Profiling of the Mechanical Properties of Ultralow- k Films Using Nanoindentation Techniques
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Profiling of the Mechanical Properties of Ultralow-k Films Using Nanoindentation Techniques Holm Geisler, Ulrich Mayer, Matthias U. Lehr, Petra Hofmann and Hans-Juergen Engelmann GLOBALFOUNDRIES Dresden Module One LLC & Co. KG, Wilschdorfer Landstr. 101, D01109 Dresden, Germany ABSTRACT Several nanoindentation techniques were applied to the surface, the reverse side and cross-sections of PECVD ultralow-k (ULK) film stacks to characterize their elasto-plastic properties quantitatively. Results showed good agreement of the reduced modulus (Er) values measured from above and on cross-sections, respectively. Er decreased by 10-22% from the upper to the lower surface of the films. This gradient suggests that UV light absorption inside the film leads to slightly reduced curing at the rear side of the films compared to the surface of the ULK layers. Both quasi-static nanoindentation and dynamic mechanical mapping showed this trend. It is demonstrated that quantitative mechanical mapping can be performed with a lateral resolution 100nm. Slight local variations of Er were detected on ULK/SiCxNy films stacked on top of Cu-low-k interconnect structures. INTRODUCTION UV curing is a suitable process to enhance the mechanical strength of ULK films which are used as interlayer dielectrics (ILD) between Cu interconnects in advanced microelectronic products [1]. However, UV light absorption inside the film material and interference and reflectivity at film/substrate interfaces should be taken into account, because they potentially lead to inhomogeneous ULK film properties [2]. The situation is supposed to be different for polychromatic UV light sources compared to monochromatic UV. The latter can additionally create a modulated stiffness across the ULK layer thickness via build-up of standing waves. A dynamic change of light absorption over time due to film density variation during curing will additionally complicate the situation. Consequently, the resulting elasto-plastic properties of ULK films have to be quantified thoroughly to guarantee that microelectronic products are mechanically reliable [3,4]. The stiffness does not necessarily scale linearly with UV cure time, and a ULK film does not implicitly need to be homogeneous from top to bottom. Care must be taken to quantify nanoindentation data measured on the surface of soft ULK if the film thickness becomes much lower than 1μm [5]. Especially, the proximity of stiff substrate materials can make nanomechanical measurements inaccurate if conventional data evaluation methods are applied. Stiffness gradients inside the films may be hidden then. Several ways were discussed to consider the influence of the substrate which can superimpose nanomechanical data measured on the surface of thin films [6-8]. In contrast, the goal of the present paper is to assess this problem by purely experimental approaches without extended data treatment or modeling. For this purpose, ULK films were mechanically probed with very high surface sensitivity using (a) stepwise layer deprocessing by chemical
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