Scanner matching based on CD or patterning contours has been demonstrated in past works. All of these published works require extensive wafer metrology. In contrast, this work extends a previously proposed optical pattern matching method that requires little metrology by adding the component requirements and the procedure for creating an automation flow. In a test case, we matched an ASML XT:1900i using a DOE to an ASML NXT:1950i scanner using FlexRay. The matching was conducted on a 4x nm process test layer as a development vehicle for the 2x nm product nodes. The paper summarizes the before and after matching data and analysis, with future opportunities for improvements suggested.
Scanner matching based on wafer data has proven to be successful in the past years, but its adoption into production has
been hampered by the significant time and cost overhead involved in obtaining large amounts of statistically precise
wafer CD data. In this work, we explore the possibility of optical model based scanner matching that maximizes the use
of scanner metrology and design data and minimizes the reliance on wafer CD metrology.
A case study was conducted to match an ASML ArF immersion scanner to an ArF dry scanner for a 6Xnm technology
node. We used the traditional, resist model based matching method calibrated with extensive wafer CD measurements
and derived a baseline scanner manipulator adjustment recipe. We then compared this baseline scanner-matching recipe
to two other recipes that were obtained from the new, optical model based matching method. In the following sections,
we describe the implementation of both methods, provide their predicted and actual improvements after matching, and
compare the ratio of performance to the workload of the methods. The paper concludes with a set of recommendations
on the relative merits of each method for a variety of use cases.
As the industry drives to lower k1 imaging we commonly accept the use of higher NA imaging and advanced
illumination conditions. The advent of this technology shift has given rise to very exotic pupil spread functions that
have some areas of high thermal energy density creating new modeling and control challenges. Modern scanners are
equipped with advanced lens manipulators that introduce controlled adjustments of the lens elements to counteract the
lens aberrations existing in the system. However, there are some specific non-correctable aberration modes that are
detrimental to important structures. In this paper, we introduce a methodology for minimizing the impact of aberrations
for specific designs at hand. We employ computational lithography to analyze the design being imaged, and then devise
a lens manipulator control scheme aimed at optimizing the aberration level for the specific design. The optimization
scheme does not minimize the overall aberration, but directs the aberration control to optimize the imaging performance,
such as CD control or process window, for the target design. Through computational lithography, we can identify the
aberration modes that are most detrimental to the design, and also correlations between imaging responses of
independent aberration modes. Then an optimization algorithm is applied to determine how to use the lens manipulators
to drive the aberrations modes to levels that are best for the specified imaging performance metric achievable with the
tool. We show an example where this method is applied to an aggressive memory device imaged with an advanced ArF
scanner. We demonstrate with both simulation and experimental data that this application specific tool optimization
successfully compensated for the thermal induced aberrations dynamically, improving the imaging performance
consistently through the lot.
The extension of ArF lithography through reduced k1, immersion and double patterning techniques makes lithography a
difficult challenge. Currently, the concept of simple linear flow from design to functional photo-mask is being replaced
by a more complex scheme of feedback and feed-forward loops which have become part of a complex computational
lithography scheme. One such novel lithography concept, called "holistic lithography", was recently introduced by
ASML, as a scheme that makes the lithography process a highly efficient solution for the scaled down geometries. This
approach encourages efficient utilization of computational lithography and the use of feed-forward and feed-back critical
dimension (CD) and overlay correction loops. As sub-nanometer feature dimensions are reached for 3x nodes, with k1
reaching the optics limitations, Mask error enhancement factor (MEEF) values grow fast, thus making mask uniformity
fingerprint and degradation throughout its life time a significant factor in printed CDU on the wafer. Whereas the
consensus is on the need for growing density of intra-field data, traditional critical dimension scanning electron
microscope (CDSEM) Feed backward loops to the litho-cell become unsuitable due to the high density CD measurement
requirements. Earlier publications proposed implementing the core of the holistic lithography concept by combining two
technologies: Applied Material's IntenCDTM and ASML DoseMapper . IntenCD metrology data is streamed in a feedforward
fashion through DoseMapper and into the scanner, to create a dose compensation recipe which improves the
overall CDU performance. It has been demonstrated that the IntenCD maps can be used to efficiently reduce intra-field
printed CDU on printed wafers.
In this paper we study the integration concept of IntenCD and DoseMapper in a production environment. We implement
the feed-forward concept by feeding IntenCD inspection data into DoseMapper that is connected to ASML's
TWSINCANTM XT:1900i scanner. We apply this concept on printed wafers and demonstrate significant reduction in
intra-field CDU. This concept can effectively replace the feedback concept using send-ahead wafers and extensive
CDSEM measurements. The result is a significant cost saving and fab productivity improvement. By routinely
monitoring mask-based CDU, we propose that all photo-induced transmission degradation effects can be compensated
through the same mechanism. The result would be longer intervals between cleans, improved mask lifetime, and better
end of line device yield.
A top challenge for Photolithographers during a process transfer involving multiple-generation scanners is tool
matching. In a more general sense, the task is to ensure that the wafer printing results in the receiving fab will match or
even exceed those of the originating fab. In this paper we report on two strategies that we developed to perform a photo
process transfer that is tailored to the scanner's capabilities. The first strategy presented describes a method to match the
CD performance of the product features on the transferred scanner. A second strategy is then presented which considers
also the down-stream process tools and seeks to optimize the process for yield. Results presented include: ASML
TWINSCANTM XT:1700i and XT:1900i scanners 1D printing results from a line-space test reticle, parametric sensitivity
calculations for the two scanners on 1D patterns, simulation predictions for a process-optimized scanner-matching
procedure, and final wafer results on 2D production patterns. Effectiveness of the optimization strategies was then
concluded.
As we move toward printing sub-100nm features using 193nm dry photolithography with high-contrast photoresists,
effects of mask transmission and pattern density start to play an important role in critical dimension uniformity (CDU).
With these two factors in existence, the linewidth for a dense feature block gradually increases from the center to the
edge of the array of the block. This change in CD is typically observed for low-transmission reticles. In this paper, we
have characterized variables, such as reticle tone and resist processing parameters, which have an effect on the CD
uniformity. Use of high-contrast photoresist can increase the effect of chemical flare and can have higher CDU. We
have further shown that by using a topcoat or by making changes in the resist bake temperature and time, the effect of
chemical flare can be reduced. We also propose a mechanism by which resists exhibit this characteristic and show that
both the photoacid generator and quencher can contribute to chemical flare.
Low k1 lithography poses a number of challenges to the process development engineer. Although polarization and
immersion lithography will allow us to create processes at lower k1 than previous paradigms allowed, the lithographer
will quickly be looking for Resolution Enhancement Techniques (RET) to push to the ultra-low k1 regime, or to extend
older generation tools and avoid the aforementioned expensive options. Reticle transmission is a RET that can enable a
low k1 process by increasing image contrast. With work performed in conjunction with our MP Mask facility, we have
been able to obtain custom-transmission EAPSM reticles. Reticle transmission optimization can be carried out through
simulation. Optimum transmission varies depending on optical parameters and feature size. Moreover, when working
with 2D patterns, reticle transmission can be optimized for weaker features, without significantly sacrificing image
contrast on primary features.
Process improvement by optimizing reticle transmission will be explored for a variety of device types using both 248nm
and 193nm lithography. Simulation, custom-transmission reticle fabrication, and empirical wafer results will be
presented.
Meeting a specific CD uniformity roadmap becomes more and more difficult as different budget components affecting CD uniformity fail to meet their requirements. For example, reticle manufacturing is at the edge of its potential, and hotplates impact CD uniformity by design. Also, etch processes must be balanced between optimal settings for varying structures. While work continues to enhance the performance of individual budget components, applying local exposure dose compensation with a scanner can provide a near-term solution for improving CD uniformity. Within the wafer processing chain, only the scanner has the unique capability to influence the final quality across-field and field-to-field in a controlled manner, making it the most effective tool for compensation. This paper describes the subsystems required for dose compensation and presents a solution that allows full integration into an automated fabrication environment. Examples will show that both the reticle contribution as well as the process-induced across-wafer fingerprint, including etch, can be improved by up to 50 percent. This improvement is demonstrated both on test structures and on memory device layers.
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