Next Generation of 2-7 Micron Ultra-Small Microvias for 2.5D Panel Redistribution Layer by Using Laser and Photolithography Technologies

Microvia is the vertical interconnect structure for multi-layer redistribution layers (RDLs) in high-density interconnect (HDI) printed circuit boards (PCBs), HDI package substrates, 2.5D interposers and fan-out packages. Three technologies such as photolithography, UV laser and excimer laser have been used to form small microvias (≤ 20 µm diameter) in polymer dielectrics. All the three above mentioned technologies are studied and compared in the work presented in this paper. Photovia was first introduced by IBM for Surface Laminar Circuit technology and it has scaled down from 125 µm then to below 10 µm today. The smallest photovia demonstrated is 2 µm in diameter by using 365 nm photolithography in 5 µm thick TOK photo-imageable dielectric (PID) (IF4605) film. Photovias of 3 µm diameter were also demonstrated in 5 µm thick Taiyo Ink dielectric dry film material (PDM) which passed 1,500 thermal cycles (-55 C to 125 C). The limitation of photovia technology is the availability and cost of photo-sensitive dielectric materials with the required electrical, mechanical, thermal and chemical properties. The state-of-the-art microvia diameter is 20 µm by using conventional high-speed UV laser technologies. Multi-layer RDL with microvias and trenches of 4 µm feature sizes are simultaneously fabricated in a 7 µm thick Ajinomoto Build-up Film (ABF) with small fillers by using excimer laser and passed 1,000 thermal cycles (-55 C to 125 C). This paper demonstrates a novel picosecond UV laser technology to push the limits of low-cost UV laser technology by optimizing laser parameters and dielectric materials. The Cornerstone picosecond UV laser tool from ESI is capable of producing output power of 16W at 355 nm wavelength. The pulse duration is 5 ps which minimizes the heat-affected zone of polymer dielectric and the high (80 MHz) repetition rate enables this laser to be used in high throughput manufacturing processes. Microvias with minimum diameter of < 7 µm were fabricated in 5 µm thick ABF with small fillers and in 7 µm thick novel Panasonic low stress dielectric film-S (PLS-S), by using 355 nm picosecond UV laser tool. These ABF and PLS-S films are non-photosensitive dielectric materials. This is the first demonstration of very small microvias (< 7 µm) in polymer dielectrics using UV laser ablation. The motivation of this work is to address the high RDL interconnect density requirements for 2.5D interposer and high density (HD) fan-out packages. The next generation of low-cost, ultra-small microvias will (1) Increase the RDL I/O density, (2) Meet fine bump pitch requirements, (3) Reduce the metal layer count for package substrate RDL, (4) Fill the gap between semiconductor back-end-of-line (BEOL) process and semi-additive process (SAP) and thereby (5) Improve the packaging performance at lower costs.