Specific Process Knowledge/Lithography/Pretreatment

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Pretreatment

Pretreatment, also known as priming, is done before spin coating in order to ensure the best conditions for adhesion between the substrate surface and the resist. Pretreatment ranges from a simple dehydration bake over etching the native oxide to vapor phase deposition of an adhesion promoter. The goal of pretreatment is to remove any moisture that may be adsorbed on the surface of the substrate, and/or to modify the contact angle of the surface to match that of the resist to be coated on the substrate.

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 spin coating. This page gives an overview of treatments available at DTU Nanolab to promote photoresist adhesion.

Dehydration

A dehydration bake immediately before spin coating removes the moisture adsorbed to the surface, and greatly improves the adhesion of resist on most surfaces. For thin hydrophilic layers, a few minutes on a hotplate at or above 100°C may suffice. For thicker layers or bulk oxide samples, a dehydration bake at 250°C over night is recommended. Due to the moisture in the cleanroom atmosphere, this priming naturally has a short shelf life, so spin coating should be done as soon as possible.

BHF dip

Stripping the native oxide using BHF only works if the native oxide of the substrate is etched by BHF, and if resist has good adhesion to the substrate material itself, which basically limits it to silicon. A BHF dip leaves the surface of a silicon wafer hydrophobic, and the dangling Si-bonds are passivated by adsorbed H⁺. This means the surface will not oxidize immediately, but the lifetime of this passivation is limited, so it is recommended to spin coat the wafer within 20, maximum 45 minutes.

HMDS

In the HMDS priming process, the -OH groups on the surface of the substrate are replaced with Si(CH₃)₃, thus changing the surface from hydrophilic to (more) hydrophobic. Substrates with surfaces of silicon, or its oxides/nitrides, all work very well with HMDS pretreatment.

Other semiconductors, insulators, or metals that form -OH groups on the surface may be suitable as well. The shelf life of substrates primed using HMDS (vapor phase) is long, maybe even several weeks.

Dip/spin-on adhesion promoter

Adhesion promoters for dip or spin-on application are commercially available. Most are solvent based with some, usually proprietary, additives. They work by first cleaning the substrate surface, and subsequently priming the surface with the additives as the solvent evaporates.

Hard bake

If the adhesion between the substrate surface and the resist is sufficient to survive the development process without delamination, the adhesion in subsequent process steps can be improved by a so-called hard bake. Baking the substrate a few minutes on a hotplate (or approx. half an hour in a convection oven) at a temperature higher than the soft bake temperature, e.g. 110-130°C, usually improves the adhesion between resist and surface. Depending on the type of resist used, and the temperature of the hard bake, reflow of the resist pattern may be a side effect.

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	Chips 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 (Tg > 150°C) Film or pattern of all 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 i.e. no resist coated wafers or crystalbonded chips!	Wafers with metal is not allowed	Resist is not allowed

Pretreatment of various surfaces

General comment regarding III-V materials:

A dehydration bake prior to spin coating can be enough, especially if adhesion is not critical, and if the resist is not left more than a day or two on the surface. Application of dip/spin-on adhesion promoter can improve the adhesion and shelf life. If adhesion is critical in the subsequent process step, application of a thin layer of SiO₂ using PECVD (followed by HMDS priming) as an adhesion layer is recommended.

Overview of what pretreatment is used for various surfaces at DTU Nanolab. Parentheses () indicate the method is not the optimal choice, or that the information is taken from product information supplied by manufacturers.

	Dehydration	BHF dip	HMDS	Dip/spin-on adhesion promoter	Comment
Silicon with native oxide	(X)	X	X	(X)
Silicon oxide	(X)		X	(X)
Silicon nitride	(X)		X	(X)
Glass (borofloat/pyrex)	X		X	(X)	Dehydration before HMDS is probably best
Fused silica (quartz)	X		X	(X)	Dehydration before HMDS is probably best
InGaAs	(X)			SurPass 3000
GaAs	(X)			SurPass 3000
GaP	(X)			?	AR800-30 is being tested (leaves residue)
InP	(X)			?	AR800-30 is being tested (leaves residue)
GaN	(X)			?
InN	(X)			?

The information on III-V materials has been provided by Kresten Yvind at DTU Fotonik June 2021.

HMDS

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 energy to promote better wetting.

The molecular formula for hexamethyldisilazane, or bis(trimethylsilyl)amine, is C₆H₁₉NSi₂.

Schematic of the HMDS vapor priming process

Comparing HMDS priming

Equipment		Oven: HMDS 2	Gamma UV/ Gamma e-beam and UV
Purpose		HMDS priming only	HMDS priming only HMDS priming and spin coating
Priming chemical		hexamethyldisilazane (HMDS)
Performance	Contact angle	Si (native oxide): 77.8° SiO2 (110 nm): 81.7° Boron Glass: 97.6°	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	120°C
	Process time	25 minutes	1-2 min / wafer
Substrates	Substrate size	Chips (using holder or silicon carrier) 50 mm wafers 100 mm wafers 150 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 (incl. Crystalbond)	Silicon and glass wafers Film or pattern of all but types, except type IV and resist/polymer (incl. Crystalbond)
	Batch	1 - 25, multiple batches possible	1 - 25

Contact angle:
The contact angle of the substrate surface can be measured using the Drop Shape Analyzer.

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 - requires login

Process information:

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

Spin Coater: Gamma UV

HMDS module (top) in Spin Coater: Gamma UV in E-5.

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

Spincoater: Gamma UV - requires login

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

Spin Coater: Gamma UV

Spin Coater: Gamma e-beam & UV

Spin Coater: Gamma e-beam & UV in E-5.

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

Spin Coater: Gamma e-beam & UV - requires login

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

Spin Coater: Gamma e-beam & UV

Buffered HF-Clean

BHF clean wetbench 04 in D-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 for new Si wafers.
The native oxide layer will be removed by 30 seconds of etching and this will promote the resist adhesion on the Si substrates. We recommend to spin coat resist as soon as possible after the procedure.

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

Buffered HF-Clean in LabManager - requires login

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

Wet Silicon Oxide Etch (BHF)

Oven 250C

Oven 250C for pretreatment in Cx-1

The oven is typically used for dehydration pretreatment, of Si and glass substrates, to promote the resist adhesion. We recommend placing the wafers in a metal carrier in the oven for at least for 4 hours, or overnight, and spin coat resist on them as soon as possible after removing them from the oven.

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

Oven 250C - requires login