Editing Ellipsometry (section)

===Imaging ellipsometry===
Ellipsometry can also be done as [[imaging ellipsometry]] by using a [[Charge-coupled device|CCD]] camera as a detector. This provides a real time contrast image of the sample, which provides information about film thickness and [[refractive index]]. Advanced imaging ellipsometer technology operates on the principle of classical null ellipsometry and real-time ellipsometric contrast imaging.  Imaging ellipsometry is based on the concept of nulling.  In ellipsometry, the film under investigation is placed onto a reflective substrate.  The film and the substrate have different refractive indexes.  In order to obtain data about film thickness, the light reflecting off of the substrate must be nulled.  Nulling is achieved by adjusting the analyzer and polarizer so that all reflected light off of the substrate is extinguished.  Due to the difference in refractive indexes, this will allow the sample to become very bright and clearly visible. The [[light source]] consists of a monochromatic laser of the desired wavelength.<ref>{{cite book | last=Tompkins | first=Harland | year=2005 | title= Handbook of Ellipsometry| url=https://archive.org/details/handbookellipsom00tomp | url-access=limited | pages= [https://archive.org/details/handbookellipsom00tomp/page/n29 13]| bibcode=2005hael.book.....T }}</ref> A common wavelength that is used is 532&nbsp;nm green laser light.  Since only intensity of light measurements are needed, almost any type of camera can be implemented as the CCD, which is useful if building an ellipsometer from parts.   Typically, imaging ellipsometers are configured in such a way so that the laser (L) fires a beam of light which immediately passes through a linear polarizer (P).  The linearly polarized light then passes through a quarter wavelength compensator (C) which transforms the light into elliptically polarized light.<ref name="auto">{{cite book | last=Tompkins | first=Harland | year=2005 | title= Handbook of Ellipsometry| url=https://archive.org/details/handbookellipsom00tomp | url-access=limited | pages=[https://archive.org/details/handbookellipsom00tomp/page/n343 329]| bibcode=2005hael.book.....T }}</ref>   This elliptically polarized light then reflects off the sample (S), passes through the analyzer (A) and is imaged onto a CCD camera by a long working distance objective. The analyzer here is another polarizer identical to the P, however, this polarizer serves to help quantify the change in polarization and is thus given the name analyzer.  This design is commonly referred to as a LPCSA configuration.

The orientation of the angles of P and C are chosen in such a way that the elliptically polarized light is completely linearly polarized after it is reflected off the sample. For simplification of future calculations, the compensator can be fixed at a 45 degree angle relative to the plane of incidence of the laser beam.<ref name="auto"/>  This set up requires the rotation of the analyzer and polarizer in order to achieve null conditions.    The ellipsometric null condition is obtained when A is perpendicular with respect to the polarization axis of the reflected light achieving complete destructive interference, i.e., the state at which the absolute minimum of light flux is detected at the CCD camera. The angles of P, C, and A obtained are used to determine the Ψ and Δ values of the material.<ref name="auto"/>
: <math>\Psi = A</math> and <math>\Delta = 2P + \pi/2,</math>
where ''A'' and ''P'' are the angles of the analyzer and polarizer under null conditions respectively.  By rotating the analyzer and polarizer and measuring the change in intensities of light over the image, analysis of the measured data by use of computerized optical modeling can lead to a deduction of spatially resolved film thickness and complex refractive index values.

Due to the fact that the imaging is done at an angle, only a small line of the entire field of view is actually in focus.  The line in focus can be moved along the field of view by adjusting the focus.  In order to analyze the entire region of interest, the focus must be incrementally moved along the region of interest with a photo taken at each position.  All of the images are then compiled into a single, in focus image of the sample.