FRONT SIDE METALLIZATION OF CRYSTALLINE SILICON SOLAR CELLS
USING SELECTIVELY LASER DRILLED CONTACT OPENINGS
Baomin Xu, Karl Littau, James Zesch, and David Fork
Palo Alto Research Center Inc., 3333 Coyote Hill Road, Palo Alto, CA 94304
E-mail: Baomin.Xu@parc.com
ABSTRACT
Selective removal of the silicon nitride dielectric layer
has been demonstrated even using nanosecond range
laser pulses, through carefully controlling the energy
density of laser pulses. It has been found that, a laser
2
pulse with a peak energy density of 4.3 J/cm or lower can
remove the nitride layer without altering the underlying
silicon while a pulse with a peak energy density of 4.8
2
J/cm or higher will substantially damage the silicon. With
the laser energy density maintained below the threshold
for silicon ablation, multiple laser pulses can be used to
more completely remove the nitride layer. Also, using high
quality blanket sputtered nickel film as metal contact layer
and screen printed silver gridlines as an etching protection
mask, a new method for front side metallization has been
developed. The specific contact resistance can be
reduced by about two orders of magnitude compared to
the conventional screen printed and fired through silver
contact, and the firing temperature can be lowered to
about 500°C.
INTRODUCTION
While screen printing silver paste and firing through
the silicon nitride dielectric passivation/antireflection layer
is the most widely used front side metallization method for
silicon solar cells, it is also one of the major factors limiting
cell efficiency, mainly due to producing a poor, very
resistive metal-silicon contact [1]. Several alternatives to
replace screen printed silver metal contact have been
studied in the past decade, among which using laser
ablation to generate contact openings in the silicon nitride
layer followed by a suitable metallization process has
received the most extensive study [2].
Laser ablation has many advantages such as being
clean (dry method), simple (no mask needed), noncontact, and fast. To utilize this technology for front side
metallization of silicon solar cells, at least two questions
need to be answered: first, if selective removal of the
silicon nitride layer can be achieved without damaging the
underlying emitter layer; second, how to form the metal
contact layer and metal gridlines after forming the contact
openings.
Several groups have reported the possibility of
selective ablation of the silicon nitride layer, but the laser
beam conditions are not consistent [2-5]. Some reports
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show that very short (picosecond) or ultra short
(femtosecond) laser pulses are needed, which makes it
more difficult for commercialization [4,5]. Also it is not
clear how to completely remove the silicon nitride in the
opening areas.
A number of methods for forming the metal contact
with the n+ emitter layer after making the laser ablated
openings have been proposed. Ink jet printing nanophase
contact metal solutions into the openings seems to be the
most elegant approach [6]. However due to technical
difficulties so far there is no successful experimental result
reported. Electroless plating nickel to form the contact
layer, followed by electroplating silver or other metal layer
to form gridlines has been broadly studied [7]. However, it
is still a big challenge to form a high quality, continuous
and dense nickel contact layer through electroless plating,
as well as to achieve high throughput for mass production.
In the first part of this paper we will demonstrate that,
by carefully controlling the energy density of the laser
pulses, selective removal of the silicon nitride dielectric
layer can be achieved even using nanosecond range laser
pulses. Moreover, maintaining the laser energy density
below the threshold for silicon ablation, multiple laser
pulses can be used to more completely remove the nitride
layer. In the second part of this paper, a new method for
metallization is presented, which uses a blanket sputtered
nickel film as the metal contact layer and screen printed
silver gridlines as an etch mask to pattern the underlying
nickel film. Compared to the conventional fired through
silver contact, the new method can reduce the specific
contact resistance by about two orders of magnitude.
SELECTIVE REMOVAL OF SILICON NITRIDE
DIELECTRIC LAYER
Experimental
A customized laser micromachining system made by
PhotoMachining Inc. is used in this work. The laser source
is a quadrupled Nd:YAG Coherent AVIA laser with 266nm
wavelength. The sketch of the optical system is shown in
Fig. 1, and a galvo scanner is used to drill the contact
openings on the samples. The distance between the focal
lens and the surface of the sample, which is defined as
stage height by the control software, can be easily
adjusted by changing the position or height of the focal
lens. This also changes the laser spot size on the surface
of the sample, with smallest spot size reached when the
sample surface is at the focal position. Hence for a given
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laser beam energy, we can change the energy density
through changing the laser spot size, which can be done
by adjusting the stage height.
Reflective mirror
AVIA laser source
2mm dia. laser beam
Galvo
scanner
Adjustable iris
Reflective mirror
Stage height, adjustable
by computer
Sample
hole depth suddenly increases to about 490nm, and also
melting and freezing of liquid Si capillary waves on the
silicon substrate can be clearly observed from Fig. 3.
When the stage height increases from 85.7 mm to
88.7 mm, the focal position, the hole depth gradually
increases while melting of the silicon substrate increases.
After passing the focal position, for the stage height from
88.7 mm to 91.7 mm, the hole depth and melting of Si
gradually decrease. When the stage height increases from
91.7 mm to 92.2 mm, the hole depth suddenly decreases
from 527 nm to 97 nm, and also there is no significant
melting of the silicon substrate. As the stage height further
increases to 94.2 mm, the hole depth is only about 44 nm,
which does not penetrate through the nitride layer.
These results demonstrate that when the stage height
is from 85.7 mm to 91.7 mm, or within ± 3.5 mm from the
focal position (the focus position is 88.7mm), the laser
drilled holes will substantially penetrate into the silicon
Stage
In order to easily characterize the profiles of the laser
drilled contact openings, in this paper we use polished
silicon wafers with an 80nm thick PECVD silicon nitride
dielectric layer. The silicon wafer is heavily arsenic doped
with a bulk resistivity of about 0.0015 Ω·cm. Hence the
whole silicon wafer functionally substitutes for the n+
emitter in the solar cells (with 50 Ω/□ sheet resistance and
0.3 to 0.4 μm layer thickness, the emitter layer is
equivalent to a bulk resistivity of 0.0015 to 0.002 Ω·cm).
Results and Discussion
For this work we have fixed the laser beam energy at
93 µJ/pulse, with a pulse length of 20 ns and a repetition
rate of 30 kHz. The galvo scan speed is 7620 mm/s, which
gives the spacing of 0.254 mm between openings. We
have changed the stage height from 84.7 mm to 94.7 mm,
with the step of 0.50 mm. With the stage height of
88.7 mm, the laser beam is focused on the sample
surface. As the stage height is moved away from this
value, the laser beam becomes more defocused, the spot
size larger, and the pulse energy density incident on the
substrate smaller.
At each stage height we drill a series of contact
openings, or simply called holes in this section, and
measure the hole profiles using a Dektak stylus
profilometer, based on which the hole depth is determined.
Fig. 2 shows a plot of hole depth as a function of stage
height, and Fig. 3 shows the optical micrographs of the
laser drilled holes at some stage heights.
It can be seen from Fig. 2 that when the stage heights
are from 84.7 mm to 85.2 mm, the hole depths are from
about 83 nm to 100 nm, which means the nitride layer is
ablated away. From Fig. 3 it can be seen that there is no
damage or significant melting of the silicon substrate.
However, when the stage height increases to 85.7mm, the
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Hole depth (nm)
1200
Fig. 1. Sketch of the laser optical system
1000
800
600
400
200
0
84
86
88
90
92
94
96
Stage height (mm)
Fig. 2. Depth of laser drilled holes vs. stage height
84.7
85.2
87.2
88.7
(Focus)
90.2
91.7
92.2
94.2
85.7
30µm
Fig. 3. Micrographs of laser drilled
holes with increasing stage height
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Energy Density per Pulse (J/cm2)
5
Fig. 5. Micrographs of laser drilled holes with
different pulses used.
80
1 pulse
4 pulses
40
0
-40
-80
-120
0
No Si damage at center
4
8 pulses
30µm
Extensive Si damage at center
4.5
4 pulses
1 pulse
Hole depth (nm)
substrate and cause damage or melting. However, when
the sample is more than 3.5 mm away from the focal
position, the silicon nitride layer can be ablated without
visibly damaging the underlying silicon substrate. This
clearly indicates that there is a threshold for the energy
density where the silicon nitride layer can be removed
without altering the underlying n+ silicon emitter layer.
In order to more accurately determine this energy
density window, the sample position is more precisely
adjusted and the positions where the underlying silicon is
not damaged and starts to be damaged are determined.
The beam powers at the surface of the sample at these
two positions are measured using a laser power meter
with a 35µm aperture. Then by approximating the beam
shape as Gaussian function the energy profiles of these
laser pulses are calculated. The results are given in Fig. 4.
It indicates that the underlying silicon layer will not be
2
damaged with the peak energy density up to 4.3 J/cm ,
and will start to be damaged with the peak energy density
of 4.8 J/cm2. To remove the nitride layer the energy
density needs to be at least 0.5 J/cm2.
50
100
150
200
250
Distance (um )
3.5
Fig. 6. Profiles of laser drilled holes with
different pulses used.
3
2.5
2
Extent of nitride
removal
1.5
1 pulse
4 pulses
1
0.5
0
0
10
20
30
40
Radius from Center
Fig. 4. Laser beam profiles with no damage to the
underlying silicon and starting to damage to silicon.
Fig. 7. EDS nitrogen field maps with
different laser pulses used.
One of the challenges using laser drilled contact
openings is that silicon nitride removal by a single subthreshold pulse is incomplete, and this reduces the contact
area. In this work we have found that, when the laser
energy density is maintained below the threshold for
silicon ablation, multiple laser pulses can be used to more
completely remove the nitride layer. The micrographs of
the laser drilled holes with different pulses are shown in
Fig. 5, the hole profiles are shown in Fig. 6, and the EDS
nitrogen maps are given in Fig. 7. These results clearly
demonstrate multiple laser pulses can more completely
remove the nitride layer, without substantially increasing
the hole depth or damaging the underlying silicon. We
have found even after 100 laser shots the silicon substrate
is still not considerably damaged.
The doping profiles in the holes with different pulses,
and in the area without holes, were measured using SIMS
and the results are shown in Fig. 8. Fig. 8(a) gives the
element analyses at the area without holes, which shows
a normal nitride layer on the top of As-doped silicon. Fig.
8(b) shows the results at the hole areas. However as the
scan area is always larger than the hole area, the results
also contain some noise level of silicon and nitride.
Nevertheless the results clearly demonstrate the more
removal of nitrogen for two pulses than one pulse. More
importantly, comparing to Fig. 8(a) it can be found the As
doping in the silicon is not changed, no matter one or two
pulses are used.
1 pulse
4 pulses
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(a)
Si->
1
1E+20
0.8
As
1E+19
0.6
N->
1E+18
0.4
1E+17
0.2
1E+16
0
50
100
150
200
250
Composition (Arb. Unit)
Concentration (atoms/cm3)
1E+21
0
300
The basic procedure is described in Fig. 9. Starting
with a solar cell silicon substrate (a) and after drilling the
contact openings through the nitride layer (b), a thin nickel
film is sputtered on the whole surface (c), followed by
screen printing silver gridlines which are aligned with the
contact openings (d). Next the uncovered nickel film is
etched away using the silver gridlines as a protective
mask, then the silver gridlines and the underlying nickel
contact layer is co-fired at high temperatures to finish the
metallization, as shown in Fig. 9(e).
(a)
Si
Depth (nm)
(b)
Ni film
1E+21
1
Si(1)->
Si(2)->
1E+20
0.8
As(1)
As(2)
1E+19
0.6
(b)
1E+18
0.4
N(1)->
1E+17
0.2
N(2)->
1E+16
0
50
100
150
200
(c)
250
Composition (Arb. Unit)
Concentration (atoms/cm3)
SiNx
n+ emitter
Contact
opening
(d)
Etched &
co-fired
(e)
Fig. 9. Metallization procedure
0
300
Depth (nm)
Fig. 8. SIMS element analyses at the area without
holes (a), or at the area with holes drilled by one or
two laser pulses (b).
METALLIZATION USING SPUTTERED
NICKEL FILM
Description of the Method
Forming a good metal contact layer is another
challenge to use laser drilled contact openings for front
side metallization. Sputtered metal films, especially the
metals which form silicide compounds with silicon such as
Ni, Co, and Ti, have been extensively used in the
semiconductor industry and can have very low specific
-7
2
contact resistance with silicon (on the order of 10 Ω·cm )
and robust adhesion to the silicon substrate with very low
stress. However, these sputtered films generally have to
be patterned using a photolithographic process, which will
increase the cost of solar cells manufacturing. In this
paper we propose using screen printed silver gridlines as
an etch mask to pattern the underlying sputtered nickel
contact layer. This approach ensures the use of high
quality nickel film as a contact layer to reduce the contact
resistance, and also avoids the use of standard
photolithographic process to reduce the cost. In addition, it
makes the nickel contact layer self-aligned with the silver
gridlines and permits immediate cell testing after firing.
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Ag lines
Results and Discussion
In order to simplify the experiment and as a first step
to verify the possibility of the new metallization method
shown in Fig. 9, again we use polished, heavily arsenic
doped, 0.50 mm-thick silicon wafer with the bulk resistivity
of about 0.0015 Ω·cm, covered by an 80 nm thick PECVD
silicon nitride dielectric layer. The experiment on the
textured, solar cell-type silicon substrates is being
conducted and the results will be reported separately.
The sample size is about 38.1 mm (1.5”) x 38.1 mm.
In order to measure the contact resistance using the
Transmission Line Method (TLM), first seven lines of
contact openings in the nitride layer were laser drilled. The
spacing between the adjacent lines varies from 2.0 mm to
7.0 mm, with the step of 1.0 mm. In each line the distance
between adjacent openings is about 0.254 mm and the
line length (from the first opening to the last opening) is
about 25.4 mm (1”), and hence 100 contact openings were
drilled in each line. After drilling the contact openings, a
100 nm-thick nickel film was blanket sputtered, followed by
screen printing silver gridlines which were aligned with the
lines of contact openings. The silver paste used is Ferro
CN33-462. A photograph of the sample at this stage is
shown in Fig. 10(a). Then we dropped the sample in a
ferric chloride etching solution for about 5 seconds, the
uncovered nickel film was removed, without any damage
of the silver gridlines, as shown in Fig. 10 (b). This verifies
the success of patterning the sputtered nickel film using
silver gridlines as the etch mask.
000520
(b)
Fig. 10. (a) Sample covered with blanket sputtered
Ni film and screen printed Ag gridlines;
(b) After uncovered Ni film etched away using
screen printed Ag lines as an etch mask.
After patterning the nickel film the silver gridlines
along with the underlying nickel contact layer, which we
call Ag/Ni lines, were co-fired at 500°C using a rapid
thermal annealer (RTA). Then the I-V curve between the
adjacent Ag/Ni lines was measured using the four probe
Kelvin method, from which the resistance value was
derived, and then the resistance between the adjacent
Ag/Ni lines and the line spacing was plotted.
Shown in Fig. 11(a) is a typical I-V curve with the line
spacing of 4.0mm. The very straight linear line indicates
the excellent ohmic contact between Ag/Ni electrode and
the silicon substrate. The reciprocity of the slope
represents the resistance between these two adjacent
lines. Fig. 12 gives the dependence of the resistance
measured between the adjacent Ag/Ni lines on the line
spacing.
0.15
0.1
(b)
0.05
-0.01
0
-0.005
0
-0.05
0.005
0.01
Current (A)
Current (A)
(a)
0.15
y = 13.679x
R2 = 0.9994
y = 0.0329x
R2 = 0.9711
0.1
0.05
0
-4
-2
0
2
4
-0.05
-0.1
-0.1
-0.15
-0.15
Voltage (V)
Voltage (V)
Fig. 11. I-V curve between adjacent, 4mm
separated Ag/Ni lines for the samples (a) having
laser drilled holes and (b) no laser drilled holes
For our sample the total resistance RT between the
Ag/Ni lines is the sum of the contact resistance RC
between the nickel contact layer and the underlying silicon
plus the silicon substrate resistance RSi between the Ag/Ni
lines, and can be expressed as:
RT = 2 RC + RSi = 2 RC + ρ Si d /( L ⋅ t )
Where ρSi is the silicon substrate resistivity, d is the
line spacing, L is the line length, and t is the substrate
thickness. Putting the numbers for each parameter we
found that the silicon substrate resistance RSi is about
0.0077Ω for the 7mm line spacing. This is about one order
of magnitude smaller than the measured resistance RT
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which is between 0.073 and 0.087Ω. Hence in our case
the measured resistance RT is mainly contributed by the
contact resistance RC and should be almost not related to
the line spacing, which is consistent with the experimental
data shown in Fig. 12. Thus the above equation can be
simplified to RT ≈ 2RC, from which RC can be derived.
Then considering the contact is through the laser drilled
openings the contact area can be calculated, and finally
the specific contact resistance can be obtained. Through
these calculations we got the specific contact resistance is
around 0.04 mohm·cm2. This is about two orders of
magnitude smaller than the specific contact resistance
between the fire-through silver gridlines and the n+ emitter
layer, which is usually between 3 to 10 mohm·cm2.
0.4
Resistance (ohm)
(a)
0.3
0.2
0.1
0
1
2
3
4
5
6
7
8
Ag/Ni line spacing (mm)
Fig. 12. Dependence of resistance on line spacing
In order to confirm all the contacts are through the
laser drilled contact openings and no possibility of firing
through nitride layer by the Ag/Ni lines. Another sample
was prepared by the same procedure but no laser drilled
contact openings, and the I-V curves between the
adjacent Ag/Ni lines were measured, with a typical result
shown in Fig. 11(b). The complicated shape of the I-V
curve, and the more than 400 times higher resistance than
Fig. 11(a), indicates that it is impossible for the Ag/Ni lines
to fire through the nitride layer and to form a good contact
with low contact resistance.
To further characterize the contact of this fired Ag/Ni
lines with silicon substrate Schottky diode samples were
prepared on a lightly doped silicon substrate with the
resistivity of about 0.5 Ω·cm, and the structure is shown in
Fig. 13. The forward bias characteristic (I-V curve) was
measured under three different conditions, and the results
are given in Fig. 14: (1) after sputtering the Ni film and
prior to screen printing Ag paste; (2) after firing the Ag/Ni
line at 500°C in air; and (3) after firing the Ni film at 500°C
in air without Ag coverage.
We have found that the bare Ni film was severely
oxidized after firing, and this greatly changed the I-V
characteristic of the diode, as shown in Fig. 14. However,
when covered by the Ag paste the Ni film was not oxidized
after firing and the I-V characteristic was almost not
changed. This is probably because the Ag paste coverage
provides the protection, and more importantly, the
000521
decomposing organics in the Ag paste generated a
localized reducing or oxygen-deficient atmosphere.
Table 1. Schottky barrier height and ideality factor
Ag I
SiO2
Ni
Lightly doped n-Si
Fig. 13. Schottky
diode structure
V
Ni
CONCLUSION
Au
1.E+00
y = 37.953x - 20.279
y = 38.155x - 21.221
1.E-02
LnI
1.E-04
As deposited Ni
After firing Ag/Ni
After firing Ni only
1.E-06
1.E-08
1.E-10
0
0.5
1
1.5
2
2.5
Voltage (V)
Fig. 14. Forward I-V curve of the Schottky
diodes under different conditions
The linear part of the I-V curve follows the thermionic
emission mechanism and can be expressed as [8]:
I = A⋅ A T e
*
2
− Φ B / kT
⋅e
qV /( nkT )
Where A is the diode area, A* is Richardson constant, ΦB
is the barrier height, and n is ideality factor, which ideally
should be equal to unity. Through fitting into the above
equation, the barrier height and ideality factor after
sputtering Ni and after firing Ag/Ni lines are calculated,
and given in Table 1, along with the reported barrier height
for Ni/Si and NiSi (nickel silicide)/Si junction [9].
After firing Ag/Ni lines the ideality factor is still very
close to 1, verifying the silver paste and high temperature
firing will not change the Ni/silicon interface. The slight
increase of the barrier height after firing may indicate the
formation of some nickel silicide. All these results
demonstrate the fired Ag/Ni lines have the similar contact
characteristic with silicon as Ni film or nickel silicide used
in semiconductor industry, and hence very low contact
resistance can be obtained.
It is worth to point out that the Ni film also enhances
the adhesion of the Ag gridlines. The firing temperature of
the paste we used, Ferro 33-462, is around 800°C. When
fired at 500°C directly on the nitride layer, it doesn’t have
any adhesion and will be peeled-off after firing. However,
with the underlying Ni layer the Ag lines can have strong
adhesion to the substrate after firing at 500°C.
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In this paper we have demonstrated the selective
removal of the silicon nitride dielectric layer using
nanosecond range laser pulses, through carefully
controlling the energy density of laser pulses. With a peak
energy density of 4.3 J/cm2 or lower the nitride layer can
be removed without altering the underlying silicon, and
multiple laser pulses can be used to more completely
remove the nitride layer. A new metallization method using
blanket sputtered nickel film as metal contact layer and
screen printed silver gridlines as an etch mask is
developed. It ensures the use of high quality nickel film as
a contact layer to reduce the contact resistance and
avoids the use of standard photolithographic process to
lower the cost. The result shows the Ag/Ni lines can be cofired at about 500°C with good adhesion to the silicon
substrate, and the specific contact resistance is about two
orders of magnitude lower than the conventional screen
printed and fired through silver contact.
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[3] P. Engelhart et al., “Laser Ablation of Passivating SiNx
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[4] K. Neckermann et al., “Local Structuring of Dielectric
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