Note: Descriptions are shown in the official language in which they were submitted.
117~13~
This invention relates to plating techniques in general, and
more particularly to methods for plating nickel onto silicon.
BACKGROUND OF THE IN~ENTION
. .. _
It is often desirable, in the manufacture of solar cells
and other semiconductor devices, to plate nickel directly onto
silicon so as to form electrodes or contacts for coupling the
semiconductor device into an electric circuit. One of the most
well known and effective ways of achieving such plating is through
electroless nickel plating.
With such a process, the first step is usually to clean the
outer surfaces of the silicon substrate where the nickel is to
be deposited so as to remove any particles of dirt or oxide which
may be present. This cleaning is desirable since the presence of
such substances on the surface of the silicon tends to interfere
with proper plating of the nickel onto the silicon. Cleaning may
be acco~plished in various ways well known to those skilled in
the art, e.g. by successively immersing the substrate in suitable
baths of hot organic solvents and hot chromic-sulfuric acid,
followed with a bath of hydrofluOric acid and rinsing with
deionized water. Once this has been done, the surface of the
silicon substrate which is to receive the nickel plating is then
pretreated with a catalyst. This is necessary since the
silicon surface will not itself support the electroless plating
process and nickel plated on an untreated silicon surface tends to
adhere poorly thereon. Normally palladium is used as the catalyst
although other activators are well known to persons skilled in
the art. U.S. Patent No. 3,489,603 shows one such alternative
to the palladium catalyst.
MTA-33 -2-
~ 37 `
Once the surface of the silicon which is to receive the nicke
has been cleaned and pretreated, the silicon is ready for electro-
less nickel plating. Plating is accomplished by immersing the
silicon substrate in a suitable acidic or alkaline bath under
appropriate conditions. A typical acidic bath might comprise
nickel chloride (at 30 g/l), sodium hypophosphite (at 10 g/l),
a third salt o~ sodlum nitrate (at 10 g/l), a pH of between 4
and 6 and a temperaiure of approximately 190F. In such a bath
the nickel chloride provides -the nickel ion which is to be re-
duced, the hypophosphite provides the reducing agent and the
third salt acts as a buffer and complexing agent for the nickel.
Alternatively, an alkaline bath may be substituted. Such
electroless nickel plating is wel] known in the art and is
described in detail in such publications as Electroplating and
Related Processes by J.B Mohler (Chemical Publishing Co., Inc.;
..
New York, 1969), Surface Preparation and Finishes for Metals edite~ 1
by James A. Murphy (McGraw-Hill Book Co.; New York, 1971) and
Handbook of Thin ~ilm Technology edited by Leon I. Maissel and
Reinhard Glang (McGraw-Hill Book Co.; New York, 1970).
Elec-troless nickel plating of this type suffers from a
number of difficulties. First, the need for catalytic pretreatmenl :
of the silicon adds an additional step to the plating process.
In addition, where the catalyst used is palladium the nickel tends
to be plated down in an uneven pattern since the palladium has a
tendency to work unevenly over the surface of the silicon. The
high cost of palladium is also a negative feature, since signifi-
cant amounts of palladium may be lost during the pretreatment
process.
MT~-33 -3-
~ i7
Furthermore, the chemistry of the electroless pla-ting bath
described above tends to result in the forma-tion of some nickel
phosphide which may be deposited on the silicon along ~ith the
nickel. The inclusion of -this nickel phosphide in the plated
nickel tends to alter the properties of the deposited nickel
and may be quite undesirable depending on the applications con-
templated. The phosphorous content in the deposited nickel may
be held down by using an alkaline rather than an acidic bath,
but the use of an alkaline bath may raise new difficulties. In
particular, alkaline baths tend to etch away any aluminum
exposed to the bath, thereby complicating plating where the silicor
has an aluminum layer thereon. In addition, the use
of an alkaline bath tends to promote the formation of an oxide
layer on the surface of the silicon so as to impede the plating
of nickel directly onto the silicon.
Thepresence of an oxide layer provides a further complica-
tion where it is desired that the deposited nickel serve as an
ohmic contact. In such case the nickel layer must be sintered
so as to diffuse into the silicon substrate ~nd form a nickel
2~ silicide at the nickel/silicon interface. When an intervening
oxide layer is present, the sintering must be carried out at a
relatively high temperature (above 350C) and/or for a relatively
long time (40 minutes or more) in order to cause the nickel to
penetrate the oxide layer and diffuse into the silicon and form
an ohmic contact. ~owever, where the substrate is a shallow
junction device such as a solar cell and the nickel layer is on
the surface of the substrate nearest the junction, there is a
tendency for the nickel to diffuse deep enough to shunt or short
out the device, especially where the oxide layer is very thin or
MTA-33
37
, .
non-existent. In this context a shallow junction silicon
device is one where the junction is abou-t 1~0 micron or
less below the surface on which the nickel is deposi-ted.
SUM~ARY OF THE INVENTION
According to an aspect of the invention there is
provided a method for plating nickel onto a silicon body
wherein the method comprises (a) immersing the silicon
body into an aqueous bath comprising nickel chloride and
arNmoni.um fluoride or ammonium fluoride and hydrofluoric
acid, (b) maintaining the silicon body in the ba-th so t~at
nickel ions in the bath will be converted to solid nickel
and deposited onto the silicon body as an adhering layer
thereon, and (c) withdrawing the silicon body from the
bath.
According to a ~urther aspect of the invention
there is provided a method of making a photovoltaic semi
conductor solar cell comprising: (1) forming a silicon
semiconductor body of a first conductivity type having a
top region of a second conductivity type, the. top region
being characterized by a top surface having first top
surface regions which are coated with silicon oxide and
second top surface regions which are free of silicon oxide,
the second top surface regions forming a grid pattern on
the top region; (2) immersing the semiconductor body in an
aqueous bath of nickel chloride and a fluoride compound which
ionizes in water; (3) maintaining the semiconductor body in
the bath long enough for a layer of nickel to be adhesively
deposited on those second surface regions; (~) withdrawing
the semiconductor body from the bath; (5) washing the semi-
conductor body with deionized wa-ter so as to remove any loose
particles from the semiconductor body; (6) sintering the
5-
, '', .~!/
cb/~
33~
adhering nickel layer so as to create a nickel silicide
junction between the silicon body and the adhering nickel
layer; and (7) removing the silicon oxide from the first
surface regions of the semiconduc-tor body.
According to a still further aspect of the invention
there is provided a method for forming an ohmic nickel contact
on a silicon body comprising the steps of: (a) immersing the
silicon body into an aqueous bath of nickel chloride and a
fluoride compound that ionizes in water, (b) maintaining
the silicon body in the bath so that nickel ions in the
bath will be converted to solid nickel and deposited onto
the silicon body as an adhering layer thereon, (c) with-
drawing the silicon body from the bath, and (d) sintering
the ~dhering layer so ac to produce a nic~el silicide at
the junction oE the silicon body and the adhering layer of
nickel.
Another aspect of the invention provides a method
for plating nickel onto a silicon body wherein the method -
comprises ~a) immersing the silicon body into an aqueous
bath of nickel chloride and hydrofluoric acid with a pH
of 2.6-2.8, (b) maintaining the silicon body in the bath
so that nickel ions in the bath will be con~er~ed to solid
nickel and deposited onto the silicon body as an adhering
layer thereon~ and (c) withdrawing the silicon body from
the bath after the layer has reached a thickness of between
about 500 and 2500 Angstroms.
BRIEF DESCRIPTION OF THE DRAWINGS
Figs. 1-3 axe cross-sectional views which illustrate
various steps in a preferred method of plating nickel onto a
silicon substrate using the present invention;
cb/~!J
7~3~3~
Figs. 4-6 are cross-sectional views which illustrate
various steps in a method of making a solar cell, using the
preerred embodiment of the present invention; and
Fig. 7 is a perspective view of a completed solar
cell formed using the preferred embodiment of the present
invention.
_ETAILED DESCRIPTION OF THE PREFERRED EMBODIMRNT
Referring first to Fig. 1, there is shown a silicon
substrate i which is to be plated with nickel in accordance
with the present invention. Substrate 1 typically comprises a
substantially silicon body 3 which may or may not have thereon
an outer layer 5 comprised of various particles of dirt and
oxides. If body 3 does have a layer 5 on its outer surfaces,
it is desirable to remove the layer 5 from those regions where
the nickel is to be plated onto the silicon. Selective or
complete removal of this exterior layer may be accomplished in
ways well known in the art, e.g., complete removal of such a
layer may be effected by successively immersing the substrate
in baths of hot organic solvents and hot chromic-sulfuric acid,
followed by a bath of hydrofluoric acid and a rinsing with
deionized water.
-6a-
cb/~-
~ 3~7
Once any unwanted layer 5 has been removed from the
silicon body 3 (Fig. 2 shows the silicon body completely devoid
of any outer layer 5), the silicon substrate is ready for nickel
plating. For this purpose the substrate is immersed in a bath
comprised of nickel chloride and a selected fluoride compound
which ionizes in water. Such immersion results in nickel ions
in the bath (donated by the nickel chloride) being acted on by
the selected fluoride compound so as to cause formation of solid
nickel by a displacement reaction. It is believed that the dis-
placement reaction is as follows: Si + 2Ni -~2Ni + Si+4
with the silicon going into solution as H2SiF6 and/or H2SiO3.
The nickel thus created is deposited onto the silicon substrate
as an exterior layer 7 (Fig. 3). While this ion reduction is
occurring, the bath chemis-try simultaneously activates the surface
of the silicon substrate so that the deposited nickel strongly ad-
heres to the silicon and forms an effective and long-lasting
plating thereon. In the case where the plated nickel is to be
an ohmic contact, after the nickel has been plated it is subjected
to sintering in order to form a suitable nickel silicide.
Preferably, though not necessarily, the selected flouride
compound is ammonium fluoride. Satisfactorily plating may be
achieved with baths containing between 10 and 642 g/l of nickel
chloride and between 10 and 40 g/l of ammonium fluoride, with a pH
of between about 2 and 6. In its preferred embodiment, the aqueou
bath comprises nickel chloride (at about 640 g/l) and ammonium
fluoride (at about 40 g/l), with a pH level of about 4 In addi-
tion, platin~ using the preferred bath embodiment has been succes-
sfully conducted at temperatures as low as 17C and as high as
100C, although a temperature of between 20 and 30C is preferred.
MTA-33 -7-
~7~3~L3~
Successful plating has also been achieved using such alter-
native fluoride compounds as hydrofluoric acid and N~14F:~F. In
the case of hydrofluoric acid, a suitable bath composition might
consist of nickel chloxide (at 200-600 g/l) and hydroflU~ic acid
(at 25-50 mls/l at a pH of 2.6 2.8 and a temperature of 23C. In
the case of NH4F:HF, a suitable bath migh-~ comprise nickel chlorid
(at 300 g/l) and NH4F:HF (at 20 g/l) at a pH of 3.2 and a temp-
erature of 23C. ~ther tempera~ures between about 17C and 100C
also may be used for such baths.
Nickel deposition rates are generally dependent upon the
specific fluoride compound in the bath, pH levels, bath tempera-
ture and impurity levels in the silicon. Additionally, the concen
trations of the bath components and surface area of the substrate
being plated may also affect deposition rates. In general,
deposition rates tend to increase as the pH level decreases, and
deposition rates generally increase with temperature. Success-
ful plating has been achieved with both P-type and N-type
silicon, with plating rates varying according to t~e type and
concentration of dopant used. In general, the deposition rate
increases as the impurity concentration in the silicon increases.
By way of example, using the preferred bath composition, pH and
temperature given above, a nickel layer having a thickness of 2100A
may be deposited on P-type silicon by immersing the silicon in
the bath for 2 minu-tes. Preferably the nickel layer is deposited
with a thickness of between about 500A and 2500A in the case of
shallow junction devices. The same or greater thicknesses may be
used for devices with no junction or with deep junctions (i.e.
greater than l micron). A thickness of about 1500A is best
preferred for shallow junction devices.
MT~-33 -B-
~`
Bath life is inversely related to the rale of deposition.
Bath pla~ing capacity is related to concentration levels of the
bath ehemicals. An eleven liter bath, eomposed of nickel chloride
(at ~40 g/l) and ammonium fluoride (at 40 g/l) has plated in ex-
cess of 7,000 square inehes of silicon to an average depth of
2000A without having to regenerate the bath. Regeneration, i.e.
the restoration of original concentrations, may be achieved simply
by adding more nickel chl,oride and ammonium fluo~ide to ihe bath.
It will be rsadily apparent to those skilled in the art that
the present plating technique is useful in a whole range of semi-
conduclor devices, particularly devices such as solar cells and
semiconductor devices which carry a lol of power and require sold-
erable pads, e.g. SCR's. In this respect it will be appreciaced
that while ~he formation of a solar eell using the present in-
vention will now be described, the choice of a solar cell as the
manufactured product is merely by way of example and not limitatior .
Figs. 4-7 illustrate the formation of a solar cell using
the nickel plating method just descrlbed. Referring first to
Fig. 4, ~here is shown a semiconductor body 8 formed in aecord-
2~ ance with ~eehniques deseribed and shown in U.S. Patent No.
4,152,824. ~he body 8 generally comprises a silicon substrate
9 formed of P-type silieon having a thickness of ~ 2.5 x 10 3cm.
A region 11 is formed on the baek side of the subscrate and com-
prises the P-lype silieon of the substrace additionally doped with ,
P-type dopant so as to comprise a high eonduetivity P~ region. A
region 13 is provided on Ihe fronc side of che substrale doped with
sufficient quantities of N-cype dopant so as to eomprise a high
eonduccivity N~ region. A top layer 15 formed of silicon dioxide
overlies region 13. Layer 15 is perforated at selected spots
so as to expose layer 13 as shown. Layer 13 is extended in depth
in those loeations immediacely below apertures 17. Layer
MTA-33 _9_
` ~ L3~
``
15 is preferably formed with a depth of about 5000 A, while layer
11 has a depth o~ ~v lO,OOOA, In addition, it is preferred tha~
layer 13 have a depth of ~ 5000A in those locations immediately
below apertures 17 and a dep~h of ~ 3500A elsewhere.
Next, -the semiconductor body 8 is immersed in a bath of
nickel chloride (a~ 640 g/l) and ammonium fluoride (at 40
g/l) at a temperature of 25C and a pH of 4.6 for approximately
1 minute. This results in the adhesive deposition of a nickel
layer 19 onto the silicon surfaces 13 adjacent apertures 17 and
adhesive deposition of a nlckel layer 21 onto the rear surface of
the silicon substrate. See Fig. 5. At the same time, however,
virtually no nickel is adhesively deposited onto ~he silicon
dioxide layer 15. Nickel layers 19 and 21 are formed of sub-
stantially pure nickel and have a depth of approximately 850A.
In addition, silicon dioxide on the side edge surfaces (not shown)
of the silicon body serves to prevent any portions of layer 19
from contacting layer 21 direclly, thereby avoiding short-circuit-
ing of the finished solar cell. In Fig. 7, the layer 19 is in ~he
form o~ a grid so as to leave portions of the solar cell region 13
as shown at 25 which are exposed to receive solar energy.
P]ating having been achieved, the semiconductor body 8
is rinsed in deionized water and dried. This serves to remove any
loose nickel or salts which may be present on the semiconductor
body. Next the body 8 is sintered in a nitrogen or hydrogen
atmosphere so as to promote the formation of a nickel silicide
layer 23 (Fig. 6) at the junctions of the nickel and silicon
regions. Formation of nickel silicide layers 23 is desirable as it
ensures that the junctions between the nickel and silicon will for~
ohmic contacts, rather than the rectifying contacts which can
MTA-33 -10-
~713~7
sometimes result when substantially pure nickel is joined di-
rectly to substantially pure silicon. Ohmic contacts are required
if the nickel is to serve as an electrode or contact. Sintering
at temperatures of between 250C and 350~C will promote the forma~
tion of Ni2Si, while sintering at temperatures of between 350~C
and 760C will promote formation of NiSi. Sintering at temperatur s
in excess of 760C will promote formation of NiSi2. For shallow
junction devices it is preferred to use the lowest practical
nickel silicide formation temperature because the rate of diffu-
sion of nickel into silicon decreases with decreasing temperature.
Furthermore the nickel silicide compound Ni2Si is generally pre~
ferred over the two other silicides since the Ni2Si tends to use
up less silicon per molecule of nickel silicide layer than the
other two silicides, thereby reducing the possibility that the N~
silicon region 13 will be completely penetrated by the nickel sili
cide layer 23 and the solar cell thereby rendered inoperable. For
shallow junction devices, sintering is carried out at between
250C and 350C for between about 15 and 40 minutes. At temper-
atures in the 250 to 350C range, approximately 30 minutes is re-
quired to attain a nickel silicide layer of Ni2Si with a depth of
approximately 300A, which is preferred for shallow junction device~
such as solar cells. For deep junction devices or for devices
having no junctions, sintering may be carried out at the same
temperatures and for the same times as for shallow junction de-
vices or at substantially greater temperatures and/or for substan-
tially greater times so as to produce one or more of the silicides
NiSi and NiSi2 and with or without some Ni2Si formation.
Once sintering has been accomplished, the semiconductor body
8 is immersed in a dilute (10%) hydrofluoric acid bath to etch awa~
the silicon oxide layer 15 and thereby yield the finished solar
cell shown in Fig. 7. This solar cell may then be used in ways
well known to persons skilled in the art to produce electricity.
M~A-33 -11-
3L7~ 7
It will readily be appreciated that various modifications
and additions to the aforementioned process ma~ be practiced with-
out changing the essence of the present invention. ~hus, for
example, the semiconductor body illustrated in Fig. 6 (showing the
semiconductor body 8 immediately after sintering but prior to
removal of the silicon oxide layer 15) may be reimmersed in the
nickel plating bath so as to plate additional nickel onto nickel
layers 19 and 21. This may be desirable since the sintering proces s
sometimes tends to make the nickel layers 19 and 21 more porous
and thereby less suitable as a contact. The deposition of this
additional nickel helps impro~e the contact quality and makes the
nickel layers 19 and 21 more receptive to solder or other contact
materials.
Another contemplated modification comprises the addition of
an aluminum backing to the rear of the solar cell. This
aluminum layer may be applied (by vacuum deposition or other well
known techniques) either before formation of the layer 11 on the
silicon substrate 9 or thereafter. Where application of the alum-
inum backing occurs beore formation of the layer 11, the layer 11
may be created by the aluminum deposition itself since
aluminum is a P-type dopant. Once the aluminum layer
has been applied, plating takes place as usual and the nickel
simply adheres to the aluminum. It should be noted that where
the nickel is deposited on aluminum, no sintering of the
aluminum-nickel junction will be required to form good ohmic
junctions. Thus, in situations where an aluminum backing is
used the sintering may be applied only to the front region
of the solar cell.
MT~-33 -12-
' ~ 3~
In making solar cells one may also subsiitu~e hydrofluoric
acid for the ammonium flouride in the bath. In such a case, the
bath may comprise nickel chloride (at 640 g/l) and hydro-
flouric acid (at 25 ml/l) at a pH of 2.9 and temperature
of 23C. Such a barh al~ernative will yield nickel plating
onto silicon in substan~ially the same manner as previously des-
cribed. However, it should be noted that such a ba~h composition
is not preferred in the manufacture of solar cells since the
hydrofluoric acid -tends to etch away the silicon o;.ide layer
15 which is used to mask the nickel deposition. Such etching
may result in undesired deposition which could deactivate
the cell via shor~ circuiting or lower the cell's efficiency
by covering over large portions of the solar collecting
portion of the cel~. Similar problems may arise in the manufactur~
of other semiconductor devices which may use a mask of silicon
oxides fox nickel deposition. In'addition, use of hydroflouric
acid in the bath tends to lead to problems in pH control. For
the'se reasons, the substitution of hydrofluOric acid for ammonium
fluoride in the bath is not preferred. Should the substitution
be preferred for plating a device which has a protective silicon
oxide coating on selected portions of one or more surfaces
thereof, the device is immersed in the bath long enough for
the plating to occur on the unprotected sur~ace areas but not long
enough to etch away the protective oxide layer(s).
There are many advantages to using the present invention in
place of electroless plaling. For one -thing, no catalyzing
pretreatment is needed to plate nickel onto silicon. ~n addition,
no nickel phosphides are produced along with nickel so as to
contaminate -the deposited metal. Furthermore, aluminum may be
used in the bath without undergoing extensive e~ching. Addi
tionally, the plating process uses a relatively uncomplicated
bath arrangement while yielding excellenl results.
r~T~-33 ~ -13-
` ~ 37
A further advantage of the invention is that it reduces
the need to form double depth junctions in solar cells due to
the reduced risk of the nickel diffusing through to the junction
and creating a short circuit. Other advantages and possible
modifications will be obvious to persons skilled in the art.
MTA-33 -14-
