Bottom-up Copper Electrodeposition in 20:1 Aspect Ratio, 20 Μm Pitch through Silicon Vias
Authors: Menk Lyle Alexander , Schmitt Rebecca Pauline , Mook William Moyer , Musick Kate , McClain Jaime , Smith Sean , Wood Michael G , Hollowell Andrew E 
Published: December 2020

Heterogeneous integration (HI) offers a path to increase bandwidth density and reducing power consumption per area for advanced microelectronic systems by minimizing fan-in/out structures and reducing parasitics. To achieve state-of-the-art areal bandwidth density, 3D HI is required with through silicon vias (TSVs) with extremely tight interconnect pitches (< 50 µm) and high aspect ratios. Fabrication and fill of these proposed TSVs pose significant challenges including substrate thinning, high aspect ratio deep reactive ion etching (DRIE), conformal thin film deposition to deposit insulating layers as well as seed metals for electrodeposition, and void-free fill of the TSVs. Here, we target an interconnect pitch of 20 µm and TSV diameter of 5 µm. At these dimensions, DRIE lag limits the TSV depth to 100 µm. The TSVs are integrated using a via-last scheme with atomic layer deposition (ALD) Al2O3 as the insulating liner and ALD Pt serving as both the barrier and electrochemical deposition (ECD) seed liner. In this work, we describe the integration scheme for these high density TSVs and focus on the ECD process for achieving a void-free fill with Cu as well as characterization approaches using focus ion beam (FIB) cross sectioning.

Recent work on Cu ECD has demonstrated the efficacy of a two-additive acid sulfate electrolyte system based on a chemistry exhibiting a S-shaped negative differential resistance voltammogram1. The electrodeposition system presented is a 1.0 mol/L CuSO4 – 0.5 mol/L H2SO4 acid sulfate electrolyte containing a polyether surfactant and halide additive which promotes a bottom-up Cu growth profile via positive feedback between localized disruption of adsorbed polyether-halide layer and metal deposition. This electrolyte has been applied to potentiostatic filling of a variety of geometries with both micro-scale1 and mm-scale2 TSVs as well as current controlled bottom-up filling from a conformally conductive seed metal3. This system relies on fine tuning of applied bias, adsorbate concentrations, convection, and deposition time to ensure a bottom-up deposition regime is maintained in a particular via geometry.

Previous work developed parameters for filling 600 µm deep 5:1 aspect ratio TSVs and the same parameters were applied to 600 µm deep 10:1 aspect ratio TSVs with drastically different filling results.2 In this work, we further adjust the relevant ECD parameters to fill 20:1 aspect ratio TSVs that are 100 µm deep.

In fine pitch, high aspect ratio ECD, characterization of the deposited fill profile is important and is difficult to obtain using mechanical cross-sectioning. Here, we use a characterization approach which relies on focused ion beam (FIB) cross-sectioning rather physical grinding and polishing. The FIB mills into the surface of the sample at a 38º angle with respect to the surface of the sample while the electron beam collects a series of images looking down at the sample, as shown in Figure 1.

The series of images collected throughout the FIB milling process are then reconstructed to generate a cross-sectional image. FIB cross-sectioning is also used to provide granular detail about deposition morphology and grain size of the ECD Cu that can be lost during mechanical cross-sectioning. Figure 2, shows a top down scanning electron microscopy image of the TSV sample after milling at 38º and the resulting reconstructed cross-sectional image.

T. P. Moffat and D. Josell, J. Electrochem. Soc., 159, D208 (2012).

L. A. Menk, D. Josell, T. P. Moffat, E. Baca, M. G. Blain, A. Smith, J. Dominguez, J. McClain, P. D. Yeh, and A. E. Hollowell, Journal of the Electrochemical Society, 166 (1) D3066-D3071 (2019).

L. A. Menk, E. Baca, M. G. Blain, J. McClain, J. Dominguez, A. Smith, and A. E. Hollowell, Journal of the Electrochemical Society, 166 (1) D3226-D3231 (2019)

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government.

This work was funded by the DARPA Photonics in the Package for Extreme Scalability (PIPES) program under award number HR001119S0004. The views, opinions, and/or findings expressed are those of the author(s) and should not be interpreted as representing the official views or policies of the Department of Defense or the U.S. Government.

Figure 1