CHEMICAL VAPOR DEPOSITION OF IR-TRANSMITTING ZINC SULPHIDE

Transcription

1 CHEMICAL VAPOR DEPOSITION OF IR-TRANSMITTING ZINC SULPHIDE Ph. Braudeau, G. Keller, J. Torre To cite this version: Ph. Braudeau, G. Keller, J. Torre. CHEMICAL VAPOR DEPOSITION OF IR- TRANSMITTING ZINC SULPHIDE. Journal de Physique Colloques, 1986, 47 (C1), pp.c1-193-c < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1986 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 JOURNAL DE PHYSIQUE Colloque Cl, supplément au n 2, Tome 47, février 1986 page ci-193 CHEMICAL VAPOR DEPOSITION OF IR-TRANSMITTING ZINC SULPHIDE Ph. BRAUDEAU, G. KELLER and J.P. TORRE Céramiques Techniques Desma.rqu.est, 2, Avenue A. Einstein, F Trappes Cedex, France Résumé - Le dépôt chimique en phase vapeur est un procédé permettant la réalisation de lentilles ou de fenêtres en sulfure de zinc dont la transmission dans la bande 8-12 microns dépasse 75%. Les facteurs jouant un rôle déterminant dans ce procédé sont la température du substrat sur lequel se dépose le sulfure de zinc et la pression à l'intérieur de l'enceinte. Abstract - Chemical vapor deposition is a suitable process for manufacturing ZnS lenses or windows with more than 75% transmittance in the 8 to 12 micron range. The most influent parameters of this process are the temperature of the substrate on which ZnS deposition takes place and the pressure within the reaction chamber. I. - INTRODUCTION Chemical vapor deposition (CVD) offers two main advantages over pressure sintering of powder for the fabrication of infrared transmitting zinc sulfide : (i) absorptions due to imputities of the powder are avoided, (ii) owing to the high density of deposits, light scattering associated with porosity is minimized. The chemical vapor deposition process can be summarized as follows in the case of zinc sulfide /1,2/.Zinc vapor and hydrogen sulfide react at the surface of a graphite substrate to form an adherent coating of zinc sulfide according to the reaction : Zn (v) + H 2 S (g) > ZnS (s) + H 2 The work reported in the present paper was undertaken in an attempt to study the influence of the temperature of the substrate and pressure within the reactor on both deposition kinetics and IR transmission of deposited parts. II - EXPERIMENTAL PROCEDURE Figure 1 is a schematic of the experimental reactor, featuring the evaporation zone, the reaction chamber, the input gas train and the device for measurement of zinc pick-up rate. Several CVD experiments were conducted in the temperature and pressure ranges respectively of C and torr. Temperature was measured in the Article published online by EDP Sciences and available at

3 cl-194 JOURNAL DE PHYSIQUE reaction chamber in the vicinity of the graphite substrate. The Zn pick-up rate to H S flow rate ratio varied between 0.7 and 2. 2 The in-line transmission from 2.5 to 50 pm was measured on polished samples cut at the top, middle and bottom of each deposit. A PERKIN-ELMER infrared spectometer was used for this purpose. I11 - RESULTS AND DISCUSSION ZnS deposition rate is given by figure 2 as a function of temperature for each pressure value (30 or 40 torr). A maximum deposition rate is always observed at 65Q C. Figure 3 demonstrates the influence of pressure on the deposition kinetics. Lowering the pressure within the reaction chamber results in an increase of the deposition kinetics. Variations of the Zn pick-up rate to H S flow rate ratio between 0.7 and 2 were found of insignificant influence on these 2 results. Comparative in-line transmission curves of typical samples deposited at different temperatures under 30 torr are shown in figure 4. The best in-line transmission is attained when deposition is conducted at 65OoC (more than 75% transmittance in the 8 to 12 micron range). Transmission curves of samples deposited at lower temperature exhibit an extrinsic band at 6 micron wavelength. This is characteristic of ZnH vibrational absorption /3/. At higher temperature the average transmittance is 2 reduced. Irrespective of temperature, most samples present an extrinsic absorption at 4.2 micron wavelength, due to the presence of CO2 bonds within the deposits. The preceding results can be explained as follows. When pressure and temperature are high enough, the frequency of molecular collisions within the gas phase increases in such a way that homogeneous.nucleation and growth of a powdery product takes place /Q/. This was evidenced when deposition was conducted at 750 C under 40 torr : under such conditions the deposition rate was found to be very low (figure 3) and the product of the reaction was essentially in the form of powder. The formation of powder and subsequent inclusions within the deposit can also explain the decrease of the average transmittance when deposition is conducted at high temperature. IV - CONCLUSION Temperature in the vicinity of the substrate and pressure are factors of major influence on the deposition rate of ZnS and on the IR transmission of the deposited products. Increasing these parameters results in the formation of powder which proves detrimental to both deposition rate and average transmittance. The optimum temperature and pressure are.respectively 650 C and 30 torr. REFERENCES /l/ HILL,J., LEWIS, K.L., Int. Conf. Chem. Vap. Deposition, 6th! 1977) 7th, (1979) / 21 DiBenedetto, B.A., Pappis, J. and Capriulo, A. J., Technical Report AF AL-TR ( 1973) / 3/ DiBenedetto, B.A., Pappis, J. and Capriulo, A.J., Technical Report AF AL-TR (1973) 141 Bryant, W.A., J. Mat. Sci g ( 1977) 1285

4 graphite substrate molten zinc i Reaction zone 1 Evaporation zone A- Electronic N2 balance Fig. 1 - Schematic of the experimental reactor DEPOSITION RATE (ptn.h-'\ Fig. 2 - ZnS deposition rate as a function of temperature.

5 JOURNAL DE PHYSIQUE DEPOSITION KINETIC o0C -. -_.- 'Q, -U, ' -,75_0 C W PRESSURE (TORR) Fig. 3 - Influence of pressure on the deposition kinetics. Fig. 4 - Comparative in the transmission curves of typical samples 1.2 mm thick deposited at different temperatures under 30 torr.