As the current photolithography moves toward the 65 and 45 nm nodes, resist blur, which is now around 50 to 90 nm full width at half maximum (FWHM), starts to limit the printability of narrow pitches by lowering image contrast, increasing mask error factor (MEF), and changing critical dimension (CD) through pitch behavior. Since such resist blur is known to originate from acid and base diffusion, which is an important process for chemical amplification, the reduction of such blur may affect the resist performance. Therefore, knowing how much diffusion can be tolerated at any given lithographic condition is critical to the success of photolithography at the 65 and 45 nm nodes. In this paper, we present a systematic study of the effect of the resist diffusion with a series of simple and accurate analytic equations and experimental data for the 193 nm lithography. We also extend our predictions to the 193 nm immersion lithography. We first show a simple way to accurately measure the effective resist diffusion length through the regular wafer exposure method for many typical resists. We then show a method to accurately quantify the contrast reduction due to such resist diffusion for both alternating phase shifting masks (Alt-PSM) and attenuated phase shifting masks (Att-PSM). We conclude that the contrast reduction is very significant with typical 193 nm resists, which have diffusion lengths of around 30 to 40 nm. In the study, we found that the mask error factor (MEF), though dependent on the illumination condition, is a strong function of the resist diffusion length at any given illumination condition. For example, in the alt-PSM case, the MEF is almost entirely determined by the resist diffusion. In the att-PSM case, however, the MEF is only partially dependent on the resist diffusion length, about 50%. In fact, a short diffusion length of less than 20 nm will be required to extend its litho-worthiness to the 45 nm node with contrast levels comparable to the current ones. Plots of the contrast and MEF through pitch for both alt-PSM and att-PSM for various diffusion lengths under typical lithographic conditions will be presented. Experimental verification of the above analytical equations will be presented.
KEYWORDS: Lithography, Etching, Photomasks, Semiconducting wafers, Metals, Critical dimension metrology, Silicon, Image processing, Back end of line, Finite element methods
This paper will present results obtained during the early development of a lithography process to meet the requirements of the 65 nm node in the BEOL. For the metal levels, an IBM/JSR jointly developed trench level resist was characterized and implemented. Resist image profile, process window, through pitch performance, image shortening and the effect of illumination conditions are discussed. Results from focus - exposure monitor (FEM) wafers are shown which were characterized for minimum resolution, process window and electrical continuity through a maze structure. For the via levels, results from another IBM/JSR jointly developed resist with high resolution and process windows are described. Process windows for nested and isolated vias are given, as well as results showing the improvement in process window and resolution due to the ARC etch. The results also include FEM measurements showing the electrical continuity through simple via chain structures versus the dimension of the via.
The use of alternating phase shifting masks (alt-PSM) can significantly improve lithographic process windows. However, the existence of phase error between the nominal 0 and 180 degree phase regions can cause printed lines to shift laterally toward each other in pairs at image planes away from the best focus. Such asymmetry, especially evident with small ground rules, challenges both overlay and critical dimension (CD) control. To minimize such effect, tight control in phase angle has been implemented, which contributes to the higher fabrication cost for an alt-PSM. Since the effect of the phase error varies with different lithographic conditions, knowing how much phase control is necessary for a given lithographic situation becomes essential to the reduction of the mask fabrication cost. Although this phenomenon has been studied in the past with a number of simulations and experiments, a systematic understanding of its mechanism, especially its interaction with CD and numerical aperture has not been reported. This paper explores the theoretical relationship between phase error and important parameters of photolithographic processes, such as CD, numerical apertures (NA), and overlay tolerance. A simple equation of the phase error is developed, which indicates that the effect of the phase error is inversely proportional to both phase error and defocus. We have compared the predictions of this theory to our first experimental results from a test mask and a good agreement is found. Based on this theory, we develop the quantity “tolerable phase error” relating the effect of the phase error to the CD, pitch, and depth of focus of the imaging system. We have found that for a system with depth of focus of +/- 300 nm, a phase error control about 2 degrees is necessary to realize a line shift control of less than 2.5% of the CD for the most aggressive feature size at any NA. We also note that the control of phase error can be relaxed at high NA. Calculations for 193 nm as well as 157 nm lithography are presented.
The shrink of semiconductor fabrication ground rule continues to follow Moore's law over the past years. However, at the 100 nm node, the fabrication cost starts to rise rapidly. This is mainly due tot he increase of complexity in the fabrication process, including the use of hard masks, planarization, resolution enhancement techniques, etc. Smaller device sizes require higher alignment tolerances. Also, higher degree of complexity makes alignment detection more difficult. For example, planarization techniques may destroy mark topography; hard masks may optically bury alignment marks, and more film layers makes the alignment signal more susceptible to process variations. Therefore in order to achieve reliable alignment, it is absolutely critical to develop an accurate and fast simulation software that can characterize alignment performance based on the film stack structure. In this paper, we will demonstrate that we have built an extremely fast alignment performance based on the film stack structure. In this paper, we will demonstrate that we have built an extremely fast alignment signal simulator for both direct imaging and diffractive detection system based on simple optical theory. We will demonstrate through examples using our advanced DRAM products that it is capable of accurately mapping the multi-dimensional parameter space spanned by various film thickness parameters within a short period of time, which allows both on-the-fly feedback in alignment performance and alignment optimization.
The continued downscaling of semiconductor fabrication ground rule has imposed increasingly tighter overlay tolerances, which becomes very challenging at the 100 nm lithographic node. Such tight tolerances will require very high performance in alignment. Past experiences indicate that good alignment depends largely on alignment signal quality, which, however, can be strongly affected by chip design and various fabrication processes. Under some extreme circumstances, they can even be reduced to the non- usable limit. Therefore, a systematic understanding of alignment marks and a method to predict alignment performance based on mark design are necessary. Motivated by this, we have performed a detailed study of bright field segmented alignment marks that are used in current state-of- the-art fabrication processes. We find that alignment marks at different lithographic levels can be organized into four basic categories: trench mark, metal mark, damascene mark, and combo mark. The basic principles of these four types of marks turn out to be so similar that they can be characterized within the theoretical framework of a simple model based on optical gratings. An analytic expression has been developed for such model and it has been tested using computer simulation with the rigorous time-domain finite- difference (TD-FD) algorithm TEMPEST. Consistent results have been obtained; indicating that mark signal can be significantly improved through the optimization of mark lateral dimensions, such as segment pitch and segment width. We have also compared simulation studies against experimental data for alignment marks at one typical lithographic level and a good agreement is found.
This paper reports an on-wafer photoacid determination technique that can be used to quickly screen materials that function as photoacid generators (PAGs). The technique includes adding a small amount of a pH-sensitive fluorophore into the resist and exposing the resist to x-rays. The acid generated during exposure reacts with the fluorophore and quenches the fluorescence. The efficiency of photoacid generation is evaluated by comparing the degree of fluorescence quenching. This technique is nondestructive, fast, and does not significantly change the resist chemical properties given the low concentration of the added fluorophore. Six compounds that can generate hydrogen halides as potential PAGs were evaluated using this on-wafer technique and the lithographic performance was evaluated for comparison. The commercial resist, Shipley SAL 605, is used as a reference for comparison. The result showed that TBBPA gave higher photoacid generation efficiency that TCBPA and PBP, but lower than that in SAL 605. The results of fluorescence measurements agree with the results obtained using normalized remaining thickness measurements. The advantages, however, of this fluorescence technique are that it is simple, fast, and requires fewer processing steps.
In this paper, we examined the saturation absorption nonlinearity of Azo doped polymers, based on which we successfully realize a novel optical filter for improvement in contrast of images.
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