We summarize the metrology and inspection required for the development of nanoimprint lithography (NIL), which is recognized as a candidate for next-generation lithography. Template inspection and residual layer thickness (RLT) metrology are discussed. An optical-based inspection tool for replica template inspection showed sensitivity for defects below 10 nm with sufficient throughput. For the RLT control, in-die RLT metrology is needed. Because the metrology requires dense sampling, optical scatterometry is the best solution owing to its ability to measure profile features nondestructively with high throughput. For in-die metrology, we have developed a new hybrid metrology that can combine key information from these complex geometries with scatterometry measurements to reduce the impact on the RLT measurement due to the layers beneath the resist. The technologies discussed here will be important when NIL is applied for IC manufacturing, as well as in the development phases of those lithography technologies.
In principal, the critical dimension (CD) of Nanoimprint lithography (NIL) pattern is determined by the CD of the template pattern. Unless one template is changed to another, NIL does not have a knob for direct control of the CD, such as the exposure dose and focus in optical lithography. Alternatively, the CD would be controlled by adjusting the thickness of the residual layer underneath the NIL pattern and controlling the etching process to transfer the pattern to a substrate. Controlling the residual layer thickness (RLT) can change the etching bias, resulting in the control of the CD of etched pattern. RLT is controllable by the resist dispense condition of the inkjet. For CD control, the metrology of RLT and feedback of the results to the dispense condition are extremely important. Scatterometry is the most promising metrology for the task because it is nondestructive 3D metrology with high throughput. In this paper, we discuss how to control CD in the NIL process and propose a process control flow based on scatterometry.
For robustness improvement of inline metrology tools, we propose an inline reference metrology system, named verification metrology system (VMS). This system combines inline metrology and nondestructive reference metrology tools. VMS can detect the false alarm error and the nondetectable error caused by measurement robustness decay of inline metrology tools. Grazing-incidence small-angle x-ray scattering (GI-SAXS) was selected as the inline reference metrology tool. GI-SAXS has high robustness capability for under-layer structural changes. VMS with scatterometry and GI-SAXS was evaluated for measurement robustness. The potential to detect metrology system errors was confirmed using VMS. Cost reduction effect of VMS was estimated for the false alarm case. Total cost is obtained as a sum of the false alarm losses and the metrology costs. VMS is effective for total cost reduction with low sampling. Also, it is important that the sampling frequency of reference metrology is optimized based on process qualities.
Optical metrology system is used as high sampling CD measurement. The optical measurement technology using Fourier image can obtain much information with various optical conditions. We evaluated Fourier image method for CD metrology. Various issues of the optical measurement technology were found for CD measurement uncertainty. Measurement uncertainty depends on the number of position on Fourier image, and measurement uncertainty is improved by using multiple positions data. Top CD value is influenced by under layer pattern CD variance and under layer thickness variance. Optical CD measurement technology is influenced by various process variation like under-layer structure, stacked film thickness, material changes and so on. If optical measurement system applies to CD metrology, Fourier image method should be used in development phase for unfixed process because high number of data and speedy process feedback in no under layer situation is needed.
For robustness improvement of inline metrology tools, we propose inline reference metrology system “Verification Metrology System (VMS)”. This system combines inline metrology tools and non-destructive reference metrology tools. VMS can detect the false alarm error and the not-detectable error caused by measurement robustness decay of inline metrology tools. GI-SAXS was selected as the inline reference metrology tool. GI-SAXS has high robustness capability for under-layer structure changes. VMS with scatterometry and GI-SAXS was evaluated for measurement robustness. The potential to detect metrology system errors was confirmed using VMS. Cost reduction effect of VMS was estimated for the false alarm case. Total cost is obtained as a sum of the false alarm loss and the metrology cost. VMS is effective for total cost reduction with low sampling. And it is important that sampling frequency of reference metrology is optimized based on process qualities.
We have created a model that uses discriminant function analysis to predict failures in etched hole patterning of the type
that induces an open-contact failure by using critical dimension scanning electron microscope (CDSEM) measurement
values of after-development resist hole patterning. The input variables of the best model were found to be the resist hole
CD, the difference in resist hole CD between that of the 50% secondary electron (SE) threshold and that of the 20% SE
threshold, and ellipticity. The model indicates that a tapered resist profile is one of the main causes of the open-contact
failure in etched hole patterning. The model is applicable not only to lithography process optimization but also to
lithography process control, where the focus center of optical exposure at resist patterning is determined not only from
the perspective of resist CD but also from the perspective of suppressing the failures of etched hole patterning.
As device structures continue to shrink and new materials are introduced, Three Dimensional (3D) Metrology becomes
more important. The creation of 3D Metrology data is defined as the generation of statistically relevant 3D information
used in R&D and/or semiconductor manufacturing. Parameters of interest are: profile shape, side wall angle, material
properties, height of the structure, as well as the variation within a die, on the wafer, between wafers. In this paper we
will show how this information is used to calibrate process control systems in a semiconductor fab. Also, results will be
shown on how this information may be used to compare different types of Metrology equipment e.g. CD-SEM and
optical CD metrology techniques like scatterometry.
Certain applications, such as the generation of profile information for 55 nm dense contact holes in photoresist, require
new technology to minimize damage to the soft photoresist. A new technique called in-situ broadband argon cleaning
will be presented. Finally, the application of the argon column for protective coating deposition of sub-45 nm photoresist
lines will be discussed.
KEYWORDS: Oxides, Scanning electron microscopy, Monte Carlo methods, Silicon, Silica, Wet etching, Inspection, Optical spheres, Electron beams, Data acquisition
As the candidates of factors to consider for accurate Monte Carlo simulation of SEM images, (1) the difference
of cross-section between an approximate shape for simple simulation and a real pattern shape, (2) the influence of native
oxide growing on a pattern surface, and (3) the potential distribution above the target surface are proposed. Each
influence on SEM signal is studied by means of experiments and simulations for a Si trench pattern as a motif. Among
these factors, native oxide of about 1nm in thickness has a significant influence that increases SEM signals at the top
edge and the slope. We have assumed and discussed models for the native oxide effect.
KEYWORDS: Monte Carlo methods, Scanning electron microscopy, Silica, Silicon, Electron beams, Dielectrics, Defect inspection, Optical simulations, Sensors, 3D modeling
CD-SEM measurement is the main measuring tool of critical dimensions (CD). CD-measurements involve
systematic errors that depend on SEM set-up and the pattern. In addition to systematic errors, charging of a
wafer plays an important role in CD-SEM and defect inspection tools. Charging dependence of secondary
electron emission coefficient which is one of the major charging parameters, was studied. Timing
characteristics were measured and then simulated using Monte Carlo model. The measurements and
simulations were done for a multiple number of frames and for imaging of a contact hole using pre-charge of
a large area. The results of simulation confirmed the measured results. The understanding of the effect helps
in tuning the settings of CD-SEM.
As design rules shrink, hotspot management is becoming increasingly important. In this paper, an automatic system of
hotspot monitoring that is the final step in the hotspot management flow is proposed. The key technology for the
automatic hotspot monitoring is contour-based metrology. It is an effective method of evaluating complex patterns, such
as hotspots, whose efficiency has been proved in the field of optical proximity correction (OPC) calibration. The
contour-based metrology is utilized in our system as a process control tool available on mass-production lines.
The pattern evaluation methodology has been developed in order to achieve high sensitivity. Lithography simulation
decides a hotspot to be monitored and furthermore indicates the most sensitive points in the field of view (FOV) of a
hotspot image. And quantification of the most sensitive points is consistent with an engineer's visual check of a shape of
a hotspot. Its validity has been demonstrated in process window determination. This system has the potential to
substantially shorten turnaround time (TAT) for hotspot monitoring.
As device feature size reduction continues, requirements for Critical Dimension (CD) metrology tools are
becoming stricter. For sub-32 nm node, it is important to establish a CD-SEM tool management system with higher
sensitivity for tool fluctuation and short Turn around Time (TAT). We have developed a new image sharpness
monitoring method, PG monitor. The key feature of this monitoring method is the quantification of tool-induced image
sharpness deterioration. The image sharpness index is calculated by a convolution method of image sharpness
deterioration function caused by SEM optics feature. The sensitivity of this methodology was tested by the alteration of
the beam diameter using astigmatism. PG monitor result can be related to the beam diameter variation that causes CD
variation through image sharpness. PG monitor can detect the slight image sharpness change that cannot be noticed by
engineer's visual check. Furthermore, PG monitor was applied to tool matching and long-term stability monitoring for
multiple tools. As a result, PG monitor was found to have sufficient sensitivity to CD variation in tool matching and
long-term stability assessment. The investigation showed that PG monitor can detect CD variation equivalent to ~ 0.1
nm. The CD-SEM tool management system using PG monitor is effective for CD metrology in production.
KEYWORDS: Monte Carlo methods, Silica, Scanning electron microscopy, Silicon, Optical simulations, Scattering, Electron beams, Laser scattering, Sensors, Electron transport
In semiconductor manufacturing, control of hotspots by optical proximity correction (OPC) requires
accurate measurements of shapes and sizes of fabricated features. These measurements are carried
out using CD-SEM. In order to measure 2D shapes, edges of features should be clearly defined in all
directions. Positions of edges are often unclear because of charging. Depending on the SEM setup and
the pattern under measurement, the effect of charging varies. The influence of measurement conditions
can be simulated and optimized. A Monte Carlo electron-beam simulation tool was developed, which
takes into account electron scattering and charging. CD-SEM imaging of SiO2 lines on Si were studied.
In experiment, an effect of contrast tone reversal was found, when beam voltage was varied. The same
effect was also found in simulations, where contrast reversal was similar to the experimental results. The
time dependence of contrast variation was also studied. A good agreement between simulation and
measurement was found. The simulation software proved reliable in predicting SEM images, which
makes it an important tool to optimize settings of electron-beam tools. Based on such simulations,
optimum conditions of SEM setup can be found.
KEYWORDS: Critical dimension metrology, Optical simulations, Electron beams, Monte Carlo methods, Silicon, Scanning electron microscopy, Scattering, Laser scattering, Oxides, Atomic force microscopy
In recent year, CD metrology is required not only precision but also accuracy for more accurate CD control.
CD bias between CD-SEM and a reference tool is the most important factor for more accurate CD measurement. CD
bias varies by many CD-SEM and pattern condition. Then, CD bias variation caused by CD-SEM should be evaluated
in detail. However, it is difficult to estimate these factors dependence on CD bias variation experimentally. Then, we
develop an electron beam simulator with charging effects. We evaluated the mechanism of CD bias variation using
electron beam simulator and CD-SEM data. As the results, CD bias variation is caused by changing of secondary
electron signal which depends on space width. There are several different points between the experimental results and
the simulation results in grayscale line profiles. Simulation data can be more similar to experimental data with charging
effects and the actual experimental conditions. Simulation has enough capability to estimate CD bias variation with the
simple structure and non-charging calculation. And mechanism of space width dependence on CD bias can be analyzed
by using electron beam simulator.
KEYWORDS: Scanning electron microscopy, Scatterometry, Metrology, Critical dimension metrology, Monte Carlo methods, Inspection, Stereolithography, Manufacturing, Time metrology, Optical proximity correction
Measurement characteristics in scatterometry and critical dimension-scanning electron microscopy (CD-SEM) for lot acceptance sampling of inline CD metrology were investigated by using a statistical approach with Monte Carlo simulation. By operation characteristics curve analysis, producer's risk and consumer's risk arising from sampling were clarified. Single use of scatterometry involves a higher risk, such risk being particularly acute in the case of large intrachip CD variation because it is unable to sufficiently monitor intrachip CD variation including local CD error. Substituting scatterometry for conventional SEM metrology is accompanied with risks, resulting in the increase of unnecessary cost. The combined use of scatterometry and SEM metrology in which the sampling plan for SEM is controlled by scatterometry is a promising metrology from the viewpoint of the suppression of risks and cost. This is due to the effect that CD errors existing in the distribution tails are efficiently caught.
CD control of hole bottom becomes more difficult with pattern size shrinkage. Since local CD variation of hole
patterns is large, CD measurement by CD-SEM is needed for measuring the local CD. Although a technique of
observing the hole bottom by CD-SEM has been reported, accuracy of bottom CD measurement is seldom examined.
We estimated the tool precision and CD bias required for highly accurate CD control. As a result, the bottom CD
measurement repeatability was examined for 0.94nm. Tool precision has sufficient capability for hp45 node. Si transfer
process was the technique used for estimating CD bias. CD bias obtained by Si transfer process was constant in the
bottom CD range of 45 nm or more. The above result indicates bottom CD measurement using CD-SEM has sufficient
capability for measuring bottom CD correctly for hp45 node.
KEYWORDS: Scatterometry, Critical dimension metrology, Metrology, Scanning electron microscopy, Monte Carlo methods, Inspection, Time metrology, Statistical analysis, Optics manufacturing, Line edge roughness
Measurement characteristics in scatterometry and CD-SEM for lot acceptance sampling of inline critical dimension (CD) metrology were investigated by using a statistical approach with Monte Carlo simulation. By operation characteristics curve analysis, producer's risk and consumer's risk arising from sampling were clarified. Single use of scatterometry involves a higher risk, such risk being particularly acute in the case of large intra-chip CD variation because it is unable to sufficiently monitor intra-chip CD variation including local CD error. Substituting scatterometry for conventional SEM metrology is accompanied with risks, resulting in the increase of unnecessary cost. The combined use of scatterometry and SEM metrology in which the sampling plan for SEM is controlled by scatterometry is a promising metrology from the viewpoint of the suppression of risks and cost. This is due to the effect that CD errors existing in the distribution tails are efficiently caught.
For advanced process control, a sampling plan for critical dimension measurement is optimized through empirical considerations concerning the nature of error and a statistical approach. The metric of the optimization is the accuracy of lot mean estimation. In this work, critical dimension errors are classified into static and dynamic components. The static component is defined as the error which repeats through lots while keeping its tendency, and the dynamic as the error whose tendency changes by lot. In the basic concept of our sampling plan, sampling positions and size are determined from the static and dynamic error, respectively. The balance of sampling number of wafer, field and pattern is obtained under the restriction of total sampling size by a statistical theory with some assumptions. Based on the concept, we could make a sampling plan for 65 nm CMOS lithography.
The measurement precision required for 65 nm technology node is 0.4 nm. However, ITRS has reported that the present CD-SEM has not had sufficient capability for 65 nm technology node. It is necessary to analyze the error factor of measurement precision thoroughly, in order to improve CD-SEM performance. Then, the items to be improved and the control method of tools for the measurement precision required for 65 nm technology node were examined. The error factors of CD measurement were divided into short-term repeatability, long-term variation, and tool matching. In factor analysis of short-term repeatability, the main factors of short-term repeatability were the image quality/measurement method and wafer load/unload. And it became clear that the interaction between local CD variation and scan shift accuracy had a remarkable effect on short-term repeatability. We established a method of monitoring tool condition in order to calculate long-term variation and tool matching with high accuracy. According to the experimental results of two tools for four weeks, the main factors of long-term variation and tool matching were initial variation and CD offset. From calculation of measurement precision using these results, measurement precision of the present CD-SEM has sufficient capability for hp90. It is reasonable to expect that improvement of these error factors will lead to the attainment of capability sufficient for hp65 measurement precision in the future.
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