Experimental matrices were conducted to determine a suitable firing schedule for fritless tin printing ink. considerable difficulties were encountered with oxidation. Best results were obtained with a firing cycle consisting of 400 C for 20 minutes in nitrogen followed by 5 minutes in air at 500 C. Elimination of oxidizing conditions impaired the adhesion of both tin and copper fritless printing inks, although adhesion of fritless copper inks was obtained when fired in nitrogen with slight oxidation

Taylor, W.

English

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https://ntrs.nasa.gov/search.jsp?R=19830006408 2020-03-21T05:05:18+00:00Z
N83-14679
Unclas
G3/44 01202
DRL. No. 159
	
DOE/JPL - 955986-3
Distribution Category UC-63
QUARTERLY TECHNICAL REPORTS NO. 4
for
DEVELOPMENT OF TECHNIQUE FOR
AR COATING AND NICKEL AND COPPER
METALLIZATION OF SOLAR CELLS
FPS PROJECT
PRODUCT DEVELOPMENT
PERFORMANCE PERIOD
April 1, 1982 to June 30, 1982
JPL CONTRACT NO. 955986
DRL NO. 159
July 15, 1982
Prepared by:
Dr. William Taylor
PHOTOWATT INTERNATIONAL, INC.
2414 West 14th Street
Tempe, Arizona 85281
(NASA-CS-169616) DEV; LOPMEN2 CF TECHNIQUE
FOR AR COATING NICKEL AND CCPPEF
!ETALLIZATION OF SCLIR CELLS FES EBOJECT
PRODUCT GEVELCEMENT Quarterly 'Technical
Report, 1 Apr. - (Phctowatt Interrational,
r	 Opp 1e8z,f
El
NAM S77 AG
? t
	
ryD 	 .<
r
k
OF PO R QUALMf
0^
DRL. No 159	 DOE/JPL - 955986-3
Distribution Category UC-63
QUARTERLY TECHNICAL REPORTS NO. 4
for
DEVELOPMENT OF TECHNIQUE FOR
AR COATING AND NICKEL AND COPPER
METALLIZATION OF SOLAR CELLS
FPS PROJECT
PRODUCT DEVELOPMENT
PERFORMANCE PERIOD
April 1, 1982 to June 30, 1982
JPL CONTRACT NO. 955986
DRL NO. 159
July 15, 1982
Submitted by:
PHOTOWATT INTERNATIONAL, INC.
2414 West 14th Street
Tempe, Arizona W8
k
- 
k1441'11Z4
Dr.wi 11 i	 ay or
Technical r'. _
!
PHOTOWATT INTERNATIONAL, INC.
/
F1
, F L_...	
- ^_1VINEW, -Moil
"The JPL Low-Cost Solar Array Project is sponsored by the U.S.
Department of Energv and forms part of the Solar Photovoltaic
Conversion Program to initiate a major effort toward the development
of low-cost solar arrays. This work was performed for the Jet
Propulsion Laboratory, California Institute of Technology by
agreement between NASA and DOE."
i
PREFACE
'E	 The objective of this program is to analyze and evaluate a base metal
system for metallizing solar cells. The system to be investigated is
printed nickel overplated with copper and applied on top of a pre-
deposited silicon nitride anti-reflective coating.
ii
1.0 SUMMARY
Experimental matrices were conducted to determine a suitable firing
schedule for fritless tin printing ink. Considerable difficulties
were encountered with oxidation. Best results were obtained with a
firing cycle consisting of 4000C for 20 minutes in nitrogen followed
by 5 minutes in air at 500
0C. Elimination of oxidizing conditions
impaired the adhesion of both tin and copper fritless printing inks,
although adhesion of fritless copper inks was obtained when fired in
nitrogen with slight oxidation.
2.0 TECHNICAL DISCUSSION
During this period attention was turned to the use of fritless
pastes in order to avoid the loss of adhesion which had been ex-
perienced when contacts formed with fritted pastes are electroplated.
Preliminary experiments were conducted with TFS 5522 Oxyfine frit-
kdss nickel. In the first experiment, samples with 0, 2 and 5%
silver fluoride were fired in oxygen for 25 minutes at 500 0C fol-
lowed by 10 minutes at 6500C in forming gas. All samples were short-
ed with no electrical output. ThP sample with no fluoride added
had good adhesion both before and after copper plating. Addition
of fluoride resulted in loss of adhesion daring plating. When
fired at 8000 and 8500 for short periods, some electrical output
was obtained, but adhesion degraded during plating. IV curves
obtained with the unmodified TFS 5522 ink are shown in Figure 1,
and as modified by 5% silver fluoride in Figure 2.
A fritless paste composed of 19.5% molybdenum, 80% tin and 0.5%
titanium hydride was obtained from Thick Film Systems. This haste
gave very high series resistance with practically no short circuit.
The firing cycle used was 25 minutes at 500°C in air followed by 5,
10 or 25 minutes at 6500C in forming gas. Similar results were ob-
tained when the prefire drying cycle was dropped to one hour at
4000C in an attempt to reduce oxidation effects. No improvement was
observed with the addition of 2% silver fluoride. These samples could
not be plated, apparently because of the presence of a fine dark color-
ed insulating powder which coated the sintered metal. This was attrib-
uted to oxidation of the molybdenum component of the paste. Ad-
hesion of the contact was poor when subjected to tape testing. This ink
was replaced with a pure tin-2% titanium hydride formulation in subsequent
Several experiments were conducted to identify a suitable firing
schedule for the tin-titanium hydride printing ink. Initial proce-
dures of baking at 4000C followed by 5-10 minutes at 675 0C in air
FILM 1. Current-voltage curves for
TFS 5522 nickle fired at 8500C
on Si 3N4
1000	
= - ;..	 ___ _.	 .. _ . ^ .	
_ . •	 _ ^	
^t	 r - _^__^
45 seconds
Y30 seconds
400-
--	 -	 ----	
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15 seconds	 i	 +
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,	
—
0	 .1	 .2	 .3	 .4	 .5	 .6	 a
ORIGINAL PAGE 18
VOLTAGE - VOLTS	 OF POOR QUALITY
3
-77
----T— 7
...	
.
FIGURE Current -voltage curves for..
TFS 55J2 nickel +5% AqF fired
at 850 on Si P4 ..-L.
low
800
I
seconds
6OO_.__
i'545	 onds
eT400 r -, 7-r-
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7
I LL Q Ll I 17t:10 .1 .2 .3 .4 .6 .6
ORIGINAL PAGE 19VOLTAGE-VOLTS
4,	 OF POOR QUALM
ORIGINAL PAGE 19
OF POOR QUALRY
F	 resulted in non-sintered, powdery deposits with extensive oxidation.
'	 Based on these observations, the following firing strategy was selected:
1. Low temperature prefire in nitrogen to initiate sintering and
agglomeration while the metal is still protected from oxidation
by the organic vehicle.
2. Fire at moderate temperature in air to burn off organics.
3. Fire at 6750C in forming gas to complete sintering and re-
duce metallic oxides.
The experimental matrix shown in Figure 3 was devised to generate
information about the firing cycle conditions. One wafer was set aside
9
from each of the matrix cells for evaluation. Observed characteristics
for cells fired in the various regions of the matrix are given in
Figure 3 and discussed below.
The tan color observed in the samples after prefire is believed to
be due to residual organics which act as a glue to hold the metal
powder together without actual sintering. The structure is thus
analogous to peanut brittle, accounting for its character of being
cohesive but brittle. Under the SEM, the deposit has the appearance
of a boulder field with tin particle dimensions in the range of 5 to
30 microns. In the sample fired at the :owest temperature of 400°C
shown in Figure 4, the tin "boulders" are quite porous and appear to
be composed of smaller particles clustered and sintered together. It
is likely that these agglomerations are present in the paste before
firing. There does not appear to be any bonding of the large tin
particles to each other.
After firing in air at 5000C for 5 minutes the appearance of the metal
powder has changed drastically (Figure 5). The substructure is no
longer visible and each particle is surrounded by a shrunken bag,
presumably of oxide. Evidence of the oxide is still present after 1
and 10 minutes of further firing at 675°C in forming gas (Figure 6
and 7). The best cohesion and adhesion of the sintered tin and least
oxide were found in the samples prefired at 400°C. Evidence of bonding
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between On particles is difficult to find in the SEMI photographs
(Figures 4 through 7).
The samples in this sequence (40&C prefire) had copeous quantities
of a white ash or residue after "iring at 675 oC in forming gas. This
white substance also slowed up after the 675 oC reducing step in sam-
ples prefixed at 4500C and WC. The white residue was believed to
be a manifestation of oxide. In subsequent experiments it became
more prevalent as precautions to exclude oxygen were increased, but
eventually wa; successfully eliminated by oxygen exclusions.
Samples prefired at 45&C and 500oC were powdery and poorly, cohesive
after the oxidation firing at 500 O and the final sintering at 6750C
in forming gas. The deposits were dark colored and believed to be
heavily oxidized.
In view of the extensive ox i dation observed, even after the forming
gas firing, a second matrix experiment was conducted, shown in Figure 8,
in which the 5000C air firing step was omitted. A prefiring temper-
ature of 4500C w.is selected with time of prefire between 5 minutes and
10 minutes. The finti1 -firing step at 675 O was tested with both nitro-
gen and forming gas atmospheres. In this experiment the samples after
f 500C prefire resembled closely those after prefire in the previous
experiment. After final sintering the deposits were powdery with large
amounts of the white residue previously described.
Based on the results of the second matrix experiment, a firing cycle
consisting of 20 minutes prefire at 4000  followed by 5 minutes sinter 	 3
in air a-L 5000C was tested. This firing cycle gave good sintering,
although with sortie evidence of oxidation. A nickel-tin paste was then
blended by mixing 10% of the tin-titanium hydride fritless paste with a
pure nickel fritless paste. In order to flux the oxides a small amount
of zinc chloride was blended -1 -to the paste. This was done by adding 2
per cent of isopropyl alcohol saturated with zinc chloride. Cells printed
9
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ORIGI14AL PAGE I`
OF POOR QUALITY
with this nickel-tin paste were then fired 20 minutes at 400°C in N2
followed by 2 minutes at 500°C in air. This deposit was very powdery
and poorly sintered.
SEM photographs of the tin-titanium hydride deposit at 400°C and then
at 5000
 are shown in Figures 9, 10 and 11. Figure 9 shows a general
view suggesting less oxid„;,ion than previously observed (Figure 5).
Sintering of the tin particles shown at larger magnification in
Figure 8 is somewhat obscured by the oxide. The photograph shown in
Figure 9 was taken in the "halo” .egion along the edge of a grid line.
This region forms from the residues of the clear organic vehicle which
segregate out after printing, carrying with it small quantities of the
solids. The numerous small particles (about 1 micron) appearing in this
photograph are believed to be the original tin particles which agglomer-
ate together to form the larger particles, some of which also appear ir
the picture. Partially consolidated clusters of the small particles can
be seen at several locations.
A SEM photograph of the blended nickel-10% tin paste after firing is
shown in Figure 12. The visual impression that the large tin particles
are wvdely separated in the fine (about 1 micron) nickel powder is con-
firmed by the tin x-ray image of same area shown in Figure 13. The tin
particles in Figure 12 do not show the oxide bag structures previously
observed (Figures 5, 6, 7 and 10) indicating that the zinc chloride was
effective in performing its fluxing function.
The observations of the tin distribution in the nickel-tin paste
suggest that the tin will not play significant role in sintering the
deposit unless the concentration is very high (about 80% Sn).
In view of the persistent oxidation problems, further attempts were made
to improve the firing procedures and equipment. At the end of the period
reducing conditions in forming gas were being obtained and fritless tin
and copper pastes were fixed in nitrogen with only minor oxidation. In
11
ORIGINAL P"G;F 3
OF POOR QUALITY
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preliminary experiments in the improved furnace atmospheres, moderate
adhesions were obtained with pure copper and silver fluoride dorm
copper on silicon nitride surfaces fired in nitrogen, but not whin
fired in forming gas. Tin-titanium hydride deposits were non-adherent
in both atmospheres and on both bare silicon and silicon nitride.
ORIONAL PAGE fS
OF POOR QUALf iY
I
15
3.0 DISCUSSION
The fritless tin and copper pastes which were fired during this period
were found to be extremely sensitive to oxidizing conditions. In the
absence of oxidizing conditions the printed deposits tend not to bond
to silicon and silicon-nitride surfaces, supporting the tentative hy-
pothesis that frits play a significant role in bonding. Titani-im hy-
dride does not appear to improve the situation. The failure of tin to
wet and spread through fine nickel powder suggests that tin-nickel mix-
tures will be limited to those having a small nickel content.
ORIGINAL PAGE 13
OF POOR QUALrTY
16


Development of technique for AR coating nickel and copper metallization of solar cells FPS project product development

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