Higher reliability with IBM glass substrate disks

IBM has recently introduced a new glass substrate disk design for its industry-leading disk drives. Glass substrate disks -- now available in 10,000 RPM hard disk drives -- are much stiffer and more robust than aluminum/magnesium substrate disks. As a result, these glass-based disks can potentially provide much higher reliability than traditional disks over both the short and long term.

A more reliable disk substrate
When selecting disk drives, end-users typically look for three key characteristics: they want disk drives that operate faster, provide better reliability, and hold an ever-increasing amount of information. To address all three of these advantages, IBM has begun to manufacture glass substrate disks that provide the following benefits:

• An approximately 20 percent improvement in the uniformity of the magnetic film surface to increase disk reliability
• A significant reduction in overall surface defects to help reduce read/write errors
• Significantly greater fly-height margin to minimize head-to-disk contact
• Better stiffness to reduce disk dynamics
• Greater ability to withstand shock and damage

Together, these benefits enable higher capacities, better reliability, improved data integrity, and longer lasting disks that are much less susceptible to physical damage -- all of which provides end-users with much more cost-effective disk drives. Read on to learn more about how glass substrate disks are enabling new levels of disk drive reliability.

	Aluminum/magnesium substrate Glass substrate Figure 1. A microscopic photo of a hard disk manufactured with a glass substrate illustrates how significantly smoother the glass substrate disk is compared to a disk manufactured with an aluminum/magnesium substrate.

Higher reliability
In an ongoing effort to reduce the possibility of disk drive errors, improve data accuracy rates, and increase data availability, IBM is now using glass substrates to manufacture disk drives. These drives feature smoother surfaces than the traditional nickel phosphorus-plated aluminum/magnesium substrates, a change that reduces the number of surface irregularities that can sometimes affect reliability and data integrity. For example, the manufacturing process that produces nickel phosphorus-plated aluminum/magnesium substrates mechanically applies a scratch pattern to the disk surface. This preparation technique increases the potential of surface irregularities, to the possible detriment of recording reliability and data integrity.

When magnetic film is applied on top of the substrate, this multilayer structure follows the same disk surface pattern of microscopic peaks and valleys -- increasing the possibility of minor read-back errors, while simultaneously requiring a higher fly-height margin to avoid disk-to-head contact. The manufacture of glass substrate disks does not require any modification of the polished substrate surface. The better surface finish and increased hardness of glass results in a more rugged disk with superior performance and reliability.

Higher areal density
The complete multilayer structure used in the IBM Ultrastar 18LZX and 36ZX disk drives with areal densities of 3.7 Gbits/sq. in., is potentially extendable to a much higher number by relatively small alloy composition changes. This enables a platform that can continue to provide outstanding disk recording capacities, higher areal densities and reliability, and increased performance over time.

Lower fly height
Minimizing the interaction between the read/write head and the disk surface can improve the accuracy of drive operation. As the distance between the head and the disk surface decreases, so does the potential for read/write errors. By reliably implementing lower fly heights in a highly error-tolerant design, IBM has become the industry leader in minimizing head-to-disk interaction in high-end server disk drives. As a result, IBM disk drives feature industry-leading soft and hard error rates (also know as recoverable and nonrecoverable error rates).

Better disk quality
The physical properties of the disk substrate also can have a significant impact on overall disk performance. As the disks rotate in the drive and the internal temperature increases, the disks are part of a dynamic environment involving many materials with different properties of thermal expansion. In addition, at the higher rotation speeds of IBM server disk drives, air friction may possibly cause slight movements of the disk outer edge. Any unpredictable movement of the disk increases the potential for track mis-registration (TMR) problems, which can delay the arrival of the head at the desired disk track.

Using a more rigid disk material -- such as glass -- reduces the amount of unpredictable movement a disk experiences, thereby increasing drive robustness. This design is especially valuable for disks that operate at a very high number of rotations per minute, such as the 10,000 RPM IBM Ultrastar disks.

Summary
By improving the types of materials -- such as glass substrates -- used in the manufacture of hard disk drives, IBM continues to lead the industry in improving quality and setting new standards for high-reliability, cost-effective disk drives.