The EUV mask is protected from particle contamination during exposure by the EUV pellicle, which is constructed from a membrane that is not attached to anything. The pellicle, which is made up of carbon nanotube (CNT) films, has excellent EUV transmission, low reflectivity, and mechanical durability. To evaluate the quality and dependability of the CNT pellicle for the purpose of optimizing the manufacturing process, porosity, bundle size, and particle size distribution are measured utilizing TEM or SEM images and specialized image processing techniques. This article presents a methodology that is specifically designed for processing TEM or SEM images of CNT membranes. Our methodology employs an auto-binarized technique to accurately determine the edge contour from images, and subsequently extract the distribution of each indicator through length or area calculations along the edge. The accuracy of our methodology has been verified through testing using a set of binarized standard reference images. Additionally, we evaluated the practical application of our methodology by comparing CNT membranes with different treatments to determine its sensitivity. We further demonstrated the feasibility of our method by comparing CNT membranes that have undergone varying degrees of hydrogen plasma treatment to the existing Raman D-band to G-band intensity ratio (D/G ratio).
The high numerical aperture EUV exposure systems aim to target a 16-nm pitch to extend Moore's law throughout the next decade. However, thinner photoresist layers and worsened stochastic effects due to photons hitting the wafer at a shallower angle is a major concern. Furthermore, the projection optics utilize an anisotropic reduction factor, which remains an open issue, requiring a dual "half-field" mask exposure sequence or a 12-inch mask for each high-NA EUV layer. Therefore, the use of attenuated phase-shift masks (APSM) to extend 0.33NA to a 28-nm pitch becomes relevant. We will discuss the prospects on optical properties refractive index (n,k) optimization with material selection, feasibility of achieving a 28-nm pitch, 3D effect mitigation and the impact of mask tonality (dark tone vs clear tone). Finally, the challenges on the needs of new APSM materials that meet the requirements of high temp thermal stability, durability under mask clean solution, its dry etching characteristics, the corresponding repair process will be addressed and the experimental results on the Ru-based candidates will be shown.
In image-based overlay (IBO) measurement, the measurement quality of various measurement spectra can be judged by quality indicators and also the ADI-to-AEI similarity to determine the optimum light spectrum. However we found some IBO measured results showing erroneous indication of wafer expansion from the difference between the ADI and the AEI maps, even after their measurement spectra were optimized. To reduce this inconsistency, an improved image calculation algorithm is proposed in this paper. Different gray levels composed of inner- and outer-box contours are extracted to calculate their ADI overlay errors. The symmetry of intensity distribution at the thresholds dictated by a range of gray levels is used to determine the particular gray level that can minimize the ADI-to-AEI overlay inconsistency. After this improvement, the ADI is more similar to AEI with less expansion difference. The same wafer was also checked by the diffraction-based overlay (DBO) tool to verify that there is no physical wafer expansion. When there is actual wafer expansion induced by large internal stress, both the IBO and the DBO measurements indicate similar expansion results. The scanning white-light interference microscope was used to check the variation of wafer warpage during the ADI and AEI stages. It predicts a similar trend with the overlay difference map, confirming the internal stress.
A new method for indicating the image quality of overlay measurement is proposed in this paper. Due to the constraint
of the overlay control tolerance, the overlay metrology requirement has become very stringent. Current indicators such as
the total measurement uncertainty (TMU) are insufficient to guarantee a good overlay measurement. This paper
describes two quality indicators, the contrast index (CI) and the asymmetry index (AI). The CI is a crucial quality
indicator that affects the overlay accuracy greatly. The AI, based on an imaging process with modified cross-correlation
operation, shows alignment mark robustness in both the x and the y directions. For determination of the best recipe, the
box-in-box overlay marks are measured to obtain the images with different conditions. The conventional TMU
indicators are used first to sieve out the better choices. Then the CI and AI can help to judge whether the overlay results
are reliable and can be applied to monitoring of process variations.
The processes to derive the associated phase of an interference signal from the data of a series of recorded frames are performed, and we find that the sampling frequency being lower than the Nyquist sampling rate can also be applied to the full-field heterodyne interferometry. Two optimal sampling conditions for a commonly used CCD camera are proposed based on the relation between the heterodyne frequency and the contrast of the interference signal under the condition that the phase error is set to be 0.05 deg.
A simple method for measuring a step-height sample is presented with the heterodyne central fringe identification
technique and a precision translation stage. This method can accurately point out the zero optical path difference position
such that the optical path lengths of two arms of the interferometer are absolutely equivalent. Thus, the two surfaces of
the step-height sample can be identified sequentially with the translation stage. The displacement of the translation stage
is equal to the step-height of the test sample. The feasibility of the technique is demonstrated. The measurable range is
not limited by the coherence length of the light source. The measurement accuracy depends on the uncertainties of the
heterodyne central fringe identification method and the translation stage. In our setup, we have a 100 mm measurable
range and a 4 nm uncertainty. The wavelength stability of the light source has a minor effect on the measurement.
Based on the Fresnel's equations and the phase-shifting method, an alternative method for measuring the refractive index
distribution of a GRIN lens is presented. A linearly/circularly polarized light in order enters a modified Twyman-Green
interferometer, in which an electro-optical modulator is used as a phase shifter. In the interferometer, the light beam is
divided by a beam-splitter into two beams, a reference beam and a test beam. After they are reflected by a plane mirror
and the tested GRIN lens, respectively, they are combined together and pass through an analyzer. The analyzer extracts
the same polarized components to interfere each other, and the
full-field interference signals produced by the
components of the s- and the p-polarizations can be obtained. The full-field interference signals are taken by a CMOS
camera. The phase differences can be obtained by using the four-step phase-shifting interferometric method. Substituting
these two groups of data into special equations derived from Fresnel equations, and the two-dimensional refractive index
distribution of the GRIN lens can be calculated. Its validity is demonstrated and has some merits such as simple optical
configuration, easy operation and high resolution.
Based on the Fresnel's equations and the heterodyne interferometry, an alternative method for measuring the refractive
index distribution of a GRIN lens is presented. A light coming from the heterodyne light source passes through a quarterwave
plate and is incident on the tested GRIN lens. The reflected light passes through an analyzer and an imaging lens;
finally it enters a CMOS camera. The interference signals produced by the components of the s- and the p-polarizations
are recorded and they are sent to a personal computer to be analyzed. In order to measure the absolute phases of the
interference signals accurately, a special condition is chosen. Then, the interference signals become a group of periodic
sinusoidal segments, and each segment has an initial phase ψ with the information of the refractive index. Consequently,
the estimated data of ψ are substituted into the special equations derived from Fresnel's equations, and the refractive
index distribution of the GRIN lens can be obtained. Because of its common-path optical configuration, this method has
both merits of the common-path interferometry and the heterodyne interferometry. In addition, the phase can be
measured without reference signals.
A novel method for full-field absolute phase measurements in the heterodyne interferometer with an electro-optic
modulator is proposed in this paper. Instead of the commonly-used half-wave voltage to drive the electro-optic
modulator, a saw-tooth voltage signal with the amplitude being lower than its half-wave voltage is used. The interference
signals become a group of periodical sinusoidal segments. The initial phase of each sinusoidal segment depends on the
phase difference induced by the test sample. In real measurements, each segment is taken by a fast camera and becomes
discrete digital points. After a series of operations, the starting point of the sampled sinusoidal segment can be
determined accurately. Next, the period of the sampled sinusoidal segments is lengthened and they can be modified to a
continuous sinusoidal wave by using a least-square sine fitting algorithm. The initial phase of the continuous sinusoidal
wave can also be estimated. Subtracting the characteristic phase of the modulator from the initial phase, the absolute
phase measured at the pixel can be obtained without the conventional reference signals. These operations are applied to
other pixels, and the full-field absolute phase measurements can be achieved. The phase retardation of a quarter-wave
plate is measured to show the validity of this method.
A collimated heterodyne light enters a modified Linnik microscope, and the full-field interference signals are taken by a fast CMOS camera. The sampling intensities recorded at each pixel are fitted to derive a sinusoidal signal, and its phase can be obtained. Next, the 2-D phase unwrapping technique is applied to derive the 2-D phase distribution. Then, Ingelstam's formula is used to calculate the height distribution. Last, the height distribution is filtered with the Gaussian filter, the roughness topography and its average roughness can be obtained and its validity is demonstrated.
KEYWORDS: Heterodyning, Interferometry, Digital cameras, Signal processing, Fourier transforms, Optical engineering, Cameras, Digital signal processing, Visibility, Transform theory
An area scan digital camera is used to record the full-field heterodyne interference signals, and the processes to derive the associated phases from the data of the recorded frames under a convenient condition are described. By calculating the possible errors under several different cases, an optimal condition to get better results is proposed.
Stylus tip reconstruction is imperative in tracing and calibration of micro and nano surface roughness measurement either for surface roughness analyzer or for scanning probe microscopy. This research is to investigate the size effects on stylus tip reconstruction in micro and nano roughness measurement. Aspect ratios within and between tips and gages, such as Tip Aspect Ratio (TAR) of tip width to height, Gage Aspect Ratio (GAR), Height Aspect Ratio (HAR) of tip height to gage height, and Width Aspect Ratio (WAR) have been formulated to develop a stylus tip reconstruction method (STRM) to estimate tip profile from the measured profile image and the traced gage profile. A simulated program has been used to test the developed STRM with different aspect ratios of tips and gages. Experiments have been conducted on a Hommelwerke T4000 surface roughness analyzer with a TKL T100 tip of radius 5 μm and a Veeco Dektak 200 surface roughness analyzer with nominal radius values of stylus tip radius 12.5 μm to measure a traced roughness gage (Mitutoyo Serial No. 0300042) of step height of 10 μm and razor blade in ISO 5436 standard. Experimental results show that the difference of STRM on step gage and razor blade measurement is about 4 % and the developed STRM can be further used to estimate the geometric size effects of tip reconstruction in scanning probe microscopy (SPM).
The precision displacement control in high-resolution instrument is influenced by non-linearity effects of PZT actuators. The capacitive sensor within PZT actuator is often used as a displacement sensor for feedback control, but the calibration and traceability of capacitive sensor are hardly to be accomplished. The optic fiber sensor is a useful high-resolution displacement sensor to perform the measurement in space-limited instrument, and it’s also capable of a non-contact function. In this paper, the structure of an optic fiber sensor was introduced, and the hysteresis characteristic of PZT actuator was evaluated. In addition, the performance of the capacitive sensor within PZT actuator for close-loop control was compared with those of the optic fiber sensor, and the differences ratio between both was less than 0.12 %. Following the scanned images by interference microscope, the images have some distortions before applying the compensated curve function for non-linearity. Thus, the optic fiber sensor could be provided a calibration service for displacement measurement of interference microscope.
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