Yongkyoo Choi

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.

The study to enhance the mask global CD uniformity by removing local CD variation

As pattern size is shrinking, required mask CD specification is tighter and its effect on wafer patterning is more severe. Recent study showed that the effect of mask local CD variation of mask on wafer is much smaller than that of global CD variation.[1] To enhance the device performance, wafer CD uniformity should be enhanced and controlled by mask global CD uniformity. Mask global CD uniformity usually can be enhanced by mask process and optimal fogging effect correction. To enhance the mask global CD uniformity on mask, resist process and FEC (Fogging Effect Correction), reliable CD measurement tool and methods are necessary. Recently, group CD using OCD(Spectroscopic Ellipsometer) or AIMS(Aerial Image Measurement and Simulation) or polynomial fitting method is introduced to represent global CD variation on mask.[2][3][4] These methods are removing local CD variation on mask. The local CD variation will be remained as residual CD after approximation. In this paper, local CD variation of mask and wafer is evaluated and 2 kinds of methods are used to measure CD on mask and wafer, and the correlation of global CD of mask and field CD of wafer are evaluated. And the repeatability of field to field CD uniformity of wafer is evaluated to correct the fields CD uniformity of wafer by controlling the selective changing of transmittance of mask or to feed back to mask process. Higher correlation between fields of wafer, more accurate correction can be possible.

Application of review-SEM to high-resolution inspection for 3xnm nodes

As the pattern size shrinkage, it becomes more important to control the critical size of various pattern shapes at a semiconductor production line. Recently, in a semiconductor process with 20 nm nodes size or less the common optical and even EB inspection tool have considerable limitation to detect critical physical defects. From these backgrounds, we have developed the high-sensitivity fixed point inspection tool based on Review-SEM as the product accomplishment judgment tool for below 10nm size defects on critical size devices. We examined the basic performance of this inspection tool, optimized inspection parameters including beam condition and image processing. Then, the defect detection performance was evaluated using various real advanced memory device containing various critical defects. In this paper, we report these results and show the effectiveness of this inspection tool to the advanced memory devices.

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 CD measuring repeatability enhancement by intensity gradient

As required CD (critical dimension) measuring accuracy is tighter, it is necessary to enhance the repeatability of CD-SEM on photo-mask, by optimizing charge up, scan speed, beam size, acceleration, current and temperature control. CD-SEM shows sparkle noise which degrades the image of CD-SEM. And defocus is also getting the source of worse gauge R&R. We evaluated the effect of defocus and noise on CD repeatability by extracting CD from gradient value of image after anisotropic nonlinear diffusion filtering on SEM image. As SEM image is measured after averaging the intensity of image on range of interest (ROI) to remove scan noise, anisotropic nonlinear diffusion (AND) which has different diffusivity according to direction, is efficient tool to get smooth pattern without averaging. This smoothing technique is effective in measuring isolated pattern on mask which is difficult to measure around corner. Some simple CD measuring algorithms are available to get better CD repeatability. Using the maximum intensity and gradient of image, we were able to measure CD on various shaped patterns with enhanced repeatability.

Improvement of develop loading effect in the FEP-171 process

CD control of FEP-171 is difficult due to abnormal CD variation in single puddle development. Fogging analysis patterns show that space CD in large open region is smaller than that in small open region. That is caused by develop loading effect, which is largely affected by developer dispensing condition in single puddle method. Double puddle development improves the abnormal CD trend and has a good CD uniformity (3sigma 5.4nm). Contrary to FEP-171, abnormal CD trend is not observed in UV-82(CAR). The low dissolution rate of FEP-171 is the cause of the abnormal CD trend. The yield of CD uniformity and MTT increases with applying double puddle method to FEP-171 process.

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

CD measurement evaluation on periodic patterns between optic tools and CD-SEM

As feature size is shrinking and MEEF (Mask error enhancement factor) is increasing, CD measurement accuracy is more important, and CD SEM is widely used to replace optic tools because of their resolution. But CD-SEM is not representing the effect of Cr profile or transmittance of light which is transferred to wafer. Recently, new OCD (optic CD) tool which use scatterometry (Spectroscopic Ellipsometry) *1) is introduced to compensate the demerit of SEM of low through-put and reflected surface information of mask. This scatterometry tool can be used only on periodic pattern like DRAM. And this tool must be calibrated on each pattern type and shape. This calibration is the barrier to use this scatterometry method to mask process where all masks are processed one time. In this work, new optical CD measurement method which use conventional optic microscope of transmitted and reflected light with high resolution lens of DUV on periodic patterns is introduced. To enhance the accuracy of measurement, interpolating method and FFT (Fast Fourier Transform) are used. CD measurement results of linearity by optic CD, SE and CD-SEM were compared on several patterns. And CD variations on full field of image were evaluated on L/S patterns and active layer of DRAM.

Enhancement of CD uniformity and throughput with KrF photomask repeater

It is intended to clarify the feasibility of 0.15μm generation mask fabrication with the photomask repeater that is based on a KrF stepper(step-and-repeat exposure system). In a photomask repeater patterning, a daughter mask is exposed to KrF light through a mother mask. Inter-field registration accuracy(3sigma) is 14.9nm in X direction and 29.1nm in Y direction within a 100mm×100mm area on a daughter mask and intra-field registration accuracy(3sigma) is 21nm in X direction and 26nm in Y direction within a 18.4mm×23.0mm field on a daughter mask. The registration error within the field on a daughter mask can be reduced to about 13.5nm(range) by compensating for the registration error to a mother mask. Inter-field CD uniformity(3sigma) is 8nm in 100mm×100mm area on a daughter mask and intra-field CD uniformity (3sigma) is 24nm within a 18.4mm×23.0mm field on a daughter mask. The intra-field CD uniformity (3sigma) can be improved into 15nm by compensating for the CD error to a mother mask . In order to satisfy the 30nm registration and 15nm CD uniformity specification of a 0.15μm generation mask, we need to reduce the inter-field registration error with removing the backside defect of a daughter mask and improve the intra-field CD uniformity by changing the dose distribution of the photomask repeater.

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.