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Pretreatment

All surfaces can be divided to hydrophilic or hydrophobic surfaces, where the oxidized surfaces such SiO2 or surface with native oxide formation on Si or Al substrates consider to be hydrophilic and have very bad wetting with hydrophobic resist.

Therefore it is very important to do the pretreatment step before the spinning. Here we will give an overview of treatments available at Danchip to promote photoresist adhesion.

Comparing pretreatment methods

	HMDS	Buffered HF-Clean	Oven 250C
Generel description	Vapor priming	Oxide strip	Dehydration
Chemical	hexamethyldisilazane (HMDS)	12%HF with Ammoniumflouride	none
Etch rate range	~75 nm/min (Thermal oxide) in BHF ~90 nm/min (Thermal oxide) in SIO Etch ~25 nm/min (Thermal oxide) in 5%HF ~3-4µm/min in 40%HF	Process dependent Tested range: ~20nm/min - ~120nm/min	Process dependent Tested range: ~230nm/min - ~550nm/min
Substrate size	50 mm wafers 100 mm wafers 150 mm wafers	100 mm wafers	100 mm wafers 150 mm wafers
Allowed materials	hmds	Silicon Poly Silicon Silicon Oxide Silicon Nitride Silicon Oxynitride Photoresist Blue film	Silicon Silicon Oxide Silicon Nitride Glass
Restrictions	None	Wafers with metal is not allowed	Resist is not allowed

HMDS

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The chemical treatment with hexamethyldisilazane (HMDS) before the spin coating can be used to promote the adhesion for photoresist. HMDS treatment leaves a mono-layer of TMS (trimethylsilyl) on the Si or SiO₂ surface.

The molecular formula for hexamethyldisilazane, or bis(trimethylsilyl)amine, is C₆H₁₉NSi₂. Here is a schematic overview of HMDS treatment of silicon-oxide surface.

Priming of oxide-forming substrates by HMDS treatment.

Comparing HMDS priming

Equipment		HMDS oven	Spin Track 1 + 2
Purpose		HMDS priming	HMDS priming only HMDS priming and spin coating
Priming chemical		hexamethyldisilizane (HMDS)
Performance	Contact angle	standard recipe 82° (on SiO₂)	60° - 90°; standard recipe 82° (on SiO₂)
Process parameters	Process temperature	150°C	50°C
Process parameters	Process time	32.5 minutes	3 min / wafer
Substrates	Substrate size	50 mm wafers 100 mm wafers 150 mm wafers	100 mm wafers
	Allowed materials	All cleanroom materials	Silicon (with oxide, nitride, or metal films or patterning) Glass (borosilicate and quartz)
	Batch	1 - 25, multiple batches possible	1 - 25

HMDS oven

At Danchip we use Star2000 model from IMTEC to do vapor deposition of hexamethyldisilizane (HMDS) under the special conditions: low pressure and high chamber temperature. The result of the dehydration bake and HMDS prime is that the wafers become hydrophobic after the treatment.

The user manual, user APV, and contact information can be found in LabManager:

HMDS oven in LabManager

Process information

Recipe 4: baseline prime process with 5 min priming time

Baseline prime process description:

1. Vacuum, 2 min

2. Nitrogen pump, 3.5 min

3. Heat- up, 10 min

4. Vacuum, 4.5 min

5. HMDS prime, 5 min

6. Vacuum chamber exhaust, 3 min

7. Nitrogen back-fill, 3.5 min

Equipment performance and process related parameters

Purpose		Promotion of photoresist adhesion by hydrophobization
Chemical		hexamethyldisilizane
Performance	Contact angle	82° (on SiO₂)
Process parameters	Process temperature	150 °C
Process parameters	Process time	32.5 minutes
Substrates	Substrate size	50 mm wafers 100 mm wafers 150 mm wafers
	Allowed materials	All cleanroom materials
	Batch	1 - 25, multiple batches possible

Buffered HF-Clean

Another commonly used method to render the surface of silicon wafers hydrophobic is the dilute HF dip.

BHF is mostly used to do pretreatment step for new Si wafers. The native dioxide layer will be removed during 30 sec etching and in this way we will promote the resist adhesion on the Si substrates. We recommend to spin resist asap after the procedure.

Oven 250C

The oven is typically used for pretreatment (dehydration) of Si and glass substrates to promote the resist adhesion. We recommend to place the wafers in metal carrier in the oven at least for 4 hours, better during the night, and spin the resist on them asap.