Critical Dimension Scanning Electron Microscope Research Articles

Line-profile and critical-dimension monitoring using a normal incidence optical CD metrology

As lithographic technology drives the minimum integrated circuit feature size toward 0.1 /spl mu/m and below, process tolerances for critical-dimension profile excursion are becoming increasingly demanding. In response, optical critical dimension metrology (OCD), an optical-wavelength light-diffraction technique, is emerging as a fast, accurate, and nondestructive sub-100-nm linewidth and profile monitor. As such, a detailed understanding of the correlation between OCD and existing metrology tools is required. Correlation between CD measurements using OCD and CD-scanning electron microscopy (SEM) techniques is investigated by measuring two types of important structures, e.g., photoresist gratings on a polysilicon gate film stack and shallow trench isolation. Intragrating CD variation is shown to account for scatter in the correlation plot. A qualitative line-profile correlation between cross-section SEM (X-SEM) and OCD is presented for photoresist gratings in a focus exposure matrix. Finally, a summary of the capability of OCD as a monitor for various processing stages is presented.

A Detection Method for a T-Topped Profile in Resist Patterns by Top-Down-View Critical Dimension Scanning Electron Microscope

A new detection method for a T-topped profile in resist patterns is proposed. This method can determine a T-topped tendency from only a single top-down critical dimension scanning electron microscope (CD-SEM) image. The method is based on the relationship between a cross-sectional pattern profile and a shape of a bright area near the pattern edge in the top-down image. Two kinds of indices for a T-topped tendency are obtained from line-edge roughness, width of the area and a correlation coefficient between the left and right borders of the area. Both indices agreed well with actual cross-sectional profiles of various resist patterns.

Quantitative Evaluation of Grid Size Effect on Critical Dimension Uniformity Improvement

In order to control the on-chip linewidth variation (OCV) in logic devices, accurate optical proximity correction (OPC) is required, and the method to enhance its result is devised. The optical proximity behaviors are severely varied according to the optical and material conditions. The change in photoresist (PR) species deteriorates the OPC rule from 9.3 nm to 15.1 nm for two kinds of PR species. The illumination condition variation also deteriorates the optical-proximity-corrected (OPCed) results from 9.3 nm to 11.6 nm. To obtain accurate OPCed results, these conditions should be fixed. For improving the correction accuracy of the optical proximity, the OPCed grid size effect on the critical dimension (CD) uniformity is evaluated quantitatively. By adopting the OPC grid size of less than 1 nm, the correction resolution limited by the grid size is enhanced. The selective bias with the assist features is applied to the line-and-space (L/S) patterns varied by the space sizes. The selective bias rule is generated with a model using the different grid sizes of 1 nm and 0.5 nm. In the nominal CD of 87 nm, the 3σ values of the optical proximity effect are measured to be 14.6 nm and 11.4 nm for 1 nm and 0.5 nm grid sizes, respectively. The improvement of 9.2 nm is achieved, corresponding to nearly 39% enhancement. The CD uniformity dependence on the grid size was characterized in two-dimensional pattern on a real static random access memory (SRAM) pattern with the different grid sizes of 1 nm and 0.5 nm. The 3σ values of the uniformity are 9.9 nm and 8.7 nm in the case of grid sizes of 1 nm and 0.5 nm, respectively. Decreasing the grid size improves the uniformity by 4.7 nm, corresponding to 22% enhancement. By considering the mask error enhancement factor (MEEF), the enhanced amount is calculated to be 3.2 nm. The pattern fidelity improvement in the mask by reducing the grid size enhances the printing images, and decreases the measurement error using the in-line CD scanning electron microscope (SEM).

Junction-isolated electrical test structures for critical dimension calibration standards

The National Institute of Standards and Technology (NIST) is developing single-crystal reference materials for use as critical dimension (CD) reference materials. In earlier work, the reference features on these reference materials have been patterned in the device layer of a silicon-on-insulator (SOI) wafers, with the buried oxide providing electrical isolation. This paper describes a new method of isolating the structures from the substrate by means of a pn junction. The junction isolation technique is expected to provide several advantages over the SOI technique including minimal susceptibility to charging when imaged in a CD scanning electron microscope (CDSEM), better edge quality, and ease of manufacture. Primary calibration of these reference materials is by imaging the cross-section of the feature with high-resolution transmission electron microscopy (HRTEM) at sufficiently high energy to resolve and count the individual lattice planes while electrical test structure metrology techniques provide the transfer calibration. Secondary calibration is performed with electrical test structure metrology, supplemented by visual techniques to verify that the features meet uniformity requirements. In this paper, we describe results for determining the electrical critical dimensions of these junction-isolated structures. This measurement and data analysis technique is a unique combination of the short-bridge variation of the cross-bridge resistor and the multi-bridge structure.

Measurements of shallow trench isolation by normal incidence optical critical dimension technique

Shallow trench isolation (STI) has emerged as one of the primary techniques for device isolation in complementary metal oxide semiconductor technologies. This device isolation technology has become extremely important to satisfy the high-density requirements of modern integrated circuits. It is of great importance to measure the critical dimensions and sidewall features of the STI structure. Currently used critical-dimension scanning electron microscopes (CD-SEMs) cannot identify the rounding typically present at the bottom or top of the profile, and it is difficult to differentiate between the top and bottom line-width values. Cross-sectional SEMs that can give the profile information require the destruction of the wafer. In this article, we present the measurement of STI by using the optical critical dimension (OCD) technique. This technique measures line or trench profiles using normal incidence polarized reflectometry with sensitivity to sub-50 nm grating lines. In the OCD technique, a broadband polarized light beam is focused onto the grating surface, and the reflected zeroth order is measured as a function of wavelength. The data obtained by measuring the grating structure give a signature of the profile, which is analyzed in real time using rigorous coupled wave analysis. Since the data are fitted in real time, there is no requirement for library generation, which makes the analysis simpler and easier to extend to other structures without the need for lengthy regeneration of a new library of profile data. Data from STI wafers before and after oxide fill will be presented. Both simulation and experimental results show great sensitivity to features such as CD, depth, sidewall angle, as well as the quality of oxide fill.

Photomask Cr–MoSi etching

Studies on photomask Cr and MoSi etch processes were carried out and etch kinetics and modeling were performed. The photomasks were etched using two different etch tools and metrology to support theoretical suppositions and process characterization was performed using profilometry and critical dimension (CD) scanning electron microscopy. A 50 keV electron-beam writing system employing positive chemically amplified resist was used for pattern creation. Loading-etch rate equations were theoretically proposed and experimentally tested. It was found that the calculated Cr and MoSi etch rates agreed well with experimental results. Local etch rates versus local loading on one photomask were studied and kinetic equations were proposed showing excellent agreement with experimental results. Cr and MoSi etch CD movements versus local load on one photomask were also investigated. It was found that load effects on Cr and MoSi etch CD movements could be controlled in opposite directions leading to a compensation strategy proposal for MoSi optimization, instead of using a point-to-point 3σ as the optimization parameter (objective function). By using this compensation method, the final MoSi CD uniformity of 100–110 nm technology node photomasks (∼30% load) was in the range of 8.5–10.1 nm (3σ). This final CD uniformity was obtained on both of the photomask etch tools tested.

Mechanism of ArF resist-pattern shrinkage in critical-dimension scanning electron microscopy measurement

ArF resist-pattern shrinkage in critical-dimension scanning electron microscopy (CD-SEM) measurement is investigated, and its mechanism is discussed. In CD-SEM irradiation damage in the resist detected by Fourier transform infrared (FTIR) spectroscopy, we propose that C=O units of pendant polymer chains decompose COOH units at the first stage and that the COOH units finally decompose CO2 outgas at the second stage. We propose the mechanism of the ArF resist-pattern shrinkage in the CD-SEM measurement from experimental and simulation results of the FTIR spectroscopy and Monte Carlo simulation of electron scattering trajectories into the resist. The first resist polymer free volume shrinkage is caused by hydrogen bonds between the COOH units, as well as decomposition of dissolution inhibitor groups. The second resist polymer free volume shrinkage is caused by the CO2 outgas within 4–14 nm in depth from the resist surface depending on the CD-SEM acceleration voltages of 400 and 800 V, respectively, with probe current of 8 pA. Based on the mechanism, we also propose an improvement in the ArF resist material design to reduce the ArF resist-pattern shrinkage in the CD-SEM measurement.

Experimental resolution measurement in critical dimension scanning electron microscope metrology

By applying the basic principles of metrology we discuss how to define the standards that any experimental method to measure resolution has to obey. Our results clearly indicate the need to apply a calibration procedure when designing algorithms to estimate resolution to satisfy accuracy requirements. Similarly, the precision of an algorithm has to be clearly specified. We compare here the performances of a variety of commonly used implementations of published methods, with that of an algorithm based on an approach known to be reliable. Our results confirm that when an algorithm is designed with the clear intent of satisfying metrology requirements, it demonstrates excellent accuracy, precision, and lack of sensitivity to the noise level, as is desirable. As a consequence, the algorithm will have the ability to measure accurately the point spread function convoluted in the image, thus paving the way for quantitative deconvolution techniques.

Optical Proximity Correction Feature Extraction Method Using Reticle Scanning Electron Microscope Images

Optical proximity correction (OPC) features are frequently used in photolithography. It is necessary to measure the shape characteristics such as height, width, area, position of center of gravity, and corner-rounding with nm-order accuracy. For this purpose, critical-dimension scanning electron microscope (CD-SEM) images are utilized. However, it is difficult to extract the shape characteristics mentioned above from a corrected pattern with conventional CD-SEM measurement. The authors developed a method to extract the shape characteristics automatically from a corrected reticle pattern with high accuracy, with combination of discriminant analysis and nonlinear regression.

Comparison of electrical and SEM CD measurements on binary and alternating aperture phase-shifting masks

Many of the recent advances in optical lithography have been driven by the utilization of complex photomasks using optical proximity correction (OPC) or phase-shifting technologies. These masks are difficult and expensive to manufacture so the ability to test and characterize the mask making process is very important. This paper examines the issues involved in the use of relatively low-cost electrical critical dimension (ECD) measurement of mask features. Modified cross-bridge test structures have been designed to allow the on-mask measurement of dense and isolated, binary and phase-shifted layouts. The results of electrical and critical dimension scanning electron microscope (CD-SEM) testing of these structures are presented and indicate the lower variability associated with ECD measurements. In particular the adverse effect of phase-shifting elements on the accuracy of SEM measurements is highlighted.

Measurement of semi-isolated polysilicon gate structure with the optical critical dimension technique

The semiconductor road map predicts the production of sub-100 nm transistor gates that push further the limits of the size and speed posed by 180 nm gates that are currently in use. With this decrease in gate size, it has become extremely critical to measure these lines accurately using nondestructive techniques. The optical critical dimension (OCD) technique is emerging as one of the most promising CD measurement techniques for sub-0.1 μm device fabrication. Compared to CD scanning electron microscopy and X-SEM, OCD has several distinct advantages. It is nondestructive, has a fast turnaround time, is sensitive to sidewall profiles, and has sensitivity to sub-100 nm linewidths. It has been successfully used in important structures such as photoresist, shallow trench isolation, and polysilicon and tungsten silicide gates. In the OCD technique, a broadband polarized light beam is focused onto a grating at angle normal to the grating surface, and the spectrum of zeroth order reflection is measured. The spectrum contains the signature of the grating profile that is analyzed in real time using rigorous coupled wave analysis. Real time curve fitting algorithms, which do not require library generation, make the analysis simple and easy to extend to a variety of grating structures. Since the OCD technique is based on specular diffraction, a primary requirement for the OCD measurement target is to have periodical grating structures with a line to space ratio typically larger than 1:1. In this article, we report use of the OCD technique to measure polysilicon gate gratings with line to space ratios as large as 1:20. Polysilicon gate grating structures with critical dimensions of 30–40 nm were measured for line to space ratios of 1:10 and 1:20. In both cases, the measurement showed extreme sensitivity to the linewidth and detailed profile, without deterioration of the repeatability. This study has significantly extended the measurement range of the OCD technique and its application to isolated line measurements.

Critical dimension guarantee for the next generation photomasks with critical dimension scanning electron microscope

Advanced critical dimension (CD)-scanning electron microscope was evaluated as a photomask CD guarantee tool. Measurement repeatability was 2.3 nm (3σ) for each measurement, and reproducibility (range of average value in five days) was 3 nm. The contamination effect was evaluated by measuring isolated Cr line and isolated space patterns. The contamination effect on measured CD value was estimated as 0.02 nm/scan (reticle scale) from the result of 500 scans at the same position. This result was quite different from the results of the aerial image of approximately 0.3 nm/scan (reticle scale) evaluated by MSM100 (λ=248 nm) and exposure results (λ=193 nm). The difference between these two evaluation results was probably due to the transmittance reduction of the electron beam scanning area of quartz substrate. This consideration was supported by atomic force microscopy and simulation results. There was no degradation on measurement repeatability and no image shift caused from the charging effect for the most severe condition (0.4 μm Cr isolated dot, global coverage of 20%, and local coverage of 0.05% under the magnification of 88 000×).

Process characterization for tapered contact etch

We report the characterization of contact etch processes which give variable sidewall taper angles. Patterning at 248 nm gave contact holes at ∼0.19 μm diameter in photoresist over organic bottom anti reflection coating (BARC). The contact stack and test structures (BARC/oxide/SiN, 5–10 kA total stack thickness) were etched in a medium-density TEL dipole ring magnetron (DRM) system. Bottom hole diameters ranging from 0.17 down to 0.10 μm could be obtained by varying the oxide etch process, which included C4F8 or C5F8, O2, and Ar. A moderate etch selectivity of ∼8:1 (oxide: SiN etch rate ratio) was determined for the main taper process. Etched patterned wafers were characterized using top-down critical dimension scanning electron microscopy (CD-SEM), cross-section SEM, and transmission electron microscopy. Ex situ surface analysis of etched blanket wafers using x-ray photoelectron spectroscopy showed only small dependence of the surface fluorocarbon film thickness and composition on taper etch process cond...