Abstract:
Methods and apparatus are provided for cleaning and passivating laser discharge chambers with plasmas. In one embodiment, an oxygen based plasma is formed in a plasma source external to the laser discharge chamber by applying a radiofrequency signal to oxygen containing gases. The oxygen based plasma is drawn into the laser discharge chamber, where it reacts with contaminants and cleans internal surfaces. After cleaning, a fluorine based plasma is formed in the plasma source and drawn into the laser discharge chamber to passivate internal surfaces. In another embodiment, cleaning with the oxygen based plasma is not used since some level of cleaning is accomplished with the fluorine based plasma. In another embodiment, oxygen based plasmas and fluorine based plasmas are formed in the laser discharge chamber by applying a radiofrequency signal to a laser discharge chamber electrode. Plasma cleaning and passivation of laser discharge chambers is safer, more efficient, and more effective than conventional thermal cleaning and passivation processes.

Description:
BACKGROUND 
     1. Field of the Invention 
     The invention relates to methods and apparatus for cleaning and passivating laser discharge chambers. More particularly, the invention relates to methods and apparatus for cleaning and passivating laser discharge chambers utilizing plasmas. 
     2. Description of the Related Art 
     Gas lasers in which the lasing medium includes fluorine or fluorine compounds are the workhorse light sources for the integrated circuit lithography industry. In fluorine based gas lasers such as krypton fluoride (KrF) excimer lasers, argon fluoride (ArF) excimer lasers, and molecular fluorine (F 2 ) lasers, a high energy electrical discharge excites a gas mixture in a discharge chamber to produce a plasma which serves as the lasing medium. 
     The performance of such lasers is degraded by the presence of impurities in the discharge chamber inadvertently introduced as contaminants during the manufacturing process, introduced by exposure to the ambient environment, or produced by reactions between the gas mixture and contaminants or chamber materials. Such impurities include HF, CF 4 , COF 2 , SiF 4 , CO 2 , various hydrocarbons, and H 2 O. Impurities can degrade the profile of the laser beam by fouling optical components, reduce the lifetime of the gas fill by reacting with and consuming the gas mixture, and reduce output power by absorbing laser light and by quenching the excited species such as ArF, KrF, and F 2  that support lasing. Also, highly reactive impurities such as HF corrode the internal surfaces of the discharge chamber. Impurities are detrimental even at low concentrations. For example, it has been observed that the presence of CO 2  at concentrations as low as 30 parts per million in a KrF excimer laser plasma can reduce the output power of the laser by 5%. Consequently, the discharge chamber must be cleaned of impurities. 
     The plasma lasing medium includes highly reactive fluorine species which also corrode unprotected internal surfaces of the discharge chamber. Consequently, the materials in the discharge chamber must be passivated to protect them from the plasma lasing medium. 
     Laser discharge chambers for fluorine based gas lasers are conventionally cleaned and passivated with a thermal process such as the following. A discharge chamber is heated to approximately 100° C. and evacuated with a vacuum pump to a pressure of approximately 20 millitorr. This temperature and pressure is maintained for at least 8 hours, during which some of the volatile contaminants in the discharge chamber, such as water, are removed by the pump. The discharge chamber is then filled with a mixture of approximately 5% F 2  and approximately 95% helium, neon, or other inert gas at a pressure of approximately one atmosphere. The temperature is maintained at 100° C. for at least 4 hours, during which a fluorine based passivation layer forms on some of the internal surfaces of the discharge chamber. 
     The fluorine gas used in conventional processes poses a safety risk, as it is highly corrosive and highly toxic. Also, the conventional process is not entirely effective. The discharge chamber must undergo a subsequent 24 hour bum-in operation period, during which the gas mixture is replaced multiple times, before laser operation is satisfactory. Furthermore, the conventional process requires at least 12 hours, more typically 24 to 48 hours, and is therefore inefficient. 
     What is needed is a laser discharge chamber cleaning and passivation process that is safer, more effective, and more efficient than conventional cleaning and passivation methods. 
     SUMMARY 
     Methods and apparatus are provided for cleaning and passivating laser discharge chambers with plasmas. In one embodiment, an oxygen based plasma is formed in an external plasma source by inductively applying a radiofrequency signal to oxygen containing gases such as O 2 , N 2 O, and mixtures thereof. The oxygen based plasma is drawn into the laser discharge chamber, where it reacts with contaminants to produce volatile species which are removed by a vacuum pump, thereby cleaning the laser discharge chamber. 
     After the oxygen based plasma cleaning process, a fluorine based plasma is formed in the external plasma source by inductively exciting fluorine containing gases such as NF 3 , F 2 , CF 4 , SF 6 , and mixtures thereof with a radiofrequency signal. The fluorine based plasma is drawn into the laser discharge chamber, where it reacts with internal surfaces to form a protective passivation layer. The fluorine based plasma also reacts with contaminants to produce volatile species which are removed by the vacuum pump. Internal surfaces of the laser discharge chamber are thereby cleaned and passivated. Since the fluorine based plasma also cleans the chambers, the oxygen based plasma cleaning process is optional. 
     In another embodiment, oxygen based plasma and fluorine based plasmas are formed in the laser discharge chamber by applying a radiofrequency signal to a laser discharge chamber electrode and thereby exciting oxygen containing gases and fluorine containing gases. The oxygen and fluorine based plasmas react with contaminants and with internal surfaces to clean and passivate the laser discharge chamber. 
     Plasma cleaning and passivation of laser discharge chambers does not require the use of dangerous F 2  gas. Also, plasma cleaning and passivation is much less time consuming, and thus much more efficient, than conventional thermal cleaning and passivation processes. Moreover, laser discharge chambers cleaned and passivated with plasmas exhibit improved performance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of an apparatus including an external plasma source for cleaning and passivating a laser discharge chamber in accordance with one embodiment of the present invention. 
     FIG. 2 is a schematic diagram of an apparatus for cleaning and passivating a laser discharge chamber with internally generated plasmas in accordance with one embodiment of the present invention. 
     FIG. 3 is plot comparing the performance of laser discharge chambers passivated according to the present invention to laser discharge chambers passivated by conventional methods. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a schematic diagram of an apparatus for cleaning and passivating a laser discharge chamber  2  with plasmas according to one embodiment of the present invention. Laser discharge chamber  2  may be, for example, a discharge chamber for a KrF excimer laser such as the model 5000 KrF excimer laser manufactured by Cymer, Incorporated. The model 5000 discharge chamber has a volume of about 20 liters and internal surfaces of brass, electroless nickel plated aluminum, and ceramic. Laser discharge chamber electrode  3 , inside laser discharge chamber  2 , is typically a conducting metal or metal alloy such as brass. Other laser discharge chambers, such as discharge chambers for ArF excimer lasers and discharge chambers for molecular fluorine (F 2 ) lasers, may also be cleaned and passivated in accordance with the present invention. 
     Laser discharge chamber  2  is placed in contact with a heater  4 , which heats discharge chamber  2  to an elevated temperature, in one embodiment about 70° C., thereby driving volatile contaminants from internal surfaces of discharge chamber  2  and also facilitating subsequent passivating chemical reactions on those surfaces. Heater  4  may be an electrically heated metal plate on which discharge chamber  2  is placed and enclosed with a cover, for example. The present invention is independent of the type of heater used. In one embodiment, the temperature of discharge chamber  2  settles at about 70° C. after about 45 minutes of heating. 
     After being installed on heater  4 , laser discharge chamber  2  is connected to a purge gas line  6  at a discharge chamber valve  8 , to an external plasma source  10  at a discharge chamber window assembly  12 , and to a vacuum line  14  at a discharge chamber window assembly  16 . Advantageously, with this geometry plasmas may be drawn the length of discharge chamber  2  to uniformly clean and passivate internal discharge chamber surfaces. Of course, depending on the design of the chamber, connections to purge gas line  6 , plasma source  10 , and vacuum line  14  may be made in many other geometries and might utilize chamber windows, valves, and ports not shown in FIG.  1 . 
     All gas lines, vacuum lines, and valves are made from high purity stainless steel to minimize introduction of contaminants into discharge chamber  2 . Internal seals in valves and flanges are preferably made of fluorine-resistant metal. Seals made of fluorine resistant perfluoro elastomers such as Kalrez® are also acceptable, however. 
     In one embodiment, external plasma source  10  is a Delta Glow™ DG 300 or DG 600 inductively coupled high energy plasma source manufactured by Manitou Systems, Incorporated. Advantageously, the antenna of such an inductively coupled plasma source is not in physical contact with the plasma and thus does not introduce contaminants into discharge chamber  2  during the cleaning and passivating process. A quartz glass reactor tube in the Delta Glow™ plasma source is replaced with a 99.8% purity alumina (Al 2 O 3 ) tube to prevent etching of the reactor tube by fluorine based plasmas. Other external plasma sources, inductively or directly coupled, are used in alternative embodiments. 
     Next, programmable controller  18  opens pressure control valve  20  to allow vacuum pump  22  to evacuate vacuum line  14 , discharge chamber  2 , and external plasma source  10 . The exhaust from vacuum pump  22 , which in subsequent process steps may contain fluorine compounds, is passed through exhaust waste gas scrubber  24  to remove corrosive or toxic exhaust constituents. The conductance through plasma source  10 , discharge chamber  2 , and vacuum line  14  is sufficiently high that they are at essentially equal pressure. 
     In one embodiment, programmable controller  18  is a SYSMAC model C200HG programmable logic controller manufactured by Omron Electronics, Incorporated. Alternative embodiments employ other programmable controllers, or manual control. 
     Conventional vacuum pumps employing oil are vulnerable to attack by corrosive gases employed or generated in the cleaning and passivating processes. In one embodiment, vacuum pump  22  is a QDP80 dry pump with a pumping capacity of 80 liters per minute manufactured by BOC Edwards, Incorporated. 
     Controller  18  reads capacitance manometer  26 , which measures the pressure in vacuum line  14 , and controls pressure control valve  20  to set the pressure in vacuum line  14 , discharge chamber  2 , and plasma source  10  to about 20 millitorr. In one embodiment, capacitance manometer  26  is a CeramiCel® capacitance manometer manufactured by Varian, Incorporated, and pressure control valve  20  is a model 651CD2S1N pressure control valve manufactured by MKS Instruments, Incorporated. Capacitance manometers provide accurate and stable absolute pressure measurements, facilitating reproducible cleaning and passivation. 
     After the pressure has reached about 20 millitorr, controller  18  closes valve  20  and monitors the pressure for about 20 minutes to test for leaks. Other conventional leak testing techniques, such as helium leak testing, are also used. After proper installation has been verified, controller  18  reopens valve  20  and vacuum pump  22  again pulls the pressure down to about 20 millitorr. 
     In one embodiment, discharge chamber  2  is next cleaned with an oxygen based plasma formed in plasma source  10  from gases including oxygen containing gases such as oxygen (O 2 ), N 2 O, and mixtures thereof, prior to being cleaned and passivated with a fluorine based plasma formed in plasma source  10  from gases including fluorine containing gases such as NF 3 , CF 4 , F 2 , SF 6 , freons, and mixtures thereof. The fluorine based plasma cleaning and passivation process may be followed by additional oxygen based plasma and fluorine based plasma cleaning and passivation processes. In another embodiment, oxygen based plasma cleaning of discharge chamber  2  is not utilized, and discharge chamber  2  is next cleaned and passivated with a fluorine based plasma. In another embodiment, discharge chamber  2  is cleaned and passivated with an oxygen and fluorine based plasma formed from a mixture of oxygen containing and fluorine containing gases. 
     In one embodiment, for example, controller  18  opens valves  28  and  30  to allow oxygen to flow from oxygen supply  32 , through mass flow controller (MFC)  34 , external plasma source  10 , discharge chamber  2 , and vacuum line  14  to vacuum pump  22 . MFC  34 , in one embodiment a model 1259C-0050GK MFC manufactured by MKS Instruments, Incorporated, regulates the flow of oxygen to a rate of typically about 10 standard cubic centimeters per minute (sccm) to about 50 sccm. The pressure registered by capacitance manometer  26  rises to about 100 millitorr to about 1.5 torr, depending on the oxygen flow rate. During an optional purge period of from about 1 minute to about 10 minutes duration, flowing oxygen displaces other gases in plasma source  10 , discharge chamber  2 , and vacuum line  14 . 
     After the optional purge period, and with oxygen continuing to flow through plasma source  10 , discharge chamber  2 , and vacuum line  14 , controller  18  turns on radio frequency (RF) power supply  36 . Power supply  36  delivers about 100 Watts to about 600 Watts, in one embodiment about 400 Watts, of 13.56 MHz continuous RF power through impedance matching network  38  to external plasma source  10 . The RF power excites the oxygen in plasma source  10  to form an oxygen plasma containing reactive oxygen ions and radicals, which flows from plasma source  10  into and through discharge chamber  2 . The oxygen plasma oxidizes hydrocarbon contaminants in discharge chamber  2  to produce volatile reaction products, such as CO 2  and H 2 O, which are removed by vacuum pump  22 . Internal surfaces of discharge chamber  2  are thereby cleaned of contaminants. 
     Since the plasma is excited externally, all internal surfaces of discharge chamber  2  are at equal electrical potential. Advantageously, externally generated plasmas consequently interact uniformly with the internal surfaces of discharge chamber  2 . 
     In one embodiment, RF power supply  36  is a model MS600 power supply manufactured by Manitou Systems, Incorporated capable of delivering up to about 600 Watts at about 13.56 MHz in pulsed or continuous mode to plasma source  10 . Though 13.56 MHz is an industry standard, other radio frequencies can also be used. In an alternative embodiment, RF power supply  36  is a model CESAR 136 power supply manufactured by Dressler HF-Technik GmbH. Impedance matching network  38 , in one embodiment a model RFS-1004 automatic impedance matching network manufactured by RF Services, Incorporated, maximizes power transfer from RF power supply  36  to the plasma load. In an alternative embodiment, impedance matching network  38  is a model ATR impedance matching network manufactured by Manitou Systems, Incorporated. 
     An oxygen plasma is formed in plasma source  10  and drawn through discharge chamber  2  for a period of about 0.5 hours to about 2.0 hours, depending upon the RF power used. Higher RF powers produce a more reactive oxygen plasma, which requires less time to clean discharge chamber  2 . In one embodiment, the period of exposure to the oxygen plasma is a predetermined period known by experiment to be sufficiently long for the oxygen plasma to satisfactorily clean discharge chamber  2 . 
     In alternative embodiments, exposure to the oxygen plasma continues until an endpoint of the cleaning process is detected. An endpoint may be defined by concentrations of one or more chemical species in the plasma or exhaust gas reaching particular values. For example, the concentration of CO 2  in the gas flowing out of discharge chamber  2  decreases as hydrocarbon contaminants are depleted. Thus, an endpoint may be defined by the concentration of CO 2  decreasing to reach a particular value which indicates that the chamber is sufficiently clean. 
     In one embodiment, an endpoint is determined by residual gas analyzer (RGA)  40 , which is a model TSPTC 100(2100) RGA manufactured by Leybold Inficon, Incorporated. RGA  40 , which is in communication with computer  42  via a conventional RS-232 interface, monitors the concentrations of the various chemical species present in vacuum line  14 . In another embodiment, an endpoint is determined by optical monitor  44 . Optical monitor  44  excites chemical species present in vacuum line  14  with an electrical discharge, and measures their optical to monitor their concentrations. In another embodiment, an endpoint is determined by monitoring the RF power reflected from plasma source  14 . The reflected RF power is known in the art to characterize the plasma. 
     At the end of the predetermined period of exposure to the oxygen plasma, or when an endpoint is detected, controller  18  closes valves  28  and  30  and turns off RF power supply  36 . Vacuum pump  22  pulls the pressure down to about 20 millitorr. 
     Next, discharge chamber  2  is cleaned and passivated with a fluorine based plasma formed in plasma source  10 . In one embodiment, for example, controller  18  opens valves  28  and  46  to allow NF 3  to flow from NF 3  supply  48 , through MFC  34 , external plasma source  10 , discharge chamber  2 , and vacuum line  14  to vacuum pump  22 . MFC  34  regulates the flow of NF 3  to a rate of typically about 5 sccm to about 25 sccm. The pressure rises to about 100 millitorr to about 1.5 torr, depending on the NF 3  flow rate. During an optional purge period of from about 1 minute to about 10 minutes duration, flowing NF 3  displaces other gases in plasma source  10 , discharge chamber  2 , and vacuum line  14 . 
     After the optional purge period, and with NF 3  continuing to flow through plasma source  10 , discharge chamber  2 , and vacuum line  14 , controller  18  turns on RF power supply  36 . Power supply  36  delivers about 100 Watts to about 600 Watts, in one embodiment about 400 Watts, of 13.56 MHz continuous RF power through impedance matching network  38  to plasma source  10 . The RF power excites the NF 3  in plasma source  10  to form a fluorine based plasma which contains reactive fluorine species such as F and F 2  radicals and ions and produces intense ultraviolet radiation. The fluorine based plasma flows from plasma source  10  into and through discharge chamber  2 . Advantageously, high concentrations of reactive fluorine species and intense ultraviolet radiation are introduced into discharge chamber  2  without requiring the use of F 2  as a precursor gas. 
     The fluorine based plasma reacts with contaminants in discharge chamber  2  to produce volatile reaction products, such as HF and SiF 4 , which are removed by vacuum pump  22 . The fluorine based plasma also reacts with the internal surfaces of discharge chamber  2  to form passivating layers which protect the surfaces from further reaction with fluorine based plasmas such as fluorine based plasma lasing media. For example, the fluorine based plasma reacts with Nickel surfaces to form stable NiF 2  layers, with stainless steel surfaces to form stable FeF 2  layers, and with alumina (Al 2 O 3 ) surfaces to form stable AlF 3  layers. This passivation process is the primary role of the fluorine based plasma. 
     The fluorine based plasma is produced in plasma source  10  and drawn through discharge chamber  2  for a period of about 0.5 hours to about 2.0 hours, depending upon the RF power used. As with the oxygen based plasmas, higher RF powers produce a more reactive fluorine based plasma, which requires less time to clean and passivate laser discharge chamber  2 . 
     In one embodiment, the period of exposure to the fluorine based plasma is a predetermined period chosen to be sufficiently long to satisfactorily clean and passivate discharge chamber  2 . In alternative embodiments, exposure to the fluorine based plasma continues until an endpoint is detected with RGA  40 , with optical monitor  44 , or with measurements of reflected RF power. The endpoint may be a particular concentration of molecular fluorine in discharge chamber  2  or vacuum line  14 , for example. As the internal surfaces of discharge chamber  2  are passivated, fluorine consumption decreases and the concentration of molecular fluorine grows to a value indicating that discharge chamber  2  is sufficiently clean and passivated. 
     At the end of the predetermined period of exposure to the fluorine based plasma, or when an endpoint is detected, controller  18  closes valves  34  and  46  and turns off RF power supply  36 . Vacuum pump  22  pulls the pressure down to about 20 millitorr. Discharge chamber  2  is checked for leaks, and controller  18  closes pressure control valve  20 . 
     Next, discharge chamber  2 , plasma source  10 , and vacuum line  14  are back filled with an inert gas such as helium, nitrogen, neon, krypton, and mixtures thereof. In one embodiment, helium from helium supply  50  flows through valve  52 , purge gas line  6 , and valve  8  to pressurize discharge chamber  2  to about 1 pound per square inch over atmospheric pressure. Under inert gas purge, which prevents ambient air from entering and contaminating discharge chamber  2 , plasma  10  and vacuum line  14  are disconnected from discharge chamber  2 , and window assemblies  12  and  16  are sealed. Valve  8  is closed, gas line  6  is disconnected, and discharge chamber  2  is removed from heater  4 . 
     Laser discharge chamber  2  may also be cleaned and passivated with internally generated plasmas. FIG. 2 is a schematic diagram of an apparatus for cleaning and passivating a laser discharge chamber  2  with internally generated plasmas in accordance with one embodiment of the present invention. Like numbers in FIG.  1  and FIG. 2 designate the same parts in the various embodiments. 
     Processes for cleaning and passivating discharge chamber  2  with internally generated plasmas differ from the processes described above utilizing externally generated plasmas primarily in the delivery of precursor gases to discharge chamber  2 , and in the coupling of RF power to the plasma. Other process steps and parameters are substantially the same as those described above. 
     Oxygen and fluorine containing gases, such as those listed above, flow through gas line  7 , valve  8 , discharge chamber  2 , and vacuum line  14  to vacuum pump  22 . RF power supply  36  delivers RF power through impedance matching network  38  to discharge chamber electrode  3 . Discharge chamber electrode  3  is used in normal laser operation of discharge chamber  2  to generate a fluorine based plasma lasing medium. Here, discharge chamber electrode  3  is used as an RF antenna. The RF power delivered to discharge chamber electrode  3  excites the gases to form oxygen based plasmas and fluorine based plasmas containing reactive chemical species which clean and passivate the internal surfaces of discharge chamber  2 . Discharge chamber  2  is subsequently purged with helium delivered through gas line  7  and valve  8 . 
     Advantageously, passivation of laser discharge chambers with plasmas in accordance with the present invention requires only about 2 to 4 hours, rather than the 24 to 48 hours required by conventional passivation processes. Moreover, the performance of lasers with discharge chambers passivated in accordance with the present invention compares favorably to that of lasers with discharge chambers passivated by conventional methods. 
     The performance of KrF excimer lasers was evaluated by measuring the normal operation discharge voltage during a series of test periods following initial turn-on of the laser. Lower discharge voltages indicate less fluorine consumption by internal surfaces of discharge chamber  2  during normal operation of the laser, and thus better performance. FIG. 3 is a plot of the normal operation discharge voltage (HV) versus test number for an average of 88 discharge chambers passivated with a conventional thermal process (diamonds), and for an average of about 10 chambers passivated with plasmas in accordance with the present invention (squares). The plasma passivated chambers were first cleaned for about 2 hours with an externally generated oxygen plasma, and then cleaned and passivated for about 2 hours with an externally generated fluorine based plasma. As FIG. 3 indicates, the performance of the plasma passivated chambers is as much as one standard deviation (sigma) better than that of the conventionally passivated chambers. 
     While the present invention is illustrated with particular embodiments, the invention is intended to include all variations and modifications falling within the scope of the appended claims.