Specific Process Knowledge/Lithography/Coaters: Difference between revisions

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 =Spin coating=
-The process of spin coating consists of a selection of the following steps:
+The typical spin coating process consists of the following steps:
-*Priming (typically HMDS)
+#Priming (typically HMDS)
-*Acceleration to a low spin speed if dynamic dispense is used
+#Resist dispense (static or dynamic)
-*Resist dispense (static or dynamic)
+#*Optional: Acceleration to a low spin speed if dynamic dispense is used
-*Resist spreading at low spin speed
+#*Optional: Resist spreading at low spin speed
-*Spin-off
+#Spin-off
-*Backside rinse (typically during spin-off)
+#Backside rinse (typically during spin-off)
-*Edge-bead removal
+#Optional: Edge-bead removal
-*Softbake (contact or proximity)
+#Softbake (contact or proximity)
-After priming, the wafer is centered on the coater chuck and held in place by vacuum, or in some cases pins. If static dispense is used, the wafer remains static during the ensuing resist dispense. In the case of dynamic dispense, the wafer rotates at low spin speed during the dispense. Using too high spin speed during dispense can cause surface wetting issues, while a too low spin speed causes the resist to flow onto the backside of the wafer. After dispense, a short spin at low spin speed may be used in order to spread the resist over the wafer surface before spin-off.
+After priming, the wafer is is transferred to the spin coater. If static dispense is used, the wafer remains static during the resist dispense. In the case of dynamic dispense, the wafer rotates at low spin speed during the dispense. Using too high spin speed during dispense can cause surface wetting issues, while a too low spin speed causes the resist to flow onto the backside of the wafer. After dispense, a short spin at low spin speed may be used in order to spread the resist over the wafer surface before spin-off.
-===Spin-off===
+==Spin-off==
 The spin-off cycle determines the thickness of the resist coating. For a given resist, the thickness is primarily a function of the spin-off speed and the spin-off time, both following an inverse power-law:
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 <math>t_1 \sdot w_1 = t_2 \sdot w_2 \rArr w_2 = w_1 \sdot \frac{t_1}{t_2}</math>
-===Backside rinse===
+==Backside rinse==
-Dependent on the spin speeds used in the various steps of the spin coating, resist may creep over the edge of the wafer and onto the backside. Also, some resists tend to leave fine strings of resist protruding from the edge of the wafer, or folded onto the backside, an effect sometimes referred to as "cotton candy". This resist will contaminate the softbake hotplate, and thus subsequent wafers with resist. In a backside rinse step, solvent administered through a nozzle to the backside of the wafer while spinning at low or medium spin speed dissolves the resist and washes it away. After the rinse, a short spin at medium spin speed dries the wafer before the softbake. During the backside rinse solvent inevitably creeps onto the front side of the wafer. This effect may be used to dissolve and subsequently remove an edge-bead, but it may also leave the rim of the wafer exposed. As an alternative to backside rinse, a wafer which is left dirty on the backside by the spin coat process may be softbaked in proximity in order to protect the hotplate from contamination. This leaves front side coating intact, but also leaves the backside dirty.
+If the spin speed is too low during resist dispense, resist may creep over the edge of the wafer and onto the backside. Some resist tend to leave fine strings of resist protruding from the edge of the wafer, or folded onto the backside, an effect sometimes referred to as "cotton candy".
-===Edge bead===
+Any resist on the edge and backside of the wafer will contaminate the end effector, softbake hotplate, and subsequent wafers.
-During spin coating, resist builds up at the edge of the wafer due to the change in surface tension at the edge. This phenomenon is called an edge-bead. Dependent on spin coating parameters, the coating may be several times thicker at the edge than in the central area. In a subsequent hard contact exposure step, this edge-bead induces an undesired proximity gap which reduces the lateral resolution, and may even cause the wafer to stick to the mask.
-In an edge-bead removal step, solvent administered through a nozzle positioned at the edge of the wafer while spinning at low or medium spin speed dissolves the resist and washes it away. After the removal, a short spin at medium spin speed dries the wafer before the softbake. Dependent on the viscosity (solvent content) of the resist at the point of edge-bead removal, this drying spin may cause the resist to re-flow and create a secondary edge-bead. In some cases, it may be necessary to (partially) softbake the resist before edge-bead removal.
+In a backside rinse step, solvent administered through a nozzle to the backside of the wafer, while spinning at low or medium spin speed, dissolves the resist and washes it away. After the rinse, a short spin at medium spin speed dries the wafer before the softbake.
-===Softbake===
+During the backside rinse solvent inevitably creeps onto the front side of the wafer. This effect may be used to dissolve and subsequently remove an edge-bead, but it may also leave the rim of the wafer exposed. As an alternative to backside rinse, a wafer, which is contaminated on the backside, may be softbaked in proximity, in order to protect the hotplate from contamination. This leaves front side coating intact, but also leaves the backside dirty.
-After spin coating, the solvent in the resist formulation must be evaporated in a baking step in order to solidify the resist. This softbake can be carried out as a contact bake or a proximity bake. In a contact bake, the wafer is held in close contact to the hotplate surface while resting on shallow bumps only 150µm above the hotplate. In a proximity bake, the wafer is first moved into proximity, e.g. 1mm, of the hotplate surface, then held there (on the lift pins) for the duration of the bake.
+==Edge bead==
+During spin coating, resist builds up at the edge of the wafer due to the change in surface tension at the edge, as well as extra drying from turbulence created by the wafer edge.
+This phenomenon is called edge-bead. Dependent on spin coating parameters, the coating may be several times thicker at the edge than in the central area. In a subsequent hard contact exposure step (mask aligner), this edge-bead introduces an undesired proximity gap, which reduces the lateral resolution, and may even cause the wafer to stick to the mask.
+In an edge-bead removal step, solvent administered through a nozzle positioned at the edge of the wafer, while spinning at low or medium spin speed, dissolves the resist and washes it away. After the removal, a short spin at medium spin speed dries the wafer before the softbake. Dependent on the viscosity (solvent content) of the resist after the edge-bead removal, this drying spin may cause the resist to re-flow and create a secondary edge-bead. In some cases, it may be necessary to (partially) softbake the resist before edge-bead removal.
+==Softbake==
+After spin coating, the solvent in the resist must be evaporated in a baking step, in order to solidify the resist. This softbake can be carried out as a contact bake or a proximity bake. In a contact bake, the wafer is held in close contact to the hotplate surface while resting on shallow bumps only 150 µm above the hotplate. In a proximity bake, the wafer is first moved into proximity, e.g. 1mm, of the hotplate surface, then held there (on the lift pins) for the duration of the bake.
 =Spin coaters at DTU Nanolab=