Thin film thickness measurement of anti corrosive paint

1 Thin film thickness measurement of anti corrosive paint M Sc in Physics of materials-2004 SUPERVISOR: Prof. K. Kandasamy NAME: Mr S. Ketheeswaren REG.NO:M.Sc/PM/FT/2003/04 Department of Physics University of Jaffna Thin film thickness measurements 2 Abstract In this experimental study, thin film measurement was made for the thin film of locally available anti corrosive paint using air wedge method. The thin film was prepared by means of a smooth rotator. Thickness of the thin film measured in the air wedge methods is t  0.94 x10 4 m Thickness of the thin film measured by means of dial gauge is 1.0 x 10-4 m. Thin film thickness measurements 3 Introduction Thin film study is in progress in the current world, as a part of it, the thin film thickness measurements are very essential. There are several methods to make measurements in thin films in the modern world up to atomic scale. Even though, we are unlucky to tool those. We here use the ordinary air wedge method for the thin film measurements. We now concentrate to discuss the Air Wedge Method. Traveling Microscope Sodium Lamp Lens Glass Plates Thin Film Sample Figure 1: Experimental set-up of Air wedge method In the set up we take an interference pattern on which we get information to measure the thickness of thin film. To get an interference pattern here we must have (1) Coherent sources There must be a constant phase difference between two interfering waves and therefore they must have the same frequency. (2) The waves that are interfering must have approximately the same amplitude. To satisfy these conditions We use monochromatic sodium lamp The arrangement of having same type of waves one reflected on the glass plate and come back to telescope after undergoing reflections in the air wedge plates another going freshly to telescope after Thin film thickness measurements 4 refraction on glass plate. In the set up Sodium lamp is placed at the focal point of the convex lens, and the glass plate is mounted 45o with vertical or horizontal. To get informative expression to do the thin film thickness measurements, we follow some theoretical steps. Thin film thickness measurements 5 Theory a b t θ l Consider an incident beam is falling on air wedge. The beam undergoes partial reflection and refraction on the front glass plate and refracted beam further undergo reflection on the base glass plate. Thus we have two beams a and b. Optical path difference between two rays a and b is 2 t cos r . Where – Refractive index of the wedge medium. t - Thickness Pn 1 θ Pn X Qn β Qn 1 Suppose the nth bright fringe occurs at Pn. The thickness of the air film at Pn  Pn Qn For bright fringe 2 t cos r   1 For air cos r  1 t  Pn Qn Then Thin film thickness measurements  2  2n  1  2 (1) 6 (1) => 2 Pn Qn  2n  1  (2) 2 The next bright fringe (n+1) will occur at Pn 1 2 Pn 1Qn 1  2n  1  1  (3) 2 (2) & (3) => Pn 1QN 1  p n Qn  Tan   2 Pn1Qn 1  Pn Qn   Qn 1  Qn 2 But Tan  t l Eliminating Tan we have  t  l 2 l =>   2t Let  n be the separation between n bright fringes then n n n n   l 2t l n 2t Y = M X n = l 2t From the gradient of graph  n Vs n, we can calculate thickness “t” of the thin film. M= Thin film thickness measurements 7 Experimental Work Thin film preparation Two thin plates were taken, and one glass plate was fitted with smooth rotator, small amount of anti-corrosive oil paint was applied on one end of glass plate using art brush, and then this system was allowed to rotate few seconds then the diffusion of paint was checked. It was again and again allowed to rotate up to a very thin film was prepared. Film was allowed to dry well, thin film was cut in a rectangular form. After that, other plate was placed carefully on this plate as shown in figure below. Glass plates Thin film Effective Length of the Wedge Figure 2: Air wedge Apparatus was arranged as shown in the figure (1) on the page 2. Na source was placed at focal point of convex lens. The glass plate, convex lens and Na source were arranged in a straight line. Glass plate was set to an angle 45˚ with horizontal as shown in the figure 1. Travelling microscope was focussed to a point on the table of travelling microscope. Then the air wedge set up was placed on the table. Adjusting the glass plate inclinaton and micrcope a good interference fringes were obtained. A particular bright fringe was pointed in the cross wire of microscope and the reading of position was noted, then next 5th bright fringe was focused by microscope and corresponding reading was also taken. This procedure was repeted until 50th bright fringe was reached. Finaly effective length of the wedge was taken by means of travelling microscope. Thin film thickness measurements 8 Results Reading of effective lengths of the air wedges: = (60.5+20x0.01mm) – (1+28x0.01mm) = 59.42 mm Readings for the graph of Number of fringes Vs fringe separation Number of the fringe Traveling Microscope Readings in mm (+ 0.01mm) Number of the fringe (n) Fringe separation (ωn) N 55.64 0 0 N+5 56.4 5 0.76 N+10 57.06 10 1.42 N+15 57.92 15 2.28 N+20 58.63 20 2.99 N+25 59.56 25 3.92 N+30 60.55 30 4.91 N+35 61.68 35 6.04 N+40 62.62 40 6.98 N+45 63.86 45 8.22 N+50 64.93 50 9.29 Thin film thickness measurements 9 Number of fringes Vs fringe separation 9 y = 0.1862x - 0.3995 8 Fringe separation in mm 7 6 5 4 3 2 1 0 0 5 10 15 20 25 Number of fringes Thin film thickness measurements 30 35 40 45 50 10 Gradient of graph: 0.1862 mm l 2t l t 2M We have gradient M= Then thickness t Where = 5893Ao 5893x10 10 mx59.42 x10 3 m 2 x0.1862 x10 3 m t  0.94 x10 4 m Thin film thickness measurements 11 Conclusion & Discussion Thickness of thin film, In the air wedge methods is t  0.94 x10 4 m By means of dial gauge is t = (1.0  0.1) x 10-4 m. The diffraction effect play a role if the size of the object is comparable with the wave length of the light used. Therefore diffraction will have no effect if the thinness of thin film is high and not comparable to the wave length of the light used. 1. Even we measure a value for thickness of thin film the value belongs to the end edge of thin film. End edge of thin film Effective Length of the Wedge Therfore we must testify that the film is perfectly flat. Then only we can predit the value of thickness for whole film. 2. The glass plate must be in perfect staight without any dumping, if not as shown below all calculation will be error and we won’t be able to get a good interference pattern. Therfore we must testify that the glass plates are perfectly flat. Thin film thickness measurements 12 References and date of referred: 1. http://hypertextbook.com/physics 11/7/2004 2. A – Level Physics. By ROGER MUNCASTER ---- FOURTH EDITION 3. Experimental physics for students By R.M.WHITTLE & J.YARWOOD Thin film thickness measurements