Abstract:
An electronic device and method of construction are disclosed that provide for a dielectric layer ( 12 ) having a low dielectric constant. A conductive sheath layer ( 18 ) is disposed adjacent to the dielectric layer ( 12 ). The conductive sheath layer ( 18 ) is operable to electrically divert etchant particles used in a plasma etch process away from the dielectric layer ( 12 ). In another embodiment of the disclosed invention, a method is provided which comprises covering an inner layer ( 40 ) with a layer of dielectric material ( 42 ). The method also comprises depositing a conductive sheath layer ( 48 ) outwardly from the layer of dielectric material ( 42 ). A photoresist layer ( 50 ) is then deposited outwardly from the conductive sheath layer ( 48 ). The photoresist layer ( 50 ) is then patterned resulting in a patterned mask composed of portions of the photoresist layer ( 50 ) disposed outwardly from the conductive sheath layer ( 48 ). Portions of the conductive sheath layer ( 48 ) not covered by the patterned mask are etched using a plasma etch process selective to the conductive sheath layer ( 48 ) relative to the photoresist layer ( 50 ). Portions of the dielectric layer ( 42 ) not covered by the patterned mask are also etched using a plasma etch process selective to the dielectric layer ( 42 ) relative to the photoresist layer ( 50 ). The photoresist layer ( 50 ) is then removed from the conductive sheath layer ( 48 ), the conductive sheath layer ( 48 ) providing mechanical and electrical shielding for the dielectric layer ( 42 ).

Description:
CROSS REFERENCE TO PRIOR APPLICATIONS 
     This application is a division of Ser. No. 09/208,082, filed Dec. 9, 1998 now abandoned which claims priority under 35 U.S.C. 120 based upon provisional application Ser. No. 60/069,002, filed Dec. 10, 1997. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     This invention relates in general to the field of electronic devices and more particularly to a system and method for avoiding plasma etch damage. 
     BACKGROUND OF THE INVENTION 
     As device geometries of integrated electronic systems become smaller, the parasitic capacitances of metal interconnects become a more stringent limiting factor than the ability to create smaller geometry features within active devices. A conductive interconnect in an integrated device exhibits a time delay which is related to the resistance and capacitance of the interconnect. One approach to reducing this time delay is to use higher conductive connectors composed of copper instead of aluminum. A separate but non-exclusive approach is to reduce the dielectric constant of the insulative materials adjacent to the interconnects by using materials other than the traditional silicon dioxide. This class of materials is generally referred to as low-K (the preferred term in this invention) or Low-E materials and typically exhibits dielectric constants less than 4.2. 
     The use of low-K materials presents problems during the processing of the conductive material used to make the interconnects. The conductive material is typically patterned and etched using high energy plasma etch processes. In other process schemes, the low-K material is patterned through the application and patterning of photoresist. The low-K material is etched through the photoresist mask, and then the photoresist removed with a high energy plasma etch process. The low-K materials are susceptible to damage from a plasma etch because they are softer, less chemically stable or more porous, or any combination of these factors. The plasma damage can manifest itself in higher leakage currents, lower breakdown voltages, and changes in the dielectric constant associated with the low-K dielectric material. 
     SUMMARY OF THE INVENTION 
     Accordingly, a need has arisen for a method of processing which addresses the damage to the low-K dielectric materials during the etching of either the conductive layers or photoresist in immediate contact with the low-K material. In accordance with the teachings of the present invention, a method and device architecture are provided that substantially eliminate or reduce problems associated with prior systems and methods. 
     According to one embodiment of the present invention, an electronic device is provided that comprises a dielectric layer having a low dielectric constant. A conductive sheath layer is disposed adjacent to the dielectric layer. The conductive sheath layer is operable to electrically divert etchant particles used in a plasma etch process away from the dielectric layer. 
     In another embodiment of the disclosed invention, a method is provided which comprises covering an inner layer with a layer of dielectric material. The method also comprises depositing a conductive sheath layer outwardly from the layer of dielectric material. A photoresist layer is then deposited outwardly from the conductive sheath layer. The photoresist layer is then patterned resulting in a patterned mask composed of portions of the photoresist layer disposed outwardly from the conductive sheath layer. Portions of the conductive sheath layer not covered by the patterned mask are etched using a plasma etch process selective to the conductive sheath layer relative to the photoresist layer. Portions of the dielectric layer not covered by the patterned mask are also etched using a plasma etch process selective to the dielectric layer relative to the photoresist layer. The photoresist layer is then removed from the conductive sheath layer, the conductive sheath layer providing mechanical and electrical shielding for the dielectric layer. 
     The disclosed invention provides several technical advantages. For example, the invention provides a process whereby damage to underlying dielectric material is substantially eliminated during the etch of overlying conductive interconnect material. A further advantage of the disclosed invention is that of preventing damage to underlying dielectric material during a resist ash process whereby the photoresist material used to pattern the dielectric layer is removed. Additionally, the invention prevents changes in the dielectric constant of the dielectric material, reduction in the breakdown voltage of the dielectric material, and higher leakage currents across the material to form parasitic electronic devices between conductive vias, for example. Other technical advantages will be readily apparent to one skilled in the art from the following figures, descriptions, and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the present invention and the advantages thereof may be acquired by referring to the accompanying figures in which like reference numbers indicate like features and wherein: 
     FIGS. 1A through 1E are greatly enlarged cross-sectional diagrams illustrating the method of constructing an electronic device according to the teachings of the present invention; 
     FIGS. 2A through 2E are greatly enlarged cross-sectional diagrams illustrating another embodiment of the present invention describing a method of constructing an electronic device; and 
     FIG. 3 illustrates the application of a high energy plasma etching process performed on a semiconductor device. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention and its advantages are best understood by referring now in more detail to FIGS. 1A-1E,  2 A- 2 E and  3  of the drawings, like numerals being used for like and corresponding parts of the various drawings. 
     FIG. 1A illustrates a substrate  10 . Substrate  10  may comprise single crystalline semiconductor material such as silicon or gallium arsenide. The processing steps involved in the methods of the present invention aid in the creation of conductive interconnects between various active portions of an integrated electronic device. As such, substrate  10  may also represent a layer of active devices that is formed outwardly from an underlying substrate. 
     Substrate  10  is covered by a layer of low-K dielectric material  12 . Dielectric layer  12  may be on the order of 5,000 angstroms in thickness and may comprise a variety of dielectric materials that display a low dielectric constgant K. For example, layer  12  may comprise xerogel, aerogel, parylenes such as AF4, benzo cyclobutenes, polyarylene ethers, porous oxides such as silicon dioxide, or silsequioxanes. It should be understood that dielectric layer  12  may comprise a homogenous layer of a single material. However, dielectric layer  12  may also comprise a heterogenous stack of various materials. Layer  12  may, for example, include barrier layers, composed of a material such as diamond-like carbon, or adhesion layers to allow the dielectric layer  12  to adhere better to materials formed inwardly from layer  12 , such as substrate  10 . 
     Referring to FIG. 1B, conventional photo-lithographic processes are used to pattern and etch openings in dielectric layer  12  as shown in regions  14  and  16 . Ultimately, conductive vias will be formed within regions  14  and  16 . The conductive vias within these regions will be surrounded by the dielectric material within dielectric layer  12 . As such, the parasitic capacitances of the vias formed within regions  14  and  16  can be reduced by using the materials mentioned previously that exhibit low dielectric constants within layer  12 . 
     Referring again to FIG. 1B, a conductive sheath layer  18  is conformally deposited outwardly from the exposed surfaces of layer  10  and layer  12 . Conductive sheath layer  18  may be on the order of 250 angstroms in thickness. Conductive sheath layer  18  may comprise, for example, titanium nitride, cobalt, titanium, graphite, platinum, ruthenium, strontium, tungsten, tungsten silicide, titanium silicide, or titanium tungsten. Ultimately, the conductive vias formed in regions  14  and  16  will comprise a suitable conductor such as tungsten or aluminum. Layer  18  must comprise a material which can withstand, on a selective basis, an etchant of the material used to construct the conductive vias within regions  14  and  16 . For example, if these conductive materials are constructed of aluminum, conductive sheath layer  18  must comprise a material which can withstand an etchant which is selective to aluminum relative to the material within conductive sheath layer  18 . Tungsten, titanium tungsten, ruthenium and platinum are examples of species which are resistant to etch processes selective to aluminum. Similarly, titanium nitride is resistant to etch processes selective to tungsten. 
     Referring to FIG. 1C, a conductive interconnect layer  20  is deposited outwardly from layer  18 . Conductive interconnect layer  20  may comprise, for example, tungsten as mentioned previously. Conductive interconnect layer  20  is deposited in such a way that it fills regions  14  and  16 , discussed previously. 
     Referring to FIG. 1D, a high energy plasma etch process is used to etch conductive interconnect layer  20  until only conductive via  22  and conductive via  24  remain, within regions  14  and  16 , respectively. As shown in FIG. 3, a chuck  26  may be used to hold a wafer  28  containing a semiconductor device  30  during the plasma etch process. The high energy plasma etch process may take place in an evacuated reactive chamber and may involve utilizing a source  32  to generate a glow discharge that produces chemically reactive species of etchant particles from a relatively inert gaseous mixture. 
     In a process utilized to etch tungsten, for example, a sulphur hexafluorine (SF6) and argon solution may comprise the relatively inert gaseous mixture. In a process utilized to etch aluminum, a chlorine, nitrogen, and BCl 3  solution may comprise the relatively inert gaseous mixture. Other gaseous mixtures may be utilized during plasma etching which will react chemically with the material to be etched. In a process for the plasma etching of tungsten, the etchant particles created from the glow discharge may include SF 5 + and F−, for example. In a process for the plasma etching of aluminum, the etchant particles created from such a discharge may include BCl 2 + and Cl−. Variations of chemical flow rates, temperatures, and chamber pressures used during such plasma processes are generally known in the art and are contemplated by the present invention. 
     Referring again to FIG.  1 D and FIG. 3, during the plasma etch process highly charged etchant particles will bombard the outer surface of conductive sheath layer  18  after conductive interconnect layer  20  is etched away. Because conductive sheath layer  18  is present, a large number of particles will not be allowed to impact the dielectric layer  12 . Conductive sheath layer  18  is electrically connected, at some part of the integrated device, to the same potential as substrate and chuck  26 . Chuck  26  is typically used as a terminal, to carry the potential that terminates the electric field lines within the plasma chamber. As such, conductive sheath layer  18  provides a conduction path for the highly charged etchant particles. In this manner, these particles are prevented from penetrating the dielectric layer  12 . If the particles were allowed to penetrate dielectric layer  12 , damage may result, including changes in the dielectric constant of the material, reduction of the breakdown voltage of the material, and higher leakage currents across the material. The etch process used to form conductive interconnect vias  22  and  24  is highly selective to aluminum or other conductive material used to form conductive interconnect layer  20  relative to the material used to form conductive sheath layer  18 . As such, the portions of conductive sheath layer  18  disposed outwardly from dielectric layer  12  will remain following this etch process. 
     In FIG. 1E, a second etch process is used to remove the portions of conductive sheath layer  18  disposed outwardly from dielectric layer  12 . This etch process results in a planar surface with conductive vias  22  and  24  ready to be interconnected with conductive interconnects or active devices to be formed outwardly from the outer surface of dielectric layer  12 . It should be understood that this second etch process may be carried out in the same evacuative chamber as the etch of conductive interconnect layer  20 , with a suitable change in gas chemistry to allow etching of the conductive sheath layer  18 . 
     A second embodiment of the disclosed invention is illustrated by the accompanying FIGS. 2A through 2E. These figures describe the disclosed invention used during the etching (commonly referred to as “ashing”) of a patterned photoresist disposed above a low-K dielectric layer. A plasma process is particularly damaging during the etching of photoresist because, like the exposed low-K materials patterned by the resist, the photoresist is itself a dielectric. Finding the particular etchant particles for use in a plasma process which are selective to the photoresist as compared to the underlying dielectric layer becomes very difficult. 
     FIG. 2A illustrates an underlying layer  40 , such as a substrate, covered by a layer of low-K dielectric material  42 . The methods of the present invention aid in the creation of conductive interconnects between various active portions of an integrated electronic device. As such, underlying layer  40  may also represent a layer of active devices that is formed outwardly from an underlying substrate. Dielectric layer  42  may be on the order of 5,000 angstroms in thickness. 
     Dielectric layer  42  may comprise a variety of electric materials that display a low dielectric constant K. For example, dielectric layer  42  may comprise xerogel, aerogel, parylenes such as AF4, benzo cyclobutenes, polyarylene ethers, porous oxides such as silicon dioxide, or silsequioxanes. It should be understood that dielectric layer  42  may comprise a homogeneous layer of a single material. However, dielectric layer  42  may also comprise a heterogeneous stack of various materials. Dielectric layer  42  may, for example, include barrier layers, such as diamond-like carbon, or adhesion layers, such as titanium nitride, to allow the dielectric layer  42  to adhere better to materials formed inwardly from dielectric layer  42 , such as substrate  40 . 
     Referring again to FIG. 2A, a conductive sheath layer  48  is deposited outwardly from the exposed surfaces of layer  40  and layer  42 . Conductive sheath layer  48  may be on the order of 250 angstroms in thickness. Conductive sheath layer  48  may comprise, for example, titanium nitride, cobalt, titanium, graphite, platinum, ruthenium, strontium, tungsten, tungsten silicide, titanium silicide, or titanium tungsten. 
     Referring to FIG. 2B, a photoresist layer  50  is deposited outwardly from conductive sheath layer  48 . Photoresist layer  50  is then subjected to the standard patterned exposure and clean, resulting in a patterned mask of photoresist directly above conductive sheath layer  48 , as shown in FIG.  2 C. The shape of this photoresist layer  50  will determine the pattern which will be etched into dielectric layer  42 . 
     Referring to FIG. 2D, a high energy plasma etch process is used to etch the conductive sheath layer  48  as described in the previous embodiment (and shown in FIG. 3) from a relatively gaseous mixture. In the plasma etch process, there is generally a high selectivity of etching conductive sheath layer  48  relative to photoresist layer  50  for conventional metal etch chemistries. The plasma etch of conductive sheath layer  48  uses techniques described previously. Only those areas of conductive sheath layer  48  not covered by photoresist layer  50  will be etched during this step, thereby beginning the formation of regions  54  and  56 . 
     Referring to FIG. 2E, a high energy plasma etch process is used to etch dielectric layer  42  completing the formation of regions  54  and  56 . The chemistry for this etch will differ from the chemistry of the etchant used to etch conductive sheath layer  48 , as dielectric materials are etched most efficiently by different etchants than those etchants used to etch conductive materials. During this etch step, there will unavoidably be plasma-induced damage to the unetched portions of dielectric layer  42 . However, this damage is partially mitigated by the presence of a conductive path for electrically charged etchant species along conductive sheath layer  48 . Conductive sheath layer  48 , as described in the first embodiment, is electrically connected to the same potential as a chuck holding the wafer during plasma (as shown in FIG.  3 ). An additional benefit of the invention is that if the etch selectivity between low-K dielectric layer  42  and photoresist layer  50  is poor, the already-patterned electrical sheath layer  48  will serve as an etch mask and maintain the integrity of the pattern. This reduces the demand for etch selectivity between photoresist layer  50  and dielectric layer  42 . 
     In FIG. 2E, the photoresist layer  50  is removed from conductive sheath  48  in a third plasma etch process, generally known as resist ash. The atmosphere for this plasma etch process is generally an evacuated chamber into which oxygen or ozone is introduced and excited into a plasma. Traditionally, this step has proven detrimental to dielectric layers with a low dielectric constant, as it allows the implantation of electrically charged and/or chemically reactive species into the dielectric layer, thus greatly increasing the ability of electrical current to travel from one conductive trench or via to adjacent trenches or vias. However, the present invention, i.e., the placement of a conductive sheath layer  48  between photoresist layer  50  and underlying low-K dielectric layer  42 , provides mechanical shielding for dielectric layer  42  against implantation of charged and/or chemically reactive species from the plasma, and also provides a conductive path to chuck  26  (see FIG. 3) for electrically charged particles to migrate away from dielectric layer  42 . Thus, conductive sheath layer  48  prevents damage to dielectric layer  42 . Regions  54  and  56  can thereafter be filled with conductive material as shown in FIGS. 1C and 1D and described above. 
     Conductive sheath layer  48  must comprise a material which can withstand on a selective basis an etch of the material used to construct the conductive vias or lines within regions  54  and  56 . For example, if regions  54  and  56  contain conductive material constructed of tungsten, conductive sheath layer  48  must comprise a material which can withstand an etch which is selective to tungsten relative to the material within conductive sheath layer  48 . Titanium nitride is an example of a species which is resistant to etchant processes selective to tungsten. Similarly, tungsten, titanium tungsten, ruthenium and platinum are resistant to etchant processes selective to aluminum. 
     Although the device of the present invention has been described with reference to the formation of conductive vias, it should be understood that the use of a conductive sheath layer is equally applicable to the formation of horizontal layers or structures. For example, the formation of an interconnect traversing the outer surface of a dielectric layer would also involve the plasma based etching of the conductive material used to form that interconnect. During this process, the dielectric layer would similarly be exposed to highly charged particles during the etch process. As such, a conductive sheath layer placed between the outer surface of the dielectric layer and the layer used to form such a traversing interconnect would similarly guard the dielectric layer against unwanted implantation of highly charged particles within the body of the dielectric layer. 
     Although the present invention has been described in detail it should be understood that various changes, alterations, substitutions, and modifications to the descriptions contained herein may be made without departing from the scope and spirit of the present invention which is solely defined by the appended claims.