Nanoindentation Analysis of Mechanical Properties of Low to Ultralow Dielectric Constant SiCOH Films

Abstract

Carbon-doped oxide SiCOH films with low to ultralow dielectric constants were prepared on a Si substrate by plasma-enhanced chemical vapor deposition (PECVD) from mixtures of SiCOH precursors with organic materials. The films have different levels of nanoscale porosity resulting in different dielectric constants and mechanical properties. The mechanical properties of the films have been characterized by continuous-stiffness nanoindentation measurements. To study the effect of film thickness, each group of samples with the same dielectric constant was composed of samples prepared with different film thicknesses. It is shown that the effective hardness and modulus of the SiCOH/Si substrate system depends significantly on indentation depth due to substrate constraint effects. The "true" film properties were determined using both an empirical formulation of the effective modulus and direct inversion based on a finite element model. The hardness and modulus of three groups of samples with different degrees of dielectric constants have been measured. The hardness increases from 0.7 to 2.7 GPa and modulus from 3.6 to 17.0 GPa as the dielectric constants change from 2.4 to 3.0. While for stiffer films the modulus measured at an indentation depth 10% of the film thickness is close to the "true" value for films thicker than 0.5 m, the measured value can give an overestimate of up to 35% for softer films. Thin film cracking and film-substrate debonding have been observed with scanning electron and atomic force microscopy at the indentation sites in softer films. The damage initiation is indicated by pop-in events in the loading curve and sharp peaks in the normalized contact stiffness curves versus indentation depth.

Mechanical Characterization of Black Diamond (Low-k) Structures for 3D Integrated Circuit and Packaging Applications

“…Researchers at Motorola [43] have concluded that passing the CMP process of low-k material is not a simple factor of modulus, hardness, adhesion or toughness, but a combination of all of these properties. As stated before, the mechanical properties of low-k films depends on chemical structure, amount of porosity and composition, Elastic modulus and hardness values of the different dielectric thin films varies from 2 to 14 GPa and 0.5 to 7 GPa respectively [41][42][43][44][45]. Volinsky et al [1] have found a linear relationship between hardness and elastic modulus for silicate low-k dielectric films in nanoindentation testing with continuous stiffness measurement (CSM) attachment.…”

“…6, where cracks are indicated by arrows on the micrographs. Many researchers have observed this kind of fracture behavior (pop-in event) in various types of low-k materials, bulk glasses and silica foams [42][43][44][45][46] Table 5 gives the information about the BD films fracture/delamination during nanoindentation testing, in terms of threshold load, threshold indentation depth and % of thickness at which film cracking occurs. For 100 nm thick film, failure is observed when the indenter tip is in the substrate and it can be seen in Fig.…”

“…This is because the elastic modulus is associated with the elastic deformation during nanoindentation, and in contrast, the hardness response of the material is associated with plastic deformation. Extensive simulation studies show that, the effective elastic modulus of a film experiences greater substrate effect than the hardness value [44]. BD -100 nm film shows significantly higher elastic modulus (E=16.48 GPa) when compared to higher thickness BD films and it is expected due to molecular restructuring in very thin BD films (≤100 nm), since the elastic modulus is an intrinsic material property, which largely depends on interatomic or molecular bonds [49].…”

“…13, it is obvious that all stacks have shown pop-in event and this event can be taken as the failure load/fracture strength of the stack. These pop-in events in the nanoindentation curves are resulting from the film cracking and delamination of the stack in the form of blisters [53]. The normal stack shows pop-in event (failure load/fracture strength of the stack) at lower loads and indentation depths, as compared to the back grinded stacks.…”

The effect of the pore topology on the elastic modulus of organosilicate glasses

“…The amphiphilic triblock copolymers act as porogen and are evaporated upon thermal or UV curing, leaving a porous silica network connected to alkyl groups as part of the network or terminal bonds. On the other hand, the CVD-OSG films 7,8 are used as reference materials to compare the elastic modulus values of several OSG materials with different pore topologies quantitatively.…”