Electric heating – Heating devices – Combined with container – enclosure – or support for material...
Reexamination Certificate
2000-01-28
2001-09-18
Walberg, Teresa (Department: 3742)
Electric heating
Heating devices
Combined with container, enclosure, or support for material...
C219S390000, C219S405000, C219S411000, C118S724000, C118S725000, C118S050100, C392S416000, C392S418000
Reexamination Certificate
active
06291799
ABSTRACT:
The invention relates to a process for heat treatment of two-dimensional objects such as semiconductor wafers, for example in the form of silicon wafers, where the objects are arranged on supports and are conveyed, in particular continuously, through a heating unit. The invention further relates to an array for heat treatment of two-dimensional objects, in particular semiconductor wafers such as silicon wafers and comprising the supports receiving the objects, a first conveyor device spanning a transport plane and conveying said objects through a heating unit, and supply and removal means for supplying and removing the objects to/from the supports.
The continuous heat treatment, in particular high temperature of semiconductor wafers, are necessary for processes such as oxidation of the semiconductor surface, sintering of layers onto the semiconductors, for example metal contacts or other antireflective coatings, or doping of the semiconductor material by diffusion.
In the application diffusion, oxidation and sintering of contacts and antireflective coatings, however, the prior art entails considerable problems in respect of the processes.
In the manufacture of semiconductor components, in particular of solar cells, thin and highly doped layers must be generated in one or more consecutive process steps on the surface of semiconductor wafers. These can either have the opposite conduction type to the base material, i.e., a highly n doped layer on p-doped base material or a highly p-doped layer on n-doped base material, or the highly doped layer can have the same conduction type as the base material.
The process used most frequently for this purpose is the diffusion of suitable dopants such as phosphorus or arsenic for n-conducting layers, or boron or aluminum for p-conducting layers.
The classic process for doping with phosphorus is diffusion in quartz tube furnace. In this process, the semiconductor wafers are placed in quartz hurdles so that the wafers are parallel and evenly spaced from one another. The hurdles are placed inside a process tube made of quartz and heated from the outside by radiation.
Inside the tube, the wafers are subjected to an oxidizing atmosphere containing phosphorus. Usually nitrogen is bubbled through a dopant in liquid form, e.g. phosphoryl chloride POCl
3
or phosphorus bromide PBr
3
. It is then passed together with a further flow of nitrogen and oxygen into the heated process tube. More rarely, the highly toxic gas hydrogen phosphide PH
2
is passed into the process tube.
At temperatures between 800° C. and 1000° C., the following reactions take place in the process tube. The dopant decomposes and phosphorus pentoxide P
2
O
2
and free Cl or Br is formed. The phosphorus pentoxide reacts with the silicon surface and forms silicon dioxide and phosphorus that is stored in the oxide.
From this silicon dioxide film, the phosphorus diffuses into the semiconductor in accordance with the concentration gradient. The properties of the n-doped layer, such as conductivity and penetration depth, depend on the concentration of phosphorus in the dopant film, on the temperature and duration of the heat treatment, and on the doping of the original material and its crystalline structure.
It is an advantage that the atmosphere in quartz tube furnaces can be kept very pure, since quartz glass can be made to a very high purity. In addition, the halogens released from the dopants contribute continuously to the cleaning of the furnace atmosphere, as they form with metals volatile halogenides that can be removed from the process tube together with the process gases.
However, there are severe drawbacks in particular for the manufacture of solar cells, since diffusion takes place on all sides, the throughput in the tube furnaces is relatively low, the process is not continuous but in individual batches, and the positioning of the wafers in quartz hurdles entails considerable expense, particularly when the upstream and downstream processes are continuous ones.
For making solar cells, therefore, a different process is being increasingly used. The semiconductor wafers are coated in a separate process step with a dopant film. This can be achieved by screen printing, spinning-on or spraying on of a film of phosphorus silicate polymers and organic solvents, or even diluted phosphoric acid.
The wafers thus coated then pass through a through-type furnace where first the organic constituents of the dopant film are evaporated or incinerated, a silicon oxide layer containing the phosphorus is generated, and finally the phosphorus is diffused from the remaining silicate film into the semiconductor material. The temperature profile of the furnace and the composition of the atmosphere in the furnace is carefully adapted to the processes here.
This process avoids the above drawbacks of the quartz tube furnace, but has another serious drawback: the furnace cannot be built completely metal-free. At least the necessary conveying device such as belt must be made of metal in view of the heavy stresses and the long operating duration required. Usually braided belts of high-temperature-proof Ni—Cr—Mo alloys are used.
It has been proven in extensive tests that impurities from the braided metal belt diffuse into the semiconductor wafers and shorten the life of the minority charge carriers or even cause short-circuits in the p n junction due to deposition, thereby diminishing the efficiency of the solar cells. This deterioration can also occur when direct contact of the semiconductor wafers with the transport belt is avoided. The degree of efficiency impairment correlates with the composition of the metal belt and with the composition of the semiconductor wafers and their crystalline structure.
For passivation of semiconductor surfaces or for making diffusion masks for photolithography, semiconductor wafers are subjected to an atmosphere containing oxygen or oxygen/steam at very high temperatures, in order to create an oxidation layer. If the process is performed in a through-type furnace, the same problems are described above occur.
Sintering of metallic coatings onto semiconductors is performed almost exclusively in through-type furnaces under a controlled atmosphere, as the specific temperature/time profiles needed for achieving a low contact resistance can only be achieved easily in through-type furnaces. During the sintering of contacts, temperatures of more than 800° C. are achieved for a short time. This can result—as is diffusion or oxidation in metals from the furnace atmosphere diffusing into the semiconductor and reducing the quality of the components produced.
A process for continuous heat treatment of semiconductor wafers is described in U.S. Pat. No. 5,449,883, where the wafers are supported on oscillating beams in a line and conveyed by these through a heat treatment zone and a cooling zone.
For thermal treatment of silicon films, they can be passed between light sources (US-Z; Fair, J., Rapid Thermal Processing for Active Matrix Devices, Solid State Technology, August 1992, p. 47-52).
The problem underlying the present invention is to develop a process and an arrangement of the type mentioned at the outset such that a targeted treatment of a two-dimensional object, in particular of a semiconductor component such as a silicon wafer, can take place on one side only, while at the same time a high throughput is achieved with incorporation as necessary into a continuous overall manufacturing process. Also, unproblematic loading of the supports with the objects should be possible. At the same time, it should be assured that there is no contamination of the objects by materials stemming from the supports, i.e. diffusing of such materials, even when high temperatures are applied
The problem is solved by the process substantially in that the objects are arranged in full-surface or almost full-surface contact on the supports, which cover the objects completely or almost completely on their underside and which in turn comprise quartz glass in particular. Here the supports are aligned and arran
Heyer Fritz
Roth Peter
Angewandte Solarenergie - ASE GmbH
Dennison, Scheiner Schultz & Wakeman
Fuqua Shaurita
Walberg Teresa
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