Reticle Enhancement Techniques Research Articles

A Novel Wire-Density-Driven Full-Chip Routing System for CMP Variation Control

As nanometer technology advances, the post chemical-mechanical polishing (CMP) topography variation control becomes crucial for manufacturing closure. To improve the CMP quality, dummy-feature filling is typically performed by foundries after the routing stage. However, filling dummy features may greatly degrade the interconnect performance and significantly increase the input data in the following time-consuming reticle enhancement techniques. It is, thus, desirable to consider wire-density uniformity during routing to minimize the side effects from aggressive post-layout dummy filling. In this paper, we present a new full-chip grid-based routing system considering wire density for reticle planarization enhancement. To fully consider a wire distribution, the router applies a novel two-pass top-down planarity-driven routing framework, which employs new density critical area analysis based on Voronoi diagrams and incorporates an intermediate stage of a density-driven layer/track assignment based on incremental Delaunay triangulation. Experimental results show that our methods can achieve a more balanced wire distribution than state-of-the-art works.

Lens aberration aware placement for timing yield

Process variations due to lens aberrations are to a large extent systematic, and can be modeled for purposes of analyses and optimizations in the design phase. Traditionally, variations induced by lens aberrations have been considered random due to their small extent. However, as process margins reduce, and as improvements in reticle enhancement techniques control variations due to other sources with increased efficacy, lens aberration-induced variations gain importance. For example, our experiments indicate that delays of most cells in the Artisan TSMC 90nm library are affected by 2--8% due to lens aberration. Aberration-induced variations are systematic and depend on the location in the lens field. In this article, we first propose an aberration-aware timing analysis flow that accounts for aberration-induced cell delay variations. We then propose an aberration-aware timing-driven analytical placement approach that utilizes the predictable slow and fast regions created on the chip due to aberration to improve cycle time. We study the dependence of our improvement on chip size, as well as use of the technique along with field blading which allows partial reticle exposure. We evaluate our technique on two testcases, AES and JPEG implemented in 90 nm technology. The proposed technique reduces cycle time by 4.322% (80ps) at the cost of 1.587% increase in trial-routed wirelength for AES. On JPEG, we observe a cycle time reduction of 5.182% (132ps) at the cost of 1.095% increase in trial-routed wirelength.

RET simulations for SLM-based maskless lithography

In this paper, we have preformed some simulations of reticle enhancement techniques (RET) for SLM-based maskless lithography. Our results show that, on the one hand, increased slope and larger process window can be simultaneously obtained by overtilting some mirrors around the printed structure; On the other hand, fully making use of the grayscaling generated by tilting a mirror across it, a maskless tool can use the same OPC model as a mask-based scanner, such as gray level serifs, sub-resolution scatter bars, transmittance controlled mask, and so on. The results also imply that optical maskless lithography (OML) provides the ability in attaining high resolution comparable to the mask-based lithography.

Analysis of the diffraction pattern for optimal assist feature placement

Assist features (AF) are an essential component of reticle enhancement techniques. Their use is indispensable in sub-100 nm technologies to ensure a maximum process window (PW) across chip, especially for critical levels. Indeed, AF, which can be binary, attenuated or phase-shifted, help in providing a larger PW to the features they assist when they are used in conjunction with off-axis illumination. The depth of focus (DOF) of an isolated structure is improved by the presence of AF by providing to the optical system a diffraction pattern close to the diffraction pattern of a dense structure. The resulting DOF and exposure latitude (EL) are dependent on the relative position of the AF from the main feature. Moreover, the PW varies while inserting one, two or more AF. The relative position of each influences the results. In this paper, a method will be detailed to optimise the placement of the AF. For the purpose of this study, the diffraction pattern induced by the insertion of one or several AF is analysed in frequency space. This analysis details the evolution of the intensity of even and odd orders during the insertion of AF. The calculation of the optimum placement is detailed, and the DOF resulting from the insertion of one or more AF is also presented.

Exploring Acidic Functionalities in the Design and Development of High Performance 193nm Photoresist Polymers

The combination of immersion lithography and reticle enhancement techniques (RETs) has extended 193nm lithography into the 45nm node and possibly beyond. In order to fulfill the tight pitch and small critical dimension requirements of these future technology nodes, the performance of 193nm resist materials needs to further improve. In this paper, we have systematically studied a variety of acidic functionalities in an effort to improve the dissolution properties of 193nm resists. The hexafluoroalcohol (HFA) and fluorosulfonamide (FSM) are found to be most effective in eliminating the swelling behavior associated with typical 193nm methacrylate resists. The HFA and FSM groups are highly transparent at 193nm. High performance 193nm resists are developed with the HFA and FSM acidic groups. Compared to the HFA group, the trifluoromethyl sulfonamide (TFSM) functionality has a lower pKa value and contains less fluorine atoms. Polymers containing the TFSM functionality have exhibited improved dissolution properties and better etch resistance than their HFA counterparts. Resists based on the FSM-containing polymers have shown superior lithographic performance for line, trench and contact hole levels under the 45nm node exposure conditions. In addition, FSM resists have also demonstrated excellent bright field and dark field compatibility and thereby make it possible to use one resist for both bright field and dark field level applications. Finally, to alleviate the adverse impact of the fluorine content in the etch resistance of the 193nm resists, we have also evaluated a non-fluorinated acidic functional alternative-naphthol.

PEB bake optimization for process window improvement of mixed iso-dense pattern

We have shown that process effects induced by extending the post-exposure bake temperature in the process flow of chemically amplified photoresists can lead to significant improvements in depth-of-focus (DOF) and exposure latitude (EL) and small geometry printing capability. Due to improved acid dose contrasts and a balanced optimization of acid diffusion in the presence of quencher, PEB temperature increase has enabled the printing of iso and semi-dense space of 0.2 µm and below with a large DOF, using binary masks and 248 nm lithography without expensing the iso-dense bias. The results and findings of a full patterning process in a device flow, with different PEB temperatures as a process enhancement, are presented. The main objective of this study is to demonstrate how, using KrF lithography with binary masks and no optical proximity correction (OPC) nor other reticle enhancement technique (RET), the process latitude can be improved. Lithographic process latitudes, intra-field critical dimension (CD) uniformity and resist profiles of different PEB processes are evaluated. Then, the after-etch profiles are also investigated to ensure the feasibility of this technique.

Optimum mask and source patterns to print a given shape

New degrees of freedom can be optimized in mask shapes when the source is also adjustable, because required image symmetries can be provided by the source rather than the collected wave front. The optimized mask will often consist of novel sets of shapes that are quite different in layout from the target integrated circuit patterns. This implies that the optimization algorithm should have good global convergence properties, since the target patterns may not be a suitable starting solution. We have developed an algorithm that can optimize mask and source without using a starting design. Examples are shown where the process window obtained is between two and six times larger than that achieved with standard reticle enhancement techniques (RET). The optimized masks require phase shift, but no trim mask is used. Thus far we can only optimize two-dimensional patterns over small fields (periodicities of ;1 mm or less), though patterns in two separate fields can be jointly optimized for maximum common window under a single source. We also discuss mask optimization with fixed source, source optimization with fixed mask, and the retargeting of designs in different mask regions to provide a common exposure level.