On the Future of Head-Based Microactuators in Hard Disk Drives
- Author(s): Wernow, Josiah Natan
- Advisor(s): Horowitz, Roberto
- et al.
Dual-stage actuation is an important technology enabler for advancing hard disk drive capacity and performance. This thesis describes the ongoing trend of decreasing track pitches required for areal density growth and increasing disturbances arising from rising spindle speeds, demonstrating that future dual-stage actuators will need to overcome the bandwidth limitations faced by the current suspension-based devices. First, the state of the art for dual-stage actuation is presented in the form of a literature review. This is followed by an overview of a new servo-mechanical design and analysis technique which is a hybrid of Galerkin beam elements and optimal linear quadratic Gaussian control analysis. This new technique is used to demonstrate the potential performance benefits of head-based actuation over suspension-based actuation. With the driving goal of designing a new head-based actuator, the constraints posed by industry are presented and discussed in detail. This is followed by an overview of the various actuation mechanisms evaluated, including several electrostatic and piezoelectric devices. The two most significant challenges identified through this analysis are the introduction of a gap at the trailing edge of the air-bearing and the necessity to actuate traces with sufficiently high actuation force.
A quasi-shear mode piezoelectric actuator is proposed as a cost-effective head-based actuator which meets the given criteria and can be implemented at the wafer-scale. A new prototype was fabricated at the Berkeley Nanofabrication Laboratory with a measured bandwidth above 50 kHz. Since the proposed head-based actuation scheme requires a gap in the air-bearing surface, a second actuation mode is also presented. It is shown that the gap width between the slider body and read/write head can be controlled independently of track-following displacement in order to minimize pressure loss at the air-bearing surface. It is also shown that the directionality along each actuation mode of this device is greater that 1:10, although the resolution of measurement tools makes it difficult to determine how much greater the directionality is.
The two key figures of merit for this project are microactuator bandwidth and static displacement gain. For the prototype presented herein, the bandwidth is shown to be greater than 50 kHz, which is the highest frequency measurable by the available equipment. The static gain is measured to be approximately 1 nm/V. With sol-gel based stacking methods, however, the static gain could be scaled as high as 20 nm/V or more, although this will require further investigation by industry to develop. The intellectual merits of this project include a newly developed servo-mechanical design tool, modified beam equations with piezoelectric energy taken into account, and several new fabrication methods. The new fabrication methods include chemical mechanical polishing of PMN-PT, eutectic bonding of PMN-PT to silicon, scribing high aspect-ratio geometry in PMN-PT with a dicing blade, epoxy drop deposition with flipchip bonding, and fabrication of tall MEMS structures with sacrificial copper plating with a wet etch release.