Liqun Han

Prospects for charged particle lithography as a manufacturing technology

Abstract Both electron beams and ion beams have unlimited resolution as far as semiconductor manufacturing is concerned. Both have serious throughput limitations because of the difficulties of engineering suitable masks and because of space charge (particle-particle repulsion) effects. Masks are a problem first because of the difficulties of generating a pattern with better than 10 nm tolerance on short range (

Performance investigation of Coulomb interaction-limited high throughput electron beam lithography based on empirical modeling

Multibeam direct writing and projection strategies have been proposed for high throughput electron beam lithography; however, the large beam current required may cause severe beam blur due to the stochastic Coulomb interaction effects between the electrons. From viewpoint of both concept proof and practical system design, evaluation of the fundamental system limits and the optimum performance in a large range of multiple system parameter variable space is critical for such high throughput tools. Based on the response surface design approach, well-fitted empirical models of image beam size as a function of various system parameters have been extracted from Monte Carlo simulations of electron interactions for different proposed multi-beam systems. The blurred imaging beam size is examined in a wide range of system parameters: beam currents from 1 to 100 μA, acceleration voltages from 25 to 100 kV, column lengths from 1 to 100 cm, field sizes from 0.5 to 2 mm, and demagnification from 4 to 16. Two general te...

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.

Multiplexed blanker array for parallel electron beam lithography

Alternative electron beam technologies to conventional single beam systems have the potential to increase beam current by an order of magnitude, into the 1–10 μA range. The technique we are investigating is to insert an aperture array at an object plane in a lithography system, thus creating an array of beamlets which are imaged as a dot matrix. Each aperture is surrounded by an independently controlled electrostatic microblanker which can modulate the beamlet passing through the aperture in conjuction with a blanking aperture placed at a crossover further down the column. An existing 20 kV shaped beam column with a LaB6 source providing 5 μA of illumination current over a 150 μm×150 μm square area is used as a test stand. The column is used to test prototype microblanker chips. Power dissipation calculations show less than a 10 K temperature rise over the prototype chips. Work has begun on a second generation prototype blanker array which will overcome many of the processing difficulties encountered with...

Field size versus column shortness in high throughput electron beam lithography

In electron beam projection systems there are competing requirements for column length. Shorter lengths lead to reduced space charge interactions while longer columns allow larger field sizes. Other parameters include the convergence angle and beam voltage. We have used a generic 4:1 reduction telecentric system as a test vehicle to study these tradeoffs. To determine the lens aberrations we employed a thick lens configuration with constant magnetic fields; space charge blur was estimated with a Monte Carlo simulation using a thin lens approximation. For a given beam voltage V, field size F, convergence angle α, and current I, there is a maximum value of column length L that satisfies a given resolution requirement. For example, if V=100 kV and I=20 μA then L must be less than 30 cm for 100 nm resolution. The use of a retarding field just upstream of the target can lead to greatly improved performance in terms of resolution, field size, and throughput; e.g., the minimum resolved feature size is approximat...

Practical approach to separating the pattern generator-induced mask CD errors from the blank/process-induced mask CD errors using conventional market measurements

With the continuing reduction of minimum feature sizes for semiconductor wafers, the manufacturing tolerances for photomask CD errors were reduced proportionately. As a result, it has become increasingly important to separate the major sources of mask CD errors so that they can be quantified and appropriately addressed. In this article we describe a reliable, convenient and inexpensive technique for separating the blank, and process CD errors from the pattern generator CD errors using conventional market measurements on cross arrays and two-dimensional spatial frequency Fourier analyses. By taking advantage of the fact that the blank/process-induced CD error spatial distributions are known to be fixed relative to the mask blank, while the pattern generator CD error distributions are known to be fixed relative to the pattern generator, a spatial frequency domain analysis of multiple sets of CD measurements on the masks, each set printed by displacing the mask relative to the previous set, is shown to allow...

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.

Performance limitations from Coulomb interaction in maskless parallel electron-beam lithography systems

Proposed high throughput maskless multi-parallel electron beam systems require a large current, which can also potentially destroy the imaging quality due to Coulomb interaction effects. By using a Monte Carlo approach, we have studied the beam current limitation set by Coulomb interaction among electrons at 0.1 micrometers resolution under a variety of column lengths and acceleration voltages for both a blanker aperture array system and a photocathode system. By taking into account the limits from the resist sensitivity and the freedom of system parameter selection, the throughput performance is evaluated in terms of an upper limit and a lower limit, and the feasible system configurations are suggested for achieving the desired throughput as well as 0.1 micrometers resolution. Some results for SCALPEL are also obtained, and the comparison of different lithography tools are discussed.

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.

Global space charge effects in high-throughput electron-beam lithography

Recently global space charge effects were found to seriously limit the performance of high throughput projection electron beam lithography systems. A fundamental analytical theory describing the global space charge lens has been developed using the variational principle. A modified paraxial ray equation and a set of aberration integrals is derived. All the space charge effects, including the defocus, the magnification variation, and the third order aberrations, are found to be proportional to the perveance, the weighted integrals of the normalized current density distribution, and a combination of the system parameters, such as: the field size and the convergence angle. A simple scaling law for the space charge aberrations, where the aberrations at the image edge scale linearly with the radial dimension, has been found for the telecentric projection configuration. Both the theoretical calculations and the Monte Carlo simulations have been done for a SCALPEL configuration with 25 (mu) A beam current and two scaled systems, whose radial dimensions are scaled from SCALPEL by twice and 3 times, and current by 4 times and 9 times respectively at 10 kV. From both theoretical and simulation results, the space charge aberrations at the edge show approximately a 63 times increase in the twice-scaled system, and about 95 times increase in the three-time-scaled system. All these aberration scaling factors are very close to those predicted from the simple scaling law. An experiment based on the simple scaling law was designed to verify the theory and evaluate the performance of the high throughput projection system. In the scaled testing column which has a 1.6 mA beam current, 5 kV beam voltage, 40 cm column length, 8 mm field diameter, and 4 mrad convergence angle at the mask, the space-charge-aberration blur to be measured will be as large as 200 micrometer. Then the final image blur of the real high throughput system can be derived by using the scaling law.