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A study on Reduced/Absent Adhesion/Cap Layers for Optimization of BEOL RC Performance
Keywords: Interfacial adhesion strength, BEOL RC performance , adhesion/graded layers
BEOL/Interconnect RC performance started to dominate circuit delay since ~0.1um tech node [1, 2]. Corresponding targets and specs for R (line resistance) and C (coupling capacitance) become cornerstones to determine critical circuit design/routing parameters such as, setup time, hold time, IR drop, etc. Lower RC delay is vital to achieve optimal and competitive circuit performance and hence the endlessly pursued goal for advanced BEOL integration scheme. So far, low-k dielectric materials, i.e., fluorine- doped oxides, carbon-doped oxide (SiCOH), to porous carbon-doped oxides (p-SiCOH) have been implemented. However, due to the process integration challenge with inherent weakness of low-k materials, the trend to pursue lower k dielectrics has come to a plateau as tech nodes scaling down to 20/14nm and thereafter. On the other hand, the trend of dimension shrinking down, such as trench CD and height, via CD and height, etc., still continues as tech node advances. Note that the adhesion/gradient layers with much higher k value were introduced to enhance interface adhesion strength between SiCOH/p-SiCOH and cap materials [3], which offset the intrinsic RC benefit from low-k dielectric material. At 7nm node and beyond, the combined adhesion and cap layers could be up to half of via or trench height, which poses a huge challenging to meet desired RC performance. Therefore, the thickness reduction of adhesion/gradient layers and cap materials becomes necessary for further tech node scaling. In this study, 4 point bending adhesion tests were conducted to evaluate the interfacial adhesion strength between OMCTS2.7, OMCTS Ex2.45, DULK, ULK2.55 on NDC and ODC with or without adhesion layers (5-6% on capacitance gain). It is shown that critical energy release rate Gc is reduced by about ~ 1J/m^2 for all these low k dielectrics without adhesion layers. TOF-SIMS signals show that failure occurred at interface. Furthermore tests on samples with thinner adhesion layers, about half of POR thickness, could recover Gc by ~0.5 J/m^2. The 4 point bending tests for OMCTS2.7 on tri- or bi-layer aESL (advanced etch stop layer, combination of AlNx, AlOx and ODC layers) with thinner thickness than POR NDC cap (3~5% on capacitance gain), showed that the interfacial strength is equivalent to the value on NDC cap. The combined RC gain with absent adhesion layer and reduced cap layer is up to 10-12%, which is quite significant to RC performance optimization for main stream 14nm tech node, especially for 7nm node and beyond when facing enormous integration challenges with dielectric materials with k less than 2.4. Advanced FEM fracture modelling/analysis with ABAQUS® is further implemented to study the application conditions with thinner/absent adhesion and cap layers. The analyses focus on the impact on interfacial adhesion strength from die size, substrate thickness, packaging material and types, etc. and provide guidelines to implement tailored thickness of adhesion and cap layers into real products/applications. Further CPI test/evaluation with full stack samples is under development.
Dewei Xu,
Globalfoundries Inc.
Malta , New York
USA


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