Constraints on removal of Si 3 N 4 film with conformation-controlled poly(acrylic acid) in shallow-trench isolation chem
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Jea-Gun Parkb) Nano-Silicon-On-Insulator (SOI) Process Laboratory, Hanyang University, Seoul 133-791, Republic of Korea (Received 28 March 2007; accepted 13 June 2007)
The effect of changes in poly(acrylic acid) (PAA) conformation on removal of Si3N4 film was investigated. PAA was used as a passivation agent by adsorption on an Si3N4 film in shallow-trench isolation chemical–mechanical planarization (STI CMP). Adsorption behavior of PAA on the Si3N4 film and the conformation transition were determined by adsorption isotherms and force measurements using atomic force microscopy (AFM) as a function of ionic strength. AFM results revealed that, as ionic strength increases, the repulsive force between the negatively charged carboxylate groups along the backbone of PAA is reduced due to counterion screening and to the changes of PAA conformation from a stretched to a coiled configuration. At high ionic strength, the coiled conformation of PAA formed a dense passivation layer on the Si3N4 film, which led to suppression of the removal rate of Si3N4 film from 72 to 61 Å/min in the STI CMP process.
I. INTRODUCTION
In complementary metal-oxide semiconductor (CMOS) technology below 0.5 m, local oxidation of silicon (LOCOS) causes problems, such as the “bird’s beak” and the “channel-stop.”1 To resolve these serious problems, shallow-trench isolation (STI) technology in conjunction with the chemical–mechanical planarization (CMP) process has replaced LOCOS. In particular, for advanced logic, dynamic random-access memory (DRAM), and other memory devices, STI technology has been adopted as the primary isolation method.2 This technology not only enables the active area to become very small but also allows for increases in the device packing density. For formation of STI, the gap-filling oxide film should be uniformly planarized3 and polishing should be stopped at the Si3N4 film by way of the CMP process.4 Therefore, oxide-to-nitride removal selectivity has been regarded as one of the most important factors for STI CMP.4,5 To achieve a high oxide-to-nitride removal selectivity, selective adsorption of polymeric molecules onto Si3N4 films is necessary.4–11 At moderate pH, the difference in the surface potential between the Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/JMR.2008.0031 J. Mater. Res., Vol. 23, No. 1, Jan 2008
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SiO2 and the Si3N4 films enables the selective adsorption of anionic polymeric molecules. This selective adsorption prevents the Si3N4 films from being polished, which results in improved removal selectivity. In particular, removal selectivity is affected mainly by the adsorption behavior of polymeric molecules adsorbed on the Si3N4 films, which is one of the key issues to be considered to achieve a high STI CMP performance. Poly(acrylic acid) (PAA) has been widely used as an organic additive in STI CMP due to its negatively charged carboxylate group at a pH above the pKa of carboxy
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