Anthony Yen

Extension of deep-ultraviolet lithography for patterning logic gates using alternating phase shifting masks

We present the combination of alternating phase-shifting masks and rule-based optical proximity correction (OPC) for the patterning of 0.13 μm polysilicon gate lines. Using the optimal process condition, a process window with 1.2 μm of depth of focus (DOF) and 15% exposure latitude is obtained for 0.13 μm isolated lines in resist. In addition, with the application of rule-based OPC, the common DOF for 0.13 μm lines of various line-to-space ratios can be improved from 0 to 0.6 μm.

Characterization and correction of optical proximity effects in deep‐ultraviolet lithography using behavior modeling

We present the characterization of optical proximity effects and their correction in deep‐UV lithography using an empirically derived model for calculating feature sizes in resist. The model is based on convolution of the mask pattern with a set of kernels determined from measuring the printed test structures in resist. The fit of the model to the measurement data is reviewed. The model is then used for proximity correction using commercially available proximity correction software. Corrections based on this model is effective in restoring resist linearity and in reducing line‐end shortening. It is also more effective in reducing optical proximity effects than corrections based only on aerial image calculations.

Illuminator design for the printing of regular contact patterns

Abstract Diffraction patterns of two different contact arrays of 0.32 μm pitch are investigated using the AIMS TM tool and analyzed using Fourier analysis. We introduce a simple method to analytically predict the pupil pattern for an arbitrary periodic mask feature. A circular illuminator is split up into components that produce varying amount of interference at the wafer plane and it is shown that contrast can be enhanced by excluding certain parts of this illuminator. The optimized illuminator depends only on stepper parameters and mask geometry.

Optical proximity effects and correction strategies for chemical-amplified DUV resists

Optical proximity effects (OPE) are narrowing the process window in the 0.25micrometers - 0.18micrometers CD range. Hence optical proximity correction (OPC) might be required. These proximity effects and correction strategies are studied in detail in this work. First, an evaluation methodology is derived for the three types of OPE (linewidth differences with pitch, end-of-line effects and corner rounding). Hence, the influence of various parameters on OPE is investigated for negative tone and positive tone resists, since clear differences exist in OPE for dark field and bright field masks. Linewidth differences with pitch are small for negative tone resists, end-of-line effects are less pronounced for positive tone materials. Obviously, optical parameters have an important influence on OPE. Also, loading effects during etch processes deserve attention. Aerial image based proximity correction is evaluated. With respect to CD variations with pitch, important improvements are obtained for some resists, but not for all materials. End-of-line effects and corner rounding are improved by the use of OPC in all our experiments. Superior proximity correction results are expected with the expansion of aerial image based OPC by implementation of resist models.

Low-k1 optical lithography for 100 nm logic technology and beyond

In this article, we present 193 nm lithography at a k1 factor of 0.37–0.40 and discuss several topics important to 100 nm logic such as optical proximity correction (OPC), control of critical dimension (CD) variation, and lowering of the mask error factor (MEF). In OPC, the best correction results can be achieved by developing accurate models and using reasonable segmentation rules. The technique of variable-dose exposures is demonstrated as a means to reduce interfield CD variation once the cause is known and can be characterized. A more than 30% reduction in CD variation is realized for variation caused by temperature nonuniformity in hot plates. The concept of two dimensional (2D) MEF is introduced to describe situations at feature ends. Higher numerical aperture or more effectively, quadrupole illumination, can be used to lower 1D and 2D MEFs. We also explore the technique of dipole illumination, which may be a candidate for taking optical lithography to k1<0.35.

Feature biasing versus feature-assisted lithography: a comparison of proximity correction methods for 0.5*(lambda/NA) lithography

The effectiveness of two methods of optical proximity correction based on feature biasing and subresolution assisting features is compared by simulation and experiments. Parameters examined are overlapping focus- exposure windows for dense lines, semi-isolated and isolated lines, and line-end shortening. Binary and phase-shifting masks containing test and real IC design features are proximity corrected either by commercial software (in the case of feature biasing) or by manual correction using optimized size and placement of assisting features. The results indicate that, while both methods are effective in reducing optical proximity effects, the feature-assisted method is more advantageous in many cases.

Effect of various ArF resist shrinkage amplitudes on CD bias

The beam parameters of CD SEM, accelerating voltage, beam current, measurement time, frame number, and magnification are evaluated to get the optimal setting for reducing the shrinkage of ArF resist. We check image resolution, resist shrinkage amplitude, CD bias between resist line and etched pattern to valuate the impact of beam parameters. On image resolution, the poly film is better resolved with the 800 V accelerating voltage. On the other hand, 300 V is more suitable for resist image. It also produces much lower resist shrinkage compared with 800 V. Beam current, measurement time, frame number, and magnification produce much less impact on resist shrinkage than the accelerating voltage. On CD bias, we also found that 300 V produces better accuracy and stability compared to 800 V. This is attributed to the lower resist shrinkage. Finally, we suggest an important concept that the optimal beam condition cannot be judged only by precision and resolution but also by the resist shrinkage and CD bias stability.

Optical proximity effects correction at 0.25 um incorporating process variations in lithography

We study the optical proximity effect and its correction using empirically derived models for DUV lithography taking into account random process variations. The sensitivity of corrected configurations to different sources of process variation (exposure dose, defocus) is evaluated. For correction at a centered condition (optimum dose, zero defocus), problems may arise in ill-conditioned areas (inside corners of T-shape features, butting line-ends, etc.), when going away from the best focus and/or exposure dose, within the exposure/defocus window. Correction for sharp corners (aggressive correction) shows a stronger sensitivity to defocus than less corner sharpening (conservative correction). Furthermore, we study what types of design configurations tend to print poorly with process variations and investigate alternative correction optimization schemes that stabilize the printing performance in such areas. Various optimization alternatives to improve performance within the process window are evaluated.

Quarter-micron lithography using a deep-UV stepper with modified illumination

We have investigated the use of annular illumination on a KrF excimer laser stepper ((lambda) equals 248 nm) working near the resolution limit of the lens. The numerical aperture of the lens was 0.48 and the illuminator-lens combination produced a partial coherence of 0.44. With a central obscuration equal to 75% of the diameter of the illuminator aperture in place and using a surface-imaging resist process, we have increased the depth of focus for 0.25 micrometers dense lines and spaces from 0.9 micrometers at one point in the imaging field to 1.5 micrometers . Performance for dense contacts was also improved. These improvements demonstrate the feasibility of 0.25 micrometers technology with deep-UV lithography.

0.25μm lithography using a deep-UV stepper with annular illumination

Abstract We have investigated the use of annular illumination on a KrF excimer laser stepper (λ=248nm). The stepper has an NA=0.44 and a partial coherence (σ) of 0.45. The central obscuration used to produce annular illumination was 75%. This illumination technique, combined with a negative tone surface-imaging resist process, has demonstrated a depth of focus (DOF) of 1.6μm for 0.25μm dense lines and spaces and 1.3μm DOF for 0.3μm dense contacts.