Selective Chemical Mechanical Polishing Using Surfactants

Abstract

Device fabrication using high density, small pattern size shallow trench isolation ͑STI͒ processes requires material removal selectivity during chemical mechanical polishing ͑CMP͒ steps for optimum product processing and quality control. To improve the selectivity of STI CMP processes, surfactants were applied to selectively polish silica as opposed to silicon nitrides surfaces. A ten-fold increase in selectivity over conventional colloidal silica slurry was achieved by the addition of sodium dodecyl sulfate ͑SDS͒ and pH adjustment. Adsorption characteristics of SDS on silica and silicon nitride were measured as a function of slurry pH and concentration of SDS. As indicated by streaming potential measurements and solution depletion adsorption experiments under acidic pH conditions, SDS adsorption on silicon nitride was significantly higher than silica primarily due to the electrostatic interactions. It was concluded that the preferential adsorption of SDS on silicon nitride results in the formation of a materialselective self-assembled passivation ͑lubrication͒ layer leading to selective polishing. Effects of different alkyl chain length of surfactants were tested. Various mixed surfactant systems were tested and it is believed that the addition of second surfactants promotes the adsorption on silica diminishing selectivity. The material-targeted boundary layer lubrication concept may be used to develop selective CMP polishing slurries.The fabrication of next-generation devices will require advances in shallow trench isolation ͑STI͒ technology. 1-3 STI chemical mechanical polishing ͑CMP͒ is particularly challenging due to the highly integrated and variable pattern density as well as smaller pattern sizes. Less than optimal CMP system can cause several defects such as dishing, nitride erosion, and failure to clear oxide that are detrimental to global planarization. To minimize such defects, current STI CMP processes comprised multistep or raw structure modification such as reverse mask, dummy active area, and additional active area. 1 For better productivity and process simplicity, a minimum number of processing steps is highly desirable. Accordingly, "high selectivity single-step" slurry designs are being widely investigated. 4,5 The term "selectivity" in this paper is defined as the ratio of material removal rate ͑MRR͒ of silica to that of silicon nitride as described below. It is the slurry property as to how much more it can polish silica than silicon nitride Selectivity = Material Removal Rate of Silica Material Removal Rate of Silicon Nitride ͓1͔In general, conventional silica-abrasive-based STI CMP slurries exhibit selectivity in the range of 3-4. 5 Besides its influence on planarization, high selectivity slurries are known to provide more reliable endpoint detection capability. Generally, if the oxide to nitride selectivity is greater than 15, monitoring wafer carrier motor current can be utilized for efficient endpoint detection. 6 In recent years, ceria-based abrasives have shown potential for high selec...

Use of anionic surfactants for selective polishing of silicon dioxide over silicon nitride films using colloidal silica-based slurries

“…Beyond pH 5, the RRs remain unaffected. A similar dependence of the nitride RRs obtained using SDS on pH was shown earlier [12]. DP also suppresses the nitride RRs to ∼1 nm/min in the pH range 2-4 but does not affect them above pH 4 while SLS suppresses the nitride RR most effectively at pH 4.…”

“…5 shows the potential values of 1 wt% silicon nitride particles dispersed in water with and without adding the surfactants. In the absence of any additives, the IEP of silicon nitride particles is ∼5, same as that reported earlier [12,14,27]. The additives reversed the charge of the silicon nitride surface (except for SLS below pH 3) for pH <5 with the potential reaching relatively high negative values, except for SLS and K 2 SO 4 .…”

“…In contrast, there is a significant amount of DBSA remaining on the silicon nitride surface at both pH 2 and 3, but the amount drops to <1 × 10 −2 mg/m 2 beyond pH 4. A similar adsorption of SDS was found on silicon nitride surface by Bu and Moudgil [12]. These data suggest that DBSA, unlike K 2 SO 4 , strongly binds to the nitride surface by adsorption driven by electrostatic interactions at pH 2 and 3.…”

“…Hence, in our case, the additive concentrations were chosen to be close to the CMCs of SDS and DBSA which are ∼0.25 wt% and ∼0.15 wt%, respectively [12,18]. Since the CMCs of DP and SLS are unknown, 0.15 wt% concentration was also chosen for them.…”

“…Even though silica slurries produce lower oxide RRs compared to ceria and, hence, lower oxide-to-nitride RR selectivity, they have several advantages such as low cost, controllable surface morphology, wide pH range stability and considerably fewer defects. A limited number of additives for silica-based slurries have been investigated to achieve the desirable high RR selectivity with minimal nitride loss [9][10][11][12]. Among them, surfactants, especially those selected from the family of sulfonic acid salts, appear to be capable of producing high oxide to nitride RR selectivity [11].…”