Semiconductor device manufacturing: process – Introduction of conductivity modifying dopant into...
Reexamination Certificate
2000-07-12
2001-06-26
Niebling, John F. (Department: 2812)
Semiconductor device manufacturing: process
Introduction of conductivity modifying dopant into...
C438S907000
Reexamination Certificate
active
06251756
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus for continuous processing of a thin planar semiconductor substrate, in particular to an open continuous diffusion furnace as well as a method of operating the same for the processing and production of semiconductor devices, in particular for the processing and production of solar cells.
2. Background of the Invention
Known solar cell fabrication processes such as described in U.S. Pat. No. 5,527,389 using a liquid based diffusion source previously applied to a semiconductor substrate make use of open conveyor belt furnaces. On the other hand vertical or horizontal closed quartz tube furnaces are typically used for diffusion from gaseous sources in the batch manufacture of semiconductor devices as described, for instance in U.S. Pat. No. 4,745,088 or U.S. Pat. No. 4,950,156. Batch manufacture is slow because of the set-up times between each batch. Attempts have been made to achieve continuous or semi-batch processing of semiconductors but all these commercially available diffusion systems yield a throughput lower than required for the future needs of the solar cell industry.
SUMMARY OF THE INVENTION
The present invention provides an apparatus for open processing of thin planar semiconductor substrates, comprising: a first part for high temperature removal of organic components from the substrates; and a second part having a plurality of processing zones, said second part being locally isolated from said first part to prevent contamination of said second part by volatile products from said first part, each zone of said second part being suitable for executing a step of a processing sequence, each zone including means for providing a gas ambient therein and for being heated to a specific temperature; and a transporter for transporting the substrates through said first and said second parts in a substantially continuous horizontal movement. Preferably, the transporter is adapted so that the area of substrates transferred per unit time exceeds the area swept by the transporter per unit time. The present invention includes use of the above apparatus for the processing of solar cells.
The present invention also provides a method of open processing of thin planar semiconductor substrates, comprising a plurality of sequential high temperature processing steps including: high temperature removal of organic components from said substrates, and further high temperature processing steps; said further high temperature processing steps being locally isolated from said organic component removal step to prevent contamination of said further high temperature processing steps by volatile products from said organic component removal step; and transporting said substrates through said plurality of sequential high temperature processing steps on a substrate carrier in a substantially continuous horizontal movement. Preferably, the area swept by the transporter in unit time is less than the area of substrates transported in a unit time.
The substrates are preferably stacked horizontally or vertically in the substrate carriers. This achieves a higher throughput than the conventional flat processing of wafers.
The apparatus in accordance with the present invention may comprise several processing zones after the burn-out zone, for example four zones, the gas ambient of the first zone comprising a gaseous diffusion source for doping said substrate and an inert gas and oxygen, the gas ambient of the second zone comprising an inert gas and oxygen, the gas ambient of the third zone comprising a gaseous diffusion source for doping said substrate and an inert gas and oxygen, and the gas phase ambient of the fourth zone comprising an inert gas and oxygen. The system is open, i.e. in contact at least indirectly with the ambient atmosphere around the apparatus. The open system preferably does not include vacuum traps or air-tight compartments.
The open continuous diffusion system that will be described in the following yields a high throughput and gives a variety of advantages over existing diffusion systems. As for conveyor belt furnaces, the system allows for the application of liquid or solid based diffusion sources such as screen printing of a phosphorous paste prior to the diffusion process. The drying and burn-out of organic components in a first part of the furnace is locally separated from the high temperature diffusion regions of the furnace in order to avoid contamination. Unlike conveyor belt furnaces, the substrates are preferably not placed horizontally on a moving belt in accordance with the present invention. The substrates are preferably loaded into quartz boats in such a way that the throughput is increased compared to the conventional flat arrangement of the wafers. For instance, the wafers may be stacked vertically or horizontally in the boats. This avoids contamination from particles on the belt and particles that might fall on the substrate surface. The vertical or horizontal stacked arrangement of the wafers in the present invention increases the throughput of the system for a given speed of the transportation mechanism. Both diffusion from solid and liquid dopants as well as sequential doping from a gaseous atmosphere may be provided by the inventive system. A variety of advantageous processes can be performed that are not possible in conventional diffusion systems. Especially when a selective diffusion is performed in only one diffusion step, the described innovative system gives additional freedom in the processing conditions.
The present invention provides a system that can be used for continuous diffusion processes by providing a furnace with several zones that can be heated individually and purged individually with various gas atmospheres. The substrates to be processed are moved sequentially through these different zones. The substrate loading and unloading occurs at locally different positions, preferably at the entrance and the exit of the system. The substrates are typically semiconductor wafers and are preferably placed vertically in quartz boats in order to avoid any direct contact with the transport mechanism. Preferably, the mechanical contact area in the slots of the clean quartz boats is reduced to a minimum. The transport of the quartz boats itself occurs ideally by a mechanism that does not create particles or any contamination in the furnace, e.g. by a walking beam. The vertical or horizontal stacks of substrates yield several advantages:
A considerably higher throughput for the same system size as compared to conventional conveyor belt furnaces can be achieved, i.e. the total area of substrates transferred per unit time exceeds the area swept by the transport means per unit time;
The substrate surfaces are protected from particles that might fall into the furnace (at exhaust outlets for instance);
Wafers can be loaded back to back in the same slot for cases in which only one major surface of the substrate to be diffused in this processing step. This minimises unwanted indirect diffusion of the back sides and doubles the throughput;
The substrate surfaces on which diffusion occurs are positioned parallel and preferably facing each other. The spacing between those surfaces can be varied in order to actively regulate the indirect diffusion from the ambient gas phase where substrates are only selectively covered with a diffusion source. The closer the spacing the higher the concentration of the doping atoms that diffuse out from the source that was applied to the substrate into the ambient gas surrounding the substrates.
The major surfaces of the substrates only touch any part of the transport mechanism on the edges thereof. Further, the edges of the substrates only touch removable and cleanable substrate carriers rather than being placed flat on a conveyor belt which first of all makes contact with one major surface of the substrate and secondly may be contaminated from previous processing steps.
The innovative diffusion furnace allows further to have an active deposition f
Horzel Jorg
Nijs Johan
Szlufcik Jozef
Interuniversitair Micro-Elektronica Centrum (IMEC vzw)
Knobbe Martens Olson & Bear LLP
Niebling John F.
Whitmore Stacy
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