KEYWORDS: Scanning electron microscopy, Line edge roughness, Carbon nanotubes, Atomic force microscope, Metrology, Atomic force microscopy, Standards development, Silicon, Manufacturing, Ion beams
The decreasing size of semiconductor features and the increasing structural complexity of advanced devices have placed continuously greater demands on manufacturing metrology, arising both from the measurement challenges of smaller feature sizes and the growing requirement to characterize structures in more than just a single critical dimension. For scanning electron microscopy, this has resulted in increasing sophistication of imaging models. For critical dimension atomic force microscopes (CD-AFMs), this has resulted in the need for smaller and more complex tips. Carbon nanotube (CNT) tips have thus been the focus of much interest and effort by a number of researchers. However, there have been significant issues surrounding both the manufacture and use of CNT tips. Specifically, the growth or attachment of CNTs to AFM cantilevers has been a challenge to the fabrication of CNT tips, and the flexibility and resultant bending artifacts have presented challenges to using CNT tips. The Korea Research Institute for Standards and Science (KRISS) has invested considerable effort in the controlled fabrication of CNT tips and is collaborating with the National Institute of Standards and Technology on the application of CNT tips for CD-AFM. Progress by KRISS on the precise control of CNT orientation, length, and end modification, using manipulation and focused ion beam processes, has allowed us to implement ball-capped CNT tips and bent CNT tips for CD-AFM. Using two different generations of CD-AFM instruments, we have evaluated these tip types by imaging a line/space grating and a programmed line edge roughness specimen. We concluded that these CNTs are capable of scanning the profiles of these structures, including re-entrant sidewalls, but there remain important challenges to address. These challenges include tighter control of tip geometry and careful optimization of scan parameters and algorithms for using CNT tips.
Surface roughness(SR) of the EUV resists exposed to EUV, ArF and KrF radiation has been investigated using three tools: spectroscopic ellipsometry (SE), AFM and SEM. The purpose of this paper is to do determine whether SE can effectively monitor the change in resist SR, and also whether we can see the effect of photon shot noise in resist patterning. EUV resists were coated on three blank wafers, and on the shot basis, exposed to different dose of each radiation. After completion of resist process, the SR was measured first with SE. Then the wafer was sliced into patches of different dose before AFM and SEM measurements were made. SE used effective medium approximation to calculate the roughness-layer thickness as a parameter for fitting to experimental data. Thus obtained thicknesses showed a monotonic correlation with AFM-measured roughness, indicating SE can be a fast, precise and nondestructive tool to evaluate resist SR, once being calibrated with AFM. In order to examine the photon shot noise effect on the resist pattern, all the steps of the resist process was kept the same except the exposure wavelength and its dose. SE results for the three exposures were compared over the full range of doses in common. The three roughness values near the dose to clear, Eclear, apparently provide an evidence that the photon shot noise played a significant role in our experiment.
The amplitude setpoint affects the critical dimension measurement with CD-AFM. The setpoint amplitude is the
amplitude of the resonant oscillation of the AFM tip maintained by the feedback loop as it scans the surface. The
Setpoint therefore decides the tip-surface distance, and the tip-surface interaction force as well. Normally, the tip moves
an unknown distance away from the sample surface. Such a tip-sample distance on the top and bottom surface is
cancelled out in height measurement. In width measurement, however, the tip-sample distance on the left and right
sidewall will add up to produce a bias in the measured CD values. The bias will appear in the opposite way and by the
same amount in line and trench measurement. We conducted the experiments to see the effect, and found out there
exists the dependence of the measured linewidths on the setpoint in the consistent behavior as our hand-waving predicts.
The effect may be a significant uncertainty source in the CD-AFM metrology.
KEYWORDS: Silicon, Carbon nanotubes, Metrology, Atomic force microscopy, Standards development, Scanning electron microscopy, Line edge roughness, Ion beams, Atomic force microscope, Platinum
The ever decreasing size of semiconductor features demands the advancement of critical dimension atomic force microscope (CD-AFM) technology, for which the fabrication and use of more ideal probes like carbon nanotubes (CNT) is of considerable interest. The recent progress in the precise control of CNT orientation, length, and end modification, using manipulation and focused ion beam processes, allowed us to implement ball-capped CNT tips and bent CNT tips for CD-AFM. Such CNT tips have been tested for the first time in a commercial CD-AFM to image a grating and line edge roughness samples. We found out that CNT tips can reasonably scan the pattern profiles including re-entrant sidewalls with the CNT tip geometries we used and with the available range of scan parameters. There still remain important issues to address - including tighter control of tip geometry and optimization of scan parameters and algorithms for using CNT tips.
KEYWORDS: Atomic force microscopy, Scanning electron microscopy, Carbon nanotubes, Ion beams, Ions, Metrology, Critical dimension metrology, Carbon, Fabrication, 3D metrology
We fabricate three kinds of carbon nanotube (CNT) probes to be employed in critical dimension atomic force microscope (CD-AFM). Despite unique advantages in its size and hardness, use of nanotube tip has been limited due to the lack of reproducible control of CNT orientation and its shape. We proposed that CNT alignment issues can be addressed based on the ion beam bending process, where a CNT free-standing on the apex of an AFM tip aligns itself in parallel to the FIB direction so that its free end is directed toward the ion source, with no external electric or magnetic field involved. The process allowed us to embody cylindrical probes of CNT diameters, and subsequently two additional types of CNT tips. One is ball-ended CNT tip which has, at the end of CNT tip, side-protrusions of tungsten/amorphous carbon in the horizontal dithering direction. The other is 'bent' CNT tip where the end of CNT is bent to a side direction. Using the former type of CNT tip, both sides of trench/line sidewall can be measured except for bottom corners, while the corners can be reached with the latter type, but the only one sidewall can be measured at a tip setting. The three types of tips appear to satisfy the requirements in both the size and accessibility to the re-entrant sidewall, and are awaiting actual test in CD-AFM.
The pitch and orthogonality of two-dimensional (2D) gratings have been calibrated by using an optical diffractometer (OD) and a metrological atomic force microscope (MAFM). Gratings are commonly used as a magnification standard for a scanning probe microscope (SPM) and a scanning electron microscope (SEM). Thus, to establish the meter-traceability in nano-metrology using SPM/SEM, it is important to certify the pitch and orthogonality of 2D gratings accurately. ODs and MAFMs are generally used as effective metrological instruments for the calibration of gratings in nanometer range. Since two methods have different metrological characteristics, they give complementary information for each other. ODs can measure only mean pitch value of grating with very low uncertainty, but MAFMs can obtain individual pitch value and local profile as well as mean pitch value, although they have higher uncertainty. Two kinds of 2D gratings, each with the nominal pitch of 700 nm and 1000 nm, were measured, and the uncertainties of calibrated values were evaluated. We also investigated the contribution of each uncertainty source to the combined standard uncertainty, and discussed the causes of main ones. The expanded uncertainties (k = 2) of calibrated pitch values were less than 0.05 nm and 0.5 nm for the OD and the MAFM, and the calibration results were coincident with each other within the expanded uncertainty of the MAFM.
Precision carbon nanotube (P-CNT) tip for atomic force microscope (AFM) was fabricated where CNT orientation and length is controlled within the precision of 1 degree and 300 nm, respectively. The orientation, diameter and length of CNT tip are crucial factors for faithful profiling of surface patterns. With a nano-manipulation while viewing scanning electron microscope live image followed by focused ion beam process, P-CNT tip could be made. P-CNT tip acts as a normal CNT tip without FIB process. Further it maintains the elasticity. P-CNT tip can, in principle, enter the trench or hole less than 70 nm, which is impossible with the current state-of-the-art silicon tip for CD-AFM. Flaring the CNT end would potentially make possible the AFM-based sub-70 nm CD metrology for these structures.
A reliable algorithm was developed for the analysis of the machined aspheric surfaces with the stylus instrument. The research has been done as a prior step, with the intent to evaluate the uncertainties in the aspheric surfaces analysis as well as to make the applications that the commercial instruments cannot provide with its own code implemented inside. The algorithm considered two important factors in the instrument-calibration and the aspheric analysis: pickup configuration (pivoted arm) and the stylus radius. It also compensates for the sample tilt and axis offset due to the setup error in the analysis of aspheric surface. The algorithm has been coded by means of C++ and MATLAB. The algorithm was also applied to the real measurement, and compared with the instrument-produced results. Our algorithm found calibration constants better fitting the calibration ball in the instrument-calibration without noticeable cost of the speed. In conclusion, the developed algorithm can cover, and further, shows better performance over the commercial one in both of the instrument-calibration and analysis of aspheric surfaces.
The characteristics of the carbon nanotube AFM tip was investigated as it is used to measure the critical dimensions in the high aspect ratio structures. The research has been done to demonstrate the limitations of the CNT probe in imaging steep or vertical sidewall. Two kinds of samples, silicon dot and the lines in the ArF resist pattern were profiled by using carbon nanotube tip in the tapping mode AFM. There is a large oscillation at the steep sidewall, which cannot be controlled by merely changing scan variables, except by slowing down the scan up to the impractical level. The interaction between the long, slim CNT probe and the vertical sidewall severely limits the usefulness of AFM as a CD metrology tool. To achieve hi-resolution and high aspect ratio imaging simultaneously, a stiffer and/or modifed probe under clever non-contact 2D feedback is needed.
Since the carbon nanotube (CNT) tip was first reported in 1996, its application to CD metrology has been an interest because of its unique properties. It is needle-like, which is the ideal shape as the scanning probe. In addition it elastically buckles, and therefore is robust and does not produce damage to the surface. It is also hydrophobic. The conventional AFM tip has the pyramidal or conical shape, so that either the steep side wall or high aspect ratio pattern like deep and narrow trench is difficult to measure, although the obtained image tends to be more stable than with the other kinds of tips. Recently, FIB(focused ion beam) tip or HDC(high dense carbon) tip with high aspect ratio structure became commercially available, improving the measurement capability of AFM. They are, however, weaker compared with the conventional tip, and still subject to the wear and contamination. In spite of the theoretical advantages, more researches seem to be needed either to make the CNT tip practicable for the precision CD measurement or even to examine its usefulness as a routine metrology tool. At KRISS, the geometry of the CNT tip is being improved by using the 3D manipulator in SEM for better performance, while commercial CNT tips are already available. Two kinds of patterns, groove and dot, have been measured with the three kinds of tips(conventional, HDC, and CNT as home-made). The AFM measurement results are compared, and the characteristics of CNT tips at the current stage, are discussed from the practical point of view.
A continuous reduction of design rule in the semiconductor industry requires more than the present state-of-the-art laser surface scanners can achieve. In this paper, a description is given of the proposed methodology which possibly enhance the sensitivity in the detection of surface defects and particles. In the methodology, concentric two beams, long-focused and short focused beams, are incident on the surface. The scattered signal of each beam is detected alternatively, and subtracted. While the haze (mocroroughness signal) is only slightly changed, the defect signal change considerably. The present scattering formula has been modified for the theoretical consideration, and the scattered signal in the incidence plane has been numerically calculated with the appropriate parameter values. The results are presented and more discussions about the advantages of the method are described.
The precision laser diffractometer of the conventional-type has been developed for the 1D grating pitch measurement, and successful precision improvement has been made. At the first stage of this improvement, the rotary arm was set to be as long as 1.2 meter, and the precision goniometer has been used. Furthermore, a focusing lens has been inserted between the grating samples and the detection plane in order to prevent the measurement error from the beam alignment. The system has been more improved to meet the need for higher accuracy can precision so far. One of the two important modifications is the use of CCD camera-based detection system instead of visual evaluation. And, the precise value of the laser wavelength, with one more digit than before, is used. These two modifications contribute considerably in improving the accuracy of the measurement comparing with the previous one. The correction of axis wearing shift has been accomplished additionally to improve the accuracy at the current stage of the development. The measured values for the grating whose pitch is certified as 288 nm and 700 nm by the manufacturer, are 287.593 and 700.762 nm. The expanded uncertainties with a coverage factor of 2, is now improved to 0.005 and 0.014 nm for the 288 and 700 nm-grating, respectively. They are significantly less, about 1.10, than previous ones.
A description is given of the scatterometer which has been developed at KRISS for BRDF and diffraction pattern measurement. Light source, goniometer, and receiver is described. As a light source, the collimated HeNe and argon ion laser is used, with which the wavelengths of 632.8, 514.5, and 488.0 nm are available. The goniometer has 6 degrees of freedom. The precision of scattering polar angle is enhanced by choosing long rotation arm (length: 1.2 meter) placed on the stepmotor-controlled rotary table whose angle is read by the angle encoder with the resolution of 0.0001 degree and accuracy 0.001 degree. The receiver has a wide dynamic range greater than 1014 in intensity without intensity attenuation and receiver aperture change, which is made by cascading three kinds of detectors: photodiode (PD), photo- multiplier in direct current mode (DC), and photo-multiplier in pulse counting mode (PC). The measured instrument signature and the sinusoidal grating BRDF at the wavelength of 488 nm is presented.
It is important to have better calibration method for the submicron dimension system as the semiconductor technology requires more accuracy on the metrology system. Gratings have been used as a standard reference for the magnification, and the precision laser diffractometer has been developed for the calibration of the grating pitch. The accuracy of the measurement may depend on the laser wavelength, the goniometer, and how to determine diffraction angles. As the accuracy of conventional diffractometry has been limited by the low resolution of diffraction angle and its determination, the rotary arm is extended and reliable high-resolution goniometer is opted, while a focusing lens is inserted between the grating sample and the detection plane to prevent the error due to the parallel shift of the diffracted beam, which might occur when the rotational axis of the arm does not coincide with the grating plane. Obtained values for all grating samples showed quite good repeatability, which is order of angstrom at worst. Incidence angle dependent fluctuation of measured value is shown to be quite small as expected. The grating pitch, certified as 288 nm by the manufacturer, is measured as 287.595 with a standard deviation ((sigma) ) of 0.017 nm at worst. The other grating, certified as 700 nm, is measured as 700.782 with a standard deviation of 0.054 nm at worst. The expanded uncertainty with the coverage factor of 2 is estimated to be 0.038 nm and 0.10 nm respectively.
Laser light scattering from holographic sinusoidal gratings has been investigated with a view to its use in the calibration of the linearity of BRDF instruments, a task that requires a wide dynamic range in the scattered intensity. An aluminum-coated grating of an amplitude of approximately 90 nm and a spatial wavelength of 6.67 micrometer was used. Measurements and calculations were performed for an angle of incidence of 6 degrees and for light incident from a HeNe laser (lambda equals 0.6328 micrometer). Experimental results are compared with the predictions of two theories: Beckmann's scalar theory and Rayleigh's vector theory applied to sinusoidal gratings. Both theories, which apply to perfectly conducting scatterers, produce nearly identical results. However, these predictions differ significantly from some of the experimental results. The measured scattering pattern has a large background of scattered light and the higher-order peak intensities are larger by several orders of magnitude than the computed ones. The measured peak intensities are polarization dependent. The large background scattering is shown to be due to the residual surface roughness. The profile of the grating was measured using a stylus instrument with a 1-micrometer-radius tip and a 0.1-micrometer-radius tip, and it appears that the profile does not contain significant harmonics that might be responsible for the large higher-order peak intensities. Scattering from a gold-coated specimen with the same specifications was also measured and compared with that from the aluminum-coated one to determine the effect of non-topographic scatter. Possible causes of the discrepancy between the measured and the computed magnitude and polarization dependence of the higher-order peak intensities are discussed.
The simple and effective optical arrangement synthesizing holographic fringe and speckle photography is presented. This optical system is based on image holography. Cantilever beam located on the precision translator is used to evaluate this system. Experimental results agree well with the actual displacements within the error of 5%. The measurement of 3-D contact deformation is demonstrated as an its application.
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