Introduction to photolithography

The following processes will explain how a mask and a positive/negative resist wafer are made. These two components are essential for the creation of our chips, as they are the master plan, the mold for the chip. This is how it works: a mask is used as a mold to make a wafer, and a wafer is used as a mold to make each of the microfluidic chip's layers (control and flow layers).

Mask

Wafers

Here are defined the two main types of photoresist. A photoresist is a light-sensitive material used in several industrial processes, such as photolithography and photoengraving to form a patterned coating on a surface:

- A negative resist is a type of photoresist in which the portion of the photoresist that is exposed to light crosslinks and thus becomes insoluble to the photoresist developer. The unexposed portion of the photoresist is dissolved by the photoresist developer.

- A positive resist is a type of photoresist in which the portion of the photoresist that is exposed to light becomes soluble to the photoresist developer. The portion of the photoresist that is unexposed remains insoluble to the photoresist developer.

Mask process and outline

Step	Process description	Machines	Cross-section after process
1			Cross section of a photolithography mask
2	Laser exposure	Heidelberg DWL200, Laser lithography system	The laser beams on the surface of the photoresist. By doing so, it imprints the pattern of the design on the PR.
3	Developing of the mask	DV10 Mask and Thick positive resist developer	A chemical treatment is used to remove the PR that was exposed by the laser in the previous step, creating the ‘holes’ on the photoresist
4	Etching of the chrome	Coillard Gravure	The chrome is removed at the sites where the resist layer is missing, using an acid bath.
5	Removal of Resist	Coillard Photolithographie	Once the chrome is removed at the precise sites (previous step), the rest of the resist is removed from the whole surface.
6	Use of mask		The mask can now be used to expose its pattern on the wafer using UV light

Control Layer Process outline

Step	Process description	Machines	Cross-section after process
1	Substrate: Wafer Clean	Tepla 300	Clean the wafer using plasma treatment
2	Photolith: Resist deposition Photo Resist : Su8 GM1070 – 30μm	Sawatec	A layer of negative photoresist is added on the wafer by spincoating
3	Relaxation time + Softbake	Sawatec	Softbake wafer using Sawatec hotplate, to solidify the photo resist
4	Photolith:UV exposure	Mask Aligner	The UV lights are exposed through the Mask on the surface of the wafer. By doing so, it imprints the pattern of the design on the PR.
5	Post exposure bake	Sawatec	Bake wafer using Sawatec hotplate
6	Relaxation delay		Wait 1 hour – overnight
7	Photolith: Develop	Wetbench plane solvent	This removes the unexposed photoresist from the wafer using chemical treatment on a wet bench
8	Hard bake	DataPlate	Bake 135°C 2 hours, using an oven

Flow layer process outline

Step	Process description	Machines	Cross-section after process
1	Substrate: Si test Priming	YES III	Dehydrate and prime with HMDS, using the oven to create hydrophobic surface on the wafer, to prepare the wafer for coating
2	Photolith: Resist deposition Photo Resist : AZ9260 – 14μm	EVG 150	A layer of positive photoresist is added on the wafer by spincoating
3	Rehydratation time		Wait minimum 1 hour, maximum 3 days
4	Photolith:UV exposure	Mask Aligner	The UV lights are exposed through the Mask on the surface of the wafer. By doing so, it imprints the pattern of the design on the PR.
5	Develop immediately		Wait maximum 1 hour before develop
6	Photolith: Develop	EVG 150	This removes the exposed photoresist from the wafer using chemical treatment with the EVG 150
7	Rinse with Deionized water	Coillard Wetbench	Rinse wafers in Quick Dump Rinse then in Ultra Clean bath, using wetbench
8	Bake to round edges	DataPlate	Bake in horizontal position, 160°C 2 hours

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