1EE129, Photolithography I, W+T 13 1Photolithography II• Basics• Nature of light• “Steppers”• Contact Printing• Alignment– Marks– Errors– Design Rules• Lithography TrendsEE129, Photolithography I, W+T 13 2Basics• Resolution– Simply put the smallest feature that can be resolved. Ultimately, it is not just home thin a line of PR we can pattern, but also how thin we can carry out our process• Diffusion under masks• Undercutting of masks• Pattern registration• ThroughputEE129, Photolithography I, W+T 13 3Basics• Pattern registration– How well can a previous mask step be fit to to following mask step.DiffusionContactWindow2EE129, Photolithography I, W+T 13 4Basics• Throughput– This is a measure of how many wafers you can pattern per unit time. • For instance: I am down to 3 minutes per 4” wafer including exposure time with 99% accuracy. What if I could go faster, but my accuracy dropped? How would you determine which was more cost effective?EE129, Photolithography I, W+T 13 5Photolithography• Nature of light– Light can behave as if it were a particle and as if it were a wave.– Depending on the type of photolithography exposure system will dictate how you have to treat light in your model.EE129, Photolithography I, W+T 13 6Photolithography3EE129, Photolithography I, W+T 13 7“Steppers”• The Mask is separated from the wafer• The image is reduced (makes it easier to make a mask)• The same pattern is stepped over the entire surface automaticallyLightMaskLensPREE129, Photolithography I, W+T 13 8“Steppers”• Expensive to maintain• Susceptible to lens errorsEE129, Photolithography I, W+T 13 9Contact Printing• Inexpensive, but not automatic.• The features have to be the same size on the mask as the are on the wafer.• The mask are in contact with the PR which can cause damage to the masks.4EE129, Photolithography I, W+T 13 10Alignment• This is the ability to successfully line up successive features on a wafer. What would happen if the gate metal were placed over the S or D contacts?EE129, Photolithography I, W+T 13 11Alignment• This is the ability to successfully line up successive features on a wafer. What would happen if the gate metal were placed over the S or D contacts?SubstrateMaskAlignedEE129, Photolithography I, W+T 13 12Lithography Trends• E-Beam– Electron beams have a large energy or small wavelength, but have to be scanned across the wafer, not flood exposed• Deep UV or Soft x-ray– Lenses can still be used to focus light as is a stepper reduction system• X-ray– Back to Contact