1. Field of the Invention
The present invention relates to multi-chamber process equipments for fabricating semiconductor devices.
2. Description of the Prior Art
In recent years, the advance in device miniaturization and IC complexity is increasing the need for more accurate and more complicated processes, and wafers of larger diameters. Accordingly, much attention is focused on multi-chamber process equipments (or systems) in view of increase of complex precesses, and enhancement of throughput in an individual wafer processing system.
FIG. 14 shows one conventional example. A multi-chamber process equipment of this example includes a wafer transfer chamber 1, a plurality of process chambers 3 connected with the transfer chamber 1 through respective gate valves 2, a load lock chamber (preliminary evacuation chamber) 5 connected with the transfer chamber 1 through a gate valve 4, and a wafer load chamber 7 connected with the load lock chamber 5 through a gate valve 6.
In the wafer transfer chamber 1 and the load lock chamber 5, there are provided wafer transfer arms 9 and 10 for carrying a wafer 8, as shown in FIG. 14. The transfer arm 10 is designed to take each wafer 8 from wafer cassettes 11, 11 placed in the wafer load chamber 7, through the gate valve 6, and bring the wafer into the wafer transfer chamber 1. The transfer arm 9 is arranged to receive the wafer 8 from the arm 10, and insert the wafer through one of the gate valves 2 into a predetermined one of the process chambers. The wafer 8 is shifted from one process chamber to another by the transfer arm 10 according to the sequence of processes.
Another conventional example is shown in "NIKKEI MICRODEVICES", May, 1990, page 47. A multi-chamber process equipment of this example includes a wafer transfer chamber, a plurality of parallel PVD or other process chambers connected with the transfer chamber, a cooling chamber, a preclean chamber, a buffer chamber, and RTP/etching/CVD chamber (or chambers), a load lock chamber, and other chambers. The pressure of each chamber is held at a predetermined degree of vacuum (base pressure) according to the object of the chamber. For example, the wafer transfer chamber is held at 10.sup.-8 Torr (1.3.times.10.sup.-6 Pa), the PVD chamber is held at 10.sup.-9 Torr (1.3.times.10.sup.-7 Pa), and the load lock chamber is held at 10.sup.-5 Torr (1.3.times.10.sup.-3 Pa).
Japanese Patent Provisional Publication (TOKKAI) No. 61-55926 shows still another conventional example.
In these equipments, the pressures of the different chambers are determined so as to ensure the clean wafer processing environment. In general, the pressures are made closer to the atmospheric pressure in the following order; (Process chamber)&lt;(Wafer transfer chamber)&lt;(Load lock chamber).
In the conventional process equipments, however, a wafer is readily affected by dew condensation especially in a low temperature etching chamber which is cooled to -20.degree. C..about.-70.degree. C. if the chamber is not evacuated sufficiently before loading of the wafer. Therefore, it is required to reduce the pressure in the chamber below a base pressure of the chamber (10.sup.-6 Torr, for example). Moreover, the degree of vacuum of the wafer transfer chamber is lower (that is, the pressure is higher) than that of the process chamber. Therefore, when the process chamber is opened, there arises a flow of residual water content from the wafer transfer chamber to the process chamber, resulting in the dew condensation. The conventional equipments cannot prevent condensation satisfactorily even if the pressure of the process chamber is decreased sufficiently below the base pressure.
On the other hand, cross contamination is caused by a flow of residual gases from a process chamber for heat treatment or photo-assisted CVD, to the wafer transfer chamber if the degree of vacuum in the wafer transfer chamber is too high.
Furthermore, the conventional equipments cannot sufficiently reduce variations of wafer properties such as sheet resistance from wafer to wafer, especially when the wafers are processed in a high temperature silicide CVD chamber. It is possible to reduce the variations of the sheet resistance by decreasing the pressure in the load lock chamber below the above-mentioned level. However, the pumping operation must be continued for three hours or more.