This invention relates generally to plasma etch processes, and more particularly to method and apparatus for process control of the etch process via valve position monitoring.
In the fabrication of integrated circuits, the removal of various layers of materials formed on a substrate to define device patterns is commonly accomplished by means of an etching process. Etching techniques in use include wet, or chemical etching, and dry, or plasma etching. The latter technique is typically dependent upon the generation of reactive species from process gases that are impinged on the surface of the material to be etched. A chemical reaction takes place between the material and these species and the gaseous reaction product is then removed from the surface.
With reference to FIG. 1, creating plasma for use in manufacturing or fabrication processes typically begins by introducing various process gases into a plasma chamber 10 of a plasma reactor, generally designated 12. These gases enter the chamber 10 through an inlet 13 and exit through an outlet 15. A workpiece 14, such as an integrated circuit wafer is disposed in the chamber 10 help upon a chuck 16. The reactor 12 also includes a plasma density production mechanism 18 (e.g. an inductive coil). A plasma-inducing signal, supplied by a plasma inducing power supply 20 is applied to the plasma density production mechanism 18. A top portion 22, constructed of a material transmissive to radiation such as ceramic or quartz, is incorporated into the upper surface of the chamber 10. The top portion 22, allows for efficient transmission of radiation from the coil 18 to the interior of the chamber 10. This radiation in turn excites the gas molecules within the chamber generating a plasma 24. The generated plasma 24 is useful in etching layers from a wafer or for depositing layers upon a wafer as is well known in the art.
An important consideration in all etch processes is control of the extent to which the wafer is etched and determining a time, referred to as the endpoint, at which to end the process. Common methods for monitoring the etch process include spectroscopy and interferometry. In the spectroscopic method, the composition of the species in the plasma chamber 10 are monitored. In the interferometric method, a light is directed through a viewing window 25 onto an area of the wafer undergoing etching within the plasma chamber 10. The intensity of the light reflected from the wafer is detected and recorded as a function of time and/or wavelength. Due to the changing optical path through the layer being etched, varying interference patterns are detected and analyzed to determine etch rate, film thickness and the process endpoint.
In an etch process, the total pressure in the plasma chamber 10 is the sum of the pressures from the gas species being delivered and pressures from the etch by-products generated. The gas species pressures are constant due to the nature of the etch recipe.
The pressure due to etch by-products changes as the amount of by-products changes. In order for the plasma chamber pressure to remain constant, as required by the etch recipe, a pressure control valve changes position to compensate for the pressure changes. By monitoring the position of the pressure control valve, information regarding the constituent species in the chamber can be determined.
A specific application of the etch process is an in-situ cleaning process of the plasma chamber 10. A polymerizing oxide etch process is commonly used to etch the oxide films on the substrate. This process leaves polymer within the chamber. The in-situ cleaning process is then carried out to remove the polymer from the inside surfaces of the plasma chamber 10 and thereby clean the inside surfaces. Conventionally this is a timed process, particularly when the number of optical channels available for observing the chamber is limited or none are available. The time set is normally exaggerated to ensure that all of the polymer is etched in a worst case scenario. Overshooting in a timed cleaning process leads to the over-cleaning of the plasma chamber which can cause hardware erosion.
It would be desirable therefore to provide a process control method and apparatus that accurately detects the endpoint of an in-situ cleaning process and thereby overcomes the aforementioned disadvantages of the prior art methods and apparatus.