Jae-Won Park

The interface between sputter-deposited gold thin films and ion-bombarded sapphire substrates

Abstract Pull-tests show that the adhesion strength of gold film sputter-deposited onto annealed and pre-sputtered sapphire is over 15 times higher than that of the film deposited onto as-received sapphire. The effects of ion beam bombardment on the sapphire substrate were investigated in situ with AES. Metallic aluminum was detected on the surface of sapphire substrates after irradiating for 3 min with 7 keV Ar + -ions. These results agree with TRIM calculations that yield preferential ion-beam etching. No metallic aluminum was formed at the surface when the ion bombardment was done at 3 keV for 3 min. The Au Al 2 O 3 interface was investigated using a new technique that takes advantage of the variable film thickness in the vicinity of a sputter-etched crater. This method not only allows for a non-destructive search of the interface but it also avoids a substantial ion damage to this region. Higher energy Auger peaks, which have higher Auger electron escape depth, revealed that a AuAlO compound formed at the interface. Formation of this compound, which is responsible for the strong metal-ceramic bonding, is due to ion-induced cleaning and reduction of the sapphire surface prior to gold film deposition.

IRRADIATION-INDUCED DECOMPOSITION OF AL2O3 DURING AUGER ELECTRON SPECTROSCOPY ANALYSIS

The effect of electron fluence on the decomposition of sapphire (Al2O3) was studied in situ by Auger electron spectroscopy (AES). The decomposition was primarily detected by monitoring the evolution of the low kinetic energy Auger transitions of aluminum in Al2O3 (54 eV) and in metallic aluminum (68 eV). The decomposition of sputter‐cleaned sapphire started at a fluence of ∼4.9×1019 electrons/cm2 (7.8 C/cm2). This fluence was independent of the electron fluxes used in this work, except the lowest, which indicates that heating due to electron bombardment does not significantly affect the decomposition behavior. Electron‐induced decomposition takes place in a minimum of the first five atomic layers of the substrate, as revealed by the evolution during irradiation of the high energy Al peaks associated with Al2O3 (1388 eV) and metallic aluminum (1396 eV). Comparison of the evolution of low and high kinetic energy Auger transitions demonstrates that the decomposition kinetics are much faster for the first mon...

The adhesion of copper films deposited onto aluminum nitride

Good adhesion between copper film and AlN substrate is obtained when the surface of AlN is laser-irradiated prior to copper film deposition and post deposition annealing is conducted. Surface chemistry of AlN substrates before and after laser irradiation and the interfacial reactions of copper film/AlN couples were studied with Auger Electron Spectroscopy (AES) to understand the adhesion mechanisms. The surface of as-received AlN substrates was covered with a thin sheath of Al2O3. Laser irradiation removed the surface Al2O3 layers, smoothened the surface, and decomposed AlN leaving metallic aluminum on the surface. The interfacial reactions in the copper film/AlN couple are affected by the amounts of oxygen and metallic aluminum available at the interface. The adhesion mechanism is the formation of a Cu-O-Al compound at the interface of copper film/AlN couple. Since copper does not react with AlN, laser induced decomposition of AlN seems to be the driving force for the formation of the compound.

VUV-light-induced deposited silica films

Abstract A novel technique to deposit dielectric films at room temperature is described. The deposition of the silica takes place inside a cylindrical glass chamber where a silent discharge is generated between two electrodes connected to a high voltage, high frequency AC source. The chamber contains two parallel glass tubes where the electrodes are located and is filled with argon or xenon at a pressure of 100 mbar. Under these conditions, it has been shown that high intensity VUV light is generated peaking at 126 nm for argon and at 172 nm for xenon. This VUV radiation seems to produce photoablation of the glass tubes that surround the electrodes. Upon operation of the lamp, polyimide, polypropylene and silicon wafer substrates lying at the bottom of the vessel became coated with silica. The films, identified using X-ray photoelectron spectroscopy (XPS), revealed that the silica is oxygen-deficient with a composition of SiO x where x is between 1.7 and 1.8. The deposition rate on silicon wafers was measured by ellipsometry. When Xe gas is used the deposition rate is much lower than when Ar is used. This result is consistent with a photoablation process since the energy of the photons generated in Ar peaks at 10 eV while those generated in Xe peaks at 7 eV. These energy values should be compared with the O–Si bond strength energy that is 8.3 eV. The morphology and structure of the films were examined by scanning and transmission electron microscopies. Deposition of carbonaceous films occurred when the glass tubes containing the electrodes were coated with carbon.

Sample charging of insulators with rough surfaces during Auger electron spectroscopy analysis

A way to circumvent the sample charging in the analysis of insulators with rough surfaces during Auger electron spectroscopy (AES) has been studied. It is very difficult to handle the charging of insulators with rough surfaces during AES because of the irregular electron accumulations on the irradiated surface. This irregularity is attributed to various incident beam angles due to the surface roughness, which produce various electron excitation volumes in an electron rastering area. The charging problem of an insulator with rough surfaces during AES could be controlled by properly selecting the analysis position and area on the top of a protrusion or a particle on the surface of the insulating sample. This simple technique uses high spatial resolution of AES and the surface topography of an insulator, aiming at reducing the electron excitation volume as well as increasing the total electron yield.

Pulsed laser ablation growth and doping of epitaxial compound semiconductor films

Pulsed laser ablation (PLA) has several characteristics that are potentially attractive for the growth and doping of chemically complex compound semiconductors including (1) stoichiometric (congruent) transfer of composition from target to film, (2) the use of reactive gases to control film composition and/or doping via energetic-beam-induced reactions, and (3) low-temperature nonequilibrium phase formation in the laser-generated plasma ``plume.`` However, the electrical properties of compound semiconductors are far more sensitive to low concentrations of defects than are the oxide metals/ceramics for which PLA has been so successful. Only recently have doped epitaxial compound semiconductor films been grown by PLA. Fundamental studies are being carried out to relate film electrical and microstructural properties to the energy distribution of ablated species, to the temporal evolution of the ablation pulse in ambient gases, and to beam assisted surface and/or gas-phase reactions. In this paper the authors describe results of ex situ Hall effect, high-resolution x-ray diffraction, transmission electron microscopy, and Rutherford backscattering measurements that are being used in combination with in situ RHEED and time-resolved ion probe measurements to evaluate PLA for growth of doped epitaxial compound semiconductor films and heterostructures. Examples are presented and results analyzed for doped II-VI, I-III-VI, and column-III nitride materials grown recently in this and other laboratories.

Mechanisms of laser activation of dielectric materials

Aluminum Nitride (AlN), Alumina, Sapphire, Magnesium Oxide (MgO), Silica and Polymide can be laser activated. The irradiation promotes near‐surface modifications that catalyze the deposition of copper when the substrates are immersed in an electroless solution. All the ceramics that exhibit laser activation have a very low absorption coefficient or are transparent to ultraviolet radiation. The damage generated during laser irradiation plays a strong role in enhancing the laser‐dielectric interaction in alumina, magnesium oxide and silica. On the other hand, after the first irradiation the interaction between the aluminum nitride and the UV radiation is controlled by the metallic aluminum on the surface. A thin layer of metallic aluminum is produced in alumina and in sapphire by laser irradiation in an Ar‐4% H2 atmosphere. This film is not present if the irradiations are conducted in air. XPS analyses of alumina and sapphire substrates show no significant changes in the positions of the aluminum and oxygen...

Auger Electron Spectroscopy of Metallic Film/Laser-Irradiated Alumina Couples

Auger Electron Spectroscopy (AES) was employed to study metal/ceramic interfaces. Adhesion pull testing showed that a strong bonding between metallic films and alumina substrates is obtained when alumina is laser-irradiated in an oxygen atmosphere before deposition and, after deposition, the couple is annealed at 300°C for 1 h. On the other hand the gold/alumina bonding is extremely weak when laser-irradiation of the substrate is performed in argon-4% H 2 . Fresh surfaces were exposed for AES at several distances from the metal/ceramic interface by sputtering the specimens in situ after each analysis. In the region of the interface the sputtering rate was slower than in the film in order to minimize any spurious effect due to ion beam bombardment. AES of a gold/alumina couple prepared in this way reveals that the Auger peaks of gold from the film and oxygen and aluminum from the alumina substrate, when the substrate was irradiated in oxygen atmosphere, shift by ∼ 1.5 eV, 1.6eV, and 1.4eV respectively when the analysis area encompasses the metal/ceramic interface. On the other hand, no shift of the Auger peaks are observed at the interface when the laser-irradiation was performed in argon-4%H 2 . In the case of copper deposited on alumina laser-irradiated in an oxygen atmosphere, an interfacial compound is formed. This compound is a double oxide of aluminum/copper or two separated oxides, and promotes strong copper/alumina bonding.

Study of substrate diffusion in epitaxial n-type CdSe films grown on GaAs (001) by pulsed laser ablation

N-type CdSe films with thicknesses of 470--630 nm were grown on (001) and 2{degree}-miscut GaAs wafers by ArF (193 nm) pulsed laser ablation of stoichiometric CdSe targets at platen temperatures (T{sub p}) of 250--425 C in vacuum and ambient Ar gas. Film-substrate interdiffusion was studied with Auger depth profiling, as well as energy dispersive x-ray fluorescent spectroscopy (EDS). Both techniques showed that extensive interdiffusion took place at the film-substrate interface for CdSe films grown at T{sub p} {ge} 355 C but was greatly reduced at T{sub p} = 250 C. Tilting the substrate to be approximately parallel to the ablation plume as well as decreasing the ambient gas pressure also reduced film-substrate interdiffusion.

On the origin of laser-induced surface activation of ceramics

Pulsed-laser irradiation of Al{sub 2}O{sub 3} and AlN surfaces promotes Cu deposition when the irradiated substgrates are immersed in an electroless bath. In this paper, the nature of the surface modification is analyzed using Auger emission spectroscopy (AES) and cross sectional transmission electron microscopy. During irradiation, AlN thermaly decomposes, leaving a discontinuous metallic film on the surface. A film of Al{sub 2}O{sub 3} is detected at the surface of the irradiated AlN substrate, much thicker when the irradiation is done in an oxidizing atmosphere than in a reducing one. Nanoparticles of metallic Al are generated during laser irradiation of Al{sub 2}O{sub 3} in a reducing atmosphere. When the Al{sub 2}O{sub 3} irradiation is done in an oxidizing atmosphere, regions containing Al or substoichiometric alumina are detected by AES. It is concluded that the presence of metallic Al is the main reason why electroless deposition can occur in both AlN and Al{sub 2}O{sub 3}. Deposition kinetics are consistent with this conclusion. It is likely that also substoichiometric alumina helps to catalyze the electroless deposition.