Overlay continues to be one of the key challenges for lithography in advanced semiconductor manufacturing. It becomes
even more challenging due to the continued shrinking of the device node. Some low k1 techniques, such as Double
Exposure and Double Patterning also add additional loss of the overlay margin due to the fact that the single layer pattern
is created based on more than 1 exposure. Therefore, the overlay between 2 exposures requires very tight overlay
Mask registration is one of the major contributors to wafer overlay, especially field related overlay. We investigated
mask registration and wafer overlay by co-analyzing the mask data and the wafer overlay data. To achieve the accurate
cohesive results, we introduced the combined metrology mark which can be used for both mask registration
measurement as well as for wafer overlay measurement. Coincidence of both metrology marks make it possible to
subtract mask signature from wafer overlay without compromising the accuracy due to the physical distance between
measurement marks, if we use 2 different marks for both metrologies. Therefore, it is possible to extract pure scanner
related signatures, and to analyze the scanner related signatures in details to in order to enable root cause analysis and
ultimately drive higher wafer yield. We determined the exact mask registration error in order to decompose wafer overlay into mask, scanner, process and metrology. We also studied the impact of pellicle mounting by comparison of mask registration measurement pre-pellicle mounting and post-pellicle mounting in this investigation.
The double patterning (DPT) process is foreseen by the industry to be the main solution for the 32 nm technology node
and even beyond. Meanwhile process compatibility has to be maintained and the performance of overlay metrology has
to improve. To achieve this for Image Based Overlay (IBO), usually the optics of overlay tools are improved. It was also
demonstrated that these requirements are achievable with a Diffraction Based Overlay (DBO) technique named SCOLTM
. In addition, we believe that overlay measurements with respect to a reference grid are required to achieve the
required overlay control . This induces at least a three-fold increase in the number of measurements (2 for double
patterned layers to the reference grid and 1 between the double patterned layers). The requirements of process
compatibility, enhanced performance and large number of measurements make the choice of overlay metrology for DPT
In this work we use different flavors of the standard overlay metrology technique (IBO) as well as the new technique
(SCOL) to address these three requirements. The compatibility of the corresponding overlay targets with double
patterning processes (Litho-Etch-Litho-Etch (LELE); Litho-Freeze-Litho-Etch (LFLE), Spacer defined) is tested. The
process impact on different target types is discussed (CD bias LELE, Contrast for LFLE). We compare the standard
imaging overlay metrology with non-standard imaging techniques dedicated to double patterning processes (multilayer
imaging targets allowing one overlay target instead of three, very small imaging targets). In addition to standard designs
already discussed , we investigate SCOL target designs specific to double patterning processes. The feedback to the
scanner is determined using the different techniques. The final overlay results obtained are compared accordingly. We
conclude with the pros and cons of each technique and suggest the optimal metrology strategy for overlay control in
double patterning processes.
The newly emerging lithographic technologies related to the 32nm node and below will require a step function in the
overlay metrology performance, due to the dramatic shrinking of the error budgets. In this work, we present results of an
emerging alternative technology for overlay metrology - Differential signal scatterometry overlay (SCOTM). The
technique is based on spectroscopic analysis of polarized light, reflected from a "grating-on-grating" target. Based on
theoretical analysis and initial data, this technology, as well as broad band bright field overlay, is a candidate technology
that will allow achieving the requirements of the 32nm node and beyond it. We investigate the capability of SCOLTM to
control overlay in a production environment, on complex stacks and process, in the context of advanced DRAM and
Flash technologies. We evaluate several metrology mark designs and the effect on the metrology performance, in view
of the tight TMU requirements of the 32nm node. The results - achieved on the KLA-Tencor's Archer tool, equipped
with both broad band bright field AIMTM and scatterometry SCOLTM sensors - indicate the capability of the SCOLTM
technology to satisfy the advanced nodes requirements.
The overlay control budget for the 32nm technology node will be 5.7nm according to the ITRS. The overlay metrology
budget is typically 1/10 of the overlay control budget resulting in overlay metrology total measurement uncertainty
(TMU) requirements of 0.57nm for the most challenging use cases of the 32nm node. The current state of the art
imaging overlay metrology technology does not meet this strict requirement, and further technology development is
required to bring it to this level. In this work we present results of a study of an alternative technology for overlay
metrology - Differential signal scatterometry overlay (SCOL). Theoretical considerations show that overlay technology
based on differential signal scatterometry has inherent advantages, which will allow it to achieve the 32nm technology
node requirements and go beyond it. We present results of simulations of the expected accuracy associated with a
variety of scatterometry overlay target designs. We also present our first experimental results of scatterometry overlay
measurements, comparing this technology with the standard imaging overlay metrology technology. In particular, we
present performance results (precision and tool induced shift) and address the issue of accuracy of scatterometry
overlay. We show that with the appropriate target design and algorithms scatterometry overlay achieves the accuracy
required for future technology nodes.
As device dimensions shrink the number of parameters influencing CD increases (PEB dispersion, development uniformity, resist thickness, BARC thickness, +/- scan focus control, scanner focus control at edge of the wafer...). Separation between all these contributors is not easy using only CD-SEM measurement, and particularly with isolated lines. For high volume manufacturing (where "time is money") and in the case of litho cluster drift, a quick and accurate diagnostic capability is an advantage for minimizing tool unavailability. An important attribute of this diagnostic capability is that its implementation is on standard production wafers. The use of production wafers enables continuous monitoring and also allows a direct correlation between monitoring measurements and the impact on product.
The technology that enables this type of diagnostic capability makes use of a compact dual tone line-end-shortening based target. A key benefit to this technology is that it provides a separation of the dose and focus parameters, which leads to quicker route cause determination.
After building a calibration model and determining minimum dose and focus sensitivity, both short term and long term stability of the model is investigated. The impact of wafer topology on model prediction is also investigated in order to assess on-product monitoring capability. The main error contributors are then identified for both track and scanner and the impact on CD control is evaluated. These cluster error contributors are then varied, first separately, and then combined. Measurement results are compared to the input parameters in order to determine error detection ability, measurement accuracy and separation capability.
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.
As design rules shrink and process windows become smaller, it is increasingly important to monitor exposure tool focus and exposure in order to maximize device yield. Economic considerations are forcing us to consider nearly all methods to improve yield across the wafer. For example, it is not uncommon in the industry that chips around the edge of the wafer have lower yield or device speed. These effects are typically due to process and exposure tool errors at the edge of the wafer. In order to improve yield and chip performance, we must characterize and correct for changes in the effective focus and exposure at the edge. Monitoring focus and exposure on product wafers is the most effective means for correction, since product wafers provide the most realistic view of exposure tool interactions with the process. In this work, on-product monitoring and correction is based on optical measurement using a compact line end shortening (LES) target that provides a unique separation of exposure and focus on product wafers. Our ultimate objective is indirect CD control, with maximum yield and little or no impact on productivity.
Due to the continuous shrinking of the design rules and, implicitly, of the lithographic process window, it becomes more and more important to implement a dynamic, on product, process monitoring and control based on both dose and focus parameters. The method we present targets lot-to-lot, inter-field and intra-field dose and focus effect monitoring and control. The advantage of simultaneous dose and focus control over the currently used CD correction by adjusting exposure dose only is visible in improvement of the CD distributions both at pre-etch and at post-etch phases. The 'On Product' monitoring and compensation is based on the optical measurement of a special compact line end shortening target which provides the unique ability to separate dose from focus on production wafers.
As the design rules shrink below 130nm it will become increasingly important to monitor and control focus and dose in-line, on product wafers to maintain the ever-decreasing process window. On process layers today, it is not uncommon to see focus related errors equaling between 50-100nm in magnitude. Today these errors go undetected and CD changes are typically corrected by making a dose correction to the exposure tool. However, corrections using dose can lead to significantly smaller process latitude and therefore, products out of spec. Using a technique that was first developed by Christopher Ausschnitt at IBM Microelectronics it is possible to monitor focus and dose on production layers with a single compact target. Extending this technology on an advanced optical tool allows for precise measurements of focus and dose errors. This paper will describe the methodology of inline focus and dose monitoring using this technique on 130nm process technology with an outlook on the expectations for future nodes. Results, including focus and dose sensitivity from multiple process steps on production wafers will be shown.