Aluminum as a vacuum chamber construction material

When learning how to use aluminum for UHV system design, a vacuum system engineer experienced in stainless steel UHV system design will first be skeptical, then interested, and then excited by the powerful capabilities that aluminum offers. Our discussion of important Al properties uses stainless steel as a convenient benchmark.

Alloys: Aluminum comes in a wide range of alloys with varying characteristics [9, 10, 11]. Wrought aluminum alloys are suitable for vacuum chamber fabrication and are designated by a four-digit number, followed by a temper specification.

The first digit specifies the primary alloy composite. Wrought alloys have been used for vacuum chamber fabrication, except for the 7000 series, which has zinc, an element with a high vapor pressure at low temperature. The 2000 series alloys are highly weldable. The 6000 series, particularly 6061 and 6063, have been used for UHV systems [9].

Machinability: One of the driving forces for fabricating vacuum systems from aluminum is that aluminum is inherently more machinable than materials such as stainless steel, which have been traditionally used for vacuum component fabrication. For example, the machining cost for 300 series stainless steel is 5.5? that of aluminum [12]. Because of this ready machinability, cluster tool components can be machined from a single plate of aluminum. These tools are exceptionally rigid, have a minimal vacuum surface area, and occupy minimal floor space.

Mechanical properties. Typical elastic modulii for aluminum alloy 6061T6 and stainless steel alloy 304 [9] are 7470 kgf/mm2 and 19700 kgf/mm2, respectively. If these values are used in mechanical formulae for standard geometries, the ratios of critical thickness for the two materials are [10]: Here, tAS(flat plate), tAS(long cylinder), and tAS(short cylinder) are the minimum thickness ratios to avoid buckling in flat plates, long cylinders, and short cylinders, respectively.

Note that the ratios are close to unity. An aluminum vacuum system will not require parts that have appreciably greater thickness than similar ones manufactured from stainless steel.

Thermal conductivity: Aluminum`s thermal conductivity, depending on the alloy, ranges between 170 W/mK and 230 W/mK. Stainless steels, by contrast, have thermal conductivities that are between 14 W/mK and 16 W/mK. High thermal conductivity is an advantage when designing systems that require temperature cycling. This is the case for vacuum systems that must be baked to reach UHV levels. An aluminum chamber may be baked and then cooled much more rapidly than a stainless chamber. Furthermore, aluminum`s high conductivity allows a complete bakeout without recondensation of gases on local cool spots, a common problem in stainless steel systems.

Weight: Aluminum is roughly 1/3 the weight of stainless steel (2.8 g/cm3 [Al] vs. 8.0 g/cm3 [stainless steel alloys]). The cost burden associated with excess weight begins when the raw materials are handled and progresses throughout the manufacturing process. It affects all production steps, including shipping, installation, and even the architectural engineering and construction of the environment surrounding a process tool. Magnetic properties. Aluminum is not magnetic, whereas stainless steel, being essentially an alloy of iron, exhibits residual magnetism. The absence of magnetic properties in aluminum is advantageous for applications involving charged particle beams, because the vacuum system will not modify the fields from the beam control magnets.

Radioactivity: Aluminum, in comparison to stainless steel, has a much more rapid decay of induced radioactivity. If both types of materials are bombarded with the same flux of charged particles, the residual radioactivity will typically be one to two orders of magnitude less for an aluminum sample than for an identical stainless steel sample [13]. The nuclear half-life of elements that make up stainless steel suggests that a-particle contamination is always present in stainless steel and a possible source of circuit damage.

Corrosion: The corrosion of both aluminum and stainless steel alloys in reactive gases is complicated. Experimental work performed on various alloys in different reactive gaseous environments shows that both aluminum and stainless steel are subject to attack by reactive gases; halogen-containing species are typically the most damaging; and the corrosion of any given compound is usually no worse than that of its halogen component alone [9, 10].

Aluminum is not a worse corroder than stainless steel. It simply has different reaction dynamics that do not serve as a source of iron and nickel contamination, one of the most significant yield-limiting factors for silicon IC production. Outgassing properties. One of the most important properties of a vacuum material is the outgassing rate, as this determines the ultimate pressure that may be obtained in the vacuum chamber. Repeatable outgassing rates of <10-13 torr liter/sec cm2 are now possible in aluminum UHV systems [13], comparable to the best outgassing rates obtainable with stainless steel [14]. This improvement in outgassing performance has been one of the principal breakthroughs that has allowed aluminum to become a competent material for the construction of UHV systems.

Conclusion: The next generation of aluminum vacuum systems will be capable of UHV performance. Surface treatment, automated welding processes, and metal-sealed flange technologies have made this possible. The impact of UHV environments for contamination-free manufacturing processing has yet to be determined in a quantitative fashion. Current studies, however, indicate that they will be essential components of semiconductor materials processing and control and key aspects of 300-mm processing systems.