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Particle removal from wafer surfaces can be accomplished by irradiation of cleaning fluid by sound waves in the MHz frequency range. Unfortunately, unless proper cleaning conditions are chosen, megasonic irradiation may also result in damage to fragile wafer features. Here, we demonstrate a strong effect of dissolved CO 2 levels on the reduction of wafer damage during megasonic cleaning. Test structures with L/S patterns were irradiated with 0.93 MHz sound waves at varying power densities and dissolved CO 2 levels, in a single wafer spin cleaning tool, MegPie ® . Dissolution of increasing amounts of CO 2 in air saturated DI water caused a significant decrease in the number of breakages to line structures and also decreased the lengths of the line breakages, at all power densities up to 2.94 W/cm 2 . This ability of dissolved CO 2 to protect against feature damage correlates well with its ability to suppress sonoluminescence in sound irradiated DI water.

No abstract

Acoustic cavitation is known to be a primary source of both cleaning and damage of wafers during their megasonic processing. Understanding the response of process fluids to variables like acoustic power recipe and dissolved gases is an important first step in achieving damage-free megasonic cleaning of wafers. This paper reports the development of a portable, UV light tight, cavitation threshold (CT) cell to measure sonoluminescence (SL) signal arising from cavitation. The closed cell, integrated with a gas sensor and contactor, allows SL measurements under very controlled conditions. Using the CT cell the effect of the concentration of dissolved O 2 , CO 2 and air on SL signal has been investigated. Results show that SL varies linearly with dissolved O 2 concentration while CO 2 is found to be incapable of supporting SL. This study also demonstrates a novel method for precise control of SL through addition of an O 2 scavenger with fast O 2 removal kinetics.

Chemical mechanical planarization (CMP) process steps are increasing in frequency and criticality.   The introduction of new, sensitive dielectric materials as well as sensitivity to residual slurry and particle contamination make the post CMP clean an integral part of a good yielding CMP step. Brush Scrubbing is the traditional method of post CMP cleaning.  With this contact cleaning method particles are removed by direct contact of a rotating brush with the wafer surface.  These particles are then swept away by the cleaning solution.  This method of cleaning has some well known challenges, among them cross contamination from wafer to wafer, brush marking and scratching as well as process feedback and control. An alternative physical cleaning method utilizes the cavitation created by the presence of Megasonic energy in the process fluid to provide a non-contact method of slurry residual and particulate removal from the planarized surface.   The Megasonic apparatus used in these experiments has been successfully applied to other single wafer cleaning processes and is implemented in many large volume production applications. Most mass production post CMP cleaning processes consist of a series of cleaning and chemistry steps as well as a mix of physical cleaning approaches.  The challenge in quantifying effect and comparing cleaning efficiency between contact and non-contact physical cleaning has been in the isolation of cleaning results attributed to a change in method. In this work we were able to provide conditions for a direct comparison of cleaning methods in a special post CMP scrubber.  This tool is configured for brush or Megasonic processes in the same chamber with the identical wafer flow from the polisher.  The CMP was done on thermal SiO2 using  two varieties of conventional slurry and a common pad type.  Fifty wafers were split processed, half with 150835 and half with D3586.  A large delta in particle removal efficiency was observed based on slurry type alone. The brush parameters were based on a long term process of record. (Table 1)  Several megasonic parameters were varied to better understand the process window.  Analysis of cleaning results was performed using an SP2 with a threshold of 90nm. (Figure 1) Table 1, Experimental process recipe details Brush Recipe Step Time (sec) Chemical Wafer RPM Brush RPM 1 40 NH4OH 2% 20 80 2 40 DIW 20 80 3 20 DIW 60 No Brush 4   Dry 2000   Meg Recipe 1 Step Time (sec) Chemical Wafer RPM W/cm2 1 52 NH4OH 2% 60 2.5 2 48 DIW 60 2.5 3   Dry 2000 0 Meg Recipe 2 Step Time (sec) Chemical Wafer RPM W/cm2 1 52 NH4OH 2% 60 1.85 2 48 DIW 60 1.85 3   Dry 2000 0 Meg Recipe 3 Step Time (sec) Chemical Wafer RPM W/cm2 1 90 NH4OH 2% 60 1.85 2 10 DIW 60 1.85 3   Dry 2000 0 These direct comparison tests prove that the non-contact Megasonic method equals or exceeds the cleaning results achieved with the traditional brush cleaning under identical process flow and conditions. Figure 1: Post CMP clean particle counts 90nm threshold with D3586 slurry. Figure 1