Active process control of reactive sputter deposition

Vacuum/volume 46lnumber 7lpages 723 to 72911995 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0042-207x/95 $9.50+.00

0042-207x(94300090-5

Active process control of reactive sputter deposition A A Voevodin, P Stevenson, C Rebholz, J M Schneider Engineering, University of Hull, Hull, HU6 7RX, UK

and A Matthews,

Research

Centre in Surface

received 25 October 1994

Control of the density, composition, ionisation rate and arrival energy of species is one of the main objectives of research in the development of reactive magnetron sputtering. The deposition of the latest generation of multilayer and multi-component coatings requires the independent control of deposited flux parameters using fast response and reliable control systems. A review of recent advances in process control showed the potential of techniques such as unbalanced magnetron sputtering in a closed magnetic field configuration, thermionically enhanced deposition and closed loop control with optical gas and metal plasma emission monitoring. These techniques were combined in an active control system. Special software was used to provide automatic computer aided process control in the deposition of multilayer and multi-component coatings. The system has been evaluated on a range of refractory and DLC coatings and recommendations on process control are given.

I. Introduction Reactive magnetron sputtering is a powerful and universal tool for the production of a wide range of coatings and thin films for various applications’,2,3, particularly for the deposition of nitrides and carbides of transition metals (Ti, Cr, W, Zr, Nb, Hf, etc.) by introducing reactive gases (N2, C,H2, CH,) into the chamber to achieve reactive sputtering conditions. Two generations of reactively sputtered coatings are already in use in industry“, and the third generation-multilayer’ and superlattice coatings’, are now becoming commercially available. Early research work on the sputtering process showed the strong influence of process parameters on the structure and properties of coatings deposited. In terms of coating formation, the density, ionisation rate, composition and energy of depositing fluxes are decisive factors, which need to be controlled. Different arrangements of sputtering sources were developed in an attempt to increase sputtering rates and control the ion/neutral flux ratio. Magnetron sputtering enhanced by electrons trapped in the intersection of magnetic and electric fields proved to be the most successful in the pursuit of these aims. Further development of this concept lead to the design of unbalanced magnetrons with different magnetic field strengths for inner and outer magnets7,“, which were soon placed in the closed magnetic field configuration”3 to trap electrons in the middle of the chamber and increase the level of ionisation within the deposited fluxes by up to 100%‘4. The high level of ionisation simplified the control of the arrival energy of particles, which is achieved by varying the negative substrate bias. Plasma composition control in reactive sputtering is more complicated because of the presence of hysteresis in the reactive gas flow-pressure curve, due to the formation of highly refractory

compounds on the target surface (the target “poisoning” effect). Several systems were proposed to control the composition of depositing fluxes, which are based on using high sensitivity pressure gauges, quadropole mass analysers (QMA)15, optical emission gas control (OGC)16 or plasma emission monitoring (PEM)“. These systems are usually placed in a closed loop feedback with a gas flow controller, thus the desired partial pressure of the reactive gas can be maintained. The deposition of multi-component and multilayer coatings of the third generation requires the ability to rapidly change deposition flux density, composition, ionisation rate and arrival energy together with a high level of repeatability of all parameters from layer to layer and from process to process. In conjunction with this demand there is a need for active automatic control of the reactive process. In the present work we report on our development of such a system using recent advances in unbalanced magnetron design, plasma composition control, optical emission monitoring and the latest generation softwares for computer aided control. In Sections 2 and 3 the existing status of process control is briefly reviewed and Section 4 presents an active control system which we have developed.

2. Control of density and ionisation rates of depositing fluxes The operation of a magnetron is based on creating a plasma discharge in the vicinity of a target and accelerating ions from the plasma to the target to sputter its surface. Electrons trapped by the electromagnetic field close to the target provide ionisation of the working gas (usually Ar). Acceleration of ions is achieved by an electrostatic field generated either by a negative bias applied to the target in d.c. mode, or by a self induced negative bias in 723

A A Voevodin et al: Control of reactive sputter deposition

material: Ti Working gas: Ar Probe-target distance: Target

*.O

2 .E

-

J

I OOmm

1.5-

variation in the chamber have a negative effect on process stability and a non direct relationship exists between working gas pressure and the ionisation of deposited fluxes (Figure 2). It was shown in studies of unbalanced magnetrons by Window and Savvides’ that by varying the strength of the magnetic poles of a magnetron (Figure 3 (a) and (b)), high or low ionisation rates of deposited fluxes can be obtained. From the Type I (low

(a)

-1

3

2

4

5

Power applied

6

7

to magnetron

0

9

(kW)

Figure 1. Ion current to a Langmuir probe, plotted against power applied to a magnetron for sputtering Ti target in an Ar atmosphere.

r.f. mode. From this the following possibilities for the control of magnetron sputtering are available : (if control of power applied to the magnetron (d.c. or r.f.), (ii) control of working gas composition and pressure in the vicinity of the target, (iii) control of the strength and configuration of the magnetic field. The simplicity of controlling the power applied to magnetrons, together with the direct relationship between applied power and deposited flux parameters, including ion current to the substrate (Figure l), make it a common tool for sputter process control. The latest generation of magnetron power supplies allow selective control of power, current or voltage, from which power and current modes are generally used, since they maintain stability of the sputter rate. However, when the power applied to magnetrons is varied, all deposited flux parameters change simultaneously and it is not possible to achieve such extremes as high deposition rate in combination with a low ionisation rate of fluxes useful for some applications. The variation of the deposition rate without influencing the ionisation ratio of deposited fluxes can be achieved by changing the working gas composition, e.g. switching from Ar to He, or by varying (in a certain range) gas pressure in the vicinity of the target. However, procedures involving gas changing or pressure

Target material: Ti Working gas: Ar Probe-target distance:

.

Type II

1OOmm

.

. .

2-

0

. 2

I

Pressure

. 3

(mbar)

Figure 2. Ion current to a Langmuir probe plotted against Ar pressure for magnetron sputtering of Ti. 724

I

4

I

I

I

I

I

I .

\

3. Different types of unbalanced magnetrons (a) Type I and (b) Type II with permanent magnets after Ref. 7 ; (c) with electromagnets to achieve magnetron variation from Type I to Type II. Figure

A A Voevodin et al: Control of reactive sputter deposition

(a)

M

Figure 4. The basic form of magnetic

field in a mirrored (a) and opposed (b) configurations”.

ionisation rate and electron density of depositing fluxes) and the Type II (high ionisation rate and electron density of depositing fluxes) magnetrons introduced in their studies, the Type II magnetrons with a permanent magnetic field (Figure 3(b)) have already found wide applications in sputter deposition. However, there is still another attractive possibility for process control. The use of electromagnets to vary the strength of the magnetic field and change the magnetron type from I to II during the deposition process (Figure 3(c)) allows the control of the ion and electron content in deposition fluxes together with high process stability. Since the strength of the magnetic field is controlled by the current in electromagnet coils, such a system is suitable for automatic computer aided control and could be a powerful tool in producing multilayer coatings with deposition parameters variable from layer to layer. Another possible way of influencing the ionisation rate of depositing fluxes is to change the magnetic field configuration in the vicinity of the substrates by placing unbalanced magnetrons in the mirrored (Figure 4(a)) or in the opposed configuration (Figure 4(b))“. If the outer magnet poles are strong enough to provide a closed magnetic field around the chamber perimeter in the opposed configuration, then electrons trapped in the middle of the chamber considerably increase the ionisation of the depositing fluxes. This has been reported by a number of researchers’ 12. The closed magnetic field configuration can however be considered as a static means of control, and from the viewpoint of active control the introduction of an additional source of ionising electrons is important. That was done in the present work using

a hot filament electron emission source placed close to the substrate. The thermionic source provides control of the ionisation of depositing fluxes independently of magnetron power and the configuration of the magnetic field. It is easy to achieve, reliable in operation and provides a direct relationship between filament negative bias and/or filament current and the ion density of the depositing fluxes (Figure 5)

6 Target material: Ti 5 _ Working gas: Ar Probe-target distance:

1OOmm

4-

3-

0

i 10

I 20 Filament

I 30 negative

I 40

I 60

I 50

bias voltage

I

0

(VI

Figure 5. Plasma ion density in the vicinity of the substrate as a function of bias voltage of a hot electron source obtained with a Langmuir probe.

725

A A Voevodin

et al: Control of reactive sputter deposition

3. Composition control of depositing fluxes in reactive sputtering

Composition control of depositing fluxes in reactive magnetron sputtering was, until recently, mainly achieved by monitoring the partial pressures of gases in the vacuum chamber using QMA based systems in combination with highly sensitive pressure gauges and gas mass flow controllers. However, such systems have some drawbacks-such as delayed response times and signal disturbance due to the differential pumping required for the QMA’“. Recently OGC and PEM systems based on optical emission plasma analysing were introduced to control gas composition in reactive sputtering. In the work by Sproul et al” an OGC system was placed in the closed feedback loop with the reactive gas mass flow controller (Figure 6). Stable and reliable partial pressure control without significant time delays was achieved with this system’6.‘9. The use of the OGC system allowed direct investigations of partial pressures in front of the target, from which the variation of partial pressure of reactive gas with the changing magnetron power was found16. Thus, correction of the partial pressure of a reactive gas during sputtering by variation of magnetron power must be carried out to maintain constant coating stoichiometry. The correction of reactive gas partial pressure depending on the magnetron power can be achieved in an automatic mode using a PEM based control system. This system, being of the same configuration as shown in Figure 6, uses the optical emission from sputtered metal atoms, rather than the emission from gas atoms as used in the OGC. Calibration of the relative intensity of a selected emission line, as a criteria of metal composition in the depositing fluxes, provides effective control of coating stoichiometry. The advantage of the PEM is the possibility of sputter control at large flows of reactive gas, when targets manufactured from transition metals develop refractory compounds on their surface leading to a considerable decrease in target sputter yield. This advantage was used by Bewilogua and Dimigen’” to deposit hydrocarbon diamond-like films with controlled tungsten incorporation, by enriching tungsten targets with carbon at large flows of acetylene. The PEM, however, has one potential drawback, which is the need to check the 100% level intensity of a monitored emission line, before introducing reactive gas into the chamber. This can be done during deposition of the thin adhesive

Control valve

metal interlayer which is usually carried out prior to the deposition of ceramic layers. Another way can be to use magnetron shutters at the beginning of the process to perform PEM calibration. 4. Closed loop active control system The advances in process control of reactive sputtering discussed in Sections 2 and 3 were used to build an active control system. In the initial stage of the system design a computer simulation of the opposed magnetron sputtering system was performed using a LabView software package. Random numbers were used to imitate the work of the magnetrons and vacuum system, and different instabilities in the process control were analysed to optimise control procedures. The results of computer simulations were taken into account in the design and development of an active control system for reactive magnetron sputtering as presented in Figure 7. A dual sputtering system was used to allow deposition by an unbalanced closed field sputtering process. Two rectangular magnetrons with targets 10.16 x 38.1 cm were installed in an opposed configuration at a distance of 440 mm apart. Magnets of NdBFe were used for the outer poles and soft iron bars for the inner poles of the magnetrons to put them in a strongly unbalanced mode with magnetic lines arranged as shown in Figure 4(b). The magnetrons are powered by MDX-10 and ENId.c. supplies. Substrate bias is achieved either by an MDX-10 power supply during the etching procedure, or by a low voltage power supply with fine regulation of negative voltage in the range O-250 V. A negatively biased hot filament tungsten wire placed above the substrate provided electron emission into vicinity of the substrate. The substrate table has variable rotating speed and a thermocouple clamped to it to monitor the temperature during coating deposition. A 0.2 m focal length Verity Instruments optical emission spectrometer was used to achieve the PEM control. The spectrometer was used in conjunction with a Reactaflo system from Megatech Ltd. and run with SCANVIEW III software either in spectral analysing mode, with a resolution of 0.2 nm in the range of 300900 nm, or in the fixed line monitoring mode. When running in monitor mode, a high speed piezo valve introduces reactive gas

Mass flow meter

Figure 6. Schematic of a reactive magnetron sputtering system with OGC control of reactive gas flow16. 726

A A Voevodin et a/: Control of reactive sputter deposition

I Hotfilament I

-

Nz

C2”2

controllers Pressure iwges

-

%

Collimator tube

Piezo valve

Computer process control

I

I

set

Figure 7. Schematic of an active control system for reactive magnetron sputtering.

into the chamber and the software controls it of the PEM signal matches a preset intensity coating stoichiometry. During the deposition nitrogen and acetylene were premixed to the being fed into the piezo-valve and Ar was ventional MFC-280 flow controller.

so that the intensity to maintain desired of carbonitrides the desired ratio before fed through a con-

Software based on the LabWindows package was developed to run the magnetrons. It allows the programming of the change of output magnetron parameters in accordance with the desired coating composition to produce multilayered structures and has a possibility to evaluate and correct magnetron status, in the case of process instability (i.e. changing arc suppression parameters,

Table 1. Coatings deposited with the developed control system, their composition, thickness and deposition rate

Coating name TiN TG 25N0 75 TG sN,, 5

Tic,, A% 25 TIC CrN TiCrN TiCrC TiCrCN Ti 150,.-DLC Ti,,,.,-DLC Ti Z,.,-DLC Ti ,,,-DLC Ti,,-DLC

Thickness (pm)

Deposition rate (nm/min)

pm) +Ti?,,-

3.2 3.0 3.0 3.0 3.1 4.1 4.4 4.2 4.2 4.1

35.56 33.34 33.34 33.34 34.44 45.56 48.89 45.68 46.67 45.56

pm) +Ti,,,,-

3.1

45.56

pm)+TizOa,a-

3.0

44.44

pm)+Ti,,,-

3.0

44.44

pm) +Ti,,-

3.1

45.56

Coating composition Ti(0.2 Ti(0.2 Ti(0.2 Ti(0.2

pm) pm) pm) pm)

+TiN(2.6 pm) +TiCN(2.8 pm) +TiCN(2.8 pm) +TiCN(2.8 pm)

Ti(0.2 pm) +TiC(2.9 pm) Cr(0.2 ,um)+CrN(3.9 pm) TiCr(0.3 pm) +TiCrN (4.1) TiCr(0.3 pm) +TiCrC (3.9) TiCr(0.2 pm)+TiCrN(O.l pm) +TiCrCN(O.lpm)+TiCrC(O.I pm) fTiCrCN(3.7 pm) Ti(0.2 pm) +TiN(O.I pm) +TiCN(0.3 pm) +TiC(0.2 pm) +Til,./,DLC(0.9 pm) +TiC(0.4 DLC(I.0 pm) Ti(0.2 pm) +TiN(O.l pm) +TiCN(0.3 pm) +TiC(O.Z pm) +Ti,,,DLC(0.9 pm) +TiC(0.4 DLC(I.0 pm) Ti(0.2 pm)+TiN(O.l pm) +TiCN(0.3 pm) +TiC(0.2 pm)+Ti,,,DLC(0.9 Fm)+TiC(0.4 DLC (0.9 pm) Ti(0.2 pm)+TiN(O.l pm)+TiCN(0.3 pm)+TiC(0.2 pm)+Ti,,,DLC(0.9 pm)+TiC(0.4 DLC(0.9 pm) Ti(0.2 pm) fTiN(O.1 pm)+TiCN(0.3 pm)+TiC(0.2 pm)+Ti,,DLC(0.9 pm)+TiC(0.4 DLC(l.0 pm)

727

A A Voevodin et al: Control of reactive sputter deposition Ti(1)

Channel

A signal

80

Ar(II) Ti(I)

60.

Ti(1)

‘,

Ti(1) Ti(1)

h I

I

400

I

/

450

500

Wavelength Figure 8. Optical emission spectra taken from the vicinity of the cathode fo; large a&J small flows of C2H2.

drift evaluation and ramp parameter corrections). A thermionic source operating independently of magnetron output provides the required level of ionisation, which is monitored as the ion current to the substrate and controlled by variation of the filament heating current and negative bias. To provide automatic control of the vacuum system, the chamber was equipped with Edwards Active Gauges, permitting direct communication with a controlling computer. Special procedures were written to control operation of valves and pumps to maintain the desired pressure in the chamber. Additionally a QMA was used to check the partial pressure of water in the chamber before starting coating deposition. The active control system developed was tested in the deposition of a number of coatings, which are listed in the Table 1 together with their composition and deposition rate. The properties of these coatings are reported in Ref. 20. Here several comments on process control during deposition of these coatings are given. Deposition of TIN, TIC, TiCN, CrN, (Ti,Cr)N and (Ti,Cr)CN coatings did not show instabilities in the control of deposition flux density, ionisation and composition. The only problem occurring was a drift of magnetron output voltage with time when magnetron power (or current) exceeded a certain level. System checks revealed a non sufficient cooling of targets, clamped to water cooled copper backplates. The direct cooling of targets is recommended to stabilise the thermal balance of the targets during sputtering. In the deposition of TiYTjO-DLCcoatings with large flows of acetylene, special attention must be given to the selection of the emission line to be monitored. Thus, when depositing Ti,,,-DLC coatings with C2H, flows of 160 seem an instability in the process was detected and more thorough studies of the emission spectra showed partial overlapping of Ti lines in the selected region with the emission from acetylene (Figure 8). Alternatively, Ti lines in the region of 336 nm and 519 nm were proposed for use and line intensities were plotted against acetylene flow (Figure 9). Results confirmed the influence of acetylene on 396-399 nm lines at flows larger then 100 cm’ s-’ and showed that emission in the region voltage

728

in the wavelength

region of 300-550

nm

of 336 nm is more favourable for control during the production of Ti,,-DLC coatings. In closer investigations the 336 nm region was found to consist of a Ti excitation line of a wavelength of 337.1 nm and a Ti” ionisation line of 334.9 nm (Figure 10). To deposit Ti,,-DLC coatings given in Table 1 the Ti excitation line was used. 5. Summary A review of recent advances in reactive magnetron sputter control showed a possibility for the development of an active control system to achieve wide variation in density, ionisation and composition of depositing fluxes. The use of unbalanced opposed magnetrons in a closed magnetic field configuration and with an additional source of electrons provides effective control of plasma ionisation rate, which is independent of the power applied to the magnetrons. The use of a closed loop system to control plasma

I q

Ti 336 nm

A Ti 396-399

nm

o Ti 519.3 nm o CH387

20

40

60

80

Acetylene

100

nm

120

140

160

flow (seem)

Figure 9. Intensities of selected lines taken from vicinity of the cathode during discharge of a Ti target with different flows of acetylene (as before for W-target in Ref. 19).

AA

Voevodin

et

al: Control of reactive sputter deposition Channel A signal Ti(1)

Ti(II) 337.1 nm --\--I

334.9 nm.

80c

i2

Ti(1) 318.5 nm

.““’ ...,...

v)

-ii 6 .-JO

300

310

320

330

340

350

Wavelength Figure 10. Ti optical emission spectra in the range of 30&350 nm with a 337.1 nm line used for process control in Ti,,-DLC coating deposition

composition based on emission spectroscopy of species in the plasma gives a direct high speed response in the control of

depositing flux composition and coating stoichiometry and allows controllable reactive sputtering with high degrees of target “poisoning”, if the emission line to be monitored is carefully selected. The combination of these advances in a computer based control system, running on specially developed software, allowed reliable control of reactive sputtering. The necessary corrections of deposition parameters are made automatically in this system according to program instructions. The developed active control system proved to be a powerful tool in the deposition of multilayer and multi-component coatings of different composition. Acknowledgements

The support of the UK Engineering and Physical Sciences Research Council (EPSRC) and industrial sources is gratefully acknowledged, The authors also express their gratitude to T Pawson and J Robson for their help in equipment modification, and S Dowey for advising on computer control system developments.

‘5 A Thornton, SurfEng, 2,283 (1986). ‘G E McGuire (ed) Semiconductor Materials and Processing Technologies, Park Ridge, N J, Noyes (1986). a A Leyland, K S Fancey and A Matthews, Surf Eng 7,207 (1991). ’ C Subramanian and K N Strafford, Wear, 165,85 (1993). ‘X Chu, S A Barnett, M S Wong and W D Sproul, Surf Coat Technol, 57, 13 (1993). ’ B Window and N Savvides, J Vat Sci Technol, A 4, 196 (1986). ’ B Window and N Savvides, J Vat Sci Technol A 4,453 (1986). 9W D Sproul, P J Rudnik, M E Graham and S L Rohde, Surf Coat Technol, 43144,270 (1990). IaS Kadlec, J Musil and W D Mtinz, J Vat Sci Technol A 8,13 18 (1990). “W-D Mtinz, F J M Hauzer, D Schulze and B Buil, Surf Coat Technol, 49, 161 (1991). I2I Efeoglu, R D Arnell, S F Tinston and D G Teer, Surf Coat Technol, 57,61 (1993). I3D Palicki and A Matthews, Finishing, 11,37 (1993). “B Window, Proc. 2nd Australian Int Conf SurfEng, University of South

Australia, Adelaide (1994). “W D Sproul, Surf Coat Technol, 33,13 (1987). I6W D Sproul, P J Rudnik and C A Cogol, Thin Solid Films, 171, 171 (1989). “S Schiller, U Heisig, K Steinfelder, J Strtimphel, R Voight and G Teschner, Thin Solid Films, 96, 235 (1982). “A Matthews, K S Fancey, A S James and A Leyland, Surf and Coat

References

Technol, 61, 121 (1993). I9K Bewilogua and H Dimigen, Surf and Coat TechnoI 61, 144 (1993).

ID S Rickerby and A Matthews (ed), Advanced Surface Coatings: A Handbook of Surface Engineering, Blackie, Glasgow (1991).

“A A Voevodin, C Rebholz, J M Schneider, P Stevenson and A Matthews. Surf‘and Coat Technol(l995), in press.

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