Specific Process Knowledge/Lithography/Coaters

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Coater Comparison Table

	Spin Coater: Gamma UV	Spin Coater: Süss Stepper	Spin Coater: Gamma E-beam and UV	Spin Coater: RCD8	Spin Coater: LabSpin 02	Spin Coater: LabSpin 03	Spin Coater: LabSpin 04	Spray Coater
Purpose	In-line substrate HMDS priming Coating and baking of AZ MiR 701 (29cps) AZ nLOF 2020 AZ 5214E	Coating and baking of BARC (DUV42S-6) KRF M230Y KRF M35G UVN2300-0.8	In-line substrate HMDS priming Coating and baking of AR-P 6200 (CSAR) AZ 5214E AZ MiR 701 (29cps) AZ 4562 Edge bead removal on novolac-based resist and SU-8	Coating of SU-8 AZ 5214E AZ 4562 AZ MiR AZ nLOF Edge bead removal	Coating of E-beam resists CSAR, ZEP, PMMA/MMA, HSQ(FOx) Coating of UV resists AZ5214E, AZ4562, AZMiR701, AZnLOF, SU-8 Coating of imprint resists		Coating of SU-8 mr-DWL other resists NB: this tool is in PolyFabLab	Spraying imprint resist Spraying photoresist Spraying of other solutions
Substrate handling	Cassette-to-cassette Vacuum handling and detection Vacuum spin chuck	Cassette-to-cassette Vacuum handling and detection Vacuum spin chuck	Cassette-to-cassette Vacuum handling and detection Vacuum spin chuck	Single substrate Vacuum chuck for 4" and 6" 4" non-vacuum chuck for fragile substrates	Single substrate Vacuum chucks for chips, 2", 4", and 6" 4" edge handling chuck		Single substrate Vacuum chucks for chips, 4", and 6"	Can handle almost any sample size and shape
Permanent medias	AZ MiR 701 (29cps) resist AZ nLOF 2020 resist AZ 5214E resist PGMEA solvent for backside rinse and spinner bowl cleaning	DUV42S-6 (BARC) KRF M230Y resist KRF M35G resist PGMEA solvent for edge bead removal and backside rinse	AR-P 6200.09 (CSAR) for 2", 4", and 6" AZ5214E for 2", 4", and 6" AZ MiR 701 (29cps) for 4", and 6" AZ4562 for 4", and 6" PGMEA solvent for edge bead removal, backside rinse, and spinner bowl cleaning	No permanent media	Only manual dispense		Only manual dispense	No permanent media
Manual dispense option	No manual dispense Syringe dispense (60cc) of PGMEA-based resist	No manual dispense Syringe dispense (60cc) of PGMEA and anisole-based resist	No manual dispense Syringe dispense (30cc) of PGMEA and anisole-based resist (2" only)	Only manual dispense	Only manual dispense		Only manual dispense	Two syringe pumps
Spindle speed	10 - 6000 rpm	10 - 6000 rpm	10 - 6000 rpm	10 - 5000 rpm (3000 rpm with non-vacuum chuck)	100 - 8000 rpm (3000 rpm with edge handling chuck)		100 - 8000 rpm	NA
Gyrset	No	No	No	Optional (max. speed 3000 rpm)	No		No	NA
Substrate size	50 mm wafers (tool change required) 100 mm wafers 150 mm wafers 200 mm wafers (tool change required)	100 mm wafers 150 mm wafers 200 mm wafers (may require tool change)	2" or 50 mm wafers 100 mm wafers 150 mm wafers	100 mm wafer 150 mm wafer	50 mm wafers 100 mm wafers 150 mm wafer small pieces down to 5x5 mm2		100 mm wafers 150 mm wafer small pieces down to 5x5 mm2	Any sample(s) that fit inside machine
Batch size	1 - 25	1 - 25	1 - 25	1	1		1	1
Allowed substrate materials	Silicon Glass No resist or crystalbond allowed in the HMDS module	Silicon Glass	Silicon III-V materials Glass No resist or crystalbond allowed in the HMDS module	All cleanroom materials except III-V materials	Silicon III-V materials Glass		All PolyFabLab materials	All chemicals to be spray coated must be approved specifically for spray coating Any non-toxic, non-particulate and non-crosslinking material is likely to be approved

Spin coating

The typical spin coating process consists of the following steps:

1. Priming (typically HMDS) followed by cooling to room temperature
2. Resist dispense (rotation: static or dynamic rotation)(arm: stationary or moving)
   • Optional: Acceleration to a low spin speed if dynamic dispense is used
   • Optional: Resist spreading at low spin speed for spreading thicker resists
3. Spin-off
4. Backside rinse (typically during spin-off)
5. Optional: Edge-bead removal
6. Softbake (contact or proximity)
7. Cooling to room temperature

After priming, the wafer is cooled to room temperature and then transferred to the spin coater. If static dispense is used, the wafer is not rotating during the resist dispense. In the case of dynamic dispense, the wafer rotates at low spin speed during the dispense. The dispense arm is normally stationary during dispense, but some substrates may require the arm to move slowly across the substrate area while dispensing. Moving arm dispensing is usually only done with a rotating substrate.

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

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:

$y=k\cdot x^{-a}$

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:

$t=k\cdot w^{-a}$

The constant, $k$, is a function of the resist viscosity and solid content, as well as 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 $t_1$ achieved at spin speed $w_1$, one can estimate the spin speed $w_2$ needed to achieve thickness $t_2$ using the relation:

$t_1\cdot w_1^{1/2}=t_2\cdot w_2^{1/2}\Rightarrow w_2=w_1\cdot \frac{t_1^2}{t_2^2}$

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:

$t_1\cdot w_1=t_2\cdot w_2\Rightarrow w_2=w_1\cdot \frac{t_1}{t_2}$

Backside rinse

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".

Any resist on the edge and backside of the wafer will contaminate the end effector, softbake hotplate, and subsequent wafers.

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 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.

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, either in direct contact on the manual hotlpates or by resting on shallow bumps 150 µm above the hotplate in the Gamma tools. 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.

Decommisioned tools

The spin track was decommissioned 2018-02-01.

Information about decommissioned tool can be found here.