This letter presents an effective approach to mitigating the prolonged issue of the insufficient etching selectivity between the sacrificial oxide and structural metal Al during the final release step of the CMOS-MEMS metal resonators. Particularly, the approach purposely slows down the etching rate not only for the sacrificial oxide but also more effectively for the structural metal by covering the resonator devices with upper metal layers in the CMOS process that contain release holes to pinch the diffusion of the etchant. Doing so realizes a $10\times $ extension in the allowable time window for over-etch, from a rather critical 5 min to over 50 min. This work further investigates the influence of the HF-based etchant temperature from 20°C to 40°C on the etch rate. The results show that the technique introduces the diffusion limiting effect at lower temperatures, leading to a less temperature sensitive etch rate compared to that of the reference counterpart. This method provides a promising and effective remedy for improving the yield during the time- and solution condition-sensitive oxide wet release step of CMOS-MEMS-based resonant devices. [2020-0299]

中文翻译：

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JMEMS Letters 通过扩散控制缓解 CMOS-MEMS 金属谐振器湿释放工艺中蚀刻选择性不足的问题

这封信提出了一种有效的方法来缓解在 CMOS-MEMS 金属谐振器的最终释放步骤中牺牲氧化物和结构金属 Al 之间蚀刻选择性不足的长期问题。特别是，该方法通过在包含释放孔的 CMOS 工艺中用上金属层覆盖谐振器器件来抑制蚀刻剂的扩散，从而有意减慢不仅对牺牲氧化物而且更有效地降低结构金属的蚀刻速率。这样做可以在过度蚀刻的允许时间窗口中实现 $10\times $ 的扩展，从相当关键的 5 分钟到超过 50 分钟。这项工作进一步研究了基于 HF 的蚀刻剂温度从 20°C 到 40°C 对蚀刻速率的影响。结果表明，与参考对应物相比，该技术在较低温度下引入了扩散限制效应，从而导致对温度敏感性较低的蚀刻速率。该方法为提高基于 CMOS-MEMS 的谐振器件的时间和溶液条件敏感氧化物湿释放步骤期间的产量提供了一种有前途和有效的补救措施。[2020-0299]