Continuing to examine presentations from the 3rd Int IEEE 3DIC Conf held in Japan in Feb 2012.
In the last several years PFTLE and IFTLE have brought copper protrusion to the forefront as an issue [see "Researchers Strive for Copper TSV Reliability" Semi Int, 12/03/2009] and reported on technical solutions as they appeared from IMEC [see IFTLE 6 "Cu-Cu and IMC Bonding Studies at 2010 ECTC..."]; TSMC [see IFTLE 34, "3DIC at the 2010 IEDM"] and others. IME has now reported on their study of 5 um x 50 um Cu TSV as they were annealed from 250 to 450C.
Cu expands vertically because it is constrained by the surrounding silicon substrate. Because it expands plastically it does not return to its original length when the sample is cooled down.
(Click on any of the images below to view the full-size version)
The effects of anneal temp, anneal time, via diameter and via depth are shown below where "room temp" refers to the protrusion present after anneal and return to room temperature and high temp refers to protrusion after anneal while still at the elevated temperature. As with previous studies they found that CMP after anneal retards any further protrusion if the temperature is again elevated.
Bottom line is that protrusion is minimized by small diameter, low aspect ratio TSV.
Samsung System LSI Division has also looked at the Cu protrusion issue and report similar results i.e that Cu protrusion can be reduced by heat treatment before CMP and that Cu protrusion and delamination strongly depend on TSV dimensions.
When the via diameter was in zone A all the vias showed high Cu extrusion and via delamination, but TSV diameters from zone B showed no problems.
Micro-cracking caused by Lateral Extrusion
Conference Chair Koyanagi and co-workers at Tohoku Univ also examined TSV dimensions and the effect of high temp annealing. An array of Cu TSV with diameters ranging from 3 to 30 um at three different pitches were annealed from 200 to 400C. Both the lateral and vertical protrusion of the copper was monitored.
Again larger diameter TSV (at a constant depth) show higher extrusion, but also that lateral extrusion (extrusion in the x-y after Cu has protruded from the surface) increased with anneal temp. For example 5 um TSV on a 10 um pitch extrude laterally 2 um at 400C. This would put them within 1 um of touching! Stresses induced by the TSV also result in microcracking "...on the periphery of the TSV array and in between the TSV." Careful choice of TSV size and pitch is recommended.
Cu-Cu Direct Bonding
Copper-copper direct bonding continues to be a popular topic due to the promise of fine pitch, low resistance interconnect which are more mechanically reliable than IMC bonding (Cu-Sn-Cu) and should show less electromigration issues. Such processes are currently limited by the required bonding time / temperature which are usually reported as 30 min / 350-400C. The holy grail appears to be a thin die Cu-Cu thermo compression bonding process which requires low bonding temp and pressure.
IMEC and TSMC have studied the direct Cu-Cu bonding of 5 x 40 um TSV with (3) different configurations ; (1) no nail head exposed (Cu CMP'd flat with the oxide surface; (2) flat nail head (cu CMP'd flat and then oxide recessed and (3) natural nail head (stop grind short of the nail head, pull back oxide revealing "dome" shaped copper protrusion. The matching landing pad is a Cu surface CMP'd flat with the oxide surface. After bonding they observed that the "no nail head exposed" and the "flat nail head" sample s delaminated even when the bonding temp and or the pressure was increased. They assumed failure was due to the low % area that is actually used to bond (less than 1%). So, what is good for the design (less than 1% of the area occupied by TSV) is not good for the strength of bonding. The dome bonding was better due to its ability to deform. IFTLE interprets this as an ability of the domed structured to deform allowing shorter TSV to now touch their pads and bond. IFTLE also thinks this is a good reason to look at hybrid bonding schemes such as proposed by Ziptronix [see PFTLE 48, "Opening the Kimono, Ziptronix gives details on DBI Process"] and CEA Leti [see PFTLE 103, "Show me the Copper"]
Stacking of Ultrathin Die
Standard 3DIC thickness has focused around 50 um for the last few years. IMEC has now shared their results of ultrathin (25 um) die stacking.
After temporary bonding and grinding, oxide is pulled back for Cu TSV reveal. The revealed "nail heads" are passivated with 3 um BCB and reconfigured with Cu/BCB RDL. Cu/Sn bumps are then fabricated on the landing pads. The 25um thick die are diced while still bonded to the carrier. They note that "this is required to have enough mechanical support during stacking"
Both NUF and WUF were looked at for underfill solutions. NUF is unfilled polymer dispensed onto the landing die prior to bonding and WUF is filled underfill film laminated to the thinned wafer while still on the carrier.
Issues with NUF were: (1) underfill trapped between the bumps;(2) voids between top and bottom die and (3) induced topography due to underfill shrinkage on cure. Shrinkage of the underfill upon curing and the CTE difference between a microbump and the underfill cause a bending of the die over the ubump connection. For an unfilled underfill and a 25um thick die a 40% increase in the drain current was observed to occur.
After several failed tries, they decided to focus on WUF with 60% filler loading. WUF was vacuum laminated onto the die and gave much better topography and the use of a filled underfill resulted in reduced stress.
They also found that increase in the die thickness from 25 to 50 um resulted in a stress reduction of 3X. Final conclusions were that 50 um thickness die were currently much better option for scalable manufacturable process and that reduction in the TSV diameter from 5 to 3 um will reduce the required KOZ by 64%.
Wireless Product with Design Partitioning
ST Micro and CEA Leti described their program to partition the digital and analog functions of a HD video transmitter onto separate die and stack them using Cu TSV and ubumps.
TSV are 10 um with a 40 um pitch and wafers are 80 um thick. Cu pillar interconnect are 25 um dia and 30 um high. Reliability tests were done at package level using JEDEC level 3. No delamination and no electrical failures were obtained after 1000 cycles.
--------------The next IEEE 3DIC Conference will be held in the fall of 2013 in San Francisco--------------
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-advanced packaging from InterNepcon Japan
-3D as the ISSCC
-detailed coverage on the IMAPS Device Packaging Conference and more
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