The deposition process, also known as the thin film process, is a process of depositing a thin and uniform thin film (a film with a thickness of 1㎛ or less) on a wafer. It is broadly divided into physical vapor deposition (PVD) and chemical vapor deposition (CVD).
PVD mainly uses the Evaporation method, which vaporizes the material source to be deposited by applying energy in a high vacuum state and deposits it on the substrate, and the Sputtering method, which strongly collides plasma ions on the source target to cause the source atoms to pop out and be deposited on the substrate.
CVD is a method for depositing a thin film by transferring a gaseous source material to a substrate. It primarily uses energy, such as heat or plasma, to induce a chemical reaction.
| PVD | CVD | |
|---|---|---|
| Advantage | – Low temperature, safe process – High vacuum environment (low contamination) – High-quality thin film deposition possible – Simple equipment structure | – Fast deposition speed – Good step coverage – Various gas sources available |
| Disadvantage | – Slow deposition rate – Adhesion issue | – Difficult to control variables – Risks when using gas – Complex field – High temperature – Limited depositable materials |
Deposition type
PVD, Physical Vapor Deposition
PVD has the following advantages:
- Because deposition occurs without chemical reaction in a high vacuum, various materials can be used as sources.
- The substrate temperature can be freely controlled. In addition, there are many other adjustable variables, such as vacuum level and voltage.
- Less particle contamination because deposition is performed in a high vacuum environment
However, it has the disadvantage of poor step coverage and thin film uniformity compared to CVD.
Evaporation
There are two main types of evaporation: thermal evaporators that use heat and e-beam evaporators that use electron beams.
Thermal evaporation deposition, performed in a high-vacuum (~10-6 torr) atmosphere, is a simple process in which the source material is vaporized by heat, diffuses, adsorbs on the substrate, and then condenses, offering the advantage of simple equipment. However, it is difficult to deposit thick films, materials with high melting points are difficult to deposit, and adhesion to the substrate is poor.
To solve this problem, an E-beam evaporator is sometimes used, which evaporates the source by applying an electron beam. Compared to a thermal evaporator, the deposition speed is faster, materials with high melting points can be deposited, and adhesion problems can be improved. However, the adhesion problem is difficult to completely solve. I deposited Pt using an E-beam evaporator, but there was an adhesion issue when depositing Pt on a bare wafer, so I deposited about 5 nm of Ti as an adhesion layer and then deposited Pt.
And in the case of the E-beam evaporator, there is a problem that X-raycan be generated. SiO2can also be deposited with the E-beam evaporator, so it was used in research, but the solar cell research team deposited SiO2with PECVD rather than the E-beam evaporator because X-rays affect the device.
Evaporation depends on the type of boat (container for the source) used, and W (tungsten) or Mo (molybdenum) is mainly used.
Sputter
Sputtering is a method in which high-energy particles (plasma) are collided with a target, and the resulting source materials are adsorbed and deposited on the substrate surface.
Plasma is mainly made using Ar, a stable inert element, which is accelerated and made into plasma. When it collides with a target with an energy of 10 eV or more, the target atoms are ejected.
The chamber where deposition takes place is essentially in a high vacuum, but Ar gas is injected to generate plasma, and the pressure is maintained at 1 to 100 mtorr during the actual deposition. This pressure is approximately 10,000 times higher than that of the evaporator, improving step coverage.
CVD, Chemical Vapor Deposition
Unlike PVD, which processes deposition in a high vacuum, CVD processes deposition in atmospheric pressure or a medium vacuum (100–10-1Pa). When a reactive gas is injected into the chamber, it adheres to the substrate and, through energy sources like heat and plasma, triggers a chemical reaction to form a thin film.
- A reactive gas flows at a constant rate over the substrate surface within the chamber.
- The velocity of gas flow is reduced to zero due to friction on the substrate surface, and this is called the boundary layer.
- The gas in the stagnant layer is adsorbed onto the wafer, lowering the gas concentration, and gas from the surrounding area with a high concentration continues to flow into the stagnant layer and is supplied to the substrate surface.
- A thin film is deposited by a chemical reaction on the substrate surface and the product is discharged out of the chamber.
Advantages and disadvantages of the CVD
| Advantage | Disadvantage |
|---|---|
| Low temperatures Good step coverage Easy to control film thickness Various gas sources available | Pprecise recipe setting is necessary Gas issue Complex equipment Not many depositable materials |
APCVD, Atmospheric Pressure Chemical Vapor Deposition
In APCVD, which operates at atmospheric pressure, the high pressure creates a large number of particles undergoing chemical reactions, facilitating reactions and enabling low-temperature processing. Furthermore, the high particle collision rate and short mean free path (MFP) result in deposition in all directions. This can lead to rapid deposition in corners with high arrival angles, potentially degrading step coverage. Furthermore, because chemical reactions occur frequently, they occur not only on the substrate surface but also within the chamber, potentially causing contamination in the form of particles.
However, it has the advantage of being simple and inexpensive as it does not require vacuum equipment.
LPCVD, Low Pressure Chemical Vapor Deposition
LPCVD, which deposits at low pressure, can form higher-quality thin films. However, because the low pressure slows the deposition rate, heat is applied to increase the process temperature, and 100 to 200 wafers are deposited at once in batches to increase productivity.
Because deposition is performed at lower pressure and higher temperature than APCVD, film quality, uniformity, and step coverage are improved. However, the additional vacuum equipment and hot wall make the equipment structure more complex and more expensive.
PECVD, Plasma Enhanced Chemical Vapor Deposition
Because LPCVD processes operate at high temperatures, CVD cannot be used on wafers containing low-melting-point metals like aluminum. In such cases, APCVD was used, but to address the aforementioned issues, PECVD was introduced.
PECVD uses thermal energy and plasma as reaction energy. This allows for rapid deposition at lower temperatures than LPCVD. However, plasma damage can occur, affecting the wafer, and the inclusion of impurities in the film reduces film quality compared to LPCVD.
However, our lab also converted APCVD equipment into PECVD equipment to improve the quality of thin films compared to APCVD.
| Division | APCVD | LPCVD | PECVD |
|---|---|---|---|
| Characteristic | Atmospheric process | Low pressure | Plasma |
| Advantage | Simple Very fast deposition rate low temperature process | High-purity thin film Good uniformity Good step coverage | low temperature process Fast deposition rate |
| Disadvantage | Bad step coverage Particle contamination | High temperature Slow deposition rate | Particle contamination |
| Application Process | PSG Passivation -> Replaced with PECVD | Pre-metal Dielectric Oxide, Nitride, W | Intermetal dielectric Passivation |
References: Samsung Electronics
