Abstract:
A method and apparatus for thermal processing of semiconductor substrates is disclosed. Each lamp of a lamp assembly is immersed in a thermally conductive atmosphere comprising oxygen. As the lamps are operated, the oxygen reacts with carbon containing species. Consumed oxygen is replaced over time until the thermal conductivity of the atmosphere falls below a tolerance threshold. The atmosphere is then evacuated and replaced.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of U.S. Provisional Patent Application Ser. No. 61/478,195, filed Apr. 22, 2011, and U.S. Provisional Patent Application Ser. No. 61/509,821, filed Jul. 20, 2011, both of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments described herein relate to thermal processing in semiconductor manufacturing. More specifically, a lamp assembly, and methods of operating the lamp assembly, are described. 
     2. Description of the Related Art 
     Rapid thermal processing (RTP) systems are employed in semiconductor chip fabrication to create, chemically alter or etch surface structures on semiconductor wafers. One such RTP system, as described in U.S. Pat. No. 6,376,804, includes a lamp assembly located on the semiconductor processing chamber. The lamps are disposed in recesses of the assembly and covered with a window to isolate the lamps from the processing environment of the chamber. 
     As the lamps are operated, trace carbon compounds adventitiously introduced into the space around the lamps may deposit and/or decompose on the lamps, leaving a carbon residue that darkens the lamps over time. Moreover, as the lamps heat and cool during process cycles, the carbon residue may accumulate on lamp walls causing overheating which ultimately cause early failure of the lamps. It is not uncommon for lamps to become unusable less than 24 hours after being put in service. 
     As semiconductor processes continually aspire to higher efficiency and lower cost, there is a need to improve the longevity and cost-effectiveness of thermal lamp assemblies. 
     SUMMARY OF THE INVENTION 
     Embodiments described herein include a method of operating a bank of lamps by immersing the lamps in a thermally conductive gas having up to about 25% of an oxidizing gas. The oxidizing gas is consumed by reaction with carbon containing species in the atmosphere around the lamps, and is made up by flowing additional oxidizing gas into the atmosphere. Air may be allowed to leak into the atmosphere to make up the oxidizing gas. When the composition of the atmosphere around the lamps is such that the thermal conductivity of the atmosphere begins to decline, the gas is replaced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above-recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
         FIG. 1  is a cross-sectional view of a lamp assembly according to one embodiment. 
         FIG. 2  is a flow diagram summarizing a method according to another embodiment. 
     
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements disclosed in one embodiment may be beneficially utilized on other embodiments without specific recitation. 
     DETAILED DESCRIPTION 
     Methods of managing fouling of process lamps, such as lamps and/or UV lamps, are described. The lamps described herein are generally energy discharge lamps that develop a temperature of at least about 300° C. during normal operation or discharge energy that may activate, dissociate, or decompose carbon containing compounds. A plurality of lamps may be disposed in a lamp assembly and operated as described herein, or a lamp may be operated singly as described herein. 
       FIG. 1  is a cross-sectional view of a lamp assembly  100  which may be used to practice methods described herein. The lamp assembly  100  has a plurality of recesses  102  in which lamps  104  are disposed. Each recess  102  may be a light pipe and may be lined by a reflector  106 . Each lamp  104  is connected to an electrical circuit  108  outside the lamp assembly  100 . More than one electrical circuit may be provided to power groups of lamps independently, if desired. The recesses  102  are open at one end to allow radiation from the lamps  104  to escape the lamp assembly  100  into a processing chamber (not shown) proximate the lamp assembly  100 . 
     Each lamp  104  may be immobilized in an electrical socket  126  by seating in a potting compound  110 . The potting compound  110  is typically a white material to reflect as much radiation as possible toward the open end of the recess  102 . The potting compound  110  is typically also porous. In some embodiments, the potting compound comprises magnesium phosphate bonded zircon, or magnesium phosphate bonded aluminum nitride. 
     The openings of the recesses  102  are typically sealed by a window  138 , which may be quartz. The window  138 , which typically faces the processing region of the chamber, allows radiation from the lamps  104  to enter the processing chamber while protecting the lamps  104  and lamp assembly  100  from the processing environment. The recesses  102  therefore form a space around the lamps  104 . This space is typically filled with an operating gas such as helium from a pressurized source  114 . In a heat lamp embodiment, the operating gas will typically be a thermally conductive gas. The pressurized source  114  is connected to the lamp assembly  100  by a port  116  and valve  118 . The operating gas is introduced into a space  120  formed between a cover  122  and the base of each lamp  104 . Because the potting compound  110  is porous, the operating gas flows through the potting compound  110  and around each lamp  104  within the recesses  102 . 
     A cooling chamber  112  may be provided surrounding the walls of the recesses  102 , allowing a cooling medium to contact the recess walls and keep the recess walls relatively cool. The cooling medium may be a liquid, such as water, or a gas and is introduced via an inlet  140  and removed at an outlet  130 , flowing between the recesses  102  and cooling the walls thereof. 
     The operating gas is provided to the recesses  102  from the pressurized source  114  into the space  120 , and may be evacuated from the recesses  102  via conduit  128 , which couples the recesses  102  to a vacuum pump  134 . Valve  136  controls exposure to the vacuum pump  134 . The recesses  102  fluidly communicate by virtue of passages  142  and the space  120 , which allow the operating gas to fill all the recesses  102 . In some embodiments, some of the passages  142  may be occluded if an end portion  140  of a wall touches the window  138 , for example due to thermal expansion. If all passages  142  for a single bulb recess  102  are occluded, fluid communication occurs through the potting compound  110  to space  120 . 
     In operation, valve  136  is closed and valve  118  is opened to provide the operating gas to the recesses  102  up to a target pressure, typically less than about 10 Torr, such as between about 2 Torr and about 7 Torr, for example about 5 Torr. The operating gas may be used to transmitform an heat away from the lamp walls to the recess walls, and ultimately into the cooling fluid circulating around the recesses  102 . An oxidizing gas is provided with the operating gas to form an operating atmosphere that oxidizes trace organics that may enter with the operating gas, outgas from the potting compound, or arrive from other sources. Such organics would otherwise deposit on the lamp walls during processing, darkening the lamp and potentially causing early lamp failure. Oxidizing the organics by reacting with an oxidizing gas in the energetic environment surrounding each lamp converts the organic material to CO 2  and H 2 O, which is harmlessly removed when the lamp assembly  100  is pumped down. 
     Initially, when the lamp assembly  100  is pressurized with the operating gas, up to about 25% oxidizing gas may be added to the operating gas to provide an oxidizing environment without reducing thermal conductivity of the gas inordinately in embodiments where thermal conductivity of the gas is desired. The oxidizing gas may be one or more gases from the group of O 2 , O 3 , H 2 O, H 2 O 2 , N 2 O, nitric oxides, and air. The concentration of oxygen species in the atmosphere may be between about 1% and about 25% by volume or by mass. 
     In one embodiment, the lamp assembly  100  is pressurized with helium to a first pressure of about 15 Torr or less, such as about 10 Torr or less, for example about 4 Torr or less. The oxidizing gas is then pumped into the lamp assembly  100  to a second pressure above the first pressure. The second pressure may be between about 5 Torr and about 10 Torr above the first pressure. 
     As the lamps  104  are operated, pressure rises in the recesses as gas, usually air, leaks into the recesses  102 . Eventually, enough gas leaks into the recesses  102  to reduce the thermal conductivity of the atmosphere surrounding the lamps, and the atmosphere needs to be renewed. The pressure in the lamp assembly rises as gas leaks in, until the pressure typically reaches a pressure of between about 10 Torr and about 25 Torr, such as between about 12 Torr and about 20 Torr, for example about 15 Torr or 20 Torr. With the valve  118  closed, the valve  136  is opened, exposing the recesses  102  to vacuum from the vacuum pump  134 . The recesses  102  are pumped down to a third pressure below about 1 Torr, for example less than about 0.1 Torr, and then the atmosphere in the recesses  102  is reconstituted with gas from the source  114  and oxidizing gas, as described above, by pumping helium into the chamber of the lamp assembly  100  to a fourth pressure of about 15 Torr or less and pumping the oxidizing gas into the chamber to a fifth pressure between about 5 Torr and about 10 Torr above the fourth pressure. The fifth pressure may be between about 10 Torr and about 25 Torr, for example about 20 Torr. 
     In the embodiment of  FIG. 1 , an optional valve  119  and line  117  may be provided to couple air from the environment into the port  116  through opening  115 . Use of a valve may improve control of the mixture of operating gas and oxidizing gas provided to the lamp assembly. The line  117  intersects the port  116  at a location selected to provide a mixing length  113  between the intersection of the line  117  and the port  116  and the entry point of the port  116  into the space  120 . A longer mixing length  113  may improve mixing of the operating gas and the oxidizing gas prior to entry into the recesses  102 . Good mixing of the operating gas and the oxidizing gas facilitates oxidative capacity in all the lamp recesses  102 . Alternately, a source of pressurized oxidizing gas may be coupled to the line  117  and valve  119  at the opening  115 . 
       FIG. 2  is a flow diagram summarizing a method  200  according to one embodiment. The method  200  may be performed on any assembly of lamps for thermal processing to prevent degradation from exposure to organic or carbon-containing species. The space around each lamp is evacuated at  202 , usually by vacuum pump. At  204 , a operating gas comprising oxygen is provided to the space surrounding each lamp. The operating gas may comprise any gas capable of regulating lamp temperature by carrying heat away from the lamps, so long as the gas does not react adversely with any materials at processing conditions. Helium is an example of a gas that may be beneficially used. Hydrogen may also be used, with appropriate safety precautions. 
     An oxidizing gas is mixed with the operating gas to scavenge carbon containing species that may enter the space around each lamp. The oxidizing gas may be a source of oxygen to react with the carbon containing species. Exemplary oxidizing gases that may be used to practice the method  200  include O 2 , O 3 , H 2 O, H 2 O 2 , N 2 O, nitric oxides, and air. The oxidizing gas is added up to a point beyond which the thermal conductivity of the gas begins to decline. In one embodiment, the atmosphere around each lamp is charged with gas to a pressure of about 5 Torr, about 25% of which is an oxidizing gas. Lower pressures may be used, and lower concentrations of oxygen containing gas may be used. The concentration of oxidizing gas in the atmosphere may be between about 1% and about 25% by volume or by mass. 
     At  206 , the lamps are activated for processing. Heat from the lamps drives a gas phase reaction between oxygen containing species and carbon containing species in the atmosphere or on heated parts of the lamp and reflector assembly. The carbon containing species are converted to CO and CO 2 , consuming the oxygen atoms from the atmosphere. As the lamps are operated, cycling on and off through a number of operating cycles, more oxidizing gas may be added to the atmosphere to make up for the lost oxygen. In one embodiment, the lamp assembly is engineered to allow air to leak into the space around the lamps as they are operated, for example through the porous potting compound. The pressure of the atmosphere grows and its thermal conductivity declines as concentration of operating gases decline in favor of CO 2 . When the thermal conductivity of the gas reaches a low tolerance threshold, the gas is replaced by pumping out. In one embodiment, the gas is replaced when the pressure around the lamps is between about 15 Torr and about 25 Torr, for example about 20 Torr. The cycle may then be repeated. In one embodiment, the method  200  is repeatedly performed at least 10 times before maintaining any of the lamps in the lamp assembly, a significant improvement over the longevity of a typical lamp. 
     In some embodiments, oxygen containing species may be supplied to the space around the lamps by including such species in the potting compound that holds the lamps in place in the lamp assembly. A volatile oxidant or a thermally releasable oxidant may be incorporated in the potting compound. White colored species such as nitrates and perchlorates may be useful in this regard. In other embodiments, gas may be continuously flowed through the space around the lamps to maintain an equilibrium of pressure, thermal conductivity, and concentration of oxidizing species in the atmosphere. As CO 2  is generated from oxidizing carbon containing species in the atmosphere, the CO 2  is continuously removed, and helium (or another inert gas) and oxidizing gas are supplied. In one embodiment, a mixture of 4 parts helium and 1 part oxidizing gas is supplied to the lamp assembly, and flow out of the lamp assembly to a vacuum pump is controlled to achieve a desired pressure in the lamp assembly, such as between about 5 Torr and about 10 Torr. 
     While the foregoing is directed to embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof.