Qunying Lin

Customized illumination shapes for 193nm immersion lithography

In this paper, a study on customized illumination shape configurations as resolution enhancement for 45nm technology node will be presented. Several new source shape configurations will be explored through simulation based on 193nm immersion lithography on 6% Attenuated Phase Shift Mask. Forbidden pitch effect is commonly encountered in the application of off axis illumination (OAI). The illumination settings are often optimized to allow maximum process window for a pitch. This is done by creating symmetrical distribution of diffraction order on the pupil plane. However, at other pitch, the distribution of diffraction order on the pupil plane results in severe degradation in image contrast and results in significant critical dimension (CD) fluctuation. The problematic pitch is often known as forbidden pitch. It has to be avoided in the design and thus limited the pitch range to be imaged for particular illumination. An approach to modify off axis illumination to minimize the effect of forbidden pitch is explored in this study. The new customized shape for one dimensional line and space pattern is modified from current off axis illumination. Simulation study is done to evaluate the performance some customized shapes. The extent of CD fluctuation and CD through pitch uniformity is analyzed to determine the performance enhancement of the new illumination shapes. From simulation result, the proposed modification have significantly improved the through pitch performance and minimized the effect of forbidden pitch.

Improvement of Rayleigh criterion with duty ratio characterization for subwavelength lithography

In this article, we have systematically investigated the dependencies of k1 on illumination conditions such as coherence setting and opening angle in off-axis illumination scheme. As result, conventional Rayleigh’s equations are not sufficient to address the effect of the numerical aperature and coherence on the depth of focus. Therefore, a metric called the coherency factor (σc) is proposed as a complementary metric of the low k1 lithography. Coherency factor (σc) is defined as the ratio of areas of captured first order and zero order light. The theory is based on simple geometrical analysis of the diffraction orders in the pupil plane. Areas of different diffraction orders captured by the pupil are evaluated as a function of wavelength, numerical aperture, and pitch. As corresponding to experimental results, a higher σc value concurs with a larger depth of focus. Extracting from Fraunhofer diffraction equation for a single slit and incorporating coherency factor σc, we have modified and extend the use o...

Forbidden pitch improvement using modified illumination in lithography

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.

Edge effects characterization of phase shift mask

The demand for steadily decreasing dimensions in semiconductor devices is driving the need for increased resolution in optical lithography. The use of phase shift masks (PSMs) is among such resolution enhancement techniques. PSM are well known to show prominent diffraction effects, which cannot be described by the assumption of an infinitely thin mask (Kirchhoff approach) that is used in many commercial photolithography simulators. A correct prediction of sidelobe printability, process window and linearity of an OPC mask requires a rigorous application of diffraction theory. Optical lithography simulation employing a time-domain finite-difference (TDFD) algorithm (TEMPEST) has been used effectively to study the problem of aerial image intensity imbalance through focus with alternating phase shift masks (altPSMs). Using Geometrical Theory of Diffraction (GTD), and the solutions to canonical problems, we obtained a relationship between mask edge and disturbance in an image space. The main objective to devel...

A comparative study for mask defect tolerance on phase and transmission for dry and immersion 193-nm lithography

193nm immersion lithography has successfully enabled numerical aperture (NA) greater than 1.0 which allows rooms for improvement in resolution as well as depth of focus. In this study, critical dimension (CD) and depth of focus (DOF) performance for the 45nm technology node for dry and immersion lithography is compared using commercial available simulation tool. The study is based on one dimensional line and space pattern with pitch vary from 150 to 500nm. The effects of mask transmission and phase angle change on CD through pitch performance and DOF are also presented in this paper. Increase in mask transmission will result in increase of CD through pitch and reduction of DOF. When phase angle for the phase shift mask is less than 180 degree, CD through pitch and DOF drop. Finally, mask defects caused by haze on several locations which include MoSi lines, line edges, and space between line ends are simulated. The influence of these defects on CD and the potential line end bridging problem is presented.

Optimization of alternating PSM mask process for 65-nm poly-gate patterning using 193-nm lithography

Alternating phase shift mask will be one of the most possible solutions for 65nm technology node as the further delay of 157nm lithography and next generation lithography. In this paper, alternating phase shift mask is used to pattern 65nm poly gate on logic device using 193nm lithography. Double exposure of dark field phase mask and binary trim mask were superimposed on wafers using 193nm scanner. Both mask making process and wafer exposure process are optimized in order to obtain maximum process margin on wafer for 65nm gate CD for pitch of 170nm. The amount of intensity imbalance on alternating phase shift mask with various mask making processes is fully characterized to improve mask making process. Furthermore, the impact of mask making process on process margin is evaluated with and without mask process optimization. The results show that with mask process optimization, large DOF of 0.50μm can be achieved for 65nm line with 170nm pitch. However, without mask process optimization, resolution is limited to 240nm pitch only due to intensity imbalance in 0 degree and 180 degree features. In addition, the study also shows that with alternating phase shift mask, intermediate NA of 0.70 is suitable for 65nm technology as high NA of larger than 0.75 will decrease DOF performance.

Sub-0.10-μm lithography technology with resolution enhancement technique

Reduction of feature sizes in semiconductor circuits has pushed photolithography to print features below the wavelength of the light source. However, severe optical proximity effects and small depth of focus for isolated lines have brought challenges to sub-wavelength lithography for application to 0.10micrometers technology using 248nm and 193nm scanners. Resolution Enhancement Technique (RET) is applied extensively for 0.10 micrometers technology. The use of attenuated phase shift masks to improve the process latitude is well known from literature, however higher transmission attPSM is limited by side-lobe printing. In order to achieve desired critical dimension control, OPC is necessary needed with PSM. Although off-axis illumination improves resolution for dense features, it degrades exposure latitude and depth of focus for sparse features. Applying scattering bars (SB), also called assisting features, to an isolated line can modify the diffraction pattern similar to that generated by dense lines. A comprehensive study on how scattering bars could affect the performance of isolated and dense lines by adjusting their bar size and placement to the main feature edges as well as changing the illumination condition and exposure energy are performed. It is showed that SB is able to increase the depth of focus for isolated line and an acceptable 0.10 micrometers patterning process is achieved. Furthermore, SB-OPC helps minimize the negative impact due to lens aberration on isolated features. The results demonstrate that SB-OPC is one of the most manufacturable solutions for sub-0.10 micrometers line patterning using current DUV lithography.

Application of attenuated phase-shifting masks to sub-130-nm lithography

In this paper, the performance of 6% and 18% attenuated phase-shifting masks (PSM) are investigated to assess their capabilities of printing 0.12μm and 0.10μm polysilicon gates, using a 248nm scanner with a high NA of 0.68. The effect of off-axis illumination on process enhancement is also investigated. Simulations were done using PROLITH/3D Version 6.1.2. Experimentation was carried out using test masks with various line pitches. The effect of optical proximity correction (OPC) to enhance the overlapping process windows for 0.12μm and 0.10μm was also studied.

Circular apertures for contact hole patterning in 193-nm immersion lithography

A novel concept of contact holes patterning for 193 nm immersion lithography is demonstrated in this study. Conventional contact holes patterning involve targeting a square printed feature on the wafer and applying optical proximity correction (OPC) such as corner serifs addition and dimensional biasing. As dimension of contact holes reduces, the resolution enhancement provided by conventional OPC methods has become limited. This is because at smaller dimension, more light is diffracted towards higher order and is not captured in the pupil plane. As a result, the corners of the printed features are rounded and features appear circular as dimension reduces. Hence, the efforts made to generate OPC assist features using a square target are inefficient. In this paper, the patterning of contact hole using circular target is demonstrated. The imaging performance of isolated and regular contact holes array is reported. Comparison with conventional approach is made. The effects of the proposed method on critical dimension (CD), depth of focus (DOF), and image contrast is investigated.

Line end shortening and corner rounding for novel off-axis illumination source shapes

Previous study has shown that off-axis illumination (OAI) which employs duplicate conventional source shape such as double dipole, double annular or double quadrupole can reduce the effect of line width fluctuation and process window degradation at the forbidden pitch. In this paper, influence of the new OAI source shape on line end shortening and corner rounding effect is studied. Despite the advantage of reduced line width fluctuation, the proximity effect at line ends and corners for new source shapes need to be examined because both lateral and longitudinal pattern fidelity is important in actual implementation. Simulation study will be used for the study of line end shortening and corner rounding effect using new source shapes and the results will be compared with those resulted from annular illumination. Line end structures such as end to end, staggered, and T-shaped patterns are used for line end shortening study. For corner rounding, L-shaped and U-shaped structure are used. The pattern density and line end separation of feature will be varied to determine the important factors that cause image distortion. Results has shown that new source shapes have similar line end shortening and corner rounding characteristic with the conventional one. Besides, the variation of new source shapes for different pattern density and line end separation is relatively smaller compared with conventional OAI source shapes.