Projection Lithography System Research Articles

Analysis and Optical Design of 1:1 Projection Lithography System

For the 1:1 laser projection lithography system used to achieve large-area patterning with higher resolutions as well as higher throughput, the key parameters such as the laser beam geometry, the numerical aperture of projection lens, the laser source power and the pulse repetition rate are theoretically analyzed. It is expounded the process of uniform exposure in hexagonal beam shape, the advantages and limitations of 1:1 projection owing to numerical apertures deciding the resolution, as well as the cause of choosing larger laser power and pulse repetition rate. Meanwhile, the projection lens for a unit-magnification, refractive imaging system is tentatively simulationdesigned using ZEMAX optical design software. The optimized three-dimensional layout is plotted. For the designed results, the maximum optical path difference is smaller thanλ /4 within entire visual field. The resolution for feature sizes 10μm can be achieved within depth of focus 400μm by evaluating MTF. The maximum field curvature is within 10μm and the maximum distortion is small than 0.000007%. This fulfills the demands in technical specifications.

Photolithographic synthesis of high-density DNA probe arrays: Challenges and opportunities

The continual need for increased manufacturing capacity in the production of GeneChip™ DNA probe arrays, and the expanding use of these arrays into new areas of application such as molecular medicine, has stimulated the development of new chemistries and production methods with higher efficiency and resolution. For current production methods based on contact photolithography, modifications in substrate materials and photoactivated synthesis reagents have provided significant improvements in array performance and information content (≥4×106 sequences∕cm2). An alternative next-generation manufacturing process is also in development, which utilizes photoacid generating polymer films, and automated projection lithography systems. This process has the ability to fabricate arrays with 1 micron feature pitch and smaller, providing an unprecedented sequence density of 108∕cm2 and greater.

Versatile maskless microscope projection photolithography system and its application in light-directed fabrication of DNA microarrays

We present a maskless microscope projection lithography system (MPLS), in which photomasks have been replaced by a Digital Micromirror Device type spatial light modulator (DMD™, Texas Instruments). Employing video projector technology high resolution patterns, designed as bitmap images on the computer, are displayed using a micromirror array consisting of about 786 000 tiny individually addressable tilting mirrors. The DMD, which is located in the image plane of an infinity corrected microscope, is projected onto a substrate placed in the focal plane of the microscope objective. With a 5× [0.25 NA (numerical aperture)] Fluar microscope objective, a fivefold reduction of the image to a total size of 9mm2 and a minimum feature size of 3.5μm is achieved. The ultrahigh pressure lamp of a video projector is a cheap, durable, and powerful alternative to the mercury arc lamps commonly used in lithography applications. The MPLS may be employed in standard photolithography. We have successfully produced patterns in 40μm films of SU-8 photoresist, with an aspect ratio of about 1:10. Our system can be used in the visible range as well as in the near UV (with a light intensity of up to 76mW∕cm2 around the 365nm Hg line). We developed an inexpensive and simple method to enable exact focusing and controlling of the image quality of the projected patterns. Our MPLS has originally been designed for the light-directed in situ synthesis of DNA microarrays. One requirement is a high UV intensity to keep the fabrication process reasonably short. Another demand is a sufficient contrast ratio over small distances (of about 5μm). This is necessary to achieve a high density of features (i.e., separated sites on the substrate at which different DNA sequences are synthesized in parallel fashion) while at the same time the number of stray light induced DNA sequence errors is kept reasonably small. We demonstrate the performance of the apparatus in light-directed DNA chip synthesis and discuss its advantages and limitations.

Electron-beam focusing in 1:1 electron projection lithography system

Factors influencing the transferred image resolution in 1:1 electron projection lithography system are considered. The influence of Coulomb interaction between electrons, gradient of magnetic field, and angle between focusing electric and magnetic fields on the electron-beam focusing is estimated. In the presence of a magnetic-field gradient in the focusing system, the radius of an electron’s orbit is changed with a change of value of magnetic field B. As a result, after completing a whole turn, the center of electron’s orbit moves in the direction perpendicular to B and the magnetic-field gradient, which results in distortion of the transferred image. Coulomb interaction between electrons during their flight from the emitter toward the e-beam resist-coated wafer also results in diffusion of the transferred image and may be considered a crucial parameter that limits the high-resolution capability in the case of high emission currents. The presence of the angle between the direction perpendicular to the wafer and emitter planes and the magnet’s field direction results in an overall displacement of the transferred image and a variation of cyclotron orbit radius that depends on the distance between the emitter and wafer.

Low energy electron beam proximity projection lithography

The low energy electron beam proximity projection lithography (LEEPL) system consists of three properties: low energy electron beam, a parallel beam, and proximity projection. The low energy electrons increase the effective resist sensitivity and greatly minimize the proximity effect. Over a 20 µm depth of focus is achieved by the parallel beam on the proximity projection. The subdeflection system of the LEEPL system is useful in correcting the mask distortion and chip distortion on the wafer by a correction data map corresponding to the field, because of the space (>30 µm) between the wafer and the mask. The overlay accuracy of the machine itself is less than 14 nm and that of mix and match is less than 25 nm. This implies that the overlay between the LEEPL system and an ArF scanner in both the x and y directions are obtained. This machine shows the 48 nm CH resist patterns as the ultimate resolution. The cost of ownership (CoO) of the LEEPL system for a 65 nm node device will be approximately less than $25/wafer/layer and the value is lower than that of an ArF scanner.

Relativistic Focus Condition for E-Beam Projection Lithography

In e-beam projection lithography (EPL) systems, the relativistic focus condition was derived and compared with the nonrelativistic focus condition to investigate the effect of a large acceleration field related to the initial energy and emission angle of an electron emitted from a cathode. The difference in confusion disk between the relativistic and nonrelativistic limits was 180 nm when 1 eV was used as the initial energy of the electron. The variation in the initial energy of the electron was more sensitive to change the confusion disk than the acceleration field. In this comparison, the focusing parameters are used as follows: axial magnetic field, 0.5 T, distance between the mask layer and the workpiece, 2 mm and emission angle of the electron, 30°.

Mask Charging Phenomena during Electron Beam Exposure in the EPL System

Charging phenomena of a mask material during electron beam exposure are analyzed in an electron beam projection lithography system. First, the three-dimensional charge deposition distribution by the electron beam irradiation is obtained. Next, in every time step, the distributions of the accumulated charge and the potential are obtained considering the current flow due to the diffusion and the drift. As a narrow bridge pattern defined in a 5 microm x 5 microm area is assumed to lay out all over the field of 1 mm x 1 mm and the potential is grounded at the circumference of the field (1 mm x 1 mm x 1 mm), the saturated potential distribution is obtained at the central 5 microm x 5 microm area in the field. The maximum potential attained is around 4.23 microV at the center of the bridge, if the accelerating voltage of the electron beam is 100 kV, the current density is 10 A/cm2, and the material of the mask is the intrinsic Si. The contribution of the charging may be negligible to the electron beam with such a high accelerating voltage, which is going through the opening beside the bridge pattern in the projection lithography.

Optimization of multilayer reflectors for extreme ultraviolet lithography

Multilayer interference coatings on reflective elements in extreme ultraviolet (EUV) projection lithography systems introduce phase and amplitude variations at reflection, which have important implications for imaging properties, e.g., resolution, depth of focus, and tolerances. We discuss the numerical results of the optical effects of multilayers (MLs) and the inclusion of these effects in optical design. This numerical study demonstrates the advantages of spatially varying (graded) MLs compared to multilayers with constant layer thicknesses. We present a new method to calculate the optimum grading of multilayers. Using this new method, we are able to fully optimize the wavefronts emerging from the projection system toward the image plane.

Monte-Carlo-Based Automatic Design of Chromeless Phase Shift Mask

The optical proximity effect correction algorithm (OPERA) program based on the Monte-Carlo method has been used for optical proximity correction (OPC) and phase shift mask (PSM) generation for low-NA projection lithography systems. To apply OPERA to high-NA systems, we adopted an imaging model based on a vector diffraction theory. Also, we developed a new convergence algorithm that does not reduce the degree of freedom and prevents the convergence process from falling into a local minimum. The new program, OPERA-V, is at least 10 times faster than the previous version. OPERA-V not only improves the computation time of traditional OPC, but can also be used as an automatic design tool for creating chromeless PSMs.

Development of dioptric projection lenses for deep ultraviolet lithography at Carl Zeiss

Advanced dioptric projection lenses from Carl Zeiss are used in some of the world's most advanced deep ultraviolet projection lithography systems. These lenses provide a resolution of better than 100 nm across the entire field of view with a level of aberration control that maximizes critical dimension uniformity and lithographic process latitude. These dioptric projection lenses are currently being used for critical layer device patterning for a wide array of complex logic, memory, and application specific integrated circuits. Zeiss' involvement in the development of ultraviolet lenses goes back to the year 1902, more than 100 years ago, when von Rohr calculated the first monochromatic ultraviolet microobjectives for ultra-high-resolution microphotography using a line-narrowed source. The modern dioptric projection lenses for lithography are influenced by the collective experience in the field of microscopy, and the more recent experience with early step-and-repeat lenses. We discuss some of the foundations of modern dioptric designs in the context of this history, demonstrating that rapid synthesis of designs is possible using combinations of monochromatic microscope objectives and early step-and-repeat lenses from the 1970s. The problems associated with ultrahigh numerical aperture objectives are discussed. Specifically, it is demonstrated that aspheres can be used effectively to reduce the volume of full field projection lenses, making the mechanical implementation of a 0.90 NA lens feasible in production. Several contemporary dioptric projection lens designs are reviewed in detail. The extension of these designs to numerical apertures greater than 1.0 using immersion techniques is demonstrated. These immersion lenses give the potential for 40-nm resolution.

Demonstrations of electronic pattern switching and 10× pattern demagnification in a maskless microion-beam reduction lithography system

A proof-of-principle ion projection lithography (IPL) system called maskless microion-beam reduction lithography (MMRL) has been developed and tested at the Lawrence Berkeley National Laboratory for future integrated circuits manufacturing and thin-film media patterning [V. V. Ngo et al., J. Vac. Sci. Technol. B 17, 6 (1999)]. This MMRL system is aimed at completely eliminating the first stage of the conventional IPL system [G. Stengl et al., J. Vac. Sci. Technol. B 10, 2824 (1992)] that contains the complicated beam optics design in front of the stencil mask and the mask itself. It consists of a multicusp rf plasma generator, a multibeamlet pattern generator, and an all-electrostatic ion optical column. Results from ion beam exposures on poly(methymethacrylate) and Shipley UVII-HS resists using 75 keV H+ are presented in this article. Proof-of-principle electronic pattern switching together with 10× reduction ion optics (using a pattern generator made of nine 50 μm switchable apertures) has been performed and is reported in this article. In addition, the fabrication of a microfabricated pattern generator [K. L. Scott et al., J. Vac. Sci. Technol. B 18, 6 (2000)] on a silicon on insulator membrane is also presented.

The Development of Dioptric Projection Lenses for Deep Ultraviolet Lithography

Advanced dioptric projection lenses from Carl Zeiss are used in some of the world’s most advanced deep ultraviolet projection lithography systems. These lenses provide a resolution of better than 100nm across the entire field of view with a level of aberration control that maximizes critical dimension uniformity and lithographic process latitude. These dioptric projection lenses are currently being used for critical layer device patterning for a wide array of complex logic, memory, and application specific integrated circuits.

Investigation of gray-scale technology for large area 3D silicon MEMS structures

Micromachining arbitrary 3D silicon structures for micro-electromechanical systems can be accomplished using gray-scale lithography along with dry anisotropic etching. In this study we have investigated two important design limitations for gray-scale lithography: the minimum usable pixel size and maximum usable pitch size. Together with the resolution of the projection lithography system and the spot size used to write the optical mask, the maximum range of usable gray levels can be determined for developing 3D large area silicon structures. An approximation of the minimum pixel size is shown and experimentally confirmed. Below this minimum, gray levels will be developed away due to an excessive amount of intensity passing through the optical mask. Additionally, oscillations in the intensity are investigated by the use of large pitch sizes on the optical mask. It was found that these oscillations cause holes in the photoresist spaced corresponding to the pitch used on the gray-scale mask and penetrate the thickness of the photoresist for thin gray levels. From the holes in the photoresist, significant surface roughness results when used as a nested mask in reactive ion etching, and the very thin gray levels are lost.

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.

Large-area high-resolution lithography and photoablation systems for microelectronics and optoelectronics fabrication

An important requirement in the production of numerous microelectronic, optoelectronic, and microsystem devices is lithographic patterning on a large area with high image resolution and precise layer-to-layer alignment. Whereas for production of semiconductor devices advances have been steadily made in steppers and other conventional lithography systems, the lithography requirements for the fabrication of large-format products, such as displays, multilayer circuits, and flexible electronics, are distinctly different, rendering various conventional lithography tools inadequate. These requirements and distinctions of large-area lithography are discussed. In the last several years, we have developed a new class of projection lithography systems that provide both high-resolution imaging and very large exposure area capability with high-precision alignment. The systems, using excimer laser sources, function as dual-mode, high-throughput production tools, capable of patterning in photoresists as well as photoablation in polymers, making them attractive for production of numerous large-format products, with feature sizes ranging from 15 /spl mu/m to below 1 /spl mu/m and substrate sizes ranging from 150 /spl times/ 150 mm to 610 /spl times/ 915 mm. We review the new lithography system technology, several completed systems, and demonstrated results.

Fast and Simplified Technique of Proximity Effect Correction for Ultra Large Scale Integrated Circuit Patterns in Electron-Beam Projection Lithography

The pattern shape modification method is an effective proximity effect correction technique for electron-beam projection lithography (EPL) systems. We used two characteristics based on the large difference between the forward and backscattering ranges of incident electrons from the 100 kV EPL system to simplify and speed up the pattern shape modification method. The first characteristic is that the amount of pattern bias becomes almost identical on opposite sides of a sufficiently small pattern compared to the backscattering range. The second characteristic is that although the equations for calculating the amount of pattern bias are simultaneous equations, they can be solved separately if the pattern is sufficiently long compared to the forward-scattering range. In addition, multithreading was used to perform parallel processing for the purpose of speeding up the pattern shape modification method. This simplification and speeding up cut the processing time to 1/3 when using four processors. We also checked the correction accuracy by repetition, and considered the use of a correction that maintains the hierarchy for compressing the amount of data.

Progress and Preliminary Results on EB Stepper

The EB stepper is an electron beam projection lithography (EPL) system. The EB stepper system is similar to an optical scanning exposure system with a reticle. Nikon has been developing EB stepper on the basis of the optical stepper design. The field size of the electron optical (EO) subsystem of EB stepper (subfield/(SF)) is 1 mm square on a reticle and 250 µm square on a wafer (demagnification: 4×). For full chip exposure, SF exposures are stitched using electron beam deflection and stage movement. A new type of stage has been developed for EB stepper using air guides and linear motors to achieve a high-throughput target. These subsystems are assembled separately at present. In this paper, preliminary results on EB stepper as an exposure system are reported. Progress on infrastructures for EPL is discussed.

A method for evaluating aberration in the crossover image in mask irradiation optics of electron beam

A method for evaluating aberration in the crossover image in a cell projection lithography system has been developed. In an electron-beam lithography system of projection-type such as a cell projection lithography system, the aberration in the crossover image causes the electron beam to pass off-axis in the electron optics. Optical simulation has quantitatively shown that the aberration in the crossover image causes an electron-beam blur and a positioning error on a writing sample. The evaluating method consists of four square apertures and a mark-detection function in a cell projection system. By measuring each position of the images of the four square apertures on the writing sample at difference focuses, the aberration can be calculated. The field curvature and the astigmatism in a cell projection system were evaluated by using this method. The field curvature agrees with the simulation. In addition, the measurement of the effect of beam alignment is also demonstrated. It is thus concluded that the method can effectively evaluate the aberration in the crossover image. This method is also useful for other projection-type lithographies of charged particles—like ion and electron beams.

Calibration and validation of projection lithography focusing by fluorescence detection of latent photoacid images in chemically amplified resist

We dope the commercial resist UVIII with the pH sensitive dye coumarin 6 (C6) to demonstrate a fluorescence microscopy technique for detecting latent photoacid images in exposed chemically amplified resist films. The spectroscopic properties of C6 in the resist matrix are investigated in order to select spectroscopic filters which optimize the fluorescence image contrast of exposed patterns. We apply this technique to focus calibration of a projection lithography system and show a significant correlation of the results to scanning electron microscope analysis of developed features. Hence, we demonstrate the utility of this technique as a fabrication line diagnostic for focus calibration and validation (i.e., proof of focus) prior to postexposure processing of the resist film.

Direct measurement of Coulomb effects in electron beam projection lithography

We describe a direct measurement of the image blur and defocus induced by Coulomb effects (stochastic and global space charge) in electron beam projection lithography. The Nikon 100 kV electron beam projection experimental column was used for the experiments. This column has similar values of electron optical parameters to those of electron beam projection lithography (EPL) systems. The Coulomb effect image blur and defocus were directly measured by a knife-edge method. The experimental data of Coulomb effect image blur and defocus as functions of beam current are shown. The experimental results show excellent agreement with our Monte Carlo simulation results.