Contamination challenges run the gamut: particles to molecules, outgassing to static
In the disk drive business today, the figure of merit is areal density.
Earlier in the decade, areal density was increasing at a rate of 60 percent per year. However, “we're going at 100 percent a year now,” says John Monroe, chief analyst for rigid disk drives at the San Jose, CA, market research concern Dataquest Inc. Monroe expects the storage explosion to continue as areal density rises from today's state-of-the-art six gigabits per square inch.
Increasing areal density translates into tighter specifications for disk drive cleanrooms. There's a corresponding effect on contamination control, as well as increased need to control static.
To further complicate matters, market researcher Disk/Trend Inc. (Mountain View, CA) forecasts the price per megabyte of storage to drop by half every year for the next few years, from 2.4 cents per megabyte to 0.6 cent from 1999 to 2001. So cost and expense control are of paramount importance. This is clearly an industry in transition.
Meeting the challenges
There are a number of ways to achieve the necessary increases in areal density. Bit lengths can shrink on disks, and heads can fly closer to the surface. Both of these increase spatial resolution and allow more data to be packed into the same area. But the enabling technology in all this change is sensor technology, which itself is undergoing a major change. Disk drive makers are deploying giant magnetoresistance (GMR) technology, which utilizes a phenomenon rooted in quantum mechanics to improve the sensing of magnetic fields.
“By the end of the year our entire product line, which is 25 million drives, will have switched over to GMR,” says Pat McGarrah, technology program director for Quantum Corp. (Milpitas, CA)
According to McGarrah, Quantum shipped its first retail GMR drive in April 1999. Others, such as Western Digital Corp. (Irvine, CA) and IBM Corp. (Armonk, NY), are also planning a switch to GMR this year. Although Seagate hadn't announced a GMR product by mid-year, it has already demonstrated a 16-gigabit-per-square-inch GMR prototype. According to Nigal Macleod, vice president for advanced concepts at Seagate, that demonstration was not of some exotic technology.
“The technology that we used in that demonstration was essentially the same technology we will be shipping this year in six gigabit products,” says Macleod.
Heading off problems
Areal density doubling, and the attendant scramble to implement new technology, has some important implications for disk drive cleanrooms.
A disk drive is a complex assembly, complete with a sensitive read/write head, magnetic media coated disks, electronic circuit boards, motors and other components. Parts are actually manufactured in several cleanrooms, which have cleanliness levels that vary from better than Class 10 to more than Class 1000.
For read/write heads, the switch to GMR is accentuating some old problems and putting some new constraints on cleanrooms. “On our advanced technologies, we are now trying to pattern things that are sub-half micron,” notes John Spangler, executive director of wafer process engineering in the Bloomington, MN, recording head manufacturing operation of disk drive maker Seagate Technology, Inc. (Scotts Valley, CA). “There are some films critical for our sensor performance that are as thin as 10 angstroms (0.001 micron).”
Spangler says that the film thicknesses found in the latest read/write head technology range from 10 angstroms to 7 microns.
The GMR effect creates heads that are three times more sensitive than previous technology, but it also requires laying down multiple thin conducting and insulating layers. This is initially done on the wafers, which are then diced to separate the individual sensing elements. But unlike semiconductors, these sensing elements cannot have built-in static protection. Therefore, today's parts can tolerate static levels, and associated electrostatic discharge (ESD) events, that are on the order of only 25 volts or so.
“It's gotten much more sensitive to ESD events as we've progressed over time,” acknowledges Spangler of Seagate. “When we have 100-angstrom films, it doesn't take much to blow them up. “
Spangler goes on to say that ESD is more of a problem than particles themselves. ESD is being tackled in several ways. Spangler notes that some manufacturers are working on sub-one-volt factories. That is, the static levels for the manufacturing process will be controlled so as to be less than a volt. This is in anticipation of tomorrow's films, which will be even thinner than today's.
One way to achieve this, while still moving the air necessary to keep down particle levels, is through the use of ionizers. Ionizing technology is being used increasingly in the disk drive industry. Sensitivity to static discharge continues throughout the manufacturing process, although resistance to ESD climbs as more components are added.
Besides ESD, Spangler foresees other contamination control issues growing in importance as film thickness shrinks. These, however, are not directly related to the cleanroom itself.
“Things that are going to be difficult for us going forward are going to be machine controls. We have much more difficult problems with equipment flaking and things like that,” he says.
Seagate recording head operation ballroom-style cleanroom.
Putting it together
After the heads are assembled into a stack, they have to be combined with the disks and base motors in another cleanroom operation. The motors and precision machine tooled parts have to be cleaned before they enter the cleanroom. Special adhesives and lubricants ensure that the mechanical sub-assemblies don't cause problems with the final system assembly or disk drive operation.
However, it's the mating of the head to the disk that requires the most care and the tightest contamination control. MKE, for instance, performs such mating in a fully automated Class 100 cleanroom. According to Mark Camenzind, a senior research chemist at Balazs Analytical Laboratory (Sunnyvale, CA), during manufacturing disk drives are relatively immune to some airborne contaminants, but that's not the case for acids, high boiling organics such as phthalates and silicones, and other reactive compounds.
“The magnetic media on the disk drive platter are very susceptible to corrosion,” says Camenzind who is active in the Santa Clara, CA, based International Disk Drive Equipment and Materials Association (IDEMA) microcontamination committee and the newly formed cleanrooms subcommittee [see “Disk drive standards on fast track,” CleanRooms, April 1999, page 1]. “They're fairly reactive metals.”
Camenzind points out that many of these metals are sputtered onto the disk. Because they are so reactive, these sputtered layers can be exposed to cleanroom air for only short periods of time. The same need for contamination control is true prior to sputtering. Airborne contaminants such as acids and organics on the surface can cause delamination of sputtered layers. In addition, acids may corrode the media after deposition or in the drive.
Manufacturing is made easier because the sputtering can take place in a chamber or in a cluster tool. Careful selection of cleanroom materials can reduce the amount of outgassing, and thus cut the amount of airborne contaminants. Camenzind notes there's still the need to clean up the air in the disk drive production area through the use of carbon, acid or base filters in return and make-up air. One reason to do so is the final manufacturing step, which involves dipping the disks in a lubricant. This leaves behind a 10- to 15- angstrom layer that provides a low-friction surface for the head to glide over. Contamination can cause the lubricant to be thin in spots, leading to media abrasion or head sticking.
For all these reasons, Camenzind reports that routine monitoring of airborne acids, bases and organic contaminants is becoming more common in disk drive cleanrooms.
Even after the disk drive is assembled and sealed, however, contamination control has to continue.
“As flying heights get lower, from a contamination perspective, you've got to keep that environment that you've now created inside the drive as clean as possible,” contends Dennis Mette, contamination control engineering manager for Quantum.
Because the drive is sealed, except for filtered breathing openings, one of the major problems is outgassing from the mixture of components, adhesives and lubricants inside the drive itself. Therefore, Mette says contamination control actually has to begin during the design phase, with the selection of materials. Not only do these have to perform a given function, but their properties and interactions during drive operation have to be well understood. Mette follows a six-stage contamination control model that looks at design, process/tooling, facilities, people, consumables, and parts. Each of these, he says, has to be considered in the final solution.
These contamination control challenges are having an effect beyond the cleanroom. For one thing, there's the IDEMA cleanrooms subcommittee. According to both Camenzind and Seth Ayers, who was IDEMA's standards program manager until August, the subcommittee is attempting to hammer out some practices that will be accepted and useful to the industry as a whole. The working title for the document is “Guidelines for construction, operation, and monitoring of disk drive cleanroom controlled environments.”
The idea is to not reinvent the wheel. Camenzind notes that where applicable, the recommended practices and guidelines for other organizations, such as the Mount Prospect, IL-headquartered Institute of Environmental Sciences and Technology (IEST), will be cited. Only those areas peculiar to the disk drive industry will be covered in full detail in the final document. The document is still being worked on and is expected to be available for balloting late in 1999.