Pulsed-bias Sputter Deposition of Chromia and Alumina films at Low T

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This work uses a novel approach of combined medium frequency (20 – 350 kHz) asymmetric bipolar pulsed magnetron sputtering (pulsed-DC) and pulsed substrate biasing to reactively deposit Cr2O3 and Al2O3 thin films at substrate temperatures of the order of 100 °C. Usually in such cases, the target and the substrate are pulsed at the same frequency [single-frequency (1F) mode] in either synchronous (master-slave) or asynchronous mode. The latter ... Shared on http://www.vacuumcoating.info

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Pulsed-bias sputter deposition of chromia and alumina films at low substrate temperature:

Pulsed-bias sputter deposition of chromia and alumina films at low substrate temperature M Audronis , A Matthews, A Leyland Department of Engineering Materials, The University of Sheffield, Sir Robert Hadfield Building, Mappin Street, Sheffield, S1 3JD, UK 1

Presentation Outline:

Presentation Outline I. Introduction Cr 2 O 3 and Al 2 O 3 coatings; Routes towards enhanced crystallisation of dielectric oxide thin films; Biased Pulsed-DC processing. II. Aims III. Experimental details Coating deposition & characterisation. IV. Results XRD, SEM and Hardness testing. V. Conclusions 2

Introduction:

Introduction 3

Cr2O3 and Al2O3 coatings:

Cr 2 O 3 and Al 2 O 3 coatings Properties High hardness (25 - 30 GPa), good wear and corrosion resistance, high dielectric constant, etc. Applications Wear and corrosion protection; Electronics, optics, etc. Problems Deposition of crystalline coatings of stoichiometric composition requires high T s . 4 for example see J.M. Schneider, W.D. Sproul, A. Matthews, SCT 98/1-3 (1998) 1473; P. Hones, M. Diserens, F. Levy, SCT 120-121 (1999) 277; J. Wang, Y.H. Yu, S.C. Lee, Y.W. Chung, SCT 146-147 (2001) 189. Chromia powder Alumina powder

Routes towards deposition of crystalline oxide films:

Routes towards deposition of crystalline oxide films Most often practiced methods : Application of negative bias [e.g. RF, (20-350kHz) pulsed-DC] to the substrate; Substrate heating; RF plasma enhancement (as in IPVD); Combination of abovementioned techniques is practised.  more complicated/expensive process. 5

Biased asymmetric bipolar pulsed-DC processing:

Biased asymmetric bipolar pulsed-DC processing One frequency (1F): synchronous / asynchronous. Time Voltage Target Substrate 0V 0V 6 J.M. Schneider, W.D. Sproul, A. Matthews, SCT 98/1-3 (1998) 1473; P.J. Kelly, R. Hall, J. O'Brien, J.W. Bradley, P. Henderson, G. Roche, R.D. Arnell, JVSTA 19/6 (2001) 2856. F t =F s Dual frequency configuration ( 2F-pulsed-DC ). Simple and easy to scale up. Voltage Time Target Substrate 0V 0V F t ≠F s

II. Aims:

II. Aims To deposit chromia and alumina coatings at low T substrate ; To investigate the structure and properties of the films; To investigate the capability of 2F-pulsed-DC configuration and to compare it to the conventional 1F/synch-pulsed-DC. 7

III. Experimental details:

III. Experimental details 8

Coating deposition:

Coating deposition Two ENI RPG-100E PS; F t = 130 kHz; F s = 250 kHz; Working pressure: ~1.3 Pa; Target- substr . dist.- 14 cm; OEM - tuned to the 521 and 396 nm lines of the Cr and Al emission spectra; I Cr,Al = 25 % & 50 %; V s = ‘Floating’ – -100V; T s = 90 – 170  C. Cr/Al 9

Coating characterisation:

Coating characterisation SEM: JEOL JEM6400 operating at 20kV; XRD: Siemens D5000 ( q -2 q + glancing angle.); Hardness testing: nanoindentation (Hysitron Triboscope) + microhardness (Knoop). 10

IV. Results:

IV. Results 11

Target and substrate voltage waveforms:

Target and substrate voltage waveforms Voltage overshoots occur at both the target and the substrate and in both, the positive and the negative, directions Depend on the impedance of the PS, the connections, etc. 12

Morphology of coatings:

Morphology of coatings T s  117  C. T s /T m = 0.15. T s  136  C. T s /T m = 0.17. 13 Cr 2 O 3 Al 2 O 3 SEM micrographs show the fracture cross-section of Cr 2 O 3 and Al 2 O 3 coatings, produced in ‘2F’ mode at I Cr/Al = 50 %, V s = -20 V.

Structure of Cr2O3 coatings I:

Structure of Cr 2 O 3 coatings I Comparison of GAXRD spectra: 2F vs. 1F/synch @ I Cr =50%. Comparison of GAXRD spectra: 2F vs. 1F/synch @ I Cr =25%. 2F vs. 1F/synch 14 Generally more crystalline coatings are produced if ‘2F’ configuration is used.

Structure of Cr2O3 coatings II:

Structure of Cr 2 O 3 coatings II GA XRD spectra of Cr 2 O 3 coatings produced in ‘2F’ mode @ I Cr = 50 %. crystalline Cr 2 O 3 coatings were produced at T s as low as 90  C; Most crystalline coatings produced at -20 V. Too low E ion results in less crystalline coatings Too high E ion , even though resulting in higher T s , causes film amorphisation . 15

Structure of Al2O3 coatings:

Structure of Al 2 O 3 coatings XRD spectra of Al 2 O 3 coatings produced at in ‘2F’ mode @ I Al = 25 %. Al 2 O 3 coatings containing crystalline constituents were produced at T s = 124-158  C; Reflection from nanocrystalline Al 2 O 3 peaks at -30 V to -40 V. Too low E ion results in amorphous coatings Too high E ion , even though resulting in higher T s , causes film amorphisation. 16

Deposition rate: chromia:

Deposition rate: chromia 17 deposition rate of Cr 2 O 3 coatings, produced in ‘2F’ mode with I Cr = 50 %, is increasing with increasing V s ; This is opposite to the trend observed when operating in 1F-synch pulsed-DC mode; Reason: enhancement of the plasma (due to ‘2F’ configuration) and associated effects. Not observed at I Cr = 25 % (target is almost fully poisoned at this value of I Cr ).

Deposition rate: alumina:

Deposition rate: alumina 18 Deposition rate (in 2F mode) is not increasing with the negative substrate bias voltage, and is rather constant until V s  -40 V; after this a conventional decrease in deposition rate is observed.

Deposition rate: overall results:

Deposition rate: overall results Improvement, when operating in 2F-pulsed-DC mode, is obtainable until a certain value of V s is reached; As this value is exceeded the conventional trend proceeds. Detailed process studies are necessary in order to understand these observations. 19

Hardness of coatings:

Hardness of coatings a -Cr 2 O 3 - nanoindentation hardness: 24 - 27 GPa; Al 2 O 3 - Knoop microhardness: ~ 30 GPa. 20

On the effectiveness of ‘2F’ processing:

On the effectiveness of ‘2F’ processing  Detailed plasma characterisation required. 21 Plasma enhancement by oscillating substrate potential; hence enhancement in deposition rate; It allows the whole range of charged species present in a discharge to be used in beneficially modifying coating growth conditions.

Conclusions:

Conclusions Up to 27 GPa hard, crystalline Cr 2 O 3 coatings were produced by biased pulsed sputtering in ‘2F’ configuration at T s as low as ~ 90  C. ~ 30 GPa hard Al 2 O 3 coatings containing some crystalline constituents were obtained at a substrate temperature in a range of 124 - 158  C. Generating optimal ion bombardment conditions at the growing film surface is a critical factor in defining the structure of Al 2 O 3 and Cr 2 O 3 coatings. ‘2F’ bipolar pulsed-DC configuration enhances the deposition process and provides better control of film structure. 22