Abstract
By theoretical calculation, the external force on the particle conveyed by pad asperities and the molecular adhesion force between particle and wafer are compared and analyzed quantitatively. It is confirmed that the molecular adhesion force between particle and wafer has a great influence on the chemical mechanical polishing (CMP) material removal process. Considering the molecular adhesion force between particle and wafer, a more precise model for the indentation of a particle into the wafer surface is developed in this paper, and the new model is compared with the former model which neglected the molecular adhesion force. Through theoretical analyses, an approach and corresponding critical values are applied to estimate whether the molecular adhesion force in CMP can be neglected. These methods can improve the precision of the material removal model of CMP.
Highlights
During the course of chemical mechanical polishing (CMP) process and as the particle’s diameter gets close to the nm scale and the particle’s indentation depth into wafer’s surface is around 0.01∼1 nm, the attractive intermolecular forces acting at the particle/wafer interface and the external force applied on the particle may be close in magnitude
In this paper in-depth study was conducted on the influential law of the molecular forces between particle and wafer surface on CMP process, and a more complete particle indentation depth model considering the effect of the molecular forces between particle and wafer surface is established
The two judicial criteria can apply to all CMP processes. It can be seen from these two criteria that the greater the wafer surface hardness HW, the Young modulus of pad EP, and the particle diameter D, the lower the particle concentration and the greater the possibility of neglecting the influence of the molecular adhesion force. It is by the inverse calculations indicated that, in typical metal CMP processes, if we take EP = 10 modulus of particle ES (MPa), Ca = 0.034, ρs = 3600 kg/m3, and ρf = 1000 kg/m3, average surface hardness of pure metal is generally less than 746 MPa, so Π1 item cannot be ignored; when the particle diameter D is greater than 64 nm, Π2 item can be neglected
Summary
During the course of chemical mechanical polishing (CMP) process and as the particle’s diameter gets close to the nm scale and the particle’s indentation depth into wafer’s surface is around 0.01∼1 nm, the attractive intermolecular forces acting at the particle/wafer interface and the external force applied on the particle may be close in magnitude. According to DLVO theory [1], the molecular forces between particle and wafer surface are mainly the electric double-layer repulsion and van der Waals attraction; other intermolecular forces are smaller than them in the order of magnitude and can be ignored. DLVO theory shows that double-layer repulsion is major, and van der Waals attraction can be ignored at large particle/wafer surface approaching distance. In the CMP process, the indentation depth of particle into wafer surface is about 0.01∼1 nm, so just van der Waals attraction is considered. It is assumed that the particle and wafer surface is completely smooth; the molecular force between the particle and the asperities of wafer surface is negligible
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