Kimitoshi Takahashi

Proximity effect correction using pattern shape modification and area density map for electron-beam projection lithography

A novel proximity effect correction algorithm using pattern shape modification and the area density map method for electron-beam projection lithography is proposed. This algorithm enables fast, accurate and self-consistent calculation of modified pattern sizes. The correctable minimum feature sizes for shape modification were investigated from two viewpoints, mask fabrication restriction and dose margin. The correctable minimum sizes are mostly determined by the dose margin requirement in the case of isolated and dense repeated patterns, implying that the tool resolution determines correctable minimum sizes. A special technique is required for isolated space patterns where the backscattering energy cannot be reduced by simple sizing. We have implemented an algorithm in which pattern densities at middle parts of large patterns are reduced by using a lines and spaces (L/S) pattern or mesh patterns for that case. Successful correction results down to 60 nm from the simulation and 100 nm from the experiment have been obtained.

Proximity effect correction using pattern shape modification and area density map

A proximity effect correction program in which forward scattering is corrected by shape modification and backscattering is corrected by dose modulation is developed. The amount of the shape modification is determined in such a manner that the full width at half maximum of the forward scattering profile is equal to the designed size. The half maximum of the forward scattering profile is equalized by the dose modulation after the amount of the backscattering is evaluated by the area density map method. This algorithm automates pattern biasing to improve the resolution and assures the resulting pattern size is as designed. The following features are included to improve the conventional methods: The area density map is smoothed iteratively to include higher order effect. Smaller meshes are used for better discretization accuracy. Auxiliary shots are generated to refine correction units where the spatial profile of the deposited energy is steep. The corrected results of 60 nm lines are also presented.

Complementary mask pattern split for 8 in. stencil masks in electron projection lithography

We have improved the M-Split complementary mask pattern split program and our electron projection lithography (EPL) data conversion system to achieve a practical data processing time and data volume. The system was designed to rehierarchicalize the data, flattened after the subfield split, by extracting polygons that all have an identical shape as a cell. The M-Split stress check function was improved by using a normalized bending moment as a criterion. A clustered computing system was used to reduce the data processing time. The processing time for a complementary mask pattern split without rehierarchicalizing was reduced to 57 min by using the stress check function and a ten PC cluster system −3–10 times as fast as with commercially available EPL data conversion systems. We successfully fabricated a full-size 8 in. Si stencil mask consisting of 8000 subfields using the data for an actual 70 nm design-rule system on chip device to demonstrate the effectiveness of M-Split. With a higher performance PC clu...

Scaled measurements of global space-charge induced image blur in electron beam projection system

Previous results on simulations of space-charge induced blur in electron projection systems indicated serious limitations on throughput particularly as minimum feature sizes (MFSs) are reduced below 100 nm. For example, a 40-cm-long 50 kV column might have a maximum throughput of only 0.2 cm2/s at MFS=50 nm [R. F. W. Pease et al., MNE, Italy, 1999]. Direct experimental verification is difficult, so we have developed a set of theory-based scaling laws for electron optics and have carried out a series of experiments for verifying these laws. Our experimental results support the earlier predictions and also confirmed that shortening the column length to the minimum allowed by the maximum practical focusing field strengths (e.g., 1 T and 107 V/m) should bring about dramatic (20- to 50-fold) improvements.

Complementary exposure of 70 nm SoC devices in electron projection lithography

We demonstrate complementary exposure of 70 nm system-on-a-chip (SoC) devices in electron projection lithography using Nikon’s EB stepper, NSR-EB1A, and a high-performance Si stencil mask (4×) fabricated by HOYA. A gate level of the SoC device pattern data called Anaheim was processed for mask fabrication using a 10 PC-clustered hierarchical data processing system in which complementary splitting was executed by the M-Split developed by Selete and ISS. Data processing times and output data volumes of the complementary split and of proximity effect correction were all drastically reduced by using our hierarchical data processing method. We optimized stitching features to compensate for the critical dimension (CD) changes that can occur with stitching errors caused by complementary exposures. The complementary stitching accuracy obtained was better than 20 nm and the CD accuracy was better than 10 nm for 100 nm line and space patterns because of the use of stitching features.

High-speed proximity effect correction system for electron-beam projection lithography by cluster processing

We have proposed the techniques of the pattern shape modification method and the pattern density reduction method as a proximity effect correction (PEC) for electron-beam projection lithography.

Stochastic Coulomb interaction effect in ion-neutralized electron-beam projection optics

The stochastic Coulomb interaction effect in positive ion-neutralized electron-beam (e-beam) projection optics was investigated using Monte Carlo simulations. Stationary ions in the projection optics neutralize e-beam space charge without causing an unwanted increase in stochastic blur. The Coulomb interaction due to the stationary positive ions only has an effect as that of a continuum space charge. We also found that asymmetric point spreads at the field corners are due to the stochastic Coulomb interaction effect and are not a result of residual space charge effect. Methods of ion cloud generation were preliminary discussed. The ion neutralization potentially increases the throughput of an exemplary electron projection tool for 30 nm devices by a factor of 2.

High-performance proximity effect correction for sub-70 nm design rule system on chip devices in 100 kV electron projection lithography

The proximity effect correction (PEC) system to achieve the practical processing time and data volume for sub-65 nm design-rule system on chip (SoC) devices is improved. The lump method, which is the technique to process several subfields at a time, is used to reduce the processing time for PEC. The hierarchical data processing for PEC is also proposed to reduce the data volume. A PC cluster system has been used to reduce the processing time for PEC. For an actual 70 nm design-rule SoC device data, the processing time has been reduced from 7.8 h to 10.3 min and the data volume has been reduced from 12.4 to 2.6 GB by using the lump method, the hierarchical data processing, and a ten PC cluster system. And, we have confirmed that the required critical dimension accuracy of ±5% is achieved for the device data in the simulation. We have successfully fabricated a full-size 8 in. Si stencil mask using the data with our PEC system for an actual 70 nm design-rule SoC device.

Simulation of space charge neutralization using ions in electron beam projection optics

We have investigated, using Monte Carlo simulations, space charge neutralization using ions in a 4:1 telecentric doublet electron beam (e-beam) projection optics. The simulations were performed for two extreme conditions; one with ions moving at the same axial velocity as the e-beam, the other with stationary ions. For both conditions, global space charge effects; defocus, field curvature and magnification change are almost completely eliminated. Although the stochastic coulomb interaction is increased the total point spread which determines resolution can be decreased, because of the elimination of space charge induced spherical aberration. This suggests that space charge neutralization can reduce the global space charge effects without inducing unacceptable stochastic effects.

Dynamic image placement accuracy of a stencil mask

Stencil masks are preferable for EPL (Electron-beam Projection Lithography)from the view point of resolution because it s free from the chromatic aberration caused by the electron energy loss in continuous membrane. However, its mechanical structure poses several concerns. Dynamic image placement (IP)accuracy is one of the essential concerns because patterns on the stencil mask are defined by free-standing Si structures. Moreover the whole pattern areas are supported by fine Si grid structures. The step-and-scan motion of EPL tools is expected to cause dynamic displacements of these fragile structures, which lead to deterioration of resolution, critical dimension (CD)and overlay (OL) accuracies. Two kinds of the dynamic displacements on an EPL stencil mask were estimated by simulations. One is the vibration of the free-standing structures and the other is the dynamic distortion of the whole pattern area. The maximum acceleration of 5 G was assumed in the simulations according to a throughput estimation. The free-standing structures are modeled into cantilever beams and both-end-fixed beams. It was found that the vibration of the structures could be suppressed below the amplitude of 1 nm by limiting the beam length. The limitations were practical ones for complementary split of mask layout. The whole pattern area was modeled into a simple grid structure. It was found that the maximum dynamic displacement was less than 7 nm. The OL accuracy was estimated including those dynamic displacements down to 35 nm node. The results show that the dynamic displacements of the EPL stencil masks would little affect the OL accuracy. The stencil mask is applicable for device fabrication at 70 nm node and below.