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IMMERSION LITHOGRAPHY ANANDHU THAMPI 3 RD SEM M.Sc. PHYSICS CUSAT , COHIN- 682022

Immersion lithography

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Page 1: Immersion lithography

IMMERSION LITHOGRAPHY

ANANDHU THAMPI3RD SEM M.Sc. PHYSICSCUSAT , COHIN- 682022

Page 2: Immersion lithography

PHOTOLITHOGRAPHY

The root words photo, litho, and graphy all have Greek origins, with the meanings 'light', 'stone' and 'writing

Lithography refers to the transfer of an image onto paper using a plate and ink-soluble grease

Photolithography is the transfer of an image using photographic techniques

Photolithography transfers designer generated information (device placement and interconnections) to an actual IC structure using masks which contain the geometrical information

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Photolithography - Application of Photo resist

First step in photolithography is to coat the surface with approx 1 μm of photo resist (PR)

PR will be the medium whereby the required image is transferred to the surface

PR is often applied to the centre of the wafer, which is then spun to force the PR over the entire surface

The spin coating typically runs at 1200 to 4800 rpm for 30 to 60 seconds

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Photolithography - Exposure

The PR is then exposed to UV (ultraviolet) radiation through a mask

The masks contains information about device placement and connection

The UV radiation causes a chemical change in the PR

The transfer of information from the mask to the surface occurs through the UV-induced chemical change - only occurs where the mask is transparent

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Exposure systems may be classified by the optics that transfer the image from the mask to the wafer

A contact printer, the simplest exposure system, puts a photomask in direct contact with the wafer and exposes it to light

A proximity printer puts a small gap between the photomask and wafer

Projection lithography derives its name from the fact that an image of the mask is projected onto the wafer

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Photolithography - Development

The PR is then developed using a chemical developer

Two possibilities: A negative PR – Unexposed

region is soluble in developer (eg- SU8 )

A positive PR – Exposed region is soluble in developer (eg- Diazonaphthoquinone )

Example (developer) - tetramethyl ammonium hydroxide (TMAH) is used in concentrations of 0.2 - 0.26 N

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Pattern Transfer

There are one basic pattern transfer approaches: subtractive transfer(etching)

Etching is the most common pattern transfer approach. A uniform layer of the material to be patterned is deposited on the substrate

The photoresist “resists” the etching and protects the material covered by the resist. When the etching is complete, the resist is stripped leaving the desired pattern etched into the deposited layer

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RESOLUTION 

The minimum feature that may be printed with an optical lithography system is determined by the Rayleigh equation

The equation is introduced by Lord Rayleigh

Minimum feature size (C D) that is projected is directly proportional to the wave length (λ) of light used and inversely proportional to numerical aperture (NA)

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 Rayleigh equation

C D – Minimum feature size –  (k1 factor) is a coefficient

λ – Wave length of light NA – numerical aperture (n

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Immersion Lithography

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Immersion Lithography

The immersion technique was first introduced by Carl Zeiss in the 1880s to increase the resolving power of the optical microscope

It attracted the IC industry's attention in 2002 when other types of lithography was delayed by several technical problems

The introduction of water into the gap between the final lens and wafer changes the optical paths of exposure light.

It is a method to increase the resolution of photolithographic method

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Resolution is determined by the wavelength of the imaging light (λ) and the numerical aperture (NA) of the projection lens according to the Rayleigh criterion

The air between the lens and the resist has the lowest index of refraction and thus is the limiting factor in NA.

Water is an ideal interface for 193nm light. It transmits over 90%

of the light and has a high index of refraction (n ~ 1.4)

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Problems with Immersion lithography

Bubbles in the water can distort the exposure image Various physical and chemical interactions between the water

and resist stack occur, leading to water immersion-related defects

Water purification Placing water on a chip can also create defects in the chip itself

merely by contaminating the backside of the chip, or neighbouring chips on the wafer

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Advantages

The introduction of water into the gap between the final lens and wafer changes the optical paths of exposure light

The NA of a "dry" exposure system does not go beyond 1.0 . With water immersion, the maximum NA approaching nwater=1.44 is possible

Immersion lithography is a stepping stone to EUV lithography

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Production

In 2007, many companies, including IBM, UMC, Toshiba, and TI are ramping for the 45 nm node using immersion lithography

 AMD's Fab 36 is already equipped for using immersion lithography for its 65 nm, 45 nm and 32 nm node technologies

For the 32 nm node in 2009, Intel will begin using immersion lithography as well

 IBM has also stated that it will be using immersion lithography for the 22 nm node

Nikon was the world’s first to develop an ArF immersion scanner for production — the NSR-S609B — a feat that broke the barrier of NA1.0 and reached NA1.07

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References

1. John Culver:, Immersion; march 12, 20062. http://www.microe.rit.edu/research/lithography/research/immersi

on.htm3. http://www.eetimes.com/news/semi/showarticle.jhtml?articleid=

1802062804. http://spie.org

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Thank You