March 11, 2021
System-in-package integrators are moving toward copper-to-copper direct bonding between die as the bond pitch goes down, making the solder used to connect devices in a heterogenous package less practical.
In thermocompression bonding, protruding copper bumps bond to pads on the underlying substrate. In hybrid bonding, copper pads are inlaid in a dielectric, reducing the risk of oxidation. In both cases, though, the surface diffusivity of copper defines the rate and temperature dependence of bond formation.
In both cases, though, the surface diffusivity of copper defines the rate and temperature dependence of bond formation. Copper crystalizes in a cubic lattice, with the exposed surface corresponding to either the face of the cube, a plane intersecting four opposite corners, or a plane intersecting three corners. Crystallographers label these faces (100), (110), and (111), respectively, based on the Miller indices of the lattice.
In copper,, oxidation is much slower and diffusivity is orders of magnitude faster: 1.22 x 10-5 cm2/sec at 250°C on the (111) surface, but only 4.74×10-9 cm2/sec on the (100) surface, and 3.56 x 10-10 cm2/sec on the (110) surface. When bonding (111) surfaces, Chien-Min Liu and colleagues at National Chiao Tung University in Taiwan achieved robust connections at temperatures as low as 150°C, while less-oriented surfaces had minimum bond temperatures closer to 350°C. Typical solder reflow processes operate at about 250°C, and many temporary adhesive compounds are designed for that temperature range.
The (111) surface also offers a higher atomic density, leading to a stronger bond. Surfaces with less than 25% of the grains oriented in this direction were prone to bond failure.
The surface orientation depends on the electroplating process used to deposit the copper features. Applied Materials process engineer Marvin Bernt explained that wide, shallow features have no significant sidewall. The bottom of the feature can serve as a template for oriented growth. As feature depth increases, a conformal seed layer helps reduce the risk of plating voids along the sidewalls.
Unfortunately, the growing copper layer tends to accumulate evenly on all of the seed surfaces. Columnar grains growing from the bottom of the feature are cut off by grains growing from the sidewalls. For aspect ratios greater than 1.5, this “pinch off” can even lead to internal voids. The plating process needs to balance the tradeoffs among orientation, deposition rate, and void-free growth.
Grain size and orientation are also affected by location within the pad array, regardless of pad size. Edge pads have smaller grains, increasing toward the inside of the array. Grain orientation depends on pad size and pad position, SeokHo Kim and colleagues at Samsung found, probably due to changes in current density during electroplating. Achieving the desired columnar grains, then, depends on the interaction between the seed layer, the current waveform supplied by the plating tool, and the chemistry of the plating bath.
When two highly oriented surfaces meet, the results are remarkable. Jing Ye Juang and colleagues at National Chiao Tung University observed a continuous lattice structure, obliterating the pre-bonding interface. In pull tests, the copper-copper interface was stronger than both the copper-silicon bond and the adhesive between the sample and the test fixture. Similarly, electrical resistance was comparable to that of bulk copper.
Successful copper-to-copper bonding depends on an electroplating process that can deliver consistent copper grain structure. Though electroplating is well-established for BEOL and TSV applications, the specific requirements of copper-to-copper bonding are new.(From Katherine Derbyshire)