Specific Process Knowledge/Lithography/Pretreatment

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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 treatment which Danchip recommend to render the surface hydrophobic.

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 coating 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.

HMDS oven

The HMDS oven is placed in Cleanroom 3.

At Danchip we use Star2000 model from IMTEC to do prime 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
• Recipe 5: baseline process without prime

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 Dehydration of substrates
Chemical		hexamethyldisilizane
Performance	Contact angle	82° (on SiO2)
Process parameters	Process temperature	150 °C
	Process time	32.5 minutes
	Process pressure	??
Substrates	Substrate size	50 mm wafers 100 mm wafers 150 mm wafers
	Allowed materials	All cleanroom materials
	Batch	1-15, multiple batches possible

HMDS on Spin Track 1 + 2

Spin Track 1 + 2 in Cleanroom 3

Specific process knowledge on Spin Track 1 + 2

Process information

General process information about HMDS priming on Spin Track 1 + 2

Standard HMDS process on Spin Track 1 and 2

Equipment performance and process related parameters

Spin Track		1	2
Purpose		HMDS priming	HMDS priming
Priming chemical		HMDS
Performance	HMDS contact angle	60° - 90°; standard recipe 82° (on SiO2)
Process parameters	HMDS priming temperature	50°C
Substrates	Substrate size	100 mm wafers
	Allowed materials	Silicon (with oxide, nitride, or metal films or patterning) Glass (borosilicate and quartz)
	Batch	1 - 25

BHF

BHF: positioned in cleanroom 3

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.

250C Oven for Pretreatment

250 degrees oven for pretreatment: positioned in cleanroom 3

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.