Abstract:
A method for in-situ cleaning of a hot wall RTP system. Internal components are heated to high temperatures above 500° C. A halocarbon gas, inert gas and oxidizing gas are flowed through the reactor for a period which may exceed 20 minutes and then purged to remove contaminants.

Description:
REFERENCE TO RELATED APPLICATION 
     The present application claims priority from U.S. provisional application No. 60/092,759 filed Jul. 13, 1998. U.S. provisional application No. 60/092,759 is hereby incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to semiconductor processing, and in particular to a system and method for in-situ cleaning for rapid thermal processing systems. 
     2. Background 
     In-situ cleaning capability, cleaning the parts within a chamber and its walls while the chamber is in its operating configuration, is an important concept in semiconductor manufacturing. Frequent in-situ cleaning removes unwanted contamination and relieves the need to shut down systems and remove parts for off-system cleaning. 
     An early example of in-situ cleaning occurs in epitaxy reactors with SiC-coated graphite parts where the cleaning process takes place at temperatures above 1100° C. in a gas environment of H 2  and high concentrations of HCl gas. Another example is the in-situ cleaning of plasma deposition systems in which inert halocarbon feed gases rendered reactive in an electric discharge plasma create radicals containing fluorine and chlorine that remove silicon-based films deposited in the reactor. The reactor temperature in such systems is generally below 600° C. because the plasma supplies the necessary additional energy to create reactive species. Yet another example is the use of trichloroethane in oxygen at nominally 1000° C. to in-situ clean difflusion furnaces to remove trace metals. A highly reactive and poisonous gas, NF 3 , has also been used at nominally 900° C. to remove deposits from reactors. 
     Conventional RTP systems consist of a transparent quartz envelope to provide a controlled atmosphere and an array of radiant heat lamps that shine through the quartz envelope to heat opaque objects within the envelope, such as a semiconductor substrate. The lamp power is turned on after the substrate is inserted and turned off to cool the substrate before it is removed from the quartz envelope. Because the quartz envelope is kept cool, conventional RTP systems are termed cold wall RTP systems. 
     Some new RTP systems, such as the Aspen RTP System sold commercially by Mattson Technology, typically incorporate one or more SiC-coated graphite heaters, a susceptor or wafer holder made of SiC-coated graphite (solid SiC or solid Si could also be used), an opaque quartz insulator surrounding the heated parts, one or more ceramic and quartz encapsulated thermocouples and a water-cooled aluminum (stainless steel is an alternate choice) vacuum enclosure with view ports, and a robotic load-lock system to control the process atmosphere. Because the substrate is inserted into an already heated enclosure, the systems are termed hot wall RTP systems. The purpose of the SiC coating on the graphite parts is to seal potential metal contamination inside the parts and prevent their migration to the semiconductor substrates. 
     In its current form, the Aspen RTP system is used for annealing semiconductor substrates at various temperatures from 400° C. to 1100° C. In the course of operating the Aspen RTP system, visible films are deposited on heated parts in proximity to the heater and downstream from the heated parts. Chemical analysis showed that these deposits contained Si, O, C, and N resulting from the purge gases (N 2 ,O 2 , or a combination, possibly with other gases such as NH 3  or H 2 ) reacting with the heated SiC-coated graphite parts to form volatile compounds that condense on cooler furnace parts. 
     Some users of the system are concerned that during the annealing of certain semiconductor substrates, out-gassing from the various layers on the substrate could condense in the system and cause problems such as cross-contamination or the creation of particles. Additionally, the build-up of trace metal contamination could allow these contaminants to enter the reactor on other wafers or could migrate from the interior of heated parts to their surface. 
     The current technology to address contamination in conventional RTP systems is to physically remove the parts and chemically clean them. This is time consuming and results in down-time for the system. An in-situ cleaning process can reduce the down-time of the system, however, in-situ cleaning may damage components of the system such as the SiC-coated graphite or solid SiC parts, the quartz insulators, the ceramic-encapsulated thermocouples, or the metal enclosure. 
     An in-situ cleaning process should remove unwanted deposits that accumulate either because of out-gassing from semiconductor substrates or because of interaction of the purge gases with the heated parts, remove unwanted metal contamination regardless of its source, be effective at temperatures below 1000IC, not require the use of gases which are reactive in air and/or toxic, and use a cleaning gas that would be acceptable with regard to environmental concerns for global-warming and high altitude ozone-depletion. Additionally, an in-situ cleaning process should not leave residual gas that would affect the properties of substrates processed after the in-situ clean. 
     SUMMARY OF THE INVENTION 
     One embodiment of the invention is a system and method for in-situ cleaning of a hot wall RTP system (or other semiconductor processing equipment), comprising heating internal components of the system at an internal pressure of less than about one atmosphere, flowing a halocarbon gas into the system for more than about 1 minute, and substantially purging the halocarbon gas from the system. In other aspects of the invention internal components are heated to a temperature above about 500° C., above about 700° C., above about 900° C., between about 500° C. and about 1000° C., between about 600° C. and about 1000° C., or between about 700° C. and about 900° C. 
     One aspect of this embodiment comprises flowing an inert gas and an oxidizing gas into the system with the halocarbon gas. Suitable inert gases include but are not limited to helium, neon, argon, krypton, xenon, and suitable oxidizing gases include but are not limited to O 2  and O 3 . Suitable halocarbon gases include but are not limited to CHCLF 2 . In another embodiment, the halocarbon gas includes a hydrogenated halocarbon gas. Another aspect of this embodiment includes flowing the gas into the system for more than about 10 minutes, and yet another embodiment includes flowing the gas into the system for more than about 20 minutes. 
    
    
     DESCRIPTION 
     Experimentally, it was found that simply baking a system at elevated temperatures and pressures below atmospheric pressure while purging with N 2  does not sufficiently remove metal contamination. While it was found that new parts, properly installed, gave surface metal contamination levels in the 2-8×10 10 /cm 2  when evaluated using silicon substrates with a thin thermal oxide and analyzed by VPD/AAS (vapor phase deposition, atomic absorption spectroscopy), such metal contamination levels are above the upper limit of the acceptable range for current processing of integrated circuits. However, after operating well above the design temperature range, processing non-clean wafers or introducing metallic instruments into the chamber higher levels of transition metals were observed and in the first case dielectric deposits. 
     Exemplary thermal processing chambers which can be used in accordance with the present invention are described in U.S. patent application Ser. Nos. 08/499,986 filed Jul. 10, 1995, for System and Method for Thermal Processing of a Semiconductor Substrate; 08/876,788 filed Jun. 16, 1997, for System and Method for Thermal Processing of a Semiconductor Substrate; and 08/923,661 filed Sep. 4, 1997, for System and Method for Rapid Thermal Processing, each of which is hereby incorporated herein by reference in its entirety. 
     Experimental Results 
     To evaluate the cleaning process, a system that had been contaminated with unwanted deposits and metals was subjected to a clean. The cleaning process used was 975° C. with 2% CHCLF 2 , 40% O 2 , and 58% N 2  for 30 minutes with pressure of 2 torr. The results were as follows: 
     i. All deposited material was removed from the system. 
     ii. No damage was evident to the heated parts. 
     iii. A small amount of unidentified material was deposited on the water-cooled reactor walls. 
     The contamination levels were as outlined in the following: 
     
       
         
               
             
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
               
               
                 Aspen RTP Contamination Levels 
               
               
                 (Control and with “dirty” system metal TCs present) 
               
               
                 (VPD/AAS) 
               
             
          
           
               
                   
                 Control (no processing) 
                 “Dirty” System* 
               
             
          
           
               
                   
                   
                 Wafer 
                 Wafer 
                 Wafer 
                 Wafer 
               
               
                 Ele- 
                 Detect 
                 Front 
                 Back 
                 Front 
                 Back 
               
               
                 ment 
                 limit 
                 ×10 10 /cm 2   
                 ×10 10 /cm 2   
                 ×10 10 /cm 2   
                 ×10 10 /cm 2   
               
               
                   
               
             
          
           
               
                 Na 
                 1 
                 67 
                 2 
                 60 
                 80 
               
               
                 Al 
                 1 
                 58 
                 13 
                 253 
                 185 
               
               
                 Ca 
                 1 
                 33 
                 22 
                 27 
                 58 
               
               
                 K 
                 0.5 
                 19 
                 9.3 
                 11 
                 27 
               
               
                 Cr 
                 0.5 
                 &lt;0.5 
                 0.8 
                 12 
                 3.4 
               
               
                 Fe 
                 1 
                 11 
                 3.5 
                 58 
                 37 
               
               
                 Ni 
                 0.5 
                 &lt;1 
                 1.4 
                 14 
                 13 
               
               
                 Cu 
                 0.5 
                 31 
                 1.7 
                 32 
                 15 
               
               
                 Zn 
                 0.2 
                 11 
                 4.0 
                 1.4 
                 1.0 
               
               
                   
               
             
          
         
       
     
     Note that the contamination levels on the frontside of the control wafer are unusually high. This may mean that spot contamination was present or that the wafer sides are incorrectly identified. 
     The in-situ clean process was tested for metal removal from a system which had been contaminated with metal thermocouples and the thermocouples then removed. The clean process was also tested for removal of Si/C/O/N dielectric compounds. 
     In a single clean step, all the Si/C/O/N dielectric compounds were removed and metal concentrations, as measured by VPD/AAS were reduced from mid-10 11 /cm 2  levels to mid-10 10 /cm 2  levels. In one experiment, the following levels were observed: 
     
       
         
               
             
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
               
               
                 Aspen RTP Contamination Levels Before and After In-situ Clean 
               
               
                 (VPD/AAS) 
               
             
          
           
               
                   
                 Wafer Front 
                 Wafer Back 
               
               
                   
                 (not same wafer) 
                 (not same wafer) 
               
             
          
           
               
                   
                   
                 Before 
                 After 
                 Before 
                 After 
               
               
                 Ele- 
                 Detect 
                 Clean* 
                 Clean 
                 Clean* 
                 Clean 
               
               
                 ment 
                 limit 
                 ×10 10 /cm 2   
                 ×10 6 /cm 2   
                 ×10 10 /cm 2   
                 ×10 10 /cm 2   
               
               
                   
               
             
          
           
               
                 Na 
                 1 
                 9.8 
                 2.4 
                 36 
                 17 
               
               
                 Al 
                 1 
                 27 
                 10 
                 52 
                 45 
               
               
                 Ca 
                 1 
                 29 
                 5.0 
                 54 
                 29 
               
               
                 K 
                 0.5 
                 11 
                 &lt;0.5 
                 17 
                 7.9 
               
               
                 Cr 
                 0.5 
                 &lt;0.5 
                 &lt;0.5 
                 1 
                 &lt;0.5 
               
               
                 Fe 
                 1 
                 7.2 
                 1.6 
                 24 
                 5.8 
               
               
                 Ni 
                 0.5 
                 0.9 
                 &lt;0.5 
                 2.0 
                 1.8 
               
               
                 Cu 
                 0.5 
                 3.3 
                 1.2 
                 3.3 
                 2.2 
               
               
                 Zn 
                 0.2 
                 1.4 
                 &lt;0.2 
                 0.5 
                 1.2 
               
               
                   
               
               
                 *Wafers processed in Mattson Aspen RTP at 975° C. after system was contaminated with metal thermocouples and a wide range of process conditions.  
               
             
          
         
       
     
     In a separate test, the in-situ clean process was evaluated for its etch rate of thermal oxide on a silicon wafer. Under the same process conditions as the in-situ clean noted above, it is believed that the oxide etch rate was less than about 5 Å/minute. This result indicates that there will be no measurable effect of any residual gas on substrates processed after the in-situ clean. The concentration of such residual gas would be only a small fraction of the recipe values; therefore, the effect on thermal silicon oxide and similar layers should be far less than 5 Å/minute. 
     The hydrogenated halocarbon, CHCLF 2 , was selected because it is known to have a short life in the atmosphere as compared with halocarbons which are not hydrogenated. It is noted that any variety of other halocarbon compounds (compound containing halogen and carbon) can be used in alternate embodiments. 
     In other embodiments of the invention increasing the temperature, the flow rate of the gases, the concentration of the reactive gases relative to the inert gas will increase the degree of reactivity, and decreasing the factors will decreases the degree of reactivity. The amount of reaction to be selected for any given situation depends on the nature and quantity of the deposits and metals to be removed. In one embodiment the relative flow rate of O 2  to CHCLF 2  is 1 to 1. The relative flow rate can be 0 to 1, ½ to 1, 1 to 1, or any ratio between 1 to 1 and 20 to 1. 
     Another embodiment of the invention includes an in-situ cleaning process useful for removing unwanted accumulations of dielectric films and metal contamination without damaging the system parts and without leaving a residual gas that would attack semiconductor substrates processed after the clean. This embodiment comprises heating the internal parts of the process chamber, once it has been evacuated, to an appropriate temperature, and then injecting the cleaning gases into the process chamber while it is hot and continuously pumping on this chamber to exhaust the reaction products. The process window is as follows: 
     
       
         
               
               
             
           
               
                   
               
               
                 Parameter 
                 Condition 
               
               
                   
               
             
             
               
                 Process Gas 
                 Nominally 60% N 2  with up to 40% O 2  and up to 4% 
               
               
                   
                 CHCLF 2 , with the O 2 /CHCLF 2  ratio being 10 or more. 
               
               
                 Temperature 
                 Nominally 900° C. Process is more effective at higher 
               
               
                   
                 temperatures, and may be effective at temperatures 
               
               
                   
                 as low as 500° C. 
               
               
                 Pressure 
                 Nominally 2 torr or less, possibly having several 
               
               
                   
                 pressure cycles between 2 and 10 torr to clean inside 
               
               
                   
                 unpurged volumes within the RTP system. 
               
               
                 Time 
                 Dependent on degree of contamination to be removed. 
               
               
                   
                 A severely contaminated system was clean to industry 
               
               
                   
                 standards in 2 hours at 975° C. at 2 torr with pressure 
               
               
                   
                 of 2 torr nominally. Lower levels of contamination might 
               
               
                   
                 permit using lower temperatures, lower reactive gas 
               
               
                   
                 concentration, shorter times and lower pressures. 
               
               
                   
               
             
          
         
       
     
     While exemplary embodiments of the invention have been described and illustrated, it will be appreciated that modification can be made to these embodiments without departing from the spirit of the invention. Thus, the invention is intended to be defined in terms of the following claims.