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1. Overview of Sputtering
The thin film deposition methods, which are largely used in the semiconductor processes, are mainly divided into PVD method and CVD method. Due to the nature of the semiconductor industry, there have been rapid progresses in the processing technologies. Diverse thin film deposition methods originated from both PVD and CVD are being applied in the semiconductor processing sites.
- LIST
- PVD
- CVD
- Principle
- Physically coated with atoms and molecules
- Chemically coated by gas decompositions
- Temperature
- Below 500℃
- 850~1100℃
- Adhesion to substrate
- Excellent
- Relatively poor
- Shape of substrate
- Not applicable for complicated shapes
- Unlimited
- Coating Thickness
- 0~4㎛
- 5~10㎛
- Warpage
- Less compared to CVD
- Possible.
- Classification
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Evaporation
Sputtering
Ion plating
Ion inplantation
Ion beam mixing
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Thermal CVD
PE-CVD
PI-CVD
MO-CVD
Laser CVD
A. PVD
i. Physical vapor deposition (PVD) is a vacuum deposition method that uses flux of atoms and/or molecules onto the surface of a substrate.
ii. Advantages and Disadvantages
1. Advantages
A. Low temperature deposition
B. Low impurity levels
C. Diverse coating materials
D. Excellent adhesion
E. Excellent thickness control2. Disadvantages
A. Low step coverage
B. Requirement for expensive equipment
C. Slow deposition rate
B. CVD
i. Chemical vapor deposition (CVD) is a chemical process using volatile precursors and their chemical reactions. It is used to deposit various materials onto the heated surfaces of various substrates.
ii. Advantages and Disadvantages
1. Advantages
A. Diverse materials can be coated.
B. Compositions of deposited layer can be controlled.
C. Fast deposition rate and mass production capability
D. Good adhesion
E. Uniform film properties
F. Uncomplicated equipment
G. Applicable to complicated shapes
H. Good step coverage2. Disadvantages
A. High reaction temperature
B. Possibility of substrate warpage
C. Complicated reaction processes
D. Toxic gas handling
E. Unsuitable for large area coating.
2. Sputtering
A. One of the major PVD methods along with Evaporation. Magnetron Sputtering method is mainly used.
B. Principle of Magnetron Sputtering
i. Cations, such as Ar+, generated by the glow discharge of inert gas, such as argon in vacuum are accelerated and collided onto the target surface. The atoms from the target surface are emitted by the momentum transfer with the cations.
ii. These emitted atoms from the target surface land and form deposited layer on the substrate located in front of the target. Since the emitted atoms obtain considerably high kinetic energy from the Ar+ ions bombarding the target surface, they form dense and firm films on the substrate.
iii. Magnetron Sputtering Method utilizes strong magnetic field to confine charged plasma particles close to the sputtering target surface. As electrons follow helical paths perpendicular to the magnetic field, the magnetic field increases the plasma density considerably. Since the magnetron sputtering method greatly improves the low deposition rate of the simple sputtering process, it is widely used in PVD coating processes.
3. Influence of Sputtering Target Properties on Sputtering Process
A. Purity
The sputtering process is used mainly for production of high quality products. Since the thicknesses of deposited thinfilms are merely 0.1micrometers to several micrometer, unwanted level of any impurity can be harmful for the desired properties of the deposited film. Therefore, it is important to select the right purity level depending on the application of the product.
B. Microstructure
i. Grain sizeWhen the target’s grain size is not uniform, particles are generated and the target life is diminished. The particle generation gives adverse effects on the quality of deposited thin films.
ii. Crystal orientationVarious closed-packed directions exist depending on the crystal structures of sputtering target materials. The emitted atoms from the collisions with argon ions during sputtering process, are likely to be ejected along the closed-packed direction. When the crystal orientation in the sputtering target is uniform, the directions of the emitted atoms will also be uniform, which makes it possible to obtain even films.
C. Bonding
A sputtering target assembly is often composed of a target of high-purity material and a backing plate of oxygen free copper. Bonding technology is required to completely bond the target and the backing plate. There are several kinds of bonding methods. The users should select the most suitable bonding method considering the quality and the application of their products.
If the bonding is not accomplished perfectly, there can be warpage of the surfaces of both the target and the backing plate and/or there can be unbonded areas at the bonding interface. The warpage can lead to uneven films as the distance between the target and the substrate becomes uneven. The unbonded areas will trap the high heat generated by the plasma on the target surface, which will result in problems such as deterioration of the film or separation of the target from the backing plate.
D. Surface
If a sputtering target is exposed to air for extended time or does not go through proper surface cleaning process, an oxidized layer can be formed or gas and water vapor can be adsorbed on the target surface. This can lead to low sputtering rate, particle generation and/or inferior film formation.
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