Semiconductor device scaling has brought about huge increases in transistor density, enabling the development of smaller, faster, and more powerful chips from computers to cell phones. In order to continue the progress of scaling, it is crucial to develop even more sophisticated deposition and patterning technology. However, the difficulties of pattern alignment during nanoscale semiconductor device fabrication have proved to be a bottleneck for scaling. In response, the semiconductor industry has developed a thin layer deposition process called area-selective atomic layer deposition (AS-ALD) which can be applied to self-aligned fabrication1 schemes. This article will explain the process of AS-ALD, its benefits, key parameters, and the challenges it needs to overcome to ensure its future development.
1Self-aligned fabrication: A patterning method using a spacer and a hardmask to create a smaller pattern that cannot be created with the wavelength of conventional UV. During a form of multi-patterning called self-aligned double patterning (SADP), the spacer is self-aligned with the hardmask in a subsequent step to double the number of patterns. For self-aligned quadruple patterning (SAQP), another round of SADP is performed to quadruple the number of patterns.
Selective Deposition of Thin Films Through AS-ALD
AS-ALD is a bottom-up fabrication process in which thin film materials are chemically deposited onto selective areas of a large wafer’s surface while controlling uniformity, conformality, and thickness at the angstrom2 level. The area covered with thin films is known as a growth area, while the area where no chemical reaction occurs during AS-ALD is called a non-growth area. The efficacy of AS-ALD is strongly influenced by precursor design, as it relies on the combination of precursor3 reactivity and strategically-sized molecules to block different precursors. It offers advantages over current self-aligned fabrication schemes by reducing the number of lithography processes and the use of toxic reagents in the patterning process to produce memory devices. This helps to not only reduce edge placement errors (EPEs) but also lower manufacturing costs. Ultimately, AS-ALD enables bottom-up and self-aligned deposition with respect to the underlying device layers. This has been proven to be a huge advancement from ALD that typically leads to uniform deposition on the entire surface without any control of the lateral arrangement of the atoms. Thus, AS-ALD provides much more precision and efficiency in this process.
2Angstrom: A unit of length used to measure the distance between atoms, equal to 10-10meters.
3Precursor: High-purity gas or liquid materials used in key steps during the manufacturing of semiconductor devices. The precursor material not only sticks to various surfaces, but it also allows only one atomic layer to be produced per ALD cycle.
Figure 1. An overview of the AS-ALD process (Source: Parsons et al., Chemistry of Materials)
Understanding ALD Technology
Before exploring AS-ALD in detail, it is important to first understand the basics of ALD, a deposition technique that has become widely used in the semiconductor industry over the past few decades. During ALD, the raw materials of films—precursors and reactants—are alternately exposed to the substrate surface during multiple cycles to build up ultra-thin film layers of atomic thickness. This process highlights the importance of ALD’s self-limiting surface reaction characteristic for AS-ALD. Since a new precursor cannot react where another precursor has previously reacted, ALD can control the thickness of the thin film at the atomic level by limiting subsequent molecular adsorption4. In other words, when the surface reaction is properly adjusted, a precursor adsorption reaction can be achieved in a desired region and a precursor desorption reaction can occur in another region. This can be considered the inherent growth characteristic of ALD.
4Adsorption: The adhesion of atoms, ions, or molecules from a gas, liquid, or dissolved solid (adsorbate) to a solid surface (adsorbent).
Figure 2. The ALD cycle
As an example, Al2O3, or aluminum oxide, films that are deposited by ALD processes using water exhibit different nucleation and growth behaviors depending on both the aluminum precursor and the substrate at a given process temperature. As shown in the comparison of aluminum precursors in Figure 3, the surface reaction and coverage rates are determined by the magnitude of the reaction between specific precursors and Lewis acids and bases5. This suggests the importance of selecting an appropriate precursor for ALD processes.
5Lewis acids and bases: Described by the Lewis theory of acid-base reactions as an electron-pair acceptor and electron pair donors, respectively. Therefore, a Lewis base can donate a pair of electrons to a Lewis acid to form a product containing a coordinate covalent bond.
Figure 3. The growth per cycle in relation to precursor exposure
Precursor Selection Key for AS-ALD
Precursor selection and design is much more critical in the AS-ALD process compared to ALD as selective growth may be achieved during one ALD process but fail in another. This is due to ALD not being able to control the area where the precursor contacts the substrate. Metal alkyl6 precursors such as trimethylaluminum (TMA) and diethylzinc (DEZ) are the most widely used precursors for ALD due to their high vapor pressure that allows them to be efficiently delivered to the deposition reactor. Consequently, a wide range of precursors including alkyls, halides, amidinates, cyclopentadienyls, β-diketonates, alkoxides, and heteroleptic precursor systems have been studied to see whether they are also adequate for AS-ALD. These precursors were found to be highly reactive, so they provide a strong thermodynamic favorability that leads to adsorption on the surface. Therefore, to control the surface reaction of not only ALD but also AS-ALD, inhibition layers such as self-assembled monolayers (SAMs) or small molecule inhibitors (SMIs) are used to block adsorption in most studies about the precursors of Al2O3 and ZnO (zinc oxide). However, TMA precursors are known to be the most difficult to use in AS-ALD. As there is a loss of selectivity with SAMs, TMA precursors experience adsorption to the SAMs after tens of cycles. In terms of growth inhibition, DEZ is more suitable than TMA as a precursor because the blocking selectivity of TMA only goes up to 6 nanometers (nm), while that of DEZ is at least 30 nm on the same SAM surface.
6Metal alkyl: Coordination complexes that contain a bond between a transition metal and an alkyl ligand.
To get a better grasp of these concepts, it is prudent to become familiar with AS-ALD’s mechanisms that are based on the characteristics of precursors. Past studies have compared a series of precursors that have the same central metal atom but different ligands7 to determine how key precursor design parameters affect the efficacy of AS-ALD. By changing the number of methyl and chloride groups in the Al(CH3)xCl3-x (x = 0, 2, 3) precursor and the chain length of the alkyl ligand in the AICyH2y+1 (y = 1, 2) precursor, the impact of precursor chemistry on selectivity can be explained. For example, the SAM-terminated substrate that serves as the non-growth surface is significantly different from a silicon substrate. As the application of SAMs on the silicon surface would be flawed, precursor molecules can penetrate the SAM structure where Lewis acidic SiOx attracts molecule adsorption. The chloride precursors adsorbed on a silicon substrate with native oxides have a higher Lewis acidity compared to the alkyl precursors. Thus, the precursors containing chlorines require a much longer purge time8 on the SAM. However, this adsorption of chloride precursors is mainly caused by physisorption as the activation energy for the chemical reaction with SAMs and/or the SiOx surface is relatively high. In other words, although an extended purge time is required, it is possible to remove the adsorbed chloride-precursor molecules from SAMs. In contrast, alkyl precursors are hardly eliminated during the chemical reaction.
7Ligand: Nonmetal atoms or groups of atoms that surround a central metal atom.
8Purge time: The time it takes to remove excessive residue.
Regarding the size of molecules, ALD of Al2O3 using the Al(C2H5)3, or triethylaluminum (TEA), precursor is most effectively blocked by a SAM inhibitor under optimal conditions. Conversely, the widely used TMA precursor is least effectively blocked among the tested precursors. Also, there is a significant difference in the energy of dimer9 formation, or dimerization, among the aluminum precursors. Only up to 1% of the AlCl3 and Al(CH3)2Cl precursors exist as dimers at 200 °C, whereas 99% of the Al(CH3)3 and Al(C2H5)3 precursors remain as monomers, which causes a difference in the average size of molecules. Through such observations, it is clear that the size of the precursor, which is governed by dimerization energy, is the most important factor in raising the selectivity of AS-ALD. In other words, the combination of precursor reactivity and the size of the effective molecules affects the blocking of different precursors. This is the reason Al(C2H5)3, which has a low Lewis acidity but a relatively large size, provides optimal blocking.
9Dimer: A substance made from the polymerization of two identical or similar molecules, which are generally hydrogen.
Figure 4. Selectivity in relation to the thickness of precursors and reactants (Source: Oh et al., Chemistry of Materials)
Main Challenges for Precursor Development in AS-ALD
The issue with all of the approaches developed for AS-ALD is that growth occurs even on surfaces where deposition was not required. This poses a problem for semiconductor manufacturers which require perfectly selective films that are only a few nanometers thick. Therefore, rather than focusing on the pattern itself during self-aligned fabrication, more attention should be placed on the fact that deposition must only occur with practical applications on 3D device structures.
The precursors that have been developed to date are designed for ALD processes to effectively form films. However, in the case of AS-ALD, thin films must not only grow in specific areas just as in conventional ALD, but their growth must be completely or partially blocked in other areas as well. In other words, the process window is extremely narrow as the adsorption and desorption of molecules occur within the same process. Ultimately, there needs to be development in new precursors that will be able to widen this process window.
The Quest for Next-Generation AS-ALD
The field of nanoscale device fabrication is witnessing a paradigm shift that is driven by the innovative approach of AS-ALD. As the semiconductor industry grapples with the challenges of IC scaling, AS-ALD has emerged as a promising solution that not only reduces edge placement errors but also slashes manufacturing costs. During this process, the precise selection of precursors is an intricate and vital task that requires a deep understanding of the surface chemistry and the characteristics of these materials. As this technology enables deposition to occur solely in specified areas which are often just a few nanometers thick, the full potential of AS-ALD lies within factors such as novel precursor designs and widening the narrow process window. If AS-ALD can realize these developments, it is set to play a key role in realizing smaller, more precise, and higher quality semiconductor products.
