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8 III-Nitride LED Chip Fabrication Techniques
8.7.1 Electroplating Technology
In a gallium nitride based vertical structure LED, a copper or copper alloy substrate is
generally fabricated by electroplating. This is because the copper plating technology
is relatively mature. The cost of copper consumables and plating solution is relatively low. The thermal conductivity and electrical conductivity of copper are ideal.
However, the use of pure metallic copper as a substrate also causes a series of problems such as thermal mismatch with gallium nitride materials, low hardness, prone
to plastic deformation, and difficult in substrate cutting. It is suggested that a copper
alloy plating with a certain composition can obtain an ideal substrate. For instance,
copper-tungsten alloy is a hard and brittle alloys. If a suitable composition can be
obtained, such an alloy could match well with the thermal expansion coefficient of
GaN. However, there is no copper-tungsten plating technology yet.
The electroplating copper process applied to the transfer substrate is discussed
below. Common copper plating solutions can be divided into cyanide plating solution and non-cyanide plating solution. The non-cyanide plating solution includes
acid copper plating solution, pyrophosphate copper plating solution, citrate copper
plating solution, tartrate copper plating solution, HEDP copper plating solution,
triacetic acid copper plating solution, diethylamine copper plating solution, and
fluorine borate copper plating solution, etc. The cyanide copper plating solution
has the characteristics of good uniformity, fine crystallizing layer and good adhesion to the seeding metal. However, the cyanide copper plating solution has strong
toxicity, and it is necessary to consider the treatment of waste water and exhaust
gas. Copper in the solution is present as a monovalent copper cyanide complex. It
is generally believed that there are mainly three forms of copper cyanide coordination compounds: [Cu(CN) 2 ]
− , [Cu(CN) 3 ]
2− , [Cu(CN) 4 ]
3− . At the cathode, monovalent copper ions are reduced and deposited by discharge. The other side reaction is
hydrogen ion reduction to hydrogen that moves out as gas.
Cu
+
+ e = Cu
(8.5)
2H
+
+ 2e = H 2 ↑
(8.6)
The anode copper dissolves into monovalent copper ions. I the anode is passivated,
oxygen can move out.
Cu − e = Cu
+
(8.7)
4OH
−
− 4e = 2H 2 O + O 2 ↑
(8.8)
The cyanide copper plating solution should be placed under ventilated conditions.
Masks and rubber gloves must also be worn during operation. The plating solution
8 III-Nitride LED Chip Fabrication Techniques
8.7.1 Electroplating Technology
In a gallium nitride based vertical structure LED, a copper or copper alloy substrate is
generally fabricated by electroplating. This is because the copper plating technology
is relatively mature. The cost of copper consumables and plating solution is relatively low. The thermal conductivity and electrical conductivity of copper are ideal.
However, the use of pure metallic copper as a substrate also causes a series of problems such as thermal mismatch with gallium nitride materials, low hardness, prone
to plastic deformation, and difficult in substrate cutting. It is suggested that a copper
alloy plating with a certain composition can obtain an ideal substrate. For instance,
copper-tungsten alloy is a hard and brittle alloys. If a suitable composition can be
obtained, such an alloy could match well with the thermal expansion coefficient of
GaN. However, there is no copper-tungsten plating technology yet.
The electroplating copper process applied to the transfer substrate is discussed
below. Common copper plating solutions can be divided into cyanide plating solution and non-cyanide plating solution. The non-cyanide plating solution includes
acid copper plating solution, pyrophosphate copper plating solution, citrate copper
plating solution, tartrate copper plating solution, HEDP copper plating solution,
triacetic acid copper plating solution, diethylamine copper plating solution, and
fluorine borate copper plating solution, etc. The cyanide copper plating solution
has the characteristics of good uniformity, fine crystallizing layer and good adhesion to the seeding metal. However, the cyanide copper plating solution has strong
toxicity, and it is necessary to consider the treatment of waste water and exhaust
gas. Copper in the solution is present as a monovalent copper cyanide complex. It
is generally believed that there are mainly three forms of copper cyanide coordination compounds: [Cu(CN) 2 ]
− , [Cu(CN) 3 ]
2− , [Cu(CN) 4 ]
3− . At the cathode, monovalent copper ions are reduced and deposited by discharge. The other side reaction is
hydrogen ion reduction to hydrogen that moves out as gas.
Cu
+
+ e = Cu
(8.5)
2H
+
+ 2e = H 2 ↑
(8.6)
The anode copper dissolves into monovalent copper ions. I the anode is passivated,
oxygen can move out.
Cu − e = Cu
+
(8.7)
4OH
−
− 4e = 2H 2 O + O 2 ↑
(8.8)
The cyanide copper plating solution should be placed under ventilated conditions.
Masks and rubber gloves must also be worn during operation. The plating solution