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Effect of ion bombardment on properties of hard reactively sputtered Ti(Fe)Nx films
Surface and Coatings Technology 177 – 178 (2004) 289–298
Effect of ion bombardment on properties of hard reactively sputtered Ti(Fe)Nx films ˇ ´ ´ J. Suna, ˇ J. Musil*, H. Polakova, J. Vlcek Department of Physics, University of West Bohemia, Univerzitnı´ 22, Plzenˇ 306 14, Czech Republic
Abstract This article reports on results of a systematic investigation of properties of hard Ti(Fe)Nx films reactively sputtered using a d.c. unbalanced magnetron. The Ti(Fe)Nx films with a low (F15 at.%) Fe content were selected as a typical single-phase material to investigate an effect of the energy Epi, delivered to them during their growth by bombarding ions, on their physical and mechanical properties. In this investigation, the energy Epi per deposited volume was varied by the magnitude of a deposition rate aD because EpiwJycm3 xsUsis yaD , where Us is the substrate bias, is is the substrate ion current density and aD is the film deposition rate. It was found that: (i) properties of sputtered films are a result of a combined action of physical and chemical processes controlled by the energy Epi and the film stoichiometry xsNy(TiqFe), respectively; (ii) Ti(Fe)Nx films can form a superhard material with hardness HG40 GPa; and (iii) superhard films with the highest hardness are: (a) formed in a transition region; (b) nearly stoichiometric with xf1; and (c) composed of a mixture of grains of different crystallographic orientations. The last finding makes it possible to explain the origin of the superhardness of single-phase materials. A special attention is devoted to mechanical properties of Ti(Fe)Nx films, particularly to relationships between hardness H, Young’s modulus E, elastic recovery We and the ratio H3yEU2, which is proportional to a resistance of the material to plastic deformation, but also to dependences of these mechanical properties on energy Epi , deposition rate aD , average size Lc of grains and microstrain Eg generated in the film during its growth; here E * sEy(1yn2 ) is the effective Young’s modulus and n is the Poisson’s ratio. Correlations between mechanical properties and modes of their sputtering are discussed in detail. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Ti(Fe)Nx; Single-phase films; Reactive magnetron sputtering; Ion bombardment; Physical and mechanical properties; Enhanced hardness
1. Introduction At present, it is well known that a correlation between properties of solids and their structure is of fundamental importance not only for materials science but also for thin films physics. However, what structure will be formed depends on process parameters and chemical composition of the film. A problem is the fact that in every deposition process there are many deposition parameters, which are mutually coupled. For instance, in magnetron sputtering these parameters are: magnetron discharge current Id and voltage Ud, substrate bias Us, substrate ion current density is, substrate temperature Ts, substrate-to-target distance dsyt, partial pressure of reactive gas pRG, total pressure of sputtering gas pTs pRGqpAr but also pumping speed of the pumping *Corresponding author. Tel.: q420-37-742-3136; fax: q420-37742-2825. E-mail address: [email protected] (J. Musil).
system, base pressure in the deposition chamber p0, location of inlets of sputtering gases, mutual orientation of the magnetron target and substrate surface (perpendicular or tilted deposition), stationary, rotating or linearly moving substrates, plasma enhancement by additional r.f., microwave or hollow cathode discharges or improvement of plasma confinement using an external magnetic field, usually produced by Helmholtz electromagnet coils, and geometry of the deposition chamber. The most important deposition parameters for every sputtering machine are: Id, Us, is, Ts, dsyt, pRG and pT. Every combination of these parameters give, however, only one discrete structure. Therefore, it is practically impossible, by changing one process parameter in this combination, to change continuously the structure of deposited film. This is a main reason why the formation of the film with a prescribed structure, i.e. with prescribed properties, is very difficult and so far a not solved problem. In our opinion, a key to the solution of
0257-8972/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2003.09.007
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this problem is a control of the energy E delivered to the film during its growth. This energy can be delivered by three ways: (1) substrate heating Ts; (2) particle bombardment Ep by: (i) ions (ion bombardment controlled by the ion energy Ei and the flux of ions ni); and (ii) fast neutrals (atom-assisted deposition controlled by pT, energy Eff and flux nff of the film forming particles); and (3) chemical reactions DH f (in exothermic reactions, when DH f -0, a heat is released and the total energy is increased and, on the contrary, in endothermic reactions, when DH f )0, the heat is consumed and the total energy decreases); here DHf is the formation energy of compounds. All three components of the total energy influence the film growth simultaneously, but the effect of individual components can be very different. For instance, in deposition of pure metals a contribution of the energy from chemical reactions is zero. On the contrary, in deposition of films using ion plating process, when the growing film is bombarded by low-energy ions, the energy Epi delivered to it by bombarding ions has a decisive effect on its growth. Therefore, the ion bombardment is very often used to control properties of deposited films, see for instance Refs. w1–15x. The energy Epi delivered to the growing film by bombarding ions has a crucial effect on its structure and so on its physical and functional properties. In a collisionless discharge this energy delivered per deposited volume can be determined from three easily measured quantities, i.e. the substrate bias Us, the substrate ion current density is and the deposition rate aD of the film, according to the following formula w16x: EpiwyJycm3z~sUsŽisyaD.Nimax x
|
at Tssconst
(1)
where Nimaxsexp(yLy li) is the amount of ions arriving at the substrate with a maximum energy eUs, e is the elementary charge, L is the sheet thickness and li is the ion mean free path for collisions leading to losses of the ion energy in the sheet. The ion mean free path can be calculated from the Dalton law as lif0.4yp wPax w17x. The high-voltage (Us4Ufl ) sheet thickness L can be expressed for a collisionless sheet near the substrate (Ly li-1) in the following form w18x: 1y4 3y4 y1y2 Ls(0.44)1y2´1y2 Us is 0 (2eymi)
(2)
where Ufl is the floating potential, ´0 is the free-space permittivity and mi is the ion mass. For instance, at ps 0.5 Pa, Ussy100 V and iss1 mAycm2 the ratio Ly lis0.116 and it means that nearly 90% of ions arrive at the substrate with maximum energy eUs. For more details see Ref. w16x. Eq. (1) clearly shows that changes in the film properties can be controlled by a combined action of Us, is
and aD. Besides, it is necessary to note that the parameters Us and (is yaD) in Eq. (1) are not physically equivalent w2,19x. This means that the films which are produced at different combinations of Us and (is yaD), which give the same value of Epi, do not exhibit the same microstructure, phase and chemical composition, i.e. such films exhibit different properties. Eq. (1) has been used by many researchers to characterize effects of low-energy ion bombardment on a microstructure of the film and its properties. For instance, the effect on: (i) grain size, lattice distortion, dislocation density in Ag, Cu, Pd films is investigated in w4x; (ii) microstructure in w5–8,11x; (iii) microhardness H, macrostress s, microstrain e in TiNx films in w5,6,9,10x; (iv) formation of ´-Ti2 N phase in TiNx films in w3,7x; (v) preferred orientation of TiN films in w12– 15x, etc. In spite of a relatively large utilization of ion plating process, there is no systematic study on correlations between the energy and properties of reactively sputtered films. Though, it is well known that the reactive sputtering of films is accompanied by the target poisoning, which results in a dramatic decrease of the film deposition rate aD, only few people realize that changes in aD, caused by a change in the partial pressure of reactive gas pRG (RGsN2, O2, CH4, etc.) under constant deposition conditions, induce huge changes in the energy Epi delivered to the film during its growth. For instance, for nitrides aD(Me)f4 aD(MeN) and for oxides even aD(Me)s(10–15) aD(MeO), where Me is the metal, MeN and MeO is the metal nitride and metal oxide, respectively. Therefore, it is necessary to expect that changes in properties of the reactively sputtered film will be the result of a combined action of two parameters; (i) chemical composition of the film, particularly the amount of reactive gas atoms incorporated in the film; and (ii) energy Epi, i.e. parameters, which both depend on the partial pressure of reactive gas. In the reactive mode of sputtering the effect of increased Epi, due to a decrease in aD with increasing pN2, is very strong. Experimental verification of these phenomena are given in this paper. This paper reports on results of a systematic study of properties of Ti(Fe)Nx films reactively sputtered using an unbalanced d.c. magnetron. The main aim of this study is to find correlations between deposition parameters, chemical composition, structure, physical and mechanical properties of these films, and energy Epi delivered to them during their growth. 2. Experimental Ti(Fe)Nx films were reactively sputter deposited in a mixture of ArqN2 using a round planar unbalanced d.c. magnetron equipped with an alloyed TiFe (90y10 at.%) target of 100 mm in diameter. The deposition chamber was an aluminium cylinder (⭋s150 mm, Ls225 mm)
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Fig. 1. (a) Deposition rate aD of Ti(Fe)Nx films and (b) energy Epi delivered to these films during their growth as a function of partial pressure of nitrogen pN2. The films were sputtered at the following three combinations of Id and is: (i) Ids1 A, iss0.5 mAycm2; (ii) Ids2 A, iss1 mAycm2; and (iii) Ids3 A, iss1 mAycm2. Constant deposition parameters: Ussy100 V, Tss300 8C, dsyts60 mm and pTs0.5 Pa.
pumped down by a diffusion pump (2000 lys) to a base pressure of approximately 10y3 Pa. The films were deposited on polished and ultrasonically precleaned, stationary, steel substrates (a disc 25 mm in diameter and 5-mm thick) made of CSN 15330 steel (the composition in wt.%: 2.47 Cr, 0.54 Mn, 0.29 C, 0.22 Mo, 0.17 V, 0.012 P and 0.012 S). The substrates were fixed to a movable and heatable substrate holder, which was electrically insulated from the grounded deposition chamber. Prior to the film deposition, a sputter cleaning of substrates was performed using a d.c. discharge (Uss y600 V, Iss20 mA, pArs15 Pa, ts5 min). The films were deposited under the following conditions: Ids1, 2 and 3 A, substrate bias Ussy100 V, substrate ion current density iss0.5 and 1 mAycm2, substrate temperature Tss300 8C, substrate to target distance dsyts 60 mm, total sputtering gas pressure pTspArqpN2s 0.5 Pa and deposition time tD ranging from 15 to 105 min. A typical thickness h of the film was 3–4 mm. The film thickness was measured with a mechanical Dektak 8 profilometer. The structure of films was characterized by X-ray diffraction (XRD) using a Dron 4.07 spectrometer in the Bragg–Brentano configuration with CoKa (ls0.179 nm) radiation. The chemical composition of the films was determined by energydispersive X-ray spectrometry (EDXS) using a SEM JEOL JXA 840 equipped with a LINK 860 analyzer. The nitrogen content was calculated as the rest to 100 at.%. The mechanical properties of the film, i.e. the microhardness, H, effective Young’s modulus, E *sEy (1yn2), and elastic recovery We, were evaluated from the load vs. displacement curves measured using a computer controlled microhardness tester Fischerscope H 100 equipped with the Vicker’s diamond indenter.
The hardness value was obtained from the average of 15 measurements taken at different positions. 3. Results and discussion 3.1. Deposition rate Dependences of the deposition rate aD of Ti(Fe)Nx films, sputtered at Tss300 8C, Ussy100 V and different values of the magnetron discharge current Id and substrate ion current density is, as a function of pN2 are given in Fig. 1a. These dependences can be divided into three regions corresponding to: (i) metallic; (ii) transition; and (iii) nitride modes of the magnetron sputtering. From Fig. 1a it can be seen that: (1) aD of the films sputtered in the metallic mode is (3–4) times higher that that of films sputtered in the nitride mode; (2) the transition between the metallic and nitride mode shifts to higher values of pN2 with increasing Id; and (3) difference in aD of films sputtered in the metallic and nitride mode decreases with increasing Id. The last phenomenon is due to broadening of an erosion zone on the surface of sputtered target with increasing Id. A strong decrease of aD in the transition mode is caused by the Ti(Fe) target poisoning, i.e. by covering of the target surface with Ti(Fe)Nx nitride. A large (3–4 times) change in aD with increasing pN2 results, in agreement with Eq. (1), in a large (3–4 times) change of the energy Epi delivered to the film during its growth by bombarding ions despite that all other deposition parameters are kept constant, see Fig. 1b. From this figure it is seen that Epi is the lowest in the metallic mode, increases in the transition mode and saturates in the nitride mode where achieves a maximum
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metallic one. This means that stoichiometric nitrides with a higher aD can be produced only in denser discharges, which are generated at higher magnetron discharge currents IdG3 A. On the contrary, Fig. 1b shows that the energy Epi, at which stoichiometric Ti(Fe)N films are produced, decreases with increasing Id. In summary, we can conclude that denser discharges are more convenient to realize reactive sputtering and to form stoichiometric nitrides. 3.3. Structure of films
Fig. 2. Elemental composition of Ti(Fe)Nx films, sputtered at Ids1 A, Ussy100 V, iss0.5 mAycm2, Tss300 8C, dsyts60 mm and pTs0.5 Pa, as a function of partial pressure of nitrogen pN2.
value Epi max. The energy Epi max in the nitride mode decreases with increasing Id due to an increase of aD. The value of Epi max can be, however, controlled by a change of the product (is=Us), which compensates, for instance, an increase in aD with increasing Id according to Eq. (1). The increase in Epi with increasing pN2 in reactive sputtering has dramatic consequences and is responsible for unexpected, very strong changes of the film structure and so its final physical properties, see next paragraphs. 3.2. Chemical composition A typical development of the elemental composition of Ti(Fe)Nx films, sputtered at the magnetron current Ids1 A, with increasing partial pressure of nitrogen pN2 is given in Fig. 2. From this figure it is seen that the nitrogen content in the Ti(Fe)Nx films increases with increasing pN2 and already at pN2s0.07 Pa the amount of N in the film achieves 50 at.%. The value of pN2 (50 at.%) shifts to higher values of pN2 with increasing Id. The last phenomenon is due to the fact that with increasing Id a number of sputtered Ti atoms also increases and so higher pN2 is needed to achieve the same amount of N in the sputtered film. Therefore, partial pressures of nitrogen pN2 (50 at.% N), at which stoichiometric Ti(Fe)Nxs1 films containing 50 at.% N are produced, also increases with increasing Id. These films are denoted by open symbols in Fig. 1. From Fig. 1a it is seen that: (i) the stoichiometric Ti(Fe)Nxs1 nitride films with xsNy(TiqFe) are formed in the transition mode and aD of these films is higher than that of overstoichiometric (x)1) nitride films produced in the nitride mode; and (ii) aD of the stoichiometric Ti(Fe)Nxs1 nitride films increases with increasing Id, i.e. shifts from the nitride mode to the
The structure of the Ti(Fe)Nx film can be easily controlled by partial pressure of nitrogen pN2, i.e. by the amount of N in the film, which decides what phase is formed. A development of the structure of Ti(Fe)Nx films sputtered at Ids3 A with increasing pN2 is given in Fig. 3. Here, for a simplicity of explanation the values of H and stoichiometry xsNy(TiqFe) are also given. The film sputtered at pN2s0 is a pure b-Ti(Fe) film. This is a high-temperature phase, which creates due to addition of Fe into Ti; for more detail see ref. w20x. With increasing pN2 the film gradually varies from a solid solution Ti(Fe,N) through a tetragonal ´-Ti2N phase at approximately 33 at.% N to a cubic d-TiNx phase. From Fig. 3 it is seen that inside an interval of pN2 corresponding to the transition mode of sputtering there is a transition region which separates Ti(Fe)Nx films with a strong (200) preferred orientation on side of lower pN2 from those with a strong (220) preferred orientation on side of higher pN2. A strong change in preferred orientation from (200) to (220) cannot be explained by a continuous, very slow increase of pN2. We believe that for this dramatic change in crystallographic orientation, the energy Epi is responsible. It increases in the transition mode of sputtering and very probably just in this region surpasses a threshold value Epi thr, which is needed to create films with (220) preferred orientation. The films produced in the transition region are composed of a mixture of grains of different crystallographic orientations. These films exhibit extraordinary properties, for instance, the highest hardness or maximum resistance to plastic deformation, see below Fig. 5 or Fig. 9. In the case when the films with extraordinary properties are required to be produced, the stoichiometry x of Ti(Fe)Nx films seems to be also very important, particularly small departures from an exact stochiometry xs1 are important. This means that the film properties are a result of a combined action of physical and chemical processes, controlled by the energy Epi and the film stoichiometry x, respectively. 3.4. Mechanical properties Mechanical properties of hard films are well described by their hardness H, effective Young’s modulus E *sEy
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Fig. 3. XRD patterns from Ti(Fe)Nx films as a function of partial pressure of nitrogen pN2 . Deposition parameters: Id s3 A, Us sy100 V, iss1 mAycm2, Tss300 8C, dsyts60 mm, pTs0.5 Pa.
(1yn2) and elastic recovery We. These quantities were evaluated from loadingyunloading curves measured by a microhardness tester Fischerscope H 100 at a load Ls20 mN of the Vicker’s diamond indenter. The measurement of H as a function of L showed that the load Ls20 mN used in these measurements gives, due to a relatively low hardness (HF40 GPa) of the Ti(Fe)Nx films, the correct value H of measured films. For more detail, see Refs. w21,22x. Measured values of H and E * permit to calculate the ratio H3 yEU2, which is proportional to a resistance of the material to plastic deformation w23x. As this is higher, the higher is the ratio H3 yEU2. Interrelationships between H, E * and process parameters ŽId, pN2. are, however, very complex to produce films with prescribed properties, and particularly to scale up the technological process of their production. This is due to the fact that properties of sputtered films depend on their: (i) chemical composition (stoichiometry xs Ny(TqFe)); and (ii) structure, which strongly depend on deposition conditions. Besides, it is well known that both the chemical composition and structure of the film,
sputtered at a given substrate temperature, can be effectively controlled by the energy Epi delivered to the growing film by bombarding ions. Therefore, it is very desirable to find relations between the mechanical properties and the energy Epi. 3.4.1. Interrelationships between mechanical properties and modes of sputtering Interrelationships between mechanical properties of sputtered Ti(Fe)Nx films, i.e. the dependences Hs f(E *), Wesf(H) and H3 yEU2sfŽH., are given in Fig. 4a, b and c, respectively. Here, films sputtered in metallic, transition and nitride modes are denoted. This representation shows a clear difference in mechanical properties of films sputtered in different modes of reactive sputtering. From Fig. 4 it can be seen that: (i) H increases with increasing E *; (ii) We and H3 yEU2 increases with increasing H in films sputtered in the metallic mode; (iii) films sputtered in the transition mode exhibit the highest values of H, We and H3 yEU2; and (iv) films sputtered in the nitride mode exhibit a lower H compared
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Fig. 4. Interrelationships between mechanical properties of reactively sputtered Ti(Fe)Nx films.
to that of the hardest film produced in the transition mode and films with the same H as those produced in the transition mode exhibit higher values of E *. This fact results in lower values of We and H3 yEU2 compared to those films produced in the transition mode. The films produced in the nitride mode are more plastic. The last fact correlates well with the amount of N incorporated in the sputtered film. The films produced in the nitride mode contain more N compared to those produced in the transition mode and are overstoichiometric Ti(Fe)Nx)1 nitrides. 3.4.2. Resistance to plastic deformation vs. film stoichiometry The effect of the film stoichiometry xsNy(TiqFe) on resistance of the Ti(Fe)Nx films to plastic deformation characterized by the ratio H3 yEU2 is displayed in Fig. 5. From this figure it is seen that the ratio H3 yEU2: (i) increases with increasing x for films sputtered in the metallic mode; (ii) is approximately constant
Fig. 5. Dependence of the ratio H3yEU2 of reactively sputtered Ti(Fe)Nx films on their stoichiometry xsNy(TiqFe).
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Fig. 6. Dependence of the hardness H of reactively sputtered Ti(Fe)Nx films on the energy Epi delivered to them by ion bombardment during their growth.
and reach the highest (0.57–0.67) values for films with x ranging from approximately 0.5 to 1, i.e. for films of sputtered in the transition mode; and (iii) is considerably lower (0.3–0.4) for films with x)1 sputtered in the nitride mode. Besides, it was found that the films sputtered in the nitride mode exhibit a lower H than the film with the highest H produced in the transition mode and the highest values of E * and so a lower: (i) elastic recovery We; and (ii) resistance to plastic deformation H3 yEU2 compared to Ti(Fe)Nx films produced in the transition mode. 3.4.3. Hardness vs. energy Epi The effect of energy Epi delivered to the growing Ti(Fe)Nx film by bombarding ions on its hardness H is displayed in Fig. 6. From this figure it is seen that superhard Ti(Fe)Nx films with HG40 GPa are produced in the transition and nitride modes of sputtering and only in the case when the energy Epi)Epi min. The value of Epi min decreases with increasing Id and Epi minf0.3 MJycm3 for Ids3 A. This experiment clearly shows that the energy Epi plays an important role in formation of hard films with H)35 GPa. To form such films the energy Epi)Epi min is necessary. A similar result was already found in formation of superhard Ti(Al,V)Nx films w16,24x. Besides, it was found that the films sputtered in the nitride mode, i.e. at the energy Epi) Epi min, have the highest values of E * and so exhibit: (i) considerably lower values of H3 yEU2, i.e. lower resistance to plastic deformation; and (ii) lower elastic recovery We, compared to those sputtered in the transition mode.
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3.4.4. Hardness and energy Epi vs. deposition rate aD The effect of deposition rate aD on: (i) the microhardness H of Ti(Fe)Nx films; and (ii) the energy Epi are given in Fig. 7. From this figure three issues can be drawn: (1) only films sputtered in the transition and nitride mode exhibit H greater than approximately 35 GPa; (2) the films sputtered in the metallic mode exhibit considerably lower H compared to those sputtered in the transition and nitride modes; and (3) the superhard films are formed only in the case when EpiGEpi minf0.3 MJycm3. The deposition rate aD of superhard films with HG40 GPa increases with increasing Id , see points A, B and C in Fig. 7a. This means that in the case when it is required to increase aD of the films, a more intensive magnetron discharge is necessary. However, the energy Epi delivered by bombarding ions to the film growing with a maximum aD is: (i) independent on the magnitude of aD; (ii) approximately constant; and (iii) equal to Epi minf0.3 MJycm3, see Fig. 7b. Moreover, the films with the highest hardness Hmax and sputtered with aD max are nearly stoichiometric Ti(Fe)Nxf1 films. This means, when films with Hmax are required to be produced, the energy Epi must be optimized, i.e. aD must be optimized, to ensure that the film stoichiometry x approaches to 1. Such a requirement can be fulfilled in the case when films are sputtered in the transition mode. 3.4.5. Hardness vs. size of grains and microstrain The correlations between hardness H and average size of grains Lc and microstrain Eg, calculated from the Scherrer formula, for Ti(Fe)Nx films are displayed in Fig. 8. The values of Lc and Eg were calculated from the most intensive reflection lines of the XRD pattern measured for a given film, see Fig. 3. In Fig. 8 arrows between points, corresponding to films produced at Ids 3 A, show a development of H with increasing pN2. From Fig. 8 it can be seen that: (i) the size of grains from which the films are composed ranges in a relatively broad interval from approximately 3 to 35 nm; and (ii) increase in H of the films produced in the metallic mode is accompanied by a continuous decrease of size Lc of grains. In the transition mode, the films are composed, at first, of small (-10 nm) grains and then a big jump in Lc from point 1 to point 2 occurs. The points 1 and 2 correspond to the film 1 and film 2, see Fig. 3. The jump correlates well with a change of the film composed of one kind of grains with the (200) orientation (film 1) to the film with (111) preferred orientation but composed of a mixture of grains with three different orientations (film 2). The last film is: (i) created in the transition region, see Fig. 3; (ii) almost stoichometric (xs1.06); and (iii) exhibits the highest hardness Hmax. We believe that for Hmax of the last film the mixture of grains of different average sizes is responsible, which
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Fig. 7. Dependences of (a) the hardness H of reactively sputtered Ti(Fe)Nx film and (b) the energy Epi delivered to it during its growth on the deposition rate aD.
makes sliding of grains more difficult. Films sputtered in the nitride mode exhibit a high hardness, which ranges in a relatively narrow interval between approximately 35 and 40 GPa. These films with approximately the same H can be, however, composed of grains of different size ranging from approximately 3 to 30 nm. The films composed of larger ()10 nm) grains exhibit a low (F0.015) microstrain Eg and those composed of small (-10 nm) grains exhibit a high ()0.015) microstrain Eg, see Fig. 8b. As it is expected, the microstrain Eg generated in films during their formation is an important parameter, which influences a mechanical behavior of the material (film). As an example, we can present films produced in the transition mode. All films produced in the transition mode, with an exception of the film 2 are composed of small (-10 nm) grains and exhibit a large
(G0.020) microstrain Eg. Just these films exhibit the largest resistance to plastic deformation compared to those produced in the nitride mode, see Fig. 9. Also, these experiments clearly show an extraordinary importance of the transition region, located between single-oriented films with different preferred crystallographic orientation, in which films composed of a mixture of grains of different orientations can be formed. This region is formed in the case when films are created in the transition mode of sputtering and usually is very narrow. Therefore, it is necessary to find deposition conditions under which a width of the transition region can be broadened. A broader transition region is formed in the case when more intensive magnetron discharges are used. Further investigations of the films produced in the transition region are highly needed and are under way in our labs.
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Fig. 8. Dependences of the hardness H on (a) average size Lc of grains and (b) microstrain Eg.
4. Conclusions The Ti(Fe)Nx films reactively sputtered under a relatively strong ion bombardment (Ussy100 V, iss1 mAycm2) in the intensive d.c. magnetron discharge (Ids3 A with average current and power target density 38 mAycm2 and 18.5 Wycm2, respectively) exhibit the following, very interesting properties: (1) the Ti(Fe)Nx film, which is a nitride of solid solution of Fe in Ti and so the one-phase material, can be superhard; (2) the superhard Ti(Fe)Nx films are nearly stoichiometric materials with xf1; and (3) the superhard films with the highest hardness are composed of a mixture of grains of different crystallographic orientations. The last finding can explain a rise of the enhanced hardness in onephase materials.
Superhard Ti(Fe)Nxf1 films are formed under special conditions. Such films are created in a transition region, which separates two groups of films with different, very strong preferred crystallographic orientations, see Fig. 3. The width of the transition region is usually very narrow; it is almost zero in weak (IdF1 A) magnetron discharges and increases with increasing Id. This is a main reason why in weak magnetron discharges it is very difficult to form superhard films, composed of grains of different crystallographic orientations, as it is almost impossible to select deposition conditions to fit the transition region. Therefore, denser magnetron discharges are more suitable to realize reactive sputtering. The transition region is formed under conditions, which correspond to the transition mode of reactive sputtering, see Fig. 3. Therefore, the transition mode of reactive sputtering is of
Fig. 9. Dependence of the ratio H3yEU2 of reactively sputtered Ti(Fe)Nx films on the microstrain Eg.
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great importance in the case when films with extraordinary properties are required to be produced. This statement seems to be of general validity. The important fact is also the finding that during a continuous increase of the partial pressure of nitrogen pN2 under all other deposition parameters constant a jump change in preferred orientation from (200) to (220) takes place. A continuous increase in pN2 cannot explain this jump. However, it can be explained by an increase of the energy Epi, because the jump in crystallographic orientation of grains occurs in the transition region in which Epi increases with increasing pN2, see Fig. 1. We believe that just in this transition region, the energy Epi delivered to the growing film by bombarding ions surpasses a threshold value Epi thr, which is necessary to create films with (220) preferred orientation. This means that extraordinary properties of reactively sputtered films, for instance the highest hardness or maximum resistance to plastic deformation, are a result of a combined action of physical and chemical processes controlled by the energy Epi and the film stoichiometry x, respectively. Acknowledgments This work was supported in part by the Ministry of Education of the Czech Republic under Projects No. MSM 235200002 and ME 529. References w1x S.M. Rossnagel, J.J. Cuomo, Vacuum 38 (2) (1988) 73. w2x R.A. Roy, D.S. Lee, in: J.J. Cuomo, S.M. Rossnagel, H.R. Kaufman (Eds.), Handbook of Ion Beam Technology, Noyes Publ, Park Ridge, NJ, USA, 1989, p. 194. w3x V. Poulek, J. Musil, R. Cerny, ´ R. Kuzel, Thin Solid Films 170 (1989) L55. w4x S.M. Rossnagel, J.J. Cuomo, Thin Solid Films 171 (1989) 143. w5x J. Musil, S. Kadlec, Vacuum 40 (5) (1990) 435.
ˇ w6x J. Musil, S. Kadlec, V. Valvoda, R. Kuzel ˇ Jr, R. Cerny, ´ Surf. Coat. Technol. 43–44 (1990) 259. w7x V. Poulek, J. Musil, V. Valvoda, L. Dobiasova, ´ˇ ´ Mater. Sci. Eng. A140 (1991) 660. w8x L. Hultman, W.-D. Munz, ¨ J. Musil, S. Kadlec, I. Petrov, J.E. Greene, J. Vac. Sci. Technol. A 9 (1991) 434. w9x J. Musil, V. Poulek, V. Valvoda, R. Kuzel ˇ Jr, H.A. Jehn, M.E. ¨ Baumgartner, Surf. Coat. Technol. 60 (1993) 484. w10x J. Musil, J. Vyskocil, ˇ S. Kadlec, Hard coatings prepared by sputtering and arc evaporation, in: M.H. Fracombe, J. Vossen (Eds.), Physics of Thin Films, 17, Academic Press Inc, 1993, pp. 145–177, Chapter 3. ˇ w11x R. Cerny, ´ R. Kuzel ˇ Jr, V. Valvoda, S. Kadlec, J. Musil, Surf. Coat. Technol. 64 (1994) 111. w12x I. Petrov, F. Abidi, J.E. Greene, L. Hultman, J.-E. Sundgren, Appl. Phys. Lett. 63 (1993) 36. w13x L. Hultman, J.-E. Sundgren, J.E. Greene, D.B. Bergstrom, I. Petrov, J. Appl. Phys. 78 (9) (1995) 5395. w14x J.-S. Chun, I. Petrov, J.E. Greene, J. Appl. Phys. 86 (7) (1999) 3633. w15x B. Rauschenbach, J.W. Gerlach, Cryst. Res. Technol. 35 (2000) 675. w16x H. Polakova, ´ ´ J. Musil, J. Vlcek, ˇ J. Allaart, C. Mitterer, Structure-hardness relations in sputtered Ti–Al–V–N films, Thin Solid Films (2003), in press. w17x J. Vlcek, ˇ ˇ ´ V. Hajek, ´ K. Rusnak, J. Appl. Phys. 86 (1999) 3646. w18x M.A. Lieberman, A.J. Lichtenberg, Principles of Plasma Discharges and Materials Processing, Wiley Interscience, New York, 1994. w19x J. Musil, Basic properties of low-pressure plasma, in: A. Paoletti, A. Tucciarone (Eds.), Proceedings of the International School of Physics, ‘Enrico Fermi’ Course CXXXV, ISO Press, Amsterdam, 1997, pp. 145–177. w20x J. Musil, A.J. Bell, J. Vlcek, ˇ T. Hurkmans, J. Vac. Sci. Technol. A 14 (4) (1996) 2247. w21x J. Musil, F. Kunc, H. Zeman, H. Polakova, ´ ´ Surf. Coat. Technol. 154 (2002) 304. w22x J. Musil, H. Zeman, F. Kunc, J. Vlcek, ˇ Mater. Sci. Eng. A340 (2003) 281. w23x T.Y. Tsui, G.M. Phar, W.C. Oliver, C.S. Bhatia, R.L. White, S. Anders, et al., Mater. Res. Soc. Symp. Proc. 383 (1995) 447. w24x J. Musil, H. Polakova, ´ ´ Structure-properties relations in hard sputtered nanostructured films, Proceedings of the International Conference on Designing of Interfacial Structures in Advanced Materials and Their Joints (DIS’02), Joining and Welding Research Institute, Osaka University, Osaka, Japan, November 25–28, 2002, pp. 149–156.
Effect of ion bombardment on properties of hard reactively sputtered Ti(Fe)Nx films ˇ ´ ´ J. Suna, ˇ J. Musil*, H. Polakova, J. Vlcek Department of Physics, University of West Bohemia, Univerzitnı´ 22, Plzenˇ 306 14, Czech Republic
Abstract This article reports on results of a systematic investigation of properties of hard Ti(Fe)Nx films reactively sputtered using a d.c. unbalanced magnetron. The Ti(Fe)Nx films with a low (F15 at.%) Fe content were selected as a typical single-phase material to investigate an effect of the energy Epi, delivered to them during their growth by bombarding ions, on their physical and mechanical properties. In this investigation, the energy Epi per deposited volume was varied by the magnitude of a deposition rate aD because EpiwJycm3 xsUsis yaD , where Us is the substrate bias, is is the substrate ion current density and aD is the film deposition rate. It was found that: (i) properties of sputtered films are a result of a combined action of physical and chemical processes controlled by the energy Epi and the film stoichiometry xsNy(TiqFe), respectively; (ii) Ti(Fe)Nx films can form a superhard material with hardness HG40 GPa; and (iii) superhard films with the highest hardness are: (a) formed in a transition region; (b) nearly stoichiometric with xf1; and (c) composed of a mixture of grains of different crystallographic orientations. The last finding makes it possible to explain the origin of the superhardness of single-phase materials. A special attention is devoted to mechanical properties of Ti(Fe)Nx films, particularly to relationships between hardness H, Young’s modulus E, elastic recovery We and the ratio H3yEU2, which is proportional to a resistance of the material to plastic deformation, but also to dependences of these mechanical properties on energy Epi , deposition rate aD , average size Lc of grains and microstrain Eg generated in the film during its growth; here E * sEy(1yn2 ) is the effective Young’s modulus and n is the Poisson’s ratio. Correlations between mechanical properties and modes of their sputtering are discussed in detail. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Ti(Fe)Nx; Single-phase films; Reactive magnetron sputtering; Ion bombardment; Physical and mechanical properties; Enhanced hardness
1. Introduction At present, it is well known that a correlation between properties of solids and their structure is of fundamental importance not only for materials science but also for thin films physics. However, what structure will be formed depends on process parameters and chemical composition of the film. A problem is the fact that in every deposition process there are many deposition parameters, which are mutually coupled. For instance, in magnetron sputtering these parameters are: magnetron discharge current Id and voltage Ud, substrate bias Us, substrate ion current density is, substrate temperature Ts, substrate-to-target distance dsyt, partial pressure of reactive gas pRG, total pressure of sputtering gas pTs pRGqpAr but also pumping speed of the pumping *Corresponding author. Tel.: q420-37-742-3136; fax: q420-37742-2825. E-mail address: [email protected] (J. Musil).
system, base pressure in the deposition chamber p0, location of inlets of sputtering gases, mutual orientation of the magnetron target and substrate surface (perpendicular or tilted deposition), stationary, rotating or linearly moving substrates, plasma enhancement by additional r.f., microwave or hollow cathode discharges or improvement of plasma confinement using an external magnetic field, usually produced by Helmholtz electromagnet coils, and geometry of the deposition chamber. The most important deposition parameters for every sputtering machine are: Id, Us, is, Ts, dsyt, pRG and pT. Every combination of these parameters give, however, only one discrete structure. Therefore, it is practically impossible, by changing one process parameter in this combination, to change continuously the structure of deposited film. This is a main reason why the formation of the film with a prescribed structure, i.e. with prescribed properties, is very difficult and so far a not solved problem. In our opinion, a key to the solution of
0257-8972/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2003.09.007
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this problem is a control of the energy E delivered to the film during its growth. This energy can be delivered by three ways: (1) substrate heating Ts; (2) particle bombardment Ep by: (i) ions (ion bombardment controlled by the ion energy Ei and the flux of ions ni); and (ii) fast neutrals (atom-assisted deposition controlled by pT, energy Eff and flux nff of the film forming particles); and (3) chemical reactions DH f (in exothermic reactions, when DH f -0, a heat is released and the total energy is increased and, on the contrary, in endothermic reactions, when DH f )0, the heat is consumed and the total energy decreases); here DHf is the formation energy of compounds. All three components of the total energy influence the film growth simultaneously, but the effect of individual components can be very different. For instance, in deposition of pure metals a contribution of the energy from chemical reactions is zero. On the contrary, in deposition of films using ion plating process, when the growing film is bombarded by low-energy ions, the energy Epi delivered to it by bombarding ions has a decisive effect on its growth. Therefore, the ion bombardment is very often used to control properties of deposited films, see for instance Refs. w1–15x. The energy Epi delivered to the growing film by bombarding ions has a crucial effect on its structure and so on its physical and functional properties. In a collisionless discharge this energy delivered per deposited volume can be determined from three easily measured quantities, i.e. the substrate bias Us, the substrate ion current density is and the deposition rate aD of the film, according to the following formula w16x: EpiwyJycm3z~sUsŽisyaD.Nimax x
|
at Tssconst
(1)
where Nimaxsexp(yLy li) is the amount of ions arriving at the substrate with a maximum energy eUs, e is the elementary charge, L is the sheet thickness and li is the ion mean free path for collisions leading to losses of the ion energy in the sheet. The ion mean free path can be calculated from the Dalton law as lif0.4yp wPax w17x. The high-voltage (Us4Ufl ) sheet thickness L can be expressed for a collisionless sheet near the substrate (Ly li-1) in the following form w18x: 1y4 3y4 y1y2 Ls(0.44)1y2´1y2 Us is 0 (2eymi)
(2)
where Ufl is the floating potential, ´0 is the free-space permittivity and mi is the ion mass. For instance, at ps 0.5 Pa, Ussy100 V and iss1 mAycm2 the ratio Ly lis0.116 and it means that nearly 90% of ions arrive at the substrate with maximum energy eUs. For more details see Ref. w16x. Eq. (1) clearly shows that changes in the film properties can be controlled by a combined action of Us, is
and aD. Besides, it is necessary to note that the parameters Us and (is yaD) in Eq. (1) are not physically equivalent w2,19x. This means that the films which are produced at different combinations of Us and (is yaD), which give the same value of Epi, do not exhibit the same microstructure, phase and chemical composition, i.e. such films exhibit different properties. Eq. (1) has been used by many researchers to characterize effects of low-energy ion bombardment on a microstructure of the film and its properties. For instance, the effect on: (i) grain size, lattice distortion, dislocation density in Ag, Cu, Pd films is investigated in w4x; (ii) microstructure in w5–8,11x; (iii) microhardness H, macrostress s, microstrain e in TiNx films in w5,6,9,10x; (iv) formation of ´-Ti2 N phase in TiNx films in w3,7x; (v) preferred orientation of TiN films in w12– 15x, etc. In spite of a relatively large utilization of ion plating process, there is no systematic study on correlations between the energy and properties of reactively sputtered films. Though, it is well known that the reactive sputtering of films is accompanied by the target poisoning, which results in a dramatic decrease of the film deposition rate aD, only few people realize that changes in aD, caused by a change in the partial pressure of reactive gas pRG (RGsN2, O2, CH4, etc.) under constant deposition conditions, induce huge changes in the energy Epi delivered to the film during its growth. For instance, for nitrides aD(Me)f4 aD(MeN) and for oxides even aD(Me)s(10–15) aD(MeO), where Me is the metal, MeN and MeO is the metal nitride and metal oxide, respectively. Therefore, it is necessary to expect that changes in properties of the reactively sputtered film will be the result of a combined action of two parameters; (i) chemical composition of the film, particularly the amount of reactive gas atoms incorporated in the film; and (ii) energy Epi, i.e. parameters, which both depend on the partial pressure of reactive gas. In the reactive mode of sputtering the effect of increased Epi, due to a decrease in aD with increasing pN2, is very strong. Experimental verification of these phenomena are given in this paper. This paper reports on results of a systematic study of properties of Ti(Fe)Nx films reactively sputtered using an unbalanced d.c. magnetron. The main aim of this study is to find correlations between deposition parameters, chemical composition, structure, physical and mechanical properties of these films, and energy Epi delivered to them during their growth. 2. Experimental Ti(Fe)Nx films were reactively sputter deposited in a mixture of ArqN2 using a round planar unbalanced d.c. magnetron equipped with an alloyed TiFe (90y10 at.%) target of 100 mm in diameter. The deposition chamber was an aluminium cylinder (⭋s150 mm, Ls225 mm)
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Fig. 1. (a) Deposition rate aD of Ti(Fe)Nx films and (b) energy Epi delivered to these films during their growth as a function of partial pressure of nitrogen pN2. The films were sputtered at the following three combinations of Id and is: (i) Ids1 A, iss0.5 mAycm2; (ii) Ids2 A, iss1 mAycm2; and (iii) Ids3 A, iss1 mAycm2. Constant deposition parameters: Ussy100 V, Tss300 8C, dsyts60 mm and pTs0.5 Pa.
pumped down by a diffusion pump (2000 lys) to a base pressure of approximately 10y3 Pa. The films were deposited on polished and ultrasonically precleaned, stationary, steel substrates (a disc 25 mm in diameter and 5-mm thick) made of CSN 15330 steel (the composition in wt.%: 2.47 Cr, 0.54 Mn, 0.29 C, 0.22 Mo, 0.17 V, 0.012 P and 0.012 S). The substrates were fixed to a movable and heatable substrate holder, which was electrically insulated from the grounded deposition chamber. Prior to the film deposition, a sputter cleaning of substrates was performed using a d.c. discharge (Uss y600 V, Iss20 mA, pArs15 Pa, ts5 min). The films were deposited under the following conditions: Ids1, 2 and 3 A, substrate bias Ussy100 V, substrate ion current density iss0.5 and 1 mAycm2, substrate temperature Tss300 8C, substrate to target distance dsyts 60 mm, total sputtering gas pressure pTspArqpN2s 0.5 Pa and deposition time tD ranging from 15 to 105 min. A typical thickness h of the film was 3–4 mm. The film thickness was measured with a mechanical Dektak 8 profilometer. The structure of films was characterized by X-ray diffraction (XRD) using a Dron 4.07 spectrometer in the Bragg–Brentano configuration with CoKa (ls0.179 nm) radiation. The chemical composition of the films was determined by energydispersive X-ray spectrometry (EDXS) using a SEM JEOL JXA 840 equipped with a LINK 860 analyzer. The nitrogen content was calculated as the rest to 100 at.%. The mechanical properties of the film, i.e. the microhardness, H, effective Young’s modulus, E *sEy (1yn2), and elastic recovery We, were evaluated from the load vs. displacement curves measured using a computer controlled microhardness tester Fischerscope H 100 equipped with the Vicker’s diamond indenter.
The hardness value was obtained from the average of 15 measurements taken at different positions. 3. Results and discussion 3.1. Deposition rate Dependences of the deposition rate aD of Ti(Fe)Nx films, sputtered at Tss300 8C, Ussy100 V and different values of the magnetron discharge current Id and substrate ion current density is, as a function of pN2 are given in Fig. 1a. These dependences can be divided into three regions corresponding to: (i) metallic; (ii) transition; and (iii) nitride modes of the magnetron sputtering. From Fig. 1a it can be seen that: (1) aD of the films sputtered in the metallic mode is (3–4) times higher that that of films sputtered in the nitride mode; (2) the transition between the metallic and nitride mode shifts to higher values of pN2 with increasing Id; and (3) difference in aD of films sputtered in the metallic and nitride mode decreases with increasing Id. The last phenomenon is due to broadening of an erosion zone on the surface of sputtered target with increasing Id. A strong decrease of aD in the transition mode is caused by the Ti(Fe) target poisoning, i.e. by covering of the target surface with Ti(Fe)Nx nitride. A large (3–4 times) change in aD with increasing pN2 results, in agreement with Eq. (1), in a large (3–4 times) change of the energy Epi delivered to the film during its growth by bombarding ions despite that all other deposition parameters are kept constant, see Fig. 1b. From this figure it is seen that Epi is the lowest in the metallic mode, increases in the transition mode and saturates in the nitride mode where achieves a maximum
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metallic one. This means that stoichiometric nitrides with a higher aD can be produced only in denser discharges, which are generated at higher magnetron discharge currents IdG3 A. On the contrary, Fig. 1b shows that the energy Epi, at which stoichiometric Ti(Fe)N films are produced, decreases with increasing Id. In summary, we can conclude that denser discharges are more convenient to realize reactive sputtering and to form stoichiometric nitrides. 3.3. Structure of films
Fig. 2. Elemental composition of Ti(Fe)Nx films, sputtered at Ids1 A, Ussy100 V, iss0.5 mAycm2, Tss300 8C, dsyts60 mm and pTs0.5 Pa, as a function of partial pressure of nitrogen pN2.
value Epi max. The energy Epi max in the nitride mode decreases with increasing Id due to an increase of aD. The value of Epi max can be, however, controlled by a change of the product (is=Us), which compensates, for instance, an increase in aD with increasing Id according to Eq. (1). The increase in Epi with increasing pN2 in reactive sputtering has dramatic consequences and is responsible for unexpected, very strong changes of the film structure and so its final physical properties, see next paragraphs. 3.2. Chemical composition A typical development of the elemental composition of Ti(Fe)Nx films, sputtered at the magnetron current Ids1 A, with increasing partial pressure of nitrogen pN2 is given in Fig. 2. From this figure it is seen that the nitrogen content in the Ti(Fe)Nx films increases with increasing pN2 and already at pN2s0.07 Pa the amount of N in the film achieves 50 at.%. The value of pN2 (50 at.%) shifts to higher values of pN2 with increasing Id. The last phenomenon is due to the fact that with increasing Id a number of sputtered Ti atoms also increases and so higher pN2 is needed to achieve the same amount of N in the sputtered film. Therefore, partial pressures of nitrogen pN2 (50 at.% N), at which stoichiometric Ti(Fe)Nxs1 films containing 50 at.% N are produced, also increases with increasing Id. These films are denoted by open symbols in Fig. 1. From Fig. 1a it is seen that: (i) the stoichiometric Ti(Fe)Nxs1 nitride films with xsNy(TiqFe) are formed in the transition mode and aD of these films is higher than that of overstoichiometric (x)1) nitride films produced in the nitride mode; and (ii) aD of the stoichiometric Ti(Fe)Nxs1 nitride films increases with increasing Id, i.e. shifts from the nitride mode to the
The structure of the Ti(Fe)Nx film can be easily controlled by partial pressure of nitrogen pN2, i.e. by the amount of N in the film, which decides what phase is formed. A development of the structure of Ti(Fe)Nx films sputtered at Ids3 A with increasing pN2 is given in Fig. 3. Here, for a simplicity of explanation the values of H and stoichiometry xsNy(TiqFe) are also given. The film sputtered at pN2s0 is a pure b-Ti(Fe) film. This is a high-temperature phase, which creates due to addition of Fe into Ti; for more detail see ref. w20x. With increasing pN2 the film gradually varies from a solid solution Ti(Fe,N) through a tetragonal ´-Ti2N phase at approximately 33 at.% N to a cubic d-TiNx phase. From Fig. 3 it is seen that inside an interval of pN2 corresponding to the transition mode of sputtering there is a transition region which separates Ti(Fe)Nx films with a strong (200) preferred orientation on side of lower pN2 from those with a strong (220) preferred orientation on side of higher pN2. A strong change in preferred orientation from (200) to (220) cannot be explained by a continuous, very slow increase of pN2. We believe that for this dramatic change in crystallographic orientation, the energy Epi is responsible. It increases in the transition mode of sputtering and very probably just in this region surpasses a threshold value Epi thr, which is needed to create films with (220) preferred orientation. The films produced in the transition region are composed of a mixture of grains of different crystallographic orientations. These films exhibit extraordinary properties, for instance, the highest hardness or maximum resistance to plastic deformation, see below Fig. 5 or Fig. 9. In the case when the films with extraordinary properties are required to be produced, the stoichiometry x of Ti(Fe)Nx films seems to be also very important, particularly small departures from an exact stochiometry xs1 are important. This means that the film properties are a result of a combined action of physical and chemical processes, controlled by the energy Epi and the film stoichiometry x, respectively. 3.4. Mechanical properties Mechanical properties of hard films are well described by their hardness H, effective Young’s modulus E *sEy
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Fig. 3. XRD patterns from Ti(Fe)Nx films as a function of partial pressure of nitrogen pN2 . Deposition parameters: Id s3 A, Us sy100 V, iss1 mAycm2, Tss300 8C, dsyts60 mm, pTs0.5 Pa.
(1yn2) and elastic recovery We. These quantities were evaluated from loadingyunloading curves measured by a microhardness tester Fischerscope H 100 at a load Ls20 mN of the Vicker’s diamond indenter. The measurement of H as a function of L showed that the load Ls20 mN used in these measurements gives, due to a relatively low hardness (HF40 GPa) of the Ti(Fe)Nx films, the correct value H of measured films. For more detail, see Refs. w21,22x. Measured values of H and E * permit to calculate the ratio H3 yEU2, which is proportional to a resistance of the material to plastic deformation w23x. As this is higher, the higher is the ratio H3 yEU2. Interrelationships between H, E * and process parameters ŽId, pN2. are, however, very complex to produce films with prescribed properties, and particularly to scale up the technological process of their production. This is due to the fact that properties of sputtered films depend on their: (i) chemical composition (stoichiometry xs Ny(TqFe)); and (ii) structure, which strongly depend on deposition conditions. Besides, it is well known that both the chemical composition and structure of the film,
sputtered at a given substrate temperature, can be effectively controlled by the energy Epi delivered to the growing film by bombarding ions. Therefore, it is very desirable to find relations between the mechanical properties and the energy Epi. 3.4.1. Interrelationships between mechanical properties and modes of sputtering Interrelationships between mechanical properties of sputtered Ti(Fe)Nx films, i.e. the dependences Hs f(E *), Wesf(H) and H3 yEU2sfŽH., are given in Fig. 4a, b and c, respectively. Here, films sputtered in metallic, transition and nitride modes are denoted. This representation shows a clear difference in mechanical properties of films sputtered in different modes of reactive sputtering. From Fig. 4 it can be seen that: (i) H increases with increasing E *; (ii) We and H3 yEU2 increases with increasing H in films sputtered in the metallic mode; (iii) films sputtered in the transition mode exhibit the highest values of H, We and H3 yEU2; and (iv) films sputtered in the nitride mode exhibit a lower H compared
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Fig. 4. Interrelationships between mechanical properties of reactively sputtered Ti(Fe)Nx films.
to that of the hardest film produced in the transition mode and films with the same H as those produced in the transition mode exhibit higher values of E *. This fact results in lower values of We and H3 yEU2 compared to those films produced in the transition mode. The films produced in the nitride mode are more plastic. The last fact correlates well with the amount of N incorporated in the sputtered film. The films produced in the nitride mode contain more N compared to those produced in the transition mode and are overstoichiometric Ti(Fe)Nx)1 nitrides. 3.4.2. Resistance to plastic deformation vs. film stoichiometry The effect of the film stoichiometry xsNy(TiqFe) on resistance of the Ti(Fe)Nx films to plastic deformation characterized by the ratio H3 yEU2 is displayed in Fig. 5. From this figure it is seen that the ratio H3 yEU2: (i) increases with increasing x for films sputtered in the metallic mode; (ii) is approximately constant
Fig. 5. Dependence of the ratio H3yEU2 of reactively sputtered Ti(Fe)Nx films on their stoichiometry xsNy(TiqFe).
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Fig. 6. Dependence of the hardness H of reactively sputtered Ti(Fe)Nx films on the energy Epi delivered to them by ion bombardment during their growth.
and reach the highest (0.57–0.67) values for films with x ranging from approximately 0.5 to 1, i.e. for films of sputtered in the transition mode; and (iii) is considerably lower (0.3–0.4) for films with x)1 sputtered in the nitride mode. Besides, it was found that the films sputtered in the nitride mode exhibit a lower H than the film with the highest H produced in the transition mode and the highest values of E * and so a lower: (i) elastic recovery We; and (ii) resistance to plastic deformation H3 yEU2 compared to Ti(Fe)Nx films produced in the transition mode. 3.4.3. Hardness vs. energy Epi The effect of energy Epi delivered to the growing Ti(Fe)Nx film by bombarding ions on its hardness H is displayed in Fig. 6. From this figure it is seen that superhard Ti(Fe)Nx films with HG40 GPa are produced in the transition and nitride modes of sputtering and only in the case when the energy Epi)Epi min. The value of Epi min decreases with increasing Id and Epi minf0.3 MJycm3 for Ids3 A. This experiment clearly shows that the energy Epi plays an important role in formation of hard films with H)35 GPa. To form such films the energy Epi)Epi min is necessary. A similar result was already found in formation of superhard Ti(Al,V)Nx films w16,24x. Besides, it was found that the films sputtered in the nitride mode, i.e. at the energy Epi) Epi min, have the highest values of E * and so exhibit: (i) considerably lower values of H3 yEU2, i.e. lower resistance to plastic deformation; and (ii) lower elastic recovery We, compared to those sputtered in the transition mode.
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3.4.4. Hardness and energy Epi vs. deposition rate aD The effect of deposition rate aD on: (i) the microhardness H of Ti(Fe)Nx films; and (ii) the energy Epi are given in Fig. 7. From this figure three issues can be drawn: (1) only films sputtered in the transition and nitride mode exhibit H greater than approximately 35 GPa; (2) the films sputtered in the metallic mode exhibit considerably lower H compared to those sputtered in the transition and nitride modes; and (3) the superhard films are formed only in the case when EpiGEpi minf0.3 MJycm3. The deposition rate aD of superhard films with HG40 GPa increases with increasing Id , see points A, B and C in Fig. 7a. This means that in the case when it is required to increase aD of the films, a more intensive magnetron discharge is necessary. However, the energy Epi delivered by bombarding ions to the film growing with a maximum aD is: (i) independent on the magnitude of aD; (ii) approximately constant; and (iii) equal to Epi minf0.3 MJycm3, see Fig. 7b. Moreover, the films with the highest hardness Hmax and sputtered with aD max are nearly stoichiometric Ti(Fe)Nxf1 films. This means, when films with Hmax are required to be produced, the energy Epi must be optimized, i.e. aD must be optimized, to ensure that the film stoichiometry x approaches to 1. Such a requirement can be fulfilled in the case when films are sputtered in the transition mode. 3.4.5. Hardness vs. size of grains and microstrain The correlations between hardness H and average size of grains Lc and microstrain Eg, calculated from the Scherrer formula, for Ti(Fe)Nx films are displayed in Fig. 8. The values of Lc and Eg were calculated from the most intensive reflection lines of the XRD pattern measured for a given film, see Fig. 3. In Fig. 8 arrows between points, corresponding to films produced at Ids 3 A, show a development of H with increasing pN2. From Fig. 8 it can be seen that: (i) the size of grains from which the films are composed ranges in a relatively broad interval from approximately 3 to 35 nm; and (ii) increase in H of the films produced in the metallic mode is accompanied by a continuous decrease of size Lc of grains. In the transition mode, the films are composed, at first, of small (-10 nm) grains and then a big jump in Lc from point 1 to point 2 occurs. The points 1 and 2 correspond to the film 1 and film 2, see Fig. 3. The jump correlates well with a change of the film composed of one kind of grains with the (200) orientation (film 1) to the film with (111) preferred orientation but composed of a mixture of grains with three different orientations (film 2). The last film is: (i) created in the transition region, see Fig. 3; (ii) almost stoichometric (xs1.06); and (iii) exhibits the highest hardness Hmax. We believe that for Hmax of the last film the mixture of grains of different average sizes is responsible, which
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Fig. 7. Dependences of (a) the hardness H of reactively sputtered Ti(Fe)Nx film and (b) the energy Epi delivered to it during its growth on the deposition rate aD.
makes sliding of grains more difficult. Films sputtered in the nitride mode exhibit a high hardness, which ranges in a relatively narrow interval between approximately 35 and 40 GPa. These films with approximately the same H can be, however, composed of grains of different size ranging from approximately 3 to 30 nm. The films composed of larger ()10 nm) grains exhibit a low (F0.015) microstrain Eg and those composed of small (-10 nm) grains exhibit a high ()0.015) microstrain Eg, see Fig. 8b. As it is expected, the microstrain Eg generated in films during their formation is an important parameter, which influences a mechanical behavior of the material (film). As an example, we can present films produced in the transition mode. All films produced in the transition mode, with an exception of the film 2 are composed of small (-10 nm) grains and exhibit a large
(G0.020) microstrain Eg. Just these films exhibit the largest resistance to plastic deformation compared to those produced in the nitride mode, see Fig. 9. Also, these experiments clearly show an extraordinary importance of the transition region, located between single-oriented films with different preferred crystallographic orientation, in which films composed of a mixture of grains of different orientations can be formed. This region is formed in the case when films are created in the transition mode of sputtering and usually is very narrow. Therefore, it is necessary to find deposition conditions under which a width of the transition region can be broadened. A broader transition region is formed in the case when more intensive magnetron discharges are used. Further investigations of the films produced in the transition region are highly needed and are under way in our labs.
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Fig. 8. Dependences of the hardness H on (a) average size Lc of grains and (b) microstrain Eg.
4. Conclusions The Ti(Fe)Nx films reactively sputtered under a relatively strong ion bombardment (Ussy100 V, iss1 mAycm2) in the intensive d.c. magnetron discharge (Ids3 A with average current and power target density 38 mAycm2 and 18.5 Wycm2, respectively) exhibit the following, very interesting properties: (1) the Ti(Fe)Nx film, which is a nitride of solid solution of Fe in Ti and so the one-phase material, can be superhard; (2) the superhard Ti(Fe)Nx films are nearly stoichiometric materials with xf1; and (3) the superhard films with the highest hardness are composed of a mixture of grains of different crystallographic orientations. The last finding can explain a rise of the enhanced hardness in onephase materials.
Superhard Ti(Fe)Nxf1 films are formed under special conditions. Such films are created in a transition region, which separates two groups of films with different, very strong preferred crystallographic orientations, see Fig. 3. The width of the transition region is usually very narrow; it is almost zero in weak (IdF1 A) magnetron discharges and increases with increasing Id. This is a main reason why in weak magnetron discharges it is very difficult to form superhard films, composed of grains of different crystallographic orientations, as it is almost impossible to select deposition conditions to fit the transition region. Therefore, denser magnetron discharges are more suitable to realize reactive sputtering. The transition region is formed under conditions, which correspond to the transition mode of reactive sputtering, see Fig. 3. Therefore, the transition mode of reactive sputtering is of
Fig. 9. Dependence of the ratio H3yEU2 of reactively sputtered Ti(Fe)Nx films on the microstrain Eg.
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great importance in the case when films with extraordinary properties are required to be produced. This statement seems to be of general validity. The important fact is also the finding that during a continuous increase of the partial pressure of nitrogen pN2 under all other deposition parameters constant a jump change in preferred orientation from (200) to (220) takes place. A continuous increase in pN2 cannot explain this jump. However, it can be explained by an increase of the energy Epi, because the jump in crystallographic orientation of grains occurs in the transition region in which Epi increases with increasing pN2, see Fig. 1. We believe that just in this transition region, the energy Epi delivered to the growing film by bombarding ions surpasses a threshold value Epi thr, which is necessary to create films with (220) preferred orientation. This means that extraordinary properties of reactively sputtered films, for instance the highest hardness or maximum resistance to plastic deformation, are a result of a combined action of physical and chemical processes controlled by the energy Epi and the film stoichiometry x, respectively. Acknowledgments This work was supported in part by the Ministry of Education of the Czech Republic under Projects No. MSM 235200002 and ME 529. References w1x S.M. Rossnagel, J.J. Cuomo, Vacuum 38 (2) (1988) 73. w2x R.A. Roy, D.S. Lee, in: J.J. Cuomo, S.M. Rossnagel, H.R. Kaufman (Eds.), Handbook of Ion Beam Technology, Noyes Publ, Park Ridge, NJ, USA, 1989, p. 194. w3x V. Poulek, J. Musil, R. Cerny, ´ R. Kuzel, Thin Solid Films 170 (1989) L55. w4x S.M. Rossnagel, J.J. Cuomo, Thin Solid Films 171 (1989) 143. w5x J. Musil, S. Kadlec, Vacuum 40 (5) (1990) 435.
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