Interconnect Bonding
created a mechanical key to prevent lateral slippage during bonding.
An SEM of an individual Cu/Sn pillar, an SEM of the 10µm pitch array and a scaled schematic diagram of the top dice pillars are shown in Fig. 1.
Fig. 2. Schematic diagram of top and bottom dice (left) and optical microscope image (right) used to fabricate Cu-Cu thermocompression test vehicles with 10µm pitch in area arrays of 512 x 640.
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Fig. 3. Graphical map of defective channels in six consecutively bonded 10µm pitch samples. Blue lines show electrical opens. Individual bond yield is >99.99%
For the Cu/Sn-Cu vehicle without a mechanical key, the bottom routing layer was formed using lithography, Ti/Cu/Ti evaporation and liftoff. Oxide was deposited by plasma enhanced chemical vapor deposition (PECVD) and patterned by reactive ion etching (RIE). Bond pads were formed by the combination of seed layer sputtering, lithography, electroplating and seed layer etching. The seedlayer etching was done with an ion mill. The top die consisted of 4µm diameter Cu pads plated 4µm thick. The bottom die consisted of 6µm diameter pads plated with 4µm of Cu and 2µm of Sn. For Cu/Sn-Cu with a mechanical key, the top die was fabricated in a process similar to the one described above, with 4µm diameter pads plated with 3.5µm of Cu and 2.5µm of Sn. The bottom die consisted of metal links terminated with 6µm diameter pads that were plated with 4µm of Cu and then coated with BCB. Vias were etched in the BCB to create openings for bonding to the bottom pads. The BCB sidewalls
For Cu-Cu thermocompression bonding of CMP’d Cu surfaces, the top and bottom pads were formed in 5µm thick SiO2 by RIE, filled with Cu by Ti/Cu seed sputtering and Cu blanket electroplating and then planarized with CMP to remove the Cu overburden. Pad diameters for top and bottom die were 4µm. After CMP, the oxide was partially etched (recessed) to expose the tops of the pillars for bonding. As noted above, in this vehicle the Cu bonding surfaces are both produced by CMP. A schematic diagram of the fabrication sequence is depicted in Fig. 2.
For Cu-Cu thermocompression bonding with CMP’d Cu to as-plated Cu, the top die was formed by CMP as above. The bottom die consisted of metal links terminated with 6µm diameter pads that were plated with 4µm of Cu and then coated with BCB. Vias were etched in the BCB to create openings for the top pads to bond to the copper bottom pads. After fabrication, roughness and uniformity were characterized as reported in [6].
Sample bonding was performed using a
SET FC150 precision bonder with split optics, allowing for alignment in “cold placement” with accuracy of ±1 µm.
Cu/Sn-Cu Temperature Optimization Prior to bonding, the Cu bottom pads were stripped of copper oxide with a dilute acid solution. The Cu/Sn top pads were fluorine plasma treated with the PADS process (Plasma Assisted Dry Soldering), which enables fluxless bonding [7]. The tin pads were flattened using an in situ coining process.
For Cu/Sn-Cu dice without mechanical key, six consecutive die pairs with pads on 15µm pitch were bonded using 275°C bonding temperature for 180s and bonding pressure of 5x106 kg/m2. All six pairs exhibited electrical opens. Die shear and inspection revealed that the top pads had slipped laterally off the bottom pads during bonding. This result confirmed the expectation that without mechanical keying, the Cu/Sn bonding process will be difficult to scale
www.euroasiasemiconductor.com Issue III 2011
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