In this paper, we present novel methods to produce structural color image for any given color picture using a pixelated generic stamp named nanosubstrate. The nanosubstrate is composed of prefabricated arrays of red, green and blue subpixels. Each subpixel has nano-gratings and/or sub-wavelength structures which give structural colors through light diffraction. Micro-patterning techniques were implemented to produce the color images from the nanosubstrate by selective activation of subpixels. The nano-grating structures can be nanohole arrays, which after replication are converted to nanopillar arrays or vice versa. It has been demonstrated that visible and invisible data can be easily stored using these fabrication methods and the information can be easily read. Therefore the techniques can be employed to produce personalized and customized color images for applications in optical document security and publicity, and can also be complemented by combined optical data storage capabilities.
Microfabricating high resolution micro-optics structures requires shape control to <1/8th wavelength (~60nm) in both
vertical and horizontal surface precision. Grayscale bimetallic photomasks are bi-layer thermal resists consisting of two
thin layers of Bi-on-Indium or Tin-on-Indium. A focused laser spot creates a thermal metal oxide with a controllably
transparency set by the beam power of optical density from ~3OD (unexposed) to <0.22OD (fully exposed). A directwrite
raster-scan photomask laser system with a CW Argon-ion laser at 514nm for the bimetallic writing and 457nm
line for measuring the OD change used a feedback-controlled Gaussian beam to achieve 256-level grayscale masks.
Setting the graylevels required to achieve uniform vertical steps in the photoresist requires adjustment in transparency
based on the exact response curves of a given resist/development process. An initial model is developed using the
classic resist threshold dose exposure D0 and dose to clear Dc creating a power law relation between the required
exposure dose for each thickness step and the mask transparency. However real resists behave differently than the
simple model near the threshold requiring careful calibrating of mask graylevel transparencies with the photoresist
response curve for a given resist/development process. Test structures ranging from steps to ramps and complex
patterns were examined via both SEM and profilometry from the resulting bimetallic grayscale masks. Secondary
corrections modify the needed bimetallic OD due to the exposure source spectrum differences from the 457nm
measurement. This enhances the patterning of micro-optic and 3D MEMS structures.
Microfabrication of high-resolution micro-optic devices requires <1/8λ (~60nm) precision both vertically and
horizontally. More critical is the creation of 256-level grayscale masks to create sufficient vertical precision in the
photoresist. Grayscale bimetallic photomasks are bi-layer thermal resists of Bismuth-on-Indium or Tin-on-Indium
become controllably transparent by varying laser power thermally producing alloy oxide ranging ~3OD (unexposed) to
<0.22OD (fully exposed). Previously, a direct-write multi-line CW Argon-ion laser writing system with feedbackcontrolled
Gaussian beam achieved 256-level grayscale masks. The feedback system effectively reduced the average
gray-level error from ±4.2 gray-levels in an open-loop approach to ±0.3 gray-levels in a closed-loop approach.
Remaining gray-level errors were due to the Gaussian beam profile creating variations in gray-levels. Preliminary results
show that a beam shaper creating a flattop beam helps reducing gray-level fluctuations. The multi-line Argon laser
enables having multiple single beams separated from a single stabilized laser source. The single 514.5nm line used for
writing gives better control of beam shape in the modulated laser beam. At the same time a lower power 457.9nm line
introduced in the beam path to characterize the grayscale pattern both during and after the writing process. Filtering the
writing laser line, sensor below the mask plate measures only the 457.9nm line enabling the high accuracy transparency
measurements of the written mask near G-line (435.8nm). One target application is the creation of micro-lens arrays,
which are lenses whose optical shape varies from lenslet to lenslet across the entire patterned surface of cm size. Laser
direct-written grayscale masks enable relatively low cost, rapid turnaround mask production needed for creating such
structures with microfabrication processes.
Grayscale photomasks are bi-layer metallic films consist of two thin layers of Bi-on-Indium or Tin-on-Indium. These
films become controllably transparent by accurately varying laser power such that the optical density changes almost
linearly from ~3 OD (unexposed) to <0.22OD (fully exposed). Previously, a direct-write raster-scan photomask system
with a multi-line CW Argon-ion laser was used with feedback-controlled Gaussian beam to achieve 256-level grayscale
masks. With the Gaussian laser spot, the feedback system was effective such that the average gray-level error reduced
from ±4.2 gray-levels in an open-loop approach to ±0.3 gray-levels in a closed-loop approach. As most of the gray-level
errors are due to the Gaussian beam profile making variations on the mask, a beam shaper was used to change the laser
spot to a flat-top beam. Raster-scanning the mask using the flat-top beam helps further reduce the gray-level errors.
Preliminary results show that the flat-top beam reduces gray-level fluctuations, and lines can be written with less
overlapped area helping to have higher resolution masks. Having lines closer with smaller overlap suggests that
accurately controlled laser power results in an accurate OD profile on the mask even with an open-loop approach. The
accuracy of the laser power is also a reason for variations as it has only 1% accuracy. Some test patterns are written on
the mask using open-loop and closed-loop approaches to demonstrate how accurate the gray-levels of the bimetallic thinfilms
are using a flat-top laser beam.
Bimetallic grayscale photomasks contain two thin layers of metals (Bismuth/Indium or Tin/Indium), 15-300nm thick on
transparent substrates. Laser exposure converts the films by thermal reaction into transparent alloy oxides. The Optical
Density changes from ~3.0OD (unexposed) to <0.22OD (fully exposed), creating grayscale photomasks. With using a
open loop direct-write raster-scan writing system with a CW Argon-ion laser, grayscale masks with 6-bit accuracy was
achieved. To reduce mask nonuniformity, sensors added to the beam path before and after the mask turning the system to
create a real-time OD and beam measurement feedback system. This feedback compensates for changes in film
parameters: film thickness reducing the mask error to ±0.3 gray-levels, yielding to a potential 256 gray-level mask. This
leaves nonuniformities due to the Gaussian shape still limits the accuracy. To remove these, a refractive field-mapping
shaper was used to create a nearly flat-top power laser spot power distribution. The OD system also allows us to profile
the grayscale levels in more detail after writing. A full 8-bit picture was written on a test mask with the feedback system
showed significant improvement in the number of gray-levels. Some fluctuations were observed at the final pattern due
to spaces between lines and nonuniformity of table speed. Some 3D test structures are created on the photoresist with the
masks written with the feedback system to demonstrate the accuracy and grayscale abilities of the bimetallic thin-films
Conventional fluorescence imaging often does not have a mechanism to remove the scattering effect in biological tissue.
We use Angular Domain Imaging (ADI) to improve the detection of smaller structures in fluorescence layer over that
can be provided by existing systems. ADI is a high resolution, ballistic imaging method that utilizes the angular
spectrum of photons to filter multiple-scattered photons and accepts only photons with small angular deviation from their
original trajectory. Advantages of the ADI technique are that it is insensitive to wavelength and the sources are not
required to be high quality, coherent, or pulse, as with OCT or time domain. Our target is to perform fluorescence ADI at
shallow tissue such as skin (≈ 1mm) with a buried collagen layer. To experimentally model shallow tissue with phantoms,
a thin layer of scattering medium with similar scattering characteristic (μs = 200cm-1, g = 0.85) is placed on top
fluorescence plastic (415nm excitation, ≈ 555-585nm emission) which is patterned by strips of non-emitting structures
(200-400μm). Positioning multiple collimated arrays with acceptance angles of 5.71° on top of the scattering medium,
test structures (200μm wide) can be detected at shallow scattering medium thickness (1mm). Monte Carlo simulation
confirms that fluorescence ADI can image structures at shallow tissue depth by using collimator array with modest
filtration angles. Results show micromachined collimator arrays provide both high spatial resolution and angular
filtration on scattered photons.
When bimetallic thin films of Bi/In and Sn/In are laser exposed, they oxidize and become variably transparent. By
controlling the writing laser power, binary and grayscale photomasks can be produced with the mask's transparency
(optical density, OD), ranging between ~3.0 (unexposed) to <0.22 OD (fully exposed). Targeting the production of
grayscale masks with 256 levels, the mask-writing system when combined with photodiode sensors obtains real-time OD
and laser power measurements and uses them to adjust the laser's writing power during the patterning process. For a
single-line stepped pattern, laser writing without OD feedback control demonstrates an average absolute error of 4.2 gray
levels, while with OD feedback control and the appropriate parameters, the same pattern is produced with an average
absolute error of 0.3 gray levels. The control parameters are shown to influence the characteristics of the resulting mask
pattern, particularly the overshoot and rise-time of the pixel transitions. With multi-line mask patterns being rasterscanned
written, the overlap of the lines combined with the laser's Gaussian profile creates variations in the mask, and
measurement problems for the OD feedback control. An interlaced raster-scan approach is proposed where a first pass
patterns non-overlapping lines using an ideal set of control parameters. A second and third pass then patterns the lines inbetween
and at the pixel boundaries using another set parameters designed to account for the overlap. The technique
allows feedback to be used for the entire mask writing process.
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