X-Ray Reflectivity to determine film thickness

Today we learned how to use an X-Ray Reflectometer to measure the thickness of the 1.4% polystyrene film we spin coated onto a glass slide. This instrument allows very precise measurements, down to several nanometers. It takes a great deal of time to set up the instrument, as there are several different stage types on which to place your specimen, and several different sizes of detection devices and X-Ray beam width calibrators that can be used. Once all of the components one needs to do one's measurements have been swapped and set into place, the zeroing process for the instrument begins.




First, the components you have placed into the instrument for use in your measurements are calibrated for their position. An X-Ray beam is sent into the empty sample area and, by viewing a computer monitor, one is able to tell if the X-Ray beam is falling on the center of the stage. The peak radiation counts should be in the 1,000,000 range in the center of the stage. If they are not, a screwdriver is used to very very gently adjust the stage position so that the beam will be centered. After each adjustment, another X-Ray beam is deployed; if the radiation readings indicate that the beam is falling very close to the center, the computer can be used to direct the instrument to reset the peak counts area to the center of the stage. This minute movement is then checked by running another scan. If the reset peak and the new peak are very close, the center of the stage has been set. At his point, the sample is introduced into the instrument.



Now, the instrument must be zeroed with the sample set onto the precise center of the stage. It takes some effort to center the slide, believe me! It is important to get the exact center of your sample in the exact center of the stage. Because spin coating is used, the thickness is pretty uniform (since the slide and sample are spinning at 12000 rpm) except near the edges, but the closer to the center of the sample you get, the less the chance of imperfection. Once the slide is in position, the number of counts obtained as a maximum number while zeroing the empty stage is halved to account for the thickness of the glass slide holding the polymer film. Again, the screwdriver is delicately employed to achieve about 500,000 counts. Better to be a bit under in counts than a bit over. Now, the instrument is set to manually scan the sample. For the first scan, the peak is compared to the peak obtained without the sample. These are usually pretty close, but if they are not then the instrument is rezeroed (by a series of computer commands) and the scan is run again. This continues two or three times until peaks obtained on two successive attempts are very close. Once this has been achieved, the instrument is set to scan.




Using the computer programs, the scan is initiated and the data saved to a designated file. During the scan, the thin polystyrene film is bombarded with tiny X-Ray beams which are reflected off of its surface. The angles of incidence are measured (known). Changes in the relationship between the amounts of incident and reflected radiation can be used to determine the thickness of the film (periodicity).




Data obtained from the scan is visually assessed; the wavelengths should be of the same amplitude (this indicates density difference and roughness of the film). Five wavelengths in succession that are uniform are selected for analysis. The thickness can be determined employing several mathematical formulas. Here is the article on X-Ray reflectometry that includes the math.