Takeo Nakano

Structure modification of radio frequency sputtered LaB6 thin films by internal stress

Thin films of LaB6 are prepared by a radio frequency (rf) magnetron sputtering technique. The effect of sputtering conditions on the film properties of internal stress and crystalline orientation are investigated. The film deposited at low pressures (<1 Pa) has a strong compressive stress as high as 109 Pa. As the pressure of Ar discharge gas decreases, the internal stress becomes greater and the preferred orientation is transformed in the sequence: (100)→(110)→(111). This behavior can be explained as an energetically favorable structure of LaB6 film by taking both the surface energy and the strain energy into account. An application of a negative bias (−20 V) to substrates results in a greater stress, so that the (110) and (111) orientation becomes more preferential.

Unambiguous detection of the adhesive failure of metal films in the microscratch test by waveform analysis of the friction signal

The vibrational microscratch tester monitors electrically the frictional response of a scratch-stylus which is forced to oscillate on the film surface. This tester is so sensitive that it is able to detect the fracture of ceramic films less than 30 nm thick in situ by catching an irregular jump of the friction. The adhesive failure of films of ductile materials, however, does not necessarily yield a sufficient fracture signal. In the present study, the waveform of the frictional signal for an oscillation period is decomposed into the frequency regime by Fourier expansion. The signal due to the solid friction is found to appear only in odd harmonics of the fundamental excitation frequency. By integrating the component of even harmonics of the observed signal, the critical failure of the adhesion has been made to observe more sensitively. The observation of the critical failure of thin copper films is demonstrated.

Mechanical and electrical properties of rf sputtered LaB6 thin films on glass substrates

Abstract LaB 6 films from 2000 to 5000 A thick were prepared on glass substrates by rf magnetron sputtering. The effects of Ar discharge gas pressure from 5.3 to 5.3 × 10 −2 Pa on the structural, electrical and mechanical properties of the films were examined. The structural characteristics were analyzed by X-ray diffraction and emission spectrochemical technique (ICP). The electrical film resistivity was measured using a four-point probe technique. The internal stress and adhesion of films were determined by the bending-plate method and the microtribometer, respectively. The deposition process of rf magnetron sputtering was studied by a quadrupole mass filter system. The results indicated that an increase in Ar gas pressure in sputtering depositions increased the atomic ratio B/La in LaB films. The most preferable Ar gas pressure for stoichiometric LaB 6 formation was found to 10 −1 Pa and it was also suitable for producing high quality films as electrodes. The main crystalline orientation of the sputtered LaB 6 films was (100) which is the orientation of lower work function (2.4–2.5 eV) in LaB 6 crystals.

The effect of 'warm' gas scattering on the deceleration of energetic atoms : Monte Carlo study of the sputter-deposition of compounds

Abstract A Monte Carlo simulation code has been developed to account for the mass transfer in the sputtering deposition. In the present work, the velocity of gas atoms at ambient temperature is taken into consideration, while the gas atoms have been regarded cold enough compared to the sputtered particle. The effect of gas motion becomes important at higher pressures since sputtered particles tend to be scattered more frequently and to be decelerated more. We present the formalism of (1) mean free path of the sputtered particle, (2) velocity distribution of the colliding gas particle, and (3) transformation of the coordinates between colliding system and laboratory one. The effect of these processes on the compositional deviation of LaB6 films that are sputter-deposited at higher gas pressures is discussed.

A hybrid simulation of high pressure sputtering, combining the Monte Carlo method and the diffusive approach

Abstract Modeling of particle transports at high pressure sputtering has been studied. We assume that the diffusion process of thermalized particles is governed by the Poissons equation, and this is solved by the boundary element method (BEM). The thermalization points are obtained by the Monte Carlo (MC) method using a ballistic approach, and they are treated as the source of particles in the Poissons equation, The result of BEM agrees fairly well with MC for the diffusion of low-energy particles under high pressures, while the computation time is extremely reduced. The hybrid method-MC and BEM, before and after the thermalization-has also been developed, and the comparison with experiments is shown.

Simulation of particle transport in high pressure sputtering

Transport properties of sputtered particles in a gas environment have been studied at finite temperatures using a recently developed new Monte-Carlo simulation code. In this code, the thermal motion of gas atoms has been incorporated to the collision frequency and the scattering process. The situation that the gas atoms are not frozen affects critically on the transport process of particles of low energies since the effect of the thermal motion of the colliding gas atom becomes important on the change in speed and direction with scattering. In this report, we show the time evolution of position and energy of sputtered particles. The manner how the mass difference of sputtered particles works is demonstrated graphically.

LaBx thin films prepared by magnetron sputtering

LaBx (x = 0–6) thin films were prepared by magnetron sputtering using a LaB6 target and Ar discharge gas. The composition of the films was investigated by the ICP method. It has been found that the composition strongly depended on the Ar discharge gas pressure. The film composition was mostly stoichiometric (x = 6) at low Ar pressures while the value of x decreased with increasing pressure. This nonstoichiometry is interpreted in terms of collision scattering of sputtered particles from the target by Monte Carlo simulation. The Ar pressure change induces the change of the crystal orientation of the films and this change is related with the work function of the film. The most appropriate Ar pressure for the preparation of the films used as electrodes is discussed.

Dielectric Breakdown Phenomena during Secondary Electron Emission Measurement of Sputter-Deposited MgO Films

The ion-induced secondary electron emission (SEE) characteristics of sputter-deposited magnesium oxide (MgO) films have been investigated. Using an RF magnetron sputtering apparatus, MgO films of 50–200 nm thickness were deposited on Si substrates from a sintered MgO target. Under irradiation of 1 keV of Ar+ ions, secondary electrons were collected at a positively biased electrode. The secondary electron current increased as the bias voltage increased, and saturated to give an SEE coefficient of 2–2.5 in all samples. The deposition condition (gas pressure 2–20 Pa) did not strongly affect this current–voltage (I–V) characteristic of the SEE, while an increase in film thickness resulted in an increase in the voltage at which the SEE current began to rise. This rising voltage depended neither on the energy nor on the current of the primary ion beam. We concluded that the rising voltage was governed by an electric breakdown phenomenon by which the positive charge accumulating on the surface was compensated from the substrate. The field strength was estimated to be as high as 109 V/m.

Transition of Roughness Evolution in Cu–In Alloy Films by the Formation of Intermetallic Compounds

The evolution of the surface morphology of Cu–In alloy film has been studied using an atomic force microscope. Samples of Cu–In were prepared by sequential vacuum deposition: copper was deposited to 10–40 nm thick first on a glass substrate, followed by the deposition of various amounts of indium to make the indium composition in the range of 0–75 at.%. Samples were prepared under three different conditions: A) deposited at room temperature (RT); B) deposited at RT and annealed at 120°C in vacuo; and C) deposited at 120°C. Samples formed by A and B (series A and B) showed a growth with a dynamical exponent β of ~0.3 while the atomic ratio In/Cu was less than 2. The formation of CuIn2 intermetallic compound was observed by X-ray diffraction as the atomic ratio approached 2. After that, β suddenly increased to ~0.7, which was attributed to the segregation of excess indium atoms to form cap-shaped islands. For samples grown by C (series C), the roughness initially increased more rapidly with β~0.7 and decreased above In/Cu~1. It showed a minimum at In/Cu=2 and finally increased again with β~0.7 by forming indium islands, as in series A and B.

Modification of film structure by plasma potential control using triode high power pulsed magnetron sputtering

We have designed a new triode configuration in a magnetron sputtering apparatus to control the plasma potential of the discharge. An additional chimney electrode was introduced above the conventional sputter gun to apply a positive voltage. The discharge power was provided by a pulse power source to achieve high power pulsed magnetron sputtering operation. We confirmed that the plasma potential increased with increasing positive electrode voltage. Copper films with substantially flatter surfaces could be obtained on a water-cooled and electrically grounded substrate at an Ar gas pressure of 5 Pa.