Sacrificial Technology Research Articles

Thermal Reliability Tests of Air-Gap TSVs With Combined Air-SiO2Liners

Using air gaps instead of conventional SiO2 liners as the isolators of through-silicon-vias (TSVs) has the potential for reducing thermal stresses and improving the thermomechanical reliability. This paper reports the thermal test results of newly developed air-gap TSVs with combined air-SiO2 insulators. Fabrication methods based on polymer sacrificial technology are reported. Thermal shock tests between −65 °C and +150 °C are performed, and the results show that the air-SiO2 liner TSVs are reliable over thermal shock with slight improvement in depletion capacitance and capacitance hysteresis. Chip curvatures caused by thermal expansion at high temperatures are measured, and it is found that the air gaps are able to mitigate plastic deformation of Cu plugs at high temperatures. These results indicate that the combined air-SiO2 insulators improve the electrical performance and the thermal stability of air-gap TSVs by isolating the organic residues originated from the sacrificial polymers, and the air-SiO2 liner TSVs have good structural integrity at high temperatures.

Low-Capacitance Through-Silicon-Vias With Combined Air/SiO2Liners

Through-silicon-vias (TSVs) with air-gap insulators have the advantages of low capacitance and low thermal stress. This paper reports the design, fabrication, and characterization of new TSVs with combined air/SiO2 insulators. Sacrificial technologies based on heat decomposition of poly(propylene carbonate) and reactive ion etching of benzocyclobutene have been developed to fabricate uniform and high aspect-ratio air gaps. Air gaps with a thickness of 0.8 $\mu \text{m}$ and an aspect ratio of 62.5:1 have been successfully fabricated. Measurement results show that the SiO2 liner is able to eliminate the impacts of residues of the sacrificial polymers, and the TSVs with air/SiO2 have low capacitances and leakage currents at both room temperature and high temperature, constant minimum capacitance, and good temperature stability.

Implementation of Air-Gap Through-Silicon-Vias (TSVs) Using Sacrificial Technology

Using air-gaps to replace conventional silicon dioxide as the insulators of through-silicon-vias (TSVs) has the possibility to improve the electrical performance and some thermal reliability issues of TSVs. This paper reports the implementation of TSVs with air-gap insulators by developing a polymer sacrificial technology. The sacrificial technology and the key fabrication processes are investigated in detail, including spin-coating of poly propylene carbonate (PPC) on the sidewalls of blind vias, copper chemical-mechanical polishing, PPC grinding, and PPC pyrolysis to form air-gaps. To address the technical challenge in coating thin and conformal PPC sacrificial claddings in blind vias, a vacuum-assisted solvent refilling technique is developed. Air-gap TSVs are successfully fabricated and the electrical performances are characterized. The accumulation capacitance of the air-gap TSVs is 48 fF, and the leakage current is as low as 1.22 pA at bias voltage of 20 V. Finite element simulation shows that air-gaps are able to reduce thermal stresses. The preliminary results demonstrate the feasibility of the sacrificial technology and the good electrical performance of air-gap TSVs.

Ultralow-Capacitance Through-Silicon Vias With Annular Air-Gap Insulation Layers

Low capacitance is critical to the electric performance of through-silicon vias (TSVs). This paper reports the development and electrical characterization of ultralow-capacitance TSVs which use air gaps to replace the conventional silicon dioxide as the insulation layers. The air-gap TSVs are successfully fabricated by developing a sacrificial technology which uses void-free filling and selective etching of an annular benzocyclobutene polymer cladding that surrounds copper plugs. The capacitance and the leakage current are tested to characterize the electrical performance. The lowest effective dielectric constant of the air enables the capacitance of the air-gap TSVs to be as low as 24 fF, and the capacitance density is more than one order of magnitude lower than that of conventional SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> liner TSVs. The leakage current to the substrate is 3 ×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-13</sup> A at 40 V, and no leakage current degradation occurs after a 40-cycle thermal shock test. The preliminary results demonstrate the new air-gap structure and the efficacy of air gaps in reducing TSV capacitance.

Air-Gap Through-Silicon Vias

This letter reports for the first time the fabrication and characterization of through-silicon vias (TSVs) using air-gap insulators to enable high-performance 3-D integration. To address the challenge in fabricating extremely high-aspect-ratio air gaps, a CMOS-compatible sacrificial technology based on pyrolysis of poly (propylene carbonate) has been developed, upon which air-gap TSVs have been successfully achieved. The measured capacitance density and leakage current density of air-gap TSVs are 1.22 nF/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and 10 nA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , respectively, about one order and two orders lower in magnitude than TSVs using SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> insulators.