Tessera’s Micro Via Pad (MVP) package, introduced in March, 2008, provides an economical package for image sensors and MEMS that avoids some of the difficulties of conventional TSV.
High aspect ratio TSV face several challenges to low-cost, high volume production. Silicon etching for high aspect ratio vias is time-consuming and requires expensive capital equipment. Lining small-diameter vias with insulating layers and filling them with conductors increases both complexity and cost.
TSV generally are blind vias ending at the insulating oxide layer of the wafer. The oxide must removed to establish a conducting path. Making a low-resistance contact to the back side of a bond pad at the bottom of this blind pipe is challenging. Verifying an acceptable contact is difficult. In addition, the sharp bends in the conductors at the ends of the via can be a weak spot in reliability over temperature.
Tessera’s MVP was developed for low density interconnects redistributing peripheral bond pads to a ball-grid area array on the back of the die. Edge connections are an alternative solution, but they require special layout rules, and increase the size of the die, giving fewer die per wafer. Blind TSV are another solution, but they face the cost and reliability limitations stated above. MVP can reduce the cost of a camera sensor module to less than $1.00.
FIG 1. MVP Die (courtesy Tessera)
A via-through-pad connection might be thought of as an edge connector that is not located on the edge of the die. Instead, openings through the silicon within the periphery of the die provide internal edges for connections. Consequently, no additional real estate is needed, and no special die layout rules are required. The process accommodates a wide range of scribe line widths, bond pad locations and bond pad sizes.
Polygonal openings through the silicon bring the redistribution conductors to the pads. The rounded corners of the openings eliminate sharp turns for the conductors. A single large opening may include connections to several pads. The conductor opening extends through the oxide layer and the bond pad, exposing a ring of metal for ohmic contact. The resulting through via eliminates the TSV step of cleaning and depositing a blind connection at the bottom of a long narrow hole.
A further MVP innovation is an unconventional, low-cost substitute for expensive conventional dielectrics: automobile paint! The well-established process of electrophoretic deposition, originally perfected to provide a glossy surface finish for automobiles, instead deposits the polymer dielectric for this assembly.
Electrophoretic materials use the principal that opposites attract. T he metal object to be coated is held at the opposite polarity from the charged non-conductive paint particles in the bath. Those particles are attracted to the metal surface, forming a thin, uniform coating which is self-limiting.
When the non-conducting particles cover all of the metal surface, they insulate the metal from the solution, terminating the deposition. The voltage applied to the bath controls the film thickness, typically in a range of 5 to 75µm. A post-bake completes the coating.
FIG 2. Conductors on die surface conformally coated by electrophoresis. (Courtesy Tessera)
In summary, Tessera’s MVP package combines through-pad vias with creative coating to produce a low cost, high reliability package for image sensors and MEMS. It combines some characteristics from TSV and some from edge connectors, while avoiding the problems of both.
FOR MORE INFORMATION
Tessera has papers, articles, technical information, and photographs of their MVP package on their site at http://www.tessera.com