In chemical mechanical polishing ͑CMP͒, the polishing rate of Ge 2 Sb 2 Te 5 ͑GST͒ films increases sharply with H 2 O 2 concentration up to 0.2 wt % and then increases slightly with a further increase in H 2 O 2 concentration up to 3.0 wt %. However, the polishing rate of SiO 2 films increases very slightly with H 2 O 2 concentration up to 3 wt %. Polishing selectivity of GST films to SiO 2 films of over 100:1 can therefore be achieved by adding H 2 O 2 to the slurry. To understand the mechanism of GST CMP with H 2 O 2 , we investigated the chemical reaction behavior on the GST film surface using potentiodynamic measurement and X-ray photoelectron spectroscopy.Phase-change memory ͑PCM͒ is one of the candidates for nonvolatile memories in the future memory market. This is because PCM offers a large signal sensing margin, moderately fast programming speed, good endurance, good scalability, and compatibility with the complementary metal oxide semiconductor logic process. 1 However, there has been some concern that problems, such as its high reset current and the thermal cross talk, could arise when a PCM is scaled down below the 30 nm design rules. 2 These problems can be solved by improving the material properties and using an appropriate device structure. 3-5 A recent confined cell structure has shown a reset current that is only half that of the conventional planar cell structure, because of its small programming volume ͑phase-change volume͒ that directly contacts to a top electrode. In the fabrication process for the confined cell structure, chemical mechanical polishing ͑CMP͒ has been adopted after the deposition of Ge 2 Sb 2 Te 5 ͑GST͒ on the bottom electrode contact followed by patterning the confined cell. 4 For the successful formation of the confined cell structure, the GST polishing should be stopped when the SiO 2 film is exposed to reduce CMP-induced defects, such as erosion and dishing. Thus, a high GST-to-SiO 2 polishing selectivity is a critical factor in the GST CMP process. In our previous work, we added tetramethylammonium hydroxide ͑TMAH͒ to colloidal silicabased slurry as the alkaline agent to etch the surface oxide of an amorphous GST film during polishing and found that the added TMAH remarkably increased the polishing rate and polishing selectivity of GST to SiO 2 film. 6 In addition, a polycrystalline GST film tends to be selected as a phase-change material for PCM instead of an amorphous GST film because a polycrystalline GST film supplies a lower material resistance than an amorphous GST film. However, the polishing rate of a polycrystalline GST film is much lower than that of an amorphous GST film. Thus, a strong oxidizing agent such as hydrogen peroxide ͑H 2 O 2 ͒ should be added in the alkaline slurries. Recently, Zhong et al. investigated the effect of H 2 O 2 in reducing surface roughness after GST polishing. 7 However, the chemical reaction between H 2 O 2 and the GST film surface during polishing and the polishing mechanism have not been clarified yet.In this work, we investigated t...

We investigated the chemical mechanical planarization (CMP) mechanism of nitrogen-doped polycrystalline Ge 2 Sb 2 Te 5 (pc-GST) using nitric acid slurry without or with 1.0 wt % hydrogen peroxide (H 2 O 2 ). Without H 2 O 2 , the pc-GST film surface undergoes selective corrosion of Ge and Sb, resulting in pitting corrosion. Otherwise, with H 2 O 2 , the pc-GST film surface produces a Ge, Sb, Te oxide layer, thereby introducing a cyclic polishing process such as chemical oxidation, chemical and mechanical polishing, and sequential chemical oxidation. As a result, the polishing rate increased up to $1000 Å /min and the surface roughness decreased to 0.858 nm.Phase change memory (PRAM) is one of the most promising nonvolatile memories due to its large signal sensing margin, fast writing speed, outstanding endurance, long data retention time, and most importantly, scalability, for commercial high density production. 1-3 Recently, PRAM has been successfully manufactured with high density, which is quite close to commercial NOR flash memory. 4,5 To facilitate mass production of the high density PRAM, chemical mechanical planarization (CMP) has been employed to develop smooth and defect-free surface of Ge 2 Sb 2 Te 5 (GST) after gap filling in the confined cell structure. 4,6,7 Here, a polycrystalline state was preferable for the deposited GST rather than an amorphous state in order to achieve a higher speed and wider sensing margin. Moreover, polycrystalline GST (pc-GST) was inevitably used to avoid incomplete contact between electrodes and amorphous GST because amorphous GST shrinks by 5-7% in volume as it switches from an amorphous to a crystalline state due to the thermal process. 8 Hydrogen peroxide (H 2 O 2 ) is commonly used as the oxidizer in various CMP slurries because it has high oxidizing ability and does not generate ionic byproducts. Our group 9,10 has reported the CMP mechanism of nitrogen-doped pc-GST film in an alkaline slurry in which H 2 O 2 has been added. We found that the polishing rate of the pc-GST film increased as the H 2 O 2 concentration was increased. However, this resulted in severe pitting corrosion because the chemical passivation layer on the pc-GST film was difficult to form in the alkaline solution with H 2 O 2 . On the contrary, Zhong et al. 8 and Wang et al. 11 have conducted amorphous GST CMP with acidic slurries added with H 2 O 2 and some surfactant, and found enhanced CMP performance such as a higher amorphous GST polishing rate and lower surface roughness without inducing pitting corrosion. However, the CMP mechanism of the GST film with H 2 O 2 in the acidic environment was not discussed in detail. Moreover, the pitting corrosion on the pc-GST film surface was much more severe than that for the amorphous GST film surface, and the polishing rate of the pc-GST film was slower than that of the amorphous GST film. 9,10,12 Thus, in our work, we investigated the detailed CMP mechanism of the pc-GST film in an acidic slurry without or with 1.0 wt % H 2 O 2 , by analyzing the corrosion...

How pH and hydrogen peroxide (H 2 O 2 ) in colloidal silica-based slurry affect chemical mechanical planarization of polycrystalline Ge 2 Sb 2 Te 5 (pc-GST) film has been investigated. The polishing rate of pc-GST film increased when the slurry pH decreased or increased from neutral pH. In addition, pc-GST polishing rate was highly affected by adding H 2 O 2 , resulting in a high pc-GST polishing rate in the entire pH ranging from 2 to 11. However, a noticeable difference in surface roughness of pc-GST film polished was observed between acidic (pH 2) and alkaline (pH 11) regions. Low RMS of roughness as well as a high polishing rate of pc-GST film was obtained in the acidic pH region with 1wt% H 2 O 2 . In contrast, in the alkaline region a high RMS of roughness of pc-GST film was observed owing to enhanced selective corrosion between Ge, Sb, and Te elements. To investigate the different polishing behavior of pc-GST film between acidic and alkaline pH regions, surface characteristics of pc-GST film dipped in acidic or alkaline media were observed by scanning electron microscopy, X-ray photoelectron microscopy, and potentiodynamic measurement.