Atomic layer deposition

Atomic layer deposition ( Atomic Layer Deposition English ALD) is a highly modified CVD process for the deposition of thin layers through two or more cycles performed by self-limiting surface reactions. The layers have a polycrystalline or amorphous structure in general. For single-crystalline ( epitaxial ) layers, the method is also known as atomic layer epitaxy (german atomic layer epitaxy ALE) known. If not individual atoms from the predecessor molecules but molecular fragments in a self-limiting reaction deposited one speaks also of molecular layer deposition (MLD, dt " molecular layer deposition ").

History

ALD was developed in the late 1970s, at that time still under the name of atomic layer epitaxy (german atomic layer epitaxy ALE). At that time was searching for a method to high-quality films on large-area substrates, for example, Dünnschichtelektrolumineszenz ads ( engl. thin -film electroluminescent, TFEL ) to produce.

In the 1980s, led the prospect to use ALD for semiconductor epitaxial layers to large investments in this area. Because of the chemical incompatibility of alkyl compounds of main group III and group V hydrides of the main ALD brought but no real advantages over the molecular beam epitaxy ( MBE) or metal organic vapor phase epitaxy ( MOVPE).

It was not until the mid-1990s ALD was again increased attention as a promising coating technique in microelectronics. Main reasons are the progressive structural miniaturization and higher demands on the aspect ratios in integrated circuits and the associated search for new materials and deposition techniques such as ALD. Only a few applications are aimed at the growth of epitaxial layers. Often the very thin layers (about 10 nm) are of an amorphous structure.

Principle

As with other CVD method is also used in the ALD film formation by a chemical reaction of at least two starting materials ( precursors, so-called precursor ) realized. In contrast to conventional CVD processes, the starting materials are cyclically inserted successively into the reaction chamber during ALD. Between the gas inlets of the raw materials, the reaction chamber is usually rinsed with an inert gas (e.g. argon). In this way the partial reactions should be clearly separated and confined to the surface. An essential feature of ALD is the self-limiting nature of the reactions, that is, the starting material is a partial reaction does not react with itself or ligands of themselves, which limits the layer growth of a partial response in any length of time and amount of gas to a maximum of one monolayer.

The ALD method is a simple two-component system, such as tantalum (V ) oxide ( Ta2O5 ), the components of tantalum pentachloride ( TaCl5 ) and water ( H2O). The two components are then as described above, alternately and separated by rinsing steps conducted in the chamber. This results in the following four characteristic steps

In summary, these four steps called a (reaction) stage, which must be repeated in the course of the coating process several times in order to achieve the desired layer thickness. In the ideal case, each action step runs completely, ie, the precursor molecules chemisorb or react with the surface groups until the surface is completely covered. Thereafter, no further adsorption takes ( self- limiting). The layer growth is self-controlling under the reaction conditions or self-limiting, i.e., the amount of the deposited material layer in each reaction cycle is constant.

Depending on the process and reactor, a cycle lasts between 0.5 and a few seconds of footage being produced per cycle 0.1 to 3 Å ( strongly dependent on the material system and the process parameters ). In reality, this means that a continuous layer of the target material can not be achieved with a cycle, the term atomic layer deposition is therefore somewhat misleading under certain circumstances. For the reduced deposition rate - usually expressed in GPC, for English growth per cycle ( dt, growth per cycle ) is - there are two main reasons:

In addition, it can also Rückätzungen example, by the reaction products of halogen-containing precursors by the ion bombardment or the use of (random ) plasma.

Pros and Cons

Despite the non-ideal growth in real processes arise summarized several advantages in the deposition of thin films by ALD. A key point is the very good thickness control of ultra-thin layers of less than 10 nm, since by the above mentioned self-limiting reaction layer per cycle grows only a determinable value that is independent of the cycle time in the saturation region. The layer increases in proportion to the number of reaction cycles, which makes a precise control of the layer thickness. An exception is the beginning of the coating can take place at the basis of a possibly different surface chemistry of the substrate material faster or slower growth.

Separate dosing of precursor substances prevents undesirable gas phase reactions in the sample space, and also enables the use of highly reactive precursors. Due to the solid dosage each reaction step is enough time to complete, this allows high-purity layers even at relatively low temperatures. In addition, the demands on the uniformity of the gas flow in comparison to other CVD method significantly lower, so the ALD is suitable for coating large areas, or more structured surfaces theoretically very good. In practice, however, economic aspects such as high throughput and low gas consumption play a crucial role in a uniform gas distribution is for these reasons still necessary.