Double Exposure Method Research Articles

Lithography Computer Aided Design Technology for Embedded Memory in Logic

We have developed an optical proximity effect correction (OPC) flow for both, the repetitive memory cell patterns by a simulation-based method and for the random logic application specific IC (ASIC) patterns by a rule-based method. Application results for a 0.28 µm static random access memory (SRAM) cell and 0.25 µm ASIC devices have shown that correction time and final data size were feasible for conventional layout design flow and mask fabrication. Also, our system has achieved significant improvement with respect to pattern printing fidelity between designed and printed resist patterns. In addition to this, an automatic alternative phase shift mask (PSM) pattern layout tool has been developed. Using the tool with a double exposure method, 0.16 µm gate patterns for both logic and dynamic random access memory (DRAM) have been obtained with a positive photoresist. The developed lithography computer aided design (CAD) technology has improved the accuracy of pattern formation for state of the art device fabrication. The fabricated patterns have satisfied the required linewidth accuracy, within ±10% of the design value, ones under the practical exposure-focus window.

Binary computer-generated holograms for security applications from a synthetic double-exposure method by electron-beam lithography

We have developed a new multiplexing method for producing binary computer-generated holograms (CGH's) for security applications. This method is based on double recording of two types of coding method upon binary CGH's. The CGH synthesized by the proposed method can have multiple image planes from the region close to the hologram (image region) to infinity (Fraunhofer region) without severe degradation of reconstructed images in the image region. A CGH containing simultaneous image- and Fourier-type holograms is fabricated by electron-beam lithography. Some experimental results are presented.

Problems of interpretation of holographic interferograms near shock wave fronts

A method of avoiding ambiguity in the interpretation of interferograms near a shock wave front is proposed. The method is based on combining the double-exposure schlieren method and holographic interferometry. Relations for calculating, on the basis of data obtained by analyzing double-exposure schlieren photographs, both the density at the shock wave front and the gradient of the density directly behind the front, which is necessary for calculating the shifts of the interference fringes near the shock wave front, are presented.

Optical study of parameters of the passage of a shock wave from air to water

A study is made of the penetration of shock waves from air into water. The shock wave in air is generated as a result of dielectric breakdown induced by pulsed CO2-laser radiation. A combination of the double-exposure shadow method and holographic interferometry is used to measure the shock-wave parameters. Density and pressure profiles behind the wave front are obtained at different times after onset of breakdown. It is shown experimentally that as the wave passes through the interface from the air to the water, there is a fourfold amplification of the pressure in the shock wave front. Estimates of the width of the shock wave front formed in the water are given in the context of studies of large-scale explosion processes. It is shown that simple empirical dependences, established in the course of studies of large-scale explosions, are also valid with certain corrections for microscopic laboratory experiments.

Theoretical basis for real-time shearography

Abstract The principle behind the formation of high resolution live fringe patterns by real-time shearography is presented. It is shown that linear and nonlinear photographic recordings both yield instantaneous fringe patterns which depict the instantaneous surface displacement gradient of the object. However, in theory, nonlinear photographic recording results in lower fringe visibility compared with linear photographic recording. In either case, real-time shearography requires a simple optical setup and eliminates the need for Fourier filtering during fringe read-out. It is also found that the fringe characteristics of real-time and double exposure methods differ by a phase angle of 180 degrees.

Theoretical basis for real-time shearography

The principle behind the formation of high resolution live fringe patterns by real-time shearography is presented. It is shown that linear and nonlinear photographic recordings both yield instantaneous fringe patterns which depict the instantaneous surface displacement gradient of the object. However, in theory, nonlinear photographic recording results in lower fringe visibility compared with linear photographic recording. In either case, real-time shearography requires a simple optical setup and eliminates the need for Fourier filtering during fringe read-out. It is also found that the fringe characteristics of real-time and double exposure methods differ by a phase angle of 180 degrees.

Overlay Accuracy Measurement Technique Using the Latent Image on a Chemically Amplified Resist

An overlay accuracy measurement technique has been devised using the latent image on a chemically amplified resist developed in-house. A latent image with step height of over 0.1 µ m was formed after KrF excimer laser irradiation. It has been confirmed that it is possible to measure overlay accuracy using this latent image with measurement accuracy within 10 nm. This technique is very useful for overlay accuracy measurement of an exposure system using a double exposure method with a chemically amplified resist. It has been verified that heterodyne holographic wafer alignment (HHWA) has the capability to detect a latent image with shallow step height. Overlay accuracy within 60 nm was obtained using overlay error correction with this developed overlay accuracy measurement technique.

Compatibility between two SR steppers

Two SR steppers, SS-1[1] and SS-2, are used at the NTT SR facility to accelerate the development of LSIs with a feature size of 0.2 microns or less. This paper presents the compatibility of the two SR steppers focusing on overlay accuracy. For stepper to stepper overlay, repeatability and offset controllability are the important factors. The stand-alone overlay repeatability of ±30nm, and offset control resolution of 10nm are achieved. The difference of run-out error between two beamlines, which have an adverse affect on compatibility, are measured. It is less than 60nm in 18mm square. The influence of chucked wafer flatness to overlay accuracy is clarified by experiments and simulations. The overlay accuracy of 60 nm(3 σ) for the two SR steppers is obtained by double exposure methods using the same mask and alignment marks formed by a photo stepper. These two steppers are now being applied to test device fabrication.

Overlay Repeatability in Mix-and-Match Exposure Using the SR Stepper: SS-1

Proximity X-ray lithography using synchrotron orbital radiation (SR) is potentially able to replicate patterns with a width of less than 0.2 µ m. We developed a die-by-die alignment basis SR stepper, which is equipped with air-lubricated lead screws for the XY stage and an optical heterodyne alignment system. An overlay repeatability of 23 nm (3σ) is obtained with only X and Y alignment when evaluated by the double exposure method. In practice, the mix-and-match scheme between the SR and optical exposures is important for reducing the cost of lithography. In the mix-and-match exposure between the SR and optical steppers, an overlay repeatability of 45 nm (3σ) is achieved with the X, Y, and θ alignment mode of the SR stepper. Analysis of the error factors in this overlay exposure experiment showed that the optically printed patterns have chip shape distortions causing overlay error of about 35 nm (3σ).

2×2 Phase Mask for Arbitrary Pattern Formation

The phase-shifting mask (PSM) extends the resolution limit of optical lithography. The major hurdle to the widespread use of PSMs is that high resolution can be obtained only for the limited pattern layouts with the PSMs. In this paper, we describe the double exposure method with a pair of 2 phase masks (“2×2 phase mask”) for arbitrary two-dimensional pattern formation. Simple mask design procedures which are based on a few rules are derived from the imaging characteristics of alternating PSMs. With a 2×2 phase mask, complicated patterns including large scale integration (LSI) patterns are successfully fabricated.

Overlay accuracy of a synchrotron radiation stepper evaluated by two-mask double exposure

The overlay accuracy of a synchrotron radiation stepper has been evaluated when different masks are used as in actual exposure. Using several masks requires both high repeatability and accurate offset controllability to perform the proper alignment of the stepper. To this end, a two-mask double-exposure method that considers mask error has been devised. It yields a 3σ deviation of 25 nm for the overlay repeatability of our SS-1 stepper. This is almost the same as in the one-mask double-exposure method, which means that the stepper effectively has an alignment repeatability of less than 25 nm. In addition, a mask stage is developed to improve the offset controllability and enlarge the stroke. The resolution and stroke of the stage are 20 nm and ±1000 μm, respectively, for x and y directions, and 0.5 and ±800 μrad, respectively, for theta. This stepper facilitates the reduction of alignment offset deviations to less than ±10 nm in the x and y directions and less than ±1.0 μrad for rotation.

Fractal properties of flocs formed by fluid shear and differential settling

The fractal properties of fine-grained sediment flocs formed by fluid shear and by differential settling were studied through floc porosity–size relationships. Flocs were produced from flocculation tests in a Couette-type flocculator in which fluid shear was the dominant collision mechanism for flocculation, and in a disk-type flocculator in which differential settling was the dominant collision mechanism. Floc samples were then introduced into a settling tube to measure their sizes and settling speeds by a double-exposure photographic method. The porosities of flocs were determined from the settling speed data using a porous sphere settling model. It was found that the flocs produced in the Couette-type flocculator could be regarded as fractals, with a fractal dimension ranging from 1.83 to 1.97. However, the flocs produced in the disk-type flocculator did not exhibit simple scaling behavior and could not be regarded as fractals. The differences in fractal property are due to the differences in floc formation schemes associated with the collision mechanisms. The similarities between experimentally obtained flocs and computer simulated clusters are discussed. The multistage growth model, a formation scheme for flocs formed by fluid shear, is consistent with the cluster–cluster model used in computer simulations. A formation scheme for flocs formed by differential settling is postulated; it is consistent with the particle–cluster model used in computer simulations.

Some problems of reconstruction in electron holography

The reconstruction method of phase information frozen in an electron hologram is discussed. The experimental results show that the double-exposure method (which does not need laser reconstruction) cannot only obtain directly the phase information, but also gives qualitative and quantitative results. This means also that one can obtain simultaneous information about both the amplitude and phase in an experiment in an electron microscope

Evaluation of heterodyne alignment technique for x-ray steppers

Registration accuracy, as well as resolution capability, is one of the critical factors to identify an appropriate lithography tool. For deep submicron devices represented by the 256 Mbit dynamic random access memory, an alignment accuracy of better than 30 nm is needed. In this article, the alignment accuracy of an x-ray stepper was evaluated using the double-exposure method with a posiresist. The alignment accuracy obtained was better than 38 nm (3σ). Moreover, making a quantitative analysis of alignment error factors, we identified the possibility of improvement toward better than 30 nm (3σ).

Formation of moiré grating in core of germanosilicate fibre by transverse holographic double exposure method

The first demonstration of the feasibility of writing a moire grating into a germanosilicate fibre using double exposure by ultraviolet fringe patterns is reported. Using this technique a resonant filter response with a bandwidth of 0.2 nm centred around 1200nm and with a finesse of 6 has been obtained.

Locating and Sizing Disbonds in Glassfibre-Reinforced Plastic Plates Using Shearography

Shearography is a double-exposure optical method which reveals defects in components from anomalies in interferometric fringe patterns. In this paper, disbonds of known shapes and sizes are deliberately created at different layers of glassfibre-reinforced plastic (GRP) laminates which are subsequently “vacuum-stressed” during the test. Experimental results show that both the shape and size of disbonds can be deduced easily from the boundaries of the anomalous fringe patterns; the depth of disbonds can also be estimated by counting the number of fringes appearing within these boundaries. The results also correlate well with those predicted from thin plate theory, suggesting that the response of disbonds in GRP plates during vacuum-stressing may be modelled with this theory.

The Structure of Intermittent Spray Injected through Diesel Injection Nozzle. Study of the Atomization in the Early Stage by Adopting the Double-Exposure Method Using a Delay System Equipped with an Optical Fiber.

A detailed observation of the intermittent spray injected through diesel injection nozzle was made by adopting a double-exposure method using a delay system equipped with an optical fiber. The results are as follows: the atomization in the early stage of the intermittent spray is clarified, various forms of fuel jets appear immediately after injection from the nozzle, and in the next moment, small fuel droplets are dispersed radially. These phenomena suggest that the following fuel jets collide with a leading part of the jets decelerated by static ambient air, and the fuel jets are atomized. In addition to the shearing interaction between the fuel jets and the ambient air, this atomization mechanism turns out to play an important role in the early stage of the intermittent spray.

An interpretation of Moiré interferometry from wavefront interference theory

The process of moiré interferometry is given mathematical description in terms of the classical theory of optical interference. The governing relationship between displacement and fringe order is derived, and the analysis is extended to cover carrier patterns, double exposure methods, and initial fringe pattern elimination. Various processes for obtaining patterns of displacement derivatives are also described mathematically.

Full-field laser metrology for fluid velocity measurement

This paper reviews the development of full-field laser metrology from its inception as a dynamic extension of scattered light speckle photography, a voherent optical technique originally developed to make full-field measurements of quasi-static displacements in transparent solids, to its present applications in fluid dynamic studies. The full-field laser velocimetry technique, including both double exposure and multiple exposure recording methods, is discussed in detail, and comparison is made of their advantages and disadvantages in applications to steady and unsteady flows. Both the coherent light speckle and the particle image modes of operation are defined and evaluated. Computer aided ‘point-wise’ (Young's fringe) and ‘global’ (Fourier filtering) methods of optically analyzing the specklegrams are also discussed. Finally, experiments in which full-field laser velocimetry has been applied to a study of convective flow in a liquid are described, thus illustrating the potential of this new, non-onvasive optical technique.

High-precision reticle making by electron-beam lithography

We have developed an electron‐beam (e‐beam) lithography system named ‘‘NOWEL’’ which utilizes very accurate pattern writing method and high‐speed data processing. Accordingly NOWEL makes it possible to manufacture reticles for 16‐Mbit dynamic random access memories (DRAM’s) or more densely integrated very large scale integrated circuits (VLSI’s). The NOWEL system has three key features: (1) Vertical landing deflection system. This consists of three yokes in series and one short‐working‐distance lens (M=0.86). The landing angle at the corner of the 5‐mm2 main field is <0.0025 rad. Therefore the butting error arising from a 10‐μ height variation of the substrate is <0.05 μ. Deflection aberration is about 0.2 μ. (2) Double exposure method called ‘‘A/B mode.’’ We adopted a variable shaped beam. The first exposure is made by serial writing with constant‐size rectangular spots, and the second one is made on the previously exposed area by shifting the beam position and reshaping the beam spots. One‐half dosage is given in each exposure. By this method the edge roughness of each pattern is improved to be <0.05 μ. (3) Pattern data compression. A hierarchical pattern data structure was developed. In the memory cell area the data can be divided into only a few pattern data for cell units and their repetition data, i.e., start positions, pitch data, and numbers of cell units in the cell area. According to the repetition data in the main field the main deflector moves the positions of cell units, written by the subdeflector, with the same pattern data. We call this exposure method ‘‘main deflection matrix mode.’’ For fine positioning especially in this mode we developed a 20‐bit digital‐analog convertor (0.005‐μ least significant bit). These reduction techniques should compress the amount of total pattern data to 1/10 for a 16‐Mbit DRAM. By using NOWEL we made a test reticle which contained 16‐Mbit cell patterns with total accuracy within 0.1 μ. In this simplest case only one cell unit with the patterns for 128‐bit cells and their repetition data were needed.We have developed an electron‐beam (e‐beam) lithography system named ‘‘NOWEL’’ which utilizes very accurate pattern writing method and high‐speed data processing. Accordingly NOWEL makes it possible to manufacture reticles for 16‐Mbit dynamic random access memories (DRAM’s) or more densely integrated very large scale integrated circuits (VLSI’s). The NOWEL system has three key features: (1) Vertical landing deflection system. This consists of three yokes in series and one short‐working‐distance lens (M=0.86). The landing angle at the corner of the 5‐mm2 main field is <0.0025 rad. Therefore the butting error arising from a 10‐μ height variation of the substrate is <0.05 μ. Deflection aberration is about 0.2 μ. (2) Double exposure method called ‘‘A/B mode.’’ We adopted a variable shaped beam. The first exposure is made by serial writing with constant‐size rectangular spots, and the second one is made on the previously exposed area by shifting the beam position and reshaping the beam spots. One‐half dosage i...