Paper MS-ThM2
Gate Oxide Process Control Optimization by XPS in a Semiconductor Fabrication Line

Thursday, October 18, 2007, 8:20 am, Room 615

The introduction of thinner films such as 15-20 Å SiON gate oxides makes in-line metrology challenges more complex. Nitrogen dose in the SiON film is strongly process deposition dependent and is one key parameter of advanced CMOS technologies. Therefore, optimized process control has to be implemented in-line to ensure process stability. Measurement techniques such as X-Ray Metrology have to move from offline characterization laboratories to fabrication lines and ultimately to in-line metrology. Among these techniques, X-ray Photoelectron Spectroscopy (XPS) is one of the best adapted metrology methods to ensure nitrogen dose monitoring of SiON gate oxides. Until recently, decoupled plasma nitridation (DPN) process monitoring has been performed on monitor wafers. However, measurements performed on monitor wafers may not fully represent actual electrical properties at the device level. Measurements on production lots allow inline detection of process excursions and provide real time feedback of process chambers’ stability. Additional productivity improvements are reducing consumption of monitor wafers and their processing as well as data acquisition for correlations with device reliability parameters. The implementation in production of SiON gate monitoring on production lots implies to fully characterize the nitridation process on patterned wafers. This study is dedicated to a comparison between monitor and patterned wafers of DPN processes for 65nm node technology and below. Measurements are carried out in a next generation XPS tool with a 35µm spot size and pattern recognition capability that enable material metrology on product wafers. XPS measurements on patterned wafers are performed in specific test structures which dimensions are 50µm x 70µm using an Al Ka monochromatic X-ray source. A larger X-ray spot providing higher signal level is available for measurements on monitor wafers. First, gate oxides will be characterized for each process and tool measurement precisions will be shown. Second, nitrogen dose and thickness uniformity over the wafer will be compared and discussed. Third, different mappings protocols will be studied to identify the best compromise between throughput and an optimized mapping representative of process distribution. Finally, we will conclude with a selection of an optimized process control strategy of gate oxides.