JPS6354072B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates generally to plating methods and, more particularly, to a method for plating nickel on silicon.

In the manufacture of solar cells and other semiconductor devices, it is often preferable to plate nickel directly onto silicon to create electrodes or contacts for connecting semiconductor devices to electrical circuits. One known and effective method for obtaining such plating is by electroless nickel plating.

The first step in this process is generally to clean the external surface of the silicon support on which the nickel is to be deposited to remove any dirt or oxide particles that may be present. This cleaning is desirable because the presence of such materials on the silicon surface tends to prevent proper plating of the silicon onto the nickel. Cleaning can be carried out by various methods well known to those skilled in the art, for example, by sequentially immersing the support in a suitable bath of hot organic solvent and hot chromic sulfuric acid, followed by a bath of hydrofluoric acid, and rinsing with deionized water. . After this has been done, the surface of the silicon support to be nickel plated is pretreated with a catalyst. Pretreatment is necessary because the silicon surface itself is not amenable to electroless plating, and nickel plated onto an untreated silicon surface tends to have poor adhesion to silicon. Palladium is commonly used as a catalyst.
Other active agents are also well known to those skilled in the art. US Patent No.
No. 3,489,603 describes an alternative to this type of palladium catalyst.

Once the silicon surface receiving the nickel has been cleaned and pretreated, the silicon is ready for electroless plating. Plating is carried out by immersing the silicon support in a suitable acidic or alkaline bath under suitable conditions. A typical acid bath is nickel chloride (30g/), sodium hypophosphite (10g/)
g/), a third salt, sodium citrate (10 g/), pH 4-6, and a temperature of about 190㎓. In this type of bath, nickel chloride supplies the nickel ions to be reduced;
The sodium hypophosphite reducing agent is provided, and the third salt acts as a buffer and complexing agent for the nickel. Alternatively, an alkaline bath may be used instead. Such electroless nickel plating is well known in the art and is described in "Plating and Related Methods" by Mohler, Chemical Publishing Co., Ltd.
New York, 1969), Surface Preparation and Finishing of Metals, edited by James A. Murphy (Mack-Grow-Hill Books, New York, 1971), and Handbook of Thin Film Technology, edited by Leon I. Meisel and Reinhard Grang. Books, New York, 1970) and other publications.

This type of electroless plating has a number of drawbacks.
First, the need to pre-treat the silicon with a catalyst adds an additional step to the plating operation.
Furthermore, if the catalyst used is palladium, the nickel tends to precipitate in a non-uniform pattern since palladium tends to act non-uniformly on the silicon surface. The high cost of palladium is also a negative point, since considerable amounts of palladium are lost during pretreatment operations.

Furthermore, the chemistry of the electroless plating bath described above tends to produce some nickel phosphide.
This has the potential to precipitate on silicon together with nickel. When this nickel phosphide is mixed into plated nickel, the properties of the precipitated nickel tend to change, which may be extremely disadvantageous depending on the intended use. Although the phosphorus content in the precipitated nickel can be reduced by using an alkaline bath rather than an acid bath, the use of an alkaline bath can create new difficulties. In particular, alkaline baths tend to corrode away the aluminum exposed to the bath, thereby complicating the plating of the aluminum layer on silicon. Additionally, the use of alkaline baths promotes the formation of an oxide layer on the surface of the silicon, which tends to prevent nickel from being directly plated onto the silicon.

The presence of the oxide layer adds further complexity where it is desired to use the deposited nickel as an ohmic contact. In such cases, the nickel layer should be sintered and diffused into the silicon support to form a nickel silicide at the nickel/silicon interface. If there is an intervening oxide layer, the temperature is relatively high (above 350°C) so that the nickel penetrates the oxide layer and diffuses into the silicon, forming an ohmic contact.
sintering must be carried out at low temperature and/or for a relatively long period of time (more than 40 minutes). If the oxide support is a shallow junction device such as a solar cell and the nickel layer is on the support surface near the junction, especially if the oxide layer is very thin or absent,
Nickel tends to diffuse deep enough to shunt or short circuit the device. In this regard, a silicon element with a shallow junction is one in which the junction exists approximately 1.0 μm or less below the surface where the nickel is deposited.

Therefore, one of the objects of the present invention is to develop a method for directly plating nickel onto silicon without using the electroless nickel plating method described above. Another object is to provide a plating method that does not require any catalytic pretreatment of the silicon surface prior to plating with nickel. Another objective is to obtain a strong adhesion of nickel to silicon. Still another object is to provide a method for directly plating nickel onto silicon without producing nickel phosphide, which is a foreign matter. Yet another object is to provide a plating method using a bath that does not corrode away the aluminum exposed to the bath. Another object is to provide a plating method that does not require the use of expensive palladium. Yet another purpose is
It is an object of the present invention to provide a method for making nickel ohmic contacts on silicon devices in which sintering can be carried out at relatively low temperatures (below 350° C.) and in a relatively short period of time (within 40 minutes). Another object is to provide a simple and reliable method of making nickel ohm contacts with good or relatively good reproducibility of high performance contacts.

These and other objects of the invention are directed to the plating of nickel onto a silicon support, consisting essentially of immersing the support in an aqueous bath of nickel chloride and a selected fluorinated compound that ionizes in water. This is accomplished by providing a method whereby nickel ions in the bath are reduced to nickel without any pretreatment of the silicon support other than washing and deposited directly onto the silicon to form a deposited layer on the silicon. The ohmic contacts are formed by firing the nickel layer. For shallow bonded devices, sintering may occur below 350°C and be completed within 30 minutes. For deep junction devices, sintering may be performed at a temperature equal to or greater than that described above and/or for a time equal to or greater than that described above.

1-3 are cross-sectional views illustrating steps in a preferred method for plating nickel on a silicon support using the present invention.

4 to 6 are cross-sectional views showing each step in a method for manufacturing a solar cell using a preferred embodiment of the present invention.

FIG. 7 is a perspective view of a completed solar cell manufactured using a preferred embodiment of the present invention.

First of all, FIG. 1 shows a silicon support to be plated with nickel according to the invention. The support 1 generally consists of a substantially silicon body 3, with or without an outer layer 5 consisting of various particles of dirt and oxides thereon. If the body 3 actually has a layer 5 on its outer surface, it is desirable to remove the layer 5 from the areas where nickel on silicon is to be plated. Selective or complete removal of this outer layer can be accomplished by methods well known in the art. For example, complete removal of such a layer can be achieved by sequentially immersing the support in a hot organic solvent bath and a hot chromium sulfuric acid bath, then in a hydrofluoric acid bath, followed by rinsing with deionized water.

Once the undesired layer 5 is removed from the silicon body (FIG. 2 shows the silicon body without any outer layer 5),
The silicon support is now ready for nickel plating. For this purpose, the support is immersed in a bath consisting of nickel chloride and a selected fluorinated compound that ionizes in water. As a result of this immersion, the nickel ions in the bath (supplied by nickel chloride) are acted upon by the selected fluoride compound to form solid nickel through a displacement reaction. The substitution reaction appears to be as follows. Si゜＋2Ni ⁺² →
2Ni゜＋Si ⁺⁴ . Silicon is H ₂ SiF ₆ and/or
Moves into solution as H ₂ SiO ₃ . The nickel thus produced is deposited as an outer layer 7 on the silicon support (FIG. 3). While this ionic reduction occurs,
The chemical action of the bath simultaneously activates the surface of the silicon support so that the precipitated nickel adheres firmly to the silicon and forms an effective permanent plating on the silicon. If the plated nickel is to be an ohmic contact, after the nickel has been plated it is sintered to form a suitable nickel silicide.

It is preferred, but not necessary, that the selected fluorinated compound be ammonium fluoride. Nickel chloride 10-642g/and ammonium fluoride 10
Using a PH2-6 bath containing ~40g/
A satisfactory finish can be obtained. In a preferred embodiment, the aqueous bath contains nickel chloride (640 g/)
and ammonium fluoride (40g/), and has a pH level of 4. Good results can be obtained by employing a more preferable bath mode and performing the testing at a temperature of 17 to 100°C. However, 20~30℃
A temperature of is preferred.

Good results are also obtained when plating is carried out using alternative fluorinated compounds such as hydrofluoric acid and NH ₄ F:HF. For hydrofluoric acid, the appropriate bath composition is nickel chloride (200-600g/
) and hydrofluoric acid (25-50 mls/), with a pH of 2.6-2.8 and a temperature of 23°C.
In the case of NH ₄ F:HF, a suitable bath would consist of nickel chloride (300 g/) and NH ₄ F:HF (20 g/) at a pH of 3.2 and a temperature of 23°C. Other temperatures within the range of 17-100°C may also be employed for such baths.

The rate of nickel precipitation generally depends on the particular fluorinated compound in the bath, the PH level, the temperature of the bath, and the amount of impurities in the silicon. Additionally, the concentration of bath components and the surface area of the support to be plated will also affect the deposition rate. Generally, the rate of precipitation increases as the PH level increases, and the rate of precipitation increases as the temperature increases. Good results have been obtained for plating using both P-type and N-type silicon, but the plating speed depends on the dopant used.
Depends on type and concentration. Generally, the deposition rate increases as the impurity concentration in the silicon increases. For example, the above preferred bath composition, PH
and temperature, a 2100 Å thick layer of nickel was deposited by dipping the silicon in the bath for 2 minutes.
- type silicon will precipitate. For devices with shallow junctions, it is preferred that the nickel layer is deposited to a thickness of 500 to 2500 Å. Equal or greater thicknesses may be used for devices with no junctions or deep junctions (ie, 1 micron or more). A thickness of about 1500 Å is most preferred for shallow junction devices.

Bath life is inversely related to deposition rate. The plating ability of a bath is related to the concentration level of chemicals in the bath. Bath 11, consisting of nickel chloride (640 g/) and ammonium fluoride (40 g/), plated over 7000 cm ² of silicon to an average thickness of 2000 Å without the need to regenerate the bath. Regeneration, or reproduction of the original concentration, can be easily accomplished by adding nickel chloride and ammonium fluoride to the bath.

It will be appreciated by those skilled in the art that the plating method of the present invention is useful in all areas of semiconductor devices, particularly solar cells, and semiconductor devices that involve large amounts of power and require solderable pads, such as SCRs. would be easily recognized. In this regard, although the manufacture of solar cells using the present invention is described below, the selection of solar cells as the manufactured product is by way of example only and not limitation.

4 to 7 illustrate the manufacture of solar cells using the nickel plating method described above. First, regarding Figure 4,
A semiconductor body is shown manufactured according to the techniques described and shown in U.S. Pat. No. 4,152,824. The body 8 generally consists of a silicon support made of P-type silicon with a thickness >-2.5x10 ^-3 cm. Region 11 is formed on the back side of the support and consists of P-type silicon additionally coated with a P-type dopant to form a high conductivity P+ region. Region 13 is provided on the front side of the support coated with a sufficient amount of N- type dopant to form a high conductivity N+ region. A top layer 15 formed of silicon dioxide covers region 13. Layer 15 is perforated at defined locations to expose layer 13 as shown. Layer 13 extends to a depth directly below opening 17 in these parts. Layer 15 is preferably formed to a depth of 5000 Å, while layer 11 has a depth of >-10000 Å. Furthermore, the layer 13 has a depth of >-5000 Å in the part immediately below the opening 17 and <-3500° in other parts.
It is preferable to have a depth of Å.

Next, semiconductor 8 is nickel chloride (640g/)
and ammonium fluoride (40 g/) at a temperature of 25° C. and a pH of 4.6 for about 1 minute.
As a result, a nickel layer 19 is deposited on the silicon surface 13 in the vicinity of the opening 17, and a nickel layer 21 is deposited on the back side of the silicon support. Please refer to FIG. 5. However, at the same time, almost no nickel is deposited on silicon dioxide. Nickel layer 19
and 21 consists of substantially pure nickel;
It has a depth of approximately 850 Å. Furthermore, the silicon dioxide on the side end faces (not shown) of the silicon body is layer 1.
9 is prevented from coming into direct contact with layer 21, thereby avoiding short circuits in the finished solar cell. In FIG. 7, layer 19 is grid-like, leaving part of the solar cell area 13 as shown at 25. In FIG.
These are exposed to receive solar energy.

After plating has taken place, semiconductor 8 is rinsed in deionized water and dried. This serves to remove any free nickel or salts that may be present on the semiconductor body. The body 8 is then sintered in a nitrogen or hydrogen atmosphere to form a nickel silicide layer 23 at the junction of the nickel and silicon regions.
(Figure 6). The nickel silicide layer allows the nickel-to-silicon junction to form an ohmic junction rather than the rectifying contact often obtained when substantially pure nickel is in direct contact with substantially pure silicon. The production of a nickel silicide layer 23 is desirable because it is guaranteed. When nickel is used as an electrode or contact,
Ohmic contacts are required. Sintering at temperatures of 250-350 °C promotes the formation of _Ni2Si , while 350-760
Sintering at a temperature of °C will promote the formation of NiSi. Sintering at temperatures above 760℃
It will promote the formation of _NiSi2 . For devices with shallow junctions, it is desirable to employ the lowest nickel silicide formation temperature actually used. This is because the diffusion of nickel into silicon decreases with decreasing temperature. Further, the nickel silicide compound Ni ₂ Si is preferred over the other two silicides. Ni ₂ Si tends to consume less silicon per molecule than the other two silicides, thus making the solar cell inoperable due to penetration of the nickel silicide layer 23 through the N+ silicon region 13. This is because the possibility becomes smaller. For shallow bonded devices, sintering is performed at 250-350° C. for about 15-40 minutes. Temperatures in the range of 250-350° C. and about 30 minutes are required to obtain a nickel silicide layer of Ni ₂ Si with a depth of about 300 Å, which is preferred for shallow junction devices such as solar cells. For deep junction devices or non-junction devices, sintering is performed at the same temperature and for the same period of time as the shallow junction devices, or at a substantially higher temperature and/or for a substantially longer period of time, and the silicide NiSi and NiSi ₂ (with or without the formation of some Ni ₂ Si).

Once sintered, semiconductor 8 is immersed in a dilute (10%) hydrofluoric acid bath to corrode away silicon oxide layer 15, resulting in the finished solar cell shown in FIG. 7. This solar cell can then be used to generate electricity in a manner well known to those skilled in the art.

It will be readily appreciated that various modifications and additions to the above method may be made without changing the essence of the invention. For example, the semiconductor shown in FIG. 6 (showing semiconductor 8 immediately after sintering but before removal of silicon oxide layer 15) may be re-immersed in a nickel plating bath to deposit additional amounts of nickel on nickel layers 19 and 21. can be plated. This may be desirable since the sintering process often tends to make the nickel layers 19 and 21 more porous and thus less suitable as contacts. This additional amount of nickel deposit improves the performance of the contacts and helps make the nickel layers 19 and 21 more receptive to solder and other adhesive materials.

Another envisaged variation consists of adding an aluminum lining to the back side of the solar cell. This aluminum layer can be applied (by vacuum evaporation or other well-known techniques) onto the silicon support 9 before the formation of layer 11 or after the formation of layer 11. If the aluminum backing is applied before forming layer 11, layer 11 can also be formed by aluminum precipitation since aluminum is a P-type dopant. After the aluminum has been applied, plating is common and nickel easily adheres to the aluminum. It should be noted that where the nickel is deposited on the aluminum, it is not necessary to sinter the aluminum-nickel joint to form a good ohmic contact. Therefore, if an aluminum lining is employed, sintering can only be applied to the front part of the solar cell.

Hydrofluoric acid may be used in place of ammonium fluoride in the bath in the manufacture of solar cells. In this case, the bath is nickel chloride (640g/)
and hydrofluoric acid (25ml/), PH
2.9 and 23°C. This alternative bath provides nickel plating on silicon in substantially the same manner as described above. However, hydrofluoric acid is a silicon oxide layer 1 used to shield nickel precipitation.
It should be noted that the bath compositions described above are not preferred for solar cell production because of their tendency to corrode away 5. Such corrosion can result in undesirable deposits that can inactivate the cell by shorting or reduce the efficiency of the cell by covering a large portion of the cell's light collection area. Similar problems may occur in the manufacture of other semiconductor devices that may use silicon oxide shielding for nickel deposition. Furthermore, the use of hydrofluoric acid in the bath tends to cause problems in PH control. because of these reasons,
It is not preferred to use hydrofluoric acid instead of ammonium fluoride. If it is necessary to employ this alternative chemical to plate a protective silicon oxide element on a specific portion of one or more surfaces of the device, the plating may occur on unprotected surface areas. The device is immersed in the bath for a period of time sufficient for this to occur, but not long enough to cause erosion of the protective oxide layer.

There are many advantages to using the present invention in place of electroless plating. First, there is no need for catalyst pretreatment to plate nickel on silicon. Further, nickel phosphide is not produced together with nickel to contaminate the deposited metal. Furthermore, extensive corrosion does not occur when aluminum is used in the bath. Furthermore, this plating method provides excellent results while using less complex bath formulations.

A further advantage of the invention is that there is less need to create double junctions in the solar cell since there is less risk of nickel diffusing through the contacts and forming short circuits. Other advantages and possible modifications will be apparent to those skilled in the art.

[Brief explanation of the drawing]

1-3 are cross-sectional views illustrating steps in a preferred method for plating nickel on a silicon support using the present invention. 4 to 6 are cross-sectional views showing each step in a method of manufacturing a solar cell using a preferred embodiment of the present invention. FIG. 7 is a perspective view of a completed solar cell manufactured using a preferred embodiment of the present invention. 1: Support, 3: Silicon main body, 5: Outer layer, 7:
Outer layer, 8: semiconductor body, 11: P- type silicon region, 13: N+ region, 15: top layer, 17: opening, 19: nickel layer, 21: nickel layer, 2
3: Nickel silicide layer.

Claims (1)

[Claims] 1. Plating nickel onto a silicon body having first and second faces facing in opposite directions for use in forming photovoltaic solar cells or other semiconductor devices. (a) plating an aluminum coating on the second surface; (b) containing a mixture of nickel chloride and ammonium fluoride or ammonium fluoride and hydrofluoric acid; (c) the nickel ions in the aqueous bath are converted to solid nickel and deposited as a deposited layer on the silicon body; A method comprising: retaining the silicon body in the aqueous plating bath for deposition; and (d) removing the silicon body from the aqueous plating bath. 2. Claim 1, wherein said bath is an aqueous bath of substantially ammonium fluoride and nickel chloride.
The method described in section. 3. said bath contains nickel chloride at a concentration of 640 g/l and ammonium fluoride at a concentration of 40 g/l;
The method according to claim 2. 4. The method according to any one of claims 1 to 3, wherein the bath has a pH of 4. 5. The method according to any one of claims 1 to 4, wherein the temperature of the bath is 20 to 30°C. 6 the silicon body has a first surface area coated with silicon oxide and a second surface area free of silicon oxide, whereby the solid nickel is coated on the second surface area free of silicon oxide; 6. The method according to any one of claims 1 to 5, wherein the method comprises depositing and adhering only. 7. Any one of claims 1 to 6 including a subsequent step of sintering the deposited nickel layer to form a nickel silicide bond between the silicon body and the deposited nickel layer. Method described. 8. The method according to claim 7, wherein the sintering is performed in a nitrogen or hydrogen atmosphere. 9. A method as claimed in claim 7, characterized in that said sintering is carried out at a temperature of 250-350<0>C to promote the formation of _Ni2Si . 10. The method of claim 7, comprising carrying out said sintering at a temperature of 350 to 760<0>C to promote the formation of 10 NiSi. 11. The method of claim 7, comprising carrying out said sintering at a temperature above 760° C. to promote the formation of NiSi ₂ . 12. The method of claim 7, comprising re-immersing the silicon body in the bath after the sintering is completed to deposit additional nickel on the nickel layer on the silicon body. 13. The method of claim 1, wherein the bath comprises ammonium fluoride and hydrofluoric acid. 14. The method according to claim 13, wherein the bath has a pH of 3.2. 15. The method according to claim 13, wherein the temperature of the bath is 17 to 100°C. 16. The method of any of claims 1-15, wherein the silicon body has a shallow P/N junction adjacent the first surface region. 17. The method of claim 16, wherein the bath comprises nickel chloride in a concentration of 200 to 600 g/l and hydrofluoric acid in a concentration of 25 to 50 g/l. 18. The method according to claim 17, wherein the temperature of the bath is 17 to 100°C. 19 The silicon body has a surface area coated with silicon oxide and a surface area free of silicon oxide before dipping, and the dipping includes a surface area coated with silicon oxide and a silicon oxide free surface area immersed in the bath. 17. The method of claim 16, comprising continuing for a period of time sufficient to cause burning, but not long enough to cause corrosion of the silicon oxide film in the bath. 20. The method of claim 19, wherein the temperature of the bath is 23°C. 21 A method of manufacturing a photovoltaic solar cell or other semiconductor device comprising: (a) first and second surfaces having a first type of conductivity and facing in opposite directions; (b) forming a coating of aluminum on the second surface; (c) dissolving the silicon semiconductor body in nickel chloride and water; (d) immersing the silicon body in an aqueous bath containing an ionizable fluoride compound and free of chemical reducing agents for a period of time sufficient to deposit a nickel layer on at least a portion of the first surface; retaining the semiconductor body in the bath; (e) removing the silicon body from the bath; and (f) depositing the deposited nickel to form a nickel silicide bond between the silicon body and the deposited nickel layer. A method consisting of sintering layers. 22 further comprising the step of coating the first surface with an electrically insulating coating, such that the first surface is covered with the electrically insulating coating.
and a second region that does not include the electrically insulating coating.
22. The method of claim 21, wherein the second top area forms a grid-like pattern on the top area. 23. The method of claim 22, further comprising the step of removing the insulating layer from the first surface after the nickel is deposited. 24. The method according to any one of claims 21 to 23, wherein the fluorinated compound is ammonium fluoride. 25. A method according to any of claims 21 to 23, wherein the aqueous bath comprises ammonium fluoride and nickel chloride. 26. A method according to any one of claims 21 to 25, wherein the bath comprises nickel chloride at a concentration of 640 g/l and ammonium fluoride at a concentration of 40 g/l. 27. The method according to any one of claims 21 to 26, wherein the bath has a pH of 4.5. 28. A method according to any one of claims 21 to 27, wherein the temperature of the bath is 23°C. 29. A method according to any of claims 21 to 28, wherein the top region has a depth of 5000 Å. 30. A method according to any of claims 21 to 29, comprising immersing the silicon body in the bath for a period sufficient to form a nickel layer having a thickness of 1500 Å. . 31 The sintering is carried out at a temperature of 250 to 350°C,
a period of time sufficient to form a nickel silicide bond, but not so long as to cause the nickel to penetrate through the top region and into the silicon body of the first conductivity type. A method according to any one of claims 21 to 30. 32. The method of claim 31, further comprising re-immersing the semiconductor body in the bath after the sintering to deposit additional nickel on the nickel layer on the semiconductor body. 33. Claim 21, wherein additional nickel is plated onto the sintered nickel layer by immersing the sintered nickel layer in an aqueous bath consisting of nickel chloride and ammonium fluoride. The method according to any one of items 32 to 32. 34. The method of claim 33, wherein said bath comprises nickel chloride at a concentration of 10 to 640 g/l and ammonium fluoride at a concentration of 10 to 40 g/l. 35 The bath has a pH of 2 to 6 and a temperature of 17 to
35. The method according to claim 34, wherein the temperature is 100°C. 36. A method according to any of claims 21 to 35, wherein the silicon body is maintained in the bath until the thickness of the nickel layer reaches between 500 and 2500 Å.