CA1178137A - Nickel plating method - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE:
A simple method for plating nickel onto silicon which renders unnecessary any catalyzing pretreatment of the silicon surface which is to receive the nickel. The method comprises the immersion of a silicon substrate in a suitable nickel bath in order that nickel ions in the bath will be reduced to solid nickel and deposited onto the substrate so as to form an adhering layer thereon. The method is especially advantageous in plating nickel onto silicon shallow junction devices for the purpose of providing ohmic contacts.

Description

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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.

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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 , .

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 , '', .~!/
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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;

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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.

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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.

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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~
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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 ~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.

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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.

Claims (42)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for plating nickel onto a silicon body wherein said method comprises (a) immersing said silicon body into an aqueous bath comprising nickel chloride and ammonium fluoride or ammonium fluoride and hydrofluoric acid, (b) maintaining said silicon body in said bath so that nickel ions in said bath will be converted to solid nickel and deposited onto said silicon body as an adhering layer thereon, and (c) withdrawing said silicon body from said bath.

2. A method according to claim 1 wherein said bath essentially comprises ammonium fluoride and nickel chloride.

3, A method according to claim 2 wherein said bath is comprised of nickel chloride at a concentration of approximately 640 g/l and ammonium fluoride at a concentration of approximately 40 g/l.

4. A method according to claim 3 wherein said bath has a pH of approximately 4.

5. A method according to claim 3 wherein said bath has a temperature of between about 20 degrees C. and 30 degrees C.

6. A method according to claim 1 wherein a layer of aluminum is applied to one side of said silicon body before said silicon body is immersed in said aqueous bath, whereby solid nickel will be deposited onto said aluminum layer.

7. A method according to claim 2 wherein said silicon body has surface regions which are coated with a silicon oxide and surface regions which are free of silicon oxide, whereby said solid will be adhesively deposited only on those surface regions which are free of silicon oxide.

8. A method according to claim 1 including the subsequent step of sintering said adhering nickel layer so as to create a nickel silicide junction between said silicon body and said adhering nickel layer.

9. A method according to claim 8 wherein sintering takes place in a nitrogen or hydrogen atmosphere.

10. A method according to claim a wherein sintering is conducted at temperatures of between 250 degrees and 350 degrees C. 50 as to promote the formation of Ni2Si.

11. A method according to claim 8 wherein sintering is conducted at temperatures of between 350 degrees and 760 degrees C. so as to promote the formation of NiSi.

12. A method according to claim 8 wherein sintering is conducted at temperatures in excess of 760 degrees C. so as to promote the formation of NiSi2.

13. A method according to claim 8 wherein said silicon body is reimmersed in said bath after sintering is complete in order to adhesively deposit additional nickel onto the nickel layer on said silicon body.

14. A method according to claim 1 wherein said bath comprises ammonium fluoride and hydrofluoric acid.

15. A method according to claim 14 wherein said bath has a pH of approximately 3.2.

16. A method according to claim 14 wherein said bath has a temperature of b between about 17 degrees C. and 100 degrees C.

17. A method according to claim 14 wherein prior to immersion said silicon body has surface regions which are coated with silicon oxide and surface regions which are of silicon oxide, and further wherein immersion is continued long enough for plating to occur on those regions which enter the bath free of silicon oxide but not long enough for the bath to etch away the silicon oxide coating.

18. A method according to claim 1 wherein said bath is completed of nickel chloride at a concentration of opproximately 200-600 g/l and hydrofluoric acid at a concentration of approximately 25-50 g/l.

19. A method according to claim 18 wherein said bath has a temperature of between about 17 degrees C. and 100 degrees C.

20. A method according to claim 18 wherein prior to immersion said silicon body has surface regions which are coated with silicon oxide and surface regions which are free of silicon oxide, and further wherein immersion is continued long enough for plating to occur on those regions which enter the bath free of silicon oxide but not long enough far the bath to etch away the silicon oxide coating.

21. A method according to claim 20 wherein said bath has a temperature of approximately 23 degrees C.

22 . A method of making a photovoltaic semiconductor solar cell comprising:
(1) forming a silicon semiconductor body of a first conductivity type having a tap region of a second conductivity type, said 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, said second top surface regions forming a grid pattern on said top region;
(2) immersing said semiconductor body in an aqueous bath of nickel chloride and a fluoride compound which ionizes in water;
(3) maintaining said semiconductor body in said bath long enough for a layer of nickel to be adhesively deposited on those second surface regions;
(4) withdrawing said semiconductor body from said bath;
(5) washing said semiconductor body with deionized water so as to remove any loose particles from said semiconductor body;

(6) sintering said adhering nickel layer so as to create a nickel silicide junction between said silicon body and said adhering nickel layer: and (7) removing said silicon oxide from said first surface regions of said semiconductor body.

23. A method according to claim 22 wherein said fluoride compound is ammonium fluoride.

24. A method according to claim 23 wherein said bath is comprised of nickel chloride at a concentration of approximately 640 g/l and ammonium fluoride at a concentration of approximately 40 g/l.

25. A method according to claim 23 wherein said bath has a pH of approximately 4.5.

26. A method according to claim 23 wherein said bath has a temperature of approximately 23 degrees C.

27. A method according to claim 22 wherein said top region has a depth of approximately 5000 Angstroms.

28. A method according to claim 22 wherein said body is immersed in said bath long enough for a layer of nickel approximately 1500 Angstroms thick to be formed.

29. A method according to claim 22 wherein said silicon body has a bottom region characterized by a bottom surface, and further including the step of applying an adherent layer of aluminum to said bottom surface before said body is immersed in said aqueous bath.

30. A method according to claim 22 wherein sintering is conducted at a temperature of approximately 250 degrees C.
to 350 degrees C. and for a period of time sufficient to create a nickel silicide junction but not so long as to cause said top region to be penetrated through to said silicon substrate.

31. A method according to claim 22 wherein said semiconductor body is reimmersed in said bath after sintering so as to deposit additional nickel onto the nickel layer on said semiconductor body.

32. A method according to claim a wherein said aqueous bath comprises hydrofluoric acid.

33. A method according to claim 22 wherein said aqueous bath comprises ammonium fluoride and hydrofluoric acid.

34. A method for forming an ohmic nickel contact on a silicon body comprising the steps o-F: (a) immersing said silicon body into an aqueous bath of nickel chloride and fluoride compound that ionizes in water, (b) maintaining said silicon body in said bath so that nickel ions in said bath will be converted to solid nickel and deposited onto said silicon body as an adhering layer thereon, (c) withdrawing said silicon body from said bath and (d) sintering said adhering layer so as to produce a nickel silicide at the junction of said silicon body and said adhering layer of nickel.

35. A method according to claim 34 further including the step of rinsing said silicon body after it is removed from said bath and before said adhering layer is sintered.

36. A method according to claim 34 further including the step of plating additional nickel onto the sintered nickel layer.

37. A method according to claim 36 wherein said additional nickel is plated onto said sintered nickel layer by immersing said sintered nickel layer into an aqueous bath of nickel chloride and a fluoride compound that ionizes in water.

38. A method according to claim 37 wherein said fluoride compound is ammonium fluoride.

39. A method according to claim 38 wherein said bath is comprised of nickel chloride at a concentration of between 10 and 640 g/l and ammonium fluoride at a concentration of between 10 and 40 g/l.

40. A method according to claim 39 wherein said bath has a pH of between 2 and 6 and a temperature between 17 degrees C. and 100 degrees C.

41. A method according to claim 40 wherein said silicon body is maintained in said bathe until the nickel layer has thickness of between about 500 and 2500 Angstroms.

42. A method for plating nickel onto a silicon body wherein said method comprises (a) immersing said silicon body into an aqueous bath of nickel chloride and hydrofluoric acid with a pH of 2.6-2.8, (b) maintaining said silicon body in said bath so that nickel ions in said bath will be converted to solid nickel and deposited onto said silicon body as an adhering layer thereon, and (c) withdrawing said silicon body from said bath after said layer has reached a thickness of between about 500 and 2500 Angstroms.