In the past 10 years, immersion lithography has been the most effective high volume manufacturing method for the critical layers of semiconductor devices. Thinking of the next 10 years, we can expect continuous improvement on existing 300 mm wafer scanners with better accuracy and throughput to enhance the total output value per input cost. This value productivity, however, can be upgraded also by larger innovations which might happen in optical lithography. In this paper, we will discuss the possibilities and the impossibilities of potential innovation ideas of optical lithography, which are 450 mm wafer, optical maskless, multicolor lithography, and metamaterial.
The perpendicularly orientated lamellar structure of the self-organized diblock copolymer is an attractive template for sub-10-nm line-and-space pattern formation. We propose a method of evaluating the neutral layer (NL) whose performance has an important bearing on the perpendicular orientation of the lamellar structure. The random copolymer of methyl methacrylate and i-butyl POSS methacrylate (MAIBPOSS) has been investigated as an NL for a polymethylmethacrylate-b-polymethacrylethylPOSS (PMMA-b-PMAIBPOSS) lamellar structure. PMMA-b-PMAIBPOSS material has the potential to form sub-10 nm line-and-space pattern, in addition to high etch selectivity due to its POSS structure. Under the free surface, PMMA-b-PMAIBPOSS film on the random copolymer layer showed horizontal orientation. However, a half-pitch of a 7-nm finger pattern structure was observed by peeling off the horizontally oriented layer. The upper portion of the PMMA-b-PMAIBPOSS film was eliminated till proximity of the random copolymer layer by CF4 gas etching. From the result, it was revealed that the PMMA-r-PMAIBPOSS works as an NL. It was confirmed that the contact angle analysis using an appropriate polymer is a suitable method for evaluation of the surface energy performance of the copolymer with the attribute of high segregation energy.
We proposed a new concept of “defect-aware process margin.” Defect-aware process margin was evaluated by investigating the energy difference between the free-energy of the most stable state and that of the first metastable state. The energy difference is strongly related to the defect density in DSA process. As a result of our rigorous simulations, the process margin of the pinning layer width was found to be: (1) worse when the pinning layer affinity is too large, (2) better when the background affinity has the opposite sign of the pinning layer affinity, and (3) better when the top of the background layer is higher than that of the pinning layer by 0.1L<sub>0</sub>.
Source mask optimization (SMO) is widely used to make state-of-the-art semiconductor devices in high-volume manufacturing. To realize mature SMO solutions in production, the Intelligent Illuminator, which is an illumination system on a Nikon scanner, is useful because it can provide generation of freeform sources with high fidelity to the target. Proteus SMO, which employs co-optimization method and an insertion of validation with mask three-dimensional effect and resist properties for an accurate prediction of wafer printing, can take into account the properties of Intelligent Illuminator. We investigate an impact of the source properties on the SMO to pattern of a static random access memory. Quality of a source made on the scanner compared to the SMO target is evaluated with in-situ measurement and aerial image simulation using its measurement data. Furthermore, we discuss an evaluation of a universality of the source to use it in multiple scanners with a validation and with estimated value of scanner errors.
Nikon’s Intelligent Illuminator, a freeform pupilgram generator, realizes a high flexibility for pupilgram control by using more than 10,000 degrees-of-freedom for pupilgram adjustment. In this work, an Intelligent Illuminator was integrated into an ArF scanner, the Nikon NSR-S621D. We demonstrate the pupilgram setting accuracy by direct correlation between on-body measured pupilgram and desired target pupilgram. We show that the Intelligent Illuminator is used for fine tuning of the pupilgram to match optical proximity effect (OPE) characteristics. We experimentally confirmed that a global source optimization software realized an improvement of lithographic process window without changing OPE characteristics by using optimized pupilgram made by Intelligent Illuminator.
Source mask optimization (SMO) is widely used to make state-of-the-art semiconductor devices in high volume manufacturing. To realize mature SMO solutions in production, the Intelligent Illuminator, which is an illumination system on Nikon scanner, is useful because it can provide generation of freeform sources with high fidelity to the target. Proteus SMO, which employs co-optimization method and an insertion of validation with mask 3D effect and resist properties for an accurate prediction of wafer printing, can take into account the properties of Intelligent Illuminator. We investigate an impact of the source properties on the SMO to pattern of a static-random access memory. Quality of a source made on the scanner compared to the SMO target is evaluated with in-situ measurement and aerial image simulation using its measurement data. Furthermore we discuss an evaluation of a universality of the source to use it in multiple scanners with a validation with estimated value of scanner errors.
Electron Projection Lithography (EPL) is considered one of promising technologies below 45nm node, especially for contact/via holes and gate layers. EPL has some nice features such as very high resolution to be applied for two device nodes, large process margin associated with large depth of focus and an expected lower CoO. Nikon has been developing an EPL tool, so-called EB Stepper. NSR-EB1A is the first EB Stepper that was designed as R&D tool for 65nm technology node and that was already delivered for Selete (Semiconductor Leading Edge Technologies, Inc.) at Tsukuba in Japan. Nikon has developed two NSR-EB1A tools so far, one system for Selete as a 300mm wafer system and the other for Nikon's development and evaluation as a 200mm wafer system. Both tools have already started to show full performance data and good stability characteristics. The latest EB1A tool performance shows very good results in such data as the resolution of 50nm 2:1 L/S and 60nm 1:1 dense contact holes patterns, stitching accuracy of around 18nm, and overlay accuracy of around 20nm(X+3sigma).
As we move technology further and further down the geometry scale we are coming upon imaging situations where our use of existing optical lithography is being questioned due to the lack of process margin in manufacturing lines. This is especially apparent in the imaging of contacts where memory devices, that generally have the densest arrays of these features, may no longer be able to print the desired features. To overcome this it is necessary to either modify the design, a very expensive and time consuming process, or find an imaging process capable of printing the desired features. Electron Projection Lithography (EPL) provides an option to print very small features with a large process margin.
In this paper we detail the performance of both memory and logic based designs in an EPL process. We detail the manufacture and results of stencil mask manufacture. Data is also presented showing the imaging results (DOF, exposure latitude, pattern transfer) of features down to 50nm imaged on Nikon’s EB1A tool.
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.
For position measurements of the EPL reticle, a new concept reticle holder is proposed. This holder clamps the same surface during measurement as during exposure in Nikon's EPL tool, the EB Stepper. Thus the holder reproduces the deformation caused by clamping in a metrology tool with that in the EB Stepper. Investigation by simulation is described. Furthermore, an experimental holder based on this concept was manufactured, and the deformation of a 200 mm EPL reticle was measured. The experimental results and simulation results show an advantage of this method.
The timely development of a Next-Generation Lithography depends upon its progress in many technology-dependent areas. Common to all high-throughput systems is the requirement of strict mask distortion control. Thus, in support of the Nikon electron-beam projection lithography tool program, three-dimensional finite element (FE) models have been developed to simulate the transient thermal and structural response of a 200-mm prototype stencil mask during electron beam exposure. Due to the relative size of pattern features, equivalent modeling techniques were employed for computational expedience. Equivalent thermal properties (conductivity and emissivity) have been calculated for perforated membranes as a function of pattern void fraction. Resulting temperature distributions were used as input for the FE structural models to characterize and quantify the local displacement fields. The structural models also utilized equivalent material properties (elastic modulus, shear modulus and Poisson's ratio). Support conditions corresponded to electrostatic chucking with four symmetrically located pad regions. The FE simulations predicted that under typical exposure conditions, the localized thermal distortions within the individual subfields are all less than 1.0 nm, which is well below the allotted error budget.
Electron Projection Lithography (EPL) has a high potential for applicability beyond the ITRS 65 nm node, especially for contacts and gate layers. The concept of synchronization control of the Nikon EB stepper is explained. The reticle stage and the wafer stage are servo controlled to target positions individually. The residual stage position errors are compensated by the electron beam deflection control. The electron beam deflection is feed forward controlled using predicted stage position data from a subsystem called “Filter/Predictor”. The performance of the stage position prediction of the Filter/Predictor is described. This paper also reports the performance of the first EB stepper tool, the NSR-EB1A, during its preliminary adjustment phase. Dynamic scanning and stitching exposure, which requires synchronization of both the beam deflection motion and the stage scanning motion, was realized. Dynamic resolution of 100 nm and dynamic subfield stitching accuracy of 25 nm (3sigma) were obtained, and further improvement is expected.
A direct means of measuring the image blur of electron beam projection lithography (EPL) tools is described. We developed an aerial image sensor using a Si membrane knife-edge and a transmitted electron detection technique. The aerial image sensor is designed to increase signal amplitude and signal contrast in order to yield a large signal to noise ratio even under a low beam current density condition. The image blur can be quantified accurate to a few nanometers because the measurement error due to the sensor is extremely small. The aerial image sensor was installed in Nikon's electron beam projection experimental column and was evaluated. The measured image blur, defined as the distance between the 12% and 88% points of the beam edge profile, under the optimum condition was 13 nm, and the measurement repeatability was 3 nm (3 sigma). The application of this technique to a system calibration is demonstrated. Focus and astigmatism were measured and the optimum settings of focus coils and stigmators were determined with excellent repeatability. The potential for this technique to provide an automated self-calibration system on EPL tools is clearly shown.
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.
A direct means of measuring an image blur of electron beam projection lithography (EPL) tools is described. An aerial image sensor used for the image blur measurement was fabricated and evaluated. The signal to noise ratio (SNR) was very high and the signal contrast was 97%. The measured image blur, defined as the distance between 12% and 88% of the beam edge profile, under the optimum condition was 13 nm and the measurement repeatability was 3 nm (e sigma). The measurement error due to the sensor was extremely small, and a quantitative measurement of the image blur can be realized using this technique. The application of this technique to a system calibration is demonstrated. Focus and astigmatism were measured and the optimum settings of focus coils and stigmators were determined with an excellent repeatability. The potential for this technique to provide an automated self-calibration system on the EPL tools is clearly 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.