Research On Key Technologies Of A Novel Through Silicon Via Based On Ultra Low Resisitivity Silicon

Due to the complexity and difficulty of traditional copper TSV fabrication process,this paper has proposed a novel Low Resistivity Silicon TSV?LRS-TSV?with a simple process to reduce manufacturing cost of TSV.In the novel TSV structure,highly doping silicon?7×10-4?cm?is applied as vertical conductor,instead of copper.Thus,the deposition of barrier layer,copper seed layer,copper electroplating and copper overburden CMP can be saved which extremely cut TSV fabrication cost.Besides,polymer with low dielectric constant is used as sidewall insulation instead of SiO₂ which can effectively reduce parasitic capacitance of TSV.This paper has described the process flow and key technologies of LRS-TSV.The mechanism of process which to fill polymer into high-aspect-ratio annular trench without any voids has been investigated.It shows that the lower viscosity of polymers,wider annular trench and higher vacuum condition lead to a better filling performance.From the basic electrical test of LRS-TSV,leakage current of LRS-TSV is measured as 8.25×10-8A/cm² which is same as leakage of Cu-TSV.When the diameter of LRS-TSV in range of 10to 30?m with height of 45?m,the DC resistance are measured in range of 1.7⁵.9?which is two orders of magnitude larger than that in Cu-TSV.As for the special silicon-insulation-silicon?SIS?structure of parasitic capacitance in LRS-TSV,this paper has deduced the capacitance formula.The results show that with the highly doping silicon substrate,depletion capacitance becomes much larger than capacitance of BCB insulation,leading to the parasitic capacitance of LSR-TSV equals to BCB insulation capacitance which does not vary with voltage.The measured capacitance of single LRS-TSV with diameter of 15?m,height of50?m and 3?m BCB thickness is a constant of 20.3fF which is one order of magnitude lower than capacitance of Cu-TSV.To evaluate the high frequency signal transmission characteristics in LRS-TSV,S parameter of LRS-TSV is extracted and compared with Cu-TSV.It shows that in middle frequency?0.15GHz⁴GHz?,the S parameter of LRS-TSV is better than that of Cu-TSV,which due to the lower parasitic capacitance.The results also show that LRS-TSV with a larger diameter,smaller height and pitch,bigger BCB thickness and lower silicon resistivity can have a better transmission characteristic.When the low resistivity silicon substrate is grounded,a kind of coaxial LRS-TSV is formed which has much better S parameter.Through the S parameter test,S11 of the single coaxial LRS-TSV?diameter of 10?m,height of 60?m and 3?m BCB thickness?is lower than-20dB from 1GHz to 50GHz,and S21 is larger than-0.9dB,which is better than that of Cu-TSV with similar size.In order to make LRS-TSV can be applied in circuit simulation software,this paper has proposed static equivalent circuit model of LRS-TSV pair and coaxial LRS-TSV.The signal integrity of LRS-TSV has also been studied.From the eye diagram test,result shows the LRS-TSV supports data transmission up to 30Gbps.This paper has also studied the vertical noise coupling between global clock interconnect in bottom chip and global channel routing wire of upper chip in 3D mixed-signal integration system.The simulation results tell us the near end and far end crosstalk is larger than-30dB beyond 3GHz.Coupling mechanism is investigated through transfer impedance.It shows the mutual inductance coupling is mainly coupling path from 0.1GHz to15GHz,and capacitance coupling comes to play after 15GHz.The factors which influence crosstalk have also been analyzed.The length of interconnect and vertical distance between interconnects have the mainly contribution to crosstalk.It can be alleviated by layout optimization,adding additional metal layer under upper chips and adopting differential interconnects.The broadband equivalent circuit model of vertical crosstalk is also proposed.And in time domain simulation,a 1.2V clock signal with 100ps falling/rising time can generate crosstalk noise with 173mV amplitude.In the last section,this paper has studied the thermal mechanical characteristic of LRS-TSV.Unlike Cu-TSV,the highest protrusion of LRS-TSV is located on BCB insulation.The increase of process temperature and BCB thickness will promote the protrusion and generate larger thermal stress.But thicker surface SiO₂ layer leads to lower protrusion and thermal stress.The thermal mechanical characteristics are also compared between LRS-TSV and Cu-TSV.It shows that the protrusion height of Cu-TSV is four times larger than that of LRS-TSV.And the interface stress of sidewall insulation of LRS-TSV is much lower than that in Cu-TSV.All in all,the thermal mechanical characteristic of LRS-TSV is much better than that of Cu-TSV.