Forbidden pitch improvement using modified illumination in lithography

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.

The existence of pitch range with depth of focus below a sustainable limit is a well known fact in lithography. Such 'forbidden pitch' range limits designers' ability to pack more functionality in a logic chip. One of the ways to increase the process window is to have a careful placement of SRAFs (Sub Resolution Assist Features) that can boost process window across the pitch range. However the standard SRAF strategy that has been followed historically is not always able to increase the process window of these 'forbidden pitches' sufficiently to allow sustainable manufacturing. With shrinking technology node, placement of SRAF is becoming rather difficult due to space limitations between concerned features and mask house's ability to manufacture mask with small assist features and smaller aspect ratios. In many cases the number of SRAF that can be inserted between main features in a symmetrical way is not enough to boost the process window. In this paper we discuss how asymmetrical placement of SRAF can increase process window for critical feature in layouts where such critical features are placed near not-so-critical patterns. We also discuss how such concepts can be extended to an array of critical features, where one SRAF is placed near a critical feature instead of placing them in the center. We finally demonstrate how wafer data confirm process window boost from such asymmetrical placement of SRAFs in gate layer for 65nm. We also show how to determine the optimal placement of SRAF in such cases and recommend some rules that can be used for 45nm node based on such results.

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%).

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.

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.

We have investited the performance of the halftone phase-shifting mask (HT PSM) with various illuminations for contact patterns of different pitches in DUV photo lithography . It was found that illumination could be optimized as a function of the pitch. Highly coherent illumination was the best for isolated contact patterns but it was the worst for extremely dense contact patterns due to optical proximity effect and interference which occurred between the primary peaks and the secondary peaks of neighboring contact holes. For extremely dense contact patterns, off-axis illumination (OAI) was found to be the most appropriate compared to conventional illuminations because extremely dense contact patterns show the optical proximity effects which was observed similarly for equal line and space patterns. We found that HT PSM combined with OAI can be used for fabricating the extremely dense contact patterns of high density devices such as 256M and 1 G bit DRAM.

OFF-AXISILLUMINATIONFOR EXTENDINGOPTICAL MICROLITHOGRAPHICSYSTEMPERFORMANCE Steve Brainerd Supervising professor: Raj Solanki Improvements in depth of focus and resolution for microlithographic projection printing systems can be achieved by illuminating the object (mask) with oblique or off axis rays. Three types of illumination as used on a waferstepper with a 0.48 numerical aperture objective lens were characterized for their effect on resolution, depth of focus, and proximity. The illumination types evaluated were partially coherent (sigma =0.64), off-axis quadrupole (inner sigma 0.26, outer sigma 0.70 and opening sigma = 0.26),and off-axisring annular(inner sigma=0.40 and outer sigma =0.70). All projected images were recorded in a modem high contrast positive photoresist. While this type of illumination offers significant performance gains. it is not without limitations. Two shortcomings are the loss of image imKlianceand proximity effects. This project will evaluate original methods to improve the later. The results show that off-axis annular and quadrupole illumination improve the depth of focus for dense 0.36 to 0.50 micron linewidths as compared to partially coherent illumination by 45% and 56% respectively. The defocus proximity effects with off-axis illumination mentioned above is where the depth of focus for an exterior line on a dense line-space group is significantly less than an interior line. Three potential solutions evaluated for improving this exterior linewidth proximity effect were: 1. The use of special sub-resolution compensating structures on the mask (object) features, 2. Contrast enhancement photoresist processing, and 3. Anti-reflective substrate coatings. The use of a modified reticle improved the depth of focus for an exterior line, while contrast enhancement and anti-reflectivecoatings did not offer any improvement. The experimental results were modeled using Silvacos SOLID 3.1 optical lithography simulator (Sjmulation of !!ptical LIthography in three Dimensions). Excellent cOlTelationbetween the SOLID simulation model calibrated to the specific photoresist process used here and the experimental results was achieved.

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.

In order to produce various kind of circuit pattern in short-term cycle, mask-less exposure using DMD(digital mirror device)-mask is useful. Although not only fine resolution but also large DOF(depth of focus) can be obtained by using PSM(phase shifting mask), there exists forbidden pitch region for PSM. While the pattern pitch of PSM becomes fine, it cannot be resolved at a certain pitch and it will be resolved again at a finer pattern pitch. This un-resolvable region is called forbidden pitch region. It has been time consuming to predict this region by numerical calculation. Applying PSM to various fields, we should predict the region simply. In this paper, we derive analytical equations which give the optimal NA(Numerical Aperture) and finest resolution and also reveal forbidden region. In addition, we confirm the validity of analytical equation by numerical calculation using PROLITHTM. This result will be useful to apply PSM in practice.

We proposed the new method for obtaining the effect of an off-axis illumination by using a mask grating formed on the photo mask. The grating structure was designed from the source distribution in the conventional off-axis illumination and the rigorous diffraction theory. Its performance was characterized with simulated Bossung curve.

In this paper, we will present comparison of attenuated phase shift mask and binary intensity mask at hyper-NA immersion scanner which has been the main stream of DRAM lithography. Some technical issues will be reported for polarized illumination in hyper-NA imaging. One att.PSM (Phase Shift Mask) and three types of binary intensity mask are used for this experiment; those are ArF att.PSM ( MoSi:Å ), thick Cr ( 1030Å ) BIM (Binary Intensity Mask), thin Cr ( 590Å ) BIM and multi layer ( Cr:740Å / MoSi:930Å ) BIM. Simulation and experiment with 1.35NA immersion scanner are performed to study influence of mask structure, process margin and effect of polarization. Two types of DRAM cell patterns are studied; one is an isolation pattern with a brick wall shape and another is a storage node pattern with contact hole shape. Line and space pattern is also studied through 38nm to 50nm half pitch for this experiment. Lithography simulation is done by in-house tool based on diffused aerial image model. EM-SUITE is also used in order to study the influence of mask structure and polarization effect through rigorous EMF simulation. Transmission and polarization effects of zero and first diffraction order are simulated for both att.PSM and BIM. First and zero diffraction order polarization are shown to be influenced by the structure of masking film. As pattern size on mask decreases to the level of exposure wavelength, incident light will interact with mask pattern, and then transmittance changes for mask structure. Optimum mask bias is one of the important factors for lithographic performance. In the case of att.PSM, negative bias shows higher image contrast than positive one, but in case of binary intensity mask, positive bias shows better performance than negative one. This is caused by balance of amplitude between first diffraction order and zero diffraction order light. Process windows and mask error enhancement factors are measured with respect to various design rules, i.e., different k1 levels at fixed NA. In the case of one dimensional line and space pattern, thick Cr BIM shows the best performance through various pitches. But in case of two dimensional DRAM cell pattern, it is difficult to find out the advantage of BIM for sub-45nm. It needs further study for two dimensional patterns. Finally, it was observed that thick Cr binary intensity mask for sub-45nm has advantage for one dimensional line and space pattern.

Improving resolution and focus latitude is needed in the manufacturing 0.18 micrometers LSI devices using the KrF lithography. In order to achieve this requirement, the attenuated phase shift mask (Att.PSM) has been proposed. 0.18 micrometers gate patterns can be attained using the Att.PSM with annular illumination. However it is expected to cause a differences in the printability of mask fabrication error by using a conventional illumination when annular illumination is used. Therefore, before applying our Att.PSM to practical use, we need to investigate the printability of Att.PSM fabrication errors when using annular illumination for 0.18 micrometers line patterns. Now, we clarify the printability of Att.PSM fabrication errors via simulation and experiments. Then we estimated the requirements of Att.PSM for 0.18 micrometers gate patterns. To apply the Att.PSM with annular illumination, we showed the feasibility of the Att.PSM fabrication. The other hand, we understood that it is necessary to improve technology of inspection and repair.

The optical resolution of photolithography is limited by the numerical aperture (NA) of lens, wavelength of light source, and <i>k</i>₁ factor. Nowadays, the low k1 process is necessary, since the tool development is delayed due to technology difficulties. In order to enhance the process latitude in the low <i>k</i>₁ region, special illumination design for specific patterns has been studied. Although illumination optimization is one of the promising solutions to develop the low <i>k</i>₁ process, specific design for each pattern has not been applied since the case-by-case illumination design is not easy. The specific layer oriented illumination design is generated using our in-house tool. A DRAM cell is composed of periodic or semi-periodic patterns, and the design of layer specific illumination is made for those patterns with the target of enlarged depth of focus (DOF). It is observed that the DOF and exposure latitude of a DRAM isolated pattern using the optimized illumination are increased in comparison with the conventional annular illumination. It is expected that the lifetime of low-grade exposure tools can be extended by this illumination optimization technique.

0.35 μm pattern fabrication using quartz-etch attenuate phase-shifting mask in an I-line stepper with a 0.50 NA and a 0.60 sigma