Temperature substrate

The optimum substrate temperature, at which a maximum diamond growth rate and a highest level of crystal perfection can be achieved for a given system, is generally in the range of 800-1000°C, with a typical low bound approaching 600°C, basically independent of deposition techniques. 

The low temperature synthesis of diamond films has been investigated in the substrate temperature range of 350-800°C in RF thermal plasma CVD, and diamond films of reasonable quality have been obtained at 550-600°C which are considerably lower than those generally considered as 

It has been reported that diamond films with crystallite size of about 200 nm form at the substrate temperatures aroimd 650°C, while 10-20 nm sized crystaUites occur at the substrate temperatures between 450 and 600°C. Another study showed that diamond grains ot 160, 13, and 10 nm were grown at the substrate temperature of 800, 600 and 415°C, respectively, in MW PACVD. Clearly, for substrate temperatures lower than 800°C, the crystallite size increases with increasing substrate temperature. These results demonstrated that the crystallite size, surface roughness and hence optical transparency as well as other properties of diamond films can be tailored by varying the substrate temperature. 

Since mushroom mycelium grows within the substrate, the substrate temperature must be monitored closely. Thermometers are placed both in the center of the substrate—the hottest region —and in the room s atmosphere. These two thermometers establish a temperature differential. Ifthe hottest point in the substrate is 80 ° F. and the air is 70 ° F. then the temperature of the total mass must lie within this range. 

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Figure Bl.19.27. AFM topographic images (7x7 pm ) of 20 epitaxial Ag films on mica prepared at five substrate temperatures (75, 135, 200, 275, and 350 °C) and four film thicknesses (50, 110, 200, and 300 mn)...

With the development of multichaimel spectroscopic ellipsometry, it is possible now to use real-time spectroscopic ellipsometers, for example, to establish the optimum substrate temperature in a film growth process [44, 42]. 

Extended x-ray absorption fine stmcture measurements (EXAFS) have been performed to iavestigate the short-range stmcture of TbFe films (46). It is observed that there is an excess number of Fe—Fe and Tb—Tb pairs ia the plane of the amorphous film and an excess number of Tb—Fe pairs perpendicular to film. The iacrease of K with the substrate temperature for samples prepared by evaporation is explained by a rearrangement of local absorbed atom configurations duting the growth of the film (surface-iaduced textuting) (47). 

Process variables that must be controlled include the power level, pressure, and flow of the arc gases, and the rate of flow of powder and carrier gas. The spray gun position and gun to substrate distance are usually preset. Substrate temperature can be controlled by preheating and by limiting temperature increase during spraying by periodic intermptions of the spray. 

In most cases, CVD reactions are activated thermally, but in some cases, notably in exothermic chemical transport reactions, the substrate temperature is held below that of the feed material to obtain deposition. Other means of activation are available (7), eg, deposition at lower substrate temperatures is obtained by electric-discharge plasma activation. In some cases, unique materials are produced by plasma-assisted CVD (PACVD), such as amorphous siHcon from silane where 10—35 mol % hydrogen remains bonded in the soHd deposit. Except for the problem of large amounts of energy consumption in its formation, this material is of interest for thin-film solar cells. Passivating films of Si02 or Si02 Si N deposited by PACVD are of interest in the semiconductor industry (see Semiconductors). 

Fig. 6. Stmctural zones in condensates at various substrate temperatures. When is the melting point (10) ...

As the substrate temperature increases, the surface mobUity increases and the stmctural morphology first transforms to that of Zone T, ie, tightly packed fibrous grains having weak grain boundaries, and then to a full density columnar morphology corresponding to Zone 2 (see Fig. 7). 

The optoelectronic properties of the i -Si H films depend on many deposition parameters such as the pressure of the gas, flow rate, substrate temperature, power dissipation in the plasma, excitation frequency, anode—cathode distance, gas composition, and electrode configuration. Deposition conditions that are generally employed to produce device-quahty hydrogenated amorphous Si (i -SiH) are as follows gas composition = 100% SiH flow rate is high, --- dO cm pressure is low, 26—80 Pa (200—600 mtorr) deposition temperature = 250° C radio-frequency power is low, <25 mW/cm and the anode—cathode distance is 1-4 cm. 

Concurrent bombardment during film growth affects film properties such as the film—substrate adhesion, density, surface area, porosity, surface coverage, residual film stress, index of refraction, and electrical resistivity. In reactive ion plating, the use of concurrent bombardment allows the deposition of stoichiometric, high density films of compounds such as TiN, ZrN, and Zr02 at low substrate temperatures. 

In PECVD, the plasma generation region may be in the deposition chamber or precede the deposition chamber in the gas flow system. The latter configuration is called remote plasma-enhanced CVD (RPECVD). In either case, the purpose of the plasma is to give activation and partial reaction/reduction of the chemical precursor vapors so that the substrate temperature can be lowered and still obtain deposit of the same quaUty. 

Processing variables that affect the properties of the thermal CVD material include the precursor vapors being used, substrate temperature, precursor vapor temperature gradient above substrate, gas flow pattern and velocity, gas composition and pressure, vapor saturation above substrate, diffusion rate through the boundary layer, substrate material, and impurities in the gases. Eor PECVD, plasma uniformity, plasma properties such as ion and electron temperature and densities, and concurrent energetic particle bombardment during deposition are also important. 

J.m/h. Because the diamond growth takes place under atmospheric conditions, expensive vacuum chambers and associated equipment are not needed. The flame provides its own environment for diamond growth and the quaUty of the film is dependent on such process variables as the gas flow rates, gas flow ratios, substrate temperature and its distribution, purity of the gases, distance from the flame to the substrate, etc. 

The effect of the substrate temperature can also be considered (Fig. 16c). As the substrate temperature iacreases, the triangular diamond domain region ia the C—H—O equiUbrium diagram shrinks to almost aline at the highest temperature. 

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