Search

The semiconductor industry is moving toward a half-pitch of 7 nm. The required metrology equipment should be one order of magnitude more accurate than that. Any metrology tool is only as good as it is calibrated. The characterization of metrology systems requires test patterns that are one order of magnitude smaller than the measured features. The test sample was designed in such a way that the distribution of linewidths appears to be random at any location and any magnification. The power spectral density of such pseudo-random test pattern is inherently flat, down to the minimum size of lines. Metrology systems add a cut-off of the spectra at high frequencies; the shape of the cut-off characterizes the system in its entire dynamic range. This method is widely used in optics, and has allowed optical systems to be perfected down to their diffraction limit. There were attempts to use the spectral method to characterize nanometrology systems such as SEMs, but the absence of natural samples with known spatial frequencies was a common problem. Pseudo-random test patterns with linewidths down to 1.5 nm were fabricated. The system characterization includes the imaging of a pseudo-random test sample and image analysis by a developed software to automatically extract the power spectral density and the contrast transfer function of the nanoimaging system.

Monte Carlo-based SEM image simulation can reproduce SEM micrographs by calculating scattering events of primary electrons inside the target materials. By using the simulated SEM images, it is possible to optimize imaging conditions prior to the specimen observation, which could save time for finding suitable observation condition. However, a recent trend of miniaturized and 3-dimentional structures of semiconductor devices, and introduction of various novel materials have created a challenge for such SEM image simulation techniques; that is, more precise and accurate modeling is required. In this paper, we present a quantitatively accurate BSE simulation and a precise parameters setting in voltage contrast simulation, for both to reproduce experimental SEM images accurately. We apply these simulation techniques to optimize the accelerating voltage of SEM for sub-surface imaging, and to analyze a charge distribution on the insulating specimen under the electron irradiation. These applications promise the advancement in developing a new device by preparing inspecting condition in a timely manner.

Experimental investigation of thermochemical laser-induced periodic surface structures (TLIPSS) formation on metal films (Ti, Cr, Ni, NiCr) at different processing conditions is presented. The hypothesis that the TLIPSS formation depends significantly on parabolic rate constant for oxide thin film growth is discussed. Evidently, low value of this parameter for Ni is the reason of TLIPSS absence on Ni and NiCr film with low Cr content. The effect of simultaneous ablative (with period ≈λ) and thermochemical (with period ≈λ) LIPSS formation was observed. The formation of structures after TLIPSS selective etching was demonstrated.

The main source of placement error in maskmaking using electron beam is charging. DISPLACE software provides a method to correct placement errors for any layout, based on a physical model. The charge of a photomask and multiple discharge mechanisms are simulated to find the charge distribution over the mask. The beam deflection is calculated for each location on the mask, creating data for the placement correction. The software considers the mask layout, EBL system setup, resist, and writing order, as well as other factors such as fogging and proximity effects correction. The output of the software is the data for placement correction. Unknown physical parameters such as fogging can be found from calibration experiments. A test layout on a single calibration mask was used to calibrate physical parameters used in the correction model. The extracted model parameters were used to verify the correction. As an ultimate test for the correction, a sophisticated layout was used for verification that was very different from the calibration mask. The placement correction results were predicted by DISPLACE, and the mask was fabricated and measured. A good correlation of the measured and predicted values of the correction all over the mask with the complex pattern confirmed the high accuracy of the charging placement error correction.

In maskmaking, the main source of error contributing to placement error is charging. DISPLACE software corrects the placement error for any layout, based on a physical model. The charge of a photomask and multiple discharge mechanisms are simulated to find the charge distribution over the mask. The beam deflection is calculated for each location on the mask, creating data for the placement correction. The software considers the mask layout, EBL system setup, resist, and writing order, as well as other factors such as fogging and proximity effects correction. The output of the software is the data for placement correction. One important step is the calibration of physical model. A test layout on a single calibration mask was used for calibration. The extracted model parameters were used to verify the correction. As an ultimate test for the correction, a sophisticated layout was used for the verification that was very different from the calibration mask. The placement correction results were predicted by DISPLACE. A good correlation of the measured and predicted values of the correction confirmed the high accuracy of the charging placement error correction.

We experimentally demonstrate a cascaded generation of a conventional dissipative soliton (DS) at 1020 nm and Raman dissipative solitons (RDS) of the first (1065 nm) and second (1115 nm) orders inside a common fiber laser cavity. The generated high-energy pulses are shown to be linearly-chirped and compressible to 200-300 fs durations for all wavelengths. Moreover, the pulses are mutually coherent that has been confirmed by efficient coherent combining exhibiting ~75 fs and

Second harmonic generation (SHG) of a random laser operating at 1308 nm via Rayleigh backscattering in a phosphosilicate fiber with Raman gain has been studied. SHG power of < 100 mW at 654 nm has been obtained in PPLN pumped by ~7 W laser power. The SHG beam is stable, herewith its spectrum has no mode structure within ~0.5 nm bandwidth that is different from conventional Raman fiber laser with linear cavity. Direct comparison of SHG in two laser configurations demonstrates better parameters of the random laser having great potential for imaging and other applications requiring low-coherent visible light.

A random lasing based on Rayleigh scattering in a passive fiber directly pumped by a high-power laser diode (LD) has been demonstrated. Owing to the random distributed feedback (RDFB) the low-quality LD beam (938 nm) is converted into the high-quality laser output (980 nm). Because of the relatively low excess above the threshold with the available LD, the RDFB laser output is limited in power on 0.5 W level. In the used gradient-index (GRIN) fiber the output beam has 4.5 lower divergence as compared with the pump beam thus demonstrating a new approach for high-power fiber lasers with high-quality output.

The semiconductor maskmaking industry imposes challenging requirements for the uniformity of critical dimensions (CD) and placement error. In electron beam lithography (EBL), electrons backscatter from the resist and substrate, reach the bottom of objective lens and come back to the resist, causing undesirable exposure and charging far away from the point of exposure. This fogging effects both CD variation and placement accuracy in EBL. The Monte Carlo software CHARIOT was upgraded to be capable of simulating this fogging effect. The results of simulations are presented. It was found that Gaussian approximation is sufficient; the approximation parameters were found for various working distances. The results were used for the correction of charging placement error. Fogging is one of the major contributing factors to the charging placement error; the DISPLACE software tool predicts the displacement map for any layout, which allows for the correction of placement error in maskmaking.

The required measurement precision for multilayered EUV mask metrology is set below 0.4 nm three sigma. In addition to limited precision of CD-SEM, there are fundamental physical factors that deteriorate the accuracy of the measurements, the most important of which is charging. It is widely believed that EUV masks are conductive. However, experiments have revealed noticeable charging in CD-SEM measurements of EUV masks that cannot be ignored. In this work, the results of the experiments and simulations of the SEM signals are presented. It was shown that charging affects the metrology in a few ways. The SEM signal shifts at each frame, changes with beam current and also depends on the wall angle of the absorber. The results of the simulations are compared to experimental results.

Placement error due to charging in electron beam lithography has been identified as the most important factor limiting placement accuracy in EBL, which is especially important in the fabrication of masks for double patterning. Published results from a few major companies demonstrated that the placement errors due to charging are far larger than 10 nm. Here, we will describe the results of predicting the charging placement error based on a significantly improved physical model. Specially designed patterns were used to characterize the details of the charging placement error. Reference marks were exposed before the exposure of the test pattern, during the exposure, and after the exposure was completed. The experimental results were used to calibrate the parameters of the physical model. Furthermore, the DISPLACE software was used to predict the placement error maps for other experiments. The results of the measurements and simulations are presented in this paper. The results produced by the software were in good agreement with the experimental measurements. When the amplitude and the direction of the placement error due to charging is predicted, it can be easily corrected using readily available software for mask data preparation, or directly in EBL writers.

Phase matching curves for parametric generation in four wave mixing (FWM) processes of different types are studied experimentally and numerically for a polarization maintaining photonic crystal fiber (PCF) pumped by a tunable continuous wave (CW) ytterbium doped fiber laser near 1 μm. Parametric frequency shifts up to 100 THz for scalar and pump-divided vector FWM processes are observed providing generation of idler wave with wavelengths as short as 765 and 758 nm for the two processes respectively. Explicit analytical solutions for scalar and polarization phase matching in vicinity of the zero dispersion wavelength have been also deduced. They are based on phase mismatch Taylor series expansion taking into account the polarization contribution. A good quantitative agreement between experimental and calculated frequency shifts is demonstrated.

A generation spectrum of a fiber laser becomes broader with increasing generation power. The spectra are rather narrow at low power and become comparable with fiber Bragg gratings (FBG) width at high power. It has been shown that the spectral broadening of a fiber laser can be described analytically if the generation spectrum is much narrower than the FBG width. The developed theory has been compared with experiment. Double clad Yb-doped fiber laser of up to 10 W output power is used in the experiment. Scanning Fabry-Perot interferometer with resolution down to 1.2 pm is applied for accurate spectral measurements. At power level less then 1 W a self-sustained pulsation regime accompanied by a narrow-line self-sweeping is observed. At higher power a quasi-CW generation regime with multiple longitudinal modes is established. Investigation of the regime shows linear increase of the generation width with generation power growth. Slope of the dependence has excellent agreement with the theory, but an additive quantity should be added to describe an absolute value that makes significant contribution at low powers. It has been shown that at low powers a spatial hole burning has to be considered. Theoretical model describing the hole burning effect for multimode cw generation is also developed. After inclusion of the hole burning effect the model starts to agree quantitatively with the linewidth measurements both at low and high powers.

Critical dimension measurement is the most essential metrology needed in nanofabrication processes and the practice is most commonly executed using SEMs for its flexibility in sampling, imaging, and data processing. In bit patterned media process development, nanoimprint lithography (NIL) is used for template replication and media fabrication. SEM imaging on templates provide not only individual dot size, but also information for dot size distribution, the location of dots, pitch and array alignment quality, etc. It is very important to know the SEM measurement limit since the feature nominal size is less than 20 nm and the dot feature size and other metrics will relate to the final media performance. In our work an analytical SEM was used. We performed and compared two imaging analysis approaches for metrology information. The SEM beam was characterized using BEAMETR test sample and software for proper beam condition setup. A series of images obtained on a 27 nm nominal pitch dot array patterns were analyzed by conventional brightness intensity threshold method and physical model based analysis using myCD software. Through comparison we identified the issues with threshold method and the strength of using model based analysis for its improvement in feature size and pitch measurement uncertainty and accuracy. TEM cross sections were performed as accuracy reference for better understanding the source of measurement accuracy deviation.

The uncertainty associated with scanning electron microscopy (SEM) metrology is significant because SEM image brightness is complexly related to the size and shape of the feature, its material, the geometry of the pattern, as well as SEM setup. While regularly used methods of extracting critical dimensions (CD) rely on image brightness analysis, the myCD software uses a physical model of the SEM in order to improve the accuracy of measurements. Metrology below 10 nm was studied in this paper. Patterns were fabricated using electron beam lithography and nanoimprint; they were imaged by SEM and examined using myCD. Factors that are important for metrology at the sub-10 nm size range were studied using advanced Monte Carlo software; the beam size, voltage, detector and linewidth were varied. SEM images were processed using myCD, which utilizes an analytic model of the SEM and so does not require any libraries. The top and bottom sizes, as well as wall angles and line width roughness were analyzed. The CD and profile results from top down SEM images were compared to the vertical crossections. The challenges of sub-10 nm metrology are discussed, mainly regarding the quality of SEM images and the physics of image formation.

The effect of broad-range self-sweeping in Yb-doped fiber laser has been demonstrated experimentally for the first time. The self-sweeping effect is observed in an all-fiber laser configuration with a double-clad Yb-doped fiber and a cavity formed by a broad-band fiber loop mirror and Fresnel reflection from one cleaved end. The sweep range is limited by the width of the broad-band reflector and reaches up to 16nm. It is found that the self-sweeping effect is related to selfpulsations. So the sweep rate is increased with an increase in pump power and is decreased with increasing cavity length. RF and optical spectra (linewidth is measured to be not more than 100 MHz) show that during the evolution of a single pulse a small number of longitudinal modes take a part in lasing. Based on these results we propose a model describing dynamics of the laser frequency. The model is based on the spatial hole burning effect and the gain saturation in Yb laser transition, and takes into account self-pulsations of the laser. Theoretical estimation for pulse to pulse change of lasing frequency is in good agreement with experimental data.

We present the results of comparative numerical study of energy deposition in single shot femtosecond laser inscription for fundamental and second harmonic of Yb-doped fiber laser. We have found that second harmonic is more efficient in absorbing energy which leads to lower inscription threshold. Hence this regime is more attractive for applications in femtosecond laser microfabrication.

Methods of extracting information regarding critical dimensions (CD) in scanning electron microscopes (SEM) are currently based on image brightness. This brings significant uncertainty of the measured results because image brightness has a complex relation to the size and shape of feature, its material, geometry of the pattern as well as SEM setup. A model based extraction of CDs out of SEM images has been developed. The analysis is based on an understanding of physical principles involved in the formation of the SEM signal. Some parameters, such as beam voltage and materials, should be known to the operator as the input data along with the SEM image. The output results of the myCD software are contours of lines, linewidth at the top, bottom, and in the middle of the line, line edge roughnessess at each edge, line width roughness, and wall angles at each edge. In addition, averaged values over all lines present in the image are also displayed. The model based analysis of SEM images may considerably improve accuracy of CD measurements in SEM.

SEM metrology involves uncertainty of the linewidth measurement because the SEM signal formation is an extremely complex process. In this work, we used an analytical SEM for CD metrology applications on quartz nanoimprint template. The SEM was tuned first to find the best reasonable condition for consistent operation. Beam characterization was done using BEAMETR beam measurement technique. SEM images of templates were taken at optimum conditions. The measurements were done using a) regular imaging processing software and b) using physical model based processing tool myCD. The quartz template was then measured using TEM crossections at selected sites to reveal profile information as metrology comparison reference. The metrology capability and fundamental limitation of analytical SEM operation with regular imaging processing was identified. Information about SEM setup and materials was used. The considerable improvement using the physical modeling imaging process was found.

BEAMETR (BEAm METRology) technique is demonstrated as an attractive solution for automatic measurement of electron beam sizes in two coordinates. The method associates one software and one specially designed pattern chip. The fabrication of new BEAMETR design is performed by electron beam lithography and metal lift-off. A specific bi-layer resist system and proximity correction is used for achieving the requirements for the "pound-key" shape of BEAMETR pattern. Beam sizes in two coordinates (x,y) of Scanning Electron Microscope are measured for various operating conditions. This method allows measuring electron beam sizes down to 2 nanometers.

SEM microscopy is a primary method for CD measurements of features on sub-micron scale. The process of feature characterization using SEM involves several steps that include image acquisition, image processing, image analysis and data analysis. Each one of these steps carries an error margin that contributes to the overall accuracy of measurements. While performing measurements at the nanoscale resolution the accuracy of the process becomes even more critical for obtaining accurate measurements, and needs to be determined and understood. Using object with known dimensions such as linewidth of an isolated polycrystalline ("poly") Si lines be useful for evaluating accuracy of the characterization process and calibrating SEM instrument. Reference materials for such evaluation are being developed by NIST. To use this approach successfully it is important to understand metrology issues involved in the image characterization process. This work will review the scope of metrology issues involved in the process of linewidth measurement using SEM imaging including image acquisition, image processing and segmentation, as well as data extraction and analysis using image analysis methods. We will review and discuss methods for evaluating accuracy throughout characterization process and obtaining reliable and repeatable measurements. The scope of study includes the use of two dimensional simulation and modeling of SEM line images, methods for grayscale image processing and segmentation, algorithms for obtaining statistically accurate linewidth measurements from the image that can be used for instrument calibration and inter-laboratory studies and are generally applicable for obtaining CD measurements of line features.

The greatly increased complexity of modern masks has in turn led to increased write times and cost of the masks. Any opportunity to decrease write time while providing the required accuracy of the fabricated pattern is highly beneficial. A writing strategy using an arbitrarily shaped beam (ASB) results in a considerably smaller number of flashes to write a complex pattern compared to other strategies. The design of an ASB system is proposed. The ASB electron-beam column is similar to that of a variable-shaped beam system, except for a modified beam-shaping block. This suggests the relatively easy integration of an ASB column. The throughput of an ASB system is a few times greater than the throughput of other systems, except for patterns with low coverage or simple geometries. In addition to the throughput advantages, an ASB system enables higher accuracy, including the feasibility of writing features according to "ideal" optical proximity correction.

Digital Planar Holography (DPH) has arrived due to progress in microlithography, planar waveguide fabrication, and theoretical physics. A computer-generated hologram can be written by microlithography means on the surface of a planar waveguide. DPH combines flexibility of digital holograms, superposition property of volume (thick) holograms, and convenience of microlithographic mass production. DPH is a powerful passive light processor, and could be used to connect multiple optical devices in planar lightwave circuits (PLCs), and if combined with active elements on the same chip, may perform not only analog operations but also logical ones. A DPH implementation of a multiplexer/demultiplexer with discrete dispersion is proposed and demonstrated, avoiding communication signal distortion inherent in multiplexers/demultiplexers with continuous dispersion. The concept of discrete dispersion leads to a device with a flat top transfer function without a loss penalty. The dispersion is created with custom-designed bandgaps for specific directions. A DPH hologram resembles a poly-crystal with long-range correlations, and it exhibits the properties of a quasi-crystal. Unlike photonic crystals, light in quasi-crystal may propagate in almost any direction. Single mode planar waveguides are specially designed to suppress parasitic reflections that appear due to mixture of TE-modes, TM-modes, and cladding modes. Demultiplexers with 2-32 channels were demonstrated on planar waveguides with binary single-layer lithography.

A novel concept of Photonic Bandgap Quasi-Crystal (PBQC) as a platform for planar integrated WDM optical devices is proposed. The PBQC can be lithographically fabricated in a planar waveguide as a computer-generated two-dimensional hologram. In this approach the spectral selectivity of Bragg gratings, focusing properties of elliptical mirrors, superposition properties of thick holograms, photonic bandgaps of periodic structures, and flexibility of lithography on planar waveguides are combined. In distinction to conventional combination of independent planar Bragg gratings, in PBQC we create multiple bandgaps by synthesizing a synergetic super-grating of a number of individual sub-gratings. The device spectral selectivity is determined by those of the sub-gratings. The super-grating comprises million(s) of dashes etched on an interface of a planar waveguide. Each dash is a binary feature placed by a computer program to serve simultaneously many channels. For realization of PBQC devices the software for generating super-gratings (GDS-II format) and 2-D simulation of its transfer function was developed. Direct e-beam writing and photolithography were used for manufacturing PBQC structures. For verification of the ideas behind the concept a number of multichannel MUX/DEMUX devices have been manufactured and experimentally tested. The results of detailed experimental study of 4- and 16-channel devices will be presented. Channel isolation ~30 dB was achieved in the 4-channel devices. The applications of PBQC platform for integrated light wave circuits are discussed.

During electron beam (e-beam) exposure, the temperature of the resist varies locally, causing critical dimension distortion. An experimental comparative study of resist heating at 50 kV for a number of writing strategies was made. Exposure, materials, and processing were equal; the same e-beam column was used, but beam deflection was modified so that raster scanning could be compared to vector scan beam (VSB) exposure at high throughput. These strategies were compared for single- pass and four-pass exposures. Simulations of temperature rise were done using the TEMPTATION (Temperature simulation) software tool. The maximum temperature rise was 38 degrees Celsius for 614-micrometer wide raster exposure, while over 220 degrees Celsius for VSB exposure. Good agreement of simulation and experiment was found. A 16% effective dose change due to resist heating was found at full coverage, single-pass exposure, while in VSB, this value was over 200%. These results indicate that raster writing has an advantage over VSB exposures with regard to resist heating.

The possibility of increasing the resolution and accuracy of the photomask fabrication process is discussed when using a laser pattern generator. The proximity effect is first taken into account in this technique. The redistribution of laser beam energy in photoresist, which results in pattern distortions, can be characterized and used for the distortion compensation. The method for measuring absorbed energy density in photoresist is proposed, and experimental results are obtained for conditions used for a standard-type of photomask manufacturing process.

The electron beam lithography application to diffractive optical elements topology generation is examined. The formula for the estimation of exposure data volume for variable shaped electron beam lithography is presented as a function of diffractive optical element parameters and approximation accuracy. Special software was developed to prepare exposure data for diffractive optical elements fabrication. Diffractive optical elements with an artificial refractive index were manufactured with a feature size much less than the wavelength. Design and experimental results on photomasks fabrication are presented for an optical element focusing irradiation into a ring with ordered parameters. The photomask set was manufactured for reflecting optical element focusing high power CO2 laser beam into two points with required parameters for laser welding.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Peculiarities of variable shaped EBL application to DOE topology generation are regarded. The estimation of exposure data volume is proposed as a function of DOE parameters and approximation accuracy. A special software was developed to prepare DOE's exposure data. Experimental results on topology fabrication are presented for DOE focusing irradiation into a ring with ordered parameters, as well as artificial index gratings.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

An experimental technique for measurement of the absorbed energy density distribution from the center of an electron beam is presented. The test structure and the way of determining critical exposure doses are constructed in such a way that the resist development process has no influence on the experimental results. Questions of `technological' and `physical' proximity functions usage are considered. The absorbed energy density function measurements were carried out for a number of accelerating voltages. The dependence of these function parameters on the electron's energy was determined.

Problems of microelectronics technology applications for diffractive optical elements fabrication have been considered. The results on x-ray diffractive focusing elements are described: amplitude and phase-contrast Fresnel zone plates as well as freestanding gold gratings. The creation of reflected Bragg-Fresnel lenses for soft x-ray range was made on the basis of multilayer x-ray mirrors. Considerable attention was paid to the correction of different distortion factors involved in the electron beam lithography process.

The effect of ion-ion scattering on the generation of plasma-based Raman laser is studied. The ion diffusion in velocity space is shown to broaden the detuning range and to increase the output power. The results are compared with the experiment made in argon laser plasma on anti-Stokes line 437 nm under 648 nm pumping. Agreement between the theory and measured generation spectra confirms that the collisions contribute to lasing processes.

The changes in the chemical and optical properties of the thin (8001200 A) layers of As2S3 after exposure with photons (1 to 40 J/cm2) electrons (3. 2. 1O to 1. 28 102 C/cm2) or soft Xradiation (0. 1 to 1 1 1 . 0 J/cm2) are studied. The structural changes occurring after exposure lead to changes in the alkaline stability of the exposed regions. The optimum exposure doses for As 25 3 layers are determined allowing their use for ebeam and xray lithographic purposes. The high stability of the layers to a great number of acid etchers used in microelectronic technologies and the stability in TUE conditions in oxygen medium make the layers accomplishing microlithographic tasks.

To improve output characteristics of CW ion lasers a transverse flow (TF) and an axial flow (AF) of atoms are applied. Both discharges have improved lasing characteristics in regard to conventional one. The effects of flow are discussed.