Gi-Sung Yeo

Automatic in-situ focus monitor using line-shortening effect

A new focus monitoring method, AIFM has been developed. Specially designed box-in-box was drawn on conventional Cr binary mask and investigate. The box-in-box were drawn to enlarge the line end shortening effect in this new method. There is a lateral shift between inner and outer box in printed feature, and it can be measured quickly at a number of locations across the field and across the wafer with a common overlay measurement system. AIFM provides a means of evaluating focus effects such as field curvature, astigmatism, and field tilt like other focus measurement method. This method has another advantage of in-situ process condition monitoring by drawing focus monitor patterns in real product masks. Experiments were performed to evaluate the effects of pattern geometry and exposure dose on the sensitivity of the focus monitor. AIFM data shows good correlation with conventional focus measurement using SEM line width measurement.

A 6F/sup 2/ DRAM technology in 60nm era for gigabit densities

A novel process technology for 6F/sup 2/ DRAM cell at 68nm design rule was for the first time developed. The cell size is 0.028/spl mu/m/sup 2/, which is the smallest cell size ever reported. ArF lithography with double expose technology and highly selective etching process were used for patterning of critical layers. This 6F cell was made of simple line/space patterns for easy patterning and self-aligned etches to improve process margins. MIM cell capacitor was developed with multi-layer high-k dielectric materials and 11A equivalent Tox and sub-lfA leakage was confirmed.

Quantitative Evaluation of Grid Size Effect on Critical Dimension Uniformity Improvement

In order to control the on-chip linewidth variation (OCV) in logic devices, accurate optical proximity correction (OPC) is required, and the method to enhance its result is devised. The optical proximity behaviors are severely varied according to the optical and material conditions. The change in photoresist (PR) species deteriorates the OPC rule from 9.3 nm to 15.1 nm for two kinds of PR species. The illumination condition variation also deteriorates the optical-proximity-corrected (OPCed) results from 9.3 nm to 11.6 nm. To obtain accurate OPCed results, these conditions should be fixed. For improving the correction accuracy of the optical proximity, the OPCed grid size effect on the critical dimension (CD) uniformity is evaluated quantitatively. By adopting the OPC grid size of less than 1 nm, the correction resolution limited by the grid size is enhanced. The selective bias with the assist features is applied to the line-and-space (L/S) patterns varied by the space sizes. The selective bias rule is generated with a model using the different grid sizes of 1 nm and 0.5 nm. In the nominal CD of 87 nm, the 3σ values of the optical proximity effect are measured to be 14.6 nm and 11.4 nm for 1 nm and 0.5 nm grid sizes, respectively. The improvement of 9.2 nm is achieved, corresponding to nearly 39% enhancement. The CD uniformity dependence on the grid size was characterized in two-dimensional pattern on a real static random access memory (SRAM) pattern with the different grid sizes of 1 nm and 0.5 nm. The 3σ values of the uniformity are 9.9 nm and 8.7 nm in the case of grid sizes of 1 nm and 0.5 nm, respectively. Decreasing the grid size improves the uniformity by 4.7 nm, corresponding to 22% enhancement. By considering the mask error enhancement factor (MEEF), the enhanced amount is calculated to be 3.2 nm. The pattern fidelity improvement in the mask by reducing the grid size enhances the printing images, and decreases the measurement error using the in-line CD scanning electron microscope (SEM).

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.

Approach to critical dimension error budget analysis and specification estimation by the Monte Carlo method

A systematic approach to determine the specifications for process latitude in a 80 nm node device is suggested based on critical dimension (CD) error budget analysis using the Monte Carlo method. Using an aerial image simulation with the Gaussian convolution model, CD error budget analysis is performed on line/space (L/S), island, and contact patterns in a 90 nm node device. The portions contributed by mask uniformity are 60%, 66%, and 50% for the L/S, island, and contact patterns, respectively, and they occupy a dominant part in comparison with ones from other factors. The contact pattern occupies 12% for focus variation, which shows the larger portion than the analysis result for L/S and island patterns. The contribution of the 1 nm grid size in the mask layout is approximately 10% of the in-field uniformity. The residual terms which include variations from aberration, inaccurate measurement, postexposure baking, development, line-edge roughness, etc. occupy 21% for L/S and contact patterns, and 16% for...

Effectiveness and confirmation of local area flare measurement method in various pattern layouts

It is well known that flare, which increases the background intensity and loses the image contrast, degrades the pattern fidelity and CD uniformity. Usually there is little mid and long-range flare at the initial exposure tool introduction except the short-range flare, so called, aberration. However, flare effect is observed in used exposure tools. To estimate the influence of flare, both lens quality of the exposure tool and mask pattern layout with various open ratios are important parameters to be considered. So it is very crucial to make a standard mask layout to measure the flare value as a tool specification. So far, CD variation of the long-range flare has been measured and reported. The long-range flare includes the average influence of the short and mid-range flare and affects more than several hundred- micron distances. Recently it is observed that lens contamination is a dominant component among sources of flare and induced by the pattern layout with its different open ratio. Being contaminated, the lens malfunctions with various types of scattering sources. These scattering sources make the mid and long range flare. This type of flare source has time dependence. If there are proper monitoring methods for the flare measurement, it is possible to maintain the lens quality within the limit of mid range flare. In addition, matching the flare value to CD distribution is not easy because there is no standard measurement method to distinguish the short and mid-range flare from the long-range one. In this paper a LOcal Area Flare Evaluation Reticle (LOAFER) method is suggested. The LOAFER is designed to measure the local area flare of the lens, that is, the short and mid-range flare and the local flare distribution of the exposure tool lens can be characterized. Then matching the result to the real device pattern will be introduced.

Assessments on process parameters' influences to the proximity correction

The on-chip variation (OCV) should be critically controlled to obtain the high speed performance in logic devices. The variation from proximity dominantly contributes to OCV. This proximity effect can be compensated by applying well-treated optical proximity correction (OPC). Therefore, the accuracy of OPC is needed, and methods to enhance its result have to be devised. The optical proximity behaviors are severely varied according to the material and optical conditions. In point of material, the proximity property is affected by species of photo-resist (PR) and change of post exposure bake (PEB) conditions. 3σ values of proximity variation are changed from 9.3 nm to 15.2 nm according to PR species. Also, proximity variations change from 16.2 nm to 13.8 nm is observed according to PEB condition. Proximity variations changes of 11.6 nm and 15.2 nm are measured by changing the illumination condition. In order not to seriously deteriorate OPC, these factors should be fixed after the OPC rules are extracted. Proximity variations of 11.4 nm, 13.9 nm and 15.2 nm are observed for the mask mean-to-targets (MTT) of 0 nm, 2nm, and 4nm, respectively. The decrease the OPC grid size enhances the correction resolution and the OCV is reduced. The selective bias rule is generated by model using grid size of 1 nm and 0.5 nm. For the nominal CD of 87 nm, proximity variations are measured to be 14.6 nm and 11.4 nm for 1 nm and 0.5 nm grid sizes, respectively. The enhancement amount of proximity variations are 9.2 nm corresponding to 39% improvement. The CD uniformity improvement for adopting the small grid size is confirmed by measuring the CD uniformity on real SRAM pattern. CD uniformities are measured 11nm and 9.1nm for grid size of 1 nm and 0.5 nm, respectively. 22% improvement of the CD uniformity is achieved.

Improving model-based OPC performance for sub-60nm devices using real source optical model

In order to satisfy high density and cost effective production, extreme illumination condition, maximum sigma and OAI, is currently implemented at low k1 process. In this condition, minimal change of optical condition results in large difference of patterning. Specifically, blurring, intensity asymmetry and tele-centricity of illumination source cause deformation of some pitch patterns and CD asymmetry of semi-isolated patterns. In conventional modeling using ideal source optical model such as top-hat shape or profile, those data are regarded as noise terms since it is difficult to fit them well and such model inaccuracy produce OPC error. This paper provided results of the OPC performance using real source optical model obtained from a scanner. Real source image was filtered and normalized for easy handling. It was shown that we improved the model accuracy and significantly reduced the number of parameters. As a result, we increased process margin for sub-60nm device.

An array cell transistor test structure for the leakage current analysis of stacked capacitor DRAMs with diagonal cell scheme

A new test structure for a stacked capacitor DRAM cell transistors with a diagonal active-area was developed to analyze the leakage current characteristics of the cell transistors. The leakage current components of the low power DRAMs with different retention fail distributions was investigated in detail using the test structure, and the important aspect of the sub-threshold leakage component was discussed for below 0.11 /spl mu/m DRAM cell transistors.

Investigation of stray light characteristic by multiple Gaussian modeling and its OPC application

Stray light is analyzed by scattering range. For the short range, stray light distributes as 1/r4 and comes from aberration. For the mid range and the long range, in the assumption of Gaussian distribution, characteristic scattering length of specific tools is estimated. EOR is proposed which contains information of layer geometry and scattering range characteristic of flare. To minimize CD errors from OPC, flare level and EOR should be considered in the OPC procedure.