Process Monitoring and Control

To provide a reproducible vacuum deposition process, the principal deposition parameters that need to be monitored and controlled are:

• Residual gas pressure and composition prior to and during deposition

• Substrate temperature and temperature variations over the substrate surface

• Deposition rate

• Angle of incidence of depositing flux

• Purity of source material

Preprocessing, such as surface preparation and substrate heating

In the case of reactive evaporation, the following parameters also need to be monitored and controlled:

• Availability of reactive gases over the substrate surfaces

• Activation of reactive gases

• Gas density distribution in the processing chamber

Substrate Temperature Monitoring. The substrate loses heat by conduction and radiation, thus often making it difficult to monitor substrate temperature. Thermocouples embedded in the substrate fixture often give a poor indication of the substrate temperature, because the substrate often has poor thermal contact with the fixture. In some cases, thermocouples can be embedded in or attached directly to the substrate material. Optical (infrared) pyrometers can be used to determine the temperature if the surface emissivity and adsorption in the optics are constant and known.

Passive temperature monitors can record the maximum temperature a substrate has approached during processing. Passive temperature monitors register color changes, phase changes (for example, melting of indium), or crystallization of amorphous materials (Ref 133).

Source Temperature Monitoring. Generally, source temperatures are very difficult to monitor or control with precision. In molecular beam epitaxy, the deposition rate is controlled by carefully controlling the temperature of a well-shielded Knudsen cell source using embedded thermocouples (Ref 7).

Deposition Rate and Deposited Mass Monitors. Quartz crystal monitors measure the frequency of the oscillations as a function of the mass added to the crystal face. By calibrating the frequency change with the mass deposited, the quartz crystal output can measure deposition rate and total mass deposited (Ref 134). Ionization rate monitors compare the ionization currents in a reference chamber and a chamber through which the evaporant flux is passing. By calibration, the differential in gage outputs can be used as a deposition rate monitor (Ref 135). A vacuum microbalance can be used to measure the deposition rate and the total amount of material deposited. In electron-beam evaporation, the ions that are formed above the molten pool can be used to monitor the vaporization rate and the mass deposited.

The total amount of deposited material is sometimes controlled by evaporating to completion a specific amount of source material. This avoids the need for a deposition controller and is used where many repetitious depositions are to be made.

Film Thickness Monitoring. There is no easy way to measure the geometrical thickness of a film during deposition because the thickness depends on the density for a given mass deposited. In general, thickness is determined from the mass deposited, assuming a density so that the mass monitor is calibrated to give thickness.

Optical Property Monitoring. In optical coating systems, in situ monitoring of the optical properties of the films is used to monitor film deposition and provide feedback that controls the evaporators (Ref 136, 137). Generally, the optical transmittance, interference (constructive and destructive), or reflectance at a specific wavelength is used to monitor the optical properties. Ellipsometric measurements can be used to monitor the growth of oxides on semiconductor materials.

Electrical Property Monitoring. An electrically conducting path between electrodes can be deposited using a mask, and the electrical resistivity of the path can be used as a deposition monitor (Ref 138).

Film Stress Monitoring. There are several techniques for measuring the film stress during the deposition process. These techniques typically measure the movement of surfaces, as monitored by changes in capacitance between two plates, or they measure the deflection of a beam by optical interferometry (Ref 139) or an optical lever arm. X-ray diffraction measurements of the lattice spacing can be used to measure film stress due to lattice deformation (Ref 140).

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