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Title:
BAKING OVEN FOR PHOTOVOLTAIC DEVICES
Document Type and Number:
WIPO Patent Application WO/2003/054975
Kind Code:
A2
Abstract:
A baking oven is described, for photovoltaic devices starting from semiconductor material characterized in that the entrainment of the semiconductor material inside the oven is effected by means of a series of rolls made of a ceramic material, which rotate on their own axis.

Inventors:
Zarcone, Mariano (Via dei Gladioli 7, Nettuno, I-00048, IT)
Romagnoli, Amerigo (Via Madrid, 110, Aprilia-Latina, I-04011, IT)
Application Number:
PCT/EP2002/013372
Publication Date:
July 03, 2003
Filing Date:
November 27, 2002
Export Citation:
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Assignee:
ENITECNOLOGIE S.P.A. (Via F. Maritano 26, San Donato Milanese-Milan, I-20097, IT)
Zarcone, Mariano (Via dei Gladioli 7, Nettuno, I-00048, IT)
Romagnoli, Amerigo (Via Madrid, 110, Aprilia-Latina, I-04011, IT)
International Classes:
F27B9/24; H01L21/00; H01L31/04; (IPC1-7): H01L31/18; H01L21/00
Foreign References:
EP0908928A21999-04-14
US3867748A1975-02-25
EP1010960A12000-06-21
US4932864A1990-06-12
US5266027A1993-11-30
US5044920A1991-09-03
US4343395A1982-08-10
US2788957A1957-04-16
Attorney, Agent or Firm:
De Gregori, Antonella (Ing. Barzanò & Zanardo Milano S.p.A, Via Borgonuovo 10, Milan, I-20121, IT)
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Claims:
CLAIMS
1. A baking oven for photovoltaic devices starting from semiconductor material characterized in that the en trainment of the semiconductor material inside the oven is effected by means of a series of rolls made of a ceramic material, which rotate on their own axis.
2. The baking oven for photovoltaic devices according to claim 1, comprising: (a) a housing delimited by curved or flat lateral walls, a top, a bottom and side passages which allow the insertion of rolls; (b) an electric engine which, by means of a shaft and belt drive, allows the contemporaneous movement of the rolls; (c) a device for keeping the temperature inside the housing (a) constant; (d) an insulation system of the housing (a); (e) a removal system of the gases formed during the baking process.
3. The oven according to claim 2, wherein the housing (a) is a muffle consisting of ceramic material.
4. The oven according to claim 1, wherein the ceramic ma terial is selected from sintered silica, quartz and other material with a low chemical interaction with the silicon.
5. The oven according to claim 1, wherein the distance between one roll and another is such that the semicon ductor material, in its rotating movement, rests on at least three of these.
6. The oven according to claim 1, wherein the heating is effected with electric resistances or with infrared lamps.
7. The oven according to claim 1, wherein the semiconduc tor material which can be used for the preparation of photovoltaic devices is selected from slices of sili con, with thicknesses of 100500 microns, or semicon ductor materials in thin layers (films) with thick nesses varying from a few microns to 100 microns.
Description:
BAKING OVEN FOR PHOTOVOLTAIC DEVICES The present invention relates to a baking oven for photovoltaic devices starting from semiconductor material, characterized in that the entrainment of the semiconductor material inside the oven is effected by means of a series of rolls made of a ceramic material which rotate on their own axis.

It is known that for the production of photovoltaic devices, commonly known as solar cells, slices of semicon- ductor material of varying thicknesses, are used.

The production of these photovoltaic devices involves, among the different types of treatment, various baking pro- cesses of the material, whose function is to fix doping elements, dispersed in solutions or in serigraphic pastes, or metallic contacts, onto the semiconductor.

In order to effect these baking processes, continuous ovens are used, i. e. ovens in which the material is charged and processed without any interruption.

The introduction of the semiconductor material into

these ovens is typically carried out using a chain which, pulled by two rolls outside the muffle, passes through the oven.

The apparatus described above, however, has various disadvantages, of which the most important are: (1) the chain in normally made of a metallic material which can cause contaminations on the part of metallic impurities in the semiconductor material treated ;.

(2) the temperature profiles at which the semiconductor ma- terial is subjected, depend on the transport chain. This generally has a thermal inertia higher than that of the semiconductor material to be baked and consequently when moving, carries with it a considerable quantity of heat.

Strongly varying profiles (for example 10-20°C/s) often necessary for optimal processes, can only be obtained with great difficulty.

Patent application EP 1010960 relates to a baking oven for photovoltaic devices which is based on the non- interaction of the entrainment system with the heating sys- tem of the oven. In particular, the rolls, which are made of quartz, are moved by means of a chain or belt, not indi- vidual, by friction.

Operating according to this patent, baking profiles are obtained by means of lamp heating, which can cause problems of stability and consequently uniformity; the en-

trainment, moreover, can be subjected to non-uniformity as the movement of the rolls is not individual. Furthermore, the quartz rolls transparent to radiation selected for the heating of the semiconductors, are extremely expensive.

It has now been found that it is possible to overcome the above drawbacks by using an oven in which the trans- porting of the semiconductor material through said oven, is effected by means of entrainment with ceramic rolls which rotate around their own axis by means of a shaft and belt drive.

The use of high purity ceramic material for the pro- duction of the rolls drastically reduces metallic contami- nation. Furthermore, as the rolls which move the semicon- ductor material are fixed in their position in the oven and the movement is transmitted by their rotation on their own axis, there are no temperature entrainment effects. In this way, even very sheer temperature profiles can be obtained.

In accordance with this, the objective of the present invention relates to an oven for the preparation of photo- voltaic devices from semiconductor material comprising: (a) a housing delimited by curved or flat lateral walls, a top, a bottom and side passages which allow the insertion of rolls; (b) an electric engine which by means of a shaft and belt drive allows the contemporaneous movement of the rolls;

(c) a device for keeping the temperature inside the housing (a) constant; (d) an insulation system of the housing (a); (e) a removal system of the gases formed during the baking process.

The housing preferably consists of ceramic material and the distance between one roll and another inside the housing, is such that the semiconductor material, during its rotating movement, rests on at least three of these.

The ceramic material used in the manufacturing of the oven is selected from sintered silica, quartz and other ma- terial with a poor chemical interaction with the silicon.

The semiconductor material which can be used for the preparation of photovoltaic devices is selected from slices of silicon, with thicknesses of 100-500 microns, or semi- conductor materials in thin layers (films) which thick- nesses varying from a few microns to 100 microns.

The heating of the oven can be effected according to the known methods, by the use of electric resistances or infrared lamps.

The temperatures inside the oven typically range from 100°C to 1200°C.

A description is provided of a typical embodiment of the invention with reference to figure 1. The embodiment is illustrative and should in no way be considered as limiting

the scope of the invention.

Figure 1 illustrates an oven consisting of a certain number of rolls (1) in series made of sintered silica and the contemporaneous movement of all the rolls is due to an electric engine (16) by means of a shaft and belt drive (13, 11). The muffle (6) and its top (5), also made of sin- tered silica, are heated by a series of electrically-fed resistances (3). The temperature is controlled with an electronic device to keep it constant. The oven is equipped with a frame (9) which encloses the insulation system (2) of the muffle, and has a removal system of the process fumes (18).

(1) Roll (2) Insulation (3) Resistances (4) Resistance support (5) Muffle top (6) Muffle (7) Ball-bearing (8) Bearing support (9) Frame (10) Shaft support (11) Crossed belt (12) Pulley (13) Shaft

(14) Pinion (15) Chain (16) Engine (17) Crown (18) Stack The devices obtained using the oven according to the present invention are free of metallic contaminants and are characterized by an excellent quality, allowing them to be used, for example, for the production of photovoltaic cells.

An example is provided hereunder for a better illus- tration of the functioning of the apparatus according to the invention.

EXAMPLE 1 An oven was produced for the diffusion of impurities of the phosphorous type with an entrainment system consist- ing of 300 rolls in series, made of sintered silica. The contemporaneous movement of all the rolls is due to an electric engine (16) by means of a shaft and belt drive (13, 11). The muffle (6) made of sintered silica is heated by a series of electrically-fed resistances (3).

The temperature is controlled with an electronic de- vice for maintaining a constant value. On selecting a set point of 900°C, the variation observed was less than 0. 5°C.

Slices of silicon on which a phosphorous doping agent

had been deposited, were passed into the oven. The passage lasted about twenty minutes.

The silicon slices thus obtained had an excellent dif- fusion of phosphorous impurities and could be used for the production of high-quality photovoltaic cells.