Image Placement Research Articles

Thin-absorber extreme-ultraviolet lithography mask with light-shield border for full-field scanner: flatness and image placement change through mask process

When a thinner absorber mask is practically applied to the extreme ultraviolet lithography for ultra large scale integration chip production, it is inevitable to introduce an extreme ultraviolet (EUV) light shield area to suppress leakage of the EUV light from adjacent exposure shots. We believe that a light-shield border of the multilayer etching type is a promising structure in terms of mask process flexibility for higher mask critical dimension accuracy. We evaluate the etching impact of the absorber and multilayer on the mask flatness and image placement change through the mask process of a thin absorber mask with a light-shield border of the multilayer etching type structure. We clarify the relation between mask flatness and mask image placement shift.

Experimental determination of image placement accuracy in extreme ultraviolet lithography

Image placement and overlay error specifications in the International Technology Roadmap for Semiconductors continue to get tighter with each successive technology node. Some of the primary contributors to this error in extreme ultraviolet lithography are reticle and chuck surface nonflatness and chucking flatness nonuniformity. In this article, the authors report on results from a set of experiments that were designed to identify and separate chuck and mask contributions to image placement error (IPE). In the first set of experiments, a reticle with large peak-to-valley (P-V) nonflatness was printed to study reticle flattening and reticle thickness variation contributions to the final IPE. The experiments were designed to partition the total IPE into tool correctable and noncorrectable components. A second set of experiments was performed with a flat reticle that can be imaged in four different orientations (0°, 90°, 180°, and 270°). Both reticles have arrays of image placement fiducials that can be printed on the wafer and then used to measure the IPE.

Hyper-thin resist system for photomask-making in double-patterning generation

Double-patterning generation at 32-nm node and beyond raises many subjects for photomask blanks. We especially focus on the resolution improvement by hyper-thin resist combined with the hardmask process called the hyper-thin resist system (HTRS). Cr-hardmask has been specially developed for the HTRS, and this Cr material shows an extremely high etching rate. Additionally, we confirmed that a 55-nm resist thickness was available to etch the Cr-hardmask and last then the resolution of MoSi-absorber patterns was improved by HTRS, such as 45-nm LS, 60-nm isolated line and hole, and 35-nm isolated space. Moreover, the Cr-hardmask showed almost no film stress, which is necessary to achieve the image placement accuracy required for the double patterning. MoSi-binary with HTRS meets the photomask technology requirements for 32-nm node and beyond.

Characterisation of baseline oscillation in congenital nystagmus eye movement recordings

Congenital nystagmus is an ocular–motor disorder that develops in the first few months of life; its pathogenesis is still unknown. Patients affected by congenital nystagmus show continuous, involuntary, rhythmical oscillations of the eyes. Monitoring eye movements, nystagmus main features such as shape, amplitude and frequency, can be extracted and analysed. Previous studies highlighted, in some cases, a much slower and smaller oscillation, which appears added up to the ordinary nystagmus waveform. This sort of baseline oscillation, or slow nystagmus, hinder precise cycle-to-cycle image placement onto the fovea. Such variability of the position may reduce patient visual acuity. This study aims to analyse more extensively eye movements recording including the baseline oscillation and investigate possible relationships between these slow oscillations and nystagmus. Almost 100 eye movement recordings (either infrared-oculographic or electrooculographic), relative to different gaze positions, belonging to 32 congenital nystagmus patients were analysed. The baseline oscillation was assumed sinusoidal; its amplitude and frequency were computed and compared with those of the nystagmus by means of a linear regression analysis. The results showed that baseline oscillations were characterised by an average frequency of 0.36 Hz (SD 0.11 Hz) and an average amplitude of 2.1° (SD 1.6°). It also resulted in a considerable correlation ( R 2 scored 0.78) between nystagmus amplitude and baseline oscillation amplitude; the latter, on average, resulted to be about one-half of the correspondent nystagmus amplitude.

STRONG PHASE SHIFT MASK MANUFACTURING ERROR IMPACT ON THE 65 NM POLY LINE PRINTABILITY

A methodology and Monte Carlo simulation flow with the integrated LSI Logic package, Molotof, was applied to the 65 nm poly line sensitivity analysis. Strong phase shift mask (sPSM) manufacturing process specification was optimized to obtain image critical dimensions (CDs) and image placement errors (IPEs) complying with technology process tolerances. Reticle manufacturing statistical variarions of phase depth, phase width, and phase intensity imbalance were used to generate a virtual sPSM for imaging poly lines on a wafer. A criterion for qualifying reticle specification is to obtain all latent image CDs and IPEs within a photo process module allowed range for a given mask specification. The approach allows for computing reticle and litho budgets into CD imaging performance. Sensitivity to a single parameter variation and full statistical analysis of the 65 nm poly line imaging performance affected by reticle manufacturing errors is presented. The optimum reticle specification, yielded 100% of critical dimensions and image placement errors, was found in simulation and confirmed by empirical data.

Analysis of pattern-dependent image placement of single-membrane stencil masks for electron-beam lithography

Large single-membrane stencil masks have been developed for electron-beam lithography. Since a large membrane induces large image placement (IP) error, which is pattern dependent, a method of correcting EB data has been studied to compensate the membrane distortion. In this study, firstly, the effect of crystal anisotropy of a Si membrane to the distortion is examined by making masks from blanks with different orientations. The influence of the anisotropy is found to be small and simulation based on isotropic modeling should be applicable. Secondly, a finite element method (FEM) called ANSYS and Pseudo-FEM are used to predict distortions for three masks with 8 mm-, 12 mm-, or 18 mm-square die of an opening ratio of 0.2 on a 24 mm-square membrane. The simulation results are compared with the results obtained in the experiment on anisotropy and a previous experiment. Qualitative agreement is observed between simulation and experiment but quantitative agreement is obtained only after introduction of adjustment factors. A suggestion is made to improve the IP correction scheme for EB data.

CD and IP accuracy in electron beam character projection technology

CD and IP accuracy in electron beam (EB) character projection (CP) was examined. We measured around 1 nm as critical dimension (CD) 3 sigma of CP line patterns without major and minor deflection. In the case of VSB, the values exceed 2 nm. This gap is based on the difference in the beam formation method of CP and VSB. Root of the difference of these two squares of 3 sigma are from 1.69 to 2.53 nm and it is caused by variation of relative position between beam and CP mask aperture in using VSB. We measured 1.64 nm as CD 3 sigma of CP line patterns with major deflection within 1520 × 680 μm and minor deflection within 80 × 80 μm and analyzed the data into local factor: 1.38 nm, major deflection factor: 0.72 nm and minor deflection factor: 0.51 nm. It seemed that CP technology for CD accuracy had sufficient potential to 45 nm half-pitch system. Then we also measured image placement (IP) accuracy of CP (position, gain and rotation of CP as to changing deflection distance for CP selection, area of CP aperture and major and minor deflection distance). The variation in the position to CP selection was observed and is considered to be due to electric or mechanical vibration of system. For our future 45 nm half-pitch system, we plan to raise the performance of each unit and to lower this variation. The dependence to change of the parameter of the system was also observed: (A) change of the rotation to CP selection deflection distance, (B) change of gain and rotation to CP area and (C) change of gain and rotation to major and minor deflection distance. We could compensate by using biased mask for (A) and (B). We are going to cope with (C) by narrowing major deflection distance.

Off-pump pulmonary valve replacement with the new Shelhigh Injectable Stented Pulmonic Valve

ignificant pulmonary regurgitation (PR) is common after surgical or percutaneous treatment of pulmonary stenosis and tetralogy of Fallot (TOF) and leads to significant late morbidity and mortality. Optimal timing of pulmonary alve replacement (PVR) and procedures minimizing surgical trauma re therefore important. The new Shelhigh Injectable Stented Pulonic Valve NR4000-PA MIS (Shelhigh Inc, Union, NJ) offers the ossibility of PVR without cardiopulmonary bypass (CPB). Here we eport our early experience with the first clinical use of this new evice.

Distortion of chucked extreme ultraviolet reticles from entrapped particles

Successful imaging of patterns with critical dimensions less than 45nm with extreme ultraviolet lithography (EUVL) requires stringent controls on all sources of image placement (IP) errors. Among the potential sources of IP error is the mechanical distortion of the patterned mask when mounted in the exposure tool. An EUVL reticle can exhibit both in-plane distortion and out-of-plane distortion due to the presence of debris lodged between the mask and the electrostatic chuck. Even particles with a compressed height as small as 100nm have the potential to consume a significant portion of the IP error budget. To alleviate this problem, a thorough understanding of the response of the reticle∕particle∕chuck system during electrostatic chucking is essential. This article describes experimental indentation testing to characterize relevant nanoscale material properties and the subsequent use of the data in finite element models that simulate the system response under typical chucking conditions.

Template Fabrication Challenges for Patterned Media

ABSTRACTThe Step and Flash Imprint Lithography (S-FILTM) process uses field-to-field drop dispensing of UV curable liquids for step and repeat patterning for applications where high-resolution mix-and-match overlay is desired. Several applications, including patterned media, photonic crystals and wire grid polarizers, are better served by a patterning process that prints the full wafer since alignment requirements are not so stringent. In this paper, a methodology for creating high resolution thin templates for full wafer (or disk) imprinting is described. The methods have been applied toward the imprinting of both patterned media and photonic crystal devices using a large area printing tool developed around the S-FIL process. Techniques for further enhancing the pattern density as well as a method for addressing feature image placement are described. Finally, a process for replicating a Master Template is discussed in detail.

Predicting electron projection lithography mask membrane image placement errors

Electron projection lithography (EPL) has been identified as a viable candidate of the next-generation lithography technologies for moderate-volume production of systems-on-a-chip for the sub-65nm nodes. EPL is especially suited for the fabrication of contact structures. The development of a low-distortion mask is essential for meeting the stringent requirements at these lower nodes. This research focuses on quantifying in-plane distortions (IPDs) due to mask fabrication and the effects of different mounting schemes in the e-beam writer and exposure tools for a 200mm EPL stencil mask. In order to quantify the IPD of the freestanding membranes, three-dimensional finite element (FE) models have been developed and benchmarked with experimentally measured data. Metrology measurements in both the membrane-side up and membrane-side down configurations can be used to correct for image placement errors (due to the fabrication and chucking of the mask) by compensating for IPD during the exposure process. To correct for higher order distortions associated with pattern specific errors, FE models were generated to simulate the entire process flow, including chucking. As a sample test case, a stencil pattern was replicated uniformly over the membrane arrays. The actual mask was fabricated and metrology measurements were taken. Selected membranes were experimentally measured to compare with FE results and the data correlated well in both direction and magnitude. Results illustrate that FE modeling and simulation can be used to predict mask distortions due to fabrication. Such techniques can be instrumental in meeting the stringent error budget required at the lower lithographic nodes.

Investigation of overlay errors due to the interaction of optical and extreme ultraviolet mask fabrication processes

Overlay tolerances are becoming increasingly severe as lithography technology drives the minimum integrated-circuit feature size below 65nm. Manufacturable solutions at the lower nodes are essentially unknown. The goal of this article is to investigate overlay errors at the 45-nm node, for a mix-and-match of optical and extreme ultraviolet (EUV) lithographies. In particular, image placement errors induced during mask fabrication are predicted for a specified test pattern, which includes the possibility of having individual layers on the device wafer patterned by either optical or EUV lithographic processes. Finite element (FE) models have been developed to simulate the response of both optical and EUV masks during fabrication and chucking. In order to track image placement errors of the actual features within the pattern area, submodeling techniques were developed for the FE simulations. In addition, equivalent material properties were determined for the individual patterns used for the different optical and EUV masks. The FE simulations clearly indicate that the most significant source of overlay error is the fabrication of the optical reticle, primarily due to differences in chucking of the mask during e-beam patterning and exposure. Modeling results are being used by the industry to establish guidelines for reticle chucking and fabrication standards.

Portable coordinate measuring tool

The ability to accurately measure the placement of features within a level is becoming increasingly critical in integrated circuit fabrication. In this article the level to be measured is patterned on a transparent wafer in poly(methylmethacrylate) (PMMA) resist to which a blue absorbing dye has been added. The dyed resist is on top of a thin partially transparent aluminum layer and is highly visible when viewed in reflected blue light in an oil immersion microscope. It is placed over a diffraction grating, which is not visible, both because the aluminum layer reflects most of the light and because the grating is far out of focus. In transmitted red laser light the grating can be seen, since various techniques may be used to increase the depth of focus of gratings in collimated monochromatic light. However, the resist pattern is not seen, because the resist is transparent to red light and the immersion oil matches its index of refraction. Measuring the separation between features reduces to measuring their location with respect to individual grating lines, and then counting the number of lines between them. The image placement accuracy of levels measured with two-dimensional gratings will be limited by the accuracy of the gratings—at present less than 2nm.

Image placement error: closing the gap between overlay and imaging

We perform experimental and theoretical studies of image placement error induced by aberrations of the projection lens. The goal is to experimentally determine the magnitude of the image placement errors, to compare experiments and simulations, and to screen possible correction strategies. The calculations and experiments are done for ArF lithography. Theoretically, we simulate image placement error using the projection lens aberration data and simulators such as Prolith or Solid-C. Features with low and high sensitivities to lens aberrations are identified, together with a reference feature that has low sensitivity for image placement error. Dedicated reticles are fabricated to print various features at different illumination conditions on the same substrate. The resulting patterns could be analyzed using top-down scanning electron microscopy (SEM), but also optically with the standard optical overlay tool KLA5200. For both techniques, the experimentally found image placement errors are in excellent agreement with simulations. In simulations, we calculate the dependency of the image placement error on pattern density, pattern orientation, and illumination conditions. These tendencies are experimentally reproduced. We conclude with a case study that demonstrates a possible correction strategy for image placement error.

Alternating phase-shifting mask design for low aberration sensitivity

The aberration present in the lenses of exposure systems can cause placement errors to the images produced by alternating phase-shifting masks (PSMs). In reality, when the aberration signature varies from one lens to another, the magnitude of placement error also varies. It remains a question of how the alternating PSM should be designed, so that the image placement error, on average, can be minimized. To achieve this goal, we are interested in optimizing the phase width of an alternating PSM with a fixed critical dimension (CD). The constraint of the optimization is the mean of root mean square (rms) aberrations for a set of interest of exposure systems. To begin the analysis, the image placement error is expressed as a function of illumination, mask spectrum, and wave aberration. A Monte Carlo technique is then applied to produce random samples of wave aberration and image placement error. This analysis shows that a global minimum of mean image placement error is likely to occur at phase widths between 0.2[/numerical aperture (NA)] and 0.4(/NA). This is further confirmed by analytically considering the expected value of the square of the image placement error. The methodology of finding the optimal phase width is applicable to the design of all alternating PSMs.

Technique for separately viewing multiple levels

It is often desired to measure the accuracy of image placement in an integrated circuit, as well as the overlay between two levels printed on the same wafer. Such measurements yield information not only about alignment, but also about wafer, mask, and imaging distortions. Image placement measurements require state-of-the-art equipment. Overlay measurements are difficult to make if the patterns overlap each other, since one pattern may obscure parts of the other. In addition, the presence of an edge in one pattern may produce an error in the location of a nearby edge in the other pattern. This article describes a method of patterning that permits optical separation of the two levels so that they can be viewed independently. The method is based on the use of a resist containing a dye, such that the resist has a high contrast at one wavelength, but is completely transparent at another. In addition, the use of a precision grating for one of the levels provides a simple way to obtain highly accurate absolute image placement measurements.

Assessment of image placement errors induced in electron projection lithography masks by chucking

Electron projection lithography (EPL) is under active development, with potential applications at the sub 65nm nodes. In order to meet the stringent error budgets in this regime, image placement (IP) errors induced by chucking the mask during e-beam patterning, metrology, and exposure must be characterized and minimized. The focus of this study is to assess the distortions induced in200-mm-diam EPL stencil masks by chucking during e-beam patterning and EPL exposure. High-throughput test masks (with 3.39mm square membrane windows) were evaluated, to assess IP errors throughout a typical mask process flow. Finite element structural models were developed to simulate the response of the test mask during each processing step. Finally, the results of the analysis were used to identify and characterize the sources of IP errors.

Biological lithography: Improvements in DNA synthesis methods

We have recently succeeded in synthesizing long oligonucleotides (90-mers) with high yield. This synthesis requires 360 virtual masks, and thus puts challenges on image placement and local contrast. We have updated our DNA synthesis modeling to Monte Carlo simulation from numerical approach. We also devised a method, called “Inverted Capping,” to remove sequence errors from edge scattering of light, which provides a large error reduction and the possibility of fabrication of higher resolutions. Finally, we have also implemented an image locking method to eliminate image drifts.

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.

Effect of electrostatic chucking and substrate thickness uniformity on extreme ultraviolet lithography mask flatness

Extreme ultraviolet lithography (EUVL) is one of the leading candidates for next-generation lithography in the sub-45 nm regime. Successful implementation of this technology will depend upon advancements in many areas, including the quality of the mask system to control image placement errors. Such errors will occur at the wafer as a result of height variations of the patterned (frontside) mask surface (i.e., its nonflatness). The Semiconductor Equipment and Materials International EUVL Mask Standard (SEMI P37) specifies that the mask frontside and backside nonflatness be no more than 50 nm peak-to-valley (P-V). Currently, the lowest level of freestanding flatness of EUVL mask substrates from production-level polishing and finishing techniques is about 200 nm P-V. The frontside and backside of typical (and representative) EUVL substrates were measured for flatness and these surfaces were represented mathematically using Legendre polynomials. The Legendre coefficients were then utilized in a finite element model to predict the effect of chucking on the flatness of these substrates. One of the significant conclusions of this research is that the substrate thickness nonuniformity has a dominant effect on the flatness of the patterned surface after chucking.