Thick-Film Substrates Go High Density
John Baliga, Contributing Editor -- Semiconductor International, 1/1/2005
At IMAPS in November, researchers from Gennum Corp. (Burlington, Canada) and DuPont Microelectronics (Research Triangle Park, N.C.) presented work on high-density, thick-film ceramic substrates designed for consumer-level stacked-chip packages.
Though multilayer thick-film ceramics have been used for years in various applications, they have been used more for high-reliability applications. While chips in cell phones are typically packaged on organic substrates, the transceiver chips in cell phone antenna stations are typically packaged on ceramic substrates. As the number of chips in stacked-chip packages increases, the reliability offered by ceramics makes them more attractive as substrate materials, but only if the interconnect density is high and the cost is low.
Using standard alumina and even AlN substrates, laser machined through-holes were filled with a silver-palladium paste to provide connections between die on either side of the substrate and to the external contacts. The cost reduction comes from using photo-imageable dielectrics and keeping the number of interconnect layers to a minimum.
The interconnect layers are made using a subtractive process starting with a thick-film screen-printed gold. The key is to make lines and spaces as small as 25 µm, which is done using readily available lithography and etching techniques. With this high wiring density in each layer, they were able to produce substrates with about one-third as many wiring layers as required for a conventional thick-film process.
The use of photo-imageable dielectrics reduces the number of mask steps per layer, which decreases cost. They demonstrated an ability to make vias as small as 50 µm in diameter. The practical minimum diameter is 100 µm, since the vias are to be filled by the same process that provides the next-level metallization. With 100 µm vias, capture pads can be as small as 200 µm in diameter.
An electrical test showed that structures such as coplanar waveguides made on this substrate had performance that was comparable to those made on conventional ceramic substrates up to 10 GHz.
