Lien-Sheng Chen

Application of Cesàro mean and the L-curve for the deconvolution problem

In this paper, the Cesaro mean technique is applied to regularize the divergent problem which occurs in ground motion deconvolution analysis in geotechnical engineering. To deal with this ill-posed problem, we use the corner of the L-curve as the compromise point to determine the optimal order of Cesaro mean so that the high frequency content can be suppressed instead of engineering judgement using the concept of a cutoff frequency. The fractional order of Cesaro mean has been derived and used to fulfill this purpose. From the examples shown, it is found that the wave form including maximum acceleration can be accurately predicted and that both the high frequency content and divergent results can be avoided by using the proposed regularization technique.

Free vibration of a SDOF system with hysteretic damping

An integral±differential equation (IDE) in the time domain is proposed for the free vibration of a single-degree-of-freedom (SDOF) system with hysteretic damping which is different from the conventional complex stiffness model as employed in the frequency domain. The integral of the Hilbert transform is embedded in the IDE and is calculated in the Cauchy principal value sense by using a numerical folding technique. Numerical experiments show that the free vibration obtained by the frequency domain approach satis®es the IDE in the time domain. A successive iteration algorithm is employed to solve the IDE subject to forced vibration, and a convergent solution for the hysteresis loop is constructed, which matches the solution found by using the frequency domain approach. Both models, the time domain and frequency domain approaches, present the noncasual effect since they are equivalent in the mathematical sense. q 1998 Published by Elsevier Science Ltd. All rights reserved.

Robust loop-shaping control for a nano-positioning stage

This paper proposes robust loop-shaping techniques for a two-axis nano-positioning piezoelectric stage. Piezoelectric transducers are usually used to drive precision mechanisms because of their favorable properties, such as high resolution, high accuracy, and large driving force. However, the nonlinear characteristics of piezoelectric materials can degrade system performance. Therefore, we model a piezoelectric stage as a linear system, and regard its nonlinear factors as system uncertainties. Because robust control can guarantee stability and performance for systems with uncertainties and disturbances, we apply loop-shaping techniques and design standard robust controllers for the stage. In addition, we consider fixed-order robust control for the system in that controllers with lower orders are preferred for hardware implementation. Lastly, the designed controllers are implemented for experimental verification. The results demonstrate the effectiveness of these robust controllers in tracking reference signals and suppressing high-frequency vibrations.

Stitching periodic submicron fringes by utilizing step-and-align interference lithography

The authors develop a step-and-align interference lithography system to fabricate large-area periodic submicron structures by stitching the unit exposure area step-by-step. A metal mask with a square transparent window in the center is used to intercept the quasi-flat-top region of the expanded Gaussian beam, and thus it serves as a beamshaper to approximate the ideal unit beam that has uniform intensity and spatial coherence. Two-dimensional precision dual-actuator motion stages could provide travel distance for full wafer exposure with 2nm high precision positioning capability for stitching the submicron patterns. The gratings with period of 700nm are successfully stitched along two directions on 100mm diameter wafers.

Line stitching in servo-assisted electron beam lithography system

Maskless electron beam direct write lithography gains more and more attention in the candidates of next-generation lithography technologies. Pattern-placement inaccuracy is a challenging topic which is vital for seamless pattern generation. In this work, a high-resolution piezo-actuated nano stage is integrated into a commercial scanning electron microscope and cooperates with a two-axis coarse stage. Three-axis laser interferometer system is setup in surrounding of the vacuum chamber to establish a global positioning scale of the composite wafer stage. Feedback control is achieved by PID controllers which are implemented in a digital control system. Coupling of the multi-axis piezo-stage is shown, and the performance of the PID controllers is also demonstrated. Stitching of sub-micron line-type pattern over millimeter-range areas is demonstrated.

Hybrid servo design for large area nano pattern stitching

Interference Lithography offers a low-cost alternative to the next-generation lithography (NGL) technology. Using ultra-violet light, the interference lithography could produce patterns with nano meter feature sizes. Because the working area of the focused laser is usually very small, it is necessary to repeat the exposure for large area applications. This research presents a precision 2D servo system to patch the successive lithography patterns together. The period of the pattern from the interference lithography is 600 nm. To achieve seamless patching, the system has to achieve the same stitching accuracy throughout the entire the length of the lithography pattern. In this research, the servo control system achieves typically less than 20 nm overall stitching accuracy to patch the patterns together.

Stitching Technology Using Hybrid Actuators in Nano Imprint

With advancements in nanotechnology and the continuing reduction of the minimum feature size in integrated-circuit technologies, there is a need for next-generation lithography (NGL) tools. The direct transfer of grating structures stitched from interference lithography to a mold for nano imprinting offers a low-cost alternative for printing sub-100nm features with great potential accuracy, high resolution, and reductivity. This research presents dual stage laser-interferometer equipment to meet these requirements. In moving forward to the stitching of a small interference area, the most important issue is alignment. If the period of this interference fringe can be guaranteed, the stage can be moved in chronological alignment with the period. This paper also presents SEM results of stitching of a 600nm periodic structure.Copyright