6 Sputtering technique

Sputtering technique (cathodic sputtering) was discovered by English physicist, W. R. Groove in 1852 and developed as a thin film deposition technique by Irving Langmuir in 1920. In order to produce stoichiometric thin films i.e. without changing the composition of the original material from target material Sputtering technique (a high-energy fabrication method) is utilized. Sputtering is also effective in producing non-porous compact films. This technique is an efficient tool to deposit multilayer films for mirrors or magnetic films for spintronics (devices using spin of electron and hole along with the charge on them) applications.

Atoms from a solid target source are ejected or sputtered via the process of momentum exchange inside the plasma by the action of high-energy ions, usually originating from some inert gas ambient like argon. The ejected particles are then deposited (condensed) on the surface of a substrate to produce a thin film. Plasma (taken from the Greek word “plassein” meaning “to mold” or “to spread”) is an ionized gas that is considered to be a distinct phase of matter. Plasma is an ionized state of matter containing ions, electrons and neutral species. Plasma is electrically conductive and is strongly influenced by electric and magnetic fields. Plasma is used to derive ionization creating a large number of ions and free electrons. Plasma is generated by applying DC or AC voltages and a bias voltage is applied to the target to promote acceleration of ions. [Gabor L. Hornyak, Joydeep Dutta, Harry F. Tibbals & Anil K. Rao; K. L. Chopra & Inderjeet Kaur; Milton Ohring and Sulabha K. Kulkarni].

1. Sputtering mechanism

Ions of Inert gas like Ar+ bombards the target surface at a very high energy. The ion-target interaction can be a complex phenomenon (i.e. the kinetic energy of the impinging particle largely dictates what event would take place) depending on the energy of ions and the ratio of ion mass to that of target atoms. The interaction of ions with the target material is dependent on the energy of the colliding ions. The incident ions may get bounced back when their energy is very low (< 5 eV), whereas for energy greater than 10 keV, the collision is nearly head-on and they get embedded in the target material (basis of ion implantation). A number of interaction mechanisms are possible depending on the value of kinetic energy i.e., if it lies in the range of the above two extremes, then it collision cascades can be created in target atoms, vacancies can be created displace some of the atoms in the target, interstitials and other defby displacing ects, adsorbates are desorbed, photons are created while losing energy to target atoms and sputter out some target atoms/molecules, clusters, ions and secondary electrons.

Figure B.3 shows a schematic picture of various possibilities.

For deposition of materials, one is interested in the sputtering yield, which is defined as the number of ejected species per incident ion and increases with the energy and mass of the bombarding ions. Sputter yield for different elements with same incident ion having same energy varies in general.

Therefore, in case of a target consisting of more than two different elements, the one element having a higher sputter yield is incorporated in large amount than the other element present. But, the material with high sputter yield also gets depleted at a faster rate and other elements present thus make a higher contribution in total. Henceforth, the required stoichiometry is achieved in the deposited film.

The deposition of thin film using Sputtering technique can be typically carried out in different ways like using Direct Current (DC), Alternating Current (AC) (or Radio Frequency (RF) sputtering) or magnetron sputtering. The glow discharge or plasma of some or the other inert gases in the vaccum chamber is utilized to create large number of ions and free electrons. The deposition is carried out in a high vacuum (HV, (10-3 –10-7 mbar or torr) since 1 mbar = 3/4 torr approximately) or ultra-high vacuum (UHV, (10-7 – 10-12 mbar or torr)) system equipped with electrodes, one of which is a sputter target (held at high negative voltage (cathode)) and the other is a substrate (held at positive, ground or floating potential with the walls of chamber (anode)), etc. Although the system during deposition is at high pressure, which ensures that the adequate purity is obtained [K. L. Chopra & Inderjeet Kaur; Milton Ohring and Sulabha K. Kulkarni].

For the highest sputter yield, a following set of conditions are valid:

(i)       Process gas should be of high atomic weight.

(ii)       Cathode material should be of low atomic weight.

(iii)     Amount of reactive gas species should be low in the vessel.

The most commonly employed process gas includes “Argon” in the sputter deposition processes as for most metals it has a high sputter yield, also it is chemically inert and non-toxic, and is relatively inexpensive and cheap as compared with the other noble gases (Krypton (Kr) and Xenon (Xe)). On the other hand, oxygen gas is known to react chemically with metals, forming metal oxides easily. Thus, in the present work, argon and oxygen gases are used in deposition of metal and metal oxide thin films respectively.

2. RF sputtering

Figure B.4 shows the basic schematic of a rf sputtering system. In rf sputtering, an alternating potential of radio frequency is applied at the target, due to which electrons exhibit to and fro motion and undergo repeated collisions with the gas atoms near the target surface which is further used to neutralize surface charges periodically with plasma electrons which have a higher mobility than the positive ions. Typically, a 13.56 MHz power supply with ~1 kW to 3 kW power and about 2 kV peak-to-peak voltage is used to couple the cathode through a matching network. There is an acceleration of positive ions from the plasma towards the surface of the target due to this bias potential and atoms are sputtered out due to the transfer of energy from them on to the target atoms. At this high frequency alternating electric field, the relatively heavier ions in the plasma cannot keep a track with the changing field anymore. Thus, only a positive charge could accumulate on the target surface during the half cycle when target behaves as a cathode. The high mobility of electrons causes a higher electron current in the plasma as compared to the current due to ion resulting in a fixed negative bias on target with respect to the plasma. This bias potential is roughly half the value of the peak-to-peak rf voltage on the target surface. Due to negative bias on the electrode, the positive ions present in the plasma accelerate towards the target and hit the target surface to sputter out the atoms.

In rf sputtering, a dark sheath (thin dark layer) is can be seen separating the luminous glow of the plasma from the surface of the target. Majority of the applied voltage drops across this sheath. The ionized atoms move randomly in the luminous part of the plasma and only those which enter the dark sheath are accelerated and bombard the target surface.

Rf sputtering vs DC sputtering:

Rf sputtering is a very versatile technique for high-rate deposition of semiconductors and insulators. In DC sputtering, deposition by reactive sputtering is limited to low rates because of the fact that a small amount of the reactive gas must be used to avoid the formation of an insulating layer at the cathode, which otherwise cannot be sputtered. However, since insulators can be rf-sputtered, the technique is ideally suited for highrate reactive sputtering.

In both DC and rf sputtering techniques, the deposition rate is quite low because the discharge expands to fill the vacuum chamber, leading to poor ionization efficiency. The heating of substrate due to bombardment of energetic species is also a problem. These problems are overcome by magnetron sputtering in which the electrons are trapped close to target surface by strong electric and magnetic fields [K. L. Chopra & Inderjeet Kaur; Milton Ohring and Sulabha K. Kulkarni].

3. Magnetron sputtering:

DC/rf sputtering rates can be further enhanced by using magnetic field. Magnetron sputtering is one of the conventional methods of increasing the ionization efficiency of electrons (figure B.5). It increases the efficiency of sputtering (sputter rate) by increasing their path length by applying both parallel and perpendicular magnetic fields to the direction of electric field. Another important advantage is the reduced operating pressure, for example, as low as 0.5 torr of Ar pressure, high sputtering rates can be achieved. Figure B4 shows the typical schematic of the magnetron sputtering system.

In magnetron sputtering, magnets are located underneath the target electrodes. Here, magnets are used to increase the percentage of electrons that take part in ionization events, increasing probability of electrons striking Ar+, increase electron path length; thereby increasing the ionization efficiency significantly [K. L. Chopra & Inderjeet Kaur; Milton Ohring and Sulabha K. Kulkarni]. In magnetron sputtering, the secondary B-10 electrons that are ejected from the cathode surface follow helical paths around the magnetic field lines undergoing more ionizing collisions with gas neutrals near the target surface. The magnetic field increases the residence time of the electrons in the plasma and hence increases the probability of ion collisions (figure B.5). So due to the presence of magnetic field, the plasma can well sustained at lower gas pressure and gives a higher deposition rate and moreover, the ionization is confined near the target thus minimizing the heating of the substrate.

Other advantages of using magnets:

(i)   Lower voltage needed to strike and sustain plasma: Magnets produce a localized magnetic field, due to which electrons move in a curved path (helical path) near the target. Curved paths are longer which promote more collisions enroute. More collisions make more ions and thus it gets easier to strike the plasma.

(ii)   Controlled uniformity: Electrons paths are more curved near stronger magnetic field. Therefore, more the ions collide with the target in regions of high magnetic field; larger is the number of sputtered target atoms. More magnets near the edge/center make the deposition thicker in that area. In the present work , two sputtering systems: (1) rf Diode and (2) rf Magnetron sputtering were used for the thin film deposition as shown in figure B.6 (a) and (b) respectively. Both the system comprises of a spherical and cylindrical deposition vacuum chamber fitted with a turbo molecular pump (Pfeiffer make) which is operated at a frequency of 833 Hz and continuously backed by a rotary pump in order to evacuate the chamber for attaining a vacuum of the order of 10-6 torr to 10-7 Torr. Also, a cold cathode gauge (Pfeiffer make) is employed to monitor the sputtering pressure over the range 1 10-2 torr to 5 10-11 torr which is fitted in the deposition chamber. Inert sputtering gases (argon, oxygen and nitrogen) can be put precisely into the deposition chamber using a mass flow controller in order to raise the pressure of the chamber to a value of the order of 10-2 torr for striking the plasma for reactive sputtering. Rf Diode sputtering system has one 3″ diameter and one 4″ diameter diode electrodes which are mounted on the base plate. This system also has a facility of side-ways sputtering by using a 6″ diameter magnetron electrode (M/S AJA International). In the B- 11 present work, diode electrodes have been used for the fabrication of SnO2 thin films. System is electrically connected with rf power supply (Make: Comdel) which can give a maximum power of 1.25 kW to the electrodes. A matching unit, which can operate either in auto or manual mode, is connected between electrode and target to reduce the reflected power. System also has two pulsed DC power supplies as shown in figure B.6 (a).

Figure B.6: (a) Rf Diode sputtering system (b) Rf Magnetron sputtering system used in the present work.

The rf Magnetron sputtering system has 1” and 2” diameter magnetron electrode guns as shown in figure B.6 (b). The sputtering system has been provided with three electrode guns and a substrate holder assembly attached with an inbuilt heater and AC/DC motors which can also be utilized for thin film deposition in confocal, Glancing Angle Deposition (GLAD) and multilayer deposition configuration. Confocal configuration could be used for doping of more than two materials, GLAD could be used for nanostructural/columnar growth of thin film, whereas multilayer configuration could be used for depositing two or more materials in multilayered thin film. In order to obtain uniform thin film substrate of different sizes (1 cm to 2.54 cm diameter) can be used on the substrate holder and also the distance between the target to substrate can be varied from 2 cm to 7 cm.

The substrate holder (or assembly) is fitted with a heater which is controlled by PID for obtaining substrate temperature up to 850o C. The maximum rf power which can be applied to the magnetron electrode is of 100 W using a water cooled rf power supply (Comdel model: CX1250S/A). To match the impedances and to keep reflected power at zero, a matching unit is used, which can operate either in auto or manual mode, is connected between the rf power supply and the target electrode. A dc supply can also be used with the sputtering system (Huttinger make, Truplasma DC 4001). The magnetron electrode and RF power supply are connected to the water cooling system (Make: Warner Finley Pvt. Ltd.) having circulated chilled water at a temperature of 10o C to dissipate the heat generated during sputtering process.

REFERENCES

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