# A Short Review of Through-Silicon via (TSV) Interconnects: Metrology and Analysis
## 2 TSV Metrology
### Metal Fill  
Silicon through-hole filling technology is a crucial technology for 3D integration and accounts for up to 40% of the overall cost of TSV preparation. The effectiveness of silicon via filling directly affects the reliability and yield of the device. The electrical interconnect material for most current integration processes is copper, and the mainstream process is plating. There are several plating methods for copper filling, including conformal plating, super-conformal plating, and bottom-up plating. Conformal plating is a method that uniformly deposits copper inside and on the surface of TSV holes. In contrast, bottom-up filling selectively deposits copper on the bottom of TSV holes, leaving the hole walls and surface almost unplated. Super-conformal filling combines both methods (see Figure 4). In addition to these plating methods, accelerators (SPS), inhibitors (PEG), leveling agents (JGB), etc., can be used to achieve seamless filling of TSV. These additives alter the deposition rate of copper at various locations along the hole depth direction, ensuring hole-free filling of TSV. However, the use of these additives increases the complexity of the process. Figure 3: FDTD schematic (adapted from [29]). Despite these various methods for copper filling, challenges still exist. For instance, the properties of copper plating may be affected by the presence of impurities, which can lead to poor filling performance. Additionally, the large aspect ratio of TSVs makes it difficult to achieve uniform filling, which can lead to voids and other defects that affect the reliability of the device. Furthermore, the deposition of copper can lead to stress and strain in the surrounding silicon, potentially causing cracking and other issues. Many non-destructive methods for void detection in TSV have been investigated by different researchers. X-ray computed tomography has been shown to be an effective non-destructive tool for detecting micro-voids within TSV. The use of high-frequency scanning acoustic microscopy (SAM) for TSV void detection was reported by [30]. In addition to the gigahertz-SAM technique, other acoustic-based techniques, such as laser-based acoustic analysis, are being investigated, in which signals obtained from isolated through-holes and dense arrays are compared with those from void-free TSVs. Locked-in-phase thermography (LIT) is another non-destructive technique that uses a lock-in amplifier to detect very small thermal changes in the sample. Thermal images can be studied to detect faults, such as voids in TSVs. Physical defects (Figure 5) caused by immature filling techniques, such as resistive open circuits and TSV leakage, often undermine the reliability of 3D-ICs. Furthermore, TSVs are vulnerable in the manufacturing process, and many physical defects, such as micro-voids and pin-holes, frequently occur [31; 32]. Table 1 signifies the percentage difference in peak noise of defective TSVs while compared with defect free case, wherein the percentage differences in the peak noise of the partially cracked and spherical air void defected TSVs (with recon and rhole as 2.1 nm) compared to defect-free condition are 24.23% and 1.15%, respectively, at an hTSV of 30 nm [33]. [PERSON] et al. proposed a method that uses probes to test TSVs simultaneously, by measuring their capacitance and resistance. However, modern cantilevered probes with pitches up to 35 \(\upmu\)m face limitations in the placement of test probes and TSVs with pitches of 4.4 \(\upmu\)m or less. To overcome this, [PERSON] et al. [34] suggested a solution that converts the RC parameter of TSV to voltage using charge sharing, and measures it using a sensitive amplifier. Figure 4: Three filling mechanisms of TSV. (**a**) Bottom-up plating; (**b**) super-conformal plating; (**c**) conformal Plating. [PERSON] et al. [31] proposed a non-invasive solution for pre-bonded TSV testing using pulse shrinkage. This method exploits the fact that defects in the TSV cause propagation delay variations.