CMP of hard disk substrate using a colloidal SiO2 slurry: preliminary experimental investigation
Introduction
With the rapid increasing of the capacity of hard disk driver (HDD), the areal recording density is growing at an astounding rate of up to 100% annually in recent years [1], this forces magnetic heads to read smaller and weaker signals. To remedy this, scientists are working on technology that enables heads to fly closer to disks to strengthen the output signals [2], [3], [4], [5]. When the areal recording density reaches 100Gb/in.2, the flight height will have to be lowered to approximately 6–7 nm [6]. Even there is work being done on heads that actually touch the media surface, referred to as contact or pseudo-contact recording [7], [8]. With heads operating so close to disks, media must be ultra-smooth to avert head crashes [9], [10], a kind of damage to read/write heads, which is usually caused by sudden contact with the disk surface, head crashes can damage the date stored in the disk’s magnetic coating [11]. Furthermore, scratches, pits and other micro defects on the media surface, which may also lead to read/write wrong, must be decreased to minimum.
In order to meet these requirements, local and global planarization for the disk substrates are becoming more urgent and necessary, here local planarization <10 μm is a result of smoothing and filling, planarization beyond 10 μm is regarded as global planarization [12]. Accordingly, the average roughness (Ra) and the average waviness (Wa), which represents the finer or shorter spatial wavelength features and the longer spatial wavelength features of the surfaces, respectively, have become the most important quality parameters for disk substrates.
Although numerous traditional planarization technologies are available, chemical–mechanical polishing (CMP) is the only technology known to provide global planarization of topography with a low post-planarization slope [13]. CMP is a surface planarization method in which a wafer rotated against a polishing pad in the presence of silica-based slurry while applying pressure [14]. CMP can dramatically reduce topographical variations to a degree not possible with any other planarizing process [13], [15]. CMP can result in a surface angle <1° compared with spin-on glass that has a surface angle of <10° [16].
However, compared with a large amount of CMP studies for IC manufacturing, very few researches on CMP of hard disk substrate have been reported to date except a few patents on slurries [17], [18], [19], [20], [21]. And in these patents, the influences of slurry parameters such as abrasive size, abrasive content, oxidizer content and pH value on the polishing performances, as well as the CMP mechanism of disk substrate, have seldom been involved. In this paper, chemical–mechanical polishing of disk substrate with nickel–phosphorous (NiP) plated in a colloidal SiO2 slurry has been studied, the CMP mechanism was also discussed.
Section snippets
Preparation of colloidal SiO2 slurry
A certain amount of colloidal SiO2 solution with a total solid content of 30 wt.%, oxidizer containing ferric (Fe3+) salt and a carboxylic acid containing more than 10-carbon alkyl chain as lubricant were added to deionized (DI) water in a container under continuously stirring. After adjusting pH value to be between 1 and 3 by adding acid or alkali, the mixture was filtrated with a 1 μm pore filter. The prepared slurry keeps stable even after having been stored for 6 months.
Polishing tests
Polishing tests were
Effect of SiO2 particle size on the polishing performances
Effect of abrasive particle size on material removal rate and surface finish has been largely reported [22], [23], [24], [25], [26], [27], [28]. Xie and Bhushan [23] observed that the polishing rate increased with an increase in particle size, while Bielmann et al. [24] claimed that a decreased alumina particle size led to higher W removal rate and the local roughness of the polished W surfaces was insensitive to alumina particle size. Zhou et al. [25] found that neither small (10 nm) nor large
Conclusions
- 1.
A novel colloidal SiO2 slurry, which introduces a carboxylic acid containing a long alkyl chain as lubricant additive, was prepared for the chemical–mechanical polishing of disk substrates with NiP plated.
- 2.
The abrasive particle size, and the contents of oxidizer, lubricant additive and abrasive particle contained in the prepared colloidal SiO2 slurry, as well as pH value of the slurry, have a strong impact on the average roughness (Ra), the average waviness (Wa) and material removal rate. Under
Acknowledgements
The authors wish to acknowledge Drs. Pan Guoshun, Tu Xifu, Kwok H. Yuen, Sima Neng, Ma Hongtao, Liu Yiwei, Gao Feng and Hu Xiaoli, as well as engineers Fang Liang, Zhang Junling, Xu Hua, Qiu Haineng and Xie Jing, for their useful support. This work was financially supported by the Nanoscience and Nanotechnology Special Foundation of Shanghai (No. 0352nm058) and the National Natural Science Foundation of China.
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