Chemistry: electrical and wave energy – Processes and products – Coating – forming or etching by sputtering
Reexamination Certificate
2002-01-30
2004-04-06
VerSteeg, Steven H. (Department: 1753)
Chemistry: electrical and wave energy
Processes and products
Coating, forming or etching by sputtering
C438S743000, C438S778000
Reexamination Certificate
active
06716324
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a transparent, conductive film forming method of forming a transparent, conductive film on a semiconductor layer stacked on a substrate, by sputtering, a semiconductor-layer defective region compensation method of compensating a defective region produced in a semiconductor layer stacked on a substrate, a photovoltaic element in which a transparent, conductive film is formed by sputtering, on a semiconductor layer stacked on a substrate, and a method of producing a photovoltaic element.
2. Related Background Art
In recent years, various studies have been and are being conducted toward practical use of photovoltaic power generation by solar cells. In order to cover some of power demands by the solar cells, they need to meet the requirements of sufficiently high photoelectric conversion efficiency of solar cells used, excellent reliability, and capability of volume production.
First, common sputtering techniques will be described below.
In general, as methods of depositing a transparent, conductive film on a substrate by sputtering, there are two types of methods proposed: one is a method of effecting sputtering in Ar gas, using an oxide of In
2
O
3
—SnO
2
or the like as a target; the other is a reactive sputtering method of sputtering an alloy of In—Sn or the like in mixed gas of Ar and O
2
. The former permits formation of a film with low electrical resistance and high transmittance immediately after sputtering, but involves the difficulty of increasing film forming rates.
On the other hand, the latter reactive sputtering method allows increase of film forming rates. Particularly, in the case of DC magnetron sputtering apparatus using a cylindrical, rotatable target, as described in U.S. Pat. Nos. 4,356,073 and 4,422,916, it is reported that the utilization efficiency of the target material is approximately 2.5 to 3 times higher than those of the general planar type (Kinouzairyou (Functional materials), Vol. 11, No. 3, pp. 35-41, March 1991). The advantages of this reactive sputtering method include saving of the target material and great decrease of production stop time for exchange of targets. Accordingly, the DC magnetron sputtering apparatus using the rotatable target is suitable for volume production.
This reactive sputtering, however, requires extremely narrow adequate ranges of film formation conditions, particularly, flow rates of gas; for example, where a transparent, conductive film was formed on a sheet-like substrate of a large area, it was difficult to control the film formation parameters such as evenness of sheet resistance and transmittance, discharge stability, and so on.
The reactive sputtering method employing a plasma emission monitor (hereinafter referred to as “PEM”) is known as a method overcoming the disadvantage.
Reference should be made to S. Schiller, U. Heisig, Chr. Korndorfer, J. Strumpfel, V. Kirchhoff “Progress in the Application of the Plasma Emission Monitor in Web Coating” (Proceedings of the 2nd International Conference on Vacuum Web Coating, Fort Lauderdale, Fla., USA, October 1988).
This PEM is a device for collecting plasma emission by a collimator, guiding the emitted light through a spectroscope to a photomultiplier tube (photomultiplier), photoelectrically converting the light into an electric signal, and monitoring the state of the plasma, based on the electric signal. The device has a function of setting the sensitivity of the photomultiplier of the PEM at a certain value and regulating the flow rate of introduction of reactive gas so as to keep the emission intensity of the plasma constant.
Japanese Patent Application Laid-Open No. 11-29863 discloses the technique of forming a film of ITO (Indium Tin Oxide) on a substrate. This technique is generally a method of setting a substrate in a film forming chamber, inducing discharge in the film forming chamber in a state in which sputter gas is introduced and reactive gas is not introduced thereinto, adjusting the sensitivity of the device for monitoring the emission intensity of the plasma such that the emission intensity of the plasma of the discharge becomes a predetermined value, and sputtering the target while controlling the introducing amount of the reactive gas so as to keep the film forming rate constant. Namely, it is the technique of forming a uniform film by regulating the flow rate of introduction of the reactive gas (O
2
) so as to keep the plasma emission intensity of In (at the wavelength=451.1 nm) constant during formation of the ITO film.
These techniques made it feasible to produce a satisfactorily good deposited film on a satisfactorily stable basis in the reactive sputtering methods.
The common defect removing techniques will be described below.
The amorphous silicon (hereinafter referred to as “a-Si”) solar cells are drawing attention, because they can be produced at lower cost and have higher mass producibility than the solar cells produced using crystalline Si and others. The reason for it is that it is possible to use readily available gas such as silane or the like as source gas, decompose it by glow discharge, and form a deposited film of a semiconductor film or the like on a relatively inexpensive, belt-like substrate such as a metal sheet, a resin sheet, or the like.
Incidentally, the output power of about 3 kW is necessary for applying the solar cells to power supply at ordinary households. With use of the solar cells having the photoelectric conversion efficiency of 10%, the area in that case is 30 m
2
, and it is thus necessary to prepare the solar cells of large area. It is, however, very hard to produce the solar cells without defects over a large area because of the production steps of solar cells.
For example, it is known that low-resistant portions appear at grain boundary regions in polycrystalline solar cells and that in the thin film solar cells such as those of a-Si, defects are produced by influence of dust or the like during formation of semiconductor layers to become the cause of shunts and decrease the photoelectric conversion efficiency and yield significantly.
Further, causes of production of defects and their effect include the following; for example, in the case of the a-Si solar cell deposited on a stainless steel substrate, the substrate surface cannot be regarded as a perfectly smooth surface, but has flaws and dents, a back surface reflecting layer of uneven structure is provided on the substrate for the purpose of effective use of incident light, it is thus difficult for thin film semiconductor layers several ten nm thick such as n- or p-layers to completely cover such a surface, defects are produced by dust or the like during film formation, and so on.
Where the semiconductor layers between the lower electrode and the upper electrode of the solar cell are lost because of the defects or the like to cause direct contact between the lower electrode and the upper electrode or where the semiconductor layers are not lost completely but themselves have a low resistance to cause shunts between the upper electrode and the lower electrode, the electric current generated by light will flow through the upper electrode into the low resistant regions of the shunt portions, resulting in loss of electric current. Such current loss will result in decrease of open circuit voltage of the solar cell.
Since in the a-Si solar cells the sheet resistance of the semiconductor layers themselves is generally high, they need to have the upper electrode consisting of a transparent, conductive film over the entire semiconductor surface. In general, the upper electrode is a transparent, electroconductive film such as films of SnO
2
, In
2
O
3
, ITO (In
2
O
3
+SnO
2
), and so on with excellent characteristics as to transparency to the visible light and electric conductivity. These transparent, conductive films are normally formed by sputtering, vacuum resistance heating evaporation, electron beam evaporation, spraying, and so on. When there exist defects in the semicon
Izawa Hiroshi
Takai Yasuyoshi
Yamashita Toshihiro
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
VerSteeg Steven H.
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