A study of HCD ion plated titanium nitride films

Vacuum/volume 44/number Printed in Great Britain

A study

0042-207X/93$6.00+.00

2lpages 99 to 104/l 993

@ 1993

of HCD ion plated

C F Ai and J Y Wu.

Physics Division,

Institute

of Nuclear

titanium Energy Research,

nitride Lung- Tan, Taiwan,

Pergamon

Press Ltd

films ROC

and C S Lee. Department received

4 April

of Physics, National

Central University,

Chuli, Taiwan,

ROC

1992

Titanium nitride films have been deposited on the surfaces of tungsten, 304 stainless steel and 20 14- T6 aluminium alloy using the hollow-cathode-discharge (HCD) ion plating technique. The characterization of titanium nitride films has been analysed, mainly by using high resolution pulsed-laser atom-probe field-ion microscopy (FIM) and the X-ray diffraction (XRD) technique. XRD indicates that the film consists predominantly of the TiN phase. The atom-probe spectrum is also found to contain molecular ions of this species. Depth profiling discloses the existence of a rather thin interface of about 15 atomic layers and also reveals that the impurity TiO is concentrated not only in this interface but also near the top film surface. Both surface temperature and nitrogen gas pressure as well as energetic ion impacts have an effect on the film deposition. Thus it is proposed that an energy enhanced and surface catalysed reaction is predominant in the formation of titanium nitride compounds during film growth.

1. Introduction Recently, the most significant advance in material engineering has been the development of a reactive ceramic film deposition process using an ion-enhanced technique’. Hollow-cathode-discharge ion plating 2.3 (HCD) is one of the most efficient methods for producing these films because it can offer a certain ionization probability of gas molecules’, leading to the promotion of the activated reaction among them and a high deposition rate. Titanium nitride films are very useful for various applications of surface engineers, especially in tribology ; this is due to the fact that they exhibit the characteristics of high abrasion resistance, a low friction coefficient, high thermal stability, and high microhardness. Using the HCD method to deposit this film is successful and is already being commercially produced“. The advantages of a HCD film coating are production of a smooth film surface and a good adhesion force,5 the latter is related to the interface properties. Analyses through bulk films and interfaces have been performed by many authors” ’ ‘, but the number focusing on TIN film production by HCD ion plating is few. Many conventional surface analysis techniques, such as AES, SEM, XPS and EDS, etc., are often utilized by these authors. At the interface area. the film only contains a rather thin layer which really belongs to the interdiffusion region consisting of film and substrate components. The high resolution atom-probe field-ion microscope (FIM), possessing single-atom and single-layer sensitivity, is able to provide a better method of exploring such a thin interface. In this paper we attempt a careful absolute depth profile of the bulk film and interface using this atom-probe FIM. The similar work on titanium nitride film deposited by the reactive magnetron sputtering process has been done by Skogsmo et al 9previously. Our partial results somewhat agree with their results. Due to the fact that the atom-probe spectrum only gives the fractures of film components, the XRD method is adopted to identify them.

In addition, using atom-probe FIM to explore oxygen incorporation during deposition is also interesting because of the amount of oxygen on the film surface correlates to the film resistivity, especially in contact resistivity applied in electronic and electrochemical devices’ *. 2. Experiments and results 2.1. HCD ion plated titanium nitride films. Titanium nitride films were coated on varied substrates by home-made HCD ion plating apparatus’” as shown in Figure I. The vacuum system is composed of a diffusion pump with a liquid nitrogen trap backed by a two-stage rotary pump adapted with a roots pump. This allowed the system to be evacuated to a base pressure of about 6 x 10 -’ torr. The HCD gun provides an electron beam with a high current of 100-200 A and a low voltage of 40 V. It is used to melt titanium pills and, simultaneously, to ionize or to excite

0

Nz r

Figure 1. Schematic of hollow-cathode-discharge

ion plating system. 99

C F Ai et al: Ion plated

Table 1. HCD deposition

TIN films

parameters

Tungsten tip Substrate bias (- ) HCD current N, + Ar pressure Substrate temperature Substrate current density

for the different 304 stainless steel

samples 2014-T6 aluminium alloy

6tLlOO V 70 v 40 v 120A 120 A 120 A 8-9 x lo-’ torr 7 x 10e3 too 6 x IO-’ torr 300-500°C 2 600°C < 200°C 5-l 0 mA/cm2

both evaporated atoms and gas molecules’4. When pure nitrogen gas, with a pressure around a few lo- 3 torr, was introduced into the chamber, the activative reaction occurred, and a titanium nitride film was formed on the substrate surface. During deposition, substrates of tungsten, 304 stainless steel and 2014-T6 aluminium alloy were located at a distance of = 30 cm from the crucible (see Figure 1) and biased negatively with 40-100 V to extract the ions. The substrate temperature was increased rapidly by these energetic ion bombardments although the thermal radiation from the HCD cathode tube, where the temperature is up to 2OOO”C, and the crucible, inside which the molten titanium was filled, has also partial contribution to this. The aluminium alloy surface temperature always remains lower than 200°C to avoid recrystallization which will reduce the surface hardness, while the stainless steel and tungsten surfaces always achieve a temperature higher than 500°C. Generally, a deposition rate of -0.5 pm/min could be obtained. To be suitable for atom-probe analysis, the specimen must be electrically conductive, and have the shape of a sharp tip. The material of the specimen must also be strong enough to support the high tensile stress produced by the electric field which is required to remove and ionize the surface atoms of the tip15. The refractory tungsten tip, a few hundred angstroms in tip radius, prepared by electropolishing, was chosen for this study. A titanium nitride film of less than 2000 8, in thickness was coated

M: Mirror P: Photo W H:

Figure 2. Schematic 100

on this tip surface using the HCD method and, thus, probing of the interface layer was possible. As the bias was always applied to the specimen tip during coating it is easy to damage the tip end where the maximum field was maintained. Therefore, careful ignition of the HCD plasma beam, without a discharge occurring, is required. HCD deposition parameters for different samples are listed in Table 1. Except the substrate surface temperature. all the other deposition parameters are quite similar. During the deposition process, we simply adjust the nitrogen flow rate slightly until the extraction ion current of the substrate reaches a maximum. In this way the microhardness Hv,, (Vickers) of coatings of titanium nitride is usually greater than 2000 kg cm-’ corresponding to a film thickness greater than or equal to 5 pm, and the colour of films is always golddyellow ’ 3. 2.2. Atom-probe FIM. The home-made linear-type high resolution time-of-flight atom-probe field-ion microscope’h, shown in Figure 2, was utilized, especially, for depth profiling titanium nitride films. There are three different detection stations away from field emitter (tip). The third station located at a distance of - 2 m from the emitter, possessing single-atom sensitivity, was chosen for this work. The sample tip was mounted on a sapphire head connected with a copper strip to the bottom of a cryogenic refrigerator which was able to cool the tip temperature down to 20 K. An internal gimble system was used to adjust the desired tip crystal orientation to align with the probe hole through which the field evaporated surface atoms passed. To avoid the influence of residual gas on the absolute quantitative study, the base pressure of the main chamber was lO_“’ torr. When the imaging gas was introduced into the chamber, the field-ion image of atomic structure of the tip surface appeared. For depth profile analysis, one shined a focusing pulsed-laser beam onto the tip on which a proper high dc electric field was applied, thus, the tip surface atoms or compounds were field evaporated and detected oneby-one and layer-by-layer. The time-of-flight detecting system recorded them accordingly.

M.P.:

Mechanical Pump Turbo molecular Pump T.P.: 0 Image gas or Reactive gas (He.Ne,Ar) . (HZ,OZ,NZ)

diode

Window Probe hole

of high resolution

pulsed-laser

time-of-flight

atom-probe

field-ion microscope

8: [xl: -$:

Angle

valve (all

Leak valve X-Y-Z-R-0 micromanipulator

metal)

C FAI ef al: Ion plated TiN films Table 2. Conditions used in the atom-probe depth profile titanium nitride film coated on the tungsten tip surfaces

analyses

I

II

60 V

60 V

HCD deposition

Bias (-) HCD current

parameters

Nz + Ar pressure

120A 8 x lo-’

Atom-probe operating conditions

Tip temperature Applied field Applied voltage

70 K -3.ovA-’ 6.619 kV

40 K -3svA3.7-9 kV

atoms/probe

S-90

2-13

Covering

surface

hole

of

120 A 8 x 10m3 torr

torr

2.3. X-ray diffraction. X-ray diffraction with Cu Kcr was used to analyse the crystal phases and their intensities of the titanium nitride film. 2.4. Atom-probe time-of-flight spectrum and depth profile. Titanium nitride film coatings on tungsten tips were processed using the conditions as listed in Table 2. Atom-probe field-ion images of the surface structures of coating films were examined, firstly, from the first detection station which was equipped with a microchannel plate and screen assembly and located at a distance of only 14 cm from the emitter. Usually, irregular and dim images appear. After removing several layers of surface atoms by dc field evaporation, one could observe slightly better smooth

Massspectrun

&co+

surfaces emerging, and, occasionally, an explicit ring pattern appeared locally. It indicated that a crystal structure existed at such a limited ring region. The depth profiling analyses of coating films were conducted with the two conditions as listed also in Table 2. For condition I, the time-of-flight mass spectrum is displayed in Figure 3. It shows that the species mainly detected correspond to the film compositions of TIN and Ti. The residual gas Hz, imaging gas Ne and the novel ions, such as H30+, N,H+/COH +. H3+, etc. were also found. The novel ions are the commonly field-enhanced reaction products”. With regard to the observation of the Ar+ peak, it is due to Ar molecules incorporating into the film, hence, during HCD deposition, a small amount of Ar gas was admitted into the HCD cathode tube to generate the ari: current. Figures 4 and 5 show the expanding mass spectra of Ti*+ and TiN’+ displayed in Figure 3, respectively. Their isotope lines are resolved clearly. The counting number of each isotope li,ne peak of Ti*+ approximates to its natural abundance. Thus, based on this fact, one could identify the existence of a small amount of TiO*+ as a few atoms overlapped with isotope peaks of TiN*+, as seen in Figure 5. It is how the oxygen distributes inside the film layer, especially at the interface region, that is our concern, since the oxide layer existing on the substrate is inhibitory to the formation of a strong adhesion between film and substrate. Here, in Figure 6, the absolute depth profile appears to show that the main distribution of TiO exists at near-surface of the film (- 20 atomic layers) while only several counts spread through bulk film randomly. Due to the jump of the sample tip, the depth profile

Nunberof Ions 188 /

Mass Spectrun

168 . TiN .I5 anu/unl

70 k 3.0 V/A

c

TiN*+

t

148 .

.u2 anu/s1ot 78 k I

t

128

80 60 .

h

0

Iltonic

Figure 3. Time-of-flight evaporation of titanium I).

/5

Mass Units

i4

(n/n)

mass spectrum of pulsed-laser promoted field nitride film coated on tungsten surface (condition

Figure 5. The expanding

IO

r

mass spectrum

of TiN’+ shown in Figure

3.

s

630 560 . 490

____~4Tio/TiN2t(5:

1J

Total Ions Figure 4. The expanding

mass spectrum

of Ti’+ shown in Figure

3.

Figure 6. Depth profile of titanium

nitride film (condition

I). 101

C F Ai et a/; Ion plated

TiN films ‘i’iN/Al :: 18O’C

36 32 28 24 20 16 12 8 4

30

film and tungsten

surface

(condition

layer between II).

50

60

70

00

II0

(=I

Total Ian5

Figure 7. Depth profile of interface

40

the titanium

nitride

through the interface layers became impossible for this condition I. It was done successfully with condition II, as shown in Figure 7, by carefully coating a rather thin film and removing the film surface atoms by properly controlling the pulsed-laser power to promote field evaporation. There are about 15 atomic layers at the interdiffusion region where species of TIN, Ti, W and even TiO are detected. Apparently, the amount of TiO is comparable with that of TIN in this region. 2.5. Diffraction pattern. On the other hand, X-ray ditrraction analyses of titanium nitride films deposited on 304 stainless steel and 2014-T6 aluminium alloy surfaces with the conditions referred to in Table I are shown in Figures 8 and 9, respectively. For both of them, only the TIN compound phases with different preferential orientations arc observed except a small amount of the TizN phase appears with a surface temperature higher than 600 C.

Figure 9. X-ray diffraction pattern of titanium nitride film coated on 2014-T6 aluminnium alloy surface with surface temperature lower than 200 C.

evaporation process even at low field - 3.0 V A ’ applied on the tip specimen. With the auxiliary analyses of X-ray diffraction, the diffraction patterns of film/304 stainless steel shows that the preferential orientation of TiN( I1 1) is predominant and a few of T&N and Ti phases also appear. On film/2014-T6 Al alloy, the main diffraction pattern of TiN(I II) is also observed but the T&N and Ti phases disappear. These results indicate that the mass peaks of TIN displayed on the atom-probe mass spectra should be main components of the titanium nitride film. Although TiN may come from the field dissociation of T&N. it should be neglected because the Ti,N phase is not formed significantly even at a higher surface temperature as shown in Figure 8. Regarding the mass peaks of Ti ions. for example Ti’+, an extensive amount of mass peaks is obtained, which is unexpcctcd considering the results of X-ray diffraction. This information indicates that the majority of detected Ti’+ ions comes from the field dissociation of TiN’ +, and the minority from the pure Ti clement.

3. Discussion 3.1. Film components. The pulsed-laser time-of-flight atomprobe mass spectrum of the titanium nitride film only shows peaks of TIN, Ti and a few of Ti2N, but it cannot determine from them what the real quantity and crystal phases are. This is owing to the fact that field dissociation may occur through the field

TiNjS. S 304 > 600’C

Figure 8. X-ray diffraction pattern of titanium nitride film coated on the 304 stainless steel surface with surface temperature greater than 600°C. 102

3.2. Film formation 3.2.1. Surface temperature effect. When the titanium nitride film was coated using the HCD method on all substrates, the performing parameters were quite similar, except the substrate temperature which was 16OO“C with 304 stainless steel, 300500-C with a tungsten tip and <2OO”C with the aluminium alloy. Here, apparently, the elevated substrate temperature is the main contributor to the formation of Ti,N and Ti crystal phases (see Figure 8). The surface temperature increase is mostly due to bombardment of energetic ions or neutrals3. Either energetic particle impact or surface temperature rise is able to desorb the adsorbed nitrogen atoms or molecules from the substrate surFace, such that a slight nitrogen deficit is crcatcd. and causes to a limited cxtcnt the formation of Ti,N and Ti crystal phases. 3.2.2. Variation of deposition rate with N, pressure decrease. As mentioned earlier. during deposition the Nz pressure is adjusted properly to ensure that the maximum ion currents (proportional to the deposition rate) are produced from a biased substrate, such that both atom-probe and XRD analyses show that the formation of the TIN compound is predominant. This fact means that the number of reactants of N atoms arc at least approximately equal to that of the Ti atoms in the reactive region inside the deposition chamber. When the Nz pressure decreases slightly

C F Ai er al: Ion plated TIN films

from the previous optimal setting, the observed substrate current decreases also. This is due to the fact that all the N$ (or N+), Ti+ and (Ti-N)+ ion counts collected by substrate decrease also. The NC (N+) or Ti+ ions are responsible for forming TiN at the solid surface whereas the (Ti-N)+ ions in the gas phase are deposited directly as the TIN compound. Where the Ti-N reaction preferentially occurs, at a solid surface or in the gas phase, our previously temperature effect results imply the preference of T&N formation on the surface. When the N, pressure is decreased to some degree, Ti,N formation becomes predominant*. This point is comparable with the deficit of absorbed N atoms on the solid surface with a higher surface temperature, and thus, it again supports TIN formation at the solid surface. It is essential to note that the total pressure of N,+Ar +Ti is 10m3 torr during the HCD ion plating process, thus the mean free path of gas particles is about a few tens of centimetres which is comparable with the distance from the crucible (Ti source) to the substrate. The collision number between Ti (Ti+) and N, (N*+) is less than or equal to unity before they arrive at the substrate surface. Once they collide with each other, the chemical reaction to form TIN in the gas phase is low due to the high dissociation energy of N, (N:) and the short lifetime of N (N+) which will recombine as N, (NC) very quickly. Therfore the formation of TIN is preferable on the substrate surface, and, i.e. the surface catalysed reaction should play an important role on this film deposition. Based on this view, the above consideration is to be judged from the kinetics points of these particles as follows. 3.2.3. Energy effect. Generally, neutral molecules of N, arriving at the substrate surface will be physically adsorbed at a location and then most of them about -3 A away from the surfaces”, will be readily desorbed by surface thermal energy or, subsequently, ion sputtering. The N *+ ions possessing an energy of a few tens of electronvolts, depending on the substrate bias, can arrive closer to the surface, and, thus, they can overcome the activation energy from the physical adsorption sites to dissociative chemisorption sites. Ti+ ions as well as N*+ ions also reach the chemisorption sites. Once both of these ions are chemically absorbed, they are readily diffused along the surface and collide with each other to form TIN. The sources for promoting the diffusion of surface adsorbed atoms are the momentum of the incident ions (or the neutral particles produced by neutralizing the energetic ions during the transferring period) and the surface thermal energy. Energetic ion or neutral particle bombardments are not removing the impurities of physical adsorption but also breaking the chemical bond between the neighbour atoms at the surface to produce displacement defects which may become a nucleation centre for crystal formation”. Indeed, as regards the effect of ion bombardment, the atomprobe depth profile throughout the bulk film (Figure 6) shows that no adsorbed impurities exist, like the C atoms which are commonly contaminating species. From the above, a suggestion for the TIN formation mechanism on a solid surface is shown in Figure 10. The energy of a few tens of electronvolts of N: (NJ or Ti+ (Ti) is not enough for penetrating deeply into the subsurface but it can overcome the activation energy for the direct T-N reaction while either only Ti or N was chemisorbed on the surface site previously (see Figure 10). It also enhanced the longitudinal diffusion of adsorbed Ti or N atoms into the subsurface. Thus, a composite interface layer is formed and a strong adhesion force between the film and substrate surface is expected.

9Ti + or

TizN

Self-diffusion

TiN Ti

Nz

Ti

or

Nz

....@z Ti

$&I+-@ Momentun

Direct

Transfer

Reaction

L

1

0:

energetic

ion

or

energetic

neutral

atom

or

molecule

Figure 10. Diagram of the reactive formation mechanism of titanium nitride compound on solid surface (energy-enhanced and surface catalysed reaction).

In fact, as shown evidence.

in Figure

7, the interface

profile gives such

3.2.4. Variation of deposition rate with N2 pressure increase. When the N, pressure is increased over the point corresponding to the maximum deposition rate, the observation of a decrease in the deposition rate accounts for the reduction of the arrival rate of Ti (Ti+) or N2 (N*+) reactants at the surface due to the shortening of their mean free path. From the other point of view, the ion energy will be reduced effectively due to the increase in the number of collisions among them. Therefore, it will lower the energy-enhanced effect of TiN formation, and, accordingly, will reduce the formation rate. 3.3. Film adhesion and oxygen incorporation. The 0 atoms always incorporate into the film especially at interface and near the top of the film surface. Figure 7 shows that there are several TiO compounds distributed throughout the interface. This fact implies that a certain amount of 0 atoms had contaminated the substrate surface before coating, otherwise the counts ratio of TiO to TiN+Ti should be neglected like the bulk film appearance. How this 0 impurity incorporation affects film adhesion with the substrate depends on the mutual solubility of Ti, TiN, W and TiO. Figure 7 also shows interdiffusion among them quite well, and which seems still to possess good adhesion unlike the fact that the pure oxide layer existing at the film interface usually gives poor adhesion. In addition, at this interface region, the detected count ratio of Ti/TiN is apparently higher than in the bulk film. That the pure Ti layers were deposited before introducing N, gas into the reaction chamber to deposit the TIN film was responsible for the higher counts of the Ti signals. The intermediate Ti layer can enhance film adhesion*’ which correlates to the fact that, as known, the binding force of TIN-Ti-W is larger than that of TIN-W. Here the pure intermediate Ti layer is not observed on account of the fact that the deposited Ti layer is very thin and readily diffuses longitudinally into TiN film and W substrate surfaces with an elevated surface temperature. TiO also concentrates in the near-surface-film with approximately 20 atomic layers (or 4 nm in thickness) as shown in Figure 103

C FAi et al: Ion plated TIN films 6. Its counts arc even comparable with those of TiN+Ti in that region, but they can be negligible beyond this. Usually, a thin oxide layer had existed on the TIN film surface. However, the formation of this layer was related to the ambient environment and created an increase in the contact resistivity2’,**. Such a completely pure oxide layer was not detected, but this does not imply that none exist. It was possible to field evaporate before depth profile analysis. The observed region of TiO appearance seems to be the sub-layers under the top oxide surface layers. We believe that the observed TiO compound mainly comes from the thermal diffusion of chemisorbed 0 atoms supplied with residual gas or impurity gas. such as H,O and O,, in the deposition chamber, interacting with TIN film componentsz2. The reason is that the deposited TIN film surface still keeps a rather high temperature within a certain period at the end of deposition. The occurrence of oxidation becomes possible at this higher temperature although it possesses good oxidation resistance at room temperature.

4. Conclusions (i) Titanium nitride films were deposited on 304 stainless steel, 20 14-T6 aluminium alloy and tungsten sample surfaces using the HCD ion plating method. For this simple coating process one of the optimal conditions was obtained by adjusting the Nz pressure in the mtorr range to ensure that the maximum ion current was collected by a biased sample. The microhardness of all these films is greater than Hv,, 2000 kg mm-‘, and their colour is goldyellow. (ii) The major component of the titanium nitride films deposited by the above process is the compound TIN; the preferential orientation of these films is the TiN( 111) plane. (iii) The higher substrate surface temperature as well as the lower Nz pressure accounts for the TizN compound/crystal formation. (iv) All the results of deposition rate with N, pressure, surface temperature effect and ion energy effect indicate that TIN forms at the solid (substrate) surface, and hence, energy-enhanced and surface catalysed reactions mainly govern TIN formation. (v) Depth profiles show TiO enrichment near the surface (- 20 atomic layers) and in the interface layers. This arises from H,O and O2 being initially chemisorbed dissociatively followed by diffusion caused by the surface temperature and reaction with film elements.

104

(vi) Both the uniform distribution of film components and the non-existence of apparent impurities throughout the bulk film support the good quality film obtained. (vii) A very thin (- 15 atomic layers) interface region exists. It exhibits a well interdiffusing region between the film and substrate and this also supports the good adhesion expected. Acknowledgements We would like to S Su for the help and atom-probe samples for depth

thank Mr C L Moh, Mr M T Chen and Mr J in setting up the HCD ion plating apparatus FIM system and for preparing the coating profile analysis.

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