on microbumps with a range of diameters and a variety of metal stacks. On wafers with a Ti/Cu seed layer, a test pattern of microbump features with diameters down to 20mm was imaged in 50 mm photoresist using an advanced packaging wafer stepper (JetStep 2300 series) which employs a low, 0.1NA, essential for creating the steep side walls required for high aspect ratio openings. These patterned wafers were then plated with a matrix of bi-layer (Cu/Ni), tri-layer (Cu/Ni/SnAg) and multi-layer Cu/Ni/Cu/SnAg stacks to form the microbumps, using a Stratus ECD tool. The PULSETM metrology was tested on these microbumps after stripping the photoresist.
We have previously discussed the use of picosecond acoustic metrology technique for characterizing under bump metallization (UBM) stacks, redistribution (RDL) layers and measurement of dielectric stacks. Using an improved experimental setup, we were able to obtain excellent signals on the samples and the measurements were unaffected by the surface roughness. The small spot size enabled measurements on 10 and 7µm diameter microbumps, thus demonstrating the readiness of the technology for the next generation of microbumps.