Baoxi Yang

Generation of trapezoidal illumination for the step-and-scan lithographic system

To meet the uniform dose exposure in optical lithography, it is desirable to get uniform illumination and trapezoidal illumination in the scanning direction on the wafer for the step-and-scan lithographic system. A design strategy for trapezoidal illumination is proposed which offers both high uniformity illumination and fixed integral energy of trapezoidal illumination in a different illumination pattern. The strategy describes a light uniform device which contains a microlens array, a microcylindrical array, and a Fourier lens and produces trapezoidal illumination directly at the scanning slit. Compared with the conventional method to obtain the trapezoidal illumination, the strategy in this paper reduces the difficulty of assembly and has the advantage of simplicity and directness without blocking energy. This method utilizes energy of the lithographic system more effectively, and it improves the throughput of the lithography. The simulation results show that this method not only maintains the uniformity of trapezoidal illumination but also improves the energy utilization.

Off-line inspection method of microlens array for illumination homogenization in DUV lithography machine

Illumination uniformity is one of the key specifications of lithography illumination system because of its strong influence on the critical dimension (CD) uniformity in optical lithography. Refractive microlens array (MLA) has been extensively adopted in lithography system to achieve highly homogeneous illumination field with less light loss relative to diffractive element. Off-line homogenization inspection of the MLA provides important data for entire system integration. It is still a challenge work to investigate the optical performance for such high-end MLA with large clear aperture and high sensitivity to the incident light parameters. In order to address these issues, subaperture stitching method has been proposed to be applied and studied in this work. The feasibility of this method has been verified by theoretical simulation of a diffracting homogenizer. In the experiment, a corresponding optical setup is constructed, and a crossed-cylindrical single-plate MLA has been tested. The experimental results are consistent with the simulation ones. It could be concluded that subaperture stitching method is a powerful method to evaluate the homogeneous performance of MLA.

Test of diffractive optical element for DUV lithography system using visible laser

Diffractive optical element (DOE) is used for off-axis illumination extensively in DUV lithographic system. A method for testing the optical performance of DOE using a visible laser is proposed to simplify the test process. In principle, the optical performance of DOE with a visible laser is analyzed with scalar diffraction theory and numerically simulated using MATLAB program. Compared with the DUV condition, the diffractive pattern distribution is enlarged proportionally with a zero-order spike under the visible laser condition. In experiments, the DOE is tested under He-Ne laser. Its far field diffractive pattern is compared and analyzed with the result tested under the working wavelength of 193.368nm. The shape, the opening angle, the azimuth angle and the pole balance coincide with the values tested in DUV condition. The usefulness of method is verified.

Method of pupil shaping for off-axis illumination in optical lithography

Off-axis illumination is one of the key resolution enhancement technologies in projection lithography systems. Phase type diffractive optical elements are adopted by most of the lithography machine manufacturers to realize off-axis illumination. In this paper, a method of pupil shaping for off-axis illumination in optical lithography is introduced which contains a zoom beam expander, circularly symmetric diffractive optical elements, and a Fourier lens. The method could produce the required illumination pattern for off-axis illumination at the pupil plane. Compared with the conventional method of off-axis illumination, the method in this paper could eliminate deterioration of the pupil thoroughly and reduces the difficulty of the optical design of the zoom lens. Based on this method, several circularly symmetric diffractive optical elements are designed for experiments, and a remarkable improvement in eliminating deterioration of the pupil is observed compared with the conventional method.

Study on the highly intelligent uniformity correction and the application for lithography

The uniformity of the illumination field in the scanning direction is an important factor that affects the lithographic overlay accuracy as well as Critical dimension uniformity (CDU). With the improvement of lithography resolution illumination integrated uniformity is also increasing. To improving illumination integrated uniformity, a highly intelligent uniformity correction is introduced, which is used to correct the illumination integrated uniformity by inserting a plurality of independent movable correction plate arrays into the illumination field. In addition, a correction algorithm based on step by step is proposed. The simulation results show that the corrected illumination integrated uniformity is better than 0.3%, which is meeting the requirements of illumination integrated uniformity for 65nm node lithography.

Measurement precision improvement for excimer laser pulse energy sensor based on a special finite impulse response filter

In deep ultraviolet lithographic tools, precise measurement of excimer laser pulse energy is essential. However, system noise of the energy sensor, which is used to monitor the laser pulse during exposure dose control, complicate pulse energy measurement. To improve measurement precision, a specific finite impulse response filter is proposed for signal denoising. Particularly, the convolution kernel of the specific finite impulse response filter is derived from the nonlinear fitting of the energy sensor output. Experimental results show that the measurement precision of the energy sensor improves 2-3 times with the specific finite impulse response filter. To verify the effectiveness of the energy sensor, it is used for transmittance measurement; the measurement result of a Corning 7980 witness sample is consistent with that measured by a commercial ultraviolet spectrophotometer and the measurement accuracy is 0.11% (k = 2).

Accurate measurement of illumination uniformity using spot sensor based 2-dimension scanning method

Illumination uniformity of the illuminator is significant for achieving stringent critical dimension (CD) control for lithography machine. In order to achieve high uniform illuminating, there is an urgent demand of accurate measurement for illumination uniformity. The difficulty for accurate measurement of illumination uniformity for the illuminator mainly lies in two aspects: the illumination plane is large; excimer laser pulse energy is variable from pulse to pulse. In this work, a spot sensor based 2-dimension scanning method for illumination uniformity measurement is proposed, where the spot sensor in combination with a 2-dimension moveable stage is located in the illumination plane of the illuminator is used to scan the illumination plane point-by-point so as to obtain the whole irradiation distribution. To improve measurement accuracy, the energy sensor of the illuminator serves as the reference and monitors each pulse in real-time showing benefit of excimer laser pulse energy fluctuation eliminating. Secondly, the used spot sensor is modified for strict synchronization control of the spot sensor and the energy sensor so that the measurement precision can be improved. Measurement results show that X direction transient non-uniformity, Y direction transient non-uniformity and X direction integral non-uniformity of the illuminator are 5.14%, 5.55% and 2.16% respectively with a measurement uncertainty of 0.31% (k=2). It is proved that the proposed method is effective and helpful for further system optimizing and alignment.

Design and simulation of double annular illumination mode for microlithography

Methods of generating various illumination patterns remain as an attractive and important micro-optics research area for the development of resolution enhancement in advanced lithography system. In the current illumination system of lithography machine, off-axis illumination is widely used as an effective approach to enhance the resolution and increase the depth of focus (DOF). This paper proposes a novel illumination mode generation unit, which transform conventional mode to double annular shaped radial polarized (DARP) mode for improving the resolution of micro-lithography. Through LightToolsTM software simulation, double annular shaped mode is obtained from the proposed generation unit. The mathematical expressions of the radius variation of inner and outer rings are deduced. The impacts of conventional and dual concentric annular illumination pattern on critical dimension uniformity were simulated on an isolated line, square hole and corner. Lithography performance was compared between DARP illumination mode and corresponding single annular modes under critical dimension of 45nm. As a result, DARP illumination mode can improve the uniformity of aerial image at 45nm node through pitch varied in 300-500 nm to a certain extent.