Dopant pastes for the production of p, p+, and n,...

Details Dopant pastes for the production of p, p+, and n,... Dopant pastes for the production of p, p+, and n,...

2001-09-12

2004-02-24

Koslow, C. Melissa (Department: 1755)

Compositions: coating or plastic

Coating or plastic compositions

Silicon containing other than solely as silicon dioxide or...

C106S287160, C106S287170, C106S287290, C252S950000, C438S542000, C438S543000, C438S923000, C438S558000, C438S561000, C438S559000, C438S540000

Reexamination Certificate

active

06695903

ABSTRACT:

The invention relates to novel boron, phosphorus or boron-aluminium dopant pastes for the production of p, p+ and n, n+ regions in monocrystalline and polycrystalline Si wafers, and of corresponding pastes for use as masking pastes in semiconductor fabrication, power electronics or in photovoltaic applications.
Boron or phosphorus doping of monocrystalline Si wafers in photovoltaic technology and in the case of power semiconductors (e.g. diodes, thyristors) is carried out via the vapour phase (e.g. POCl
3
, phosphine, BBr
3
) or with solids (films or solid slices) or with liquid media such as, e.g. phosphoric acid, and other organic B or P systems.
Directly patterned application is not possible with these techniques. Defined regions cannot be selectively doped without using elaborate, e.g. photolithographic processes, to mask the areas that are not to be doped.
Directly patterned application is not possible with these techniques. Defined regions cannot be selectively doped without using elaborate e.g. photolithographic processes to mask the areas that are not to be doped.
In photovoltaic technology, an extensive or local back surface field (BSF) can be achieved by p++ doping on the back. Existing production lines currently operate only with an extensive back surface field, which is obtained by an Al metal layer. Local BSFs have currently only been realized on a laboratory scale, for reasons which will be explained in more detail below.
An extensive back surface field is especially advantageous, inter alia, in the case of thin Si wafers (≦250 &mgr;m), since otherwise the efficiency of the cell turns out to be significantly lower. Such a BSF is currently obtained in the case of Si wafers of >300 &mgr;m thickness by extensive printing with an aluminium paste. This case is advantageous in that, on the one hand, p+ doping, i.e. a BSF, is built up by the aluminium and, on the other hand, the electricity generated in the cell is also dissipated by the aluminium. In the case of thin wafers, however, curvature of the wafer occurs after the aluminium paste has been burned in. The further handling and processing of such wafers therefore requires special measures since there are pronounced difficulties involved with the incorporation of such bent cells into the usually planar-designed PV modules.
In DE 195 08712 C2 and in K. A. Münzer, R. R. King, R. E. Schlosser, H. J. Schmidt, J. Schmalzbauer, S. Sterk, H. L. Mayer, 13
th
European Photovoltaic Solar Energy Conference, 23-27 10.1995, p. 1398, attempts are made to solve this problem by a spin-on process with boron. In this case, p++ doping is produced by boron and an aluminium framework is implemented in a subsequent print by means of aluminium paste.
Disadvantages of this process are
a) the large material requirement of the spin-on process
b) the considerable equipment outlay, with which angular wafers are to be coated uniformly using the spin-on process
c) the high throughput and the expensive handling, which are to be realized in mass production only with great difficulty, and
d) that selective patterning of the wafers is not possible, as described above.
The object of the present invention was therefore to provide improved, inexpensive dopant pastes usable in semiconductor technology, which do not have the disadvantages described above and can be used in a straightforward way. The object of the present invention was also to provide corresponding screen-printable pastes.
The object is achieved by dopant pastes for the selective patterning and for the extensive printing of Si wafers for the production of p, p+, p++, n, n+, n++ regions in the Si wafer, containing
a) one or more components with dopant action,
b) an SiO
2
matrix,
c) solvents,
d) a thickening agent or wetting agent,
e) optionally an acid and water, and optionally
f) additives,
the total composition having impurities in the form of metal ions in respective concentrations of less than 200 ppb, preferably less than 100 ppb.
Through tests, it was found that the disadvantages mentioned above are eliminated by the screen-printable dopant pastes found here.
The pastes according to the invention may contain a dopant source selected from the group boron salt, boron oxide, organic boron compounds, boron-aluminium compounds and phosphorus salt, phosphorus oxide, phosphorus pentoxide, phosphoric acid, organophosphorus compounds and organic aluminium compounds.
In principle, the compounds known to the person skilled in the art primarily from main groups III and V of the periodic table are suitable.
The corresponding pastes furthermore contain an SiO
2
, matrix consisting of one or more SiO
2
precursors.
The dopant pastes which have been found may contain one or more dopant sources selected from the group boron oxide (B
2
O
3
), phosphoric acid (H
3
PO
4
), phosphorus pentoxide (P
2
O
5
) and aluminium salt, it being possible for the doping sources to be used in different concentrations.
The present invention also relates to dopant pastes which contain one or more dopant sources from the group of organic boron, phosphorus and aluminium compounds, it being possible for the doping sources to be used in different concentrations.
The object is furthermore achieved by dopant pastes, containing one or more dopant sources selected from the group boron oxide (B
2
O
3
), phosphoric acid (H
3
PO
4
), phosphorus pentoxide (P
2
O
5
) and aluminium salt, and optionally one or more dopant sources from the group of organic boron, phosphorus and aluminium compounds, it being possible for the doping sources to be used in different concentrations.
The SiO
2
matrix, which the dopant paste contains, may according to the invention be formed by an organic silicon compound of the general formula R′
n
Si(OR)
4−n
, in which
R′ denotes methyl, ethyl or phenyl,
R denotes methyl, ethyl, n-propyl or i-propyl, and
n denotes 0, 1 or 2.
The present invention furthermore relates to a dopant paste that contains an SiO
2
matrix, which is formed by silanes, selected from the group tetramethoxysilane, tetraethoxysilane, tetrabutoxysilane, and tetrapropoxysilane, individually or as a mixture.
The present invention also relates to the use of the described new dopant pastes in semiconductor technology, photovoltaic technology or in power electronics.
Besides the dopant pastes, the invention also relates to masking pastes, containing
a) an SiO
2
matrix,
b) solvents,
c) a thickening agent or wetting agent,
d) optionally an acid and water, and optionally
e) additives,
the total composition having impurities in the form of metal ions in respective concentrations of less than 200 ppb, preferably less than 100 ppb.
The masking pastes, like the dopant pastes mentioned above composed in the same way, except that the masking pastes are free from dopants, are used according to the invention in semiconductor technology, photovoltaic technology and in power electronics.
It is likewise possible to achieve extensive or selective (selective emitter) phosphorus doping inexpensively and with high throughput by means of screen-printable pastes.
Patterning stages, which are currently made possible by photolithographic processes, can be replaced inexpensively by the use of screen-printable dopant paste.
The masking paste, which can be described as pure SiO
2
matrix paste without dopant additives, can be employed for intentionally defined protective layer formation. For this purpose, the paste my be applied extensively to the Si wafer, or else in patterned fashion. This makes it possible to protect defined regions from doping during the diffusion process.
The object is achieved by novel pastes for the selective production of silicate layers doped with phosphorus, boron and boron-aluminium. These pastes are suitable for use in technical printing processes in the electronics industry. These include, especially, screen-printing technology or pad printing.
Depending on the desired field of application, both the concentrations of the individual components a

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