Short-pulse Ultraviolet Laser Research Articles

Study on the interfacial adhesion property of low-k thin film by the surface acoustic waves with cohesive zone model

The cohesive zone model being increasingly used in discrete fracture processes simulation is adopted to study the interfacial adhesion property of low dielectric constant film deposited on the silicon substrate in this work. The two parameters, maximum normal traction and normal interface characteristic length in cohesive zone model, are taken into account to calculate the theoretical surface acoustic wave dispersion curves. Broadband surface acoustic wave signals with effective frequency up to 200MHz are generated by short pulse ultraviolet laser source and detected by a piezoelectric transducer. The interfacial adhesion properties of dense and porous films determined accurately by matching the experimental dispersion curves with the calculated theoretical dispersion curves are 10.7PPa/m and 2.8PPa/m, respectively. The results show that the adhesion quality of dense low dielectric constant film is better than that of the porous. The study exhibits that the adhesion properties determined by improved laser-generated surface acoustic wave technique have the same trends with the test results of the nanoscratch technique, which indicates that the surface acoustic wave technique with cohesive zone model is a promising and nondestructive method for determining interfacial adhesion properties between low dielectric constant film and substrate.

Nondestructive determination of interfacial adhesion property of low-k/Si by the surface acoustic waves

Nondestructive surface acoustic wave (SAW) technique promoted for interfacial adhesion investigation is employed to study the adhesion properties of both the porous and the dense low dielectric constant (low-k) films deposited on the silicon substrate. Spring boundary model is employed by introducing the parameters of distributed shear and normal spring constant per unit area (KT and KN) between the film and the substrate to take the adhesion property into account to calculate the theoretical SAW dispersion curves. Broadband SAW signals are generated by short pulse ultraviolet laser source and detected by a piezoelectric transducer. Interfacial adhesion properties of the low-k samples can be obtained by fitting the experimental dispersion curve with the theoretical ones. The smooth experimental dispersion curves with maximum frequency up to 150 and 200MHz are achieved for the porous and dense low-k samples, respectively. The determination result indicates that the adhesion property of dense low-k film is much better than that of the porous low-k sample used in this study. The determined adhesion result by SAW exhibits the same trend with the test of the nanoscratch.

Acceleration of a solid-density plasma projectile to ultrahigh velocities by a short-pulse ultraviolet laser

It is shown by means of particle-in-cell simulations that a high-fluence (≥1 GJ/cm2) solid-density plasma projectile can be accelerated up to sub-relativistic velocities by radiation pressure of an ultraviolet (UV) picosecond laser pulse of moderate values of dimensionless laser amplitude a0∼10. The efficiency of acceleration by the UV laser is significantly higher than in the case of long-wavelength (λ ∼ 1 μm) driver of a comparable value of a0, and the motion of the projectile is fairly well described by the “Light Sail” acceleration model.

Experimental and numerical optimization of beam shapes for short-pulse ultraviolet laser cutting processing

In this work the effect of elliptical beam shapes on cutting performance of silicon is studied experimentally using a Diode Pumped Solid State Q-switched UV laser operating at the wavelength of 355 nm. Elliptical beams are investigated by varying scanning speed, repetition rate and average output power. It is shown that a short elliptical beam with 6×230 μm dimension forms a deeper groove at higher cutting speed and lower average output power compared to a longer elliptical beam. A numerical model of the laser cutting process is also described. Some validation results for single and multi-pulse cutting are shown.

Surface machining characteristics of sapphire with fifth harmonic YAG laser pulses

In order to reduce the thermal influences in laser machining of sapphire surface such as reattachment of molten material, cracks, etc., effects of a short-pulse ultraviolet laser were investigated. For the ultraviolet laser, the Fifth harmonic generation (Fifth HG) wave of an Nd:YAG laser (wavelength: 213 nm) was utilized. It was shown that significant reduction of thermal damages on the surface was demonstrated with the Fifth HG pulses compared to longer wavelengths of the Nd:YAG laser, such as fundamental (1064 nm), Second HG (512 nm), Third HG (355 nm). Also hole-drilling and surface grooving characteristics were investigated. It was found that the control of depth of the hole and groove with reduced thermal influences was possible in lower fluence cases (less than 40 J/cm 2) with a homogenized beam. Moreover, it was shown that smooth bottom surfaces with their roughness Ra <1 μm could be obtained.