Specific Process Knowledge/Lithography/Coaters: Difference between revisions

=Coater Comparison Table=

[[Category: Equipment |Lithography coaters]]

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|style="background:WhiteSmoke; color:black"|<b>[[Specific_Process_Knowledge/Lithography/Coaters#Manual_Spin_Coaters|Spin Coater: LabSpin 02]]</b>

|style="background:WhiteSmoke; color:black"|<b>[[Specific_Process_Knowledge/Lithography/Coaters#Manual_Spin_Coaters|Spin Coater: LabSpin 03]]</b>

|style="background:WhiteSmoke; color:black"|<b>[[Specific_Process_Knowledge/Lithography/Coaters#Manual_Spin_Coaters|Spin Coater: Manual All Purpose]]</b>

* Coating of

** ESPACER

** Experimental resists

* Coating on

* Vacuum chuck

* Can handle almost any sample size and shape

* No permanent media

|colspan="2" rowspan="2" align="center" style="background:WhiteSmoke; color:black"|Only manual dispense

|colspan="1" rowspan="2" align="center" style="background:WhiteSmoke; color:black"|Only manual dispense

* No permanent media

* pneumatic dispense for SU-8 resist and EBR solvent

* Two syringe pumps

*10 - 6000 rpm

*10 - 6000 rpm

*10 - 6000 rpm

*10 - 5000 rpm (3000 rpm with non-vacuum chuck)

*100 - 5000 rpm (3000 rpm with edge handling chuck)

*100 - 7000 rpm

*no

*no

*no

*optional (max. speed 3000 rpm)

*no

*no

*small pieces down to 10x10 mm2

*small pieces down to 3x3 mm2

*Any sample(s) that fit inside machine

*1 - 25

*1 - 25

*1 - 25

*1

*1

*1

*1

No resist or crystalbond allowed in the HMDS module

No resist or crystalbond allowed in the HMDS module

*All cleanroom materials except III-V materials

*All cleanroom materials

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)

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.  

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 (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^-a. 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 t₁ achieved at spin speed w₁, one can estimate the spin speed w₂ needed to achieve thickness t₂ using the relation: <br> t₁*w₁^½ = t₂*w₂^½ => w₂ = w₁ * t₁²/t₂². <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> t₁*w₁ = t₂*w₂ => w₂ = w₁ * t₁/t₂. <br>

===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.

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.

===Softbake===

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.

The spray coating process as well as major features of the three nozzles are described into more detail in the manual which can be found via the Equipment Info page in LabManager under the Documents sections. The manual can also be found by clicking [http://labmanager.dtu.dk/d4Show.php?id=2523&mach=293 this direct link].

[http://labmanager.dtu.dk/function.php?module=Machine&view=view&mach=293 Spray coater in LabManager]'''

*[[/AZ4562|Spray coating using AZ4562]]

*[[/TISpray|Spray coating using TI Spray]]

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