242

30. E v a p o r a t i o n and Sput ter ing 3o

227. Optically Active Fluorite Films. United Kingdom. Letter by Niels O. Young and Jakym Kowal, Nature 183, No. 4654, 104, 10 Jan. 1959.

30 228. The Production of Thin Films by Thermal Evaporation of Inorganic Materials in High Vacuum. Survey of present day techniques and evaporation plant design. Discussion concerning the measurement of thickness of coating during deposition and the selection of materials to be evaporated and evaporation sources.

Albert Ross, Vakuum-Technik 8, 1-11, 1959.

30 229. Thin Magnetic Films. United Kingdom. The preparation by vacuum deposition of thin films of magnetic alloys with specific characteristics is complicated by the large number of parameters involved. The relation between film thickness and coercivity has already been studied by other workers for film thicknesses in excess of domain wall thickness. Small variations in thickness were shown to give rise to energy changes as domain walls were moved such that the relation Hc~t -4~3 is obtained where Hc is the coercivity and t the average thickness. This dependence has been investi- gated more fully in this report. An apparatus was built which enabled the low frequency hysteresis loop to be monitored both during deposition of the film and afterwards. A diagram of the apparatus is shown. Various nickel iron alloys were evaporated from an induction heated source on to glass substrates held at temperatures in the region 250-400 °C. An alternating (50 c/s) aligning field in the plane of the substrate was used. The measuring apparatus was sensitive enough to give a recognisable hysteresis loop for films only 150A_ thick. In most evaporations the coercivity changed only slightly as the thickness increased from 150 to 1500A. Up to about 10,000A a steady decrease of coercivity from 2 oersteds to about 0.5 oersteds is seen. A sudden increase to about 20 oersteds may occur at a thickness of about 2000~ and this is attributed to atomic arrangements within the layer. On other substrates the behaviour of the coercivity may be different. If the glass is coated with a layer of heat-oxidized aluminium a few hundred angstrom units thick, an initial coercivity of 10-20 oersteds may be obtained, which steadily decreases to 1-5 oersteds as the thickness increased to 1500A. In general these results show that the variation of coercivity with thickness is not a simple relation but is influenced strongly by the substrate. Certainly it is possible to make magnetic specimens with a well defined magnetic axis and relaxation time less than 200 m~ in an applied field of twice the coercivity. G.S.

Letter by R. S. Webley, Nature 183, No. 4666, 999, 4 April 1959.

30 230. Calculation of Absorption Peaks in Thin Metal Films. United Kingdom. Letter by R. M. Hill, Nature 182, No. 4643, 1150, 25 Oct. 1958.

30 231. Errors in the Measurement of Film Thickness by Multiple- Beam Interferometry. United Kingdom. When making measurements of film thickness by multiple-beam interferometry it is necessary to produce a sharp step in the film. This can be done by scribing a line with a pointed instrument, across the film surface. The authors show that serious errors may arise in this method due to marring of the substrate surface. G.S.

Letter by C. Weaver and P. Benjamin, Nature 182, No. 4643, 1149, 20 Oct. 1958.

Abstracts 227-235

Vacuum Applications

30 232. Mechanical Properties of Whiskers and Thin Films. United Kingdom. This is a review of the nineteen papers which were presented at a convention on 17-19 March 1958 held at the Cavendish Laboratory, Cambridge and Tube Investments Research Laboratories. The main purpose of the meeting was to review the present state of knowledge on the high mechanical strength observed when one or two dimensions of a test specimen are reduced to a low value. Papers were invited from universities and from industrial research laboratories in Great Britain and the United States of America.

Review by J. E. Gordon and J. W. Menter, Nature 182, No. 4631, 296, 2 August 1958.

3O 233. Measurement of the Optical Constants of Thin Dielectric Films by Means of Frustrated Total Reflexion. New Zealand. It is shown that the transmission of light through a thin dielectric film under conditions of frustrated total reflexion can be used to measure the refractive index and thickness of the film. The refractive index and thickness have been measured for each of several magnesium fluoride films. The thickness was checked in each case by multiple beam interference and the refractive index compared with the results obtained by other authors for similar films. It appears that many present concepts on which the theory of the optical properties of thermally evaporated films is based are inadequate for the problems involved. (Author)

B. P. Sanford, J. Opt. Soc. Amer. 48, 482-486, July 1958.

30 234. Use of Fe 55 for Measuring Titanium Coating Thickness. United States. Note by P. D. Zemany, Rev. Sci. lnstrum. 30, 292, April 1959.

30 235. Two-layer Anti-reflexion Coating for Low Refractive Index Glass. Gt. Britain. A single-layer anti-reflexion coating on glass of index about 1.5 would require a coating material of index about 1.23, but available materials which form durable coatings all have indices greater than 1.3. Various multi-layer coatings have been proposed, but have met difficulty with absorption, and lack of durability, in the high-index layer needed. The authors propose the use of Bi~O3 as the high-index material in a two-layer coating, the layer being obtained by sputtering Bi in 02. The oxide film has a refractive index of about 2.45 and is used with MgF2 (1.38), the high-index layer being made 0.049 A thick and the low-index 0.33 A. The apparatus is described in some detail, with a sectional drawing. The evaporation source and sputtering electrodes are mounted in the same vacuum chamber. The electrodes are 60 °, 6~ in. r sector plates, made from ~ in. Cu, with Cu cooling pipes brazed to the underside. Each is mounted, on a vertical pillar (which encloses pipes for the cooling water), in a horizontal plane with the apex near the centre of the chamber. One is 4 in. below the surface of the glass to be coated, which is mounted in a rotary holde, the other 6 in. below. The electrodes are connected to the secondary of an h.t. transformer (not earthed) so that each becomes the cathode in turn. Each plate is coated with Bi and earthed shields are arranged to prevent sputtering of the Cu fittings. A photometer system projects a beam of light on to the underside of the glass plate being coated and receives the reflected light, to allow the reflectance to be monitored during coating. To obtain Bi~O3 films with low light absorption, it was found necessary to sputter at a high current density (10 mA/cm ~) and to allow 02 to flow continuously through the chamber. The film of oxide was deposited at 1 A/sec. The Bi203 is deposited until the reflectance has increased from 4 per cent (clear glass) to 8 per cent : the MgF2 is then evaporated until the combined reflectance