Atomic Layer Deposition: An introduction and application to Micro- and Nanoelectronics
Atomic Layer Deposition is the conformal, homogeneous and surface-controlled ultra thin-film deposition technique. ALD involves two self-limiting and complementary reactions sequentially and alternatingly for the depositing of a perfectly homogeneous and ultra-pure very thin-film. In fact, the ALD incorporates angstrom (A°) or monolayer film-deposition control possiblity.
Historically, ALD was introduced in year 1970. It used to be termed as Atomic Layer Epitaxy (ALE). Inital research on ALE was carried out by the Research Group from University of Helsinky, Finland led by Prof. Dr. Tuomo Suntola. In year 1977, Pro. Suntola filed a patent for the production of compound thin-films, which was the first patent in the development of ALD process . The first pioneering research work on ALE from Prof. Suntola was published in Materials Science Reports in 1989 . The inital phase of ALE was entirely dedicated towards the growth of single crystal of III-V and II-VI compounds and ordered heterostructures like superlattices and superalloys. Those days ALE was studied to meet the requirement for the improvement of ZnS thin-films and dielectric thin-films for electrolumniscent thin-film displays .
Fig. (1): Figure 3. Cross-sectional SEM image of an Al2O3
ALD ﬁlm with a thickness of 300 nm on a Si-wafer with a trench structure. (Source: From ref 42. Copyright 1999 John Wiley & Sons.)
Conformal Sub-monolayer ALD of Al2O3 by TMA-precursor and Water reactant
In recent year, Atomic Layer Deposition (ALD) has evolved as the most applicable thin-film deposition technique. To get highly conformal, ultra-pure and homogeneous thin-film on the substrate, ALD is the best process for micro- and nanofabrication. Experimentally, ALD involves two i.e. self-limited and complementary reactions of different compounds on the substrate to produce sub-monolayer of film growth. One ALD cycle is hence the combination of two half-cycle reactions (Fig. 1 and 2).
In the first half-cycle, precursor molecule is supplied into the deposition chamber. In this reaction, the precursor molecule adsorbs on the substrate forming co-valent bond with the surface molecules (step 1, fig. 1). After some time the substrate is saturated with precursor molecules i.e. no-more precursor molecule can further adsorbs on the surface. Then, the ALD chamber is purged to remove reaction by-product of the first-half cycle along with the unreacted precursor molecules (step 2, fig. 1) from chamber.
In the second half-cycle, the reactant molecule is supplied into the deposition chamber. In this cycle, the reactant molecules react with the precursor molecules depositing very thin-film of desired material. The second half-reaction is such that, in one hand first-layer of desired material is deposited on the substrate and in the other hand surface state evolves same as the initial state (step 3, fig. 1). This phenomenon is termed as the complementary reaction. The deposition chamber is again purged to exhaust non-reacted reactant molecules along with the second-half reaction by-products (step 4, fig. 1) from the chamber.
Fig. 1: Schematic diagram depicting a comple ALD cycle (4 steps-precursor dose, purge, reactant dose and purge), separated into individual half-reactions and purge cycles, on the Substrate .
Fabrication of solid-state nanopores with sub-10 nm diametres (d < 10 nm)
One of the possible field of interest for a bio-medical engineer or a biologist would be the label-free detection of a molecule directly at the site (sample source) with the lab-on-chip technology. This technology demands cheaper and portable bio-sensors. Current measurement through nanopore is one of the approaches towards label free detection of moleule which opens a broader perspective towards development of cheaper bio-sensors. Resistive pulse sensing technique (RPS) is applied to detect ingoing molecules through a nanopore sensor. A nanopore conventionally used to be the biological nature, which includes a micele or α-Haemolysin pore. However, the diametre of a pore is not small enough to detect a single DNA sequence, despite the fact that they are quite noise-resistant. After rapid development of different micro and nanofabrication techniques, stable and size-controlled solid-state nanopores are possible. Solid state nanopores are fabricated on the crystalline materials like silicon dioxide, silicon nitride and gold (Au) substrates. These substrates are stable compared to biological nanopores.
Scopes of solid state nanopores are broader in range, like: DNA sequencing , detection of proteins or polypeptides or nucleic acids or antibody-virus binding[2,3], nanoparticles , synthetic polymers , antibody-antigen [6,7] interaction and ions . In RPS technique, the incoming molecule displaces the conductive liquid flowing through the nanopore, which gives decrement in current or increment in resistance as the electrical signal. In this article, two major solid-state nanopore fabrication techniques would be discussed. The two techniques are: Focused electron beam method (introduced at Kavli Institute of Nanoscience, Delft University of Technology) and Ion Sculpting Method (introduced at Harvard University). Nanopores with the size in the orders of nanometres (d = 1-10 nm) can be fabricated on the substrates with these methods.
Microelectronics: Fabrication of micro/nano structures on Silicon Surface
In microelectronics, fabrication of metal structures (micro/nano) are state-of-art research due to their broad range of applications. On of the major application of microfabrication is the production of thin-film transistors (TFTs). In one of the production process of TFT is by depositing metal structures on the silicon wafer (i.e. above the insulator layer). Channel region between these metal structures is the region where semiconductor is deposited to produce a working field-effect TFT. Major interest for scientists all over the world is the production of large area printable, low-cost and low-temperature processable TFTs. For the production of low-cost TFTs, the fabrication technique too demands cheaper and the faster processes.
Photolithograpy is such a cheaper and faster process, that yields TFTs with channel length down to 1um. However, the process is unreliable in nanometre scale. For futher, below 1um e-beam lithography is applied. E-beam lithograpy is very reliable, yields the TFT with channel in the orders of nanometers. But, the process is expensive and slow. For sub-100 nm shadow based deposition is also beneficial. However, this is rather cubersome process and needs highly-skilled manpower. Apart from transistors, these structures could be also applied as the sensors and the thin micro/nano fluidic-channel in the field of Lab-on-Chip Technology, plamonics for Surface Enhancement in thin film Solar Cells, Surface analysis esp. in Surface Enhanced Raman Scattering Measurement e.t.c.