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Laser Safety

Lasers refer to devices that are based on the use of amplified light which is generated from the stimulated emission of radiation. Lasers are used for a variety of purposes including micro-machining, cutting, welding, sealing, alignment, spectroscopy and surgery. Lasers have been extensively used in the semiconductor industry for a wide variety of applications including; mapping of wafers, wafer marking, scanning of barcodes, lithography, and alignment of optical beam paths. Lasers can emit radiation in a variety of wavelengths from the Infrared spectrum to the Ultraviolet Light spectrum. Laser beams can be colored or be invisible based upon the type of laser and the wavelength of the laser beam.

A Laser consists of a Laser media, a source of excitation energy and a beam collimator (optical resonator). The laser media is the solid, liquid, or gas (usually gas) that sustains the stimulated emission of energy initiated by the excitation source. As more and more atoms of the lasing media are stimulated, the intensity of the resulting emitted photons is increased.

The excitation mechanism or source of energy excites or "pumps" the electrons in the active media from a lower to a higher energy state. When the electronics fall back to the lower state they emit a photon of a specific wavelength. When the photons are collected they form a source of single wavelength radiation from the laser. Typically, lasers used in high-tech applications are based upon the use of electric current as the excitation energy source. An optical pumping mechanism such as xenon lamps are also occasionally used. Optical resonator is typically a pair of mirrors located at either end of the laser media. One mirror is a total reflector and the other is a partial reflector. This allows the beam to be reflected back and forth through the lasing media (stimulating more and more atoms), while still allowing a portion of the light energy being produced to escape and be emitted as a laser beam.

The light produced by lasers is very different from light produced by conventional sources. The light produced by a Laser is monochromatic (single wavelength), directional and coherent.

Monochromatic:

The color of light is determined by the wavelength of the light waves. Ordinary light is a mixture of many different wavelengths (white light contains all or most of the colors of the visible spectrum). Even ordinary colored light typically is made up of a rather broad spectrum of wavelengths. Lasers, on the other hand, typically emit radiation of only a single wavelength which is determined by the photons emitted by the laser media.

Directionality:

Ordinary light sources emit light in all directions. This light can be focused somewhat (e.g. spot lamps) but will still spread rather quickly. Lasers emit light in a single direction with a very narrow cone of divergence. Lasers have been developed which spread to cover only a half a mile when pointed at the moon, which is over 250,000 miles away.

Coherence:

Coherent light means that all waves are in phase with each other at every point. Ordinary light is not coherent.

Laser Hazards/Health Effects

When a laser beam impacts the human body, a portion of the energy from the light is absorbed by body tissues. If the laser is powerful enough, this absorbed energy can cause damage to the tissue. The two parts of the body most susceptible to injury from lasers are the eyes and the skin. The potential for injury to the eyes is far greater than for any other part of the body. Acute exposure may cause corneal and retinal burns. Also, eye injuries may occur at relatively lower power levels. Chronic exposure may cause cataracts. Injuries to the eye are wavelength dependant. Particular wavelengths are more damaging to certain parts of the eye. Wavelengths which are focusable on the retina are much more hazardous to the retina, while wavelengths not focusable are more hazardous to the outer surface or lens of the eye. The primary injury to the skin resulting from laser exposure is from burns. High power lasers are capable of producing very severe burns and even starting fires.

Maximum Permissible Exposure (MPE)

The MPE is the level of laser radiation to which a person may be exposed without suffering hazardous effects or adverse biological changes in the skin or eyes. The MPE is calculated based upon the power level of the laser beam, the duration of exposure, the wavelength of the laser beam and the type of beam (i.e., continuous or pulsed). The primary reference for the determination of MPE is the ANSI Standard Z 136.1, Safe Use of Lasers. The calculated MPE is specific to the individual laser and application being considered.

The classification scheme used in the United States and Europe for lasers is based upon the power level of the laser beam and the wavelength of the laser beam. The classification is based upon the following four classes of lasers:

• Low Power

Class 1: Lasers that are considered incapable of producing damaging radiation levels are exempt from control measures. These are the lowest powered lasers which are not considered hazardous under normal conditions. Class 2 and Class 2a: These are low power/low risk lasers in the visible light spectrum, for which the normal aversion reflex (blinking or turning away) is sufficient to provide eye protection. Overcoming this aversion reflex and looking directly into the beam can cause eye damage, however this is not a skin or fire hazard.

• Medium Power

Class 3a: These medium power lasers may be hazardous from direct viewing or from specular (mirror like) reflections. Aversion reflexes will not protect the eye from damage from the visible or invisible light source. This laser is not capable of causing skin injuries or hazardous diffuse reflection under normal usage nor is it considered a fire hazard. Class 3b: These medium power lasers may be hazardous from direct viewing or from specular reflections. Aversion reflexes will not protect the eye from damage from the visible or invisible light.

• High Power

Class 4: These high power lasers can produce serious hazards for the eyes from direct viewing of visible or invisible light due to specular reflections and diffuse reflections. Class 4 lasers are also capable of causing injury (burns) to the skin. High power lasers may also be a fire hazard.

Laser Safety Guidelines

Guidelines for the safe operation and use of lasers are set forth by the American National Standards Institute (ANSI); Z 136.1-1991, Safe Use of Lasers. These guidelines address the safety measures recommended for protecting employees working with lasers. Product Safety Design requirements for equipment which incorporate lasers are established by several countries: United States of America -US Federal Food and Drug Administration (US FDA),Center for Device and Radiological Health (CDRH). -Federal Regulations under 21 CFR 1000 - 1050. -CDRH Office of Compliance http://www.access.gpo.gov/nara/cfr/cfr-table-search.html Europe; IEC -EN 60825-1 -International Electrotechnical Commission http://www.iec.ch Japan -Japanese Industrial Standard; C 6801 -Japanese Standards Association http://www.jsa.or.jp/eng/catalog/frame.html SEMI S2 Guideline includes requirements for equipment using lasers -Must meet CDRH requirements -Provide information on lasers in the manuals (wavelength, power level and class of laser) Apply correct hazard warning labels based upon class of lasers present

US CDRH Laser Safety Design Requirements

Requirements depend on the class of laser being used: Class 1 laser: Certification of laser; no other requirements Class 2, 3, 4 laser: Certification, Interlocked protective housing, Labeling, Beam attenuator, Emission indicator Class 3b and 4 lasers (only): Remote interlock connector; e.g., signal to laser when room door opened to disable beam