Reactor uniform deposition

CVD reactors can have one of several configurations. Each has particular advantages and disadvantages. Reactors that support wafers horizontally have difficulty controlling the deposition uniformity over all the exposed wafers. Reactors having vertical wafer support produce uniform deposition, but are mechanically complex. Barrel reactors are not suited for extended operation at temperatures greater than 1200°C. 

Manufacturing economics require that many devices be fabricated simultaneously in large reactors. Uniformity of treatment from point to point is extremely important, and the possibility of concentration gradients in the gas phase must be considered. For some reactor designs, standard models such as axial dispersion may be suitable for describing mixing in the gas phase. More typically, many vapor deposition reactors have such low L/R ratios that two-dimensional dispersion must be considered. A pseudo-steady model is... 

The gas motion near a disk spinning in an unconfined space in the absence of buoyancy, can be described in terms of a similar solution. Of course, the disk in a real reactor is confined, and since the disk is heated buoyancy can play a large role. However, it is possible to operate the reactor in ways that minimize the effects of buoyancy and confinement. In these regimes the species and temperature gradients normal to the surface are the same everywhere on the disk. From a physical point of view, this property leads to uniform deposition - an important objective in CVD reactors. From a mathematical point of view, this property leads to the similarity transformation that reduces a complex three-dimensional swirling flow to a relatively simple two-point boundary value problem. Once in boundary-value problem form, the computational models can readily incorporate complex chemical kinetics and molecular transport models. 

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. 

Finally, we can comment on the influence of the reactor type on the films that can be deposited. Evidently, the hot-wall reactor tends to deposit very Ta-rich films. Although it may be possible to alter the stoichiometry in this type of reactor, the choices are limited. One must operate under conditions where uniform depositions are achieved both on each wafer and from wafer to wafer, because this is a batch system. In the cold-wall reactor, it was possible to obtain the proper stoichiometry at high deposition rates. Since the higher deposition rates permit development of a single-wafer reactor, there are more choices in the process conditions to be used. 

Typically, deposition uniformity depends on the reactive gas mixtures, as is illustrated in Figure 11.1 Clearly, much trial and error is required to establish reactor operating conditions that will yield wafers with uniform deposits. 

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. 

There are two general types of CVD reactors, one is the chamber type and the other is the tube type. The tube type reactor is typically a hot wall reactor and has been used in the semiconductor industry for the deposition of simple binary thin films such as SijN. This type of deposition reactor usually has quite large throughput because a few hundred wafers can be loaded and processed. However, the CVD precursors should have large diffusivities in the gas phase and be stable over the homogeneous reactions to produce uniform deposition on a large number of wafers. For tube type reactors, as for all hot wall type reactors, the CVD reaction occurs on the wall of the reactor as well as on the wafers. This increases the consumption of the precursors. Therefore, CVD reactors for BST thin films are the other type, except for a very recent report from Toshiba of Japan. They reported CVD of BST thin films utilizing a tube type reactor which had a rotatory wafer holder to improve the uniformity of deposited films. Details of the CVD reactor have not been reported yet, thus, in this section only the details of chamber type reactors are discussed. 

Another type of reactor using a simple injector is a reactor with a dome type cover.In this case, the injector is installed on the bottom plate of the reactor. Figure 11 shows the typical configuration of this type of reactor. In this case the dome is usually heated to a proper temperature by a separate heater to prevent precursor condensation on it. It is rather difficult to expect uniform deposition of a thin film from this asymmetrical gas injection geometry. However, it has actually been proved that good thickness and compositional uniformity are obtained over the 8 wafer surface for Ta O thin films. Figure 12 shows the typical variation in thickness of a Ta O thin film using this reactor. 

Ihe equipment used in this study (Figure 1) consisted of a capacitively-coupled plasma reactor similar to the apparatus described by Poulsen 3) for the plasma etching of integrated circuits. This arrangement resulted in uniform depositions over a range of flow rates and improved utilization of monomer. 

Equations 5.3.b-9 and 5.3.b-ll or 5.3.b-12 form a set of simultaneous equations that clearly shows that the coking of the catalyst not only depends on the mechanism of coking, but also on the composition of the reaction mixture. Consequently, even under isothermal conditions, the coke is not uniformly deposited in a reactor or inside a catalyst particle whenever there are gradients in concentration of reactants and products. This important conclusion will be quantitatively developed in a later section. 

Although the flow in the microchannels is laminar, a uniform radial concentration profile and consequently a narrow residence time distribution were obtained. Depending on the method used for manufacturing the microchannels, the Boden-stein number was found to be Bo = ud/Dj 70 and consequently the microreactor behaves almost like a plug-flow reactor. The catalytic coating had no influence on this distribution, indicating a uniform deposition of the catalyst within the microchannels (see Figure 14.3). 

For this process, the reactive gases (see Table 1) are passed from one end of the reactor and pumped out through the quartz tube reactor chamber (Fig. 6). The fabricated devices which ai e in wafer form (from 50-200 wafers per run) are vertically stacked in the reactor chamber. Since the deposition rate is a function of both reactive gas concentration and temperature, there is a reactive gas concentration gradient in the reactor chamber — being rich at the beginning of the reactor and poor at the end of the reactor. Therefore, the oxide or nitrite tends to deposit faster at the beginning of chamber and progressively less as they move down from the reaction chamber. A non-uniform thickness deposit could result. To resolve this non-uniform deposition problem, a... 

The study of reaction kinetics in flow reactors to derive microkinetic expressions also rehes on an adequate description of the flow field and well-defined inlet and boundary conditions. The stagnation flow on a catalytic plate represents such a simple flow system, in which the catalytic surface is zero dimensional and the species and temperature profiles of the estabhshed boundary layer depend only on the distance from the catalytic plate. This configuration consequently allows the application of simple measurement and modehng approaches (Sidwell et al., 2002 Wamatz et al., 1994a). SFRs are also of significant technical importance because they have extensively been used for CVD to produce homogeneous deposits. In this deposition technique, the disk is often additionally forced to spin to achieve a thick and uniform deposition across the substrate (Houtman et al., 1986a Oh et al., 1991). 

The CVD method usually employs pyrolysis or hydrogen reduction of a suitable chemical, such as palladium hexafluoroactylacetonate Pd(C5HFg02)2 as the palladium precursor. The precursor is vaporized at around 60°C and fed to the CVD reactor using nitrogen as carrier gas. Then, it is decomposed at a higher temperature ( 200°C) and Pd is deposited on the cleaned substrate. This method has the advantages of fast deposition rate, easy control of the membrane thickness, uniform deposition... 

Polysilicon. Polysihcon is used as the gate electrode material in MOS devices, as a conducting material for multilevel metallization, and as contact material for devices having shallow junctions. It is prepared by pyrolyzing silane, SiH, at 575—650°C in a low pressure reactor. The temperature of the process affects the properties of the final film. Higher process temperatures increase the deposition rate, but degrade the uniformity of the layer. Lower temperatures may improve the uniformity, but reduce the throughput to an impractical level. 

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