The Effect of Interactions Between Water and Polishing Pads on Chemical Mechanical Polishing Removal Rates

Abstract

The elastic properties of polishing pads critically affect polishing results during chemical mechanical polishing ͑CMP͒ of integrated circuit substrates. The ability of water to plasticize polishing pads and the resulting effects on pad performance were investigated. Water is hypothesized to penetrate the surface of the polishing pads, disrupting hydrogen bonds between adjacent polymer molecules within the pads and altering the pad elastic modulus. Infrared spectra obtained using an attenuated total reflection technique were used to confirm the effect of water on the polishing pad structure. Contact with deionized water reduced the pad elastic modulus due to the formation of a hypothesized soft layer on the pad surface. Removal of the soft layer from the pad, as occurs during conditioning, increased observed CMP removal rates. Experiments on a commercial dual-axis polisher using a noncommercial process showed decreased oxide removal rates as the duration of the pad exposure to water increased. A pad asperity-wafer contact model was developed. This model indicates that if the surface of the pad asperities is softened by the penetration of water compared to the bulk pad, then the asperities will deform to a greater degree under the applied load than in the absence of the softening. As a result, the effective pad-wafer contact area for a given applied load will increase and the average pad-wafer separation will decrease, causing the applied load to be carried by a larger number of asperity-wafer contacts and reducing the local load on the wafer surface. The reduced local loads will contribute to reduced polishing rates, consistent with experimental observation. The maximum depth of penetration of water into the pad surface was estimated to be ϳ20 m.Chemical mechanical polishing ͑CMP͒ has become an essential step in the manufacturing of advanced integrated circuits ͑ICs͒. During CMP, material is removed from the surface of a wafer when the wafer is rubbed against a polymeric polishing pad coated with aqueous slurry. Although this technology has developed rapidly in recent years, understanding of the fundamental chemical and mechanical processes involved in polishing is still developing. In this work, the effects of water penetration into the pad are considered, including effects on the pad physical properties and the resulting polishing performance.A combination of mechanical and chemical processes during polishing results in material removal from the surface of both the wafer and pad. It has been observed that as the number of wafers polished by a pad increases, pad morphology is altered by wear and glazing. 1,2 Pad glazing occurs as debris from the pad and wafer accumulates in the pad pores and as the pad surface is smoothed by the polishing. 3 In the absence of steps to counteract wear and glazing, material removal rates decrease compared to the original removal rate with increases in the number of wafers polished by a pad. 4 To maintain stable removal rates over the life of the pad, pad conditioning is widely uti...

Energy dissipation and constitutive modeling for a mechanistic description of pad scratching in chemical–mechanical planarization

“…These results show that the pad material relaxes stress more over time as water enters the IC1000 material structure. It has been reported in the literature that the pad material decreases in stiffness when it is soaked in water [17][18][19][20]. It is suggested by Castillo-Mejia, et al that the decrease in stiffness is a result of the water breaking hydrogen bonds within the polyurethane.…”

Novel ceria–polymer microcomposites for chemical mechanical polishing

“…In literature reports, interactions between a soft pad, hard abrasive particles, and the wafer surface have been considered using a variety of approaches that extend from modeling [49,50] to experiments [51]. In an experimental study, Castillo-Mejia and coworkers found that reducing the elastic modulus of a IC 1000 pad surface led to lower polishing rates.…”

Pads for IC CMP

“…The reduced elastic and shear moduli of the pad could lead to lower polishing rates. The material removal rate is strongly influenced by the mechanical properties of the pad surface (although the bulk material properties of the pad are nearly unchanged), which may be affected significantly by the time of immersion in water [19,[21][22][23]. In tungsten CMP, polished materials and debris (tungsten), abrasive particles like alumina, oxidizer ion like iodate, and other ions could accumulate in the pad after polishing.…”

“…However, these models did not consider the effect of pad surface roughness (asperity height distribution) on polishing removal rate. There are quite a few other publications considering the effect of pad roughness and other pad properties on polishing removal rate [21,[47][48][49][50][51][52][53][54].…”

“…21 Dishing as a function of (a) linewidth and time and (b) pattern density. Symbols are for data from (a)Figure 5.18 of Tugbawa[78] and (b)Figure 4.12 of Tugbawa[78].…”