3.1.1.

Grain-structure investigation of filled TSVs

In this case study,¹² plasma-FIB milling was evaluated and optimized for SEM electron backscatter diffraction (EBSD) cross-section analysis of Cu-filled TSV interconnects. For quantitative EBSD grain structure analysis of the Cu filling, it is essential to achieve a perfectly flat cross-section and not to modify the Cu grain structure. Additionally, for small-scaled TSV geometry with high aspect ratios and diameters of $<10\mu \text{m}$, an almost perpendicular cross-section through the TSV center is required. Because milling rates do vary locally and massive curtaining may occur, particularly in deep cross-sections, these milling artifacts can make subsequent SEM analyses of the barrier and sidewall isolation as well as the Cu grain structure difficult or even impossible. To overcome this problem, advanced milling strategies based on automated specimen rocking were evaluated and optimized for TSV cross-sectioning. In this study, the surface quality resulting from these new milling strategies was analyzed in detail and evaluated for further EBSD grain structure analyses.

The plasma-FIB milling and EBSD grain structure analyses were investigated using blind TSVs of 50 μm diameter and 150 μm depth, fabricated in 300-μm-thick Si wafers. The specimens were analyzed after Cu filling process (as deposited) and after annealing at 400°C for 3 h in an ${\mathrm{N}}_2$ atmosphere to reveal changes in the Cu microstructure induced by thermal cycling. The test samples were first prepared by cleaving the Si specimen along an array of TSVs. Then, mechanical polishing was used to approach the edge of several TSVs, enabling optimal access for later plasma-FIB processing and for the EBSD analysis.

To minimize curtaining artifacts caused by the surface topography of the TSV prior to plasma-FIB milling, a protective Pt layer was deposited on top. Following this, rapid plasma-FIB milling was performed at each TSV until reaching the center of the TSVs using beam currents of 570 nA for 10 min (step 1) and 230 nA for 7 min (step 2). After this coarse milling, a final plasma-FIB polishing step was additionally performed to improve the surface quality, using different milling parameters on each TSV. These parameters were the beam current (27 or 74 nA), the milling pattern (box or line by line), and the specimen that was polished either in static or in automated rocking mode. In the rocking mode, the specimen is alternating tilted (plus and minus 8 deg) laterally in between the polishing steps (Fig. 2). The benefit of using different tilt angles is that curtaining effects from one direction are removed by the milling from the second direction, resulting in a much smoother section face. It could be shown that the massive curtaining occurring after 27-nA standard perpendicular polishing ($\sim 1\mu \text{m}$ surface topography) can be removed almost completely ($\sim 50\mathrm{nm}$ surface topography) by further stepwise rocking beam polishing (Fig. 3).

Fig. 2

Plasma-FIB milled cross-section of a Cu-filled TSV after final polishing with 30 keV, 27 nA ion-beam without specimen rocking (a), P-FIB milled cross-section of a Cu-filled TSV after final polishing with 30 keV, 27 nA ion-beam with specimen rocking at $\pm 8\mathrm{deg}$ (b). (Image widths 170 μm).

Fig. 3

AFM profile scans across TSV bottom surface polished by rocking beam technique (upper line scan) compared to TSV bottom surface polished by standard cleaning cross-section method (lower line scan).

Additionally, for the different milling regimes, vertical profile line scans along the polished TSV center were measured by AFM to compare the achievable slope angles. For the TSV polished with a 27-nA ion beam current, a slope angle of $\sim 3\mathrm{deg}$ was measured. This angle was found to be independent from the other milling conditions. The profiles were almost linear. As a result, optimal conditions for TSV cross-sectioning and further EBSD grain structure analysis requiring a low surface roughness could be achieved by 27-nA final polishing applying the rocking/polishing technique—with a 3-deg overtilt included to give a vertical final result.

In order to optimize the TSV processing steps, the relationship between microstructure, e.g., the local texture, and the reliability properties under temperature cycling have to be investigated. EBSD is an efficient and practical technique for grain structure analyses due to its excellent spatial resolution. The specimens were investigated by scanning the electron beam across a selected surface area. Due to the high surface sensitivity of EBSD, the quality of the obtainable results is directly related to the quality of the prepared specimen surface. In this case study, the geometrical requirements for an EBSD analysis could be fulfilled by specific removal of material along the direction of the diffracted beam, enabling an artifact-free texture analysis with spatial resolution in the range of nanometers. Figure 4 shows quantitative EBSD orientation maps of Cu-filled TSV cross-sections made by plasma-FIB. Comparing TSVs as fabricated with additionally annealed TSVs, the change of grain size and grain distribution due to the temperature processing can clearly be obtained.

Fig. 4

Electron backscatter diffraction (EBSD) cross-section analysis of the Cu grain structure and grain size distribution. TSV specimen after Cu deposition (a) and after 400°C anneal for 3 h (b). Measurements and calculations are performed using an Oxford Instruments Channel 5 EBSD system.

Overall, it was demonstrated that plasma-FIB milling enables time-efficient grain structure analysis and sidewall characterization of Cu-filled TSV interconnects, offering significant advantages compared to common mechanical or Ga-FIB-based preparations. Furthermore, plasma-FIB polishing provides a fast and reproducible method, which allows cross-sections without significant damage or modification of the microstructure. This is essential for EBSD-based process characterization to study Cu protrusions or other microstructural changes that are related to annealing of TSVs and cause reliability issues.

3.1.2.

Inspection and failure analysis of open TSVs

A specific open TSV interconnect technology has been developed as an alternative to Cu-filled TSVs. The Bosch etching process combined with further IC processing steps is used to form TSV structures with sidewall isolations and metallization with Ti/TiN, W, and Al. Finally, the TSVs are capped by a silicon dioxide film and silicon nitride layers. For process optimization and failure analysis, the sidewall layer structure has to be imaged and measured. To get access for SEM and TEM imaging, cross-sections of the open TSV structure have to be made. In most cases, wafer cutting through the TSV is not applicable because of the bonded Si/Si oxide/Si substrate. Alternatively, large area mechanical grinding or ion beam polishing can be applied. For precise cross-sectioning to get access to specific TSV areas of interest or to defect sites within the TSV structure, rapid plasma-FIB milling has been evaluated. In a first step, high-current plasma-FIB cross-sectioning followed by rocking beam polishing of the whole TSV structure was applied to get access to the entire sidewall layer. Strong curtaining effects caused by the open TSV geometry could not completely be removed, limiting the SEM inspection of the sidewall layer. An alternative approach would be to fill the TSV structure before milling, but was not attempted.

For failure analysis of sidewall issues, TSV filling would often not be a good option. Geometrical deviations or located electrical shorts at the sidewall have to be directly accessed to allow for site-specific FIB cross-sectioning. Depending on the defect size and position within the TSV, accuracy of the defect localization, and the request for SEM or TEM, the milling strategy has to be adapted. The following examples demonstrate possible preparation flows.

A crucial requirement is the localization of electrical shorts on TSV sidewalls combined with further target preparation via TSV cross-sectioning and high-resolution electron microscopy analysis. In a first case study, electrical shorts of the TSV sidewall metallization to the Si-substrate were localized by EBAC imaging technique¹⁶ (Fig. 5). In this case, a rough cross-section of the TSV was made by mechanical grinding.

Fig. 5

Overlay of secondary electron (SE) and electron beam absorbed current (EBAC) signals: EBAC analysis on complete TSV structure to localize relevant defect position (red spot) as leaking path.

Then standard FIB/SEM cross-sectioning was applied to screen through the EBAC spot. A rough inhomogeneous sidewall was present at the TSV bottom with abnormal thickness deviations of the isolation oxide layer between substrate and sidewall metallization. Additionally, inclusive voids were observed at that interface with more or less 3-D Si artifact probably caused by insufficient Si etch process at TSV bottom. A 3-D-formed Si residue was detected within the EBAC active area, Fig. 6. It shorts the substrate with the TSV metallization at the bottom sidewall, generating a high-ohmic leakage path. Due to its 3-D shape, the complete leakage path was only observable by reviewing several cross-sections. This kind of failure can occur when last Si etch steps were processed insufficiently and, thus, distinctive layer thickness variations were caused. The conducted case study¹³ showed the feasibility of these localization and preparation methods to characterize complex 3-D defects at interconnect structures.

Fig. 6

SEM image of tilted sample 45 deg with cross-section at the localized defect position (a). SEM image of cross-section at middle of EBAC spot position after further FIB polishing (b).

A second case study was performed on defective TSV structures with localized delaminated sidewall layers. To find the root cause, the delaminated side wall interface should be analyzed by SEM and TEM imaging. In this case, mechanical grinding was not an option for rough cross-sectioning because of the high risk to fully detach the delaminated sidewall layer. Therefore, rapid plasma-FIB milling with 1.3-μA Xe beam was applied to get a cross-section through the middle of the TSV. A box of $700\times 300\mu \text{m}$ and 500 μm was milled within only 5 h (Fig. 7).

Fig. 7

Plasma-FIB cross-sectioning of an open TSV structure and SEM imaging of the delaminated sidewall layers.

Then local sidewall polishing at 70 nA was done at the upper left of the TSV followed by SEM imaging of the delaminated sidewall layer. As a second analysis step, a TEM lamella was made (using a Ga-FIB-based FIB-SEM system) from the plasma-FIB cross-section exactly at the transition between the delaminated and faultless sidewall layer (Fig. 8).

Fig. 8

TEM lamella preparation and imaging of the TSV sidewall with delaminated sidewall layers (top). Energy dispersive spectroscopy measurement at Si-oxide/Si interface showed no contamination detectable (bottom).

The defect site was analyzed by TEM imaging and energy dispersive spectroscopy (EDS) analysis. A delamination between the Si substrate and the Si oxide isolation could be found, but interface contamination as a possible root cause for a poor adhesion could not be identified, with Si and O being the only significant EDS peaks noted.

3.2.1.

Face-to-face die bonding

The first example is a face-to-face bonding layout, where a series of Kelvin structures had been created in a reliability test structure. In such a structure, the part of the device stack accessible to the ion beam is the silicon substrate of the upper die. As this is an essentially featureless expanse of polished silicon, the FIB needs an alignment scheme to locate the area of interest. Several methods of navigation are possible, including dead-reckoning from die edges or opening up test holes to orient the sample, but in this example, a more direct and accurate method was employed: using an in situ IR microscope to image through the silicon substrate to locate the area of interest [Fig. 9(a)]. For this sample, the die had been polished to 50 μm of silicon substrate remaining. Thicker samples can also be processed—the IR microscope is capable of imaging through full-thickness silicon substrates if necessary—while fast silicon removal with ${\mathrm{XeF}}_2$ gas etching can be utilized to create a local trench into which the cross-section is then cut. ${\mathrm{XeF}}_2$ is a common etch gas employed with Ga-FIBs and provides even greater volume removal rates when combined with the higher currents from the plasma-FIB.⁵

Fig. 9

Cross-section through face-to-face bonded dies. (a) In situ IR microscope images were used to locate region of interest through silicon substrate (two images from adjacent areas—total image width 170 μm). The dashed line shows the target position of the section—cutting through 20- and 40-μm-pitch structures. (b) Overview of the cross-section through the microbumps, with void in underfill (image width 250 μm). (c) Detail of one of the 40-μm-pitch interconnect stacks (image width 18.4 μm). Sample courtesy IMEC.

After locating the area of interest in the in situ IR microscope, the plasma-FIB preparation took 41 min to expose the section (using beam currents from 1.3 μA to 59 nA) (Fig. 9). Like earlier figures, these images show the utility of ion beam microscopy on the cross-section face. In particular, the different components [intermetallic compounds (IMCs)] that make up the interconnect stack can be observed as layers of differing contrast and grain structure (e.g., large, small, or no grains visible), which will be characteristic of each material composition. By characterizing a comparable device with EBSD or similar technique, a translation from FIB contrast and grain structure to metallurgical composition can often be made, allowing analysis of the bonding process, including layer thicknesses, to be obtained directly from the FIB image.

3.2.2.

Face-to-back die bonding

Face-to-back bonding schemes incorporating TSVs were also investigated with the plasma-FIB (Fig. 10). Here a series of 31 TSVs and associated die-to-die bonds were sectioned. The sample preparation took $\sim 2.5\mathrm{h}$. In a subsequent step, the cross-section face was milled further to expose the next set of TSVs; this took an additional 61 min of milling. The FIB images again show the details of the bonding process as well as some voids in the underfill around the bonds. Such images could also be used to assess die-to-die alignment or make other critical measurements that can only be assessed once the completed 3-D structure has been assembled. In addition, the localized nature of the sectioning means that multiple sections can be made, even at different angles, which allows more data to be collected from a single device.

Fig. 10

Cross-section through face-to-back bonded dies. (a) Overview of the complete cross-section through 31 TSVs (image width 590 μm). (b), (c), and (d) Progressively higher magnification views through the TSV array (image widths 77, 36, and 15 μm, respectively). Sample courtesy IMEC.

3.2.3.

Silicon interposer—burn-in test failure analysis

The final example in this section is a failure analysis investigation on an interconnect stack that included packaging bump onto a silicon interposer with TSV, which was then microbumped to the lower die (Fig. 11). The sample was part of a reliability burn-in test, and the sectioning was targeted at microbumps thought to be responsible for sample failures.

Fig. 11

Die-to-interposer bonding investigation. (a) Overview of the complete cross-section through 31 TSVs (image width 330 μm). (b) Detail of microbumps (image width 118 μm). (c) Plasma-focused ion beam (PFIB) and (d) SEM images of delamination found between bump and underlying (image widths 7.3 and 2.5 μm, respectively).

The main section took $<2\mathrm{h}$, and when the microbumps were imaged with the FIB, evidence of a problem at one of the interfaces was noted [Fig. 11(c)]. Higher-resolution SEM imaging of a subsequent slice through the defect showed a clear delamination [Fig. 11(d)]. Further delamination examples on other microbumps were also found with the plasma-FIB.