Atomic layer deposition (ALD) enables controlled deposition of dielectric materials in complicated device structures. In order to satisfy the requirements of a broad range of devices and application fields that can benefit from these dielectric films, there is a continuous attraction towards new ALD processes and improved material properties such as a higher dielectric constant. Also for a number of applications, there is a need to move towards lower temperatures, while maintaining a high material quality.
Controlled Deposition of Dielectric Materials
It is possible to apply several oxidants in the ALD cycle such as ozone, water and oxygen plasma that differ in oxidizing strength.
Common dielectric materials are shown in Figure 1 and also whether they have been grown using Oxford Instrument systems (OpAL and/or FlexAL). Using oxygen plasma, the deposition of the widest range of oxides has been made possible and it has also been proven by the Eindhoven University of Technology that deposition at room temperature for certain Al203, SiO2 and TiO2 is possible.
Figure 1. Common dielectric materials and possible oxidants by which they have been deposited. Materials that have been grown at room temperature are also indicated. Processes in blank spaces may have been demonstrated in literature but have not yet been directly demonstrated by Oxford Instruments. Material list is not exhaustive.
A weaker oxidant or low power plasma is desired for deposition on sensitive substrates (such as lll-V materials). Other than binary oxides, the desire for deposition of multi-component oxides using ALD is present. For instance stoichiometric strontium titanate oxide (STO) can have very high k values.
As the ALD process has a cycle- wise nature and the OpAL and FlexAL have recipe-based software, easy mixing of materials is possible by alternating ALD cycles of the binary compounds.
For example, Figure 2 shows the technique of tuning the optical characteristics of STO deposited on a FlexAL system by modifying the [SrO]/[TiO2] ALD cycle ratio. This example also shows how in situ ellipsometry can be used to determine the stoichiometry for these materials.
Figure 2. (a) Real and imaginary part of the dielectric functions ε1 (a) and ε2 (d), respectively, of as-deposited Ti02, SrO and of STO The corresponding [Sr]/([Sr]+[Ti]) content ratio from RBS and [SrO]/[TiOJ cycle ratio are indicated for the STO films
It was observed that post annealing at 600/650°C for 10min under N2 gas resulted in crystallization into the high-k perovskite phase. 15 nm polycrystalline stoichiometric SrTiO3 films showed a very high capacitance density and resulted in a capacitor equivalent thickness (CET) of about 0.7 nm.
Key Benefits of ALD
One of the key benefits of ALD is its inherent high conformality which can be used to deposit dielectric films for trench capacitors. Since the ALD surface reactions are self-limiting, complicated 3D structures can be covered with equal thickness films throughout the structure as long as sufficient flux of precursor and oxidant has reached all surfaces.
Conformal coating of a very high aspect ratio (>1000:1) was reported for thermal ALD processes. In this case, techniques for enclosing the precursor in the reactor chamber such as the automatic pressure control valve (APC) are beneficial to restrict the precursor usage per cycle. One must also understand that the APC which is standard on the FlexAL system is now also offered as an alternative for the OpAL system.
Based on radical type and surface material, conformality for plasma processes can be highly challenging due to recombination of plasma radicals at surfaces. Nevertheless, plasma deposition enables conformal coating of high aspect ratio structures.
For example for plasma ALD of SiO2, conformal coating of 30:1 trenches using an OpAL has been reported. Also for plasma ALD of HfO2, it is possible to obtain high conformality as shown in Figure 3.
Figure 3. SEM image of Si trenches (AR = 25:1) with conformal coating by plasma ALD of Hf02 (20 nm) as shown by insets of SEM images on the right at the trench corner and the trench bottom.
Here 20nm HfO2 coat the 25:1 trenches. Generally, Oxford Instruments ALD tools are equipped for deposition of a wide range of dielectric materials under challenging conditions, such as room temperature deposition, stoichiometry control of ultrahigh k materials and plasma deposition in 3D structures.
This information has been sourced, reviewed and adapted from materials provided by Oxford Instruments Plasma Technology.
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