Electron Projection Lithography Research Articles

Stress and image-placement distortions of 200mm low-energy electron projection lithography masks

Low-energy electron projection lithography (LEEPL) is a candidate for next generation lithography and thus the 1× LEEPL mask requires a stringent local image placement (IP) error budget. Applying a doping method with silicon-on-insulator substrates, 700-nm-thick membranes were investigated for stress control and in-plane distortion (IPD), which are the main contributors to local IP errors. Stress control results show that at a dopant concentration of 6.74×1019∕cm3, the membrane stress in a 10mm test structure is 8.4MPa. Also stress variation is excellent at 0.3MPa across a 200mm complementary stencil mask on support strut-type LEEPL mask. The IPD results indicate that small membrane window size, low void fraction, as well as low membrane stress is the proper strategy to allow a stencil mask with dense stencil patterns to meet required IPD. Additionally, the local IP errors of large scale integrated 90nm hole pattern were demonstrated.

A Promising Lithography Gets Stuck - Once aimed at the mainstream, electron projection lithography will have a diminished role-if any.

A Promising Lithography Gets Stuck - Once aimed at the mainstream, electron projection lithography will have a diminished role-if any.

A promising lithography gets stuck

Once aimed at the mainstream, electron projection lithography will have a diminished role-if any.

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.

First dynamic exposure results from an electron projection lithography tool

Electron projection lithography (EPL) is one of the promising technologies below the 65 nm node, especially for contact hole and gate layers. Nikon is developing an EPL exposure tool as an electron beam (EB) stepper and the first generation EB stepper is now being manufactured. The voltage of 100 kV is adopted for electron beam acceleration. The subfield size is 0.25 mm×0.25 mm on the wafer and the deflection width of the electron beam is 5 mm on the wafer. The magnification of the projection optics is 1/4. A 5 mm×25 mm area from the φ200 mm reticle can be exposed by the combination of beam deflection and stage scanning motion (dynamic exposure). This area is called “a mechanical stripe.” After one mechanical stripe exposure, the reticle and wafer stages turn around and the next exposure of the adjacent mechanical stripe starts as a scan and stitch stage motion. Finally, a 20 mm×25 mm exposure field from the φ200 mm reticle is exposed. We report the first dynamic exposure in the history of EPL although only a φ100 mm reticle was used. A 5 mm×10 mm area was used as the mechanical stripe and 10 mm×10 mm exposure fields were exposed. 100 nm nested lines were resolved in the entire exposure field and stitching accuracies of 17–18 nm (3σ) are obtained. There remain systematic errors, and stitching accuracy less than 15 nm will be achieved after fine adjustment of the subfield positions. We feel the reality of EPL is now sufficiently proven.

Wafer heating analysis for electron-beam projection lithography

Electron projection lithography (EPL) is one of the principal next-generation lithography technologies for the sub-65 nm regime. To satisfy the stringent resolution requirements, all image placement errors must be characterized and minimized. These include the distortions of the device wafer during exposure. The wafer absorbs beam energy which produces temperature increases and thermomechanical strains that directly contribute to pattern-placement errors, stitching errors between adjacent subfields, and image blur. Thus, CD control and pattern overlay will be directly affected. In this article, the thermomechanical distortions caused by wafer heating in the EPL system have been simulated using finite element models that include the effects of the interaction between the wafer and chuck.

Benchmarking stencil reticles for electron projection lithography

Electron projection lithography (EPL) is one of the leading candidates for next-generation lithography at the 65 nm lithography node, particularly for contact levels. This article describes the results of an experimental effort to benchmark the current state of EPL stencil mask making. In this article, we report on the current status of the data handling software needed to pattern an EPL stencil reticle, EPL stencil reticle repair techniques, and EPL stencil mask stability following prolonged electron-beam irradiation.

Fabrication of a continuous diamondlike carbon membrane mask for electron projection lithography

Fabrication of 8 in. high-performance continuous diamondlike carbon (DLC) membrane masks for electron projection lithography is described. The mask substrate materials and structures were optimized by evaluating the lithographic performance of the mask. The optimum mask consists of a sandwich structure, consisting of a thicker DLC scatter/a CrNx etching stopper/and a thin DLC support membrane on a bulk silicon wafer. The internal stress of each film component can be controlled by adjusting the film deposition conditions. A DLC film can be easily etched by oxygen gas, and the CrNx etching stopper has a high etching durability. Highly accurate pattern properties can be obtained while also meeting performance requirements. The critical dimension accuracy of a DLC scatterer was less than ±5% with a 280-nm-feature size in a 135×43 mm field. The electron aperture transmittance of a 44-nm-thick DLC membrane, measured by energy and angular distribution analysis for membrane, was 13 times as high as the 150-nm-thick SiNx membrane.

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.

Ferroelectric emission studies for electron emission lithography applications.

Ferroelectric switching emission, dielectric switching emission, and pyroelectric emission were studied by patterning images on electron resist for electron emission lithography applications. It was observed that the pyroelectric emission is most acceptable for a high throughput 1:1 electron projection lithography application. A 1:1 electron projection lithography was demonstrated by patterning images with line widths of 30 microm and using pyroelectric emission. A degradation of the pyroelectric emission property of the material was observed during repeated heating cycles below the phase-transition temperature of the ferroelectric material. Annealing excursions above the phase transition temperature prevented the degradation of the pyroelectric emitter.

Stencil Mask Technology for Electron-Beam Projection Lithography

The development of silicon stencil masks for cell projection lithography (CPL), electron projection lithography (EPL), low-energy electron-beam-proximity-projection lithography (LEEPL), and low-energy electron-beam lithography (LEB) is described. To satisfy the required pattern accuracy of the stencil masks, such as critical dimension and image placement, we developed a high-aspect stencil pattern formation technique to achieve a side-wall angle of over 90 deg and a membrane stress control technique using a stress-correction layer. In addition, for fabricating low-magnification stencil masks, we developed a new mask substrate with a sputtered scattering silicon membrane and a sputtered intermediate stopper layer as an etching stopper. We selected CrNx as the intermediate layer material, which demonstrated high performance in stencil mask fabrication. By improving the CrNx film qualities, its durability to dry and wet etching of backside silicon was improved. In order to improve the pattern image placement accuracy caused by membrane stress changes during stencil pattern formation, we developed a new and simple technique using a stress correction layer, which also functions as a silicon deep-etching mask. By using this functional layer, the total stress change in the pattern field in a mask pattern formation process was reduced to within 5 N/m, which corresponds to less than a 10 nm pattern image placement (IP) error.

Electron Optics Properties of Electron Beam Stepper

Electron projection lithography (EPL) is one of the most reliable lithography tools for 65 nm node generation and below. An electron optics (EO) subsystem, which has been developed in collaboration with IBM, and Nikon's original stage/body are integrated into an electron beam (EB) stepper. The latest EO properties of the EB stepper are discussed. Experimentally, the curvilinear variable axis lens (CVAL) adjustment is established. Therefore, good resolution (better than 80 nm) and low nonlinear distortion (approximately 10 nm) is obtained at the maximum (2.5 mm) deflected sub-field on the test stand. After docking with body and stages, good resolution (better than 90 nm) is achieved for the on-axis beam.

EPL electron optics performance on test stand.: 2. Field distortion and stitching results

Nikon in collaboration with IBM has been developing electron projection lithography (EPL) as a promising candidate for 65 nm node technologies. In this paper, the latest performance of electron optics for a 65 nm R&D tool will be described with emphases on field stitching and distortions. For evaluation, data measured by a Nikon XY5i metrology tool, SEM pictures of visual vernier patterns, and other data will be presented to show field stitching results. These represent the first full main field stitching results achieved. To obtain stitching accuracy between any two adjacent subfields, it is critical to eliminate subfield distortion at each deflected position. Subfield distortion is mainly reduced by a combination of optimized curvilinear variable axis lens (CVAL) deflection optics and dynamic correction lenses adjustment. We will discuss an experimental method to minimize subfield distortion by adjusting the CVAL deflection optics at deflected positions. In addition, dynamic correction of the subfield image shape eliminates linear components of subfield distortion. Controllability of subfield shape will be discussed. As another important topic, we will present results of nonlinear distortion correction by a higher order dynamic correction lens system. By combining this with the above mentioned electron optical adjustment, we achieved less than 10 nm residual subfield distortion at the most deflected position. We will also present the wide ‘tolerance’ or ‘margin’ of nonlinear distortion correction adjustment.

Predicting overlay performance for electron-projection-lithography masks

Minimizing mask-level distortions is critical to the success of electron projection lithography (EPL) in the sub-100-nm regime. A number of possibilities exist to reduce mask-fabrication and pattern-transfer distortion including subfield correction, dummy patterns, pattern splitting, and film stress control. Finite element modeling was used to illustrate the advantages and capabilities of these correction schemes for a 100-mm stencil mask with 1-mm×1-mm membrane windows. Static-random-access-memory-type circuit features, including both the interconnect and contact levels, were used, to simulate realistic circuit layouts with both cross-mask and intra-membrane pattern density gradients. With such correction techniques, it is possible to reduce the EPL mask-level distortions for worst-case mixed pattern types to less than 1.0 nm.

Characterizing Thin-Film Stress Fields by Resonance of Membrane Arrays

ABSTRACTControlling thin-film stress magnitudes and nonuniformities is a persistent and pervasive challenge for applications ranging from the semiconductor industry to nanotechnology. Consequently, the ability to accurately measure the intensity and spatial distribution of film stress is essential for fabrication process optimization. This paper describes novel extensions of the membrane resonance method to characterize in-plane film stress gradients. Arrays of freestanding membrane windows are fabricated by first depositing thin films over a silicon substrate. Back-etching then creates an array of membrane windows which are sequentially resonated with a compatible drive electrode. The technique was used to determine stress uniformity across electron projection lithography masks, which incorporate membrane window arrays. In addition, a procedure is presented for identifying the two principal stresses in individual membranes.

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 cluster system and the rehierarchicalizing, we expect to further reduce the M-Split processing time to 10 min.

Evolution of electron projection optics from variable axis immersion lenses to projection reduction exposure with variable axis immersion lenses

The optics of the electron projection lithography (EPL) system PREVAIL (projection reduction exposure with variable axis immersion lenses) is retraced from its conceptual and practical foundation in IBM’s shaped beam probe-forming tools via the EPL proof-of-concept system to the implementation of the complex three-dimensional curvilinear variable axis lens optics in the Alpha column, part of the first-of-a-kind electron-beam stepper presently under construction by the IBM alliance partner Nikon Corporation. The significant elements of the optics are described, as well as the innovations, which were required to establish an advanced lithography tool viable in an integrated circuit production environment.

Electron projection lithography mask format layer stress measurement and simulation of pattern transfer distortion

Electron projection lithography (EPL) is one of the leading candidates for the sub-65 nm lithography node. The development of a low-distortion mask is critical to the success of EPL. This article proposes and analyzes two new EPL mask formats described as a “corrugated-continuous membrane mask” and a “carbon-continuous membrane mask.” Novel process flows for the manufacture of these masks have been developed at Team Nanotec GmbH. Resonant frequency stress measurements of the ultrathin membrane bilayers were completed and subsequently used in the finite element simulation of the mask fabrication and pattern transfer. The new mask types have the benefits of the lower distortions of a typical continuous membrane mask, but maintain the advantage of the higher throughput stencil format because of the ultrathin films. In addition, the proposed masks remove the need for pattern splitting typically used with complementary systems.

Ultraviolet and direct ultraviolet inspection of next generation lithography reticles

The optical inspection of next generation lithography (NGL) patterned reticles, multilayer-coated blanks, and uncoated substrates is particularly challenging. The difficulties arise not only because of the higher sensitivity necessary at the smaller design rules, but also due to the specifics of the NGL mask materials and structures. Our research program is investigating the theoretical and practical operational limitations facing optical inspections of patterned and unpatterned NGL masks. We are constrained by the necessity to inspect only in reflected light, limitations in mask contrast, and interference effects caused by partially coherent illumination. We present inspection results and images of several types of NGL masks, blanks and substrates obtained on high resolution ultraviolet (UV) and direct ultraviolet (DUV) optical mask inspection systems. While electron projection lithography (EPL) stencil masks can be inspected in reflection, limited transmission through the mask suggests that subsurface defects will be detected with reduced sensitivity, compared to defects on the surface. For unpatterned inspections, we have demonstrated UV system sensitivity to 88 nm PSL spheres on quartz substrates and 117 nm PSLs on silicon substrates. On extreme ultraviolet masks we have achieved DUV system sensitivity down to defects as small as 80 nm in size, both in die-to-die inspections and in simulated die-to-database inspections. In addition to advancing the development of optical inspection systems for NGL reticles, these inspection results provide feedback to NGL mask developers.

Fabrication of complete 8 in. stencil mask for electron projection lithography

We fabricated an 8 in. stencil mask having the complementary pattern of the 70 nm rule system-on-chip device. The 8 in. stencil mask was realized from the development of a mask substrate fabricated by using the sputtering method to form a scattering silicon membrane and an intermediate stopper layer. The intermediate layer material, which functions as an etching stopper, was CrNx. This material has demonstrated high performance in stencil mask fabrication, which is described in detail. The stress in the CrNx could be controlled within ±20 MPa by adjusting the deposition condition. The deposited silicon membrane stress could be easily adjusted in the range of 0–10 MPa. The etching selectivity, when the substrate backside etching was performed, was over 1000 under the low bias power. When the deep etching process was performed using SF6 and CHF3 etching gases for the mask pattern formation, the Si/CrNx etching selectivity was over 100 under the low bias power condition. The mask substrate, which is made up of a 2-μm-thick deposited silicon membrane and 0.35-μm-thick CrNx stopper layer, enabled an 8 in. stencil mask to be fabricated for use with electron projection lithography. In complementary mask split, we used “M-Split” which was developed by Selete and ISS as a pattern split software.