PRODUCTION CVD REACTOR SYSTEMS

In Chapters 1 and 2, we not only covered the basics of thermal and plasma-enhanced CVD, but we described the general reactor configurations that researchers have explored over the years. From these concepts have come a few production CVD reactors that satisfy the commercial needs of the integrated circuit manufacturing process. 

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. 

The cost per wafer will depend on many factors. First, the reactor can be quite expensive, so it is a capital item and must be amortized. Also, if the reactor has to be cleaned very frequently or is unreliable and experiences a lot of down time, then this will also add to the capital cost. If the reactants are expensive and not utilized efficiently, then this is another expense item. Energy requirements can be high for heating either the chamber or the susceptor. So, a system with high wafer throughput leads in the direction of lower cost per wafer, provided film quality is acceptable. 

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 

It is important to recognize that a production reactor is not simply a reaction chamber. If it is a low pressure unit, there will be a vacuum system which can be quite complex. There will be a gas panel which regulates gas mass flow into the chamber. The method of heating the wafers and/or the entire chamber has to be chosen carefully. Wafer transport involves many tradeoffs, and for batch systems if any degree of automation is required, will be quite involved. Finally, most production reactors these days operate under microprocessor control, and quite a lot of software must be developed. 

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Features common to all CVD reactors include source evaporators with an associated gas handling system to control input gases and gas-phase precursor concentrations, a reactor cell with a susceptor heated by either radio frequency or infrared radiation, and an exhaust system to remove waste products (which may include a vacuum pump for low-pressure operations). Substrate temperatures can vary from less than 200 °C to temperatures in excess of 1000 °C, depending on the nature of the material layer and precursor used. Schematic diagrams of some simple CVD reactors are shown in Figure 4. 

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. 

In reality, these are only two of many arrangements that could be devised to create and deliver to a substrate large quantities of reactive species using a plasma. Since there are many shortcomings to existing commercial plasma-enhanced CVD reactors, it will be useful to explore other reactor concepts that are under development, but have yet to be widely developed commercially. Whether or not they will lead to practical production systems remains to be seen. 

Nanopowders have been of particular interest in recent years as they can be used to make high-performance products due to their superior properties. Manufacturing of these powders is therefore important. Such a system using a laser-induced cold-wall CVD reactor is shown in Figure 3.28. It mainly consists of five parts a reactor with two reaction zones, an oven for vaporising hexamethyl disilylamine (HMDS-... 

At the basic level CVD reactors fall into two classifications—open and closed reactor systems. In the closed CVD system the precursors are loaded into the CVD chamber together with the specimens to be treated. The system is then closed and the temperature increased to initiate the chemical process. The process continues for a time sufficient to produce the required effect. The temperature is then reduced to ambient so that the reactor may be opened and the specimens removed. This reactor design is frequently used for the purification of metals and chromizing parts. The dominant type of CVD reactor is the open type. Flowing precursors continuously enter this reactor and the gaseous by-products of the chemical process are continuously removed from the reactor (usually with a vacuum pump) and appropriately treated for discharge into the environment. 

Typical Reactor Design. Table 5.1 lists typical CVD production reactors which include cold-wall and hot-wall reactors operating at low or atmospheric pressures. The decision to use a given system should be made after giving due consideration to all the factors of cost, efficiency, production rate, ease of operation, and quality. 

CVD can be performed in a so-called closed or open system. In closed CVD, both reactants and products are recycled for reuse within an enclosure [11], The closed reactor CVD method is primarily used for metal purification and protection coating for aero-engine blades (e.g. pack cementation for aluminising, chromising, etc.). In most cases, CVD happens in an open state which is described in Chapter 1 and Figure 1.1. 

Chemical vapor deposition includes various systems, and they are low-pressure CVD (LPCVD), atmospheric pressure CVD (APCVD), plasma enhanced CVD (PECVD), and others. Each type of CVD system has its own advantages and limitations. For instance, in LPCVD, the reactor is usually operated at 1 torr. Under this condition, the diffusivity of the gaseous species increases significantly compared to that under atmospheric pressure. Consequently, this increase in transport of the gaseous species to the reaction sites and the by-products from the reaction sites in LPCVD will not become the rate-limiting steps. This leads to the surface reaction step to be the rate limiting one. 

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