Nanometer deposition processes create new challenges for cryopumps
10/01/2005
Every time cryogenic pumps are pushed to lower pressures in semiconductor manufacturing, new and often unexpected phenomena occur. Such is the case with advanced processes needed for magnetoresistive memory, or magnetic random-access memory (MRAM) - which promises to be one of the most significant memory technology breakthroughs of this decade - as well as for high-density hard-disk drive (HDD) read/write heads.
MRAM technology combines the best attributes of three mainstream memory devices: the density of DRAM, the speed of SRAM, and the nonvolatility of flash storage. But to put high-density MRAMs into volume production with leading-edge processes, manufacturing challenges must be addressed, especially in vacuum deposition tools, which coat wafers with alternating layers of nanometer-thick magnetic films and insulating materials for bit storage. The stacked layers used to build MRAM’s magnetic tunneling junction structure (Fig. 1) can range from 1-30nm thick, with uniformity requirements within a few angstroms across a 200mm wafer.
 Figure 1. Layers making up the magnetic tunneling junction structure used in MRAM devices. |
Successfully manufacturing MRAMs requires vacuum deposition with a base pressure of 10-9torr and a process pressure of 10-7torr, with quick recovery to base pressure. Manufacturing also presents challenges such as minimizing O2, H2, and long-chain hydrocarbon contaminates in deposition. In contrast, today’s PVD tools typically use a base pressure of 10-8torr and process pressure of 10-3torr to layer semiconductor thin films. Most sputtering-type deposition processes require vacuum levels to be maintained in the 10-3torr range, although some newer, more exotic processes may require pressures as high as 10-1torr.
Similar challenges exist in the production of read/write heads for high-density, small HDDs. These heads use giant magnetoresistance (GMR) technology to read data from platters. New vacuum deposition systems are needed to deposit ever thinner and purer films of magnetic and insulating materials on smaller GMR heads, which are needed to read data from smaller recording tracks on platters. Often referred to as “spin-valve technology,” GMR is usually defined in terms of spin-dependent scattering provided by a sandwich of very thin layers of two ferromagnetic-type materials around a metallic spacer layer (Fig. 2).
 Figure 2. Stack of layers used to build GMR heads for disk drives. |
Standard cryopumps have served GMR head production lines, but the need for lower base and processing pressures, high pumping speeds, and greater temperature control have resulted in a growing need for changes in vacuum system designs. In many cases, new vacuum deposition systems being prepared for GMR processes can be leveraged for the production of next-generation microprocessors and MRAMs that require nanometer-thin material layers.
The requirement for 10-9torr pressure in MRAM and GMR deposition applications limits the options for vacuum pumping. Turbopumps, which are similar to small turbines that physically drive gases out of the vacuum system, require minimal tool downtime under normal circumstances, but they have low pumping speed for a given hole size and are especially limited when pumping long-chain hydrocarbon contaminates and light gases such as hydrogen. Alternatively, ion pumps, which chemically and physically trap gas, need very little maintenance at low vacuums, but they are not suitable for higher-pressure operations because they are limited-capacity capture pumps.
Vacuum system requirements
Since building MRAMs and GMRs requires maximum speed, cleanliness, and quick recovery of vacuum following process steps in ion-beam deposition and sputtering, cryopumps (see sbove) are the preferred technology for ultrathin-layer materials coating. Cryopumps have significantly higher speed than other pumps, particularly for removal of water vapor, which is one of the most common and harmful gases handled in vacuum processes [1].
 A worker checks the On-Board IS 320F cryopump on a VSEA tool. |
Ever since cryopumps were first introduced to commercial semiconductor manufacturing roughly 30 years ago, they have been required to adapt to an unending stream of ever-more daunting production challenges.
The following list includes key challenges for cryopumps used in the latest memory technologies’ production (MRAM and GMR):
- The cryopumps must readily achieve low vacuum pressure, into the ultrahigh-vacuum range, to assure low contamination.
- Vacuum pumping speeds need to be high to ensure low contamination and rapid recovery of vacuum during the transition between process steps.
- High vacuum consistency must be achieved to ensure stable process conditions and maximize productivity in terms of processing the maximum possible number of useful substrates.
- High levels of tool and vacuum system integration are necessary to maximize all metrics and optimize productivity.
- Vacuum system availability must be maximized for users to realize optimal production levels.
Design considerations
Following major maintenance and servicing of tools, a vacuum level of 10-9torr must be achieved as soon as possible to resume MRAM fabrication. It could take many hours to achieve the desired vacuum level, but the 10-9torr specification has implications for every piece of the vacuum system, especially for the sealing. Almost any rubber or nonmetallic seal in the vacuum system will have a leak or permeation rate sufficient to violate this requirement, unless very large and expensive vacuum pumps are used. The implication is that either the seals must be all metal or dual nonmetallic seals with interstage vacuum pumping. Both of these are viable approaches, but they do have hardware and maintenance costs associated with implementation.
Modern cryopumps destined for use in MRAM and GMR fabrication have an adaptive control system with a high level of system integration. The original equipment manufacturer integrates a set of high-level software commands in its system controller that communicate directly with the cryopump controller to provide real-time control and data acquisition of timing-critical measurements. The cryopump’s control system monitors key pump parameters including helium compressor supply, return pressure, AC line voltage, and frequency; and cryopump operational parameters such as refrigerator stage temperatures.
Such data is fed into an adaptive control algorithm that resides in the pump controller. In turn, the controller varies the operational cycle of the cryopump to adjust its consumption of helium. All this yields a system that adapts to changing heat loads and operational constraints and to variations in AC line voltage and frequencies while maintaining vacuum performance and consistency, thus improving MRAM product yield and lowering the cost-of-ownership.
An example of a cryopump system with intelligent control is the CTI-Cryogenics On-Board IS 320F from Helix Technology Corp. The intelligent system controls leverage real-time system knowledge to manage motor speed and cryogenic temperature, enabling this pump to adjust automatically to changing heat/gas loading conditions. The result is enhanced vacuum consistency and improved recovery time. The system also adjusts for accumulation of process-related coatings without compromising reliability and productivity and offers second-stage-only rapid regeneration to ensure maximum system uptime and productivity.
Summary
Producing smaller feature sizes that deliver more powerful system performance necessitates denser features. In turn, manufacturing to desired densities has driven requirements for vacuum system performance to higher, cleaner, more reliable and consistent vacuums. Although cryopumps are the technology of choice for this application because of their inherent high pumping speed and cleanliness, the need for higher vacuum and higher tool productivity has pushed pump makers to re-evaluate pump performance capabilities. The resulting next-generation pumps are closely tailored to these new requirements and tightly integrated into the tool control system.
References
- Tsang, et al., “Design, Fabrication, and Performance of Spin Valve Read Heads for Magnetic Recording Applications,” IBM Jour. of Res. and Dev., Vol. 42, No. 1, 1998.
- J. Harvell, P. Lessard, “Managing Water Vapor in a Vacuum Process,” Semiconductor International, June 1991.
- Bartlett, et al., US Patent #5,301,511, “Cryopump and Cryopanel Having Frost Concentration Device,” Apr. 12, 1994.
Michael Eacobacci received his BS in mechanical engineering and his MS in materials science from Northeastern U. in Boston, and has completed the Technology Management Program at Babson College in Wellesley, MA. He is a senior technologist at Helix Technology Corp.; e-mail [email protected].
Paul Amundsen received his BS in electrical engineering from the U. of Massachusetts at Lowell, and is a product applications manager at Helix Technology Corp.