Evolution in technical TPB deposition on policarbonate using

"Evolution in technical TPB deposition on policarbonate using different procedures and possible results. . . " L. Periale 06 July 2012

Coating Method • The coating is prepared by evaporation of cristalline TPB from a molibdenium boat which is heated resistively up to 200 °C in vacuum evaporator ( 2. 10 -6 bar). • The evaporation method is calibrated by means on float glass plates of known size and weight. • During the evaporation, the duration and heating current are registered toghether with reading a quartz crystal microbalance (QCM) placed at a certain distance from the boat. • The thickness of the evaporated subtance is measured by weighing the glass plates before and after coating.

Cern TPB Evaporation Chamber

Quartz Crystal Microbalance • A quartz crystal microbalance (QCM) takes advantage of the piezoelectric effect found in quartz crystals. • Application of an electric potential across the quartz crystal induces mechanical shear strain in the crystal. • If the polarity of this electric potential is reversed, the strain direction reverses. • Rapid oscillation of the electric potential polarity leads to vibrational motion of the quartz crystal. • Under the proper conditions, this vibration can induce an acoustic standing wave between the two crystal faces. • The frequency of the standing wave is proportional to the thickness of the quartz crystal. • If additional material is uniformly deposited on the face of the crystal, the additional thickness will decrease the resonant frequency of the acoustic wave.

This frequency shift due to mass deposition may be correlated to the absolute mass deposited via the following substituted form of the Sauerbrey equation: • Where: • ρq is the density of quartz, • Aq is the area of resonance, • Nq is a frequency constant for AT-cut quartz crystals • (1. 668 105 Hz cm), • Fq is the frequency of quartz prior to deposition, and • F is the frequency at any point during the deposition process.

Set up evaporation system in Turin a) The coating setup consists of a vacuum chamber with evaporator enclosing 3 ceramic crucibles which may melt simultaneously a TPB crystals, plus a quartz crystal microbalance (QCM) Inficon XQI- 8010 G in changeable position. b) The vacuum system is composed of a diaphragm pump and a turbo-molecular pump that allow vacuum levels close to 10 -7 mbar in the evaporator;

How it work • The crucibles are heated by a cartridge with an adjustable current that allows to monitor the temperature and control the evaporation rate, essential to prevent bubbling and sputtering of the TPB on the substrate. • The substrate is positioned on a sample-holder fixed on a spinning disk located 25 cm above the crucibles. A shutter located under the holder allowed to mask the exposed surface when required.

The relevant coating parameters… a) the deposition rate and thickness, b) the temperature in the crucible and the vacuum level are displayed in the deposition control units. c) this allowed a constant monitoring of the evaporation process.

and procedure When the deposition rate is stabilizing around a constant value, typically between 1. 5 and 2. 5 Å/s, a steady evaporation process of the TPB is establishing • The shutter is then opened and the spinning of the sample-holder is initiating • to insure a uniform TPB deposition on the substrate. •

…and • When the desired thickness is reached, the shutter is closed, • the evacuation is stopped and the evaporator was opened. • Then, the coated samples must be stored in N 2 atmosphere or in vacuum…

This facility is located in a class 10. 000 clean-room due to the stringent cleanliness conditions

Evaporation chamber: 12” plate

Next step… a) Samples degassing, before deposition b) temperatures monitoring during the evaporation (close to the samples) c) optical characterization: absolute quantum efficiency, transmission and diffusion.

How makrolon reacts ? in these conditions no one know… this is the challange