Implant Stripping
T
he stripping of heavily implanted photoresist (PR),
while also minimizing material loss and surface damage, is a challenging process. The aggressive treatments required to remove the tough carbonized, or cross-linked, surface layer formed on the photoresist surface by the ion implantation process also remove material from other exposed surfaces resulting in unwanted damage. As the number of photoresist layers has increased and acceptable levels of material loss, related non-uniformity and surface damage have simultaneously decreased, this process is becoming even more difficult. One commonly used approach first exposes the wafer surface to the aggressive chemical/physical action of plasma ashing, followed by a wet chemical clean. The material losses caused by this process have become unacceptable as feature sizes have continued to shrink and junction depths have become shallower. An alternative approach uses steam injection to increase the reactivity of a sulphuric acid hydrogen peroxide mixture (SPM), so that it can break up and dissolve the surface layer and achieve maximum photoresist removal. Steam injection achieves high temperatures on the wafer surface, while avoiding excessive dilution of the chemicals, both of which are required to achieve sufficient reactivity. This article will discuss the benefits of using a single-wafer steam- injected SPM process to improve uniformity and minimize material loss during the photoresist removal process.
Advanced Process Challenges
At 32 nm and smaller design nodes, new materials and design structures are creating new challenges for the photoresist stripping process. At the same time, acceptable levels of material loss and surface damage have
significantly decreased. Ultra-shallow implants and FinFET structures, for example, are extremely sensitive to material loss.
These new challenges are causing the industry to reconsider the use of plasma-based ashing as an acceptable photoresist removal process. Each ash/clean cycle can result in a material loss of more than 5Å, while the steam- injected SPM (ViPR) process has achieved less than 0.2 Å (2 keV, 2x1015 ions/cm2 As implant). Achieving adequate PR removal and reduced material loss with an all-wet SPM process is dependent on increasing the chemical reactivity of the cleaning solution enough to dissolve the tough surface layer by elevating its temperature without excessively reducing the concentration of the reactive chemical species. Figure 1 illustrates the differences in material loss experienced using an ashing clean approach vs. an all-wet clean approach.
Wet Clean Processes Are Not All Equal
Batch immersion SPM cleaning tools typically operate at approximately 110-
150ºC to limit the breakdown of the hydrogen peroxide used in the cleaning chemistry. Single-pass (fresh dispense) systems are not limited by the breakdown of the hydrogen peroxide, but rather by the temperature limits of the fluoropolymer components of the process hardware, typically around 150 ºC. This issue can be overcome using point-of-use (POU) mixing, where the heat generated when sulphuric acid is mixed with hydrogen peroxide and water, can increase SPM temperatures at the wafer surface to approximately 180ºC, while limiting the heat experienced by the hardware to acceptable levels. The reactivity achieved with this approach is ultimately limited to some optimal value determined by the competing contributions increasing temperature and decreasing concentration. Steam injection overcomes these limitations. When the steam is injected into the SPM solution it releases its latent heat of vaporization, permitting greater increases in temperature without excessively diluting the reactive species. Steam-injected SPM not only removes implanted photoresist, it does
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Figure 1: A comparison of the material and dopant lost during photoresist cleaning during ashing and wet clean processes
www.euroasiasemiconductor.com Issue II 2010
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