4.2  Design of TSVs

Through Silicon Vias are large structures in comparison to the components inside the device. The TSVs radius can span from 5m up to 300m and the height usually follows aspect ratios from 1:1 up to 1:10 in relation to the radius [29][78]. Therefore, if CMOS 90nm technology is considered, a TSV would solely occupy the area of, at least, several thousands of transistors. Such a large metal structure in the middle of the device generates considerable stress, which can lead to device malfunction and cracks in the silicon.

Thermal and residual stresses account for several reliability issues in TSVs. Consequently, they have become a major concern in mechanical stability designs [79][80][81]. The thermo-mechanical stress arises from the difference between the coefficient of thermal expansion (CTE) of silicon – which surrounds the via – and the interconnection metal. Meanwhile, the residual stress results from different physical mechanisms which take place during metal deposition.

The impact of each stress component can be controlled by the choice of materials and the geometry forming the TSV. A good design should manage the mechanical issues while not compromising the electrical properties of the device. One of the most common and well documented layouts is the cylindrical copper TSV. The good electrical properties of copper and the ease of fabrication are an advantage of this technology. However, the difference of more than one order of magnitude between silicon CTE and copper CTE compromises its mechanical reliability.

CTE mismatch can be compensated with various strategies, for instance by the use of polymer liners around the TSV [82]. The liners work as a barrier which absorbs the stress and hinder its propagation towards the silicon. Another approach is the use of an open (unfilled) TSV instead of a filled via [82][83]. This scheme reduces the amount of material in the structure and provides room for the metal to expand freely towards the axis of the via, leading to an overall stress reduction (depending on the metal properties as will be discussed in the next section). Additionally, the stress induced by the TSVs in the silicon can be attenuated by their particular placement [83]. A device usually has several vias close to each other, which can be arranged in such a way that the stress is mutually cancelled or reduced between them.

Circular shaped TSVs are preferred over other polygonal geometries. Such designs results in stress accumulation points in the corners without bringing any particular advantage to the overall process complexity or device performance.

This work attempts to cover two of the most frequent types of TSVs, filled TSVs and unfilled (open) TSVs. However, special attention is given to the open TSVs due to a lack of documentation and available information in the literature. Although the TSVs can generally be classified in two types, each TSV technology has minor modifications to handle local stress problems or even to treat processing challenges such as layer delamination, material diffusion or to improve layer adhesion. In order to establish a common ground for discussion, during this work two standard TSVs are considered, as shown in Fig. 4.1: one for filled vias and another for unfilled vias. Any deviation from the standard discussion of TSV performance and reliability will be properly addressed and explained in the text. The design is based on a previous TSV technology [8][29], for which experimental data were available.

Figure 4.1.:

Schematic of the standard TSVs considered in this work for unfilled (a) and filled (b) technology. The vias have a 100m diameter and 250m height. Filled TSVs are usually smaller than the one presented here, however, the dimensions were chosen in order to allow for a fair comparison between both technologies.