Combined illumination sources for hyper-NAcontact hole printing
11/01/2007
EXECUTIVE OVERVIEW
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Patterning of contact layers with minimal half pitch below 90nm has proven to be a very difficult task. In general, difficulties in contact patterning are driven by the low depth-of-focus (DoF) toward isolated contacts and/or the higher mask error enhancement factor (MEEF) for denser contact arrays. As for the illumination source shape, the traditional choice is between conventional illumination and off-axis illumination (OAI). Recent results obtained at IMEC on immersion scanners at 1.2 and 1.35 numerical aperture (NA), however, show that combined illumination sources, called Soft Annular and Soft QUASAR, are advantageous for through-pitch contact printing, in terms of MEEF and process latitude, without the use of assist features.
The continuous downscaling of devices leads to smaller pitches and targets on the wafer. Using single-patterning ArF lithography, downscaling requires increasing the NA at an aggressive pace. For the patterning of contacts at half-pitches below 90nm, hyper-NA immersion imaging is required. However, the increase of NA unavoidably comes together with a general decrease in process latitude. In particular, for the contact layer, obtaining sufficient process margin through pitch has become a very difficult task. In the following, we demonstrate how contact patterning can benefit from source optimization.
Conventional vs. OAI
The standard and traditional way of printing contact holes is using a 6% attenuated phase-shift mask and conventional illumination with coherence settings around 0.7. Although this type of illumination typically leads to moderate need for optical proximity correction (OPC) and good process latitude toward isolated contacts, it suffers from limited dense performance. In particular, the MEEF becomes the limiting factor, such that the lowest k1 value is limited to approximately 0.45.
To go to smaller k1 values, higher OAI is required. Namely, high-sigma illumination greatly improves the printing of dense contacts. The drawback of high-sigma OAI, in practice annular or QUASAR, is a prominent decrease of process latitude for the semi-dense to isolated pitches. The latter is enhanced again by using assist features around the more isolated contacts on the reticle, but this comes at the expense of increasing mask cost due to more complex OPC, mask technology, mask writing, and repair.
Combined illumination sources
In order to increase the options in making the trade-off between performance and cost, recently developed Soft QUASAR or Soft Annular sources can be applied [1]. These are a combination of off-axis (QUASAR or annular) and a central conventional pole that enhances exposure latitude (EL) and DoF for the isolated holes.
Examples of Soft Annular (SA) and Soft Quasar (SQ) sources as measured at 1.2NA on an ASML XT:1700Fi scanner are shown in Fig. 1.
Since the illumination pupils of Fig. 1 consist of incoherent, individual source points, the combined sources can basically be considered as an incoherent sum of annular/QUASAR and conventional illumination. The final characteristics are also a balance of their respective pros, cons, and optimizations that are already well-known in optical lithography. Therefore, by tuning the sigma values and the relative intensities, one is able to tune the balance between off-axis and conventional, and design an illumination shape that is most suited for specific requirements.
To investigate the potential of these alternative illumination shapes, the Belgian research center, IMEC, experimentally compared the performance of the combined sources to QUASAR and conventional illumination on regular XY arrays. The exposures were performed on an ASML XT:1700Fi, at 1.2NA, and using XY-polarization for all sources but the conventional ones. The litho target is a 65nm contact hole through pitch. On the reticle, the appropriate biases were applied to print on target, but no assist features were used.
The sources in Fig. 1, which were used in the exposures, have different balances of the off-axis and conventional component. The sources are organized from more off-axis (left) to more conventional on-axis illumination (right). Figure 2 shows the EL through-pitch, for the respective sources of Fig. 1. At the dense pitch of 140nm (k1 = 0.43), QUASAR shows a performance that is ~2× that of conventional illumination.
 Figure 2. Maximal exposure latitude obtained through pitch for QUASAR, conventional, and the combined sources as shown in Fig. 1. (ASML XT:1700Fi at 1.2NA.) |
Toward the isolated side, the trend above reverses: QUASAR illumination is underperforming, and conventional gives the better contrast. The combined Soft Annular and Soft QUASAR sources show intermediate performance for all pitches, and therefore may be the preferred option for through-pitch printing. At the isolated pitch, the different balances for the different combined sources come about in the variation of the EL between that of QUASAR and that of conventional. One can also note that the combined sources can have an EL with very little variation through pitch (e.g., SA3, or SQ) (Fig. 2).
Besides high EL, low MEEF and high DoF are also desirable through pitch. In Fig. 3, the MEEF and DoF results of only Soft Annular (SA3) and Soft QUASAR (SQ2) are shown, and compared to QUASAR and conventional (Conv1). The limiting factor for contact hole printing toward the dense side is MEEF. As seen in Fig. 3, using Soft Annular and especially Soft QUASAR provides a clear reduction compared to the common approach using conventional illumination. Typically, a MEEF value <4 can be considered as acceptable.
DoF at 8% EL was considered using an elliptic fit to the process window (by using a fixed focus constraint through pitch). Figure 3 illustrates how QUASAR illumination (without assists on mask) performs very well toward dense, but poorly toward isolated pitches. The conventional, Soft Annular, and Soft QUASAR sources on the other hand, perform rather comparable to one another at first sight. At the dense side, however, the SA and SQ illuminations are beneficial. Toward isolated, Soft QUASAR appears preferable with respect to DoF. In this work, a minimum DoF through pitch of 120nm at 8% EL was set as a criterion.
Choosing between Soft Annular and Soft QUASAR
If one has to make a choice of illumination source based on the above results, the preference may be Soft QUASAR, as it has the better EL, MEEF, and DoF through pitch. The second choice may be Soft Annular, which mainly has a little higher MEEF at dense, and a somewhat less DoF toward isolated. However, these considerations for Soft QUASAR and Soft Annular are only valid for regular XY arrays.
In practice, a contact hole design may have a random layout. One has to distinguish between contact hole layouts on-grid and not on-grid. In an on-grid design, the minimum pitch (140nm in this case) can only be present in the X or Y direction. For designs not on-grid, the minimum pitch may be found in any direction. The latter is illustrated in Fig. 4, where in all cases the smallest pitch is 140nm. On the reticle, all contacts were equally biased to 96nm (1×). Using Soft Annular, the minimum pitch prints well in all directions, while for Soft QUASAR, this is only the case for the on-grid design (Fig. 4).
 Figure 4. SEM images of generic 2D clips at P140, comparing Soft QUASAR and Soft Annular. |
State-of-the-art: Contact printing at 1.35NA using Soft QUASAR
Using water-based immersion ArF lithography, the highest physically attainable NA is 1.35. This record NA has recently been achieved in ASML’s XT:1900Gi full-field scanners. Preliminary exposures using Soft QUASAR (SQ1 as in Fig. 1) confirm the scalability of the above observations toward 1.35NA.
 Figure 5. Experimental EL and MEEF obtained through pitch for Soft QUASAR (SQ1 as in Fig. 1). (ASML XT:1900Gi at 1.35NA.) |
Namely, acceptable MEEF and EL through pitch have been found starting at a k1 value of ~0.42, or 60nm half-pitch at this NA, as shown in Fig. 5. As for DoF at 8% EL, 140nm is obtained at the dense side, and 100nm at the isolated side (not shown).
Conclusion
Soft Annular and Soft QUASAR source shapes provide a balanced trade-off between conventional and higher OAI, such as Annular or QUASAR, and offer a benefit for assist-free contact hole printing.
Soft QUASAR may be the preferred option in the case of on-grid contact hole designs, while Soft Annular is applicable in designs where the minimum pitch is not XY restricted. In comparison to the conventional approach, a gain in resolution is obtained while maintaining performance through pitch. These combined sources prove to be the preferred solution for through-pitch contact hole printing, in terms of MEEF and process latitude, without the need for assist features.
Acknowledgments
Soft Annular and Soft QUASAR are registered trademarks of ASML. The authors are grateful to ASML for providing the customized Soft Annular and Soft QUASAR DOEs, and the assistance in the installation of the elements in the ASML XT:1700Fi and XT:1900Gi scanners.
Reference
1. Jo Finders et al., SPIE Proc. of the Optical Microlithography Conference, 6154-12, 2006.
For more information, contact Joost Bekaert, a researcher, optical extensions, in the lithography group at IMEC, Kapeldreef 75, B-3001, Leuven, Belgium; e-mail [email protected].
Also in IMEC’s lithography group, Vincent Truffert works as an R&D engineer for backend applications, Patrick Willems is a process engineer and metrology expert, and Lieve Van Look and Eric Hendrickx are researchers, optical extensions. Maaike Op de Beeck is a member of IMEC’s lithography group for backend applications.
Geert Vandenberghe is the leader of the optical extensions and reticles group, and manages the immersion and hyper-NA lithography program at IMEC.