Specific Process Knowledge/Lithography/Pretreatment

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Pretreatment

All surfaces can be divided to hydrophilic or hydrophobic surfaces. Oxidized surfaces such as SiO2 or surfaces with native oxide (e.g. Si or Al substrates) are considered to be hydrophilic and have very bad wetting with hydrophobic resist. The adhesion of most resists on hydrophilic surfaces is deteriorated if moisture is present on the surface.

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

Comparing pretreatment methods

	HMDS	Buffered HF-Clean	Oven 250C
Generel description	Vapor priming	Native oxide strip	Dehydration
Chemical	hexamethyldisilazane (HMDS)	12%HF with Ammoniumflouride	none
Substrate size	50 mm wafers 100 mm wafers 150 mm wafers	100 mm wafers	100 mm wafers 150 mm wafers
Allowed materials	Silicon, glass, and polymer substrates Film or pattern of all but types, except type IV and resist/polymer	Silicon Poly Silicon Silicon Oxide Silicon Nitride Silicon Oxynitride Photoresist Blue film	Silicon Silicon Oxide Silicon Nitride Glass
Restrictions	Type IV and resist/polymer on polymer substrate	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. Vapor priming with HMDS leaves a mono-layer of TMS (trimethylsilyl) on the Si or SiO₂ surface. The process dehydrates the substrate surface, and lowers the surface tension.

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		Oven: HMDS 2	Spin Track 1 + 2	Gamma UV
Purpose		HMDS priming only	HMDS priming only HMDS priming and spin coating	HMDS priming only HMDS priming and spin coating
Priming chemical		hexamethyldisilizane (HMDS)
Performance	Contact angle	Si (native oxide): 77.8° SiO2 (110 nm): 81.7° Boron Glass: 97.6°	Track 1 [Track 2]: Si (native oxide): 74.9° [73.3°] SiO2 (110 nm): 81.8° [73.4°] Boron Glass: 85.4° [84.4°]	Fast recipe [standard recipe]: Si (native oxide): 72.9° [81.0°] SiO2 (110 nm): 71.0° [79.7°] Boron Glass: 90.7° [96.2°]
Process parameters	Process temperature	150°C	50°C	120°C
	Process time	25 minutes	3 min / wafer	1 min / wafer
Substrates	Substrate size	50 mm wafers 100 mm wafers 150 mm wafers	100 mm wafers	50 mm wafers (tool change required) 100 mm wafers 150 mm wafers 200 mm wafers (tool change required)
	Allowed materials	Silicon, glass, and polymer substrates III-V materials on silicon carrier Film or pattern of all but types, except type IV and resist/polymer	Silicon and glass wafers Film or pattern of all types	Silicon and glass wafers Film or pattern of all but types, except type IV and resist/polymer
	Batch	1 - 25, multiple batches possible	1 - 25	1 - 25

HMDS oven

The HMDS oven is placed in Cleanroom C-1.

At DTU Nanolab 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. Pump-down, 2 min

2. Nitrogen purge, 3.5 min

3. Heat-up, 10 min

4. Pump-down to 3 mBar

5. HMDS prime, 5 min

6. Chamber exhaust, 3 min

7. Nitrogen purge, 3.5 min

Oven: HMDS 2

The Oven: HMDS 2 oven is located in E-5.

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

Oven: HMDS 2 in LabManager

Process information

• Recipe 1: baseline prime process with 5 min HMDS priming time

Spin track 1 + 2

HMDS module in spin track 1 + 2 in C-1.

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

Spin track 1 + 2 labmanager page

Additional information about the spin track and processes can be found in Labadviser:

Spin track 1 + 2 labadviser page

Spincoater: gamma UV

HMDS module in gamma UV spincoater in E-5.

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

Spincoater: Gamma UV labmanager page

Additional information about the spin coater and processes can be found in Labadviser:

Spincoater: Gamma UV labadviser page

Buffered HF-Clean

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Buffered HF-Clean in C-1

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.

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

Buffered HF-Clean in LabManager

Process information

For more information on this BHF bath take a look here: Wet Silicon Oxide Etch (BHF)

Oven 250C

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Oven 250C for pretreatment in C-1

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.

The user manual, and contact information can be found in LabManager: Oven 250C