Susan D. Allen

Laser‐assisted micron scale particle removal

A novel laser‐assisted particle removal (LAPR) technique capable of removing micron scale particles from semiconductor substrates is presented. In our preliminary experiments the contaminated substrates were dosed with water which preferentially adsorbs in the capillary spaces under and around the particles and were subsequently irradiated with transverse, electric, atomspheric CO2 laser pulses. At the CO2 laser wavelength the beam energy is mainly absorbed in the water and not the substrate. The subsequent explosive evaporation of the adsorbed water molecules produces forces many orders of magnitude larger than the adhesion forces between the particle and the substrate which propel the particles off the substrate surface. LAPR is inherently clean and can easily be incorporated into current or planned wafer processing systems.

Enhancement of Nd:YAG LIBS emission of a remote target using a simultaneous CO(2) laser pulse.

For the first time to the best of our knowledge, a simultaneous 10.6 mum CO(2) laser pulse has been used to enhance the Laser Induced Breakdown Spectroscopy (LIBS) emission from a 1.064 mum Nd:YAG laser induced plasma on a hard target. The enhancement factor was on the order of 25 to 300 times, depending upon the emission lines observed. For an alumina ceramic substrate the Al emission lines at 308 nm and Fe impurity line at 278 nm showed an increase of 60x and 119x, respectively. The output energy of the Nd:YAG laser was 50 mJ/pulse focused to a 1 mm diameter spot to produce breakdown. The CO(2) laser pulse had a similar energy density of 40 mJ/mm(2). Timing overlap of the two laser pulses within 1 microsecond was important for enhancement to be observed. An observed feature was the differential enhancement between different elemental species and also between different ionization states, which may be helpful in the application of LIBS for multi-element analysis.

Shock wave analysis of laser assisted particle removal

Laser assisted particle removal (LAPR) is an innovative laser cleaning technique which can remove various particles from solid surfaces via laser induced explosive evaporation of a chosen energy transfer medium, e.g., water. An Ar+ ion continuous‐wave laser (488 nm) was used to study the CO2 laser pumped explosive evaporation of water adsorbed on a Si substrate. The probe laser beam was parallel to the sample surface at different displacements and interacted with the ejected material upon pulsed CO2 laser irradiation in analogy with the time resolved laser beam deflection experiments on laser induced vaporization of copper by Guo et al. [Opt. Commun. 77, 381 (1990)]. Using CO2 laser energies which are much greater than the LAPR thresholds, we observed the generation and propagation of a shock wave at supersonic speeds followed by a water vapor/aerosol/particle cloud at a much slower speed. From the evolution of the shock wave, the total conversion efficiency of the incident laser beam into the shock wave ...

high deposition rate laser direct writing of Al on Si

We report, for the first time, the direct write laser patterning of highly conductive Al from a liquid precursor, triisobutylaluminum (TIBA). Al wires were written on Si with a scanned Ar+ laser from liquid TIBA at speeds of up to several mm/s. Wires 3 μm wide by 1 μm high with a resistivity of 5.6 μΩ cm were routinely achievable.

Excimer laser machining and metallization of vias in aluminum nitride

Laser machining of ceramics is used extensively in the microelectronics industry for scribing and via hole drilling. Scribing involves laser ablation of a groove or row of holes that form perforation lines to separate a large substrate into individual circuits. Via machining is generally followed by a metallization step to create three-dimensional (3-D) interconnections in a multilayer circuit board. Aluminum nitride (AlN) is a desirable substrate material for high power, high frequency applications because of its high thermal conductivity and low thermal expansion coefficient than Al/sub 2/O/sub 3/. In this paper, an excimer laser is used to machine high aspect ratio, straight walled via holes in aluminum nitride with or without a metallization layer deposited on the via walls. Via diameters range between 60 and 300 /spl mu/m through substrates 635 /spl mu/m thick. Through hole machining can cause damage to the back surface of the substrate, however, attachment of a second substrate or metal sheet will prevent damage. Ablation of the attached metal backing with subsequent redeposition on the via walls produces a metallized via with a resistance of less than 1 /spl Omega/ per via. Single and multilayer via structures are described. Substrate damage at through hole exits results from shock wave propagation and reflection in the substrate. The attached backing material reduces reflection of the shock wave at the back surface of the substrate to prevent damage. Shock wave analysis, via cross sections, and resistance measurements are discussed.

CO2 laser assisted particle removal threshold measurements

Laser removal of particulates from solid surfaces was achieved using an energy transfer medium which preferentially adsorbs in the capillary spaces under and around the particulates on the contaminated surface. Subsequent laser irradiation causes explosive evaporation of the energy transfer medium and propels the particles off the substrate much like a small rocket engine. In our experiments, a TEA CO2 (10.6 and 9.6 μm) laser was used to remove 9.5 μm Al2O3, 5 μm Al2O3, and 1 μm polystyrene particles from Si surfaces using water as the energy transfer medium. At these wavelengths the laser energy is absorbed predominantly in the water not in the substrate. The threshold fluence for particle removal was found to follow a degenerate threshold model with measured thresholds significantly below the substrate damage threshold. The temperature rise in the energy transfer medium was estimated using energy conservation, suggesting that superheating of the adsorbed water is a reasonable mechanism for water assiste...

Optical transmission measurements of explosive boiling and liftoff of a layer of micron-scale water droplets from a KrF laser-heated Si substrate

Water plume velocities were measured in air by optical transmission as a function of laser fluence using a KrF laser for explosive boiling and liftoff of a layer of micron-scale water droplets from a laser-heated Si substrate of interest for laser particle removal. The thickness of the superheated water layer near the water/Si interface determines acceleration and removal of the water droplets from the Si substrate.

Submicrosecond Dynamics of Water Explosive Boiling and Lift-Off from Laser-Heated Silicon Surfaces

Explosive boiling and lift-off of a thin layer of micron-sized transparent water droplets from an absorbing Si substrate heated by a nanosecond KrF laser were studied using a contact photoacoustic technique. The compressive photoacoustic response increases steeply to an asymptotic value on the order of the water critical pressure starting at a threshold laser fluence of 0.20J∕cm2, where lift-off of the water layer also occurs. Above this threshold, several reproducible discrete multimegahertz components are revealed in Fourier spectra of the acoustic transients, corresponding to nanosecond oscillations of steam bubbles inside the water droplets on the microsecond time scale of the lift-off process. The acoustic pressure buildup, bubble dynamics, and the subsequent lift-off of the thin water layer are interpreted as relaxation stages after near-spinodal explosive boiling of the superheated interfacial water.

Photoacoustic study of explosive boiling of a 2-propanol layer of variable thickness on a KrF excimer laser-heated Si substrate

The dynamics of explosive boiling of a 2-propanol layer of variable thickness on a Si substrate heated by a nanosecond KrF excimer laser was studied using a contact photoacoustic technique. The transition from acoustic generation at a free Si boundary to that at a rigid alcohol/Si boundary accompanied by a sharp increase of acoustic generation efficiency was found above a laser fluence threshold of 0.17 J/cm2 and a liquid layer thickness greater than 0.25 μm due to subnanosecond near-critical explosive boiling of the superheated liquid layer near the hot absorbing Si substrate. The gradual increase of the photoacoustic response of the superheated alcohol with increasing thickness of the liquid film at fluences above the explosive boiling threshold was attributed to a diffraction effect due to the fluence- and time-dependent increase of the area undergoing explosive boiling. A model describing photoacoustic generation and subsequent lift-off of the entire liquid layer in this experimental “thin transparent...

Nanosecond-laser plasma-assisted ultradeep microdrilling of optically opaque and transparent solids

A mechanism of ultradeep (up to tens of microns per pulse, submillimeter total hole depths) plasma-assisted ablative drilling of optically opaque and transparent materials by high-power nanosecond lasers has been proposed and verified experimentally using optical transmission and contact photoacoustic techniques to measure average drilling rates per laser shot versus laser intensity at constant focusing conditions. The plots of average drilling rates versus laser intensity exhibit slopes which are in good agreement with those predicted by the proposed model and also with other experimental studies. The proposed ultradeep drilling mechanism consists of a number of stages, including ultradeep “nonthermal” energy delivery into bulk solids by the short-wavelength radiation of the hot ablative plasma, bulk heating and melting, accompanied by subsurface boiling in the melt pool, and resulting melt expulsion from the target.