Reliability Assessment of Ultra-low-K dielectric Material and Demonstration in Advanced Interposers

Polymer dielectrics have been used as build-up layers in 2.5D interposers with multiple redistribution layers (RDL). To achieve high interconnection densities with good electrical performance, there is always a need to explore newer materials with lower dielectric constants (k) to enable improved data rate, reduced crosstalk, and signal integrity (low latency). With short wiring lengths, the capacitance of the interconnects plays a critical role in determining high bandwidth density and low energy per bit (EPB). Since the capacitance scale with dielectric constant, lowering the dielectric constant of the build-up films used in RDL layers in future interposers becomes very important.Most common polymer dielectric materials that are presently used in the redistribution layers have dielectric constants in the range of 3-3.5. Lowering the dielectric constant to 2.4, which will help reduce the capacitance and thereby the EPB by up to 40%. In this paper, a material with a dielectric constant 2.4 has been studied. In addition, different aspects of reliability of a new ultra-low k dielectric film that include, adhesion strength, capacitance, and leakage current before and after highly accelerated stress test (HAST-85% RH, 130 °C for 100 hours) are presented. The paper also presents the results of process optimization to achieve < 3 μm line width/ space (L/S) using this build-up layer material along with an advanced photoresist. Semi-additive processing (SAP) is employed for this purpose and results of the minimum line/space (L/S) feature sizes obtained by employing different lithography techniques such as projection stepper and maskless aligner will also be presented. This paper will also present the results of advanced characterization tools for measuring the critical dimensions (CDs).