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Applied Optics

APPLICATIONS-CENTERED RESEARCH IN OPTICS

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Early Posting

Accepted papers to appear in an upcoming issue

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April


Circularly polarized light with high degree of circularity and low azimuthal error sensitivity

  • Jose Luis Vilas, Eusebio Bernabeu Martinez, Luis Miguel Sanchez-Brea, and Rafael Espinosa-Luna
  • received 01/30/2014; accepted 04/15/2014; posted 04/15/2014; Doc. ID 205744
  • [full text: PDF (742) KB)]
  • Abstract: Abstract We analyze how to obtain circular polarized light with a high polarization degree and a high stability using a system composed by two waveplates. The configurations studied involve half and quarter waveplates. We show how the high polarization degree is achieved using a λ/2+λ/4 configuration. However the highest stability under small variations in the azimuths of the waveplates is obtained employing a λ/4+λ/2 configuration. Analytical calculus are particularized for quartz and MgF₂ waveplates are presented.

Keywords (OCIS):

  • (160.1190) Materials : Anisotropic optical materials
  • (220.4830) Optical design and fabrication : Systems design
  • (260.0260) Physical optics : Physical optics
  • (260.1440) Physical optics : Birefringence
  • (260.5430) Physical optics : Polarization

A simple far-field radiative thermal rectifier using Fabry-Pérot cavities based infrared selective emitters

  • E. NEFZAOUI, J. Drevillon, Y. Ezzahri, and K. Joulain
  • received 02/04/2014; accepted 04/15/2014; posted 04/15/2014; Doc. ID 205934
  • [full text: PDF (1021) KB)]
  • Abstract: We present a thermal rectification device concept based on far-field radiative exchange between two selective emitters. Rectification is achieved due to a large contrast between the two selective emitters thermo-optical properties. A simple device constituted by two Fabry-Pérot cavities made of metallic (Au) and semiconductor (Si and HDSi) thin films is proposed. This device is the first rectifier based on photonic structures.It shows a rectification ratio increasing with temperature up to 19% for a temperature difference ΔT = 370 K. Presented results might be useful for energy conversion devices, smart radiative coolers / insulators engineering and thermal logical circuits development.

Keywords (OCIS):

  • (050.2230) Diffraction and gratings : Fabry-Perot
  • (160.6840) Materials : Thermo-optical materials
  • (290.6815) Scattering : Thermal emission
  • (010.5620) Atmospheric and oceanic optics : Radiative transfer

Design of resonant cavity structure for efficient room temperature operation of single photon avalanche photodiodes

  • Mahdi Zavvari, Kambiz Abedi, and Mohammad Karimi
  • received 02/10/2014; accepted 04/14/2014; posted 04/15/2014; Doc. ID 206174
  • [full text: PDF (657) KB)]
  • Abstract: A novel design of single photon avalanche photodiode (SPAD) is proposed based on resonant cavity (RC) structure and its performance is studied. In the proposed structure, InAlAs/InGaAs distributed Bragg reflectors (DBR) are employed as top and bottom mirrors and the quantum efficiency (QE) of absorption region is calculated for absorption region considering the effect of RC. Results show that using 12 periods of DBRs as bottom reflector without incorporation of top mirror can enhance the QE to about 90% at room temperatures. For this RC-enhanced SPAD, a single photon quantum efficiency (SPQE) is obtained about 0.35 at T=300K. For temperatures lower than T=260K, SPQE is about 1. Results show that although the RC doesn’t affect the dark current, however for a given SPQE the dark count rate is lower for RC-SPAD.

Keywords (OCIS):

  • (040.3060) Detectors : Infrared
  • (230.1480) Optical devices : Bragg reflectors
  • (040.1345) Detectors : Avalanche photodiodes (APDs)

Portable multispectral imaging system based on light-emitting diodes for spectral recovery from 370 to 1630 nm

  • Jorge Herrera-Ramírez, Meritxell Vilaseca, and Jaume Pujol
  • received 02/13/2014; accepted 04/14/2014; posted 04/15/2014; Doc. ID 206391
  • [full text: PDF (2287) KB)]
  • Abstract: To expand and investigate the potential of spectral imaging, we developed a portable multispectral system using light-emitting diodes. This system recovers spectral information from the ultraviolet to the near infrared over a large area using two different image sensors synchronized with 23 bands of illumination. The system was assessed for spectral reconstruction through simulations and experimental measurements by means of two methods of spectral reconstruction and three different evaluation metrics. The results over a Macbeth color checker chart and other samples, including pigments usually employed in art paintings, are compared and discussed. The portable multispectral system using light-emitting diodes constitutes a cost-effective and versatile method for spectral imaging.

Keywords (OCIS):

  • (110.0110) Imaging systems : Imaging systems
  • (120.4820) Instrumentation, measurement, and metrology : Optical systems
  • (230.3670) Optical devices : Light-emitting diodes
  • (110.4234) Imaging systems : Multispectral and hyperspectral imaging

Novel calibration method for structured light system with an out-of-focus projector

  • Beiwen Li, Nikolaus Karpinsky, and Song Zhang
  • received 02/13/2014; accepted 04/14/2014; posted 04/15/2014; Doc. ID 206416
  • [full text: PDF (3179) KB)]
  • Abstract: A structured-light system with binary defocusing technique has potential to have more extensive application due to its high speeds, gamma calibration free and no rigid synchronization requirement between the camera and projector. However, the existing calibration methods fail to achieve high accuracy for a structured light system with an out-of-focus projector. This paper proposes a method that can accurately calibrate a structured light system even when the projector is not in focus, making it possible for high-accuracy and high-speed measurement with the binary defocusing method. Experiments demonstrate that a root-mean-square (rms) error of 77 $\mu$m can be achieved with a calibration volume of $150(H)\times 250(W)\times 200(D) mm^3$

Keywords (OCIS):

  • (100.2650) Image processing : Fringe analysis
  • (120.0120) Instrumentation, measurement, and metrology : Instrumentation, measurement, and metrology
  • (120.5050) Instrumentation, measurement, and metrology : Phase measurement

Design of an Omni-directional Optical Antenna of UV Communication

  • Xuebin Zhang, Yi Tang, Heqing Huang, Lijun Zhang, and Tingzhu Bai
  • received 01/03/2014; accepted 04/12/2014; posted 04/15/2014; Doc. ID 203359
  • [full text: PDF (497) KB)]
  • Abstract: In this paper, an omni-directional large field optical antenna structure for ultraviolet (UV) communication is proposed and demonstrated. Theoretical analyze shows it’s suitable for short-range UV communication. Simulation indicates that the optical gain is 32.06, and the system has a good spot uniformity, incident angle of incident ray satisfy the requirement of the interference filter(±10°). Outdoor experiments show that the Angle of FOV is about ±20°~±80° and the SNR increased 31dB compared with bare tube, validated that it is useful in free-space UV communication.

Keywords (OCIS):

  • (060.4510) Fiber optics and optical communications : Optical communications
  • (120.4570) Instrumentation, measurement, and metrology : Optical design of instruments
  • (220.0220) Optical design and fabrication : Optical design and fabrication

Investigation of pitch and angle in the gradual-triangle Lenticular lens for point-blank LED fog lamp

  • Hsi-Chao Chen and Chi-Hao Yang
  • received 03/12/2014; accepted 04/11/2014; posted 04/15/2014; Doc. ID 208004
  • [full text: PDF (4056) KB)]
  • Abstract: The effects of different pitch and angle of gradual-triangle Lenticular lens for the point-blank LED fog lamp was investigated under the standard of ECE R19. The novel LED fog lamp was assembled of point-blank LED light source, parabolic reflector and a gradual-triangle Lenticular lens. A light tracing analysis was used for the design of the gradual-triangle Lenticular lens. The pitch, which varied from 1 to 6 mm, and the apex angle, which changed from 5 to 32 degrees, were both investigated in regard to the gradual-triangle Lenticular lens. The optimum pitch was 5mm, and the efficiency of the lamp system and Lenticular lens could reach 93% and 98.1% by simulation, respectively. The results of experiment had over 94% similar to that of simulation by normalized cross correlation (NCC) for the light intensity. © 2014 Optical Society of America

Keywords (OCIS):

  • (220.0220) Optical design and fabrication : Optical design and fabrication
  • (220.4830) Optical design and fabrication : Systems design
  • (230.3670) Optical devices : Light-emitting diodes
  • (220.2945) Optical design and fabrication : Illumination design

A modified dwell time optimization model and its applications in subaperture polishing

  • Zhichao Dong, Haobo cheng, and Honyuen Tam
  • received 03/12/2014; accepted 04/11/2014; posted 04/15/2014; Doc. ID 208062
  • [full text: PDF (4565) KB)]
  • Abstract: The optimization of dwell time is an important procedure in deterministic subaperture polishing. We present a modified optimization model of dwell time by iterative and numerical method, assisted by extended surface form and tool path for suppressing the edge effect. Compared with discrete convolution and linear equation models, the proposed model has essential compatibility to arbitrary tool paths, multiple tool influence functions (TIFs) in one optimization and asymmetric TIFs. The emulational fabrication of a Ф200mm workpiece by the proposed model yields a smooth, continuous and nonnegative dwell time map with a root-mean-square (RMS) convergence rate of 99.6%, and the optimization cost much less time. By the proposed model, influences of TIF size and path interval to convergence rate and polishing time are optimized respectively for typical low and middle spatial frequency errors. Results show that (i) the TIF size is nonlinear inversely proportional to convergence rate and polishing time. A TIF size of ~1/7 the workpiece size is preferred; (ii) the polishing time is less sensitive to path interval, but increasing the interval markedly reduces the convergence rate. A path interval of ~1/8-1/10 of the TIF size is deemed to be appropriate. The proposed model is deployed on a JR-1800 and MRF-180 machine. Figuring Results of Φ920mm Zerodur paraboloid and Φ100mm Zerodur plane by them yield RMS of 0.016λ and 0.013λ (λ=632.8nm) respectively, thereby validate the feasibility of proposed dwell time model used for subaperture polishing.

Keywords (OCIS):

  • (220.4610) Optical design and fabrication : Optical fabrication
  • (220.5450) Optical design and fabrication : Polishing
  • (240.5450) Optics at surfaces : Polishing

On the Residual Dispersion Compensation Over S + C + L + U Wavelength Bands Using Highly Birefringent Octagonal Photonic Crystal Fiber

  • M. Samiul Habib, Redwan Ahmad, Md. Selim Habib, and M. Imran Hasan
  • received 12/18/2013; accepted 04/11/2014; posted 04/15/2014; Doc. ID 203170
  • [full text: PDF (506) KB)]
  • Abstract: An octagonal photonic crystal fiber (O-PCF) with elliptical shape in the centre core is numerically investigated for residual dispersion compensation in the wavelength range of 1460 – 1675 nm. The designed fiber exhibits flattened negative dispersion over S + C + L + U wavelength bands and average dispersion of – 465.5 ps/(nm.km) with absolute dispersion variation of 10.5 ps/(nm.km). Besides the proposed PCF shows a high birefringence of 2.68×10-2 at the operating wavelength 1550 nm which meet the requirement of high birefringence. Moreover, variation of two air-holes in the first ring up to 5% ensures average dispersion of – 491.5 ps/(nm.km) with dispersion variation of 13 ps/(nm.km) and birefringence reaches up to 3×10-2. Furthermore, to evaluate the sensitivity of the fiber dispersion properties, ±5% variation in the optimum parameters is studied.

Keywords (OCIS):

  • (060.2310) Fiber optics and optical communications : Fiber optics
  • (060.2330) Fiber optics and optical communications : Fiber optics communications

Rugate filters used in slit lamp delivery to improve color rendering of illumination for retinal photocoagulation

  • Yi Li, Jiao Chen, Hai Feng, Hao Chen, and Qin Wang
  • received 02/20/2014; accepted 04/11/2014; posted 04/15/2014; Doc. ID 206842
  • [full text: PDF (2602) KB)]
  • Abstract: Laser filter and color filter used in slit lamp delivery have been developed by rugate approach to improve color rendition of illumination for retinal photocoagulation. The double-notch laser filter is designed according to the required laser lines and incident angle. Based on the transmission spectrum of double-notch laser filter and the spectral power distribution (SPD) of the light source of slit lamp, a scanning search algorithm is applied to find the optimal transmission spectrum of multi-notch color filter for the best color rendition index of illumination. An open-source software is used to design the refractive index profile of all the rugate filters which are deposited by the reactive pulse magnetron sputtering. The SPD of slit lamp delivery which will use the deposited double-notch laser filter and multi-notch color filter is also calculated to demonstrate that the color rendition property of illumination can be improved remarkably, thus making fundus observation and retinal laser photocoagulation more reliable and efficient.

Keywords (OCIS):

  • (310.1620) Thin films : Interference coatings
  • (310.1860) Thin films : Deposition and fabrication
  • (330.1715) Vision, color, and visual optics : Color, rendering and metamerism

Optical behavior and structural property of CuAlS2 and AgAlS2 wide-band-gap chalcopyrites

  • Ching-Hwa Ho and Chia-Chi Pan
  • received 02/27/2014; accepted 04/10/2014; posted 04/15/2014; Doc. ID 207250
  • [full text: PDF (1134) KB)]
  • Abstract: Single crystals of CuAlS2 and AgAlS2 were grown by chemical vapor transport method using ICl3 as the transport. The as-grown CuAlS2 crystals reveal transparent and light-green color. Most of them possess a well-defined (112) surface. The AgAlS2 crystals essentially show transparent and white color in vacuum. As the AgAlS2 was put into atmosphere, the crystal surface gradually darkened and brownish because the surface reaction with humidity or hydrogen gas. After a long-term chemical reaction process, the AgAlS2 will transform into a AgAlO2 oxide with yellow color. From X-ray diffraction measurements, both CuAlS2 and AgAlS2 as-grown crystals show single phase and isostructural to a chalcopyrite structure. The (112) face is more preferable for the formation of the chalcopyrite crystals. The energies of interband transitions of the CuAlS2 and AgAlS2 were determined accurately by thermoreflectance measurements in a wide energy range of 2-6 eV. The valence-band electronic structures of CuAlS2 and AgAlS2 have been detailed characterized using polarized-thermoreflectance measurements in the temperature range between 30 and 300 K. The band-edge transitions belonging to the E|| and E⊥ polarizations have been respectively identified. The band edge of AgAlS2 is near 3.2 eV while that of CuAlS2 is about 3.5 eV. On the basis of the experimental analyses, optical and sensing behaviors of the chalcopyrite crystals have been realized.

Keywords (OCIS):

  • (160.4760) Materials : Optical properties
  • (160.6000) Materials : Semiconductor materials
  • (260.5430) Physical optics : Polarization
  • (300.6470) Spectroscopy : Spectroscopy, semiconductors

Development of an All-optical Temperature Insensitive Non-pendulum Type Tilt Sensor Employing Fiber Bragg Gratings

  • R. Aneesh, Sunil Khijwania, and Pathi Munendhar
  • received 02/12/2014; accepted 04/10/2014; posted 04/15/2014; Doc. ID 206316
  • [full text: PDF (522) KB)]
  • Abstract: A novel design strategy for the development of an all-optical, temperature insensitive, non-pendulum type, tilt sensor employing Fiber Bragg Gratings (FBGs) is proposed and experimentally demonstrated. Being non-pendulum type, there is no possibility of any inherent friction and the limiting effects at mechanical joints. Sensor is observed to be capable of resolving magnitude as well as the direction of inclination from horizontal strategically more effectively and without any inherent instability. This is reflected in a relatively better resolution (better than 0.004o), a better accuracy (~ ±0.05o) and lesser maximum discrepancy (~ ±0.001 nm) during the forward as well as the reverse tilt. Sensitivity of the proposed sensor, theoretically proportional to the half of the mass employed in the sensor design, is observed to be 0.012 nm/o with a comparatively smaller mass. Importantly, sensor response is characterized with a very high degree of reversibility and repeatability over the designed/observed dynamic range.

Keywords (OCIS):

  • (060.2370) Fiber optics and optical communications : Fiber optics sensors
  • (060.3735) Fiber optics and optical communications : Fiber Bragg gratings

Improved algorithm based on a fiber loop: applications for optical water vapor sensor

  • C Zhu, J. Chang, P. Wang, Q. Wang, W. Wei, C. Tian, and S. Zhang
  • received 12/17/2013; accepted 04/09/2014; posted 04/15/2014; Doc. ID 203135
  • [full text: PDF (543) KB)]
  • Abstract: An improved algorithm based on a fiber loop optical structure is proposed for measurement of water vapor concentration. Compared to the traditional algorithm, the intrinsic loss of optical devices in the loop no longer needs to be accurately measured, but instead be directly eliminated by normalization. The insertion loss requirement of the optical devices could also be greatly reduced because the calculation of water vapor concentration can be achieved by only two pulses. In addition, a background absorption problem caused by the water vapor existing inside internal end-face gaps of optical components can also be solved. The gas concentration can be directly calculated by using the Beer-Lambert law. By using the improved algorithm, the mean absolute error is reduced from 861 ppm to 29.4 ppm.

Keywords (OCIS):

  • (010.7340) Atmospheric and oceanic optics : Water
  • (060.2370) Fiber optics and optical communications : Fiber optics sensors
  • (300.1030) Spectroscopy : Absorption
  • (150.5495) Machine vision : Process monitoring and control

The Derivative Matrices of a Skew-Ray for Spherical Boundary Surfaces and Their Applications in System Analysis and Design

  • Psang Dain Lin
  • received 01/22/2014; accepted 04/08/2014; posted 04/09/2014; Doc. ID 205161
  • [full text: PDF (2103) KB)]
  • Abstract: In a previous paper [P. D. Lin, Appl. Opt. 52, 4151 (2013)], we presented the first- and second-order derivatives of a ray for a flat boundary surface to design prisms. In this paper, that scheme is extended to determine the Jacobian and Hessian matrices of a skew-ray as it is reflected / refracted at a spherical boundary surface. The validity of the proposed approach as an analysis and design tool is demonstrated using an axis-symmetrical system for illustration purpose. It is found that these two matrices can provide the search direction used by existing gradient-based schemes to minimize the merit function during the optimization stage of the optical system design process. It is also possible to make the optical system designs more automatic, if the image defects can be extracted from the Jacobian and Hessian matrices of a skew-ray.

Keywords (OCIS):

  • (080.0080) Geometric optics : Geometric optics
  • (080.2740) Geometric optics : Geometric optical design
  • (080.3620) Geometric optics : Lens system design
  • (080.1753) Geometric optics : Computation methods

Three-dimensional inline inspection for substrate warpage and ball grid array coplanarity using stereo vision

  • Takeshi Nakazawa and Ayman Samara
  • received 01/30/2014; accepted 04/08/2014; posted 04/09/2014; Doc. ID 205491
  • [full text: PDF (2634) KB)]
  • Abstract: We present a method for full field 3D measurement of substrate warpage and ball grid array coplanarity, which is suitable for inline back-end inspection and process monitoring. For evaluating the performance of the proposed system, the linearity between our system and a reference confocal microscope is studied by measuring a particular substrate sample with an area of 38 mm x 28.5 mm. The correlation coefficient is 0.965 and the 2 sigma difference in the two methods is 26.9 um for the warpage measurement. 1 sigma repeatability is 4.2 um based on 35 measurements. For BGA coplanarity inspection the correlation coefficient is 0.952 and the 2 sigma difference is 31.2 um. 1 sigma repeatability is 3.7um. Data acquisition takes about 0.2 seconds for full field measurements.

Keywords (OCIS):

  • (120.4630) Instrumentation, measurement, and metrology : Optical inspection
  • (150.0150) Machine vision : Machine vision
  • (150.6910) Machine vision : Three-dimensional sensing
  • (150.3045) Machine vision : Industrial optical metrology
  • (150.5495) Machine vision : Process monitoring and control

Probing micro-structural information of anisotropic scattering media using rotation independent polarization parameters

  • Minghao Sun, Honghui He, Nan Zeng, E Du, Yihong Guo, Cheng Peng, Yonghong He, and Hui Ma
  • received 01/07/2014; accepted 04/07/2014; posted 04/08/2014; Doc. ID 204267
  • [full text: PDF (1644) KB)]
  • Abstract: Polarization parameters contain rich information on the micro- and macro-structure of scattering media. However, many of these parameters are sensitive to the spatial orientation of anisotropic media, and may not effectively reveal the micro-structural information. In this paper, we take polarization images of different textile samples at different azimuth angles. The results demonstrate that the rotation insensitive polarization parameters from rotating linear polarization imaging and Mueller matrix transformation methods can be used to distinguish the characteristic features of different textile samples. Further examinations using both experiments and Monte Carlo simulations reveal that the residue rotation dependence in these polarization parameters are due to the oblique incidence illumination. This study shows that such rotation independent parameters are potentially capable of quantitative classification of anisotropic samples such as textiles or biological tissues.

Keywords (OCIS):

  • (160.2290) Materials : Fiber materials
  • (290.4210) Scattering : Multiple scattering
  • (290.5855) Scattering : Scattering, polarization

Analysis of the relationship between fringe angle and three-dimensional profilometry system sensitivity

  • Ping Zhou, Xinran Liu, and Tongjing Zhu
  • received 12/06/2013; accepted 04/06/2014; posted 04/07/2014; Doc. ID 202594
  • [full text: PDF (2380) KB)]
  • Abstract: The relationship between projector-camera baseline and the phase variation direction of fringe patterns is one of essential characteristics in three-dimensional profilometry system, although it has been ignored. This paper indicates that the three-dimensional profilometry system will be most sensitive to the object depth change when the phase variation direction of fringe patterns is parallel to the baseline, which is analyzed in the systems based on both triangulation and stereovision principles. An efficient method is proposed to achieve the most sensitivity by projecting a set of fringe patterns of different phase variation directions. Experimental results demonstrate our analysis and the proposed determination method.

Keywords (OCIS):

  • (100.5070) Image processing : Phase retrieval
  • (120.0120) Instrumentation, measurement, and metrology : Instrumentation, measurement, and metrology
  • (120.2650) Instrumentation, measurement, and metrology : Fringe analysis

Encrypted imaging based on algebraic implementation of double random phase encoding

  • Kazuya Nakano, Masafumi Takeda, Hiroyuki Suzuki, and Masahiro Yamaguchi
  • received 01/09/2014; accepted 04/04/2014; posted 04/08/2014; Doc. ID 204270
  • [full text: PDF (1419) KB)]
  • Abstract: The security of important information captured by sensors and cameras is currently a growing concern as information theft via techniques such as side channel attacks become increasingly more prevalent. Double random phase encoding (DRPE) is an optical encryption method based on optical Fourier transform that is currently being used to implement secure coherent optical systems. In this paper, we propose a new DRPE implementation for incoherent optical systems based on integral photography that can be applied to “encrypted imaging (EI)” to optically encrypt an image before it is captured by an image sensor. Because the proposed incoherent DRPE is constituted from conventional DRPE by rewriting the optical encryption via discretization and Euler’s formula, its security level is the same as that of conventional DRPE. The results of an experiment in which we encrypted a plaintext image optically and then decrypted it numerically demonstrate that our proposed incoherent optical security system is feasible.

Keywords (OCIS):

  • (070.4560) Fourier optics and signal processing : Data processing by optical means
  • (110.1758) Imaging systems : Computational imaging
  • (060.4785) Fiber optics and optical communications : Optical security and encryption

Bistability and all-optical flip-flop with active microring-resonator

  • Qiliang Li, Hongliang Yuan, and Xianghong Tang
  • received 02/27/2014; accepted 04/04/2014; posted 04/08/2014; Doc. ID 207331
  • [full text: PDF (713) KB)]
  • Abstract: In this paper, we study the bistability of an active nonlinear microring resonator and design the flip-flop based on the active micro-ring resonator. In the presence of nonlinear loss and linear loss, we use the erbium-doped gain medium in micro-ring to obtain gain to compensate the loss of the resonator. Both analytical and numerical methods are used to solve the propagation in the microring with double couplers, and we obtain the hysteresis loops of the microring. These results obtained by two methods all reveal that in the presence of nonlinearity in micro-ring resonators, the system exhibits the bistability, and the gain in microring leads to a decrease of the bias power when the active microring is taken as a bistable switcher. Basing on the bistability of the microring, we realize the set-reset flip-flop through adding a positive or negative feedback onto the bias. We also find that the duration of the set and reset pulses must exceed the field build up time of microring if we want to achieve the switching of the bias signal. In our design the duration time is about 2 ps.

Keywords (OCIS):

  • (190.0190) Nonlinear optics : Nonlinear optics
  • (190.1450) Nonlinear optics : Bistability
  • (190.3270) Nonlinear optics : Kerr effect
  • (230.1150) Optical devices : All-optical devices
  • (230.3990) Optical devices : Micro-optical devices

Corrective finishing of EUV photomask blanks by precessed bonnet polisher

  • Anthony Beaucamp, Yoshiharu Namba, and Phillip Charlton
  • received 03/04/2014; accepted 04/04/2014; posted 04/08/2014; Doc. ID 207559
  • [full text: PDF (1371) KB)]
  • Abstract: The progressive transition from Excimer to EUV lithography is driving a need for flatter and smoother photomask blanks. It is however proving difficult to meet the next generation specification with the conventional chemical mechanical polishing technology commonly used for finishing photomask blanks. This paper reports on the application of sub-aperture CNC precessed bonnet polishing technology to the corrective finishing of photomask substrates for EUV lithography. Full-factorial analysis was used to identify process parameters capable of delivering micro-roughness below 0.5 nm rms, whilst retaining relatively high removal rates. Experimental results show that masks pre-polished to 300~600 nm P-V flatness by CMP can then be improved down to 50~100 nm P-V flatness using the automated technology described in this paper. A series of edge polishing experiments also hints at the possibility of increasing the quality area beyond the 142 mm square defined in the official EUV photomask specification.

Keywords (OCIS):

  • (220.3740) Optical design and fabrication : Lithography
  • (220.5450) Optical design and fabrication : Polishing
  • (340.7480) X-ray optics : X-rays, soft x-rays, extreme ultraviolet (EUV)

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