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OPTICAL EXPOSURES
BY
AJAL.A.J
When employees are trained to work safely
they should be able to anticipate and avoid
injury...
Eye and Face Protection
Thousands of people are blinded each
year from work-related eye injuries.
According to the Bureau ...
EYEWEAR LABELS
All eyewear must be labeled with
wavelength and optical density.
CDRH CLASS WARNING LABELS
CLASS II LASER PRODUCT
Laser Radiation
Do Not Stare Into Beam
Helium Neon Laser
1 milliwatt max/...
LASER PROTECTIVE BARRIER
INTERNATIONAL LASER
WARNING LABELS
Symbol and Border: Black
Background: Yellow
Legend and Border: Black
Background: Yellow...
CLASS 4 LASER
ND:YAG 1064 nm
100 Watts Max. Average Power
VISIBLE and/ or INVISIBLE LASER
RADIATION-AVOID EYE OR SKIN
EXPO...
SKIN BURN FROM CO2 LASER EXPOSURE
Accidental exposure to partial reflection of 2000 W CO2 laser beam
from metal surface du...
HUMAN EYE
Choroid
Aqueous
Cornea
Macula
Optic Nerve
Sclera
Vitreous
Retina
Lens
Laser-Professionals.com
25 µ
Photo courtesy of U S Air Force
THERMAL BURNS
ON
PRIMATE RETINA
Laser-Professionals.com
MULTIPLE PULSE RETINAL INJURY
Laser-Professionals.com
• Most beam injuries occur during alignment.
• Only trained personnel may align (NO EXCEPTIONS!)
• safety eyewear is requi...
Photo courtesy of U S Army Center for Health Promotion and Preventive Medicine
EYE INJURY BY Q-SWITCHED LASER
Retinal Inju...
Photo-litho-graphy
• Photo-litho-graphy: latin: light-stone-writing
• Photolithography: an optical means for transferring
...
Overview of the
Photolithography Process
1. • Surface Preparation
2. • Coating (Spin Casting)
3. • Pre-Bake (Soft Bake)
4....
1] Surface Preparation
Wafer Cleaning
• Typical contaminants that must be removed prior to photoresist coating:
• dust fro...
Wafer Priming
• Adhesion promoters are used to assist resist coating.
• Resist adhesion factors:
• moisture content on sur...
2] Coating (Spin Casting)
Photoresist Spin Coating
• Wafer is held on a spinner chuck by vacuum and resist is coated to
uniform thickness by spin co...
Spin Coater
3] Pre-Bake (Soft Bake)
Prebake (Soft Bake)
• Used to evaporate the coating solvent and to densify the resist after spin
coating.
• Typical therma...
Mask Aligner
4] Alignment
Mask to Wafer Alignment - 1
– 3 degrees of freedom between mask and wafer: (x,y,q)
– Use alignment marks on mask and wafer...
Mask to Wafer Alignment - 2
• Normally requires at least two alignment mark sets on
opposite sides of wafer or stepped reg...
Some more Alignment Marks
Optical Exposure
• Projection Optics
• Numerical Aperture
• Raleigh Criterion
• Coherence
• Optical Correction & Phase Shi...
4] Alignment
5] Exposure
6] Development
7] Post-Bake (Hard Bake)
8 ] Processing Using the Photoresist as
a Masking Film
9 ] Stripping
10 ] Post Processing Cleaning (Ashing)
STEPPER
The Head of a Stepper
Projection Lithography Requirements
– b = minimum feature size (spot or line)
– 2b = minimum period of line-space pattern
...
• Short exposure wavelengths can create standing waves in a layer of photoresist.
Regions of constructive interference cre...
WHAT IS A PHOTOMASK?
Photomasks are high precision plates containing microscopic images of
electronic circuits. Photomasks...
MATERIAL USED TO MAKE PHOTOMASKS:
There are four types of material used to make photomasks; quartz (the most commonly
used...
Standard Elements of PhotoMask
Defects in Photomask
Postbake (Hard Bake) - 1
• Used to stabilize and harden the developed photoresist prior to processing
steps that the resis...
Postbake (Hard
Bake)
• Firm postbake is needed for acid
etching, e.g. BOE.
• Postbake is not needed for processes
in which...
Photoresist Removal (Stripping)
• Want to remove the photoresist and any of its residues.
• Simple solvents are generally ...
PhotoResist
Advantages of Positive Photoresists
• Most commonly used in the IC industry.
• Superior to negative photoresists because:
...
Requirements: the Photoactive Component
• Need an overlap of the absorption spectrum with the emission spectrum of the
exp...
Bleaching of a Positive Photoresist
– The solution to the coupled Dill equations predicts a sharp boundary
between exposed...
Primary Components of a Positive Photoresist
• Non-photosensitive base phenolic resin
– usually novolac
• Photosensitive d...
Negative Photoresist Ingredients
• 1. Non-photosensitive substrate material
• 2. Photosensitive cross-linking agent
• 3. C...
Negative Photoresist Development - 1
• The unexposed (uncross-linked) areas of resist as well as polymer chains that have ...
The Gel Point
– All sites for cross-linking (chromophores) are equally likely; thus, larger
polymer chains are more likely...
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Optical exposures

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When employees are trained to work safely they should be able to anticipate and avoid injury from job-related hazards.
• Surface Preparation
• Coating (Spin Casting)
• Pre-Bake (Soft Bake)
• Alignment
• Exposure
• Development
• Post-Bake (Hard Bake)
• Processing Using the Photoresist as a Masking Film
• Stripping
• Post Processing Cleaning (Ashing)

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Optical exposures

  1. 1. OPTICAL EXPOSURES BY AJAL.A.J When employees are trained to work safely they should be able to anticipate and avoid injury from job-related hazards.
  2. 2. Eye and Face Protection Thousands of people are blinded each year from work-related eye injuries. According to the Bureau of Labor Statistics (BLS), nearly three out of five workers are injured while failing to wear eye and face protection.
  3. 3. EYEWEAR LABELS All eyewear must be labeled with wavelength and optical density.
  4. 4. CDRH CLASS WARNING LABELS CLASS II LASER PRODUCT Laser Radiation Do Not Stare Into Beam Helium Neon Laser 1 milliwatt max/cw CLASS IV Laser Product VISIBLE LASER RADIATION- AVOID EYE OR SKIN EXPOSURE TO DIRECT OR SCATTERED RADIATION Argon Ion Wavelength: 488/514 nm Output Power 5 W Class II Class IIIa with expanded beam Class IIIa with small beam Class IIIb Class IV Laser-Professionals.com
  5. 5. LASER PROTECTIVE BARRIER
  6. 6. INTERNATIONAL LASER WARNING LABELS Symbol and Border: Black Background: Yellow Legend and Border: Black Background: Yellow Laser-Professionals.com INVISIBLE LASER RADIATION AVOID EYE OR SKIN EXPOSURE TO DIRECT OR SCATTERED RADIATION CLASS 4 LASER PRODUCT WAVELENGTH 10,600 nm MAX LASER POWER 200 W EN60825-1 1998
  7. 7. CLASS 4 LASER ND:YAG 1064 nm 100 Watts Max. Average Power VISIBLE and/ or INVISIBLE LASER RADIATION-AVOID EYE OR SKIN EXPOSURE TO DIRECT OR SCATTERED RADIATION. Controlled Area Warning SignLaser-Professionals.com
  8. 8. SKIN BURN FROM CO2 LASER EXPOSURE Accidental exposure to partial reflection of 2000 W CO2 laser beam from metal surface during cutting Laser-Professionals.com
  9. 9. HUMAN EYE Choroid Aqueous Cornea Macula Optic Nerve Sclera Vitreous Retina Lens Laser-Professionals.com
  10. 10. 25 µ Photo courtesy of U S Air Force THERMAL BURNS ON PRIMATE RETINA Laser-Professionals.com
  11. 11. MULTIPLE PULSE RETINAL INJURY Laser-Professionals.com
  12. 12. • Most beam injuries occur during alignment. • Only trained personnel may align (NO EXCEPTIONS!) • safety eyewear is required for class 3B and class 4 beam alignment. • ANSI REQUIRES approved, written alignment procedures for ALL class 4 laser alignment activities and recommends them for class 3B. SAFE BEAM ALIGNMENT
  13. 13. Photo courtesy of U S Army Center for Health Promotion and Preventive Medicine EYE INJURY BY Q-SWITCHED LASER Retinal Injury produced by four pulses from a Nd:YAG laser range finder. Laser-Professionals.com
  14. 14. Photo-litho-graphy • Photo-litho-graphy: latin: light-stone-writing • Photolithography: an optical means for transferring patterns onto a substrate. • Patterns are first transferred to an imagable photoresist layer. • Photoresist is a liquid film that is spread out onto a substrate, exposed with a desired pattern, and developed into a selectively placed layer for subsequent processing. • Photolithography is a binary pattern transfer: there is no gray-scale, color, nor depth to the image.
  15. 15. Overview of the Photolithography Process 1. • Surface Preparation 2. • Coating (Spin Casting) 3. • Pre-Bake (Soft Bake) 4. • Alignment 5. • Exposure 6. • Development 7. • Post-Bake (Hard Bake) 8. • Processing Using the Photoresist as a Masking Film 9. • Stripping 10. • Post Processing Cleaning (Ashing)
  16. 16. 1] Surface Preparation Wafer Cleaning • Typical contaminants that must be removed prior to photoresist coating: • dust from scribing or cleaving (minimized by laser scribing) • atmospheric dust (minimized by good clean room practice) • abrasive particles (from lapping or CMP) • lint from wipers (minimized by using lint-free wipers) • photoresist residue from previous photolithography (minimized by performing oxygen plasma ashing) • bacteria (minimized by good DI water system) • films from other sources: – solvent residue – H 2O residue – photoresist or developer residue – oil – silicone
  17. 17. Wafer Priming • Adhesion promoters are used to assist resist coating. • Resist adhesion factors: • moisture content on surface • wetting characteristics of resist • type of primer • delay in exposure and prebake • resist chemistry • surface smoothness • stress from coating process • surface contamination • Ideally want no H 2O on wafer surface – Wafers are given a “singe” step prior to priming and coating 15 minutes in 80-90°C convection oven
  18. 18. 2] Coating (Spin Casting)
  19. 19. Photoresist Spin Coating • Wafer is held on a spinner chuck by vacuum and resist is coated to uniform thickness by spin coating. • Typically 3000-6000 rpm for 15-30 seconds. • Resist thickness is set by: – primarily resist viscosity – secondarily spinner rotational speed • Resist thickness is given by t = kp 2 /w 1/2 , where – k = spinner constant, typically 80-100 – p = resist solids content in percent – w = spinner rotational speed in rpm/1000 • Most resist thicknesses are 1-2 mm for commercial Si processes.
  20. 20. Spin Coater
  21. 21. 3] Pre-Bake (Soft Bake)
  22. 22. Prebake (Soft Bake) • Used to evaporate the coating solvent and to densify the resist after spin coating. • Typical thermal cycles: – 90-100°C for 20 min. in a convection oven – 75-85°C for 45 sec. on a hot plate • Commercially, microwave heating or IR lamps are also used in production lines. • Hot plating the resist is usually faster, more controllable, and does not trap solvent like convection oven baking.
  23. 23. Mask Aligner 4] Alignment
  24. 24. Mask to Wafer Alignment - 1 – 3 degrees of freedom between mask and wafer: (x,y,q) – Use alignment marks on mask and wafer to register patterns prior to exposure. – Modern process lines (steppers) use automatic pattern recognition and alignment systems. • Usually takes 1-5 seconds to align and expose on a modern stepper. • Human operators usually take 30-45 seconds with well-designed alignment marks.
  25. 25. Mask to Wafer Alignment - 2 • Normally requires at least two alignment mark sets on opposite sides of wafer or stepped region. • Use a split-field microscope to make alignment easier:
  26. 26. Some more Alignment Marks
  27. 27. Optical Exposure • Projection Optics • Numerical Aperture • Raleigh Criterion • Coherence • Optical Correction & Phase Shift •Selection
  28. 28. 4] Alignment
  29. 29. 5] Exposure
  30. 30. 6] Development
  31. 31. 7] Post-Bake (Hard Bake)
  32. 32. 8 ] Processing Using the Photoresist as a Masking Film
  33. 33. 9 ] Stripping
  34. 34. 10 ] Post Processing Cleaning (Ashing)
  35. 35. STEPPER
  36. 36. The Head of a Stepper
  37. 37. Projection Lithography Requirements – b = minimum feature size (spot or line) – 2b = minimum period of line-space pattern – l = exposure wavelength – Using b = f qmin , obtain that b » l/2NA. – The depth of focus can be shown to be d f = ± l/2(NA)2 – A “voxel” is a volume pixel. – For highest resolution lithograpy, desire the tallest aspect ratio voxel. – Thus, wish to maximize the ratio d f /b = 1/NA. – SO: it all depends upon the NA of the lens!
  38. 38. • Short exposure wavelengths can create standing waves in a layer of photoresist. Regions of constructive interference create increased exposure. • These can impair the structure of the resist, but can be eliminated by: – use of multiple wavelength sources – postbaking • Effects are most noticeable at the edge of the resist. • Standing waves are enhanced by reflective wafer surfaces. • If the wafer or substrate is transparent, reflections from the aligner chuck can create standing wave patterns, also. – This can be eliminated by using: • a flat black chuck (anodized aluminum) • an optical absorber under the wafer (lint free black paper) • a transparent glass chuck (used on Karl Suss MJB3) • Exposures can be greatly miscalculated by the presence of standing waves and reflective wafers or chucks.
  39. 39. WHAT IS A PHOTOMASK? Photomasks are high precision plates containing microscopic images of electronic circuits. Photomasks are made from very flat pieces of quartz or glass with a layer of chrome on one side. Etched in the chrome is a portion of an electronic circuit design. This circuit design on the mask is also called geometry.
  40. 40. MATERIAL USED TO MAKE PHOTOMASKS: There are four types of material used to make photomasks; quartz (the most commonly used and most expensive), LE, soda lime, and white crown. The mask sizes can range from 3 inches square to 7 inches square and 7.25 inches round. The thickness of the masks ranges from 60 mils to 250 mils. Currently the most common sizes of masks used are 5 inches square 90 mils thick and 6 inches square 250 mils thick. The quartz or glass (or substrate) has a layer of chrome on one side. The chrome is covered with an AR (anti-reflective) coating and a photosensitive resist. The substrate with chrome, AR, and resist is known as a blank.
  41. 41. Standard Elements of PhotoMask
  42. 42. Defects in Photomask
  43. 43. Postbake (Hard Bake) - 1 • Used to stabilize and harden the developed photoresist prior to processing steps that the resist will mask. • Main parameter is the plastic flow or glass transition temperature. • Postbake removes any remaining traces of the coating solvent or developer. • This eliminates the solvent burst effects in vacuum processing. • Postbake introduces some stress into the photoresist. • Some shrinkage of the photoresist may occur. • Longer or hotter postbake makes resist removal much more difficult.
  44. 44. Postbake (Hard Bake) • Firm postbake is needed for acid etching, e.g. BOE. • Postbake is not needed for processes in which a soft resist is desired, e.g. metal liftoff patterning. • Photoresist will undergo plastic flow with sufficient time and/or temperature: – Resist reflow can be used for tailoring sidewall angles.
  45. 45. Photoresist Removal (Stripping) • Want to remove the photoresist and any of its residues. • Simple solvents are generally sufficient for non-postbaked photoresists: – Positive photoresists: • acetone • trichloroethylene (TCE) • phenol-based strippers (Indus-Ri-Chem J-100) – Negative photoresists: • methyl ethyl ketone (MEK), CH 3 COC 2 H 5 • methyl isobutyl ketone (MIBK), CH 3 COC 4 H 9 • Plasma etching with O 2 (ashing) is also effective for removing organic polymer debris. – Also: Shipley 1165 stripper (contains n-methyl-2-pyrrolidone), which is effective on hard, postbaked resist.
  46. 46. PhotoResist
  47. 47. Advantages of Positive Photoresists • Most commonly used in the IC industry. • Superior to negative photoresists because: – They do not swell during development. – They are capable of finer resolution. – They are reasonably resistant to plasma processing operations. Positive PhotoResist
  48. 48. Requirements: the Photoactive Component • Need an overlap of the absorption spectrum with the emission spectrum of the exposure source, e.g. a Hg lamp. • Need bleachability at the exposure wavelength so that the photoreaction is able to reach the resist-substrate interface. • Need compatibility with the base resin (novolac) so that the two form a single, miscible phase. • Need thermal stability so that the photoactive dissolution inhibitor does not break down at prebake temperatures. • Photoactive dissolution inhibitors are often modified to alter their spectral absorption, thermal stability, and miscibility characteristics.
  49. 49. Bleaching of a Positive Photoresist – The solution to the coupled Dill equations predicts a sharp boundary between exposed and unexposed regions of the resist. The boundary is the front of a bleaching edge which propagates downward to the substrate as the resist is exposed. This makes the wall angle more dependent upon the {A,B,C} Dill parameters than upon the exposure wavelength, and gives positive photoresists very high resolution.
  50. 50. Primary Components of a Positive Photoresist • Non-photosensitive base phenolic resin – usually novolac • Photosensitive dissolution inhibitor – usually a DQ-derived compound • Coating solvent – n-butyl acetate – xylene Secondary Components of a Positive Photoresist • Antioxidants • Radical scavengers • Amines to absorb O 2 and ketenes • Wetting agents • Dyes to alter the spectral absorption characteristics • Adhesion promoters • Coating aids
  51. 51. Negative Photoresist Ingredients • 1. Non-photosensitive substrate material • 2. Photosensitive cross-linking agent • 3. Coating solvent • 4. Other additives: (usually proprietary) – antioxidants – radical scavengers – amines; to absorb O2 during exposure – wetting agents – adhesion promoters – coating aids – dyes Negative PhotoResist
  52. 52. Negative Photoresist Development - 1 • The unexposed (uncross-linked) areas of resist as well as polymer chains that have not been cross-linked to the overall network of the gel must be dissolved during development. • Negative photoresist developers are solvents which swell the resist, allowing uncross- linked polymer chains to untangle and be washed away. • A sequence of solvents is often used to keep the swelling reversible. • The swelling of the resist during development is the largest contributor to loss of features and linewidth limitations.
  53. 53. The Gel Point – All sites for cross-linking (chromophores) are equally likely; thus, larger polymer chains are more likely to bind together than small ones. – A many-branched supermolecule results from increased exposure. – This supermolecule permeates the irradiated area forming a lattice which solvent atoms can penetrate, but not disperse. – The polymer chains have at this point been rendered insoluble to the solvent, and the exposure required to produce this is called the Gel Point.

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