The invention relates to a method and a device for regulating a pressure difference between a wafer handling chamber and a process chamber of an epitaxy reactor.
Epitaxy reactors are used to produce epitaxial layers on single-crystal materials, for example for producing an epitaxial coating on semiconductor wafers, particularly silicon wafers. A reaction medium, generally a reaction gas, reacts on a surface which is to be coated to deposit further material in single crystal form, which generally also forms gaseous byproducts.
The epitaxy reactors which are used, substantially comprise a wafer handling chamber (WHC), a process chamber (PC) and a gas-supply system. The wafer handling chamber forms a connection between the process chamber and the environment, generally a clean room. The wafer handling chamber is continuously purged with inert gas, such as nitrogen, to keep it clean.
The workpieces, or semiconductor wafers, which are to be epitaxially coated, are transferred from a clean room via a lock into the wafer handling chamber, where these workpieces may be temporarily stored. These workpieces are then generally transferred individually, via a robot arm, through a gate valve (GV) into the process chamber. Then, the gate valve is closed and the epitaxial coating process is performed. During the epitaxial coating process, various gases, including at least a carrier gas and a reaction gas, are supplied via a gas supply and passed in laminar flow at high temperatures over the semiconductor wafer.
During this process, the reaction gas reacts on the wafer surface. Excess gas and gaseous byproducts are discharged via an exhaust and an off-gas scrubber.
After the process has ended, the gate valve is opened again, and the wafer is transferred back into the wafer handling chamber and is replaced by an untreated wafer.
A crucial factor regarding the quality of the epitaxially coated semiconductor wafers is the number of unwanted particles which settle on the wafers. To reduce the number of particles, it is crucial for there to be no gas turbulence or pressure surges which activate inactive particles lying, for example, at the bottom of the wafer handling chamber.
If a wafer is being conveyed from the wafer handling chamber into the process chamber, the connection between the two chambers, through the gate valve, has to be opened. It is crucial for the two chambers to be at approximately the same pressure level before the gate valve is opened. Otherwise, a sudden pressure compensation may cause particles to be swirled up.
However, the pressure in the wafer handling chamber must always be at a slightly higher level than the pressure in the process chamber, so that when the gate valve is opened there is a gentle flow in the direction of the process chamber. The pressure differential is provided to prevent substances such as gases or particles from being able to move from the gas outlet of the process chamber back into the process chamber, or from the process chamber into the wafer handling chamber.
While the wafer handling chamber can be kept at a constant pressure without problems, the pressure conditions in the process chamber are subject to considerable changes over the course of time. These changes are caused by the off-gas pipe being occupied, or by changes in the state of the off-gas scrubber.
The wafer handling chamber can now be kept at a constant pressure which is so high that this pressure is higher than the variable pressure in the process chamber. If there is a constant incoming flow of inert gas, the outgoing flow of inert gas from the wafer handling chamber can be regulated by a backpressure regulator so that the pressure in the wafer handling chamber remains constant. However, this so-called dynamic pressure regulation leads to a strong pressure surge, and therefore to turbulence when the gate valve is opened if the pressure in the process chamber is low at the time. A further drawback is the high inertia of the system, with the result that only slight pressure differences can be compensated for within a short time period. Regulation of the incoming flow of inert gas to the wafer handling chamber via the power of a delivery system also has the same drawbacks.
To regulate the incoming flow, a compensation line is provided between the wafer handling chamber and the process chamber, which can be opened using a valve. The wafer handling chamber can then be set to a relatively high, constant pressure as described above, wherein this constant pressure is always higher than the fluctuating pressure of the process chamber. Before the gate valve is opened, the valve of the compensation line is opened, so that the pressure in the two chambers can be matched in a regulated manner. However, this results in a relatively higher pressure gas stream being formed in the compensation line, and this in turn generates turbulence, with the associated risk of particles being activated. Moreover, this pipeline and the valve have been designed to be very small, to limit the gas stream and thereby avoid turbulence, leading to a relatively high inertia. Furthermore, when the compensation line is open, back-flows from the process chamber into the wafer handling chamber are possible, which is, as stated above, preferably to be avoided.
A further known possible solution consists in applying an additional pressure to the wafer handling chamber via a regulated inert gas purge shortly before the gate valve is opened, to prevent a gas stream from flowing from the process chamber into the wafer handling chamber. In this way, the wafer handling chamber can be kept at a low pressure throughout the majority of the time required for a process. This method, unlike the dynamic-pressure regulation, offers the possibility of discharging inert gas through an outlet valve, with the result that particles can escape from the wafer handling chamber without obstacle. However, this method also leads to considerable pressure fluctuations during the opening and closing of the gate valve. Therefore, it is impossible to prevent particles from being activated.
One object of the invention is to provide a system that ensures that when the gate valve is opened, gas does not flow back out of the off-gas system of the wafer handling chamber and gas does not flow out of the process chamber into the wafer handling chamber even with pressure fluctuations in the process chamber. Another object is to avoid a high pressure difference—even if only temporary—between the two chambers, which leads to considerable turbulence and therefore, to particles being activated when the gate valve is opened. Another object is to provide a system having a low inertia wherein the two chambers are completely separated with the gate valve closed.
The object is achieved by providing a method for regulating the pressure in an epitaxy reactor, which has a wafer handling chamber, a process chamber and a gate valve connecting the two chambers. The wafer handling chamber is continuously purged with inert gas, wherein the pressure difference between the wafer handling chamber and the process chamber is measured, and the resulting measurement signal is used in a control circuit to regulate the pressure in the wafer handling chamber. The pressure in the wafer handling chamber is reduced if the pressure difference is above a predetermined value. The pressure in the wafer handling chamber is increased if the pressure difference is below a predetermined value. With this method, the pre determined pressure difference, which is defined as the pressure in the wafer handling chamber—pressure of the process chamber which is between 5 and 500 Pa.
The method according to the invention makes it possible to maintain a constant pressure difference between the wafer handling chamber and the process chamber. Since the pressure difference is always kept constant by means of the regulation, even when the pressure in the process chamber fluctuates, it is possible to forego with a high excess pressure in the wafer handling chamber. Therefore, the set value for the pressure difference can be fixed between 5 and 500 Pa, but preferably between 10 and 100 Pa, according to the invention, depending on the pressure conditions prevailing in the process chamber. This defined, low pressure difference ensures a gentle stream of gas from the wafer handling chamber toward the process chamber when the gate valve is opened, whereas flows in the opposite direction are avoided. Since the pressure difference is very low, when the gate valve is opened, the gas turbulence with the associated risk of particles being activated, is minimized. Under these conditions, it is possible to dispense with a compensation line between the two chambers. The chambers always remain separated when the gate valve is closed. The pressure-difference stabilization according to the invention can be achieved over a relatively wide pressure range.
The object of the invention is also achieved by providing a device for regulating the pressure in an epitaxy reactor which has a wafer handling chamber, a process chamber, a gate valve connecting the two chambers and in each case, one gas feed line and one gas discharge line for each of the two chambers. This device has at least one pressure-measuring appliance for measuring the pressure difference between the chambers and a control unit or circuit for regulating the pressure in the wafer handling chamber.