Abstract:
A low dielectric constant material and a process for controllably reducing the dielectric constant of a layer of such material is provided and comprises the step of exposing the layer of dielectric material to a concentration of an oxygen plasma at a temperature and a pressure sufficient for the oxygen plasma to etch the layer of dielectric material to form voids in the layer of dielectric material. The process may also include the step of controlling the reduction of the dielectric constant by controlling the size and density of the voids. The size and density of the voids can be controlled by varying the pressure under which the reaction takes place, by varying the temperature at which the reaction takes place, by varying the concentration of the oxygen plasma used in the reaction or by varying a combination of these parameters. The process of the present invention is particularly useful in the fabrication of semiconductor devices.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. application Ser. No. 09/384,668, filed Aug. 27, 1999 now U.S. Pat. No. 6,140,249. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a process for forming a dielectric film having a low dielectric constant. In the process of the present invention, a dielectric film is etched with an oxygen plasma to form voids in the dielectric film. These voids serve to reduce the dielectric constant of the dielectric film. The present invention also relates to semiconductor devices which incorporate such reduced dielectric constant dielectric films. 
     In very large scale integrated (VLSI) circuit devices, several wiring layers connect the active and/or passive elements in a semiconductor chip. The interconnection structure comprises thin conductive lines which are separated by an insulation material in one layer or level and which are connected through vias or studs from contacts of the elements of the semiconductor chip or to a similar layer in another level of interconnections. This interconnection structure is similar to a transmission line because signals-being transmitted in these wiring layers experience propagation delays. Propagation delays are referred to as being RC delays because they depend upon the resistance (R) of the material of the conductive line or runner and the capacitance (C) between adjacent lines. With the present trend toward more and more levels of integration on these circuit devices, the spaces between adjacent lines in which to insert insulation material are being narrowed. These narrower spaces increase the capacitance between adjacent lines and place increased demands on the insulative properties of the insulation material formed between the conductive lines. 
     The capacitance (C) of a material is calculated by multiplying the dielectric constant (κ) of the material by the area (A) of the opposing faces of the conductive lines and dividing the product by the distance (D) between the conductive lines. A decrease in the distance between the conductive lines results in an increase in capacitance. Since signal delay of a signal transmitted on the conductive lines is controlled by the RC constant, an increase in capacitance degrades the performance of the integrated circuit. 
     Currently in the art, silicon compounds are used as the insulation material between the conductive lines. Silicon dioxide, for example, has a dielectric constant between 3.5 and 4.0. In comparison, a vacuum has a dielectric constant of 0 and provides the basis for measurement of the dielectric constant of materials. As another example, air and other insulating gases each have a dielectric constant of about 1 or slightly-less than 1. 
     Organic films, such as polyimides, silosesquioxanes and hydroxysilsequioxanes, have been proposed for use as insulation layers between adjacent conductive lines. While most organic dielectric films have a dielectric constant (κ) which is lower than that of silicon dioxide, organic dielectric films are more subject to contamination and shrinkage than silicon dioxide films. Further, organic dielectric films are hydroscopic and any absorbed moisture has the potential to corrode the metal lines. Additionally, organic films are also not as thermally and structurally stable as silicon dioxide films. Thus, it is not currently desirable to use organic based dielectric films in the formation of VLSI circuit devices. 
     One solution has been proposed by Avanzino et al. in U.S. Pat. No. 5,691,573. Avanzino et al. propose conventionally depositing an insulating layer on and between conductive lines and then masking the conductive lines and removing the insulation in the gap between the lines. A nonconformal silicon containing insulating layer is then deposited in the narrow gap and, due to the nonconformal step coverage, only a thin layer is coated on the vertical walls of the conductive lines on opposite sides of the narrow gaps. However, a thick layer forms on the top and shoulders of the conductive lines. As the insulation layer grows, the thick layer reduces the width of the gap at the top of the lines until it bridges across the narrow gap and creates a void between the lines. At least one-third of the gap is filled so that the effective dielectric constant of the combined insulating material and the void is at or below about 3. However, the process described by Avanzino et al. requires a considerable amount of time to complete because of the number of steps involved in the process. 
     U.S. Pat. No. 5,598,026 to Kapoor et al. proposes forming a porous layer of a dielectric material to serve as an insulation layer between two conductive layers of an integrated circuit. Kapoor et al. teach that the porous layer of dielectric material is formed by depositing a composite layer comprising a mixture of two or more materials. At least one of the materials which forms the mixture is extractable from the mixture after the layer is formed and at least one other material which forms the mixture will remain after the extraction as a porous matrix comprising a low dielectric constant insulation material. The low dielectric constant material will have a dielectric constant of less than 3.9, the dielectric constant of SiO 2 . The extractable material is removed from the layer to remain by means of a solvent which removes the extractable material and produces the low dielectric constant insulation material. To avoid contamination problems in the low dielectric constant insulation material, one or more layers of nonporous low dielectric constant insulation materials are preferably also formed above and/or below the porous matrix comprising the low dielectric constant insulation material. However, the Kapoor et al. process may cause contaminants to be introduced into the VLSI circuit device fabrication process through the use of a solvent to remove the extractable material. Further, the Kapoor et al. process can not be performed in situ. 
     Another solution is provided in U.S. Pat. No. 5,723,368 to Cho et al. Cho et al. describe a method for making a porous dielectric material for use in semiconductor devices. In the method taught by Cho et al., surface hydroxyl groups are removed from the surface of a porous substrate by baking the substrate in the range of 100° C. to 490° C. The porous substrate is formed from a gel which requires generally about one day of aging before the water content of the gel is removed to create the pores. The method may farther comprise baking the substrate in a reducing atmosphere. More preferably, the reaction is carried out at or below ambient pressure. However, this process requires several additional steps, some of which can not be performed in situ and others which require an extended amount of time to complete. 
     Thus, a need still exists in the art for a method for forming a dielectric layer for a semiconductor device in which the dielectric layer has a reduced dielectric constant. Desirably, this method would be simpler, faster, and involve fewer steps than currently used processes and would be less subject to contamination. A need also exists for semiconductor devices which incorporate a layer of dielectric material which has a reduced dielectric constant. 
     SUMMARY OF THE INVENTION 
     The present invention provides a solution to the current needs in the art by providing a method for forming a layer of a dielectric material between adjacent conductive lines such that the dielectric constant of the dielectric material is reduced. This reduction in the dielectric constant of the dielectric material results in a corresponding decrease in the capacitance of the dielectric material thus reducing RC delays. The method of the present invention is simpler and involves fewer steps than conventional processes. The method of the present invention is less subject to contamination because it does not employ organic films or use solvents to remove a component of the dielectric layer to cause the dielectric constant of the layer of dielectric material to be reduced. 
     One aspect of the present invention is directed to a process for controllably reducing the dielectric constant of a layer of dielectric material. The process includes the step of exposing the layer of dielectric material to a concentration of an oxygen plasma at a temperature and a pressure sufficient for the oxygen plasma to etch the layer of dielectric material to form voids in the layer of dielectric material. The process may also include the step of controlling the reduction of the dielectric constant by controlling the size and density of the voids. The size and density of the voids can be controlled by varying the pressure under which the reaction takes place, by varying the temperature at which the reaction takes place, by varying the concentration of the oxygen plasma used in the reaction or by varying a combination of these parameters. 
     Another aspect of the present invention is directed to a process for forming a semiconductor device. The process includes the steps of: providing a semiconductor substrate; forming a layer of a dielectric material on at least a portion of the semiconductor substrate, the dielectric material having a dielectric constant; and exposing the layer of dielectric material to a concentration of an oxygen plasma at a temperature and a pressure, sufficient for the oxygen plasma to etch the layer of dielectric material to form voids in the layer of dielectric material, the voids reducing the dielectric constant of the dielectric material. The process may also include the step of forming a layer of a second dielectric material on the layer of dielectric material. 
     Still another aspect of the present invention is directed to a process for the formation of a dielectric layer of a semiconductor device. This process comprises the steps of: depositing a layer of a first dielectric material between a pair of conductive lines of a semiconductor device; exposing the layer of first dielectric material to a concentration of an oxygen plasma at a temperature and a pressure sufficient for the oxygen plasma to etch the layer of first dielectric material to form voids in the layer of first dielectric material, the voids reducing the dielectric constant of the first dielectric material; and forming a layer of a second dielectric material on the layer of first dielectric material. 
     Yet another aspect of the present invention is directed to a process for the formation of a dielectric layer of a semiconductor device comprising the steps of: providing a semiconductor substrate; forming a first conductive line and a second conductive line on the semiconductor substrate, the first conductive line and the second conductive line defining a space therebetween; forming a layer of a first dielectric material between the first conductive line and the second conductive line in the space; and exposing the layer of dielectric material to a concentration of an oxygen plasma at a temperature and a pressure sufficient for the oxygen plasma to etch the layer of dielectric material to form voids in the layer of dielectric material, the voids reducing the dielectric constant of the dielectric material. 
     An additional aspect of the present invention is directed to a process for the formation of a dielectric layer of a semiconductor device. The process comprises the steps of: providing a semiconductor substrate; forming a first conductive line and a second conductive line on the semiconductor substrate, the first conductive line and the second conductive line defining a space therebetween, each of the first conductive line and the second conductive line having a surface; forming a layer of a first dielectric material between the first conductive line and the second conductive line in the space, the layer of first dielectric material being substantially coplanar with the surfaces of the first conductive line and the second conductive line; exposing the layer of first dielectric material to a concentration of an oxygen plasma at a temperature and a pressure sufficient for the oxygen plasma to etch the layer of first dielectric material to form voids in the layer of first dielectric material, the voids reducing the dielectric constant of the first dielectric material; and forming a layer of a second dielectric material on the layer of first dielectric material such that the layer of second dielectric material covers the surfaces of the first and second conductive lines. 
     Another aspect of the present invention is directed to a process for forming a semiconductor device. The process comprises the steps of: providing a semiconductor substrate having a layer of a conductive material formed thereon; forming a layer of a dielectric material on at least a portion of the layer of conductive material, the layer of dielectric material having a dielectric constant; and exposing the layer of dielectric material to a concentration of an oxygen plasma at a temperature and a pressure sufficient for the oxygen plasma to etch the layer of dielectric material to form voids in the layer of dielectric material, the voids reducing the dielectric constant of the dielectric material. This process may also include the step of forming a layer of a second dielectric material on the layer of dielectric material. 
     Yet another aspect of the present invention is directed to a process for forming an integrated circuit. The process comprises the steps of: providing an integrated circuit including a plurality of active devices and multilevel interconnections, the integrated circuit further including at least one interconnection level and at least one pair of conductive lines; forming a layer of a dielectric material between the conductive lines, the layer of dielectric material having a dielectric constant; exposing the layer of dielectric material to a concentration of an oxygen plasma at a temperature and a pressure sufficient for the oxygen plasma to etch the layer of dielectric material to form voids in the layer of dielectric material, the voids reducing the dielectric constant of the dielectric material; and forming at least one additional layer including an active device, the additional layer being formed over the layer of dielectric material. 
     Still yet another aspect of the present invention is directed to a process for forming a semiconductor device. The process for forming a semiconductor device comprises the steps of: providing an integrated circuit including a plurality of active devices and multilevel interconnections, the integrated circuit further including at least one interconnection level and at least one pair of conductive lines; forming a layer of a first dielectric material between the conductive lines, the layer of first dielectric material having a dielectric constant; exposing the layer of first dielectric material to a concentration of an oxygen plasma at a temperature and a pressure sufficient for the oxygen plasma to etch the layer of first dielectric material to form voids in the layer of first dielectric material, the voids reducing the dielectric constant of the first dielectric material; forming a layer of a second dielectric material on the layer of first dielectric material such that the layer of the-second dielectric material covers the conductive lines; and forming at least one additional layer including an active device, the additional layer being formed over the layer of second dielectric material. 
     An additional aspect of the present invention is directed to a process for forming a memory array which comprises a plurality of memory cells arranged in rows and columns, each of the plurality of memory cells comprising at least one field effect transistor. The process comprises the steps of: providing a semiconductor substrate; forming sources, drains and gates for each of the field effect transistors on the semiconductor substrate; forming a plurality of conductive lines to interconnect the memory cells, at least two of the plurality of conductive lines defining a space therebetween; forming a layer of a dielectric material in at least one of the spaces, the layer of dielectric material having a dielectric constant; and exposing the layer of dielectric material to a concentration of an oxygen plasma at a temperature and a pressure sufficient for the oxygen plasma to etch the layer of dielectric material to form voids in the layer of dielectric material, the voids reducing the dielectric constant of the dielectric material. This process may also include the step of forming a layer of a second dielectric material over the layer of dielectric material such that the layer of second dielectric material covers the conductive lines. 
     Still another aspect of the present invention is directed to a process for fabricating a wafer. The process comprises the steps of: providing a wafer including a semiconductor substrate; forming a repeating series of sources, drains and gates for a plurality of field effect transistors on each of a plurality of individual die over the semiconductor substrate; forming a plurality of conductive lines to interconnect the field effect transistors, at least two of the conductive lines defining a space therebetween; forming a layer of a dielectric material in the space, the dielectric material having a dielectric constant; and exposing the layer of dielectric material to a concentration of an oxygen plasma at a temperature and a pressure sufficient for the oxygen plasma to etch the layer of dielectric material to form voids in the layer of dielectric material, the voids reducing the dielectric constant of the dielectric material. 
     Another aspect of the present invention is directed to a semiconductor device. The semiconductor device includes a semiconductor substrate having a plurality of active devices disposed thereon. A plurality of multilevel interconnection layers are also disposed on the substrate, at least one interconnection layers has at least one pair of conductive lines. The conductive lines define a space therebetween. A layer of a first dielectric material is formed in the space. The dielectric material includes a plurality of voids having been formed therein by treating the layer of dielectric material with an oxygen plasma. The voids are provided to reduce the dielectric constant of the layer of dielectric material. This semiconductor device may also include a layer of a second dielectric material formed over the layer of first dielectric material such that the layer of second dielectric material covers the conductive lines. 
     Still yet another aspect of the present invention is directed to a memory array. The memory array includes a semiconductor substrate on which a plurality of memory cells are arranged in rows and columns. Each of the plurality of memory cells comprises at least one field effect transistor which comprises a source, a drain and a gate formed on the semiconductor substrate. A plurality of conductive lines interconnect the memory cells. A portion of the plurality of conductive lines define spaces therebetween. A layer of a dielectric material is formed in at least a portion of the spaces. The layer of dielectric material has a dielectric constant and the layer of dielectric material has a plurality of voids formed therein. The voids are formed by treating the layer of dielectric material with an oxygen plasma and the voids reduce the dielectric constant of the dielectric material. The memory array may also include at least one layer of a second dielectric material formed on at least a portion of the layer of dielectric material. 
     Yet another aspect of the present invention is directed to a semiconductor wafer. The wafer includes a semiconductor substrate. A repeating series of sources, drains and gates for a plurality of field effect transistors on each of a plurality of individual die are formed over the semiconductor substrate. A plurality of conductive lines interconnect the field effect transistors, at least two of the lines defining a space therebetween. A layer of a dielectric material is formed in the space. The dielectric material has a dielectric constant and has a plurality of voids formed therein. The voids are formed by exposing the layer of dielectric material to an oxygen plasma and the voids reduce the dielectric constant of the layer of dielectric material. The semiconductor wafer may also include a layer of a second dielectric material formed on at least a portion of the layer of dielectric material. 
     Objects and advantages of the present invention will be apparent from the following detailed description, the accompanying drawings and the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 presents a cross sectional view of a semiconductor device of the present invention; 
     FIGS. 2-4 present cross section views of the sequence of steps for making the semiconductor device shown in FIG. 1; 
     FIG. 5 shows a top view of the semiconductor device shown in FIG. 1 with the top layer of dielectric material being cut away; 
     FIG. 6 presents a cross sectional view of an alternate embodiment of a semiconductor device of the present invention; 
     FIG. 7 presents a schematic view of a memory array of the present invention; 
     FIG. 7A shows a schematic view of a memory cell of the memory array shown in FIG. 7; and 
     FIG. 8 presents a view of a wafer of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     For the purposes of the present application, the preposition “on” refers to formation in contact with the surface of a particular layer, while “over” refers to a formation above and/or in contact with a surface of a particular layer. 
     FIG. 1 presents a cross sectional view of a semiconductor device  10  of the present invention. The device  10  is formed on a semiconductor substrate  12 . A layer  14  of an insulation material  16  is formed on the substrate  12 . A first layer  18  of a conductive material  20  is formed in the layer  14  of insulation material  16  over the substrate  12 . A second layer  22  of a conductive material  24  is also formed in the layer  14  of insulation material  16  over the substrate  12 . A first via  26  and a second via  28  are formed in the layer  14  of insulation material  16  and provide access, respectively, to the first layer  18  of conductive material  20  and the second layer  22  of conductive material  24 . A first-conductive line  30  is formed in the first via  26  and a second conductive line  32  is formed in the second via  28 . The first conductive line  30  has a surface  34  and the second conductive line  32  has a surface  36 . The first conductive line  30  contacts the first layer  18  of conductive material  20  and the second conductive line  32  contacts the second layer  22  of conductive material  24 . The first conductive line  30  and the second conductive line  32  are provided to interconnect various active devices, such as capacitors or field effect transistors, and/or passive devices, such as other conductive lines, on the semiconductor substrate  12  with each other. Often the first conductive line  30  and the second conductive line  32  will interconnect devices through multiple layers on the semiconductor substrate  12 . 
     With continuing reference to FIG. 1, a layer  38  of a first dielectric material  40  is formed on the layer  14  of insulation material  16  at least between the first conductive line  30  and the second conductive line  32 . Desirably, the layer  38  of first dielectric material  40  is formed so that it is coplanar with the surfaces  34  and  36  of the first conductive line  30  and the second conductive line  32 , respectively. The layer  38  of first dielectric material  40  has a plurality of voids  42  formed therein. The voids  42  are not drawn to scale and shown out of scale solely for ease of illustrating the invention. The voids  42  are formed in the layer  38  of first dielectric material  40  to lower the dielectric constant of the first dielectric material  40 . By lowering the dielectric constant of the first dielectric material  40 , RC delays can be reduced, if not eliminated. The process by which the voids  42  are formed is discussed below. A layer  44  of a second dielectric material  46  is formed on the layer  38  of first dielectric-material  40  and the layer  44  of second dielectric material  46  covers the surfaces  34  and  36  of the conductive lines  30  and  32 , respectively. 
     The substrate  12  is formed from any material currently in use in the art which is used to form substrates for semiconductor devices such as semiconductor device  10 . Exemplary materials useful to form the substrate  12  include, but are not limited to, doped or undoped silicon, doped or undoped polycrystalline silicon, gallium arsenide, gallium phosphide, indium arsenide and indium phosphide. 
     The layer  14  is desirably an insulating material  16  which can be any material currently in use in the art as an insulator or a dielectric. Materials useful for insulating material  16  include, but are not limited to oxides, nitrides, and carbon-containing oxides, and especially silicon or metal oxides and nitrides. 
     The conductive material  20  and  24  can be any material which conducts an electric current. Exemplary materials useful for forming the conductive material  20  and  24  include, but are not limited to, polycrystalline silicon, aluminum, copper, aluminum-copper alloys, aluminum-titanium alloys and gold. Desirably, the layer  18  of conductive material  20  and the layer  22  of conductive material  24  are formed from using physical vapor deposition (PVD) or chemical vapor deposition (CVD) techniques which are known in the art. The layer  18  and the layer  22  can be formed from the same or different conductive materials. 
     The conductive lines  30  and  32  are formed from any material which conducts an electric current. Exemplary materials useful for forming the conductive lines  30  and  32  include, but are not limited to, polycrystalline silicon, aluminum, copper, aluminum-copper alloys, aluminum-titanium alloys and gold. Desirably, the conductive lines  30  and  32  are formed using PVD or CVD techniques. 
     The first dielectric material  40  is desirably silicon dioxide although other ceramic oxides such as aluminum oxide, zirconium oxide, and yttrium oxide can also serve as the first dielectric material  40 . 
     The second dielectric material  46  can be formed from the same material as the first dielectric material  40  or from a different material. Desirably, the first dielectric material  40  and the second dielectric material  46  are silicon dioxide. Other materials which can be used to form the second dielectric material  46  include, but are not limited to oxides, nitrides, and carbon-containing oxides. 
     The semiconductor device  10  of the present invention is formed by the following process which is illustrated in FIGS. 2-4. With reference to FIG. 2, the layer  14  of insulation material  16  is formed on at least a portion of the substrate  12  by any conventional deposition or forming process. Next, the layer  18  of conductive material  20  is formed in the layer  14  and is positioned over the substrate  12 . The layer  18  of conductive material  20  is formed so that it is surrounded by the layer  14  at least on three sides, or is surrounded on all sides if the layer  18  of conductive material  20  does not contact the substrate  12 . 
     For example, the layer  18  of conductive material  20  can be formed in the following manner. After the layer  14  has been formed on the substrate  12 , a layer of a photoresist material is formed on the layer  14 . The photoresist layer is then patterned and etched by conventional processes to provide an area in which the layer  18  of conductive material  20  can be formed. The layer  18  of conductive material  20  is then formed in the area by any conventional deposition process. After the layer  18  of conductive material  20  has been formed in the area, the layer  14  is formed over the layer  18  of conductive material  20  to surround the layer  18  of conductive material  20 . The layer  22  of conductive material  24  is formed in the same manner as and, desirably, simultaneously with the layer  18  of conductive material  20 . 
     With continuing reference to FIG. 2, the first via  26  and the second via  28  are formed by a conventional masking and etching technique. A layer of a photoresist material is deposited on the layer  14  of insulation material  16  and then the layer of photoresist material is patterned to provide for the via openings. The layer  14  of insulation material  16  is then etched to form the vias  26  and  28  and, after the etching has been completed, the layer of photoresist material is removed. 
     The first conductive line  30  and the second conductive line  32  are then formed in their respective vias  26  and  28 . The conductive lines  30  and  32  are formed by a blanket deposition of a layer of conductive material onto the surface of the layer  14  of insulation material  16 . The conductive lines  30  and  32  are formed by any conventional deposition process. The layer of conductive material is then patterned and etched to separate the conductive lines  30  and  32  from each other to prevent short circuits from forming in the circuitry of the semiconductor device  10 . One skilled in the art will appreciate that, although conductive lines  30  and  32  are shown as each having a T-shaped cross section, each of the conductive lines  30  and  32  may have any cross section, such an I-shaped cross section, which is currently used in the art. Further, the conductive lines  30  and  32  do not have to have the same cross section. Once conductive lines  30  and  32  have been etched, they define a space  48  therebetween. The space  48  was occupied by the conductive material prior to the etching of the layer of conductive material  20 . 
     As shown in FIG. 3, the layer  38  of first dielectric material  40  is formed between the first conductive line  30  and the second conductive line  32  in the space  48 . Desirably, the layer  38  of first dielectric material  40  is formed so that it is substantially coplanar with the surface  34  of the first conductive line  30  and the surface  36  of the second conductive line  32 . However, the layer  38  of first dielectric material  40  may be formed to cover the surfaces  34  and  36 . The layer  38  of first dielectric material is nonporous and  40  desirably has a thickness of up to about 1000 Å. 
     After the layer  38  of first dielectric material  40  has been formed on the layer  14  of insulation material  16 , the layer  38  of first dielectric material  40  is etched so that nanopores or voids  42  are formed in its surface. These voids  42  are shown in FIG. 4 in a side cross sectional view and in FIG. 5 in a top cross sectional view. The voids  42  are not drawn to scale in FIGS. 4 and 5 and are shown out of scale for ease of illustration. Desirably, the voids  42  will have an effective diameter of from about 2 nm to about 50 nm with the average effective diameter being from 10 nm to about 20 nm. 
     The voids  42  are formed by exposing the semiconductor substrate  12  to an oxygen plasma at a low partial pressure and a high temperature. To form the voids  42 , the substrate  12  is placed into a high density plasma system in which it is exposed to a high density oxygen plasma. A useful high density plasma system is an inductively coupled high density plasma system, such as, for example, a high density plasma system from Applied Materials of Santa Clara, Calif., U.S.A., under the product name “Ultima.” Other useful high density plasma systems include electron cyclotron resonance plasma systems or remote microwave plasma systems. 
     In the high density plasma system, an oxygen plasma desirably having a concentration density of greater than or equal to about 1×10 11  ions/cm 3  is employed to etch the voids  42  in the layer  38  of dielectric material  40 . The plasma is desirably generated by a power source of from about 5000 Watts to about 6000 Watts. In the high density plasma system, the oxygen plasma typically heats the substrate  12 . Desirably, the substrate  12  is heated to as high a temperature as possible without damaging or melting the conductive lines  30  and  32 . More desirably, the oxygen plasma heats the substrate  12  from about 400° C. to about 450° C. The pressure in the system is maintained as low as possible to drive the reaction toward void formation. Desirably, the pressure is maintained between about 1×10 −3  Torr and about 100×10 −3  Torr and is more desirably maintained at about 10×10 −3  Torr. This treatment results in layer  38  of dielectric material  40  having a porosity which is preferably greater than about 50%. 
     Under these particular conditions, when the layer  38  of dielectric material  40  is, for example, SiO 2 , the following reaction takes place: 
     
       
         Si+O−→SiO(g). 
       
     
     This reaction etches the surface of the silicon dioxide dielectric layer  40  and forms voids  42  in the surface of the layer  38  of dielectric material  40 . Desirably, a large number of nano-sized voids are formed through this process. As the voids increase in size, the structural integrity of the layer  38  of dielectric material  40  is threatened. By forming a large number of small voids instead of a small number of larger voids, the structural integrity of the layer  38  of dielectric material  40  is preserved. 
     In the process of the present invention, the rate of void formation is directly proportional to the concentration of the plasma. The rate of void formation is also directly proportional to the temperature at which the reaction is conducted. Desirably, the reaction is driven by the plasma concentration rather than the temperature at which the reaction takes place so that the temperature does not damage any of the conductive lines, such as conductive lines  30  and  32 . By using the plasma to drive the reaction, the temperature at which the reaction takes place can be maintained so that it will not damage the various layers of the semiconductor substrate  12 . However, the temperature can also be used to control the void formation in the reaction as long as the temperature is maintained at a point at which it would not damage other components, such as the conductive lines  30  and  32 , or other layers on the substrate  12 . That is, an increase in the reaction temperature will produce a corresponding increase in the rate at which the voids  42  are formed. 
     The reaction is conducted for a period of time sufficient to form voids of the desired size. One skilled in the art will appreciate that, as the reaction proceeds, the size of the voids will increase. As the size of the voids increases, the layer  38  of dielectric material  40  loses structural integrity, as described above. Thus, the reaction is desirably allowed to proceed for a period of time until the voids attain a desired size. Desirably, the reaction will proceed until the voids  42  attain the effective diameters set out above. 
     Further, by using an oxygen plasma to etch the layer  38  of dielectric material  40 , layers of substantial thickness maybe etched. The process of the present invention can etch voids in layers of up to about 1000 Å in thickness. This is superior to a process which uses molecular oxygen to etch dielectric materials because processes which employ molecular oxygen as an etching tool are only capable of etching layers having a thickness of less than 100 Å. 
     Once the voids  42  have been formed in the layer  38  of dielectric material  40 , a second layer, such as layer  44  of second dielectric material  46 , can be formed on the layer  38  of first dielectric material  40 , as shown in FIG.  1 . The layer  44  of second dielectric material  46  is formed so that it encapsulates the surfaces  34  and  36  of the conductive lines  30  and  32 , respectively, to insulate the conductive lines  30  and  32 . The layer  44  of second dielectric material  46  is formed by any conventional deposition process. The layer  44  of second dielectric material  46  covers the layer  38  of first dielectric material  40  and seals the voids  42  to ensure that no materials, which would increase the dielectric constant of the first dielectric material  40 , become deposited in the voids  42 . Because the voids  42  have such a small effective diameter, they do not become filled by the layer  44  of the second dielectric material  46 . 
     After the layer  44  of second dielectric material  46  has been formed on the layer  38  of first dielectric material  40 , the semiconductor device  10  can be further processed or prepared for shipment if its processing is completed. Additional processing steps include forming additional layers on the layer  44  of second dielectric material  46 . These additional layers can contain additional active devices such as capacitors, transistors, resistors, conductive lines, etc. The additional layers may also be passivation materials, insulative materials and/or contain nonactive devices. 
     FIG. 6 presents a cross sectional view of an alternate semiconductor device  60  of the present invention. The semiconductor device  60  is formed similarly to the semiconductor device  10 . The semiconductor device  60  includes a semiconductor substrate  62  which is formed from the same materials and in the same manner as described above in regard to substrate  12 . A layer  64  of a conductive material  66  is formed on the substrate  62  by conventional deposition and etching processes. The conductive material  66  is formed from the same materials used to form the conductive material  20  and  24 , which are described above. A layer  68  of a dielectric material  70  is formed over the layer  64  of conductive material  66 . The layer  68  of dielectric material  70  is formed by a conventional deposition process and is formed from the same material which is used to form the first dielectric material  40 , which is also described above. The layer  68  of dielectric material  70  has a plurality of voids  72  formed therein. The voids  72  are formed in the same manner as the voids  42 , which process is described above. That is, the voids  72  are formed by exposing the layer  68  of dielectric material  70  to a concentration of an oxygen plasma at a temperature and pressure sufficient for the oxygen plasma the etch the layer  68  of dielectric material  70  to form the voids  72  such that the voids  72  reduce the dielectric constant of the dielectric material  70 . Finally, an optional layer  74  of a second dielectric material  76  may be formed on the layer  68  of dielectric material  70 . The second dielectric material  76  is formed from the same material which is used form the second dielectric material  46 , which is also described above. 
     The process of the present invention may be used to form a typical memory array, such as, for example, a static random access memory (SRAM) array or a dynamic random access memory (DRAM) array  100 , which is shown in FIG.  7 . The DRAM array  100  comprises a plurality of memory cells  102  arranged in rows and columns. As shown in FIG. 7A, each of the memory cells  102  includes at least one field effect transistor  104  and one capacitor  106 . Each field effect transistor  104  is coupled to a capacitor  106 . The gate of the field effect transistor  104  is coupled to a word line  108  via an interconnect structure, such as conductive lines  30  and  32 , described above. The process described above in connection with FIGS. 1-5 can be used to form dielectric layers, such as layer  38  of dielectric material  40 , during the fabrication of the memory array  100 . It should be apparent that other devices such as other transistors, bipolar transistors, resistors, other capacitors and the like, may be interconnected by conductive lines  30  and  32 . 
     The process of the-present invention may also used in the fabrication of a wafer W, as is shown in FIG.  8 . The wafer W includes a plurality of individual die  150  formed on a semiconductor substrate, such as substrate  12 . Wafer masks (not shown) are used to apply a desired circuit structure on each of the individual die  150 . The desired circuit structure may comprise any of the above described structures, e.g., the DRAM array  100  or an SRAM array. The wafer W is processed using standard wafer fabrication techniques. Again, the process described above in connection with FIGS. 1-5 can be used form layers of dielectric material having reduced dielectric constants on the wafer W. 
     One skilled in the art will appreciate that the method of the present invention can be carried out as a stand-alone process, clustered as part of the semiconductor manufacturing process or as an in situ treatment. 
     One skilled in the art will also appreciate that, although the process of the present invention has been primarily described herein as forming layers of dielectric material between vertically positioned conductive lines, the layers of dielectric material formed by the process of the present invention can be used to form insulative layers between horizontally positioned conductive materials, such as conductive lines  30  and  32 . 
     Having described the invention in detail and by reference to desired embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention which is defined in the appended claims.