GB2229314A - High pressure gas laser - Google Patents
High pressure gas laser Download PDFInfo
- Publication number
- GB2229314A GB2229314A GB9005397A GB9005397A GB2229314A GB 2229314 A GB2229314 A GB 2229314A GB 9005397 A GB9005397 A GB 9005397A GB 9005397 A GB9005397 A GB 9005397A GB 2229314 A GB2229314 A GB 2229314A
- Authority
- GB
- United Kingdom
- Prior art keywords
- lasing
- lasing region
- excimer laser
- laser
- region
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Withdrawn
Links
- 230000005684 electric field Effects 0.000 claims description 4
- 238000000034 method Methods 0.000 claims description 3
- 239000007789 gas Substances 0.000 description 47
- 230000005855 radiation Effects 0.000 description 5
- 239000000203 mixture Substances 0.000 description 4
- 229910052736 halogen Inorganic materials 0.000 description 3
- 150000002367 halogens Chemical class 0.000 description 3
- 239000012535 impurity Substances 0.000 description 3
- 238000004519 manufacturing process Methods 0.000 description 3
- 230000035939 shock Effects 0.000 description 3
- XKRFYHLGVUSROY-UHFFFAOYSA-N Argon Chemical compound [Ar] XKRFYHLGVUSROY-UHFFFAOYSA-N 0.000 description 2
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 description 2
- CURLTUGMZLYLDI-UHFFFAOYSA-N Carbon dioxide Chemical compound O=C=O CURLTUGMZLYLDI-UHFFFAOYSA-N 0.000 description 2
- 238000001816 cooling Methods 0.000 description 2
- 238000009826 distribution Methods 0.000 description 2
- 238000003754 machining Methods 0.000 description 2
- 238000001228 spectrum Methods 0.000 description 2
- ZAMOUSCENKQFHK-UHFFFAOYSA-N Chlorine atom Chemical compound [Cl] ZAMOUSCENKQFHK-UHFFFAOYSA-N 0.000 description 1
- PXGOKWXKJXAPGV-UHFFFAOYSA-N Fluorine Chemical compound FF PXGOKWXKJXAPGV-UHFFFAOYSA-N 0.000 description 1
- KTQPXWUHSJNRDF-UHFFFAOYSA-N [Ar].[Cl].[F] Chemical compound [Ar].[Cl].[F] KTQPXWUHSJNRDF-UHFFFAOYSA-N 0.000 description 1
- 230000002411 adverse Effects 0.000 description 1
- 238000013459 approach Methods 0.000 description 1
- 229910052786 argon Inorganic materials 0.000 description 1
- 229910002092 carbon dioxide Inorganic materials 0.000 description 1
- 239000001569 carbon dioxide Substances 0.000 description 1
- 239000000919 ceramic Substances 0.000 description 1
- 238000006243 chemical reaction Methods 0.000 description 1
- 229910052801 chlorine Inorganic materials 0.000 description 1
- 239000000460 chlorine Substances 0.000 description 1
- 238000004140 cleaning Methods 0.000 description 1
- 230000008602 contraction Effects 0.000 description 1
- 230000002596 correlated effect Effects 0.000 description 1
- 238000007599 discharging Methods 0.000 description 1
- 230000007613 environmental effect Effects 0.000 description 1
- 229910052731 fluorine Inorganic materials 0.000 description 1
- 239000011737 fluorine Substances 0.000 description 1
- 229910052734 helium Inorganic materials 0.000 description 1
- 239000001307 helium Substances 0.000 description 1
- SWQJXJOGLNCZEY-UHFFFAOYSA-N helium atom Chemical compound [He] SWQJXJOGLNCZEY-UHFFFAOYSA-N 0.000 description 1
- 230000001939 inductive effect Effects 0.000 description 1
- 229910052743 krypton Inorganic materials 0.000 description 1
- DNNSSWSSYDEUBZ-UHFFFAOYSA-N krypton atom Chemical compound [Kr] DNNSSWSSYDEUBZ-UHFFFAOYSA-N 0.000 description 1
- 239000007788 liquid Substances 0.000 description 1
- 229910052751 metal Inorganic materials 0.000 description 1
- 239000002184 metal Substances 0.000 description 1
- 150000002739 metals Chemical class 0.000 description 1
- 238000012544 monitoring process Methods 0.000 description 1
- 229910052754 neon Inorganic materials 0.000 description 1
- GKAOGPIIYCISHV-UHFFFAOYSA-N neon atom Chemical compound [Ne] GKAOGPIIYCISHV-UHFFFAOYSA-N 0.000 description 1
- 229910052757 nitrogen Inorganic materials 0.000 description 1
- 229910052756 noble gas Inorganic materials 0.000 description 1
- 230000003287 optical effect Effects 0.000 description 1
- 229920002120 photoresistant polymer Polymers 0.000 description 1
- 231100000614 poison Toxicity 0.000 description 1
- 230000007096 poisonous effect Effects 0.000 description 1
- 229920000642 polymer Polymers 0.000 description 1
- 238000000275 quality assurance Methods 0.000 description 1
- 239000004065 semiconductor Substances 0.000 description 1
- 239000007787 solid Substances 0.000 description 1
- 230000003068 static effect Effects 0.000 description 1
- 239000000126 substance Substances 0.000 description 1
- 239000013077 target material Substances 0.000 description 1
- 230000035899 viability Effects 0.000 description 1
- 229910052724 xenon Inorganic materials 0.000 description 1
- FHNFHKCVQCLJFQ-UHFFFAOYSA-N xenon atom Chemical compound [Xe] FHNFHKCVQCLJFQ-UHFFFAOYSA-N 0.000 description 1
Classifications
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/02—Constructional details
- H01S3/03—Constructional details of gas laser discharge tubes
- H01S3/036—Means for obtaining or maintaining the desired gas pressure within the tube, e.g. by gettering, replenishing; Means for circulating the gas, e.g. for equalising the pressure within the tube
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/14—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range characterised by the material used as the active medium
- H01S3/22—Gases
- H01S3/223—Gases the active gas being polyatomic, i.e. containing two or more atoms
- H01S3/225—Gases the active gas being polyatomic, i.e. containing two or more atoms comprising an excimer or exciplex
Landscapes
- Physics & Mathematics (AREA)
- Electromagnetism (AREA)
- Engineering & Computer Science (AREA)
- Plasma & Fusion (AREA)
- Optics & Photonics (AREA)
- Lasers (AREA)
Description
HIGH PRESSURE GAS LASER ' -. i a The present invention relates to high
pressure gas laserst that is to say, to gas lasers in which the lasing action takes place in a gas at a pressure approximately equal to or greater than atmospheric pressure, and more particularly to excimer lasers.
Excimer lasers have as their lasing medium a mixture of small amounts of fluorine chlorine argon, xenon, or krypton in a bulk gas consisting of helium, neon, or argon, or a mixture of these bulk gases.
High pressure gas lasers can be made to operate at many different wavelengths. For example, carbon dioxide gas lasers emit radiation in the infra red region of the electromagnetic spectrum. Excimer lasers on the other hand emit radiation in the ultra violet region of the electromagnetic spectrum.
Excimer lasers can be used in variety of industrial applications as a noncontact alternative to mechanical machining and marking of polymers, ceramics and certain metals. As opposed to infra red lasers, the photons of the light from an excimer laser interact directly with the chemical bonds within the atomic or molecular structure of the work piece. The short wavelength of excimer laser radiation makes it possible to achieve a machining method which produces therefore no heat affected zones. Other applications can be found in the semi-conductor industry, where excimer lasers can replace conventional UV lamps for the exposure of light sensitive photo resist for the manufacture of integrated circuits. Alternatively, they can be used to generate Xrays when focused on a suitable target material. The economic viability of excimer lasers in these applications is closely correlated with the production volume which can be reached with a given laser system. This depends strongly on the amount of energy that can be delivered to the work piece in each laser pulse, and the laser pulse repetition rate. A growing demand exists for excimer lasers which can supply a time-averaged power of UV radiation in the order of 1 kW at a pulse repetition frequency of or above approximately 1 kHz.
Excimer laser output power is closely related to the concentration of the halogen component in the laser gas but also to the concentration of impurities caused by the violent electric discharge between the electrodes and the chemical reaction of internal components of the laser with the halogen. Halogen is consumed slowly the during operation of the laser and impurity levels increase accordingly. There is a need for a gas quality monitoring system which continuously recycles the gas, cleaning it and adding components as necessary. Such systems are available commercially and their operation is based on cooling the gas to liquid nitrogen temperatures, when impurities will condense. Recycled gas is brought to operating temperature and pressure and is re-injected in the laser.
A pulse of laser light from an excimer laser is generated by applying an electric voltage across two electrodes in the laser cavity. After initial liberation, by other means (preionisation), of electrons in the laser gas an electric current will flow in a glow discharge between both electrodes, thus creating the population inversion required for lasing action. This electric current heats the gasp creating shocks, acoustic waves and thermal pertubations immediately after the electric discharge.
The laser light intensity distribution in a plane across the generated pulsed laser beam is closely related to the homogeneity of the medium between the electrodes. Best possible beam quality is achieved with a perfectly homogeneous laser medium, when intensity variations are caused solely by diffraction of the laser beam due to finite apertures in the laser. Refractive index variations in the laser gas from shocks, waves and thermal pertubations can cause severe deviation from the ideal diffraction-limited laser beam intensity distribution.
The pertubations in the laser cavity after each laser pulse will decay with a time constant which depends on the gas composition, discharge current and duration and a number of other factors. For increasing pulse repetition rate the time between pulses becomes too small for the gas in the laser cavity to become homogeneous again for the next laser pulse. This makes it necessary after each pulse to replace the gas in between the electrodes with a continuous flow of fresh, undisturbed laser gas through the laser cavity. The velocity of the gas flow through the laser cavity depends principally on the repetition rate at which the laser will be required to operate and the dimensions of the cavity.
Shocks and acoustic waves caused by the electric discharge travel away from the discharge area at up to supersonic velocities. It is not sufficient to replace just the body of gas geometrically between the electrodes in order for the next pulse to take place in a homogeneous medium. This can be expressed in the clearing ratio or flush factor having a value above unity. In order to reach a high clearing ratio with a given gas volume flow, the gas flow in an excimer laser is transverse to the laser cavity. Excimer lasers require a clearing ratio of up to 4 for pulse repetition rates of up to at least 1.5 kHz. It is possible that higher pulse repetition rates would require a larger clearing ratio. Generally, excimer lasers operated at several kHz pulse repetition rates require a flow velocity transverse to the discharge area of at least loom/S.
As opposed to low pressure gas flow lasers, which have a Reynolds number in the discharge area that is far below the limit above which the flow becomes turbulent, the high pressure laser described here has a Reynolds number of typically 105-106 in the discharge area and as a consequence the flow is turbulent. Care must be taken in the design of the laser that the amount of turbulence in the flow is sufficiently low as not to cause significant variations in the refractive index of the laser gas.
A gas flow through the discharge area can be obtained by discharging a pressurized supply of laser gas through the laser cavity into the surrounding atmosphere. The pressure in the laser cavity is determined by the supply pressure and the nature of the gas. This type of gas flow can be applied if burst mode operation only is required but is unsuitable for industrial, or indeed, any application which requires to some extent a continuous operation of the laser. It is exceedingly wasteful in laser gas and may present an important health and environmental hazard in the case of excimer lasers. An industrially viable technique is to circulate the laser gas in a closed circuit which includes the laser cavity with its discharge electrodes and a circulator. It can also include a heat exchanger for removal of heat energy from the electrical discharge and circulator,'hnd one or more settling chambers to achieve a sufficiently low level of turbulence.
The circulator in the gas flow circuit can be of a number of different designs. Cross-flow fans have a shape suitable for transverse gas flow through the laser cavity ' and can be used to deliver moderate gas flow volume and dynamic pressure. However, they have a narrow operating range. For more demanding requirements one or more propellers can be used, but these induce significant adverse rotational turbulence. Also they must be operated at high rotational speeds. As with a cross-flow fan, a propeller has a narrow operating regime, unless the blades have a variable pitch. Centrifugal or radial fans on the other hand, are capable of delivering large flow volumes at high dynamic pressures without the disadvantages or cross-flow fans and propellers. They have a comparatively high efficiency. Gas flow can enter a single inlet centrifugal fan from one side only; it can enter a double inlet centrifugal fan from either side.
The mechanical power that is required to accelerate a gas is proportional to the mass density of the gas and to the cube of the velocity. In a practical excimer laser the power required can be as much as several tens of W. In a highly efficient flow circuit, similar to a wind-tunnel, the losses due to friction and turbulence can be kept small so that the gas still has a large proportion of kinetic energy as it re- enters the circulator. This will reduce the power requirement of the circulator. An excimer laser, having an average power above 1 kW and a fast gas flow of 100 m/s through its cavity, will be very heavy, large and expensive if it is designed to windtunnel standards. Furthermore, the operating pressure of up to several times atmospheric pressure, and the use of a corrosive, highly poisonous gas component (chlorine or fluorine) make strict quality assurance of all components of the vessel essential during manufacture, which greatly adds to cost. Another disadvantage of such a vessel is the large volume of pressurized laser gas, since the noble gas components are very expensive.
The invention is an alternative to the wind-tunnel concept.
According to the present invention there is provided An excimer laser as hereinbefore defined comprising an elongated pressure vessel enclosing a closed system of ducting including a lasing region of rectangular cross-section, an electrode assembly extending longitudinally of the lasing region parallel to the major axis thereof and adapted to produce a transverse electric field such as to excite a gaseous lasing medium to lasing action, and at least one centrifugal or radial fan adapted to circulate the lasing medium through the lasing region in a direction perpendicular to both the longitudinal axis of the lasing region and the exciting electric field.
There may be provided also means for cooling the gaseous lasing medium after its passage through the lasing region and means for maintaining a desired composition of the lasing medium. In addition there may be provided means for preionising the gaseous lasing medum prior to its admission to the lasing region.
The invention will now be described, by way of example, with reference to the accompanying drawings in which, Figure 1 is a cut-away perspective view of an excimer laser embodying the invention and Figure 2 is a diagrammatic longitudinal section of the laser shown in Figure 1 illustrating the circulation path of the gaseous lasing medium.
Referring to the drawings, an excimer laser embodying the invention consists of a cylindrical pressure vessel 1 within which there is a lasing region 2 the optical axis 35 of which is parallel to that of the pressure vessel 1. Extending along the lasing region 2 are opposed anode and cathode electrodes 3 and 4, respectively. Electrical feed throughs 5 enable a high potential to be applied to the 1 anode 3 and current returns 6 complete the electrical circuit. A viewing port 7 enables the gas discharge in the lasing region 2 to be observed and an exit window 8 for laser radiation is provided. The lasing region 2 also is situated between the orifices of two inlet ducts 9 of equal rectangular cross-sections and a rectangular exhaust manifold which feeds two side exhaust ducts 10, of equal area, and a central rectangular cross-section bifurcated exhaust duct 11. The direction of gas flow across the lasing region 2 is shown by the arrow 12. A circulator housing 13 encloses two centrifugal fans 14, only one of which is shown. The centrifugal fans 14 are mounted on a common shaft 15 which emerges from the pressure vessel 1 through rotating seals, which are not shown, and is driven by an hydraulic motor 16. As can be seen from figure 2, the arrangement is such that the gaseous lasing medium exhausted from the lasing region 2 enters both sides of each of the centrifugal fans 14 and leaves them through the single outlets 21, which become the respective inlet ducts 9. The inlet ducts 9 have smoothly contracting cross-sections. In figure 2, the inlet gas flow to the lasing region 2 is shown as dotted arrows 22 and the exhaust gas flow from the lasing region 2 is shown as solid arrows 23. The cross-hatched regions 24 of figure 2 are regions which do not form part of the gas flow circuit.
1 Following an idealised path of an imaginary slug of lasing medium as it leaves one of the fans 14, its velocity vector changes direction smoothly through 1800 as it approaches the lasing region 2, without it acquiring a significant component parallel to the longitudinal axis of the lasing region 2. After the lasing medium has traversed the lasing region 2, the velocity vector is changed through 90-1800. depending on the exact location in the exhaust manifold at which the beginning of the return flow path of the slug of lasing medium occurs. The velocity vector of the slug of lasing medium then develops a component parallel to the axis of the pressure vessel 1 until when it reaches the appropriate inlet to one of the fans 14 the gas flow direction is completely axial with respect to the pressure vessel.
The steep contraction gradients of the sections of the flow loops leading to the lasing region 2 have negative (static) pressure gradients and so render that part of the gas flow stable. On the other hand, the diverging sections of the ducting leading from the lasing region to the inlets of the fans 14 act as diffusers with positive pressure gradients. The divergence gradients of the diffusers are kept small enough to prevent boundary layer detachment, which could cause energy-absorbing vortices to occur.
is- If desired, flow straightening blades and micro-turbulence inducing grids can be incorporated into the inlet ducts 9.
9 t
Claims (6)
1. An excimer laser as hereinbefore defined comprising an elongated pressure vessel enclosing a closed system of ducting including a lasing region of rectangular cross-section, an electrode assembly extending longitudinally of the lasing region parallel to the major axis thereof and adapted to produce a transverse electric field such as to excite a gaseous lasing medium to lasing action, and at least one centrifugal or radial fan adapted to circulate the lasing medium through the lasing region in a direction perpendicular to both the longitudinal axis of the lasing region and the exciting electric field.
2. An excimer laser according to claim 1 including two symmetrically disposed centrifugal fans each connected to a respective rectangular inlet to-the lasing region the combined widths of the inlets being less than the major transverse dimension of the lasing region, the shapes of the inlets being such as to promote stable flow of the input lasing medium to the lasing region.
3. An excimer laser according to claim 2 wherein the outlet from the lasing region.divides into a plurality of symmetrically disposed outlet ducts having a total ' cross-sectional area greater than that of the lasing region and so shaped as to act as diffusers.
4. An excimer laser according to claim 3 wherein the pressure gradient along the diffusers is insufficient to cause boundary layer detachment of the lasing medium.
5. An excimer laser according to any preceding claim wherein the system of ducting includes flow modifying devices prior to the lasing region.
0
6. An excimer laser substantially as hereinbefore described and with reference to the accompanying drawings.
X 1 Published 1990 atThe Patent O:Mce. State House.6671 High Holborn. London WC1R 4TP. Further copies maybe obtLmedfroM The Patent =ice Wee Branch, St Mary Cray. Orpington, Kent BRS 3RD. Printed bY Multiplex techniques ltd. St Mary CrAY, Kent. Con. 1.187
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB898905937A GB8905937D0 (en) | 1989-03-15 | 1989-03-15 | High pressure gas laser |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| GB9005397D0 GB9005397D0 (en) | 1990-05-02 |
| GB2229314A true GB2229314A (en) | 1990-09-19 |
Family
ID=10653367
Family Applications (2)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB898905937A Pending GB8905937D0 (en) | 1989-03-15 | 1989-03-15 | High pressure gas laser |
| GB9005397A Withdrawn GB2229314A (en) | 1989-03-15 | 1990-03-02 | High pressure gas laser |
Family Applications Before (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB898905937A Pending GB8905937D0 (en) | 1989-03-15 | 1989-03-15 | High pressure gas laser |
Country Status (2)
| Country | Link |
|---|---|
| DE (1) | DE4008138A1 (en) |
| GB (2) | GB8905937D0 (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2415287A (en) * | 2004-05-21 | 2005-12-21 | Istitut Teoreticheskoi I Prikl | Recirculating Gas Laser |
Citations (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4096449A (en) * | 1977-01-14 | 1978-06-20 | Universal Laser Corp. | Apparatus for producing an electric glow discharge in a flowing gas |
| US4457000A (en) * | 1982-03-03 | 1984-06-26 | Rockwell International Corporation | Shock wave suppressing flow plate for pulsed lasers |
-
1989
- 1989-03-15 GB GB898905937A patent/GB8905937D0/en active Pending
-
1990
- 1990-03-02 GB GB9005397A patent/GB2229314A/en not_active Withdrawn
- 1990-03-14 DE DE19904008138 patent/DE4008138A1/en not_active Withdrawn
Patent Citations (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4096449A (en) * | 1977-01-14 | 1978-06-20 | Universal Laser Corp. | Apparatus for producing an electric glow discharge in a flowing gas |
| US4457000A (en) * | 1982-03-03 | 1984-06-26 | Rockwell International Corporation | Shock wave suppressing flow plate for pulsed lasers |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2415287A (en) * | 2004-05-21 | 2005-12-21 | Istitut Teoreticheskoi I Prikl | Recirculating Gas Laser |
| GB2415287B (en) * | 2004-05-21 | 2008-05-07 | Istitut Teoreticheskoi I Prikl | Recirculating gas laser |
Also Published As
| Publication number | Publication date |
|---|---|
| GB9005397D0 (en) | 1990-05-02 |
| DE4008138A1 (en) | 1990-09-20 |
| GB8905937D0 (en) | 1989-04-26 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| WAP | Application withdrawn, taken to be withdrawn or refused ** after publication under section 16(1) |