Munsik 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.

New critical dimension uniformity measurement concept based reticle inspection tool

The Critical Dimension Uniformity (CDU) specification on photo-mask is getting increasingly tighter which each successive node. The ITRS roadmap for optical masks indicates that, the CDU (3 sigma) for dense lines for binary or attenuated phase shift mask is 3.4nm for 45nm half-pitch (45HP) node and will go down to 2.4nm for 32HP node. The current variability in mask shop processes results in CDU variation across the photo-mask of ~2-3nm. Hence, we are entering in a phase where the mask CDU specification is approaching the limit of the capability of the current POR (process on record). Hence, mask shops have started exploring more active mechanisms to improve or compensate for the CDU of the masks. A typical application is in feeding back the CDU data to adjust the mask writer dose and compensate for non-uniformity in the CDs, resulting in improved quality of subsequent masks. Another option is to feed the CD uniformity information forward into the wafer FAB and adjust the scanner dose to correct for reticle non-uniformity. For these purposes mask makers prefer a dense measurement of CDs across the reticle in a short time. Mask makers are currently using the CD-SEM tool for data collection. While the resolution of SEM data ensures its position as the industry standard, an output map of CDU from a reticle inspection tool has the advantage of denser sampling over larger areas on the mask. High NA reticle inspection systems scan the entire reticle at high throughput, and are ideally suited for collecting CDU data on a dense grid. In this paper, we describe the basic theory of a new, reticle inspection-based CDU tool, and results on advanced memory masks. We discuss possible applications of CDU maps for optimizing the mask manufacturing and wafer production processes.

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.

Investigation of subpellicle defect formation at KrF lithography

In this study we investigated the defect due to pellicle frame materials for repeating exposure in months. Defects were found in the sub-pellicle and the defect density was high in the 4 corners compared to the center of the mask. The defects grew on MoSiON or the interface Quartz and MoSiON film, and the defect size was below 0.5 um. By analyzing with Raman Spectroscopy, defects consist of Ammonium Sulfates, Melamine Formaldade Resin and KClO3. The evaluation method for cleaning process and pellicles was Ion Chromatography. According to Ion Chromatography analysis, the main composition of defect was substances of pellicle frame materials. Also we confirmed the pellicle frame effect with the exposure test.

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.

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.

Advanced repair solution of clear defects on HTPSM by using nanomachining tool

As the mask specifications become tighter for low k1 lithography, more aggressive repair accuracy is required below sub 20nm tech. node. To meet tight defect specifications, many maskshops select effective repair tools according to defect types. Normally, pattern defects are repaired by the e-beam repair tool and soft defects such as particles are repaired by the nanomachining tool. It is difficult for an e-beam repair tool to remove particle defects because it uses chemical reaction between gas and electron, and a nanomachining tool, which uses physical reaction between a nano-tip and defects, cannot be applied for repairing clear defects. Generally, film deposition process is widely used for repairing clear defects. However, the deposited film has weak cleaning durability, so it is easily removed by accumulated cleaning process. Although the deposited film is strongly attached on MoSiN(or Qz) film, the adhesive strength between deposited Cr film and MoSiN(or Qz) film becomes weaker and weaker by the accumulated energy when masks are exposed in a scanner tool due to the different coefficient of thermal expansion of each materials. Therefore, whenever a re-pellicle process is needed to a mask, all deposited repair points have to be confirmed whether those deposition film are damaged or not. And if a deposition point is damaged, repair process is needed again. This process causes longer and more complex process. In this paper, the basic theory and the principle are introduced to recover clear defects by using nanomachining tool, and the evaluated results are reviewed at dense line (L/S) patterns and contact hole (C/H) patterns. Also, the results using a nanomachining were compared with those using an e-beam repair tool, including the cleaning durability evaluated by the accumulated cleaning process. Besides, we discuss the phase shift issue and the solution about the image placement error caused by phase error.

The method of quartz damage recovery in the photomask repair process

As the pattern size became gradually smaller, the defect detectability of the photomask inspection tool was more improved. For these reasons, we have to repair various defects more precisely. By improving the mask yield through the repair process, we can reduce the cost of mask fabrication. In this study, we studied the defect called quartz damage which distorts the AIMSTM (Arial Image Measurement System) intensity of the repaired pattern and causes the scrap of the photomask. The quartz damage is generally observed when the abnormal defects like particles were repaired in the poor repairing condition. The quartz damage occasionally results in repair errors and affects the AIMS intensity. Currently there is no clear solution for recovering the quartz damage. As a result, it is very difficult to get the high quality photomask if the quartz damage is generated on the photomask. Therefore, it is important to find a method of recovering the quartz damage for producing the high quality photomask. In this paper, we demonstrated that the quartz damage can be recovered through the TEOS (Tetraethoxysilane) gas deposition. Also we investigated the effect on the recovery of the quartz damage of various parameters such as the type and the depth of the quartz damage as well as the repair conditions of the TEOS gas deposition.

Defects caused by blank masks and repair solution with nanomachining for 20nm node

As the number of masks per wafer product set is increasing and low k1 lithography requires tight mask specifications, the patterning process below sub 20nm tech. node for critical layers will be much more expensive compared with previous tech. generations. Besides, the improved resolution and the zero defect level are necessary to meet tighter specifications on a mask and these resulted in the increased the blank mask price as well as the mask fabrication cost. Unfortunately, in spite of expensive price of blank masks, the certain number of defects on the blank mask is transformed into the mask defects and its ratio is increased. But using high quality blank mask is not a good idea to avoid defects on the blank mask because the price of a blank mask is proportional to specifications related to defect level. Furthermore, particular defects generated from the specific process during manufacturing a blank mask are detected as a smaller defect than real size by blank inspection tools because of its physical properties. As a result, it is almost impossible to prevent defects caused by blank masks during the mask manufacturing. In this paper, blank defect types which is evolved into mask defects and its unique characteristics are observed. Also, the repair issues are reviewed such as the pattern damage according to the defect types and the repair solution is suggested to satisfy the AIMS (Arial Image Measurement System) specification using a nanomachining tool.