FABRICATION
TECHNIQUES

CLOSED SPACED SUBLIMATION (CSS)

Purpose: To deposit thin polycrystalline films of device quality.

The CSS technique is characterized by the short distance between the source and the substrate, which is usually less than 1 mm. The reaction chamber consists of a source and a substrate, separated by spacers. They are held between two graphite susceptors and are placed in a quartz reactor. Heating is either provided by visible or infrared lamps facing the graphite blocks outside the reactor. It can also be provided by the Joule Effect (resistance heating), using the two graphite blocks as resistances. A combination of the two can also be adopted.

Cooling of the substrate block can also be provided (some researchers have done that). This enables low substrate temperatures to be achieved and permits rapid cooling of the substrate block. The graphite block temperature is monitored by thermocouples, one each for the source and substrate. The aim of the CSS technique is to provide a temperature difference of a several 10s of degrees centigrade between the source and the substrate. This temperature difference enables a diffusion controlled transport mechanism.

The films are usually grown in a flow of gas, such as hydrogen or inert gas such as Argon or Helium. Some CSS systems also work under vacuum conditions of about 1 Torr.

It has been found that the transport conditions are independent of the gas flow rates and the geometrical factors of the reactor; the main parameters acting on the deposition rates are the spacing, the source and the substrate temperature, and the nature of the ambient gas. This deposition is done at the EDL lab at Burges Hall B-104.

THERMAL EVAPORATION

This is done inside a vacuum chamber where the material, usually in a boat is heated typically to its melting point and the substrate to be deposited on is positioned facing the source a couple feet away. A high current flowing through the boat heats it up and causes evaporation. A crystal monitor is mounted close to the substrate, which provides an estimate of how much and how fast the material is being deposited. The distance between the source and the substrate is wide to prevent solid particles reaching the substrate. This deposition is done at the EDL lab at Burges Hall B-104.

CHEMICAL BATH DEPOSITION (Aqueous Solution Method)

This deposition takes place in a beaker with DI water where it is heated between temperatures of 85 C to 90 C. Chemicals, namely Cadmium Acetate, Ammonium Acetate, Ammonium Hydroxide, and Thiourea are poured individually and its chemical reaction results in the deposition of CdS onto all surfaces in the bath, including the surfaces of the ITO coated glass. Typically, for a 20-minute run, a thickness of 800-1000 Å can be achieved. The substrate is usually annealed before another deposition of other materials takes place. This deposition is done at the EDL lab at Burges Hall B-104.

SPUTTERING

Sputtering is performed in a vacuum chamber where the substrate is mounted on a plate that is negatively charged and the source is released by impinging high-energy noble gas atoms, commonly Argon, provided by a high voltage dc or rf glow discharge. The high-energy impact of the ions induces atoms at the surface of the substrate to be ejected or sputtered. These sputtered atoms form a thin film coating after condensing. This deposition is done at the EDL lab at Burges Hall B-104.

SPRAY PYROLYSIS

In spray pyrolysis, the source is deposited to the substrate in droplets of liquid sprayed onto the surface of the substrate to form a coating. The substrate is heated to about 350-500 C, which results to films that are complex in structure. The liquid that is sprayed is made of the material diluted in either water or alcohol, or both. This deposition is done at the EDL lab at Burges Hall B-104.

ELECTRON BEAM EVAPORATION

This is a typical physical vapor deposition (PVD) process that is also performed in a vacuum chamber. A high dc voltage is applied to a tungsten filament that causes electrons to be discharged. The stream of electrons emitted excites the targeted solid and turns it into vapor, which travels to the substrate. As they reach the surface, they condense and form a thin film coating. This process is done at the EDL lab at E-322.

 

 

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