One annoying problem that degrades high-fidelity pattern images in mask-defect inspection systems is the generation of ghost images in the imaging process. Ghost images arise from spatial coherence periodicity on the mask plane, which is due to periodic and discrete arrangements of fly-eye elements in mask inspection optics. By considering the contrast of ghost images under partial coherence illumination, we can derive the condition that represents the necessary number of fly-eye elements to substantially suppress ghost images in the image field. In addition, we confirm this theoretically derived condition of suppressing ghost images by numerical calculations. As a result, we prove that this suppresing condition is effective, and that the nonuniformity in distribution of image intensity can also be reduced in this way.
Due to the feature size shrinking, the application of 193nm-ArF scanner systems with high numerical aperture (NA)
and the use of resolution enhancement technologies (RET) have been essential for obtaining the desired pattern
accuracy on a wafer. Thus the complexity and volume of data required for masks have been rapidly increasing.
Moreover, the complexity of mask pattern makes mask inspection increasingly more difficult. The most annoying
problem relating to the sensitivity of inspection system is the encountering of false signals arising from nuisance
defects. Setting up thresholds in the defect detection algorithm is a difficult task between high sensitivity and less false
defect detection. In addition, the effect of variations in defect printability which is strongly dependent on defect types
and position must be considered in order to correctly evaluate mask defect inspection procedures.
In order to overcome the problems we have previously proposed new algorithm for die-to-wafer-like image (D-to-WI) in real time. This inspection method compares the die, i.e. the wafer image calculated from CAD data, with the
wafer-like image calculated from the mask images detected by the mask inspection system.
This paper described optimum mask inspection optics for the D-to-WI mask inspection. We verify the optimum mask
inspection optics with numerical simulation for various NA and partial coherence of illumination (σ) in the mask
inspection optics. The simulated result shows that the optimum mask inspection optics has NA 0.9 and σ=1 for ArF-6%-PSM (Phase Shift Mask) 65/65 nm Line/Space pattern of 193nm-ArF scanner with NA 0.92. In this case the difference
of the critical dimensions (CDs) found by D-to-WI and rigorous simulation results from CAD data was less than 1.5nm.
In this work, a newly developed optical lens cementing technology is reported. A fluoride material is used as an optical cement that can reduce damage from deep-ultraviolet (DUV) radiation. The degradation of transmittance and the surface quality of the cemented optical elements, including adhesive used for cementing, are evaluated after prolonged DUV irradiation. It is shown that with a 248-nm wavelength, this cement works quite well, up to 1000 h of operation, and the change in transmittance is negligible where average irradiation power is within 27 to 37 mW/cm2. Hence for all practical purposes, the use of this cement in microscope objectives is quite acceptable for 248-nm applications, thus confirming that this cementing technology is satisfactory and meets the performance requirement of DUV inspection systems.
The most annoying problem accompanying production of high-fidelity pattern images in mask defect inspection
systems is the generation of virtual images in the imaging process. The focused image pattern on the image acquisition
sensor has two images, one true and one virtual. The virtual images are generated under Kohler's illumination using
an integrator. The theoretical cause of this virtual image is the periodicity of the integrator.
The improvement of image quality gives the mask defect inspection system higher defect detection sensitivity. To
reduce virtual images, the double integrator method is applied to the illumination optics. By adopting the double
integrator illumination method, virtual images disappear in the imaging field. Further, since this also lowers the power
density at bright spots, the interference of lenses in working environments at the aperture stop position between
objective imaging lenses is greatly reduced.
This paper reports a method by which the ill effects of image quality improvement in the mask defect inspection
system can be dramatically reduced. The simulation results when this method is applied to an advanced mask defect
inspection system are shown.
The concept of defect printability, i.e., mask error enhancement factor (MEEF), should be integrated into mask defect inspection procedures, and thus avoid the huge burden of defect detection algorithm development. It is necessary to simplify the difficult task of defining defect size which is caused by nonlinear transfer of killer defects, and which is strongly dependent on defect types. One solution to the problem is to incorporate defect printability study using aerial image based inspection into the existing mask inspection system. This paper shows how the measured mask pattern images obtained from mask inspection system are transformed into wafer-like images by simulation-based software. It is important that wafer-like images (WI) from measured mask images are created within a reasonable calculation time and the result has sufficient accuracy. The paper also introduces calculation of aerial images using perturbation approach and demonstrates the possibility of D-to-WI inspection. The paper points out that the technique of generating wafer-like image from measured mask pattern is well established for attenuated PSMs and Cr binary masks.
In this paper, a newly developed optical lens cementing technology is reported. Here, a fluoride material is used as an optical cement which can reduce damage from DUV radiation. The degradation of transmittance and the accuracy of surface of the cemented optical elements including adhesive used for cementing have been evaluated after prolonged DUV irradiation. It has been shown that with 248 nm wavelength this cement works quite well, and moreover, even with 193 nm wavelength, when used for 1000 hours, the change in transmittance was negligible where average irradiation power was kept within 300mW/cm2. Hence for all practical purpose the use of this cement in microscope objective is quite acceptable for 248 nm applications, thus confirming that this cementing technology is satisfactory and meets the performance requirement of DUV inspection systems.
In 65nm node and some more technology node probably may go with current optical lithography and industry has predicted many challenges. In patterning point of view, quality and cost of mask became more and more important than ever. Particularly, mask defect engineering technology is key area not only inspect the defects but also mask process monitoring and improvements. In mask inspection technology there were a lot of new progresses to enhance the defect inspection sensitivity and stability. The key solution to achieve better sensitivity may be short inspection wavelength and adequate detection algorithm. In this paper, we will propose defect size specifications of 65nm and beyond optical mask with various OPC and RET environments. In addition, we will present initial data of newly developed 198.5nm inspection wavelength system. Through this study, we found future optical mask faces new challenges in defect inspection and to solve these problems, we need advanced mask inspection system and collaborations among patterning related fields.
A novel high-resolution mask inspection platform using DUV wavelength has been developed. This platform is designed to enable the defect inspection of high quality masks for 65nm node used in 193nm lithography. In this paper, newly developed optical system and its performance are reported. The system is operated at wavelength of 198.5nm, which wavelength is nearly equal to 193nm-ArF laser exposure tool. Some defect image data and defect inspection sensitivity due to simulation-base die-to-die (D/D) inspection are shown on standard programmed defect test mask. As an initial state D/D inspection performance, 20-60 nm defects are certified. System capabilities for 65nm node inspection and beyond are also discussed.
As semiconductor integration goes down to nano-meter scale, finer patterning technology is inevitable. Therefore it is more and more important not only new lithographic development but also mask quality enhancement. Particularly, due to the delay of NGL technology, optical lithography is growing candidate for 65nm and beyond node device. In that case, mask CD uniformity and defect control issues are more important than ever. In mask inspection technology, there were a lot of new progresses to enhance the defect inspection sensitivity and stability via short-wavelength and advanced defect inspection algorithms. In this paper, we will present a concept and on going status of newly developed short-wavelength DUV inspection tool that is co-worked by Selete, Toshiba, and NEC. Moreover, we will discuss defect specifications that is required 65nm node and beyond technology node by simulations. This will include relations between defect inspectability and printability in the case of ArF, ArF immersion, and F2 lithography in various layouts and patterns. Through this study, we can conclude stable short-wavelength inspection tool and proper inspection algorithms are essential for future generation mask to cope with low k1 lithography.
The mask blank surface inspection system for the electron beam mask writing system (EB mask writer) has developed. This system, that has the small vacuum chamber attachable to EB mask writer, inspects a mask blank that is just before EB writing in vacuum environments. It can inspect whole area of the 230mm mask at 0.3micrometer sensitivity. It also can perform fast inspection by applying the original scanning algorithm for the laser beam. It has the wide detective range from 0.3 to 2.0 micrometers of particle size. It can distinguish sizes of particles in that range. The auto focus function is most important factor for maintaining the sensitivity.
A stage tracking function has been developed for a mask-scan EB mask writer. Position error of EB mask on an EB-mask-stage induces position error of projection beam on the EB-mask and the position of a writing pattern. The position of the EB-mask is measured by a laser interferometer. The shift from the aimed position is fed back to a mask selection deflection and a main deflection. The velocity of EB-mask stage and specimen-stage is also fed back to the deflection. The deflection control unit for the stage tracking has been made and the tracking function confirmed from the test memory of the unit. Using the unit, scanning writing patterns have been obtained with step and repeat stage mode.
We proposed a new adjustment method for the beam-intensity distribution. On the 2nd shaping aperture plate, a small aperture is positioned sufficiently apart from the aperture for variable shaping. The center of the 1st shaping aperture image is moved to the small aperture by adjusting an alignment coil. The beam current, which passes through the small aperture, is measured by the Faraday cup on the target while the 1st shaping aperture image is scanned over the small aperture by operating the shaping deflector. Using this method the beam-intensity distribution of the 1st shaping aperture image on the 2nd shaping aperture plane is obtained. The beam-intensity distribution obtained is ideally a series of concentric circles and the maximum value is reached at the center circle. The center is shifted when the alignment of the limit aperture at the illumination or at the 1st shaping aperture is incorrect. The position of the maximum in the beam intensity is adjusted to the center of the 1st shaping aperture image. At the beam-current-density of 20 A/cm2, the slope of the distribution is usually under 1 percent. To evaluate the adjustment accuracy, resist profile of drawn pattern is measured by AFM. The resist profiles in a beam shot coincide well with the beam intensity distribution.
Meeting the latest requirements of aggressive users for the advanced mask for optical lithography will be difficult. In addition, improving the productivity and throughput of the advanced mask with high-density pattern data is necessary. To overcome these hurdles, Toshiba Corp. and Toshiba Machine Co., Ltd. have developed the new advanced mask writer, the EBM- 3000, shown in Figure 1. The EBM-3000 especially takes measures against airborne contamination in the load-lock chamber. Three components of the mask blank handling system have a function as mini environment. To link each of these components, it also employs a standard mechanical interface, SMIF, based on the concept of local cleaning technology. This paper is intended to describe the design concept of the new mask blank handling system for the EBM-3000, and prove these measures to be effective against airborne contamination by the experimental results.
Toshiba and Toshiba Machine have developed an advanced electron beam writing system EX-11 for next-generation mask fabrication. EX-11 is a 50 kV variable-shaped beam lithography system for manufacturing 4x masks for 0.15 - 0.18 micrometer technology generation. Many breakthroughs were studied and applied to EX-11 to meet future mask-fabrication requirements, such as critical dimension and positioning accuracy. We have verified the accuracy required for 0.15 - 0.18 micrometer generation.
Background exposure of a resist caused by scattered electrons (the fogging effect) degrades critical dimension accuracy when the pattern density changes over the specimen. We measured the fogging effect in an electron beam optical column. In order to reduce the fogging effect, a scattered electron absorber plate having a converging holes structure was attached to the lower surface of the objective lens. When the most severe pattern for the fogging effect was applied, we achieved the size variation caused by the fogging effect less than 8 nm. The converging holes effectively trap the scattered electrons and greatly reduce the fogging effect.
Meeting the latest requirements of aggressive users for advanced masks for optical lithography will be difficult. In addition, improving the productivity and throughput of advanced masks with high-density pattern data is necessary. To overcome these hurdles, Toshiba and Toshiba Machine have developed a new advanced mask writer, the EX-11, shown in Figure 1. The EX-11 takes measures against airborne contamination before drawing is started. It also employs a standard mechanical interface (SMIF) based on the concept of local cleaning technology. This paper describes the design concept of the new mask blank handling system for the EX-11, and the efficiency of these measures was confirmed by the experimental results.
We have newly designed and constructed a unique electron optical column installed with an in-situ cleaning system, applying the down-flow ashing process with a mixture O2 and CF4. We carried out in-situ cleaning using designed system, and confirmed that beam drift which is caused by charging up of a contamination layer was reduced.
For mask defect inspection in 256 Mbit and 1 Gbit DRAMs, it is necessary to have high sensitivity of 0.2 - 0.1 micrometer. A new die-to-database mask inspection system MC-2000 for 256 Mbit and 1 Gbit DRAMs has been developed. This system has high resolution optics with i-line light and high NA lens, and high speed and high accuracy data processing circuit by new multilevel bit map pattern generator, so the system has both high detectability and high throughput. This paper describes system configuration which include optical system and mechanical system, the defect inspection method, and inspection performance including defect sensitivity.
Improvement of pattern placement accuracy is an important factor for the development of the electron beam (EB) lithography system for the next-generation photomask. It has been qualitatively pointed out that pattern shift error is induced by surface distortion of photomask. In this paper, we quantified pattern shift error induced by mask process and have identified aeolotropic magnification error and negligible orthogonality error. These results obtained by experiment and simulation indicate that attention must be paid to pattern shift error induced by mask process in fabrication of the next-generation photomask. Thus, a more rigid and stiffer photomask will be required to reduce pattern shift error induced by coating and developing.
Pattern measurement repeatability of metrology tools must be evaluated precisely to warrant higher pattern placement accuracy, according to a budget of pattern shift errors effected by the initial deformation of a substrate, clamping conditions, etc. As first steps, we focused on our metrology tool, Nikon XY-3i. Pattern measurement repeatability was usually evaluated to measure a referential pattern of a single mask repeatedly. For taking tilting variations on each of mask setting into account, we divided the coordinates of measured data into some error factors. Besides, we proposed sag correction method to eliminate tilting variation for precisely evaluation. This method was effective to unify each of the referential planes on measuring. Sag correction was effective to diminish in variations of orthogonality error factor and trapezoid error factors and deviations (3(sigma) ) of measurement repeatability. Therefore, we succeeded to get the quantitative budget of measurement repeatability for our metrology tool.
This paper presents a method for estimating the influence of reticle flexure on pattern positioning accuracy, and evaluates the method by measuring patterned reticles. Reticle flexure causes the pattern shift which occurs by stretching or compression of the reticle surface. A height-mapping function of an electron beam (EB) writing system and a measuring machine are used to calculate the pattern shift due to reticle flexure. The bent shape of a reticle on the EB-writing system differs from that on the measuring machine, so that the patten shifts on the two machines are different. The pattern shifts caused by the bent shape difference were excluded from the measurement result of pattern positioning errors. The values of pattern positioning accuracy evaluation parameters, x, y-scaling and orthogonality, are calculated among several reticles (5 inches, 0.09 inches thick). The deviations of these three values are reduced to less than 50% of their uncompensated values.
Masks and their fabrication technologies are keys to the further advancement of optical lithography. A stable SiNx single layer attenuated masks for DUV have been developed. A 0.2 micrometers contact hole pattern was fabricated using a KrF stepper with the SiNx attenuated mask. Toshiba mask fabrication system, including an electron beam writing system, a data base inspection system, and a data conversion system, has been developed for 64 Mbit DRAM class. Required mask improvements for increasing optical lithography resolution include better critical dimension (CD) uniformity, higher mask writing system resolution, and automatic shifter patten generation of alternating phase shifting masks. In addition, improved mask pattern positioning accuracy is also required. In this paper, experimental CD uniformity and resolution improvements, automatic phase shifter assignment method, and improvement in positioning accuracy, are described. The future development of masks will incorporate these key technologies.
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