In leading edge patterning processes, overlay is now entangled with CD including OPC residuals and stochastics. This combined effect is a serious challenge for continued shrink and can be characterized with an Edge Placement Error (EPE) budget containing multi-domain components: global and local CD, local placement errors, overlay errors, etch biases and OPC. EPE defines process capability and ultimately relates to device yield. Understanding the EPE budget leads to efficient ways to monitor process capability and optimize it using EPE based process control applications. We examine a critical EPE use case on a leading edge DRAM node. We start by constructing and verifying the EPE Budget via densely sampled on-product in-device local, global CD and Overlay metrology after the etch process step. EPE budget contributors are ranked according to their impact to overall EPE performance and later with simulated EPE performance improvements per component. A cost/benefit analysis is shown to help choose the most HVM-friendly solutions.
Introduction and problem statement
Given that EUV lithography allows printing smaller Critical Dimension (CD) features, it can result in non-normal distributed CD populations on ADI wafers [Civay SPIE AL 2014], leading to errors in predicted failure rates [Bristol SPIE AL 2017]. As a result, there is a need to quantify the actual behavior of the CD population extremes by means of massive metrology [Dillen EUVL 2018]. Not only allows this to study the CD distribution, we can in parallel also evaluate pattern quality and the failure mechanisms leading to defects. This massive metrology method provides an accurate failure rate based on CD, and enables new possibilities to define a failure rate based on different metrics in a single measurement.
Method
We analyze the CD uniformity of pillars in polar coordinates using a global waveform based thresholding strategy. In conjunction with this CD information, we also evaluated the print quality of each individual measured feature.
Fig 1. In line detected anomalies and failure definitions
As we gather this information during the measurement of CD, we can limit the additional measurement overhead to neglectable levels.
Application and outlook
We will show how we can leverage this to determine a defect based process window and relations of failure mechanisms through process conditions (see figure 2). When we take failures in a CH dataset into account, we illustrate the effect on the shape of a large dataset distribution in figure 3.
Fig 2. Defect identification for a through exposure dose experiment of pillars. For each condition >13k pillars where measured. The plot clearly shows an asymmetric behavior due to different failure mechanisms at low and high energy. The 2 vertical lines at relative energies 0.93 and 1.05 times nominal indicate the low defect process window.
Fig 3. A distribution of measured regular grid dense CH. The red line is the unfiltered CD data, the blue line is the shape of the distribution after filtering individual CH measurements that have a much lower contrast than expected.
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