Plasma-Enhanced Chemical Vapour Deposition

8.1.3. Plasma-Enhanced Chemical Vapour Deposition

The Plasma-Enhanced Chemical Vapor Deposition (PECVD) is widely used to produce thin films. The volume of literature on the subject is quite large, since this technique is used to produce a variety of materials for diverse applications. Here, a brief description is given.

The PECVD is the family of deposition processes, which are broadly defined as ‘chemical vapour deposition’ (CVD). In conventional CVD, the ‘precursor gases’ are thermally decomposed by the temperature of 500-1000°C. However, in the PECVD processes, they proceed at temperatures that are much lower because the energetic electron gas of a plasma is capable of highly dissociating the feed gas. Even when the feed gas and the substrate are near room temperature, the PECVD is useful for deposition on sensitive substrates that are damaged by high temperature or, in the case of semiconductor production, where dopant redistribution is an important concern. So, the rise of temperature is not desir- able for such a process. The concurrent ‘ion bombardment’ of the PECVD film by the plasma may also modify the properties of the film during the deposition process (see the excellent review by Hopwood in ref. [2]).

There are two classes of PECVD process, which are commonly practiced : (a) Direct method, and (b) Remote method. In the ‘direct’ method PCVED, the precursor gases, inert gas dilutants, and the substrates are

directly in the region. The formation of reactive species and deposition precursors occurs in the active plasma region by many possible ‘reaction pathways’. These pathways include electron impact dissocia- tion, dissociative collisions with electronically excited atoms, and the reactions involving the products of recombination, as well as the by-products of the film formation.

In the ‘remote’ PECVD techniques, which generate a plasma separately from the region of the deposition. The inert or non-depositing gases are introduced into the plasma region. The excited species and radicals diffuse ‘downstream’ to the substrate area, where additional reactants are supplied. The

long-lived metastable excited states of He supply the energy required for the dissociation of SiH 4 and subsequent deposition of amorphous silicon. The number of reaction path is greatly reduced compared with direct PECVD, making control of the deposition a simpler task. However, the deposition rates for the remote PECVD are generally lower than that of the direct method.

One of the greatest successes of PECVD is the deposition of polycrystalline films of diamond and cubic boron nitride. Many methods have been used for the decomposition of hydrocarbon precur- sors, including dc hot filament, oxygen-acetylene flame, and the microwave direct and remote diamond deposition. Typically, highly diluted mixtures of methane (~ 1%) in hydrogen are used to create PECVD diamond films. The substrate temperatures for the production of PECVD diamond fall in the range of 400-900°C, with the lower temperature of depositions requiring the addition of oxygen.

NANO MATERIALS

At lower temperature (< 400°C), the PECVD of methane and other hydrocarbons produces amor- phous films of hydrogenated carbon (C:H). The film exhibits a high degree of four-fold bonding, similar to that of diamond. Typical hydrogen concentrations are 20-40%. Many of the properties of the films are similar to those of the ‘actual diamond’, and therefore the material is referred to as diamond like carbon (DLC). The hardnesses are normally in the range of 20-50 GPa, compared to greater than 100 GPa for crystalline diamond. Although plasma decomposition of methane results in the deposition of thin films, DLC hardnesses are quite low (~ 5 GPa) without energetic (> 50 ev) ‘ion bombardment’ by substrate biasing. However, the excessively energetic ion bombardment of the film degrades the diamond like properties. It is also observed that 'ion bombardment' reduces the atomic hydrogen concentration in the film.

The silicon dioxide and silicon nitride films have long been mainstay insulators in the electronics industry. The PECVD of SiO 2 at 300°C was accomplished by introducing oxygen radicals from a weak, inductively coupled, plasma source in a tube furnace. SiH 4 dilute in N 2 was introduced down stream from the discharge at the inlet of the furnace. An advantage of a remote PECVD process in the deposi- tion of thin-films is a reduction in the damage caused by the energetic electrons and ions found in the plasma.

The electron cyclotron resonance (ECR) plasma sources are frequently used in the deposition of silicon oxide. ECR plasma produce highly dissociated radical fluxes at very low pressure (~ 10 -4 torr), thus proving usable deposition rate (20 nm-40nm/min) at sufficiently low pressure in order to reduce

gas-phase nucleation of particles. Since SiO 2 is used as an inter-metal dielectric insulating layer in the fabrication of integrated circuits, the planarisation of the film that is deposited over thin pattern “wires” or an inter-connection is needed. The application of a bias to the substrate during the deposition of SiO 2 results in the re-sputtering and planarisation of the film.

However, there are also disadvantages of the PECVD processes, such as incorporation of impu- rities, i.e. the particles of silicon are found to contain large amounts of hydrogen from the silane precur- sor. These impurities can be detrimental for many applications [2].