Chengwu An

Anomalous Hall effect and origin of magnetism in Zn1−xCoxO thin films at low Co content

A small anomalous Hall effect signal of a Zn1−xCoxO (x=0.05) thin film was observed, indicating that there presents sp-d interactions in the Zn1−xCoxO thin film. However, under a self-held setup, we did not obtain observable differences between the M-H curves when the near band electrons were changed. Integrated with our previous results, possible mechanisms for the magnetism of Zn1−xCoxO thin films were proposed based on a picture of spins in a carrier sea.

Electron field emission from polymer films treated by a pulsed ultraviolet laser

The poly(phenylcarbyne) polymer films were coated on silicon substrates and then irradiated by a pulsed ultraviolet laser (λ=248 nm) with various fluences (1–60 mJ/cm2) at an atmospheric pressure of nitrogen. The structures of the resulted films were investigated by Raman spectroscopy. The morphologies of the films were examined by scanning electron microscopy (SEM). The electron field emission properties of the films as cathodes were studied. Raman spectrum analysis and SEM results indicate that the polymer film is converted to nanoparticle carbon film with the laser fluence from 10 to 60 mJ/cm2. The conversion mechanism from the polymer to nanoparticle carbon and electron field emission mechanism from the converted carbon film is discussed. The converted carbon film showed better field emission properties, i.e., lower turn-on threshold emission field, higher emission current density, and higher emission light spot density with increasing laser fluence from 10 to 50 mJ/cm2. By increasing the laser fluenc...

Observation of species distribution in laser induced plasma

The species distribution and evolution in laser induced plasma are optically studied by using time-of-flight spectra extracted from time-resolved spectra and newly introduced space-of-flight spectra extracted from space-resolved spectra. The plasma consists many ionic and atomic species. At the head of the plasma plume, ions are always the dominant species. However, there are two distribution peaks for the atoms. One is at the head, and another is at the tail of the plume. If the laser fluence used is high enough, the atoms reside mainly in the tail of the plume. The distribution changes with time and space, which implies an evolution of the species in the course of plasma flying. It is also observed that the proportion of ions to atoms in the plasma decreases with decreasing laser fluence.

CO2-Laser-Induced Regular Periodic Structures on Glass Substrates

A CW CO2 laser was used to induce regular and tidy periodic structures on glass substrates. It is experimentally shown that the wave vector of the ripples is perpendicular to the recording polarization irrespective of the scanning direction. The occurrence of periodic structures is highly sensitive to laser power level and scanning velocity. To obtain appropriate periodic patterns, a combined condition of laser power and scanning velocity must be satisfied. Different directions of laser scanning lead to different occurrence areas. Different kinds of materials seem to develop the occurrence areas with almost similar shape, but their relative positions and dimensions may be different. The maximum laser scanning velocity range to form regular and tidy periodic structures appears at the optimum power condition. Moreover, the maximum period could be obtained at this optimum condition.

Self-organization of silicon nanocone array induced by pulsed CO2 laser irradiation

The periodic microfringe structures were induced on the front silicon surface by pulsed CO2 laser irradiation with the existence of a painting or coating layer on the bottom surface of silicon substrate. The microfringes were formed under single pulse irradiation due to the laser induced periodic surface structure (LIPSS) effect. The nanodot were self-organized in chains under cumulative laser pulse irradiation, and located along the microfringe ridges. The silicon surface was patterned with a nanocone array of two different average sizes at even higher laser intensity. The larger cones distributed in rows, and the smaller cones scattered between the two adjacent rows of the larger cones. The far-infrared absorption of silicon is promoted by the increase of free carrier concentration and thermal energy resulting from the laser interaction with the coating/painting materials at the substrate bottom surface.

Laser applications in integrated circuit packaging

Laser processing has large potential in the packaging of integrated circuits (IC). It can be used in many applications such as laser cleaning of IC mold tools, laser deflash to remove mold flash form heat sinks and lead wires of IC packages, laser singulation of BGA and CSP, laser reflow of solder ball on GBA, laser marking on packages and on SI wafers. During the implementation of all these applications, laser parameters, material issues, throughput, yield, reliability and monitoring techniques have to b taken into account. Monitoring of laser-induced plasma and laser induced acoustic wave has been used to understand and to control the processes involved in these applications.

Laser singulation of IC packages

The separation of IC packages from a BGA board is realized by means of laser multi-scan method. The laser used in the study is a double frequency Nd-YAG laser with wavelength of 532 nm. The big problem in the laser processing approach mainly arises from the multi-layer materials of BGA board with copper, polyethylene and epoxy glass fiber, because of their different absorption coefficient to the laser beam and their different absorption coefficient to the laser beam and their different heat conductivity. In the experiment approach, the effects of laser parameters, such as wavelength, on the dicing efficiency has been investigated for choosing laser. The influence of sample side for laser incidence on cut profile and, the influence of the focused extent of laser beam on singulation speed are discussed. The experimental results show that laser singulation of IC packages is efficient and reliable.

Observation of temporary degeneration of material transmittance under strong laser irradiation

Laser induced temporary degeneration of transmissivity in optical materials has been observed under strong laser irradiation. In the experiment, a green light laser beam was focused onto a transparent sample, an ultra fast photo-tube was used to detect the laser signals behind the sample, an oscilloscope was used to record the amplitude change of the signals. When the laser power density on the sample increases to some levels, the detected signal starts to decrease relatively, which means the transmissivity of the sample commences to degenerate. In this case, if the laser power density is reduced back a little bit, the transmittance of the sample can restore. This means that the degeneration of the transmittance is temporary. When the laser power density increases further more after the degeneration has commenced, damage to the material will happen. Therefore, there is a transition course in material before it is damaged. The transition course is from the beginning of transmittance degeneration to damage rising. The influence of laser repetition rate on the damage formation has been also investigated.

Microstructures formed on silicon wafer by CO 2 laser irradiation

Laser bumps have been formed on the silicon surface with anisotropic patterns induced by the pulsed CO2 laser under the backside effect conditions. The microstructures are formed with periodic patterns, and vary with the laser parameters. The parallel narrow fringes have period around 2 micrometers , and the fringe orientation is in parallel with the laser polarization direction. The circular fringes with spacing around 1 micrometers appear when the laser energy increase. The fringe patterns were found to be independent on the laser pulses, therefore are re-writable. The bump formation mechanism is considered to be thermal capillary wave effect during the material melting and resolidification processes under laser irradiation. Whereas, the laser absorption may be induced by the hot electrons, or the thermal energy resulting from the laser interaction with the backside coating material.

Laser marking on IC package by micro-encapsulated tapes

In this paper, we are reporting a new way to do marking on IC package. In this way, white ink is wrapped in microcapsules that are coated on a transparent tape. Laser is irradiated on the tape surface, the microcapsules are broken and the ink is released onto the IC package surface. After an UV light treatment, the ink will stick on the IC surface, forming a high contrast marking. It is found that the quality of the marking depends on tape configuration, tape-IC distance, laser peak power, scan speed of laser irradiation and other laser parameters.