Patent Application
20070002918
The present invention relates in general to excimer lasers. The invention relates in particular to damping of acoustic shock waves generated during high-repetition frequency, pulsed operation of such lasers.
During operation of an excimer or molecular fluorine (F2) laser, particularly when operating the laser at high pulse repetition frequency (PRF), for example, about 4 kilohertz (kHz), acoustic shock waves are generated at discharge electrodes of the laser. The acoustic shock waves propagate through the lasing (excimer or F2) gas and reach walls of the laser chamber in which the electrodes are located and in which the lasing gas is confined. The acoustic shock waves are reflected back into a discharge area between the electrodes in which optical gain in the lasing gas is generated by the discharge.
The acoustic shock waves are unwanted pressure changes in the gas that, when reflected back into the discharge area, disturb the performance of the laser system. The degree to which the energy efficiency and energy stability of the laser system are affected depends upon the PRF, as this frequency can interact with natural acoustic modes of the chamber.
In order to stabilize the operation and the energy efficiency of the laser it is necessary to damp these disturbances acoustic shock waves. Several approaches to such damping are described in the prior-art. One approach is to use angled reflectors in the laser chamber to assist in dissipating the acoustic shock waves. These reflectors may have different configurations. By way of example, the angled reflectors may have grooves and holes defined in the reflective surface, which scatter acoustic shock waves incident thereon as well as generate interference within the waves. The angled reflectors may also be covered with an acoustic shock-wave absorbing material, such as felt metal. Further, angled reflectors may have layers thereon that absorb incident acoustic shock waves. For example, a layered baffle-stack of multiple perforated plates may be used as layered angled reflectors.
In addition, the walls of the laser chamber may be configured to assist in the dissipation of the acoustic shock waves through absorption, scattering, and by generating interference within the reflected waves. For example, the layered baffle stack may be used along the walls of the laser chamber to absorb and scatter incident waves. The walls of the laser chamber may also be covered with an acoustic shock-wave absorbing material, such as felt metal. Alternatively, the walls of the laser chamber may have grooves, such as triangular or rectangular grooves, which scatter incident waves and generate interference within the waves.
It is believed that none of the prior-art proposed methods provides adequate suppression of these acoustic shock waves generated by the high pulse-repetition frequency operation. Accordingly, there is a need for more effective acoustic shock-wave suppression scheme than has hitherto been proposed.
The present invention is directed to damping gas-discharge-initiated acoustic disturbances in laser gas of an excimer or F2 laser. In one aspect, the invention comprises an electrode assembly including first and second electrodes arranged face-to-face, leaving a gap therebetween. When electrical power is applied to the electrodes a gas discharge is struck in laser gas in the gap. At least one of the electrodes is partly covered by a ceramic foam for damping an acoustic disturbance in the laser gas that is initiated by the striking of the gas discharge.
In one experiment, refractive index variations in the laser gas resulting from firing a discharge pulse were measured, from the time of firing a discharge pulse, for a prior-art electrode arrangement in which neither electrode had a foam covering, and for the same arrangement in which only one electrode had a foam covering in accordance with the present invention. Even with only the one electrode covered by ceramic foam, refractive index variations were significantly reduced compared with those of the prior-art electrode arrangement.
The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the present invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain the principles of the present invention.
Referring now to the drawings, wherein like components are designated by like reference numerals,
An electrode assembly 14 is located in upper portion 12B of chamber 12. Electrode assembly 14 includes an elongated upper electrode 16 (cathode) and an elongated lower electrode 17 (anode). The upper electrode 16 is attached to a cathode plate 36. Cathode 16 has a central, conductive, ridge or nose portion 16A extending from shoulder portions 16B disposed on opposite sides thereof. Cathode 17 has a central conductive ridge portion 17A extending from shoulder portions 17B. In this example, the electrodes have a cross-section in which shoulder portions thereof are rounded. This prior-art laser configuration is described in detail in U.S. Pat. No. 6,546,036, assigned to the assignee of the present invention, and the complete disclosure of which is hereby incorporated herein by reference.
Electrodes 16 and 17 are separated by a gap or discharge area 27 through which a gas mixture is flowed, as indicated by arrow B. The cathode plate 36 and a ceramic frame 34 are sealed by an O-ring 35 and form an upper portion 12B of laser chamber 12. A second O-ring 35 seals the ceramic frame 34 to the laser chamber 12. Located below electrode 17 in the lower portion of the laser chamber is a gas flow guide 23. A fan 22 is located close to the gas flow guide, which has a cut-away section 25 to accommodate the fan. Rotation of the fan indicated by arrow A, and the form and positioning of the gas flow guide, causes the laser gas to flow toward and into a channel 29A between an elongated spoiler 24A and anode 17 as indicated by arrows B. The gas from channel 29A passes through gap 27, flows through another channel 29B between another spoiler 24B, then returns to the fan for recirculation as indicated by arrow C. Spoilers 24A and 24B are attached to the ceramic frame 34. A dust precipitator 33 is used to clean the laser gas mixture of dust particles.
When a high voltage pulse is applied across the electrode assembly 14, a discharge 25 occurs in gap 27 between conductive ridge portions 16A and 17A of cathode 16 and anode 17, respectively. Typically a discharge pulse is one of a series of discharges (pulses), repeatedly fired. By way of example, a discharge pulse may have a duration between about 3.9 and 100.0 nanoseconds (ns), and the discharge pulses may be repeated at a frequency between about 1.0 and 8.0 kilohertz (kHz). The gas mixture is naturally heated as it is excited by the electrical discharge in gap 27. Heat exchangers 20 cool the heated gas after the gas exits gap 27.
One, two, or more pre-ionization units 32 are located in upper chamber-portion 12B and are used to pre-ionize the laser gas in the gap 27 before discharge 25 is initiated. Such pre-ionization helps make initiation of discharge repeatable in response to the applied voltage, and provides for a faster rise-time of the discharge than would be the case without pre-ionization. Pre-ionization units may include ultraviolet light emitting tubes (represented here in cross-section) extending along the length of the laser chamber. Alternatively, pre-ionization may be accomplished by a plurality of pin electrodes extending through (and insulated from) cathode plate 36. As pre-ionization arrangements are well known in the gas laser art, no further description thereof is presented herein.
It should be noted, here, that it is not necessary to cover all of the surfaces discussed above with the ceramic foam material to achieve shock wave damping in accordance with the present invention. At a minimum, however, at least one electrode, preferably the high-voltage electrode, must have a portion thereof covered by the ceramic foam. Indeed, in one experimental arrangement, covering only shoulder portions of cathode 16 with a ceramic foam resulted in a significant reduction of acoustic disturbances compared with those present in a corresponding prior-art laser. This result discussed is in more detail further hereinbelow. While covering other surfaces with ceramic foam can provide additional damping, other damping measures, including above-discussed prior-art damping measures, may be applied to these other surfaces without departing from the spirit and scope of the present invention.
The ceramic foam material comprises a matrix of pores or voids and sintered ceramic material. The pores or voids have a random size distribution about some nominal average size. The material can be characterised as having a median number of pores or voids per linear inch. Preferably, the material is selected to have an average number of pores (voids) per inch between about 20 and 80. One preferred ceramic foam is commercially available from Fraunhofer Institut, Keramische Technologien und Sinterwerkstoffe, Dresden, Germany, as material PPI 20-80, the name PPI, here, referring to the above-discussed porosity in pores per inch (ppi). This preferred material is formed from alumina (Al2O3) having a purity of about 99.5%. Ceramic materials with a high fraction of silicon (Si), carbon (C), or phosphorus (P) are to be avoided in an excimer laser as these materials are incompatible with the laser gases. Preferably, any ceramic material used should have a content of Si, C, or P less than about 1%. In any ceramic foam material, there should be a minimum of closed pores or voids. Such closed pores or voids could trap gas that could eventually leak therefrom into the laser chamber, and thereby possibly, eventually contaminate laser gases in the chamber. Preferably, the content of closed pores should be less than about 0.1% of the total number of pores. The ceramic foam can be attached to an electrode body with metal screws or clamps or by integrated, solid ceramic elements.
Another possible ceramic material for ceramic foam is zirconia (ZrO2). Zirconia ceramic foam material is commercially available from Drache-Umwelttechnik GmbH of Diez, Germany. In certain experiments a 60-ppi ZrO2 foam provided adequate damping but unfortunately had poor compatibility with the laser gases. It was not possible to passivate the laser chamber properly. The gas lifetime was 4 times less then for a prior-art laser, and the energy per laser output pulse was ½ of the expected value. It was not possible, because of lack of available resources, to determine whether the ZrO2 chemical composition, closed-pore content, purity of the material, or cleanliness of the material caused the observed problems.
In one method of manufacture, the ceramic foam is formed by first immersing polyester foam having a porosity about the same as the porosity of ceramic foam desired in a ceramic slurry, in a manner such that all surfaces of the polyester foam are covered with a thin layer of the slurry. The ceramic-covered polyester is then heated to a temperature sufficient to sinter the ceramic slurry. The polyester material, at this sintering temperature, is vaporized, leaving behind the sintered ceramic and voids forming the ceramic foam.
In ceramic foam manufactured in this manner, the average dimension of pores or voids is usually inversely proportional to the average pore-count in pores per inch. The relationship between porosity in ppi and the average pore size in millimeters (mm) can be approximated by the formula
ppi=1.6*25.4/Φpore, (1)
wherein Φpore is the average pore size in mm, 1.6 is a geometrical factor, and 25.4 is the numerical conversion factor required to reconcile the pore size specified in mm with the porosity per inch. Accordingly the above-discussed preferred range of porosity of between about 20 and 80 ppi transforms to a preferred range of average pore size between about 2.0 mm and 0.5 mm. One particularly preferred porosity is 60 ppi. A preferred thickness of a layer of the ceramic foam is about between about 1.0 mm and 10.0 mm. This provides adequate shock wave damping, consistent with adequate mechanical strength. Such a 60-ppi ceramic foam would have an average pore size of about 0.7 mm.
As the surface of the ceramic foam material is not smooth, laser gas flow through channel 29A and gap 27 can be adversely influenced depending on which surfaces are covered by the foam. The adverse influence may result, for example, from increased turbulence in the flowing gas or reduction of volume flow. This can reduce the maximum pulse repetition rate at which the laser can be operated.
Continuing with reference to
Pre-ionization units 32 produce acoustic shock waves in addition to those produced by discharge 25 between ridge portions 16A and 17A of electrodes 16 and 17. While the shock waves from the pre-ionization units are generally of lesser magnitude than the shock waves from the main discharge 25, it is nevertheless preferable to damp the pre-ionization shock waves in addition to damping the primary shock front coming from the main discharge. Such damping is achieved to some extent in laser 11 of
Another electrode configuration 80 in accordance with the present invention capable of minimizing arcing between electrodes is schematically depicted in
In electrode 80 the ceramic inserts act as an insulating barrier over the electrode body for minimizing arcing, similar to the insulating (ceramic) layers 69 of electrodes 66 and 67 in the lasers of
In above-discussed embodiments of the present invention, electrodes are depicted, for convenience of illustration, as being un-cooled, and having solid, conductive, body portions. While such a configuration will be adequate for many conditions of operation of an excimer laser, under some conductions, for example at high pulse repetition rates, it may be found advantageous to cool the electrodes. Cooling can be effected, for example, by providing tubes or channels with an electrode body and passing a cooling fluid through the tubes or channels. One skilled in the art may substitute a cooled electrode body for an un-cooled electrode body depicted herein, or substitute an electrode body having more than one component for any “one-piece” electrode body depicted herein without departing from the spirit and scope of the present invention.
As noted above the acoustic disturbances, to the damping of which this invention is directed, are unwanted pressure changes in the lasing gas. At the instant of igniting a discharge between the electrodes there is a rapid heating and expansion of the gas in the discharge. This causes an acoustic disturbance (shock front) to propagate outward from the discharge area. This disturbance is reflected from chamber walls and other components, including the electrodes themselves, absent any measures to prevent such reflection.
Reflected portions of the disturbance (shock fronts) return into the discharge area, through which the laser beam being generated by the discharge is propagating. The reflected shock fronts interfere with each other, causing a complex and rapidly changing distribution of pressure in the discharge area. The changes and distribution of pressure cause corresponding changes or disturbances in the refractive index of the laser gas, and the refractive index changes affect, among other parameters, the pointing or propagation direction of the laser beam. In an experiment to measure the effectiveness of the ceramic foam for damping acoustic disturbances, an effect representing variation of beam pointing of an excimer laser as a function of time was measured for a laser with and without the inventive shock-front damping.
In the experiment, the discharge chamber of the laser being evaluated was located at a distance of about 0.1 meters from an aperture, behind which was located a photodiode detector. The beam of a red pilot-laser was passed through the gap between the electrodes to provide a refractive index diagnostic laser beam. The diagnostic laser beam was directed through the chamber between the electrodes and was incident on the aperture. Because of the relative dimensions of the beam and the aperture, any change in beam pointing due to a change in the gas refractive index was manifest in a change in the signal from the photodiode in response to the incident beam. This change was used to provide a measure of the effectiveness of the ceramic foam in damping acoustic disturbances, i.e., gas refractive index disturbances.
One result of the experiment is presented in
The laser tested was a Lambda Physik® Model A4003 laser wherein the (uncovered) electrode configuration is similar to that of the electrodes of laser 10 of
It can be seen that even with only this minimum application of the ceramic foam to only one electrode, stability of the beam pointing at about 130 microseconds after the firing of the discharge pulse was dramatically improved. In numerical terms, between 130 and 300 microseconds (μs) the standard deviation of the upper trace is about 2.7 times greater than that of the lower trace. Equally, if not more important, is the fact that the range of disturbance is reduced more rapidly in the inventive arrangement. In the prior-art arrangement there is still significant disturbance in the gas after 250 μs. In a laser operating at 4 kHz, this is the time at which another discharge pulse would be fired. In the inventive arrangement, the range disturbance after 150 μs (about 6 kHz) has been reduced to less than that in the prior-art arrangement after 250 μs. Accordingly, the inventive arrangement should be capable of providing more stable laser operation at the same PRF as the prior-art arrangement, and the same stability of operation at a higher PRF than is possible in the prior-art arrangement.
In summary, the present invention is described above in terms of a preferred and other embodiments. The invention is not limited, however, to the embodiments described and depicted. Rather the invention is limited only by the claims appended hereto.
