immersion lithography
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IMMERSION LITHOGRAPHY
ANANDHU THAMPI3RD SEM M.Sc. PHYSICSCUSAT , COHIN- 682022
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
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
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
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
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
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
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)
Rayleigh equation
C D – Minimum feature size – (k1 factor) is a coefficient
λ – Wave length of light NA – numerical aperture (n
Immersion Lithography
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
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)
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
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
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
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
Thank You
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