Wave Polymerization During Vapor Deposition of Porous Parylene-N Dielectric Films
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Mat. Res. Soc. Symp. Proc. Vol. 565 01999 Materials Research Society
run, the chamber was pumped to a base pressure of 0.01 Torr as measured with a capacitance manometer pressure gauge. For our deposition rate runs, the pressure was then brought to 300 Torr with nitrogen to limit the amount of premature sublimation of the dimer. After the sublimator operating temperature was reached, the pressure was reduced to 0.09 Torr by an automatic feedback pressure controller driving a butterfly valve on the deposition chamber exhaust line. During the runs for which we report deposition rates, the pressure was maintained constant at 0.09 Torr. In the runs in which we monitored the 'wave' polymerization, the butterfly valve was manually held open and pressure was allowed to rise on its own during the course of the run. For a typical 'wave' polymerization run, the maximum chamber pressure achieved ranged from 0.04 to 0.06 Torr. Substrates were cut from polished silicon (100) wafers obtained from Silica Source. Samples were approximately ¾" x 2 ½", and were employed without pretreatment (e.g., no adhesion promoter was employed). The samples were weighed before and after each run. Following deposition, the substrate was freeze fractured by immersing it in a container of liquid nitrogen until it reached liquid nitrogen temperature, then removing it and breaking it with a diamond scribe. Cross-sectional SEMs were taken with a JEOL JSM-840 scanning microscope, operating at an accelerating voltage of 15 keV. Prior to SEM imaging, samples were coated with gold in a Denton sputter coater for 180 seconds at 20 mA and 2.5 kV in Argon in order to minimize electron beam charging. Wave polymerization videos were taken with a Sharp View Cam VL-E750 with a 32X digital zoom, operating at 15 frames per second. RESULTS During vapor deposition polymerization (VDP) of Parylene-N at sample temperatures above 217 K, a transparent polymer thin film is continuously formed as evidenced by color changes associated with light diffraction as the thickness of the film steadily increases. In contrast, we observe a very unique phenomenon for deposition at liquid nitrogen temperatures. Upon initiation of monomer flow into the chamber, the originally polished, shiny surfaces of the sample holder and the substrate become cloudy. The intensity of the cloudiness increases as the run progresses. We interpret this visual change as an increase in the amount and thickness of monomer condensed on the substrate and sample holder. At a certain point in the run, which varies depending upon process conditions, a very rapid, dramatic change takes place in the observable film properties. These changes are initiated by a moving 'wave front that propagates from one side of the sample surface to the other, as shown in Figure 1. In this figure a sequence of pictures of the sample holder and substrate surface are shown for a given run, where the first frame in the figure is at a deposition run time just before the 'wave' started, and the last frame is just after the 'wave' mov
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