the problems of over - cleaning and incomplete passivation can be eliminated by using the cleaning and preconditioning method of the present invention . fig1 shows a typical cvd system 100 comprising a dome 101 , wafer susceptor 102 , process gas injector 103 , cleaning gas injector 104 , and exhaust manifold 105 . dome 101 is typically made of an insulating dielectric , such as al 2 o 3 . dome 101 includes an induction coil 110 , which is provided in an expanding spiral pattern . fig2 shows a detail of the expanding spiral pattern of induction coil 110 . rf power 111 is applied to induction coil 110 to generate a reactive plasma inside the chamber . induction coil 110 is typically made of copper tubing , within which a cooling fluid , such as cooling water , is flowed . cooling water may enter the copper tubing at connection 112 at the bottom of the dome , passing through valve 113 , and then exit the copper tubing at connection 114 at the top of dome , passing through valve 115 . alternatively , cooling water may flow through the copper tubing from connection 114 at the top of the dome to connection 112 at the bottom of the dome . a typical cvd process begins with heating of the cvd chamber . a semiconductor substrate is placed on susceptor 102 , which is shown in fig1 in an upper position 102 a and a lower position 102 b . a semiconductor wafer is placed on susceptor 102 while in lower position 102 b , and then susceptor 102 is raised into the chamber to upper position 102 a . next , reactant gases are introduced into the chamber through process gas injector 103 . often , as many as eight process gas injectors may be present in the chamber , although only one injector 103 is shown for simplicity . the process gases react in the chamber to form a deposition layer , such as sio 2 or sin , on the surface of the wafer . the deposition process may be assisted or enhanced by the generation of a reactive plasma inside the chamber . a plasma maybe generated by applying rf power 111 to induction coil 110 . in addition to forming a deposition layer on the surface of the wafer , the cvd process also forms a film on the interior surfaces of dome 101 . during deposition , if cooling water is flowed through induction coil 110 from connection 112 at the bottom of the dome to connection 114 at the top of the dome , then the deposited film on dome 101 will be thicker at the bottom of the dome than at the top of the dome . following deposition of a film , such as sio 2 or sin , the cvd chamber is generally cleaned using a conventional cleaning process wherein at least one fluorine - containing cleaning gas is introduced into the chamber through cleaning gas injector 104 . more than one cleaning gas injector may be present in the chamber , although only one injector 104 is shown for simplicity . as discussed previously , in areas of the dome where the film deposition is thinner , some of the al 2 o 3 dome is often converted to alf 3 during a conventional cleaning . this nonuniformity in film thickness results from a temperature gradient in the dome during deposition caused by the flow of cooling water through the induction coil . to achieve complete cleaning of areas on the dome having thickest deposition coverage , areas with thinner coverage will be over - cleaned , resulting in conversion of al 2 o 3 to alf 3 . the same temperature gradient during cleaning results in nonuniform cleaning efficiency , thus exacerbating the problem of over - cleaning . for example , if cooling water flows through induction coil 110 from connection 112 at the bottom of the dome to connection 114 at the top of the dome , then a temperature gradient will be created wherein the bottom of the dome is slightly cooler than the top of the dome . as a result , the deposition layer will be thickest at the bottom of the dome , but the cleaning efficiency also will be poorer at the bottom of the dome . to achieve complete cleaning of the thickest films at the bottom of the dome , the top of the dome will be over - cleaned , resulting in conversion of some al 2 o 3 to alf 3 at the top of the dome . this problem of over - cleaning can be addressed by the cleaning method of the present invention , which comprises reversing the direction of flow of the cooling water . for example , if during deposition the cooling water flows through induction coil 110 from the bottom of the dome to the top of the dome , then during the cleaning method of the present invention the cooling water flows in the opposite direction , from the top of the dome to the bottom of the dome . reversing the direction of flow of the cooling water can be accomplished , for example , by simply disconnecting the water supply from connection 112 and reconnecting it to connection 114 . alternatively , reversing the direction of cooling water flow can be accomplished by using the apparatus shown in fig3 . in fig3 two cooling water connections 112 and 114 are provided which are in fluid communication with induction coil 110 . two valves 113 and 116 are provided between and in fluid communication with connection 112 and induction coil 110 . valve 113 is in fluid communication with the bottom of the dome , and valve 116 is in fluid communication with the top of the dome . similarly , two valves 115 and 117 are provided between and in fluid communication with connection 114 and induction coil 110 . valve 115 is in fluid communication with the top of the dome , and valve 117 is in fluid communication with the bottom of the dome . if it is desired to flow cooling water from the bottom of the dome to the top of the dome during deposition , this can accomplished by either of two arrangements . first , cooling water may be supplied through connection 112 , with valves 113 and 115 in the open position , and valves 116 and 117 in the closed position . during cleaning , valves 113 and 115 are closed , and valves 116 and 117 are opened , thereby flowing cooling water from the top of the dome to the bottom of the dome . in the second arrangement , cooling water may be supplied through connection 114 , with valves 117 and 116 in the open position , and valves 115 and 113 in the closed positions . during cleaning , valves 117 and 116 are closed , and valves 115 and 113 are opened . using either of these arrangements , the cooling water supply need not be disconnected and reconnected between deposition and cleaning steps . when the flow of cooling water is reversed , the temperature gradient in the dome is also reversed , so that the bottom of the dome is slightly warmer than the top during cleaning . as a result , the cleaning efficiency is slightly greater at the bottom of the dome where the deposition film is thickest . the thicker film at the bottom of the dome will be removed at a faster rater than the thinner film at the top of the dome , resulting in reduced over - cleaning of areas with thinner deposits , and reduced conversion of al 2 o 3 to alf 3 . nevertheless , even with the flow of cooling water reversed during cleaning , to ensure complete cleaning it is inevitable that some al 2 o 3 will be converted to alf 3 . preconditioning of the dome to convert the alf 3 species back to al 2 o 3 prior to further cvd processing is therefore necessary . as discussed previously , conventional preconditioning processes produce undesirable contaminants such as water and aqueous hf , therefore precluding overpassivation and often resulting in incomplete passivation . “ overpassivation ” in this context can be defined as passivation past the stoichiometric equivalence point in localized areas within the dome , but not necessarily past the overall average endpoint throughout the dome . in the preconditioning method of the present invention , no undesirable contaminants are produced . therefore , the passivation reaction need not be limited , and overpassivation is possible . after evacuating the cleaning gases from the chamber , the preconditioning method of the present invention begins with an initial h 2 passivation step comprising introduction of h 2 through cleaning gas injector 104 , and generation of a reactive plasma of the h 2 by applying rf power 111 to induction coil 110 . it is believed that this initial h 2 passivation step produces some al 2 o 3 as well as intermediate al y o x and al y o x f z species . then , a mixture of h 2 and n 2 is introduced into the chamber through cleaning gas injector 104 , and a plasma of this mixture is generated by applying rf power 111 . it is believed that the intermediate al y o x and al y o x f z species react with n 2 to form al y o x n z and aln . all other byproducts of the reaction , such as nh 3 , nh 4 f , nf 3 , and various excited nf x and alf y species , are gaseous and are therefore removed from the chamber through exhaust manifold 105 by the vacuum pump . no undesirable water or other aqueous byproducts are formed . thus , the passivation reaction need not be limited to prevent production of undesirable byproducts , and the dome can be completely passivated . an added advantage of this preconditioning method can be realized during subsequent deposition of sin films . it is believed that following preconditioning by the method of the present invention , the dome is comprised of an outer al 2 o 3 layer , an intermediate al y o x n z layer , and an inner aln layer exposed to the chamber . when a sin film is subsequently deposited on the exposed aln layer of the dome , it is believed that an intermediate si y al x n z species is formed between the aln and sin , thereby providing a stronger bond between the aln and sin . this stronger interface reduces the likelihood that portions of the sin film will be released , causing particulate contamination . the initial h 2 passivation step of the preconditioning method should be performed for a time , at a flowrate , and using an rf power sufficient to partially passivate the alf 3 species on the dome and form intermediate al y o x and al y o x f z species . specifically , the h 2 gas may be flowed into the chamber for a time ranging from about 20 seconds to about 240 seconds , at a flowrate ranging from about 200 standard cubic centimeters per minute ( sccm ) to about 1000 sccm . preferably , the h 2 gas is flowed into the chamber at a flowrate of about 600 sccm for about 120 seconds . in addition , sufficient rf power should be applied to generate a plasma of the h 2 gas . specifically , an rf power ranging from about 1000 watts to about 3600 watts , preferably about 2400 watts , may be applied . the h 2 / n 2 passivation step should be performed for a time , at a flowrate , and using an rf power sufficient to completely convert the intermediate al y o x and al y o x f z species to al y o x n z and aln . specifically , the h 2 gas may be flowed into the chamber for a time ranging from about 20 seconds to about 240 seconds , at a flowrate ranging from about 200 sccm to about 1000 sccm . the n 2 gas may be flowed into the chamber for a time ranging from 30 seconds to about 600 seconds , at a flowrate ranging from about 200 sccm to about 1000 sccm . preferably , both the h 2 and n 2 are flowed into the chamber at a flowrate of about 600 sccm for about 120 seconds . the h 2 and n 2 gases may be mixed either prior to introduction into the chamber , or upon introduction into the chamber . in addition , sufficient rf power should be applied to generate a plasma of the h 2 / n 2 mixture . specifically , an rf power ranging from about 1000 watts to about 3600 watts , preferably about 2400 watts , may be applied . during preconditioning in accordance with the method of the present invention , the direction of cooling water flow may be the same as during cleaning . that is , if during deposition the direction of cooling water flow is from the bottom of the dome to the top of the dome , and during cleaning the direction is reversed such that cooling water flows from top to bottom , then during preconditioning the cooling water may remain flowing from top to bottom . however , it is preferred that the direction of cooling water flow is again reversed during preconditioning , such that it is opposite the direction during cleaning and the same as the direction during deposition . as discussed previously , if the direction of cooling water flow during deposition is from the bottom of the dome to the top of the dome , then the film coverage will be thinner at the top of the dome . during a subsequent cleaning step , even with the direction of cooling water flow reversed , over - cleaning is more likely to occur at the top of the dome where film coverage is thinner . that is , conversion of al 2 o 3 to alf 3 will be greater at the top of the dome . during preconditioning to convert alf 3 to al 2 o 3 and aln , flowing the cooling water from the bottom of the dome to the top of the dome will result in a slightly greater rate of passivation at the top of the dome where more alf 3 species are located . thus , during preconditioning , it is preferred that the direction of cooling water flow is opposite the direction during cleaning and the same as the direction during deposition . if the direction of flow during deposition is from the bottom of the dome to the top of the dome , then the direction during cleaning will be from top to bottom , and the direction during preconditioning is preferably from bottom to top . while the present invention has been particularly described in conjunction with a preferred embodiment and other alternative embodiments , it is evident that numerous alternatives , modifications and variations will be apparent to those skilled in the art in light of the foregoing description . it is therefore intended that the appended claims embrace all such alternatives , modifications and variations as falling within the true scope and spirit of the present invention .