The development of Electron Projection Lithography (EPL) has proceeded for more than 10 years since its first description. EPL is regarded as a practical technology for 65 nm technology node and below. Nikon has been developing an EPL tool, named as the EB stepper. NSR-EB1A is the first EPL tool that has full functions for practical R&D use such as dynamic exposure by combination of electron beam deflection and stage scanning, wafer alignment, and so on. Some features of the EB stepper, which uses a 100 kV electron beam, are high resolution, and a large process margin associated with large depth of focus (DOF). Large DOF is a major feature of electron beam lithography.
In the previous paper, we reported data of dynamic resolution and subfield stitching accuracy as preliminary performances that were obtained by NSR-EB1A. Recently the development of EPL reticle is significantly progressed. Today, high quality 200 mm diameter EPL reticle is available from plural mask suppliers. Using 200mm EPL reticle, we achieved subfield stitching accuracy about 20nm (3s). And we also evaluated total performance such as CD uniformity, overlay accuracy. This paper reports the latest performance of NSR-EB1A.
The Electron Projection Lithography (EPL) has already presented high resolution capabilities and been developed as one of the candidates of post optical lithography. However, much discussion has not been made for resist chemistry, especially on outgassing during exposure, regardless of utilizing high acceleration voltage and applying vacuum system. Moreover, two types of resist system, positive and negative tones, are required for a complete device manufacturing due to its stencil mask structure. Both resist tones with chemically amplified system were experimentally formulated to examine the partial and total pressure changes after exposure. The mass number of outgassing species was also measured in vacuum. The positive tone resist sample indicated many peaks at high mass numbers, in contrary to that negative tone resist sample showed strong peaks at low mass numbers. In addition, it was found that there was a clear trend between the total exposure doses and the total pressure changes in a certain positive-tone resist formulation. The fact may suggest the necessity of high sensitivity resists for EPL from the different standpoint of high throughput in mass production. The dependency of resist base polymer backbone was also examined under an accelerated exposure condition. The resist comprising of methacrylate base polymer indicated high amount of outgassing than that of poly(hydroxystyrene) (PHS) base polymer, with the same resist formulation. The polymer decomposition other than deprotection was considered since the exposure energy in EPL was much greater than that of optical lithography. We developed a new resist adopting the low outgassing concepts such as high sensitivity, non-methacrylate part, and low protecting ratio. The resist presented 56nm 1:2 contact resolution with resist sensitivity of 5.7μC/cm2.
We investigated the defect printability of hole patterns in electron projection lithograpy (EPL) using a diamond reticle with a programmed defect pattern. The reticle was fabricated by NTT-AT and wafer exposure was performemd using Nikon's EB projection experimental column. We simulated the defect printability to udnerstand in greater detail. We found that the mask error enhancement factor (MEF) of the size shift defect category exceeded the value of one and was degraded by the amount of beam blur. On the other hand, the printability of the dot defect category was lower than the shift category. In particular, pint hole defects smaller than 100 nm were not printed. However, the defect types of under size shift, truncation, edge intrusion, and corner intrusion (they decreased the opening area), actually increased the defect size because the defect was too small for hole patterns to print. In general, the defect printability of hole patterns depends on the beam blur, and the printed error size at the hole patterns getting larger than the line patterns. We have to pay clsoer attention to the hole pattern defect than to the line patterns.
In Electron Projection Lithography (EPL) that is desigend for 65nm production tool, proximity effect correction (PEC) is an important issue for an accurate feature size control. Reticle resizing meethod is adopted for its correction. High controllability (ΔCD within ±10%) of critical dimension (CD) is required after proximity effect correction. For estimating the CD controllability, we have evaluated its dependency on exposure dose and beam blur in resizing method for the first time in EPL using Nikon's EPL experimental column (acceleration voltage=100kV, magnification=1/4, sub field size=250×250μm). Evaluating patterns were the target size of 100nm isolated line and twin lines. Beam blur was controlled by changing focus and was measured by Aerial Image Sensor (AIS). Groups of different biased patterns were located at different distances from large pattern on the wafer respectively. As a result, CD variation by proximity effect was 35-40 nm for 100nm-isolated. Under our recommended condition that resizing range in puls side was equal to that in minus side (±20%), blur latitude and dose latitude satisfied our CD uniformity budget and mask enhanced error factor (MEF) was around 1, then reticle fabrication CD controllability from budget requirement was satisfied. Therefore it was shown that proposed proximity effect correction method achieved high CD controllability.
A single-layer resist process for a technology nodes at or below 65nm utilizing a novel supercritical dry technique and Electron-beam Projection Lithography (EPL) technology is discussed. EPL is inhernelty advantageous in imaging sub-65nm geometries with high aspect ratios. Pattern collapse of these high aspect ratio (resist) structures, however, is a critical and limiting issue. By employing our novel supercritical carbon dioxide (SCCO2) dry technique, 70nm and 60nm lines and spaces patterns with a resist thickness of 250nm, whose aspect ratio is 3.5 and 4.2 respectively, have been successfully demonstrated without resist pattern collapse.
Mask defect printability of electron projection lithography (EPL) by using a diamond reticle with programmed defect pattern was investigated. Th reticle was obtained from NTT- AT and wafer exposure was performed by Nikon's EB projection experimental column. In general, the printability of the defects of 'dot' categories are lower than 'shift' categories and the allowable defect size of 'dot' categories in 70 nm node are larger than 100 nm on mask with +/- 10 percent critical dimension (CD) tolerance criteria except edge extension defect. On the other hand, the printability of the defects of 'shift' categories is higher than 'dot' categories. According to the defects CD dat of mask and resist patterns, CD error or placement error caused by the defects of 'shift' categories show a linear correlation between the defect size of mask x reduction ratio and printed defect size of resist pattern. SO the allowable defect size of 'shift' categories in 70 nm node should be less than 28 nm on mask.
Nikon, in collaboration with IBM, has been developing EB stepper, which is the electron beam projection lithography (EPL) system for 70 nm node generation and below. As the standard reticle for EB stepper, the scattering silicon stencil type is used to obtain highest performance. The EB reticle has thin silicon membranes of thickness 2 micrometers and membrane size 1.13 mm square with stencil opening patterns, which are supported by a grid-grillage structure. The development of the EB reticle is one of key issues in the EB stepper development. We had accomplished 76nm reticle development using silicon-on-insulator wafer with a stress- controlled membrane. Now we are in the 200 mm reticle development phase. We have curried out experiments in cleaning, inspection and repair for the EB reticle, which are very important issues for the EB reticle fabrication. We showed possibilities of Ar aerosol cleaning, a reticle repair using Focused Ion Beam, pattern defect inspection with DUV microscope and so on.
The latest development status of EB Stepper is reported. The experimental data include the latest resist image data exposed by 100keV electron beam, mask error factors and dosage margins at several backscattered electron levels, transmission data of continuous membrane reticles, and recommended structures for alignment marks, etc. The basic studies related to system design are also explained, those are the strategy for the management of reticle deformation and the stitching accuracy in overlaid layers, etc. Through these data, the resolution capability down to 50nm technology node is clearly shown and alignment/stitching capability is also described. The requirement to a continuous membrane reticle is indicated from experimental data.
For electron beam projection lithography system, it is one of the most important issues to stitch desired patterns accurately. We have found a way to stitch the patterns with high accurate critical dimension by a pattern edge deformation that moderates a stitching error by as much as 2.5 times compared with no-deformed edge.
The stencil reticle is one of masks for EPL (Electron beam Projection Lithography). The ability to repair pattern defects in such masks is an essential requirement for their use. For clear defects, repair issues include deposition on the stencil pattern, electron scattering properties of the deposition material, stability of the repair against 100 kV electron beam (EB) bombardment and etching and pattern profile accuracy. For opaque defects, ion milling of the stencil material to high accuracy is required. In this paper, we discuss the repair deposition of clear defects up to 0.15 micrometer using a carbon-compound precursor and the critical dimension (CD) controllability of the exposed resist pattern with various thickness of repair depositions using Nikon's EB experimental projection column (100 kV). In addition, using a Monte Carlo simulation of the electron scattering in the silicon membrane and the repair deposition, the beam contrast is estimated for EPL at contrast aperture (CAP) size. The resist CD can be controlled using more than 2 micrometer-thick Diamond like Carbon (DLC) deposited on the stencil reticle and shaped by focused ion beam (FIB). Profiles, pattern sizes and the electron scattering properties of DLC repairs are stable against 100 kV EB irradiation (about 2 C/cm2 dosage; corresponding to half-year dosage). These results show the possibility of DLC as the repair material for clear defects. The possibility of repairing opaque defect, which involves FIB milling of sizes from 0.2 to 0.4 micrometer, is also demonstrated.
An electron projection lithography (EPL) system which projects reticle patterns onto a wafer will be applied to sub 100 nm lithography. Requirements for line width accuracy are very strict as feature sizes are less than 100 nm. For electron beam lithography, proximity effect corrections have always been an important issue for accurate feature width control. In this paper characteristics of several correction methods are examined, and appropriate correction methods for 100 kV EPL are introduced. Employing the shape correction method burdens the reticle pattern preparation system much more than other methods. Therefore a calculation method suitable for 100 kV EPL where the backscatter radius is very wide ((beta) b approximately equals 30 micrometer) and the forward scatter radius is narrow ((beta) f approximately equals 7 nm) has been developed. The calculation of deposition energy by the backscattered electron beam is carried out with a coarse grid but wide range. The calculation of the combined effect of the electron scattering blurs from the features is carried out only within a narrow range. The correction calculation is carried out using both of these results. Using this method, accurate and fast calculations can be achieved. Employing the GHOST correction method increases total exposure cost. The practical GHOST correction methods may also be improved. An additional correction method named shape correction with GHOST is also shown.
Nikon is developing an Electron Beam (EB) stepper as one of the next-generation lithography systems for feature sizes of less than 100 nm. As a reticle for the EB stepper using a high power EB (acceleration voltage: 100 kV, current on reticle: 100 (mu) A), a scattering stencil reticle with a grid-grillage structure has been investigated, EB projection experimental column which operates a high power EB was constructed. Some experimental data of scattered electron characteristics using the EB projection experimental column are given as follows: (1) Scattering contrast of 99.9% can be obtained using 100 kV electron beam (membrane thickness; 2 micrometer, aperture half angle onto reticle; 2 mrad). (2) Changes of resist pattern width of 1:1 and 1:2 lines and spaces are around 40% and around 20% respectively due to the proximity effects by backscattered electrons form the silicon substrate. (3) Contrast of EB mark detection for the system calibration, the reticle alignment, and the wafer registration is obtained. Comparing with the values that be obtained by theoretical calculation, some of experimental data gave good agreement.
The imaging concept of electron projection lithography (EPL) with silicon stencil reticle is explained. A silicon membrane thickness of 1 - 4 micrometer is suitable for the reticle. A scattering contrast of greater than 99% is expected. Nikon EB stepper's dynamic writing strategy of discrete exposure on a sub-field by sub-field basis with deflection control of the electron beam is explained. The basic system configuration of EB stepper is introduced. Examples of error budget for CD variation and Overlay/Stitching are shown. Nikon's policy for countermeasures for critical issues such as proximity effect correction, sub-field/complementary stitching and wafer heating influence are explained. For extensibility down to 70 nm and below, both exposure tool and reticle should be improved.
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