3σ Value Research Articles

EUV pattern defect detection sensitivity based on aerial image linewidth measurements

As the quality of EUV-wavelength mask inspection microscopes improves over time, the image properties and intensity profiles of reflected light can be evaluated in ever-greater detail. The SEMATECH Berkeley Actinic Inspection Tool (AIT) is one such microscope, featuring mask resolution values that match or exceed those available through lithographic printing in current photoresists. In order to evaluate the defect detection sensitivity of the AIT for dense line patterns on typical masks, the authors study the linewidth roughness (LWR) on two masks, as measured in the EUV images. They report the through-focus and pitch dependence of contrast, image log slope, linewidth, and LWR. The AIT currently reaches LWR 3σ values close to 9nm for 175nm half-pitch lines. This value is below 10% linewidth for nearly all lines routinely measured in the AIT. Evidence suggests that this lower level may arise from the mask’s inherent pattern roughness. While the sensitivity limit of the AIT has not yet been established, it is clear that the AIT has the required sensitivity to detect defects that cause 10% linewidth changes in line sizes of 125nm and larger.

Hyperbranched Polymers for Photolithographic Applications – Towards Understanding the Relationship between Chemical Structure of Polymer Resin and Lithographic Performances

A chemically amplified resist based on a hyperbranched polymer resin is demonstrated for the first time. The hyperbranched polymer is synthesized using the atom-transfer radical polymerization (ATRP) technique, and resists prepared from this hyperbranched polymer present good pattern profiles and line-edge roughness (3σ) values comparable to those of the reference (commercial) resist.

A New Statistical Timing Analysis Using Gaussian Mixture Models for Delay and Slew Propagated Together

In order to improve the performance of the existing statistical timing analysis, slew distributions must be taken into account and a mechanism to propagate them together with delay distributions along signal paths is necessary. This paper introduces Gaussian mixture models to represent the slew and delay distributions, and proposes a novel algorithm for statistical timing analysis. The algorithm propagates a pair of delay and slew in a given circuit graph, and changes the delay distributions of circuit elements dynamically by propagated slews. The proposed model and algorithm are evaluated by comparing with Monte Carlo simulation. The experimental results show that the accuracy improvement in μ + 3σ value of maximum delay is up to 4.5 points from the current statistical timing analysis using Gaussian distributions.

NONLINEAR MID-FREQUENCY DISTURBANCE COMPENSATION IN HARD DISK DRIVES

In single-stage hard disk drives (HDDs), it is difficult to suppress mid-frequency narrow band disturbances caused by disk flutter or windage effectively. This article presents a nonlinear compensation scheme to deal with narrow band disturbance around 1 kHz. The nonlinear compensator that provides additional phase lead without degrading the magnitude characteristic is proposed. This scheme is used to compensate for phase loss due to narrow band filter and enhance the mid-frequency disturbance suppression capability. Settling performance is verified not to be degraded significantly due to harmonics from the nonlinear element. Our results show that the peak value of power spectra of NRRO due to disk vibration are decreased by 99% and position error signal 3σ value is improved 68%.

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).

A priori method of evaluating uncertainties in qualitative chromatographic analysis: probabilistic approach

Probabilistic algorithms to evaluate result reliability in qualitative chromatographic analysis are discussed in the paper. The elementary uncertainty (P0), concerned with a single test (comparison of sample and reference peak positions), is treated as the sum of misidentification and omission probabilities. Both constituents are calculated separately using the simplified model and Laplace functions. In the model, the main reasons for elementary uncertainties are random normally distributed deviations during retention characteristic measurement. Algorithms to calculate both constituents of P0 have to take into account real measurement precision, supposed composition of the sample, content of the database, chosen coincidence criterion and other factors. At a high selectivity of retention, the 3σ value is recommended as the most convenient coincidence criterion. It leads to more reliable and unambiguous attribution of peaks in the chromatogram. For cases that are more complicated, the probabilistic algorithms based upon Bernoulli theorem are proposed to calculate the summary uncertainty of identification, concerned with the multiple test. They take into account P0 value, the number of repeated single tests (n) in the similar or different conditions, and chosen identification criterion K (minimal number of coincidences). The above-mentioned algorithms lead to a priori optimisation of the mode of operation of any identification software system associated with the chromatograph. They can be useful during a metrological validation of corresponding qualitative analysis methods.

Development of an Amperometric Biosensor for β‐N‐Oxalyl‐L‐α,β‐Diaminopropionic Acid (β‐ODAP)

AbstractAn amperometric method for the determination of the neurotoxic amino acid β‐N‐oxalyl‐L‐α,β‐diaminopropionic acid (β‐ODAP) using a screen printed carbon electrode (SPCE) is reported. The electrode material was bulk‐modified with manganese dioxide and used as a detector in flow injection analysis (FIA). The enzyme glutamate oxidase (GlOx) was immobilized in a Nafion‐film on the electrode surface. The performance of the biosensor was optimized using glutamate as an analyte. Optimum parameters were found as: operational potential 440 mV (vs. Ag/AgCl), flow rate 0.2 mL min−1, and carrier composition 0.1 mol L−1 phosphate buffer (pH 7.75). The same conditions were used for the determination of β‐ODAP. The signal was linear within the concentration range 53–855 μmol L−1 glutamate and 195–1950 μmol L−1 β‐ODAP. Detection limits (as 3σ value) for both analytes were 9.12 and 111.0 μmol L−1, respectively, with corresponding relative standard deviations of 3.3 and 4.5%. The biosensor retained more than 73% of its activity after 40 days of on‐line use.

Sequential injection spectrophotometric determination of phenylephrine hydrochloride in pharmaceutical preparations

A fully automated sequential injection spectrophotometric method for the determination of phenylephrine hydrochloride in pharmaceutical preparations is reported. The method is based on the condensation reaction of the analyte with 4-aminoantipyrine in the presence of potassium ferricyanide. The absorbance of the condensation product was monitored at 503nm. A linear relationship between the relative peak height and concentration was obtained in the range 0.5–17.5mgl−1. The detection limit (as 3σ value) was 0.09mgl−1 and repeatability was 0.8 and 0.6% at 2.5 and 5mgl−1, respectively. Results obtained by this method agreed very well with those obtained by the AOAC official method.

Peak Coincidence Tests in Qualitative Chromatographic Analysis: Taking into Account the Reproducibility of Retention Characteristics

Probabilistic algorithms are proposed for calculating the values of chromatographic peak coincidence. These algorithms take into account the standard deviation of normally distributed retention characteristics (σ) and the selectivity of the tabulated values. At a high selectivity, the 3σ value is recommended for the assignment of well-reproducible and widely spaced peaks. In more complicated cases, it is desirable to use values that take into account the number of repeated tests and minimize the integrated probability of errors of first and second kinds, which are associated with a random shift of peaks in chromatographing a sample.

Patterning of 0.175 μm platinum features using Ar/O2 chemically assisted ion-beam etching

Argon/oxygen based chemically assisted ion-beam etching has been investigated for the patterning of stacked capacitor platinum electrodes at ground rules of 200 nm and below. Titanium nitride and bilayers of titanium on top of titanium nitride were used as hard mask layers in the patterning of the platinum. The ion-beam platinum etch process relies on physical sputtering by Ar ions with oxygen being added to the chamber during the etch to provide passivation of the Ti or TiN hard mask material. Pt:Ti etch selectivities of up to 20 have been achieved on blanket wafer samples. Sidewall profile angles greater than 80° (measured from the horizontal) were obtained for tightly spaced platinum features with a pitch of 350 nm using a multiple-angle ion-beam etch process. The uniformity of the etch process across 200 mm diam blanket oxide wafers was measured to be 3.5% (3σ value).

Magnetic Neutral Loop Discharge (NLD) Plasma and Application to SiO2 Etching Process

The applicability of the magnetic neutral loop discharge (NLD) plasma etching apparatus to the 0.1 µm wafer process was experimentally verified. Variation of the SiO2 etch rate was obtained to within 1.3% (2% as 3σ value) on a 200 mm wafer, by applying a functional current to the magnetic coil with 0.1 Hz. A superfine trench pattern with a width of 20–50 nm and a depth of 800 nm was also successfully fabricated with an electron beam resist mask in a C4F8 + CH2F2 plasma at 0.3 Pa.

Economical sampling algorithm using Fourier analysis for mapping wafer critical dimension variations

Characterizations of critical dimension (CD) errors on wafers are usually based on the assumption that the errors are random and normally distributed, and that they therefore can be fully described by a mean and 3σ value. However many sources of CD errors are known to be primarily systematic and are often spatially correlated. In order to establish an efficient measurement sampling scheme and data analysis algorithm that accounts for such nonrandom errors, we collected 900 CD data points using electrical test structures on each of nine wafers that were fabricated in two different fabrication facilities. Wafers using both phase-shifting mask and conventional mask lithography were measured. Fourier analysis was then used to study the errors in the spatial frequency domain. It was found that in all cases systematic and spatially correlated errors, in particular a set of errors that were largely repetitive with stepper field periodicity, dominated over random CD errors. By taking advantage of the correlated nature of these errors, an economical two-step sampling algorithm is defined that significantly reduces the amount of sampling needed (by approximately fourfold). By combining the results of the two-step sampling in the spatial frequency domain, an accurate map of actual error spatial distribution can be extracted. The accuracy of the scheme is verified by using subsets of the 900 data point measurements, and comparing the results to those from the full set of data.

Electron-Beam Study of Nanometer Performances of the SAL 601 Chemically Amplified Resist

The SAL 601 chemically amplified resist has been characterized for electron-beam (e-beam) exposure with feature size down to 75 nm. The main resist process parameters such as the pre and post exposure baking time and temperature, the resist thickness and its development conditions, have been investigated and calibrated for sub-100 nm resolution. Various writing strategies making use of test patterns comprising different nominal feature-size were also investigated. Dense lines, as fine as 75 nm, are achieved in a 350 nm thick resist with exposure latitude of 0.4 nm/µC/cm2 at 50 kV accelerating voltage. The sub-100 nm resolution was achieved with a 3σ value of 12 nm.

Fine-pattern lithography for large substrates using a holographic mask-aligner

A new holographic mask aligner with an imaging resolution of 0.5 μm and an exposure field of 250 × 300 mm has been realised. It has been employed to print a 10.5″-diagonal pattern of 1-μm-diameter microholes for field-emission displays. The CD uniformity achieved over the complete pattern was 8.3% (3σ value). The system, which can handle substrates of up to 370 × 470 mm, offers a powerful alternative for high-resolution flat-panel display manufacture.

0.1 μm WSiN-gate fabrication of GaAs metal-semiconductor field effect transistors using electron cyclotron resonance ion stream etching with SF6–CF4–SiF4–O2

We investigate etching characteristics of WSiN gates using electron cyclotron resonance ion stream etching with a SF6–CF4–SiF4–O2 gas mixture, and we fabricate 0.1 μm WSiN gates for ultrahigh speed GaAs metal-semiconductor field effect transistors (MESFETs). A vertical WSiN gate etched pattern is obtained as a result of CF4 addition. Moreover, uniform etching over the entire wafer can be attained with high selectivity between the WSiN and the GaAs and with accurate control of the gate length. A current-gain cutoff frequency (fT) of 131.4 GHz with a 3σ value of 5.0 GHz in whole wafer has been obtained for 0.1 μm gate GaAs MESFETs.

X-ray mask distortion correction technology using pattern displacement simulator

A new method to correct x-ray mask distortion was developed. This method uses the correction method of previous analysis of distortion and transformation of coordinates (PAT) and a simulator to calculate pattern displacement caused during backetching. The key advantages of using PAT with a simulator are that no extra time, substrates, or labor are required to fabricate send-ahead masks. Using this correction method, it was confirmed that the 3σ value of the pattern placement error was reduced to below 76 nm. This indicated that PAT with a pattern displacement simulator was as useful for improving the pattern placement accuracy of x-ray masks as PAT with send-ahead masks. In addition, analysis of the remaining error made it clear that the correction error was mainly caused by the lack of e-beam writing accuracy.

Submillimeter Line Search in Jupiter and Saturn

A Fourier transform spectrometer was used to search for the rotational transitions of a number of molecular species in the atmospheres of Jupiter and Saturn in all available submillimeter atmospheric windows between 300 and 1000 GHz (1000 and 300 μm). The molecules which were searched for include the saturated molecules PH3and H2S, the hydrogen halides HCl and HBr, and the alkali hydrides LiH and NaH, as well as HCN and HCP. A strong absorption feature at 800.5 GHz corresponding to theJ= 3–2 transition of PH3was detected in both Jupiter and Saturn, but the detailed analysis of these results will be presented elsewhere. A feature coincident with theJ= 1–0 transition of HCl was also tentatively observed in Saturn. No other molecules were detected in either planet, setting stringent limits on most of the species listed above.Our measurements set an upper limit of 0.3 ppb on the HCN mole fraction in Jupiter if HCN is distributed with a constant mixing ratio. This concentration is significantly lower than the purported infrared detection of Tokunagaet al.(1981). A conservative upper limit of 0.4 ppb is obtained for Saturn, although the molecule may actually be present in Saturn at this abundance. If condensation of HCN is included, the deep tropospheric HCN limits are increased to 2 ppb for Jupiter and ∼15 ppb for Saturn. We set a tropospheric HCl upper limit of 5 ppb in Jupiter, and have tentatively detected this molecule at a mole fraction of 1.1 ppb in Saturn. We set HBr upper limits of 7 ppb in Jupiter and 1.1 ppb in Saturn. To our knowledge, these are the first spectroscopic limits on the halides in any jovian planet. They indicate that Cl is strongly depleted relative to the solar abundance in the upper tropospheres of Jupiter and Saturn, but the tentative detection of HCl in Saturn suggests that vertical transport is rapid compared to the chemical lifetime of HCl. We also obtain upper limits of 29 ppb (Jupiter) and 16 ppb (Saturn) for H2S (the latter is an order of magnitude improvement over existing measurements), and 22 ppb (Jupiter) and 5 ppb (Saturn) for HCP. For Saturn, this is nearly a factor of 2 smaller than the value suggested by photochemical models of Kaye and Strobel (1984). Finally, we obtain upper limits of 11 ppt (Jupiter) and 12 ppt (Saturn) for LiH, and 8 ppt (Jupiter) and 5 ppt (Saturn) for NaH. These are the first tropospheric upper limits to be placed on alkali metals in the troposphere of the Jovian planets, and indicate that both Li and Na are strongly depleted relative to meteoritic abundances. All limits are 3σ values.

Exposure latitude and CD control study for additively patterned X-ray mask with GBit DRAM complexity

40 kV electron beam lithography has been used to pattern gold plated x-ray masks containing GBit DRAM complexity layouts. The two commercial e-beam resists used, namely PMMA and SAL 601, both showed 0.12 μm resolution capability in dense and large layouts patterning and also, under optimised exposure and development conditions, exhibited good exposure latitudes which were also evaluated for two different beam spot sizes. Futhermore, a study of development technique and effect of e-beam spot size indicated a marked dependence of ultimate resolution and exposure latitude on such parameters. A statistical analysis of 0.12 μm resolution SAL patterning on large chip dies (30 × 30 mm2) resulted in a dimensional control of 10 nm (3σ value).

Patterning accuracy improvement of the electron beam direct writing system EX-8D

To improve patterning accuracy, a feedback compensation technique for the magnetic field was applied to EX-8D. The beam position deviation induced by low-frequency ambient magnetic field changes and 50 Hz magnetic interference has been minimized by using a feedback system for the interference source signals. As the result of the feedback technique and magnetic shielding applied to EX-8D, the subfield position deviation measured on written patterns has been improved to be 22 nm as a 3σ value without room shielding. The main deflection calibration method was also improved by utilizing an axially displaceable mark together with a fixed-height mark. On the fixed mark, the main deflection coefficients were determined with no concern to height sensor outputs. Height-dependent coefficients were then calibrated by evaluating the field size for various heights using the axially displaceable mark. The calibration accuracy was estimated to be 9.7 nm at the main field edge. It was also confirmed that height-dependent coefficients would be calibrated accurately enough by the use of a piezodriven mark. As a result, a stripe stitching error of less than 30 nm has been obtained.

Intrafield linewidth variances in 0.25 μm i-line lithography

We have studied variations of the widths of polysilicon lines patterned to nominal dimensions of 0.25 μm with an i-line stepper. The analysis is based on measurements of electrical resistance of the etched polysilicon patterns. Systematic dependence of linewidths on exposure field coordinates accounts for a large fraction of their total variance. We have expressed the systematic intrafield variance as a sum of three terms: one attributed to the stepper, another to the reticle, and a third to both. The systematic intrafield dependence is most significant for small isolated lines; this term alone had a 3σ value of 25 nm at a nominal dimension of 0.25 μm.