Abstract:
A region is locally modified so as to create a zone that extends as far as at least part of the surface of the region and is formed from a material that can be removed selectively with respect to the material of the region. The region is then covered with an insulating material. An orifice is formed in the insulating material emerging at the surface of the zone. The selectively removable material is removed from the zone through the orifice so as to form a cavity in place of the zone. The cavity and the orifice are then filled with at least one electrically conducting material so as to form a contact pad.

Description:
PRIORITY CLAIM 
     The present application claims priority from French Application for Patent No. 04 11123 filed Oct. 20, 2004, the disclosure of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     The present invention relates to integrated circuits, especially their fabrication, and more particularly to the production of a contact pad on a region of an integrated circuit. 
     2. Description of Related Art 
     At the present time, a contact pad is generally produced on a region of an integrated circuit, for example on the source, drain or gate region of a transistor, or else on a polysilicon line forming a resistor, by silicidation (i.e., the formation of a metal silicide) of the upper part of the region and then, after an insulating material, for example silicon dioxide, has been deposited on the region, by making an opening, commonly called a “via” by those skilled in the art, and filling the via with a metal that will come into contact with the silicided zone. This via will thus allow an electrical connection to be made between the silicided semiconductor region and a metallization level of the integrated circuit. 
     Such a process has many drawbacks. Among these, mention may for example be made of the complexity and the cost of production, but also the problem of the resistance of the silicide layer and the problem of the interface between the metal silicide and the silicon. This is because, if this interface is not correct this may result in a puncture of the junctions, an increase in the resistance, or even debonding of the metal silicide. The interface between the silicided zone and the metal that fills the via must be perfectly controlled. It is therefore necessary to clean the bottom of a contact after opening the vias, so as to minimize the risks of obtaining an interface of poor quality. Consequently, this further increases the complexity and the production cost. 
     A need accordingly exists in the art to remedy these drawbacks. A process for producing a contact pad that is radically different from that currently used in the prior art is needed. 
     There is further a need in the art for a process for producing a contact pad that is extremely simple to carry out, with a very good yield, while in particular dispensing with the step carried out in the prior art of cleaning the bottom of the contact. 
     There is still further a need in the art for a contact pad having a very low resistance and in any case lower than that obtained in the prior art by silicidation. 
     There is also a need in the art to decrease the contact resistance without increasing the aperture of the via in order to maintain the integration density of the interconnections. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention, what is proposed is a process for producing at least one contact pad on the at least one region of an integrated circuit, in which the region is locally modified so as to create a zone that extends as far as at least part of the surface of the region, this zone being formed from a material that can be removed selectively with respect to the material of the region. The region is covered with an insulating material and an orifice emerging at the surface of the zone is formed in the insulating material. The selectively removable material is removed from the zone through the orifice so as to form a cavity instead of the zone. The cavity and the orifice are filled with at least one electrically conducting material, for example a metallic material. Thus, for example, it is possible to obtain an entirely metallic contact pad on the region. 
     According to this aspect of the invention, what is thus provided is in particular the simultaneous production, on the semiconductor region of the integrated circuit, of a metallization and of the interconnection via for connecting this metallization to a higher metallization level of the integrated circuit. This is particularly simple to carry out, for little cost. The yields are very good as there is a very large choice of electrically conducting materials and temperatures. In this regard, it is possible to envisage, without limitation, filling the cavity and the etching orifice with one and the same metal, but also with several other, different metals or metallic compounds, or else with one or more metal oxides, such as for example an indium oxide, such as that known by those skilled in the art by the term ITO (Indium Tin Oxide). 
     Moreover, the problem of cleaning the bottom of the contact after opening of the vias poses no problem here since, according to one method of implementing the invention, the removal of a layer of selectively removable material is provided, and therefore the interface thus obtained is virtually perfect. 
     Moreover, the metallized layer located on the surface of the region on which the contact pad is made may have a very low resistance, and in any case lower than that of a metal silicide. 
     Moreover, the thickness of this metallized layer can be perfectly determined and can be easily adjusted depending on the intended application. 
     Furthermore, the metallization is carried out at an instant late in the process for fabricating the components on which it is necessary to produce a metal contact. Consequently, the thermal budget is lower. The surface junctions may be produced over a very small depth because the thermal budget is low and because the metallization of the junction may be carried out over a very small thickness. 
     Moreover according to a particularly advantageous embodiment, the aperture of the cavity is bigger than that of the orifice, which permits to increase the area of the electrically conducting contact (i.e., the zone filled with the electrically conducting material) and consequently to decrease the contact resistance without increasing the dimensions of the via, i.e., those of the orifice. And this is possible because the definition of the location and the dimensions of the cavity is performed before the etching of the orifice and because the actual formation of the cavity is performed after said etching and through the etched orifice. 
     The invention applies in particularly to contact pads on regions of active components, such as transistors, or passive components, such as resistors, for example resistive lines made of polysilicon. The invention thus makes it possible in particular to reduce the resistances for access to all kinds of devices, in particular bipolar transistors. 
     The invention may also apply to any element, whether passive or active, of an integrated circuit on which it is necessary to have a contact pad. 
     The thickness of the zone resulting from the local modification of the region may have any thickness, which a person skilled in the art will know how to choose depending on the envisaged application. This being the case, it is particularly advantageous for the zone to be a very thin surface zone, which is easily made possible by a method of implementing the process according to the invention. In this regard, the thickness of the surface zone is for example less than 50 nm. 
     The material of the region on which it is desired to make the contact pad is for example a silicon-based material, for example single-crystal silicon or polycrystalline silicon. In this case, the selectively removable material may be a silicon-germanium alloy. 
     For example, the zone is formed by the implantation of dopants, for example by implantation of germanium, it being possible, for example, for this implantation to be localized by masking with a resist, or else to be self-aligned with respect to a pre-existing feature of an integrated circuit. It would also be possible in certain cases to implant oxygen or nitrogen. 
     The orifice may be formed in the insulating material by chemical or plasma etching. 
     According to one method of implementation, the material is removed from the zone by selective etching. 
     In certain cases, the entire surface of the region may be metallized. In other words, according to such a method of implementation, the zone extends over the entire surface of the region. 
     The region may be a source, drain or gate region of a field-effect transistor, or an emitter, collector or base region of a bipolar transistor. 
     When the invention is applied to the fabrication of an MOS transistor, this fabrication comprises, according to one method of implementation: 
     a) the formation of the gate, source and drain regions of the transistor; and 
     b) the simultaneous production of the respective metal contact pads on the source and drain regions as defined above. The insulating material, in which the orifices emerging in the source and drain regions are formed, then covers the structure obtained in step a). 
     Thus, it is possible to obtain, according to one method of implementing the invention, metallizations that are self-aligned with respect to the source and drain regions of the MOS transistor and to do so without any problem of short-circuiting the subjacent junctions. 
     It is also possible to produce the metallization and the metal contact pad on the gate region simultaneously. 
     Thus, in the case of an MOS transistor for example, the invention is noteworthy in that the source, drain and gate metallizations and the vias that interconnect these metallizations to the upper metallization level of the integrated circuit are produced simultaneously and in a self-aligned manner. 
     Another aspect of the invention is an integrated circuit comprising at least one region equipped with an entirely metallic contact pad obtained by the process as defined above. 
     This region may form part of a passive or active component. 
     This component may thus be a resistor formed from a polysilicon line or else a transistor obtained by a fabrication process as defined above. 
     In accordance with an embodiment of the invention, an integrated circuit comprises a silicon-based region on the surface of which was formed and removed a sacrificial alloyed silicon region so to define a cavity. An insulating layer overlies the silicon-based region, the insulating layer including an opening therein aligned with the cavity on the surface of the silicon-based region. A metallic material fills both the opening in the insulating layer and the cavity on the surface of the silicon-based region so as to form a contact pad with that silicon-based region. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the method and apparatus of the present invention may be acquired by reference to the following Detailed Description when taken in conjunction with the accompanying Drawings wherein: 
         FIGS. 1 to 10  illustrate schematically the main steps of one method of implementing the process according to an embodiment of the present invention; 
         FIG. 11  shows schematically a transistor obtained by such a method of implementation, within an integrated circuit; and 
         FIG. 12  illustrates schematically another component of an integrated circuit equipped with a contact pad according to one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The main steps of one method of implementing the process according to the invention will now be described in greater detail. 
     In  FIG. 1 , an active zone  1  is formed between two isolation zones  2  on a substrate SB, for example a silicon substrate, it being possible for the isolation zones to be isolation trenches. These isolation trenches may for example be deep trenches of the DTI (Deep Trench Isolation) type or shallow trenches of the STI (Shallow Trench Isolation) type. 
     An initial gate region  4 , which may also be made of polysilicon, is produced by means known per se on the active region  1 . 
     Next, the source and drain regions are produced ( FIG. 2 ). These are produced, for example, by carrying out two implantations  40  of dopants respectively before and after the formation of spacers  3  surrounding the gate. The nature of the dopants is chosen depending on whether the transistor on the circuit IC is of the n type or p type. Since the initial gate  4  acts as a mask for the ion implantations, source and drain regions that are self-aligned with the initial gate region  4  and with the active zone  1  bounded by the isolation regions  2  are obtained. 
     Dopants are also implanted into the gate. 
     These various implantations make it possible to produce, as illustrated in  FIG. 3 , a diffused source region  51 , a diffused drain region  61  and a diffused gate region  41 . Of course, it should also be noted that the gate of the transistor is in this case an isolated gate, separated from the substrate  1  by a gate oxide, this oxide not being shown for the purpose of simplification. 
     For example, a germanium implantation  400  is then carried out in the integrated circuit IC ( FIG. 4 ). This is accordingly a “full sheet” implantation. It will be recognized that a localized implantation of the dopant could alternatively be provided. 
     As illustrated in  FIG. 5 , the energy of this implantation  400  is chosen so as to form surface zones  410 ,  510  and  610  in the respective regions  41 ,  51  and  61 . These surface zones  410 ,  510  and  610  are then formed from a material that can be selectively removed with respect to the silicon forming the regions  41 ,  51  and  61 . More precisely, in the present case, these zones  410 ,  510  and  610  are formed from a silicon-germanium alloy SiGe. 
     An annealing step may furthermore be carried out so as to recrystallize the materials and thus better define the boundaries between the material forming the regions  51 ,  61  and  41  and the selectively removable material in the source zones  510 ,  610  and  410 . This annealing may for example be carried out at a temperature of 800 to 900° C. 
     The surface of the circuit is then covered by known means with a layer of an insulating material  7 , the layer  7  consisting for example of silicon dioxide ( FIG. 6 ). This layer  7  is surmounted by a resist layer  8  ( FIG. 7 ), also obtained by known means, in which orifices  9 , that will form the position  9  of the future connection vias, are formed in a known manner. 
     Next, the insulating material  7  is anisotropically etched through the orifices  9  in the resist mask so as to obtain etching orifices  90  that extend down to the source zone  510 , the drain zone  610  and the gate zone  410  ( FIG. 8 ). 
     It is through the orifices  90  that the selectively removable material, present in the zones  510 ,  610  and  410 , is selectively removed ( FIG. 9 ) in order to obtain cavities  520 ,  620  and  420  surmounting the diffused source region  52 , drain region  62  and gate region  42 . 
     It is noted here that the dimension of the cavities is greater than the aperture of the orifice. 
     This selective removal may be carried out by any known means, for example by means of an oxidizing chemical agent such as a solution comprising 40 ml of 70% HNO 3 +20 ml of H 2 O 2 +5 ml of 0.5% HF, or by isotropic plasma etching. 
     Next, one or more electrically conducting materials are deposited by known methods, allowing the previously obtained cavities to be simultaneously filled ( FIG. 10 ) in order to obtain the source region  52 , the drain region  62  and the gate region  42 , these regions being surmounted by electrically conducting zones, for example metallizations  53 ,  63  and  43  and connection vias  91 , which are also electrically conducting, between these metallizations and the surface of the integrated circuit. 
     Since the aperture of the cavities is greater than that of the orifices, the areas of the metallizations  43 ,  53 ,  63  are greater than the vias aperture. This permits a decrease in the contact resistance without increasing the lateral dimensions of the vias, which permits one to maintain the integration density of the interconnections. 
     In this regard, the cavities  420 ,  520 ,  620  and the etching orifices  90  may be filled with tungsten (W), for example by chemical vapor deposition (CVD) known per se, for example within the 500° C. to 600° C. temperature range, or else by ALD (Atomic Layer Deposition), also known per se, for example within the 200° C. to 300° C. temperature range. 
     It is also possible to use copper (Cu), cobalt (Co) or nickel (Ni) by electrochemical deposition at room temperature. In particular in the case of copper, this is preferably deposited prior to a barrier layer, for example made of titanium nitride (TiN), by chemical vapor deposition, or made of tantalum (Ta). 
     It is also possible to use tantalum nitride (TaN) by ALD-type deposition, for example within the 200° C. to 300° C. temperature range. 
     It is also possible to use highly doped polysilicon by LPCVD (low-pressure CVD) in a furnace, for example at between 700° C. and 800° C., or else ALD-type deposition, for example within the 200° C. to 300° C. temperature range. 
     It is also possible to use a metal compound comprising aluminum (Al) and a small percentage, for example 5%, of silicon. 
     It is also possible to use metal oxides, such as for example an indium oxide, such as the one known by the name ITO (Indium Tin Oxide). 
     The excess metal is removed for example by chemical-mechanical polishing, and a metallization level M 0  is produced, by known means, in order to obtain the integrated circuit of  FIG. 11 . 
       FIG. 11  therefore shows an integrated circuit IC comprising a transistor according to one embodiment of the invention. 
     The integrated circuit comprises a substrate SB in which the active zones  1  have been produced between the isolation zones  2  of the substrate. A transistor T has been produced in and on the active zone  1  of the substrate SB. The electrodes (source  52 , drain  62  and gate  42 ) of the transistor are covered with electrically conducting contact pads, these being for example entirely metallic and formed from surface metallizations  52 ,  63  and  43  and from metal vias  91  connected to the metallization level M 0 . 
     The invention is not limited to the production of self-aligned contact pads on a transistor, rather it applies to any passive or active component of an integrated circuit on which it is necessary to make a contact pad, whether or not self-aligned. By way of indication,  FIG. 12  illustrates schematically a polysilicon line  75 , forming a resistor, on which an entirely metallic contact pad is produced, for example in accordance with the method of implementation described above. 
     More precisely, this polysilicon resistive line  75 , resting for example on a subjacent insulating material  70 , is provided locally with an entirely metallic contact pad. This contact pad comprises, as explained above, a metallic surface zone  750  and a metallic interconnection via  91  made within an upper insulating material  7 . The via  91  makes it possible to electrically connect the metallization  750  for the resistor  75  to the upper metallization level Mi of the integrated circuit. The metallic surface zone is obtained using the above method of implementation, for example by germanium implantation localized by a resist mask. 
     Although preferred embodiments of the method and apparatus of the present invention have been illustrated in the accompanying Drawings and described in the foregoing Detailed Description, it will be understood that the invention is not limited to the embodiments disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the spirit of the invention as set forth and defined by the following claims.