Microcantilevers for Atomic Force Microscope Data Storage

The information age imposes an ever-increasing demand on the capacity and speed of data storage devices. Magnetic disk drives form the bulk of today’s non-volatile storage devices, with bit areal densities in the Gbit/in² regime in high-end commercial disk drives. Read/write data rates reach into the 100 Mbit/s range. In the laboratory, bit densities as high as 10 Gbit/in² have been reported [1.1]. Many technological advances, such as giant magnetoresistive (GMR) read heads and improved materials for storage media, promise to keep magnetic storage on its steep growth curve, which has achieved an annual 60% rise in bit areal density in recent years [1.2]. Compact disk (CD) technology has also made substantial inroads into the computer/consumer product market, mostly in the form of read-only memory (CD-ROM) devices but with some rewritable devices as well. Conventional CDs provide a storage density of approximately 0.6 Gbit/in², and the latest CD-ROM drives feature readback rates on the order of 10 Mbit/s.

This chapter describes a micromachined cantilever with an integrated heating element in the form of a lightly doped, high-resistivity region near the tip of the cantilever. The rest of the cantilever is heavily doped and has low electrical resistance (Fig. 2.1). When electric current flows through the cantilever, power is dissipated mainly within the high-resistivity heater region, leading to a localized temperature rise at the tip. Thermal writing can be accomplished when the tip is brought into contact with a polycarbonate substrate.

One important consideration in the design of the heater-cantilever is how fast it can be heated and cooled, which ultimately determines the maximum data writing rate. The cooling rate of the heater region can be represented by a figure of merit called the thermal time constant. Methods of measuring and reducing the thermal time constant of a heater-cantilever were studied. Time-domain and frequency-domain analyses were performed. Finite-element models were developed to help explain the experimental results. Finally, the dynamic thermal behavior of cantilevers of different geometries was evaluated, and a mathematical optimization was performed in order to determine the optimal design with respect to thermal time constant and mechanical resonant frequency.

The piezoresistive cantilevers in our AFM data storage system are subject to several design requirements. First, the cantilevers need to have sufficiently low mechanical stiffness to enable low-wear operation on polycarbonate. Second, the cantilevers need to have sufficiently high piezoresistive sensitivity to be able to detect topographic steps on the order of 100 Å. Third, the cantilevers need to have as high a resonant frequency as possible for high readback rate. To satisfy these requirements, cantilevers of thickness 1 μm or below have been fabricated. This is at least two times thinner than previously reported piezoresistive cantilevers, such as those of Tortonese, et al. [4.1, 4.2]. The fabrication process of Tortonese, et al., had to be modified before it could be applied to 1 μm thick cantilevers. The modifications involved, as well as the measured performance of the fabricated cantilevers, will be described.

In the preceding chapters it was shown how reading and writing in AFM data storage can be accomplished with simple planar cantilevers incorporating the appropriate functionality. To form a complete storage system, however, a data tracking method is required as well. This is because of the inevitable runout that occurs when a data disk is not precisely centered with respect to the spindle axis (see Fig. 5.1). In this situation, the read head needs to be able to move from side to side in order to follow a particular data track on the spinning disk. In addition, there needs to be a mechanism by which the read head senses any deviation from the track. Finally, the deviation signal—usually called the tracking error signal (TES)—needs to be processed by a servo controller that constantly adjusts the position of the read-head actuator in order to maintain tracking.

This chapter describes two main applications of the dual-axis piezoresistive cantilever: (a) AFM data tracking, for which the device was originally designed, and (b) lateral force microscopy (LFM). For AFM data tracking, the cantilever was mounted on a modified compact-disk actuator controlled by a two-channel servo controller. The lateral force signal from the cantilever, generated by the interaction between the cantilever tip and the data pit sidewall, was used as input to a negative feedback loop within the servo controller in order to keep the tip on track.

In this work, the design, fabrication and testing of advanced silicon micromachined cantilevers for high-density AFM thermomechanical data storage was accomplished. Three types of cantilevers were fabricated to provide the functionality of three major components in a AFM thermomechanical data storage system: a vertically sensitive readback sensor, a thermal writing head, and a tracking mechanism based on a lateral force sensor. These three components are based on micromachined elements with integrated sensing/heating capability, and eliminates the need for bulky external accessories such as lasers. The results of this work represent a major step towards the development of a compact, low-cost, high-density data storage device.