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SPUTTERING

ASSIGNMENT

SUBMITTED BY, M.RANJITHKUMAR 2009104044

What is Sputtering?

Sputtering is a technique used to deposit thin films of a material onto a surface. By first creating a gaseous plasma and then accelerating the ions from this plasma into some source material , the source material is eroded by the arriving ions via energy transfer and is ejected in the form of neutral particles - either individual atoms, clusters of atoms or molecules. As these neutral particles are ejected they will travel in a straight line unless they come into contact with something - other particles or a nearby surface. If a "substrate" such as a Si wafer is placed in the path of these ejected particles it will be coated by a thin film of the source material.

First discovered in 1852, and developed as a thin film deposition technique by Langmuir in 1920.

Sputtering is the preferred vacuum deposition technique used by manufacturers of semiconductors, CDs, disk drives, and optical devices. Sputtered films exhibit excellent uniformity, density, purity and adhesion. It is possible to produce alloys of precise composition with conventional sputtering, or oxides, nitrites and other compounds by reactive sputtering.

In sputtering, the target material and the substrate is placed in a vacuum chamber. A voltage is applied between them so that the target is the cathode and the substrate is attached to the anode. A plasma is created by ionizing a sputtering gas(generally a chemically inert, heavy gas like Argon). The sputtering gas bombards the target and sputters off the material we’d like to deposit.

Sputtering is a process whereby atoms are ejected from a solid target material due to bombardment of the target by energetic particles.It only happens when the kinetic energy of the≫ incoming particles is much higher than conventional thermal energies ( 1 eV). This process can lead, during prolonged ion or plasma bombardment of a material, to significant erosion of materials, and can thus be harmful. On the other hand, it is commonly utilized for thin-film deposition, etching and analytical techniques

Sputtering is a term used to describe the mechanism in which atoms are ejected from the surface of a material when that surface is stuck by sufficiency energetic particles. It is an alternative to evaporation.

Reasons for Sputtering :

Use large-area-targets which gives uniform thickness over the wafer.

Control the thickness by Dep. time and other parameters.

Control film properties such as step coverage (negative bias), grain structure (wafer temp), etc.

Sputter-cleaned the surface in vacuum prior to deposition.

Steps in Sputtering :

1.Ions are generated and directed at a target.

2.The ions sputter targets atoms.

3.The ejected atoms are transported to the substrate.

4.Atoms condense and form a thin film.

Applications of Sputtering :

Film deposition

Sputter deposition is a method of depositing thin films by sputtering that involves eroding material from a "target" source onto a "substrate" e.g. a silicon

wafer. Resputtering, in contrast, involves re-emission of the deposited material, e.g. SiO2 during the deposition also by ion bombardment.Sputtered atoms ejected into the gas phase are not in their thermodynamic equilibrium state, and tend to deposit on all surfaces in the vacuum chamber. A substrate (such as a wafer) placed in the chamber will be coated with a thin film. Sputtering usually uses

an argon plasma.

Etching

In the semiconductor industry sputtering is used to etch the target. Sputter etching is chosen in cases where a high degree of etching anisotropy is needed and selectivity is not a concern. One major drawback of this technique is wafer damage.

For analysis

Another application of sputtering is to etch away the target material. One such example occurs in Secondary Ion Mass Spectrometry (SIMS), where the target sample is sputtered at a constant rate. As the target is sputtered, the concentration and identity of sputtered atoms are measured using Mass Spectrometry. In this way the composition of the target material can be determined and even extremely low concentrations (20 µg/kg) of impurities detected. Furthermore, because the sputtering continually etches deeper into the sample, concentration profiles as a function of depth can be measured.

In space

Sputtering is one of the forms of space weathering, a process that changes the physical and chemical properties of airless bodies, such as asteroids and

the Moon. It is also one of the possible ways that Mars has lost most of

its atmosphere and that Mercury continually replenishes its tenuous surface- bounded exosphere.

Sputter Yield :

S depends on the type of target atom, binding energy of target atoms, relative mass of ions and atoms , incident ion energy, pressure and angle of incidence of ions.

S can range from 0.1 to 10. Sputter yield is defined as the number of atoms ejected per incident ion.It determines the deposition rate.

Deposition :

a)Sputtered atoms from the target make their way on to the substrate through diffusion.

b)Ions and neutralized gas atoms may also embed on the substrate as impurities.

c)The ions incident on the substrate may also re-sputter the surface.

d)Chemical reactions may occur.

Deposition Rate :

•It is proportional to the sputtering yield.

•An optimum pressure exists for high deposition rates.

Higher pressure means more collisions and ions.

Lower pressure means less scattering

Glow Discharge Formation :

•Initially the current (charge flow) is small. As charges multiply the current increases rapidly but the voltage, limited by supply, remains constant.

•Eventually, there are enough ions and charges for the plasma to be self- sustaining.

•Some of the electron-atom collisions will produce light instead of electrons and ions and the plasma will also glow accompanied by a voltage drop (normal glow)

•If the input power is increased further, the current density becomes uniform across the cathode and we’ll be in the abnormal discharge regime. This is where sputtering operates.

The Glow Discharge

Few methods in Sputtering :

Reactive sputtering

In order to avoid any chemical reaction between the sputtered atoms and the sputtering gas, the sputtering gas is usually an inert gas, such as argon. However, in some applications, such as the deposition of oxides and nitrides, a reactive gas is purposely added to argon so that the deposited film is a chemical compound. This type of sputtering is called “reactive sputtering”.

RF sputtering

When the plasma ions strike the target, their electrical charge is neutralized and they return to the process as atoms. If the target is an insulator, the neutralization process results in a positive charge on the target surface. This charge may reach a level where bombarding ions are repelled and the sputtering process stops. To continue the process, the polarity must be

reversed to attract enough electrons from the plasma to eliminate surface charge. This periodic reversal of polarity is done automatically by applying a radio-frequency (RF) voltage on the target assembly. Thus, this type of sputtering is known as “RF sputtering”.

Magnetron sputtering

The "diode sputtering" has proven to be a useful technique in the deposition of thin films when the cathode is covered with source material ("sputtering target"). Diode sputtering however has two major problems - the deposition rate is slow and the electron bombardment of the substrate is extensive and can cause overheating and structural damage.

The development of magnetron sputtering deals with both of these issues simultaneously. By using magnets behind the cathode to trap the free electrons in a magnetic field directly above the target surface, these electrons are not free to bombard the substrate to the same extent as with diode sputtering. At the same time the extensive, circuitous path carved by these same electrons when trapped in the magnetic field, enhances their probability of ionizing a neutral gas molecule by several orders of magnitude. This increase in available ions significantly increases the rate at which target material is eroded and subsequently deposited onto the subtrate.

In order to increase the efficiency of the sputtering process, it is common for the sputtering source to have some magnetic confinement through a magnetron source. The effect of the magnetic field is to spiral the electrons so that they have more chance of undergoing an ionizing collision thus enabling the plasma to be operated at a higher density. This type of sputtering is called “magnetron sputtering” and it can be used with DC or RF sputtering.

Collimated sputtering

During the PVD process, metal atoms are sputtered at all angles. The standard process deposits metal on all areas of the process kit and at various angles on the wafer.A small range of arrival angles during deposition can cause nonuniform film.

This leads to poor bottom coverage of small geometry, high aspect ratio contacts and vias as the “holes” seal off at the top before filling up at the bottom.One way to improve this by having a narrow range of arrival angles, while atoms arriving perpendicularly to the wafer. This method called collimated sputtering (first proposed in 1992).

Hot sputtering

Hot sputtering is a method used to fill spaced during deposition as well as to improve overall coverage.The basic idea is to heat the substrate to 450- 500ºC during deposition.Surface diffusion is significantly increased so thatfilling in spaces, smoothing edges and planarization are accomplished, driven by surface energy reduction.The temperature in Via planarization processes is generally lower than that in contact to protect previously deposited Al layers.The lower power in the hot aluminum step increases the length of time that the Al atoms can diffuse, increasing the distance that they travel before they stop.

Usually, a thin “cold” deposition is done first with substrate at room temperature, which has better adhesion to the underlying material. Then is followed by hot PVD deposition. Main drawbacks is the relatively high temp. (reaction, thermal-budget, etc).

Comparison of evaporation and Sputtering :

 

 

 

 

EVAPORATION

 

 

SPUTTERING

 

 

 

low energy atoms

 

higher energy atoms

 

 

 

high vacuum path

 

low vacuum, plasma path

few collisions

 

 

many collisions

line of sight deposition

 

less line of sight deposition

little gas in film

 

 

gas in film

 

 

 

 

 

 

 

larger grain size

 

smaller grain size

 

 

 

fewer grain orientations

 

many grain orientations

 

 

 

poorer adhesion

 

better adhesion

 

 

 

 

Conclusion :

In conclusion, sputtering has the following distinct features:

It is a versatile technique that can be used for almost all material types.

It has many process variables that may be used to tailor and modify the properties of the materials.

The use of solid targets makes it possible to control the type and composition of the material. In addition, this avoids the use of complicated process chemistry.

Compared with other advanced thin film deposition technologies, such as MOCVD or MBE, sputtering has the following advantages:

Simple apparatus

Low deposition temperatures

Affordable price

Advantages

•Not a line of sight method

–Can use diffusive spreading for coating

–Can coat around corners

•Can process alloys and compounds.

– High temperatures are not needed

– Even organic compounds have been sputtered.

•Can coat large areas more uniformly.

•Large target sources mean less maintenance.

References :

1)http://www.uccs.edu/~tchriste/courses/PHYS549/549lectures/sputtertech.html

2)http://faculty.kfupm.edu.sa/phys/kuhaili/TSF/sputtering.pdf

3)http://users.wfu.edu/ucerkb/Nan242/L07-Sputtering_a.pdf

4)http://www.ajaint.com/whatis.htm

5)http://en.wikipedia.org/wiki/Sputtering