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Jan 7th, 2014
Convergence of FE, BE and FPD manufacturing technologies: a closer look
At the recent IWLPC Conference in San Jose, Klaus Ruhmer of Rudolph Technologies addressed the convergence of Front End (FE), Back End (BE) and flat panel Display technologies that is happening in order to meet the requirements of mid end packaging technologies such as 2.5D on interposer, 3D stacked chips or very thin fan-out packages. I-Micronews thought it was worth… A closer look.
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While cost remains the single most important aspect of back-end advanced packaging, typical back-end interconnect technologies such as Copper Pillars, microbumps and RDL (redistribution layers) require ever finer resolution requirements. For 2.5D Interposer fabrication, back-end patterning demand is well into the single digit micron range (< 5μm L/S) for current high density applications even though 20μm was state-of-the art only a few years ago.
Such miniaturization is clearly observed in the FC-BGA roadmap shown below. 

FC BGA Roadmap (Courtesy of Yole Développement)

In the early days of wafer level back-end manufacturing, screen printing could be used to deposit solder. With decreasing bump pitch, full field lithography combined with electroplating into a photoresist became necessary. For 300mm applications, aligners quickly reached their resolution and overlay limitations and forced the use of stepper technology. Older 1X steppers or more advanced 2X steppers are used today.

One way of reducing cost is to take advantage of economy of scale. It is proposed that larger formats will enable lower cost per chip for interposers and fan-out packages.

Glass interposers for 2.5D lend themselves to diverge from traditional round wafer form-factors and move to small rectangular panel sizes used early on by the FPD industry. It is thought that processing such panels will require manufacturing techniques which have previously been utilized for Flat Panel Display manufacturing.

The lithography requirements of advanced packaging and LCD are compared below:

Comparison between Advanced Packaging & Flat Panel Display lithography requirements

A cost analysis by Rudolph concludes that a 1.7X cost reduction in lithography can be achieved  by going from a 1X stepper and 300mm glass wafers to a 550 x 650mm glass panel (gen 3 LCD panel) using their panel lithography systems (all other things being equal).

Lithography cost/die decrease by going to larger substrate size.

Yole has previously reported that such panel sized substrates will be required to lower the cost of fan-out packaging.

Cost/die based on substrate size for Fan-Out packages (Courtesy of Yole Développement)



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