Implant Stripping
Figure 2: Process time comparisons between standard POU SPM and steam-injected POU SPM cleaning chemistries
so very quickly. Figure 2 demonstrates the 80% reduction in time achieved in a single wafer FSI ORION system when the steam-injected process is compared to a lower temperature point-of-use (POU) mixing process.
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The SPM blend ratio can be adjusted to optimize the reactivity of the solution. Injecting steam has little dilution effect because of the tremendous amount of energy that is released when the steam is dissolved into the SPM mixture on the wafer’s surface. Since so much energy is released at this point, relatively little steam is required to substantially increase the SPM temperature. Figure 3 demonstrates the effects of mixing steam and liquid water with sulfuric acid.
The high chemical reactivity achieved by steam-injected SPM enables the stripping of photoresist by wet chemical reaction alone in most
cases, thus eliminating the need for ashing in all but the most extreme implant conditions. When implemented on a single wafer platform, process time is reduced by as much as 80% percent, while material loss is reduced by a factor of ten or more. At the same time, this approach reduces process cycle time, process complexity, the number of process steps and the number of tools required, all of which help to lower overall cost of ownership. Steam-injected SPM requires a closed chamber design to safely contain the aggressive, high-temperature cleaning chemistries. A closed chamber also provides control of both atmospheric and dissolved gases in the process environment. Single wafer cleaning systems with either open or partially controlled chambers cannot deliver the high temperatures achieved in a closed chamber, thereby limiting the efficiency of their cleaning processes. Open chambers may also pose a safety hazard, since there may be a danger of acid exposure to chemical spray or fume leakage. Partially closed systems also run the risk of increased particle defectivity due to potential contamination of the top plate during the cleaning process. This contamination can later result in particles precipitating onto the wafer.
Chemistry Delivery
The methodology by which the cleaning chemistries are delivered to the wafer can significantly affect the uniformity of the cleaning process. A central dispensing nozzle cannot ensure an
Figure 3: Enthalpy-concentration diagram for aqueous sulfuric acid showing the difference between mixing liquid water and steam with sulfuric acid
even distribution of the cleaning chemistry across the wafer, creating potential uniformity problems during the resist removal process. The dispensing system should provide a range of capabilities that can be customized to meet the requirements of a specific application. A central nozzle may be sufficient in certain situations. Generally, however, a linear spray bar will provide more even distribution of the chemical solution and surface temperature, resulting in more uniform chemical action across the wafer surface.
Atomization enables energetic delivery of the chemical solution which can enhance particle removal without damaging delicate device structures. Figure 4 compares the particle removal
The SPM blend ratio can be adjusted to optimize the reactivity of the solution. Injecting steam has little dilution effect because of the tremendous amount of energy that is released when the steam is dissolved into the SPM mixture on the wafer’s surface. Since so much energy is released at this point, relatively little steam is required to substantially increase the SPM temperature
www.euroasiasemiconductor.com Issue II 2010
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