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PLANAR MAGNETRON SPUTTERING OF TITANIUM NITRIDE ON HIGH SPEED STEEL
J. Skogsmo, P. Lindblad, H. Norden
To cite this version:
J. Skogsmo, P. Lindblad, H. Norden. PLANAR MAGNETRON SPUTTERING OF TITANIUM
NITRIDE ON HIGH SPEED STEEL. Journal de Physique Colloques, 1986, 47 (C7), pp.C7-251-C7-
255. �10.1051/jphyscol:1986743�. �jpa-00225937�
JOURNAL DE PHYSIQUE
Colloque C 7 , suppl6rnent au n " 11, Tome 47, Novembre 1986
PLANAR MAGNETRON SPUTTERING OF TITANIUM NITRIDE ON HIGH SPEED STEEL
J. SKOGSMO, P. LINDBLAD and H. NORDEN
D e p a r t m e n t o f P h y s i c s , C h a l m e r s U n f v e r s i t y o f T e c h n o l o g y , S-412 96 G d t e b o r g , S w e d e n
Abstract
A method to prepare needle-shaped high speed steel specimens and to deposit thin coatings of TiN on them has been developed. The specimens were examined by analytical electron microscopy and atom-probe field-ion microscopy.
Introduction
Physical vapour deposition (PVD) is a very versatile technique for the application of a variety of coatings to many different substrate materials. The PVD method enables, for instance, high speed steel tool materials to be coated with wear resistant TiN at temperatures which do not affect the structure of the steel.
During recent years TiN coatings have been produced by activated reactive evaporation, reactive ion plating, various sputtering processes and several other methods. The factor common to all these methods is that the coatings are deposited in the reactive mode where titanium is sputtered, evaporated or ion plated in a nitrogen atmosphere. Several papers (Sundgren et al 1983, Jacobson et a1 1984, Steinmann and Hintermann 1985, Gabriel and Kloos 1984 ) have dealt with the influence of process parameters on deposition rate, wear resistance and hardness but not so much has been published about the chemical composition, microstructure, morphology and initial growth of the coatings. One reason for this is probably the difficulty in preparing thin foil specimens, suitable for transmission electron microscopy, of these multiphase materials.
The present work was initiated with the objective of investigating the possibility of using the PVD method of planar magnetron sputtering to deposit very thin TiN coatings (less than 100 nm) on needle-shaped high speed steel substrates. Such substrates can be used for both analytical electron microscopy (AEM) and atom-probe field-ion microscopy (AP-FIM).
Experimental
Apparatus The TiN coatings produced in the present work were made in a high vacuum planar magnetron sputtering system. The vacuum system was composed of a turbo-molecular pump backed by a rotary vane pump. This allowed the system to be evacuated to a pressure of about 5 nbar before the gases (argon and nitrogen) were admitted to the chamber. There were two water cooled planar magnetrons (13x21 cm) placed opposite each other in the reaction chamber. Polycrystalline titanium targets were screwed onto the magnetrons and the distance between the targets was 10 cm, see Fig. 1. In the figure only one magnetron is shown. The magnetrons were DC powered by current controlled magnetron power supplies. Inside the reaction chamber there was also an extra DC powered electrode to provide the possibility of discharge cleaning of substrates and chamber walls.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986743
C7-25 2 JOURNAL DE PHYSIQUE
The substrate holder, see Fig. 2, was placed at one end of the substrate table. By rotating the substrate table the specimen holder could be moved from the magnetrons. A power supply was connected to the table so that a bias voltage could be applied. Sputter cleaning could also be performed using the same power supply. There was also a substrate shutter to protect the substrate from deposition during target cleaning.
,
-Voltage Front SideTarget I I U
I I Back
b
.
SubstrateFig. 1. A one-sided planar Fig. 2. The specimen holder for the magnetron sputtering apparatus. needle-shaped specimens.
Substrates Thin rods (01x30 mm) of M7 high speed steel with composition according to Table 1 were used. The specimens were electropolished into sharp needles in a three step sequence.
The electrolytes used were of two types with compositions according to Table 2.
First, the rods were thinned by electropolishing in a layer,20-25 mm thick, of electrolyte A on top of mchlorethylene at a potential of 20-25 volts DC at room temperature. The polishing was interrupted when the middle of the polished section had a thickness of about 0.3 rnrn.
In the second step the two thicker ends were cut off and the thin middle section was divided into two thin rods, 8-10 mrn long. These rods were attached to aluminium handles (01x10 mm with 0.5 rnrn hole) and electropolished in a layer, 5-6 rnm thick, of electrolyte A on trichloroethylene at 30 V DC at RT.
When a sufficiently thin waist was obtained the third step commenced and the specimen was electropolished in electrolyte B, at a voltage of 20 V DC and a current of 7-8 mA, until the lower part fell off.The specimen was washed in methanol and inspected in a transmission electron microscope (TEM). Specimens with a tip radius less than 50 nm were chosen for the deposition experiments.
One reason for using this three step procedure was to reduce the amount of magnetic M7 material which otherwise caused a difficult astigmatism in the TEM.
Substrate holder Due to the unusual shape of the specimens, a special substrate holder had to be constructed. The holder could carry five specimens which would be exposed to identical deposition fluxes and uniform ion and atom bombardement. In order to avoid arcing of the sputtering discharge and high currents through the needles, the substrate holder should elecmcally shield the specimens as much as possible without excessively decreasing the deposition rate. Therefore the only way the substrates could see the discharge during deposition were through a hole in the front of the holder, Fig.
2, since only the target facing this side was used.
De~osition ex~eriments The sputtering depositions were made under the conditions in Table 3.
Results a n d discussion
The thickness of the coatings could easily be controlled by varying the deposition time. Very short deposition times (5-30 seconds) gave very thin coatings which were difficult to study in TEM.
Deposition times of 50 seconds to two minutes gave coatings with thickness of 50-100 nm which were more suitable for TEM. The variation in coating thickness between substrates from the same deposition was very small. The coating was smooth but the thickness was non- uniform and the side of a substrate facing the magnetron had a much thicker coating than the side not directly exposed to it, Figs. 3 and 4.
The substrate shape was very little influenced by the deposition process, as could be seen by comparison between an uncoated specimen and the same specimen after deposition. The TiN grains showed a columnar growth for all depositions, and most columns reached from the substrate to the coating surface. This indicates zone 1 growth behaviour according to the structure zone scheme.
Fig. 3. TEM bright field micrograph of a Fig. 4. TEM dark field micrograph of a specimen specimen coated for 1 min. coated for 2 min.
The crystal structure of a deposited TiN coating is strongly influenced by its nitrogen content.
Below 15 at.% N only single phase a-Ti structures have been observed (Sundgren 1983). Above 15 at.% N the tetragonal E-Ti2N phase starts to appear. This phase is present up to about 35 at.% N and within a narrow range at about 33 at.% N is the only phase present. The 6-TiN phase starts to appear above 30 at.% N and is the only phase present above 35 at.% N. The interplanar spacings of the e-Ti,N and 6-TiN phases change with nitrogen content. In the case of 6 T i N its lattice parameters increases with increasing nitrogen content and reaches a maximum at about 50 at.% nitrogen (Sundgren 1983).
To examine the influence of the nitrogen one experiment was carried out without any nitrogen flow.
Coatings with very small grains and a dense structure were obtained, see Fig. 5. The grains did not show so pronounced columnar growth and the columns did not usually reach through the coating as in the experiments with nitrogen flow. Electron diffraction showed that the crystal structure was different to those which contained nitrogen. Probably the hexagonal a-Ti had formed.
Fig. 5. TEM dark field micrograph of a specimen Fig. 6. EDS depth profile taken at the coated for 2 min. without nitrogen flow. marked positions in Fig. 3.
C7-254 JOURNAL DE PHY SIOUE
Energy dispersive X-ray analysis was performed on some specimens. The analysis was taken in steps across a specimen, and the distance given in the diagram in Fig. 6 is measured from the outermost part of the coating and towards the middle of the specimen. This analysis is taken at the marked positions on the specimen in Fig. 3. Due to broadening of the electron beam in the specimen the spatial resolution of the EDX method is not sufficient to give the exact location of the detected elements. Fig. 6 indicates however how the concentrations of the main elements Ti, Fe, Mo and Cr vary in the coating.
In the field-ion microscope it was possible to image coated specimens using neon as the image gas.
Fig. 7 shows the image of the specimen shown in Figure 3. The image shows a rather irregular contrast due to the small crystallites which give an irregular end form and hence overlapping images.
In connection with field-ion microscopy, atom-probe analysis was carried out. Mass spectrum obtained from ions collected from the specimen after the outermost atomic layers had been removed is shown in Fig. 9. There are no significant peaks above m/q equals 60. Apart from the expected Ti and N also major peaks from Fe, Ti(N,O,C), C, Ar, Cr, Mo, V, W were found. The presence of Fe, Cr, V, Mo and C in the coating can be explained as arising from the substrate through diffusion and sputtering processes. The HSS used as substrate material contained these elements. The amount of Ar present in the coating is rather interesting.The substrates were biased during deposition, which means that they were bombarded with Ar ions. A portion of the Ar ions thus became incorporated in the coating. Depth profiles through the TiN coating and the interface TiN-HSS are shown in Fig. 8.
Significant amounts of Fe, 0, Ar and alloying elements from the high speed steel are dissolved in the coating.
NUMBER OF I O N S
Fig. 7. FIM micrograph from the specimen Fig. 8. Atom-probe depth profile from the
shown in Fig. 3. specimen in Fig. 3.
" 6 0 . ~e.*+
C
.-
0'C 0 L
n 0)
E
3 C
20 40 60
a t o m i c m a s s unit ( m/q ) Fig. 9. Atom-probe mass spectrum from the specimen in Fig. 3.
Only one process parameter, the deposition time, has been varied systematically during the present work. However, most of the processparameters in the reactive magnetron sputtering process can be varied using the needle shaped specimens. Problems could occur if the bias is set to a very high negative voltage. This could cause resputtering of substrate material which would change the shape of the substrate. A relatively high current density through the substrates could also cause substrate heating with a resulting temperature rise.
Conclusions
*
The described method is potentially very useful for preparation and examination of very thin PVD coatings.*
All coatings showed columnar growth.*
The TiN grains had the cubic 6-TN smcture.*
The grain size and the amount of voids increased with increasing deposition time.*
AEM revealed that considerable amounts of Fe, Cr and Mo were present in the coatings.*
Atom-probe analysis showed ,apart from the expected Ti and N, peaks from Fe, Cr, Mo, V, W, and C, all of which are present in the HSS material.*
Ar and 0, which are present in the atmosphere during deposition, were also incorporated in the coatings.Acknowledgement
The authors wish to thank Sandvik Hard Materials AB for financial support and for provision of deposition facilities. Kloster Speedsteel AB is thanked for providing the substrate material.
References
Sundgren J.-E., Johansson B.-0.. Karlsson S.-E., and Hentzell H.T.G.
Thin Solid Films 105 (1983) 353
Jacobson B.E., Deshpandey C.V., Doerr H.J., Karim A.A., and Bunshah R.F.
Thin Solid Films 118 (1984) 285 Steinmann P.A. and Hintermann H.E.
J. Vac. Sci. Technol. A 3(6) (1985) 2394 Gabriel H.M. and Kloos K.H.
Thin Solid Films 118 (1984) 243
Table 1. Composition of M-7 HSS.
Electrolyte A : 20 % glycerol,
10 % perchloric acid and 70 % ethanol.
Electrolyte B : 2 % perchloric acid in ethylene-glycol-monobutyl ether.
Table 2. Composition of electrolytes.
Total pressure: 3 pbar Argon flow rate: 100 sccm Nitrogen flow rate: 12 sccm Substrate bias voltage: -50 V Substrate bias current: 140 mA Magnetron current: 4 A Magnetron voltage: 400 V Substrate-target spacing: 5 cm Deposition times: 5 s - 2 m i n
