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In-situ thermal deformation measurement of low-k layer using nano-pattern recognition and correlation technique

Transition from aluminum to copper interconnects for integrated circuit applications, concomitant with the shift to a lower resistance conductor, is well underway. A progression to lower permittivity (low K) dielectrics replacing silicon dioxide has been required in order to take advantage of significant reduction in signal delay, cross-talk and power dissipation, enabling the semiconductor industry to continue device scaling. The integration of these new conductors and dielectric materials, within the framework of aggressively scaled feature sizes, presents a formidable challenge. The implementation of low K materials requires new reliability learning for copper metallization systems and low K layers. The need becomes more imminent as Cu-low K technology is pushed to its limits for the 50 nm technology node and beyond. The reliability of devices with copper circuitry is determined largely by thermal and mechanical deformations of low K layers. Because of its low modulus, the low K layer can deform significantly during the interconnect manufacturing process. The problems of deformation and stress are exacerbated during operation as a result of thermal expansion mismatch between the chip and the substrate. Numerous studies to characterize the intrinsic mechanical properties of low K materials, including material and interfacial strength, have been conducted in recent years; yet it is not known how much deformation the low K layers will be subjected to during device manufacturing and operation. It is an issue of failure and reliability. The deformation information is crucial to a low K design and material selection guideline. The study requires measurement sensitivity on the order of a few nanometers on a sub-micron length scale. Several techniques have been proposed to document nano-scale deformations. However, none of the existing techniques including electron-beam moiré (EBM) and SEM/TEM/AFM digital image correlation (DIC) [1-5] can provide both the displacement measurement sensitivity/accuracy and the spatial resolution required for the analysis. Recently, a new technique called Nano-Pattern Recognition and Correlation Technique (N-PRCT) has been developed to cope with the limitations of the existing techniques. The N-PRCT uses regularly oriented nano-scale structures that are fabricated on the surface of the specimen. Special benefits accrue from the regularity, relative to the random markings used for existing techniques. For the regularly oriented patterns, the inherent random noise produced during SEM imaging, which was the main reason why the applicability of the existing techniques could not be extended to domains with extreme magnifications, can be eliminated effectively by using an approach combining the conventional low pass filter with an advanced band-pass filter (Wiener Filter). Then, the images before and after loading are documented by an SEM. A novel image processing algorithm for pattern recognition and correlation is applied to the images to produce a whole-field nano-scale displacement field. The implementation of N-PRCT includes nano-pattern fabrication, SEM imaging under thermal loading using a custom designed conduction stage, and application of the pattern recognition and correlation technique to produce deformation-induced displacement fields. Area E-Beam Lithography was employed to fabricate the nano-scale patterns on the surface of specimen. A thin PMMA layer was coated on the specimen surface and exposed by E-beam through a nano-pattern mask that was fabricated using a nano-hole template. By controlling the exposure dose precisely, both negative and positive patterns were obtained. As a simple alternative solution, direct transfer of a thin (≈ 20 nm) disposable gold membrane onto a specimen surface was also implemented. Figure 1 shows the negative (a) and positive (b) patterns using the area E-beam lithography and the transferred thin gold membrane.