Abstract:
A chemical vapor deposition system that includes a housing configured to form a processing chamber, a substrate holder configured to hold a substrate within the processing chamber, a gas distribution system configured to introduce gases into the processing chamber, a plasma generation system configured to form a plasma within the processing chamber, a processor operatively coupled to control the gas distribution system and the plasma generation system, and a computer-readable memory coupled to the processor that stores a computer-readable program which directs the operation of the chemical vapor deposition system. In one embodiment the computer-readable program comprises instructions that control the gas distribution system to flow a process gas comprising silicon, oxygen and a halogen family member into the chamber to deposit a halogen-doped silicon oxide film on a substrate positioned on the substrate holder and instructions that control the gas distribution system and plasma generation system to densify the halogen-doped silicon oxide film by bombarding the film with ionic species from a plasma of an argon-containing gas source. In another embodiment, the computer-readable program comprises instructions that control the gas distribution system to flow a process gas comprising silicon, oxygen and a halogen family member into the chamber to deposit a halogen-doped silicon oxide film on said substrate and instructions that control the gas distribution system and plasma generation system to form a plasma from a hydrogen containing source gas to bombard the halogen-doped silicon oxide film with hydrogen ions to remove loosely bound halogen atoms from the film.

Description:
This application is a divisional patent application of Ser. No. 09/330,247, filed Jun. 10, 1999 now U.S. Pat. No. 6,103,601, which is a divisional application of Ser. No. 08/548,391, filed Oct. 26, 1995 now abandoned, the disclosures of which are incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to the deposition of dielectric layers during wafer processing and more specifically to a method and apparatus for forming halogen-doped layers having a low dielectric constant and high film stability. 
     One of the primary steps in the fabrication of modern semiconductor devices is the formation of a thin film on a semiconductor substrate by chemical reaction of gases. Such a deposition process is referred to as chemical vapor deposition or “CVD”. Conventional thermal CVD processes supply reactive gases to the substrate surface where heat-induced chemical reactions take place to produce a desired film. The high temperatures at which some thermal CVD processes operate can damage device structures having metal layers. Plasma enhanced CVD (frequently referred to as PECVD) processes on the other hand, promote excitation and/or disassociation of the reactant gases by the application of radio frequency (RF) energy to a reaction zone proximate the substrate surface, thereby creating a plasma of highly-reactive species. The high reactivity of the released species reduces the energy required for a chemical reaction to take place, and thus lowers the required temperature for such CVD processes. The relatively low temperature of a PECVD process makes such processes ideal for the formation of insulating layers over deposited metal layers. 
     Semiconductor device geometries have dramatically decreased in size since such devices were first introduced several decades ago. Since then, integrated circuits have generally followed the two year/half-size rule (often called “Moore&#39;s Law”) which means that the number of devices which will fit on a chip doubles every two years. Today&#39;s wafer fabrication plants are routinely produced 0.5 and even 0.35 micron feature size devices, and tomorrow&#39;s plants soon will be producing devices having even smaller geometries. 
     As device sizes become smaller and integration density increases, issues which were not previously considered important by the industry are becoming of paramount concern. With the advent of multilevel metal technology in which three, four, or more layers of metal are formed on the semiconductors, one goal of semiconductor manufacturers is lowering the dielectric constant of insulating layers such as intermetal dielectric layers deposited by PECVD methods. Low dielectric constant films are particularly desirable for intermetal dielectric (IMD) layers to reduce the RC time delay of the interconnect metallization, to prevent cross-talk between the different levels of metallization, and to reduce device power consumption. 
     Many approaches to obtain lower dielectric constants have been proposed. One of the more promising solutions is the incorporation of fluorine or other halogen elements, such as chlorine or bromine into a silicon oxide layer. An example of halogen incorporation is described in U.S. Ser. No. 08/344,283 commonly assigned to Applied Materials, Inc. filed on Nov. 24, 1994 and incorporated herein by reference. Fluorine, the preferred halogen dopant for silicon oxide films, lowers the dielectric constant of the silicon oxide film because fluorine is an electronegative atom that decreases the polarizability of the overall SiOF network. Fluorine-doped silicon oxide films are also referred to as fluoro silicate glass films or FSG for short. 
     In addition to decreasing the dielectric constant, incorporating fluorine in intermetal silicon oxide layers also helps solve common problems encountered in fabricating smaller geometry devices, such as filling closely spaced gaps on semiconductor structures. It is believed that because fluorine is an etching species, fluorine doping introduces a deposition/etch/deposition effect on oxide formation. The deposition/etch/deposition effect allows FSG films to have improved gap filling capabilities such that the films are able to adequately cover adjacent metal layers having an aspect ratio of 1.8 or more. 
     Thus, manufacturers desire to include fluorine in various dielectric layers and particularly in intermetal dielectric layers. A problem with the incorporation of fluorine in silicon oxide or similar films, however, has been keeping the fluorine in the film. Experimental have shown only a certain level of fluorine is retained in a silicon oxide layer, even when the gas flow of the fluorine containing source gas is increased during the CVD process. 
     At least two separate forces effect the fluorine retention rate. The first is that FSG films absorb moisture easily. Clean rooms have ambient moisture in the air. When a wafer is exposed to the ambient in a clean room, for example, when the wafer is passed to a new processing chamber after deposition of the oxide layer, the FSG layer absorbs moisture, thus increasing the film&#39;s dielectric constant. The absorbed moisture (H 2 O) also reacts with the fluorine to form hydrofluoric acid (HF). 
     Another problem with FSG films occurs when the film is exposed to a thermal process such as an anneal process. The high temperature of the thermal processes can move the fluorine atoms out of the oxide layer through metal or other subsequently deposited layers. The excursion of fluorine atoms in this manner is referred to as outgassing. 
     From the above, it can be seen that an oxide film having a low dielectric constant is necessary to keep pace with emerging technologies. It can also be seen that a method is needed to prevent moisture absorption and outgassing in fluorine-doped oxide films. 
     SUMMARY OF THE INVENTION 
     The present invention solves the above problems of the prior art by providing an insulating layer having a low dielectric constant that is resistant to moisture absorption and outgassing of dopant atoms. The invention also provides a method and apparatus for forming such an insulating layer. 
     In one embodiment of the method of the present invention, a capping layer is formed on a halogen-doped silicon oxide film by introducing a separate silicate glass layer over the oxide layer. In a preferred version of this embodiment, the separate layer is an undoped silicate glass (USG) layer formed in a plasma enhanced CVD deposition chamber. The USG layer in this embodiment is relatively thin and thus does not effect the bulk film properties of the halogen-doped silicon oxide layer. In a still further preferred version of this embodiment the halogen-doped silicon oxide film is an FSG film. 
     Another embodiment of the method of the present invention forms a USG layer over an FSG or similar halogen-doped silicon oxide layer by stopping the flow of the halogen-containing source before deposition of the oxide layer is completed. Without the halogen-containing source present at the end of formation of the halogen-doped silicon oxide layer, a small USG cap is formed on the layer that is sufficient to prevent moisture absorption and outgassing without effecting bulk film properties. The halogen-doped silicon oxide layer comprises an FSG layer in a preferred embodiment. 
     A further embodiment of the method of the present invention treats an FSG or similar halogen-doped silicon oxide layer with a plasma post-treatment step to improve film stability. In one version of this embodiment, a plasma from a nitrogen containing source is used to treat the FSG film. It is believed that the nitrogen plasma treatment forms an overlying, layer on the FSG layer densifying that layer and preventing moisture absorption and outgassing without effecting bulk film properties. In another version of this embodiment, a plasma from an inert gas such as argon is employed to densify the bulk FSG layer. In still another version of this embodiment, a plasma from a hydrogen containing source gas is used to bombard an FSG film with hydrogen ions. It is believed that the hydrogen ions remove loosely bound fluorine atoms from the FSG film so that the loosely bound fluorine atoms do not outgas during subsequent processing. In another version, the plasma post-treatment step includes forming a plasma from a hydrogen containing and nitrogen containing source such as NH 3 . These and other embodiments of the present invention, as well as its advantages and features are described in more detail in conjunction with the text below and attached figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a vertical, cross-sectional view of one embodiment of a simplified chemical vapor deposition apparatus according to the present invention; 
     FIG. 2 is a simplified cross-sectional view of a semiconductor device manufactured according to the method of the present invention; 
     FIG. 3 is a graph showing the effect of fluorine dopant flow on the dielectric constant of FSG film; 
     FIG. 4 is a graph illustrating experimental results showing the dielectric constant of FSG films produced according to one method of the present invention; 
     FIG. 5 is a graph of HF evolution for an uncapped FSG film and an FSG film capped with 1000 Å of USG immediately after the films were deposited; 
     FIG. 6 is a graph of H 2 O evolution for an uncapped FSG film and an FSG film capped with 1000 Å of USG immediately after the films were deposited; 
     FIG. 7 is a graph of HF evolution for an uncapped FSG film and an FSG film capped with 1000 Å of USG 48 hours after the films were deposited; 
     FIG. 8 is a graph of H 2 O evolution for an uncapped FSG film and an FSG film capped with 1000 Å of USG 48 hours after the films were deposited; 
     FIG. 9 is a graph illustrating experimental results showing the dielectric constant of FSG films produced according to another method of the present invention; and 
     FIG. 10 is a graph comparing H 2 O evolution for an FSG film subject to hydrogen bombardment and an FSG film not subject to hydrogen bombardment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     I. Exemplary CVD Reactor Chamber 
     FIG. 1 illustrates one embodiment of a simplified, parallel plate plasma enhanced chemical vapor deposition (PECVD) reactor  10  having a vacuum chamber  15  in which the dielectric layer according to the present invention can be deposited. Reactor  10  contains a gas distribution manifold  11  for dispersing deposition gases to a wafer, not shown, that rests on a susceptor  12 . Susceptor  12  is highly thermally responsive and is mounted on supports  13  so that susceptor  12  (and the wafer supported on the upper surface of susceptor  12 ) can be controllably moved between a lower loading/off-loading position and an upper processing position  14  which is closely adjacent manifold  11 . 
     When susceptor  12  and the wafer are in processing position  14 , they are surrounded by a baffle plate having a plurality of spaced holes  23  which exhaust into an annular vacuum manifold  24 . Deposition and carrier gases and liquids are supplied through lines  18 , having control valves not shown, into a gas mixing chamber  19  where they are combined and then sent to manifold  11 . During processing, gas inlet to manifold  11  is vented toward and uniformly distributed radially across the surface of the wafer as indicated by arrows  22  and  21 . The gas is then exhausted via ports  23  into the circular vacuum manifold  24  and out an exhaust line  31  by a vacuum pump system (not shown). The rate at which gases are released through exhaust line  31  is controlled by a throttle valve  32 . 
     A controlled plasma is formed adjacent to the wafer by RF energy applied to manifold  11  from RF power supply  25 . Gas distribution manifold  11  is also an RF electrode, while susceptor  12  is grounded. RF power supply  25  can supply either single or mixed frequency RF power (or other desired variation) to manifold  11  to enhance the deposition of reactive species introduced into chamber  15 . 
     A circular external lamp module  26  provides a collimated annular pattern of light  27  through quartz window  28  onto susceptor  12 . Such heat distribution compensates for the natural heat loss pattern of the susceptor and provides rapid thermal and uniform susceptor and wafer heating for effecting deposition. 
     A motor, not shown, raises and lowers susceptor  12  between a processing position  14  and a lower, wafer-loading position. The motor, control valves connected to lines  18 , throttle valve and RF power supply  25  are controlled by a processor  34  over control lines  36  of which only some are shown. Processor  34  operates under the control of a computer program stored in a memory  38 . The computer program dictates the timing, mixture of gases, chamber pressure, chamber temperature. RF power levels, susceptor position, and other parameters of a particular process. 
     Typically, any or all of the chamber lining, gas inlet manifold faceplate, support fingers  13 , and various other reactor hardware is fabricated from material such as anodized aluminum. An example of such a PECVD apparatus is described in U.S. Pat. No. 5,000,113 entitled “Thermal CVD/PECVD Reactor and Use for Thermal Chemical Vapor Deposition of Silicon Dioxide and In-situ Multi-step Planarized Process,” which is commonly assigned. 
     The above reactor description is mainly for illustrative purposes and should not be considered as limiting the scope of the present invention. Variations of the above described reactor such as variations in susceptor design, heater design, location of RF power connections and others are possible. Additionally, other plasma CVD equipment such as electron cyclotron resonance (ECR) plasma CVD devices, induction coupled RF high density plasma CVD devices, or the like may be employed. Additionally, thermal CVD equipment may also be used for formation of the halogen-doped bulk film layer. The dielectric layer and method for forming such a layer of the present invention is not limited to any specific apparatus or to any specific plasma excitation method. 
     II. Exemplary Fluoro Silicate Glass Deposition 
     According to the process of the present invention, the bulk of a halogen-doped silicon oxide layer may be formed using any of several different processes. The process recipes of two FSG films, one using triethoxyfluorosilane (hereinafter referred to as “TEFS”) as a fluorine source and one using C 2 F 6  as a fluorine source, are set forth below as examples. Further details on this portion of the process are also set forth in U.S. Ser. No. 08/344,283 previously mentioned. 
     The exemplary process may be performed in PECVD reactor  10 . Of course, those of ordinary skill in the art would understand that other process recipes and other reaction chamber types may also be used to deposit the bulk FSG films. 
     A. TEFS-FSG Film Deposition 
     The exemplary TEFS-FSG bulk film is deposited by heating the wafer and susceptor to a temperature of between 200-500° C., preferably to a temperature within the range of 350-500° C. and most preferably to about 400 degrees, and maintaining this temperature range throughout the deposition. The reaction chamber is maintained at a pressure within a range of about 1-50 torr. Preferably, the pressure is maintained within the range of about 3-16 torr, and most preferably it is maintained at about 5 torr. The susceptor is positioned about 200-600 mils from the gas distribution manifold and is preferably positioned about 250 mils from the manifold. 
     A mixture comprising TEFS as the source of fluorine, tetraethoxysilane (hereinafter referred to as “TEOS”) as the source of silicon and one or more gaseous sources of oxygen is formed. Being liquids, the TEFS and TEOS sources are vaporized and then combined with an inert carrier gas such as helium. The flow rate of TEFS is between about 200-1500 mgm and is preferably between about 300-900 mgm and is most preferably at about 850 mgm, while the TEOS flow rate is between about 400-1500 mgm and preferably about 260 mgm. The vaporized TEOS and TEFS gases then are mixed with a helium carrier gas flowing at a rate of between 400-1500 sccm and are preferably flowing at a rate of about 560 sccm. Oxygen in the form of O 2  is introduced at a flow rate of between about 100-5000 sccm and is preferably introduced at a flow rate of about 1200 sccm. This mixture is introduced into the reaction chamber from the gas distribution manifold and excited into a plasma state. The plasma is formed using a frequency of 13.56 MHz at between 0-500 Watts, and preferably at about 155 Watts, and a low radio frequency of between 10 KHz to 2 MHz, preferably about 350 KHz powered at between about 0-900 Watts and preferably at about 230 Watts. 
     The ratio of TEFS to TEOS is between 0.33-10:1 and preferably between about 1-3:1. The total flow rate of all gaseous sources is between 500-6500 sccm and is preferably between about 1500-2500 sccm. 
     The above conditions result in an FSG film deposited at a rate of between 3000-7000 Å/minute having a compressive stress level of between −0.5 to −3.0*10 9  dynes/cm 2 , a wet etch ratio with respect to the thermal oxide film of between 3-5 for 6:1 BOE etchant and an atomic fluorine concentration of between 0.5-15. Preferably, the fluorine concentration of the resulting film is between 0.5-6% SiF as measured by Fourier transform infrared (FTIR) spectroscopy of the peak ratio of SiF bonds to SiF+SiO bonds. Most preferably the fluorine concentration of the resulting film is between 1.0-4.0% SiF. 
     B. C 2 F 6 -FSG Film Deposition 
     The exemplary C 2 F 6 -FSG bulk film is deposited by heating the wafer and susceptor to a temperature of between 200-500° C., preferably to a temperature within the range of 350-500° C. and most preferably to about 400 degrees, and maintaining this temperature range throughout the deposition. Electrode spacing ranges from about 200-600 mils, and is preferably at about 250 mils. 
     A gaseous mixture comprising C 2 F 6  as the gaseous source of fluorine, TEOS as the source of silicon, and one or more gaseous sources of oxygen is introduced into the reaction chamber from the gas distribution manifold and an RF plasma is formed. The plasma is formed using a high radio frequency of 13.56 MHz at between 0-500 Watts, and preferably at about 200 Watts, and a low radio frequency of between 10 KHz to 2 MHz, preferably about 350 KHz at between about 0-900 Watts and preferably at about 430 Watts. 
     The TEOS source is first vaporized and then combined with an inert carrier gas such as helium. The flow rate of TEOS is between about 400-1500 mgm and preferably at a rate of about 600 mgm. After being vaporized, the TEOS gas source then is mixed with a helium carrier gas at a rage of between 400-1500 sccm and preferably at a rate of about 760 sccm. C 2 F 6  is introduced at a flow rate of between 100-900 sccm and is preferably introduced at a rate of about 400 sccm. Oxygen in the form of O 2  is introduced at a rate of between about 100-5000 sccm and preferably at a rate of about 1200 sccm. 
     The reaction chamber is maintained at a pressure within a range of about 1-50 torr, preferably at a pressure within the range of about 3-16 torr and most preferably at a pressure of about 5 torr. The ratio of C 2 F 6  to TEOS is between about 1-25:1 and preferably between about 5-15:1. While the total flow rate of all gaseous sources is between 500-6200 sccm and is preferably between about 2000-3000 sccm. 
     The above process conditions result in an FSG film deposited at a rate of between 3000-6000 Å/minute having compressive a stress level of between 0 to −2.0*10 9  dynes/cm 2 , a wet etch ratio with respect to the thermal oxide film of between 3-5 for 6:1 BOE etchant and an atomic fluorine concentration of between 0.5-15%. Preferably, the fluorine concentration level is between about 0.5-6% SiF and is most preferably between about 1.0-4.0% SiF. 
     The parameters is the above processes should not be considered limiting to the claims. One of ordinary skill in the art can also use other chemicals, chamber parameters, and conditions to produce similar films. 
     The present invention is also applicable to silicon oxide films doped with members of the halogen family other than fluorine. Specifically, the invention is applicable to silicon oxide films doped with chlorine or bromine. Such films may be deposited, for example, from a plasma of oxygen, TEOS and a reagent from the class of haloorganosilanes of the formula SiX n R 4-n  where n is an integer in the range of 1-3, X is a halogen, and each R is either a hydrogen or an alkoxy group with the proviso that at least one R is an alkoxy group as described in U.S. Ser. No. 08/344,283 mentioned above. Of course, such films may be deposited by other known methods as understood by one of ordinary skill in the art. 
     C. The Dielectric Constant of the Exemplary FSG Films 
     Experiments have shown that the dielectric constant of the above TEFS-FSG and C 2 F 6 -FSG films ranges from between about 4.2 to 3.2 depending on the flow rate of the fluorine dopant and other factors. These experiments, however, also show that, for a given set of process conditions, increasing the fluorine dopant flow alone may not achieve an FSG film having the lowest dielectric constant possible. One result of these experiments is shown in FIG. 3, which is a graph showing the effect of fluorine dopant flow on the dielectric constant of TEFS-FSG and C 2 F 6 -FSG films deposited under specified process conditions as described below. The dielectric constant measurements performed in these experiments were done with a mercury capacitance-voltage measurement on an MOS capacitor structure as is well known to those skilled in the art. 
     The TEFS-FSG film having characteristics shown in FIG. 3 was deposited on a low resistivity silicon wafer at a pressure of 5 torr and a temperature of 400° C. using a high frequency of 13.56 MHz at 155 Watts and a low frequency of 350 KHz powered at 230 Watts. The susceptor was positioned 250 mils from the gas distribution manifold. TEOS was introduced at a flow rate of 260 mgm while oxygen was introduced into the chamber at a rate of 1250 sccm. TEFS was introduced at a rate of between 0-1500 mgm from an ampule, vaporized and mixed with a helium carrier gas flowing at a rate of 560 sccm. 
     The C 2 F 6  film of FIG. 3 was deposited on a low resistivity silicon wafer at a pressure of 5 torr and a temperature of 400° C. A plasma was formed from the C 2 F 6  source gas, TEOS and oxygen using a frequency of 13.56 MHz at 200 Watts, then a lower frequency of 350 KHz at 430 Watts. The susceptor was positioned 250 mils from the gas distribution manifold. TEOS was introduced at a flow rate of 260 mgm, oxygen was introduced at a rate of 1000 sccm and C 2 F 6  was introduced at a flow rate of about 0-400 sccm. 
     III. Decreasing the Dielectric Constant of the Halogen-doped Silicon Oxide Layer 
     It is believed that film stability is a limiting factor in the reduction of the dielectric constant of halogen-doped silicon oxide films such as FSG. Several methods to improve film stability and lower the dielectric constant of these films, as described below, were investigated. Each method described may be performed on a halogen-doped silicon oxide layer, after the layer is deposited using a recipe such as one of the exemplary FSG deposition recipes described above. Furthermore, although each method is described below with respect to treatment of an FSG film, the methods are equally applicable to any halogen-doped silicon oxide film. 
     A. Deposition of an Undoped Silicate Glass Capping Layer 
     According to one embodiment of the present invention, the deposited FSG layer is capped with a thin, separate undoped silicate glass (USG) layer. The USG capping layer is a stable film that does not readily absorb moisture. Thus, the USG capping layer provides a hydrophobic surface on top of the FSG layer that prevents moisture present in the ambient from being absorbed into the FSG film. Furthermore, the USG capping layer is a relatively dense film that impedes fluorine evolution. 
     The USG layer can be deposited in a separate processing chamber from the FSG layer, but preferably is done as an in-situ process. The wafer and susceptor are heated to a temperature within the range of about 100 to 500° C., but preferably from about 360 to 440° C. and then maintained at this temperature throughout the deposition. The susceptor is positioned about 200-400 mils away from the gas distribution manifold and preferably about 250 mils away. 
     A gaseous mixture comprising oxygen and silicon sources is introduced into the deposition chamber. In a preferred embodiment, the silicon source is TEOS and the oxygen source is O 2 , but those skilled in the art will recognize that additional silicon sources such as silane (SiH 4 ), TMCT or similar sources, and other oxygen sources such as ozone, H 2 O, N 2 O and similar sources and mixtures of the same also can be employed. When TEOS is used as a silicon source, a carrier gas such as helium, argon or nitrogen is employed. The ratio of O 2  to TEOS may range from about 0.1-1000:1, but is preferably between about 0.3-30:1. Typically the ratio will be about 10:1. 
     The optimal total flow of the gaseous reactants will vary according to the geometry and design of the deposition chamber. The gas flow also can be varied to control the deposition rate. Typically, TEOS is introduced at a flow rate of between about 10 and 10,000 mgm and is preferably introduced at a flow rate of between about 500 and 1500 mgm. O 2  is introduced at a flow rate of between about 10 and 10,000 sccm and is preferably introduced at a flow rate of between about 500 and 1500 sccm. Helium is a typical carrier gas that is introduced at a flow rate of between 100-100,000 sccm and preferably between about 1000-10,000 sccm. Usually, the total flow of gases into the deposition chamber will vary from about 1000 to about 10,000 sccm. 
     Chamber pressure is maintained at a pressure of between 0.5 and 20 torr, and is preferably at a pressure of between 3 and 10 torr. Such a low pressure provides increased deposition rate and film uniformity. The plasma is formed using a frequency of 13.56 MHz at between 0-900 Watts, and preferably at about 350 Watts, and a lower frequency of between about 10 KHz to 2 MHz, preferably about 350 KHz at between about 0-500 Watts and preferably at about 100 Watts. 
     Under the above conditions, deposition rates of between 5000 and 10,000 Å/minute are attainable. At such deposition rates, a USG cap layer of 1000 Å can be deposited in approximately 5-20 seconds. Typical applications will have USG cap layers of between 100 and 2000 Å, and preferably between 500 and 1000 Å. A person or ordinary skill in the art, however, will realize that capping layers of different thickness can be employed depending on the specific application and device geometry size. Depending on the application in which the deposited film is used, it may be preferable, although not essential, that the capping layer be relatively thin compared to the underlying FSG layer so as to not change the bulk film properties of the FSG layer. Also, depending upon the application, a thick USG capping layer may be deposited and etched back using a technique such as chemical mechanical polishing (CMP). 
     A person of ordinary skill in the art will also recognize that different capping layers such as thermally grown USG films, plasma enhanced or thermally grown phosphosilicate glass (PSG) or similar, stable oxide films may be utilized. In fact, if a particular application requires the use of CMP techniques, a PSG capping layer may be preferential to a USG capping layer because PSG films may be etched away faster. Preferably, the capping layer should not be limited in effectiveness by the same factors that limit the stability of the underlying FSG layer. Thus, the capping layer preferably should not be an FSG layer or similar halogen-doped film unless the fluorine or halogen content of the capping layer is substantially lower than that of the capped, bulk layer. The FSG layer according to the above embodiment of the present invention is a conformal film, having a low pin hole count, high density, compressive stress and a high wet etch rate. 
     B. Premature Cutoff of the Fluorine Source 
     According to another embodiment of the present invention, a USG or similar cap layer is formed on an FSG film by turning off the fluorine-containing gas source just before completion of deposition of the FSG layer. In this embodiment, the initial FSG layer is formed as described above. Flow of the fluorine dopant source into the reaction chamber then is stopped while the plasma remains on for an additional period of between 0.5-20 seconds. Preferably, the plasma remains on for about 0.5-10 seconds and most preferably the plasma remains on for an additional 2-5 seconds. The preferred time period should be greater than the average residence time of an individual atom (the time an atom spends in the reaction chamber) and preferably is at least twice the average residence time. In one embodiment where reactor pressure is low and reactor volume is small (approximately 5 liters), the average residence time of an individual atom is about 1.0 seconds. In this embodiment, the fluorine source is stopped by closing a valve on the source&#39;s supply line so that the plasma is maintained without a fluorine dopant for at least 2 seconds. 
     Of course, stopping of the fluorine source must be coordinated with the time it takes the gas to travel from the point of the valve to the gas mixing chamber and then through the faceplate of manifold  11 . In most CVD machines several seconds is required for gas to flow from the injection valve to the deposition chamber, so the valve should be closed sufficiently in advance to allow for these delays. 
     Thus, if TEFS is the fluorine-containing gas source, closing the valve on the TEFS supply line several seconds before the completion of depletion of the FSG layer results in a thin USG cap which prevents occurrence of the previously described moisture absorption and outgassing phenomenon. Similar, if C 2 F 6  is the fluorine-containing gas source, closing the valve on the C 2 F 6  supply line several seconds before the completion of deposition of the FSG layer produces a equally effective USG cap. 
     C. Plasma Post-Treatment Process 
     Still another embodiment of the present invention treats the deposited FSG layer with a post-treatment plasma step. Various gas sources can be employed for this post-treatment plasma step. In one version of this embodiment, the plasma consists of a nitrogen gas source such as nitrogen, N 2 O or a similar gas or combination of gases. In another version the plasma consists of a nonreactive gas source such as argon. When the plasma layer is formed from a nitrogen containing source gas, it is believed that nitrogen atoms bond in the silicon lattice to form traces of silicon nitride. It is also believed that the silicon nitride forms a hermetic seal over the underlying FSG layer, thus forming a hydrophobic film that prevents moisture absorption and is resistant to fluorine outgassing. Forming the plasma layer with a heavy gas such as argon, densifies the film. The densified film is more resistant to moisture absorption and is more resistant to fluorine outgassing. 
     The plasma treatment can be carried out in a separate processing chamber from the FSG layer, but preferably it is deposited in an in-situ process immediately after deposition of the FSG layer. The post-treatment plasma layer is deposited by maintaining the wafer and susceptor temperature within the range of about 25 to 600 degrees C, but preferably from about 360 to 440 degrees C. Chamber pressure is maintained at a pressure of between 0.1 and 20 torr, and is preferably kept at a pressure of between 0.5 and 5 torr. The susceptor is positioned about 200-400 mils away from the gas distribution manifold and preferably about 250 mils away. 
     In a preferred version of this embodiment, the plasma consists of a nitrogen containing source gas such as N 2  or N 2 O. The gas is introduced into the deposition chamber at rate of between about 10 and 20,000 sccm and is preferably introduced at a flow rate of between about 500 and 3000 sccm. The optimal flow of the gas will vary according to the geometry and design of the deposition chamber and the gas may be combined with a carrier gas as appropriate. 
     With the high frequency (13.56 MHz) power source, power ranges from about 0 Watts to about 2000 Watts, and is preferably at about 50 Watts. In the low frequency (between about 10 KHz-2 MHz and preferably about 350 KHz) power source, power ranges from about 0 Watts to about 2000 Watts, and is preferably at about 450 Watts. Experiments have shown that use of a mixed frequency RF power source is preferable to a single, high frequency source. A mixed frequency source results in ions bombarding the film with a higher energy level, which in turn results in a denser and more stable film. Of course, the particular gases, processing parameters, and power sources used will vary according to the particular application. 
     In typical applications, a plasma post-treatment step of between 10 and 500 seconds, and preferably of between 20 and 100 seconds is used. The plasma post-treatment process does not change the bulk film properties of the underlying FSG layer. 
     The FSG film can be further densified by connecting the low frequency RF power supply to the susceptor rather than to the electrode. The ions generated in the plasma are not mobile enough to follow the high frequency RF source applied to the manifold, but are mobile enough to follow the low frequency source. Thus, ion bombardment is controlled by the low frequency source at the susceptor. Placing a low frequency DC bias on susceptor  12  moves the voltage sheath closer to the receptor and increases ion bombardment at the surface substrate. The generated ions are accelerated through the electrical gradient across the wafer at higher kinetic energy levels, resulting in formation of a denser film. 
     Still another version of this embodiment of the method of the present invention treats the FSG film with a post-treatment hydrogen plasma step. The plasma step bombards the FSG film with ions from a hydrogen containing source gas such as H 2 . The hydrogen ions from the plasma act as a reducing agent removing loosely bound fluorine atoms from the FSG film. The small size of the hydrogen ions allows the ions to penetrate deep into the FSG layer and remove loosely bound fluorine atoms from both the surface and interior of the film. It is believed that the bombarded hydrogen atoms are not bonded into the FSG film. After being subjected to the hydrogen plasma, the fluorine atoms remaining in the FSG film are generally tightly bound within the film. 
     This method may be performed on a FSG layer, after the layer is deposited using a process such as one of the exemplary FSG deposition processes described above. Furthermore, although the hydrogen bombardment step is described below with respect to treatment of an FSG film, it is equally applicable to any halogen-doped silicon oxide film. 
     The hydrogen bombardment post-treatment process is performed by maintaining the wafer and susceptor temperature within the range of about 300 to 500° C., but preferably from about 400 to 480° C. Chamber pressure is maintained at a pressure of between 1 and 100 torr, and is preferably kept at a pressure of between 50 and 100 torr. The susceptor is positioned about 250-500 mils away from the gas distribution manifold and preferably about 400 mils away. 
     In a preferred version of this embodiment, the plasma consists of a hydrogen containing source gas such as H 2 . The gas is introduced into the deposition chamber at rate of between about 250 and 2000 sccm and is preferably introduced at a flow rate of between about 500 and 1000 sccm. The gas may be combined with a carrier gas such as nitrogen or helium, argon, or other inert gas introduced at a rate of between about 50 and 500 sccm. The optimal flow of the gas will vary according to the geometry and design of the deposition chamber. 
     With the high frequency (13.56 MHz) power source, power ranges from about 0 Watts to about 2000 Watts, and is preferably at about 250 Watts. In the low frequency (between about 10 KHz-2 MHz and preferably about 350 KHz) power source, power ranges from about 0 Watts to about 2000 Watts, and is preferably at about 100 Watts. The use of a mixed frequency RF power source is preferable to a single, high frequency source. A mixed frequency source results in ions bombarding the film with a higher energy level, which in turn is more likely to remove loosely bound fluorine atoms from the bombarded FSG layer. Of course, the particular gases, processing parameters, and power sources used will vary according to the particular application. 
     In typical applications, a hydrogen bombardment post-treatment step is used for between 10 and 300 seconds, and preferably for between 20 and 60 seconds. Because no measurable deposition occurs, the plasma post-treatment process does not change the bulk film properties of the underlying FSG layer. 
     In still another version of this embodiment, the plasma step bombards the FSG film with ions from a nitrogen containing source and a hydrogen containing source. In such a step, it is believed that the hydrogen ions remove loosely bound fluorine atoms as described above and it is also believed that a nitrogen capping layer is formed sealing the FSG layer. In a preferred version of this embodiment, the hydrogen containing source and nitrogen containing source are form a single source gas such as ammonia (NH 3 ). 
     IV. Exemplary Structure 
     FIG. 2 illustrates a simplified cross-sectional view of an integrated circuit  200  according to the present invention. As shown, integrated circuit  200  includes NMOS and PMOS transistors  203  and  206 , which are separated and electrically isolated from each other by a field oxide region  220 . Each transistor  203  and  206  comprises a source region  212 , a drain region  215  and a gate region  218 . 
     A premetal dielectric layer  221  separates transistors  203  and  206  from metal layer M 1  with connections between metal layer M 1  and the transistors made by contacts  224 . Metal layer M 1  is one of four metal layers, M 1 -M 4 , included in integrated circuit  200 . Each metal layer M 1 -M 4  is separated from adjacent metal layers by respective inter-metal dielectric layers IMD 1 , IMD 2  or IMD 3 . Adjacent metal layers are connected at selected openings by vias  226 . Deposited over metal layer M 4  are planarized passivation layer  230 . 
     While the dielectric layer of the present invention may find uses in each of the dielectric layers shown in integrated circuit  200 , physical properties of the film such as its low dielectric constant and good gap-fill properties along with the PECVD process used in formation of the preferred embodiments make it most useful as an insulation layer between adjacent metal layers as shown by IMD layers IMD 1 -IMD 3 . Typically, such IMD layers are between 0.2 and 0.3 microns thick. 
     The dielectric layer of the present invention may also be used in damascene layers which are included in some integrated circuits. In damascene layers, a blanket FSG layer is deposited over a substrate, selectively etched through to the substrate and then filled with metal and etched back or polished to form metal contacts such as M 1 . After the metal layer is deposited, a second blanket FSG deposition is performed and selectively etched. The etched areas are then filled with metal and etched back or polished to form vias  226 . 
     It should be understood that the simplified integrated circuit  200  is for illustrative purposes only. One or ordinary skill in the art could implement the present method for fabrication of other integrated circuits such as microprocessors, application specific integrated circuits (ASICS), memory devices, and the like. 
     V. Test Results and Measurements 
     A. Deposition of an Undoped Silicate Glass Capping Layer 
     To demonstrate the operation of the apparatus and method according to the embodiment of the present invention employing a USG capping layer, experiments were performed measuring the dielectric constant of FSG films manufactured without a USG capping layer and measuring the dielectric constant of FSG films with the capping layer. The uncapped FSG films were 1.0 micron thick and the capped FSG films were 1.0 micron thick with a 1000 Å USG cap. Both films were deposited on a low resistivity silicon wafer. As shown in Table 1 below, two different FSG films are illustrated: one in which the fluorine source was TEFS and one in which the fluorine source gas was C 2 F 6 . Both films were deposited in a Precision 5000 chamber manufactured by Applied Materials, Inc. 
     The TEFS-FSG film was deposited using a frequency of 13.56 MHz at 155 Watts and a lower frequency of 350 KHz at 230 Watts. Chamber pressure was maintained at 5 torr and the susceptor was heated to 400° C. and positioned 250 mils from the gas distribution manifold. 
     TEOS was introduced at a rate of 600 mgm into a liquid injection valve and mixed with a helium carrier gas flowing at a rate of 380 sccm. TEFS stored in an ampule was combined with a helium carrier gas flowing at 150 sccm. The flow rate of oxygen was 1250 sccm. 
     The above conditions resulted in an FSG film deposited at a rate of 6000 Å/minute, and a refractive index of 1.44. The film had a compressive stress of −1.0*10 9  dynes/cm 2  and contained 1.0% SiF bonds. 
     The C 2 F 6 -FSG film was deposited using a high frequency of 13.56 MHz at 200 Watts and a low frequency of 350 KHz at 430 Watts. The pressure in the processing chamber was maintained at 5 torr and the susceptor and wafer were heated to 400° C. and positioned 250 mils from the gas distribution manifold. 
     TEOS was introduced at a rate of 600 mgm into a liquid injection valve and mixed with a helium carrier gas at a rate of 740 sccm, while the flow rate of C 2 F 6  was 350 sccm. Oxygen was introduced at a rate of 1000 sccm. 
     In each case, the USG capping layer was formed in an in-situ process immediately after the bulk FSG layer was deposited. The susceptor was heated to a temperature of 400° C., the chamber was maintained at a pressure of 5 torr, and the susceptor was positioned 250 mils from the gas distribution manifold. TEOS, oxygen and helium were introduced into the deposition chamber at flow rates of 600, 840, and 560 sccm, respectively, and a plasma was maintained using a 13.56 MHz and 350 MHz RF power supplies at 350 and 100 Watts, respectively. 
     The above conditions resulted in an FSG film deposited at a rate of 4500 Å/minute, having a uniformity rate of 4.0% and a refractive index of 1.42. The film had a compressive stress of −0.6*10 9  dynes/cm 2  and contained 2.5% SiF molecules. 
     The parameters used in the experiments should not be limiting to the claims as described herein. One of ordinary skill in the art also use other chemicals, chamber parameters, and conditions to produce similar films. 
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 EFFECT OF CAPPING AND ANNEALING ON 
               
               
                 DIELECTRIC CONSTANT 
               
             
          
           
               
                   
                 TEFS-FSG Film 
                 C 2 F 6 -FSG Film 
               
               
                   
                 (FTIR % SiF: 2.5) 
                 (FTIR % SiF: 2.5) 
               
               
                   
                   
               
             
          
           
               
                 Plain Film deposited 
                 3.76 
                 3.84 
               
               
                 at 400° C. 
               
               
                 Film with 1000 PE USG cap 
                 3.51 
                 3.42 
               
               
                 Film annealed at 450° C. 
                 3.85 
                 3.93 
               
               
                 Film annealed at 850° C. 
                 3.61 
                 [Not Tested] 
               
               
                 Film deposited at 440° C. 
                 3.74 
                 [Not Tested] 
               
               
                 Susceptor temp. 
               
               
                   
               
             
          
         
       
     
     As evident from the table, the FSG films having a 1000 Å USG cap deposited over the FSG layer had a lower dielectric constant than the non-capped FSG films. For the TEFS-FSG film, the dielectric constant was 3.51 as compared to 3.76, and for the C 2 F 6 -FSG film the dielectric constant was 3.42 as compared to 3.84. 
     Table 1 also shows that other attempts at obtaining a reduced dielectric constant film were performed. These experiments concluded that subjecting the film to a 30 minute anneal step at 450° C. had no beneficial effect on the dielectric constant for either the TEFS or C 2 F 6  film. Furthermore, while a TEFS-FSG film subjected to a 30 minute anneal at 850° C. had a decreased dielectric constant of 3.61 as compared to 3.76, IMD layers cannot be subjected to a thermal step of 850° C. without damaging underlying metal layers. Also, depositing an initial TEFS-FSG layer at an increased temperature of 440° C. resulted in a negligible decrease in the dielectric constant. Data for an 850° C. anneal for the C 2 F 6  film and the C 2 F 6  film deposited at 440° C. was not measured. 
     FIG. 4 provides additional information about the effect of the USG cap on the dielectric constant. The diamonds in FIG. 4 represent the dielectric constant for wafers having a 1.0 micron C 2 F 6 -FSG layer. Measurements were performed with a mercury probe capacitance-voltage measurement on an MOS capacitor structure, as is well known to those skilled in the art. The squares represent the measured dielectric constant for wafers having a 1000 Å USG cap deposited on top of a 1.0 micron FSG layer. 
     As evident from FIG. 4, the dielectric constant for the capped layers was lower than the uncapped FSG layers in every case. The dielectric constant for wafers having a 1000 Å USG cap varied from between about 3.4-3.6 and averaged about 3.5. Based on simulations, an insulation layer having a dielectric constant of about 3.5 can increase device speed by about 10% over an insulation layer having a dielectric constant of about 4.2. Additionally, 3.5 dielectric constant layer film reduces device power consumption and significantly reduces cross talk. 
     Further evidence of the benefits of a USG cap on an FSG film is evident from FIGS. 5-8, which are graphs comparing the evolution of HF and H 2 O as a function of temperature in capped and uncapped FSG films based on thermal desorption spectroscopy (TDS) as well understood by those of ordinary skill in the art. FIG. 5 compares the evolution of HF (electron mass of 20) in an uncapped film FSG to the evolution of HF in a film capped with a 1000 ÅUSG layer immediately after deposition. The TDS data plotted in FIG. 5 shows that HF evolution begins occurring in the uncapped FSG film at approximately 400° C. which is about 75 degrees before HF evolution occurs in the capped FSG film. This difference is significant in that many thermal cycles take place at temperatures of between 400-450 degrees. Such thermal cycles, which are particularly common for IMD layers, result in outgassing of HF in an uncapped FSG layer, but do not result in outgassing in a FSG layer capped according to this embodiment of the present invention. 
     In FIG. 6, which compares the evolution of H 2 O (electron mass of 18) in an uncapped film FSG to the evolution of HF in a film capped with a 1000 Å USG layer immediately after deposition, the plotted TDS data shows that H 2 O evolution begins occurring in the uncapped FSG film at approximately 320° C. which is approximately 130° C. before H 2 O evolution occurs in the capped FSG film at 450° C. 
     Some of the conclusions drawn from FIGS. 5 and 6 along with other comparison data proving the benefits of an FSG film having a USG cap layer appear in Table 2. Table 2 summarizes the TDS and FTIR data for a TEFS-FSG film. 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 TEFS-FSG: FILM STABILITY FOR AS DEPOSITED FILMS 
               
             
          
           
               
                   
                 As Dep FTIR Data 
                 Thermal Desorption Spectrum Data 
                   
               
             
          
           
               
                   
                 SiOH/SiO 
                 H 2 O 
                 H 2 O 
                 HF 
                 HF 
                 Stress Hysteresis 
               
               
                   
                 Ratio 
                 Start 
                 Peak 
                 Start T 
                 Peak T 
                 Over 25 to 450° C. 
               
               
                 TEFS-FSG Film Type 
                 (%) 
                 (C.) 
                 (C.) 
                 (C.) 
                 (C.) 
                 (x-E8 dynes/cm 2 ) 
               
               
                   
               
             
          
           
               
                 Plain Film deposited at 
                 0.048 
                 320 
                 380 
                 400 
                 550 
                 2.5 
               
               
                 400° C. 
               
               
                 Film with 1000 Å PE 
                 0.0009 
                 450 
                 515 
                 475 
                 550 
                 2 
               
               
                 USG cap 
               
               
                 Film with 450° C. 
                 0.048 
                 450 
                 520 
                 440 
                 560 
                 1 
               
               
                 anneal 
               
               
                 Film deposited at 
                 0.032 
                 No 
                 No 
                 480 
                 600 
                 2 
               
               
                 440° C. 
                   
                 Peak 
                 Peak 
               
               
                 Susceptor temp. 
               
               
                 Film at 440° C., capped 
                 0.015 
                 500 
                 560 
                 525 
                 600 
                 2 
               
               
                 and annealed 
               
               
                   
               
             
          
         
       
     
     Further experiments were performed to evaluate the thermal desorption characteristics of the films after the films were exposed to ambient conditions for 48 hours. FIGS. 7 and 8 show the results of these experiments, and that the benefits derived from the USG cap layer continue, or even increase, over time. FIG. 7 is a graph comparing HF evolution in an uncapped FSG film to HF evolution in an FSG film capped with a 1000 Å USG cap after the films have been deposited for 48 hours. The TDS data plotted in FIG. 6 shows that after being deposited 48 hours, HF evolution begins occurring in the uncapped FSG film at approximately 120° C. before HF evolution occurs in the capped FSG film (360° C. as compared to 480° C.). 
     Similarly, in FIG. 8, which compares the evolution of H 2 O in an uncapped film FSG to the evolution of HF in a film capped with a 1000 Å USG layer after the films have been deposited for 48 hours, the plotted TDS data shows that H 2 O evolution begins occurring in the uncapped FSG film at approximately 380° C. before H 2 O evolution occurs in the capped FSG film (40° C. versus 420° C.). 
     Some of the conclusions drawn from FIGS. 7 and 8 along with other comparison data proving the benefits of an FSG film having a USG cap layer is summarized in Table 3 below. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 TEFS-FSG: THERMAL DESORPTION OVER TIME 
               
             
          
           
               
                   
                 H 2 O Start Temp 
                 H 2 O Peak Temp 
                 HF Start Temp 
                 HF Peak Temp 
               
             
          
           
               
                   
                 (C.) 
                   
                 (C.) 
                   
                 (C.) 
                   
                 (C.) 
                   
               
               
                   
                   
                 After 
                   
                 After 
                   
                 After 
                   
                 After 
               
               
                 TEFS-FSG Film Type 
                 As Dep 
                 48 Hrs. 
                 As Dep 
                 48 Hrs. 
                 As Dep 
                 48 Hrs. 
                 As Dep 
                 48 Hrs. 
               
               
                   
               
             
          
           
               
                 Plain Film deposited 
                 320 
                 40 
                 380 
                 160 
                 400 
                 360 
                 550 
                 480 
               
               
                 at 400° C. 
               
               
                 Film with 1000 Å PE 
                 450 
                 420 
                 515 
                 500 
                 475 
                 480 
                 550 
                 580 
               
               
                 USG cap 
               
               
                 Film with 450° C. 
                 450 
                 40 
                 520 
                 160 
                 440 
                 390 
                 560 
                 500 
               
               
                 anneal 
               
               
                 Film deposited at 
                 No 
                 440 
                 No 
                 480 
                 480 
                 440 
                 600 
                 575 
               
               
                 440° C. 
                 Peak 
                   
                 Peak 
               
               
                 Susceptor temp. 
               
               
                 Film at 440° C., 
                 500 
                 450 
                 560 
                 500 
                 525 
                 500 
                 600 
                 550 
               
               
                 capped and annealed 
               
               
                   
               
             
          
         
       
     
     B. Premature Cutoff of the Fluorine Source 
     To demonstrate the operation of the apparatus and method according to the embodiment of the present invention that prematurely terminates the fluorine containing source, experiments were performed measuring the dielectric constant of C 2 F 6 -FSG films manufactured without cutting off the fluorine source and measuring the dielectric constant of C 2 F 6 -FSG films having a USG capping layer formed by shutting off the fluorine source. The uncapped FSG films were 1.0 micron thick and the capped FSG films were 1.0 micron thick with a USG cap approximately 200 Å thick. Both films were deposited on a low resistivity silicon wafer in a Precision 5000 chamber manufactured by Applied Materials, Inc. 
     The C 2 F 6 -FSG film was deposited using a high frequency of 13.56 MHz at 200 Watts and a low frequency of 350 KHz at 430 Watts. The pressure in the processing chamber was maintained at 5 torr and the susceptor and wafer were heated to 400° C. and positioned 250 mils from the gas distribution manifold. TEOS was introduced at a rate of 260 mgm into a liquid injection valve and mixed with a helium carrier gas introduced into the chamber at a rate of 740 sccm. The flow rate of C 2 F 6  was 350 sccm, and oxygen was introduced at a rate of 1000 sccm. 
     The process conditions for the capped film were identical to the uncapped film except that the flow of the C 2 F 6  source was stopped 2 seconds prior to completion of deposition of the FSG layer. 
     The results of these experiments are shown in FIG. 9, which is a graph illustrating experimental results showing the decreased dielectric constant of FSG films produced according to this embodiment of the present invention. In FIG. 9, the diamonds represent the dielectric constant for wafers having a 1.0 micron FSG layer. The x&#39;s represent the measured dielectric constant for wafers having approximately a 200 Å USG cap deposited over a 1.0 micron FSG layer. 
     As evident from FIG. 9, except for two instances which represent normal noise in such tests, the dielectric constant for the capped layers was lower than the uncapped FSG layers. The dielectric constant for wafers having a 200 Å USG cap varied from between about 3.3-3.8 and averaged about 3.55. 
     C. Post-Treatment Plasma Process 
     To demonstrate the operation of the apparatus and method according to the embodiment of the present invention that subjects the halogen-doped film to a hydrogen plasma treatment, experiments were performed measuring the evolution of H 2 O from FSG films treated with a hydrogen plasma step and those not treated with such a step. Both films were deposited on a low resistivity silicon wafer and were 0.1 micron thick. Both films were deposited in a Precision 5000 chamber manufactured by Applied Materials, Inc. 
     The FSG films were C 2 F 6 -FSG films deposited using a high frequency of 13.56 MHz at 200 Watts and a low frequency of 350 MHz at 430 Watts. The pressure in the processing chamber was maintained at 5 torr and the susceptor and wafer were heated to 400° C. and positioned 250 mils from the gas distribution manifold. TEOS was introduced at a rate of 260 mgm into a liquid injection valve and mixed with a helium carrier gas at a rate of 740 sccm, while the flow rate of C 2 F 6  was 350 sccm. Oxygen was introduced at a rate of 1000 sccm. 
     The hydrogen bombardment step was formed in an in-situ process immediately after the bulk FSG layer was deposited. The susceptor was heated to a temperature of 400° C., the chamber was maintained at a pressure of 90 torr, and the susceptor was positioned 400 mils from the gas distribution manifold. Hydrogen was introduced into the deposition chamber at a flow rate of 700 sccm mixed with a carrier gas introduced at a flow rate of about 50 sccm, and a plasma was maintained using a 13.56 MHz and 350 KHz RF power supplies at 250 and 0 Watts, respectively. 
     Evidence of the benefits of a hydrogen post-treatment plasma step on an FSG film is evident from FIG. 10, which is a graph comparing the evolution of H 2 O as a function of temperature in treated and untreated FSG films based on thermal desorption spectroscopy (TDS) as well understood by those of ordinary skill in the art. FIG. 10 compares the evolution of H 2 O (electron mass of 18) in an untreated FSG film to the evolution of H 2 O in a film treated with a hydrogen plasma post-treatment step of 60 seconds. The plotted TDS data shows that H 2 O evolution occurs at two intervals in the untreated FSG film. The first interval, which represents evolution of moisture absorbed by the film after deposition of the FSG layer occurs at approximately 150° C. It is believed that the H 2 O evolving at this temperature is H 2 O absorbed by the FSG layer while the film was exposed to ambient conditions for 48 hours prior to having the TDS data plotted. 
     The second interval in FIG. 4 represents the evolution of loosely bound H 2 O from the bulk of the FSG film. H 2 O evolution at this interval begins at approximately 400° C. and peaks at approximately 450° C. 
     In contrast, the FSG film treated with hydrogen post-treatment plasma only has one instance of H 2 O evolution. The TDS data for this film shows that moisture absorbed by the film after the hydrogen plasma treatment step is released as approximately 150° C. As above, it is believed that this moisture is present in the film because the film was exposed to ambient conditions for 48 hours prior to having the TDS data plotted. As shown in FIG. 4 though, there is no second interval of H 2 O evolution. The hydrogen plasma treatment previously removed the loosely bound fluorine atoms from the FSG layer. 
     Having fully described several embodiments of the present invention, many other equivalent or alternative methods of depositing the low dielectric constant oxide layer according to the present invention will be apparent to those skilled in the art. For example, while most of the illustrated embodiments concerned fluorine-doped oxide layers, the present invention is also applicable to fluorine-doped nitride and similar films. These equivalents and alternatives are intended to be included within the scope of the present invention.