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080602: IITC shows the way to 3D
Ed’s Threads 080602
Musings by Ed Korczynski on June 2, 2008

IITC shows the way to 3D
The 11th International Interconnect Technology Conference (IITC) started today in Burlingame near the San Francisco airport. Once again, the leading-edge of on-chip interconnect technology developments were presented, with details on new materials, processes, and structures. 3D interconnects and through-silicon vias (TSV) were discussed in serious detail, while work continues on air-gap dielectrics and carbon nanotubes (CNT) along with new copper barrier materials.

3D with TSV may be considered as the ultimate interconnect concept, since stacked chips provide optimal functionality/volume, and provide for relatively low-cost heterogeneous integration of diverse technologies such as sensors. TSV and the many variations thereof have been hot topics in 3D for many years, with “via-last” being done today in production for memory stacks needing typically <100 TSV per chip. In contrast, “via-first” TSV processing flows may produce thousands per chip, and there are many integration schemes possible. IITC publicity chairman (and IBM researcher) Michael Shapiro commented that, “3D is such a ‘silicon-centric’ process technology that the IITC is really the place to have the discussion, because all the experts of silicon etching and planarization are here and have always been here.”

Fraunhofer IZM (Institute for Reliability and Microintegration) in Munich has been leading the world in 3D-IC work for over ten years, and researchers from there have been developing detailed system-level heterogeneous integration schemes for wireless applications (for the European 3D integrated sensor program “e-CUBES”). Their target is die-to-wafer (D2W) stacking of a tire pressure monitoring system (TPMS). The wafer has the microcontroller chips, onto which are stacked chips for the RF transceiver, pressure sensor, and bulk acoustic resonator (BAR). For TSV, they integrate chips with both solid metal trenches (typically W filled ~20 µm deep) or hollow vias coated with doped poly-silicon (through the 300 µm thick pressure sensor).

Researchers from Georgia Tech built upon work they first showed three years ago at IITC, and together with IBM and Nanonexus showed real results of using integrated microchannel cooling to remove heat from 3D-IC stacks. Fluidic microchannels were fabricated at the wafer-level using four lithography steps, and the resulting chips showed thermal resistance of just 0.24°C/W compared to 0.6°C/W for equivalent 65nm node air-cooled chips. With reduced thermal resistance, significant advances in speed, power, and/or operating temperature can be achieved; for example, power could be reduced ~20% at the same frequency, or the frequency could increase 10% at the same power.

Basic materials integration challenges of 3D integration were shown in two presentations by IMEC. Micro-Raman spectroscopy (µRS) was used to determine the plastic yield criterion for an accurate finite element modeling (FEM) of the stress near Cu-filled TSV. Due to the inherent mismatch between CTE of Cu (16.7 ppm/°C) and Si (2.3 ppm/°C), some strain will be inherent, and it may degrade electrical carrier mobility. Defining an “exclusion zone” of transistors from the TSV such that mobility degrades <5%,>
IMEC researchers also looked at reliability in a presentation on “Resistance to electromigration of purely intermetallic micro-bump interconnections for 3D-device stacking.” Both Cu-Sn and Co-Sn were shown to withstand 1000 hours of testing at the extremely aggressive conditions of 150°C and 0.63mA/µm2).

Scott Pozder of Freescale Semiconductor showed an excellent poster on Cu-Sn microconnects formed simultaneously with an adhesive dielectric bond using thermal compression bonding of flipped dice on a wafer. After D2W bonding using Cookson F602 material at micropad pitches of 59, 64, and 69µm, the robustness of the bond was shown by grinding the bonded dice to 50µm thin using a Disco Hi-Tec tool. While no TSV are used in this die-to-wafer stack, this pragmatic approach based on standard unit-processes which can be found in the open foundry market shows one clear way forward toward 3D today.

Tohoku University researchers showed one way to cut costs in D2W bonding: use a rough lithographic step to form hydrophobic and hydrophilic areas on the wafer, add an aqueous coating and then roughly place the dice. The surface tension of the liquid induces the dice to self-align, and control of the ambient can allow for the liquid to evaporate which temporarily bonds the dice to the wafer. The average alignment accuracy on 100 dice was ~0.5µm, with most dice aligned within <1µm and all <1.5µm.

D2W stacking of 3D chips allows for the used of known good dice (KGD) and the associated minimization of yield losses anticipated with wafer-to-wafer (W2W) stacking. D2W stacking technology will first follow Freescale’s lead by flipping the top die for two levels of silicon, but TSV and three or more levels will certain follow.

Much of the limitation in the use of TSV today remains with the designers; lacking EDA tools, it is not only difficult to optimize a design for 3D, it is challenging to just try to quantify the potential benefits in advance. Until EDA tools are ready the greatest potential value of 3D stacking will not be seen, and most commercial TSV will continue to be used for memory stacks and CMOS image sensors.

This is the last year in which interconnect technologists living in the San Francisco bay area have the exclusive luxury of the International Interconnect Technology Conference being local. Next year (June 1-3, 2009), the 12th IITC will occur in Sapporo, Japan at the Royton Sapporo hotel. The 2010 meeting will be back in the San Francisco bay area, and then the 2011 meeting is expected to occur somewhere in Europe.


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080602: IITC shows the way to 3D

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Ed's Threads is the weekly web-log of SST Sr. Technical Editor Ed Korczynski's musings on the topics of semiconductor manufacturing technology and business. Ed received a degree in materials science and engineering from MIT in 1984, and after process development and integration work in fabs, he held applications, marketing, and business development roles at OEMs. Ed won editorial awards from ASBPE, including interviews with Gordon Moore and Jim Morgan, and is not lacking for opinions.