1. Field of the Invention
The present invention relates generally to the field of semiconductor processing and, more specifically, to thermal annealing during semiconductor device fabrication.
2. Description of the Related Art
Rapid thermal processing (RTP) is a process for annealing wafers during semiconductor fabrication. During this process, thermal radiation is used to rapidly heat a wafer in a controlled environment to a maximum temperature of over nine hundred degrees above room temperature. This maximum temperature is maintained for a specific amount of time ranging from less than one second to several minutes, depending on the process. The wafer is then cooled to room temperature for further processing. High intensity tungsten or halogen lamps are commonly used as the source of thermal radiation. Conductively coupling the wafer to a heated susceptor provides additional heat.
The semiconductor fabrication process has several applications of RTP. Such applications include thermal oxidation (a wafer is heated in oxygen or a combination of oxygen and hydrogen which causes the silicon substrate to oxidize to form silicon dioxide); high temperature soak anneal (different gas mixtures such as nitrogen, ammonia, or oxygen are used); low temperature soak anneal (typically to anneal wafers deposited with metals); and spike anneal (primarily used in processes where the wafer needs to be exposed to high temperatures for a very short time). During a spike anneal, the wafer is rapidly heated to a maximum temperature sufficient to activate a dopant and cooled rapidly to end the activation process prior to substantial diffusion of the dopant.
A method and apparatus for spike anneal is described in U.S. patent application Ser. No. 2003/0183611, published Oct. 2, 2003, which disclosure is incorporated by reference. During a spike anneal, the wafer is heated using thermal radiation from an array of lamps. Typically, the wafer is heated at a ramp rate of up to 250° C./sec to temperatures greater than 1000° C. The wafer is then cooled by conductively coupling the hot wafer to the cold reflector plate using a blanket of inert gas such as helium gas. This forced cooling facilitates a faster cooling rate, achieving ramp down rates of up to 80° C./sec.
The object of spike annealing is typically a substantially uniform temperature profile across the wafer. This is achieved by controlling an array of lamps or other heat sources which uniformly heat the wafer on the front side while a reflective surface on the back side of the wafer uniformly reflects heat back to the wafer. Emissivity measurement and compensation methodology have been used to improve the uniform temperature gradient across the substrate. However, uniform control of a spike anneal chamber often does not provide substantially uniform temperature profiles across the diameter of the wafer because the chamber components do not behave uniformly at all operating conditions. Thus, apparatus and methods to improve uniformity remain of interest.