The wide application of grating in engineering puts forward higher requirements for fabrication efficiency and precision. Maskless lithography based on digital micromirror device (DMD) is considered to be a promising technology with the advantages of high efficiency, low cost, and good flexibility. However, DMD-based digital lithography has been implemented mostly for micron-scale fabrication, which restricts its application to microstructures with submicron feature sizes. In this study, to realize the rapid fabrication of the grating of variable submicron line width, we adopt two misaligned DMDs to collaboratively modulate the exposure dose. By flexibly adjusting the misalignment parameter and the feature size parameter of the original grating mask, the gratings of variable micrometer and submicron line width are fabricated. We also evaluate the fabrication results by the photomicrographs and the optical diffraction modulation effects of the gratings. DMDs collaborative modulation lithography may be a feasible solution for DMD-based digital lithography, which can balance the lithography resolution, the fabrication efficiency and the manufacturing cost.
At present, the resolution of digital lithography based on digital micromirror devices is constantly improving. The assembly and debugging of traditional autofocusing devices are complicated, and the focus detection precision and range are difficult to meet the requirements of high precision digital lithography. Therefore, it has become an important research topic to explore the matching digital lithography autofocusing method. In this paper, a set of high resolution digital lithography system is established, which integrates digital lithography autofocus algorithm with variable step size peak search autofocus algorithm. Various common image sharpness evaluation functions and objectives at a variety of magnifications are used to realize the autofocus algorithm, and the performance of the image sharpness function is analyzed comprehensively. The experimental results show that by using algorithm fused with the autofocusing device in this paper, focus detection ranges of 10x, 20x and 40x objective lenses can reach 280μm, 160μm and 105μm, respectively, and the autofocusing accuracy can reach 0.625μm, achieving the submicron precision and large range autofocus at various magnifications. The experimental results of this paper can provide important reference value for autofocus of submicron scale digital lithography.
The wide application of spiral phase plate has promoted the rapid development of its production technology. Traditional spiral phase plate fabrication techniques such as electron beam lithography, focused ion beam lithography, laser direct writing have the disadvantages of high cost and low efficiency due to expensive equipment and point-by-point exposure. In this paper, we propose a fast preparation method for spiral phase plates based on digital grayscale mask. The 3D contours of the spiral phase plate are mapped into a grayscale mask by interval quantization, and the criteria for interval quantization are explored in detail. For the continuous spiral phase plate and step spiral phase plate, gray scale continuous digital mask and gray scale step digital mask are designed and used for digital lithography respectively, and the experimental results are tested and analyzed. The results show that spiral phase plates with different topological charges for specific wavelength can be fabricated by controlling the gray-scale digital mask. The fabricated spiral phase plates have smooth surfaces with well-defined edge contours and are capable of initially modulating the laser beam.
Brillouin scattering lidar is a new frequency modulation detection technology, which has a broad application prospect in marine environment monitoring and underwater target detection. This paper introduces three remote sensing techniques used in practical application to realize Brillouin scattering lidar. Through the comparison of the measurement principle, measurement method and detection accuracy of three technologies, the main application characteristics of three detection techniques and the practical process in the field of ocean exploration are further understood, so as to improve the Brillouin scattering lidar system.
High power laser bean can give rise to nonlinear change in the distribution of acoustic wave field and excite scattering Brillouin scattering (SBS) in water. At present, the uniform grating theory is used to analyze the spectral distribution of stimulated Brillouin scattering, which is inconsistent with the experiment. In this paper, three kinds of refractive index distribution model for reflection spectra were simulated by using the method of multilayer dielectric film, and the changes of the reflection spectra of different refractive index distribution models were analyzed.
In order to understand the stimulated Brillouin scattering (SBS) process in water, it is necessary to analysis theoretically the factors which to determine the SBS gain coefficient and threshold. The coupled wave equation is used to investigate the effects of temperature, salinity and attenuation coefficient on SBS gain coefficient and threshold. The results indicate that the higher the temperature and salinity, the greater the SBS gain. And the threshold is negatively correlated with the temperature. The effects of attenuation coefficient on threshold of steady-state SBS is greater than transient SBS.
Measurement of sound velocity in water by Brillouin scattering method was proposed in this paper. According to the relationship between Brillouin scattering and ocean parameters, the method for measuring sound velocity by stimulated Brillouin scattering frequency shift in water was analysed theoretically and experimentally. The theoretical calculation value of sound velocity in pure water and different salinity seawater were given. Based on this, the sound velocity in pure water and 35‰ salinity seawater has been measured experimentally. The results show that sound velocity in water is obtained accurately by using Brillouin scattering and the measured results are in good agreement with the theoretical results.
The optical scattering characteristics of marine suspended particles not only directly affect the propagation characteristics of laser in seawater, but also serve as an important theoretical basis for optical monitoring of marine environment. In this paper, different experimental schemes and theoretical calculation methods were designed for the Au and ZnO nanoparticles dispersed in seawater with different sizes and shapes according to the angle distribution of scattering intensity. The results show that the angular distribution of scattering intensity of Au nanorod is different from the numerical results based on Mie scattering or Rayleigh scattering theory. In addition, at a specific wavelength, the angular distribution of scattering intensity of the seawater dispersion solution of ZnO nanospheres shows an obvious forward propagation characteristic, and when the particle diameter increases from 10nm to 200nm, the forward scattering intensity gradually dominates.
To improve the transverse resolution of digital mask lithography pattern, we present a novel dynamic fractal digital lithography to fabricate microlens array. The basic idea of this technique is to divide a high-frequency digital mask into a group of low-frequency masks by using equal-height-rounding quantization. Consequently, dynamic fractal digital lithography has the advantage in decrease of mask quantity and mask quantization error. Then these low-frequency masks are exposed in sequence and the superimposed exposure of multiple masks takes the place of single exposure of original high-frequency mask. We demonstrate the feasibility of dynamic fractal digital lithography by experimental fabrication. The uniform microlens array with smooth surface and clear edge profile are achieved.
Digital coding-mask technique based on digital micro-mirror devices (DMD) is proposed in this paper. The fundamental rule of digital coding-mask technique is to modulate the incident light intensity by adjusting the transmittance of the units on the coding-mask. The transmittance is controlled by the apertures on the units of the coding-mask. Lohmann’s III coding method and error diffusion coding method are employed to coding mask, and wavelet transformation is used to suppress the background noise of the mask image. Real-time control on the image of the digital coding mask can be realized by loading the coded mask image to DMD, which is driven by a computer. Digital coding-mask technique gives full play of the advantages of DMD, such as real time and flexibility. In addition, the digital coding-mask technique is helpful to deal with the problem of mask aberration, which is caused by the nonlinear effect in the process of projection and exposure. This technique can also make use of optimization algorithm to suppress the background noise of the digital coding-mask images so that the quality of the relief structure of photoresist is improved.
Based on DMD-lithography system, the mapping relation between gray scale and photoresist relief has been investigated.
We obtain DMD reflectivity of different gray scale by experiment. Testing curve shows the light modulation of DMD is
nonlinear from 0 to 255. However, the testing curve shows local linearization as well. The modulation of DMD is
approximately linear from 40 to 130 and from 160 to 230. When designing the gray-scale mask, we should choose the
gray scales in the same linear region to satisfy the requirement of multi-step relief. After being modulated by DMD, the
reflection with mask information passes through the reduction projection system and images on the photoresist. After
development, the photoresist relief can be formed. By comprehensively considering the influence of all parts on
lithography, the exposure model is set up. We calculate the gray tones of 4-step and 8-step elements respectively
according to the model. Experimental results show that the exposure model is reasonable and correct. The establishment
of exposure model has reference value for the precise control on relief depth.
An approach, which is used to fabricate diffractive optical elements (MOEs), is presented by applying reduction
projecting system based on digital micro-mirror device (DMD). In this paper, coding-mask is combined with DMD
which is real-time, flexible and easy for alignment. Pre-distortion according to the non-linear effect in the aerial
image and resist course can be implemented. The accumulation of exposure energy can be obtained through
modulating rotating velocity of mask graphics, and finally distribution of exposure, which is required, can be
achieved. It shows that the method is practical and feasible on the grounds of the results of MATLAB simulation and
experiments.
This paper presents a novel method to fabricate circular gratings based on binary optics. A smart digital micro-mirror device (DMD) laser writing system is used to fabricate circular gratings. By use of the DMD system, we are capable of making low-cost and useful circular gratings easily. Their levels and radii can be controlled and be suited for many optical systems. Experiment results have been obtained with two and four levels. In this paper, it will also show a method of simulating circular gratings' diffraction. Using the simulation method, we can value and check the diffraction effect of circular gratings to rectify the design parameters.
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