14JR Hollahan and AT Bell Ed Techniques and Applications of Plasma Chemistry John Wiley 1974 Types of Pecvd Systems

As with thermal CVD reactors, PECVD systems can be either the hot-wall type, in which the reactor walls, the substrate, and the reactant gases are all at the same temperature, or the cold-wall type, in which only the substrate is heated to the desired temperature. In addition, PECVD reactors can be of direct or remote plasma type, where consideration is given to the plasma generation and what is coupled to this plasma. Numerous articles in the published literature (Ref 15, 16, 17, 18, 19, 20, 21) describe in detail the different geometries of commercial PECVD reactors and their use in the deposition of specific films.

In a direct PECVD system, all the reactant gases as well as the substrate are exposed to the rf plasma glow. A schematic of a direct plasma cold-wall reactor is shown in Fig. 2. The upper electrode is connected to the high-frequency rf power generator through an impedance-matching network. The reactants are introduced through the upper electrode, which is perforated, and directly enter the plasma region. The gases are pumped out at the bottom of the chamber. The lower electrode is a continuous plate and has a resistive heating element to heat the substrates. Substrates (wafers) are placed on the lower electrode and are thus immersed in the plasma as well as heated to the desired temperature of deposition.

Fig. 2 Schematic of a direct plasma cold-wall reactor. Source: Ref 17, 22

Remote PECVD Systems. In a direct PECVD system, the substrate is exposed to the plasma discharge environment, which may cause radiation damage in the film being deposited. In a remote PECVD system, the substrate is removed from the plasma glow, and the reactant gases may be selectively excited by the plasma. The excited species are then carried to the substrate surface, where they react with other adsorbed gaseous reactants and the desired chemical reaction for deposition takes place. Keeping the substrate out of the plasma glow eliminates the deleterious effects of radiation. Moreover, this technique allows independent optimization of plasma, reactant gas chemistries, and wafer parameters.

One example of a remote PECVD system is shown in Fig. 3. This reactor was designed and built at the Massachusetts Institute of Technology (Ref 23) for metallo-organic CVD (i.e., CVD where some of the source gases are metallo-organic compounds) of gallium arsenide. The reactor consists of three vertically aligned concentric regions. The group III source gases enter through the center tube, the group V source gases enter between the inner and center tubes, and the reaction byproducts are exhausted between the inner and outer tubes. The substrate rests horizontally on a SiC-coated graphite disk, which is heated from below by a 750 W quartz halogen lamp with an elliptical reflector. The reactor can be equipped with internal electrodes fabricated of either aluminum or tantalum that can be connected via feedthroughs to an external power source to generate the plasma. The electrode configuration consists of an aluminum cylinder, 5 cm (2 in.) in diameter and 4 cm (1.6 in.) long, placed parallel to the gas stream. The distance between the wafer and the plasma can be varied by moving the electrode up or down in the inner quartz tube. In this design, the substrate receives a uniform distribution of plasma species and uniform infrared heat.

Fig. 3 Schematic of a remote plasma-enhanced chemical vapor deposition reactor for depositing compound semiconductor films. TMG, trimethylgallium. Source: Ref 23

Hybrid PECVD systems are a combination of the direct and remote PECVD systems. One example of such a system is a reactor designed for the deposition of amorphous hydrogenated silicon (Ref 24). In this reactor, one of the electrodes is replaced by a grid and the substrate is positioned directly under it. The substrate is thus shielded from the direct plasma glow while the reactant gases are subjected to the plasma discharge.

In some PECVD systems, microwave power is used to generate plasma. The plasma is excited by the resonance of microwaves and electrons through a microwave discharge across a magnetic field of 800 to 1200 G. This process, called electron cyclotron resonance, allows the film to grow at high rates of deposition at very low gas pressures (• 10-4 torr) and at very low temperatures (<275 °C, or 525 °F) (Ref 25). The plasma is contained magnetically in the system and the deposition occurs outside the glow region, while all the gases are subject to plasma excitation.

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