Drive makers embrace new contamination control practices
By John Haystead
As recording heads get smaller, flying heights lower, and disk densities greater, clean manufacturing facilities and practices have become an increasingly critical requirement for disk drive manufacturing. In particular, the industry`s transition to magnetoresistive (MR) head drives has brought renewed attention to particulate control as well as molecular-level contamination in critical manufacturing steps, while electrical contamination (ESD control) continues to pose challenges. In addition to incorporating stricter contamination control requirements on all aspects of the manufacturing process from tooling, protocols, components, and consumables, the industry is responding with increased use of automation and robotics to reduce human-source contamination on manufacturing lines as well as improve cycle times.
MR heads set new cleanliness standards
There`s no question that the disk drive industry`s transition from thin-film-inductive to higher-density MR head technology has been the principle driver behind its escalating contamination control requirements. In 1997, IBM began manufacturing 3 Gbit/square inch drives and reports its MR heads have increased in areal density at a 60 percent compound annual growth rate (CAGR). If this rate continues, densities of 10 Gbit/square inch could be achieved by 2001 and 40 Gbit/square inch by 2004. MR-based products have also already become the largest segment of the drive market. According to the International Disk Drive Equipment and Materials Association (IDEMA; Sunnyvale, CA), shipments of MR heads and MR media will exceed 50 percent of the total worldwide drive volume this year.
Unlike the somewhat nebulous relationship of the past, cleanliness levels have a direct and clearly measurable impact on the yield levels of MR drives. To perform reading operations, MR heads use a bottom-mounted magneto-resistive stripe which detects changes in resistance caused by minute fluctuations in magnetic fields on the recording medium. If a particle becomes caught between the head and the disk, hitting the stripe, there is an instantaneous high rise in temperature changing its resistance characteristics and rendering it unable to read data.
This phenomenon, combined with reduced head/disk geometries, poses a whole new level of cleanliness challenges for drive manufacturers. Quantum Inc. (Milpitas, CA) is already manufacturing 100 percent MR drives, and “The technology has required an order of magnitude increase in cleanliness levels and dedication to controlling both particle and ESD contamination,” describes Paul Webb, Quantum`s director of process engineering.
“With flying heights approaching 1 microinch (25 nm), even half-micron (500 nm) particles will now be over ten times the size of the read/write gap,” explains Douglas Cooper, director of contamination control for The Texwipe Company (Upper Saddle River, NJ).
Yet, at the same time that MR technology has made the job of contamination control professionals exponentially more challenging, it has also provided them with the ammunition they need to justify their solutions to management. “Prior to the introduction of MR-head technology,” recalls Jack Murphy, senior contamination control engineer at Seagate`s Twin City Operations (Bloomington, MN), “it was inevitably a frustrating process trying to justify increased investment in cleanliness levels since it was difficult to generate a detailed cost analysis of the impact of contamination.” Now, “with MR heads,” says Murphy, “this problem has gone away. Today, the impact of contamination is very obvious and can be clearly demonstrated in the test area.”
Even as the industry gets its arms around MR technology, however, a next-generation technology is already being prepared for introduction that will raise the contamination control bar yet again. Disk drives with Giant Magnetoresistive (GMR) heads are expected to support areal densities over 10 Gbit/square inch and will be more than twice as sensitive as MR devices. Their greater sensitivity to the magnetic fields of disks, will not only make GMR heads better able to detect and read smaller recorded bits at higher data rates, but their higher signal levels will also help overcome electronic noise.
Over the next couple of years, GMR-drives will begin to enter volume production. In fact IBM`s Storage Systems Division (San Jose, CA) has already begun shipping GMR heads with an areal density of 3.38 Gbytes per disk in its high performance desktop products. The 1-inch-high “Deskstar 16GP” disk drive series offers capacities ranging from 3.2 Gbytes to 16.8 Gbytes with sustained data rates up to 12 Mbytes per second.
While there`s no doubt that GMR heads will put further pressures on contamination control systems and practices, the industry has not yet been able to fully predict the magnitude of their impact. Fortunately, however, while acknowledging that a significant additional level of improvement will be necessary, most observers don`t expect anywhere near the same level of impact as the initial transition to MR heads. “We`re probably already about 80 percent there with the upgrades introduced for MR technology,” according to Seagate`s Murphy.
Cleanroom requirements
With the arrival of MR and GMR head technology, drive makers are now beginning to take a second look at their overall cleanroom classification requirements. Even as Class 100 environments remain the norm for disk-drive assembly, the scaling down of head sizes, flying heights, and increased media densities have nevertheless required the industry to be concerned with smaller and smaller particles. “We`re already dealing with bacterial-sized particles, and it`s unclear whether overall cleanroom requirements will have to be upgraded or if there will be more use of minienvironments,” Quantum`s Webb observes. Like the semiconductor industry, the drive industry is now evaluating the tradeoffs between more easily controllable minienvironments and the flexibility of larger, open facilities.
Already, however, certain critical manufacturing areas are being operated at higher than Class 100 cleanliness levels. As explained by Seagate`s Murphy, drive makers generally try to create Class 10 islands around critical assembly steps such as the disk pack, the head-stack assembly (HSA) and the head-disk interface (HDI). Critical areas generally include any step where the media is being handled. The HSA refers to the head together with its electrical connections to the interface card, while the HDI is where the HSA and disk pack are merged into the drive.
While acknowledging the potential benefits, Murphy doesn`t see the overall class-level requirements for disk-drive cleanrooms changing anytime soon. “For this industry it will be very difficult to establish a Class 10 environment throughout the assembly process.” At the same time, however, he sees the use of minienvironments as still in the experimental stage. This view is shared by Albert Ong, director of advanced manufacturing development, Western Digital (San Jose, CA). “We have considered minienvironments, but right now we don`t see any clear indication that this is the way to go,” he says.
Molecular contamination
As new requirements for particulate contamination control are being dealt with, the drive industry must now also pay increasing attention to molecular-level contamination as well. Although ionic and molecular contamination have always been a concern for drive manufacturers, today`s HDIs are increasingly susceptible to outgassing of organic materials with many potential sources. Drive components are composed of a number of different materials including surface-treated alloys, polymers, elastomers, and ceramics. These parts often need to be cleaned with aqueous solutions, non-ozone depleting solvents, CO2, or other methods. In addition, a variety of lubricants and adhesives are used, all potential sources of outgassed contamination.
The impact of molecular contamination can be severe. Contaminants from vapors and gases can condense on disk surfaces causing “stiction,” wear, and other performance problems. Stiction is the lateral force required to separate the HDI during the startup of the disk. High stiction levels can prevent the drive from starting causing damage to both the head and disk. At high rotation speeds, stiction will also cause excessive local heating and increased wear at the HDI which can then lead to particle contamination.
To protect the magnetic layer from damage, a sputtered or chemical vapor deposition carbon overcoat and a lubricant layer are customarily applied above the data (magnetic) layer of the disk. Given that a “disk surface will travel on average of 100,000 miles/year, poor lubricant coverage caused by molecular contaminants can dramatically accelerate wear rates,” describes Texwipe`s Cooper.
Molecular contamination can also lead to corrosion. For example, residual anions from chlorides and sulfates can absorb water ultimately destroying component surfaces. The principal anions of concern to the disk drive industry are chloride, sulfate, nitrate, phosphate, oxalate and formate, while cations include ammonium, cobalt and nickel.
As disk drives continue to shrink in size, lower and lower levels of contaminants become problematic. Since the volume of airspace within the drive is also shrinking, the concentration levels of volatile contaminants around critical components also becomes greater.
Molecular-level contamination must be addressed all the way from initial plant design to final product testing. A number of technologies are employed, including ion chromatography, to test for inorganic anions and cations, and nonvolatile residue analysis for organics, as well as conduct outgas testing for volatile contaminants. Adhesives are a particular and growing concern. “The level of attention paid to the contamination contribution of adhesives has increased fourfold over the last five years, with the list of approved adhesives getting shorter every year,” Murphy says.
Electrostatic discharge
With the industry`s transition from inductive to MR heads, a lot has also been learned about the importance of ESD control. “The level of susceptibility to ESD damage is extremely high in MR heads, and we soon realized that the acceptable practices of the past were no longer adequate, resulting in dramatic head failures and shifts in magnetics,” Webb observes.
However, “When it comes to controlling ESD,” Murphy describes, “there are very few silver bullets out there, and up to this point most controls have been pretty much common sense.” Approaches include new grounding techniques as well as ion-generation and ion-based airflow systems. In addition, however, manufacturers try to eliminate as much as possible the generators of triboelectric (electricity generated by friction) charging. Since non-conductive materials such as plastics are primary sources of triboelectric charges any fixtures which come in contact with sensitive components are instead made from other highly con ductive materials.
Since it is extremely difficult to completely inhibit the phenomenon of triboelectric charging, drive makers instead focus most of their attention on eliminating its effects by keeping the voltage potential on surfaces as low as possible. This is done by creating a highly conductive environment in the assembly area which gives any developing charges a direct path to ground. In addition to grounding all equipment and fixtures, human operators are required to wear conductive garments and/or wrist straps.
Although most agree that GMR heads will be more susceptible to ESD than MR heads, as with particulate and molecular contamination, the magnitude of the additional ESD challenge is not clear at this point. “We`re all pretty much targeting ESD-control at the 5V level as we move into GMR,” explains Quantum`s Webb. He believes these levels are achievable with today`s technology, but points out that “in some areas the increment will be tougher to reach than others perhaps requiring new material selections for both tools and conveyance systems.”
On the other hand, Murphy thinks some of the early projections for GMR`s ESD requirements may be excessive. “Many of these same people initially oversold the requirements for MR and, right now, we don`t have the necessary familiarity with GMR technology to know just how difficult it will be in terms of ESD.”
Ong tends to agree. “As we move into GMR, we`re still debating what the appropriate target should be,” says Ong, pointing out that they didn`t experience many of the ESD issues they heard about during their ramp-up to MR technology. “While we`re now working toward a limit of 20V with MR, which is adequately controllable with available technology and materials, it`s still unclear whether today`s technology can cost-effectively achieve the 5V-or-less levels being proposed.”
Regardless of the final levels targeted, however, Murphy says ESD considerations will have to be dealt with from the very beginning of the design and implementation of the manufacturing line, incorporating expertise from the tool designers, as well as industrial and process engineers. “Four years ago our ESD goal was a 10V environment. Now, this requirement has been halved and will eventually probably be halved again. If you rely on after-the-fact fixes to tools or rooms, you`re doomed to failure.”
Focus on automation
Increasingly demanding particle and ESD-control requirements are pushing the disk drive industry to incorporate greater levels of automation wherever possible. The main objective is simple — reduce the number of people in the cleanroom. “Anyone who has designed a cleanroom will tell you there are major differences in contamination control requirements between a room to be populated with one person per 125 square feet and one with one person per 300 to 500 square feet,” Seagate`s Murphy observes.
In addition to the amount of organic material people introduce to the cleanroom environment, Murphy points out they also provide a greater opportunity for second-hand distribution of contamination into the product. “In manual operations, you have just as many different processes as you have people doing them,” observes Murphy. Another side benefit to the improved cycle times provided by automation, is that the amount of time that components are exposed to the cleanroom environment and potential contamination is reduced.
Where people must be present, the universal goal in the drive industry is to keep them as far away from the actual assembly process as possible. “While there may be people present to actuate the equipment, or to move pallets of components or finished assemblies, the objective is always to minimize their contact with the components themselves,” Western Digital`s Ong stresses.
Quantum has made a major commitment to automation in its manufacturing facilities, according to Webb. In May of this year, Quantum formed a joint venture company with Matsushita-Kotobuki Electronics, Ltd. (MKE; Vancouver, WA) to design, develop, and manufacture recording heads. MKE is Quantum`s manufacturing partner, building all of its disk drives in what Webb describes as basically a lights-out, continuous-flow robotic manufacturing operation, including handling and assembly steps.
“Because of our emphasis on automation and robotics,” says Webb, “people-related cleanroom protocols are becoming less of a factor. ESD-control is also somewhat easier, because processes are more uniform and repeatable.” The automated process reduces the total environmental exposure times of some components to less than two minutes, according to Webb. Quantum is also encouraging it parts suppliers to make increased use of automated assembly tools, which Webb says “goes a long way to minimizing variation and stabilizing the product.”
While the benefits are clear, automation is also expensive, however, requiring manufacturers to make major upfront capital investments in facilities and robotic equipment. Given that the size of the production run largely determines the value of automation, these investments are often difficult to justify from an overall cost/unit standpoint. “Today the market lifetime of a drive can be measured in months or even weeks, and it will be a lot easier to justify the cost of automating a line that`s producing 20,000 units/day over an 18-month period than one that`s only producing 3,000 units/day in the same period,” notes Seagate`s Murphy.
Monitoring systems
In addition to control systems, the advent of MR technology has also increased requirements for both particulate and ESD monitoring. With automation beginning to play a larger role in these systems, Seagate`s Minneapolis facility, for example, bought its first on-line particle counters in 1986 but since then has doubled that number, now employing some 22 laser particle counters in each 8,000 square feet of cleanroom space. Data is recorded once every 30 seconds at an air flow rate of 1/10 cubic foot/minute. In some cases the air sampling is done at workstations within inches of the HDIs, and “the monitoring system is pivotal to understanding and designing not only the cleanroom but the workstations as well,” Murphy explains.
While particulate monitoring is not new, Western Digital is now working to integrate automated real-time particulate monitoring with ESD-monitoring in one system. Developed in conjunction with Lighthouse Associates (Milpitas, CA), the system has been successfully beta-tested at the company`s San Jose development facility and will now be phased into its production facilities. Ultimately, “The system will provide the baseline against which we can evaluate the effectiveness of all of our cleanroom practices both in terms of particulate reduction and ESD-control,” Ong says.
Individual monitoring stations will be located at key workstations throughout the manufacturing facility. The objective is not to monitor every workstation, Ong explains, but only those assembly steps where either the head or media is physically exposed to the environment from an ESD or particulate standpoint. Depending on the product line this would be between five or six locations per module with anywhere from 15 to 18 modules per cleanroom.
Real-time monitoring is expected to help identify specific instances of contamination such as when an operator has come in contact with a product, during material movement, or following a maintenance or wipedown session. “The tricky part,” says Ong, “will be to develop appropriate SPC charting methods based on realistic and practical techniques that will not only provide data but help drive the business.”
The value of ESD-monitoring systems is not yet universally proven or accepted, though other drive manufacturers may keep an eye on Western Digital`s progress. Seagate`s Murphy, for example, isn`t yet convinced that ESD-monitoring systems will provide a good return on investment, noting that in their evaluations to date, they haven`t yet identified a system that was effective in the manufacturing environment. “Right now, we see them as useful tools for theoretical measurements, but not in the workplace during actual assembly. For the most part, they don`t tell you anything you didn`t already know before.”
Parts and protocols
Increased concern for particle contamination manifests itself not only in the manufacturing process, but also in the design of parts, material selections, and design for cleanability. “A total systemized approach to contamination control is essential to ensuring that the end product is sufficiently clean that it doesn`t create problems,” Webb observes. Robotics and automation are also being applied to parts-cleaning processes. At Quantum, for example, all parts are passed through high-precision robotic cleaning systems before entering the cleanroom.
Seagate`s Murphy points out that over the years not only has the real-world definition of what a Class 100 cleanroom actually entails changed dramatically, but along with it the minimum expectations for the cleanliness of both components and finished products. “Today the parts we receive from our vendors are cleaner than what would have been assembled into the final drives seven to 10 years ago,” observes Murphy.
As with most contamination-sensitive product manufacturers, the disk drive industry tries to drive as much of the cleanliness load as is feasible and cost effective back onto its suppliers. “Today, our approach to cleanliness starts with our parts suppliers` first assembly steps,” Murphy says.
Similarly, Western Digital takes a multi-phase approach to contamination control. In addition to monitoring the assembly environment, the company also has development programs underway to use liquid particle counters (LPCs) to evaluate the cleanliness of drive components before they enter the cleanroom.
In addition to measuring particles on incoming component parts or assemblies, LPCs are useful for baseline characterization, process optimization, supplier evaluation and process monitoring at suppliers` facilities. Particle sizes of interest for components and assemblies are typically from 0.1 or 0.2 microns to a few tens of microns. “Together with our real-time monitoring capabilities, these systems provide a complete picture of the manufacturing environment as well as an understanding of the major sources of contamination,” Ong says.
Consumables
The advent of MR drives has perhaps most dramatically impacted the types and requirements of consumable products used in disk drive cleanrooms. “Contamination concerns have now extended down to all types of consumables from janitorial products to tweezers,” Webb observes. For example, although the industry had been using latex gloves for most of its early existence, these have now been universally discarded in favor of higher conductivity nitrile materials. Similarly, concerns for better static-dissipative and conductivity characteristics have also led to changes in garment design.
Since many manmade fabrics such as polyesters tend to easily tribocharge, drivemakers are now using garments with carbon-fiber threads. The threads are woven into the material in a crisscross fashion from top-to-bottom including boots with a conductive floor provided in ESD-sensitive areas to provide a path to ground for the garment.
In addition to providing better ESD control, the new garments also provide greater freedom of motion for operators. Previously operators were required to wear wrist straps which restricted their hand movements. In addition, particulate levels are also reduced since there is no longer a need for the skin around the strap to be exposed to the environment. Today, the only exposed skin on Western Digital`s operators is the area around the eyes which they are also looking at addressing through an integrated face shield, according to Ong.
There have also been major changes in the requirements for swabs and wipes used in disk drive cleanrooms, clearly the largest being the trend toward their complete prohibition from the cleanroom during actual assembly. Although, to address the changing needs of their biggest customer — swab makers are working hard to design and manufacture new types of static-dissipative, ultra-low-contamination products — the disk drive industry appears to have found a different solution. “Our approach to ESD-concerns for certain swabs is to ensure that they are not within the work area during assembly. Instead they are brought into the workstations between builds, usually at shift changes or breaks, and usually by different, dedicated cleaning operators,” Ong says.
Likewise Murphy says swabs are also no longer a part of their assembly processes. “Where we once used swabs to do spot cleaning of parts prior to assembly, this is now taken care of by the cleanliness requirements imposed on the part suppliers.” Where swabs and wipes are still used, such as in housekeeping cleaning operations, however, Murphy says current swabs and wipes are adequate. “While not as absorbent as we`d like them to be, we recognize it`s a compromise, and don`t expect to be making new demands.”
John Haystead is a freelance techno logy writer and former Editor of CleanRooms magazine.
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This image shows an example of particulate damage. It depicts media wear due to embedded alumina.
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Residual anions from chloride and sulfate can cause corrosion. This image shows corrosion of pole tips due to chloride attack.
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At right: A view of Quantum`s servo light section of the production/manufacturing line. Robots, which swing in over the lines to drop in the servo light mechanisms, can be seen on the left and right sides of the image.
Below: A view of Quantum`s HDA assembly line. White plastic racks hold extra parts for robots to prevent humans from needing to enter to get parts.