Molecular dynamics analysis of adhesion strength of interfaces between thin films
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We have developed a molecular-dynamics technique for determining the adhesion strength of the interfaces between different materials. This technique evaluates the adhesion strength by calculating the adhesive fracture energy defined as the difference between the total potential energy of the material-connected state and that of the material-separated state. The extended Tersoff-type potential is applied to calculate the adhesive fracture energy of metal/dielectric interfaces as well as metal/metal interfaces. We used the technique to determine the adhesion strength of the interfaces between ULSI-interconnect materials (Al and Cu) and diffusion-barrier materials (TiN and W). It was also applied to determine the adhesion strength of interfaces between the interconnect materials and a dielectric material (SiO2). Because the adhesion strength determined by this technique agrees well with that measured by scratch testing, this technique is considered to be effective for determining the adhesion strength. I. INTRODUCTION
Highly integrated semiconductor devices and magnetoresistive heads are made up of thin-film layers of various materials. Following the trend toward increased integration density in these devices, the number of interfaces between different materials in a given area is becoming larger. Because diffusion at interfaces is more active than that in the bulk, the problems related to diffusion such as voiding and adhesive fracture are becoming more serious. In ULSIs, for example, it is well known that the adhesion of Cu-interconnect films to TiN underlayers is not very good, whereas that of Al-interconnect films to TiN is good.1,2 It is time-consuming to find appropriate underlay materials for Cu interconnects by experimental trial and error. Therefore, we have developed a molecular-dynamics (MD) technique for determining the adhesion strength of the interfaces between thin films. This technique was used to quantitatively analyze the adhesion strengths of interfaces between ULSIinterconnect materials (Cu and Al) and diffusion-barrier materials (TiN and W) and those of interfaces between interconnect materials (Cu and Al) and a dielectric material (SiO2). To compare the results of this analysis with the measured adhesion strength, scratch testing was conducted. II. ANALYSIS METHOD
The adhesion strength is determined by calculating the adhesive fracture energy defined as the difference between the total potential energy of the materialJ. Mater. Res., Vol. 16, No. 6, Jun 2001
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connected state (Fig. 1) and that of the material-separated state (Fig. 2). Figures 1 and 2 show the models used for adhesion analysis of an interconnect/underlayer interface. Total potential energy UT is given by UT ⳱ ⌺i
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