1.12 50 years journey of ic technology_appendix iii_alternatives  (Page 2/2)

Irradiates the supersonic jet of Xenon Gas

↓reradiates

EUV(13.4nm equivalent to 100eV energy)

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Complex Optical System

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Through the optical system, EUV is reflected to Reflective Mask with pattern

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From the Reflective Mask, EUV is reflected to the substrate’s photo resist coating with 4 times reduction creating the correct size chip pattern in the photo resist for further processing

1. X-Ray Proximity Lithography:

Here the wavelength used is Soft X-Ray of wave length one order of magnitude lower i.e. of 1.34nm = 13.4A° (1000eV)

Synchrotron Radiator is used as X-Ray Source

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Thin stripes of radiation generated

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The stripe of radiation is bounced off a low-incidence angle oscillating mirror

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Bounced off beam is scanned across the mask area

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Chip pattern is imprinted on the Photo Resist coating of the wafer

Here the mask pattern is of the same size as that of the pattern imprinted on the wafer. Mask is made of SiC , 2µm thick and 5 cm wide. The mask is kept thin to make it transparent to Soft X-Ray. These are fragile and because the mask pattern is of the same size as the actual pattern hence electron beam generation of the pattern on the mask becomes difficult. Soft X-Ray has deeper penetrating power hence existing photo-resist suffice X-Ray proximity lithography requirement.

1. Projection Electron Beam Lithography:

Electron’s wave-particle duality allows its wave nature to be utilized for electron beam lithography where electron wave corresponds to a wave length λ. This wavelength is determined by its linear momentum p = mv.

From deBroglie wave particle duality: λ = h/p = h/ (mv);

If an electron beam is accelerated through 100kV it acquires a kinetic energy of 100keV.

Therefore (1/2)mv ² = 100kev = (100×10 ³ ×q)

Therefore mv = √(200×10 ³ ×q×m) where m = mass of an electron.

Therefore λ = h/[√(200×10 ³ ×q×m)]

Substituting the numerical values of h, q and m:

λ = 3.9 pico meter.

This wave length is more than sufficient for resolving features of nm size. By using differentially scattered electron beam, electrons through the transparent area exposes the photo resist whereas in pattern area the electron beam is prevented from transferring its energy to the photo resist. This system is called “Scattering with angular limitation projection electron beam lithography” known by its acronym “SCALPEL”. Diagram Figure III.1 describes the SCALPEL system.

Here 4-to-1 reduction is allowed hence the mask is conveniently generated but fragility of the mask remains a problem.

1. Ion Projection Lithography:

This is the least developed of all the NGL. It uses accelerated hydrogen or helium ions instead of electrons and focuses them using electrostatic field. This has a mask which has holes punched for allowing the ions to pass through. The holes are the transparent equivalent of the conventional optical masks. Hence this is stencil like mask. The stencil mask does not allow closed path of conductors . This is the donut problem.

1. Retaining Optical Lithography:

Next step in Moore’s scaling aims at 100nm and 70 nm generation chips. Here Excimer Laser source also known as Flouride Lasers will generate 157nm monochromatic light. Calcium Fluoride lenses could be used for focusing. Resolution enhancement could make optical lithography viable for 100nm and 70nm generation chips.