This is the latest in a series of flip chip tutorials intended for new flip chip users, potential users, and those interested in specific flip chip processes and applications. Tutorial #1 presents the basics: an overview of what flip chip is and does, and how it is made. The other tutorials cover a wide range of topics in more detail. Concurrently, FlipChips Dot Com’s Technology Updates present industry experts describing the newest developments in their fields; our Literature and Photo pages give supplemental material.

Filmetrics, Inc.

Single interface reflection occurs whenever light crosses the interface between different materials.   The fraction of light that is reflected by an interface is determined by the discontinuity in n and k. For light reflected off of a material in air as shown by To see how spectral reflectance can be used to measure optical constants, consider the simple case of light reflected by a single nonabsorbing material (k=0). Clearly, n of the material can be determined from a measurement of R. In real materials, n varies with wavelength (that is to say, real materials exhibit dispersion), but since the reflectance is known at many wavelengths, n at each of these wavelengths is also known, as shown here. n and k Definitions Optical constants (n and k) describe how light propagates through a film. In simple terms, the electromagnetic field that describes light traveling through a material at a fixed time is given by where x is distance, lambda is the wavelength of light, and n and k are the film's refractive index and extinction coefficient, respectively. The refractive index is defined as the ratio of the speed of light in a vacuum to the speed of light in the material (n=c/v). The extinction coefficient is a measure of how much light is absorbed in the material. Multiple Interfaces Consider now a thin film on top of another material. In this case both the top and bottom of the film reflect light. The total amount of reflected light is the sum of these two individual reflections. Because of the wavelike nature of light, the reflections from the two interfaces may add together either constructively (intensities add) or destructively (intensities subtract), depending upon their phase relationship. . Their phase relationship is determined by the difference in optical path lengths of the two reflections, which in turn is determined by thickness of the film, its optical constants, and the wavelength of the light. Reflections are in-phase and therefore add constructively when the light path is equal to one integral multiple of the wavelength of light. For light perpendicularly incident on a transparent film, this occurs when where d is the thickness of the film and i is an integer (the factor of two is due to the fact that the light passes through the film twice.) Conversely, reflections are out of phase and add destructively when the light path is one half of a wavelength different from the in-phase condition, or when     The qualitative aspects of these reflections may be combined into a single equation: . From this, we can see that the reflectance of a thin film will vary periodically with 1/wavelength, which is illustrated below. Also, thicker films will exhibit a greater number of oscillations over a given wavelength range, while thinner films will exhibit fewer oscillations, and oftentimes only part of an oscillation, over the same range.   For more information: The complete tutorial is available at www.Filmetrics.com , both on-line and as a downloadable file. Filmetrics designs and manufactures thin-film measurement equipment capable of measuring silicon layers up to 160 microns thick. Great for flip-chip thinning and MEMS applications! Filmetrics, Inc.        9335 Chesapeake Dr.      San Diego      CA 92123 Phone 858-573-9300     FAX 858-573-9400    Email info@filmetrics.com FlipChips Dot Com 210 Park Ave #300 Worcester, MA 01609 USA Phone 508-753-3572 FAX 508-757-0220 editors@flipchips.com Site Map   |   Contact   |  Privacy