Etch selectivity between layers is an important parameter in the fabrication of microelectronics and microsystems. This is particularly true in the case of isotropic gas/vapour etching methods used to release free standing structures through the selective etching of sacrificial layers. Commonly used structural materials have been reported to be largely inert when exposed to a given vapour etchant, indicating high selectivity when measured against typical sacrificial layers. However, there is growing evidence that these structural layers are actually etched at an enhanced rate if they are located in the proximity of the sacrificial layer being removed. Hence, removal rates given in the literature, which have resulted from measurements of layers that have been etched in isolation, can no longer be trusted to characterize critical etch processes in device fabrication. In this paper, a test structure is reported that enables a far more appropriate determination of the etch selectivity between sacrificial and structural materials. The method is demonstrated with the two most common vapour etch processes. Firstly, the XeF ₂ vapour etch of a polysilicon sacrificial layer located above a silicon nitride structural layer, and secondly, the HF vapour etch of silicon dioxide placed above a silicon nitride structural layer. Both test structure datasets are presented. The polysilicon and silicon nitride layers, etched with XeF ₂ show a selectivity of 5:4. The silicon dioxide and silicon nitride layers etched with HF, show a selectivity of 6:1 to 8:1.

中文翻译：

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用于测量 XeF2 和 HF 气相蚀刻工艺中的选择性的测试结构

层间蚀刻选择性是微电子和微系统制造中的一个重要参数。在用于通过牺牲层的选择性蚀刻释放独立结构的各向同性气体/蒸汽蚀刻方法的情况下尤其如此。据报道，常用的结构材料在暴露于给定的蒸汽蚀刻剂时大部分是惰性的，这表明在对典型的牺牲层进行测量时具有高选择性。然而，越来越多的证据表明，如果这些结构层位于被去除的牺牲层附近，它们实际上会以更高的速率被蚀刻。因此，文献中给出的去除率是对隔离蚀刻层的测量结果，不能再信任表征器件制造中的关键蚀刻工艺。在本文中，报告了一种测试结构，它可以更合适地确定牺牲材料和结构材料之间的蚀刻选择性。该方法用两种最常见的气相蚀刻工艺进行了演示。首先，XeF ₂位于氮化硅结构层上方的多晶硅牺牲层的气相蚀刻，其次，位于氮化硅结构层上方的二氧化硅的HF气相蚀刻。提供了两个测试结构数据集。用 XeF ₂蚀刻的多晶硅和氮化硅层 显示出 5:4 的选择性。用 HF 蚀刻的二氧化硅和氮化硅层显示出 6:1 到 8:1 的选择性。