Film Deposition
Etching is a fabrication process in which material is removed from a substrate. Deposition is a fabrication process in which thin films of various materials are deposited on the surface of the wafer. There are many ways to deposit different types of thin film materials on a substrate.
Two important film deposition process issues:
(1) Step coverage:
(a)Uniform or conformal step coverage (ideal case) whereby the film thickness on top and along the walls of a step is the same as that at the bottom of the steps. This uniform step coverage results from rapid surface migration whereby the energetic atoms or reactants adsorb on the surface and then rapidly diffuse along the surface for some distance before reacting to form part of the film.
(b) Non-conformal step coverage (practical case) whereby the film thickness on top is thick and uniform but is thin and non-uniform along the walls and bottom of a step. This non-conformal step coverage results from the rapid reactions between the arriving atoms or reactants with one another and the substrate surface to form part of the film without significant surface migration.
(i) If the mean free path of the target atoms or reactant is much larger than the dimensions of the step, with the arrival angle of the atoms 180 degree at the top of the horizontal surface whereas it is only 90 degree at the top of the vertical step, there could be some crack at the bottom of the steps due to self-shadowing resulting in part of the bottom of the step having a thicker film than other parts of the bottom.
(ii)If the mean free path of the target atoms or reactant is close to the dimensions of the step, and with the arrival angle of the atoms is 270 degree at the top of the vertical step and only 90 degree at the bottom of the vertical step, there could be thick cusp at the top of the step and thin layer at the bottom of the step.
(2) Film stress:
A thin film deposited onto a substrate can be either in tensile stress in which the film would relax by contracting or it may be in compressive stress in which the film would relax by expanding. If the stress is too large, the deposited film may eventually peel off the surface of the substrate. There are 2 major components in the stress experienced by the deposited film. One component is the thermal stress (sth),which is due to the mismatch in the thermal expansion coefficients of the film with respect to the substrate material especially at elevated temperature during deposition. The second component of the stress is the intrinsic stress(sbi). During film deposition, deposited film may form single crystal grains. In polycrystalline films, stress can arise when the deposition is carried out at elevated temperature where the larger grains grow at the expense of the smaller grains. This increase in crystalline order leads to an internal stress (within the film) which depends on substrate temperature deposition rate, film thickness and chamber ambient. Thus in general, the overall stress s = sth + sbi
(ii)If the mean free path of the target atoms or reactant is close to the dimensions of the step, and with the arrival angle of the atoms is 270 degree at the top of the vertical step and only 90 degree at the bottom of the vertical step, there could be thick cusp at the top of the step and thin layer at the bottom of the step.
(2) Film stress:
A thin film deposited onto a substrate can be either in tensile stress in which the film would relax by contracting or it may be in compressive stress in which the film would relax by expanding. If the stress is too large, the deposited film may eventually peel off the surface of the substrate. There are 2 major components in the stress experienced by the deposited film. One component is the thermal stress (sth),which is due to the mismatch in the thermal expansion coefficients of the film with respect to the substrate material especially at elevated temperature during deposition. The second component of the stress is the intrinsic stress(sbi). During film deposition, deposited film may form single crystal grains. In polycrystalline films, stress can arise when the deposition is carried out at elevated temperature where the larger grains grow at the expense of the smaller grains. This increase in crystalline order leads to an internal stress (within the film) which depends on substrate temperature deposition rate, film thickness and chamber ambient. Thus in general, the overall stress s = sth + sbi
No comments:
Post a Comment