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INTEGRATED CIRCUITS. Dr. Esam Yosry. Lec . #6. Lithography. Introduction Lithography Photolithography Photoresist Process Types of Photoresist Photomask Clean Rooms. Introduction ( Chip Fabrication Cycle). Introduction ( Processes). Oxidation Diffusion Ion Implantation Deposition

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Integrated circuits

INTEGRATED CIRCUITS

Dr. EsamYosry

Lec. #6


Lithography

Lithography

  • Introduction

  • Lithography

  • Photolithography

  • Photoresist Process

  • Types of Photoresist

  • Photomask

  • Clean Rooms


Introduction chip fabrication cycle

Introduction(Chip Fabrication Cycle)


Introduction processes

Introduction(Processes)

  • Oxidation

  • Diffusion

  • Ion Implantation

  • Deposition

  • Etching

  • Lithography

  • Deposition

  • Removal

  • Patterning

  • Modification of electrical properties


Lithography1

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.


Lithography2

Lithography

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


Lithography3

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).


Lithography4

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.


Lithography5

  • 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).

Lithography


Photolithography

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

Basics of Photolithography

Silicon Wafer with Silicon Dioxide

SiO2

To be etched

Silicon Wafer


Basics of photolithography1

Basics of Photolithography

Apply photo resist

Photoresist

SiO2

Silicon Wafer


Basics of photolithography2

Basics of Photolithography

Apply photomask

Photomask

SiO2

Silicon Wafer


Basics of photolithography3

Basics of Photolithography

Apply Ultraviolet Light

Photomask

SiO2

Silicon Wafer

Decreasing feature size require the use of shorter λ


Basics of photolithography4

Basics of Photolithography

Develop photoresist

SiO2

Silicon Wafer


Basics of photolithography5

Basics of Photolithography

Etch Silicon Dioxide

SiO2

Silicon Wafer


Basics of photolithography6

Basics of Photolithography

Strip Photoresist

SiO2

Silicon Wafer


Integrated circuits

Photoresist Process

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


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.


Photoresist process1

Photoresist Process

2. Wafer

Coating

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

Before spin After spin

PR Wafer PR

Chuck


Photoresist process2

Photoresist Process

3. Pre-Bake

(Soft-Bake)

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°C

    Ovens

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

  • IR oven - Most popular. 3-4 min.

  • Hot plate - very effective for single wafers.


Photoresist process3

Photoresist Process

4. Expose

Photomask

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

SiO2

Silicon Wafer


Photoresist process4

Photoresist Process

5. Develop

SiO2

Silicon Wafer

Development removes the unwanted photoresist.

  • Positive Resist - Area exposed is removed

  • Negative Resist - Area not exposed is removed


Photoresist process5

Photoresist Process

6. Post-Bake

100-120°C

30 minutes

  • 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.


Photoresist process6

Photoresist Process

7. Inspection

  • 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.


Photoresist process7

Photoresist Process

8. Etch

SiO2

  • 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.

Silicon Wafer


Photoresist process8

Photoresist Process

9. Strip

SiO2

Silicon Wafer

  • 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.


Photoresist process9

Photoresist Process

PHOTORESISTAREA


Types of photoresist

Types of Photoresist


Photomask

Photomask

  • The pattern to be

    etched on the wafer

    surface is drawn on

    the pohtomask

  • Photomasks are made from chromium

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


Integrated circuits

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 ppft3 (0.5µ)

Class 1 < 1 ppft3 (0.5µ)


Integrated circuits

Thanks

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


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