Crystallization Behavior and Thermal Analysis of CoFeB Thin Films

We examined two targets containing Co 40 Fe 40 B 20 and Co 60 Fe 20 B 20 . We deposited Co 40 Fe 40 B 20 and Co 60 Fe 20 B 20 monolayer thin films of various thicknesses on glass substrates through DC magnetron sputtering; the thicknesses ranged from 25 to 200 Å. The thermal properties of the Co 40 Fe 40 B 20 and Co 60 Fe 20 B 20 thin films were determined using a differential scanning calorimeter (DSC). The thermal properties included the glass transition temperature (T g ), onset crystallization temperature (T x ), and glass-forming ability, which were determined according to these values. Using the Kissinger formula revealed that the activation energy of the Co 60 Fe 20 B 20 with a thickness of 75 Å is the highest, implying that crystallization was the lowest and the Co 60 Fe 20 B 20 film showed anticrystallization properties. However, the energy of 75 Å Co 40 Fe 40 B 20 thin films was the lowest, which is opposite to that of Co 60 Fe 20 B 20 . This observation can be reasonably explained based on the concentration of Co or Fe. Therefore, a thickness of 75 Å is critical.


Introduction
In recent years, amorphous CoFeB has been found to have characteristics that can be exploited in scientific research and engineering applications, such as magnetic recording media, magnetoresistance random access memory (MRAM), and gauge sensors.CoFeB thin films with thicknesses in the range 10-50 Å are used in magnetoresistance (MR) devices, and annealed CoFeB thin films exhibit perpendicular or inplane anisotropy and B diffusion [1][2][3][4].CoFeB thin films demonstrate excellent magnetic and electrical properties because of their amorphous structure and high spin polarization.An as-deposited ferromagnetic CoFeB thin film is typically inserted into a spin-valve structure to form a free/ pinned layer of magnetic tunneling junction (MTJ).The formation of the junction leads to increased tunneling magnetoresistance (TMR) and the development of ferromagnetism (FM)/antiferromagnetism (AFM) exchange-biasing anisotropy, which makes the structure suitable for both magnetoresistance random access memory and gauge sensor applications.The increased TMR and exchange-biasing anisotropy are caused by the mean free path of spin being shorter in amorphous materials compared with that of crystalline materials [5][6][7][8][9][10][11].The concentration ratio of Co affects the stability of the amorphous state [12].
In this study, we investigated the crystalline behavior of amorphous CoFeB thin films through thermal analysis.The thermal characteristics and glass-forming ability (GFA) index of amorphous CoFeB thin films are worthy of research.The structure of CoFeB films was determined using Xray diffraction (XRD) patterns.Nonisothermal differential scanning calorimetry (DSC;   Instruments DSC 2920) was used to determine thermal properties at a heating rate of 20 K/min.These properties included the glass transition temperature (  ), onset crystallization temperature (  ), and liquid temperature (  ).Crucial related parameters were also measured to estimate the thermal performance, including the range of temperatures at which the film was in the supercooled region (Δ  =   −   ) and the GFA index,

Experimental Details
CoFeB thin films were sputtered on a glass substrate by using DC magnetron sputtering at room temperature (RT) to obtain films with thicknesses ranging from 25 to 200 Å.The base chamber pressure exceeded 2 × 10 −6 Torr, and the Ar working pressure was 5 × 10 −3 Torr.The atomic compositions of the two CoFeB targets were 60 at % Co, 20 at % Fe, and 20 at % B and 40 at % Co, 40 at % Fe, and 20 at % B. XRD with a CuK 1 line (Philips X'pert) was used to determine the amorphous structure.Thermal performance was investigated using nonisothermal DSC at a heating rate of 20 K/min.Heating rates in the range 10-40 K/min were applied in nonisothermal DSC analysis to determine the crystallization behavior.Several temperatures between   and   were applied in isothermal DSC analysis to examine the crystallization kinetics.DSC measurement provides qualitative and quantitative data in endothermic (heat absorbing) and exothermic (heat releasing) processes; nonisothermal and isothermal heating methods can be used to obtain information on changes in physical and/or chemical properties.Figures 3(a) and 3(b) show the  of the crystallization of the CoFeB films, derived using the Kissinger plot [16,17]:

Results and Discussion
where B denotes different heating rates in the range 10-40 K/min for the nonisothermal DSC analysis performed to determine the crystallization behavior. is the gas constant and  is the activation energy. is the specific measured where  is the volume fraction transformed as a function of time ,  denotes the rate constant, which is sensitive to temperature because of the dependence of nucleation and growth rates, and  is a dimensionless constant that depends on the combination of nucleation and growth mechanisms for the involved transformation.The volume fraction of crystallization , which is obtained by measuring the partial area under the peak up to time  as a function of annealing time, is illustrated in Figure 5.Because the high annealing temperature thermally induces a high driving force that causes the amorphous structure to become crystalline, the duration of the high annealing temperature is shorter than

Conclusions
In summary, the thermal performance, crystallization behavior, calculated anticrystallization, and structure of Co 40 Fe 40 B 20 and Co 60 Fe 20 B 20 thin films were investigated using DSC, XRD, and the Kissinger fitting method.The GFA index, defined using  and   , increased as the thickness decreased.Moreover, the Kissinger fitting indicated that the critical thickness of the CoFeB thin film was 75 Å.The performance of Co 60 Fe 20 B 20 thin films is more suitable for amorphous magnetic thin-film applications because of a high GFA index, , and Δ  .A critical result is that amorphous CoFeB thin films can be used in the magnetic recording industry and crystalline applications because of their crystalline behavior.Finally, based on the nonisothermal and isothermal analyses, the thermal stability and incubation time of Co 60 Fe 20 B 20 films were more favorable than those of Co 40 Fe 40 B 20 films.

Glass/Co 40 Figure 2 :
Figure 2: (a) DSC plots of amorphous Co 40 Fe 40 B 20 thin films heated at 20 K/min.(b) DSC plots of amorphous Co 60 Fe 20 B 20 thin films heated at 20 K/min.

The
Kissinger formula was used to calculate the dependence of the  of crystallization on the thickness of the Co 40 Fe 40 B 20 and Co 60 Fe 20 B 20 films, as shown in Figure 4.The two curves exhibit a cross feature and share a critical thickness of 75 Å.These activation energies yielded a critical thickness of 75 Å for amorphous Co 40 Fe 40 B 20 and Co 60 Fe 20 B 20 thin films.For the amorphous Co 40 Fe 40 B 20 film thickness of 75 Å, the lowest activation energy was approximately 45.78 kJ/mol.By contrast, at the same film thickness, the highest activation energy of Co 60 Fe 20 B 20 was approximately 83.44 kJ/mol.This result was consistent with the calculated DSC results.A higher activation energy corresponds with a higher resistance to crystallization.The thermal performance of the Co 60 Fe 20 B 20 thin film was higher than that of the Co 40 Fe 40 B 20 thin film with various Co and Fe concentrations [18-20].Figures 5(a) and 5(b) show the crystallization fraction as a function of the annealing temperature based on isothermal analysis of Johnson-Mehl-Avrami (JMA) for 75 Å-thick Co 60 Fe 20 B 20 and Co 40 Fe 40 B 20 films [21].The volume fraction () versus time () relationship is based on the following equation:

Figure 3 : 20 Co 40 Fe 40 B 20 CoFeBFigure 4 :
Figure 3: (a) Kissinger plot of DSC peaks for crystallization of amorphous Co 40 Fe 40 B 20 thin films.(b) Kissinger plot of DSC peaks for crystallization of amorphous Co 60 Fe 20 B 20 thin films.

Figure 6 :
Figure 6: Incubation time of 75 Å-thick Co 40 Fe 40 B 20 thin film and 75 Å-thick Co 60 Fe 20 B thin film as a function of isothermal annealing temperature.

Table 1 :
(a) Thermal parameters of amorphous Co 40 Fe 40 B 20 thin films and (b) thermal parameters of amorphous Co 60 Fe 20 B 20 thin films.Fe 20 B 20 and Co 40 Fe 40 B 20 thin films, and   is the peak temperature of Co 60 Fe 20 B 20 and Co 40 Fe 40 B 20 thin films.1000/  is easier to fitting data than 1000/.The  of crystallization was determined according to the slope of a plot of ln(B/ 2  ) as a function of 1/. represents the energy barrier that must be overcome before crystallization can occur.A high  value indicates that transforming the atomic amorphous state to the crystalline state is difficult.By contrast, a low  value indicates that the atomic amorphous state easily transforms to the crystalline state.The  values of Co 60 Fe 20 B 20 thin films were higher than those of Co 40 Fe 40 B 20 thin films at the film thickness of 75 Å.