ArF lithography is still the main technology in the most advanced processes of semiconductor fabrication. Being able to
reliably measure and characterize these lithographic processes in-depth is becoming more and more critical. Critical
Dimension-Scanning Electron Microscope (CD-SEM) continues to be the work horse tool for both in-line critical
dimension (CD) metrology and characterization of ArF photoresist pattern. CD shrink of ArF photoresist has been one of
the major challenges for CD-SEM metrology, and it becomes more difficult to measure shrinkage accurately for smaller
feature size than ~50nm. The authors have developed a new measurement technique of photoresist shrinkage which
measures CD difference between shrunk and non-shrunk sites after etching.
There are many imaging and image processing parameters in CD-SEM which need to be optimized to obtain small
shrinkage and good precision. There is a trade-off relationship between shrinkage and precision, and a comprehensive
and systematic methodology is required for optimization of parameters. The authors have developed an optimization
method that uses Taguchi method, where only 18 experiments are required. We can predict shrinkage, precision and
relative CD offset for any combination of measurement parameter settings used in the 18 experiments by Taguchi
method, and these predicted data can be used for optimization. A new concept of secondary reference metrology is also
introduced in this paper to reduce the number of measurement by a reference metrology tool.
OPC technique is getting more complicated toward 32nm and below technology node, i.e. from moderate
OPC to aggressive OPC. Also, various types of phase shift mask have been introduced, and then the
manufacturing process of them is complicated now. In order to shorten TAT (Turn around time) time, mask
technique need be considered in addition to lithography technique.
Furthermore, the lens aberration of the exposure system is getting smaller, so the current performance of it is
very close to the ideal. On the other hand, when down sizing goes down to 32nm technology node, it starts to
be reported that there are cases that size cannot be matched between a mask pattern and the corresponding
printed pattern. Therefore, it is very indispensable to understand the pattern sizes correlation between a mask
and the corresponding printed wafer in order to improve the accuracy and the quality, in the situation that the
device size is so small that low k1 lithography had been developed and widely used in a production.
Then it is thought that it is one of the approaches to improve an estimated accuracy of lithography by using
contour that was extracted from mask SEM image in addition to mask model.
This paper describes a newly developed integration system in order to solve issues above, and the applications.
This is a system which integrates CG4500; CD-SEM for mask and CG4000; CD SEM for wafer; using
DesignGauge; OPC evaluation system by Hitachi High-Technologies.
It was investigated that a measurement accuracy improvement by executing a mask-wafer same point
measurement with same measurement algorithm utilizing the new system. At first, we measured patterns
described on a mask and verified the validity based on a measurement value, picture, measurement parameter
and the coordinate. Then create a job file for a wafer CD-SEM using the system so as to measure the same
patterns that were exposed using the mask. In addition, average CD measurement was tried in order to
improve the correlation. Also, in order to estimate very accurate pattern shape, a contour was calculated from a mask SEM image, the
result and the design data was used in a litho simulation. This realizes verification including mask error.
It is thought that it is beneficial for both mask maker and device maker to use this system.
OPC (Optical Proximity Correction) technique is getting more complicated towards 32 nm technology node and beyond,
i.e. from moderate OPC to aggressive OPC. Also, various types of phase shift mask have been introduced, and their
manufacturing process is complicated. In order to shorten TAT (Turn around time), mask design technique needs be
considered in addition to lithography technique.
Furthermore, the lens aberration of the exposure system is getting smaller, so its current performance is very close to the
ideal. On the other hand, when down sizing of device feature size reaches the 32nm technology node, cases begin to be
reported where the feature dimension is not matched between a mask pattern and the corresponding printed pattern.
Therefore, it is indispensable to understand the pattern size correlation between a mask and the corresponding printed
wafer in order to improve the processing accuracy and the quality in the situation where the device size is so small that
the low k1 lithography is widely used in production.
One of the approaches to improve the estimated accuracy of lithography is the use of contour data extracted from mask
SEM image in addition to the application of a mask model.
This paper describes a newly developed integration system that aims to solve the issues above, and its applications. This
is a system that integrates mask CD-SEM (Critical Dimension-Scanning Electron Microscope) CG4500, wafer CD-SEM
CG4000, OPC evaluation system DesignGauge, all manufactured by Hitachi High-Technologies.
The measurement accuracy improvement was examined by executing a mask-wafer same point measurement, i.e.
measurement of the corresponding points, with same measurement algorithm utilizing the new system. First, we
measured mask patterns and verified the validity based on the measurement value, the image, the measurement
parameter and the coordinates. Then a job file was formulated for a wafer CD-SEM using the new system so as to
measure the corresponding patterns that were exposed using the mask. In addition, the average CD measurement was
tried in order to improve the capability.
Furthermore, in order to estimate the pattern shape with high accuracy, a contour was calculated from a mask SEM
image, and the result was used with the design data in a litho simulation. This realizes a verification that includes mask
This system is expected to be beneficial for both mask makers and device makers.
Measurement uncertainty requirement 0.37 nm has been set for the Critical Dimension (CD) metrology tool in 32 nm
technology generation, according to the ITRS. The continual development in the fundamental performance of Critical
Dimension Scanning Electron Microscope (CD-SEM) is essential, as in the past, and for this generation, a highly precise
tool management technology that monitors and corrects the tool-to-tool CD matching will also be indispensable.
The potential factor that strongly influences tool-to-tool matching is the slight difference in the electron beam
resolution, and its determination by visual confirmation is not possible from the SEM images. Thus, a method for
quantitative evaluation of the resolution variation was investigated and Profile Gradient (PG) method was developed. In
its development, considerations were given to its sensitivity against CD variation and its data sampling efficiency to
achieve a sufficient precision, speed and practicality for a monitoring function that would be applicable to mass
semiconductor production line. The evaluation of image sharpness difference was confirmed using this method.
Furthermore, regarding the CD matching management requirements, this method has high sensitivity against CD
variation and is anticipated as a realistic monitoring method that is more practical than monitoring the actual CD
variation in mass semiconductor production line.
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.
In the previous study, we reported on the CD measurement of multi gate field effect transistors (MuGFETs) by using CD-SEM. We focused on the etching residue at the fin-gate intersection, which causes gate length variation and affects the device performance. Therefore we proposed a technique to quantify the amount of etching residues from CD-SEM top-down images. The increment of the gate linewidth at the fin sidewall was introduced as the "residue index". In this
study, to validate the residue index measurement technique, experiments were carried out. First, the actual shape of the
etching residue was verified in detail by high-resolution experimental-SEM and STEM cross-sectional imaging techniques. Next, the measurement capability of CD-SEM image was confirmed by comparing with the high-resolution experimental-SEM measurement results. Finally, the proposed technique was applied to the layout dependency
evaluation of the residue index, and it was confirmed that the residue index has enough sensitivity to quantify the systematic residue size variation related to fin A/R. Then, we confirmed the reliability of the proposed technique. The residue index measurement technique is expected to be useful for the evaluation of the gate etching process of the MuGFET.
Multiple Gate Field Effect Transistors (MuGFETs) have been proposed to enable downsizing, when scaling
the transistors to the 32nm technology node. The dimension of the gate on the surface of fin determines the
effective channel length of the device. So, the characterization of the gate profiles at fin sidewalls becomes
extremely critical. It is especially important to quantify the rounded intersection (etch residual) at the
intersection of the fin and gate.
In this report, we show top down images of a MuGFET taken with critical-dimension scanning electron
microscopy (CD-SEM) and the results that were measured and characterized by measuring various portions of
the pattern which will impact the MuGFET performance i.e. gate length, fin width. We will introduce a
quantified relation between fin length and "its effect on the etch residue at the intersection of fin and gate".
Next we discuss our approaches to analyze the variation of the shape of the gate at the fin sidewall.
With the recent introduction of immersion lithography, optical systems with numerical aperture (NA) reaching 1.0 or
larger can be realized. Various Resolution Enhancement Techniques (RET) such as various phase shift mask approaches
have been used to push even further the resolution limit by reducing k1 scaling factor, including Double Patterning
Technology. However, with the improved resolution by Hyper-NA and Low-k1, lithographers face the problem of
decreasing Depth of Focus and in turn reduced process latitude. Throughout the industry, Process Window has been
widely used as an analytical tool to evaluate process latitude for a given design feature size; therefore, the ability to
accurately and efficiently derive a Process Window within which a process can run on target and in control is
fundamental to Low-k1 lithography. Accuracy of Process Window derivation is based on the ability to accurately
measure and model the physical dimension of the design feature and how it changes in response to changes in process
parameters. In the case of lithography, the Process Window of a desired critical dimension target is bounded by
changes in exposure energy and defocus. To be able to accurately measure the physical dimension of the design
feature remains a big challenge for metrologists especially in the presence of other process noise. In this work, it is
shown that the precision of PW measurement can be enhanced by using CD-ACD (Average CD) function to measure a
FEM (Focus-Exposure matrix) wafer. ACD is a function, which simultaneously measures several points, thus
providing higher precision measurement in comparison to the conventional single point measurement. As seen in this
work, by using ACD measurements to derive the Process Window, there is a significantly improvement in the stability
of the derived Process Window. Also reported is the MPPC (Multiple Parameters Profile Characterization) *1), a
function which provides the ability to extract pattern shape information from a measured e-beam signal. This function
together with the ACD function enables PW measurement with high precision, which also takes into account the actual
pattern shape. PW derived from conventionally measured data was compared with PW derived from ACD and MPPC
measurement and we were able to demonstrate an improvement of more than 30% in precision of PW determination.
There are many factors to consider when monitoring the stability of CD-SEM tools in the semiconductor manufacturing environment. With decreasing feature size and high aspect ratio dimensions, metrology tool calibration, stability, monitoring and matching play a more significant role in obtaining consistent CD measurements. It is not easy to separate the cause of outlier CD measurements. Tool owners need to consider all possible factors when matching across toolsets. For example, the tool should demonstrate repeatable electrical beam alignments in order to minimize the contribution of CD-SEM drift to measurement error. In order to overcome error in CD measurement caused by CD-SEM tool drift, it is important to monitor critical tool parameters that can produce shifts in CD measurements.
Probe current is a critical CD-SEM parameter that affects CD measurement precision. Drifts in probe current can be the result of instabilities in the emission current, accumulation of contamination on the objective aperture, or misalignment of the SEM optics. Since measurement precision is impacted by drifts in probe current, Hitachi and HP began monitoring probe current on HP’s S9000 CD-SEMs in an effort to understand Ip drift effect on CD measurements.
HP and Hitachi utilized an Information Server system, which was developed by Hitachi High Technologies America, Inc., to facilitate data collection. Information server is a web-based program which will archive and monitor many parameters of Hitachi CD-SEM tools. Hitachi Applications Engineers worked with HP Metrology Engineering to put the capability in place.
In this paper, we will address probe current instability and its impact on CD measurements. We will explore the relationship between probe current, CD data, and errors in pattern recognition caused by probe current and alignment drift.
A prototype of a digital video storage system (CD-watcher) has been developed and attached to a Hitachi S-9380 CD-SEM. The storage system has several modes that are selectable depending on the phenomenon of interest. The system can store video images of duration from a few seconds to a few weeks depending on resolution, sampling rate, and hard disc drive capacity.
The system was used to analyze apparent focusing problems that occurred during the execution of automated recipes. Intermittent focusing problems had been an issue on a particular tool for a period of approximately three months. By reviewing saved images, the original diagnosis of the problem appeared to be auto focus. Two days after installation, the CD-watcher system was able to record the errors making it possible to determine the root cause by checking the stored video files. After analysis of the stored video files, it was apparent that the problem consisted of three types of errors. The ability to record and store video files reduced the time to isolate the problem and prevented incorrect diagnosis.
The system was also used to explain a complex phenomenon that occurred during the observation a particular layer. Because it is sometimes difficult to accurately describe, and to have others easily understand, certain phenomena in a written report, the video storage system can be used in place of manual annotation.
In this report, we describe the CD-watcher system, test results after installing the system on a Hitachi S9380 CD-SEM, and potential applications of the system.
Besides feature size control of advanced semiconductor device manufacturing, critical dimension (CD) measurement SEMs are also indispensable tools for the development of advanced semiconductor manufacturing equipment or new semiconductor manufacturing materials. Especially in the case of advanced stepper and resist development for ultra micro patterns where the role of CD-SEMs is particularly important for evaluation specific samples, such as focus exposure matrix (FEM). An FEM sample is a wafer that has hundreds to thousands of patterns created with varying resist exposure dosage and Stepper focuses. As a result, the pattern shape and the line width vary dramatically within one wafer and the number of CD-SEM measurement points necessary to evaluate such FEM samples also increases drastically with decreasing semiconductor design rules. Thus, a CD-SEM that can measure FEM samples with high throughput and high reliability is strongly desired. For this purpose Hitachi has developed a new pattern detection algorithm. This algorithm detects a target and judges the quality of the actual pattern by using criteria similar to those a human operator might use when measuring the sample. With this method implemented on a Hitachi CD-SEM S-9200 we achieved a highly automated, fast and accurate measurement of FEM samples on which conventional algorithms failed.