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Surface and Thin Film Analysis
The interaction  particles& thin layer

The interaction  X Ray with thin layer

The interaction   Light with thin layer

SEM:  Scanning Electron Microscopy

TEM:Transmission Electron Microscopy
SPM :  Scanning   probe   Microscopy

AFM :  Atomic   Force      Microscopy

STM :  Scanning  Tunneling Microscopy

Sims: Secondary  ion  mass  Spectroscopy

RBS: Rutherford Backscattering Spectroscopy

 XRD:      X_Ray Diffraction

XPS:   X_Ray Photoelectron
 
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  Electron_Beam  Gun
Vacuum          About

Rotary               Pump

Turbomolecular   Pump

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The barometer History

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Sputtering


(Sputtering)

Sputtering, first applied in 1852. At that time, a person that his name was گرو, could coat the metal layer on the cold cathode using electrical discharge. Sputtering at first, was used mainly for refractory metal coating because their coating was not possible by thermal method and with some modifications was used for the coating of dielectric materials. One of these changes was the use of radio frequency waves, which allowed to dielectrics were coated directly.
It expanded, use of the waves Rf since 1960, and Maiseel & David were used of Rf waves to build dielectrics layers in 1966. In 1968, the Hohenstein Company coated the glass with Rf waves and metals, nickel, copper and aluminum using DC direct current at the same time.
Then, using of magnetic field to improve the coating and enhance the growth rate of layer, which was truly a revolutionary in the development of this technology and caused to be discussed between the experts, comparison between sputtering with other methods such as evaporation method for coating of metals, alloys, dielectrics, and other materials, and considered as a reliable method for the coating.
In fact, sputtering is the transferring momentum process of incident particles (usually ions of inert gases), to the collision surface. Some parameters such as power, angle, and the mass of particles incident, as well as binding energy between the atoms are effective in efficiency. In this process incident particles which are used to bombard a substance target are formed usually from published ions from an inert gas, such as He, Ne, Ar, Xe and CF4, because they can accelerate easily, by creating an electric field.
The effect of incident energy is shown in Figure 1. It can be seen that ions with the voltage less than 5 eV, attracted to the surface or are reflected. By increasing the energy of the incident ions, the destruction of surface is started, so that the atoms are driven into the target sample and when the amount of energy increased to more than the threshold (usually from 10 to 30 eV) and atoms will be forced to dislodge from
Sputtering occurs, in ion energy range between 104-102 eV. However, at higher energies, ion Implant done.
  

                                   
Figure 1: The required energy of incident ions for doing different reactions.

Dislodge atoms from the surface depends on the threshold energy (E0), which is equal to the lowest required energy for dislodge an atom from the surface. In sputtering, this bonding energy must be greater than the bonding energy of surface atoms, which is shown by Eb. E0 amount is usually about 3 to 4 times greater than the amount of Eb. Therefore, assuming that Eb is the equivalent of 4 to 8 eV, the energy threshold is usually around 15-40.
By creating an electrical discharge between the source (negative) and anode (which are, in fact, the walls of the chamber and the substrate that are connected to the earth), neutral gas that is existed in the chamber (such as Ar) is ionized in accordance to the following formula, and the produced ions are accelerated toward the target source that is placed in negative potential. To generate plasma in the range of 10-1 - 10-3 torr, the potential difference of order several hundred volts (300 volts) is used between the anode and cathode.
If the transferred energy by the incident positive ions to surface target atoms, be bigger than their binding energy to the surface, the initial dug-out atoms collide to other atoms and share their energy through the waterfall collisions and plasma is maintained. The energy of Ar positive ions, which is usually more than 100, is proportional to the potential difference between the plasma and the target.
In this conditions, sputtering process starts and atoms, clusters or molecules are dug-out from the target surface of source, and after a few collisions pass, more or less, through the plasma and reach to the substrate with remained energy until deposition taken. As a result of the interaction between the surface and incident ions, according to figures 2 and 3, occurred several new reactions that will vary according to the energy of the incident, the mass of the ions, and the nature of target surface atoms.


                                                              

Figure 2: The initial processes, in the ions collisions to surface of target substance.


                                                            

Figure 3: Schematic view of the sputtering deposition process
  

One of them is the emission of secondary electrons from the target surface. This secondary electrons can collide with Ar gas atoms and produce Ar ions and in this way, contribute to the stability of the discharge light derived from the plasma and ion collisions.



Figure 4: A view of the sputtering deposition systems.



The main components of this system are as follows:
1. Rough and high vacuum pumps
2. Input connection and inert gas control
3. Input connection and reactive gas control
4. Cathode and target substance
5. The substrate holder
6. The power supply (in the RF case, an adaptive system is added, which called Matching network).
The target substance, as seen in Figure 5, is in the form of a flat disk, which is located usually in a metal plate (which is usually copper) that called the Back Plate and cooled with water.
Note that more than 80% of the incident kinetic energy of Ar ions, is converted into heat during sputtering. Thus, the substance target should be made in a form that be above its thermal conductivity.
 

  
Figure 5: View of sputtering sources




RF sputtering
In metals sputtering, according to their conductivity can be absorbed to the target, the existing plasma in chamber by applying a DC voltage, and sputtering performed. But if target material is non-conductive with using direct current, due to the lack of charge transfer between the ions and surface, the negative potential of cathode surface reduced gradually due to the accumulation of positive ions on it and finally, the sputtering process stops.
To solve this problem use from RF sputtering and reactive sputtering methods. As noted above, for dielectric materials sputtering instead of DC voltage should be used the DC voltage. Each frequency more than MHz 10 can be used to do it, but common frequency equals 56/13 MHz.
In Figure 6 is shown the schematic view of the sputtering RF. A simple explanation of this process is concerned to the difference mass between electrons and generated positive ions and thus their mobility. Because, according to the following equation, the acceleration of charged particle has an inverse relation with its mass and therefore the acceleration of electron motion is more than the ions motion of ions.
 

         

 Figure 6: Schematic example of the RF sputtering system. 



As shown in Figure 7, assuming the target potential is positive in the first half cycle. While electrons with high velocity go toward the target (anode) and concentrate in there. Ions due to their high weight (compared to electrons) can’t follow the potential fluctuations and thus in the first half cycle, the concentration of ions near the cathode (relative to the concentration of electrons near the anode) became low. In return, and in the second half of the cycle because of ions existing, the electrons motion became slower (in this condition composition of electrons and ions build a neutral zone that called the black area).
On the other hand, because the mass of electrons is much less than the mass of the corresponding ions, the target is positive only for a very little time. In this way, the potential of target surface remains negative until the duration of cycle voltage be less than the needed time for discharge a negative charge, on its surface (by positive ions). Thus, is made an efficiently negative voltage, so that we can also coat the dielectrics with it.

Critical point, in distinction between the cathode and anode, is that the area of the substrate electrode, (cathode), is more smaller than the substrate electrode, which is connected to the body and thus to the ground (anode) and thus, the density of electrons in the surface of target electrode is higher than the density of electrons on the substrate electrode, which could cause that formed a negative bias on the target electrode.



                                                      


Figure 7: The process, in the alternative, for a sputtering system RF, Region 1: the accumulation of electrons, region 2: The concentration of ions, region 3: the concentration of ions, region 4: the accumulation of electrons.
Unfortunately, the sputtering rate, in this case, is limited due to the low thermal conductivity of non-conductive targets, and therefore in many cases it is preferred that the insulating layer be prepared in the reactional form, from metal sources. On the other hand, the use of large RF power is not cost-effective due to their high prices. Also, the high temperature caused in the insulating materials, due to the high voltage of bias, depending on the RF power. According to the same reasons, for business applications, is used less from RF sputtering method.





Reactive Sputtering
The word reactive use namely of a reactive gas in combination with source material, which is used commonly for the formation of non-metallic layers. According to the type of used source, it is used usually desired gases in two forms:

1. Dielectric sources
The formed layers from dielectric sources are usually different with source material, In other words, their stoichiometry changes after the coating. To more understand, we explain a sample. For example, a obtained film by sputtering of alternative TiO2 source material don't keep fully the stoichiometry of source and produce a layer, in which x and y have different values than 1 and 2. To solve this problem, we will try to amended the materials stoichiometry by injecting reactive gas (here oxygen) under controlled conditions.
2. Metal sources
It was said earlier that for the formation of dielectric layers by applying direct current, is used from reaction gases. To do this process, the metal part of dielectric material is placed on the cathode as a target material and the other part, which include gases such as nitrogen or oxygen are injected into the chamber with direct current during the sputtering. In this case, the formed layer is a combination of metal target material and reactively injected gas.
For example, as shown in Figure 8, to form a TiN layer by DC sputtering method, is used from Ti as a target matter and from nitrogen as a reactive gas. The formed material will be TiN dielectric layer.

It is notable that the correct stoichiometry for the formed layer is achieved by injecting the correct amount from desired gas that be achieved usually experimental and in different methods.



                                              
Figure 8: TiN sputtering coating using reactive gas.

It should be noted, that in some applications, is also use from reactive gas only to add an impurity to change the nature of accumulated layers.


Magnetron Sputtering

We know that it is possible, using magnetic fields to control and restrict the movement of electrons. These magnetic fields may exist with permanent or electric magnets, which are placed behind the cathode, in various shapes and sizes, and for this reason called magnetron, this type of sputtering systems.
Thus, in magnetron sputtering, as shown in Figure 9, in addition to the electric field, a magnetic field use in parallel with the target in order to trap the secondary electrons (emitted from target during its bombardment) near the target surface. Thus, an electronو before it disappeared and by combining in chamber walls can create multiple ionization of Ar.
According to Lorentz law, Force F on the particle with charge q and velocity v and from a magnetic field B, is calculated by the following equation:
 F=qV×B
That causes electrons travel in a helical path with a radius r. Lightweight electrons are influenced severity by the magnetic field,  and move in a radius, which is much smaller than the dimensions of the plasma and system. In contrast, much heavier ions move with radius much larger than size of the system and plasma. As a result, is not so important the effect of the magnetic field on ions.
Therefore, magnetic fields controls obtained secondary electrons and apply them to make plasma. The existing plasma, in the vicinity of target surface, increases in a result of the impact of electrons with inert gas molecules, and therefore increase sputtering rate.
Because, the most of isolated atoms are relatively heavy and without charge, they are not influenced by the magnetic field and move toward the substrate. In addition, in this case, the substrate bombardment is reduced by electrons. This also means that the plasma can be kept in a lower pressure.
The sputtering atoms are usually neutral and not influenced by the magnetic trap. Thus, plasma can be kept stable at low pressure (Pa 1-1 / 0).


                                      

Figure 9: Schematic design from a magnetron cathode.


A flaming can be seen from transition of excited ions to a lower energy level and thus, excess energy is emitted as radiation T. Due to this fact that, depending on the type of elements, the visible light radiate in different wavelengths, it can be seen different color. Also coatings with high quality (dense and sticky) is produced by magnetron sputtering help.

In magnetic sputtering, relation between current and voltage expressed unusually. In which, I is discharge current, V is voltage, and K and n are constants depending on the systems material and the used gas.
For example, for a typical magnetic sputtering, the n value varies between 5 to 10, while for DC DIODE  (non-magnetic), this amount is less than 2.




Unbalanced Magnetron
This method that todays is regarded highly, refers to a kind of magnetron that in it, some of the electrons are allowed to escape from the dense plasma area. Magnetrons have varying degrees of this condition. The extracted electrons from magnetic traps move to the substrate and as a result of it, ion bombardment of the substrate, also, increase considerably and thus, they cause to, produce a new plasma away from the target substance and close to substrate. Schematic view from a magnetron disequilibrium sputtering system is shown in Figure 10.
                                                        

Figure 10: Schematic view of a disequilibrium magnetron sputtering system.
1- Target sample, 2- cooler, 3 & 4-magnet, 5- back plate of target, 6- magnetic field lines, 7- vacuum chamber.



Disequilibrium magnetrons are made sometimes in a binary arrangement, in which output field begins from the north pole of first magnet (which are stronger than the South Pole) and proceed toward the South Pole of opposite magnet. This leads to trapping more electrons. Of course, this trapping is increased with use of one negative bias at the top and bottom of the magnetron, as a result of it, the ions are accelerated from this new plasma toward the substrate and bombarded it, which increases the quality of the layer.
By applying a non-equilibrium magnetron advanced, it can be controlled ion current, which is moving toward the substrate and thus, can increase dramatically the quality of the coating. In summary, the main advantages of using magnetic sputtering outlined as follows:
1. The increase in the rate of deposition.
2. Reducing the sputtering process, the substrate and the body.
3. Reduce the process of heating the substrate, during the coating.
4. Reducing the required gas pressure.
The most common of them are cylindrical magnetron springs and a circular screen.
Cylindrical magnetron has this advantage that target is not consumed rapidly. Electrons trapped in the cover of the plasma and surround the cylindrical cathode in same radius with radius of the anode. The main difference between the magnetron and diode sputtering is method of production and maintaining a stable plasma, which affects the quality of the coating.
       

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