The reflection of back-scattered electrons (BSE) at the objective lens of an electron beam writer leads to a diffuse resist
exposure which extends over several millimetres. The deposed energy of this unintentional exposure is much lower than
the direct one. However, if the area of the direct electron beam exposure is large enough the accumulated energy is no
longer negligible and may cause significant CD variations. Therefore, it is of crucial importance to study possible ways
of reducing this dose contribution to a minimum and in order to perform a correct proximity correction targeting to
determine its radial distribution.
In this work a model of a 50kV E-Beam writer was developed, consisting of a resist-coated silicon wafer and an opposing low-reflection disk mounted at the pole piece of the objective lens. In order to improve the low-reflection disk, different material compositions as well as an optimized surface topography of the disk are modelled.
We present the Monte Carlo simulation program MCSEM, developed at the Physikalisch-Technische Bundesanstalt
(PTB), Germany, for the simulation of Scanning Electron Microscopy (SEM) image formation at arbitrary specimen
structures (e.g. layout structures of wafers or photomasks).
The program simulates the different stages of the SEM image formation process: the probe forming, the probe-sample
interaction and the detection process. A modular program structure is used for an easy adaptation of the program to new
Arbitrarily shaped 3D structured specimen models can be applied and different electron probe shapes are modeled.
Various physical models for electron scattering in solid state material are included.
Secondary electron (SE) detection modeling is based on SE raytracing, detectors for backscattered electrons (BSE) and
transmitted electrons (TE) are also available. An electromagnetic field solver is used to simulate charging of the
specimen and the transport of the SE within the electromagnetic field. Some examples of simulation results are presented
together with comparisons with experimental results.
Proc. SPIE. 7638, Metrology, Inspection, and Process Control for Microlithography XXIV
KEYWORDS: Edge detection, Detection and tracking algorithms, Image processing, Scanning electron microscopy, Printing, Monte Carlo methods, Image filtering, Photomasks, Algorithm development, 3D image processing
We present a comparison of different methods to extract area information from images. Two different physical-based
algorithms were tested which determine the areas of arbitrarily shaped 3D nano-structures on wafers or photo-masks
(e.g. contact holes) using secondary electron images of scanning electron microscopy (SEM). One of these algorithms,
called NANOAREA, was developed by the PTB. The other one is the software package MaskEXPRESS, which was
developed by Toppan Printing Co., Ltd.
In addition to real SEM images we used Monte Carlo generated SEM images of contact holes of different shapes and
sizes. For this, the Monte Carlo simulation program MCSEM, developed at PTB, was applied. MCSEM simulates the
electron diffusion and secondary electron generation and transport in solid state material and provides simulated SEM
images of arbitrary 3D specimen structures.
NANOAREA uses basic image processing routines to estimate the edge position of a structure. Then, one-dimensional
profiles which intersect the structure boundary perpendicularly are extracted. A one-dimensional edge detection
algorithm determines the edge position on each profile. Finally these detected edge positions are used to calculate
the polygon area using the triangle method. NANOAREA showed a very small underestimation of the area of about
0.3 % with regard to the Monte Carlo simulations (i.e. sub-pixel deviation).
MaskEXPRESS has a similar approach, however employs a different edge detection algorithm. For quadratic contact
holes a very high correlation coefficient r larger than 0.99 of the CDs was seen with an offset of about 0.3 nm for the
two tested programs. Here the critical dimension (CD) is defined as the square root of the area. The deviations from
the mean offset were smaller than 1 nm over the whole investigated range. For analysis of arbitrarily shaped features
we used a double T-shaped structure. Also here almost perfect correlation was found (r = 0.98). The observed mean
offset in this case was also about 0.3 nm. The offsets depend on the length of the edge and can vary with the shape of
the structure, too.
Here we report the excellent correlation of the investigated algorithms and programs to determine area parameters
from SEM images. The results found are an important prerequisite for harmonized area measurement based on
independent algorithms and pave the way to a standardized approach to area determination and reporting of
Scanning electron microscopy (SEM) is widely used as a fast and high resolution measurement method capable to per-form characterizations of the smallest isolated and dense features which are to be specified and produced on photomasks and wafers down to the 32 nm node and below. Furthermore, electron beam writing tools for mask or direct wafer patterning need electron beam based metrology capabilities for the required high precision alignment purposes. All of these applications benefit from a proper physical understanding of the electron interaction processes in the measured features of interest and suitable simulation capabilities in order to model the measured SEM image or signal contrasts.
In this contribution we will report on a new Monte Carlo based modular simulation package, developed at the PTB and called MCSEM, which allows to model secondary as well as backscattered electron image contrasts on 3-dimensional object features. The fundamentals, basic features as well as first applications of the new simulation package MCSEM in the nanometrology field will be explained. Where appropriate, also other existing Monte Carlo based simulation pack-ages still are in use at the PTB, examples and comparisons with the new MCSEM simulation will be given.
In this work, we investigated possible geometry optimizations of backscattered electron (BSE) detectors in order to significantly improve the signal to noise ratio (SNR) of shallow Si-topographic marks. To achieve this, Monte Carlo simulations of the BSE angular distribution as well as of the BSE exit position were performed. A comparison of some theoretical calculations with the respective experimental results allowed us to qualify the theoretical results. Based on these results, we are able to present an optimized BSE detector design featuring a significant improvement of the measured SNR.
In preparation of the international Nano1 linewidth comparison on photomasks between 9 national metrology institutes,
NIST and PTB have started a bilateral linewidth comparison in 2008, independent of and prior to the Nano1 comparison
in order to test the suitability of the mask standards and the general approach to be used for the Nano1 comparison. This
contribution describes the rationale of both comparisons, the design of the mask comparison standards to be used and the
measurement methods applied for traceable photomask linewidth metrology at NIST and PTB.
Scanning electron microscopy (SEM) is used today and will be used also in the near future as a fast and high resolution
measurement method capable to perform characterizations of the smallest isolated and dense features which are to be
specified and produced on photomasks down to the 32 nm node and perhaps below. It has been demonstrated however,
that SEM based CD metrology results on different mask absorber stacks may show systematic dependencies on the absorber
materials and other stack parameters between different CD-SEM equipment. These mask dependent effects have
to be taken into account for proper cross-correlation of different CD metrology tools, which is required in mask manufacturing
In this contribution we will report on systematic investigations of simulated and measured SEM profiles of line and
space features present on current and future mask generations as well as on suitable test structures. The investigations
comprise variations of SEM primary energy settings and the response of different edge operators on these variations as
well as variations of edge angle of the mask features. The simulations are based on Monte Carlo methods which are
supplemented by additional phenomenological approaches where appropriate. Simulations of SEM edge contrasts and
the response of different CD-SEM edge algorithms will be described for feature sizes down to 32 nm, because the
smallest assist-features on the mask which have to be controlled with respect to their dimensions are about the same size
as the smallest feature sizes on the wafer.
Proc. SPIE. 6921, Emerging Lithographic Technologies XII
KEYWORDS: Signal to noise ratio, Lithography, Sensors, Silicon, Monte Carlo methods, Optical alignment, Electron beam direct write lithography, Semiconducting wafers, Signal detection, Vestigial sideband modulation
New types of alignment marks to be applied in electron beam direct write (EBDW) have been studied theoretically and
The dependence of signal contrast and signal form on such mark properties like step height, mark pitch and stack
material has been investigated in detail using Monte Carlo simulations.
The different alignment marks were etched in Si to different depths and the respective alignment repeatability was determined
with a Vistec SB3050 DW lithography tool. Finally, for the most promising mark, test exposures were performed
and the overlay determined.
We report on the traceable calibration of linewidth (CD) photomask standards which are used as reference standards for
production masks of the 65 nm node. Two different types of masks with identical layout were produced and calibrated,
namely a binary mask (CoG) and a half-tone phase shifting mask (193MoSi PSM). We will in particular describe the
applied calibration procedures and cross-correlate the results from different high resolution metrology tools, like SEM,
UV microscopy and AFM. The layout of the CD photomask standard contains isolated as well as dense line features in
both tones with nominal CD down to 100 nm. Calibration of the standards was performed at PTB by UV microscopy and
LV-SEM, supported by additional AFM measurements. For analysis of the measured high resolution microscopy images
and the deduced profiles appropriate signal modeling was applied for every metrology tool, which allows a meaningful
comparison of geometrical parameters of the measured calibration structures. By this approach, e.g. the deduced feature
widths at the top of the structures and the widths at 50% height of the structures can be related to the measured edge angles.
The linearity e. g. of the measured top CD on different type of structures on the CoG CD standard was determined
to be below 5 nm down to line feature dimensions well below 200 nm.
A new algorithm for SEM CD evaluation of trapezoidal line structures is presented. It is based on the physical modeling
of SEM image formation and allows the assignment of top and bottom structural edge positions to the SEM signal. The
SEM image profile is described by a set of piecewise continuous functions which is convoluted with the electron probe
intensity profile. The resulting function is fitted to the measured signal profile by a least squares algorithm. The fit
returns both top and bottom edge positions as well as the electron probe diameter. The algorithm is verified against three
different Monte Carlo simulation programs using different physical models of elastic and inelastic electron scattering and
secondary electron generation and transport. The effect of the physical modeling on the evaluated critical dimension is
discussed and the absolute CD deviation of the algorithm is determined for different sets of specimen and tool
parameters like edge slope angle, beam energy, and electron probe diameter.
Different type of CD metrology instrumentation is in use today for production control of photomasks, namely SEM,
AFM as well as optical microscopy and optical scatterometry is emerging, too. One of the challenges in CD metrology is
to develop a system of cross calibration which allows a meaningful comparison of the measurement results of the different
systems operated within a production environment. Here it is of special importance to understand and also to be able
to simulate the response of different metrology instrumentation to variations in sidewall profile of features on photomasks.
We will report on the preparation of a special COG test mask with an intended variation of sidewall features
and the subsequent metrological characterization of this mask in different type of CD instrumentation. The discussion of
the measurement results will be accompanied by a discussion of the simulation of instrument response to feature sidewall
The PTB has developed a new 6025 photomask standard for calibration of CD metrology tools in a joint project with partners from mask industry in Germany . We report on the design of the standard, its calibration procedures and the results of recent round robin measurements on this standard in which different CD metrology tools of the project partners were involved.
The layout of the CD photomask standard (COG and 193 nm halftone PSM) contains isolated as well as dense features in both tones with nominal CD down to 100 nm. Calibration of the standards was performed at PTB by UV microscopy and LV-SEM, supported by additional AFM measurements for edge slope characterization. For analysis of UV microscopy as well as SEM images appropriate signal modeling was applied, which allowed to extract the feature widths at the top of the structures as well as the widths at 50% height of the structures.
In this contribution we will also discuss results of a recent round robin comparison measurement performed on up-to-date metrology tools available for CD metrology today by means of one of the newly developed CD standards. We used PTB calibrated CD standards in order to provide a set of CD references for the different tools which then should be used in a "blind" comparison to calibrate an unknown CD mask of the same design as the standards. Different type of CD metrology instrumentation, namely standard UV and DUV optical as well as DUV water immersion CD microscopes, a new UV dark field optical microscope, CD-SEM, and AFM/SFM were applied. The outcome of this comparison on cur-rently applied metrology instrumentation provides a valuable source of information for cross calibration issues which are discussed in mask industry today.
We report on the results of a recent round robin comparison on new linewidth or CD photomask standards in which several partners from different companies and institutes in Germany were involved. The round robin activity is at the end of a joint project targeting at the development of a new CD mask standard and it was intended to show the performance of the CD mask standard and to test its application in cross-calibration processes. Different type of CD metrology instrumentation was used, namely optical transmission microscopy including water immersion CD microscopes with NA of 1.2 and scanning electron microscopy, supported by additional scanning probe microscopy (SPM/AFM) characterizations. A set of differently processed CD mask standards with smallest line and space structures down to 0.1 μm and based on different mask blanks was produced with identical layout. At the PTB this set of CD standards was calibrated by UV transmission microscopy and by CD-SEM as well. For the round robin an unknown CD mask of the same design as the standards was used and the participants were asked to provide measurement data with their CD metrology tools, referred to their respective PTB calibration standards. It will be shown, that the agreement of measurement data between different CD metrology tools can be significantly improved if proper definitions of the measurand and a metrologically sound approach to signal modelling and interpretation of CD measurement values is applied. The outcome of this comparison provides a valuable source of information for cross calibration issues which are discussed in mask industry today and, moreover, it proves the performance of the newly developed CD mask standard, which now is available to other interested parties, too.
Proc. SPIE. 5752, Metrology, Inspection, and Process Control for Microlithography XIX
KEYWORDS: Electron beams, Detection and tracking algorithms, Silicon, Scanning electron microscopy, Monte Carlo methods, Photomasks, Extreme ultraviolet, Extreme ultraviolet lithography, Critical dimension metrology, Signal detection
For extreme ultraviolet lithography (EUVL) the absorber binary mask is until now the most promising mask type. Since at EUV only reflective masks are possible, EUVL will introduce new materials for mask manufacturing. In addition it is likely that the pattern of an EUV mask will consist of a structured double layer system. Therefore, mask CD-SEM metrology for EUVL has to deal with the contrast of rather new materials and has to face a more complex mask pattern topography situation. Using a Monte Carlo model, we simulate the SEM-signals emerging from a given EUV mask pattern topography while scanned by the electron beam of a SEM. The simulation is tuned to closely match the experimental situation of a commercial CD-SEM. Generated SEM images are analyzed by means of a commercial CD-algorithm and a peak detection CD-algorithm. Knowing the exact pattern shape that are fed into the simulation, we determine the effect of specific pattern profile changes on SEM-signal and algorithm specific CD.
We report on the metrological characterization of photomask standards which were developed within a project with industrial partners in Germany to be used as reference standards for different type of linewidth or CD metrology instruments. It was the objective to develop high quality mask standards which would allow to perform CD calibrations with smallest possible uncertainties and to use the standards within the project consortium as well as to make the standards available to interested third parties. The design of the standards consists of line as well as box structures, isolated as well as group patterns, each in different tones and in x/y-orientation. The structures are all placed on a regular measurement grid to be easily accessible by automated CD metrology systems. Target CD values are going down to 0,2 μm or even below with smallest CD steps of 20 nm and line to space ratio within groups is varied between 1:1 and 1:5. Additionally, there are larger CD structures and clear fields and different pitch structures on the mask. This contribution will concentrate on the description of the methods and instrumentation used at the PTB for investigation and calibration of the standards. Different methods of optical transmission microscopy as well as low energy scanning electron microscopy and scanning probe microscopy were applied which all were developed to provide traceable linewidth calibration values. This implies, that suitable physical models for the interaction of the different probes with the photomask structures had to be developed for a meaningful interpretation of the measurement signals for all type of CD metrology methods. The analysis also has to take into account the influences of structure imperfections, like edge slope and line edge roughness.
Proc. SPIE. 5375, Metrology, Inspection, and Process Control for Microlithography XVIII
KEYWORDS: Microscopes, Metrology, Interferometers, Calibration, Electron microscopes, Atomic force microscopy, Scanning electron microscopy, Very large scale integration, Line edge roughness, Phase shifts
We report on investigations including calibration of a 100 nm pitch structure, the NanoLattice by VLSI Standards, with a special metrological scanning electron microscope (SEM) and a scanning force microscope (SFM). The SEM used is called electron optical metrology system (EOMS) and basically consists of a dedicated low voltage e-beam column which is mounted on top of a large vacuum chamber with an integrated, laser-controlled precision 2D stage. The key feature of this instrument is the advantage to combine sub-nm-resolution object position measurement by vacuum laser interferometry with a high resolution e-beam probe of about 5-10 nm. Correlation methods combining the laser interferometer and secondary electron intensity profile data are used to analyze global pitch as well as local pitch deviations. The EOMS measurements confirm an excellent pitch uniformity. Preliminary estimations yield sub-nanometric mean pitch uncertainties for the 100 nm grating period over the whole active area of 1 mm x 1.2 mm. Additional SFM investigations were performed by a modified NanoStation III (SIS GmbH, Germany) which has been especially adapted for high stability measurements. In this way, the instrument allows to determine pitch homogeneity and line edge roughness (LER) of the structures with high reproducibility.
Preliminary results show a good agreement with EOMS measurements.
We report on the metrological characterization of photomask standards which were developed within a project with industrial partners in Germany to be used as reference standards for different type of linewidth or CD metrology instruments. It was the objective to develop high quality mask standards which would allow to perform CD calibrations with smallest possible uncertainties and to use the standards within the project consortium as well as to make the standards available to interested third parties. The design of the standards consists of line as well as box structures, isolated as well as group patterns, each in different tones and in x/y-orientation. The structures are all placed on a regular measurement grid to be easily accessible by automated CD metrology systems. Target CD values are going down to 0.2 μm with smallest CD steps of 20 nm and line to space ratio within groups is varied between 1:1 and 1:5. Additionally, there are larger CD structures and clear fields and different pitch structures on the mask. This contribution will concentrate on the description of the methods and instrumentation used at the PTB for investigation and calibration of the standards. Different methods of optical transmission microscopy as well as low energy scanning electron microscopy and scanning probe microscopy were applied which all were developed to provide traceable linewidth calibration values. This implies, that suitable physical models for the interaction of the different probes with the photomask structures had to be developed for a meaningful interpretation of the measurement signals for all type of CD metrology methods. This analysis also has to take into account the influences of structure imperfections, like edge slope and line edge roughness.
We report on comparison measurements for linewidths of test structures on chrome on glass photomasks by means of different types of light optical transmission microscopy and low voltage scanning electron microscopy (LVSEM). The investigated linewidth or critical dimension range was chosen to be between 0.3 micrometers and 5 micrometers on isolated and dense as well as clear and opaque line structures. After offset correction of the commercial i-line CD metrology tool we observed overall agreement between the results from light optical microscopy and the results from the LVSEM in the range of 20 nm. No significant polarization dependencies of transmitted light results were observed. The edge detection algorithms used for extraction of edge position from the measured profiles will be discussed for the types of instruments involved.