Why do we need high vacuum for deposition of thin films and what does vacuum do?

Thin films are deposited by a number of processes, including sputtering (DC, AC, diode, magnetron), thermal and electron beam evaporation, molecular bean epitaxy (MBE), chemical vapor deposition (CVD), pulsed laser deposition (PLD) and atomic layer deposition (ALD).  Although the mechanism for deposition for these processes differ significantly, they all have one thing in common:  they require a high vacuum environment.  Vacuum deposition is a family of processes used to deposit layers of material atom-by-atom or molecule-by-molecule on a solid surface. These processes operate at pressures well below atmospheric pressure (i.e., vacuum). The deposited layers can range from a thickness of one atom up to millimeters, forming freestanding structures. Multiple layers of different materials can be used, for example to form optical coatings. The process can be qualified based on the vapor source; physical vapor deposition uses a liquid or solid source and chemical vapor deposition uses a chemical vapor.

 

Here V = volume of vessel or vacuum chamber, n = number of particles (atoms, molecules), R = universal gas constant and T = temperature.  Thus, we see that pressure decreases when the number of particles in the chamber decreases.  Why is this important?  All deposition processes essentially move an atom or molecule from a source to a substrate and condense atoms on a substrate.  These processes will not work if the pressure is above a certain value, i.e., there are too many gas atoms or molecules in the chamber which degrade the transport of atoms from the deposition source.   As will be addressed in the future, the quality and microstructure of the thin film depends critically on a low pressure.

The vacuum environment serves one or more purposes:

*. Reducing particle density so that the mean free path for collision is long, thus preserving the energy of the particles
*. Reducing particle density of undesirable atoms and molecules (contaminants)
*. Providing a low pressure plasma environment
*. Providing a means for controlling gas and vapor composition
*. Providing a means for mass flow control into the processing chamber.

Sputtering processes employ a plasma to bombard and eject atoms from the source.  Atoms are ejected with a specific energy.  Collisions of atoms ejected by the deposition source with gas atoms in the chamber reduce the energy of the ejected atoms, which results in poor film adhesion to the substrate and with high porosity.  Atmospheric gas atoms (oxygen, nitrogen, hydrogen, etc.) present in the deposition chamber can contaminate the thin film and even change its composition if not eliminated to very low levels.  Deposition processes such as sputtering employ a plasma to eject particles from the source.  A plasma can only form at low pressures (~ mTorr).   In the reactive deposition process a reactive gas (oxygen, nitrogen, hydrogen, fluorine, hydrocarbon, etc.) is introduced into the deposition chamber to react with the ejected source particles to form compounds such as oxides, nitrides, fluorides, carbides, etc.  These reactive gases must be held at a pressure low enough to form a plasma and conform to the above criteria.  High vacuum in the deposition chamber also provides the means to introduce and control gases located externally at a higher pressure.

Can any atmospheric process (i.e., non-vacuum) achieve high quality thin films?  The answer is generally “no”.  The only possible exception is electroplating, where metal ions dissolved from a source are transported in an ionic solution from the source to the substrate.  Atmospheric plasmas are used for a variety of applications, except for deposition of thin films.  Atoms or molecules cannot be ejected from a source or transported from a source to a substrate by any other method.

(Note: The above section contains corrected equations as of 5/5/17)