FABRICATION OF SILICON

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FABRICATION OF SILICON ON INSULATOR (SOI)
DONE BY, A.POOJA SHUKLA ECE B
(190912066) SAVEETHA UNIVERSITY
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AIM

• To fabricate PHOTONIC CRYSTAL in SOI.
• To use 248-nm deep UV Lithography for
  fabrication.
• To use metal-oxide-semiconductor process.

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ABSTRACT

• Demonstration of wavelength-scale photonic
  nanostructures, including PHOTONIC CRYSTALS.
• Fabrication of silicon on insulator using deep UV
  Lithography.
• Comparing UV lithography with E-BEAM lithography.

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INTRODUCTION

1. Overview of photonic crystals, using deep UV
   Lithography.
2. Use in optical waveguides.
3. Current Lithography techniques for fabrication of
   PICs.

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PROCESS INVOLVED

1. PHOTONIC CRYSTALS
2. SOI FOR INTEGRATED OPTICS
3. LITHOGRAPHY FOR PHOTONIC CRYSTALS
4. FABRICATION
5. LITHOGRAPHY ISSUES

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1. PHOTONIC CRYSTALS

• PHOTONIC CRYSTALS are periodic optical
  nanostructures that are designed to affect the
  motion of photons in a similar way that
  periodicity of a semiconductor crystal affects
  the motion of electrons.
• They have separate high dielectric and low
  dielectric regions.
• Periodic spacing for relevant light frequency.

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ADVANTAGE OF PHOTONIC CRYSTAL

• Reduce Band Gap
• Reduce defects
• example if there is a LINE DEFECT in
  structure, it will act as a waveguide
• Avoids Propagation of a Material

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CONSTRUCTION STRATEGIES

• The fabrication method depends upon the number of
  dimensions that the photonic band gap must exist
  in.
• 1-D Photonic Crystals
• 2-D Photonic Crystals
• 3-D Photonic Crystals

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PHOTONIC CRYSTAL SLAB

• Any type of dimension can be used.
• High refractive index contrast gives high
  diffractive property.
• PBG bounds defects in crystal.

• Completely lossless and allows short bends
  without radiation loss.

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2. SOI FOR INTEGRATED OPTICS

• SOI was first used in CMOS application to reduce
  the parasitary capacitance to the silicon
  substrate.
• The top layer of SI acts as an optical waveguide
  due to high vertical index contrast.
• SOI uses large cores i.e., top SI layers of upto
  10um thick but we use 205nm.
• SOI wafer bonding of a buried oxide is 400nm.
• Due to leakage slab waveguide remains single
  mode for a silicon thickness upto 268nm.
• The minimum loss of 6 db/mm.

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SILICON LAYER THICKNESS CHART
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3. LITHOGRAPHY FOR PHOTONIC CRYSTALS

• Photolithography (or "optical lithography") is a
  process used in microfabrication to selectively
  remove parts of a thin film or the bulk of a
  substrate.
• It uses light to transfer a geometric pattern
  from a photomask to a light-sensitive chemical
  "photoresist", or simply "resist," on the
  substrate.
• For example, in complex integrated circuits, a
  modern CMOS wafer will go through the
  photolithographic cycle up to 50 times

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GENERAL COMPARISON

• E-BEAM

• DEEP UV

• Size within 10nm.
• Most used for research purpose.
• Structure is not defined.
• Defines extremely small features.
• Not suitable for large volume because the process
  is very slow.

• Size of any illuminated wavelength.
• Widely used for CMOS fabrication.
• Mostly used for structure defining.
• Reduced wavelength become fuzzy.
• High end deep lithography

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4. FABRICATION

• Steps involved in the fabrication of the PHOTONIC
  CRYSTALS are given below,
• FABRICATION PROCESS
• 1. Lithography
• 2. Etching
• DENSE SQUARE LATTICES
• SUPERDENSE TRIANGULAR LATTICES

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FABRICATION PROCESS
1. LITHOGRAPHY



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• The deep uv lithography facilities we use
  5500/300 deep uv stepper with an illumination
  wavelength of 248 nm.
• The stepper uses 200 nm wafers.
• It is used in resist coating, baking, and
  development.
• Steps for lithography,
• 1. Wafer illuminated in stepper
• 2. Post exposure bake resist
  impurity is removed
• 3. Development

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2. ETCHING
The etching of the SOI wafer is done using a
double etch, in different chambers
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TOP AND BOTTOM VEIW OF SILICON ETCHING

• No air is exposed when two chambers are etched.
• The top layer of silicon is etched using LOW
  PRESSURE and HIGH DENSITY.
• The top layer of silicon can be replaced by
  AMORPHOUS SILICON.

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DENSE SQUARE LATTICES
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• The first lithography test were carried out
  using a CMOS process evaluation mask with dense
  contact holes.
• For perfect crystal we expose LARGER HOLES but
  SAME PITCH (400 to 600 nm), ratio is (0.25 to
  0.35).
• Vertical sidewalls show roughness in square
  lattices so we prefer SUPERDENSE TRIANGULAR
  LATTICES

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SUPERDENSE TRIANGULAR LATTICES
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• The triangular lattice provides various pitch
  and hole size, both in top-down and cross-section
  view.
• Holes are very uniform through out the lattice.
• There is an strong effect of side lobes from the
  crystal wall.

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5. LITHOGRAPHY ISSUES

• A common problem in dense structures are,
• The size and shape of a structure is changed
  with the presence of a neighboring structures.
• The various structures on photonic Ics each
  require different lithography conditions.
• The effects are,
• (a) Optical Proximity
  Effects
• (b) Line Hole Bias

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OPTICAL PROXIMITY EFFECTS
BASIC PRINCIPLE

• Photonic crystals are superdense periodic
  structures with feature sizes close to the
  illumination wavelength.
• During lithography, neighboring holes interfere
  with eachother.
• Due to this the holes get larger or smaller
  during the print.
• This phenomenon is called as OPTICAL PROXIMITY
  EFFECTS (OPE).

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EFFECT IN SUPERDENSE LATTICE

• The denser the structures and the smaller the
  pitch, the stronger the OPE becomes

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• The border holes are smaller than the holes in
  the bulks.
• The hole in the inner corner prints more smaller
  than the border holes.
• EXAMPLE
• When the OPE of the
  lattice with a relatively large pitch of 530 nm,
  but with holes targeted at 420 nm.

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LINE HOLE BIAS

• Different geometrics are on the same level of
  the chip, and preferably printed together.
• Small holes needs a much higher illumination
  than the larger holes i.e, a few hundreds of nm
  in width.

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RESULTS

• The new mask structures should be included to
  study the effect of OPE in photonic crystals.
• The lithography should target the features with
  the highest exposure.
• The bias should be applied on the mask to the
  features that need less energy to print on target.

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CONCLUSION

1. Deep uv lithography has potential for the mass
   fabrication of ultra compact photonic Ics based
   on photonic crystal.
2. SOI shows well defined holes with very little
   edge roughness.
3. The neighboring of the holes can be avoided by
   using OPTICAL PROXIMITY CORRECTION (OPC) method.
4. Thus, deep uv lithography is suitable for
   providing mass-manufacturing capabilities to the
   ultracompact photonic Ics.

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REFERENCES

• Fabrication of photonic crystals in SOI using
  248 nm deep uv lithography, IEEE.
• SOI Photonic Crystal Fabrication Using Deep UV
  Lithography, IEEE.
• www.google.com
• www.wikipedia.com

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