Tracks in Hard Disks
Storage capacity also depends on the narrowness of the tracks, and so far manufacturers have been able to cram up to 20,000 tracks per inch. This number is limited by various factors, such as the ability of the recording head to resolve the different tracks and the accuracy of its position-sensing system. Squeezing in additional tracks will require significant mprovements in several areas, including the design of the head and the actuator that controls that head. To achieve an overall density of I 00 gigabits per square inch, the industry must somehow figure out a way to fit about 150,000 tracks or more per inch.
With the existing technology, tracks must be separated by gaps of 90 to 100 manometers, according to analyists. Most write heads look like a horseshoe that extends across the width of a track. Recording is in the longitudinal direction [that is, along the circular track], but they also generate fringe fields that extend radially. If the tracks are spaced too closely, this effect can cause information on adjacent tracks to be overwritten and lost.
One solution is to fabricate the recording head more precisely to smaller dimensions. “You can use a focused ion beam to trim the write head and to narrow the width of the track that a writer writes,” one researcher says. But the read head, which is a complex sandwich of elements, poses a harder manufacturing problem. Furthermore, for 150,000 tracks or more per inch to be squeezed in, the tracks will have to be less than about 170 manometers wide. Such microscopically narrow tracks will be difficult for the heads to follow, and thus each head will need a secondary actuator for precise positioning. (In current products, just one actuator controls the entire assembly of heads.)
Last, smaller bits in thinner tracks will generate weaker signals. To separate those signals from background noise, researchers need to develop new algorithms that can retrieve the information accurately. Today’s software requires a signal-to-noise ratio of at least 20 decibels. According to some analyists, current industry is at least six decibels short of being able to work with the signal-to-noise ratio that would apply when dealing with the bit sizes entailed in disks with areal densities of 100 to 150 gigabits per square inch.
Nevertheless, such problems are well understood, many industry experts concur. In fact, analyists assert that the improvements in materials, fabrication techniques and signal processing already being studied at IBM and elsewhere will, over the next few years, enable the manufacture of disk drives with areal densities in the range of 100 to 150 gigabits per square inch.
The introduction of thin-film heads took nearly 10 years. The transition from that to magnetoresistive technology required six more years because of various technical demands, including separate read and write elements for the head, a manufacturing technique called sputter deposition and different servo controls.
But the switch to giant magnetoresistive drives is occurring much faster, taking just between 12 and 18 months. In fact, IBM and Toshiba began shipping such products before the rest of the industry had fully converted to magnetoresistive heads.
The quick transition was possible because giant magnetoresistive heads have required relatively few changes in the surrounding disk-drive components. According to researchers, the progression to drive capacities of 100 gigabits per square inch will likewise be evolutionary, not revolutionary, requiring only incremental steps.