Different microelectronic processes require very clean surfaces in terms of particulate contamination. Among them, direct wafer bonding has very aggressive requirement in terms of particulate cleanliness. Current work describes development, testing and verification of a single wafer megasonic cleaning method utilizing a transducer design that meets extreme particle neutrality, particle removal efficiency and repeatability requirements of production scale wafer bonding and other very low particle applications. Wafer bonding will also be used as a characterization technique.
Substrate cleaning is a very important process step in direct wafer bonding. Current work describes development, testing and verification of a single wafer megasonic cleaning method utilizing a transducer design that meets the extreme particle neutrality, Particle Removal Efficiency (PRE), and repeatability requirements of production scale wafer bonding and other applications requiring extremely low particle levels. The results were obtained using 300 mm diameter Si wafer which were processed as received, without any wet bench cleaning process. These experiments simulated real case production scenario in which the particle counts on incoming wafers are typically 0.1 LPD/cm2 and lower.
The increased complexity of microelectronic and sensor applications that have emerged over the past decade have driven wafer bonding to mature as a production technology for both MEMS and 3D stacked die manufacturing. The use of CMOS device wafers has restricted the types of viable bonding processes to low temperature fusion bonding, adhesive bonding and metal bonding. Due to the specific requirements of CMOS technology the allowed bonding processes had to be adapted in order to fulfill the specific CMOS-compatibility demands. This paper presents a novel single wafer Megasonic based cleaning method with enhanced process control capabilities. This cleaning process is integrated in low temperature fusion bonding process and enables higher bonding yields and a reduction in process fluid usage.
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
MEMS-specific processes required to adapt standard IC manufacturing technologies in order to accommodate its custom requirements. Photolithography is one of the standard techniques which had to adapt particularly to the use of thick resists needed for high aspect ratio features fabrication. In order to work with resist thickness varying between few tens and up to few hundreds of micrometers the standard process flow had to be changed to multiple-layers coating, customized (and very long) baking times, special exposure techniques and development. This paper introduces a megasonic-enhanced method used for developing resists. In case of MEMS applications this method brings not only a major yield improvement (better quality features) but also a major decrease of process time.
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