CMP of hard disk substrate using a colloidal SiO₂ slurry: preliminary experimental investigation

Abstract

With magnetic heads operating closer to hard disks, the hard disks are forced to be ultra-smooth. At present, chemical–mechanical polishing (CMP) has become a widely accepted global planarization technology. In this paper, the effects of SiO₂ particle size, and the contents of SiO₂ particle, oxidizer and lubricant additive in the prepared slurry, as well as pH value of the slurry, on the polishing performances in the CMP of hard disk substrates with nickel–phosphorous plated were investigated. Results indicated that the average roughness (R_a) and the average waviness (W_a) of the polished surfaces as well as material removal amount were much dependent on all of the factors above. For comparison, CMP of hard disk substrates with a kind of commercial SiO₂ slurry was conducted under the same polishing conditions. Based on Auger electron spectrogram (AES) examinations of the chemical changes in the polished surfaces with the prepared slurry, the CMP mechanism was deduced preliminarily.

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 (R_a) and the average waviness (W_a), 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.