Dimensiones: Comprensión lectora: plano de la narración

F ig u r e 2 .2 s h o w i n g a to p v i e w o f g a s f l o w s a s t h e y e n te r th e r e a c t io n c h a m b e r fr o m th e b r a s s b a f f le .

A side view o f the reaction cham ber is shown in figure 2.3.

glass sub strate top plate

m ain carrier gas flow

heated carbon block

brass block

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2.1.3 Pipe layout

Figure 2.4 shows a pipe layout diagram o f the CVD reactors. The routing o f the gas flow s through the bubblers has already been discussed and can also be seen in figure 2.4. A flow m eter is attached to the beginning o f each o f the gas lines to enable be the gas flow to be controlled and m onitored through each o f the lines. The four w ay valve is used so that an exact deposition tim e for the reaction can be made. This is possible because it allow s the sim ultaneous diversion o f all the precursor gas flow stream s into the reaction cham ber at the start o f the deposition and into the w aste exhaust at the end o f the deposition. As can be seen the syringe line jo in s up to the bubbler lines a short distance (10 cm) before entering the reaction cham ber. This is necessary to m inim ise the num ber o f blockages that can occur in the pipew ork due to pre-reaction o f the m etal halide precursors w ith the co reactant in the syringe line.

N , flow meter

N-> " ^ { flo w m e te r ) ^ -

N î ~ ^ { flowmeter

N : flow meter

N2- ^ ( flowmeter

1 1 = four way valve

^ = valve A = bubbler

1

1

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From figure 2.4 it can be seen that it is possible to use two bubblers sim ultaneously w ith the apparatus, thus m aking it possible to deposit films containing tw o different m etals as a solid solution.

2.1.4 F our-w ay valve

T he four-w ay valve has two inlets A and B and tw o outlets C and D to enable either inlet to be diverted to either outlet. In figure 2.5a below A is connected to C and B to D, but i f the central section is rotated 90°, as in figure 2.5b, then A is connected to D and B to C.

c

c

F ig u re s 2 .5 a and 2 .5 b s h o w in g fo u r w a y v a lv e arrangem ent

T he valve itse lf is quite large, 100 m m x 100 m m , and m ade o f steel. This valve w ould act as a significant heat sink to the gases passing through it, and to com pensate for this it is heated by a flat disc heater placed on the rear side o f the valve and controlled b y a eurotherm 808 electronic controller and a nickel chrom ium therm ocouple. This heating in turn results in the nylon plastic inserts o f the valve

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softening and m elting and leads to the valve either blocking or stopping it diverting the gases properly. To rem edy this the nylon inserts w ere replaced b y V espel© plastic inserts that have a m uch higher m elting p oint and do not suffer from the sam e problem .

2.2

Operation o f the CVD apparatus

W hen preparing a piece o f glass for coating the follow ing procedure w as followed. Firstly, the glass substrate w as cleaned and prepared to provide, as m uch as possible, the sam e conditions for nucléation and grow th in each film that is grow n, thus allow ing easy com parison o f films at a later date.

The glass used in this w ork as the substrate w as supplied by Pilkington G lass, and had been pre-treated on one side w ith a barrier coating to stop diffusion o f ions from w ithin the glass bu lk into the film. This anti-diffusion coating took tw o form s during the course o f this work. In the first h a lf o f the w ork the coating w as a patented Si-C-O containing layer, (all vanadium and chrom ium oxide films w ere deposited on glass using this barrier coating) w hile all TiO ] film s w ere grow n on glass that w as coated w ith an SiO ] layer. This change in substrate w as necessary due to the discontinuation o f m anufacture o f the Si-C-O coating b y Pilkington glass.

To ascertain w hich side o f the glass had been coated w ith the barrier layer and therefore identify the side to be coated, a 254 nm uv fluorescent tube light w as used. This is possible due to the w ay in w hich the glass is m anufactured. G lass is

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produced on a float glass plant, w hich is a process that uses a m olten tin bath onto w hich the m olten glass is poured and then allow ed to cool. This process results in a very flat sheet o f glass that is largely free o f thickness im perfection. H ow ever, tin ions are absorbed into the bottom surface o f the glass during this process. T in ions fluoresce under uv light, therefore the bottom surface o f the glass is the side w hich fluoresces under uv light. Thus, the side onto w hich depositions are carried out is the side w hich does not fluoresce.

O nce the correct side has been found the glass m ust be cleaned. This is carried out b y initially scrubbing the surface w ith lab tissue that has been w etted w ith 40°- 60° petroleum spirit to rem ove any grease, fingerprints etc. The glass is then rinsed w ith propan-2-ol that is allowed to evaporate slow ly w hile standing so that the surface is left clean. O nce dry the glass is placed onto the carbon block. The top plate is inserted to contain the gas flows in the cham ber, and the steel end plates attached to seal the reactor. The gas lines are turned on so that a small dinitrogen flow is present in each o f the lines (total flow o f 1 dm^ m in‘‘). This flow is used to flush the system so that no oxygen is present during the deposition. The heater tapes are turned on and set to 150 °C so that the steel pipe w ork starts to heat up and the carbon block heaters are turned on w ith a ram p rate o f 12 °C min'* to ram p up to the desired tem perature. The four-w ay valve is set so that the plain line is passing through the reaction cham ber. The bubbler hotplates are turned on and left to heat up to the correct tem perature. The apparatus is then left for one hour to heat up and equilibrate.

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W hen the apparatus is up to tem perature the gas flows are set to the correct level for the deposition studies. The syringe driver is turned on and left for 30 seconds to let the CO precursor volatilise into the gas phase and to ensure that there is a flow o f co precursor reaching the reactor before the reaction is started. The bubbler valves are switched to divert the carrier gas through the bubbler. This is carried out in the following order to prevent any pressure build-up pushing liquid precursor into the pipew ork o f the apparatus. Firstly, the outlet valve is opened, then the inlet valve and finally the bypass valve is closed. Finally the four-w ay valve is turned to divert the bubbler flow s into the reaction cham ber, it is at this point that the deposition is considered to have started and is tim ed until the m om ent at w hich the four-w ay valve is returned to its original position. The syringe driver is then turned o ff and the bubbler valves also returned to their original positions in the following order; bypass, inlet and then outlet. Gas flows are returned to m inim al levels, all the heaters are turned o ff and the reaction cham ber allow ed to cool to 70 °C at that point the glass substrate are rem oved from the cham ber. T he glass is then left to cool to room tem perature and stored by w rapping in tissue paper.

2.3

Transmission reflection spectrometer

For this project an optical spectrom eter w as constructed to look at the optical transm ission and reflection properties o f the glass and coatings that w ere produced over the visible and near I.R. regions.

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This spectrometer was based around two solid-state devices supplied by Ziess, the