As a highly precise and controllable thin film manufacturing process, atomic layer deposition (ALD) is being used in an increasing number of applications. But many friends still ask about the difference between chemical vapor deposition (CVD) and atomic layer deposition (ALD). Today, we will explain from three aspects: reaction efficiency, uniformity, and reaction temperature.
Atomic layer deposition (ALD) is a thin film deposition technique based on continuous use of gas-phase chemical processes; It is a subclass of chemical vapor deposition. Most ALD reactions use two chemicals called precursors (also known as reactants). These precursors react with the material surface one by one in a sequential and self limiting manner. By repeatedly exposing to individual precursors, the film slowly deposits. ALD is a key process for manufacturing semiconductor devices and a part of the toolkit for synthesizing nanomaterials.
In chemical vapor deposition (CVD), precursors are simultaneously and continuously introduced into the reactor, and these precursors react with each other on the surface of the hot substrate. The deposition rate may be higher than ALD, but the adhesion of the coating is poor, not dense enough, and uneven.
Due to the lack of self passivation in CVD, it is also impossible to form a uniform high aspect ratio coating. The CVD process results in a much lower thickness than the substrate surface due to the lower concentration of precursor in the trench or hole. CVD typically requires higher substrate temperatures.
Advantages of ALD atomic layer deposition
1. By controlling the number of deposition cycles, the thickness of the film can be controlled with sub nanometer precision, demonstrating excellent repeatability.
2. The coating has very low roughness and fully follows the curvature of the substrate.
3. Perfect 3D conformity and 100% step coverage: a uniform and smooth coating around flat, internally porous, and granular samples.
4. The coating may even grow underneath the dust particles on the substrate to prevent pinholes from appearing.
5. Due to covalent bonds with the surface or sometimes even penetration (of polymers), it has excellent adhesion. It even sticks to polytetrafluoroethylene!
6. Easy to batch expand (many substrates can be stacked and coated simultaneously, with perfect coating thickness uniformity).
7. Large area with uniform thickness, even exceeding the meter size.
8. The mild deposition process of sensitive substrates usually does not require plasma.
9. Wide process window (insensitive to temperature or precursor dose changes).
10. Low defect density
11. It can be amorphous or crystalline, depending on the substrate and temperature
12. Customize material properties through digital control of sandwiches, heterostructures, nanolaminates, mixed oxides, gradient layers, and doping.
13. Standard and easily replicable formulas for oxides, nitrides, metals, semiconductors, etc.
14. All types of objects can be coated: wafers, 3D components, film, porous materials, and even powders ranging from nanometers to meters in size.
15. The coating equipment is sturdy, durable, easy to operate, and expandable, without the need for ultra-high vacuum. Even atmospheric ALD is possible.
Efficiency of Atomic Layer Deposition Process
As is well known, the growth process of atomic layer deposition (ALD) technology is quite slow, requiring about 1 second per cycle for 1 atomic layer. However, some variants are much faster, especially the rapidly optimized flow reactor (1-5 nm/s) and spatial ALD (1-10 nm/s).
However, due to the inherent self passivation properties of ALD technology, thousands of substrates can be loaded into the reactor, resulting in extremely fast, uniform, and repeatable coating speeds for each component! Alternatively, roll to roll ALD can be used, where the roll speed can be very high (compared to spatial ALD) when using many coating heads.
But when ALD is applied to powder substrates with high specific surface area, the growth time per cycle will be longer, even up to 1 hour, due to the time required for blowing.
Temperature required for atomic layer deposition
In ALD, the suitable substrate temperature range for deposition is from room temperature to 800 ℃, but most deposition occurs around 100-200 ℃. When the temperature is above 100 ° C, water vapor, which is usually used as one of the reactants, evaporates rapidly from the substrate and walls. Therefore, using temperatures above 100 ° C will result in faster circulation rates between precursors.
At high temperatures, certain materials can achieve epitaxial growth. If the deposited layer matches the substrate crystal structure, a single crystal coating can be formed, which is called atomic layer epitaxy!
Coating types supported by atomic layer deposition process
1. Oxide: Al2O3,CaO,CuO,Er2O3,Ga2O3, HfO2,La2O3,MgO,Nb2O5,Sc2O3,SiO2 ,Ta2O5,TiO2,VXOY,Y2O3,Yb2O3,ZnO Wait;
2. Nitrides: AlN, GaN, TaNX, TiAlN, TiNX, etc;
3. Carbides: TaC, TiC, etc;
4. Metals: Ir, Pd, Pt, Ru, etc;
5. Sulfides: ZnS, SrS, etc;
6. Fluorides: CaF2, LaF3, MgF2, SrF2, etc;
7. Biomaterials: Ca10 (PO4) 6 (OH) 2 (hydroxyapatite), etc;
8. Polymers: PMDA-DAH, PMDA-ODA, etc;
ALD can also be used for doping and mixing different structures to form metal organic hybrids.
The potential of ALD continues to expand
For example, a highly promising application is the deposition of selective regions using existing selective membranes. Researchers are currently developing methods for depositing metals and dielectrics at specific locations, which is essentially a different graphical approach.
Selectivity has become the most important membrane property for the first time and is crucial for the integration of 5nm to 3nm technology nodes. ALD is also being explored to improve coverage control or to accurately align new patterns with existing patterns.
Any offset or misalignment of the lower level electrical contacts will reduce conductivity and have a negative impact on the performance of the chip.
It is expected that atomic layer technology will play an increasingly important role in promoting advanced semiconductor manufacturing. As a key technical support, ALD will continue to develop and be integrated into the next generation of devices to address the challenges of new structures and scaling strategies.
