disclosed is a method of formation of a stacked film of silicon nitride and silicon oxide . the two films are preferably formed within the same plasma processing chamber . in another aspect , the two films contain a silicon oxynitride layer between the two films , where the oxygen percentage in the silicon oxynitride is graded to avoid an abrupt silicon nitride and silicon oxide interface . the invention will be described first in terms of the stacked film structure . then , a method of forming the stacked film will be described . any reference to dimensions are purely for illustrative purposes , and are determined by the design rules of a particular semiconductor device . any reference to “ approximate ” or like term , should be construed as a target manufacturing specification , plus or minus variation within reasonable manufacturing tolerances , unless otherwise specified . the detailed description will refer to silicon oxide , but it should be construed very generically , so that silicon oxide includes any oxides of silicon in varying compositions , and various additives that may be used . for example , a “ silicon oxide ” film may actually be fluorinated silicon oxide . the context of the description will be in semiconductor processing , and more specifically , in the use of copper for metallization in a damascene process . although the context is copper damascene process , the invention is not limited to that context in any way . nor is the invention limited to semiconductor devices or semiconductor processing . a person of ordinary skill in the art is expected to apply the invention to various contexts as reasonably can be expected of a person of such skill ; the description below should not be construed to limit the scope of the invention to the aspects described . [ 0018 ] fig1 illustrates in cross - section , a preferred embodiment of the stacked silicon nitride and silicon oxide film . there is a substrate 10 , which may be a silicon wafer or a silicon wafer with silicon oxide deposited on top of the wafer . a silicon nitride film 20 is formed on substrate 10 . silicon nitride film 20 may be approximately 100 angstroms to 2000 angstroms in thickness . a silicon oxide film 30 is held a distance above silicon nitride film 20 . silicon oxide film 30 may be approximately 5000 angstroms to 3 microns in thickness . a layer of graded silicon oxynitride 40 separates silicon nitride film 20 from silicon oxide film 30 . silicon oxynitride 40 may be approximately 300 to 2000 angstroms thick . the percentage of oxygen is graded in silicon oxynitride 40 so that the bottom of silicon oxynitride 40 at the silicon nitride interface 50 contains approximately zero percentage of oxygen . the top of silicon oxynitride 40 at the silicon oxide interface 60 contains an oxygen composition that is approximately 50 to 70 % oxygen , preferably approximately 60 % oxygen . the minimum thickness of silicon oxynitride 40 is determined by the desired oxygen and nitrogen concentration gradients . the amount of oxygen in silicon oxynitride 40 can be measured in estimate using depth profile sims or similar technique . because in semiconductor manufacturing it is advantageous to minimize processing time , the maximum thickness of silicon oxynitride 40 is preferably determined by the minimum thickness required for the desired concentration profiles plus added thickness for manufacturing tolerances including film thickness uniformity across substrate 10 . [ 0019 ] fig2 shows a graphical description of the gradation of oxygen in silicon oxynitride 40 of fig1 . at approximately zero film thickness , the oxygen percentage is about zero . at approximately 500 to 1000 angstroms distance from silicon nitride , the oxygen percentage is about 60 %. note that varying the process for forming silicon oxynitride 40 can vary the composition of oxygen with respect to film thickness . a description of a process is provided further below . [ 0020 ] fig3 shows in cross section an application of the present invention to a copper metallization semiconductor device . there is a semiconductor wafer substrate 100 . transistor structures 110 are built on substrate 100 . a first dielectric layer 120 is disposed above transistor structures 110 . first dielectric layer 120 may be silicon oxide , at a thickness of approximately 1 to 2 microns . first dielectric layer 120 may be deposited over transistor structures using a chemical vapor deposition , to create a blanket layer of first dielectric 120 . contact holes 140 within first dielectric 120 are filled with a metal such as tungsten , aluminum or copper , to enable electrical interconnection between transistor structures 110 and electrical wiring above the device . contact holes 140 may be created in first dielectric layer 120 by plasma etching a hole pattern into first dielectric layer 120 , and filling the pattern with the desired metal using vapor deposition techniques . directly above first dielectric layer 120 containing filled contact holes 140 , there is a second dielectric layer 125 . second dielectric layer 125 surrounds a first metallization 145 . second dielectric layer 125 may be a fluorinated silicon oxide , so that the dielectric constant of the film is lower than that of pure silicon dioxide . first metallization 145 may be made of copper , and is patterned in a series of lines for forming a first wiring plane in the semiconductor device . first metallization 145 is formed by opening a pattern in second dielectric layer 125 by plasma etching , and then filling the openings in the pattern with copper or other preferred metal , and then planarizing the surface to remove any excess metal . this “ single damascene ” technique effectively fills contact holes 140 with the desired metal . first metallization 145 and second dielectric 125 may be approximately 5000 angstroms to 1 micron in thickness . the next series of films is a silicon nitride / silicon oxynitride / silicon oxide stack . there is a first silicon nitride etch stop 150 that is directly above the surface of second dielectric 125 and first metallization 145 . directly above the surface of first silicon nitride 150 is a first graded silicon oxynitride 160 . directly above the surface of first graded silicon oxynitride 160 is a third dielectric layer 170 , which may be a fluorinated silicon oxide (“ siof ”) so that the dielectric constant of third dielectric layer 170 is lower than that of pure silicon dioxide . first silicon nitride 150 is formed directly over second dielectric 125 and first metallization 145 preferably by vapor deposition , to a thickness of approximately 40 nanometers to 500 nanometers , or 400 angstroms to 5000 angstroms . it is important to make first silicon nitride 150 as thin as possible within manufacturing feasibility to avoid dielectric constant increase effects . first graded silicon oxynitride 160 is preferably formed using vapor deposition , within the same processing chamber used for forming first silicon nitride 150 , by combining the process gases used for forming first silicon nitride 150 and siof 170 . first graded silicon oxynitride 160 is preferably a thickness of approximately 300 to 2000 angstroms . siof 170 is preferably formed using vapor deposition , within the same processing chamber used for forming first silicon nitride 150 and graded silicon oxynitride 160 , to a thickness of approximately 5000 angstroms to 3 microns . within the plane of siof 170 there is a dual structure of a first via 155 and a second metallization 165 . first via 155 is formed by etching a hole within siof 170 , and , after forming a larger opening within siof 170 to create the second metallization 165 , extending the hole through the thickness of siof 170 , first graded silicon oxynitride 160 and first silicon nitride 150 so that the hole extends to first metallization 145 . first via 155 is in direct metal - to - metal contact with first metallization 145 . second metallization 165 is formed by etching line patterns into siof 170 , preferably in - situ with cutting the hole for first via 155 as explained above . the line patterns for second metallization 165 are aligned with the hole patterns for first via 155 to enable direct metal - to - metal contact between first via 155 and second metallization 165 . after creating the hole openings and line openings within the stack of siof 170 , first silicon oxynitride 160 and first silicon nitride 150 , first via 155 and second metallization 165 are completed by filling the openings with copper or other desired electrically conductive material , utilizing a vapor deposition process . the next structure layer of the semiconductor device may be similar to the above . there is a second silicon nitride 180 , formed directly above the surface of siof 170 and second metallization 165 . directly above the surface of second silicon nitride 180 is a second graded silicon oxynitride 190 . directly above the surface of second graded silicon oxynitride 190 is a fourth dielectric layer 200 , also referred to as second siof 200 , so that the dielectric constant of second siof 200 is lower than that of pure silicon dioxide . second silicon nitride 180 is formed preferably by vapor deposition , to a thickness of approximately 40 nanometers to 500 nanometers , or 400 angstroms to 5000 angstroms . it is important to make second silicon nitride 180 as thin as possible within manufacturing feasibility to avoid dielectric constant increase effects . second graded silicon oxynitride 190 is preferably formed using vapor deposition , within the same processing chamber used for forming second silicon nitride 180 , by combining the process gases used for forming second silicon nitride 180 and second siof 200 . second graded silicon oxynitride 190 is preferably a thickness of approximately 300 to 2000 angstroms . second siof 200 is preferably formed using vapor deposition , within the same processing chamber used for forming second silicon nitride 180 and second graded silicon oxynitride 190 , to a thickness of approximately 5000 angstroms to 3 microns . within the plane of second siof 200 there is a dual structure of a second via 175 and a third metallization 185 . second via 175 is formed by etching a hole within second siof 200 , and , after forming a larger opening within second siof 200 to create the third metallization 185 , extending the hole through the thickness of second siof 200 , second graded silicon oxynitride 190 and second silicon nitride 180 so that the hole extends to second metallization 165 . second via 175 is in direct metal - to - metal contact with second metallization 165 . third metallization 185 is formed as explained above , by etching line patterns into second siof 200 , preferably in - situ with cutting the hole for second via 175 . the line patterns for third metallization 185 are aligned with the hole patterns for second via 175 to enable direct metal - to - metal contact between second via 175 and third metallization 185 . after creating the hole openings and line openings within the stack of second silicon nitride 180 , second silicon oxynitride 190 and second siof 200 , second via 175 and third metallization 185 are completed by filling the openings with copper or other desired electrically conductive material , utilizing a vapor deposition process . the process for forming the stacked silicon nitride and silicon oxide film can be carried out in a commercial chemical vapor deposition chamber . an example process is described in reference to fig4 . substrate 300 is placed within a plasma processing chamber 310 . the preferred plasma processing chamber utilizes microwave frequency source 320 for forming plasma in an electron cyclotron resonance . other sources of plasma such as inductive coupling can be used . the processing chamber 210 is pumped down to a minimum pressure for processing chamber 310 . then , silicon nitride - forming gases are introduced , preferably these are silane ( sih 4 ) 330 and nitrogen ( n 2 ) gas 340 . the gases flow through an inlet 350 ( ammonia may be used if the system is not a high density plasma system capable of dissociating diatomic nitrogen ). the gas flow rates depend on the size of the processing chamber 310 and the size of the substrate 300 ; an example of a total gas flow rate may be approximately 80 standard cubic centimeters per minute ( sccm ) to 200 sccm . a ratio of approximately ¼ to ½ of sih 4 to n 2 may be used . the temperature of substrate 300 is regulated at approximately 375 to 450 degrees celsius . the substrate temperature is regulated by securing the substrate 300 against a helium - cooled chuck 360 preferably by electrostatic force or a mechanical clamp . the plasma is formed by applying a microwave frequency to the process gases , at a power level of approximately 450 to 750 watts . substrate 300 may be biased using radio frequency at about 3000 watts . the various process parameters may be adjusted to achieve a desired refractive index and deposition rate for the silicon nitride . silicon nitride is formed to a desired thickness . next , a graded silicon oxynitride is formed . without modifying the electrical or thermal parameters described above , the flowrate of the gases used for forming silicon nitride is reduced by approximately one half , and gases for forming siof ( since it can be desirable to use siof in lieu of pure silicon dioxide ) are added to the mixture , for an example total gas flow rate of approximately 130 to 250 sccm . such gases for forming siof may be silicon tetrafluoride ( sf 4 ) 370 and oxygen ( o 2 ) 380 . sif 4 370 may be approximately 10 to 20 % of the total gas flow , and o 2 380 may be approximately 50 to 75 % of the total gas flow , and the ratio of sih 4 330 to n 2 340 remain approximately the same as for depositing silicon nitride . silicon oxynitride is formed to a desired thickness . next , without removing substrate 300 from processing chamber 310 , the siof is formed . the flow rates of sif 4 370 and o 2 380 from the previous step are approximately doubled , and sih 4 330 is preferably turned off . n 2 340 may remain on but preferably at a flowrate reduced by 75 to 50 % from the previous step , for an example total gas flow rate of approximately 200 to 300 sccm . power levels for substrate bias and microwave power may be adjusted to obtain the desired refractive index and deposition rate for silicon oxide . when the desired amount of silicon oxide is formed , the plasma is turned off , and substrate 330 is removed from process chamber 310 . a process has been described for forming a silicon nitride , silicon oxynitride and siof films all within the same plasma processing chamber . forming a stacked silicon nitride and silicon oxide film structure in the same chamber has the advantage of avoiding contamination to the silicon nitride surface that may result if the substrate is removed from the processing chamber and inserted into a separate chamber for forming the silicon oxide . additionally , a graded silicon oxynitride can be formed by having a step in between the formation of silicon nitride and silicon oxide , where the gases for forming the two films are merged together . a graded silicon oxynitride is advantageous in that it helps the silicon oxide adhere to the silicon nitride by having a gradual , rather than abrupt , interface . while details have been provided on the process for formation of a silicon nitride , silicon oxynitride and silicon oxide stacked film , they are provided for an example in a semiconductor fabrication context only . gas chemistries , power levels , bias , pressure , and temperature may differ depending on the processing chamber used , the type of plasma that is formed , the size or type of substrate , and the desired refractive indices and deposition rates . other details that may have not been provided may be obtained by experimentation by a person of ordinary skill in the art . while particulars of the structure and process have been provided here , such details should not be construed to limit the present invention in any way , as the invention is limited only by the claims below .