Shaolin Zhou

Initiator-Integrated 3D Printing Enables the Formation of Complex Metallic Architectures

Three-dimensional printing was used to fabricate various metallic structures by directly integrating a Br-containing vinyl-terminated initiator into the 3D resin followed by surface-initiated atomic-transfer radical polymerization (ATRP) and subsequent electroless plating. Cu- and Ni-coated complex structures, such as microlattices, hollow balls, and even Eiffel towers, were prepared. Moreover, the method is also capable of fabricating ultralight cellular metals with desired structures by simply etching the polymer template away. By combining the merits of 3D printing in structure design with those of ATRP in surface modification and polymer-assisted ELP of metals, this universal, robust, and cost-effective approach has largely extended the capability of 3D printing and will make 3D printing technology more practical in areas of electronics, acoustic absorption, thermal insulation, catalyst supports, and others.

Four-quadrant gratings moiré fringe alignment measurement in proximity lithography

This paper aims to deal with a four-quadrant gratings alignment method benefiting from phase demodulation for proximity lithography, which combines the advantages of interferometry with image processing. Both the mask alignment mark and the wafer alignment mark consist of four sets of gratings, which bring the convenience and simplification of realization for coarse alignment and fine alignment. Four sets of moiré fringes created by superposing the mask alignment mark and the wafer alignment mark are highly sensitive to the misalignment between them. And the misalignment can be easily determined through demodulating the phase of moiré fringe without any external reference. Especially, the period and phase distribution of moiré fringes are unaffected by the gap between the mask and the wafer, not excepting the wavelength of alignment illumination. Disturbance from the illumination can also be negligible, which enhances the technological adaptability. The experimental results bear out the feasibility and rationality of our designed approach.

SU-8-Induced Strong Bonding of Polymer Ligands to Flexible Substrates via in Situ Cross-Linked Reaction for Improved Surface Metallization and Fast Fabrication of High-Quality Flexible Circuits.

On account of in situ cross-linked reaction of epoxy SU-8 with poly(4-vinylpyridine) (P4VP) and its strong reactive bonding ability with different pretreated substrates, we developed a simple universal one-step solution-based coating method for fast surface modification of various objects. Through this method, a layer of P4VP molecules with controllable thickness can be tethered tightly onto substrates with the assistance of SU-8. P4VP molecules possess a lot of pyridine ligands to immobilize transitional metal ions that can behave as the catalyst of electroless copper plating for surface metallization while functioning as the adhesion-promoting layer between the substrate and deposited metal. Attributed to interpenetrated entanglement of P4VP molecules and as-deposited metal, ultrathick (>7 μm) strongly adhesive high-quality copper layer can be formed on flexible substrates without any delamination. Then through laser printer to print toner mask, a variety of designed circuits can be easily fabricated on modified flexible PET substrate.

Moiré-Based Phase Imaging for Sensing and Adjustment of In-Plane Twist Angle

We explore the feasibility of a controllable and easy-to-implement moiré-based phase-sensitive imaging scheme for in-plane twist angle sensing and adjustment between two parallel planes, e.g., the optical alignment in nanoimprint lithography, photolithography, and X-ray lithography. The fundamental derivation of in-plane twist angle detection is given. Any angle variations can be readily sensed and monitored by the phase shift of optical field that occurs at the surface of two overlapped gratings. The performances of a circular grating and a specially designed composite linear grating were tested. Computational and experimental results show that the in-plane twist angle can be readily sensed and remedied within the magnitude of 10-4 rad even in a manual operation mode by this method.

Moiré fringe alignment using composite circular-line gratings for proximity lithography.

We explore the feasibility of a controllable and easy-to-implement moiré-based composite circular-line gratings imaging scheme for optical alignment in proximity lithography. One circular grating and four line gratings located on both the mask alignment mark and wafer alignment mark are used to realize the coarse alignment and fine alignment respectively. The fundamental derivation of coarse alignment employing circular gratings and fine alignment employing line gratings are given. Any displacement of misalignment that occurs at the surface of two overlapped gratings can be sensed and determined through subsequent fringe phase analysis without the influence of the gap between the mask and the wafer or wafer process. The design and manufacture process of the alignment marks are presented. The experimental results validate and demonstrate the feasibility of the proposed approach.

Moire interferometry with high alignment resolution in proximity lithographic process

Moiré interferometry is widely used as the precise metrology in many science and engineering fields. The schemes of moirés-based interferometry adopting diffraction gratings are presented in this paper for applications in a proximity lithographic system such as wafer-mask alignment, the in-plane twist angle adjustment, and tilts remediation. For the sake of adjustment of lateral offset as well as the tilt and in-plane twist angle, schemes of the (m,-m) and (m,0) moiré interferometry are explored, respectively. Fundamental derivation of the moiré interferometry and schemes for related applications are provided. Three pairs of gratings with close periods are fabricated to form the composite grating. And experiments are performed to confirm the moiré interferometry for related applications in our proximity lithographic system. Experimental results indicate that unaligned lateral offset is detectable with resolution at the nanometer level, and the tilt and in-plane twist angle between wafer and mask could be manually decreased down to the scope of 10(-3) and 10(-4)u2009u2009rad, respectively.

Spectrum-Integral Talbot Effect for UV Photolithography With Extended DOF

A route of spectrum-integral Talbot lithography (SITL) with extended depth-of-focus (DOF) for microfabrication of periodic structures was explored under broadband incoherent illumination in this letter. The transmitted diffraction fields by different spectral components integrate together to generate the successive periodicity since certain distance along the direction of propagation. The mechanism of DOF extension was derived and numerically elucidated using the spectrum of a practical ultraviolet source. Experiments of proximity lithography in term of the numeric results were performed to record the intensity distributions within the DOF area. Finally, the results reveal the validity of SITL and its potentials in high-fidelity lithography of periodic micropatterns with almost unlimited DOF and thus enhanced resolution.

Depth-of-Focus Determination for Talbot Lithography of Large-Scale Free-Standing Periodic Features

The depth-of-focus (DOF) is an important factor to influence the pattern quality but seldom treated in conventional contact or proximity lithography. One explicit formula for DOF prediction is analytically derived in the context of photolithography of periodic features (i.e., the Talbot lithography), in which wavefront aberrations contributed by all diffraction orders are considered. Besides the numerical elucidations, a direct photolithographic process is also customized for experimental demonstration. With strict DOF estimation based on the governing equation for thickness control of the resist coating, large-scale freestanding features of micromeshes with well-controlled perforation and steepness are fabricated.

Scalar-Based Analysis of Phase Gratings Etched in the Micro/nanofabrication Process

The diffraction characteristics of several types of phase gratings often etched on the substrate by the micro/nanofabrication techniques are analytically explored using scalar-based analysis in this paper. The process of an incident wave being reflected or transmitted by the diffraction grating is regarded as a process of modulation, and the reflectance or transmittance can be unified as the modulation index. The mechanisms of phase modulation, amplitude modulation, and the amplitude-phase hybrid modulation in different situations are discussed. Analytical results indicate that the diffraction efficiency is directly determined by the phase difference of adjacent features, i.e., the cyclically distributed ridges and grooves that induce different phase and amplitude variations. The absolute phase grating with phase difference equivalent to π has the maximum diffraction efficiency among all types of gratings. The conclusions could, in general, provide guidance for the design and micro/nanofabrication of phase gratings for many diffraction-based applications of optical metrology or imaging.

The Anomaly of Periodicity Doubling in Projection Photolithography of Periodic Features

In the case of duplicating periodic features by UV projection lithography, the unwanted phenomenon of periodicity or frequency doubling somehow occurs due to far-field imaging of the near-field diffraction. In this letter, we fundamentally explored the mechanism of frequency or periodicity doubling in an analytical and numerical way, and reconfirm the validity by experiments of projection lithography. In this process, both the Talbot self-image and π-phase-shifted image were synchronously captured to generate the overlapped pattern with doubled periodicity within the depth-of-focus (DOF) area. The Fresnel diffraction theory was used to further derive the relationship between the mask and the objective lens parameters (i.e., the resolution and DOF). Finally, experimental results showed the self-image and π-phase-shifted image, both clearly recorded on wafer when optical parameters were well adjusted.