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8 III-Nitride LED Chip Fabrication Techniques
non-optical lithography machine uses light sources that comes from other components of the electromagnetic spectrum (X-rays, electron beams, etc.). Exposure equipment typically consists of two systems for alignment and exposure: one is to accurately position the pattern on the surface of the wafer (different alignment systems of
different alignment types); the other is the exposure system (including an exposure
source) and a radiation beam directed onto the wafer. The performance of the alignment machine is reflected in the resolution and registration capabilities. Resolution
is the ability of a machine to produce a specific size. The higher the resolution, the
better the performance of the machine is. Registration capability is the ability to
accurately locate graphics. Light for exposure must pass mirrors and lenses, so it
is converted into parallel light beam so as to ensure the required feature size. The
most widely used exposure source is a high-pressure mercury lamp which produces
ultraviolet light (UV). To achieve higher resolution, the photoresist is designed to
react with only a narrow wavelength of light in the spectrum of the mercury lamp
(Called the deep ultraviolet region or DUV).
• Development
The development of the photoresist is to dissolve the soluble region of the photoresist by a chemical agent so called developer. The main purpose of the photoresist
is to accurately transfer the mask pattern into the photoresist. There are generally
three steps in this process: development, rinsing, and drying. Common developing
solutions are NaOH (Shipley 351), KOH (Shipley 606), TMAH (Shipley CD-26,
MF-321, OCG 945) and so on.
• Post-bake
When the light is irradiated onto the interface between the photoresist and the wafer,
a part of the reflected light and the incident light are superimposed to form a standing
wave. Post-baking partially eliminates this effect. At the same time, the photoresist
side wall can be smoothened to improve the resolution.
• Hard-bake
The hard bake process is to make the film adhere to the surface of the wafer more
firmly by heating and baking. Such a process can increase the etching resistance of
the photoresist. Hard bake is not a necessary process. This step is to improve the
stability in the etching process. It can also improve the adhesion of the photoresist
to the wafer surface, which is beneficial to the subsequent wet etching process.
It can also improve the pinholes present in the photoresist. Of course, there are
certain drawbacks resulted from this step: it may cause the photoresist to reflow,
which will reduce the accuracy of the pattern transferred. Also, it will increase the
difficulty of removing the photoresist. The post-baking time is therefore critical. For
a given photoresist, the hard-bake temperature is usually higher than the pre-bake
temperature.
8 III-Nitride LED Chip Fabrication Techniques
non-optical lithography machine uses light sources that comes from other components of the electromagnetic spectrum (X-rays, electron beams, etc.). Exposure equipment typically consists of two systems for alignment and exposure: one is to accurately position the pattern on the surface of the wafer (different alignment systems of
different alignment types); the other is the exposure system (including an exposure
source) and a radiation beam directed onto the wafer. The performance of the alignment machine is reflected in the resolution and registration capabilities. Resolution
is the ability of a machine to produce a specific size. The higher the resolution, the
better the performance of the machine is. Registration capability is the ability to
accurately locate graphics. Light for exposure must pass mirrors and lenses, so it
is converted into parallel light beam so as to ensure the required feature size. The
most widely used exposure source is a high-pressure mercury lamp which produces
ultraviolet light (UV). To achieve higher resolution, the photoresist is designed to
react with only a narrow wavelength of light in the spectrum of the mercury lamp
(Called the deep ultraviolet region or DUV).
• Development
The development of the photoresist is to dissolve the soluble region of the photoresist by a chemical agent so called developer. The main purpose of the photoresist
is to accurately transfer the mask pattern into the photoresist. There are generally
three steps in this process: development, rinsing, and drying. Common developing
solutions are NaOH (Shipley 351), KOH (Shipley 606), TMAH (Shipley CD-26,
MF-321, OCG 945) and so on.
• Post-bake
When the light is irradiated onto the interface between the photoresist and the wafer,
a part of the reflected light and the incident light are superimposed to form a standing
wave. Post-baking partially eliminates this effect. At the same time, the photoresist
side wall can be smoothened to improve the resolution.
• Hard-bake
The hard bake process is to make the film adhere to the surface of the wafer more
firmly by heating and baking. Such a process can increase the etching resistance of
the photoresist. Hard bake is not a necessary process. This step is to improve the
stability in the etching process. It can also improve the adhesion of the photoresist
to the wafer surface, which is beneficial to the subsequent wet etching process.
It can also improve the pinholes present in the photoresist. Of course, there are
certain drawbacks resulted from this step: it may cause the photoresist to reflow,
which will reduce the accuracy of the pattern transferred. Also, it will increase the
difficulty of removing the photoresist. The post-baking time is therefore critical. For
a given photoresist, the hard-bake temperature is usually higher than the pre-bake
temperature.