Cold-wall tube

The first study of CVD tungsten for application to integrated circuits was done by Shaw and Amick,18 working with the hexafluoride. They carried out their depositions in an atmospheric-pressure horizontal cold-wall tube reactor (see Figure 17, Chapter 1), where the susceptor that held the wafers was inductively heated. 

Near drum wastage is the most prevalent type of wastage documented. It cart be found anywhere in the generating bank but is most common on the wall tubes, in tlic row immediately next to soot blower lanes and in the hot and cold row s. Three types of near drum w astagc have been documented. 

The second indication is a faint smoke-like cloudiness in the zone of the tube which is being heated by the Bunsen this is readily visible as the interior of the tube is normally quite clear and bright. This is a later stage of development of the flash-back than the rise of pressure, already mentioned, and should be counteracted by moving the Bunsen immediately to the point of the combustion tube where heating was commenced. In either case the Bunsen should then be moved slowly forwards as before. A flash-back is attended by the deposition of carbon particles, carried back by the explosion wave, on the cold walls of the tube. Care should be taken that these are completely burnt off as the Bunsen is slowly moved forward again. 

If the cooling bath for the first trap is lowered much below — 15°, plugging of the trap is likely to occur. These traps are connected so that the vapor enters the larger, annular space, impinging on the cold wall before entering (and possibly plugging) the smaller inner tube. 

The apparatus used for the CVD processes [19-21] described below is a cold wall reactor, the principal features of it are described in a former publication [22]. It consists of a vacuum line to which a cyhndrical glass tube is connected, the tube being enrolled by a copper wire, which is used as... 

The principal use of LDPE and LLDPE in building products is as a film water barrier under below-grade floors as a wall vapor barrier, though PVC is typically preferred and as temporary enclosure film during construction. The film is made either by extruding a thin-walled tube, which may be slit or wound up direcdy, or by extmsion through a slot die and cast direcdy on to a cold roll, cooled, then wound up. The former method is more widely used. A much smaller use for low density polyethylene is in piping. 

The allyl bromide solution was allowed to cool efficiently by dripping it against the cold walls of the flask. It is important that allyl bromide reach the reaction mixture at the lowest possible temperature in order to obtain an optimal stereoselective alkylation. The cannula was protected against heat exchange with air by coating it with a fine rubber tubing. 

Polytetrafluoroethylene powders can be extruded but not by the methods applied to conventional thermoplastics. The polymer must not be subjected to excessive mechanical working above the melting point or cracking of the finished piece will result. For this reason, the polymer is worked while it is still cold. The powder is compacted with the aid of a screw or a ram and forced through a hot die in which it is sintered (James). This method is used to fabricate rods or heavy wall tubing or to apply heavy coatings to electrical conductors. 

It is interesting to compare the present results with data obtained in a hot wall furnace18 tube, even though the present calculations are for one hot and one cold wall and a different physical arrangement. For one case, deposition rate was measured at p = 532 mTorr, Y = 2.3% and T = 898°K. Without running the exact case numerically, we can estimate from Figures 11 and 12 a calculated value of 2.5 A/min compared to a measured value of 4 A/min. 

One of the problems with cold wall systems is the difficulty in maintaining a very uniform temperature on the wafers. Such concern can be eliminated if the entire reactor chamber is placed within a furnace maintained at a very uniform temperature. An ideal candidate for such a furnace is the standard diffusion tube furnace already in wide use for integrated circuit fabrication. If in addition, wafers could be loaded vertically as in a diffusion furnace, the reactor throughput could be substantial. 

The problem of assuring uniform depositions on many wafers closely spaced in a long uniform tube was solved when operation of the reactor at low pressure was considered.22 Normally, in an atmospheric pressure cold wall CVD system, the reactant gas is heavily diluted in N2 in order to reduce gas phase nucleation. At the pressures used for low pressure CVD (0.5-1.0 Torr), this is less of a problem so less diluent is needed. The net effect then is that deposition rates only fall by a factor of five. However, as many as 100 wafers can be processed in such a reactor at one time (see Figure 26), and this more than compensates for the lower deposition rate. In addition, due to the low pressure, diffusion occurs at high rates and the deposition tends to be controlled by the surface temperature. Given the very uniform temperatures available in a diffusion furnace, the deposition uniformity tends to be excellent in such a system. 

The second study was done in a cold-wall reactor12-13 using the same reactants. The reactor was a single-wafer system, similar to the tube reactor of Figure 18 in Chapter 2, with the wafer heated by an electrical resistance heater in the pedestal. In this case, the sublimator was operated at 88°C with a 10 seem flow of H2. The influence of SiH4 flow rate on the film stoichiometry and resistivity (after anneal) are shown in Figure 11. 

The lowest resistivity silicide film of the four we are considering is the TiSi2 film, so such films have always been of interest. A recent study14 has shown that these films can also be deposited by low-pressure CVD. For these experiments, a cold-wall reactor similar to the parallel-flow tube reactor sketched in Figure 17 of Chapter 1 was used. The wafer was heated by heating the susceptor from below by optical radiation. 

The research at MIT has been done in the cold-wall vertical tube reactor shown in Figure 14. The wafer is aligned almost parallel to the flow on a vertical silicon carbide-coated susceptor. The wafer is heated by optical radiation from high-intensity lamps to a temperature of 775°C. Silane was introduced... 

Figure 1 illustrates conventional CVD reactors. These reactors may be classified according to the wall temperature and the deposition pressure. The horizontal, pancake, and barrel reactors are usually cold-wall reactors where the wall temperatures are considerably cooler than the deposition surfaces. This is accomplished by heating the susceptor by external rf induction coils or quartz radiant heaters. The horizontal multiple-wafer-in-tube (or boat) reactor is a hot-wall reactor in which the wall temperature is the same as that of the deposition surface. Therefore, in this type of reactor, the deposition also occurs on the reactor walls which presents a potential problem since flakes from the wall deposit cause defects in the films grown on the wafers. This is avoided in the cold-wall reactors, but the large temperature gradients in those reactors may induce convection cells with associated problems in maintaining uniform film thickness and composition. 

It is, however, possible by utilizing a cascade of such separations to multiply the separation. Clusius and DickeP devised a column that consists of a long tube that is externally cooled and contains a heated wire along the axis. Convection causes a fractionation effect, since hot gas hitting the cold wall falls, cycles, and is reheated at the wire. The multiple effect of such convection cycles is such that it was possible to make an almost 100 per cent separation of IT2-CO2 mixtures between the top and bottom of the column. (Twiro = 600°C, length of tube = 1 m.) The column has since been extensively developed for isotopic separation. 

The Clusius-Dickel column is shown schematically in Figure 2. A wire is mounted at the axis of a cylinder. The wire is heated electrically and the outer wall is cooled. This sets up a radial thermal gradient which leads to a thermal diffusion separation in the x direction. As a result of the radial temperature gradient, a convection current is established in the gas, which causes the gas adjacent to the hot wire to move up the tube with respect to the gas near the cold wall. The countercurrent flow leads to a multiplication of the elementary separation factor. For gas consisting of elastic spheres, the light molecules will then concentrate at the top of the column, while the heavy molecules concentrate at the bottom. The transport theory of the column has been developed in detail (3, iS, 18) and will not be presented here. In a later section we shall discuss the general aspects of the multiplication of elementary separation processes by countercurrent flow. 

Nevertheless, one substantial advantage of the tube systems is that the tube can be considered more or less an isothermal system. This is very advantageous since now the determination of the real wafer temperature is not a problem. This is, as we will see, in contrast with cold wall systems where the real wafer temperature is very difficult to measure and sometimes difficult to control. 

Figure 3.28. Schematic of laser-induced cold-wall CVD reactor [57] (1 C02 laser, 2 reflector, 3 laser beam, 4 GaAs lens, 5 cooling water, 6 reactor, 7 nozzle, 8 reaction flame, 9 particle plume, 10 board, 11 window, 12 throttling valve, 13 powder collector, 14 pump, 15 pressure gauge, 16 water-cooled Cu block, 17 temperature controller, 18 oven, 19 heater, 20 precursor vessel, 21 liquid HMDS, 22 needle valve, 23 flow meter, 24 preheating tube, 25 co-axial protection gas, 26 lens protective gas)...

For simplicity, we have chosen long Schlenk tubes (Figure 3), without the conventional cold finger condenser. Deposition of crystals takes place on the cold walls of the sublimation flask 1-2 cm above the conical reflector. 

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