Deposition Explained!

Deposition

• Different kinds of films are used in microcircuit technology. Functions of these films are also different and there are different techniques used for deposition of these films. Insulating or protective cover films, Films for ohmic contacts and interconnections. Most films can be deposited/formed by more than one method.

Film deposition methods:

1. -       Chemical Vapor Deposition (CVD)
2. -       Vacuum Evaporation
3. -       Sputtering

Chemical Vapor Deposition:

• In VPE (Vapor Phase Epitaxy) emphasis is on the growth of single crystal films with a high degree of crystal quality. Whereas when CVD is used for thin film deposition uniformity and high throughput are important factors. Films are generally polycrystalline or amorphous so higher growth rates can be achieved. Level of purity and freedom from contaminants these requirements are more stringent in VPE (Vapor Phase Epitaxy) than in CVD.

Various types of CVD:

1. Atmospheric pressure – APCVD
2. Low pressure – LPCVD
3. Plasma enhanced – PECVD
4. High-density plasma - HDPCVD

Steps in CVD:

1. Transport reactants via forced convection to reaction region
2. Transport reactants via diffusion to wafer surface
3. Adsorb reactants on surface
4. Surface processes: chemical decomposition, surface migration, site incorporation, etc.
5. Desorption from surface
6. Transport byproducts through boundary layer
7. Transport byproducts away from deposition region

Vacuum Evaporation:

• It is the simplest technique.
• Materials to be evaporated are heated in an evacuated chamber – it attains gaseous state.
• Vapors traverse from source to substrate and will eventually get deposited on the substrate.
• Principally this is similar to MBE (Molecular Beam Epitaxy). Difference between the two is, as follows – vacuum evaporators operate at higher base pressures, handle multiple slices and have much higher deposition rates than MBE.
• Typical rates for metal deposition are 0.5 µm/min. This necessitates equilibrium pressure of the evaporant is in the 10 mtorr range. This can be used for materials like aluminum and gold.
• Path taken by evaporant to the substrate should be collision-free. Thus its length should be less than mean free path. Mean free path depends upon pressure. Thus it limits the choice of materials which can be deposited by this method.
• Substrates are not heated except for radiation from the evaporant source.
• The energy associated with thermal evaporation is in the range of 0.1 to 0.15 eV. Thus this process results in no damage to the substrate surface. This is a significant advantage in the deposition of films over gate oxides. However, in e-beam evaporation, we do not get this advantage. Here high voltages are used for beam formation and direction which produces X-rays which cause damage to the substrate on which film is deposited. In gate oxides, there is the increase in fixed oxide charges & interface trap density.
• Vacuum evaporation of alloys with accurately controlled composition is extremely difficult due to vapor pressure dissimilarity of their separate components.
• In-situ thermal substrate cleaning used in MBE but not practical in vacuum evaporators due to their multiple slice capability. Due to this adhesion of the film to the substrate is sometimes imperfect. With noble metals like gold, it is practice to first evaporate a thin film layer of a reactive material like chromium, titanium which acts like „glue‟ between evaporant and substrate.

Electron beam (e-beam) Evaporation:

• An intense beam of energy is applied locally to a target.
• Evaporation occurs at a highly localized point, while its bulk remains solid.
• There is no contamination from crucible if e-beam is well designed and only strikes the material during this process.
• Electron gun results in to point source results in non-uniform film thickness across the substrate.
• In multiple slice systems, this non-uniformity can be minimized by rotating these slices during evaporation.
• In addition, the point of impact of e-beam is moved around electronically during evaporation so as to simulate a large area source.
• The method can be used for refractory as well as non-refractory materials.
• Multiple materials each kept if different water-cooled hearth can be used to
• deposit multilayer films in a sequential manner without the need to break the vacuum and with very little system contamination.

Physical Vapor Deposition - (PVD):

Advantages:

• Versatile – deposits almost any material
• Very few chemical reactions
• Little wafer damage

Limitations:

• Line-of-sight
• Shadowing
• Thickness uniformity
• Difficult to evaporate materials with low vapor pressures

Film Characteristics:

• Smooth morphology
• Free from various voids and pinholes
• Adhere well to the surface on which they are placed
• Step coverage in a conformal manner
• Electrical properties of the film should be optimized as per their function. Example insulating films must have higher resistivity and high break down electric field strength whereas conducting films should have low resistivity and should be capable of operating at high current densities without failure.
• These properties should be sustained during many subsequent processes to which slice are subjected to.

 ~Jay Mehta

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