Examples for the role of elementary photochemical processes for laser induced modification of surfaces by deposition - using aluminum-alkyls -, and etching - using excimer induced ablation - are described.

ArF (193 nm) and KrF (248 nm) laser-induced dissociation of Al2(CH3)6 (TMA) adsorbed on Si(100) surfaces have been investigated using x-ray photo-electron, ultraviolet photoelectron, and high-resolution electron energy loss spectroscopies (XPS, UPS, and HREELS). UPS and XPS valence spectra from the adlayer contained three broad peaks near 4.5, 7.5, and 14.5 eV below the Fermi level whose positions were in agreement, based upon molecular orbital calculations, with those expected for TMA dimers. Angle-resolved XPS measurements indicated a splitting in both the Al 2p and Si 2p levels which was interpreted as evidence for TMA dimer molecules being adsorbed with the long (i.e. Al-Al) axis perpendicular to the surface. The HREELS adlayer spectrum consisted of three clusters of peaks located near 86 meV (methyl rocking and Al-C stretch), 170 meV (methyl deformation), and 362 meV (C-H stretch). Changes in the XPS, UPS, and HREELS spectra were monitored as a function of ArF and KrF laser irradiation. No-changes were observed for KrF irradiation with intensities up to 200 mJ-cm . However, ArF laser irradiation resulted in changes in both the XPS and HREELS spectra which indicated TMA dissociation and the desorption of methyl groups. Nevertheless, even after 104 20 mJ-cm-2 ArF pulses most of the carbon-containing species remained. Higher intensities, up to 200 mJ-cm-2, resulted in the formation of compounds which were stable on the surface.

Trimethyl aluminum vapor is found to chemisorb on the surface of silicon dioxide and aluminum oxide via reaction with surface hydroxyl groupe. In addition to the chemisorbed layer, trimethyl aluminum also forms a weakly bound multilayer consisting of molecular dimers.

Laser processing conditions have been discovered which provide high-quality aluminum deposits from the photodecomposition of TMA (trimethylaluminum). Using an ArF excimer laser, AI can be deposited on Si and glass in well defined patterns at high rates (600 A/min) at low TMA pressure (≤ 30 mtorr) if substrate temperature is kept low (≤ 25C). Deposit composition is characterized by x-ray photoelectron spectroscopy (XPS) which shows that deposit carbon content depends upon laser pulse intensity varying nearly quadratically from < 1% at 8 mJ/cm2 to nearly 25% at 80 mJ/cm2. Mass spectroscopie analysis during laser processing identifies methane, ethylene and ethane as primary hydrocarbon photoproducts. Thermal desorption measurements indicate that many adlayers of adsorbed TMA leave the sample surface with only slight heating (35C) providing an explanation for observing higher growth rates at room temperature. Because of the high deposition rate and deposit quality observed, this low temperature process provides an attractive processing alternative for interconnect repair on Si and glass.

An in situ XPS study is made of the AI film growth from TIBA using an ArF pulsed laser on Si (100) substrates. It is found that the film is formed by the photochemical decomposition of the organometaliic on the surface. A metallic film is formed by island growth. These islands are covered with an organometaliic fragment layer of partially decomposed TIBA. The consequences of these observations toward understanding how laser processing can create prenucleation regions that catalyze film growth at temperatures below standard CVD temperatures are discussed.

We have studied the deposition of aluminum films by triisobutylaluminum (TIBA) pyrolysis on clean and oxidized Si(lll) (7×7), Si(100) (2×1), and an evaporated aluminum film on silicon. On all of the silicon surfaces, HBA has a very low reaction probability (<0.01). On aluminum, the reaction probability is much higher, and the rate of steady state aluminum deposition is limited by β-hydride elimination from surface-bound isobutyl species to give isobutene and hydrogen. In the steady state deposition of aluminum on top of silicon, gas phase silanes such as isobutyl- and diisobutylsilane are also evolved. This clearly indicates that hydrogen and alkyl ligand transfer from aluminum to silicon (which has diffused up through the growing aluminum film) can occur. We have also found that alkyl species generated on aluminum surfaces by. the adsorption of alkyl iodides have surface chemistries analogous to the aluminum alkyl ligands.

Tri-isobutylaluminum decomposition occurs preferentially on aluminum rather than silicon oxide surfaces. This preference might be exploited to fill via holes for multi-level integrated circuit metallizations. Initially a KrF excimer laser (248 nm) was used to sensitize aluminum pads in the base of the via holes. However, depending on the chemical treatment of the surface, it was discovered that laser enhanced activation was not required to obtain nucleation. At 240”C spontaneous aluminum growth can be made to occur exclusively in the via holes simply by utilizing appropriate cleaning methods. This technique was used successfully to fill via holes 1 μπι in diameter and 0.7 μπι deep with aluminum plugs. Selectivity is excellent, but chamber contamination can result in random nucleation. Technical problems still to be over come include grain growth morphology, 100 % via hole activation and better control of nucleation selectivity.

Partially ionized beam (PIB) deposition technique was used to deposit Al thin film on Si(n) substrate. The fabricated Schottky diodes showed an anomalous C-V characteristics for the Al films deposited with a bias potential equal to 2.5kV. The 1/C2 vs. V plot showed a drastic decrease in the slope as compared to the diodes deposited without the bias potential. Further measurements showed a frequency dependence in the C-V characteristics. This anomalous C-V characteristics can be explained by the formation of a p-n junction diode underneath the Si surface. A model of self-ion implantation which explains the formation of this surface p-layer on the Si(n) substrate is proposed and tested.

Our previous experiments have shown that Al and Au films deposited on Si substrates by ionized cluster beams (ICB) exhibit high thermal stability and .long electromigration life time. To understand these characteristics, in-situ XPS measurements during ICB deposition of Au on Si(111) substrate have been made. The results show that at low coverages, the splitting between the two prominent structures in the Au-5d peaks in the valence band becomes closer to those of pure bulk gold when a film is deposited at high acceleration voltages. In contrast to this an appreciable change of the binding energy of the Si-2p state at low coverage and high acceleration voltage could not be observed. These results suggest that a high density layer is formed and any Au-Si interaction is suppressed at the initial stage of film deposition. This could be one of the reasons why ICB-deposited films exhibit high thermal stability.

The laser-assisted chemical vapor deposition (LCVD) of Fe and W on Si(lll)-(7×7) was investigated under ultrahigh vacuum conditions at 120 K. Multiple internal reflection Fourier transform infrared and Auger electron spectroscopies and temperature programmed desorption and low energy electron diffraction were used to study the adsorption and decomposition of Fe(C0)5 and w(C0)6 on the surface. Neither of the molecules thermally reacts in temperature programmed desorption experiments. Reversible molecular desorption is exclusively observed. Decomposition of both molecules via electronic excitation was induced by ultraviolet, but not visible photons. This was shown by measuring the photodecomposition yield as a function of wavelength. Visible photolysis (λ=720 nm) had no observable effect on either molecule. No surface stable, partially decarbonylated, Fe(C0), x<5, fragments observable with infrared spectroscopy were produced by photolysis. Also, no new features were observed in temperature programmed desorption experiments and only iron was detected on the surface after photolysis. In contrast, initial results indicate that photolysis of W(C0)fi does produce surface stable, W(C0), x<6, fragments.

A quartz crystal microbalance has been used to measure the rates of focused uv laser-induced deposition from the Group 6 hexacarbonyls in real time. The experimental configuration employs acwuv laser beam focused onto the microbalance crystal surface at normal incidence to deposit material by decomposition of a metal complex vapor. Simple equations are given for determining absolute deposition rates in terms of the total mass deposited per unit time. Kinetic data obtained with this system have been used to study the mechanism of laser photodeposition from Cr(CO)e, Mo(CO)6, and W(CO)é induced by a frequency-doubled argon ion laser. The rate-determining deposition step involves condensation of products formed upon single-photon dissociation of the metal carbonyls in the gas phase. Additional observations indicate that the adsorbed photoproducts undergo further photo-initiated dissociation on the substrate surface.

Cr containing deposits were formed by focussing a chopped (2000Hz) 325nm CW He-Cd laser on the inner surface of the quartz window of a cell containing 110-150 mTorr of Cr(C0)6 precursor. The deposit thickness and the chemical identity of contaminants were determined by stylus profilometry and attenuated total reflectance IR spectrometry. In certain regimes of incident laser power density, deposits exhibit a dual morphology, with a narrow inner deposit up to several microns in thickness on top of a wider deposit less than 0.16 microns in thickness. Changes in the morphology with incident power density and changes in transmitted and reflected light during the deposition enables assignment to photothermal and photochemical mechanisms. A deposition rate for each mechanism and an optical attenuation coefficient for the photochemical deposit at 325nm were determined from the deposit thicknesses as a function of time.

Excimer laser photolysis at 248 nm of trimethylgal1ium (TMG) was studied by probing gallium atoms by laser induced fluorescence (LIF) technique. From the dependence of LIF intensity on the excitation wavelength, temperature and pressure, a thermally assisted photodissociation in the chemisorbed state was concluded.

Laser-stimulated deposition from the acetylacetonates of Pt and Ir has been found to take place on silicon surfaces. The gas phase photoreactions have been observed in a supersonic free jet TOF mass spectrometer and above silicon. The bare metal ions and metal cluster ions are formed from the complexes in the gas by multiphoton excitation. On the substrates deposits are detected by microprobe analysis with a metal content higher than 95 %. Also from condensed complexes laser-induced metal deposition, and as competitive process photodesorption, takes place.

Laser light (the second or third harmonics from an Nd:YAG laser) was focussed on the metal substrate in vacuum. A pulsed molecular beam of an organometallic compound, a metal carbonyl, or an organic molecule was introduced in the entrance region (the region in front of the ion-lens assembly) of a quadrupole mass filter which was located near the metal substrate. The reaction of the molecules in the beam with either the electrons or ions from the metal substrate was studied by monitoring the product ions by the mass spectrometer.

The laser induced forward transfer (LIFT) technique employs a pulsed laser to transfer material, originally precoated as a thin film on a transparent support, from that support onto a selected target substrate. LIFT has been used to deposit a variety of thin films, thick films through the repetitive transfer of thin films, multilayer structures through the transfer of appropriately layered precoated films, and metal silicon alloys.

A gas phase process for large area depositions on an ambient temperature substrate using laser-induced dielectric breakdown of gas phase precursors has recently been developed.1 Deposits of nickel alloys show excellent grain refinement (<10 nm) and metastable phase incorporation due to rapid quenching from the gas phase. Particle size distribution and compositional variance within the deposited films have been studied using electron microscopy and electron diffraction. Kinetic expressions to explain homogeneous gas phase nucleation and growth of the deposited materials have been developed in an effort to better understand this process. The effect of system variables on film and powder grain sizes has been studied. This analysis gives insight into the fluid flow/heat transfer patterns involved in the system and their effect on the final deposited material. The effect of system pressure, gas phase composition, and laser pulse energy, on particle size, surface area coverage, and deposition thickness are discussed.

A modulated surface temperature technique was used to study the surface chemical reactions occurring during the laser-assisted deposition of gold from dimethyl gold hexafluoroacetylacetonate. An argon ion laser was chopped and used to modulate the surface temperature on an area roughly 1 mm in diameter on an alumina substrate. Reactant and products of the photothermal reaction were sampled through an orifice located at the center of the deposit and were then introduced directly into a quadrupole mass filter. Since the reactant concentration at the surface decreased when the laser turned on while the product concentration at the surface increased when the laser turned on, modulating the surface reaction rate and employing phase-sensitive detection allowed the identification of reaction products.

A beam of ions incident on a substrate can cause adsorbed gas molecules to break up, resulting in material deposition. We have previously demonstrated deposition of gold from a gas of dimethyl gold hexafluoro acetylacetonate (C7H7F6O2Au) using both focused and broad ion beams. Here we investigate growth at various substrate temperatures and examine micro-structure using transmission electron microscopy. Films grown at room temperature were discontinuous even up to the thickness of 250μπι while films grown at higher substrate temperatures were continuous even at lower thicknesses. Deposition carried out on substrates at 100°C and 160°C using 70 kev Ar+ ions resulted in resistivities approaching the bulk value and a deposition yield of 60 to 75 atoms/ion. The relationship between growth conditions, micro-structure and resistivity will be discussed.

Tin oxide deposition was performed by focused argon ion laser irradiation in a tin tetrachloride gas atmosphere. Etching of gallium arsenide and tin oxide deposition were observed in a single scan of a laser beam under specific conditions. The center of the irradiated area was etched and then covered with deposit. Density ratio of 0/Sn obtained from the deposit by AES measurements was about 1.4 - 1.6. Additional hydrogen gas in ambient SnCl4 gas reduced both tin oxide deposition and gallium arsenide etching. Deposition, at first, occurred at the outer side of the irradiated area and gallium arsenide etching occurred at the center of it. The amount of deposit depends on beam dwell time. Additional oxygen in ambient SnCl4 gas improved deposited film quality: the deposited film with additional oxygen proved to be tin dioxide, which is an conductor.