Data storage cleanroom design takes the next step
11/01/2002
by Thomas M. Coughlin
Enormous cost-control pressures in the industry have led to novel approaches to reducing the cost and required size of cleanroom manufacturing and assembly operations
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While maintaining some similarities to the cleanroom requirements of the semiconductor industry, the data storage industry has its own unique set of cleanroom requirements.
Disk drives use small electromechanical components such as actuators and motors as well as electrostatic discharge (ESD)-sensitive heads for writing and reading the information on the magnetic disks. Disk drive heads are actually two orders of magnitude more sensitive to ESD voltages than dynamic random access memory (DRAM) and are also sensitive to particles and gaseous contaminants that can cause heads to crash on the disks or corrosion and chemical reactions with the head and media materials, leading to the growth of defects on the disk.
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In addition, the enormous cost-control pressures in the industry have led to novel approaches to reducing the cost and required size of cleanroom manufacturing and assembly operations.
ESD: The culprit
The giant magnetoresistive (GMR) head used in modern disk drives is a sensitive electrical device prone to failure in the presence of excessive ambient voltages combined with a path to ground through the sensor.
Voltages can build up in the GMR sensor through contact with a charged object or via close proximity through induction. There is also evidence of related electromagnetic interference (EMI)-induced problems with uninstalled GMR heads due to the operation of everyday devices such as cell phones.1
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If the GMR sensor is grounded, the voltage drives a current through the sensor, which, if large enough, can cause serious damage to the device.
In addition to actual destruction of part of the GMR sensor, a smaller static voltage discharge can generate sufficient heat to raise the temperature of the complex magnetic layers in a GMR head. These layers and their placement in a magnetic recording head are shown in Figure 1.2
If the temperature is high enough—about 400 degrees Celsius—the anti-ferromagnetic layer that pins the free layer in the sensor will change its magnetic orientation from local ambient magnetic fields. Changes in the pinning layer reduce the sensitivity of the GMR head and lead to loss of signal-to-noise (SNR).
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At an even higher temperature—about 1,400 degrees Celsius—the sensor can start to melt, leading to loss of part or all of the magnetic sensor and either severe damage to or destruction of the GMR head. Even a non-destructive ESD event can leave the GMR sensor more susceptible to future ESD damage.
As the areal density of magnetic recording increases, the track width of the magnetic heads decreases. This leads to additional sensitivity to ESD voltages. Figure 2 shows the sensitivity of GMR head sensors to magnetic (loss of pinning layer orientation) and resistive (melting of the sensor layers) damage with ESD energy levels as a function of magnetic recording areal density and projected time.3
Figure 3 shows voltage levels for ESD failure of GMR heads using several ESD voltage models as a function of time and data capacity per 95mm disk.4 Note that ESD failure voltages using the Human Body Model will be less than 10V by 2003.
Gaining control
The increasing ESD sensitivity of GMR heads places significant requirements on the equipment and practices used in a disk drive cleanroom as well as the design of the disk drive and drive components.
One rule of thumb is to eliminate all floating conductors in the assembly areas. Gloves should allow static dissipation and tweezers should not be electrically conductive. Personnel should use grounding straps and test them before entering a cleanroom assembly area where unmounted GMR heads reside.
In addition, ionization of the cleanroom air discharges the conduction build-up on surfaces and particles in the cleanroom. Make sure that all grounding is adequate using ESD certification and testing of the facilities.
The magnetic recording heads or sliders are mounted on a gimbaled suspension that attaches to the head actuator to position the heads over the data on the magnetic recording disk during drive operation.
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The combination of the head and gimbaled suspension is called a head gimbal assembly (HGA). Electrical leads for the GMR head are also attached to the suspension. To avoid ESD-induced damage in the head during drive assembly and testing, the manufacturers of suspensions have included a shunt that can disconnect the leads of the test pads from the GMR sensor while the head suspension is placed in a digital electrical tester. Figure 4 shows such a shunt and the resulting protection from static voltages near the test pads.3 Figure 5 shows greater detail of the shunt.5
Static-dissipative coatings can be used to reduce the build up of static charge that can damage the GMR sensor. Figure 6 shows a static-dissipative coating that is applied to the trace flex-circuit on the head suspension to keep voltages from building up on the flex-circuit and possibly damaging the GMR head. The surface resistance with this coating is (5)106 to (6)108, while without the coating the surface resistance is at least four orders of magnitude higher. Thus, the average voltage decay time with the coating is <0.1 second, while without the coating the voltage decay time is >60 seconds.
The coating also provides protection from outgassing. With the coating the outgassing rate is 1.2 ng/cm2, while without it the outgassing rate is 46.6 ng/cm2.
Advanced design saves the day
People are a source of contamination, and anything that can be done to remove people from contamination-sensitive manufacturing environments will add greatly to product yield and performance.
The assembly of magnetic recording heads and media involves cleanroom practices as advanced as those in semiconductor manufacturing and more advanced in some areas such as ESD control, mentioned previously.
Photolithographic tools at the cutting edge of semiconductor manufacturing are used for magnetic head production to obtain today's narrow track widths. It may be that the lack of smaller line width photolithographic tools for head production will slow the growth in magnetic recording areal density over the next few years. In recent years, this growth has exceeded 100 percent annually—as much as twice the current Moore's Law for semiconductor density growth.
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An interesting aspect in the growth of magnetic recording areal density has been the increase in the time required to servo-write the disk drives due to the higher track density. If left unchecked, this would result in significant increases in the requirements for cleanroom space for drive servo writing because a disk drive is open to the ambient environment during the servo writing process.
Figure 7 shows projections for servo writers, servo-write time and cleanroom space required for disk drive production based on reasonable projections of track density growth over the next few years.6 However, due to the high cost of servo writers and particularly cleanroom space, disk drive manufacturers are looking for ways to do servo writing without the expense of additional cleanroom space.
This need has led to development of servo pattern printing of media, media servo writing (at the media manufacturing plant during the disk testing) and in-drive servo writing which can be done on drives outside of the cleanroom environment because the drive remains sealed.
Future data storage
ESD and cleanliness requirements will become ever more critical in disk drive component and drive manufacturing. This plus the need to reduce costs through better use of manufacturing space will lead to greater degrees of automation in manufacturing.
Removing the human element will also result in greater yields due to less contamination as well as less ESD and handling damage. Many of the tools developed for disk drive assembly are specialized but some of the techniques developed for this industry may find application in other assembly operations as well. One area that may benefit is the assembly of micro-electrical-mechanical systems (MEMS) devices that also involve complex electromechanical systems.
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Look for disk drive and drive component assembly and cleanroom practices to blaze new processes and tools that can be used in many other cleanroom assembly processes.
Thomas M. Coughlin, president of Coughlin Associates (Atascadero, CA), has degrees in physics and electrical engineering and 25 years of experience in the data storage industry. Coughlin has 40 articles, reports, and technical presentations to his credit and 10 patents granted or pending. He can be reached at [email protected].
References
- Kraza, Vladimir and Wallash, Albert, "The effects of EMI from cell phones on GMR magnetic recording heads and test equipment," Journal of Electrostatics, February 2002.
- Himle, Jenny Ariane, "Electrostatic Discharge Considerations for GMR Heads," Maxtor Corp.
- Wallash, Albert, "Electrostatic Discharge (ESD) in Magnetic Recording: Past, Present and Future," Quantum, www.wallash.com.
- Adams, Stan, "ESD Strategies: Past, Present and Future," Maxtor Corp.
- Hutchinson Technology Inc. ZFP Switch Shunt.
- "Data Storage Test/Process Equipment Report," Peripheral Research Corporation, 2002.