Predicting and measuring scummed contact holes

CD-SEM after develop inspection (ADI) and measurement of contact holes can detect missing or undersized holes but may have trouble detecting a hole with an acceptable critical dimension (CD) that is scummed (that is, where the resist is not completely cleared out of the bottom of the hole). For a modestly high aspect ratio (height over width of the hole), the very bottom of the hole is often not sufficiently visible in the CD-SEM image. In fact, a previous SEM simulation study showed that the aspect ratio, rather than the hole width and/or height individually, and the sidewall angle control the SEM signal emanating from the bottom of a contact hole. In an inevitably noisy SEM image, the signal coming from the bottom of the hole must be strong enough to be reliably detectable over the variations in that signal due to SEM noise. This leads to the important question of what circumstances lead to the reliable detection of scummed holes using ADI SEM images. While the previous study was theoretical, this work will address this question experimentally, while also validating the results of that prior simulation study. Here we collect post-develop CD-SEM images of contact holes under a variety of mask biases with varying dose to generate varying aspect ratios, and likely different levels of scumming. To develop a best-known method for scum detection ADI, we evaluate the contrast detected in the CD-SEM images.