We report our development of fogging effect correction method aimed for EBM-8000, our newest series of EB mask
writers for mask production of 22nm half-pitch generation and for mask development of 16nm half-pitch generation. We
refined the method of fogging effect correction by taking account of dose modulation for proximity effects correction
and loading effect correction into fogging effect correction, greatly reducing theoretical error. Writing experiment has
shown that our method based on the threshold dose model is effective, though deviation from the model is observed.
We previously proposed a new method to correct critical dimension (CD) errors appearing in large-scale integrated circuit (LSI) fabrication processes, such as long range loading effect, local flare, and micro loading effect. The method provides high accuracy correction dimensions when using the pattern modulation method (method correcting CD errors by controlling figure sizes of LSI patterns). Now the case that several processes cause CD errors when a layer of an LSI pattern is fabricated on a wafer is discussed. These CD errors are corrected by generalizing the method proposed previously and taking the sequence of processes into account. It is shown from numerical calculation that the method can suppress the CD error to less than 0.01 nm with three iterations, under the condition that the maximum CD errors by micro loading effect and flare are 10 nm and 20 nm, respectively. It is strongly suggested that our methods will provide the necessary CD accuracies in the future.
In our previous paper, we proposed a new method to obtain
accurate pattern dimensions for correcting global critical dimension CD
errors, which are defined as errors of CD uniformity in a region of several
millimeters to several centimeters. The method is based on the pattern
modulation method a method of correcting CD errors by controlling figure
sizes of large-scale integration LSI patterns. An essential point of
our method is to take into account the difference between the pattern
density of the original LSI pattern and that of the corrected pattern which
has a pattern dimension different from original one after pattern modulation
for correction to provide an accurate correction. In this paper, we
apply the proposed method to correct CD errors caused by flare and
microloading effects. It is shown from numerical calculations that our
method can suppress the correction error to less than 0.1 nm for both
cases by three iterations. It is strongly recommended that our method be
used for a wide range of applications to provide the necessary CD accuracies
of the future.
Optical lithography will be extended down to 65nm to 50 nm. However, a mask with high accurate CD uniformity and resolution enhancement technology (RET) such as optical proximity effect correction (OPC) and phase shifting mask (PSM) are required to achieve resolution by exposure wave length. The mask technology is the key of the optical
lithography extension. We developed the electron beam mask writer EBM-3000 for 180-150nm design rule 1), 2), and EBM-3500 for 150-130nm design rule 3), to achieve high accuracy CD uniformity mask and small OPC pattern writing. They were variable shaped electron beam mask writing system with continuous moving stage, at 50kV acceleration
voltage, and had the functions of multi-pass field shift writing, real-time proximity effect correction, grid matching correction, and automatic adjustment for election optical column.The LSI road map calls for such small minimum feature size as that so close to optical resolution limitation where increasingly complex optical proximity corrections (OPC) as well as extremely good mask CD uniformity are required. What is making the challenge even more difficult is that writing time is exponentially increasing as the shot number is exploding to primarily cope with the complex and voluminous OPC and extremely good CD uniformity requirements. Thus the newly developed electron beam mask lithography system EBM-4000 is designed to overcome all these difficult problems associated with 100nm as well as 70nm node masks. In order to increase throughput, triangle/rectangle beam optical column, high current density/high resolution lens, and high speed DAC amplifiers have been developed. To achieve accurate CD uniformity, foggy electron correction/loading effect correction functions are developed.
The electron beam (EB) writing system with high acceleration voltage must be used for the mask fabrication because of its fine resolution. In this case, the resist heating effect becomes one of the serious problems in CD control. This paper discusses the controllability of the resist heating effect and shows that; (1) The CD variation caused by the effect increases with higher pattern coverage and larger shot size, which supports qualitatively results of temperature simulation based on Ralf's model. (2) The multiple exposure is effective to suppress the temperature rise in a substrate and the CD variation. The shifting-type exposure is more effective than the non-shifting-type exposure for suppression of the effect. (4) The CD variation for ZEP7000 can be suppressed to less than 5.0 [nm] (range) provided the shot size is less than or equal to 1.0 [micrometer] and the shifting-type exposure is adopted. Thus, the resist heating effect can be controlled and the CD variation by the effect can be suppressed enough for fabricating the masks to produce 0.15 micrometer devices and beyond.
Toshiba and Toshiba Machine have developed an advanced electron beam writing system EX-11 for next-generation mask fabrication. EX-11 is a 50 kV variable-shaped beam lithography system for manufacturing 4x masks for 0.15 - 0.18 micrometer technology generation. Many breakthroughs were studied and applied to EX-11 to meet future mask-fabrication requirements, such as critical dimension and positioning accuracy. We have verified the accuracy required for 0.15 - 0.18 micrometer generation.
In electron beam writing with high accelerating voltage on photomask blanks, resist heating effect, which is the main factor of CD error in a localized area, is one of the serious problem that must be solved or ameliorated. In this study, the dependence of CD error on the types of resists and the dependence of CD error on the writing conditions of EB writer, were investigated. In this experiment, ZEP7000 (Nihon Zeon), a typical standard of non-chemically amplified resist for electron beam and two chemically amplified resists (CARs) were selected. As a result, the CD error caused by the resist heating effect for the CARs was smaller than that for ZEP7000. The efficiency of multi-pass writing for all of the evaluated resists was observed. The multi-pass writing was very effective in reducing the CD error for both ZEP7000 and the CARs, and especially so for ZEP7000. The dependence of the CD error caused by the resist heating effect on the various writing parameters was investigated using Ralf's model simulation, which is the calculation tool of the temperature rise during the exposure of electron beam including the heat diffusion equation. The CD error for the CARs was smaller and more stable than that for ZEP7000 in various writing conditions. Current density and shot size influenced CD error in sub-field strongly, however, settling time of each shot don't almost influence CD error in sub-field for ZEP7000. The fact that the results for CARs, which have high sensitivity, didn't depend on the current density and shot size indicates the ability to fabricate more accurate mask with higher throughput.
Recently, next-generation mask fabrication processes have been actively examined for application with Electron Beam writing tools and chemically amplified resists. In this study, we used a variable shaped electron beam writing system with an accelerating voltage and chemically amplified resist to investigate the dependence of the CD error in a localized area of a 6025 mask on the process factors, with the goal of fabricating more accurate masks with improving sensitivity. Our results indicated that CD error in a localized area did not depend on the resist thickness. Higher sensitivity and CD uniformity were achieved simultaneously. Moreover, we could isolate the CD error caused by the resist heating effect is more apparent for higher doses than lower doses. However, a higher dose gives rise to a small CD change rate. In this experiment, the effect of the lower CD change rate at a higher dose counterbalances the resist heating effect. By decreasing CD error in a localized area, we obtained a CD uniformity of 14 nm in a 100 mm area on the mask.
Proximity effect correction is a key technology for fabricating reticles by electron beam writing systems. To write patterns of 1 Gbit or higher-capacity DRAMs, the dimensional accuracy required for the correction is better than about 10 nm. Conventional methods do not have sufficient accuracy at the position where pattern density changes sharply. We propose a new correction method with higher accuracy for various patterns and show that we can achieve corrections accurate to about 5 nm.
The critical dimension uniformity required in the fabrication of photomasks for 1 gigabit DRAMs will be more stringent that 20 nm in terms of 3 sigma. High-voltage variable-shaped e-beam (VSB) writing is advantageous because of its high resolution, linewidth stability, and throughput performance. However, stitching errors in VSB writing have been a critical problem in the fabrication of advanced photomasks. In this paper, an improved method to calibrate the size of a VSB shot and reduce shot stitching errors is proposed. The accuracy of the calibration method depends on that of the linewidth measurement system, and shot-size calibration with an accuracy of +/- 10 nm can be achieved using existing measurement systems. The positioning accuracy of VSB shots was enhanced by a multiple pass exposure scheme. With these procedures applied to a 50 kV VSB system, the linewidth variation of a photomask in a local area such as a square region of 200 micrometers X 200 micrometers was reduced to less than 20 nm.
Proximity effect correction for reticle making is discussed. The algorithm for calculating optimum dose is based on the dose formula method and the representative figure method. Its main feature is that dose evaluation points are fixed at individual small regions whose size is sufficiently small compared with the back scattering range. The time for preparing the representative figures for a 1G DRAM and an 8 M gate array was evaluated as 10 and 1.5 minutes, respectively, when a 4 CPU system with a calculation speed of 100 MIPS was used. On the other hand, the correction time for a 10 X 10 cm reticle area was evaluated as 2 - 3 hours by using a 100 MIPS 1 CPU system. It is considered that real-time proximity effect correction will be possible when a parallel processing system with multiplicity of 4 - 7 is used.
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