GB2476925A

GB2476925A - Printing screens and method of fabricating the same

Info

Publication number: GB2476925A
Application number: GB0916617A
Authority: GB
Inventors: Tom Falcon
Original assignee: DTG International GmbH
Current assignee: ASM Assembly Systems Switzerland GmbH
Priority date: 2009-09-21
Filing date: 2009-09-21
Publication date: 2011-07-20
Also published as: GB0916617D0

Abstract

A printing screen for printing elongate structures on substrates is disclosed. The screen comprises first upper layer 3 and second lower layer 5 of different material. The first layer includes a plurality of elongate apertures 7 across which extend a plurality of bridges 9. The second layer includes plurality of elongate apertures 15 in registration with the apertures in the first layer. The first layer may be formed of metal, preferably electroformed nickel. The second layer may be photo-imageable and formed of a screen emulsion. A method of fabricating the screen is also provided.

Description

PRINTING SCREENS AND METHOD OF FABRICATING THE SAME

The present invention relates to printing screens, often alternatively referred to as stencils, in particular for printing narrow, elongate structures on substrates, such as the front side conductors on silicon solar cells, and a method fabricating such printing screens.

In silicon photovoltaics, a significant barrier to continued development is the shadowing of the surface of the silicon wafer by the frontside metallisation grid, which will typically block light from reaching about 8% of the wafer.

This shadowing loss cannot simply be mitigated by reducing the number of conductors or by spacing the conductors more widely, as this would result in further efficiency losses, primarily due to the increase in resistance across the silicon between the conductors. Although this trade-off between shadowing and resistance seems to leave little room for improvement, the present inventor has recognized that refinements can still be made to give credible efficiency gains.

It is an aim of the present invention to provide a method of minimising conductor shading losses, by enabling narrower conductor lines, whilst maintaining or improving their cross sectional area uniformity, and .. : therefore their electrical conductivity. * *** * * ****

Improving aspect ratio and cross sectional area uniformity together have several advantages, in that narrower, taller conductors will expose more ** * ...

* silicon to sunlight, with a resultant increase in efficiency. Another benefit *:*. can be had by simply printing more of these conductors on a finer pitch, which will reduce the resistance loss, whilst maintaining the shading loss, again resulting in an efficiency gain. A further benefit is that a high aspect ratio conductor with a uniform cross-sectional area will make the most efficient use of silver, thereby reducing the final cost per watt'.

In one aspect the present invention provides a printing screen for printing elongate structures on substrates, the printing screen comprising first and second layers of different material, the first layer providing a surface over which a printing element is in use traversed and including a plurality of elongate first printing apertures across which extend a plurality of bridges at spaced intervals, and the second layer in use overlying a substrate and including a plurality of elongate second printing apertures through which printing medium is in use printed onto the underlying substrate, each of the second printing apertures being located in registration with respective ones of the first printing apertures in the first layer.

In one embodiment the first layer is a metal layer.

In one embodiment the first layer is an electroformed layer, preferably an electroformed nickel layer.

In one embodiment the first printing apertures are formed in the first layer prior to application of the second layer thereto.

In one embodiment the second layer is a photo-imageable layer. *..*

: In one embodiment the second layer in a non-metal layer.

S * S ****

In one embodiment the second layer is formed of a screen emulsion. 1*5*

***. ** * In one embodiment the second layer is applied as a wet, liquid film. ** . * * . * **

In one embodiment the second layer is formed by application of one or more wet films to the first layer, preferably to both sides of the first layer.

In another embodiment the second layer is formed as a dry film.

In one embodiment the first printing apertures have a width of about 200 pm.

In one embodiment the bridges have a width (in the lengthwise direction of the respective first printing aperture) of from about 10 pm to about 60 pm, preferably from about 20 pm to about 50 pm and more preferably from about 20 pm to about 40 pm.

In one embodiment the bridges have a pitch (in the lengthwise direction of the respective first printing aperture) of at least about 150 pm, preferably at least about 250 pm, more preferably at least about 400 pm, and still more preferably at least about 500 pm.

In one embodiment the first layer includes keying apertures to which the material of the second layer keys.

In one embodiment the keying apertures are arranged in rows parallel to the first printing apertures.

In one embodiment the second printing apertures have a width which is smaller than the width of the first printing apertures, whereby the second .. : printing apertures define the width of the elongate structures to be printed on the substrate.

In one embodiment the second printing apertures have a width of at least * about 50 pm less than the width of the respective first printing apertures. * . * * *

:. In one embodiment the second layer extends into the first printing apertures in the first layer.

In one embodiment the second layer extends to the upper surface of the first layer at the first printing apertures therein, whereby the second printing apertures in the second layer extend the full height of the stencil.

In one embodiment the second printing apertures have substantially vertical sidewalls.

In one embodiment the second printing apertures have a width of less than about 150 pm, preferably less than about 100 pm, more preferably less than about 50 pm, and even more preferably less than about 30 pm.

In one embodiment the second printing apertures have an open area of at least about 80%, preferably at least about 90% and more preferably at least about 95%.

In one embodiment the thickness of the main body of the first layer is not less than the thickness of the main body of the second layer.

In one embodiment the printing screen has a thickness of not more than about 60 pm, preferably not more than about 50 pm, more preferably not more than about 40 pm, and still more preferably not more than about 30 pm.

In one embodiment the substrate is a silicon solar cell and the elongate S...

: structures are metallization lines. * . S...

In another aspect the present invention provides a printing screen for printing elongate structures on substrates, the printing screen comprising S.....

* first and second layers, the first, upper layer providing a surface over which a printing element is in use traversed and including a plurality of elongate first printing apertures across which extend a plurality of interconnecting elements at spaced intervals, and the second, lower layer in use overlying a substrate and including a plurality of elongate second printing apertures through which printing medium is in use printed onto the underlying substrate, each of the second printing apertures being located in registration with respective ones of the first printing apertures in the first layer.

In a further aspect the present invention provides a method of fabricating the above-described printing screen.

In one embodiment the second layer is applied to the first layer as a wet, liquid film.

In one embodiment the second layer is formed of a screen emulsion.

In one embodiment the second layer is applied to extend into the first printing apertures in the first layer.

In one embodiment the second layer is formed by application of one or more films to the first layer.

In one embodiment the second layer is formed by application of films to both skies of the first layer.

: In one embodiment the film applied to one side of the first layer is removed I...

prior to curing of the second layer.

In another embodiment the second layer is formed as a dry film. S.S.e S *

In one embodiment the second layer is laminated to the first layer.

A preferred embodiment of the present invention will now be described hereinbelow by way of example only with reference to the accompanying drawings, in which: Figure 1 illustrates a fragmentary perspective view of a stencil in accordance with a preferred embodiment of the present invention; Figure 2 illustrates a first sectional view (along section I-I) of the stencil of Figure 1; Figure 3 illustrates a second sectional view (along section 11-Il) of the stencil of Figure 1; Figure 4 illustrates a third sectional view (along section 111-Ill) of the stencil of Figure 1; Figure 5 illustrates the mean aspect ratios of print lines achieved using the test stencil of the Example; Figure 6 illustrates a 3D map of one measurement site for the print line with the highest aspect ratio achieved using the test pattern of the Example; and Figure 7 illustrates the mean aspect ratios of print lines achieved using the comparative stencil of the Example. *.ee

: The stencil comprises first and second layers 3, 5, the first, upper layer 3 providing a surface over which a printing element (not illustrated), such as a squeegee, is traversed and the second, lower layer 5 being in contact with an underlying substrate S.

S

In this embodiment the first layer 3 is a metal layer, here an electroformed nickel layer, which is fabricated prior to the application of the second layer 5 thereto. In an alternative embodiment the first layer 3 could be formed by chemical etching or any other suitable cutting technology.

In this embodiment the second layer 5 is a photo-imageable layer, here formed of a screen emulsion, one example of which being a photopolymer. -7-.

In this embodiment the screen emulsion is applied as a wet, liquid film, here by the repeated application of thin films to both sides of the first layer 3, with the film applied to the upper, contact side of the first layer 3 being removed, typically by a squeegee, before curing. In this way, the screen emulsion fills apertures 7, 11 in the first layer 3, which apertures 7, 11 will be described in more detail hereinbelow, with the screen emulsion being held in the apertures 7, 11 by surface tension.

In an alternative embodiment the second layer 5 could be formed from a dry film, such as a dry film resist, which in one embodiment is laminated to the first layer 3.

In this embodiment the substrate S is a silicon solar cell, but it should be understood that the present invention has application to any kind of substrate.

The first layer 3 includes a plurality of printing apertures 7, in this embodiment narrow, elongate linear apertures, through which printing medium is delivered in printing onto the underlying substrate S, across which extend a plurality of bridges 9, as interconnecting elements, at spaced intervals. For ease of illustration, only one printing aperture 7 is illustrated *..* . : in Figure 1. * * * ***

In this embodiment the printing apertures 7 have a width of about 200 pm. **** * I

The bridges 9 act to maintain the integrity of the first layer 3, and in * particular the spacing between opposite edges of the respective printing apertures 7, when the stencil is under tension and during movement of a I..

printing element thereover.

In this embodiment the bridges 9 have a width (in the lengthwise direction of the printing aperture 7) of about 30 pm and a pitch (in the lengthwise direction of the printing aperture 7) of about 500 pm.

In a preferred embodiment the bridges 9 have a width of from about 10 pm to about 60 pm, preferably from about 20 pm to about 50 pm and more preferably from about 20 pm to about 40 pm.

In a preferred embodiment the bridges 9 have a pitch of at least about 150 pm, preferably at least about 250 pm, more preferably at least about 400 pm, and still more preferably at least about 500 pm.

In this embodiment the first layer 3 includes keying apertures 11, here arranged in rows parallel to the printing apertures 7, to which the material of the second layer S keys. In this embodiment the keying apertures 11 have a diamond shape, but it will be understood that the keying apertures 11 could have any suitable shape, such as square, rectangular or circular.

In an alternative embodiment the keying apertures 11 could be omitted. In one embodiment the lower surface of the first layer 3 could be treated, such as to have a surface roughness, in order to promote bonding of the first and second layers 3, 5. In another embodiment the lower surface of the first layer 3 could be provided with an intermediate bondcoat, in order to promote bonding of the first and second layers 3, 5.

The second layer 5 includes a plurality of printing apertures 15, here narrow, elongate apertures, through which printing medium is printed onto the underlying substrate S, each located in registration with the printing apertures 7 in the first layer 3. * S S...

In this embodiment the printing apertures 15 in the second layer 5 have a width which is smaller, preferably significantly smaller, than the width of the *:*. printing apertures 7 in the first layer 3, whereby the printing apertures 15 . define the width of the elongate structures to be deposited on the substrate S. In this embodiment the second layer S extends to the upper surface of the first layer 3 at the printing apertures 7 therein, such that the printing apertures 15 in the second layer 5 extend the full height of the stencil and define completely the contact surface with the printing medium. In this embodiment the printing apertures 15 have substantially vertical sidewalls.

In this embodiment the printing apertures 15 have a width of less than about 150 pm. In a preferred embodiment the printing apertures 15 have a width of less than about 100 pm, preferably less than about 50 pm, and even more preferably less than about 30 pm.

With this configuration, the requirement for precise alignment of the imaging equipment in forming the printing apertures 15 is reduced, as the smaller width of the printing apertures 15 as compared to the printing apertures 7 in the first layer 3 provides for significant tolerance in the position at which the printing apertures 15 can be formed relative to the printing apertures 7 in the first layer 3, that is, the printing apertures 15 merely have to be formed within the material of the second layer 5 that is present in the printing apertures 7 of the first layer 3. This compares to configurations where edges of printing apertures have to be aligned exactly.

In this embodiment the printing apertures 15 have an open area of about 94%. The present invention advantageously provides a much greater open area than compared to conventional mesh screens, which typically have an open area of less than 65%. * *

. In a preferred embodiment the printing apertures 15 have an open area of at least about 80%, preferably at least about 90% and more preferably at least about 95%. ** * * * S * S.

In this embodiment the first layer 3 and the major part of the second layer 5 each have a thickness of about 30 pm, excepting at the through apertures 7 -10 -in the first layer 3, where the second layer 5 extends the full height of the stencil.

In one embodiment the thickness of the main body of the first layer 3 is not less than the thickness of the main body of the second layer 5.

In a preferred embodiment the stencil has a thickness of not more than about 60 pm, preferably not more than about 50 pm, more preferably not more than about 40 pm, and still more preferably not more than about 30 pm.

Example

The present invention will now be described with reference to the following non-limiting Example.

In this Example, the above-described stencil was prepared with a test pattern including printing apertures 15 having nominal widths of 30, 40, 50, 60, 70 and 80 pm, repeated across the stencil, and with bridges 9 having a nominal width of 35 pm and nominal pitches of 180, 280, 380 and 480 pm.

A silicon solar cell was then printed with a silver paste using the above-described test stencil, and the resulting prints measured using a measuring microscope (Nikon VMR3O2O Type 2). S...

Width and height measurements were each taken at eight measurement sites (excluding intersections) for each printed line type, that is, for each S...

combination of printing aperture width and bridge pitch. Each individual width measurement is the mean width over an 800 pm sampling length.

Each individual height measurement is the mean height of the centre of the ** printed line above the plane of the surrounding silicon surface over a 400 pm sampling length. The height measurements were performed at higher magnification for greater resolution. All measurements were taken on wet -11 -paste soon after printing, in order to remove effects associated with drying and firing.

From these measurements, mean aspect ratios and a cross-sectional area uniformity (CSAU) index were determined.

Mean Aspect Ratio = Mean Width/Mean Height CSAU Index (Minimum Height/Mean Height + Minimum Width/Mean Width)/2 The CSAU index provides a reliable indication of uniformity, in characterizing a relationship of the minimum height and width to the mean height and width, which is not always provided by the mean aspect ratio. For example, it is quite possible for a measured sample to have an excellent mean aspect ratio but still have low or narrow points, which will exhibit increased electrical resistance. The closer the CSAU index to 1, the closer the minimum height of the printed line is to the mean height.

Figure 5 illustrates the mean aspect ratios achieved using the test stencil of this Example for printing aperture widths of 50, 60, 70 and 80 pm and bridge pitches of 180, 280, 380 and 480 pm.

As will be observed, all of the mean aspect ratios are above 0,3, with the highest at 0.431 for a printing aperture width of 50 pm and a bridge pitch of 480 pm. This compares to measurements determined from conventional mesh screens which exhibited mean aspect ratios of about 0.125, with the highest at 0.147 for a 280 mesh screen with an emulsion thickness of 13 pm and a printing aperture width of 75 pm.

S S * S

IS

Figure 6 illustrates a 3D map of one measurement site for the print line with the highest aspect ratio. This print line exhibits a CSAU index of 0.930, which compares to 0.834 for the best performing 280 mesh screen.

-12 -By way of comparison, a two-layer metal stencil was fabricated to the same test design, that is, having the same printing aperture widths, bridge widths, bridge pitches and layer thicknesses.

Figure 7 illustrates the mean aspect ratios achieved using the comparative metal-metal stencil for printing aperture widths of 50, 60, 70 and 80 pm and bridge pitches of 180, 280, 380 and 480 pm.

As will be observed, the mean aspect ratios obtained using this equivalent, comparative metal-metal stencil are much reduced as compared to the stencil of the present invention.

As will be clearly seen, the stencil of the present invention exhibits markedly improved performance as compared to mesh screens and metal-metal stencils. The present invention not only achieves superior mean aspect ratios, but also exhibits an excellent cross-sectional area uniformity for a narrow printed conductor.

Finally, it will be understood that the present invention has been described in its preferred embodiment and can be modified in many different ways without departing from the scope of the invention as defined by the appended claims. * * * ** S * S.. * S *SSS *.S. * S

S

*5S*.* * S SS * * . S S.

S S.. a

Claims

-13 -CLAIMS1. A printing screen for printing elongate structures on substrates, the printing screen comprising first and second layers of different material, the first layer providing a surface over which a printing element is in use traversed and including a plurality of elongate first printing apertures across which extend a plurality of bridges at spaced intervals, and the second layer in use overlying a substrate and including a plurality of elongate second printing apertures through which printing medium is in use printed onto the underlying substrate, each of the second printing apertures being located in registration with respective ones of the first printing apertures in the first layer.
2. The printing screen of claim 1, wherein the first layer is a metal layer.
3. The printing screen of claim 2, wherein the first layer is an electroformed layer, preferably an electroformed nickel layer.
4. The printing screen of any of claims 1 to 3, wherein the first printing apertures are formed in the first layer prior to application of the second layer thereto.
5. The printing screen of any of claims 1 to 4, wherein the second layer is a photo-imageable layer.SS
6. The printing screen of any of claims 1 to 5, wherein the second layer in a non-metal layer.SS.....*
7. The printing screen of claim 6, wherein the second layer is formed of a screen emulsion.S **

S
8. The printing screen of any of claims 1 to 7, wherein the second layer is applied as a wet, liquid film.

-14 -
9. The printing screen of claim 8, wherein the second layer is formed by application of one or more wet films to the first layer, preferably to both sides of the first layer.
10. The printing screen of any of claims 1 to 7, wherein the second layer is formed as a dry film.
11. The printing screen of any of claims 1 to 10, wherein the first printing apertures have a width of about 200 pm.
12. The printing screen of any of claims 1 to 11, wherein the bridges have a width (in the lengthwise direction of the respective first printing aperture) of from about 10 pm to about 60 pm, preferably from about pm to about 50 pm and more preferably from about 20 pm to about 40 pm.
13. The printing screen of any of claims 1 to 12, wherein the bridges have a pitch (in the lengthwise direction of the respective first printing aperture) of at least about 150 pm, preferably at least about 250 pm, more preferably at least about 400 pm, and still more preferably at least about 500 pm.
14. The printing screen of any of claims 1 to 13, wherein the first layer includes keying apertures to which the material of the second layer keys. * * **
15. The printing screen of claim 14, wherein the keying apertures are arranged in rows parallel to the first printing apertures. ** * * * S * S*
16. The printing screen of any of claims 1 to 15, wherein the second printing apertures have a width which is smaller than the width of the -15 -first printing apertures, whereby the second printing apertures define the width of the elongate structures to be printed on the substrate.
17. The printing screen of claim 16, wherein the second printing apertures have a width of at least about 50 pm less than the width of the respective first printing apertures.
18. The printing screen of any of daims 1 to 17, wherein the second layer extends into the first printing apertures in the first layer.
19. The printing screen of claim 18, wherein the second layer extends to the upper surface of the first layer at the first printing apertures therein, whereby the second printing apertures in the second layer extend the full height of the stencil.
20. The printing screen of claim 19, wherein the second printing apertures have substantially vertical sidewalls.
21. The printing screen of claim 20, wherein the second printing apertures have a width of less than about 150 pm, preferably less than about pm, more preferably less than about 50 pm, and even more preferably less than about 30 pm.
22. The printing screen of claim 21, wherein the second printing apertures have an open area of at least about 80%, preferably at least about 90% and more preferably at least about 95%. * S *S..
23. The printing screen of any of claims 1 to 22 wherein the thickness of * the main body of the first layer is not less than the thickness of the main body of the second layer.S S..
24. The printing screen of any of claims 1 to 23, wherein the printing screen has a thickness of not more than about 60 pm, preferably not -16 -more than about 50 pm, more preferably not more than about 40 pm, and still more preferably not more than about 30 pm.
25. The printing screen of any of claims 1 to 24, wherein the substrate is a silicon solar cell and the elongate structures are metallization lines.
26. A method of fabricating the printing screen of any of claims 1 to 25.
27. The method of claim 26, wherein the first printing apertures are formed in the first layer prior to application of the second layer thereto.
28. The method of claim 27, wherein the second layer is applied to the first layer as a wet, liquid film.
29. The method of claim 28, wherein the second layer is formed of a screen emulsion.
30. The method of claim 28 or 29, wherein the second layer is applied to extend into the first printing apertures in the first layer.
31. The method of claim 30, wherein the second layer is formed by application of one or more films to the first layer.
32. The method of claim 31, wherein the second layer is formed by *.* S application of films to both sides of the first layer.S S S...
33. The method of claim 32 wherein the film applied to one side of the *5SS** first layer is removed prior to curing of the second layer.
34. The method of claim 26 or 27, wherein the second layer is formed as a dry film.

-17 -
35. The method of claim 34, wherein the second layer is laminated to the first layer.
36. A printing screen substantially as hereinbefore described with reference to any of Figures 1 to 4 of the accompanying drawings, optionally in conjunction with Figures 5 and 6 of the accompanying drawings.
37. A method of fabricating a printing screen substantially as hereinbefore described with reference to any of Figures 1 to 4 of the accompanying drawings, optionally in conjunction with Figures 5 and 6 of the accompanying drawings. e.. * * S ** S S... * S S... S'S. * . **.* * SS * S * * .SS *SeS