As the requirements for photomask linewidth control continue to tighten, the necessity for performing proximity correction for electron beam mask exposure will increase. GHOST proximity effect correction is one method that can be used to ensure that critical dimension linearity over a large range of feature sizes meets mask user requirements. The GHOST strategy uses an additional exposure to correct for the backscatter component of the primary exposure. Because of the way the image using a GHOST correction is constructed, image contrast will be lower than exposures done without GHOST. This paper uses simulation to examine the process window that is available when GHOST is used and this process window is compared to that without GHOST. The effect of resist contrast on the process window is examined by simulating ZEP 7000 resist and comparing it to resists with other contrasts. The effects of dose, develop time, data bias and spot size on the process window are also examined.

JEOL has developed an E-Beam lithography system JBX-9000MV with a vector scan and variable shaped beam (VSB) electron optics for the manufacture of 180nm - 150nm devices masks. This system employs 50kV accelerating voltage, low space charge effect column and in-lens deflector system. Beam current density is 10A/cm², maximum mask size is 230 mm (9'), beam address size is 2 nm. Extended evaluation of the system shows pattern placement accuracy of 30 nm or better, field stitching accuracy of 20 nm or better, critical dimension (CD) accuracy of 20 nm or better.

CD uniformity is one of the most critical parameters for mask making today. The mask error factor (MEF) in lithography for features that are smaller than the stepper wavelength means that any CD error is transferred to the wafer to a greater extent than the stepper reduction factor would indicate. CD results form a new laser pattern generator, the Omega6000 product line, will be presented. The system features an acousto-optic deflection architecture specifically designed to meet the CD requirements of 180 nm photomasks. A 0.86 NA final lens provides the high resolution of the system. The CD control and the high resolution makes the system well suited for today's advanced photomasks.

This paper describes improvements in column design and writing strategy that, together, enable mask production for the 130 nm technology node. The MEBES^R 5500 system employs a new high-dose electron gun and column design. We summarize experiments relating lithographic quality to increased dose and the effects of spot size and input address on lithography. These experiments are performed with ZEP 7000 resist and dry etch. A new graybeam writing strategy, Multipass Gray-II (MPG- II), is described in detail. This strategy creates eight dosed gray levels and provides increased writing throughput (up to 8X, compared to single-pass printing) without loss of lithographic quality. Significantly, critical dimension (CD) uniformity, butting, and other important specifications are improved with MPG-II. Lithographic results and throughput data are reviewed. A consequence of the improvement in CD control and throughput is greater productivity for 180 nm devices.

Modifications have been made to the ALTA^R 3500 system to improve critical dimension (CD) control in three ways. First, the mean-to-target performance has been improved by increasing the repeatability of the measurement of optical efficiency, thereby more precisely setting the dose delivered to the photoresist. Second, the compressed dry air (CDA) used by the focus subsystem has been replaced with pressurized air drawn from the print head of the writing system. By using the humidity-controlled air from the print head, the water content of the photoresist is not affected by the focus subsystem. As a result, variability in CD uniformity that is dependent on some aspects of the pattern size and density are eliminated. Other pattern-dependent CD uniformity issues arising from process effects are also addressed. Finally, an option to allow the system to print with eight rather than four averaging passes has been evaluated. With an increased averaging of errors, improvement is seen in several performance parameters, particularly stripe butting, CD uniformity, and composite overlay.

I-line optical pattern generators using non-chemically amplified resists have become the workhorses for high throughput mask fabrication. The demand for smaller and more uniform features on photomasks has driven the development of a 257 nm optical pattern generator. A non-chemically amplified resist is being developed to maximize the performance of this new 257 nm mask tool. Resist characterization and lithography simulation are being used to formulate a non-chemically amplified resist for 257 nm optical pattern generators. Non- chemically amplified resists are advantageous for us in mask fabrication due to their storage and post-exposure stability. Chemically amplified resists may provide higher performance but they also require environmental mini-environments and a post-exposure bake equipment not commonly present in mask houses. Diazonaphthoquinone (DNQ)/novolak resists have not been used for DUV Integrated Circuit (IC) applications mainly due to the low sensitivity and the strong absorbance of the DNQ photoactive compound (PAC) at 248 nm. However, a 2,1,4 DNQ based resist has been characterized that bleaches at 257 nm and inhibits novolak. The photoproduct of the 2,1,4 DNQ PAC is much more transparent at 257 nm than 248 nm. Novolak resin is too strongly absorbing for use in formulating efficient 248 nm resists, but novolak has an absorbance minimum at 257 nm that provides transparency similar to poly (hydroxystyrene). Lithography simulation is being used to develop a non- chemically amplified resist to minimize the expensive iteration of manufacturing trials. An exposure system using a 257 nm frequency double Ar laser system has been constructed to study the resist photokinetics. Dill exposure parameters (A, B and C) have been extracted for a 2,1,4 DNQ/novolak based resist. Dissolution rate measurements have been made with a DRM developed at the University of Texas at Austin. Simulation is used to determine the optimal resist absorption, bleaching, dose and dissolution properties to maximize resolution. It is possible to formulate a high performance resist for 257 nm if care is taken in optimization of the formulation.

The use of Plasma Etch in patterning Binary Cr layers for modern reduction reticles has seen dramatic increase in the past two years. The drive towards the 0.25 micrometer and 0.18 micrometer technology has rendered wet etch of Binary Cr inadequate for the demanding gate level designs of most advanced devices. The use of dry etch for these patterns is studied closely through the pattern loading within a mask (Exposed Cr Load). It has been seen that Cr Load strongly affects several plasma etch responses, e.g.: resist selectivity, Cr etch rate, overall CD Uniformity and within- Mask CD Uniformity pattern.

Process optimizations have been done to produce 'Fine Pattern' reticles whose minimum target sizes are under 720 nm. 'Zero Bias' process for binary Cr reticles can be achieved with our dry etching process using ZEP-7000 blanks. MEBES-4500 exposure on resist films of 300 nm and dry etching with Magnetic Enhanced Reactive Ion Etching (MERIE) system are adopted. It is shown that adjusted develop condition weakens thinning effect of resist in sub-micron area due to proximity effect of exposure, and MERIE system with Gas Assist Etching (GAE) also improves pattern resolution. CD shift due to 'Loading Effect' is small and resist patterns are perfectly printed as Cr pattern with negligible deterioration of CD linearity. Much improved CD distribution in the area of 132 mm square can be obtained. In production of MoSiON-based attenuated-PSM (Att- PSM), CD shifts between Cr and MoSiON except OPC patterns (such as serif and scattering-bar) can be estimated about 30 nm. It is also shown that there are few defects after dry etching, and 'Zero Defect' reticles are obtained in most cases. Through the all results, validity and probability of our process to produce 'Fine Pattern Reticles' near the half- micron are discussed.

Decreasing feature sizes combined with high mask error enhancement factors (MEEF) are rapidly causing tighter defect and CD uniformity specifications on photomasks. In general, dry etching photomasks improves feature fidelity but also tends to increase defectivity. Since the first automated mask defect inspection usually occurs after chrome etch, it is difficult to determine if a defect originated with the photoblank or during one of the mask patterning steps (write, develop, and etch). To understand and optimize the dry etch process, After Develop Inspection (ADI) has been developed to isolate the cause of photomask defects. In this study, ADI was used to inspect Cr photomasks incorporating iP3600 and ZEP7000 resists at several thicknesses. The detected defects were analyzed and compared to defects found after etch. A test mask with programmed defects was also created and tested to characterize the sensitivity of this new capability.

Manufacturing of reticles, which combine both OPC and PSM, is becoming more and more challenge. Materials cost is high, several accurate writing processes are needed and repair is almost impossible. This makes inspection a critical and very complicated process. This study describes an inspection of a test vehicle consisting of 55 cells targeted for sub- wavelength design rule technology. This study describes an inspection of the 55 cells test plate targeted for 0.17 micrometer design rule technology. The plate is written on a MoSi layer with 18% transmission for 248 nm lithography. The MoSi has higher transmittance in I-line and G-line that reduces the contrast between the MoSi and the glass (relative to the usual contrast in binary plates). The technique for inspection by Applied Materials RT8000ES 436nm die-to-database is described. The technique is based on expansion of the reduced dynamic range of gray level that results from the lower contracts, re-gaining the inspection capability. This paper reviews the results of G-line versus I-line inspection of high transmission PSM and describes the method of the sensitivity verification including CD defects analysis.

Alternating Phase-Shift Mask (Alt-PSM) is one of the key technologies for 0.15 micrometer or below rule device fabrication. But it is not yet widely utilized because of difficulty on phase controllability and defect controllability. Through more than 7 years at our commercial base operation and feedback from customers, we have improved our Alt-PSM both on its performance on wafer resist image and defect minimization. We have focused on the two elements, defects detection and repair, that made it difficult to control defect quantity on Alt-PSMs. In this paper, we describe results of experiment and optimization method that aims to assure zero defect on Alt-PSM for DUV lithography. We prepared evaluation plates. The plates contain series of programmed quartz defect on 0.6 micrometer line & space, each has phase errors of 60, 120 and 180 degrees at KrF wavelength. We used several latest models of inspection tools to evaluate phase shift quartz defect detectability, which are KLA353UV, STARlight, 9MD84SR(i). Micrion8000 was used as the repair tool. MSM-100/AIMS was used to evaluate wafer CD error of defect area before and after repair. As results, we found that inspection by short wavelength, especially by 9MD84SR(i), was effective for detection of phase shift defects, and that if this method were combined with STARlight inspection, detectability of the phase shift defect would be improved. With combination of this inspection method and our FIB repair, which is optimized for premeasured height of each phase shift defect by use of AFM, we would be able to supply zero defect Alt-PSMs for 0.15 micrometer design rule devices.

In order to perform mask inspection with the high reliability for 150 nm-rule and below devices, the inspection system with high resolution is indispensable. The phase shift masks like DUV HT masks must also be inspected with high sensitivity. A next generation mask inspection system MC-3000 which used DUV optics has been developed, in order to achieve these requirement. The wavelength of this optics is 257 nm that is shorter than that of current UV inspection systems, and is nearly equal to that of current DUV lithography systems. Short wavelength light and high NA optics obtain high resolution, so the defect detection of 130 nm or less is attained. The special issues for the DUV optics were solved by several new techniques. This paper reports the system configuration, basic characteristics for defect detection and inspection performances.

The manufacturing implementation of alternating aperture PSM's (AltPSM) has been gated by the impacts these techniques have on reticle manufacturing, specifically reticle defect inspection and repair. Die-to-die inspection techniques have been achieved for some clearfield multiphase alternate phase reticles, but the required die-to-database solutions are not currently available with defect inspection systems. In response to these mask manufacturing issues and IC design layout issues, resolution enhancing techniques based on Darkfield Alternate Phase (DAP) reticle designs are now of growing importance. A DAP Programmed Evaluation Reticle, DAPPER, was fabricated and inspected on a new high numerical aperture ultraviolet reticle inspection system. The results show reasonable defect sensitivity performance in the presence of both reticle geometry and quartz etch topography characteristic of 130-nm node advanced logic circuit DAP reticles.

As DUV lithography becomes more ubiquitous in the manufacture of semiconductors, the importance of detecting mask anomalies that can be attributed to the exposure of mask materials to 248 nm exposure becomes necessary. The requirement to find and eliminate the sources of these types of defects becomes even more important with low k₁ lithography. The authors wish to report a new class of defects that can significantly impact mask performance and semiconductor chip manufacturing yields. This paper will discuss the techniques and defect detection systems used to identify the presence of these sub-pellicle (or pellicle-related) defects. Additionally, the mechanism of defect formation and micro-analytical results identifying both the composition and possible sources of the defects will be presented.

Low-k₁ lithography requires enhancement techniques like phase shift and OPC. These techniques impose new and challenging specifications on photomasks. A development to establish means and methods to verify corner rounding, line end shortening, defect printability and the size of jogs, serifs and assist lines in a production worthy manner is based on the assessment of mask production data through a new cluster software tool which combines the output data of a mask defect inspection system, a CD metrology system, an AIMS based mask review station and printing simulation results. Possible definitions of new type photomask quality criteria are discussed and measurement procedures are proposed. As a key application the review of critical features on reticles (OPC, classical defects, contact printability, etc.) at changing stepper conditions ((lambda) , N.A., (sigma) ) is discussed. The concept and the development status of a Photomask Qualification Cluster is presented and early performance results are examined against the target values which are a defect detection sensitivity of 125 nm, optical resolution of 200 nm lines for assist line assessment, CD measurement on lines, contacts and OPC structures with 5 nm repeatability and mask pattern fidelity assessment at printing conditions down to 500 nm lines at reticle level.

The Mask Error Enhancement Function (MEEF) serves to amplify reticle errors. This can lead to proximity effects and bias problems that are much larger than would be expected from the normal reduction factor of the imaging system. The economic impact on reticle specifications can be severe. This paper examines the theoretical description of the MEEF for dark features: isolated lines, isolated posts, and dense 1:1 line/space features. MEEF for dense features is found in general to be smaller than 1 over a wide range, while MEEF for isolated features is always greater than 1. This 'MEEF Gap' between isolated and dense features may help to explain the sensitivity of OPC to isolated and dense bias.

Deep Ultraviolet Lithography is slowly gaining widespread use. Although the introduction of 248 nm excimer laser steppers and step&scan systems has been slower than anticipated, all of the advanced wafer fabs are now using two of the three technology platforms in mix-and-match with I-line steppers or scanners. But the aggressive acceleration of the roadmaps, both for device technology as well as for equipment, brings some new and challenging questions into maskmaking, one of the key segments of today's IC manufacturing. Long having been in the discussion as one of the potential roadblocks, we are rapidly approaching the limits of manufacturability, and new ways to write, process, inspect and qualify a mask need to be developed. Although a lot of different efforts are under way, the area that still leaves the most doubts in the end user is mask qualification: does the mask meet the CD requirements, and is it free of any printable (CD) defects? This paper will give an insight on new tools that will be needed to help fully characterizing a mask, and will also try to investigate some of the printability concerns.

With increasing mask error enhancement factors (MEEF), detecting and analyzing photomask critical dimension (CD) uniformity errors is critical for understanding how photomasks can be manufactured to afford high wafer yields. Using UV pattern inspection tools, recent improvements in automated inspection algorithms can now provide CD error detection below 50 nm. This level of sensitivity is necessary in order to provide a clear picture of the reticles' contribution to the final wafer image and possibly function. However, dispositioning CD errors less than 100 nm is very challenging at defect review which has previously resulted in misclassification of true CD errors. Classifying very small errors require the high precision and resolution of a metrology tool. A process has been developed for detecting very small CD errors with a UV inspection tool and the coordinates of areas of concern transferred to a CD SEM for sizing, review and disposition. In this study, we have characterized the sensitivity and false defect performance of a new algorithm with production masks and a new programmed defect test mask. The inspection results were transferred to a CD SEM for analysis. SEM measurements were taken to validate the sensitivity of the algorithm and to quantify the calibration accuracy of the review tools of the inspection system.

In the photolithographic process, critical dimensions (CD) of exposed features in photoresist need to be controlled to within a specified tolerance related to the nominal feature size. A portion of this tolerance budget is consumed by variations in CD on the photomask. At low k₁ factor, a number of parameters in the lithography system impact linearity including lens aberrations, defocus, exposure, partial coherence, and photoresist contrast. The combined effect of these parameters is that errors in the mask CDs are not transferred to the wafer in direct proportion to the optical reduction value of the lithography system. This Mask Error Factor (MEF) becomes a significant problem as it consumes a larger than anticipated portion of the CD tolerance budget. This paper will discuss experimentally evaluated MEF using a 4X i-line stepper for a range of feature sizes from subwavelength to approximately twice the exposure wavelength. A test reticle was built with isolated lines from 200 nm to 600 nm in 12.5 nm increments at 1 X. CD measurements on the reticle were compared to corresponding CD measurements on the wafer in order to establish both linearity and MEF curves for the lithography system. MEF values were also determined across a process window for multiple feature sizes. The MEF was observed to be less than 1.4 for CDs greater than 330 nm (k₁ equals 0.5) throughout the process window. However, the MEF rises rapidly to over 3 for CD values smaller than 300 nm (k₁ equals 0.45) at nominal focus and exposure. Changes in exposure were not observed to have a noticeable impact on MEF while focus offsets were observed to result in significant increases in MEF. These results indicate that MEF has a much larger impact on focus latitude than on exposure latitude. As a result the process window will be compressed more in focus than in exposure.

System architecture choices for an advanced mask writer (100 - 130 nm) have been evaluated. To compare and contrast variably shaped beam vector architecture with raster-based architecture, factors such as beam accelerating voltage and its effects on lithographic performance and system throughput for complex patterns have been studied. The results indicate that while both architectures have strengths and weaknesses, in the final analysis, raster-based systems offer the best combination of benefits to the user in terms of versatility and overall system throughput. Furthermore, other system requirements needed to support the challenges of the next generation mask writers are discussed. An architecture that includes a 50 kV raster graybeam (RGB), based architecture, a new writing strategy, a new stage system, an advanced environmental/thermal control management system, an automated material handling system, and a new resist and process is proposed.

Standardizing on reticle size is critical for semiconductor tool manufacturers and the semiconductor industry as a whole. The advantages of large reticles are well known: larger die and increased throughput. Although predictions of extremely large die have not yet been realized, the throughput implications remain valid. As large reticles have the potential to increase throughput, some proponents view them as potential cost savers. This work examines the implications of migrating from today's standard 6-inch reticles to 9-inch reticles. It explores the factors that drive reticle cost, and describes why larger reticles should cost more. The paper also describes the cost benefit of implementing larger masks. Comparing the expected cost of building the reticle to the potential cost savings of using the reticle in production provides significant insight into the problem of selecting the optimal reticle size. Finally, the paper presents an analysis of the impact of 6X reduction systems on the selection of reticle size.

Membrane masks are commonly adopted for the next generation lithography (NGL) such as proximity X-ray lithography (XRL), ion beam projection lithography (IPL), and SCALPEL. Since the NGL membrane masks have lower mechanical stability than the conventional optical mask, it is necessary to study the distortions of membrane masks during mask fabrication and application. Using various kinds of test patterns such as line and space patterns, contact hole arrays, rectangular island arrays, and square island arrays, the mechanical stiffness changes due to pattern transfer were investigated. The in- plane distortions of the membrane mask due to pattern transfer during mask fabrication are dependent on not only void fraction but also pattern shapes. The membrane masks with various kinds of patterns were fabricated and the in-plane distortions were measured.

Three 4X test reticles for 0.15 - 0.18 micrometer lithography are characterized using a critical-dimension scanning electron microscope (CD-SEM). The reticles, which contain metrology cells of isolated and nested lines, are manufactured with an i-line (365 nm) laser patterning tool and wet etch (reticle A), an e-beam patterning tool and dry etch (reticle B), and an e-beam patterning tool and wet etch (reticle C). We investigate the nature of pattern density-induced CD variations by studying groups of metrology cells with systematically varying linewidths and line spacings. The CD bias between nested and isolated lines is a smoothly increasing function of pattern density on reticles B and C. The CD signatures on reticle B exhibit strong microloading and loading signatures related to the plasma etch process. The bias characteristic on reticle A demonstrates a threshold effect, increasing sharply as the space between adjacent nested lines becomes less than approximately 600 nm. Matching images taken with secondary and backscattered electron spectra support the conclusion that the laser write process is resolution-limited at this pattern density. The SEM data is also compared to data from broad-band optical metrology tools to show that the optical response for dense features becomes unreliable when the feature spacing is less than approximately 600 nm.

In this paper we discuss some of the problems and solutions discovered when implementing 2-mask strong phase shifter designs for the poly gate level in logic designs. Experimental results are presented showing pattern fidelity for different reticle designs. Simulations are presented indicating the improvement in pattern fidelity that can be expected from using OPC. Simulations, PSM assignment and model-based OPC correction are performed by the Calibre WORKbench, Calibre DRC, Calibre PSMgate and Calibre OPCpro tools from Mentor Graphics. In conclusion, we show that while fairly simple designs can be used to achieve 250 nm design rules (approximately 150 nm gates), in order to achieve both pattern fidelity as well as small feature size it is necessary to use 3-layer/phase-aware model-based OPC to correct for pattern distortion for design rules of 180 nm and below (approximately 100 nm phase-shifted gates).

New simplified process is developed successfully for KrF excimer half-tone mask with chrome shielding method. Process times are reduced to half compared with conventional process in which resist coating and exposure is performed twice. New simplified process is characterized by adopting half-exposure of electron beam, thin chrome film on a half-tone film, and endpoint detection in dry etching process. It is confirmed in this paper that half-tone masks fabricated by simplified process have mask quality and optical capability, which are required in 0.18 micrometer generation devices.

Critical dimension (CD) control and resolution requirements of advanced photomasks require a new class of fabrication processes. These include the use of higher contrast resists and low etch bias processes such as plasma etching for patterning chrome films. Previous work has shown that ZEP 7000 resist and ICP dry etching of chrome provide the process latitude needed to meet 180 nm mask requirements and beyond. However, due to the loading effects, the deviation of the CD from the target value is a function of the chrome loading on the plate when using dry etching. Therefore, CD control must occur by varying the exposure dose or the develop time based on the pattern loading of a particular mask level. By understanding the relationships between the change in CD with respect to dose, develop time and pattern loading, models can be created which accurately predict the required parameters to tightly control CD performance independent of dry etch loading effects. In this paper a production process is described which utilizes ZEP 7000 and ICP dry etching. A series of experiments have been run to characterize the change in CD based on both dose and develop time. Then a matrix of experiments were run to determine the effect of pattern loading on CD. A predictive model was generated from the DOE data which accurately predicts the dose and develop time needed to meet the CD targeting requirements for any given mask level regardless of pattern density. The model was then verified on production mask levels of randomly varying pattern density.

The downscaling of critical dimensions (CD) in semiconductor circuits has been pushing photolithography to print features below the wavelength of the light source. However, severe proximity effect and small DOF for isolated lines have brought challenges to sub-0.18 micrometer lithography in manufacturing using 248 nm scanners. To improve proximity effect and DOF for isolated lines, assisting features (AF) on masks are considered. However, the practical application of this technique has been limited because of difficulties in mask fabrication. In this paper, we discuss items that concern both photolithographers and mask-makers as AF is applied in manufacturing. These items include mask error factor (MEF), depth of focus (DOF) improvement, AF line width control, lithographic impact caused by the drift of the mean value of mask CD, defect printability in resist, and defect sensitivity during mask inspection.

Optical proximity effect correction (OPC) now is a prerequisite technique to improve common process windows of the present optical lithography process. It will be also applied to 0.13 micrometer device processes. Although the technology is now widely used, there are still issues to be solved in terms of choice of OPC methods, pattern fidelity specs, software/data processing limitations, mask manufacturing equipments and manufacturing yields with costs. Photomask Japan '99 Symposium (PMJ '99) had a panel discussion this year covering such issues. For 0.13 micrometer generation, critical dimension (CD) accuracy of plus or minus 4 nm on wafer is given for error budget derived from masks, which OPC masks must also achieve. Mask defects with a size of 70 nm must be repaired, which also requires mask repair accuracy of OPC geometry copy features with high performance. As OPC is sometimes applied to the corner part of figure shapes, it is also required to clarify CD specifications for such portions depending on desired pattern layouts. A mask process with zero defects will also be required as a countermeasure of such mask making difficulty. As a consensus, it is also proposed that acceleration of collaboration between device makers, equipment/software makers and mask shops is much required.

Image imbalance refers to the phenomenon that printed image sizes are different between etched and unetched openings if the effective phase and transmission of an alternating phase shifting mask (PSM) are not ideal. The root cause of the phenomenon is the presence of a 0-th diffraction order. When the 0-th order interacts with plus or minus 1 orders, depending on location, it constructively or destructively interferes with the ideal sinusoidal fields generated by the plus or minus 1 orders, causing aerial image difference between the etched and unetched openings. Furthermore, the image imbalance affect changes through focus because optical path length of the 0-th order differs from that of the plus or minus order. An analytical solution of the aerial image has been derived to quantitatively understand the phenomenon. It can be shown that the contributions of phase error and transmission error are orthogonal, thus both errors must be eliminated to eliminate the 0-th diffraction order. The biased etchback approach proposed by Ferguson et al. has been shown to be effective in correcting transmission errors. Detailed implementation of the technique are explored in this presentation. The technique consists of multiple steps of uniform wet etch coupled with aerial image measurements after each etch step. After transmission error is corrected, a phase trimming process may be necessary to improve through-focus image balance.

We propose zirconium silicon oxide (ZrSiO) film as a powerful candidate for attenuated phase-shift mask (Att-PSM) materials. A bi-layer structure of this material with an absorptive film (AF) and a transparent film (TF) can effectively control the transmittance and phase. We confirmed the durability of the ZrSiO film in ArF laser irradiation. The lifetime with the change in transmittance and phase after irradiation at 30 kJ(DOT)cm^-2 is equivalent to a total dose of 3 years in future ArF exposure systems. We investigated the resolution performance of the Att-PSM with ZrSiO film for a 130 nm pattern. The depth of focus with the Att-PSM was larger than that of the binary mask. Therefore, an Att-PSM with ZrSiO is promising for developing a 130-nm-technology node with ArF lithography. Controllability of the critical dimension on the wafer is discussed from the viewpoint of the mask error factor (MEF).

Alternating Phase Shift Masks (alt.PSM) became one of the primary options to keep optical lithography on its fast shrinking path. They promise -- and demonstrated by numerous examples -- lithography with k1 factors down to 0.25, which is equivalent to print structures below 100 nm using established DUV exposure tools. However, lithography at that small k1 factors is a highly non-linear process. So proximity effects are expected, when realistic chip structures have to be printed. In addition to the well-known proximity effects at the use of conventional chromium masks or half-tone phase shift masks, the width of a line printed by an alt.PSM depends also on the geometry of the surrounding phase shifters. The paper confirms, that a simulation-based OPC-tool is able to describe this effect correctly and to correct for it. However, experimental data and simulation results suggest, that there is an additional proximity effect arising from the 3- dimensional geometry of the phase edges. This proximity effect changes the effective phase of a given phase edge from its intended value. It is possible to correct this effect for one geometry, but for arbitrary geometries, phase changes as function of the proximity of phase edges must be taken into account. Based on simulation results of 3-dimensional alt.PSM geometries, we developed a strategy to include all proximity effects of alt.PSM in order to print realistic chip layouts with design rules down to 130 nm.

A 17% transmittance embedded attenuated phase shift mask (ESPM) has been prepared and evaluated in terms of its optical printability of 180 nm to 150 nm hole features at KrF excimer laser light. A 6% transmission ESPM has also been evaluated for comparison. Contact hole features on test reticles were written by a laser writer with the laser proximity correction (LPC). The following effects of the LPC were found: (1) The mask CD is controlled within plus or minus 20 nm from the target for both dense and isolated features. (2) The CD was shifted over -20 nm for isolated eatures. (3) An improvement in CD linearity was observed where the CD error was kept within 10 to 70 nm even when the feature size and pitch were varied. A similarity in the results by Aerial Image Measurement Software (AIMS) and the optical simulator was found especially for isolated features, and this allows us to use the optical simulator instead of AIMS for evaluating the optical properties of EPSMs in some cases. By the Log-slopes of the optical intensity profiles of 180 nm contact hole features, we found that the high transmission EPSM has better Log-slope especially for isolated features. This means that the manufacturability of 180 nm contact holes will be achieved by using the high transmission EPSM. By similar evaluation we found that good manufacturability of 150 nm contact holes will be more difficult to achieve even by using the high transmission EPSM. The variation of CD-focus curves was analyzed for isolated contact hole features using AIMS data. The high transmission EPSM improves the exposure latitude for 180 nm features even with only 20 nm 4x resizing and also for 150 nm features but cannot improve the focus latitude. The Mask Error Factor (MEF) is evaluated by using the optical simulation. It is found to range in 1.1 - 1.5.

Dual trench alternating phase shifting masks with an optimized value of the so-called shallow trench depth represents an interesting approach to overcome aerial image imbalances. In order to get a better understanding of the possibilities and limits of this approach, especially for 5X reduction, theoretical and experimental investigations were accomplished. In this paper experimental data obtained from 5X dual trench type alternating PSMs, using DUV-lithography are introduced and compared with 3D-mask simulations. The masks were fabricated with different etch depths and contain parts of typical DRAM patterns. Besides the transmission balancing also the phase balancing has an important influence on the effective process window of an alternating PSM. The effective phase error can be measured with an AIMS-system (MSM100). The comparison with simulated data allows the determination of the phase error. In a second step the influence of different balancing methods on phase and transmission were investigated with the TEMPEST mask simulator for unpolarized light. The optimization of the balancing with respect to the CD-bias, undercut and etch depth will be shown and a first approach of a sensitivity analysis will be presented.

157 nm lithography is a strong contender for 100 nm and 70 nm technology. Although feasibility studies did not identify any strong evidence that 157 nm lithography was not viable, several technological challenges still must be faced. CaF₂ has been found to be an optical material that is practical for lens and illumination elements. Initially, the use of CaF₂ materials for the reticle substrate was investigated. Its high thermal expansion, however, virtually excludes its use as a mask material. Recent measurements of modified fused silica have shown sufficient transmission of 157 nm for its use in lithographic masking. The residual absorption in modified- quartz masks is expected to give rise to thermal effects, but the choice of a non-Cr-based absorber may reduce the reticle heating effect. When considering mask design rules of 100 nm and 70 nm lithography, CD requirements become more critical. Even with NA values exceeding 0.7, the k₁ value for 70 nm resolution exposed with 157 nm wavelengths will be low. Consequently, the mask error factor (MEF) is expected to increase further resulting in a larger reticle contribution to the overall CD budget. A counter measure for this would be a change in stepper reduction ratio from 4X to 6X. For 6' reticles only, this would result in smaller field sizes; but with 7' or 9' reticles, field sizes could become comparable to those available in current 4X Step & Scan Systems. The advantages and disadvantages of a change in the reduction ratio are discussed.

Modified fused silica is the primary candidate material for 157-nm photomask substrates. Standard UV excimer grade silica, such as Corning HPFS^R, does not transmit below about 175- nm because of its high OH content. In contrast, we have prepared fused silicas with low OH contents and low levels of fluorine with measured transmissions up to 73.8%/6.4 mm and internal transmittances up to 87.9%/cm at 157-nm. Refractive index measurements at 157-nm are presented from which we calculate a theoretical limit for the measured transmission (reflection losses only) of about 88.5% at 157-nm. Modified fused silica is shown to have high resistance to laser-induced color center formation. The thermal and mechanical properties of modified fused silica are shown to be similar to those of standard excimer grade fused silica but not identical. For example, the thermal expansion and Young's Modulus of modified fused silica are slightly lower than that of Corning HPFS^R, while thermal conductivity is the same. Modified fused silica substrates have been shown to behave similarly to standard fused silica substrates in mask-making processes such as polishing and Cr film deposition. In summary, our property and process results support the selection of modified fused silica for the 157-nm photomask application.

In this paper, Dry & F doped fused silica (modified fused silica) expected as material of photo-mask substrate in the 157 nm (F₂ laser) lithography has been evaluated with respect to the optical properties of transmittance, internal loss, F₂ laser durability, index homogeneity and stress birefringence, etc. Obtained internal loss coefficient at 157.6 nm was approximately 0.015/cm (base 10), which was measured by several samples with different thickness. From this coefficient, internal loss was calculated as approximately 3% per 1 cm, 2% per quarter-inch. The uniformity in transmittance inside the sample diameter (120 mm) was nearly less than plus or minus 0.5% per 1 cm, plus or minus 0.3% per quarter-inch. Laser durability test was made by MIT/LL. The samples (3 mm, 20 mm and 40 mm thickness) were exposed to F₂ laser light up to 80 - 520 million pulses with energy density of 0.1 to 1.4 mJ/cm²/pulse. No significant transmittance change was observed (change was less than 1.0% per quarter-inch at 0.1 to 1.4 mJ/cm²/pulse, 520 million pulses). We measured the index homogeneity by using interferometer and the stress birefringence by using phase modulation method. Inside the sample diameter (120 mm), index homogeneity was 150 ppm at 632.8 nm, the distribution configuration of relative refractive index has a central symmetry property. The stress birefringence was less than 5 nm per quarter-inch at 632.8 nm. As the result, we concluded that this new material had enough capability for the mask substrate of 157 nm lithography.

Constraints of the photomask are beginning to play dominate roles in the advancement of new microlithographic technology. Mask substrate materials are being explored by several groups. In this work, we explore the thin film masking layers themselves and identify potential solutions for binary and phase-shift masking films for use at 157 nm. The chromium based absorbing films used for binary masking are likely to meet the required needs. Modification of the composition will be required. Attenuated phase shift masking films become challenging as few transparent host thin film materials exist at the 157 nm wavelength.

The mask is deemed one of the areas that require significant research and development in EUVL. Silicon wafers will be used for mask substrates for an alpha-class EUVL exposure tool due to their low-defect levels and high quality surface finish. However, silicon has a large coefficient of thermal expansion that leads to unacceptable image distortion due to absorption of EUV light. A low thermal expansion glass or glass-ceramic is likely to be required in order to meet error budgets for the 70 nm node and beyond. Since EUVL masks are used in reflection, they are coated with multilayers prior to patterning. Surface imperfections, such as polishing marks, particles, scratches, or digs, are potential nucleation sites for defects in the multilayer coating, which could result in the printed defects. Therefore we are accelerating developments in the defect reduction and surface finishing of low thermal expansion mask substrates in order to understand long-term issues in controlling printable defects, and to establish the infrastructure for supplying masks. In this paper, we explain the technical requirements for EUVL mask substrates and describe our efforts in establishing a SEMI standard for EUVL masks. We will also report on the early progress of our suppliers in producing low thermal-expansion mask substrates for our development activities.

We report on the comparison of defect printability experimental results with at-wavelength defect inspection and printability modeling at extreme ultraviolet (EUV) wavelengths. Two sets of EUV masks were fabricated with nm- scale substrate defect topographies patterned using a sacrificial layer and dry-etch process, while the absorber pattern was defined using a subtractive metal process. One set of masks employed a silicon dioxide film to produce the programmed defects, whereas the other set used chromium films. Line-, proximity- and point-defects were patterned and had lateral dimensions in the range of 0.2 micrometer X 0.2 micrometer to 8.0 micrometer X 1.5 micrometer on the EUV reticle, and a topography in the range of 8 nm - 45 nm. Substrate defect topographies were measured by atomic force microscopy (AFM) before and after deposition of EUV-reflective Mo/Si multilayers. The programmed defect masks were then characterized using an actinic inspection tool. All EUVL printing experiments were performed using Sandia's 10x- reduction EUV Microstepper, which has a projection optics system with a wavefront error less than 1 nm, and a numerical aperture of 0.088. Defect dimensions and exposure conditions were entered into a defect printability model. In this investigation, we compare the simulation predictions with experimental results.

Along with the rapid generation change of lithography and widened application of OPC, the process for High Accelerated Voltage E-Beam reticle is changing from Wet Etching to Dry Etching. Establishment and optimization of Dry Etching process becomes urgent need for mask manufacturers. However, the current resist commonly used for Dry Etching has a problem, which is resist film thickness loss on high pattern density area. Unless we can solve this problem, we can not say the Dry Etching process is suitable for next generation lithography. We selected and evaluated Chemically Amplified (CA) Resist to improve Dry Etching process of high-accelerated E-Beam. At first, as a reference of present quality, we evaluated both Dry Etching and Wet Etching process with a non-CA resist, which is commonly used. As a result advantages of Dry Etching over Wet Etching are confirmed in Linearity, pattern profile and resolution. But the problem of Dry Etching, which is pattern dependent CD variation caused by the uneven loss of resist thickness was also materialized. It is also confirmed that the amount of resist thickness loss is strongly related to pattern density. Then we did basic evaluation about several CA resists. As a result, it was confirmed that CA resist had advantages over non-CA resist on almost all aspects such as Contrast, film thickness loss and cross-sectional profile. And CA resist also solved the problem of resist thickness loss related to pattern density, which we observed when we used non-CA resist. Among several candidates, we selected one CA resist which has high contrast, vertical cross sectional profile, and enough remaining film thickness. We decided that the resist is suitable for Dry Etching optimization, then we did patterning evaluation using this resist. As a result the reticle quality was satisfactory. We could confirm big improvement on both pattern dependent CD variation and pattern profile compared to Dry Etching process using non-CA resist.

The 50 kV electron-beam (EB) writing system HL-800M (Hitachi Co. Ltd.) was developed for 0.25 - 0.18 micrometer design-rule mask fabrication and widely applied. Chemically Amplified Resist (CAR) has merits of high sensitivity, high resolution and dry-etching durability. The combination of 50 kV EB and CAR is one of the best solutions to improve accuracy and throughput of next generation reticles such as 0.18 micrometer design-rule mask and beyond. The purpose of this study is to optimize the exposure and process conditions of the combination of 50 kV EB and CAR for improving Critical Dimension (CD) accuracy. At first, new positive-type CAR; RE515OP (Hitachi Chemical Co. Ltd.) has been evaluated. This resist shows the high resolution of 0.25 micrometer. Because of the vector-scanning EB such as HL-800M, the use of negative-type resist improves throughput of exposure. Negative-type CAR; NEB-22A (Sumitomo Chemical Co. Ltd.) has been also evaluated. This resist shows also the high resolution of 0.14 micrometer. It is clarified that both resists have the characteristic to meet the 0.18 - 0.15 micrometer design-rule mask fabrication. Besides, in order to improve CD accuracy with HL-800M, Proximity Effect Correction (PEC) condition has been optimized. Especially, as parameters of mesh-size and times of smoothing area-density, CD errors are investigated. As a result, CD linearity of 18 nm is obtained in the pattern-widths from 0.7 micrometer to 3 micrometer.

The national metrology institute of Germany, PTB, offers traceable, two-dimensional pattern placement calibrations of microstructures on masks. Whereas the measurement uncertainty for the calibrated pattern positions in terms of traceable coordinate values is only comparable to the position tolerances actually required for mask patterning, i.e. about 35 nm, the deviations from the design grid can be determined with smaller uncertainties. In calibration certificates offered by PTB, these differences are taken into account and the uncertainties for the length of the grid and the uncertainties for the position deviations from the design grid are quoted separately as 30 nm and 10 nm respectively. Developments to further reduce the uncertainties are under way. An analysis of photomask recalibration results shows the long-term reproducibility of PTB length calibrations to be better than 15 nm or 1*10^-7, including substrate stability effects. Details of the recalibration results will be given. Leica Microsystems Lithography runs a laboratory for two-dimensional pattern placement calibrations, formally accredited by PTB within DKD, the German calibration service. Results of recent comparison measurements between the DKD laboratory and PTB on masks of different size will be given and discussed as an example of the process of dissemination of the length unit to industry.

The inorganic antireflection coating (AR3-chromium oxide) commonly used on photomask blanks was designed to minimize flare in h-line (405 nm) lithography steppers. The reflection of light (flare) off this coating (air-photomask) increases with shorter exposure wavelengths. High levels of flare occur in 248 nm and 193 nm IC steppers due to reflections off of the photomask surface. The reflections (standing waves) in photomask resists also increase during exposure using lower wavelengths of light. Lithography simulations and photomask manufacturing trials have led to resist processes that can generate sub-0.30 micron resist features on photomasks with I- line optical pattern generators (ALTA 3500). Lower developer concentration, higher exposure doses and the minimization of standing waves by incorporating a post-exposure bake and/or organic antireflection coatings (ARC) maximizes resolution. High resolution photoresists show standing waves on photomasks fabricated with optical pattern generators. Low contrast resist processes show only small standing waves or in certain cases resist 'footing.' The use of organic antireflection coatings can minimize standing waves and allow the use of high contrast resist processes. ARCs reduce the swing ratio, which improves linewidth uniformity. ARCs also improve the adhesion of the resist to the photomask surface. Simulations reveal that the optimal ARC coating thickness is around 46 nm for typical I-line systems and around 50 nm for 257 nm non- chemically amplified photomask resists. Preliminary I-line photomask manufacturing trials have been done with bottom antireflection coatings at the DPI Reticle Technology Center. The first process trials reveal that the standing waves in high resolution resists were reduced but not removed. Very precise control of the ARC thickness must be exercised to completely extinguish standing waves and careful film thickness optimization appears to be necessary to minimize these reflections.

The IBM EL-4+ maskmaker, installed in the Advanced Mask Facility in Burlington site has been undergoing extensive evaluation of its ability to fabricate advanced 4X chrome on glass (COG) reticles. In previous years on 1X x-ray masks, the EL-4+ maskmaker has demonstrated image placement of 20 nm 3(sigma) , minimum feature size to 100 nm, image size uniformity to below 10 nm while maintaining availability of 97% during the last 6 months of 1998. So far, in 1999, the EL- 4+ system has demonstrated minimum etched features to 100 nm in COG using ZEP7000 resist and dry etching. Promising results have also been achieved using a gate level 0.18 micrometer ground rule mask as a learning vehicle. COG learning is continuing, with indications that we should be able to achieve 35 nm image placement. There are a number of significant advantages to be realized in COG reticle fabrication on the EL-4+. Resolution, coupled with a 1/640 tool grid permits features to be patterned with sharp corners and precisely located edges without any impact on writing speed. An advanced proximity correction algorithm exploiting the systems dose modulation capabilities insures image fidelity across a wide spectrum of pattern density.

Photocathodes have been proposed as a technology for achieving multiple independently modulated electron beams for advanced electron-beam lithography applications. Thin-film gold photocathodes are of particular interest because they offer the potential of practical multi-beam electron sources that can be easily fabricated using thin-film deposition techniques without special environmental or handling techniques to achieve the desired stability and performance. A series of gold thin films were deposited on sapphire, using a developed baseline process and subsequently studied for photoemission in the 200 to 300 nm range. The study focused on the impact of surface chemistry, thermally driven microstructural changes, and adsorbed gases on photoemission. We have achieved current densities between 1 - 4 amp/cm² with electron energies spread of less than 0.5 eV (laser illumination wavelength of 257 nm). The maximum current values, stability and minimum emitter spot size are limited primarily by excess temperature rise in the film caused by the laser illumination. In this paper we propose time, temperature and environmental dependent degradation mechanisms of gold thin film photocathodes which verifies their potential for advanced electron-beam lithography applications.

The incorporation of laser proximity correction into mask production is presented. The ALTA 3000 has been chosen for demonstration. The goal is to improve pattern fidelity of the ALTA 3000 to a level comparable to the ALTA 3500. This provides the possibility to shift production from the ALTA 3500 to the ALTA 3000 and extends the lifetime of an ALTA 3000. The paper focuses on demonstrating different applications and the incorporation into the standard mask production flow.

Analysis of pattern placement errors has shown a pattern and exposure sequence dependent component of placement error exists that cannot be accounted for by beam and stage positioning errors alone. The interaction of the electron beam (e-beam) with the resist can cause displacement of the e-beam from its desired position. This is commonly referred to as mask charging. A special pattern was created to enhance this effect in order to study its functional dependencies, including resist thickness, resist type, and exposure sequence. These errors are noticeable when writing in multipass strategies and where there are large gradients in pattern density. Customer and acceptance test patterns are included in the matrix to determine the magnitude of the errors with more production-oriented patterns. To further characterize this placement error phenomenon, the MEBES e-beam column was modified to minimize the distance between the exit location of the electrons from the electron optics and the surface of the resist-coated mask. Preliminary test results indicate pattern placement error is reduced by approximately 25% with this 'reduced gap' design. We are currently assessing the long-term effects of this new design. Of more importance, choice of resist and process are key components in reducing the charging effect. Reduction of more than 50% in placement errors using ZEP 7000 resist is detailed in this paper.

Residual linear birefringence is an important property for quality control of optical components used in optical lithographic instruments. We present in this paper a sensitive instrument to measure both the magnitude and angular orientation of residual linear birefringence in optical components, including photomask blanks. A variety of optical components are studied using this instrument. We will focus on the birefringence measurement of photomask blanks in this report. We have identified different patterns and levels of residual linear birefringence in these samples. We hope that the data provided in this paper will be helpful to the suppliers of photomask blanks, photomask manufacturers, and producers of optical microlithography instruments.

We have developed two puddle developers for ZEP7000. These developers are the mixtures of solvents having almost the same boiling points at higher temperature. Developer A and B are vaporized much slower than ZED500. Excellent CD uniformity is expected with them. Developers A and B are a stronger and a milder developers than ZED500, respectively. In this report, we focus on Developer B, taking the request of high resolution into consideration. We report the evaluation results of Developer B in comparison with ZED500.

The film thickness estimation is one of most important subject for the design up of photomask blanks. For control of photomask specifications, the thickness measurement technology is the key of well-designed photomask blanks. The grazing- incidence X-ray reflectivity technology is very useful in order to measure thickness, density and interface roughness of photomask blank. In this paper, we reported the adaptation of the X-ray reflectivity technology to photomask evaluation. A Cr and ZrSi oxide thin photomask blanks were prepared with DC sputtering method. The X-ray reflectivities of those photomask blanks were measured with RIGAKU ATX-E diffractometer system. The thicknesses and densities of the photomask blanks were calculated with RIGAKU XDD degree(s) program. The coincidence between the calculated X-ray reflectivity curves with surface modified model which was defined with a surface oxidized layer is better than the calculated results without a surface oxidized layer. These results indicate that the photomask blanks have a surface oxidized layer. Thicknesses of the same position with same films were measured by ordinary technologies, for example, microscopic observation and thickness measurement technologies. The accuracy of those layer parameters which were calculated with X-ray reflectivity curves were better than those parameters measured with other microscopic and thickness measurement technologies.

As optical lithography will be extended to device generations below 150 nm of design rule, the critical dimension (CD) uniformity on a photomask is required to be as small as 13 nm (3(sigma) ). The relationship between development method and dark erosion is discussed in view of CD variations. The temperature variation on a photomask is found to be more than 4 degrees Celsius for a conventional spin-spray development. An optimized process using a 'puddle process' to minimize developer temperature effect and non-uniform spray pressure effect on CD variation. The CD uniformity using the optimized process brings 11 nm in 3(sigma) over 127 mm X 127 mm area on a 6-inch mask.

We have been developing novolak-based chemically amplified positive resists for the next generation photomask fabrication. In this paper, we report two different types of EB resists: RE-5150P and RE-5160P. Our resist materials consist of four components: a novolak matrix resin, a polyphenol compound, an acid generator and a dissolution inhibitor. We applied two different types of dissolution inhibitors to our resist materials. RE-5150P and RE-5160P employed respective a high and a low activation energy type of a dissolution inhibitor. RE-5150P has high contrast and RE- 5160P has wide process window. As a result, we confirmed RE- 5150P could achieve 0.24 micrometer line-and-space vertical resist pattern profiles at 8 (mu) C/cm² using the 50 kV EB- writer HL-800M, and RE-5160P has wide process window: post exposure delay stability is over 24 hrs. and post coating delay stability is over 30 days.

To further extend the lifetime of optical lithography, stronger requirements are placed on the optical materials used in stepper systems. For example, photomask requirements from mask makers and users include good transmission uniformity, low induced absorption, low defect density and low thermal expansion. However little emphasis and attention has been placed on the magnitude of birefringence. In this paper we present results on the dispersion of birefringence in fused silica photomask substrates measured from the visible to DUV wavelengths. An examination of the wavelength dependent stress-optic response that produces birefringence is performed and found to increase from 633 nm to the DUV. Measurements were performed on substrates with levels of birefringence that varied by an order of magnitude. Birefringence in photomasks results in a change in the polarization of an optical beam and for some lithography tools this can influence system performance and degrade image quality. Therefore, we believe an emphasis on low birefringence photomask substrates can possibly improve system performance.