Abstract:
In accordance with the objectives of the invention a new method is provided for creating air gaps in a layer of IMD. First and second layers of dielectric are successively deposited over a surface; the surface contains metal lines running in an Y-direction. Trenches are etched in the first and second layer of dielectric in an X and Y-direction respectively. The trenches are filled with a layer of nitride and polished. A thin layer of oxide is deposited over the surface of the second layer of dielectric. Openings are created through the thin layer of oxide that align with the points of intersect of the nitride in the trenches in the layers of dielectric. The nitride is removed from the trenches by a wet etch, thereby opening trenches in the layers of dielectric with both sets of trenches being interconnected. The openings in the thin layer of oxide are closed, leaving a network of trenches containing air in the two layers of dielectric.

Description:
BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The invention relates to the fabrication of integrated circuit devices and more particularly, to a method of creating a network of air gaps that is located in a layer of Inter Metal Dielectric of a semiconductor device. 
     (2) Description of the Prior Art 
     The increased device density that is part of semiconductor device fabrication leads to the placing of numerous devices on one monolithic substrate. The individual devices are electrically isolated from each other and are interconnected to other semiconductor devices of the total package to perform certain desired circuit functions. Decreased device dimensions result in a reduction of electrical energy that can be stored in a device. This reduction of device size further results in a reduction of the internal device capacitances such as parasitic capacitances at source and drain regions of a gate electrode of MOS devices. At these reduced devices capacitances, the relative parasitic capacitance of the device interconnect lines become more important in the design of the semiconductor device and in the ability of the device to perform the required functions at the required frequency and the required power level. The interconnect capacitance becomes an important fraction of the capacitance of each circuit, an effect that is especially pronounced as the operating frequency of the device is increased. With more device function packed in smaller device geometry, more signal lines are in closer physical proximity than ever before resulting in an increased impact of parasitic capacitances of the signal lines. This problem of interconnect parasitic capacitance must therefore be addressed whereby processes or solutions that result in reducing this capacitance are the objective. 
     Coupling capacitances between adjacent lines are dependent on a number of factors such as the length of the lines, the proximity of the lines and the distance over which the lines runs adjacent to lines in close proximity. Longer lines result in increased capacitive coupling, as does closer proximity between lines. Of equal importance in determining the capacitive coupling that occurs between adjacent lines is the material that is used to separate adjacent lines. Materials (that are used between adjacent lines) that have a high dielectric constant increase the capacitive coupling between the adjacent lines while materials with low dielectric constant decrease the capacitive coupling between adjacent lines. It is therefore important to select materials (dielectrics) that are used to separate interconnect lines that have as low a dielectric constant as possible. Conventional semiconductor fabrication typically uses silicon dioxide as a dielectric; this has a dielectric constant of about 3.9. Other examples are silicon with a dielectric constant of 11.7, silicon nitride with a dielectric constant of 7.0, Spin-On-Glass with a dielectric constant of 4.0. All of these examples are of dielectric materials that have a dielectric constant that is too high for application with sub-micron device feature sizes and that would impose serious limitations on device performance both in the frequency range in which the device can be used and in the power or switching level of the device. For high frequency operation, power consumption increases proportionally with frequency. For a given interconnect layout, both power consumption and crosstalk decrease, and performance increases, as the dielectric constant of the insulator decreases. 
     The use of many of the low dielectric constant materials, such as aerogel with a dielectric constant of 1.2, is not feasible due to the fact that equipment is not available to properly process the new dielectric material in various integrated circuits or because these materials require critical steps of processing which significantly increase the manufacturing cost. Also, the chemical or physical properties of many low dielectric constant materials are usually difficult to make compatible with or integrate into conventional integrated circuit processing. For instance, aerogel requires a very sensitive drying step that is not well suited for a high volume, low cost process of semiconductor device manufacturing. 
     The lowest possible and therefore the ideal dielectric constant is 1.0, this is the dielectric constant of a vacuum whereas air has a dielectric constant of slightly larger than 1.0. 
     The formation of air gaps between conducting lines of high speed Integrated Circuits (IC&#39;s) is typically a combination of the deposition of a metal layer, selective etching of the metal layer to form the desired line patterns, the deposition of a porous dielectric layer or a disposable liquid layer which is then selectively removed to form the desired air-gaps. 
     The presence of a high level of capacitive coupling between adjacent signal lines can result in an increase of capacitive crosstalk between adjacent conductor lines of a semiconductor circuit, that is the voltage on a first conductor line alters or affects the voltage on a second conductor line. This alteration in voltage can cause erroneous voltage levels in the Integrated Circuit making the IC increasingly prone to faulty operation. It becomes therefore imperative to reduce the resistance capacitance (RC) time constant and the crosstalk between adjacent conducting lines. 
     The invention addresses the creation of air gaps that are created in a layer of Inter Metal Dielectric (IMD). For Prior Art applications of a layer of IMD, the layer of IMD has to be thick in order to significantly reduce crosstalk and the RC delay of the adjacent metal lines. However, this thickness by its very nature limits the aspect ratio of the contact openings that can be established in the layer of IMD, which is contrary to the requirement of sub-micron device features. In addition, dielectric materials that have the required low-k value do not have the mechanical strength to support overlying device features if high aspect ratio openings are created in the dielectric. It is therefore required to provide a method that reduces the capacitive coupling through the layer of IMD while at the same time providing a mechanically stable support structure within the semiconductor device. 
     U.S. Pat. No. 5,828,121 (Lur et al.) shows an air gap between metal lines at different levels by etching the dielectric layers between the metal line levels. This is close to the invention. 
     U.S. Pat. No. 5,783,864 (Dawson et al.) shows air gaps and pillars between metal layers. This is close to the invention. 
     U.S. Pat. No. 5,561,085 (Gorowitz et al.) shows a method for forming air gap bridges. 
     U.S. Pat. No. 5,461,003 (Havemann et al.) and U.S. Pat. No. 5,527,137 (Jeng) show other air gap processes. 
     U.S. Pat. No. 5,908,318 (Wang et al.) shows an air gap in an ILD by etch out. 
     SUMMARY OF THE INVENTION 
     A principle objective of the invention is to provide a method to form air gaps between metal lines thereby reducing capacitive coupling through a layer of Inter Metal Dielectric. 
     Another objective of the invention is to provide a structure of air gap formation in a layer of IMD that is mechanically stable. 
     In accordance with the objectives of the invention a new method is provided for creating air gaps in a layer of IMD. First and second layers of dielectric are successively deposited over a surface; the surface contains metal lines running in an Y-direction. Trenches are etched in the first and second layer of dielectric in an X and Y-direction respectively. The trenches are filled with a layer of nitride and polished. A thin layer of oxide is deposited over the surface of the second layer of dielectric. Openings are created through the thin layer of oxide that align with the points of intersect of the nitride in the trenches in the layers of dielectric. The nitride is removed from the trenches by a wet etch, thereby opening trenches in the layers of dielectric with both sets of trenches being interconnected. The openings in the thin layer of oxide are closed, leaving a network of trenches containing air in the two layers of dielectric. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 a  shows an expanded view of a surface over which a first layer of dielectric has been deposited. FIG. 1 b  shows a corresponding cross section taken in the X-direction. 
     FIG. 2 a  shows an expanded view of a surface after trenches have been etched in the X-direction in the first layer of dielectric. FIG. 2 b  shows a corresponding cross section taken in the X-direction. 
     FIG. 3 a  shows an expanded view of a surface after the trenches in the first layer of dielectric have been filled with nitride. FIG. 3 b  shows a corresponding cross section taken in the X-direction. 
     FIG. 4 a  shows an expanded view of a surface after a second layer of dielectric has been deposited and after trenches have been etched in the Y-direction in the second layer of dielectric. FIG. 4 b  shows a corresponding cross section taken in the X-direction. 
     FIG. 5 a  shows an expanded view of a surface after the trenches in the second layer of dielectric have been filled with nitride. FIG. 5 b  shows a corresponding cross section taken in the X-direction. 
     FIG. 6 a  shows an expanded view of a surface after a first thin layer of oxide has been deposited over the surface of the second layer of dielectric and after openings have been etched in this first thin layer of oxide. FIG. 6 b  shows a corresponding cross section taken in the X-direction. 
     FIG. 7 a  shows an expanded view of a surface after the nitride has been removed from the trenches in the first and the second layer of dielectric and while a second thin layer of CVD oxide is being deposited to close off the openings in the first thin layer of oxide. FIG. 7 b  shows a corresponding cross section taken in the X-direction. 
     FIG. 8 a  shows an expanded view of a surface after metal lines have been deposited and patterned over the surface of the second thin layer of oxide. FIG. 8 b  shows a corresponding cross section taken in the X-direction. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now specifically to FIG. 1 a , there is shown an expanded three-dimensional view of a surface  10  over which a first layer  14  of dielectric has been deposited. Metal contacts  12  have been provided in the layer  10 . Layer  10  can typically be the surface of a dielectric layer. The surface  10  is however not limited to the surface of a substrate but can be any surface that is used in the creation of a semiconductor device. 
     Layer  14  of Inter Metal Dielectric can contain any suitable dielectric such as for instance silicon dioxide (“oxide”, doped or undoped) or silicon nitride (“nitride”), silicon oxynitride, fluoropolymer, parylene, polyimide, tetra-ethyl-ortho-silicate (TEOS) based oxides, boro-phosphate-silicate-glass (BPSG), phospho-silicate-glass (PSG), boro-silicate-glass (BSG), oxide-nitride-oxide (ONO), a low dielectric constant material, such as hydrogen silsesquioxane and HDP-FSG (high-density-plasma fluorine-doped silicate glass. 
     The most commonly used and therefore the preferred dielectrics are silicon dioxide (doped or undoped), silicon oxynitride, parylene or polyimide, spin-on-glass, plasma oxide or LPCVD oxide. The preferred dielectric material to be used for the invention is SiO 2 . 
     The deposition of the layer  14  of dielectric uses conventional deposition technology and can, for instance, be deposited using PECVD procedures at a temperature of between about 350 and 450 degrees C. to a thickness between about 5000 and 10,000 Angstrom using TEOS as a source. 
     FIG. 1 b  shows a cross section of surface  10 , the metal contact points  12  in layer  10  and the layer  14  of dielectric that has been deposited over the surface  10 . The cross section that is shown in FIG. 1 b  is a cross section in the X-direction of the surface  10 . 
     FIG. 2 a  shows an expanded view of the surface  10  after the first layer of oxide has been masked and etched to form trenches  15  in the first layer  14  of IMD. These trenches run in the X-direction of the surface  10 . Standard photolithography and RIE procedures, using CF 4 /CHF 3  as etchant gas, are used to create the trenches in the first layer  14  of dielectric. 
     FIG. 2 b  shows a cross section of surface  10 , the metal contact points  12  in layer  10  and the layer  14  of dielectric that has been deposited over the surface  10 . The cross section that is shown in FIG. 2 b  is a cross section in the X-direction of the surface  10  and does therefore not show the trenches  15  that have been created in the first layer of dielectric  14 . 
     FIG. 3 a  shows an expanded view of the surface  10  after the trenches in the first layer  14  of dielectric have been filled with nitride ( 16 ). The layer  16  of nitride can be deposited using PECVD procedures at a pressure between about 200 mTorr and 2000 mTorr, at a temperature between about 350 and 450 degrees C., using NH 3  and SiH 4  as source, to a thickness that is adequate to fill the trenches in the first layer of dielectric and to slightly overlay the exposed surface of the first layer of dielectric. The deposited nitride is polished using standard CMP to at least the surface of the first layer  14  of dielectric. 
     FIG. 3 b  shows a cross section of surface  10 , the metal contact points  12  in layer  10  and the layer  14  of dielectric that has been deposited over the surface  10 . The cross section that is shown in FIG. 3 b  is a cross section in the X-direction of the surface  10  and does therefore not show the nitride that has been deposited in the trenches in the first layer of dielectric  14 . 
     FIG. 4 a  shows an expanded view of the surface  10  after a second layer  18  of dielectric has been deposited and after trenches  19  have been etched in the Y-direction in the second layer  18  of dielectric. The methods and processing conditions for this step can be the same as previously have been highlighted for the first layer  14  of dielectric under FIG. 2 a  and do therefore not need to be further highlighted at this point. 
     FIG. 4 b  shows a cross section of surface  10 , the metal contact points  12  in layer  10  and the layer  14  of dielectric that has been deposited over the surface  10 . The cross section that is shown in FIG. 4 b  is a cross section in the X-direction of the surface  10  and shows the second layer  18  of dielectric that has been deposited over the first layer of dielectric  14  and the trenches  19  that have been created in the second layer of dielectric  18 . 
     FIG. 5 a  shows an expanded view of the surface  10  after the trenches in the second layer  18  of dielectric have been filled with nitride ( 20 ). The methods and processing conditions for this step can be the same as previously have been highlighted for the nitride deposition under FIG. 3 a  and do therefore not need to be further highlighted at this point. 
     FIG. 5 b  shows a cross section of surface  10 , the metal contact points  12  in layer  10  and the layer  14  of dielectric that has been deposited over the surface  10 . The cross section that is shown in FIG. 4 b  is a cross section in the X-direction of the surface  10  and shows the second layer  18  of dielectric that has been deposited over the first layer of dielectric  14  and the nitride  20  in the trenches ( 19 , FIGS. 4 a  and  4   b ) that have been created in the second layer of dielectric  18 . 
     FIG. 6 a  shows an expanded view of the surface  10  after a first thin layer  22  of oxide has been deposited over the surface of the second layer  18  of dielectric thereby including the nitride ( 20 ) that has been deposited in the trenches of the second layer of dielectric. Openings  24  have been etched in this thin layer  22  of oxide. The nitride that has been used to fill the trenches ( 15 , FIG. 2 a ) that have been created in the first layer  14  of dielectric intersect is in contact with the nitride that has been used to fill the trenches ( 19 , FIGS. 4 a  and  4   b ) that have been created in the second layer  18  of dielectric. The angle of intersection of the trenches in the first (etched in the X-direction of the surface  10 ) and second (etched in the Y-direction of the surface  10 ) layer of dielectric is 90 degrees. Openings  24  that have been created in the thin layer  22  of oxide align with the intersections of these trenches. 
     The deposition of the layer  22  of thin oxide uses conventional deposition technology and can, for instance, be deposited using PECVD procedures at a temperature of between about 350 and 450 degrees C. to a thickness between about 1000 and 4000 Angstrom using TEOS or SiH 4  as a source. 
     Standard photolithography and RIE procedures, using CF 4 /CHF 3  as etchant gas, can be used to create the openings.  24  in the thin layer  22  of oxide. 
     FIG. 6 b  shows a cross section of surface  10 , the metal contact points  12  in layer  10  and the layer  14  of dielectric that has been deposited over the surface  10 . The cross section that is shown in FIG. 6 b  is a cross section in the X-direction of the surface  10  and shows the first layer of dielectric  14 , the second layer  18  of dielectric that has been deposited over the first layer of dielectric  14  and the nitride  20  in the trenches that have been created in the second layer of dielectric  18 . Also shown is a cross section of the deposited thin layer  22  of oxide. 
     After the openings  24  have been created in the thin layer  22  of oxide, the nitride that has been deposited in two layers of dielectric is now accessible for removal. 
     FIG. 7 a  shows an expanded view of the surface  10  after the nitride has been removed from the trenches in the first layer ( 14 ) and the second layer ( 18 ) of dielectric and while a second thin layer  26  of PECVD oxide is being deposited to close off,the openings  24  in the first thin layer  22  of oxide. This thin layer  26  of oxide is required to close the openings  24  thereby making the structure a self contained and complete structure and thereby furthermore enclosing the trenches that have been created in the first and the second layer of dielectric. Methods and techniques that can be used to deposit this layer  26  of oxide have previously been detailed and do therefore not need to be discussed any further at this point. The thickness of the thin oxide layer  26  is to be optimized such that adequate surface tension can be created such that the oxide of this layer  26  does not penetrate into openings  25  or into the trenches that have been created by the removal of the layers of nitride. 
     FIG. 7 b  shows a cross section of surface  10 , the metal contact points  12  in layer  10  and the layer  14  of dielectric that has been deposited over the surface  10 . The cross section that is shown in FIG. 7 b  is a cross section in the X-direction of the surface  10  and shows the first layer of dielectric  14 , the second layer  18  of dielectric that has been deposited over the first layer of dielectric  14  and the trenches  19  that have been created in the second layer of dielectric  18 . Also shown is a cross section of the deposited thin layer  22  of oxide. Trenches  19  together with the trenches (not shown in FIG. 7 b ) that have been created in the first layer  14  of dielectric are interconnected and, after the removal of the nitride layers, filled with air thereby creating an excellent low-k layer of IMD. The nitride layers can be removed from the trenches created in the IMD by dipping the structure into hot phosphoric acid (H 3 PO 4 ) (standard wet nitride removal). 
     FIG. 8 a  shows an expanded view of the surface  10  after metal lines have been deposited and patterned over the surface of the second thin layer  26  of oxide. Before the metal pattern  28  is deposited that layer of oxide that overlays the created IMD is grown to a thickness of between 3000 and 6000 Angstrom, this to complete the desired thickness of the layer of IMD. The thickness of the IMD is, among others, determined by the aspect ratio of the openings that may have to be created in this layer of IMD. The overall thickness of the layer of IMD, that is the combined thickness of layers  14 ,  18 ,  22 ,  26  and  30 , may therefore have to be extended to between about 12,000 and 14,000 Angstrom. 
     Conventional methods can be used for the creation of the metal line pattern  28 ; a Ti/TiN/AlCu/TiN process is the preferred method to form this line pattern. Electrically conductive materials that can be used for the metal lines  28  include but are not limited to Al, Ti, Ta, W, Mo, Cu or a combination of these materials. 
     Although the invention has been described and illustrated with reference to specific illustrative embodiments thereof, it is not intended that the invention be limited to those illustrative embodiments. Those skilled in the art will recognize that variations and modifications can be made without departing from the spirit of the invention. It is therefore intended to include within the invention all such variations and modifications which fall within the scope of the appended claims and equivalents thereof.