Specific Process Knowledge/Lithography/TIspray

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Resist Description

TI Spray is specifically designed to be used for spray coating, and is a positive UV photoresist with image reversal capability. It is considered to have good adhesion when wet etching.

Image reversal

Typical image reversal parameters:

• Reversal bake temperature: 110°C
• Reversal bake time: 60 - 120 s
• Flood exposure: 200 - 500 mJ/cm²

If TI Spray is baked after exposure, the exposed resist will cross-link, making it insoluble in the developer. This is called the "Reversal bake". The reversal bake activates cross-linking of the exposed areas, which "reverses" the polarity of the design. When the substrate is flood-exposed after the reversal bake, the previously unexposed areas become soluble, and will be removed in the subsequent development. The reversal procedure effectively makes TI Spray a negative resist.

The image reversal process greatly increases the sensitivity of TI Spray, and the dose of the image exposure is a critical parameter, especially if negative angled resist sidewalls are desired.
Half dose of the normal, positive process is a good starting point for optimization. Similarly, the reversal bake is also a critical step, and must be tightly controlled in order to achieve consistent results. If negative sidewalls are desired, 60 - 120 s at 110°C is recommended (as well as a resist thickness above 2 µm), but if straight sidewalls are desired, 60 - 120 s at 120°C can be used.

The flood exposure, on the other hand, is uncritical, and 2-5 times the normal positive process dose is generally used.

Development

Development speed:

• Puddle development in 2.38% TMAH (AZ 726 MIF): ~2 µm/min