RU2784453C1 - METHOD FOR OBTAINING A FILM OF PERMALLOY NITRIDE FeXNi1-XN - Google Patents

Abstract

FIELD: magnetic films production.

SUBSTANCE: invention relates to a method for producing magnetic films used in spintronics and magnonics devices. The method for obtaining a film of permalloy nitride Fe_xNi_1-xN, with 0.23≤x≤0.27, includes deposition of the said film on the substrate by the method for reactive magnetron sputtering of the target. A target consisting of two plates is used. A magnetron is installed in the vacuum chamber, on the body of which the inner target plate of iron and the outer target plate of nickel are rigidly fixed with slots in the form of holes with a total area in the range (0.95-1.71)⋅10^-3 m², and fixed substrate with ensuring the distance to the target in the range (33-38)⋅10^-3 m. The vacuum chamber is evacuated to a residual pressure of (1-5)⋅10^-6 Torr, the plasma-forming argon gas is introduced to ensure pressure in the vacuum chamber (3–6)⋅10^-3 Torr. Then, a nitrogen flow is introduced into the vacuum chamber at a rate in the range of 3–10 cm³/min and the magnetron discharge current density is provided in the range (350–700) A/m² for reactive magnetron sputtering with argon ions of iron and nickel nitrides formed on iron and nickel plates at a temperature of the said plates, which is 1800-2000 K.

EFFECT: obtaining on the substrate a film of permalloy nitride Fe _x Ni_{1– x} N, which has a coercive force of less than 1 Oe.

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Description

The invention relates to methods for producing magnetic films on an arbitrary substrate by reactive magnetron sputtering, used in spintronics and magnonics devices. These methods contain a sequence of actions, the main of which is the direct deposition of films during the operation of the magnetron.

Iron nitride (Fe–N) films have attracted much attention due to their excellent magnetic properties and ability to improve surface hardness and wear resistance for magnetic device applications [N. Pandey, M. Gupta, R. Rawat et al., Phys. B: Cond. Matter. 572 (2019) 36-41]. However, they had poor thermal stability, which led to the disappearance of high magnetic properties with increasing temperature. The addition of a third element to Fe–N with the formation of binary solid solutions of nitrides, for example, Fe–Me–N (where M is a transition metal atom) made it possible to solve this problem [LL Wang, WT Zheng, T. An et al., J. Alloys Comp. 495 (2010) 265–267]. Among such films, permalloy nitride Fe _x Ni _{1– xN} , which has high magnetic properties, was of great interest [R. Loloe, J. Appl. Phys. 112 (2012) 023902]. Some publications indicate that permalloy nitride films Fe _x Ni _{1– xN} at 0.23 < x < 0.27 with a thickness of more than 200 nm have the minimum coercive force H _c < 1 Oe.

Known methods for producing films of binary solid solutions of nitrides based on reactive magnetron sputtering. The deposition of nitride films using magnetron systems consists in sputtering the necessary metals in a mixture of argon and nitrogen at reduced pressure. Sputtering is carried out by inert gas ions formed in an anomalous glow discharge when crossed electric and magnetic fields are applied to it.

The closest in technical essence to the claimed solution is the method described in [X. F. Wang, P. Wu, X. L. Li, et al., J. Magn. Magn. mater. 312 (2007) 147–152]. In this work, reactive magnetron sputtering of a target in a mixture of argon and nitrogen was used directly for film deposition.

The method consists in the fact that in the vacuum chamber where the magnetron is installed, the substrates are fixed on the object table. The target is an iron plate, on the surface of which nickel plates are fixed. The chemical composition of the film is determined by the total area of these plates. Next, the chamber is closed and pumped out, creating a high vacuum in it. Then the plasma-forming gas argon is introduced into the chamber and its pressure is brought to the working value. In the next step, nitrogen is introduced into the chamber. Next, a DC source is connected to the magnetron. After the onset of the discharge, the current is brought to the operating value and the partial pressure of nitrogen is set at the level of 5–10% of the total pressure of the Ar + N ₂ gas mixture. The deposition process itself is carried out for a predetermined time interval, which makes it possible to form a film of a predetermined thickness having the necessary magnetic properties. Then the magnetron is turned off and the samples, after exposure for about one hour in a vacuum, are removed from the chamber.

The method implemented in the prototype has a number of disadvantages:

• the obtained films have unstable uniformity in chemical composition, characterized by a change in the stoichiometric coefficient x in the formula Fe _x Ni _{1– x} N during deposition;

• film thickness uniformity does not exceed 0.95.

The problem to be solved by the invention is to create a method for obtaining on an arbitrary substrate a film of permalloy nitride Fe _x Ni _{1– x} N, which has a coercive force of less than 1 Oe, which is achieved with a stably homogeneous chemical composition in the range of 0.23 ≤ x ≤ 0.27 and a high relative uniformity, which should not go beyond the interval 0.99 ≤ L _l / L ₀ ≤ 1.01. Here L _l is the film thickness at the point on the edge of the substrate furthest from the center, and L ₀ is the film thickness at the center of the substrate.

The problem is solved due to the fact that, unlike the known method for obtaining a permalloy nitride film, which includes its deposition on an arbitrary substrate in a gas mixture of argon and nitrogen by reactive magnetron sputtering of a target made of iron and nickel, in the proposed method the target is made of two plates operating in hot mode, the inner one is made of iron, and the outer one is made of nickel with slots in the form of holes with a total area in the range (0.95 – 1.71)⋅10 ^–3 m ² and located at a distance from the target to the substrate in the range ( 33 - 38)⋅10 ^-3 m at a current density value in the range (350 - 700) A / m ² .

The achieved technical result is the creation of a method for obtaining on an arbitrary substrate a film of permalloy nitride Fe _x Ni _{1– x} N, which has a coercive force of less than 1 Oe, which is achieved with a stably homogeneous chemical composition in the range of 0.23 ≤ x ≤ 0.27 and high relative uniformity, which should not go beyond the interval 0.99 ≤ L _l / L ₀ ≤ 1.01.

The essence of the invention is illustrated by drawings:

fig. 1 – target design;

fig. 2 – temperature dependences of external T _Ni and internal T _Fe plates on current density;

fig. 3- dependences of iron nitride fluxesQ _FeNsp,Q _FNeev andQ _FeNtot on the current density at α =s ₂/s = 0.5;

fig. four- nickel nitride flux dependenciesQ _NiNsp,Q _NiNev andQ _NiNtot on the current density at α =s ₂/s = 0.5;

fig. 5 – dependences of the stoichiometric coefficient x in the Fe _x Ni _{1– x} N film on the area of the slits s ₂ and the current density;

fig. 6 – area on the plane of independent variables j – s ₂ , which corresponds to the deposition of Fe _x Ni ₁ – _x N films with stable chemical composition at 0.23 ≤ x ≤ 0.27. _The dashed line connects the points ^of regimes at which a stably homogeneous film ^of composition Fe _0.25 Ni _0.75 N is deposited.

fig. 7 - dependence of the coercive force H _c on the film thickness L ;

fig. 8 – dependence of the coercive force H _c on the deposition time t ;

fig. 9 - dependences of the relative uniformity of the film on geometric factors.

In the proposed solution, the technical result is achieved by the fact that a specialized reactive sputtering system has been created to implement the method for producing a permalloy nitride film. It includes a vacuum system with a chamber on which a magnetron is fixed with a target consisting of two plates rigidly fixed on the magnetron housing and operating in a hot mode. The inner plate is made of iron, the outer plate is made of nickel and has slots in the form of holes through which the inner plate is sprayed. The vacuum chamber is provided with the input of the gas mixture Ar + N ₂ necessary to obtain a nitride film by reactive magnetron sputtering. In addition, it is provided that the residual pressure in the chamber can be increased to (1–5) ^⋅10–6 Torr; the partial pressure of argon can be set in the range (3–6) ^⋅10–3 Torr; the magnetron discharge current density can be set in the range (350–700) A/ ^m2 , the nitrogen flow introduced into the chamber can be changed in the range (3–10) ^cm3 /min; A film that is stably uniform in chemical composition and highly uniform in thickness can be obtained by choosing independent parameters of the sputtering system, which include the discharge current density, the area of slots in the outer plate, and the distance between the target and the substrate.

The invention is based on the following physical phenomena:

• magnetron sputtering occurs when both plates are in the target in a hot mode, in which their temperature can be brought up to 1800–2000 K, so the deposition process can be affected by plate evaporation and thermionic emission;

• in the most general case, during the operation of the magnetron on the surface of each plate, the processes of formation of a nitride film and its removal due to sputtering and evaporation compete;

• during film deposition, both plates must operate stationary in the nitride mode, in which their surfaces are coated with the corresponding nitride (iron or nickel). This mode for each of them arises at a set of values of the main independent parameters of the deposition process, which provide a higher film growth rate on its surface;

• a permalloy nitride film on the substrate is formed due to flows of iron and nickel nitrides, which are sputtered by argon ions from the surface of the corresponding targets;

The proposed method is carried out as follows. In a vacuum chamber with an installed magnetron, substrates of a given material are fixed on the object table. It is removed from the target at a given distance h . The target contains an inner plate made of iron and an outer plate made of nickel, operating in a hot mode. In this case, cuts in the outer target can have a total area in the range (0.008–0.0018) ^m2 . Next, the chamber is closed and pumped out, creating a high vacuum in it with a residual pressure of (0.5–1.0) ^⋅10–5 Torr. Then the plasma-forming gas argon is introduced into the chamber and its pressure is brought to the working value (3–5) ^⋅10–3 Torr. At the next step, nitrogen is introduced into the chamber and its flow is set in the range ^of 4–6 cm3/min. Next, a DC source is connected to the magnetron. After the discharge occurs, the magnetron discharge current density is adjusted to the operating value in the range (300–800) A/m ² . The deposition process is carried out for a predetermined time interval, which makes it possible to form a film of a predetermined thickness having the required magnetic properties. After that, the magnetron is turned off and after exposure for about one hour in a vacuum, the samples are removed from the chamber.

Let us consider an example of using the method of obtaining a film of permalloy nitride Fe _x Ni _{1– x} N stably homogeneous in chemical composition and uniform in thickness at 0.23 ≤ x ≤ 0.27. The example is based on a numerical process simulation. In the simulation, a cylindrical balanced magnetron with a diameter of 130 mm was used. The sputtered assembly (Fig. 1) is bolted to the magnetron housing 1 cooled by running water. The unit contains on one axis a cooling plate 2 4 mm thick and a target consisting of two plates: the inner 3 is made of Fe, the outer 4 is made of Ni. The thickness of each is 1 mm. Washers 1 mm thick are installed between the plates, providing a gap between them. This design provides heat removal from both plates only due to radiation and through the fastening elements. The erosion zone 5 of the nickel plate has the shape of a ring with an area of s = 36 cm ² . Its outer radius is 3.9 cm, and its inner radius is 1.9 cm. Slots in the form of holes 6 are made in this zone, located symmetrically with respect to the center of the target. The total area of the slots s ₂ specifies the area of the erosion zone of the inner iron plate 7 . For a nickel plate, the area of a similar region is s ₁ = s - s ₂ . The value of s ₂ is an independent process parameter that affects the growth rate, the chemical composition of the film, and its thickness uniformity.

The process of film deposition proceeds as follows (see Fig. 1). By setting the required values of the current density of the magnetron discharge and the flow of nitrogen, both plates are transferred to the nitride mode. In this case, the temperatures of the plates take on the values T _Fe and T _Ni . The surfaces of the plates become sources of nitride flows, which, deposited on the substrate, form a film of the required composition. The inner plate 3 through the slots 6 in the nickel plate 4 having an area s ₂ generates a flow of iron nitride Q _FeNtot consisting of evaporated Q _FeNev and pulverized Q _FeNsp components. The outer plate 4 generates a flow of nickel nitride Q _NiNtot , also consisting of vaporized Q _NiNev and sputtered Q _NiNsp components.

To determine the values of Q _FeNtot and Q _NiNtot , the temperature estimates of both plates were used in the form of exponentials shown in Figs. 2. The ratio of the FeN and NiN components in the Fe _x Ni _{1– x} N film sets the value:

.

. (one)

At the same time, each of the components in (1) in a known way depends on the value of s ₂ . For example, the flow of iron nitride Q _FeNtot , consisting of two components, is equal to:

, с^–1,

, s ^–1 , (2)

where for iron nitride:S _FeN – spray coefficient; γ_FeN is the coefficient of ion-electron emission;A _FeN andB _FeN are constants that specify the pressure of saturated steam;m _FeN is the mass of the molecule;k = 1.38 10^-23 J/K is the Boltzmann constant;j is the discharge current density on the target, A/m²;e= 1.6 10^-19 Cl is the electron charge.

The nickel nitride flux Q _NiNtot , also consisting of two components, is equal to

, с^–1,

, s ^–1 , (3)

where for nickel nitride:S _NiN – spray coefficient; γ_NiN is the coefficient of ion-electron emission;A _NiN andB _NiN are constants that specify the pressure of saturated steam;m _NiN is the mass of the molecule.

In FIG. Figures 3 and 4 give examples of the dependences of the FeN and NiN fluxes on the current density, which were obtained at α = s2/s = 0.5 using expressions ( ₂ ) and (3), respectively. From FIG. 3 and 4, it can be seen that there are areas in which the effect of the evaporation process of each plate can be neglected. Such regions will exist for any values of α.

Expressions (1) - (3) were used for further analysis. In FIG. Figure 5 shows the dependencies x = f ( s ₂ , j ). It follows from this that the chemical composition of the Fe _x Ni _{1– x} N film can be uniquely controlled by varying the discharge current density and the parameter s ₂ . In addition to the curves x = f ( s ₂ , j ) in FIG. The dashed lines in Fig. 5 mark the composition levels of Fe _0.23 Ni _0.77N and Fe _0.27 Ni _0.73 N. The points of intersection of the x = f ( s ₂ , j ) lines with the x = 0.23 and x = 0.27 lines set the region on the plane of the independent variables j – s ₂ , which corresponds to the deposition of Fe _x Ni _{1– x} N films at 0.23 ≤ x ≤ 0.27 (see solid lines in Fig. 6). The current density j region is limited by values of 350 and 700 A/m ² . At j = 350 A/ ^m2 , _the slot area s2 is limited to 0.95 ⋅ ^10–3 and 1.04 ⋅ ^10–3 . At j = 350 A/ ^m2 , 1.6 ⋅ ^10–3 and 1.71 ⋅ ^10–3 . The dashed line in Fig. 6 combines the conditions for the deposition of Fe _0.25 Ni _0.75 N films. The dot at j = 525 A/m ² and s ₂ = 0.00137 m ² marks the center of the region _.

A model of the proposed device was used to evaluate the magnetic properties of Fe _0.25 Ni _0.75 N films. FIG. Figure 7 shows the dependence of the coercive force H _c on the film thickness L . The dots represent the experiment, the solid line shows the result of the approximation in the form

, Э.

, E. (four)

From FIG. It follows from Fig. 7 that the coercive force of the films decreases to an asymptotic value of 0.17 Oe with increasing thickness. 8, this value is reached within about 10 minutes.

Fig. 9 reflects the effect of geometric factors on the uniformity of the film over the thickness, which is set by the ratio L _l / L ₀ , where the film thickness L _l at the point farthest from the center on the edge of the substrate is given by the expression

,

, (four)

M ₀ = 4π( R ₁ ² – R ₂ ² ) J _m t is the mass of the substance deposited on the substrate; ρ is the density of the film material; h is the distance between the target and the substrate; l is the coordinate on the substrate; R ₁ is the radius of the outer circumference of the erosion zone on the outer plate; R ₂ is the radius of the inner circle of the erosion zone on the outer plate. The film thickness at the center of the substrate L ₀ is obtained from (4) by substituting l = 0 into it:

.

. (5)

The dashed lines in Fig. 9 shows the specified uniformity limits 0.99 ≤ L _l / L ₀ ≤ 1.01 of the deposited films. The best uniformity is achieved with the ratio R / h = 1.1. With an outer radius of the sputtered region of the considered magnetron of 3.9 cm, this ratio leads to a distance between the target and the substrate equal to 3.5 cm. At larger distances ( R / h < 1.1), a convex film will be deposited on the substrate, which will have a maximum thickness in the center of the substrate. At smaller distances ( R / h > 1.1) the film will be concave. From FIG. It follows from Table 9 that, at h = 3.5 cm, a film satisfying the condition 0.99 ≤ L _l / L ₀ ≤ 1.01 can be obtained on a substrate whose size specifies the inequality l / h ≤ 0.58. Considering that the total size of the substrate is 2 l , we finally obtain the value of the largest substrate size, which is 4.1 cm.

Fig. 5 - 9 indicate that the goal has been achieved. Since the total area of the slots in the outer plate, the discharge current density, and the distance between the target and the substrate remain unchanged during deposition, the proposed method makes it possible to obtain films that are chemically uniform in the range 0.23 ≤ x ≤ 0.27 and have a high relative uniformity that cannot go beyond interval 0.99 ≤ L _l / L ₀ ≤ 1.01.

Claims (1)

A method for producing a film of permalloy nitride Fe _x Ni _1-x N, moreover, 0.23≤x≤0.27, including deposition of the said film on the substrate by the method of reactive magnetron sputtering of the target, characterized in that a target consisting of two plates is used, while a magnetron is installed in the vacuum chamber, on the body of which the inner target plate made of iron and the outer target plate made of nickel are rigidly fixed with slots in the form of holes having a total area in the range (0.95-1.71)⋅10 ^-3 m ² , and the substrate is fixed to ensure the distance to the target in the range (33-38)⋅10 ^-3 m, the vacuum chamber is pumped out to a residual pressure of (1-5)⋅10 ^-6 Torr, the plasma-forming gas argon is introduced to ensure the pressure in the vacuum chamber (3- 6)⋅10 ^-3 Torr, then a nitrogen flow is introduced into the vacuum chamber at a rate in the range of 3-10 cm ³ /min and the magnetron discharge current density is provided in the range (350-700) A/m ² for reactive magnetron sputtering with argon ions of the emerging on plates from iron and nickel of iron and nickel nitrides at the temperature of the mentioned plates, which is 1800-2000 K.

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