Specific Process Knowledge/Lithography/Coaters: Difference between revisions
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==Spin coating== | ==Spin coating== | ||
The process of spin coating consists of a selection of the following steps: | |||
*Priming (typically HMDS) | |||
*Acceleration to a low spin speed if dynamic dispense is used | |||
*Resist dispense (static or dynamic) | |||
*Resist spreading at low spin speed | |||
*Spin-off | |||
*Backside rinse (typically during spin-off) | |||
*Edge-bead removal | |||
*Softbake (contact or proximity) | |||
The wafer is first centered on the spindle chuck and held in place by vacuum. If static dispense is specified in the process, the spindle remains static during the ensuing resist dispense. In the case of dynamic dispense, the spindle is accelerated to a low spin speed before the resist is dispensed. 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. The resist is dispensed through the dispense arm, positioned over the center of the wafer. The resist pump administers a fixed volume of resist (4 ml), but multiple dispenses may be used. 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. | |||
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 soft bake 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 soft bake. 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 soft baked in proximity in order to protect the hotplate from contamination. This leaves front side coating intact, but also leaves the backside dirty. | |||
===Spin-off=== | ===Spin-off=== | ||
The spin-off cycle determines the thickness of the resist coating. The thickness is primarily a function of the spin-off speed and the spin-off time, both following an inverse power-law (y=k*x^-a). The acceleration to the spin-off speed also influences the thickness, but the effect is dependent on previous steps. The spin-off is usually a simple spin at one speed, but it may be comprised of several steps at different spin speeds. After spin-off, the wafer is decelerated. | |||
The coated thickness, t, as a function of the spin-off speed, w, follows an inverse power-law, t = k * w<sup>-a</sup>. The constant, k, is a function of the resist viscosity and solid content, and the spin-off time. The exponent, a, is dependent on solvent evaporation, and is typically ~½ for UV resists. This means that from the thickness t<sub>1</sub> achieved at spin speed w<sub>1</sub>, one can estimate the spin speed w<sub>2</sub> needed to achieve thickness t<sub>2</sub> using the relation: <br> t<sub>1</sub>*w<sub>1</sub><sup>½</sup> = t<sub>2</sub>*w<sub>2</sub><sup>½</sup> => w<sub>2</sub> = w<sub>1</sub> * t<sub>1</sub><sup>2</sup>/t<sub>2</sub><sup>2</sup>. <br> For thick SU-8, however, a is observed to be ~1 (probably due to the low solvent content and/or the formation of skin). In this case, the relation simply becomes: <br> t<sub>1</sub>*w<sub>1</sub> = t<sub>2</sub>*w<sub>2</sub> => w<sub>2</sub> = w<sub>1</sub> * t<sub>1</sub>/t<sub>2</sub>. <br> | |||
===Edge bead=== | ===Edge bead=== | ||
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 soft bake. 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) soft bake the resist before edge-bead removal. | |||
===Softbake=== | ===Softbake=== | ||
After spin coating, the solvent in the resist formulation must be evaporated in a baking step in order to solidify the resist. This soft bake 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. | |||
==Spin Track 1 + 2== | ==Spin Track 1 + 2== | ||