Going deep: New capacitor trench etching enables sub-70nm DRAMs

by Debra Vogler, Senior Technical Editor

The need to control DRAM cell capacitance and ensure this parameter is uniform across all cells on the wafer becomes more critical to memory manufacturers as they hit 70nm structures requiring ~80:1 aspect ratios. For DRAM applications, the volume of the trench (i.e., the depth and width) determines the capacitance of the DRAM cell, so the deeper the trench, the higher the capacitance. The requisite across-wafer uniformity is enabled by having good selectivity to the hard mask, which allows the etching to go deep. Applied Materials is targeting both these requirements with its Centura Mariana Trench Etch tool.

The new etch chamber is based on traditional MERIE (magnetically enhanced reactive ion etch) technology, with some additions, such as a 60MHz bottom source as opposed to placing the source in the lid. Declan Scanlan, deep trench KPU head within Applied’s conductor etch division, told that the company’s plasma studies research answered two key questions: 1) whether putting the source power through the bottom, instead of the top, would be more efficient at coupling the power into the plasma; and 2) whether it could be made uniform. The studies revealed “a significant improvement in electron density uniformity when we went from a top source to a bottom source,” Scanlan said, and with respect to the efficiency, the modeling also showed that the energy of the species in the plasma “was basically the same between top and bottom sources,” with bottom source uniformity of 2% vs. 8% for top-source uniformity.

Process control is provided by Applied’s interferometric endpoint detection technology (trademarked as “EyeD”), which measures the amount of hard mask that remains. “Because we can measure the remaining mask, we can squeeze every inch of capacitance and trench depth out of each wafer because we can stop exactly where needed, so we’ve eliminated the safety margin that had been needed,” Scanlan noted.

The etch system’s gas injection has two process uniformity tuning knobs; dual-zone gas injection allows the user to flow different gases into the center of the wafer vs. the edge of the wafer. Additionally, an independent gas injection enhancement provides for the direct flow of one or two gases to the edge of the wafer — without flowing any into the center of the wafer — making up for the differences between the center and the edge of the wafer. — D.V.


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