Simulation and Improvements of Two-Photon Polymerization Fabricating Process
Date Issued
2012
Date
2012
Author(s)
LIN, Chin-Te
Abstract
Two-photon polymerization (TPP) is a non-linear optics technology based on two-photon absorption (TPA) and able to produce three-dimensional micro-objects with arbitrary shapes and sub-micron resolutions. Owing to the development of micro-chip lasers and the discovery of various photopolymers, the TPP systems are not only sold with affordable prices, but also have wider and wider fields of the applications. For the urgent requirements of the applications, TPP fabrication studies for better quality controls and higher resolutions have become interesting topics during the last decade.
A virtual fabrication program for TPP is developed to predict the TPP polymerized shapes. During the TPP fabrications, a laser gives an exposure to form a voxel, which is the TPP basic element. After all designed voxels are scanned with the laser and the insufficiently compact polymer is removed in a rinsing process, a micro-object composed of the voxels can be obtained. The virtual fabrication program computes the shapes of micro-objects by determine whether the polymerizing level is over a threshold value. The photo-polymerizing level is linear to the initiating radical species concentration, which is proportional to the square the laser intensity. In the TPP free-radical mathematic model without the consideration of diffusion effects, the local initiating radical species concentrations can be summed up to the global concentration. Thus, the global polymerizing level can be defined by accumulating all results of the local voxels. Through this manner, the boundary of the polymerized solid will be found out, and the produced shapes of micro-object can be determined.
The effect of the intensity of the focused laser is studied because the polymerized region of the TPP strongly depends on the laser intensity. The setup of the TPP system makes a collimating laser beam go through an objective lens with a high numerical aperture, immersion oil, and a coverslip before the beam focuses inside the photopolymer. The real intensity distribution of the focused beam can be calculated with the analytical expressions of the point spread function (PSF), and the result indicates that the passage causes the beam imperfect. In order to study the effect of the real beam, a series of the nano-lines are fabricated. With the measurement of the high resolution scanning electrical microscopy (SEM) and the simulation of the virtual fabrication program, it can be observed that the produced shapes of these samples can be quantified by the real beam model. Therefore, it can be concluded that the polymerized behaviors of the TPP would be defined by the characteristics of the real beam.
Laser power correction approach of angular defects is carried out to improve the product quality fabricated by TPP. During TPP fabrication, the laser follows calculated trajectories and exposures all voxels to give rise to complex microstructures. When the trajectories have sharp angles, the laser will give the angles more exposure and produce large structures, called angular defects. The angular defects are diagnosed by the help of the virtual fabrication program, and the diagnosis is verified by the atomic force microscopy (AFM) measurements of the samples with angular defects. To correct angular defects, a method of the laser power correction and formulas for the correcting power are proposed. The formulas are derived from the comparison of both exposure conditions of the line region and the defect region. Samples are fabricated with the laser correction approach and then measured, and the results show that the proposed method is effective to remove the angular defects. This manner will benefit the TPP fabrications using the contour scanning methods.
Subjects
two-photon polymerization
fabricating simulation
micro-chip laser
beam quality
laser correction method
angular defect
Type
thesis
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