SCREEN PRINTING FORM FOR USE IN A SCREEN PRINTING METHOD, SCREEN PRINTING DEVICE, AND SCREEN PRINTING METHOD

Abstract

A screen printing form (1, 1′) for use in screen printing, in particular for producing a metallic contact structure of a photovoltaic solar cell, having a woven screen printing fabric (1b) with a plurality of elongate woven fabric elements, which are arranged in a first element direction and a second element direction perpendicular thereto, and a stencil (1c), arranged on the woven screen printing fabric (1b) that has at least one opening formed as straight channel with a channel width BK. The woven fabric elements have a spacing AF in the first element direction and a spacing which deviates by less than 5% from AF in the second element direction, and the woven fabric elements have a diameter DG in the first element direction and a diameter which deviates by less than 5% from DG in the second element direction. A screen printing device and screen printing form are also provided.

Claims

1. A screen printing form (1, 1′) for use in a screen printing method, the screen printing form comprising: a screen printing fabric (1b) with a multiplicity of elongate woven fabric elements, which are arranged in a first element direction and a second direction perpendicular thereto; a stencil (1c) arranged on the screen printing fabric (1b), which has at least one opening formed as a straight channel with a channel width BK; wherein the woven fabric elements have a first spacing (AF) in the first element direction and a spacing which deviates by less than 5% from the first spacing (AF) in the second element direction, and the woven fabric elements have a diameter (DG) in the first element direction and a first diameter which deviates by less than 5% from the first diameter (DG) in the second element direction; the channel (1d) encloses with the first element direction an angle (φ) with a tolerance of +/−0.1°, and the channel width BK of the channel is less than or equal to a maximum width (BKmax) associated with the angle (φ), wherein the angle (φ) is one of the following: the angle φ[°] from the group 11.31°, 14.04°, 18.44°, 26.57°, 45° with an associated maximum width (BKmax) [μm] according to
BK.sub.max=(2DG+AF)sin φ+DG cos φ or the angle φ[°]=33.69°, with an associated maximum width (BKmax) [μm] according to
BK.sub.max=(2DG+AF)cos φ+DG sin φ, and the channel width (BK) is smaller than the spacing between two woven fabric elements in the second thread direction and in the first thread direction.

2. The screen printing form (1, 1′) as claimed in claim 1, wherein at least one of the woven fabric elements have the first spacing (AF) in the first element direction and the spacing which deviates by less than 2% from the first spacing (AF) in the second element direction or the woven fabric elements have the first diameter (DG) in the first element direction and the diameter which deviates by less than 2% from the first diameter (DG), in the second element direction.

3. The screen printing form (1, 1′) as claimed in claim 1, wherein the stencil (1c) includes a plurality of straight parallel channels.

4. The screen printing form (1, 1′) as claimed in claim 3, wherein the channels have a spacing which corresponds to an integer multiple of the spacing of the woven fabric elements in at least one of the first or second thread direction.

5. The screen printing form (1, 1′) as claimed in claim 1, wherein the channel (1d) encloses with the first element direction the angle (φ) with a tolerance of +/−0.1°, and the channel width (BK) of the channel is less than or equal to the maximum width (BKmax) associated with the angle (φ), as follows: for the angle (φ)[°] from the group 11.31°, 14.04°, 18.44°, 26.57°, 45°, the maximum (width BKmax) [μm] is according to
BK.sub.max,21=AF sin φ+DG cos φ or for the angle (φ)[°]=33.69°, the associated maximum width (BKmax) [μm] is according to
BK.sub.max,21=(2DG+AF)(cos φ−sin φ).

6. The screen printing form (1, 1′) as claimed in claim 5, wherein the first distance (AF) and the first diameter (DG) are selected such that with a condition BKmax=BK.sub.max1 and the angle (φ)[°] from the group 11.31°, 14.04°, 18.44°, 26.57°, 45°, the following $\phi <{\tan }^{-1}\left(\frac{\mathrm{DG}}{\mathrm{DG}+\mathrm{AF}}\right)$ additionally applies, and with the angle (φ)[°]=33.69°, the following $\phi >{\tan }^{-1}\left(\frac{\mathrm{AF}}{\mathrm{AF}+\mathrm{DG}}\right)$ additionally applies, with a condition BKmax=BK.sub.max0 and the angle (φ)[°] from the group 11.31°, 14.04°, 18.44°, 26.57°, 45°, the following $\phi >{\tan }^{-1}\left(\frac{\mathrm{DG}}{\mathrm{AF}}\right)$ additionally applies, and with the angle (φ)[°]=33.69°, the following $\phi >{\tan }^{-1}\left(\frac{\mathrm{AF}}{\mathrm{AF}+\mathrm{DG}}\right)$ additionally applies.

7. A screen printing device for applying a screen printing paste (2) to a substrate, the screen printing device comprising: the screen printing form (1, 1′) as claimed in claim 1, and at least one squeegee (3), wherein the screen printing device is configured to apply screen printing paste (2) to a substrate through the screen printing form (1, 1′) using the squeegee.

8. A screen printing method for applying a screen printing paste (2) to a substrate, the method comprising: applying the screen printing paste (2) to the substrate through the screen printing form (1, 1′) according to claim 1 at least one squeegee (3).

9. (canceled)

10. The screen printing form (1, 1′) as claimed in claim 5, wherein the channel (1d) encloses with the first element direction the angle (φ) with a tolerance of +/−0.1°, and the channel width (BK) of the channel is less than or equal to the maximum width (BKmax) associated with the angle (φ), as follows: for the angle φ[°] from the group 11.31°, 14.04°, 18.44°, 26.57°, 45°, the associated maximum width (BKmax) [μm] is according to
BK.sub.max,1=(2DG+AF)sin φ−DG cos φ or for the angle φ[°]=33.69°, the associated maximum width (BKmax) [μm] is according to
BK.sub.max,1=AF(cos φ−sin φ).

11. The screen printing form (1, 1′) as claimed in claim 10, wherein the channel (1d) encloses with the first element direction the angle (φ) with a tolerance of +/−0.1°, and the channel width (BK) of the channel is less than or equal to the maximum width (BKmax) associated with the angle φ, as follows: for the angle (φ)[°] from the group 11.31°, 14.04°, 18.44°, 26.57°, 45°, the associated maximum width (BK.sub.max) [μm] s according to
BK.sub.max,0=AF sin φ−DG cos φ or for the angle (φ)[°]=33.69°, the associated maximum width (BKmax) [μm] is according to
BK.sub.max,0=AF cos φ−(2DG+AF)sin φ.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Further advantageous features and preferred embodiments will be explained in more detail below by using exemplary embodiments and the figures, in which:

[0057] FIG. 1 shows a first embodiment of a screen printing device according to the invention for flatbed screen printing;

[0058] FIG. 2 shows a second embodiment of a screen printing device according to the invention for rotary screen printing;

[0059] FIG. 3 shows a sectional illustration of an exemplary embodiment of a screen printing form according to the invention of the screen printing device according to FIG. 1;

[0060] FIG. 4 shows a plan view from above of the screen printing form according to FIG. 3;

[0061] FIG. 5 shows an enlargement of a detail from FIG. 4;

[0062] FIGS. 6A-6D show partial illustrations to explain exclusion criteria of the presence of junction components; and

[0063] FIG. 7 shows partial details of several further exemplary embodiments of screen printing forms according to the invention.

DETAILED DESCRIPTION

[0064] The figures show schematic illustrations that are not to scale. The same designations in the figures designate the same or equivalent elements.

[0065] FIG. 1 illustrates a first exemplary embodiment of a screen printing device according to the invention in side view. The device is designed to carry out a flatbed screen printing method.

[0066] The device has a screen printing form 1 which is designed as a flatbed screen printing form. The screen printing form 1 is partially permeable to a screen printing paste 2 and partially impermeable to the screen printing paste 2, in order to form a predefined structure. This will be explained in more detail below using FIGS. 3 and 4.

[0067] The printing paste 2 in the present case is a printing paste containing metal particles, which is used, following thermal treatment, to form a metallic contact structure in the form of a contact grid known per se on the front side of a photovoltaic solar cell.

[0068] The device has a squeegee 3, which can be moved by motorized means, not illustrated, along the direction indicated by the arrow depicted above the squeegee. As a result, the screen printing paste 2 is swept over the screen printing form 1 and passes through the screen printing form 1 at the permeable points, so that the structure 4 of screen printing paste is applied to a substrate 5.

[0069] In the present case, the substrate 5 is formed as a silicon wafer, which already has p-doped and n-doped areas for forming emitter and base. The substrate 5 thus constitutes a solar cell precursor; to finish the solar cell, it further requires the arrangement of the metallic contact structure on the front side of the semiconductor substrate 5.

[0070] The device has a feed unit for feeding and discharging semiconductor substrates, which has a conveyor belt (not illustrated), on which a plurality of shuttles are arranged. By way of example, a shuttle 6 with a substrate 5 lying thereon is illustrated in FIG. 1.

[0071] A second exemplary embodiment of a printing device according to the invention is illustrated in FIG. 2 as an alternative embodiment. This printing device is designed to carry out a rotary screen printing method. As a comparison of FIGS. 1 and 2 shows, some elements are designed and arranged identically. However, it is important that, in the second exemplary embodiment according to FIG. 2, the screen printing form 1′ is designed as a round screen with a cylindrical form. The squeegee 3 is arranged in the interior of the cylindrically formed screen printing form 1′, with the result that the printing paste 2 is forced outward through the screen printing form from the interior of the screen printing form 1′, in order to form the structure 4 of printing paste on the substrate 5.

[0072] To this end, the screen printing form 1′ formed as a round screen has an axis of rotation 1a and is rotatable by motorized means in the direction identified by the circularly curved arrow. The axis of rotation 1a is thus perpendicular to the drawing plane in FIG. 2.

[0073] At the same time, by means of the shuttle 6, the semiconductor substrate 5 is moved in the direction illustrated as a rectilinear arrow, in such a way that the relative speed between substrate 5 and lateral surface of the screen printing form 1′ at the contact point between the screen printing form 1′ and the substrate 5 is zero or negligibly low.

[0074] On the other hand, the squeegee 3 carries out no rotational movement, so that the printing paste 2 is pressed against the squeegee 3 in the interior of the screen printing form 1′ because of the rotational movement of the screen printing form 1′ and, by means of the squeegee, is applied to the substrate 5 through the screen printing form.

[0075] The screen printing forms 1 and 1′ are in principle constructed in the same way; only the screen printing form 1 has a flat, rectangular form, whereas the form of the screen printing form 1′ corresponds to the lateral surface of a cylinder.

[0076] FIG. 3 illustrates a cross section through the screen printing form 1 of the device according to FIG. 1. The screen printing form 1 has a rectangular frame 1a, in which a screen printing fabric 1b is stretched. The screen printing fabric 1b has a multiplicity of woven fabric elements, which are arranged in a first element direction and a perpendicular element direction. The first element direction is perpendicular to the drawing plane according to FIG. 3 and, correspondingly, the second element direction according to the illustration in FIG. 3 lies parallel to the drawing plane.

[0077] Arranged on the screen printing fabric 1b is a stencil 1c, which formed as an emulsion in a manner known per se. The stencil 1c has a multiplicity of openings, which are each formed as a straight, elongate channel 1d. The channels of the screen printing form 1 run parallel to one another and perpendicular to the drawing plane according to FIG. 3 and, in the present case, have identical widths. By way of example, three channels 1d are identified in FIG. 3.

[0078] If, then, screen printing paste is pressed onto the screen printing form 1 from above by means of the squeegee in FIG. 3, then the screen printing paste can pass through the screen printing form only in the area of the channels 1d, so that a structure of screen printing paste corresponding to the positive form of the channels is produced on a substrate lying under FIG. 3, which structure consists of a corresponding multiplicity of lines of screen printing paste arranged parallel to one another.

[0079] FIG. 4 shows a rear view from below of the screen printing form 1 according to FIG. 3. It is illustrated schematically that, in the area of the openings of the stencil 1c, which means in the area of the channels 1d, the screen printing form is not completely open, since woven fabric elements of the screen printing fabric 1b run underneath the channels 1d.

[0080] The screen printing forms according to the invention are distinguished by the fact that in the areas of the channels 1d there are no or at least a reduced number of junctions of the screen printing fabric 1b, so that a homogenous structure of screen printing paste can be produced. Nevertheless, the woven fabric elements of the screen printing fabric 1b do not run perpendicular to the longitudinal extent of the channels 1d, which means not horizontally in FIG. 4, as would be the case in a 0° screen printing form. This will be explained in more detail in FIG. 5 by using a detail from FIG. 4.

[0081] FIG. 5 shows a horizontal illustration of the detail from FIG. 4 identified by A. The length dimensions of the channel width BK, of the diameter of the woven fabric elements DG and of the spacing between two parallel woven fabric elements (open space) AF and the angle φ between woven fabric elements and channel edge are identified.

[0082] Different types of overlaps are illustrated in FIG. 6, in order to indicate the presence of a junction, that is to say a crossed area of two woven fabric elements extending perpendicularly to each other, in the opening area of the stencil 1c, that is to say within a channel 1d.

[0083] In FIGS. 6A-6D, the outer limits of the junctions K are identified by thickened lines.

[0084] As can be seen in FIG. 6A, a junction K can be arranged in such a way that all four corners are located within the channel 1d. This represents an unfavorable arrangement, since the junction K leads to high area coverage within the channel 1d, so that there is a high risk of an inhomogeneous application of paste in the longitudinal direction of the channel 1d. The situation according to FIG. 6A is avoided or at least considerably reduced by the conditions named in claim 1.

[0085] However, it is advantageous if a reduced number of the corner points of the junction K is present in the area of the channel 1d. Thus, in the arrangement according to FIG. 6B, although one corner of the junction K (the upper left corner) is already arranged underneath the stencil 1c, three corners of the junction K nevertheless remain within the channel 1d, so that there is a reduced risk but still a risk of an inhomogeneous application of paste.

[0086] In a corresponding way, according to the illustration in FIG. 6C, it is further advantageous to choose the arrangement in such a way that only two of the corner points of the junction K lie within the channel 1d, whereas the other two corner points are located under the stencil 1c. It is also advantageous, according to FIG. 6D, if only one corner point of the junction K is still located in the channel 1d.

[0087] Accordingly, it is particularly advantageous if there is even no corner point of the junction K within the channel 1d, so that the arrangement can be designated “junction-free”.

[0088] Various alternative exemplary embodiments of a screen printing form according to the invention are shown in FIG. 7 with associated parameters.

LIST OF DESIGNATIONS

[0089] 1, 1′ Screen printing form [0090] 1a Frame [0091] 1b Screen printing fabric [0092] K Junction [0093] 1c Stencil [0094] 1d Channel [0095] D Axis of rotation [0096] 2 Screen printing paste [0097] 3 Squeegee [0098] 4 Structure of printing paste [0099] 5 Semiconductor substrate [0100] 6 Shuttle