Study On The Material Adhesion Removal Mechanism In Chemical Mechanical Polishing Of Silicon Wafers

Chemical mechanical polishing combines aqueous corrosion chemistry and contact mechanics to obtain material removal and planarization of wafer surface. Chemical mechanical polishing (CMP) is superior to other planarization technologies in producing excellent local and global planarization at low cost, and thus is widely adopted in IC fabrication. However, the fundamental mechanisms of material removal and the interactions of the chemical and mechanical effects are not well understood, which limits a better control and improvement of the CMP process. With the rapid development to smaller feature size, higher resolution, denser packing and multi-layer interconnects of ultra large scale integrated (ULSI) circuits, the CMP techonology now is facing a stringent challenge. The present challenge of the CMP processing is to develop physical models to explain these processes which then will lead to the development of new and more robust processes.In this study, fundamental understandings of the mechanics of material removal during CMP are established based on the modeling of wafer-pad-particle interactions. First, On the basis of the analysis of the molecular dynamics simulation of sliding contact between a single micro-particle and a smooth flat surface, a new approach for the material removal in CMP process is put forward in this paper, which deems that the amorphous material is stripped off the wafer surface to be the wear debris due to the adhesion of the abrasive particles when sliding across the wafer surface. Meanwhile, in accordance with this approach, combined with the equivalent beam bending model for a three body contact among pad/wafer/particles, a new mathematical model about the material removal rate in CMP process is developed, The model comprehensively considers the influence of most valuables in CMP process including pad properties, operational conditions and slurry characteristics. Especially, the large deformation and the super elasticity of the pad while in contact with wafer surface are involved in the model. What's more, a new important parameter k, named adhesion coefficient, which represents the ability to remove the amorphous layer on wafer surface by a single particle, is firstly put forward in the model. It is the comprehensive parameter incorporating the mechanical effect and the chemical effect. After the validation of some teams of experimental data, it is found that the removal rates predicted by the model agree well with those experimental value under same CMP conditions.Then, through theorical calculation, the importance of the molecular adhesion force between a particle and wafer surface to CMP material removal rate is confirmed. And then, based on the force balance equation of particles embedded into the interface between the pad and wafer surface during CMP, a new model considering the molecular adhesion force of the indentation depth of a particle into wafer surface is achieved. In addition, the result of the new model is compared with the old one which did not consider the effect of the molecular adhesion force. After theorical analysis and deduction,the estimation criterions and critical value which deside whether the molecular adhesion force should be neglected or not are introduced. Utilizing these theorical results, the aftermentioned amorphous material removal model in CMP is reviced by considering the molecular adhesion force. After validation of experimental data, it is concluded that when the abrasive diameter is smaller than the critical value, the effect of molecular adhesion force on CMP material removal rate is noticeable and can not be neglected, when larger than the critical value, the effect of molecular adhesion force can be neglected.Furthermore,according to the theoretical and experimental analysis, the CMP process is divided into two phases:chemical effect dominant phase and mechanical effect dominant phase. Then, from the balance point of the two phases, a new equation, which can quantitatively describes the generation rate of oxidized layer on wafer surface in CMP is developed. The modeling of the generation rate of oxidized layer in CMP will further the research of the CMP material removal mechanism and offer a direction to control the CMP process more accurately. Based on the theorically calculating result of generation rate of oxidized layer, it is authenticated that the hypothesis of molecular scale material removal mechanism in CMP is right.Finally, on the basis of the dynamical equilibrium of oxidation and removal of wafer surface atoms/molecules, the adhesion coefficient k is deduced quantitively from the scope of energy. And then the effects of lots of physical and chemical variables on the adhesion coefficient k are analysed qualitatively. Utilizing the equation of k, the aftermentioned amorphous material removal model in CMP is reviced. The reviced model is based on the chemical-mechanical synergetic effects, and not only incorporates the mechanical effect of the slurry particles, the chemical role of the slurry, other important factors on CMP process such as wafer material properties, pad surface profiles and operating variables, but also includes the influence of molecular adhesion force between wafer surface and abrasive particle, the large deformation and super elactricity of the contact of pad and wafer surface and the identation depth of abrasive into wafer surface. Particularly, the oxidant concentration in slurry is firstly integrated in the model, which causes that the influence law of the oxidant concentration in slurry on material removal rate MRR can be calculated quantitatively. The model predictions of abrasive diameter, oxidant concentration as well as pad elasticity modulus are presented in graphical form and show good agreement with the published experimental data.In general, the optimization of the controlling parameters involved in the polishing process can be obtained through model predictions. Optimization scheme of high performance slurries and operation parameters to reduce wafer scale variation can be developed on the basis of this study. In addition, the value of the experimental and modeling efforts of current study can provide guidelines for the design of novel polishing processes and for the identification of unexplored process parameters. The reported findings in this study can also provide a fundamental understanding of the polishing mechanisms and can provide guidelines for controlling the CMP process more accurately.