Oscar Han

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.

Novel cleaning techniques to achieve defect-free photomasks for sub-65-nm nodes

The ability to eliminate the critical source of haze contamination which can be derived from the cleaning chemistry residues and mass production environment has become a major challenge for 193 nm photolithography in semiconductor industry. Furthermore, as the specification for pattern generation on photomask becomes tighter, it is getting harder and harder to eliminate defects with both minimal structural damage and preservation of photophysical properties. We designed for the smart cleaning strategy to achieve the defect-free photomasks as a concern of above current issue with a combination of well-known cleaning technology, such as using the collective effects of ozonated water (DIO3) for the alternative to conventional clean (SPM/SC1) and UV/O3 treatment for the control of sulfate concentration. In addition to photomask clean, these strategies are also used for photoresist stripping. As well as the final cleaning process, it is a rational strategy that judicious modification of inter-process clean. Specially, that kind of view is focused on the after-development clean (ADC) process which mainly eliminated the source of fatal defects on the mask, such as pattern bridge following dry etch process. In this paper we will propose a novel cleaning strategy for the elimination of potential source of haze formation and fatal defects.

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.

Mask CD correction method using dry-etch process

In this study, the method for achieving precise CD MTT (critical dimension mean to target) in manufacturing attenuated PSM (phase shift mask) was investigated. As the specification for photomask becomes tighter, more precise control of CD is required. There are several causes to result in CD MTT error. In general mask patterning processes which are from blank material to dry etch, it is difficult to detect CD MTT error before final CD measurement and correct it. It is necessary to apply new process to mask production to correct CD error and control CD MTT precisely. Reducing number of factors which can have an effect on CD and introducing reliable method to correct CD error are important to achieve accurate CD MTT. For the correction of CD error, the reliability of CD in each measurement step such as resist CD or Cr CD before and after resist removal and effect on items related with CD like CD uniformity, isolated-dense CD difference, etc should be considered and evaluated. In this method to correct CD MTT error, Cr CD after removing resist was measured before MoSiN dry etch and additional corrective Cr dry etch using Cr CD information was applied to cancel CD error and then MoSiN dry etch was followed. In this case, factors affecting final CD are additional corrective Cr etch and MoSiN etch. The relationship between CD shift and corrective Cr etch time for masks with various pattern densities was found and necessary corrective Cr etch time was applied to CD correction process. The CD MTT error is canceled by additional corrective Cr dry etch step. As a result, accurate CD control and significant decrease of CD MTT error for attenuated PSM is achieved through the use of this CD correction method.

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.

Survey of SO4 out gas on mask storage environment

To countermeasure the haze problem on a reticle, we investigated the mask storage environment of wafer manufacturing Fab and mask manufacturing Fab. Through IC (Ion chromatography) and AIM system, we measured the outgas quantities of Fab environment, SMIF pod, mask carrier boxes and pellicle. With the evaluation result, the environmental factors around the production mask do not meet the level of its residual SO4 ion. We suggested the imminent priority to improve the environment surrounding the production masks. Additionally, we adopted a new process to decrease the SO4 outgas of pellicle frame up to 90%.

Effect of UV/O3 treatment on mask surface to reducing sulfuric residue ions

The critical source of haze contamination which mainly occurred on MoSiN surface and the interface of MoSiN and quartz is known as sulfuric ions remained after mask process. In this experiment, the UV treatment with oxygen gas was carried out before and after wet cleaning process for reducing residue ions from mask surface, and the effect with the sequence of UV treatment and wet cleaning was investigated. The composition of amorphous MoSiN layer was slightly modified by 172nm UV treatment with oxygen gas, and the amount of chemical residue ions after wet cleaning which use the piranha and SC-1 was reduced according to the transformation of surface composite. And also the relation of the surface transformation and the phase shift after SC-1 cleaning was evaluated.

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.