8.2 Photolithography
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the substrate so that the photoresist is not infiltrated by the liquid developer during
development process.
• Spin coating
In the lithography process, spin-coating is generally used for coating photoresist.
The wafer is placed on the vacuum chuck, and the photoresist is sprayed at the center
of the wafer. The photoresist spreads outwardly at a high spin speed and is uniformly
coated on the surface of the wafer. A photoresist film is finally formed as a result of
the surface tension of the photoresist and the centrifugal force of rotation. The higher
the rate of rotation, the more uniform of the coating. During the coating process, the
photoresist spreads outward on the surface of the rotating wafer. Usually, there is
a different rate to determine the thickness of the photoresist. First, there will be a
low speed rotation (500 rpm), and then slowly rise to (~3000 to 7000 rpm). The rate
is adjusted according to the needs of the photoresist thickness. The final thickness
of the photoresist film is determined by the viscosity of the photoresist, the rotation
speed, the surface tension, and the dryness of the photoresist.
• Pre-bake
Since the as coated photoresist is fresh and soft, it cannot be directly exposed after
the coating. It must be baked to evaporate the solvent in the photoresist. The baked
photoresist remains “soft”. However, it will adhere to LED wafers more firmly. The
purpose of pre-baking is to evaporate the organic solvent component to cure the
photoresist on the surface of the LED wafer. The role of the solvent in the photoresist is to make the applied photoresist thinner, but it also absorbs heat and affects the
adhesion of the photoresist. After pre-baking, the solvents are substantially removed,
thereby the as-coated photoresist becomes thin thickness (a reduction of approximately 25%). This baking process must avoid excessive degree of baking since
over-baking will cause polymerization of photoresists and decrease photosensitive
sensitivity. This will thus influence the adhesion and exposure.
• Alignment and exposure
Alignment is the positioning or alignment of the desired pattern from the photomask on the surface of the LED wafer. The exposure of a substrate coated with a
photoresist to the UV light or other radiation is to initiate a photochemical reaction
so that the optical properties of photoresist are changed by the received light. Such
a process allows the transfer of the pattern from the photo-mask to the photoresist
coating layer. If the photoresist is the “material” core of the lithography process, then
alignment and exposure are the “device” core of the process. Accurate alignment is
one of the decisive factors in ensuring proper operation of the LED device.
The initial exposure equipment was a contact lithography machine and a proximity lithography machine. The lithography machines have currently evolved into
two major types: optical lithography machines and non-optical lithography machines.
The optical lithography machine uses ultraviolet light as the light source, while the
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the substrate so that the photoresist is not infiltrated by the liquid developer during
development process.
• Spin coating
In the lithography process, spin-coating is generally used for coating photoresist.
The wafer is placed on the vacuum chuck, and the photoresist is sprayed at the center
of the wafer. The photoresist spreads outwardly at a high spin speed and is uniformly
coated on the surface of the wafer. A photoresist film is finally formed as a result of
the surface tension of the photoresist and the centrifugal force of rotation. The higher
the rate of rotation, the more uniform of the coating. During the coating process, the
photoresist spreads outward on the surface of the rotating wafer. Usually, there is
a different rate to determine the thickness of the photoresist. First, there will be a
low speed rotation (500 rpm), and then slowly rise to (~3000 to 7000 rpm). The rate
is adjusted according to the needs of the photoresist thickness. The final thickness
of the photoresist film is determined by the viscosity of the photoresist, the rotation
speed, the surface tension, and the dryness of the photoresist.
• Pre-bake
Since the as coated photoresist is fresh and soft, it cannot be directly exposed after
the coating. It must be baked to evaporate the solvent in the photoresist. The baked
photoresist remains “soft”. However, it will adhere to LED wafers more firmly. The
purpose of pre-baking is to evaporate the organic solvent component to cure the
photoresist on the surface of the LED wafer. The role of the solvent in the photoresist is to make the applied photoresist thinner, but it also absorbs heat and affects the
adhesion of the photoresist. After pre-baking, the solvents are substantially removed,
thereby the as-coated photoresist becomes thin thickness (a reduction of approximately 25%). This baking process must avoid excessive degree of baking since
over-baking will cause polymerization of photoresists and decrease photosensitive
sensitivity. This will thus influence the adhesion and exposure.
• Alignment and exposure
Alignment is the positioning or alignment of the desired pattern from the photomask on the surface of the LED wafer. The exposure of a substrate coated with a
photoresist to the UV light or other radiation is to initiate a photochemical reaction
so that the optical properties of photoresist are changed by the received light. Such
a process allows the transfer of the pattern from the photo-mask to the photoresist
coating layer. If the photoresist is the “material” core of the lithography process, then
alignment and exposure are the “device” core of the process. Accurate alignment is
one of the decisive factors in ensuring proper operation of the LED device.
The initial exposure equipment was a contact lithography machine and a proximity lithography machine. The lithography machines have currently evolved into
two major types: optical lithography machines and non-optical lithography machines.
The optical lithography machine uses ultraviolet light as the light source, while the