Heat treatment delivers high strength sputter targets

09/01/1997

Heat treatment delivershigh strength sputter targets

Sputtering target manufacturers, like many other equipment suppliers, are beginning to come to grips with the requirements of 300-mm systems. Johnson Matthey Electronics (JME), Spokane, WA, recently introduced a high strength aluminum backing plate, named TITAN, for diffusion-bonded sputtering targets.

Flat panel display and 300-mm sputtering systems require larger targets, so the backing plate must support more weight and still prevent bow and flex. According to sputtering targets product manager Keene Little, JME`s conventional diffusion-bonded titanium-aluminum target design flexed by up to 6 mm at the larger diameter. Minor flexing can flake off TiN deposited on the backing plate, generating particles. Major flexing has even more severe consequences, as the altered target-to-wafer spacing degrades uniformity or, in extreme cases, causes shorts between the target and the wafer. Monolithic titanium targets are stronger, but are also substantially more expensive.

Little said that Applied Materials and other OEMs have long specified T6 temper for aluminum backing plates. T6 tempering first uses "solution heat treatment" to allow alloying elements to diffuse into a uniform solid solution. Then, rapid quenching produces a super-saturated solution of alloying elements in aluminum. The alloy is not stable in this condition, and, after "artificial aging" at an elevated temperature, metastable phases precipitate out and harden the material. This temper has been difficult to achieve in finished targets because the heat of diffusion bonding reverses the tempering process and softens the backing plate. A modified fabrication process for the new backing plate allows this final heat treatment to occur during diffusion bonding itself.

High materials purity and tight process control are essential in semiconductor fabrication, but relatively unknown in extractive metallurgy. Harry Rosenberg, of the Alta group, a wholly-owned subsidiary of JME, noted that additional advances in titanium refining and process control allow JME to offer "designer targets" with the precise impurity levels and metallurgy requested by the user. First, a proprietary electrolysis process further refines electronics grade titanium sponge to reduce impurities. Metallic impurities (Fe, Al, Cu) increase resistivity in the deposited film, and excess oxygen can degrade target particle performance. Second, an e-beam furnace specially designed for impurity control melts the sponge into ingots. The furnace allows higher vacuums than conventional equipment, for more effective water removal, and is designed to allow replenishment of the charge without breaking vacuum. Once a titanium ingot with the desired impurity levels is obtained, Rosenberg said, proprietary metal-working methods allow close control of grain size. Finally, to maintain consistent performance, the Alta Group is implementing improved statistical process control methods. - K.D.