Yongdae Kim

Evaluation of shadowing and flare effect for EUV tool

One of the major issues introduced by development of Extreme Ultra Violet Lithography (EUV) is high level of flare and shadowing introduced by the system. Effect of the high level flare degrades the aerial images and may introduce unbalanced Critical Dimension Uniformity (CDU) and so on. Also due to formation of the EUV tool, shadowing of the pattern is another concern added from EUVL. Shadowing of the pattern will cause CD variation for pattern directionality and position of the pattern along the slit. Therefore, in order to acquire high resolution wafer result, correction of the shadowing and flare effect is inevitable for EUV lithography. In this study, we will analyze the effect of shadowing and flare effect of EUV alpha demo tool at IMEC. Simulation and wafer testing will be analyzed to characterize the effect of shadowing on angle and slit position of the pattern. Also, flare of EUV tool will be plotted using Kirks disappearing pad method and flare to pattern density will also be analyzed. Additionally, initial investigation into actual sub 30nm Technology DRAM critical layer will be performed. Finally simulation to wafer result will be analyzed for both shadowing and flare effect of EUV tool.

Feasibility of EUVL thin absorber mask for minimization of mask shadowing effect

Conventional EVUL mask has 80nm absorber height which brings considerable shadowing effect. H-V CD bias of 40nm line and space by shadowing effect is more than 4nm, and that is expected to increase much more for narrower patterns by simulation. However various reports have been presented on mask shadowing bias correction, experimental results are not reliable to derive required mask bias correctly. Even more difficulty will arise when complex 2D structures are taken into account. Therefore minimization of shadowing effect by reducing absorber thickness is desirable. To transfer EUV lithography from experimental stage to HVM era, we need to find optimum absorber height of EUVL mask which allows us less shadowing effect with minimum loss of process window. In this paper, we present optimal absorber height of EUV mask which has been found in terms of shadowing effect and process window by simulation and exposure. To find minimized absorber height experimentally, we will compare the printing result of conventional and thin mask stack using simple 1:1 line and space and island patterns. Simulated H-V CD bias and process window will be presented.

Performance in practical use of actinic EUVL mask blank inspection

A high-volume manufacturing (HVM) actinic blank inspection (ABI) prototype has been developed, of which the inspection capability for a native defect was evaluated. An analysis of defect signal intensity (DSI) analysis showed that the DSI varied as a result of mask surface roughness. Operating the ABI under a review mode reduced that variation by 71 %, and therefore this operation was made available for precise DSI evaluation. The result also indicated that the defect capture rate was influenced by the DSI variation caused by mask surface roughness. A mask blank was inspected three times by the HVM ABI prototype, and impact of the detected native defects on wafer CD was evaluated. There was observed a pronounced relationship between the DSI and wafer CD; and this means that the ABI tool could detect wafer printable defects. Using the total DSI variation, the capture rate of the smallest defect critical for 16 nm node was estimated to be 93.2 %. This means that most of the critical defects for 16 nm node can be detected with the HVM ABI prototype.

A Study of Flare Variation in Extreme Ultraviolet Lithography for Sub-22 nm Line and Space Pattern

One of the critical issues in extreme ultraviolet lithography (EUVL) is flare, which is an integrated light scattering from surface roughness in the EUVL optical system. Flare degrades the control of critical dimension (CD) uniformity across the exposure field. Also, it generates larger CD sensitivity as line and space (L/S) half pitch size decreases. Therefore, we discussed the calculation of accurate flare maps to compensate for flare variation. The influence of three-dimensional (3D) mask topography on flare was investigated with different absorber thicknesses, off-axis illumination angles, and azimuthal angles. Some types of dummy patterns were found to be effective in controlling the flare variation within a L/S patterned target and the average flare of a L/S patterned target. Our studies has definitely made progress in an effective flare variation compensation using a rule-based correction for sub-22 nm L/S half pitch node and beyond.

EUV mask inspection tool using high NA DUV inspection tool

A new inspection system with DUV laser beam and high NA optic for EUV mask has been developed to inspect defects on EUV blank mask and defects by process and handling. The development of new reflective image and optics has increased inspection speed on EUV mask before absorber etch and after absorber etch. Defect classification and operation has increased the productivity of inspection and particle control on EUV mask process. With this new inspection system, defects on blank mask, after resist develop and after etch processed mask were classified and evaluated to install EUV mask process. And defect sensitivities according to various pattern size and process steps were evaluated with required defect size of simulated printing effect on wafer. Designed defect pattern of 46nm node were prepared. Blank masks from Hoya were used. Patterns were exposed using 50KeV electron beam writer. After resist develop, patterns with program defect were inspected. After absorber etching, defects were inspected and evaluated. According to sub film, inspection condition was optimized. Using simulation tool, defects printability were simulated and compared with sensitivity of this inspection tool. Our results demonstrate that this inspection tool is very effective to detect and identify defects and their sources on EUV mask process. In this paper, mask inspection performance of high NA, DUV optic with short working distance was evaluated and described on programmed EUV mask.

The source of carbon contamination for EUV mask production

As the semiconductor industry requires lithography suitable for 32-nm node, extreme ultraviolet lithography (EUVL) has the potential to provide this capability for the mass fabrication of semiconductor devices. But because an extreme ultraviolet (EUV) lithography exposure system is operated in vacuum, during irradiation by EUV light, hydrocarbons are decomposed in vacuum1-3, for example, by the out-gassing from EUV mask, and contaminate the surface of imaging optics which is coated with Mo/Si multi-layers with carbon. Thus, this contamination not only reduces the reflectivity of the Mo/Si multi-layers of imaging optics and degrades the exposure uniformity, but also degrades the resolution of the imaging optics. In this study, as we examined the volume of the out-gassing and the species from EUV mask after every process for EUV mask production, we will control the carbon contamination of EUV mask. Keywords: EUV, carbon contamination, reflectance, out-gassing

Analysis of phase defect effect on contact hole pattern using a programmed phase defect in EUVL mask

Influence of phase defect on printed images of mask pattern was experimentally investigated by printing contact hole (CH) pattern of hp 32 nm on wafer. And the experimental results were compared with the simulation results. A test mask prepared for this experiment contained programmed phase defects of 92 nm ~ 34.8 nm in width and of around 0.68 nm ~ 1.65 nm in depth. The defects were arrayed in a way such that the pitch of the array would differ from the pitch of the absorber contact hole pitch. Therefore, the phase defects were placed at different positions relative to those of the CH patterns. Mask patterns were printed on wafer using an exposure tool NXE3100 with a numerical aperture (NA) of 0.25 and a reduction of 4X. To evaluate the printed patterns affected by the phase defects, circular illumination was employed. The incident angle of mask illumination chief ray was 6 degrees. The printed CH patterns were measured by SEM. An influence of resolution limit of the resist pattern did seem to appear in this experiment, to be a quantitative difference between the simulation and experimental results, the relative location dependence was quite noticeable and the effect of a phase defect was mitigated by covering the defect with an absorber pattern.

Optimized Multigrid Strategy for Accurate Flare Modeling with Three-Dimensional Mask Effect in Extreme-Ultraviolet Lithography

Extreme-ultraviolet (EUV) lithography has many critical challenges regarding its implementation in the semiconductor industry. One of the main challenges is flare, the unwanted total integrated light scattering at the wafer level, which reduces the critical dimension and imaging performance. Therefore, EUV flare has been intensively studied and has been compensated by a rule-based method for many years. However, there are few results with regard to developing more accurate and feasible flare-modeling techniques to enable us to satisfy the criteria of the sub-22 nm half pitch (HP) technology node and beyond. In this work, we studied an improved flare-modeling technique considering the interaction of scattered EUV light with a three-dimensional EUV mask topography in order to obtain an accurate flare distribution and an optimized multigrid strategy for generating a flare map over the full-field scale. Also, we proposed a flare-modeling technique based on the pedestal model, which we developed using novel effective reflection coefficients in order to achieve sufficient accuracy. Such an approach is thought to be needed instead of the conventional pattern density approach in preparation for upcoming advanced HP technology nodes or for different absorber materials and illumination angles. Lastly, the need for a flare map shift to compensate for the mask defocus error is introduced and some flare evaluations of mask patterns used in the exposure-dose-monitoring technique were performed.

Feasibility of EUVL thin absorber mask for sub-32nm half pitch patterning

EUV lithography is the leading candidate for sub-32nm half-pitch device manufacturing. EUV Pre-Production Tool (PPT) is expected to be available at the end of 2010. As EUVL era comes closer, EUVL infrastructure has to get mature including EUVL mask stack. To reduce HV CD bias which comes from shadowing effect, thin mask stack has been considered. We presented that EUVL mask with 58nm absorber height shows same printing performance with conventional EUVL mask with 80nm absorber height in our previous work. CD change and pattern damage at the exposure field edges due to light leakage from the neighboring fields were also demonstrated. In this paper, optimal mask stack which shows lower H-V CD bias than conventional structure using 70-nm-thick absorber is proposed. To find minimized absorber height for sub-32nm pattering experimentally, printing result of conventional mask and thin mask stack with 1:1 L/S patterns will be compared. Further-on, we demonstrate the printing result of the reticle which is designed to minimize CD error at the exposure field edges due to mask black border reflectivity by reducing reflectivity from the absorber. All the wafers are exposed at ASML Alpha Demo Tool (ADT) and Pre-Production Tool (PPT) S-litho EUV is used for simulation.

An explosive haze formation under low energy exposure

Haze formation on reticle continues to be a significant source of concern for the photolithography. Possible sources and causes continue to be investigated. This paper provides a haze source evaluation result under the sub-pellicle defect on the mask. It is well known that there are several sources to produce the haze. One is inorganic molecules such as SOx, NH3, H2O and CO2. The haze formation of inorganic sources is promoted for growing defect size by the exposure energy in time. The other is organics that are prevalent Fab and storage environment. In this paper, we deal with the haze that is immediately generating with a low energy exposure. Especially, this study treats the haze source during the mask packaging method.