Chapter 2 Modified Dense Plasma Focus for Nanofabrication and Characterization Techniques

Transcription

1 Modified Dense Plasma Focus for Nanofabrication and Characterization Techniques

2 2.1. Introduction Plasma has been playing a major role in the field of nanoscience and nanotechnology. Previously plasma based techniques and processing contributed half of all the processes in the semiconductor and microchips manufacturing for ultra large scale integration industry. The fast development in microelectronic industry in which the circuit dimensions have shrunk steadily has brought about many challenges. One of the foremost is the challenge for fabrication of nanomaterials with tailored and improved properties such as optical and electronic properties, hardness, wear and corrosion resistant, low friction or a combination of these properties. The increased requirements of novel nanomaterials have also opened up the need for development of fabrication technique which will increase ion energies resulting in enhancement of adatom mobility. Plasma provides environments which is suitable for growth and nucleation of nanostructures. If the size has to be reduced than the presently achievable dimension, self assembly is the only possible means to control the formation of nanostructures and their self organization into patterns. Nanofabrication using non-plasma methods usually rely on self organization and control in specific environments. However, plasma methods of nanofabrication have better ability to guide the self assembly of building units onto a substrate or solid surfaces. Furthermore, if the controllability of plasma methods in self assembling the building units into nanostructures becomes competitive with other non-plasma methods of nanofabrication, future beholds bright for plasma based nanofabrication. Moreover, the advantage of using charged particles in plasma methods with electromagnetic fields has the ability to control the energy and motion in the plasma while the directionality of particles is an issue in other processes. The simplest device for producing laboratory plasma is the glow discharge. In this device, a dc voltage is applied between two metal electrodes inside the chamber to generate discharge of a gas. Plasma produced in such type of device has density ~ m -3 and temperature ~ 1-5 ev. A glow discharge is modified slightly in RF discharge by applying alternating electric field between the electrodes to cause discharge. RF discharge can produce plasma having densities ~ m -3 and electron temperature ~ 10 ev. Another plasma device used in laboratories is Quiescent- 27

3 machine (Q-machine) which produces very low temperature ~ 1000 K but electron density ~ m -3, steady state and fully ionized plasma (99 %) having a number of instabilities. Ionization occurs when alkali atoms come in contact with hot tungsten plate which is heated to a temperature of about 2300 K so as to ionize the atoms. Electrons from the hot plate are also emitted through thermionic emission adding to the constituents of plasma formed. A strong magnetic field is applied to confine the flow of ions and electrons constituting the plasma. Another device which has been very commonly used in Plasma Research Laboratory at Delhi University for phase change, thin films preparation and now for nanofabrication is a dense plasma focus (DPF) device. This device produces high density and high temperature plasma, highly energetic and high fluence ions, soft X-rays and near relativistic electrons. We will discuss this device in the next section Modified dense plasma focus device A DPF is like a Z-pinch discharge produced by a high voltage pulse when applied to a low-pressure gas between coaxial electrodes generating a short duration, high density and high temperature plasma. It comprises of a plasma chamber which has coaxial electrode arrangement with the anode at centre surrounded by cathode. The plasma focus was independently developed by Filippov, Filippova and Vinogradov [1] at Kurchatov Institute in Russia and by Mather [2] at Los Alamos Scientific Lab. in USA. The main difference between two major designs viz. Filippov type and Mather type plasma focus is in their geometries. The anode aspect ratio (length/diameter) of central electrode in Filippov type is less than 1. The outer electrode is formed by the outer wall of the chamber and the central electrode has an insulating sleeve over its whole length. On the other hand, the anode aspect ratio of Mather type DPF is greater than one and there is a distinct current sheath acceleration phase along the electrode. However, the dynamics of current sheath in both the designs are similar. DPF of both Mather and Filippov types with few kj to hundreds of kj are operational worldwide. DPF of Mather type which are self fabricated including the one at Delhi University, India and other systems, namely, PF-3, POSEIDON and Julich II, PF-360, and DPF-3 have been 28

4 operational in Italy, Germany, Poland and USA respectively whereas Filippov type DPFs are operational in Russia, Egypt, Iran and other countries. One of the most significant characteristic of DPF is its ability to produce intense neutron pulse (~10 12 per shot) when operated in deuterium atmosphere. The DPF is a device in which plasma is confined by self generated magnetic field. The increase in magnetic field with rise in the discharge current compresses the plasma to high density and high temperature. The basic principle of DPF device is the conversion of stored electrical energy in the capacitor bank to magnetic energy, a part of which is rapidly converted into plasma thermal energy. The dynamics leading to the formation of focused plasma are highly influenced by the occurrence of macroscopic and microscopic instabilities. In addition, DPF device is well known for production of energetic ions, soft X-rays and near relativistic electrons. Studies of high energy ions emitted from plasma focus are of great importance for technological applications of plasma focus. It has been established for the first time by plasma research group at University of Delhi that high temperature, high density and strongly non-equilibrium plasma generating energetic argon ions can be successfully employed for material processing such as phase change of the as-grown thin films [3-7]. It has also been established for the first time by the group with modifications [8,9] to the device that this hot dense and extremely non-equilibrium plasma could be used for deposition of thin films [8,10-13], hybrid materials such as titanium carbide [14] and more recently, fabrication of nanomaterials and nanostructures [9,15-17] by modifying the device. Plasma Research Laboratory at University of Delhi has a modified 3.3 kj DPF of Mather type which has been operational since mid 1980s. The modified DPF device in our laboratory consists of (i) cylindrical plasma chamber with input flanges, (ii) capacitor (30 µf, 15 kv) assembly with swinging cascade spark gap arrangement (iii) high voltage power supply and (iv) triggering electronic circuit assembly. The block diagram of DPF with its sub systems are shown in fig The design and working of DPF and subsystems and subsequently its modifications for nanofabrication are discussed in the following subsections. 29

5 Plasma chamber with input flanges Plasma chamber of DPF device consists of coaxial copper electrode assembly, glass insulating sleeve in between the electrodes, two viewing ports, gas inlet, pressure gauge and bottom flanges supporting back wall plate, rubber gasket and perspex sheet. Fig. 2.1: Block diagram of DPF device. Schematic of the plasma chamber with input flanges is shown in fig The chamber is cylindrical in shape with two hollow side arms fixed perpendicularly to the main body of the chamber. The cylindrical portion has diameter and length of 165 mm and 336 mm respectively. The chamber has a volume of about 7 litres. The hollow side arms are covered with glasses using O-rings for achieving vacuum. The electrode assembly consists of a single hollow copper anode rod at the centre surrounded by six solid cathode rods in a squirrel-cage like structure. The anode is kept hollow so as to reduce Joule heating and metal erosion. The anode has outer and inner diameters ~ 19 mm and 16 mm respectively and length ~ 167 mm. The cathodes are solid and six in number, each is having a diameter~ 10 mm and length~ 150 mm. They are arranged in a squirrel cage like structure around the anode. 30

6 Fig. 2.2: Schematic of plasma chamber consisting of electrode assembly with input flanges. Photographs of plasma chamber and coaxial electrode assembly are shown in fig This design of having discrete number of copper rods instead of having a continuous one as cathode has an advantage as it facilitates optical diagnosis of current sheath in the acceleration phase. The annular spacing of anode and cathode is optimized so as to provide region for uniform breakdown and symmetrical current sheath propagation along axial direction of the system. (a) (b) Fig. 2.3: Photograph of (a) plasma chamber and (b) coaxial electrode assembly in a squirrel cage like structure of a modified DPF. 31

7 The operation of this device demands the central electrode to be at positive potential with respect to the outer electrode. Therefore, the central electrode is connected to a plate of thickness~ 6 mm having positive polarity whereas the outer electrode is connected to 12 mm thick brass plate which is grounded. Electrical insulation between central anode and ground flange is maintained by a perspex sheet and three mylar sheets wrapped together using a polythene sheet. A silicone rubber gasket with a hole at the centre is designed in such a way that it fits into the groove formed by the ground flange and the perspex sheet. This rubber gasket acts as vacuum sealing to maintain vacuum in the focus chamber. This also damps out and absorbs any mechanical stress caused by asymmetrical discharge during the operation. The input flanges act as an interface between the capacitor and plasma chamber. It also provides uniform current distribution to the plasma chamber by connecting capacitor to the anode through sixteen coaxial cables High voltage power supply High voltage power supply consists of (a) a transformer with an output rating of 15 kv, 50 ma and ac mains as input, (b) a series of fifty IN 4007 diodes having breakdown voltage of 400 V, (c) a V variac, (d) a solenoid plunger and (e) two high wattage current limiting resistors (100 kω, 200 W each). The circuit diagram of high voltage power supply is shown in fig. 2.4 and photograph of the top view of the high voltage power supply is shown in fig The transformer has a set up ratio of 1:70 and is filled with transformer oil so as to enhance its electrical insulation. A series of fifty diodes is inserted into a plastic hose which is filled with transformer oil in order to protect from exposure to air and enhance electrical insulation. The plunger connects the output load to earth through two current limiting resistors of 100 kω and 200 W rating and acts as a dumping switch. When the switch S 2 is close, current starts flowing through the solenoid plunger due to which an upward magnetic force is induced. This upward magnetic force lifts up the plunger detaching the earthing point. The variac is then used to increase the input of transformer. The output of transformer after being rectified using a chain of diodes is fed to the output load through two current limiting resistors connected in series. The purpose of current limiting resistors and plunger as dumping system is basically for safety reasons. 32

8 - - Fig. 2.4: Schematic of high voltage power supply. 33

9 Fig. 2.5: Photograph of top view of high voltage power supply Capacitor with swinging cascade spark gap (SCSG) Capacitor assembly comprises of a single 30 µf, 15 kv oil filled capacitor (~ 3.3 kj energy) and two conducting plates. The cross sectional view of the capacitor assembly depicting capacitor, conducting plates and SCSG is shown in fig Fig. 2.6: Side view of capacitor with swinging cascade spark gap arrangement. Conducting plates are used to conduct charge from capacitor to input flanges of DPF device. Each conducting plate is made of copper of 3 mm thickness and 380 mm width. One of the conducting plates is grounded and the other plate which also holds outer electrode of SCSG assembly is connected to the positive terminal of the capacitor. Electrical insulation between two conducting plates is obtained by two sets of four mylar sheets (each of 5 mil thickness) and a nylon sheet of 10 mm thickness. A hollow 34

10 cylindrical nylon cap with one end dipped in oil enclosing the positive terminal of capacitor and the other flatten end is sandwiched between the two sets of mylar sheets. This is to insulate the positive terminal of capacitor from ground plate. A modified SCSG from the one developed by Mather and William [18] is used to transfer the stored energy of capacitor to focus chamber. SCSG is preferred over other high voltage switches because of its fast switching time (< 10-7 s), low inductance and ease of construction. It is also capable of transferring high voltage without jittery. Fig. 2.7 shows the top view photograph of SCSG showing horizontal arrangement of two outer electrodes, e 1 and e 2 and the third central electrode which is also known as trigger electrode placed in between the outer electrodes with air gap of 3:2 ratio. The air gap between electrode e 1 and trigger electrode is kept at 4.5 mm using a brass disc or slot (4.5 mm thickness) and that between electrode e 2 and trigger electrode is at 3.0 mm using another brass disc or slot (3.0 mm thickness). The outer electrodes are made of copper plates of 0.5 inch thick and the trigger electrode is of copper rod of 0.5 inch diameter. One of the outer electrodes, e 1 is connected to the positive terminal of capacitor and the other electrode, e 2 is connected to the input flanges of plasma chamber through sixteen coaxial cables (each of 1 m length). The connecting plates which hold two outer electrodes and trigger electrodes are fixed by perspex holders. A series of five 33 MΩ resistors is connected between electrode e 1 and trigger electrode which has larger gap and a series of three 33 MΩ resistors is connected between electrode e 2 and trigger electrode having smaller gap. Fig. 2.7: Photograph of the top view of SCSG. 35

11 When the capacitor is charged upto 15 kv, the potential drops across 5 x 33 MΩ resistors and 3 x 33 MΩ resistors are 9 kv and 6 kv respectively. Therefore, electrode e 1, trigger electrode and electrode e 2 are at 15 kv, 6 kv and 0 kv potentials respectively. Each mm of dry air gap between rounded electrodes needs 3 kv to break down. When a negative voltage pulse of maximum amplitude kv is applied to the trigger electrode, the potential difference across the larger gap increases instantaneously whereas that across the smaller gap drops. Subsequently, the potential across the larger gap attains a value which causes the breakdown leading to transfer of the capacitor voltage to the trigger electrode. The potential of the trigger electrode swings past its initial negative value thereby creating a high voltage across the smaller gap. Eventually the smaller gap also breaks down which allows the transfer of high capacitor voltage to the input flanges of plasma chamber Triggering circuit assembly The triggering assembly consists of (a) a low voltage silicon controlled rectifier (LVSCR) unit, (b) a high voltage silicon controlled rectifier (HVSCR) unit, (c) a high voltage TV transformer and (d) an isolation capacitor. The LVSCR unit produces a voltage pulse of 20 V which is used to trigger the HVSCR and the oscilloscope. The LVSCR circuit diagram is shown in fig A dry cell of 22.5 V is used to charge capacitors, C 1 and C 2. Charging current of capacitors, C 1 and C 2 is controlled by resistors, R 1 and R 2. When the triggering switch is pressed manually, the capacitor C 1 discharges through resistor, R 4. The momentary flow of current through resistor, R 4 causes a voltage pulse of 20 V across resistor, R 4 which is used to trigger the oscilloscope. At the same time, the voltage at the gate of silicon controlled rectifier (SCR) starts rising until it reaches the value at which SCR starts conducting. Capacitor C 2 discharges through resistor, R 3. The momentary flow of current through resistor, R 3 produces a voltage pulse of about 20 V which is used to trigger the HVSCR unit. The HVSCR unit is used to generate a negative pulse of 680 V. Fig. 2.9 shows the circuit diagram of HVSCR unit which is similar to that of LVSCR unit except that a step up transformer, rectifying diodes (D 1, D 2 ), high voltage SCR and capacitor C 2 having higher current ratings are used. 36

12 Fig. 2.8: Circuit diagram of LVSCR. Fig. 2.9: Circuit diagram of HVSCR. 37

13 The HVSCR can be triggered externally by LVSCR as well as manually. The step up transformer produces a voltage of 680 V which charges up the capacitor C 2. The output voltage pulse of LVSCR unit is used to trigger the gate of high voltage SCR. When the high voltage SCR starts conducting, the capacitor C 2 discharges through resistor R 3. The momentary flow of current through R 3 generates a negative voltage pulse of 680 V at its output. A high voltage TV transformer is used to step up the output of HVSCR unit to a negative voltage pulse with maximum amplitude of about kv. This negative voltage pulse is applied to the trigger electrode through an isolation capacitor as shown in fig The purpose of introduction of the isolation capacitor is for safety reason as the isolation capacitor obstructs backward flow of charges from the capacitor to the triggering system in case when breakdown does not occur in the plasma chamber. The high negative voltage pulse is responsible for activation of the spark gap switching system and effective transfer of the stored capacitor energy to the input flanges of the plasma chamber. Fig. 2.10: Triggering circuit assembly. 38

14 The modifications to the DPF device for material processing and nanofabrication are discussed in the next subsection Modifications in DPF for material processing and nanofabrication The DPF device is modified for phase change experiments of as-grown thin film in such a way that the top flange of the chamber has two inlets through which brass rods can be inserted. One of the brass rods has perspex substrate holder attached on its end. The substrate holder mounted with the as-grown thin film can be suspended at a distance from the top of anode using this brass rod. The distance between substrate and the top of anode can be varied along the axis of the anode by moving this brass rod axially. An aluminium shutter in the form of a flat rectangular plate screwed to one end of another brass rod is introduced below the substrate so as to prevent ions produced from unfocused plasma hitting the substrate. The photograph of the arrangement is shown in fig This arrangement has been used for phase change [3-7] of as-grown thin films. Substrate holder Brass rod Shutter Fig. 2.11: Substrate holder and shutter arrangement in DPF device. In addition to the above mentioned modifications, the anode of DPF is modified for thin film deposition and nanofabrication. In the modified configuration of DPF, the top portion of the central anode is made detachable as is shown in fig (a) and is fitted with the material to be deposited in the form of disc as is shown in fig (b). This 39

15 detachable anode top arrangement helps in easing technical difficulty of fixing any type of solid target material in the form of disc. Substrates are mounted on a substrate holder which is suspended at an axial distance from the top of anode. Fabrication of thin film is achieved by deposition with multiple DPF shots whereas few shots are used for fabrication of nanostructures. The deposition rate achieved is about 45 nm per shot for graphite target and differs for different target materials. (a) (b) Fig. 2.12: (a) Detachable anode with (b) its top portion fitted with the target material Working of modified DPF device Plasma focus is a fast dynamic z-pinch like device which produces short lived dense and hot plasma. Dynamics of plasma in DPF can be studied by dividing into three distinct phases such as: (i) breakdown and inverse pinch phase, (ii) acceleration phase or run down phase and (iii) radial collapse phase. We discuss these phases leading to production of focus plasma in the following subsections Breakdown and inverse pinch phase The Plasma chamber is filled with argon gas. As soon as the capacitor is charged to about 15 kv high capacitor, voltage is discharged through the electrode assembly in the 40

16 plasma chamber and argon gas breakdown occurs between anode and cathode along the insulating sleeve [19]. This phase begins with the breakdown of gas between the anode and back wall plate over the insulating sleeve as shown at (a) in fig The breakdown should occur at the close end of the back wall plate connected to cathode rods for efficient operation of the device. That is the reason why the insulating sleeve is placed at the close end of the cathode. Image charges on glass insulating sleeve initialize discharge between the anode and the closest region of the back wall plate forming weak filaments which have dominantly negative axial components of current density. The flow of current along the anode induces an azimuthal magnetic field. The negative axial component of current density along with the azimuthal magnetic field gives rise to a radially outward Lorentz force due to which current filaments move radially outward. During this process, the current filaments have one end fixed at the same region of anode and the other tail ends move radially outward. When the tail ends of current filaments reach the inner surface of cathode rods shown as (b) in fig. 2.13, current filaments unite to form a uniform current sheath between the anode and the cathode. This process of current filaments moving radially outward and subsequently forming a uniform current sheath is also called inverse pinch phase. The formation of uniform current sheath indicates beginning of axial acceleration phase of plasma dynamics. Cathode Anode (b) (a) Inverse Pinch Phase Insulator Fig. 2.13: Schematic of (a) initial breakdown and (b) inverse pinch dynamics. 41

17 Axial acceleration phase or run down phase The current sheath has both radial as well as axial components [20] at this instant, out of which radial component is more dominant. This dominant radial component of current sheath along with the azimuthal magnetic field gives rise to an axially directed Lorentz force which accelerates the current sheath towards the open end of the electrode assembly as is shown in fig However, the axial component of Lorentz force varies across the annulus between the electrodes having highest magnitude near the anode surface gradually decreases towards the cathode surface [21]. Thus the current sheath near the anode surface moves with higher velocities towards the open end of the electrode assembly. This phase along with inverse pinch phase has been photographed by Mather and Bottoms [22] in which the current sheath takes a paraboloid shape and seems to progress radially outward first and then accelerates axially upward. During the last stage of this phase, the frontal part of current sheath sweeps around the end of anode whereas its rarer part continues to accelerate along the cathode. Then the current sheath dynamics enters radial collapse phase which is discussed in the following subsection. Axial Acceleration Phase Insulator Cathode Anode Fig. 2.14: Schematic of axial acceleration phase. 42

18 Radial collapse phase Towards the end of acceleration phase, the current sheath sweeps around the open end of anode and possesses upward axial component of current density. This axial component of current density along with the azimuthal magnetic field gives rise to a radially inward Lorentz force due to which the current sheath gets pinched towards the axis of anode and eventually collapses forming a thin column of dense plasma above the top of anode. The dense plasma column has a diameter and length of the order of 5 mm and 10 mm respectively. The schematic of the radial collapse phase resulting in formation of dense plasma is shown in fig The collapse of current sheath in DPF is different than that of z pinch devices. In case of z pinch devices, the electrical energy stored in the capacitor bank is transformed to plasma energy but the duration of pinching is few microseconds. However, in DPF device the energy stored in capacitor bank is first converted into magnetic energy in the form of moving current and this magnetic energy is then converted into plasma energy within few nanoseconds. Thus, the pinching in DPF device is much faster than that in z pinch device. Hot and Dense Plasma Radial Collapse Phase Anode Insulator Cathode Fig. 2.15: Schematic of radial collapse resulting in hot and dense plasma. 43

19 The plasma dynamics in this phase is very complex and most of the plasma phenomena occur in this phase. In order to understand this phase in a better way, we further divide this phase into five sub-phases. They are (a) compression phase (b) very dense phase (c) quiescent phase (d) unstable phase and (e) decay phase. I. Compression phase This phase starts with sweeping of current sheath around the open end of anode and ends when the plasma density attains its maximum value. The radial collapsing velocity of current sheath increases first and then decreases. The velocity of current sheath during this phase is in the range of 1-4 x 10 5 ms -1 [23,24]. The current sheath takes the form of a hollow cylindrical fountain in the beginning. The heating inside the plasma is due to shock heating before the front of the current sheath meets. Later, when the current sheath is transformed to plasma column, the heating mechanism [25] changes to Joule heating. Rayleigh Taylor instabilities are observed to be present during the compression phase [26] and known to cause disruption of plasma column and limiting the confinement time of plasma. The onset of Rayleigh Taylor instabilities is characterized by fluting of the boundary of the plasma column as confirmed from the shadowgraph and interferometric measurements by Peacock et al. [26]. It has been reported that Rayleigh Taylor like instabilities commence about 30 ns before this phase ends [24]. These instabilities damp out with the decrease in the radial velocity. The plasma resistance [24] is observed to increase at the end of this sub phase. This enhancement of plasma resistance leads to diffusion of magnetic energy to the plasma column. The inductance of the system as inferred from the spike in the voltage oscillogram of the digital storage oscilloscope is observed to be increasing during this phase. The electrons are rather cold as compared to the ions. But this is understood as the heating mechanism is mainly due to shock heating. The electron temperature is measured to be in the range of ev whereas the ion temperature is measured to be of the order of 300 ev [9]. Soft X-rays are also observed to be emitted during the later part of this phase [27]. II. Very dense phase This phase begins with the attainment of maximum plasma density when the minimum radius of the final plasma column is about 0.5 mm. The dimension of the plasma 44

20 column remains unchanged throughout this phase. It is estimated that the number of ions (or electrons) ~ 2 x in a plasma volume of about 5 mm 3 [25] gives rise to a peak number density of about 4 x m -3. In the beginning of this phase, the electron temperature is lesser than the ion temperature due to shock heating. Thus electrons exchange energies with ions through collisions. But before the equilibrium temperature is reached the plasma column starts expanding. The magnetic field has completely diffused into the plasma resulting in anomalously high resistance of plasma. The main heating mechanism of plasma during this phase is Joule s heating. The soft X-rays have become harder indicating rise in the electron temperature [27]. X-ray emission in DPF has been studied by many researchers [27-34]. The spectrum of X-rays covers a wide range from IR to hard X-ray regions. X-rays are continuum radiation generated from the process of Bremsstrahlung during free-free collisions of electrons with ions. The X-rays of non thermal origin are attributed to a cloud of heavy metal ions leaving the anode and drifting along the axis and are characteristics line radiations of the anode material and the working gas. These radiations lie in hard X-ray region and are generated due to bombardment of energetic electrons on the anode surface and neutral gas atoms. III. Quiescent phase This phase starts after the very dense phase and lasts about 30 ns. During this phase, the plasma column is expanding both radially and axially till it reaches to a typical radius ~ 1.2 mm and volume ~ 50 mm 3 [25]. Consequently, the number density drops to about 4 x m -3 even though the number of ions and electrons in the whole plasma column remains unaffected. The ion temperature attains a value of about 700 ev [25] during this phase. The rate of radial expansion is affected by the confining magnetic pressure whereas the axial expansions remains unhindered due to the fountain like geometry of the column resulting in formation of an axial shock front. Shadowgraphs of formation of diffused bubble in this phase, along with those of axial acceleration and radial collapse phases have been taken by Rawat, Srivastava and Mohanty [35] The increase in the plasma inductance during the compression phase induces an electric field which accelerates the ions and electrons in opposite directions. The relative drift velocity between the electrons and ions increases. The moment the relative drift approaches and 45

21 surpasses the thermal electron velocity, the condition for onset of micro-instabilities such as electron cyclotron and various forms of beam plasma instabilities is satisfied. The onset and later growth of m=0 instabilities indicates the end of the quiescent phase and marks the beginning of unstable phase. IV. Unstable phase The plasma inductance attains its maximum value and anomalous plasma resistance reaches to a value of 0.2 Ω during this phase [38] which lasts about 20 ns. This anomalous resistance which appears at the end of the compression phase causes a Joule heating. This leads in the further growth of m=0 instabilities resulting in enhancement of an induced electric field. This enhancement in electric field along with the magnetic field causes acceleration of more electrons towards the anode and ions in the opposite direction. The plasma column appears to be sausaged locally due to m=0 instabilities. At the same time an axial accelerating ionization wave is observed in time resolved intereferometric photographs by Bernard et al. [25] and Mather and Bottoms [22]. This accelerating ionization wave quickly overtakes the axial shock wave which was produced in the axial expansion of the plasma column. The ionization wave attains a maximum velocity ~ 1.2 x 10 5 ms -1 [36]. The onset of the ionization wave corresponds to the beginning of hard X-rays and ion pulses. The ionization wave develops onto a bubble like structure having several density gradients [25,35]. Bernard et al. [25] interpreted the first density gradient as the ionization wave that separates the ambient non ionized gas from the completely ionized bubbles. The ionized particles namely ions and electrons are accelerated in opposite directions due to the induced electric field. The accelerated electrons strike the anode surface resulting in the release of large amount of Z impurities into the plasma as well as emission of hard X-rays from the anode surface. The Z impurities in the plasma cause the increase in Pease-Braginski current causing the plasma column to neck off near the inner electrode [37]. The disruption continues till the whole plasma column is broken up completely. Consequently the plasma density decreases and the drift velocity of electrons attain a typical value of about 2 x 10 7 m s -1 [25] which is higher than electron thermal velocity. This gives rise to onset of microinstabilities resulting in strong plasma turbulent heating. As a result, the electron temperature increases which is indicated by large amount of Bremsstrahlung. The 46

22 decrease in plasma density below 2 x m -3 indicates the final phase of focus i.e. decay phase. The occurrence of instabilities such as Rayleigh Taylor and m=0 instabilities which are confirmed in a numbers of experiments [25,26,38] are responsible for the disruption of the plasma column and the limitation of plasma confinement in DPF. In addition to these macroscopic instabilities, microinstabilities and turbulence in DPF have been confirmed by many researchers [36,39] through observations of various phenomena observed as a result of their existence. The onset of microinstabilities is triggered when the drift velocity of electrons exceeds the thermal electron velocity. The resulting electromagnetic turbulence will interact with the drifting electrons causing them to scatter. As a result, an anomalous resistivity is generated inside the plasma. Some of the microinstabilities and turbulence induced phenomena are anomalous resistivity of pinch plasma, burst of highly energetic electrons and energetic ions as well as emission of non thermal radiation in microwave range. The origin of high energy electron beam is related with the growth of m=0 instabilities and the electrons gain their energies from large electric field succeeding the violent collapse and oscillation of the current column [40]. The existence of high energy relativistic electron beam in DPF has been studied extensively by many researchers [40-44]. The growth of m=0 instabilities during the unstable phase enhances an induced electric field. This large electric field along with the magnetic field accelerates the electrons (towards the anode) which attain relativistic kinetic energy. However, m=0 instabilities being the cause of acceleration of relativistic electron beam has been ruled out by Choi et al. [44] and Yamamoto et al. [45] according to whom the occurrence of relativistic electrons is before m=0 instabilities and is caused when plasma column is stable. The production of accelerated ions in DPF device is associated with the growth of m=0 instabilities during the unstable phase. These instabilities cause the enhancement of induced electric field which together with magnetic field accelerate the ions towards the top of the plasma chamber. The measurement of energy spectra of ions is of great importance due to the aspect that the energy spectra are helpful for 47

23 introducing DPF as a charge particle accelerator and for technological applications including material processing. Several techniques such as nuclear activation analysis [46,47], time of flight technique using Faraday cups [41,48-53], nuclear track detectors [41,54-56], Thomson spectrometers [57-60], etc. are commonly used for studying ion beam spectrum. V. Decay phase During this phase, large, hot and thin plasma cloud is formed due to complete disruption of the plasma column. This plasma cloud emits energetic and high Bremsstrahlung. The emission starts in the beginning of the unstable phase and peaks 30 ns after entering into the decay phase Observation of focused plasma The entire discharge phenomena beginning with the initial argon gas breakdown across the electrode assembly to the post focus phase in DPF device takes place in few microseconds. Moreover, the voltage pulse produced is very high which needs to be attenuated in order to get measured. Hence we make use of a resistive divider for measuring a transient voltage. The resistive divider is connected between positive flange and ground flange of DPF device. It consists of ten 510 Ω resistors (R) which are connected in series with a shunting resistor of 51 Ω and the whole arrangement is enclosed in a copper tube as shown in fig A PVC pipe is used to provide electrical insulation between the high positive voltage and the ground. The shunting resistor yields an output attenuated to about 100 times. The attenuation factor, K v can be expressed as K v = r/(r+r). The main limitation of resistive divider is that the measured voltage consists of an extra voltage due to the anode, in addition to the true voltage of plasma. However, this offers an important tool of diagnostics for formation of focused plasma. A high voltage is developed as a result of the rapid rise in the plasma inductance during radial collapse phase. This is recorded as a sharp spike in the voltage signal probe viz. digital storage oscilloscope through an attenuator circuit with attenuating factor of 10. A typical voltage signal recorded in the digital oscilloscope is shown in fig indicating a good focusing of a DPF discharge at about 80 Pa argon 48

24 gas pressure. The response time of the resistive divider is about 14 ns which is short enough to register the radial collapse phase typically of ns duration. Fig. 2.16: Schematic of resistive divider. Breakdown Focusing signal Fig. 2.17: A typical voltage signal recorded on the digital storage oscilloscope. 49

25 We make use of the modified DPF device which produces high density, high temperature and strongly non-equilibrium plasma to generate high fluence and highly energetic material ions for nanofabrication. We discuss the experimental set up for material processing and nanofabrication in the next section Experimental detail for nanofabrication using modified DPF device The schematic of modified DPF device is shown in fig Prior to the fabrication process, some procedure are to be followed. This includes preparation of the modified anode, fixing the material to be deposited in the form of disc or pellet and cleaning of the substrate on which deposition is to be made. Fig. 2.18: Modified DPF configuration showing target, substrate and shutter arrangement. The detachable anode is prepared from a solid copper rod having diameter equal to the outer diameter (19 mm) of the anode. The rod is cut to a length of about 3.0 cm with one end having threads for tightening onto the central hollow anode. Further, the detachable anode is made hollow in order to prevent metal erosion. The other end of the 50

26 rod is designed in such a way that a disc or pellet of material of high purity can be fitted just inside it. If the material is in the form of rod, plate or foil, then it is cut to the desired diameter using necessary machining. If the material is in powder form then a pellet is made. The procedure for making pellet which includes: grinding, addition of the binder such as polyvinyl alcohol, making ingot by applying hydraulic pressure and sintering the ingot at temperature near to melting point of the material; is followed. Substrates are cleaned thoroughly with acetone and ultra-sonicated. The cleaned substrates are mounted onto the perspex substrate holder which is placed right above the anode by a movable brass rod inserted from the top plate of plasma chamber. The distance between the top of anode and substrates can be varied axially above the anode by moving the brass rod from outside the chamber. The native oxide on silicon substrate surface is removed by plasma cleaning. For this, we fix a hollow detachable anode. We place a shutter in between the anode top and the substrate using another brass rod so as to avoid impact of ions produced by unfocused plasma. Initially, the plasma chamber is evacuated by a rotary pump to a pressure of about mm of Hg for few hours and flushed with argon gas several times. It has been optimized in earlier experiments of our group that good focusing is obtained at argon pressure of about 80 Pa and charging voltage on capacitor to be about 14 kv. We have therefore maintained the argon gas pressure to be 80 Pa in plasma chamber and charged the capacitor to about 14 kv. After achieving good focus, the shutter is removed. The stored energy is transferred to the electrode assembly using triggering circuits and fast switching electronic system. The breakdown of the gas occurs between the anode and the back wall plate over the insulating sleeve forming a current sheath. This current sheath undergoes inverse pinch phase, axial acceleration phase and finally, radial collapse phase to form focused plasma of density (~ m -3 ) and temperature (~1-2 kev) above the anode. The formation of focused plasma is indicated as a spike in voltage probe signal on digital storage oscilloscope and is referred to as a DPF shot. After observing this signature of the good focusing, the shutter is removed. We make use of the argon plasma generated in one or two DPF shots to remove the native oxide from silicon substrate surface. Subsequently, the hollow detachable anode is replaced with another anode with the 51

27 material disc fitted inside its top portion. The whole process is repeated to obtain hot, dense and extremely non-equilibrium plasma. This hot and dense argon plasma ionizes the material fitted inside the top of anode. These highly energetic material ions along with argon ions move vertically upward in a fountain like structure due to the large electric fields generated by instabilities in the post focus phase. These high fluence and highly energetic ions lose their energies on hitting the substrate and subsequently get deposited as nanostructures depending upon the number of shots. Fabrication of nanostructures is achieved with one to three DPF shots while film is obtained by multiple shots. As we increase the number of shots, thickness of film increases Characterization techniques Characterization of nanostructures is important as fabrication of these nanostructures. Structural and morphological properties such as size, shape etc. and their correlation with their physical properties such as optical and electrical is equally important. We shall discuss basic working principle and physical attributes of these techniques used for characterization of the nanostructures Structural characterization Nanostructures are too small to be visualized with conventional optical microscope. It is important to characterize their structure and surface at the atomic and molecular level. We shall discuss some of these techniques used to study the structural and surface properties such as X-ray diffraction (XRD), electron microscopy, namely, scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and scanning probe microscopy such as atomic force microscopy (AFM). I. X-ray Diffraction XRD is an important technique based on Bragg s law of diffraction. When a collimated beam of X-rays is incident on a sample, it is diffracted by the crystalline planes in the specimen according to Bragg s law, λ = 2d sin θ where λ is the wavelength of the incident X-rays, θ is the angle of diffraction and d is interplanar spacing. Schematic of a typical X-ray diffractometer is shown in fig which consists of an X-ray source, a sample stage and a detector. The X-ray is focused on the sample at some angle θ, while 52

28 the detector reads intensity of the X-ray it receives at 2θ away from the source path. The diffraction pattern is a plot of the intensity of the diffracted X-rays and the diffraction angle. The XRD is used for determination of crystallinity, crystal structure and lattice constants of nanostructures. It is a non destructive technique and does not require elaborate sample preparation. The mean crystallite size can be estimated from the peak 0.9λ width with the Debye Scherrer s formula D = where β is the full width at half β cosθ maximum of the diffraction peak. This is very useful for characterizing nanoparticles as this estimation works for only very small particles. However, the size estimated using Scherrer s formula may be different from the true size of nanoparticle as they can be of twinned structures. Diffraction peak position can be used for characterizing homogeneous and inhomogeneous strains. Homogeneous or elastic strain shifts the peak position whereas inhomogeneous strain can cause a broadening of the diffraction peak. Moreover, as size of the nanocrystals decreases, the linewidth is broadened due to loss of long range order as compared to bulk. Fig. 2.19: Schematic of X-ray diffractometer. II. Scanning electron microscopy SEM is used for obtaining topographical feature of thin films and nanostructures. It is based on the principle that when a focused electron beam impinges onto a specimen, it generates secondary electrons, backscattered electrons, Auger electrons and X- rays 53

29 which provide information about topological features, crystal structures and elemental composition of the sample. The resolution of the SEM can be increased to view 1 nm objects and it can operate at magnifications ranging upto The image resolution achievable by SEM depends on the property of the electron probe and also the interaction between the probe and the specimen. One such interaction between the incident electrons and the specimen produces secondary electrons with energies typically smaller than 50 ev. The emission efficiency depends on the surface geometry, topology as well as chemical composition of the specimen. Schematic of a typical SEM as shown in fig employs an electron source which produces a focused beam with a fine spot ~ 5 nm and energy ~ few hundred ev to 50 kev. The beam of electrons is rastered over the surface of the specimen by deflection coils. The electrons on striking the specimen surface causes electron specimen interactions resulting in the emissions of secondary electrons, backscattered electrons, X- rays and Auger electrons. These emitted species are collected using different detection techniques. For instance, the emitted secondary electrons are collected in a cathode ray tube to give the images. In case of energy dispersive X-ray spectroscopy (EDX) which is online with SEM, X-rays are detected. EDX gives information of the chemical composition of the specimen. Fig. 2.20: Schematic of scanning electron microscope. 54

30 III. Transmission electron microscopy TEM is one of the powerful tool which employs a highly focused electron beam transmitting through a very thin slice of the specimen. TEM can be used to obtain structural and morphological information of the sample. The schematic of a typical TEM is shown in fig TEM consists four main components namely, electron source, electromagnetic lens system, sample holder, and imaging system. The electron source consists of a cathode which is a tungsten filament and an anode. The electron beam from the cathode is then accelerated to typical energy ~ 100 kev- 1 MeV towards the specimen by the positive anode. The electromagnetic lens is used to focus the accelerated electron beam resulting in an intense beam of small energy range and comprises of magnetic lens and metal aperture. The magnetic lens acts like an optical lens to focus the electrons by generating a circular magnetic field. Aperture is used to restrict the electron beam and filter out unwanted electrons before hitting the specimen. The sample holder consists of a platform equipped with a mechanical arm for holding the specimen and controlling its position. (a) (b) Fig. 2.21: (a) Schematic of a typical transmission electron microscope and (b) its ray diagram. 55

31 The imaging system is made up of another electromagnetic lens system and a phosphor screen. The electromagnetic lens system is used for refocusing the electrons after they pass through the specimen, for enlarging the image and projecting it onto the screen. The screen has a phosphorescent plate which glows when being hit by electrons. Thus an image is formed on the screen. The typical magnification achievable by TEM is upto 2.5 x The main advantage of TEM is that lattice imaging of 0.05 nm can be achieved by combining central and diffracted beam to form an image. The photograph of transmission electron microscope shown in fig is FEI Technai G 2 T30, U- TWIN which is at Delhi University. The intensity of the diffracted beam is much stronger as compared to XRD as the primary electrons are scattered strongly by the nucleus and also by the electron potential of the sample. The disadvantage of TEM is that it requires highly elaborate and difficult procedure of sample preparation to attain less thickness (less than 200 nm) of sample for observation. The high voltage giving rise to MeV electrons in the microscope greatly overcome the sample thickness limitation. Fig. 2.22: Photograph of transmission electron microscope at Delhi University. 56

32 TEM has essentially three modes of operation namely (i) image mode, (ii) scanning mode and (iii) diffraction mode. In normal imaging mode, an area of the sample is irradiated with accelerated electron beam and the image of the sample is formed in the image plane of the objective (as the objective plane of the intermediate lens and projector lens) as shown in fig which is then magnified by a series of intermediate and projection lenses and focused onto the screen. The final magnified image thus formed is reproduction of topography of the sample. This mode of operation provides micrograph showing structures and shape of the nanoparticles and helps to identify stacking faults, dislocations, grain boundaries and interphase boundaries. This method is referred to as high resolution transmission electron microscopy. In scanning mode, a small electrode probe (formed by condenser system) is scanned across a part of the sample and the intensity of the electrons scattered to different angles are measured as a function of position of the probe. Atomic level resolution is achieved both in imaging as well as in scanning mode. Analytical methods such as energy X-ray dispersive spectroscopy (EDX) and electron energy loss spectroscopy (EELS) are typically employed in scanning mode of transmission electron microscopy to obtain information about the local elemental composition at selected locations of the sample. In diffraction mode, the diffracted beam forms diffracted pattern in the back plane of the objective which is then magnified by the intermediate and projection lenses and finally focused onto the screen. A selected area electron diffraction (SAED) pattern of the sample is obtained with the help of selected area apertures. The analysis on electron diffraction patterns is used to obtain information on lattice parameters, crystal symmetry and the arrangement of atoms in the unit cell of a crystal. IV. Scanning probe microscopy (SPM) Since early 1980s, SPM developed by Binnig [61] has become a powerful technique for characterizing surface morphological features at ambient conditions. SPM is a general term for a family of microscopes. A common characteristics of this family is that an atomic sharp tip scans across the specimen surface and images are formed by either measuring the current flowing through the tip or the force acting on the tip. The schematic of SPM is shown in fig and the photograph of Digital Instruments CP II scanning probe microscope which is at Delhi University is shown in fig SPM is 57