Optimal lithographic process control involves a closely coupled combination of test wafer and
product wafer characterization. It has been shown in previous work that MPX (Monitor Photo
Excursion) optical technology for line-end-shortening metrology of focus and dose provides reliable
and low cost product monitor solution. In this work we apply MPX technology to litho cell monitor
and control on test wafers. Focus-exposure matrix (FEM) wafers are measured and analyzed
automatically on a routine basis. Process window parameters are tracked over time by scanner,
including spatial analysis of results across the scanner field such as tilt and curvature.
Improvements in litho cell control are discussed.
A method has been developed for calculating the statistical effects of spatial noise on the overlay measurement extracted from a given overlay target. Previously, this metric has been shown to correlate well to the random component of Overlay Mark Fidelity (OMF), and that OMF is a significant contributor to the noise hierarchy. Quantitative diagnostic methods are required in order to assess the capabilities of overlay metrology and provide visibility into root-causes of potential inaccuracies intrinsic in the processing of overlay targets. We explore the use of this target noise metric for improved process control including improved overlay modeling, process fault detection, and process troubleshooting. In this paper we demonstrate the use of this additional metric for multiple layers in a large volume of production data utilizing existing production sampling.
CD control is one of the main parameters for IC product performances and a major contributor to yield performance. Traditional SEM metrology can be a challenge on particular layers due to normal process variation and has not proven to provide sufficient focus monitoring ability. This in turn causes false positives resulting in unnecessary rework, but more importantly missed focus excursions resulting in yield loss.
Alexander Starikov, Intel Corporation, alludes to the fact that focus and exposure "knobs" account for greater than 80% of CD correctible variance1. Spansion F25 is evaluating an alternative technology using an optical method for the indirect monitoring of the CD on the implant layer. The optical method utilizes a dual tone line-end-shortening (LES) target which is measured on a standard optical overlay tool. The dual tone technology enables the ability to separate the contributions of the focus and exposure resulting in a more accurate characterization of the two parameters on standard production wafers. Ultimately by keeping focus and exposure within acceptable limits it can be assumed that the CD will be within acceptable limits as well without the unnecessary rework caused by process variation.
By using designed experiments this paper will provide characterization of the LES technique on the implant layer showing its ability to separate focus-exposure errors vs. the traditional SEM metrology. Actual high volume production data will be used to compare the robustness and sensitivity of the two technologies in a real life production environment. An overall outline of the production implementation will be documented as well.
We explore the implementation of improved overlay mark designs increasing mark fidelity and device correlation for advanced wafer processing. The effect of design rule segmentation on overlay mark performance is studied. Short loop wafers with 193 nm lithography for front-end (poly to STI active) as well as back-end (via to metal) were processed and evaluated. A comparison of 6 different box-in-box (BiB) overlay marks, including non-segmented, multi bar, and design-rule segmented were compared to several types of AIM (Advanced Imaging Metrology) grating targets which were non-segmented and design rule segmented in various ways. The key outcomes of the performance study include the following: the total measurement uncertainty (TMU) was estimated by the RMS of the precision, TIS 3-sigma and overlay mark fidelity (OMF). The TMU calculated in this way show a 40% reduction for the grating marks compared to BiB. The major contributors to this performance improvement were OMF and precision, which were both improved by nearly a factor of 2 on the front-end layer. TIS-3-sigma was observed to improve when design rule segmentation was implemented, while OMF was marginally degraded. Similar results were found for the back end wafers. Several different pitches and segmentation schemes were reviewed and this has allowed the development of a methodology for target design optimization. Resulting improvements in modeled residuals were also achieved.
Although the subject of frequent concern, criticism, and attention in the modern semiconductor fabrication facility, human after develop inspection (ADI) does not catch the major scrap and yield events early enough, if at all. The overall success of scrap and photo redo reduction programs over past years has resulted in residual problem levels which are difficult to improve upon -- yet still very costly. Detected 'events' are few and far-between, although evidence of their prevalence is frequently seen at subsequent inspections, or finally at probe. In the ASIC fab, they put on-time delivery to customers at risk, because individual wafer lots in an ASIC facility have a designated customer. The sampled area is limited by human throughput to less than 10% of the wafers in a lot. The visual ADI process step is unpopular among manufacturing technicians. It is often a bottleneck in the photo area. Statistically, in a photo area with capacity of 5000 wafer starts per week, only a few wafers processed per day are destined for scrap. Since wafer events occur in sporadic clusters, the photo area experiences only a few significant incidents per month. The typical operator can expect to intercept such an event less than once during several months of otherwise uneventful ADI inspection haystack.' Hence the stubbornness of our residual problem. Going beyond the statistical problem, our current manual macro-inspection equipment is engineered appropriately to ancient IC generations. A collimated, oblique-oriented light was an effective darkfield illumination source, when line widths were much larger than the wavelength of light. When line width is comparable to, or smaller than, the wavelength, the collimated light source produces scintillating diffracted colors on the wafer. Thus diffraction 'noise' significantly buries the defect 'signal' in the typical bright light visual macro inspection. In addition, there is the problem of variability between human inspectors, and the impossibility of accurate classification and recording of defect types, locations, and layer of occurrence. In this paper, we discuss a pilot implementation of an automated macro inspection system at Motorola, Inc., which has enabled the early detection and containment of significant photolithography defects. We show a variety of different types of defects that have been effectively detected and identified by this system during production usage. We introduce a methodology for determining the automated tool's ability to discriminate between the defect signal and process noise. We indicate the potential for defect database analysis, and identification of maverick product. Based upon the pilot experience, we discuss the parameters of a cost/benefit analysis of full implementation. The costs involve tool cost, additional wafer dispositions, and the engineering costs of recipe management. The most tangible measurable benefit is the saved revenue of scrapped wafers. An analysis of risk also shows a major reduction due to improved detection, as well as reduced occurrence because of better containment. This reduction of risk extends both to the customer -- in terms of field failures, OTD, maverick product -- as well as to the production facility -- in terms of major scrap incidents, forced inking at probe, redo, and containment.
The alignment and overlay metrology issues for various damascene process architectures were studied and optimized. The Buried Hard Mask, Via First and Trench First approaches are studied comparatively for the Via to Metal1 and Metal2 to Via alignments. Alignment capability was studied by the alignment signal strength and alignment repeatability. Overlay metrology capability was studied by the overlay target appearance, static repeatability, target correlation and Tool Induced Shift. Final overlay measurement and long term overlay stability were used as a mean to verify the result. A Design of Experiment was done with splits in the hard mask material/thickness and the degree of copper CMP. It was found that for Metal2 alignment to Via, the Buried Hard Mask approach possess a showstopper unless one align Metal2 to Metal1 instead. The effect of CMP to the alignment to Metal1 level seems to have less trouble than the Tungsten CMP counterpart except the overlay target acquisition might be difficult for underpolish case. The choice of which damascene approach to take depends also on the trade off between overlay and CD control and other process performance and should be customized by individual's requirement.