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. 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. 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. EYEWEAR LABELS All eyewear must be labeled with wavelength and optical density.
- 6. 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
- 8. 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
- 10. 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
- 11. 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
- 13. 25 µ Photo courtesy of U S Air Force THERMAL BURNS ON PRIMATE RETINA Laser-Professionals.com
- 14. MULTIPLE PULSE RETINAL INJURY Laser-Professionals.com
- 15. • 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
- 16. 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
- 17. 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.
- 18. 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)
- 19. 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
- 20. 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
- 21. 2] Coating (Spin Casting)
- 22. 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.
- 23. Spin Coater
- 27. 3] Pre-Bake (Soft Bake)
- 28. 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.
- 30. 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.
- 31. 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:
- 32. Some more Alignment Marks
- 33. Optical Exposure • Projection Optics • Numerical Aperture • Raleigh Criterion • Coherence • Optical Correction & Phase Shift •Selection
- 34. 4] Alignment
- 36. 5] Exposure
- 37. 6] Development
- 38. 7] Post-Bake (Hard Bake)
- 39. 8 ] Processing Using the Photoresist as a Masking Film
- 40. 9 ] Stripping
- 41. 10 ] Post Processing Cleaning (Ashing)
- 44. STEPPER
- 45. The Head of a Stepper
- 51. 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!
- 54. • 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.
- 59. 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.
- 61. 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.
- 62. Standard Elements of PhotoMask
- 66. 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.
- 67. 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.
- 68. 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.
- 69. PhotoResist
- 75. 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
- 77. 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.
- 78. 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.
- 80. 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
- 82. 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
- 83. 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.
- 84. 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.
Notas del editor
- All eyewear must be labeled with the optical density and wavelength for which it provides protection. In many cases the same eyewear will provide a different optical density at different wavelengths. Optical Density curves for all eyewear is available from the manufacturers. In research situations it is sometimes necessary to use eyewear that is not labeled for the specific wavelengths in use. In these cases, eyewear data must be available in the laboratory.
- A CAUTION label means that a laser is visible and that it cannot deliver more than 1 mW through the pupil of the eye. Only the aversion response is needed for protection. A DANGER label means that the laser is a class 4, a class 3b, or a class 3a that has a small beam that can deliver more than 1 mW through a 7-mm pupil. The class of the laser is stated in the lower right corner of the class warning label. Laser products are always labeled according to the requirements of the federal standard. This means that many low power IR diodes that have danger labels stating they are class 3b actually produce no hazard and may be treated as class 1 lasers under the ANSI Standard. Examples of this include diodes with wavelengths of 1.55 microns used in fiber optic communications systems. A 1 mW diode is labeled as class 3b under the federal standard, but it is not really a hazard and can be treated as a class 1 under the ANSI Standard. In fact, the ANSI class 1 limit is 9.6 mW. It is important that workers understand the actual hazard associated with the lasers they are using. The power levels stated on class warning labels are often greater than the laser can actually produce. The correct values may be found in the printed product information.
- Laser protective barriers are often used to enclose laser hazards when an industrial laser system must be operated with the beam exposed during maintenance or service. Laser protective barriers and curtains can also be used to limit the NHZ inside laser controlled areas. These barriers are often used to protect entryways, computer work stations, and workbenches where workers are likely to remove laser protective eyewear. It is especially important that no direct optical path exist between laser optics tables and computer stations in laser laboratories.
- The international label is now acceptable on lasers sold in the United States. The class 1 limits under the international standard are almost the same as the ANSI Standard. Many low power IR lasers that are class 3b under the U S FLPPS are class1 under the international standard. The information in the yellow rectangle always includes the class of the laser product and usually contains the laser type, wavelength, and power.
- A danger sign must be posted at each entryway to a class 4 laser controlled area. If the hazard in a class 3b controlled area is limited, the warning sign may be posted inside the controlled area instead of on the door to the area. Only authorized personnel may enter a laser controlled area unless approval is given by an authorized laser operator. When authorized laser operators enter the laser controlled area, they must first determine the status of laser operations inside the controlled area and then follow approved procedures.
- Most laser skin injuries are thermal in nature. Exposure to high power beams at all wavelengths can result in skin burns. These burns usually do not lead to long-term disability, but they can have painful short-term consequences. Far IR light, such as that from CO2 lasers, is absorbed strongly by water in the skin and results in a surface burn. Near IR light with wavelengths close to 1 m, such as that from a Nd:YAG laser, penetrates more deeply into tissue and can result in deeper, more painful burns. If a high power laser beam is focused on the skin, it can vaporize tissue and drill a hole or produce a cut if the beam or tissue is moving. CW beams will cauterize the tissue preventing bleeding. Focused short pulses form repetitive pulse Q-switched lasers vaporize tissue without heating the surrounding tissue enough to cauterize. Exposures to focused Q-switched beams can result in cuts several millimeters deep that bleed freely. Photochemical skin injuries include sunburn and the possibility of promoting skin cancer by repeated low level exposures over long periods of time. The best way to avoid this issue is to enclose high power UV beams.
- The cornea of the eye is the outer layer. It is a transparent protein chemically similar to the white of an egg. When long wavelength CO2 laser light strikes the cornea, it heats it much like a hot frying pan heats an egg white. Ultraviolet exposure of the cornea produces a photochemical effect called photokeratitis, also known as welder’s flash. This is a painful but temporary condition. The lens of the eye absorbs ultraviolet light. The long term effect is the formation of scar tissue on the surface of the lens. This is called a cataract. Reducing UV exposure is important in preventing cataracts later in life. Polycarbonate shop glasses block the wavelengths most likely to cause cataracts. The macula is the area of the retina with the greatest concentration of cones for color vision and high visual acuity. Damage to the macula will result in the greatest loss of vision. The fovea is a dip in the center of the macula.
- This image shows the macula of the eye of a rhesus monkey. It has a diameter of about 2 mm. (Yours is a little bigger.) The light spots on this retina were produced by 0.25 s exposures to a green laser beam with a power of 10 mW. Each of these exposures heated the retinal tissue to the point that the protein cooked, producing a “white burn”. This is the most common type of laser eye injury in humans. It is likely that thousands of people have received these small retinal burns. They are permanent blind spots. If the burn is outside the macula, the effect on vision is small. If the burn is inside the macula, the effect is much greater. One such burn in the center of the macula will mean that you cannot thread a needle using that eye. A slightly larger spot or multiple spots will make reading difficult. This type of injury can be prevented by wearing laser safety eyewear.
- This is an image of the retina of a human who experienced an eye injury from a repetitive pulse near infrared laser. The beam was invisible. In such cases people do not usually realize they are being exposed until their vision has been severely effected. The person’s eye was moving during this exposure. This resulted in a line of laser burns on the retina. This is a color enhanced image to better show the laser damage. The macula of the eye is located out of the photo to the lower left. This individual was lucky that the damage did not extend into the macula. The laser safety eyewear would have prevented this injury.
- Safety during beam alignment is of critical importance. Most eye injuries occur when untrained personnel attempt beam alignment without approved, written procedures and laser safety eyewear. Most of those injured are students. Only personnel who have completed laser safety training should ever perform laser alignment. Alignment of many research systems requires specific training on the system by experienced personnel. Written alignment procedures are required for class 4 laser alignment and are recommended for class 3b alignment. Alignment procedures should be written by experienced laser personnel and approved by the LSO. These procedures should identify beam hazards during alignment and specify the control measures and eyewear to be used during alignment.
- This is a human eye injury resulting from four pulses into the macular region from an AN/GVS-5 Nd:YAG laser rangefinder. The pulse duration was about 20 ns and the pulse energy was about 15 mJ. The safe exposure limit for this pulse duration is 2 J per pulse. Thus, this exposure was 7500 times the safe level. Short pulse lasers produce the greatest eye hazards. Each short pulse results in a tiny explosion in the retina. The resulting shockwave causes severe damage to the retinal tissue. This photo was taken three weeks after the exposure. It shows the permanent destruction of the macular region. Visual acuity in the eye is approximately 20/400 and will not improve. This injury could not have occurred if the individual had been wearing the appropriate laser safety eyewear.
- 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 2 O on wafer surface – Wafers are given a “singe” step prior to priming and coating • 15 minutes in 80-9°C convection oven