integrated circuits

INTEGRATED CIRCUITSDr. Esam Yosry

Lec. #6

Litho

grap

hy Introduction

Lithography

Photolithography

Photoresist Process

Types of Photoresist

Photomask

Clean Rooms

Intro

duct

ion

(Chi

p Fa

brica

tion

Cycle

)

Intro

duct

ion

(Pro

cess

es)

Oxidation

Diffusion

Ion Implantation

Deposition

Etching

Lithography

Deposition Removal Patterning Modification of

electrical properties

Lithography

• Lithography is the process of transferring patterns drawn on a mask to a thin layer of radiation sensitive material (resist) covering the surface of the material to be etched (oxide or metal).

• The main equipment is optical ultraviolet unit (UV wavelength 200 to 400 nm).

• In this case the mask is called photomask (PM), the resist is called photoresist (PR) and the process is called photolithography.

Lithography

• Patterning process consists of mask design, mask fabrication and wafer printing.

Lithography

• In case of X-ray (wavelength 0.2 to 5 nm) the resist is called X-ray resist and the process is called X-ray lithography.

• In electron-beam (EB) lithography (wavelength ≈ 0.1 nm) using EB resist.

• The shorter the radiation wavelength the smaller the feature size that can be produced.

• Since the EB radiates very small spots, no mask is needed and direct exposure of the EB resist is carried out (serial exposure of the resist or direct writing on the resist).

Lithography

• When masks are used (photo- and X-ray- lithography), we irradiate the whole resist surface using parallel exposure.

• EB lithography is used when the product volume is small because masks are very expensive to fabricate.

• Masks are themselves fabricated by EB lithography for shaping chromium metallic layer deposited on quartz transparent supports.

• Masks are only used to reproduce very large number of wafers exceeding 10000.

Lith

ogra

phy

• As processing equipment and technologies become more advanced, transistor size shrinks

• • In the 70’s, 8

• Now, 0.02

• This is due to advancement in lithography

• .• Each layer of dopant or

material has its own layout (15 – 20 mask).

Photolithography

Photolithography is the heart of integrated circuit processing. It is the method used to transfer the individual circuit design knowledge onto the silicon wafer.Probably one half of the wafer fabrication costs go into obtaining proper photolithography.The basic elements consist of:• An Align and Expose Tool• Masks containing design information• Photo-sensitive Resist

Basics of Photolithography

Silicon Wafer with Silicon Dioxide

Silicon Wafer

SiO2

To be etched


Apply photo resist

Silicon Wafer

SiO2

Photoresist


Apply photomask

Silicon Wafer

SiO2

Photomask


Apply Ultraviolet Light

Silicon Wafer

SiO2

Photomask

Decreasing feature size require the use of shorter λ


Develop photoresist

Silicon Wafer

SiO2


Etch Silicon Dioxide

Silicon Wafer

SiO2


Strip Photoresist

Silicon Wafer

SiO2

This flow chart shows the typical process used for VLSI Lithography.

SUBSTRATE CLEANING SPIN COAT

DEVELOP

PRE-BAKE

EXPOSE

INSPECTION

POST-BAKE

ETCH STRIP

Photoresist Process

Photoresist Process

1.Substrate Cleaning and Preparation• The surface is as clean as possible

immediately after a high temp operation.

• Acid cleaning may be necessary if wafers sit for a long time.


DEVELOP

PRE-BAKE

EXPOSE

INSPECTION

POST-BAKE

ETCH STRIP

The goal of coating is to apply a uniform, defect free film of photoresist over The entire wafer. Spin Coating is used for VLSI• accelerate the wafer - fast• spin at constant speed to get uniform

thickness

2. Wafer Coating

Before spin After spin

PR Wafer PR

Chuck


DEVELOP

PRE-BAKE

EXPOSE

INSPECTION

POST-BAKE

ETCH STRIP

Photoresist Process

The Soft-Baked is used to:• Drive off solvent from resist. (reduced to ~5%)• Improve adhesion and anneal stresses in resist

Typical process Temp is 90-100°COvens

• Convection oven - Very stable over time. ~ 30 min

• IR oven - Most popular. 3-4 min.• Hot plate - very effective for single wafers.

3. Pre-Bake (Soft-Bake)


DEVELOP

PRE-BAKE

EXPOSE

INSPECTION

POST-BAKE

ETCH STRIP

Photoresist Process

To produce the desired photochemical effects in the shortest time possible.

Time is important because:• Wafers are individually processed• Align machines are expensive

However, short times usually mean lower resolution.

Exposure to UV light chemically changes the resist

4. Expose


DEVELOP

PRE-BAKE

EXPOSE

INSPECTION

POST-BAKE

ETCH STRIP

Photoresist Process

Silicon Wafer

SiO2

Photomask

Development removes the unwanted photoresist.• Positive Resist - Area exposed is removed• Negative Resist - Area not exposed is

removed

5. Develop


DEVELOP

PRE-BAKE

EXPOSE

INSPECTION

POST-BAKE

ETCH STRIP

Photoresist Process

Silicon Wafer

SiO2

• Hardens Resist Prior to Etch• Temperature is critical• Too high a Temp (>120°C) causes resist to flow

• Extra high Temp (>180°C) causes resist to lift.

6. Post-Bake 100-120°C 30 minutes


DEVELOP

PRE-BAKE

EXPOSE

INSPECTION

POST-BAKE

ETCH STRIP

Photoresist Process

Photoresist Process

• The object of this inspection is to verify the photoresist is acceptable prior to etch.

• A complete rework can now be performed if the photoresist is not acceptable.

7. Inspection


DEVELOP

PRE-BAKE

EXPOSE

INSPECTION

POST-BAKE

ETCH STRIP

Photoresist Process

• Dry Plasma Etch is commonly used in commercial manufacturing lines.

• We use Wet Etch with acids because of the cost and flexibility of the process.

8. Etch


DEVELOP

PRE-BAKE

EXPOSE

INSPECTION

POST-BAKE

ETCH STRIP

Silicon Wafer

SiO2

Photoresist Process

9. Strip


DEVELOP

PRE-BAKE

EXPOSE

INSPECTION

POST-BAKE

ETCH STRIP

• It is a complete removal of the Photoresist. • Dry Plasma Strip in O2 is commonly used in

commercial manufacturing lines.• We use Wet Chemical Strip in special solvents

because of the cost and flexibility of the process.

Silicon Wafer

SiO2

Photoresist ProcessPH

OTOR

ESIS

T AR

EA

Types of Photoresist

Photomask• The pattern to be

etched on the wafersurface is drawn onthe pohtomask

• Photomasks are made from chromium

• Many masks are needed in recent CMOS technologies. The # of masks depends on the process complexity

Clean Rooms

Photolithography must be carried out in a clean room otherwise dust particles causing fabrication errors.The total number of dust particle are controlled with temp and humidity.

Clean rooms standards:Class 100 < 100 pp ft3

(0.5µ)Class 10 < 10 pp ft3 (0.5µ)Class 1 < 1 pp ft3 (0.5µ)

Many thanks to Prof. Hany Fikry and Prof Wael Fikry for their useful materials that help me to prepare this presentation.

Thanks

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