Research on transmission and crosstalk characteristics of a new type of through silicon via and multilayer through silicon via

In this work, a new type of bar-coaxial ring TSV (B-coaxial ring TSV) structure was proposed. This study used three-dimensional electromagnetic simulation software for analysis. Firstly, the conventional Bar-TSV (B-TSV) structure and the B-coaxial ring TSV structure were modeled, and their S-parameter results were compared. Our results indicated that the B-coaxial ring TSV structure had better transmission characteristics and crosstalk resistance. Secondly, by simulating of the B-coaxial ring TSV structure and the coaxial ring-tapered TSV structure, we proposed how to choose different structures for different application scenarios. Thirdly, the influence of some structural parameters of the B-coaxial ring TSV structure on the transmission performance was studied. Finally, the multilayer TSV structures of conventional B-TSV, B-coaxial ring TSV and coaxial ring-tapered TSV were studied respectively, and their performance was compared and analyzed.


Introduction
Since Moore's Law was proposed, it has been leading the development of the integrated circuit industry.That is, the number of transistors on a microchip doubles every 18 to 24 months [1].In other words, we can expect the speed and capability of our computers to increase every couple of years, and we will pay less for them [2].However, Moore's Law is facing more and more challenges due to the size of semiconductor fabrication process is reduced to the deep submicron level [1].The cost of the new process is higher and higher, but the improvement of performance is smaller and smaller.For this reason, the International Technology Roadmap for Semiconductors (ITRS) introduced the concept of "post-Moore's Law" in 2005 [3].The emergence of three-dimensional integration technology based on Through Silicon Via (TSV) interconnection has brought earth shaking changes to the integrated circuit industry.Three-dimensional integration allows multiple dies to be stacked, which realizes the high integration and multi-function of chips, and TSV plays an important role in it [4].Figure 1 is a schematic diagram of TSV interconnection.So how to design a TSV structure with good performance has become an urgent technical problem.
In order to obtain better electrical characteristics, many researchers have made unremitting efforts on the structural design of TSV.The most basic structure of TSV is cylindrical TSV, which is the most widely studied due to its simple structure, and thermal mechanical and high frequency performance are the main problems.Then researchers proposed the tapered TSV at MIT Lincoln Laboratory [5].Although this structure did not solve the problems faced by the cylindrical TSV, it was not easy to produce the void  To reduce footprint while maintaining good transmission characteristics, the bar-coaxial ring TSV (B-coaxial ring TSV) was studied in this paper.The proposal of this structure provides more options for the field of 3D integrated circuits.Subsequently, the performance differences between the improved structure and other structures were further compared, and the multi-layer TSV structure was analyzed.This research result also has good reference value for work needs in different fields.

Establishment and analysis of the bar-coaxial ring through silicon via model
In this section, the model of B-coaxial ring TSV structure was proposed.And it was compared with the conventional B-TSV in HFSS.Then, the advantages and disadvantages of the coaxial ring-tapered TSV structure and the B-coaxial ring TSV structure were investigated.Finally, the selection of different structures was discussed.

Structure of bar-coaxial ring TSV
The structure of B-coaxial ring TSV was proposed, as shown in Figure 2(a).This structure mainly combines the conventional bar structure and coaxial ring structure, which should be able to obtain superior electrical characteristics in theory.The corresponding simulation analysis will be performed in the next section.

Electrical performance comparison of bar-coaxial ring TSV and conventional Bar-TSV
As shown in the following Figure 3  Compared with the conventional B-TSV structure, S11 has been optimized to a certain extent.S21 was slightly worse on the low frequency region (<3.75GHz), but greatly improved on the high frequency region (>3.75GHz).Because this novel structure adds a layer of metal shield compared to the conventional B-TSV, it can suppress the crosstalk between signals and improve the signal integrity.Thus S31 and S41 became better.All in all, the B-coaxial ring TSV structure not only ensures good transmission performance, but also improves the signal crosstalk, which has a good application prospect.

Advantages and disadvantages of coaxial ring-tapered TSV and bar-coaxial ring TSV
Due to the space limitation of application scenarios, some scholars have proposed the B-TSV.For the tapered TSV, its floor area is the same as that of the cylindrical TSV with the same upper surface dimensions.In this regard, we studied the advantages and disadvantages of coaxial ring-tapered TSV and B-coaxial ring TSV, and analyzed how to make a choice for different application scenarios.
As shown in the following Figure 5, a three-dimensional model of a coaxial ring-tapered TSV was established in HFSS [12].The corresponding structural dimensions of the coaxial ring-tapered TSV could be accurately calculated while keeping the circumference of the cross-section constant.The sidewall angle of TSV was 88.8°.Then the same frequency region was used for the simulation.The Sparameter followed the change in the frequency shown in Figure 6.It could be seen from Figure 4 and Figure 6 that the coaxial ring-tapered TSV was slightly better than the B-coaxial ring TSV, but it was similar.In fact, they can meet the requirements of the current three-dimensional integrated circuit field.
In summary, on the premise of similar performance, if the project has no excessive restrictions on the floor area of TSV, we can choose the coaxial ring-tapered TSV structure firstly.On the contrary, we can choose the B-coaxial ring TSV structure.

Influence of the main structural parameters of bar-coaxial ring through silicon via on transmission performance
The TSV structure parameters given in this study were only a set of parameters based on satisfying the existing process [10], not the most favorable parameters for the electrical performance.In order to obtain better performance, it was necessary to analyze the basic parameters.For the coaxial structure, its anti-crosstalk performance will be relatively good, we mainly analyzed S11 and S21.

Single change of a parameter size
Firstly, if the thickness of the inner layer of copper material was changed, the result was shown in Figure 7.With the decrease of the inner ring metal thickness, the S11 parameter decreased.The S21 parameter became worse with the decrease of the metal thickness on the low frequency region, but the overall difference was very small.The S21 parameter became better with the decrease of the inner ring metal thickness on the high frequency region.Due to the skin effect, the increase of metal thickness will lead to the increase of parasitic capacitance and inductance of TSV on the high frequency region, which will reduce the transmission performance [16].But the skin effect is not obvious on the low frequency region.The increase of metal thickness has a greater impact on the parasitic resistance of TSV.With the increase of thickness, the parasitic resistance will decrease.
Secondly, if the thickness of the outer layer of copper material was changed, as shown in Figure 8. S-parameter did not change significantly.Because the outer metal mainly plays the role in electromagnetic shielding and is the path of current return, it has little impact on the transmission performance.Thirdly, if the thickness of inner and outer copper materials was changed at the same time, the result was shown in Figure 9.It combined the analysis of case 1 and case 2 in this section.The increase of inner and outer metal thickness was not conducive to signal transmission.
Fourthly, if the thickness of the middle (or outer) BCB material was changed or both were changed at the same time, as shown in Figure 10.With the increasing thickness of BCB material, the capacitance of the insulation layer decreases significantly [16], so its S-parameter becomes more and more ideal.

Changing parameters while ensuring the same floor area
Firstly, if the ratio of inner and outer copper material thickness was changed, the result was shown in Figure 11.The results showed that the return loss S11 and the insertion loss S21 were improved to a certain extent as the ratio of the inner copper thickness to the outer copper thickness decreased.It was shown in Figure 8 that the change of the outer copper thickness had no effect on the transmission performance, so this situation could be equivalent to that in Figure 7. Therefore, the simulation result should be similar to it, which has been proven to be correct.Secondly, if you increased the thickness of the outer copper material and decreased the diameter of the innermost silicon layer, as shown in Figure 12.With the increasing of the outer copper thickness, the S11, S21 parameters were optimized.Because the change of the outer copper thickness had no effect on the transmission performance, it could be equivalent to that the S-parameter was optimized with the decrease of the innermost silicon diameter.This is mainly because the side area of the inner conductor also decreases with the diameter of the innermost silicon layer decreases, resulting in the reduction of parasitic capacitance and conductance.Thirdly, if you increased the thickness of the inner copper material and decreased the diameter of the innermost silicon layer, as shown in Figure 13.S11, S21 did not change much when the parameters were changed.This situation could be equivalent to changing the volume of the filler metal.Because the impedance of TSV is capacitive, which depends on the outer surface of the inner conductor and is independent of the filling volume.
Fourthly, if you increased the thickness of the inner and outer copper material at the same time, and decreased the diameter of the innermost silicon layer, we could analyze it in combination with case 2 and case 3 in this section.As shown in Figure 14, the S11, S21 parameters were optimized.Fifthly, if the ratio of middle and outer BCB material thickness was changed, the result was shown in Figure 15.We found that the magnitude of the ratio had almost no effect on its transmission performance.Combined Figure 10, we could obtain some conclusions.The performance was only related to the total thickness of the middle and outer BCB Figure 15.S-parameter when the ratio of middle and outer BCB material thickness changes.Sixthly, if the thickness of the middle (or outer) BCB material was increased or both were increased at the same time, and the diameter of the innermost silicon layer was decreased, the result was shown in Figure 16.According to the previous analysis, both reducing the diameter of the innermost layer silicon and increasing the thickness of BCB material could promote the transmission performance, so the experimental result was correct.

Analysis and research of multilayer through silicon via
Much work has been done to explore the electrical property of different TSV structures.Figure 17 showed three types of multilayer TSV structures [17,18], which were modeled in HFSS respectively.Different layers of TSVs were connected by 0.5um copper columns (or copper rings), as shown in Figure 18.And the rest of the dimensions were the same as the structures in the previous section.Figure 19 showed the simulation results of three structures.B-coaxial ring TSV and coaxial ringtapered TSV share the similar performance characteristics as single-layer TSVs, and they still have good electrical performance.For different 3D packaging applications, designers can choose according to different needs.

Conclusion
In this paper, we first established the bar-coaxial ring TSV model and studied its electrical characteristics.Based on previous studies on the different structures of TSV, some results have been achieved.1.The electrical characteristics of the novel TSV and the conventional Bar-TSV were compared.2.The advantages and disadvantages of the novel structure and the coaxial ring-tapered structure were analyzed, and presented a methodology for selecting different structures.3.The influence of different parameters on transmission performance was studied.4.Finally, the electromagnetic performance of the multilayer TSV structure was analyzed.
The improvement of the TSV structure is very significant in the post-Moore era and provides a theoretical guidance for the development of integrated circuits.

References
[5].The reliability has been improved.In 2006, Paul S. Andry, a researcher at IBM, proposed the ring TSV structure [6, 7].After testing, it could be found that the structure had better thermal mechanical stress while ensuring the transmission characteristics.Because TSV is prone to signal leakage when transmitting high frequency signals, which affects the system performance, Sparks et al. proposed the coaxial TSV [8, 9].In addition, many research papers have been published in recent years.In 2017, Shikang Wang of Xidian University proposed the ring-tapered TSV structure [10].In 2018, Zheng Mei of Xidian University proposed the coaxial ring TSV structure [11].In 2021, researchers at Nanjing University of Posts and Telecommunications proposed the coaxial ring-tapered TSV [12].However, many researchers have made innovations in filling materials.BCB material is usually used to replace SiO2, and it has low dielectric constant and good fluidity.If BCB material is used as the insulation layer, the TSV structure can obtain better transmission characteristics.As the size of semiconductor continues to shrink, the TSV structure needs to be applied to different scenarios, so the production of TSV faces severe space limitation.In 2018, Zhensong Li of Beijing University of Posts and Telecommunications proposed the Bar-TSV (B-TSV) structure [13], which could reduce the floor area of the TSV.But it still has the same problems as the cylindrical TSV, and needs to be improved.

Figure 1 .
Figure 1.Schematic diagram of TSV interconnection.To reduce footprint while maintaining good transmission characteristics, the bar-coaxial ring TSV (B-coaxial ring TSV) was studied in this paper.The proposal of this structure provides more options for the field of 3D integrated circuits.Subsequently, the performance differences between the improved structure and other structures were further compared, and the multi-layer TSV structure was analyzed.This research result also has good reference value for work needs in different fields.

Figure 2 .
Figure 2. Structure of B-coaxial ring TSV.(a)Side view (b)Top view.
, a three-dimensional model of a conventional B-TSV was established in HFSS[13].This model was a ground-signal-signal-ground-type TSV.The conventional B-TSV structure had the same height and diameter as the B-coaxial ring TSV, the thickness of SiO2 around the copper conductor as an insulator was 0.1um.

Figure 3 .Figure 4 .
Figure 3. Structure of conventional B-TSV.For signal integrity, the S-parameter plays a pivotal role, meaning that we view the transmission channel as a black box, and the S-parameter describes the frequency domain characteristics of the black box itself [14].S-parameter is a measure of reflected power and transmitted power in a network as a function of frequency [15].For the purpose, we have simulated the conventional B-TSV and Bcoaxial ring TSV respectively, and obtained the S-parameter to compare whether the electrical performance has been improved.The simulation result was demonstrated in Figure 4.

Figure 7 .
Figure 7. S-parameter when the inner ring metal thickness changes.

Figure 8 .
Figure 8. S-parameter when the outer ring metal thickness changes.Thirdly, if the thickness of inner and outer copper materials was changed at the same time, the result was shown in Figure9.It combined the analysis of case 1 and case 2 in this section.The increase of inner and outer metal thickness was not conducive to signal transmission.Fourthly, if the thickness of the middle (or outer) BCB material was changed or both were changed at the same time, as shown in Figure10.With the increasing thickness of BCB material, the capacitance of the insulation layer decreases significantly [16], so its S-parameter becomes more and more ideal.

Figure 9 .Figure 10 .
Figure 9. S-parameter when the inner and outer ring metal thickness changes.

Figure 11 .
Figure 11.S-parameter when the ratio of inner and outer metal thickness changes.

Figure 12 .
Figure 12.S-parameter when the outer ring metal thickness increases and the innermost silicon diameter decreases.Thirdly, if you increased the thickness of the inner copper material and decreased the diameter of the innermost silicon layer, as shown in Figure13.S11, S21 did not change much when the parameters were changed.This situation could be equivalent to changing the volume of the filler metal.Because the impedance of TSV is capacitive, which depends on the outer surface of the inner conductor and is independent of the filling volume.Fourthly, if you increased the thickness of the inner and outer copper material at the same time, and decreased the diameter of the innermost silicon layer, we could analyze it in combination with case 2 and case 3 in this section.As shown in Figure14, the S11, S21 parameters were optimized.

Figure 13 .
Figure 13.S-parameter when the inner ring metal thickness increases and the innermost silicon diameter decreases.

Figure 14 .
Figure 14.S-parameter when the inner and outer ring metal thickness increases and the innermost silicon diameter decreases.Fifthly, if the ratio of middle and outer BCB material thickness was changed, the result was shown in Figure15.We found that the magnitude of the ratio had almost no effect on its transmission performance.Combined Figure10, we could obtain some conclusions.The performance was only related to the total thickness of the middle and outer BCB

Figure 16 .
Figure 16.S-parameter performance.(a)When the middle BCB material thickness increases and the innermost silicon diameter decreases.(b)When the middle and outer BCB material thickness increases and the innermost silicon diameter decreases.

Figure 19 .
Figure 19.Comparison of the simulation results.
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Table 1 .
Model symbol and physical parameter.