Abstract:
Black diamond films, deposited using PECVD at low substrate temperatures, have been effectively stabilized by immersing them in de-ionized water at a temperature of about 90° C. for about 20 minutes or in a hydrogen peroxide solution (typically at a concentration of 10%) for about 60 minutes. Since it has been observed that the dielectric constant of the stabilized film increases with both immersion time and/or peroxide concentration, this effect may be used as a means for adjusting the dielectric constant of a black diamond film. Standard analytical techniques confirm that these low temperature stabilized films have electrical properties that are at least as good as those of films stabilized using high temperature heat treatments.

Description:
FIELD OF THE INVENTION 
     The invention relates to the general field of low dielectric constant layers for use in integrated circuits with particular reference to methods for forming and stabilizing such films at or near room temperature. 
     BACKGROUND OF THE INVENTION 
     The internal dimensions within integrated circuits continue to shrink, including the thicknesses of the dielectric layers used to separate various layers of wiring from one another. However, as these wiring levels are brought closer together, the possibility of cross-coupling between them starts to rise as well as the incidence of parasitic capacitances. One way to minimize this problem is to reduce the dielectric constants of these inter-metal layers. Thus there is considerable interest in developing low k materials as well as deposition methods for them that are compatible with integrated circuit technology. 
     For our purposes we will define a low k dielectric as one that has a dielectric constant dose to or less than about 3. Several such materials are known to exist but they have the disadvantage that they are organic rather than inorganic compounds. Examples include hydrogen silsesquioxane, fluorinated polyimide, polyarylene ether, fluorinated arylene ether, polytetrafluoro-ethylene, and benzocyclobutene. Because of their organic nature these materials are innately soft, which physical property can give rise to problems during semiconductor processing, particularly during planarization. 
     The present invention is concerned with a low dielectric constant material that is essentially inorganic in nature, referred to by practitioners of the art as black diamond, or BD. BD is silica that has been doped with about 10 mole % methane. When formed as will be described below, BD is porous, about 36% of its volume being in the form of pores having a diameter between about 8 and 24 Angstroms. 
     To be of acceptable quality, the resistivity of an inter-metal dielectric layer needs to exceed about 10 5  ohm-cm. In practice an indication of the ‘leakiness’ of any given film can be obtained by measuring its flat-band voltage. This is the gate voltage required tp bring about the flat-band condition, so the higher the flat-band voltage, the greater the leakage current associated with the film., 
     Two processes have been used in the prior art to form BD films of acceptable quality. In the hot film method, a mixture of trimethyl silane, silane, nitrous oxide, and oxygen is used at about 400° C. to form a plasma out of which the BD deposits. In the cold film method, deposition of the BD film takes place from a room temperature plasma of a mixture of helium, methyl silane, and nitrous oxide. 
     It has been found that films deposited by the cold film method are, as deposited, of poor quality. To raise the quality to acceptable levels, the practice of the prior art has been to first stabilize them by heating in oxygen for about 40 minutes at about 200° C. followed by further heating in nitrogen for about 30 minutes at about 400° C. This post-deposition treatment adds to the overall manufacturing cost, both because of the extra energy that must be expended (increased thermal budget) as well as because of the associated reduction of product throughput in the manufacturing line. 
     A routine search of the prior art was performed but no references that teach the exact processes and structures of the present invention were discovered. Several references of interest were, however, encountered along the way. For example, in U.S. Pat. No. 5,504,040, Moslehi shows a dielectric oxide process that uses a chuck chilling device while in U.S. Pat. No. 4,992,306 Hochberg teaches a deposition process for silicon oxide low k using organic reactants. Reference to BD was found on the web-site for Applied Materials in the form of a press release dated Feb. 28, 2000 describing this material. 
     SUMMARY OF THE INVENTION 
     It has been an object of the present invention to provide a process for stabilizing black diamond films at a temperature at or near room temperature. 
     Another object of the invention has been to provide a method for adjusting the dielectric constant of black diamond films as part of the stabilization process. 
     These objects have been achieved by immersing the as-deposited films in de-ionized water at a temperature of about 90° C. for about 20 minutes or in a hydrogen peroxide solution (typically at a concentration of 10%) for about 60 minutes. Since it has been observed that the dielectric constant of the stabilized film increases with both immersion time and/or peroxide concentration, this effect may be used as a means for adjusting the dielectric constant of a black diamond film. Standard analytical techniques confirm that these low temperature stabilized films have electrical properties that are at least as good as those of films stabilized using high temperature heat treatments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows the apparatus used to deposit the films. 
     FIG. 2 compares Infrared spectra for prior art films and those of the present invention. 
     FIG. 3 is a flow chart summarizing the various stabilizing options offered by the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As indicated above, BD films formed by the so-called cold method have (in the prior art) required a post-deposition heat treatment to stabilize the films. We now disclose several methods by which the films may be stabilized by treating them at or close to room temperature. Additionally, the dielectric constant of any given stabilized film has been found to vary, over a small range and in a predictable way, with the exact treatment given. We therefore also disclose application of this observation to a method for controlling dielectric constant in BD films. 
     First Embodiment 
     Referring now to FIG. 1, we show there a vacuum tight chamber  11  suitable for performing plasma enhanced chemical vapor deposition (PECVD). A gaseous mixture of helium, methyl silane, and nitrous oxide is admitted into the chamber through a gas manifold (not shown). The helium has a flow rate between about 800 and 2,000 SCCM, the methyl silane has a flow rate between about 50 and 90 SCCM, and the nitrous oxide has a flow rate between about 350 and 400 SCCM. RF power is then applied at RF electrode  14 . This stimulates the gas to form plasma  15  initiating chemical vapor deposition of a layer of black diamond on substrate  13  which can be heated by heater  12 . The substrate is at a temperature between about 15 and 25° C. while the layer is depositing. When the thickness of the film is between about 2,000 and 6,000 Angstroms, CVD is stopped and the film is removed from the chamber. 
     Post-deposition stabilization of the BD film now follows. As a key feature of the invention, said stabilization treatment is performed near room temperature. Specifically, the layer of black diamond is immersed in de-ionized water at a temperature between about 70 and 90° C. for between about 10 and 60 minutes, thereby stabilizing the layer. 
     Second Embodiment 
     Referring once more to FIG. 1, we show there a vacuum tight chamber  11  suitable for performing plasma enhanced chemical vapor deposition (PECVD). A gaseous mixture of helium, methyl silane, and nitrous oxide is admitted into the chamber through a gas manifold (not shown). The helium has a flow rate between about 800 and 2,000 SCCM, the methyl silane has a flow rate between about 50 and 90 SCCM, and the nitrous oxide has a flow rate between about 350 and 400 SCCM. RF power is then applied at RF electrode  14 . This stimulates the gas to form plasma  15  initiating chemical vapor deposition of a layer of black diamond on substrate  13 . The substrate is at a temperature between about 15 and 25° C. while the layer is depositing. When the thickness of the film is between about 2,000 and 6,000 Angstroms, CVD is stopped and the film is removed from the chamber. 
     Post-deposition stabilization of the BD film now follows. As a key feature of the invention, said stabilization treatment is performed at or near room temperature. Specifically, the layer of black diamond is immersed in an aqueous solution of between about 5 and 10 weight % hydrogen peroxide, for between about 10 and 60 minutes, thereby stabilizing the layer. 
     Third Embodiment 
     This, and the following two embodiments, are concerned with applying our observed variation of dielectric constant with stabilization specifics to a method for controlling the dielectric constant of BD films. As above, we show in FIG. 1 a vacuum tight chamber  11 , suitable for performing plasma enhanced chemical vapor deposition (PECVD). A gaseous mixture of helium, methyl silane, and nitrous oxide is admitted into the chamber through a gas manifold (not shown). The helium has a flow rate between about 800 and 2,000 SCCM, the methyl silane has a flow rate between about 50 and 90 SCCM, and the nitrous oxide has a flow rate between about 350 and 400 SCCM. RF power is then applied at RF electrode  14 . This stimulates the gas to form a plasma initiating chemical vapor deposition of a layer of black diamond on substrate  13 . The substrate is at a temperature between about 15 and 25° C. while the layer is depositing. When the thickness of the film is between about 2,000 and 6,000 Angstroms, CVD is stopped and the film is removed from the chamber. 
     Low temperature post-deposition stabilization of the BD film, as disclosed in the first two embodiments, now follows. As in the first embodiment, the layer of black diamond is immersed in de-ionized water at a temperature of about 90° C. By adjusting the length of time for which immersion takes place to be between 5 and 60 minutes, it is possible to select a dielectric constant value for the BD film that is between 3.0 and 4.0. The dielectric constant changes linearly with time while it is immersed so any intermediate value between the end points can readily be obtained. 
     Note that the invention is not limited to the above-specified immersion temperature of 90° C., it being chosen only as an example. It will be readily understood by one skilled in the art that any immersion temperature within the range given in the first embodiment may be used. If this is done, the time and k value end points would change accordingly with the dielectric constant changing more rapidly with time at the higher immersion temperatures. 
     Fourth Embodiment 
     Preparation of the BD film is as described for the third embodiment. As before, this is followed by a low temperature post-deposition stabilization treatment. Unlike the third embodiment, this is performed at room temperature with immersion being in an aqueous solution of 10 weight % hydrogen peroxide. By adjusting the length of time for which immersion takes place to be between 10 and 60 minutes, it is possible to select a dielectric constant value for the BD film that is between 3.0 and 4.0. The dielectric constant changes linearly with time while it is immersed in the hydrogen peroxide so any intermediate value between the end points can readily be obtained. 
     Fifth Embodiment 
     Preparation of the BD film is as described for the other embodiments. Also as before, this is followed by a low temperature post-deposition stabilization treatment. As in the fourth embodiment, this is performed at room temperature with immersion being in an aqueous solution of hydrogen peroxide. Immersion is for about 20 minutes. By adjusting the concentration of the hydrogen peroxide to be between 5 and 10 weight %, it is possible to select a dielectric constant value for the BD film that is between 3.0 and 3.9. The dielectric constant increases linearly with hydrogen peroxide concentration, so any intermediate value between the end points can readily be obtained. 
     Note that the invention is not limited to the above-specified immersion time, it being chosen only as an example. It will be readily understood by one skilled in the art that any immersion time within the range given in the first embodiment may be used. If this is done, the concentration and k value end points would change accordingly with the dielectric constant increase with concentration being larger for longer immersion times. 
     A flow chart summarizing all five embodiments is provided in FIG.  2 . 
     BD layers stabilized in accordance with any of the above-described embodiments have been found to be of equal or better quality than BD films stabilized using the methods of the prior art. Evidence for this can be seen in the FTIR (Fourier Transform Infrared) spectra shown in FIG. 3 which plot % absorbance as a function of the wave number in cm −1 . Curve  21  is for a BD film stabilized according to the methods of the prior art. Curve  22  is for a BD film stabilized, in accordance with the present invention, by dipping in de-ionized water while curve  23  is for a BD film stabilized, also in accordance with the present invention, by dipping in hydrogen peroxide. As can be seen, key peaks such as that due to Si—O stretching at 1100 wave numbers and that due to Si—H at 2190 are present at full amplitude, indicating high quality films in all cases. 
     Finally, in TABLE I below we summarize the results of several different ways of performing the stabilization step for the as-deposited BD film. 
     
       
         
               
               
               
               
               
             
           
               
                   
                 TABLE I 
               
               
                   
                   
               
               
                   
                 Stabilization step 
                 k value 
                 Flat-band voltage 
                 RI 
               
               
                   
                   
               
             
             
               
                   
                 Furnace O 2 , 200° C. 
                 3.10 
                 −41.0 V 
                 1.421 
               
               
                   
                 DI water 90° C. 20 
                 2.99 
                 −11.0 V 
                 1.440 
               
               
                   
                 min. 
               
               
                   
                 10% H 2 O 2  r.t. 10 min. 
                 3.03 
                  −1.4 V 
                 1.425 
               
               
                   
                 10% H 2 O 2  r.t. 60 min. 
                 3.46 
                  −0.5 V 
                 1.447 
               
               
                   
                 5% H 2 O 2  r.t. 30 min. 
                 3.21 
                  −2.1 V 
                 1.430 
               
               
                   
                   
               
             
          
         
       
     
     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.