Yoshinori Momonoi

Investigation of Bowing Reduction in SiO2 Etching Taking into Account Radical Sticking in a Hole

The bowing mechanism in high-aspect-ratio contact hole (HARC) etching was investigated by taking into account reactive sticking on the sidewall of the hole. Sticking coefficients of radicals on the sidewall have been estimated by comparing the observed deposition profile with the calculated one. It was found that the coefficients of C rich radicals and CFx radicals were 0.5 and 0.004, respectively, and that F radical reaction probability to the fluorocarbon polymer is 0.07. These coefficient values were deduced that the excessive flux of O and F onto the sidewall of a hole causes bowing during HARC etching. It was also indicated that the bowing can be suppressed by reducing of the flux of oxygen. These findings were confirmed by the results of experiments using an ultra-high frequency-electron cyclotron resonance (UHF-ECR) plasma.

Dry cleaning technique for particle removal based on gas-flow and down-flow plasma

A dry-cleaning technique—consisting of down-flow plasma and utilizing gas-flow viscosity—has been developed. This technique is expected to be used for particle cleaning during semiconductor manufacturing. Particles are removed by both the chemical effects of the plasma and the mechanical effects of the viscous friction of the gas flow. Regarding the mechanical effects, the viscous friction is increased by narrowing the gas-flow space along the wafer surface. As for chemical effects, fluorine radical relaxes the adhesion force of particles on the silicon wafer. It has been experimentally found that the removal efficiency of Al2O3 particles on a Si wafer is 98% in 60 s and the etching depth of polysilicon is 0.17 nm. It has been also found that the technique causes no charge-buildup damage.

Methodology for establishing CD-SEM robust metrology algorithm for development cycles applications

ArF lithography is still the main technology in the most advanced processes of semiconductor fabrication. Being able to reliably measure and characterize these lithographic processes in-depth is becoming more and more critical. Critical Dimension-Scanning Electron Microscope (CD-SEM) continues to be the work horse tool for both in-line critical dimension (CD) metrology and characterization of ArF photoresist pattern. CD shrink of ArF photoresist has been one of the major challenges for CD-SEM metrology, and it becomes more difficult to measure shrinkage accurately for smaller feature size than ~50nm. The authors have developed a new measurement technique of photoresist shrinkage which measures CD difference between shrunk and non-shrunk sites after etching. There are many imaging and image processing parameters in CD-SEM which need to be optimized to obtain small shrinkage and good precision. There is a trade-off relationship between shrinkage and precision, and a comprehensive and systematic methodology is required for optimization of parameters. The authors have developed an optimization method that uses Taguchi method, where only 18 experiments are required. We can predict shrinkage, precision and relative CD offset for any combination of measurement parameter settings used in the 18 experiments by Taguchi method, and these predicted data can be used for optimization. A new concept of secondary reference metrology is also introduced in this paper to reduce the number of measurement by a reference metrology tool.

Three-dimensional profile extraction from CD-SEM top-view image

Emerging three-dimensional (3D) transistor structures have increased the demand for an easy and practical method to measure pattern feature metrics (such as CD, line-edge roughness, etc.) as a function of height (z coordinate). We have examined 3D pattern-profile extraction from a top-view image obtained using a critical-dimension scanning electron microscope (CD-SEM). An atomic force microscope (AFM) was used to measure 3D pattern profiles as a reference. In this examination, line-edge positions were firstly obtained from a CD-SEM image at various threshold levels, and the result was compared with the reference profile measured using the AFM. From this comparison, a mapping function from threshold levels of CD-SEM image-processing to z coordinates is obtained. Using this mapping function, 3D pattern profiles were reconstructed from CD-SEM signal profiles, and the obtained profiles were similar to the directly obtained cross-sectional profile. Put simply, a 3D pattern-profile was extracted from a top-view image successfully. Though the results are not sufficient to confirm the validity of our method yet, the method may feasibly be introduced for quick and easy 3D measurement.

CD-SEM metrology of spike detection on sub-40 nm contact holes

In this work, for the purpose of contact-hole process control, new metrics for contact-hole edge roughness (CER) are being proposed. The metrics are correlated to lithographic process variation which result in increased electric fields; a primary driver of time-dependent dielectric breakdown (TDDB). Electric field strength at the tip of spoke-shaped CER has been simulated; and new hole-feature metrics have been introduced. An algorithm for defining critical features like spoke angle, spoke length, etc has been defined. In addition, a method for identifying at-risk holes has been demonstrated. The number of spike holes can determine slight defocus conditions that are not detected though the conventional CER metrics. The newly proposed metrics can identify contact holes with a propensity for increased electric field concentration and are expected to improve contact-hole reliability in the sub-40-nm contact-hole process.

Mechanism for difference in etched depth between isolated and dense via holes of SiOCH film

We investigated the mechanism for differences in etched depth between isolated and dense via holes of a damascene structure using a SiOCH film (k=2.8). In Ar∕CHF3∕N2 and Ar∕C4F6∕N2 plasmas, the depth of an isolated (4.8μm pitch) hole was more than 40% varied from a dense (200nm pitch) hole. The difference between dense and isolated holes was found to become larger with increasing air exposure time of the wafer. The amount of H2O in the SiOCH film also increased with increasing air exposure time based on thermal desorption spectroscopy (TDS) analysis. Therefore, we investigated the effects of H2O addition to the plasmas. The investigation revealed that the etched depths of dense holes became similar to those of isolated holes with H2O addition. Based on these results, we concluded that the mechanism for the difference between isolated and dense holes was as follows. When the capping layer on SiOCH is etched, the H2O contained in SiOCH desorbs in etched holes and affects etching reactions. The etching chara...

High-precision CD measurement using energy-filtering SEM techniques

Abstract. Voltage contrast (VC) images obtained using an energy filter (EF) were used to measure the bottom surface of high-aspect-ratio structures. The VC images obtained using the conventional EF were sensitive to variations in wafer potential. Since CD-SEM metrology requires precise EF voltage control when using VC images, we developed an EF voltage correction method to be used at each measurement point. Consequently, bottom-edge measurement, independent of the wafer potential fluctuations, was achieved using the newly developed EF. Our developed technique is effective for CD-SEM metrology using VC images.

Scanning electron microscope line-profile analysis of less-than-10-nm patterns

Scanning electron microscopes (SEMs) are widely used in various fields and have contributed to advances in nanotechnology. To analyse SEM images, it is necessary to consider the size of the probe electron (PE) beam and the range of PE scattering inside the pattern because they affect edge sharpness in SEM images. As the feature size of the sample drops to less than 10 nm, their effects become even more noticeable. In this study, we utilized Monte-Carlo simulation to clarify the effects of the size of the PE beam and the range of PE scattering for patterns with a line width less than 10 nm. Using a reference pattern with a known cross-sectional shape, we determined the PE beam full-width at half maximum (FWHM) for the simulation by fitting the simulation line profile to the experimental one. The resultant simulation SEM images matched the experimental ones for various PE beam FWHM conditions at acceleration voltages of 500 to 2400 V, even though the pattern dimensions differed from those of the reference pattern. These results indicate that analysis using Monte-Carlo simulation is an effective approach to clarifying SEM image formation and that the optimum observation conditions for patterns with a line width even less than 10 nm can be explored using simulation.