Off-axis illumination for extending optical microlithographic system performance

Markets continuously demand higher resolution and higher productivity in flat panel display (FPD) exposure systems. Our solution to improve resolution and productivity is the use of broadband wavelength illumination. A larger depth of focus (DOF) is also very important to achieve higher productivity because inadequate DOF can cause product defects. To obtain higher resolution and larger DOF, off axis illumination (OAI) conditions have been widely used. OAIs using a narrowband wavelength illumination are well documented and sufficiently studied. On the other hand, Canon FPD exposure tools use a broadband illumination source to achieve higher resolution and productivity. To obtain sufficient OAI effects in broadband exposure lithography, new technology should be developed with consideration of broadband wavelength because OAI effects are different between broadband and narrowband illumination. In this paper we introduce divided spectrum illumination (DSI), a new design concept proposed to achieve both high resolution and large DOF by optimizing the broadband illumination source wavelength band depending on the illumination angle. Experimental imaging results of line and space patterns with a line width of 1.0-μm and pitch of 2.0-μm showed that the DSI design improved resolution. Results showed that test patterns imaged using traditional narrowband OAI could not be resolved at the top of the resist even at the best focus, however resist profiles using DSI were sharp enough to retain pattern fidelity at the top of resist. DOF with DSI also improved 21 % compared to traditional OAI.

Combining assist features with appropriate off-axis illumination conditions can significantly improve depth of focus and uniformity of critical dimensions of contact windows. It is known that sub-resolution assist features modify the environment of isolated features in a fashion that they appear dense. In recent years the impact of assist features was mostly studied for gate-level lithography. In this paper the placement and dimension control of assist features for contact windows are examined and analyzed using 193 nm lithography in conjunction with a state-of-the-art single layer resist. Our study is primarily done for 160 nm contact windows, and it is based on experimental data obtained from critical dimension measurements with varying focus, exposure dose, and in different environments. Along with optical proximity corrections we use off-axis illumination technique which increases depth of focus of contact windows and improves the overall process latitude. Specifically for this study we have designed a test photomask with different geometries and pattern densities of contact windows with and without assist features to be used at 193 nm wavelength. To study the proximity effects, different sizes of assist features were used as well as the distances of assists from the main feature were varied. The results have shown that while increasing the process latitude for the primary feature using assist slots in combination with off-axis illumination, the resist thickness and contrast are limiting the assist feature dimensions that can be used. Assist features appear to significantly increase critical dimension uniformity of the contact windows when using both conventional and off-axis illumination techniques, and they dramatically increase the common exposure dose latitude for contact windows with densities from near-isolated to dense, decreasing the proximity effects. Assist feature technique combined with quadrupole illumination demonstrates about 0.3 micron improvement in depth of focus for every type contact window pattern used.

In lithography, forbidden pitch refers to pitch that suffers degradation in the process window due to the application of off-axis illumination (OAI). Destructive light field interference of diffracted light from a mask at forbidden pitch causes reduction in image contrast and depth of focus (DOF) and limits the pitch range to be patterned. In this paper, a modification to conventional OAI shape is proposed to minimize the effect of forbidden pitch. The modification is based on the interaction of illumination source with pattern density. The modified source employs double annular illumination. Simulation is carried out to investigate the effect of the modified source for one dimensional line and space pattern with a pitch varying from 130to500nm. Results shows that the maximum critical dimension fluctuation is around 3% compared to 13% in conventional annular illumination. Furthermore, the degradation in DOF is within 21% of DOF compared to 49% in conventional annular illumination.

For 100nm-level patterning using optical lithography, high NA system and various RETs such as PSM, off-axis illumination and OPC are obviously required. In particular, assistant feature (AF)-OPC is indispensable to overcome narrow depth of focus (DOF) caused by iso-dense bias and to compensate for linearity difference under the given OAI condition. Previously we reported the application of AF-OPC in DRAM process with 120nm design rule. The extraction of OPC rule and the feasibility of AF-OPC were successfully confirmed by experimental method in real process. In this paper, more comprehensive and aggressive AF-OPC rule is investigated. The old rule is modified in order to obtain larger common DOF. TO avoid dead zone that means discontinuity between dense line and semi-dense line, we apply a comprehensive rule such as insertion of AF between the neighboring main patterns as many as possible. As a result, the discontinuity of OPC application, which is used with or without AF in the boundary region, is effectively minimized. Also, polygon-shaped AF is used to improve DOF of special main pattern. And then, the mask specification and the behavior of isolated line pattern are predicted in case of very high NA KrF and ArF lithography by simulation result. Considering 100nm design rule, the decrease of common DOF is expected to be severer than now. Finally, the optimum AF-OPC rules such as AF size, space and shape are available and shown in case of very high NA KrF and ArF lithography.

We have studied the optical proximity effect, the depth of focus (DOF) and the mask-error-enhancement factor (MEF) of ArF attenuated phase shift masks (att. PSMs) for the application to 100-nm logic gate patterns. In the previous work, we have compared the performances of the binary mask with those of the 6% transmittance ArF att. PSM for printing 100-nm lines, and we obtained the better performances on the att. PSM with the annular illumination. In this paper, we reported the evaluation results of the higher transmittance att. PSMs. We fabricated them with 6-20% transmittance (quartz reference). Those masks have TaSiO bi-layer shifter with two tones. We did not use assist bars. An ArF scanner with NA 0.60 and a positive chemically amplified ArF resist with the thickness of 0.4 um are used. 100 nm semi-dense lines with a pitch of 250 nm are exposed without bias and 100 nm isolated lines are exposed with biases of 5 -15 nm (on wafer). Our results show that the higher transmittance att. PSMs can improve the DOF and the MEF on both semi-dense lines and isolated lines. In our experiments, the highest transmittance of 20% gives the broadest DOF of 0.4 um and the smallest MEF of 1.78 and 1.11 for semi-dense lines and isolated lines respectively. However, some higher transmittance PSMs may create the side lobes on the large patterns. From these results, we demonstrated that that 10% transmittance is suitable for the ArF att. PSMs for the fabrication of 100 nm line patterns.

Patterning of contact holes using KrF lithography system is one of the most challenging tasks for the sub-90nm technology node,. Contact hole patterns can be printed with a KrF lithography system using Off-Axis Illumination (OAI) such as Quasar or Quadrupole. However, such a source usually offers poor image contrast and poor depth of focus (DOF), especially for isolated contact holes. In addition to image contrast and DOF, circularity of hole shape is also an important parameter for device performance. Sub-resolution assist features (SRAF) can be used to improve the image contrast, DOF and circularity for isolated contact holes. Application of SRAFs, modifies the intensity profile of isolated features to be more like dense ones, improving the focal response of the isolated feature. The insertion of SRAFs in a contact design is most commonly done using rule-based scripting, where the initial rules for configuring the SRAFs are derived using a simulation tool to determining the distance of assist features to main feature, and the size and number of assist features to be used.. However in the case of random contact holes, rule-based SRAF placement is a nearly impossible task. To address this problem, an inverse lithography technique was successfully used to treat random contact holes. The impact of SRAF configuration on pattern profile, especially circularity and process margin, is demonstrated. It is also shown that the experimental data are easily predicted by calibrating aerial image simulation results. Finally, a methodology for optimizing SRAF rules using inverse lithography technology is described.

Projection and proximity printings are compared in terms of linewidth tolerance. Computer simulated exposure-defocus and gap-exposure diagrams are used to characterize the two techniques respectively. The depth of focus for projection printing in terms of the universal depth parameter k2 is evaluated as a function of the universal width parameter k1, for five long and short representative lithographic features, then three long features. Similarly, the depth of focus and working distance for proximity printing are evaluated for a O.25-jm system in x-ray and 2.5-,um system in uv and some preliminary comparison to experimental results is shown. The image contours and depth of focus of the two imaging techniques are compared. Proximity effects are present for both techniques. The distinction of working distance and depth of focus for proximity printing is observed for the first time. Against intuition, a shorter mask-to-wafer gap is not always better.

Fan-Out wafer level packaging has seen rapid adoption over the last few years due to its form factor, performance, and cost advantages compared to 3D packaging techniques. Redistribution layers (RDL) are used to route the very high density connections on the chip to the much lower density connections of the substrate. Multiple layers of RDL are required in order to match the line density of the chip. This in turn increases the cost of the total package. Decreasing the metal line width for RDL supports reducing the number of redistribution levels and decreasing the total packaging cost. Reducing the RDL width requires tightening of requirements for the lithography tool and the photoresist process. Also, electroplating and etching processes need to be enhanced. This study investigates creation of RDL structures using a through resist electroplating process. A test vehicle was designed that contains various test structures used to characterize the process. This includes metrology structures for in-line monitoring, CD-SEM measurements, and comb and serpentine electrical test structures. The process starts with the deposition of a Cu seed layer needed for electroplating followed by lithography, electroplating, resist strip and Cu seed removal. The minimum thickness of the seed layer is constrained since the uniformity of the electroplating process also depends on the resistance and thus thickness of this layer. This thickness in turn has a large impact on line fidelity after removal of the Cu seed at the end of the process. In order to fabricate RDL lines with sufficient low resistance the plating thickness needs to be maximized. This translates to maximizing the aspect ratio of the photoresist pattern. In order to minimize the CD loss by Cu seed etching the wet process has to be controlled carefully. The lithography process window must have a large depth of focus in order to accommodate local height variation due to wafer topography in Fan-Out packaging. The optical lithography tool used in this study has a variable numerical aperture (NA) that can be optimized to maximize depth of focus while maintaining resolution performance in photoresist. After completing the development of the 1 μm RDL process the electrical performance was validated with meandercomb electrical test structures. The results show that lithography needs to be performed at the isofocal setpoint in order to obtain the optimal depth of focus of 7.5μm. Seed etch will reduce the Cu linewidth by 300 to 500nm and an offset in lithography CD may be required to compensate this loss. With optimal conditions good electrical yield is obtained for 1.0μm L/S meandercomb test structures.

Many research teams have begun pursuing optical micromachining technology in recent years due to its associated noncontact and fast speed characteristics. However, the focal spot sizes and the depth of focus (DOF) strongly influenced the design requirements of the micromachining system. The focal spot size determines the minimum features can be fabricated, which is inversely proportional to the DOF. That is, smaller focal spot size led to shorter DOF. However, the DOF of the emitted visible or near-infrared light beam is typically limited to tens of nanometers for traditional optic system. The disadvantages of using nanosecond laser for micromachining such as burrs formation and surface roughness were found to further influence the accuracy of machined surfaces. To alleviate all of the above-mentioned problems, sub-wavelength annular aperture (SAA) illuminated with 780 nm femtosecond laser were integrated to develop the new laser micromachining system presented in this paper. We first optimized the parameters for high transmittance associated with the SAA structure for the 780 nm femtosecond laser used by adopting the finite difference time domain simulations method. A lateral microscope was modified from a traditional microscope to facilitate the measurement of the emitted light beam optical energy distribution. To verify the newly developed system performance the femtosecond laser was used to illuminate the SAA fabricated on the metallic film to produce the Bessel light beam so as to perform micromachining and process on silicon, PCB board and glass. Experimental results were found to match the original system design goals reasonably well.

With the ever-increasing demand for reducing the size of devices on a chip, one frequently meets the lithographic challenge of printing very small lines on a wafer. The rapidly shrinking process latitudes, especially the depth-of-focus (DOF), really become a burden when trying to print sub-80nm resist lines on a wafer with 193nm lithography. In this paper, we report on a method that is capable of enlarging the process windows for printing small lines, while also reducing a line collapse issue, by using an enhanced resist processing procedure. In this procedure, the PEB time duration, in combination with the exposure dose, is used as a tuning process parameter. It is shown that, by using this procedure, a significant increase in DOF is obtained for printing small (down to 40nm) isolated and semi-dense lines, while the pitch is not scaled down. While using a binary mask and annular 193nm illumination with a NA of 0.63 and s of 0.87/0.57, we show that it is possible to print 50nm lines on a 240nm pitch with 0.8 micron DOF, whereas the standard process, using the vendor recommended PEB, can only print 80nm lines with a comparable DOF. The large process enhancement, among which the reduced curvature in the Bossung plots, is explained by the more efficient use of the acid and quencher present in the photoresist, as well as by the peculiarities of acid diffusion near regions where quencher is remaining. The results obtained are explained by a compact resist model in which acid-quencher reaction-diffusion is incorporated.

The insertion point for the first scattering bar is a key point in the development of a process using assist features, because this semi dense feature will determine the overall depth of focus of the process. A study of the parameters, which influence the choice of this insertion point, has been performed using a 0.63 NA 193 nm scanner for a 100 nm CD target after litho. The impact of the scattering bar on: Depth of Focus, Energy Latitude, Mask Error Enhancement Factor, printability, and the effect of scattering bar line width variation on main feature described by a parameter called AFMEEF will be discussed in this paper. The optimal insertion point for the first scattering bar will strongly depend on the litho-graphic process and the mask parameters. A model is proposed to determine the optimal insertion point, as function of the dose, focus budget, minimal allowed scatterbar width, and mask CD dispersion for both scattering bars and main features.

Attenuated phase shift mask (APSM) has been considered a viable technique for contact patterning. For 250 nm lithography technology, 300 nm contact patterns are to be defined. In this paper, Solid-C has been used for aerial image simulations. Conventional and annular illumination settings are optimized for better focus latitudes, that is, large depth-of-focus (DOF), for sufficient throughput. In addition, mask transmission is optimized for different illumination settings. From our simulations of aerial images, it has been shown that a DOF of 1.35 micrometer is achieved when conventional illumination is combined with APSM at high mask transmission (approximately 8 - 10%). However a larger DOF of 1.55 micrometer can be obtained when annular illumination is used with APSM at low mask transmission (approximately 3 - 4%).

NA and (sigma) will be optimized to establish 0.35 micrometers i-line single layer resist process without use of super resolution techniques. Resolution, depth of focus (DOF), and proximity effect are evaluated using a variable NA and (sigma) stepper. NA is varied by an aperture stop in a projection lens. (sigma) is varied by not only an aperture stop (mechanical (sigma) ) in an illumination optics but also intensity distribution of illumination at the aperture stop (effective (sigma) ). Optimized NA and (sigma) are applied to a newly developed high resolution resist. Obtained results show that high NA and high (sigma) stepper has a great availability for 0.35 micrometers device fabrication.

xIn order to address some specific issues related to gate level printing of the 0.09μm logic process, the following mask and illumination solutions have been evaluated. Annular and Quasar illumination using binary mask with assist feature and the CODE (Complementary Double Exposure) technique. Two different linewidths have been targeted after lithography: 100nm and 80nm respectively for lowpower and high-speed applications. The different solutions have been compared for their printing performance through pitch for Energy Latitude, Depth of Focus and Mask Error Enhancement Factor. The assist bar printability and line-end control was also determined. For printing the 100nm target, all tested options can be used, with a preference for Quasar illumination for the gain in Depth of Focus and MEEF. For the 80nm target however, only the CODE technique with Quasar give sufficient good results for the critical litho parameters.

Currently, the fundamental relationships which establish the limits to optical imaging appear useful only as comparative metrics for microlithographic applications. Furthermore, with the continual advances in optical lithography, the validity of such 'rules-of-thumb' become suspect. A set of universal equations would be useful but additional description and expansion beyond the Rayleigh definitions is needed for application to current technologies. Relationships which govern the limits of resolution and focal depth are presented here in order to develop a more complete description for various lithographic imaging situations. Expanded Rayleigh definitions can then allow for insight into fundamental limitations and scaling for conventional partially coherent imaging, imaging with off-axis illumination, and phase-shift masking. Depth of focus relationships are also expanded to describe the distribution of diffraction orders over the lens pupil and are modified to accommodate feature pitch and partial coherence. Such analysis can allow for a more thorough understanding and prediction of performance for a given lithographic technology.