#### Transcript Light Sources

Light Sources 1 Xe Lamps UV LEDs Photo Mask Fabrication 4 Photolithography Photomasks and Reticles Examples of 5X Reticles: Mask Alinement 10 Photolithography Photomasks and Reticles Photomask This is a square glass plate with a patterned emulsion of metal film on one side. The mask is aligned with the wafer, so that the pattern can be transferred onto the wafer surface. Each mask after the first one must be aligned to the previous pattern. Difraction Problem 16 Phase-Shifting Phase-shift Mask Photolithography-Diffraction • At smaller dimensions, diffraction effects dominate • If the aperture is on the order of l, the light spreads out after passing through the aperture. (The smaller the aperture, the more it spreads out.) • If we want to image the aperture on an image plane (resist), we can collect the light using a lens and focus it on the image plane. • But the finite diameter of the lens means some information is lost (higher spatial frequency components). PhotolithographyDiffraction • Image formed by a small circular aperture (Airy disk) as an example • Image by a point source forms a circle with diameter 1.22lf/d surrounded by diffraction rings (airy pattern) • Diffraction is usually described in terms of two limiting cases • Fresnel diffraction - near field. • Fraunhofer diffraction - far field. PhotolithographyDiffraction PhotolithographyDiffraction • Rayleigh suggested that a reasonable criterion for resolution (R = distance between A and B) is that the central maximum of one point source lies at the first minimum of the Airy pattern of the other point (R = diameter of circle) • The numerical aperture (NA) of a lens represents the ability of the lens to collect diffracted light and is given by NA = n sin a in this expression n is the index of refraction of the medium surrounding the lens and a is the acceptance angle of the lens ( n = 1 for air) PhotolithographyDiffraction • In the latter expression k1 is an experimental parameter and depends on resist properties and the lithography system ( 0.60.8) • You may remember that, for a plane wave incident on a grating of period d, the angles q at which the intensity maxima in the image occur are given by: sin q = N l/d, where N= 0,1,2,…. Photolithography-Diffraction • The angle q in the figure is the maximum angle for which diffracted light from the mask will be collected for imaging by the lens. • With sin q = N l/d now, only those values of N for which the term on the right is less than sin q are allowed. Thus, as the period d gets smaller (l/d gets larger), N gets smaller (i.e. lower diffracted orders). • The figure on the right shows the spread of the diffracted orders for a decrease in relative slit width (b). • Because of this spreading effect, fewer diffracted orders form the image. This means that information about the pattern is being lost. PhotolithographyDiffraction • The figure on the right shows the effect of including increasing numbers of diffracted orders on the image of a slit of width w. You can think of the aperture as truncating these diffracted orders at some small number. • The value of sin a for an optical system is the numerical aperture, or NA. If the value of the NA is small for a system, fewer orders will be imaged, and the grating may not be resolved. • It has been shown that the depth of focus, DOF, or the range of focus for which a feature can be resolved, is given by: DOF = k2 l/(NA)2 • The R and DOF equations sum up all of the problems and the promise of optical lithography using projection tools: The way to increase resolution is to decrease the wavelength at which the machine can operate, and to increase the numerical aperture of the lens. However, both of these options have the effect of decreasing the depth of focus. Electron Beam Lithography 26 مطالب دیگر در موضوع لیتوگرافی Sychrotron radiation • Emitted when a fast electron interacts with a magnetic field. This causes the electron to change direction. As a result, the electron will be accelerated, causing it to radiate electromagnetic energy which could be be in the form of X-rays. X-Ray Production • Electrons jump off the heated cathode. • Free electrons are accelerated at the target by high voltage. • Electrons hit the target & give off x-rays.