Electric heating – Metal heating – By arc
Reexamination Certificate
1997-11-07
2001-01-09
Leung, Philip H. (Department: 3742)
Electric heating
Metal heating
By arc
C219S746000, C219S686000, C219S761000, C118S7230MW, C156S345420
Reexamination Certificate
active
06172322
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to a method and an apparatus for annealing amorphous films, and more particularly to a method which may be used to anneal amorphous silicon films (“a-Si”) so that they are transformed into polysilicon films (“p-Si”). The method uses microwave energy to perform the heating for the anneal.
BACKGROUND OF THE INVENTION
Plasma-assisted chemical reactions, such as plasma-enhanced chemical vapor deposition (“PECVD”), have been widely used for film deposition in the semiconductor and flat panel display industries in the manufacture of thin film transistors (“TFT”s) for active-matrix liquid crystal displays (“AMLCD”s), e.g. In accordance with PECVD, a substrate is placed in a vacuum deposition chamber that is equipped with a pair of parallel plate electrodes. One of the electrodes, e.g., the lower electrode, generally referred to as a susceptor, holds the substrate. The other electrode, e.g., the upper electrode, functions as a gas inlet manifold or shower head. During deposition, a reactant gas flows into the chamber through the upper electrode and a radio frequency (RF) voltage is applied between the electrodes to produce a plasma within the reactant gas. The plasma causes the reactant gas to decompose and deposit a layer of material onto the surface of the substrate.
Two types of materials often deposited are p-Si and a-Si. Both a-Si and p-Si are used to fabricate TFTs, such as those used in AMLCDs. Other applications include solar cells, etc. Typically, p-Si is deposited from a silane source at a substrate temperature above about 600 degrees Celsius (° C.). If the substrate temperature is lower, the initially deposited structure is a-Si; i.e., it has no definite arrangement of atoms.
Thus, it is often easier to deposit a-Si because less substrate heating is required. However, devices made with a-Si often exhibit poorer performance when compared to the same devices made with p-Si. E.g., field effect mobility, which is generally accepted to be one of the most important device characteristics, may be better for p-Si than for a-Si by orders-of-magnitude. Thus, using p-Si films instead of a-Si films may increase the performance of devices such as flat panel displays. Fortunately, a p-Si film may be obtained from an a-Si film by annealing the a-Si film.
Annealing, however, may be difficult in some cases. One reason for this concerns the quality of the substrate used. For a commercially-viable application of flat panel displays, e.g., the glass substrates used should be inexpensive. However, the maximum processing and deposition temperature of an inexpensive glass substrate may be relatively low, e.g., below 400° C. At these temperatures, only a-Si can be deposited.
Current annealing methods encounter difficulties with such substrates. These techniques heat the substrate and the film at the same time, and thus are problematic for the reason noted above.
Conventional methods of performing annealing include, among others, laser annealing, thermal annealing, and lamp annealing. These approaches have certain inherent drawbacks. Laser annealing requires a complicated scanning laser configuration. Thermal annealing requires, e.g., a source of hot gas in addition to the gas sources already required for deposition. Lamp annealing requires a complicated multiple-lamp system to ensure temperature uniformity.
SUMMARY OF THE INVENTION
In one aspect, the invention is directed to a system for annealing a film on a substrate in a processing chamber, including a microwave generator disposed to provide microwaves to an area within the interior of the chamber. The microwaves have a frequency in a range such that the film is substantially absorptive at some frequency within the range but the substrate is not substantially absorptive at the frequencies in the range. A waveguide distributes the microwaves over the surface of the film to provide a substantially uniform dosage of microwaves over the surface of the film.
Implementations of the invention may include one or more of the following. The substrate may be glass and the film amorphous silicon. The microwave frequency may be about 2.45 GHz.
In another aspect, the invention is directed to a method of annealing a film in a processing chamber. The method includes depositing a film on a substrate in the processing chamber. During at least a portion of the time of the depositing step, microwaves are generated having a predetermined frequency. The film has an absorption peak at or near the predetermined frequency but the substrate lacks a substantial absorption peak at or near the frequency. The microwaves are directed towards the film.
Implementations of the invention may include one or more of the following. The deposition step may include the step of forming a silane plasma within the processing chamber.
In another aspect, the invention is directed to a method of annealing an amorphous film of silicon in a thermochemical processing chamber to form a film of polysilicon. An amorphous silicon film is deposited on a glass substrate in the processing chamber. Microwaves are generated having a frequency such that the amorphous silicon film is substantially absorptive at or near the frequency but the glass is not substantially absorptive at or near the frequency. The microwaves are guided to the surface of the amorphous silicon film such that the amorphous silicon film is transformed into a polysilicon film.
In another aspect, the invention is directed to a method similar to the above. A further step includes heating the film for a predetermined time period using the microwaves.
In another aspect, the invention is directed to a method of annealing a film in a processing chamber. In this method, a film is provided on a substrate in the processing chamber, and a gas is provided in the processing chamber. Microwaves are generated having a frequency such that the microwaves induce a plasma of at least a portion of the gas. The microwave-induced plasma has a substantial thermal energy transfer to the film.
In another aspect, the invention is directed to a method of depositing and annealing a film in a processing chamber. A substrate is provided in the processing chamber. A gas is flowed into the processing chamber. Microwaves are generated having a such that a portion of the microwaves induce a plasma of at least a portion of the gas. A film is deposited on the substrate from the plasma. The microwaves are directed to the surface of the film, the microwaves having a frequency such that the film is substantially absorptive at or near the frequency but the substrate is not substantially absorptive at or near the frequency.
Advantages of the invention include the following. The invention achieves an anneal of a-Si films, transforming a-Si films into p-Si films, without the complexity of, e.g., laser annealing, thermal annealing, or lamp annealing systems. The invention also achieves a higher quality anneal by not heating the substrate during the anneal, instead only heating the deposited film. In this way, lesser quality substrates may be accommodated if necessary. Finally, the system of the invention may be easily scaled for large or small substrates.
Other features and advantages of the invention will be apparent from the following description, including the drawing and the claims.
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Law Kam S.
Robertson Robert McCormick
Shang Quanyuan
Sun Sheng
Takehara Takako
Applied Technology, Inc.
Leung Philip H.
Morris Birgit E.
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