Hidetoshi Morokuma

Development of a portable mass spectrometer characterized by discontinuous sample gas introduction, a low-pressure dielectric barrier discharge ionization source, and a vacuumed headspace technique.

The present study has attempted to downscale a mass spectrometer in order to make it portable and enable onsite analysis with it. The development of a small mass spectrometer required the use of a compact pump whose displacement was small, decreasing the sensitivity of that spectrometer. To get high sensitivity with a small mass spectrometer, we have integrated novel techniques: a highly sensitive ionization source and efficient extraction of sample vapor. The low-pressure dielectric barrier discharge ionization (LP-DBDI) source made it possible to increase the conductance between the source and the mass analyzer, compared with ambient ionization sources, enhancing the efficiency of the ion transfer from the ionization source to the mass analyzer. We have also developed a vacuumed headspace method efficiently transporting the sample vapor to the ionization source. The sensitivity was further enhanced by also using a discontinuous sample gas introduction technique. A prototype portable mass spectrometer using those novel techniques was found to be sensitive enough to detect 0.1 ppm methamphetamine, 1 ppm amphetamine, 1 ppm 3,4-methylenedioxymethamphetamine, and 10 ppm cocaine in liquid.

A new matching engine between design layout and SEM image of semiconductor device

Optical proximity correction (OPC) plays a vital role in the lithography process development of current semiconductor devices. OPC is utilized to achieve the ideal pattern shape because of the limitations of optical resolution. However, the lithography process design has become increasingly more complex due to the abundant use of OPC features. Hence, metrology requests for CD-SEM have also become more complex and diverse in order to characterize the critical OPC models. The number of measurement points for OPC model evaluation has increased to several hundred points per layer, and metrology requests for realized pattern shapes on the wafer are no longer simple one-dimensional measurements. Metrology requests include not only the traditional line width measurements, but also edge placement error (EPE) and corner rounding to identify line end shortening. Several researchers have proposed using the design layout as a template instead of the SEM image for the recipe creation of CD-SEM and EPE measurement. However, it is very difficult to achieve good matching results between the design layout and the SEM image in practical processing times. Hitachi High-Technologies has developed a robust and quick matching engine between the design layout and SEM image bitmap. The new system, incorporating this new matching engine, can automatically create a practical recipe from the coordinate information of measurement point and the design layout information, such as GDSII. As a result, the new system can vastly reduce the amount of time and number of operations required to generate a several-hundred point CD-SEM recipe for OPC evaluation. This study demonstrates the capability and presents evaluation results of this new matching engine. This new capability has proven to be a viable solution for OPC evaluation, and its efficiency will allow for quicker information turns between design and manufacturing.

Characterizing cross-sectional profile variations by using multiple parameters extracted from top-down SEM images

This paper describes a new approach towards monitoring the semiconductor lithography process using critical dimension scanning electron microscopy (CD-SEM). In the lithography process, there are two important process parameters, exposure dose E and focus F. To monitor both the E and F variation, a new method for characterizing the cross-sectional profile of the photoresist pattern from the secondary electron (SE) waveform has been developed. An innovative feature of this method is that it can quantify the degree of top rounding (TR) and bottom rounding (BR) of the cross-sectional profile separately.

Dose and focus estimation using top-down SEM images

As design rules shrink and process windows become smaller, strict process control is becoming increasingly important. The two primary process parameters in the photolithography process, exposure dose and focus, require strict control in order to maintain the photoresist profile. This paper presents the second stage of an approach towards monitoring the semiconductor photolithogprhay process by using critical dimension-scanning electron microscopy. In the former paper, we propsed a method that quantifies the photoresist pattern profile variation caused by dose or focus variation. In this paper, a new method for estimating the variation in exposure dose and focus is presented. Top-down SEM imagse are intrinsically limited in the inability to observe the re-entrant profile. This limitation has been overcome through the use of two tyeps of common patterns: island patterns and window patterns. Island patterns, such as isolated line patterns, have a tapered profile for negative defocus, while window patterns, such as isolated spaces patterns, have an inverse tapered profile for negative defocus. Using both types of patterns allows the focus deviation to be monitored, whether positive or negative defocus. The behavior of the two types of patterns is considered here based on photolithography simulation, and a new algorithm for estimating the exposure dose and focsu variation is proposed.

Automated CD-SEM recipe creation: a new paradigm in CD-SEM utilization

As the trends in integrated circuit fabrication follow Moores Law to smaller feature sizes, one trend seen in lithographic technology is the continually increasing use of optical enhancements such as Optical Proximity Correction (OPC). Small size perturbations are designed into the nominal feature shapes on the reticle such that the intended shape is printed. Verifying the success of OPC is critical to ramp-up and production of new process technologies. CD-SEMs are imaging tools which are capable of measuring feature sizes in any part of a chip, either in a test structure or within a circuit. A new trend in CD-SEM utilization is the implementation of automated recipe generation of complex CD-SEM recipes. The DesignGauge system uses design-to-SEM recipe creation and data collection. Once the recipe creation flow is implemented, the task of recipe creation can be accomplished within minutes. These applications enable a CD-SEM to be utilized to collect data for very complex OPC CD-SEM recipe runs which measure many different unique linewidths, spaces, and pattern placements within a circuit to check OPC success and lithographic fidelity. The data collection can provide accurate data results that can be utilized for comparing achieved feature measurements to nominal values from the design layout. This new application adds much value to the CD-SEM compared to other technologies such as OCD, as it completes the evaluation of in-circuit behavior to test structures in a scribe lane, something OCD currently cannot do. The present work evaluates the capabilities of DesignGauge, which is available for the latest-generation Hitachi S-9380II CD-SEMs. The evaluation includes rigorous tests of navigation, pattern recognition success rates, SEM image placement, throughput of recipe creation and recipe execution.

MPPC technique for gate etch process monitoring using CD-SEM images and its validity verification

The effectiveness of multiple parameter profile characterization (MPPC) as a three-dimensional measurement technique for etched gates is examined by comparison of shape indices with device performance. The MPPC method derives shape indices from top-down, critical-dimension scanning electron microscopy (CD-SEM) images to characterize the sidewall angle and footing roundness of the gate, which are considered to be the structural features that have a great effect on device performance. The capabilities of the proposed method are evaluated through experiments using processed gate wafers etched under different conditions, comparing the shape indices with the cross-sectional profiles obtained by atomic force microscopy. The relationship between the MPPC indices and threshold voltage is also investigated, confirming that variations in sidewall angle and footing roundness have several times the impact on threshold voltage as line width variation. This study confirms the importance of three-dimensional measurement of gate profiles for process monitoring through the use of a method such as MPPC.

Probe Heating Method for the Analysis of Solid Samples Using a Portable Mass Spectrometer

We previously reported on the development of a portable mass spectrometer for the onsite screening of illicit drugs, but our previous sampling system could only be used for liquid samples. In this study, we report on an attempt to develop a probe heating method that also permits solid samples to be analyzed using a portable mass spectrometer. An aluminum rod is used as the sampling probe. The powdered sample is affixed to the sampling probe or a droplet of sample solution is placed on the tip of the probe and dried. The probe is then placed on a heater to vaporize the sample. The vapor is then introduced into the portable mass spectrometer and analyzed. With the heater temperature set to 130°C, the developed system detected 1 ng of methamphetamine, 1 ng of amphetamine, 3 ng of 3,4-methylenedioxymethamphetamine, 1 ng of 3,4-methylenedioxyamphetamine, and 0.3 ng of cocaine. Even from mixtures consisting of clove powder and methamphetamine powder, methamphetamine ions were detected by tandem mass spectrometry. The developed probe heating method provides a simple method for the analysis of solid samples. A portable mass spectrometer incorporating this method would thus be useful for the onsite screening of illicit drugs.

Cross-sectional gate feature identification method using top-down SEM images

This study presents a method of extracting 3D metrological information for etched gate structures from top-down SEM images for use in critical dimension analysis. The variations in sidewall angle and bottom corner roundness are quantified as feature indices by multiple parameter profile characterization (MPPC), and are used as the main indicators of device performance. A stable algorithm developed based on simulation and experimental results partitions the SEM image signal into the sidewall and footing based on the first derivative of the image signal. The width of the sidwall is used as an index of the sidewall angle, and the width of the footing is used as an index of the footing roundness. The validity of the MPPC method is confirmed through experiments using actual poly-Si gate wafers, and is shown to have a 3σ accuracy of ±0.9° for sidewall angles deviating by mroe than 2°. The sidewall angle index and its distribution map are useful for evaluating the etching process, and are particularly effective for revealing subtle macro variations like asymmetry, while the footing roundness index is useful for screening out bad wafers. As MPPC employs only top-down SEM images, no throughput loss will be incurred in comparison with conventional CD measurements.

Development of beam-tilt angle calibration method for CD-SEM

An image processing technique for estimating the incidence angle of an electron beam (beam-tilt angle) of a critical dimension scanning electron microscope (CD-SEM) has been developed. The estimation and correction of the error of the beam-tilt angle are indispensable for high precision measurement of CD and/or three-dimensional profiles of semi-conductor device patterns. In this technique, a pyramidal-shaped crystal sample made by anisotropic etching is used for calibration. From the top-down and tilted views of the sample, x and y directional beam-tilt angles relative to the top-down view are estimated simultaneously, with the geometrical variations of the pyramid ridge lines detected by image processing. Exact positioning of the sample is not required because the inclination and rotation of the sample towards the wafer surface are estimated separately from the beam-tilt angles. Evaluation of 40 sample images, including 4 directional tilt angles, indicated that deviations of the estimated x and y beam-tilt angles were 0.13 and 0.12 degree respectively (3 sigma). It will also be shown that the technique is robust against characteristic SEM image distortion and low S/N. This technique has achieved high precision and quantitative estimation for the beam-tilt angles, and will provide a method for high precision measurement of CD and three-dimensional profile for semi-conductor process monitoring and control in the future.