Optical surface topography measuring instruments are used more and more widely for surface quality control in industry by enabling fast, areal and non-destructive surface topography measurements. However, due to the complexity of the interaction between the surface properties to be measured and the measuring system, their capability to accurately reproduce topographical features of a surface under test is quite often questionable. To understand and investigate the topographic measurement accuracy of different optical surface topography measuring instruments, a physical measurement standard has been developed at PTB which is intended to be used to determine the metrological characteristics of surface topography measuring instruments such as topographic spatial resolution and topography fidelity. The physical standard, fabricated by a diamond turning process, containing nine sinusoidal structures with different amplitudes from 50 nm to 10 μm and varying spatial wavelengths from 2.6 μm to 82.8 μm, is suitable for the characterization of optical instruments with different magnification and numerical apertures. The design of the chirp structures, including the wavelength series, the smallest wavelength for different amplitudes, the slope distribution and the layout are detailed in this paper. The tool path for accurately positioning the cutting tool in fabrication is also described. First measurement results of the instruments response in terms of the features’ aspect ratio, slopes and curvatures, the homogeneity of the field of view of a confocal microscope are presented.
The demand for growing precision of shrinking structures on photolithographic masks makes fast, reliable, and robust testing tools necessary. Scanning electron microscopes (SEM) therefore are standard metrology tools for critical dimension (CD) measurements. An algorithm that is independent of `a priory knowledge of material parameters of the nanostructure is employed to determine CD. The analysis procedure is optimized in particular for fast scanning and small current probing beams to measure line structures on photolithographic masks. The edge characterization with SEM has been complemented with AFM measurements. In particular, the width of the edge transition estimated from height and slope of AFM topography information has been compared to that obtained from SEM scans. Height and slope are unidirectional parameters, so their determination does not need tip deconvolution. To characterize corner roundings the convolution process has to be understood. Therefore, simulations of physical processes of atomic force microscopy have been carried out to understand the influence of force gradients on the probing process causing a double convolution of tip and sample geometry, firstly of the geometries directly and secondly because of the changing interaction due to changing geometries. The analysis method to estimate edge parameters from SEM images works stable for line widths down to 50 nm. A long term stability down to ±0.6 nm has been observed.
KEYWORDS: Atomic force microscopy, 3D metrology, Critical dimension metrology, Metrology, Digital signal processing, Photomasks, Calibration, Signal processing, Signal detection, Line width roughness
A new three-dimensional atomic force microscopy (3D-AFM) for true 3D measurements of nanostructures has been developed at Physikalisch Technische Bundesanstalt (PTB), the national metrology institute of Germany. In its configuration, two piezo actuators are applied to drive the AFM cantilever near its vertical and torsional resonant frequencies. In such a way, the AFM tip can probe the surface with a vertical and/or a lateral oscillation, offering high 3D probing sensitivity. For enhancing measurement flexibility as well as reducing tip wear, a vector approach probing (VAP) method is applied. The sample is measured point by point using this method. At each probing point, the tip is approached toward the surface in its normal direction until the desired tip-sample interaction is detected and is then immediately withdrawn from the surface. Preliminary experimental results show promising performance of the PTB system. The measurement of an IVPS 100 sample using a flared AFM tip showed a repeatability of its 3D profiles better than 1 nm (p-v). A single crystal critical dimension reference material having features with almost vertical sidewalls was also measured using a flared AFM tip. These results show that the feature has average left and right sidewall angles of 89.5 and 89.4, respectively. However, the nonuniformity of the feature width within the measurement window of 1 μm may be up to 10 nm. The standard deviation of the average middle CD values from 10 repeated measurements is 0.1 nm. In addition, an investigation of long-term measurement stability was performed on a PTB photomask. The results changed at a rate of about 0.00033 nm per line, which confirms the high measurement stability and the very low tip wear of the system.
KEYWORDS: Atomic force microscopy, 3D metrology, Critical dimension metrology, Digital signal processing, Photomasks, Metrology, Calibration, Line width roughness, Signal processing, Crystals
A new 3D-AFM for true 3D measurements of nano structures has been developed at Physikalisch
Technische-Bundesanstalt, the national metrology institute of Germany. In its configuration, two piezo actuators are applied to drive
the AFM cantilever near its vertical and torsional resonant frequencies. In such a way, the AFM tip can probe the surface
with a vertical and/or a lateral oscillation, offering high 3D probing sensitivity. For enhancing measurement flexibility
as well as reducing tip wear, a so called "vector approach probing" (VAP) method has been applied. The sample is
measured point by point using this method. At each probing point, the tip is approached towards the surface in its normal
direction until the desired tip-sample interaction is detected and then immediately withdrawn from the surface.
Preliminary experimental results show promising performance of the developed system. The measurement of a line
structure of 800 nm height employing a super sharp AFM tip is performed, showing a repeatability of its 3D profiles of
better than 1 nm (p-v). A single crystal critical dimension reference material (SCCDRM) having features with almost
vertical sidewall is measured using a flared AFM tip. Results show that the feature has averaged left and right sidewall
angles of 88.64° and 88.67deg;, respectively. However, the feature width non-uniformity may reach 10 nm within the
measurement range of 1 μm. The standard deviation of the averaged middle CD values of 7 repeated measurements
reaches 0.35 nm. In addition, an investigation of long term measurement stability is performed on a PTB photomask. The
results shows that the 3D-AFM has a drift rate of about 0.00033 nm per line, which confirms the high measurement
stability and the very low tip wear.
With the increasing amount of applications in the field of nanotechnology there is a growing demand for a detailed inspection of surface areas of millimeter sizes. The geometry of silicon micro structures is as well of interest as the detection and shape characterization of defects on optical surfaces. It is state of the art to be able to measure topographies within 2 1/2 D with nanometer resolution by using scanning probe microscopes. So far they are usually restricted to area sizes of 100 square microns. Furthermore it is state of the art to build positioning systems covering several millimeters and resolving nanometers. Those systems are restricted in their positioning uncertainty, which can be estimated within several ten nanometers.
Very few research labs and one or two industrial sites are involved in developments and investigations on combining large area positioning systems with atomic force probe heads. Systems being able to cover 6 decades (mm...nm) are highly sensitive to the choice of control parameters. We are investigating a prototype of such a system employing a calibration standard representing a lattice with 1 micron pitch width. Some "real life" semi conductor structures have been measured as well. Up to now the response of atomic force probe heads to the scanning motion of the table is not fully understood.
The talk will reveal obstacles, their overcoming, and it will probe that realizing large area topography measurements with high resolution is possible. Futhermore, the need of strategies of selecting areas such that the amount of data can be handled in a reasonable way will be shown.
Scanning Probe Microscopy is a powerful tool in nanometrology. SPMs are now widely used in research institutions, and even in some branches of high-tech industry. However, there is a severe drawback: these instruments provide mainly images, not measurements of micro- and nano-objects. Therefore the European Commission decided to establish a network to investigate the possibility of calibration procedures traceable to the national standards of the partners involved. This paper deals with the objectives of the network, the partners, some round robin tests, the calibration structures used and the conclusions drawn from the work of the SPMet group. Consequently, it is a summary of the deliverables for the European Commission authored by the members of the network.
It is well known that functional surfaces in microelectronics and optics can be inspected very fast with scatterometers or scatter sensors. However, there is an increasing interest in looking at specific sample areas in more detail, i.e., the micro topography of the surface has to be measured. Since far-field optical profilers are diffraction limited, the means of choice are very often Scanning Probem Microscopes (SPMs), and namely, Atomic Force Microscopes (AFMs). Various methods for processing data obtained by AFMs will be discussed. An AFM measurement should deliver the surface micro topography to enable the determination of characteristic values, in particular rms roughness. Normally, a superposition of the surface characteristics and artifacts due to the AFM is obtained.
To measure microroughness, defects and contamination on surfaces such as wafers or optical instruments stray light sensors are a fast means. In order to obtain a traceable quantitative, i.e. metrological, measure of roughness (rms) the relation between rms from BRDF of a stray light sensor and rms from topography has to be given. The quantification of stray light sensor signals can well be done with smooth surfaces that have no defects, since forward simulation of the bidirectional reflectance distribution function (BRDF) from smooth surfaces obeying Rayleigh-Rice approximation is possible. We have measured the topography of large areas up to 315 X 315 micrometer2 with an atomic force microscope (AFM) by patching several scans (up to 25) with overlap to obtain bandwidth limits compatible to our stray light sensor. In profilometry roughness usually is evaluated after detrending, i.e. subtraction of surface figures. Hence for an evaluation of the roughness parameter rms by integrating the BRDF of a stray light measurement, the integration limits need to be chosen carefully. This paper gives a detailed discussion on a quantification of roughness measures.
For quality inspection of polished surfaces as applied in semiconductor and optical industry, various methods are used for a fast detection of microroughness, defects, and contaminations. With the aid of stray light sensors the intensity distribution of the reflected and scattered light, i.e. the BRDF, is measured. The probability distribution of values of a BRDF is parametrized to obtain a measure for roughness and for classes of defects. There is still need for justifying the choice of statistical moments to characterize and finally to classify different surfaces. Of course, a basic quantitative, i.e. metrological understanding of stray light sensors is necessary. The power spectrum of surface topographies sufficiently smooth to obey Rayleigh-Rice approximation is proportional to the BRDF. Therefore a comparison was only carried out with sample surfaces obeying this approximation. Defects and contaminations with lateral sizes smaller than the wavelength of the illuminating light employed in the stray light sensor, however, could not be analyzed within this investigation. We have measured the topography of large areas up to 600 micrometer X 100 micrometer with an AFM by patching several scans (up to 8) with overlap. BRDFs evaluated from AFM measurements agree well with BRDFs measured with a stray light sensor.
For monitoring processes of semi conductor or optical industry automatically, stray light sensors are employed for a fast surface san to measure rms, defects, and contamination on surfaces. Surfaces can be characterized by the BRDF. The BRDF is parametrized to classify different surfaces. Classification may be done with various pattern recognition tools, but up to now no proof exists that justifies any choice of classes found empirically. Of course, a basic quantitative, i.e. metrological understanding of stray light sensors is necessary, which could successfully be obtained after comparing BRDFs evaluated from AFM topography scans with smooth surfaces. The power spectrum of surface topographies sufficiently smooth to obey Rayleigh-Rice approximation is proportional to the BRDF. Surfaces obeying this approximation, however, may not include defects and contamination with lateral sizes smaller than the wavelength of the illuminating light employed in the stray light sensor. Thus the comparison was only carried out with specially prepared samples. We have measured the topography of large areas up to 600 micrometer X 100 micrometer with an AFM by patching several scans (up to 8) with overlap. BRDFs evaluated from AFM measurements agree well with BRDFs measured with a stray light sensor.
It is pointed out that the von-Mises distribution can replace the Gaussian distribution for circular or spherical vector fields, i.e. BRDF data obtained from a variety of technical surfaces by stray light measuring or sensing. For the purpose of in line quality control formulae for the parameters corresponding to mean and variance in Gaussian distributions as well as parameter tests and confidence intervals for circular unimodal vector fields will be given. A family of scatter sensors is introduced. Finally, measurement results will be compared to circular statistical inference.
Especially for wafers, hard disks and flat panel displays fast and accurate technical means for roughness measurement, texture modeling, defect detection and classification are needed. However, speed and accuracy are often contradictory in these fields. It is shown that by using scatter (ARS/BRDF) data a very fast acquisition of surface microtopography information is possible. Furthermore, it is pointed out that the von-Mises-distribution can replace the Gaussian distribution for circular or spherical vector fields, i.e. BRDF data obtained from a variety of technical surfaces by stray light measuring or sensing. For the purpose of in line quality control formulae for the parameters corresponding to mean and variance in Gaussian distributions as well as parameter tests and confidence intervals for circular unimodal vector fields will be given. Finally, measurement and simulation results will be compared to circular statistical inference.
It is pointed out that the von-Mises-distribution can replace the Gaussian distribution for circular or spherical vector fields, i.e. BRDF data obtained from a variety of technical surfaces by stray light measuring or sensing. For the purpose of in line quality control formulae for the parameters corresponding to mean and variance in Gaussian distributions as well as parameter tests and confidence intervals for circular unimodal vector fields are given. Finally, measurement and simulation results are compared to circular statistical inference.
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