Shoshana Tamir

Pre-bonding technology based on excimer laser surface treatment

Abstract The application of ArF excimer laser for surface pre-treatment of polycarbonate, polyetherimide, polyaryl ether–ether–ketone (PEEK) composite, fiberglass, aluminum, copper and fused silica was investigated. Various substrates were tested with excimer laser irradiation using various parameters, such as: intensity, repetition rate, and number of pulses. The optimal laser treatment parameters were found for each material needed for achieving maximum adhesional strength of the corresponding bonded joints. Experimental results indicated that UV laser surface treatment improved significantly the adhesion strength compared to conventional treated substrates for all the materials tested. The improved adhesion was correlated with the roughening of the irradiated surface, chemical modification and removal of contamination.

Growth mechanisms of amorphous SiOx nanowires

Amorphous SiOx nanowires (NWs) 10–50nm thick and tens of microns long were grown by laser ablation of silicon containing targets onto different substrates held at elevated temperatures. The influence of the growth parameters on the NWs growth and structure was studied. Deposition of a metal catalyst on the substrates was found essential for the SiOx NW growth. The morphology and structure of the NWs were studied using high resolution scanning and transmission electron microscopes with their accessories. Possible growth mechanisms of these nanowires were suggested and discussed.

Growth modes of ZnO nanostructures from laser ablation

ZnO nanowires (NWs) and other nanostructures were grown by laser ablation of a ZnO containing target onto different substrates with and without the presence of an Au catalyst. The morphology and structure of the NWs were studied using high resolution scanning and transmission electron microscopes [including imaging, selected area electron diffraction (SAED), and energy dispersive x-ray spectroscopy (EDS)]. The different growth modes obtainable could be tuned by varying the Zn concentration in the vapor phase keeping other growth parameters intact. Possible growth mechanisms of these nanowires are suggested and discussed.

Correlation between microstructure and photoluminescence of nanocrystalline silicon powder prepared by laser-induced CVD

Abstract Nanocrystalline silicon powder was prepared by decomposition of SiH4 gas using an excimer laser beam. The laser was operated at 193 nm, a repetition rate of 10–70 Hz, and a pulse duration of 24 ns. The base vacuum in the chamber was 10−6 Torr and the pressure during deposition was 1–5 Torr. Si particles were formed in the gas phase and deposited on Si(100) and also on SiO2 substrates. The microstructure of the Si powder was studied using analytical and high-resolution transmission electron microscopy. The photoluminescence of the powder was measured in wavelengths of 400–900 nm using excitation light of 488 nm and 330 nm. The powder was found to consist of nanocrystalline circular grains. The average size and size distribution of the grains were found to be strongly dependent on the pressure and flow rate of the gas mixture and on the repetition rate of the laser beam. Grains as small as 5 nm with a narrow size distribution were obtained. Photoluminescence (PL) emission was observed at room temperature from the Si powders in the following wavelengths: 610–670 nm, 510–550 nm, and 430–510 nm. The width and position of the PL peak were found to depend on the size distribution and average size of the Si grains.

Electroluminescence and electrical properties of nano-crystalline silicon

Nc-Si formed by laser induced chemical vapor deposition (LCVD) showed photoluminescence behavior similar to porous silicon. We have tried to measure electroluminescence signals by applying contacts to the nc-Si made of materials such as Au, ITO, Al. A three-layer structure Me/nc-Si/Me was used and the contacts were found to be unstable during measurements and failed at relative low bias. The total resistance of the structure was very high and therefore the current flow was very low and only few EL measurements could be done at an integral form. The I-V behavior was studied also on nc-Si produced by pulsed laser deposition (PLD).

Laser induced chemical vapor deposition of optical thin films on curved surfaces

Laser induced chemical vapor deposition (LCVD) of silicon nitride and silicon dioxide single and double layers has been investigated using excimer laser operating at a wavelength of 193 nm. Single layers of silicon nitride were formed from SiH 4 /NH 3 gas mixture with Si/N atomic ratio of 0.6-0.7. The layers that contained a small amount of hydrogen had a refractive index and extinction coefficient of n = 2, k = 0015 at 600 nm. Deposition of silicon dioxide was investigated using SiH 4 /N 2 O. Using this gas mixture the film composition depended strongly upon the SiH 4 /N 2 O ratio. At high ratio the film formed was silicon oxynitride, which contained both Si-N and Si-O bonds. The film also contained small a amount of Si-H bonds. Decreasing the SiH 4 /N 2 O ratio led to the formation of pure silicon dioxide with a refractive index of 1.45. A two layer coating of silicon nitride and silicon dioxide resulted in the formation of an antireflection coating with a reflectivity of about 0.5% at 750 nm.

Laser induced deposition of nanocrystalline Si with preferred crystallographic orientation

Abstract Laser induced chemical vapor deposition (LCVD) has been utilized for formation of thin films on various substrates. The quality of the films depends on the irradiation and process parameters. Under high beam energy and repetition rate powder particles can be obtained instead of continuous film. In the present work we studied the conditions required for obtaining Si powder particles composed of nanocrystalline grains having uniform size and spherical shape. The Si was deposited from a SiH 4 /Ar gas mixture on a thermally grown amorphous Si-oxide layer on a single crystalline Si(100) substrate by using an excimer laser. The laser was operated at 193 nm with a pulse duration of 24 ns. It was found that at a laser energy density between 60 and 300 mJ/cm 2 , a repetition rate above 20 Hz, and a total pressure above 60 Torr, powdered Si was formed. SEM studies of the deposited Si show large clusters (1–10 μ m) of Si powder particles. The clusters have characteristic shapes depending on their size. Microstructural studies by analytical and high resolution transmission electron microscopy show that the Si particles consist of nanocrystalline grains having perfect circular shape and narrow size distribution. In addition, the grains have a preferred crystallographic orientation even though deposited on amorphous Si-oxide film. The microstructural observations of the Si powder particles are explained by homogeneous nucleation of nano-size Si grains in the gas phase, followed by agglomeration of the nano-size grains to clusters in the gas phase, and finally deposition of the Si clusters, while conserving the initial size and microstructure, on the substrate and formation of Si powder particles.

Optical properties, microstructure and composition of SixN1−x prepared at low temperatures by laser induced CVD

Abstract Thin films of silicon nitride were deposited on optical glasses and Si(100) substrates at temperature as low as 170°C, by using laser induced CVD (LCVD). ArF laser was used for dissociation of gas mixture composed of SiH 4 NH 3 . The effect of lowering substrate temperature on microstructure, composition and optical properties of the films has been investigated. Amorphous silicon nitride films having uniform composition and thickness, which are transparent in the visible range were obtained. The index of refraction of the film decreases by decreasing substrate temperature from 1.98 at 320°C to 1.75 at 170°C. The decrease is associated with the formation of silicon oxide on the substrate at temperatures lower than 200°C. The deposition of the silicon nitride was found to be limited by the dissociation rate of the ammonia gas and the substrate temperature.

Laser-induced nickel silicide formation

Abstract Nickel silicide compounds were formed via excimer laser irradiation (193 nm) of a (111) silicon substrate coated with 85 nm thick nickel films. Irradiation was carried out in a laser energy range from 2 J cm -2 to 0.2 J cm -2 at a repetition rate of 2 Hz and scanning speed of 0.2 mm s -1 . Two distinct processes of silicide formation were observed. At laser energies in the range 1.28-0.5 J cm -1 the silicides were formed via melting and recrystallization of the nickel coating and part of the silicon substrate. The main phase observed was NiSi 2 . At laser energies below 0.5 J cm -2 the nickel coating was unaffected and only the inner part was consumed. The main phases observed were Ni 5 Si 2 and Ni 3 Si 2 . The microstructure and composition of the silicide formed were analysed by scanning electron microscopy, transmission electron microscopy, X-ray diffraction and Auger spectroscopy.

Growth mechanisms of silicon films produced by laser-induced chemical vapor deposition

Abstract We have studied chemical vapor deposition (CVD) of Si from an SiH 4 + Ar mixture, using excimer laser beam excitation parallel to a Si substrate. The growth mechanisms were studied by analyzing the effects of the temperature, pressure, laser repetition rate and intensity on the deposition rate and on the film microstructure. In addition, absorption measurements and gas analysis were performed during the deposition. This paper discusses the results of thickness measurements and calculations of the activation energy. A Gaussian-shaped transverse thickness distribution was obtained with a maximum corresponding to the center of the laser beam. This distribution depended on the deposition parameters and was attributed to the diffusion process of the silane decomposition products in the gas phase to the substrate. An Arrhenius plot of the deposition rate vs. the substrate temperature could be divided into two regimes associated with different activation energies. Between 340 and 460 °C, the activation energy is 0.25−0.3 eV, while it is 1.1 eV between 500 and 560 °C. The activation energy in the higher temperature regime is similar to that found for thermal CVD without the use of a laser. However, in the lower temperature regime, the deposition process is mainly laser induced, and the value of the activation energy was attributed to the process of adsorption of the gas species onto the substrate.