Showing papers on "Pulsed laser deposition published in 1990"

Use of ion beam assisted deposition to modify the microstructure and properties of thin films

Abstract: Ion beam assisted deposition, the bombardment of a thin film with a beam of energetic particles during deposition, provides a powerful technique for modifying the microstructure and properties of thin films and coatings. Various experimental approaches used for ion beam assisted deposition are described and the physical basis for the effects is examined. Observations on modification of nucleation and growth behaviour, microstructure development, compound synthesis, and applications to the modification of properties such as intrinsic stress, adhesion, surface mechanical properties, corrosion and oxidation resistance, optical properties, and electrical properties, are reviewed.

Epitaxial yttria‐stabilized zirconia on hydrogen‐terminated Si by pulsed laser deposition

Abstract: Epitaxial yttria‐stabilized zirconia films were grown on Si (100) and Si (111) by pulsed laser deposition. Rutherford backscattering spectroscopy indicates a high degree of crystalline perfection with a channeling minimum yield of 5.3%. A necessary predeposition process is removal of native silicon oxide from the Si prior to film growth. This is done outside the deposition chamber at 23 °C using a wet‐chemical hydrogen‐termination procedure. Epitaxial YBa2Cu3O7−δ films have been grown on these films.

High critical currents in strained epitaxial YBa2Cu3O7−δ on Si

Abstract: Epitaxial YBa2Cu3O7−δ (YBCO) films were grown on Si (100) using an intermediate buffer layer of yttria‐stabilized zirconia. Both layers are grown via an entirely in situ process by pulsed laser deposition. All films consist of c‐axis oriented grains as measured by x‐ray diffraction. Strain results from the large difference in thermal expansion coefficients between Si and YBCO. Thin (<500 A) YBCO films are unrelaxed and under tensile strain with a distorted unit cell. Rutherford backscattering spectroscopy indicates a high degree of crystalline perfection with a channeling minimum yield for Ba as low as 12%. The normal‐state resistivity is 280 μΩ cm at 300 K; the critical temperature Tc (R=0) is 86–88 K with a transition width (ΔTc) of 1 K. Critical current densities of 2×107 A/cm2 at 4.2 K and 2.2×106 A/cm2 at 77 K have been achieved.

Theoretical model for deposition of superconducting thin films using pulsed laser evaporation technique

Abstract: We have theoretically and experimentally analyzed the laser‐induced evaporation process for deposition of superconducting thin films from bulk targets. The spatial thickness variations have been found to be significantly different from a conventional thermal deposition process. Unlike a cos θ thickness variation expected from a thermal evaporation process, the laser evaporation process is characterized by a forward‐directed deposit with a sharp variation in its thickness as a function of distance from the center of the deposit. We have studied in detail the interactions of nanosecond excimer laser pulses with bulk YBa2Cu3O7 targets leading to evaporation, plasma formation, and subsequent deposition of thin films. A theoretical model for simulating the pulsed laser evaporation (PLE) process has been developed. This model considers an anisotropic three‐dimensional expansion of the laser‐generated plasma, initially at high temperature and pressure. The forward‐directed nature of laser deposition has been found to result from anisotropic expansion velocities of the plasma edges arising due to the density gradients in the gaseous plasma.The physical process of the laser ablation technique for deposition of thin films can be classified into three separate interaction regimes: (i) interaction of the laser beam with the bulk target, (ii) plasma formation and initial isothermal expansion, and (iii) adiabatic expansion leading to deposition of thin films. The first two regimes occur during the time interval of the laser pulse, while the last regime initiates after the laser pulse terminates. Under PLE conditions, the evaporation of the target is assumed to be thermal in nature, while the plasma expansion dynamics is nonthermal as a result of interaction of the laser beam with the evaporated material. The expansion velocities of the plasma edges are related to the initial dimensions and temperature of the plasma, and the atomic weight of the respective species present in it. Preliminary calculations have been carried out on spatial thickness variations as a function of various parameters in PLE deposited thin films. The effects of the various beam and substrate parameters including energy density and substrate‐target distance affecting the nature of deposition of superconducting thin films have been theoretically examined. Experimental results have been obtained from thin films deposited on silicon substrates by XeCl pulsed excimer laser (λ=308 nm, τ=45×10−9 s) irradiation. The spatial thickness and compositional variations in thin films have been determined using Rutherford backscattering technique and the results compared with the theoretical calculations.

Silicon dioxide deposition method using a magnetic field and both sputter deposition and plasma-enhanced cvd

Abstract: In plasma enhanced chemical vapor deposition (PECVD) of silicon dioxide on a substrate, voids and discontinuities are reduced by first depositing silicon dioxide in a sputter each chamber (22) in which a magnetic field is produced within the rf plasma for depositing the silicon dioxide. Simultaneous sputter etch and deposition occurs which inhibits net deposition at the corners of metal conductors over which the silicon dioxide is deposited. The substrate is then removed and transferred through a load lock (27) to a conventional PECVD deposition chamber (23).

a‐axis oriented epitaxial YBa2Cu3O7−x‐PrBa2Cu3O7−y heterostructures

Abstract: We have grown YBa2Cu3O7−x‐PrBa2Cu3O7−y heterostructures with the perovskite a axis normal to the surface of a variety of (001) oriented substrates using the pulsed laser deposition technique. X‐ray diffraction studies indicate little or no formation of the traditional c‐axis normal orientation usually nucleated on these substrates, while ion channeling, transmission electron microscopy (TEM), and Raman scattering studies reveal a highly ordered crystalline structure similar in quality to that obtained in the best c‐axis oriented films. Superconducting transition temperatures, for current transport in the plane of the films, consistently exceed 80 K, but are probably lower than the optimum 92 K transition expected for YBa2Cu3O7−x due to the presence of growth twin boundaries between grains with the c axis oriented along the two substrate directions.

The effect of oxygen in diamond deposition by microwave plasma enhanced chemical vapor deposition

Abstract: High quality diamond thin films were deposited on different substrates at temperatures from 300 to 1000 °C by the microwave plasma enhanced chemical vapor deposition (MPCVD) system. The quality of deposited diamond films was improved by adding oxygen in the gas mixtures. Different ratios of methane to oxygen concentration in hydrogen at different temperatures have been studied. At high temperatures (800–1000 °C), the addition of oxygen will not only enhance the growth rate of deposited films but also extend the region of diamond formation. At low temperatures ( 900 °C) were either graphitic or diamond containing a large amount of graphitic or amorphous carbon and at low temperatures (<500 °C) were white, soot-like coatings which were easily scraped off. The quality of the deposited films was characterized by Raman spectroscopy and scanning electron microscopy.

Origin of surface roughness for c‐axis oriented Y‐Ba‐Cu‐O superconducting films

Abstract: One problem with the growth of high quality c‐axis oriented YBa2Cu3O7−x films is the tendency of the film surface to become rough. We studied the film growth mechanism as a function of deposition rate using pulsed laser deposition. These films form by the classic nucleation and growth process; the thickness at which the nucleated islands coalesce increased with decreasing deposition rate. The film has pinholes prior to coalescence and nucleates outgrowths during coalescence. The outgrowths enlarge rapidly because they contain materials and crystallographic directions with growth rates faster than that of the c‐axis film. A smooth surface is obtained if the substrate temperature and deposition rate are chosen such that coalescence is just completed at the final film thickness. We observed the outgrowths nucleating at coalescence and propose that certain defects, related to the c‐axis growth habit, may be the fundamental cause of outgrowth formation. Outgrowths have not been observed in a‐axis films. Outgrowths are easily confused with the particulate deposition problem associated with laser deposition. In these experiments, the particulate problem was essentially eliminated by using freshly polished targets for each run.

Effects of beam parameters on excimer laser deposition of YBa2Cu3O7−δ

Abstract: Broad angular distributions have been observed for XeCl laser ablation plumes used in the deposition of YBa2Cu3O7−δ thin films. Distributions (inferred from film thickness) and film stoichiometry were measured as a function of laser fluence, beam shape, and oxygen pressure. Parallel to the long axis of the laser spot, plumes exhibited a cos3.5(θ) spread and composition varied with angle; in the perpendicular direction, more diffuse [cos1.5(θ)] plume distributions were associated with stoichiometric deposition. The observed phenomena are consistent with formation of a Knudsen layer near the target surface.

Microwave properties of highly oriented YBa2Cu3O7−x thin films

Abstract: We have performed intra‐ and extra‐cavity microwave frequency (1–100 GHz) measurements on high quality Y1Ba2Cu3O7−x superconducting thin films on (100) LaAlO3 substrates. The ∼0.3 μm thin films fabricated by the pulsed laser deposition technique exhibit superconducting transition temperatures >90 K, as determined by resistivity and ac susceptibility measurements, and critical current densities of 5×106 A/cm2 at 77 K. Moreover, ion beam channeling minimum yields of ∼3% were measured, indicating the extremely high crystalline quality of films grown on the LaAlO3 substrate. Microwave surface resistance values at 77 K for these films are found to be more than one to two orders of magnitude lower than for copper at 77 K for almost the entire frequency range explored. We postulate that the reason we observe such low surface resistances in these films is the virtual absence of grain and phase boundaries coupled with the high degree of crystallinity. Furthermore, we believe that the residual resistance measured b...

Epitaxial growth of ferroelectric bismuth titanate thin films by pulsed laser deposition

Abstract: Epitaxial thin films of ferroelectric bismuth titanate Bi4Ti3O12 have been grown by pulsed laser deposition on single‐crystal [100] SrTiO3 substrates. Bismuth titanate has a high Curie temperature (675 °C) and saturation polarization values of 4 and 50 μC/cm2 along the c and b axis, respectively. Its a,b lattice parameters allow thin‐film growth on substrates such as SrTiO3, LaAlO3, MgO, etc. These single crystalline films exhibit good quality as evidenced by x‐ray diffraction, Rutherford backscattering, and transmission electron microscopy. Applications for these films include memory devices and optical displays.

Physical vapour deposition techniques for the growth of YBa2Cu3O7 thin films

Abstract: The growth and characterisation of YBa2Cu3O7 thin films are reviewed. The discussion of growth is limited to the physical vapour deposition techniques of laser ablation, sputtering and evaporation. Emphasis is placed on the practical problems of growing epitaxial films in situ and characterising them.

Microstructure of YBa2Cu3O7−x thin films deposited by laser evaporation

Abstract: YBa2Cu3O7–x thin films deposited by laser evaporation were studied by transmission electron microscopy. The crystal structure and microstructure of the films depend sensitively on the oxygen partial pressure during deposition. Cooling conditions affect primarily the oxygen sublattice and thus determine electrical properties in the superconducting state. Deposition of technologically relevant thin films requires an oxygen partial pressure of 0.3 mbar during deposition and slow cooling in an oxygen atmosphere. Such films have a Tc of 90 K and a transition width of 0.6 K. Electron diffraction patterns yielded a relative lattice parameter difference (b – a)/b of 1.6%, from which the oxygen content can be determined with considerable accuracy. Extended crystal defects and second phases were analyzed by analytical and high-resolution electron microscopy. Films deposited at lower oxygen partial pressures (≈10–2 mbar) exhibit a strongly strained crystal structure with lattice planes heavily bent on an atomic scale. In these films evidence for a decomposition reaction of YBa2Cu3O7–x was found by the simultaneous presence of BaCu2O2 grains and Y2O3 precipitates. The films deposited at low oxygen partial pressure become superconducting when cooled slowly in an oxygen atmosphere. The onset of the broad transitions lies at 50 K.

Laser transfer deposition

Abstract: The present invention is directed to a method for producing a smooth and continuous pressure bonded layer of a material on a base substrate through the driving force of a pulsed laser. The method comprises the steps of selecting a laser absorptive material. Then applying thereto a thin layer of the material for pressure bonding to the base substrate. Next, placing the base substrate and the thin layer of material on the laser absorptive material in a vacuum, then placing the base substrate in close proximity to the thin layer of material. Finally, directing pulsed laser energy toward the laser absorptive material and the layer to be pressure bonded, whereby the laser energy is absorbed by the laser absorptive material causing heating and vaporization thereof to a controlled depth of the laser absorptive material, leaving remaining portions of the laser absorptive material unvaporized after the vaporization to act as a support for the material to be bonded. After bonding, the thin material adheres to the base substrate when the remaining continuous absorptive material is peeled away, the vaporization having provided the necessary pressure for pressure bonding on to the base material.

Downstream microwave plasma‐enhanced chemical vapor deposition of oxide using tetraethoxysilane

Abstract: In this paper, results for dielectric oxide films deposited using downstream microwave plasma‐enhanced chemical vapor deposition in the temperature range between 250 and 400 °C are presented. The deposition of oxide using TEOS (tetraethoxysilane)+O2 and TEOS+N2O chemistries are studied. In the reactor, the TEOS is injected directly into the deposition chamber without passing through the discharge. Only He, O2, or N2O are fed through the microwave cavity where the discharge is generated. In addition, no ions but chemically active species are present in the deposition chamber during the deposition. The deposition rate is found to decrease with increasing temperature. In addition, it appears that the deposition rate increases with increasing concentration of active oxygen species in the deposition chamber. These suggest that the generation of intermediate species of TEOS and adsorption/desorption of the reactant on the surface are the key steps that determine the deposition rate. The stress of the deposited oxide films is found to be tensile and less than 2×109 dyn/cm2. The Si‐OH concentration in the films is found to be low and can be below the detection limit of infrared spectrometry by increasing the flow ratio of O2/TEOS during the deposition. The step coverage of the oxide films over the Al runners is found to be excellent due to the long diffusion time available for TEOS surface species before forming SiO2. The mechanisms of oxide deposition using TEOS+O2 and SiH4+N2O chemistries are studied and compared. The details of oxide step coverage versus different deposition processes are also discussed.

Effect of deposition rate on properties of YBa2Cu3O7−δ superconducting thin films

Abstract: Superconducting thin films of YBa2Cu3O7−δ on (100) SrTiO3 are prepared in situ by a pulsed laser deposition technique at deposition rates from 1 to 145 A/s. Crystallinity of the films is examined by Rutherford backscattering in the channeling mode using a 2.2 MeV He+ ion beam. The backscattering minimum yield (χmin) increases with the deposition rate. A χmin of 3% is observed in the films deposited at the lowest deposition rate. Even at a deposition rate of 145 A/s, the films show good crystallinity with χmin of 15%, indicating epitaxial growth. The x‐ray diffraction measurements show that the films have strong c‐axis orientation normal to the substrates. The films have metallic resistance versus temperature behavior with zero resistance temperatures of 90 K. The results indicate that the pulsed laser deposition technique could be used to deposit large‐area films efficiently with adequate substrate movement.

Metal buffer layers and Y‐Ba‐Cu‐O thin films on Pt and stainless steel using pulsed laser deposition

Abstract: A versatile pulsed laser deposition chamber was employed for fabricating metal buffer layers and superconducting YBaCuO thin films on metallic substrates. Ag buffer layers were found to improve the resistive transition behavior for superconducting films on Pt and stainless steel. As‐deposited YBaCuO films with Tc (R=0) at 84 K were produced on stainless steel using in situ laser‐deposited Ag buffer layers. The critical current density was measured to be approximately 103 A/cm2 at 67 K.

Electrical properties of Bi2Sr2CaCu2Ox thin films prepared in situ by pulsed laser deposition

Abstract: Thin Bi2Sr2CaCu2Ox films were prepared onto SrTiO3 single crystalline substrates by pulsed laser deposition. The inductively measured Tc, onset reached values up to 83 K. Critical current densities and resistive transitions were measured in external magnetic fields up to 7 T with the field direction parallel and perpendicular to the c-axis of the film. The measured jc values reached 1×107 A/cm2 at 4.2 K and 1×106 A/cm2 at 5o K, though pattering by wet chemical photolithography caused serious degradation of Tc.

Fluorine‐doped tin dioxide thin films prepared by chemical vapor deposition

Abstract: Transparent conductive tin dioxide thin films were prepared by a low‐temperature atmospheric‐pressure chemical vapor deposition method in air. The raw material was tin(II) trifluoroacetate. At a reaction temperature above 250 °C, polycrystalline thin films were obtained with a high deposition rate. This chemical vapor deposition method effectively incorporates F atoms into a crystalline structure and consequently maximizes the carrier concentration, yielding fluorine‐doped SnO2 films of very low resistivity. For the 1260‐nm‐thick film deposited at 400 °C, the resistivity was 5.92×10−4 Ω cm, and the sheet resistance was 4.69 Ω/⧠. The deposition condition, structure, and characteristics of films were compared to the corresponding values in the chemical vapor depositions of SnO2 from tin(II) acetate.

Laser ablation for materials synthesis

Abstract: The large number of acronyms that are used in the literature to describe the laser deposition process that was the subject of Symposium N at the Materials Research Society 1990 Spring Meeting, Laser Ablation for Materials Synthesis, has reduced the overall visibility of this technique. nevertheless, this two day symposium attracted considerable attention with 44 contributed papers which covered a wide range of topics including fundamental processes, novel applications, and process optimization. Roughly half of the contributed papers in this symposium concerned the deposition of high T{sub c} superconductors -- an application for which this technique has been repeatedly proven. The remaining half of the submitted papers demonstrated the versatility of Pulsed Laser Deposition in the synthesis of semiconductor epitaxial layers, band gap engineered layers, diamond-like carbon, ferroelectrics, SiC, and metallic thin films.

Excimer laser ablation and etching

Abstract: The characteristics of excimer lasers and the nature of ablation and etching processes are described. The two kinds of laser ablation mechanisms, thermal and electronic, are discussed. Thermal processes all rely on an intense laser pulse to heat a surface very rapidly. Electronic mechanisms do not rely on heating. Two of these quantum-type processes have been discussed widely. In the first, laser photons directly excite and break the bonds of the solid, causing ejection of material. In the second, photoexcitation creates electron-hole pairs, the potential energy of which can be coupled directly into kinetic energy of the atoms via a radiationless process. The energized atoms are able to overcome the surface binding energy, and again material is ejected. Pulsed laser etching has many of the same physical-interaction mechanisms as laser ablation but requires an active chemical medium to be in contact with the solid because laser-induced chemical reactions serve as the driving force for material removal. Issues that must be addressed prior to the successful implementation of an excimer-laser fabrication tool are discussed. The use of patterned beams for direct patterning of surfaces is considered. Microelectronic applications suitable for excimer-laser ablation and etching are examined. >

Selective vapor-phase deposition on patterned substrates

Abstract: Vapor growth techniques such as evaporation, sputtering, molecular beam epitaxy (MBE), and chemical vapor deposition (CVD) are usually used for large-area thin film deposition. However, local, or so-called selective, deposition onto desired regions of patterned substrates can be attained. Principally, there exist several categories of selective growth systems. In area-selective growth, the deposition takes place only on one substrate material, and no deposition is obtained on the other. This selectivity is based on a difference in the interfacial reactions between the different substrate materials and the vapor. The interfacial reactions on one substrate material should be inhibited completely to avoid nucleation, whereas the deposition reactions should be stimulated on those substrate areas where the depositon will occur. In phase-selective deposition, different phases are deposited simultaneously and selectively on the different substrate materials. In analogy to the phase-selective deposition,...

Laser sputtering apparatus

Abstract: A laser sputtering apparatus comprising: a laser oscillator for emitting a pulsed laser beam; an optical unit for uniforming intensity of the laser beam; a vacuum tank which contains a target; and an optical system for irradiating the laser beam onto the target through the optical unit.

Digital chemical vapor deposition and etching technologies for semiconductor processing

Abstract: A novel concept on digital chemical vapor deposition (CVD) and etching technologies is described. ArF excimer laser irradiation to a condensed Si2H6 layer on a substrate cooled to −70 °C has resulted in spatially selective poly‐Si film growth. The adsorbate thickness control by gas pressure and substrate temperature allows shot by shot atomic layer growth of Si. Digital CVD of SiO2 is also achieved by a repetitive cycles of silane radical deposition and subsequent oxidation. This reaction is promoted by an alternate introduction of pulsed microwave‐discharged SiH4 and O2 beams. The deposition species ejected with supersonic velocity into the high vacuum reactor fills SiO2 into a deep trench. Also, digital etching of Si monolayers has been studied for the goal of damage‐free etching. A preliminary result obtained by repeating the reaction cycle consisting of adsorption of fluorine atoms on a Si surface cooled to −100 °C and subsequent Ar+ ion irradiation has realized atomic layer etching of Si(100).

Temperature dependence of growth of GexSi1−x by ultrahigh vacuum chemical vapor deposition

Abstract: We report the deposition of epitaxial films of GexSi1−x on (100) silicon by the ultrahigh vacuum chemical vapor deposition technique Epitaxial films grown at temperatures ranging from 577 to 665 °C have been characterized with respect to growth rate and germanium content The results show features which have not been previously reported including an incubation time and a peak in the growth rate as a function of GeH4/H2 flow

Growth of epitaxial PrO2 thin films on hydrogen terminated Si (111) by pulsed laser deposition

Abstract: A new epitaxial oxide, PrO2, has been grown on Si (111) by pulsed laser deposition. X‐ray diffraction shows that films are oriented with the PrO2[111] direction parallel to the substrate [111]. The full width at half maximum for the omega rocking curve on the PrO2 (222) peak is as low as 0.75°, while phi scans indicate in‐plane epitaxial alignment to better than one degree. In the best quality films, epitaxy is almost pure type‐b epitaxy which is characteristic of epitaxial CaF2 on Si. To achieve epitaxy, it is essential to remove the native silicon oxide from the substrate prior to film growth. This is done at room temperature using a wet‐chemical hydrogen‐termination procedure.

Rapid thermal chemical vapor deposition of germanium on silicon and silicon dioxide and new applications of Ge in ULSI technologies

Abstract: In this study, low pressure chemical vapor deposition of pure germanium on silicon and silicon dioxide has been considered for new applications in future ultra large scale integration (ULSI) technologies. Germanium depositions were performed in a lamp heated cold-wall rapid thermal processor using thermal decomposition of GeH4. It is shown that Ge deposition on Si can be characterized by two different regions: a) at temperatures below approximately 450° C, the deposition is controlled by the rate of surface reactions resulting in an activation energy of 41.7 kcal/mole. b) Above this temperature, mass transport effects become dominant. The deposition rate at the transition temperature is approximately 800 A/min. It is shown that Ge deposition on SiO2 does not occur, even at temperatures as high as 600° C, resulting in a highly selective deposition process. Selectivity, combined with low deposition temperature makes the process very attractive for a number of applications. In this work, it is shown for the first time that selective Ge deposition can be used to eliminate silicon consumption below the gate level during the silicidation of the shallow source and drain junctions of deep submicron MOSFETs. In addition, a new in situ technique has been developed which allows polycrystalline germanium (poly-Ge) deposition on SiO2. In this work poly-Ge has been considered as a low temperature alternative to polycrystalline silicon (poly-Si) in the formation of gate electrodes in single-wafer manufacturing where low-thermal budget processes are most desirable.

Superconductivity and crystallinity of Ba2Y1Cu3O7−δ thin films prepared by pulsed laser deposition with substrate bias voltage

Abstract: Substrate bias voltages were found to be significantly effective in preparing high‐quality laser‐deposited superconducting Ba2Y1Cu3O7−δ films at reduced substrate temperatures. The zero‐resistance temperature of the biased films, positive bias being more effective than negative, decreased very slightly when the substrate temperature was lowered, whereas that of the unbiased films decreased considerably. In addition, the surface morphology and c‐axis orientation have been improved by applying substrate bias voltages. Bias voltages within ±500 V hardly affect the composition of the resulting films so that stoichiometric films have been obtained from a stoichiometric target. A time‐resolved optical observation revealed that a short time emission, probably being oxygen plasma, occurred in a few μs after the laser pulse impingement. The improvement in crystallinity of the resulting films is attributed to this emission. The velocity of emissive species in the plume was determined to be 6×105 cm/s. Under positiv...