Conference-ISSS-5-Effect of Excess Energy with Plasma Process on Nanostructure of Fe-Mg ∗

Ion-plating (IP) system as plasma process is expected to apply high excess energy into thin films. Excess energy of particles on IP process was defined as difference in temperature of evaporation particles and deposition particles of substrate. In this study, Fe-Mg thin films were prepared by IP process and discussed with effect of excess energy with IP process on nanostructure of thin films. Fe-Mg thin films were prepared by Bunshah’s triode type IP system. Excess energy of IP process was controlled by applied bias voltage of positive electrode. Excess energy of IP process has estimated by kinetic energy and ionization rate in evaporation particles. Kinetic energy of ion and ionization rate in evaporation particles were measured by Langmuir probe and Faraday cup respectively. Excess energy of IP process was increased with increasing of applied bias voltage of positive electrode. The value of excess energy on IP process was sufficient to exceed mixing enthalpy of Fe and Mg. In X-ray diffraction analysis, crystal structures of all samples were α-Fe bcc structures of Fe-Mg solid solution alloy. This is because lattice expansion was caused by excess energy on IP process. Nanostructure of Fe-Mg thin films was changed by Excess energy on IP process. Thus, IP process can control solubility limit and nanostructure in thin films. [DOI: 10.1380/ejssnt.2009.855]


I. INTRODUCTION
Fe-Mg system is immiscible in room tempature, since mixing enthalpy of Fe-Mg system shows positive value [1].Recently, Hotta et al. have reported that Fe-Mg body-centered cubic (bcc) alloys were synthesized by mechanical alloying (MA) process [2].They described that nanostructure of Fe-Mg was changed with the lattice distortion as mechanical energy accumulated into the α-Fe bcc lattice by MA process.
In this study, Bunshah's triode type ion plating (IP) system was used for the film preparation [3].In this process, dense plasma flux (∼2.5 A) of ionized source materials can be dosed and deposited on a substrate.The flux of the source vapor is ionized by thermal electrons accelerated from molten pool to a positive electrode.In this study, excess energy of particles on IP process was defined as difference in temperature of evaporation particles and deposition particles of substrate.Excess energy on IP process is able to control by applied bias voltage of positive electrode.
We have reported on magnetostrictive thin films of Fe-Al and Fe-Ga prepared by IP process [4][5][6][7].The lattice distortion was increased by substitution of Al or Ga into Fe (bcc).In those samples, solid solubility limit was extended and kept α-Fe bcc structure.The lattice distortion in samples for exceed solid solubility limit was caused by excess energy of IP process.
In this study, Fe-Mg thin films were prepared by an IP Figure 1 shows the schematic diagram of IP system.Since vapor pressure of Mg has about 8 orders differences compared with that Fe [8], Since the vapor pressure of Mg is much higher than Fe, Fe and Mg were evaporated by electron beam and resistance heating, respectively.Com  by crystal oscillator of "ULVAC CRTM-5000" individually.Applied bias voltage of positive electrode defined as V b was controlled at 50 V, 100 V and 150 V. Substrate potential was set as −10 V for accumulating of kinetic energy on ions in evaporation particles.Table I shows a deposition condition of the film formed by IP process.

B. Sample analysis
The composition of the film samples was measured by using energy dispersive X-ray spectroscopy (EDX).The film nanostructures were analyzed by X-ray diffraction (XRD).The surface observations of the film samples were used by scanning electron microscope (SEM) and transmission electron microscope (TEM).Internal stress of the film samples measured by optical lever method.

C. Method of measuring excess energy
Excess energy of particles on IP process was defined as difference in temperature of evaporation particles and deposition particles on substrate as E ε .
E ε is as follows Here, R is a gas constant.T vap and T sub are temperature of evaporation particles and deposition particles on substrate respectively.RT vap is as follows Here, E ion and E met are energy of ions in evaporation particles and evaporation particles which are not ionized respectively.E ion is as follows N A is Avogadro's constant, E Ki is kinetic energy of ion in evaporation particles and i is ionization rate in evaporation particles.E Ki is as follows V P and V sub were potential of plasma in evaporation particles measured by Langmuir probe and substrate respectively.n i and n vap were the flux of evaporation particles and ions measured by crystal oscillator and Faraday cup respectively."i" was estimated by ratio between ions and evaporation particles impinging unit area of substrate.
i is as follows E met is as follows T met is temperature of evaporation particles which are not ionized decided to 2000 K of molten pool temperature.RT sub is as follows T sub is temperature of substrate decided to 300 K from room temperature.Thus, E ε from a series of formulas (2) to ( 7) is as follows

III. RESULTS AND DISCUSSION
A. Structure of Fe-Mg films prepared by IP process impressed 50 V Figure 2 shows X-ray diffraction patterns of film samples at V b of 50 V.In all samples, α-Fe bcc structure of Fe-Mg alloy thin film was observed by X-ray diffraction patterns.At the increasing of Mg content, diffraction peaks of samples were shifted to lower angle and broadened.This is because the lattice expansion was increased with increasing of substitution of Mg into α-Fe bcc lattice.samples were the same at various V b .Diffraction peak of sample at V b of 100 V was sharpened.This is because the lattice expansion was decreased by increasing of mobility in deposition particles on substrate.

C. Concern of excess energy Eε and structure
Table II shows T i at various V b ."i" was increased with increasing of V b (Fig. It is thought that increasing of E ε is increase of impinging energy of evaporation particles to substrate and mobility of deposition particles on substrate.
Impinging energy of evaporation particles influences both solid solubility limit and crystallographic characterization in film.So, accumulated lattice distortion energy as lattice expansion of α-Fe bcc lattice was increased by impinging energy of evaporation particles.
Mobility of deposition particles on substrate influences only crystallographic characterization in film.At V b of 100 V, crystallographic characterization in film was reformed by increasing of mobility of deposition particles.In Thornton's Zone Model, lattice distortion of film was increased with increasing of impinging energy in evaporation particles and mobility of deposition particles on substrate [9,10].Figure 6 shows ratio of lattice distortion energy and excess energy impressed 50 V.The majority of deposition particles energy might be converted heat.It was found that energy more than mixing enthalpy of Fe and Mg was applied.

IV. CONCLUSION
In this study, Fe-Mg thin films were prepared by IP process and discussed with their nanostructure and E ε .E ε were increased with increasing of V b .E ε were inputted more than the mixing enthalpy of Fe and Mg.Accumulated lattice distortion energy was increased with increasing lattice expansion of α-Fe bcc lattice increased by impinging energy of evaporation particles.Crystallographic characterization in film was changed by mobility of deposition particles on substrate and impinging energy of evaporation particles.Thus, these results were shown that in IP process, solid solubility limit and crystallographic characterization of film are able to control by E ε .

FIG. 2 :
FIG.2: X-ray diffraction patterns of Fe-Mg films prepared by IP process at V b of 50 V.

BFigure 3 2 FIG. 3 :FIG. 4 :
Figure 3 shows X-ray diffraction patterns of samples of Mg content about 36% at various V b .Diffraction angles of

TABLE I :
Condition of the film formation by IP process.