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8 III-Nitride LED Chip Fabrication Techniques
Although the principle of the evaporator has been mastered in the country, and the
laboratory evaporator can be manufactured in China, it has not entered the large
production line due to challenges such as automation, process repeatability and
uniformity. In view of the fact that the electrodeposition for LED electrode deposition
is less difficult than the semiconductor PVD device, the domestic equipment manufacturer with experience in semiconductor PVD equipment will be able to realize the
localization of the LED vapor deposition station.
Since the multilayer metal material used for the LED chip electrode is not easy
to form an electrode pattern thereon by etching, it is necessary to form a metal
electrode pattern by a lift-off process. First, a layer of photoresist is coated on the
LED chip, and then photolithography is performed to form a desired pattern. The
metal electrode material is then evaporated, and finally the residual photoresist and
the metal deposited on the photoresist are deposited. The photoresist is removed to
obtain an electrode pattern. There are two methods to remove positive and negative photoresists. When using positive photoresist stripping, a layer of polymer film
that is insensitive to ultraviolet light and can be corroded by an alkaline solution is
applied to the substrate before coating, followed by spin coating a thicker lithography on the polymer film. When the photoresist is developed by using an alkaline
solution, the bottom of the photoresist is undercut due to the corrosive action of the
alkaline solution on the polymer film, thereby facilitating the stripping process. After
development, a metal layer is evaporated on the photoresist. Since the evaporation
process does not have a strong covering ability to the step, the vapor-deposited metal
film naturally breaks at the edge of the undercut photoresist. Finally, the solution is
used to dissolve the photoresist, resulting in the natural stripping of the metal film
deposited on the photoresist layer. The metal film deposited on the substrate is the
pattern of the electrode. In the manufacturing process of the metal pads of the LED
chip where the metal electrode adopts the negative photoresist stripping process, the
undercut is naturally formed during the exposure due to the characteristics of the
negative photoresist itself. The shape thus facilitates the formation of the peeling.
8.4.2 SiO 2 Passivation Layer
Since dust, moisture, and various impurity ions may adhere to the surface of the
LED chip, the number of external electrons at the surface of the chip is not saturated.
Dangling bonds, which form surface state energy level in the bandgap, can thereby
cause leakage. Moreover, these dangling bonds are highly active and easily adsorb
other molecules, atoms and ions. When the adsorbed impurities are ionized, a current
path is formed. The leakage problem caused by such surface contamination can be
solved by making an insulating protective layer on the surface and sidewall of the
LED chip. The insulating layer materials commonly used in the LED chip fabrication process are mainly silicon dioxide (SiO 2 ) and silicon nitride (Si x N y ) materials.
Usually PECVD deposition of SiO 2 insulating layer on the LED and sidewalls of the
chip is used to improve LED chip reliability.
8 III-Nitride LED Chip Fabrication Techniques
Although the principle of the evaporator has been mastered in the country, and the
laboratory evaporator can be manufactured in China, it has not entered the large
production line due to challenges such as automation, process repeatability and
uniformity. In view of the fact that the electrodeposition for LED electrode deposition
is less difficult than the semiconductor PVD device, the domestic equipment manufacturer with experience in semiconductor PVD equipment will be able to realize the
localization of the LED vapor deposition station.
Since the multilayer metal material used for the LED chip electrode is not easy
to form an electrode pattern thereon by etching, it is necessary to form a metal
electrode pattern by a lift-off process. First, a layer of photoresist is coated on the
LED chip, and then photolithography is performed to form a desired pattern. The
metal electrode material is then evaporated, and finally the residual photoresist and
the metal deposited on the photoresist are deposited. The photoresist is removed to
obtain an electrode pattern. There are two methods to remove positive and negative photoresists. When using positive photoresist stripping, a layer of polymer film
that is insensitive to ultraviolet light and can be corroded by an alkaline solution is
applied to the substrate before coating, followed by spin coating a thicker lithography on the polymer film. When the photoresist is developed by using an alkaline
solution, the bottom of the photoresist is undercut due to the corrosive action of the
alkaline solution on the polymer film, thereby facilitating the stripping process. After
development, a metal layer is evaporated on the photoresist. Since the evaporation
process does not have a strong covering ability to the step, the vapor-deposited metal
film naturally breaks at the edge of the undercut photoresist. Finally, the solution is
used to dissolve the photoresist, resulting in the natural stripping of the metal film
deposited on the photoresist layer. The metal film deposited on the substrate is the
pattern of the electrode. In the manufacturing process of the metal pads of the LED
chip where the metal electrode adopts the negative photoresist stripping process, the
undercut is naturally formed during the exposure due to the characteristics of the
negative photoresist itself. The shape thus facilitates the formation of the peeling.
8.4.2 SiO 2 Passivation Layer
Since dust, moisture, and various impurity ions may adhere to the surface of the
LED chip, the number of external electrons at the surface of the chip is not saturated.
Dangling bonds, which form surface state energy level in the bandgap, can thereby
cause leakage. Moreover, these dangling bonds are highly active and easily adsorb
other molecules, atoms and ions. When the adsorbed impurities are ionized, a current
path is formed. The leakage problem caused by such surface contamination can be
solved by making an insulating protective layer on the surface and sidewall of the
LED chip. The insulating layer materials commonly used in the LED chip fabrication process are mainly silicon dioxide (SiO 2 ) and silicon nitride (Si x N y ) materials.
Usually PECVD deposition of SiO 2 insulating layer on the LED and sidewalls of the
chip is used to improve LED chip reliability.