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).

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

In this study, we investigated the impact of the molecular weight of the base polymers of negative-tone photosensitive dielectric materials (PSDMs) for fine pitch redistribution layers (RDLs) on the swelling and resulting pattern profile.We prepared two PSDMs: samples A and B, with different molecular weights of the base polymer. We monitored swelling during development using a quartz crystal microbalance (QCM), and observed that the samples exhibited significantly different swelling behaviors. It has been suggested that controlling the molecular weight of the base polymer is important for preventing swelling and achieving fine patterning. We controlled the molecular weight of the base polymer and realized 0.8-μm line/space (L/S) patterning with an aspect ratio of 3.8 formed using an i-line stepper with 0.5 numerical aperture (NA). In addition, we show that the PSDM has insulation reliability.

Swelling Analysis of Negative-tone Photosensitive Dielectric Materials for Fine Pitch Redistribution Layers

In this letter, the RF reliability of the redistribute layer (RDL) under different environment stresses was investigated and the degradation mechanism of the RDL was analyzed. The inter-connect performance was characterized by the S parameters from 10MHz to 40GHz. The test vehicles of the RDL with coplanar waveguide (CPW) and microstrip (MS) were fabricated on Si and epoxy resin substrates. It was found that the insertion loss of CPW on Si substrate degraded seriously after reliability tests, especially at the low frequency range of 10MHz-30GHz. For other samples, the humidity stress has more impacts on the RDL reliability compared with thermal stress. Warpage was found for the RDL on the epoxy substrate, while the warpage of the RDL on the Si substrate was under detection limits. The 3D-Xray and SEM of the cross section of the samples were characterized. No declamation was found in the RDL after reliability tests. The surface of the Cu contact pad were oxidized and an obvious declamation was found in the contact region, which results in the insertion loss changing.

Investigation of the RDL reliability based on RF characterization

Low-stress and low-warpage dielectrics are gaining importance as we move towards large-body Multi-chip Modules (MCMs). This paper demonstrates fabrication of redistribution layer (RDL) with 5 μm linewidth/spacing using a novel low-stress dielectric dry film- PLSF (Panasonic low stress film) as a build-up layer. Microvias formation down to 7 μm diameter has also been demonstrated in this paper. The main feature of PLSF is its low- stress and low-warpage. In this paper, we have studied residual stress characteristics of PLSF and its comparison with the industry-standard dielectric (ISD). The lower tensile modulus of PLSF as compared to ISD results in significantly lower residual stress and warpage on the substrate. Electrical and thermomechanical reliability of RDL with PLSF as build-up layer has also been studied and the results are presented in this paper.

Fukuya Naohito

Papers

Fabrication and reliability demonstration of 5μm redistribution layer using low-stress dielectric dry film