Hee-Sun Yoon

Manufacturability evaluation of model-based OPC masks

A systematic method for the model-based optical proximity correction in presented. This is called optical proximity effect reducing algorithm (OPERA) and has been implemented to TOPO, an in-house program for optical lithography simulations. Comparing simulational results as well as experimental results, we found that OPERA is not only suitable for shape restoration but also for resolution enhancement. However, the resulting optimized patterns have a high degree of complexity and this brought up a number of issues for mask manufacturing. First, data volume and exposure time were dramatically increased for conventional e-beam file formats. This was solved by using the MODE6 format that preserves data hierarchy. Second, due to excessive shot divisions, a variable-shaped beam machine could not finish the exposure process. A raster-scan beam machine successfully finished the exposure. Finally, a die-to-die inspection was performed but many false defects that do not affect wafer printing were defected. This will be solved by a new type of tool that inspects a mask by evaluating its aerial image.

Study of characteristics and control of haze contamination induced by photochemical reaction

Haze is a kind of surface contamination on photomask and lithography optics that made by photochemical reaction. There are many problems in photomask manufacturing, inspection and lithography process because of slowly growing feature of haze. In the photolithography process, the wafer damage has been occurred due to the time dependent growth of haze. In this study, we identified the origin and formation mechanism of haze using accelerated contamination experiments, also developed control method for haze, in which the removal efficiency was confirmed by mass production of photomask. From these results we expect that the photocontamination control technology should be developed and been an important part of NGL technology.

Evaluation of the attenuated PSM performance as the shifter transmittance and illumination systems

Phase shift mask (PSM) with optical proximity effect correction (OPC) is the efficient tool of the resolution enhancement technology (RET). Particularly, it is commonly known that the attenuated PSM(att.PSM) has some merits in the memory device with a repeated cell patterns. But there are only a few results of what illumination systems affect the performance of the attPSM and how much the transmittance of the attPSM affects the lithography performance -- such as resolution and depth of focus. In this paper, we will present the optimized illumination systems for patterning with the attPSM, and the relationship between the transmittance of the attPSM and the lithography performance by using simulations and experiments. The resolution of KrF lithography with the Hi-T att.PSM can be extended to 130 nm through the simulation. We extend the resolution of KrF lithography with the Hi-T att.PSM to 150 nm through the experiment.

Improvement of shot uniformity on a wafer by controlling backside transmittance distribution of a photomask

CD (critical dimension) uniformity on a wafer is affected by several factors such as resist coating, bake, development, etch processes, scanner performance, and photomask CD uniformity. Especially, shot uniformity or in-field CD uniformity is strongly dependent on scanner and photomask. CD error of a photomask and imaging error of a scanner lead to nonuniformity of in-field linewidth distribution. In this paper we propose and demonstrate a shot uniformity improvement method. The shot uniformity improvement method described in this paper utilizes the original shot uniformity map and dose latitude to determine the distribution of illumination intensity drop suitable for correcting CD error on the wafer. The distribution of illumination intensity drop is realized by controlling pattern density of contact hole pattern with 180° phase on the backside of the photomask. We applied this technique to several masks and it was found that global CD uniformity could be excellently improved by the method.

Simulation of optical constants range of high-transmittance attenuated phase-shifting masks used in KrF laser and ArF laser

Phase-shifting masks (PSMs) have provided us a breakthrough in the future semiconductor industry by extending lithography further to the submicrometer order. PSMs have been used over the past several years, and their requirements have changed due to the development of semiconductor technology. We investigated high-transmittance attenuated PSMs (HT-Att-PSMs) that satisfy the requirements of 20§ 5% transmittance and 180 ‐ phase shift at the exposure wavelength and less than 40% transmittance at the inspection wavelength. Regarding the wavelength, we targeted the inspection wavelength of 248 nm for ArF laser (exposure wavelength of 193 nm) HT-Att-PSM, and 365 nm for KrF laser (exposure wavelength of 248 nm) HT-AttPSM. In this study, we developed refractive index-extinction coefficient-thickness (n-k-d) charts showing optimum optical constant ranges for HT-Att-PSM using the matrix method. The simulation was verified by comparing calculated transmittance data with measured ones.

Impact of mask CD error on wafer CD error at low-k1 photolithography

With the reduction of feature size, until probably 0.13 micrometer generations, it is going to be patterned using a DUV illumination source at 248 nm. In these conditions, device development and manufacturing are occurring at ever decreasing k1 factor [k1 equals CD(NA/(lambda) )]. In this low-k1 region, degradation of image contrast brings about nonlinear amplification of mask CD error during image transferring on wafer. This phenomenon (ER; Error Ratio) is severely occurred when critical dimension is smaller than 0.5((lambda) /NA). In this paper, we investigated this phenomena with various condition such as critical dimension, density of L/S, mask type, and phase shift mask. Error ratio at defocused condition is also investigated. From this viewpoint, we discussed effect of mask CD error on wafer CD error and presented newly revised CD control criteria in each device generation. In addition, several strategy which should be considered for reducing error ratio and a photomask process technology for making next generation photomask satisfying high resolution and good CD control requirements has been described in view of low k1 lithography era.

Analysis of mask CD error by dose modulation for fogging effect

Mask critical dimension (CD) errors are analyzed in case fogging effect is corrected by dose modulation method with comparison of measurement and simulation. In the test mask, an extreme condition from pattern density 0% to 100% is applied for making fogging effect. On the ground of the utmost pattern densities which is one of the factors of fogging effect, various mask CD errors are observed with optical measurement in spite of fogging correction. Each error factor is distinguished from whole mask error using electron beam simulator which is adopting Monte Carlo (MC) calculation for electron scattering modeling, proximity effect correction (PEC) and even fogging effect correction. From error analysis, 3 kinds of mask error are observed. The first CD error is from an inaccurate modeling of fogging effect, the second is from fogging correction program. The third is error from development loading effect. The two formers are comparatively less important than the latter because they can be soluble problems by careful selection of fogging model or improvement of computing systems. However, error from develop loading effect is hard to solve so that not only chemical but also fluid mechanical approach is needed.

Fogging Effect Consideration in Mask Process at 50 KeV E-Beam Systems

To achieve higher resolution and critical dimension (CD) accuracy in mask fabrication, 50KeV E-beam systems are used widely. However, as a high acceleration system is adapted, the degree of fogging effect caused by multi-scattering electrons becomes more serious. Although considerable efforts have been made, fogging effect cannot be removed perfectly, therefore several compensation techniques are applied instead. Fogging effect not only deteriorates CD uniformity but also makes mean to target (MTT) control difficult. Moreover, Fogging effect causes proximity effect correction (PEC) error according to PEC methods such as dose modulation type usually used in variable shaped beam (VSB) system and GHOST type commonly used in Gaussian beam system. In this paper, we investigated the fogging effect under the various exposure conditions at raster scan Gaussian beam system and VSB system experimentally and analytically.

Chromium aluminum oxide films for ArF line high transmittance attenuated phase shifting mask application

Phase shifting masks (PSMs) provide us with a breakthrough in the future semiconductor industry by extending submicrometer lithography further. PSMs have been used for several years, and their requirements are changing due to the development of other technologies. First, it became possible to use a high transmittance attenuated PSM (HT-Att- PSM) because of improvement in the photoresist technology. Second, continuous improvement in the inspectional equipment technology lowered the inspection wavelength. Now, we are provisionally targeting to make HT-Att-PSM having 20±5% transmittance at the ArF line and to choose the KrF line as the inspection wavelength. We simulate the optimum conditions for HT-Att-PSM using the matrix method. The simulation is performed to find the optimum range of the variables that yield 180 deg phase shift and 20±5% transmittance at the exposure wavelength, and less than 40% transmittance at the inspection wavelength. Based on the simulation results, we find the optimum fabrication condition of HT- Att-PSM. Using measured optical constants as a function of film composition, we determine the optimum film composition yielding 20±5% transmittance and 180 deg phase shift at the exposure wavelength and below 40% transmittance at the inspection wavelength. As a result, Cr2Al8O15 film with thickness of 124 nm is found to be suitable for an ArF line HT-Att-PSM.

Identification of defect source to control reticle defect density for CAR and dry etching in the photomask process

As the design rule of lithography becomes smaller, printability of reticle defect to wafer is critical for the photomask manufacturing technology. In order to improve the controllability of reticle defects, inspection and repair systems are expanding their capability by continuously modifying hardware and software. This is a good solution to detect and review the defect but it is indirect approaching to reduce the defect in the photomask process. To produce the photomask of defect free or low defect density, effort is needed to improve the capability of defect control in the mask-making process and to evaluate the source of hard defect as well as soft defect. In this paper, we concern the defect source and the feature of printed defects in photomask manufacturing steps. We also discuss the efforts to eliminate the defect source and to control the mask-making process with low defect density. In order to eliminate the source of defects, we partition the mask-making process with defect inspection system, SLF27 TeraStar and Lasertec MD2000, and review a defect shape with CD SEM and AFM. And we compare printed defects, which exist in each process steps, after dry etching process.