Pattern Transfer Reading Chapter 7 Fabrication Engineering at the Micro and Nanoscale OutLine Photolithography Phase shifting optical lithography E beam lithography X ray lithography Focused ion beam lithography Neutral atomic beam lithography Photolithography System Areal Image of the Mask The dividing line is the pattern of radiation that strikes the surface of the wafer Aligners they reproduce the image of a particular layer they must also align that layer to the previous ones Measures of Aligner Performance There are three primary measures of performance for an aligner Resolution the minimum feature size that can be exposed Determined by optical system resist etch process Registration a measure of the overlay accuracy from layer to layer Determined by optical system and aligner Throughput how many wafers hour Determined by optical system resist Photolithography Optical System Geometry size like lens is larger than wavelength of light light can be treated as a article traveling in straight lines between the components Feature sizes on the mask approach the optical wavelength we must consider properties such as diffraction and interference Without diffraction and interference knowledge it is difficult to understand the lithographic limitations and the reason that one system is superior to another Interference Two slit interference As the two waves propagate outward from the slits they will interfere There will be constructive interference at places where the two waves are in phase where the path lengths from the two slits differ by an integral multiple of the wavelength And there will be destructive interference at places where the two waves are 180 out of phase where the path lengths from the two slits differ by an odd multiple of half of the wavelength For example there is constructive interference at point A in Fig 3 and destructive interference at point B Two slit interference Interference What is the interference pattern on a screen that is located very far to the right of the wall Assume that the screen is parallel to the wall Far field limit is D d Two slit interference Interference In this d case we effectively have a single light source from a single slit interference from the two slits is irrelevant because the waves can never be much out of phase The function in the first plot is simply the intensity we would see from a single slit The d 0 5 plot gives the cutoff case when there is barely destructive interference at x infinite The d 5 and d 50 plots exhibit noticeable interference The local maxima occur where the two pathlengths differ by an integral multiple of the wavelength The local minima occur where the two pathlengths dffer by an odd multiple of half of the wavelength Two slit interference In practice we re usually concerned only with small x and values in which case Eqs and this small angle approximation N slit interference Interference Fig 11 shows the actual intensity on the screen Diffraction when a plane wave impinges on just one wide slit with width a if the width a isn t negligible compared with the wavelength the interference pattern will depend on a in a particular way we can consider the wide slit to consist of an infinite number of line sources or point sources if we ignore the direction perpendicular to the page next to each other each creating a cylindrical wave the diffraction pattern from one continuous wide slit is equivalent to the N infinite limit of the N slit result diffraction refers to a situation with a continuous aperture inter ference refers to a situation involving two or more apertures whose waves interfere diffraction is simply the N infinite limit of interference A specific kind of pattern arises so it makes sense to give it its own name We can combine interference and diffraction by constructing a setup with waves coming from a number of wide apertures Diffraction Pattern We ll keep working in the far field limit which here means that D a we imagine the slit of width a to consist of N infinitesimal slits separated by a distance d a N Width of the Diffraction Pattern Note that this is inversely proportional to a The narrower the slit the wider the diffraction pattern The farther away from the screen larger L the wider the pattern of light becomes The narrower the opening smaller a the wider the pattern of light becomes Near Field Fresnel Diffraction Diffraction When waves encounter an obstacle or a slit The waves will change direction and bend around the corners of an obstacle or aperture into the region of geometrical shadow of the obstacle These characteristic behaviors are exhibited when a wave encounters an obstacle or a slit that is comparable in size to its wavelength The diffraction phenomenon is based on the interference of waves Diffraction On the mask is a single feature a long narrow aperture of width W and length L Any local disturbance in an optical system such as a photomask can be considered to generate a large number of spherical wavelets that propagate outward from the point of disturbance The mask can be divided into a large number of differential rectangular elements of width dx and length dy Each rectangular element generates a wavelet The effect of wave on the surface of wafer is the sum of all of the wavelets Diffraction When W and L the summation of an infinite series of spherical waves that exactly reproduces the undisturbed wave When the aperture is of finite extent the electric field is the superposition of plane waves with different phases In the simplest case the aperture has been divided into only two elements The cross term due to interference between the wavelets It gives rise to the oscillations that are a characteristic part of the diffracted image Near Field Fresnel Diffraction In the case r is the radial distance between the center of the diffraction pattern and the observation point generally r W The result is near field or Fresnel diffraction The edges of the image rise gradually from zero and the intensity of the image oscillates about the expected intensity The oscillations decay as one approaches the center of the image The oscillations are due to constructive and destructive interference of wavelets from the aperture in the mask The amplitude and period of these oscillations depend on the size of the aperture Near Field Fresnel Diffraction When W is small enough the oscillations are large When W is very large the oscillations rapidly die out no obvious diffraction For longer wavelength