Study of metal adhesion on porous low-k dielectric using telephone cord buckling

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Abstract

The interfacial adhesion energy between metal and porous low-k dielectrics is an important parameter for the reliability study of back-end of line integration. In this work, we have observed the spontaneous film delamination with telephone cord morphology after 130 nm thick Ta was sputtered onto methyl silsesquioxane (MSQ) low-k dielectric. The highly compressive stress inside the Ta film is the driving force for the spontaneous buckling. The adhesion failure was identified to be at Ta/MSQ interface by using focused ion beam and scanning electron microscopy. Pinned circular blister model was applied to fit the buckling morphology. The interfacial adhesion energy was extracted to be 7.90 J/m2 at 87° phase angle. The Cu/MSQ interface was evaluated in a similar fashion by using a stressed overlayer Ta/Cu. The fracture energy was calculated to be 3.34 J/m2 with the similar phase angle. The results suggest that an adhesion promoter between Cu and low-k dielectrics is essential for a mechanically stable structure.

Introduction

In microelectronic industry, the continued scaling in device size led by Moore’s law has caused significant challenges in the back-end of line (BEOL) structure, such as the increase in resistance–capacitance (RC) delay, crosstalk noise and power consumption [1]. To alleviate these issues, more conductive metal – Cu is currently used for interconnects; also lower dielectric constant (low-k) insulators with certain porosity have replaced SiO2 as the main interlayer dielectrics (ILD) [2], [3], [4]. However, it is challenging to integrate metal onto porous low-k dielectrics, not just due to their easy penetration into porous structure [5], but also the poor adhesion. The Cu debonding from ILD is deteriorated by the residual stress in multilayer device structures and thermal stress from temperature cycling during processing and operation [6]. Therefore, a metallic adhesion promoter between Cu and low-k dielectrics is necessary to ensure a reliable mechanical structure meanwhile preserving the high conductivity within interconnects. Various metal adhesion layers have been proposed [7], [8]. To justify these adhesion promoters, their interfacial adhesion energy on low-k dielectrics needs to be evaluated.

Many techniques have been developed in an attempt to measure the interfacial adhesion, such as Scotch tape, indentation and four-point bending [9], [10]. Recently, the study of post-buckling morphology has been used to calculate the interfacial energy release rate and provide an in-depth understanding of the buckling mechanism [11], [12]. The film buckling is due to the release of strain energy from thermal or lattice mismatch between different materials of the films. This leads to the delamination of the film with straight, circular or zigzag (telephone cord) patterns. The straight-sided blister is well studied and modeled as a two dimensional Euler column which was characterized by von Karman non-linear plate theory with fully clamped edges. The circular blister is an axis-symmetric counterpart to the straight-sided buckle, and can be simulated with similar non-linear plate theory with clamped boundary at the blister edge [11]. Telephone cord buckle happens more commonly, and has also been observed when integrating metal on low-k dielectrics [13], [14]. However, due to the challenge in modeling the telephone cord buckling, simplified models are often adopted, such as the straight buckle model and the asymptotic solution of the circular blister [13], [15]. The accuracy of straight buckle model for telephone cord structure is yet to be determined, since the mode mixity in their curved crack fronts is different; while for the asymptotic solution of circular blister, it is an approximation for buckling deflection when it is much smaller than the film thickness, which is not always the case for thin film buckling. Recently, a pinned circular blister analysis was used to model wavy telephone cord buckling [16]. It is based on the circular blister model, with an additional clamping boundary at the center of the blister. This model has also been compared with the four-point bending technique, both of which produce similar results in the interface adhesion study [17].

In this article, we report the telephone cord buckle structures of Ta (a widely proposed adhesion layer) and Cu on low-k dielectric. After the delaminated interfaces were identified by focused ion beam (FIB) and scanning electron microscopy (SEM), the morphology of the buckled surface was imaged using atomic force microscope (AFM). Pinned circular blister model was used to simulate the telephone cord buckling surface morphology. The interfacial energy release rate and phase angle for metal/low-k dielectric interface were extracted and compared based on the simulation.

Section snippets

Experiment

The porous low-k dielectric is methyl silsesquioxane (MSQ) with dielectric constant 2.4, provided by Freescale Semiconductor, Inc. The 300 nm film was spin-coated on 6.8 nm thermal oxide on n-type Si(1 0 0) substrate. The porosity is 33%, with 3.6 nm average pore diameter. The nominal stoichiometry of bulk/dense MSQ is SiO1.5(CH)0.5 and a dielectric constant of 2.8–3.0 [18]. Ta and Cu films were deposited on MSQ using a CVC® DC Magnetron Sputterer. The film delamination was triggered by indenting

Analytical model

The surface profile of the buckled surfaces is modeled by numerically integrating the coupled non-linear axis-symmetric von Karman equations for pinned circular blisters [16],ddr˜ddr˜r˜d2w˜dr˜2-dw˜dr˜r˜N˜+1r˜=pr˜ddr˜r˜3dN˜dr˜+r˜dw˜dr˜2=0,where r˜,w˜,N˜ are the normalized radial distance, normal deflection and radial membrane force in the plate, respectively. p is a normal pressure chosen to be sufficiently small during calculation. This model is developed from the well-studied circular blister

Results and discussion

Fig. 1a shows an optical image of the film buckling after Ta deposition on MSQ. This Ta film is around 130 nm thick and was deposited through a shadow mask with 1 mm diameter apertures. These circular metal/MSQ/Si capacitors were originally for electrical test purpose. When the tungsten probe was in contact with the top metal gate, the buckling was triggered. It is similar to indentation, that an indenter penetrates through the films, causing film debonding and blister formation to release the

Conclusion

Telephone cord buckling was observed in Ta and Ta/Cu stressed overlayer on MSQ. The FIB-SEM was used to identify the delaminated interfaces, which are at metal–MSQ interface. The AFM measured buckling surface morphologies were well fitted by the pinned circular blister model, and the adhesion energies for Ta/MSQ and Cu/MSQ interfaces were evaluated to be 7.90 J/m2 and 3.34 J/m2, respectively, both with 87° phase angle.

Acknowledgements

This work was partially supported by Semiconductor Research Corporation (SRC) and National Science Foundation (NSF) of USA under Grant Number 0506738. We thank Greg S. Spencer for providing the porous MSQ films.

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