PLASMA PROCESSING UPDATE

A newsletter from the

Facilitation Centre for Industrial Plasma Technologies,

Institute for Plasma Research

Issue 47 September - December 2004

Contents

Editor's Note

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Zirconium Thin Films Deposited by the DC Magnetron Sputtering Technique

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Plasma Nitriding for Textile Industries

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Atmospheric Pressure Glow Discharge Plasma

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Plasma Nitriding and its Variant Processes

Editor's note

Sputtering is a process, in which atoms are removed from the solid surface by the bombardment of energetic ions. Although thin films of many materials have been successfully deposited by sputtering technique, this process has low deposition rates, low ionisation efficiency of the plasma, and substrate over-heating effects. However, these limitations have been overcome by the so-called magnetron sputtering deposition technique. Dr. Jay Chakraborty describes the preparation and characterization of Zr films by dc magnetron sputtering method.

Textiles account for 20 per cent of India’s industrial production and around 27 per cent of its export earnings. From growing its own raw material (cotton, jute, silk and wool) to providing value added products to consumers (fabrics and garments), the textile industry covers a wide range of economic activities, including employment generation in both organized and unorganized sectors. The growth of the textile industries is largely dependent on the availability of good textile machineries. Since, the wear of the textile machinery parts is inevitable with time, there are many techniques adopted to enhance the life of the components. Application of plasma based diffusion processes is one such solution to combat wear and Alphonsa Joseph sheds more light on the beneficial aspects of these processes.

Glow discharge plasma find applications in various material processing and surface engineering technologies. However, the creation and sustaining of glow discharge plasma requires lower operating pressures and hence calls for vacuum compatible chambers and vacuum pumps. These requirements set a limitation on the overall size of the components to be treated. Though atmospheric pressure corona discharge alleviates these problems, the process itself suffers from non-uniformity of treatment and doesn’t allow much control of the discharge physics and chemistry. Hence, a relatively recent invention of an apparently uniform, non-thermal, glow discharge that can be created at atmospheric pressure has received much attention. However, it is done, if one uses only reactive gases diluted with large amounts of helium, a high frequency generator more than 1 kHz and electrodes whose surfaces are covered with dielectric plates, an atmospheric pressure glow plasma can be generated stably and uniformly at room temperature. Mr. Anand Kumar Srivastava reports on various applications of the atmospheric pressure glow discharge plasma.

Engineered surfaces are critical to the performance of many commercial products. The vast majority of engineered surfaces are developed empirically for specific components and materials, and tend to represent an incremental improvement. Plasma nitriding and its variant processes are also aimed to achieve enhanced surface properties. The unique property of these methods is that all these processes can be done in the same vacuum system but by using different gas compositions. Mr. Ganshyam Jhala gives a brief description of all the variant processes.

Alphonsa Joseph

Editorial Assistance: A. Satya Prasad

Facilitation Centre for Industrial Plasma Technologies

The Institute for Plasma Research (IPR) is exclusively devoted to research in plasma science, technology and applications. India’s  first high temperature plasma device `Aditya Tokamak’, built at IPR, produces plasmas at 5 million degrees temperature - comparable to that of the sun.  An advanced fusion device with superconducting magnets, capable of steady state operation is under fabrication. IPR is also active on a broad front of fundamental studies. It has engineering groups skilled in technologies of Superconducting Magnetics, Ultra High Vacuum, Pulsed Power, Microwave and RF, Computer-based Control and Data Acquisition, theory and computer simulation complement experimental programmes.

The Facilitation Centre for Industrial Plasma Technologies (FCIPT) links the Institute with the Indian industries and commercially exploits the IPR knowledgebase. FCIPT interacts closely with entrepreneurs through the phases of development, incubation, demonstration and delivery of technologies. Complete package of a broad spectrum of plasma-based industrial technologies and facilitation services is offered.

Some of the recent FCIPT achievements are: plasma nitriding of industrial components to increase wear resistance and hardness, coating of quartz-like films on brassware to inhibit oxidation and tarnishing, ceramic synthesis and processing, plasma ion implantation and ion plating for surface engineering, thermal plasma technologies for smelting of minerals and waste treatment etc.

The Centre has process development laboratories, jobshops and material characterisation facilities. The process development laboratory exploits the areas of expertise in plasma and other allied fields of the institute in developing new plasma based technologies for the industry. The jobshop executes job work for surface and material treatment on an industrial scale to promote the acceptance of plasma based technologies and to generate techno-commercial data relevant to entrepreneurs. The advanced instruments in the material characterisation facility are open to users from industry, research establishments and universities.

This newsletter is designed to help you keep abreast with the developments in the important field of plasma assisted manufacturing and  to look for new industrial opportunities. We would be  very happy to have you write to us on ways of improving this service or visit us for further discussions.

Please visit our website: http://www.plasmaindia.com

Zirconium Thin Films Deposited by DC Magnetron Sputtering Technique

Introduction

Thin films are extensively used in many technical devices (for example in microelectronics). Device performance depends on the crystallographic structure and microstructure (instead of ‘microstructure’, one should rather say ‘nanostructure’) of the films. Therefore, deposition of thin films and their structural characterization is extremely important for the development of many technical devices. Apart from the technological importance, thin films are the key materials for studying diffusion, phase transformations and many interfacial phenomena of fundamental interest. Many of these phenomena are thermodynamically irreversible in nature. Thin films often can sustain very high amount of residual stress (~1GPa) and they also have crystallographic texture. Presence of residual stress and crystallographic texture in thin films are due to the very nature of nucleation and growth process during their deposition on the substrates. Among the various techniques of film deposition, magnetron sputtering is one of the most frequently used methods for the deposition of films of various thicknesses with varied microstructures (grain size, shape and their distribution, residual stress, crystallographic texture etc.). Both dc (direct current) and rf (radio frequency) magnetron sputtering techniques can be used for thin film deposition. During deposition, substrates can be electrically biased either by a continuous dc (or rf) voltage or by pulsed dc (or rf) voltage which in turn may give rise to thin films of various crystallographic structures and microstructures. The present work consists of an investigation in this area. Investigation of thin films deposition of zirconium metal and their structural characterizations have been performed. DC magnetron sputtering technique has been used for Zr films deposition. X-ray diffractometer (XRD) and secondary electron microscope (SEM) have been used for the structural and microstructural characterization of the films. There exist several reports [1-3] on this particular topic although they lack a systematic and detail analysis of their experimental results.

In the present work, a thorough analysis of the XRD results has been performed. Apart from phase analysis, attempt has been made to determine the crystallite size and microstrain in Zr fims from the x-ray diffraction pattern. High precision diffraction measurements have been performed for the determination of residual stress and crystallographic texture. Diffraction results have been correlated with the microstructure (nanostructure) observed by SEM. In this article, only phase analysis, crystallographic textures and residual stresses in the Zr films have been reported.

Experimental details:

A. Deposition of Zr thin films by dc magnetron sputtering

DC magnetron sputtering technique has been used to deposit zirconium thin films on glass substrates. The experimental setup consists of vacuum chamber and accessories, (see Fig.1) a planar magnetron, power supplies, etc. Argon gas is used to produce plasma and Ar⁺ ions are used for sputtering of the target atoms. A current density of 12 mA/cm² is attained at the cathode surface. The zirconium target has been biased with -470V dc Voltage. The substrate is kept at room temperature during deposition. Zirconium films of various thicknesses (a few nanometers to a few micrometers) have been deposited by varying the deposition time from 5min to 20min. Precise determination of thicknesses of the films are s

Chamber

Substrate

Holder

RF Supply

Pirani Gauge

Penning Gauge

Magnetron

Magnetron Bias

- DC 0 - -1000V

Pirani Gauge

Baffle Valve

Diffusion Pump

Rotary Pump

Substrate

Plasma

Gas Dosing Valve

till under investigation.

Fig.1: Schematic of the magnetron sputtering deposition system

B. Characterization of Zr films by X-ray diffraction

1. X-ray diffraction of Zr thin films: phase analysis

X-ray diffraction is widely used as a non-destructive technique for the structural and microstructural characterization of materials. In the present investigation, a Seifert X-ray diffractometer (PTS 3000 goniometer) has been used in the Bragg-Brentano geometry for characterizing the Zr films. CuKα (strictly not monochromatic, a mixture of Kα₁and Kα₂ radiations) radiation has been used as an x-ray source and a scintillation detector has been used for the detection of the diffracted beam. Diffraction is governed by the well known Bragg’s law which is as follows [4]:

where ‘ ’ is the order diffraction. l is the wavelength of characteristic X-radiation (for example, CuKα₁ radiation l =1.54056Å). d is the distance between a set of parallel lattice planes (interplanar spacing). ‘ ’ is the angle between the incident X-ray beam and the set of parallel lattice planes inside the crystal. Each set of parallel plane has a particular d-spacing from which the reflection occurs. The position of the peak i.e. value, depends on this particular ‘d’ value according to Bragg’s law. Therefore the XRD patterns consist of a series of peaks from the various sets of parallel planes of Zr appearing at different s. The peak intensity of a diffraction peak depends on atomic position (and therefore the crystal structure) inside the material.

Experimentally obtained diffraction patterns of zirconium films deposited for various deposition times have been matched with the standard database (so called JCPDS data base) corresponding to the bulk Zr of different crystal structure to confirm the formation of single phase Zr .

2. Diffraction analysis of residual stress in Zr thin films

In a polycrystalline material, if the strain is uniform over a large range, then the interplanar spacing (for a particular set of plane hkl) in the grains change from their stress-free value to newer value corresponding to the stress applied. This new spacing is essentially constant from one grain to another for any particular set of planes. This uniform strain causes a macroscopic shift of the diffraction peak to new 2θ position. This macrostrain can be determined by measuring the peak shift by the diffraction technique. Then it is possible to determine the stress within the film by multiplying the strain by the so-called x-ray elastic constants [5]’ of the polycrystalline materials. However, it should be noted that the precise determination of x-ray elastic constants of a polycrystalline material is dependent on several models (like Voigt, Reuss etc.) and the assumptions already implied in these models. X-ray elastic constants should not be used in case the polycrystalline materials are textured. A more sophisticated determination of elastic constants needs the use of so-called ‘stress factor’. In the present work, diffraction stress analysis is being carried out by using x-ray elastic constants of Zr.

3. Measurements of crystallographic texture in Zr thin films

Crystallographic textures in the Zr films have been measured by conducting ‘psi’ scans for and reflection of Zr using CuKα radiation in the point focus mode of the X-ray tube in the Seifert diffractometer equipped with an Eulerian cradle and a point collimator in the primary beam optics. This kind of texture measurement (only psi scans) is only valid if the Zr films have so-called fiber textures. Complete pole figure measurements for these reflections are necessary to confirm the complete nature of the texture which is presently being carried out.

4. Microstructural investigation by SEM

Microstructural investigation of Zr thin films has been performed by Scanning Electron Microscopy (SEM). Study of thin film growth, surface topography (roughness etc.) grain size, shape and their distribution can be performed by this technique.

Results

1. X-ray diffraction of Zr thin films: phase analysis

Analysis of diffraction patterns of Zr thin films deposited for various times shows that almost single phase polycrystalline hexagonal zirconium has been produced. Presence of small amount of tetragonal ZrO₂ has also been detected. Fig.2 shows the plots of the diffracted intensities versus for Zr films deposited for various durations.

Fig.2: Diffraction patterns of Zr films deposited for various durations. A: 5 min. deposition. B: 10 min. Deposition. C: 20 min deposition D: 30 min deposition. E: 40 min deposition.

It is also clear that there is a strong overlapping between two main reflections of zirconium. This can be also due the presence of small amount of ZrO₂ having ‘d’ values somewhere intermediate between these reflections. All diffraction patterns show that the Zr films have preferred orientations (crystallographic texture).

2. Residual stress in Zr films

Significant amount of shifts in the peak positions for both 0002 and reflections of Zr films have been observed. This shows that significant amount of residual stresses are present in Zr films. Stress analysis by x-ray diffraction has been carried out by the so called method [5]. Fig.3 shows plots for the peak shifts for and 0002 reflections for Zr thin film deposited for 5 minutes. It can be easily seen that due to the presence of crystallographic texture, intensities from only selected orientations have been obtained. Neerfeld-Hill X-ray elastic constants for polycrystalline Zr have been calculated from single crystal elastic constants of Zr using Voigt and Reuss model [5]. However, this detail analysis is time consuming and at present it is being carried out.

Fig.3: Macroscopic peak shifts (measures of residual stresses) in Zr films at various tilt angles.

3. Texture in Zr films

Fig.4 shows psi ( ) scans , plots of normalized intensities versus sample tilt angles (this is the angle between the sample (Zr) normal and the diffraction vector) for 0002 reflections for Zr films deposited for various durations. Presence of peak at and near shows that all Zr films have 0002 orientations i.e. basal planes of hexagonal Zr grains in all the films are parallel to the films (sample) surfaces. Pole widths (half width at half maximum) for different Zr films can also be seen from the plots. It can be seen that pole width for 5min deposition is considerably high (16.26^o) and it decreases in the subsequent deposition time up to 30 minute. However, for 40 min. deposition the pole width is considerably high which is 23.46^o. Pole width is a measure of the strength of the texture. Strongest texture means lowest pole width and vice versa. Therefore, initially for 5min. deposition, more grains are randomly oriented (other than 0002) compared to the subsequent period of deposition 10min., 20min.,30 min. etc. although 0002 orientation was always preferred.

Another important observation is the presence of an hump close to 70o psi angle which indicates the presence of other orientation. Theoretical calculation indicates that this is from which 0002 reflection has come.

Fig. 4: Crystallographic texture in Zr films: Psi ( ) scan (see the text) for reflection.

4. Microstructure of Zr films

Fig.5 shows the surface of the Zr thin film deposited for 20minutes. Nanometer size islands are clearly visible on the surface of the film. Detailed study of the surface roughness, island distribution, heights etc. are presently being carried out.

Fig.5: SEM micrograph of Zr thin film (for 20minutes deposition) Average island size is 40nm

Conclusion

Zirconium thin films have been deposited for various durations by dc magnetron sputtering technique. Resulting thin films have been characterized by XRD and SEM for structural and microstructural characterization. Crystallite sizes and microstrains in the films have been determined by Williamson-Hall method although they are not presented. All the films have crystallographic textures within them. Mainly 0002 and orientations are there although other orientations are also possible which can only be determined after complete pole figure measurements. All Zr thin films have large amount of residual stresses in them. Residual stress analysis is presently being carried out by the so-called method.

References:

1. Aouadi S.M. et.al., Surface & coatings Technology, 187 (2004), 177-184

2. Mitsuo A. et.al. Nuclear Instruments and Methods in Physics Research B 206 (2003) 366

3. Aouadi S.M. et.al., Vacuum, Article in Press.

4. X-ray diffraction, A Practical Approach, Suryanarayana C. and Norton G.M., Plenum Presss. New York

5. Residual Stress, Noyan and Cohen.

Plasma Nitriding for Textile Industries

Textiles, one of the oldest industries in India, are the backbone of the national economy. It accounts for about 20% of the nation’s industrial production and contributes almost one third of its foreign exchange earnings. The industry is the largest employer in the private sector and the second largest in the country after Railways.

Fig. 1 Textile machinery

In textile machines, as shown in Figure 1, minute fiber particles can increase abrasive wear by several orders of magnitude. The profitable operation of modern, high productive textile machines depends, to a large extent, on how well the key components that make contact with the fibers and threads resist wear and tear.  At many stages in the processing line, the surface finish has a direct influence on the quality of the final product. Hence, in order to improve the quality of the final product and the life of the key components, the surface layers of these components must be smooth and wear resistant. Any technique that would modify these surfaces should ensure:

· Uniform thread quality guaranteed by the components smooth surface that remains unchanged through out its service life

· Increase in processing speed met by an improved wear resistance

Plasma nitriding is one such technique that improves the surface hardness without impairing the surface finish, making it an ideal method to apply for such components like the weft clamps of looms, yarn guides, grippers and cutting edges. In plasma nitriding process the components are negatively biased with respect to the vacuum chamber which acts as an anode. The nitrogen-hydrogen gas mixture introduced into the chamber gets ionized when certain dc voltage is applied across the electrodes at an optimum background pressure. These ions accelerate towards the cathode and bombard the surface with high kinetic energy thereby increasing the temperature of the components and involving diffusion of nitrogen to higher depths with time. As a result the top few layers are nitrided and have a maximum hardness on the surface. Typically for En8 material