Role of lenses: captures diffracted light

• the bigger the lens, the better the image

Types of lithography:

• Defined by the distance between mask and wafer:

• Contact - no gap
• Proximity - small gap between mask and wafer
• Projection - larger gap

• Contact
  • Pressure is applied for a better contact
  • Because there is no gap, there are no diffraction effects and the method has high resolution
  • Problem: due to contact, defects can be generated on the mask or wafer
• Proximity
  • Gap of 10-50 micron
  • Increasing the gap decreases the resolution, thus worse resolution than for contact printing
  • Preblems with Fresnel diffraction
• Projection
  • Light is recolected by projector
  • Problems with Fraunhofer diffraction (the distance is much larger than in the proximity case)
  • Numerical aperture (NA) defines the capacity of the projector to collect light: , where α is the angle shown in the picture and n is the refractive index 

• Resolution is as high as for contact printing
• The minimum feature dimension (resolution) that can be reproduced with projection is:
  • The bigger the lenses, the higher NA, thus the better the resolution
  • The smaller the wavelength, the better the resolution, thus DUV excimer laser is preferred for smaller structures, in comparison to the mercury lamp
  • Another method to obtain better resolution, is to decrease k - Rayleigh constant. Usually k is around 0.5 - 0.6, but can be decreased until 0.25. RET (resolution enhancement techniques) are used to obatin a better image for low k values:
    • OPC (optical proximity corrextion) in which masks are modified, taking into consideration the proximity effect
    • PSM (phase-shift masking) in which masks are modified , so that some openings are coated with a material with different refractive index. The material modifies the pahse of the light, and distructive interference occurs on the wafer surface. The contrast is increased, thus smaller features can be created.
    • OAI (off-axis illumination) in which light is tilted and doesn't hit normally the mask.