Cross-Section Nano-Indentation for Rapid Adhesion Evaluation
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CROSS-SECTION NANO-INDENTATION FOR RAPID ADHESION EVALUATION S. H. Brongersma,1 Dominiek Degryse,1 Jerome Souiller,1 Bart Vandevelde,1 and K. Maex1,2 1 2
IMEC, Kapeldreef 75, B-3001 Leuven, Belgium, K.U.Leuven, ESAT, Kasteelpark Arenberg 10, B-3001 Leuven, Belgium,
ABSTRACT Cross-section nano-indentation is a technique that consists of cleaving a wafer, indenting behind the stack on the side surface causing delamination, and measuring the delaminated area in an optical microscope. It is shown to be a reliable rapid technique for adhesion evaluation and process optimization of the SiO2/barrier/copper stack. INTRODUCTION
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With the introduction of copper and low-k materials in back-end-of-line processing, adhesion issues have regained importance. Especially the properties of copper diffusion barriers are of critical importance, as they need to prevent diffusion into the dielectric as well as ensure copper adhesion to the underlying stack. The rapid succession of new low-k and barrier materials necessitates a strong effort in terms of characterizing integratability. Thus the optimization of barrier layers, e.g. TaN, TiN, and WCN, depends critically on rapid feedback from adhesion measurements. Over the last few years the 4-point bend technique1 has become the generally accepted choice for reliable quantitative adhesion determination. However, sample
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Figure 1. Four stages of cross-section nano-indentation. The initial indent is determined by the silicon properties only. The unloading curve shows how much of this indent is elastic. Above a critical depth cracks running from the indent to the interface are induced and then deflected into the weakest interface.
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preparation is cumbersome and time consuming. As an alternative, cross-section nano-indentation provides a rapid alternative as a process optimization tool that was first introduced by Sánchez et al.2 Here the sample preparation is limited to cleaving the wafer and mounting it on its side in a nano-indentation set-up. Then, a series of indents is performed over a range of distances behind the stack of interest. In contrast to indentation for modulus determination vertical thermal drift of the indent column is not critical and measurements can start within minutes after loading the samples. Finally, the resulting delamination is measured in an optical microscope with 100x objective. Typically, reliable data can be obtained within the hour. Quantification of the technique depends on a combination of 4-point bend measurements and finite element simulations. EXPERIMENTS AND RESULTS Samples are made by blanket layer depositions on Si(001) substrates. For the measurements described here these layers are 50 nm Si3N4, 500 nm SiO2, barrier layer, and 600 or 1000 nm copper. The barrier is either a WCN or Ta/Tan layer. For indentation a Nano-indenterXP from MTS with precision x,y-stage (0.1 µm) is used. Indents are made up to 10 µm’s behind the interface of interest. During indentation several stages can be identified, a
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