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Plasma etching requires the nanometer scale precision control of the etched structures without particle generation which is a key factor of the defect and the yield ratio. Nevertheless, process drifts [1] and particles flaking off from the chamber wall are frequently observed in case of non-volatile metal accumulation on a chamber wall, because cleaning of metal by-product is more difficult than other by-products such as silicon and carbon. R Ramos et al [2] have reported the effective cleaning to reduce AlFx residue. However, sometimes, an etching of non-volatile chamber wall materials such as Al2O3 and Y2O3 at the same time by metal cleaning chemistries becomes the root cause of the particle and decreasing MTBC(mean time between clean). Therefore, it is important not to generate particles flaking off in mass production.
In this study, effective AlFx cleaning was evaluated without chamber wall damage by microwave ECR plasma etching system. At first, effective AlFx cleaning was quantified by using in-situ FTIR equipment attached to the chamber side wall. The FTIR data showed that BCl3/Cl2 chemistry was higher cleaning efficiency than other chemistries such as Cl2, SiCl4/Cl2 and SF6/O2 for removing AlFx residue as mentioned previous study [2]. Second, Al2O3 etch rate distribution were measured using Al2O3 coupon sample simulating AlFx deposition. In this experiment, Al2O3 coupons were set in different locations such as top plate, sleeve and susceptor near the wafer stage. One of the unique characteristics of ECR plasma is that plasma generation position (ECR height) is movable by changing solenoid coil setting. After exposing BCl3/Cl2 plasma with two kinds of ECR heights at 146.8 and 197.2 mm from ESC (Electrostatic chuck) position, Al2O3 etch rates were measured. High ECR plasma (197.2 mm) showed higher Al2O3 etch rate at high chamber location such as top plate, while low ECR plasma (146.8 mm) showed higher Al2O3 etch rate at lower chamber location such as sleeve and susceptor. The scanning ECR height improves clean uniformity in the chamber. Also, ECR plasma has an advantage in reducing chamber wall damage by spattering non-volatile chamber wall materials with plasma to be able to control ECR height away from chamber walls.
To evaluate long-term process stability of BCl3/Cl2 cleaning, marathon tests of 500 alumina wafers were executed. The Al2O3 wafers were etched by fluorine-based gas chemistry to generate AlFx residue. The two kinds of plasma cleaning every three Al2O3 wafers were evaluated. One was BCl3/Cl2 cleaning and the following SF6/O2 cleaning. The other was only SF6/O2 cleaning. As a result, particle count was stable during 500 wafers with BCl3/Cl2 cleaning while particle increased after 200 wafers without BCl3/Cl2 cleaning. The increased particle component was mainly Al. Therefore, the ECR plasma with scanning BCl3/Cl2 metal cleaning which could remove Al2O3 at the every corner, enables us to increase MTBC in etching mass productions including metal residues.
References.
[1] Kosa Hirota, et al, J. Vac. Sci. Technol. A 32(6)，61304 (2015).
[2] R Ramos, et al, Plasma Sources Sci. Technol. 16, 711-715 (2007).
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