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Tuesday, November 15, 2016

What is Sputtering?

If a surface is bombarded with an energetic particle it is possible to cause ejection of surface atoms. This process is called sputtering. These ejected atoms could be condensed on to a substrate to form a thin film; this process can be realized by forming positive ions of heavy neutral gas such as Argon gas and bombarding the surface of target material by making the surface of cathode in an electrical circuit.

Basic Principle
When a charged particle strikes a surface a variety of interactions are possible. The most important reactions are shown in figure.

(a) Ejection of neutral atoms of surface material.

(b) Ejection of small no of charger atoms of surface material.

(c) Ejection of free electrons, the no. of free electrons usually greaker than io for each arriving incident ion.

Observed points

1) The first major effect in this method is that the neutral ejected atoms can be collected on a suitably placed substrate to form a film.

2) The second most important effect is that the electrons ejected can be accelerated from target cathode to a suitably anode.

3) On the way to anode they can cause for their ionization which helps to enhance sputtering head.

a) Cathode to Anode spacing : 15 cm

b) Pressure: 10-2 Torr.

Summary of observed salient features

i) The sputtering yield defined as the average no. of atoms ejected from target per incident ion, increases with increasing energy of incident ion.

ii) Also depends on atomic weight of target.

iii) Sputtering threshold exist within 5eV and 25ev for most of materials. Yield increased rapidly beyond the threshold shows some linearity in beginning and then approaches saturation.

iv) The yield depends on angle of incidence. It increases approximately as cosent angle between normal to the target surfaces and beam direction.

v) The yield as a function of atomic number of target displays an unduality behavior.

vi) The yield is rather in sensitive to target temperature.

vii) The ejected atoms have considerable energy. The distribution is Maxwellian.

viii) With increasing bombarding energy, the peak of curve shifts slightly towards high energy.

ix) The peak energy of ejected atoms increases with angle of ejection. However a decrease occurs for large angles greater than angle of ejection.

x) The ejected particles are considerably single atoms however in case of copper or silver at high energies some percent of ejected particles, clusters consists of two or three atoms.

xi) The ejected atoms will be in excited state and show characteristic recombination light emission.

sputtering offers many advantages over other deposition techniques such as

1) You can get film of multi component material such as Alloys.

2) Irrespective of melting point, refractive materials films can also be prepared.

3) Insulating materials.

4) Good adhesion

5) Low temperature epitaxy

6) Thickness uniformity over large plane of areas.


i) The source material must be available in sheet form.

ii) Deposition rates are usually less than 2000 per minute.

iii) Substrate material should be cooled.


1) Transfer of energy

2) Transfer of momentum

3) Radiation

The hot spot of evaporation theory yield at experimental evidence. Later developments showed that sputtering is not an energy transfer but rather a momentum transfer process. It considers sputtering as resulting of double or triple collisions of ion with in lattice atoms followed by its back reflection by lattices.

More sophisticated theories consider the sputtering as essentially a radiation damage phenomenon. Accordingly the incident ion displaces a no. of atoms (knocks on) during its passage thru material and thus loses its energy. Some fraction of knocked – on atoms will diffuse to surface and emerge as sputtering atoms.

The Knocked on atoms may also have sufficient energy to produce additional atoms which contribute to total sputtering yield.

The exact details of interaction of an ion with target atom depends on

a) Target atom

b) Momentum Transfer

c) Collision means free path.

Different Kinds of Sputtering techniques:

1) Glow discharge sputtering

2) Reactive sputtering

3) Bias sputtering

4) Triode sputtering

5) Ion beam sputtering

6) RF sputtering

The deposition of metal film by sputtering form a cathode by glow discharge method was first observed by groove.

The ejection of atoms from cathode surface by impinging of energetic positive ions of noble gases such as helium, argon, neon, krypton at a reduced pressure under high dc voltage gives rise to sputtering phenomenon.

It is now possible to make various resistive, semiconducting, superconducting and magnetic films by this technique in better way.

If the process doesn’t involve any chemical reaction between bombarding gas ions and the cathode is known as Physical sputtering.

On the other hand, some reactions are involved then it is termed as Reactive sputtering.

Both the types of sputtering are carried out in poor vacuum and are known as high pressure sputtering.

The mechanism of process involves a momentum-transfer between the impinging energetic ions and cathode surface atoms as a result of which physical removal of atoms takes place.

Sputtering yield increases with the energy and mass of bombarding ions and also with decrease of angle of incidence to the target. A minimum energy is required to start the sputtering process. Sputtered atoms have much higher energies than those of the thermally evaporated ones. Sputtering yield also decreases with large increases of ion energy because of deeper penetration of ions or neutrals inside the lattice.

Sputtering is also accompanied by the emission of secondary electrons from cathode surface. Auger transitions (radiation less) also take place along with emission of secondary electrons.

Sunday, November 13, 2016


We know that a conventional PN Diode is doped by impurity atoms in the concentration 1 part in 108 . With this order of doping, there exists a potential barrier of order 5 microns which restrains the flow of majority carriers to the side of where they constitute minority carriers.
If the concentration of impurity is greatly increased, (by about 1000 times) in a PN Junction diode then the potential barrier width reduces from order of 5 microns to less than 100 A0  . This thickness is only about one-fiftieth of wavelength of visible light. And thus the devices characteristics are completely changed.

“Esaki” introduced a new device, and it is names as “Tunnel diode” as it uses the phenomenon of “Tunneling”.

“A Tunnel diode is a high conductivity two terminal P-N diode doped heavily about 1000 times higher than a conventional diode”.

The heavy doping produces following effect:
(i)                  Width of Depletion layer is reduced.
(ii)                Reverse breakdown voltage reduces to a very small scale.
(iii)               It produces a negative resistance section on V-I Character

From above fig, Tunnel diode is an excellent conductor in Reverse direction i.e when it is Reverse biased.For as soon as forward bias is applied, significant current is produced. The current quickly reaches its peak value ’Ip’ when the applied forward voltage reaches a value ` Vp’.
At the peak current ‘Ip’ corresponding to voltage ‘Vp’, the slope dI/dV of characteristic is zero.

This current variation in the vicinity of origin is due to quantum mechanical tunneling of electrons through narrow space charge region of the Junction.
When ‘V’ is increased beyond ‘Vp’, the current decreases. As a consequence, the dynamic conductance
g = dI/dV is negative. The current reduces to its minimum value ‘Iv’ (valley current) at ‘Vv” (valley voltage).

Tunnel diode exhibits ‘ Negative resistance characteristic’ between Ip & Iv.

At valley voltage Vv at which I=Iv, The conductance is again zero, and beyond this point the resistance becomes and remains positive.

At the so called peak forward voltage ‘Vf’ the current again reaches the value ‘Ip’ and for larger voltages the current increases beyond this value.

Standard circuit symbol for a Tunnel diode is given by
 Uses of a Tunnel diode:-
è         For currents whose values are between ‘Iv’ and ‘Ip’, the curve is triple valued, because each current can be obtained at three different applied voltages. It is this multi valued featured which makes Tunnel Diode useful in “Pulse and Digital circuitry”.
     It can be used as a very high speed switch. Since Tunneling takes place at speed of light, the transient response is limited only by a shunt capacitance (Junction Plus stray wiring capacitance) and peak driving current switching times of order of  a nano second are reasonable, and times as low as 50 pico sec have been obtained.

       Tunnel diode can be used as a very high frequency (microwave) oscillator.

Manufacturing of Tunnel diodes:
è        The most Common commercially available tunnel diodes are made from “Germanium” or “Gallium Arsenide”. It is difficult to manufacture a silicon tunnel diode with a high ratio of peak to valley current Ip/Iv.
      The below table summarizes important static characteristics of these devices. The voltage values are almost independent of current rating.


  • Gallium Arsenide has highest ratio Ip/Iv and lagest voltage swing Vf-Vp ⩯͌ 1.0 V as against 0.45V for Ge.
  • The peak current ‘Ip’ is determined by impurity concentration (resistivity) and Junction area.
  •  The peak point (Vp, Ip) which is in tunneling region is not a very sensitive function of Temperature.
  • The temperature coefficient of ‘Ip’ may be positive or negative depending upon impurity concentration and operating temperature, but temperature coefficient of ‘Vp’ is always negative.
  • Valley point ‘Vv’ which is affected by injection current is temperature sensitive. ‘Iv’ increases rapidly with temperature.
  •  Tunnel diodes are found to be several orders of magnitudes less sensitive to nuclear radiation than are transistors.

  •   Low cost, Low Noise, simplicity
  •   High speed, environmental immunity, low power
  • Low output – voltage swing
  • Fact that is a two terminal device, as there is no isolation between input & output and this leads to serious circuit design difficulty.
  • Transistors are preferred for switching times longer than several nano seconds.