Abstract:
The document explains, inter alia, a method in which a titanium nitride layer is removed by wet chemical means ( 106 ). Following removal of the titanium nitride, further metalization strata are produced ( 114 ). The result is an integrated circuit arrangement having connections which have a low electrical resistance. The circuit arrangement is particularly suitable for the purpose of switching high powers.

Description:
PRIORITY 
   This application is a continuation of International Patent Application No. PCT/DE2003/004045, filed Dec. 9, 2003, which claims the benefit of priority to German Patent Application DE 10257681.5, filed on Dec. 10, 2002, both of which are incorporated by reference herein. 

   TECHNICAL FIELD 
   The invention relates to a method in which the following steps are carried out: 
   an initial metalization stratum covered by an intermediate layer is produced, 
   an insulating layer is applied after the initial metalization stratum has been produced, and 
   the insulating layer is patterned to form at least one cutout using a dry etching process. 
   BACKGROUND 
   A metalization stratum or a metalization plane contains a multiplicity of interconnects, for example made of aluminum or copper. Various metalization strata are electrically insulated from one another by a respective insulating layer. The insulating layer is penetrated by “vias” containing connecting sections. In the connecting sections, current flows between various metalization strata during operation of the integrated circuit arrangement. 
   The intermediate layer is electrically conductive and serves, by way of example, as: 
   a diffusion barrier, 
   to increase the mechanical adhesion, and/or 
   as an antireflection layer. 
   Suitable materials for the intermediate layer are, by way of example, titanium nitride or tantalum nitride, or double or multiple layers with these materials, e.g. a double layer made of a titanium layer and of a titanium nitride layer. 
   Typical dry etching processes are: 
   physical dry etching, 
   chemical dry etching, or 
   chemicophysical dry etching. 
   If the insulating layer is made of silicon dioxide, for example, then dry etching can be carried out using fluorine-containing chemicals, for example. 
   SUMMARY 
   It is an object of the invention to specify a simple method for fabricating an integrated circuit arrangement, which method can be used, in particular, to produce electrical connections having a low electrical resistance. The intention is also to specify an integrated circuit arrangement. 
   The object relating to the method is achieved by a method having the method steps specified in patent claim  1 . Developments are specified in subclaims. 
   The invention is based firstly on the consideration that the intermediate layer often has a higher electrical resistance than a metal layer in the metalization stratum. For this reason, the intermediate layer should be removed particularly at points where there is a large flow of current, i.e. particularly at the via bottoms. 
   In addition to the method steps cited at the outset, the inventive method therefore involves the following methods steps being carried out: 
   the cutout is extended using a wet chemical etching process or using a dry etching process, with material from the intermediate-layer being removed in the region of the cutout, and 
   at least one further metalization stratum is produced after the cutout has been extended, with the cutout being filled with a metal or with a metal alloy. 
   A dry etching process for extending the cutout makes it possible to etch the cutout in one etching operation. By contrast, a wet chemical etching process can be carried out more easily, particularly in a shorter time. First, a wet chemical etching process does not involve the production of any polymers making the etching operation more difficult, as is the case with many dry etching processes. 
   Removing the intermediate layer, which, although electrically conductive, has a higher electrical resistance than the material in the metalization strata, produces a connection between the metalization strata which has a lower electrical resistance than would be the case if the intermediate layer were retained. Particularly in the case of vias with a maximum diameter smaller than 1 μm (micron) and/or in the case of vias through which relatively high switching currents flow, the effect of reducing the electrical resistance is a significant reduction in the power loss. This in turn results in low demands on cooling the integrated circuit arrangement. 
   In one development, the intermediate layer is made of titanium nitride or the intermediate layer contains titanium nitride. Titanium nitride can be deposited in a simple manner, for example using a CVD (Chemical Vapor Deposition) method or by sputtering. 
   In one development with an intermediate layer which contains titanium nitride, the material in the intermediate layer is removed in the region of the cutout using a hot alkali metal hydroxide. 
   Thus, titanium nitride is etched in an aqueous, alkaline peroxide-containing system, for example. In a first step, titanium nitride is oxidized in an alkaline solution to form titanium oxide ions:
 
2TiN+4OH − →TiO 2+ +N 2 +H 2 O.
 
   The titanium cations TiO 2+  react further to form titanium dioxide:
 
TiO 2+ +2OH − →TiO 2 +H 2 O.
 
   Titanium dioxide dissolves by reacting further with hydrogen peroxide to form titanium peroxide:
 
TiO 2 +H 2 O 2 →[Ti(O 2 )] 2+ ═H 2 O.
 
   By way of example, the alkali metal hydroxide has a temperature of 60° C. (degrees Celsius). Alternatively, a layer containing titanium nitride can also be removed using a base or using a solution which has a basic action. In one refinement, aqueous ammonia is used as the solution with a basic action. In a subsequent refinement, the aqueous ammonia is between 20% strength and 35% strength, preferably 28% strength. 28% strength aqueous ammonia is commercially available and is also frequently used for fabricating integrated circuits. In one refinement, the etching solution is aqueous and alkaline and contains an oxidizing agent, e.g. hydrogen peroxide H 2 O 2 . Choline is also suitable as an alkaline component. Aqueous solutions can be disposed of easily. 
   In a subsequent development, the initial metalization stratum contains a metal layer which is made of aluminum or of an aluminum alloy. By way of example, the initial metalization stratum is made of an aluminum/silicon alloy, of an aluminum/silicon/copper alloy or of an aluminum/copper alloy. The additional components for the aluminum are usually below 5 wt % (percent by weight). Although aluminum has a higher electrical resistance than copper, it can be patterned more easily. In particular, no contamination problems arise with aluminum. 
   In a subsequent development with a metal layer containing aluminum in the initial metalization stratum, a liquid used for wet chemical etching has an oxidizing agent admixed with it which oxidizes the metal layer. In one refinement, this oxidizing agent is hydrogen peroxide H 2 O 2  or an H 2 O 2  solution. The oxidation layer on the metal layer prevents the metal layer from being attacked by the means for removing the intermediate layer and dissolves oxides from the intermediate layer, e.g. titanium dioxide. In one refinement, the hydrogen peroxide solution is between 30% strength and 40% strength. In particular, a 35% strength hydrogen peroxide solution is commercially available and is frequently used for fabricating integrated circuits. As an alternative to the hydrogen peroxide, other oxidizing agents can be used, for example liquids containing ozone O 3 , such as water. 
   In one development with an etching bath comprising aqueous ammonia and hydrogen peroxide solution, these constituents are mixed in a ratio by volume of between 10:1 and 30:1. Good results can be achieved with a ratio by volume of 20:1. 
   In a subsequent development, the oxide layer which forms during wet chemical etching is removed from a metal layer in the initial metalization stratum. This measure prevents the electrical resistance from being increased in the region of the vias. 
   In a subsequent development, the oxide layer is removed using a back sputtering process before the cutout is filled. If the cutout is filled using a sputtering method, back sputtering means only a small amount of additional complexity. In addition or alternatively to back sputtering, the oxide layer can also be removed using a wet chemical etching process. By way of example, an aluminum oxide layer Al 2 O 3  can be removed very well using choline or using a choline solution. Choline is a base having the structural formula:
 
HO—CH 2 —CH 2 —N + —(CH 3 ) 3 ,
 
   where the nitrogen atom is positively charged and is connected to four carbon atoms. In one refinement, the choline solution is between 0.05% strength and 0.5% strength. Good results can be achieved with a 0.1% strength choline solution. 
   In a subsequent development, the intermediate layer is made of tantalum nitride. Tantalum nitride has similar properties to titanium nitride and can also be removed using similar chemical methods. 
   In a subsequent development, the initial metal stratum contains a metal layer which is made of copper or of a copper alloy. By way of example, the initial metalization stratum is produced using a “damascene” or dual “damascene” method. 
   In another development, the further metalization stratum contains a metal layer made of aluminum or of an aluminum alloy, particularly an aluminum/silicon alloy, an aluminum/silicon/copper alloy or an aluminum/copper alloy. The additional components to the aluminum are usually below 5 wt % (percent by weight). 
   Alternatively, the further metalization stratum contains a metalization stratum made of copper or of a copper alloy. 
   In a subsequent development, the cutout contains tungsten, aluminum, an aluminum alloy, copper or a copper alloy. Tungsten can be introduced in a good filling ratio. 
   For producing the initial metalization stratum or the further metalization stratum, one development involves the following steps being carried out: 
   a metal layer is applied, 
   the intermediate layer is applied, and 
   the metal layer and the intermediate layer are patterned, particularly using a lithographical method or using the damascene technique. 
   In another development, the method is used to produce a contact having a contact area larger than 10 μm 2  (square microns), larger than 100 μm 2 , larger than 1000 μm 2  or larger than 5000 μm 2 . Such large contact areas are used in “smart power” circuits in order to be able to switch very large currents, e.g. currents which are larger than 1 mA (milliamps), larger than 10 mA or else larger than 100 mA for the contact area. Particularly in the case of the associated switching powers, the technical effects of the inventive method are found to be particularly intense. By way of example, motors are actuated using these circuits. 
   Alternatively, the method is also used for contact areas which are smaller than 10 μm 2  or smaller than 1 μm 2 , however. 
   The invention also relates to an integrated circuit arrangement which contains a plurality of metalization strata. Two metalization strata are connected by at least one electrically conductive connecting section which is situated in a cutout. The cutout extends through an insulating layer between the metalization strata and through an intermediate layer. At the boundary between insulating layer and intermediate layer, the cutout has a projecting edge which is formed, by way of example, on the basis of an undercut at the boundary between intermediate layer and metal layer. In other words, the cutout extends at the boundary abruptly as compared with the rest of its profile. Such a circuit arrangement is produced when the inventive method or one of its developments is applied. The technical actions cited above thus also apply to the integrated circuit arrangement. 
   In one development, the circuit arrangement is suitable for the purpose of switching powers higher than 100 W, i.e. it is a power circuit arrangement, also referred to as a power device. 
   BRIEF DESCRIPTION OF THE DRAWING 
   Exemplary embodiments of the invention are explained below with reference to the accompanying drawings, in which: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A to 1C  show fabrication stages in the fabrication of an integrated circuit arrangement, and 
       FIG. 2  shows method steps in a method in which titanium nitride is removed. 
   

   DETAILED DESCRIPTION 
   As  FIG. 1A  shows, an integrated circuit arrangement  10  has been fabricated by first applying an aluminum layer  12  to a prefabricated portion of the integrated circuit arrangement  10 . The prefabricated portion contains active semiconductor elements in a silicon substrate. The aluminum layer  12  has had a titanium nitride layer  14  applied to it which, by way of example, has a thickness of 45 nm (nanometers) and serves as an antireflection layer, for example. 
   Following application of the titanium nitride layer  14 , the titanium nitride layer  14  and the aluminum layer  12  have been patterned using a photolithographic method including an etching method, with, for example, BCl 3 , and an interconnect  16  in a metalization stratum  18  has been produced. 
   The patterned metalization stratum  18  has then had a silicon dioxide layer  20  applied to it, for example using a silane-based method (SiH 4 ). Above the remainder of the titanium nitride layer  14 , the silicon dioxide layer  20  has a thickness of between 0.5 μm and 1.5 μm, for example. 
   The silicon dioxide layer  20  has then had a photoresist layer  22  applied to it. The photoresist layer  22  has been selectively exposed using a mask, particularly above the remainder of the titanium nitride layer  14 , see arrow  24 . The photoresist layer  22  has then been developed, and a cutout  26  has been produced in the photoresist layer  22  above the titanium nitride layer  14 , the bottom of said cutout being on the silicon dioxide layer  20 . 
   As  FIG. 1B  shows, a dry etching process has then been carried out in which the cutout  26  has been extended to form a cutout  26   a  which has the same diameter as the cutout  26  but is deeper. The bottom of the cutout  26   a  is on the titanium nitride  14  or in the titanium nitride layer  14 . The dry etching is performed using carbon tetrafluoride CF 4 , for example. The dry etching process is carried out either using end-point identification or under time control. 
   Suitable dry etching methods are plasma etching, reactive ion etching or electron beam etching. 
   In another exemplary embodiment, the dry etching process is ended under time control before the silicon dioxide layer  20  has been etched through. That portion of the silicon dioxide layer which remains at the bottom of the cutout is subjected to wet chemical etching together with the intermediate layer, i.e. the titanium nitride layer  14  in the exemplary embodiment. 
   As  FIG. 1C  shows, a wet chemical etching process is then carried out in order to remove the titanium nitride layer  14  in the region of the cutout  26   a . In this case, a cutout  26   b  whose bottom is approximately at the boundary between the titanium nitride layer  14  and the aluminum layer  12  is produced from the cutout  26   a . The wet chemical etching bath used is a mixture of a 35% strength aqueous hydrogen peroxide solution and of an approximately 28% strength aqueous ammonia solution. The ratio by volume of hydrogen peroxide solution to ammonia solution is 20:1. The wet chemical etching is performed at room temperature. 
   The wet chemical etching is isotropic, which means that the silicon dioxide layer  20  is undercut by etching. The width B of the undercut has approximately the same value as the layer thickness D of the titanium nitride layer  14 , i.e. the width B is approximately 80 nm in the exemplary embodiment. The side faces of the cutout  26   b  in the region of the titanium nitride layer  14  are inclined, with the distance between the side faces increasing toward the boundary between titanium nitride layer  14  and silicon dioxide layer  20 . At the bottom of the cutout  26   b , the diameter is at its original value again, in the exemplary embodiment, i.e. it is approximately equal to the diameter of the cutout  26  or  26   a . Instead of inclined side faces, rounded side faces also appear. 
   In another exemplary embodiment, particularly with a high level of overetching, the diameter at the bottom of the cutout  26   b  is larger than in the top region of the cutout  26   b.    
   Depending on the thickness of the titanium nitride layer  14 , the etching time for the wet chemical etching is between 3 min (minutes) and 20 min. 
   The wet chemical etching process also involves the formation of an aluminum oxide layer  30  on the remainder of the aluminum layer  12 , said aluminum oxide layer  30  being a few nm thick and preventing the aluminum layer  12  from being attacked by the etching bath. 
     FIG. 2  shows method steps in the fabrication of the integrated circuit arrangement  10 . The method starts in a method step  100 . a method step  102  corresponds to the application of the photoresist layer  22  and patterning thereof which have been explained above with reference to  FIG. 1A . 
   A method step  104  has been explained in more detail above with reference to  FIG. 1B  and concerns the dry etching of the silicon dioxide layer  20  using fluorine-containing chemicals, which proceeds in line with the following formula, for example:
 
Si+4F→SiF 4 .
 
   A method step  106  relates to the wet chemical etching of the titanium nitride layer  14 . The method step  106  has already been explained above with reference to  FIG. 1C  and proceeds in line with the following formulae, for example:
 
2TiN+4OH − →TiO 2+ N 2 +H 2 O
 
4Al+3H 2 O 2 →2Al 2 O 3 +3H 2 .
 
   The wet chemical etching also involves removal of the photoresist layer  22 . 
   Method step  106  is followed by a method step  108  in which the integrated circuit arrangement  10  is washed with water and is then dried. 
   In a subsequent method step  110 , the aluminum dioxide layer  30  is removed using a wet chemical method. The etching bath used this time is a dilute basic aqueous solution, e.g. a 0.1% strength choline solution. By way of example, the integrated circuit arrangement  10  is dipped into the choline solution for 30 s (seconds) at room temperature. 
   In a subsequent method step  112 , the integrated circuit arrangement  10  is washed, e.g. in water, and dried. 
   Method step  112  is followed by a method step  114  in which removal of the remainders of the photoresist layer  22  is followed by the start of sputtering of a next metalization stratum. In the process, the cutout  26   b  is also filled with aluminum. 
   The method is ended in a method step  116  after contact areas for connecting external connecting wires or for bearing solder balls have been exposed on the integrated circuit arrangement  10 . 
   In another exemplary embodiment, instead of method steps  110  and  112 , method step  108  is followed by the performance of a method step  122  in which the aluminum oxide layer  30  is removed by back sputtering in argon, see also arrow  120 . The back sputtering is again followed by method step  114 , in which the next aluminum layer is sputtered on, said next aluminum layer being used to produce the next metalization strata. 
   In another exemplary embodiment, in addition to method steps  110  and  112 , method step  122  is performed, see arrow  130 . In the case of this variant, it is not necessary for the aluminum oxide layer  30  to be removed completely by wet chemical means. On the other hand, a thin oxide layer which forms upon washing and drying is also removed again on the exposed metal, for example if washing with water. 
   The method explained prevents particle formation in a plasma etching system when removing the titanium nitride layer. The result of this is longer endurance times between chamber cleaning operations, which means that throughput increases and costs fall. In addition, the process speed for wet chemical removal of the titanium nitride layer is higher than when a dry etching method is used. Furthermore, the etching rate in the case of wet chemical removal of the titanium nitride is not dependent on the exposed area of the titanium nitride. 
   In another exemplary embodiment, the photoresist layer  22  is removed before the wet chemical etching in method step  106  but after the dry etching in method step  104 . This means that the etching solutions are not unnecessarily contaminated. 
   Particularly in the case of a copper layer instead of the aluminum layer  12 , the wet chemical etching solution used can also be a solution of nitric acid HNO 3  and hydrofluoric acid HF in a mixture range of between 3:1 and 1000:1, e.g. nitric acid HNO 3  at a concentration c of 10 mol/l and hydrofluoric acid HF at a concentration c of 3 mol/l. 
   The wet chemical etching is carried out by means of immersion etching, spray etching or rotation etching, for example.