Large Depth Of Focus Research Articles

Die Placement Error management for Fan out Applications using Projection Lithography

Abstract Photolithography has long been the key patterning technology for structuring organic materials used in advanced packaging applications like flip-chip wafer bumping, electroplated gold, solder bumps, copper pillar technologies and redistribution layers. Photolithography is a key manufacturing process and cost contributor, the careful selection of the right exposure solution is critical to achieve the best possible cost structure in today's industrial lithography applications We will introduce a projection exposure technology that uses a 1:1 projection lens; the projection exposure technology provides high resolution patterning performance. With the capability of varying NA from 0.07 to 0.14, a large depth of focus is obtained, that is required to ensure high pattern fidelity when working with thick resists. The projection exposure can also be applied to FOWLP applications, when the die placement data is fed into a software optimization algorithm to correct for it. We will present the projection exposure technology capability in managing the die placement error for FOWLP. In addition, details on the overlay capability of the projection exposure in thin resist are described and a Cost-of-Ownership comparison between different photolithography solutions is discussed.

A comprehensive model for sub-10 nm electron-beam patterning through the short-time and cold development

In this study, we propose a set of single-spot experiment to construct a comprehensive model of electron-beam lithography to describe the relation among the incident electrons, resist, and the development conditions such as durations and temperatures. Through the experiments, small feature can be achieved by performing a short-time development due to the high acceleration voltage and large depth of focus of electron-beam system. The singular point in the beginning of the development is also observed in our model and supported by the experimental data. In addition, we verify the characteristic region of each incident spot induced by the point spread function of the electron-beam system. We further fabricate the single line with narrow groove width by utilizing the results from single-spot experiment at low developing temperatures. The line is formed by arranging a series of incident points with a distance close to the characteristic radius. This method can eliminate the proximity effect effectively and thus the groove width is scaled down to 8 nm. By adopting the successful experience in the single line formation, dense array with narrow linewidth is also demonstrated under well suppression of the proximity effect. The minimum groove width of 9 nm with 30 nm pitch is achieved with 5 s development time at −10 °C. Finally, the exceptional capability of pattern transfer is presented due to the high aspect ratio of the resist.

Synthetic aperture focusing technique for photoacoustic endoscopy.

Photoacoustic endoscopy (PAE) is a promising tool for the detection of atherosclerotic plaque. In this work, we propose a novel design of a side-viewing PAE probe based on a synthetic aperture focusing technique (SAFT) to enable high transverse resolution over large depth of focus (DOF) along the radial direction. A point-like ultrasonic detector is used to ensure a wide detection angle and thus a large synthetic aperture for SAFT. We first perform numerical simulation to optimize the PAE probe design, which involves the placement of the point-like detector and the diameter of a reflection rod mirror. Then, experiments are conducted based on the optimized probe design. High transverse resolution of 115-190 µm over large DOF of 3.5 mm along the radial direction is experimentally obtained. The SAFT-based PAE holds promise for endoscopic imaging with a high transverse resolution for both the surface and deep regions of tissue.

Planar binary-phase lens for super-oscillatory optical hollow needles

Optical hollow beams are suitable for materials processing, optical micromanipulation, microscopy, and optical lithography. However, conventional optical hollow beams are diffraction-limited. The generation of sub-wavelength optical hollow beams using a high numerical aperture objective lens and pupil filters has been theoretically proposed. Although sub-diffraction hollow spot has been reported, nondiffracting hollow beams of sub-diffraction transverse dimensions have not yet been experimentally demonstrated. Here, a planar lens based on binary-phase modulation is proposed to overcome these constraints. The lens has an ultra-long focal length of 300λ. An azimuthally polarized optical hollow needle is experimentally demonstrated with a super-oscillatory transverse size (less than 0.38λ/NA) of 0.34λ to 0.42λ, where λ is the working wavelength and NA is the lens numerical aperture, and a large depth of focus of 6.5λ. For a sub-diffraction transverse size of 0.34λ to 0.52λ, the nondiffracting propagation distance of the proposed optical hollow needle is greater than 10λ. Numerical simulation also reveals a good penetrability of the proposed optical hollow needle at an air-water interface, where the needle propagates through water with a doubled propagation distance and without loss of its super-oscillatory property. The proposed lens is suitable for nanofabrication, optical nanomanipulation, super-resolution imaging, and nanolithography applications.

Aspherical Lens Design Using Computational Lithography

Optical lithography is often applied to fabricate three-dimensional (3D) patterns because of its capability on the scale of optical wavelengths. An aligner, a laser writer, and a stepper with an optical projection system are used as the exposure tool. The focusing capability must enable exposure of step heights of ≥50μm, which is normally achieved using an optical projection system with a large focal depth. Recently, a projection lens was developed that can produce a variable step height using an adjustable numerical aperture, which has led to the fabrication of micro-lens arrays (MLAs) with step heights as high as 100μm. These MLAs are used in various fields but mostly only as condensing lenses. However, wider applications await the realization of optical aberration reduction. The introduction of aspherical lenses would be a major step in this direction; however, aspherical lenses are difficult to produce because of the limitations in processing and polishing the aspherical surfaces of optical elements made of glass. Alternatively, the fabrication of 3D patterns using optical lithography involves a photomask whose optical density distribution corresponds to the 3D profile after exposure and development. A density distribution corresponding to the desired 3D shape can be achieved using a computer to control the dot density of a microdot mask pattern. In this study, we present a design method for low-aberration aspherical MLAs that satisfies the Rayleigh standard using computational lithography.

Analysis of fracture roughness parameters of S355J2 steel and EN AW-2017A-T4 aluminium alloy

This article presents an analysis of fracture surface for steel S355J2 and EN AW-2017A-T4 aluminium subjected to fatigue bending, torsion, and bending with torsion. Fracture surfaces were observed using the focus variation microscope, which allows for the acquisition of data sets with large depth of focus. The authors focus on presenting the features of profile roughness parameters on the example of differences between fracture propagation and rupture areas. The researchers are looking for a correlation or ratio between different profile roughness parameters, especially arithmetical mean deviations of the roughness profile Ra and maximum height of the roughness profile Rz, for this case.

Synchronous scan-projection lithography on overall circumference of fine pipes with a diameter of 2 mm

The scan-projection exposure of small-diameter pipe surfaces was investigated using a newly developed prototype exposure system. It is necessary to secure a very large depth of focus for printing thick resist patterns on round pipe surfaces with a roughness larger than that of semiconductor wafers. For this reason, a camera lens with a low numerical aperture of 0.089 was used as a projection lens, and the momentary exposure area was limited by a narrow slit with a width of 800 µm. Thus, patterns on a flat reticle were replicated on a pipe surface by linearly moving the reticle and rotating the pipe synchronously. By using a reticle with inclined line-and-space patterns, helical patterns with a width of 30 µm were successfully replicated on stainless-steel pipes with an outer diameter of 2 mm and coated with a 10-µm-thick negative resist. The patterns replicated at the start and stop edges were smoothly stitched seamlessly.

Focus modulation of cylindrical vector beams through negative-index grating lenses

A cylindrically symmetric negative-index grating lens composed of unitary material is proposed as an effective method to modulate the focusing of cylindrical vector beams (CVBs). The grating parameters are designed to obtain an appropriate negative index, and the lens profile is tailored to realize the constructive interference. The plano-concave lens is parameterized to achieve desired focal length and the plano-cone lens is proposed to obtain large depth of focus. An optical needle is generated with radially polarized incidence, and an optical tube is achieved with incidence of azimuthal polarization. Moreover, the presented modulation methods can be applied for any arbitrary polarized CVBs. This work offers a more flexible and effective approach to design negative-index lenses for subwavelength focusing of CVBs, which has potential application value in related areas, such as optical trapping, and other nano-optics fields.

A Fast Three-Dimensional Image Reconstruction With Large Depth of Focus Under the Illumination of Terahertz Gaussian Beams by Using Wavenumber Scaling Algorithm

In this paper, a fast three-dimensional wavenumber scaling algorithm (3-D WSA) was proposed to reconstruct the wideband terahertz (THz) holographic image under the illumination of THz Gaussian beams. By taking the cross-range range coupling term into account, which is usually neglected in some fast algorithm, such as the enhanced phase shift migration (EPSM), the focusing depth for image reconstruction is greatly improved in the proposed fast algorithm. To deal with the cross-range range coupling term, a wavenumber scaling operator (WSO) was developed in the proposed algorithm which only uses chirp multiplications and FFTs with high computation efficiency and free of interpolation. Theoretical analysis on the relationship of the WSO parameters to the resulted maximum non-ambiguous range in holography image reconstruction was carried out with a quantitative criterion derived to appropriately determine the WSO parameters. Simulation results validate the proposed focusing algorithm and indicate that 3D WSA offers almost the same focusing capabilities as the most rigid phase shift migration (PSM) algorithm, meanwhile with computation efficiency greatly improved. Proof-of-principle experiments in 0.2 THz band were also performed based on a monostatic prototype imager. The experimental results demonstrate the effectiveness and the efficiency of the 3D WSA proposed in this paper.

Focus modulation of cylindrical vector beams by using 1D photonic crystal lens with negative refraction effect.

Sub-wavelength focusing of cylindrical vector beams (CVBs) has attracted great attention due to the specific physical effects and the applications in many areas. More powerful, flexible and effective ways to modulate the focus transversally and also longitudinally are always being pursued. In this paper, cylindrically symmetric lens composed of negative-index one-dimensional photonic crystal is proposed to make a breakthrough. By revealing the relationship between focal length and the exit surface shape of the lens, a quite simple and effective principle of designing the lens structure is presented to realize specific focus modulation. Plano-concave lenses are parameterized to modulate the focal length and the number of focuses. An axicon constructed by one-dimensional photonic crystal is proposed for the first time to obtain a large depth of focus and an optical needle focal field with almost a theoretical minimum FWHM of 0.362λ is achieved under radially polarized incident light. Because of the almost identical negative refractive index for TE and TM polarization states, all the modulation methods can be applied for any arbitrary polarized CVBs. This work offers a promising methodology for designing negative-index lenses in related application areas.

Electron properties of a microcolumn with a CNT tip

We investigated a possibility of the CNT tips as an electron emitter tip of a miniaturized microcolumn. For the purpose, we analyzed potential distribution, field distribution and electron beam properties of the conventional tip, single-walled nanotube (SWNT) and double-walled nanotube (DWNT). From the result, we made certain that the nanotube emitters have advantages in small beam divergence angle and beam current enhancement. Especially, the SWNT has the largest field strength at the tip apex among the three tips, thus it can emit enough electron current even at a low extract bias voltage. On the other hand, the DWNT has the smallest electron beam divergence angle. It means that the tip has the largest depth of focus among the three investigated tips.

Optimized lithography process for through-silicon vias-fabrication using a double-sided (structured) photomask for mask aligner lithography

Through-silicon vias (TSV) are very important for wafer-level packaging as they provide patterning holes through thick silicon dies to integrate and interconnect devices which are stacked in the z-direction. For economic processing, TSV fabrication primarily needs to be cost effective, especially for a high throughput. Furthermore, a lithography process for TSV has to be stable enough to allow patterning on prestructured substrates with inhomogeneous topography. This can be addressed by an exposure process which offers a large depth of focus. We have developed a mask-aligner lithography process based on the use of a double-sided photomask to realize aerial images that meet these constraints.

Shaping a Subwavelength Needle with Ultra-long Focal Length by Focusing Azimuthally Polarized Light.

Flat optics, which could planarize and miniaturize the traditional optical elements, possesses the features of extremely low profile and high integration for advanced manipulation of light. Here we proposed and experimentally demonstrated a planar metalens to realize an ultra-long focal length of ~240λ with a large depth of focus (DOF) of ~12λ, under the illumination of azimuthally polarized beam with vortical phase at 633 nm. Equally important is that such a flat lens could stably keep a lateral subwavelength width of 0.42λ to 0.49λ along the needle-like focal region. It exhibits one-order improvement in the focal length compared to the traditional focal lengths of 20~30λ of flat lens, under the criterion of having subwavelength focusing spot. The ultra-long focal length ensures sufficient space for subsequent characterization behind the lens in practical industry setups, while subwavelength cross section and large DOF enable high resolution in transverse imaging and nanolithography and high tolerance in axial positioning in the meantime. Such planar metalens with those simultaneous advantages is prepared by laser pattern generator rather than focused ion beam, which makes the mass production possible.

Lensless three-dimensional integral imaging using variable and time multiplexed pinhole array.

We present a multivariable coded aperture (MVCA) for lensless three-dimensional integral imaging (3D II) systems. The new configuration is based on a time multiplexing method using a variable pinholes array design. The system provides higher resolution 3D images with improved light intensity and signal to noise ratio as compared to single-pinhole system. The MVCA 3D II system configuration can be designed to achieve high light intensity for practical use as microlenslets arrays. This new configuration preserves the advantages of pinhole optics while solving the resolution limitation problem and the long exposure time of such systems. The three-dimensional images are obtained with improved resolution, signal to noise ratio, and light efficiency. This integral imaging lensless system is characterized by large depth of focus, simplicity, and low cost.

Subwavelength anti-diffracting beams propagating over more than 1,000 Rayleigh lengths

Researchers take advantage of the nonlinear response of a dipolar glass and observe non-paraxial propagation of a sub-micrometre-sized beam over 103 diffraction lengths. Propagating light beams with widths down to and below the optical wavelength require bulky large-aperture lenses and remain focused only for micrometric distances1,2. Here, we report the observation of light beams that violate this localization/depth-of-focus law by shrinking as they propagate, allowing resolution to be maintained and increased over macroscopic propagation lengths. In nanodisordered ferroelectrics3,4 we observe a non-paraxial propagation of a sub-micrometre-sized beam for over 1,000 diffraction lengths, the narrowest visible beam reported to date5,6,7,8. This unprecedented effect is caused by the nonlinear response of a dipolar glass, which transforms the leading optical wave equation into a Klein–Gordon-type equation that describes a massive particle field9. Our findings open the way to high-resolution optics over large depths of focus, and a route to merging bulk optics into nanodevices.

Creation of super-length optical tube by phase modulated azimuthally polarized beam with multi-zone phase filter

We suggest a new approach to utilize a multi-zone phase filter along with a high NA lens axicon to accomplish a subwavelength focal hole with large uniform focal depth by means of azimuthally polarized Bessel–Gaussian beam. The intensity distributions are obtained and calculated based on the vector diffraction theory, the recommended system generates a subwavelength super-length optical tube with a sub diffraction beam size of 0.494 λ and long focal depth is approximately 63.6 λ in free space. We also established that the focal depth and sub diffraction beam size is dependent relative on focusing condition of the numerical aperture. The authors presume such a high intense tube beam may perhaps find the applications in atom optics, optical manipulations, optical trapping and optical processing technologies.

Chip-on-the-Tip Projection Design with Large Depth of Focus for Structured Light 3D Endoscopic Imaging

Chip-on-the-Tip Projection Design with Large Depth of Focus for Structured Light 3D Endoscopic Imaging

Confocal soft X-ray scanning transmission microscopy: setup, alignment procedure and limitations.

Zone-plate-based scanning transmission soft X-ray microspectroscopy (STXM) is a well established technique for high-contrast imaging of sufficiently transparent specimens (e.g. ultrathin biological tissues, polymer materials, archaeometric specimens or magnetic thin films) with spatial resolutions in the regime of 20 nm and high spectroscopic or chemical sensitivity. However, due to the relatively large depth of focus of zone plates, the resolution of STXM along the optical axis so far stays unambiguously behind for thicker X-ray transparent specimens. This challenge can be addressed by the implementation of a second zone plate in the detection pathway of the beam, resulting in a confocal arrangement. Within this paper a first proof-of-principle study for a confocal STXM (cSTXM) and an elaborate alignment procedure in transmission and fluorescence geometry are presented. Based on first confocal soft X-ray micrographs of well known specimens, the advantage and limitation of cSTXM as well as further development potentials for future applications are discussed.

Parametric source-mask-numerical aperture co-optimization for immersion lithography

Source mask optimization (SMO) is a leading resolution enhancement technique in immersion lithography at the 45-nm node and beyond. Current SMO approaches, however, fix the numerical aperture (NA), which has a strong impact on the depth of focus (DOF). A higher NA could realize a higher resolution but reduce the DOF; it is very important to balance the requirements of NA between resolution and the DOF. In addition, current SMO methods usually result in complicated source and mask patterns that are expensive or difficult to fabricate. This paper proposes a parametric source-mask-NA co-optimization (SMNO) method to improve the pattern fidelity, extend the DOF, and reduce the complexity of the source and mask. An analytic cost function is first composed based on an integrative vector imaging model, in which a differentiable function is applied to formulate the source and mask patterns. Then, the derivative of the cost function is deduced and a gradient-based algorithm is used to solve the SMNO problem. Simulation results show that the proposed SMNO can achieve the optimum combination of parametric source, mask, and NA to maintain high pattern fidelity within a large DOF. In addition, the complexities of the source and mask are effectively reduced after optimization.

Creation of super-long bright channel using high NA lens axicon with dedicated multibelt binary phase mask

We provide a theoretical approach to use a multibelt binary phase mask in combination with a high numerical aperture lens axicon to achieve a subwavelength focal spot with large uniform focal depth when illuminated by an azimuthally polarized Laguerre–Gaussian beam. The intensity distributions are derived and evaluated based on the vector diffraction theory. Here we report a new method that generates a transversely polarized light beam with sub diffraction beam size (0.384 \(\lambda \)) that propagates without divergence over a long distance (of about 14 \(\lambda \)) in free space. The author expect such investigation is worthwhile for the applications of the optical manipulation, optical microscopy and optical micromachining.