Proceedings Article | 5 April 2012
KEYWORDS: Plating, Ions, Metrology, Copper, Semiconducting wafers, Plasma, Inspection, Xenon, Scanning electron microscopy, Ion beams
3D IC integration continues to increase in complexity, employing advanced interconnect technologies such as throughsilicon
vias (TSVs), wafer-to-wafer (W2W) bonding, and multi-chip stacking. As always, the challenge with developing
new processes is to get fast, effective feedback to the integration engineer. Ideally this data is provided by nondestructive
in-line metrology, but this is not always possible. For example, some form of physical cross-sectioning is still
the most practical way to detect and characterize TSV copper plating voids. This can be achieved by cleaving, followed
by scanning electron microscope (SEM) inspection. A more effective physical cross-sectioning method has been
developed using an automated dual-beam focused ion beam (FIB)-SEM system, in which multiple locations can be
sectioned and imaged while leaving the wafer intact. This method has been used routinely to assess copper plating voids
over the last 24 months at SEMATECH. FIB-SEM feedback has been used to evaluate new plating chemistries, plating
recipes, and process tool requalification after downtime.
The dualbeam FIB-SEM used for these studies employs a gallium-based liquid metal ion source (LMIS). The overall
throughput of relatively large volumes being milled is limited to 3-4 hours per section due to the maximum available
beam current of 20 nA. Despite the larger volumetric removal rates of other techniques (e.g., mechanical polishing,
broad-ion milling, and laser ablation), the value of localized, site-specific, and artifact-free FIB milling is well
appreciated. The challenge, therefore, has been to reap the desired FIB benefits, but at faster volume removal rates. This
has led to several system and technology developments for improving the throughput of the FIB technique, the most
recent being the introduction of FIBs based on an inductively coupled plasma (ICP) ion source. The ICP source offers
much better performance than the LMIS at very high beam currents, enabling more than 1 μA of ion beam current for
fast material removal. At a lower current, the LMIS outperforms the ICP source, but imaging resolution below 30 nm has
been demonstrated with ICP-based systems. In addition, the ICP source allows a wide range of possible ion species, with
Xe currently the milling species of choice, due to its high mass and favorable ion source performance parameters. Using
a 1 μA Xe beam will have an overall milling rate for silicon some 20X higher than a Ga beam operating at 65 nA.
This paper will compare the benefits already seen using the Ga-based FIB-SEM approach to TSV metrology, with the
improvements in throughput and time-to-data obtained by using the faster material removal capabilities of a FIB based
on an ICP ion source. Plasma FIB (PFIB) is demonstrated to be a feasible tool for TSV plating void metrology.
