2.2 Metallization technologies
2.2.2 Screen-printing
Screen-printing constitutes a fast and reliable metallization technology with the advantage that the front side structure is printed in one process step. The contact formation takes place in the following high temperature process. For solar cell metallization three printing steps are performed. In the first step two busbars are printed using Ag/Al paste onto the rear, followed by printing the remaining area
with Al paste8. The front structure is either printed before or after the rear side printing process. In-between the printing steps, the drying of paste take place in a conveyor belt furnace. A single industrial screen-printing line has a throughput of about 1000 wafers/hour. This implies that one printing process, including transfer of the wafer, alignment and the printing step takes less than 3 seconds.
Printing Step: The printing step in which the paste is forced through the openings of the emulsion layer onto the surface of the wafer, can be subdivided into three consecutive phases (see Fig. 2.5). In the filling phase, the open areas of the screen are flooded by moving a squeegee (floodbar) over the surface of the screen without applying a vertical force. Depending on the application, the filling phase can also be carried out directly before the contact phase by the paste bead lying in front of the printing-squeegee (Fig. 2.5 (1)). A floodbar would not be necessary in this case. However, direct flooding after the print through process, as typically performed for solar cell metallization, has the advantage that a drying of not released paste in the screen openings is significantly reduced. In the contact phase a vertical force is applied to the printing-squeegee, pressing the screen onto the wafer and forcing paste through the screen openings. The paste sticks to the substrate due to adhesion forces. In the final phase, the paste is released out of the screen [41]. The quality of the print image depends mainly on the screen, the paste and on the printing parameters.
Fig. 2.5: Screen-printing process: (1) The openings in the screen are filled with paste; (2) the squeegee brings the screen in intimate contact with the substrate and presses paste through the openings. (3) While the screen is lifted up, paste is released from the screen and sticks to the substrate.
8 For interconnection of the solar cells into a module, a tab is soldered onto the Ag/Al busbar on the rear. Soldering on a pure aluminum rear is not yet possible.
As illustrated in Fig. 2.6-a, the screen consists of an aluminum frame, a mesh of wires being clamped to the frame and an emulsion layer, which is photolithographically structured with the desired printing image. The size of the frame needs to be large enough that the mesh releases from the substrate and paste during the snap-off (see below). In order not to damage the mesh, the screen tension must be smaller than the elasticity limit of the used wire material.
a)
frame wire mesh emulsion layer frame frame wire mesh emulsion layer frame
b)
Fig. 2.6: Structure of a screen illustrating the mesh, the emulsion and the frame as well as characteristic parameters: ws is the thickness of the screen mesh, wst the thickness of the emulsion
layer, wsg total thickness of the screen, c the cross section of the wire and d the wire separation
distance.
Fig. 2.6-b shows a side view (left-hand) and top view (right-hand) of a wire mesh illustrating parameters for characterization. The screen opening fraction a0,
describing the ratio of the screen opening area to the total screen area, is defined by:
(
)
2 0 ² d c d a + = (2.1)Especially for fine-line printing being the case for the front side metallization process of solar cells, a mesh with fine wires and a high mesh separation distance is desired, as this leads to a high screen opening fraction. However, the danger of screen breakage rises strongly with reduced finger cross section area and increased finger separation distance. That is the reason why mainly steel wires are used possessing a high tensile strength. In addition electrostatic charging of steal wires does not occur.
The minimum line width wf_min that can be achieved was defined by Scheer [42]
in dependence of the wire cross section c and wire separation distance d by an empirical equation:
(
)
d d c c wf_min = 2 + 2 (2.2)For a screen with a wire thickness of c = 25 µm and a 280 mesh9, the wire separation distance is equal to d = 65.7 µm. Applying the latter equation results in a minimum finger width of wf_min = 62 µm. To achieve very fine contacts of e.g.
wf_min = 30 µm width, a wire thickness of e.g. c = 14 µm and a 370 mesh would be
necessary.
The theoretical paste volume vpaste being transferred can be calculated by [42]:
²
d w
Vpaste = st ⋅ (2.3)
This theoretical value neglects that the area above and below the wire is also flooded with paste, which would increase the paste transfer slightly. On the other hand some of the paste is not released during the printing process and adheres to the emulsion layer and wire, reducing the paste transfer. As a high volume of paste is desired, a thick emulsion layer is preferred. However, with increasing emulsion layer thickness, the release is reduced.
From the theoretical paste volume, the transferred silver amount and thus the expected finger conductivity can be estimated. The amount of silver in a paste is typically given in weight percent and not in volume percent. The volume percent of silver vag is a screen-printing paste can be calculated by:
1 , * − + + ⋅ = s sol gl gl ag ag ag ag ag paste m m m m v o o o o ρ ρ ρ ρ (2.4)
with mag, mgl, msol being the mass percent of silver, glass frit and solvent in a paste,
respectively,
ρ°
ag,ρ°
gl,ρ°
sol are the appropriate densities10 . Assuming a density
ρ°
gl = 5 g/cm 3 [43], ofρ°
sol = 1 g/cm 3 and ofρ°
ag = 10.5 g/cm 3and a mass percent of mag = 84%, mgl = 3% and msol = 13% results in a silver volume of vag = 37% in
the paste. The minimum achievable line resistance Rline (resistance per finger
length) can then be estimated by:
0 a v w w R ag st sc ag line ⋅ ⋅ ⋅ ≈ ρ [Ω/m] (2.5)
with
ρ
ag being the specific resistivity of silver (1.6·10-8 Ω m) and w
f the finger
width in the screen. Assuming a finger width of wf = 100 µm, a wire thickness c of
25 µm, a wire separation distance d of 65.7 µm (280 mesh), an emulsion layer thickness wst of 50 µm and a silver content in the paste vag of 33% leads to a finger
resistance of Rline = 18 Ω/m. This calculated value is slightly lower than
9 The acuteness of a screen with a steal wires is usually specified in mesh (mesh = inch-1)
10ρ
experimentally determined values being in the range of 30 Ω/m for the appropriate screen design. Possible explanations are the incomplete release of paste out of the screen and the difference of the specific conductivity of the sintered silver structure to the one of bulk silver.
Snap-off distance: The snap-off distance is defined as the distance between the bottom of the printing form and the solar cell surface. The snap-off determines the upward movement of the screen, after the squeegee has passed. For too small snap- off distances the paste will not detach from the screen. If the distance is too large, the tension in the screen is decreased, which reduces its lifetime.
Printing speed: The printing speed is defined as the speed of the squeegee moving over the screen during the print through process. The choice of printing speed depends on the past rheology, the screen thickness, the screen itself, the positioning of the squeegee and the printing image. Due to the increased applied force with elevated printing speed, the viscosity of the paste is reduced, simplifying the print through process. For industrial application a fast printing speed is required, leading to high throughput rates. The printing speed in industrial environment is in the range of 100 cm/s to 200 cm/s.
Print squeegee and print squeegee pressure: The squeegee, as central paste transfer tool, fulfils a number of functions [42]. Most importantly the squeegee brings the screen and the solar cell surface into intimate contact, which allows paste transfer. Dependent on the hardness of the squeegee material and its geometry an adaptation of the substrate surface occurs. Additionally the squeegee removes excessive paste from the print-image area.
The intimate contact between screen and substrate surface is achieved by applying an appropriate vertical force to the squeegee. The pressure should be slightly higher than necessary for the kissprint-point11, allowing tolerances in the process. An overestimated pressure results in a reduced printing quality of fine-line printed contacts and due to the increased abrasion, the lifetime of the screen- printing form and the squeegee is shortened.