Overlay is one of the key factors which enables optical lithography extension to 1X node DRAM manufacturing. It is natural that accurate wafer alignment is a prerequisite for good device overlay. However, alignment failures or misalignments are commonly observed in a fab. There are many factors which could induce alignment problems. Low alignment signal contrast is one of the main issues. Alignment signal contrast can be degraded by opaque stack materials or by alignment mark degradation due to processes like CMP. This issue can be compounded by mark sub-segmentation from design rules in combination with double or quadruple spacer process. Alignment signal contrast can be improved by applying new material or process optimization, which sometimes lead to the addition of another process-step with higher costs. If we can amplify the signal components containing the position information and reduce other unwanted signal and background contributions then we can improve alignment performance without process change. In this paper we use ASML's new alignment sensor (as was introduced and released on the NXT:1980Di) and sample wafers with special stacks which can induce poor alignment signal to demonstrate alignment and overlay improvement.
Overlay metrology target design is an essential step prior to performing overlay measurements. This step is done through the optimization of target parameters for a given process stack. A simulation tool is therefore used to improve measurement performances. This work shows how our Metrology Target Design (MTD) simulator helps significantly in the target design process. We show the role of film and Optical CD measurements in improving significantly the fidelity of the simulations. We demonstrate that for various target design parameters we are capable of predicting measured performance metrics by simulations and correctly rank various designs performances.
In this paper, set of wafers with separated processes was prepared and overlay measurement result was compared in two methods; IBO and DBO. Based on the experimental result, theoretical approach of relationship between overlay mark deformation and overlay variation is presented. Moreover, overlay reading simulation was used in verification and prediction of overlay variation due to deformation of overlay mark caused by induced processes. Through this study, understanding of individual process effects on overlay measurement error is given. Additionally, guideline of selecting proper overlay measurement scheme for specific layer is presented.
In this study, we proposed the concept of high order field-by-field correction for Matched Machine Overlay (MMO)
error minimization and we have validated it through experiments. Because scanners have unique grid fingerprint, MMO
value between machines is higher than the one of Single Machine Overlay (SMO). In some cases, the localized grid
distortion mainly contributes to the MMO value. However, this localized grid distortion cannot be flatten by a normal
correction method such as 10-parameter correction. Until now, in order to flat the localized grid distortion, ultimate
correction capability can be realized by combining 6-parameter field-by-field correction and intra-field high order
correction methods. However 6-parameter could be not enough to follow the diversity of local distortion. In this study,
for further improvement of MMO, high order field-by-field correction capability was investigated and the results were
compared. Base on simulation, we found that the field-by-field correction was a successful way to lower the MMO value
of EUV vs. ArF immersion scanners. By experimental demonstration, it showed that field-by-field correction was more
effective to correct localized grid distortion and the gain via high order model was about 0.5 nm. These results will be
helpful to achieve the MMO specification for the next generation device.
The shrinkage of design rule necessitated corresponding tighter overlay control. However, in advanced applications, the
extension of current technology may not be able to meet the control requirement, consequently, additional breakthroughs
are required. In this study, we investigated methods to enhance the overlay control, approaches by extraction of real
overlay error out of overlay measurement. So far, only the destructive inspections like vertical SEM have enabled us to
measure real misalignment. But, a concept of non-destructive method is proposed in this paper, extracting vertical
information from the results of multiple measurements with various measurement conditions, keys or recipes. With this
proposed method, the measurement accuracy can be improved and we can enable a new knob for overlay control.