DIFFRACTION

• Notions of diffraction are needed, because when light passes through small openings of the mask, it can be diffracted (light goes behind the corners), and the light intensity on the resist will change
• The light can be characterised by:

• Huygen's principle says that a local disturbance generates spherical wavelets that will interfere
• Diffraction occurs when light hits edges of objects. Thus it has a higher impact when the openings of the masks are small (for making small devices), and a lot of edges will be in the light's way
• Diffraction bands are observed when light waves pass through narrow slits.
• When the light passes through the small windows of the mask, it will be diffracted, and the light intensity that will fall on the photoresist will differ in the created diffracted bands (maximum intensity in the middle, then the intensity has peaks - first order, second order, etc. of interference - see picture below)

• We will calculate the light intensity after light passes through an window, and how the window dimension influences it: (S - source, D - distance source -mask, g - distance mask - wafer, W, L - dimensions of the opening)

• The total effect can be calculated by integrating over the whole surface of the window
• For W, L → ∞, we can speek about an undisturbed wave and the intensity of the light is:
• For W, L finite, the total intensity depends on the light phase of the secondary sources placed at the opening (see Huygen's principle):
• The interference can be constructive if the waves have similar phases and the intensity will increase,
• or it can be destructive in the waves are out of phase, and the intensity will decrease
• Because the general solution is quite complicated, there are two special cases, which are the most important in lithography:
  • Fresnel diffraction - near-field diffraction - occurs when there is a small distance light-mask-wafer  ()
  • Fraunhofer diffraction - far-field diffraction - occurs when there is a larger distance light-mask-wafer  ()