Single wafer reactor

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. 

Cold wall reactors are the other major category of CVD reactors. In such systems, the substrates are heated but the walls are cooled. Figure 9 shows an example of a cold wall rotating disk CVD reactor.This system has water-cooled quartz walls, with a rotating holder for (silicon or compound semiconductor) wafers that is resistively heated from below. Other commercial cold-wall reactors include lamp heated single-wafer reactors that are widely used in microelectronics fabrication, and inductively heated horizontal flow reactors. Cold-wall reactors are often run at relatively high pressures, several hundred torr to atmospheric total... 

Figure 7.10. Sketch of a single wafer reactor. In this case the wafer is heated by lamps.

The problem of isolating intrinsic rates of reaction from reaction rates measured in a single-wafer reactor appears to have been reduced but not necessarily solved. If relative intensities of absorption bands exhibited by reactants or reaction intermediates can be ascertained as a function of time, it may be possible to check rate expressions based upon a single step being rate-controlling. 

Other transport limitations, such as diffusion-controlled reactions, can lead to localized depletion of etchant, which results in a number of observable etch effects. The size and density of features can influence the etch rate at different locations on a single wafer, thus producing "pattern sensitivity." Depletion across a wafer produces a "bulls eye" effect, while depletion across a reactor is indicated by the fact that the leading wafer edge etches faster than the trailing edge. Similar effects are noted when product removal is transport-limited. Most of these effects can be reduced... 

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. 

When considering a production reactor, we first assume that the requisite quality film can be made at least one at a time. The challenge then is to develop a reactor that is capable of acceptable wafer throughput with each wafer having film thickness within an acceptable tolerance. For example, we may want a reactor that can process 30 wafers per hour with thickness uniformity on a single wafer, and from wafer to wafer, of 5%. In addition, we may impose other conditions such as permissible number of particles per cm2, or for epi silicon films, the allowable number of defects per cm2. When we speak of wafer throughput, we are concerned with the actual cost per wafer for this process step. 

One way to achieve high wafer throughput is to pack many wafers into each reactor load. This is what is referred to as a "batch" system. It is for this reason that most production CVD reactors today operate as batch systems. One alternative is to operate a single-wafer system, but do it in a continuous... 

A second approach to a cold wall system is the single-wafer CVD reactor developed by Varian-Torrex. A schematic of the reaction chamber is shown in Figure 25. Again, tungsten silicide is deposited in this cold-wall reactor. Other conducting films such as blanket and selective tungsten can also be deposited. 

Figure 25 Varian-Torrex 5101 single-wafer CVD reactor system.

A final point should be made concerning the single-wafer CVD reactor concept. This approach only makes sense if each wafer can be processed in 1 to 2 minutes, so reasonable throughput can be achieved. In many applications, conducting films can be thin, 2000 A, so deposition rates of 1000 to 1500 A/min would be suitable. Such rates are not unreasonable, for example, for WSi2 films. 

FIGURE 13.1 I Application of computational flow model to simulate single-wafer CVD reactor (from Komiyama et al., 1999). (a) Standard geometry, (b) evaluation of outlet configurations. 

Figure 1.10 Top view of a single-wafer CVD reactor single-wafer processing generally employs duster tools that permit a wafer to be shuttled between the different reactipn/process chambers w.ithout exposing it to contamination or ambient conditions the top view of one reactor shows the pins on which a wafer rests during the CVD reaction. Courtesy of Applied Materials.

The whole system is constructed from two silicon wafers, fabricated using photoresist by deep reactive ion etching (DRIB) [21]. The wafers were thermally bonded. Thereafter, inlet and outlet ports were machined and the single reactors isolated by DRIB. 

The steam reformer is a serpentine channel with a channel width of 1000 fim and depth of 230 fim (Figure 15). Four reformers were fabricated per single 100 mm silicon wafer polished on both sides. In the procedure employed to fabricate the reactors, plasma enhanced chemical vapor deposition (PECVD) was used to deposit silicon nitride, an etch stop for a silicon wet etch later in the process, on both sides of the wafer. Next, the desired pattern was transferred to the back of the wafer using photolithography, and the silicon nitride was plasma etched. Potassium hydroxide was then used to etch the exposed silicon to the desired depth. Copper, approximately 33 nm thick, which was used as the reforming catalyst, was then deposited by sputter deposition. The reactor inlet was made by etching a 1 mm hole into the end... 

Fig. 9. Sketch of cut and uncut single crystal wafers ofMoS2, and a twin reactor used for activity test.

In this section we will review the various types of CVD reactors scientists and engineers have used for the development of thermal CVD processes. This will be distinct from the commercial reactors used for production which will be covered in a later chapter. A similar review of reactors for development of plasma-enhanced CVD processes will be made at the end of the next chapter. We will cover the so-called cold wall systems for either single or multiple wafers first, followed by a discussion of continuous belt systems. Finally, we will review the hot wall reactor approach. 

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