Their transport in the atmospheric condition to the substrate, and 3. Eventual condensation on the substrate

2.23) where a is the distance between the spins, A is called the exchange stiffness constant having

Chapter 3 Experimental Methods

2. Their transport in the atmospheric condition to the substrate, and 3. Eventual condensation on the substrate

There are four different types of sputtering process [direct current (DC), Radio- frequency (RF), Magnetron and Reactive sputtering] used for thin film deposition. We have used DC magnetron sputtering for depositing the films of present investigation.

Figure 3.02: Schematic arrangement of DC sputtering technique [HTTP3].

3.2.1.1. DC sputtering technique

Figure 3.02 displays a typical arrangement used for DC sputtering. The target and substrate

oppose each other in the vacuum chamber having a distance of few centimeters to few tens

of centimeters. The target is connected to a negative output of a DC power supply and hence

acting as the cathode. The substrate and chamber walls act as anode. After the creation of

required argon atmosphere with a pressure of about 1 - 100 mTorr, the gas discharge is ignited by applying a DC voltage. The created Ar

⁺

ions are now accelerated towards the target and eject atoms from the target. These atoms travel in the controlled atmospheric conditions and subsequently are deposited on the substrate. At low pressures, the mean free path between collisions is large, the ionization efficiency is low and self-sustained discharges cannot be maintained below few mTorr. As the pressure increases at a fixed voltage, the electron mean free path is decreased, more ions are generated and large current flow. If the pressure is too high, the sputtered atoms undergo increased collisional scattering resulting a low deposition process. The deposition rate is proportional to (a) power consumed, (b) square of current density and (c) 1/(electrode spacing). DC sputtering works with all types of target materials which are conductive in nature.

However, DC sputtering suffers from two major drawbacks as compared to conventional evaporation: (i) low deposition rates and (ii) high thermal load of the substrate due to bombardment of secondary electrons on the cathode surface. In order to increase the deposition rate and to control the thermal load, magnetron sputtering as described below in Figure 3.03 is utilized for the fabrication of the films.

Figure 3.03: Schematic presentation of magnetron sputtering gun assembly [HTTP3].

3.2.1.2. Magnetron sputtering technique

In magnetron sputtering, electrons ideally don't reach the anode but are trapped near the

target by the presence of magnetic field and thereby enhancing ionization efficiency. This

is accomplished by employing magnetic field oriented parallel to the target and perpendicular to the electric field. Practically, this is achieved by placing bar magnets behind the target as shown in Figure 3.03. The magnetic field lines emanate first normal to the target and then bend with a component parallel to the target surface and finally return to magnet completing the magnetic circuit. Electrons emitted from cathode are initially accelerated toward the anode, but undergoes a helical motion when they encounter the region of the parallel components of magnetic field. Therefore, they are bent in an orbit back to the target. The chief reasons of its success are (1) increased sputtering rates (~ 5 - 10 times) due to high plasma density around target, (2) low discharge voltages of 300 to 1000 V due to the reduced plasma impendence and (3) low thermal load of the substrate due to deflection of secondary electrons by the magnetic field.

Figure 3.04: Photographic view of the magnetron sputtering system used in the present work.

Figure 3.04 depicts the typical set up of magnetron sputtering used in the present

thesis work for fabricating single and multilayer structured thin films. The chamber is

equipped with four different guns for making multilayer films. The substrate was loaded on

the rotating substrate holder. Subsequently, the chamber was pumped to high vacuum (< 10

^-

4

Pa) using diffusion pump and rotary pump combination. The argon gas of fixed pressure was permitted into chamber continuously using mass flow controller (MFC) and the argon gas pressure in the chamber was maintained by adjusting the MFC and processing valve.

The optimized sputtering Ar gas pressure for the deposition of CoFeB, Ta and Cr films was fixed at 10 mTorr, respectively. After stabilizing the set argon gas pressure in the chamber, a constant DC power was applied to commence the sputtering process. The deposition of the films was carried out after stabilizing the plasma and completing the pre-sputtering process. The nominal thickness of the films is controlled by sputtering time ranging from 0.2 nm to 6 nm for Ta/Cr spacer layer and from 7 nm to 200 nm for CoFeB film. The deposition rate for the films was calibrated by using an ex-situ surface profilometer (Vecco, Dektak-150) as described in the next section.

Figure 3.05: Photographic view of Vecco-Dektak 150 surface profilometer.

3.2.2. Calibration of deposition rate

To optimize the properties of the films at different thicknesses, it is very much essential to control the thickness of the film deposited under controlled sputtering conditions.

Deposition rate of the films studied in the present investigations was calibrated with surface

profilometer (Vecco, Dektak-150), as illustrated in Figure 3.05. Stylus profilers are versatile

measurement tools for studying surface topography. Their primary function is to measure

film thickness by scanning step heights and trench depths. The stylus profilers typically rely

on a small-diameter stylus moving along a surface either by movement of the stylus or

movement of the surface of interest. A true stylus profiler moves linearly to obtain the

measurement. As the stylus encounters surface features, the stylus moves vertically to

measure various surface features, such as deposited film and irregularities. The stylus

profiler used in the present work was sponsored by Defence Research & Development Organisation (DRDO), New Delhi [ERIP/ER/0900363/M/01/1185 Dated 16 November 2009]. To monitor the thickness of the deposited films, a proper marking using permanent marker was made on top of the cleaned substrate. Subsequently, the deposition was done under controlled sputtering environment (constant Ar gas pressure, DC power and target to substrate distance, etc.) for a given time at ambient temperature. After the completion of the deposition, the film was cleaned through sonication in acetone. As a result, the film deposited on the substrate reveals a clear step.

Figure 3.06: A typical scan profile of the Dektak surface profilometer.

The step size was evaluated using surface profiler as displayed in Figure 3.06 and the average deposition rate was eventually calculated by dividing the average thickness measured at various locations on the substrate with deposition time. In order to confirm the reproducibility, more films were subsequently made under the same sputtering conditions and analyzed using surface profilometer.

Figure 3.07: Schematic ray diagram of diffraction of X-rays by a crystal [THOR1993].

(a) (b)

Figure 3.08: (a) Photographic view of Rigaku TTRAX III 18 kW X-ray diffractometer and (b) Bragg-Brentano diffraction geometry of a powder X-ray diffractometer.

Similar procedures were followed for all the films prepared under different sputtering conditions. The thickness of the films for selected thickness range is also monitored using X-ray reflectivity technique discussed later in this chapter.

3.3. Structural property characterization 3.3.1. X-ray diffraction

X-ray diffraction (XRD) technique is useful to identify the existence of various phases,