Patent Publication Number: US-2011049639-A1

Title: Integrated circuit manufacturing method and integrated circuit

Description:
The present invention relates to a method of manufacturing a transistor, the method comprising providing a substrate comprising a source and drain region connected by a channel region, said channel region being covered by a gate stack separated from the channel region by a dielectric layer, the gate stack comprising a metal portion over the dielectric layer and a polysilicon portion over the metal portion. 
     The present invention further relates to an integrated circuit comprising a plurality of transistors, each transistor comprising a channel region connecting a source region to a drain region, the channel region being covered by a dielectric layer and a gate stack comprising a metal portion and a silicide portion. 
     The shrinkage of feature sizes in integrated circuits (ICs) such as CMOS ICs is accompanied by a large number of challenges that have to be overcome in order to provide an IC that can operate in accordance with demanding operating requirements. For instance, the reduction in the transistor gate dielectric thickness increases the direct tunneling of carriers through the ultra-thin gate dielectric. This is becoming a major obstacle in further CMOS scaling. 
     Several solutions have been proposed to reduce such tunneling effects. Most solutions focus on replacing the conventional dielectric layer with a high-k dielectric layer. The high-k layer has higher dielectric constant than SiO 2  so that it can be physically thicker, thus increasing the tunneling energy barrier, and reducing leakage as a consequence. However, the combination of a high-k dielectric and a poly-Si gate is not considered to be feasible. For this reason, it has been proposed to replace the SiO 2 /polysilicon (poly-Si) gate stack by a high-k dielectric/metal gate stack. The metal gate also avoids the effect of poly-Si depletion, resulting in higher inversion capacitance and hence more performance. 
     The replacement of the poly-Si layer entirely with a solid metal layer would solve the aforementioned problems as long as a metal with a suitable work function is selected. However, the patterning of solid metal gates is far from trivial. Alternatively, a poly-Si gate may be formed on the gate dielectric, which is replaced by a metal gate using a Damascene process. A drawback of such an approach is that it is requires a relatively large number of process steps, thus making the gate forming process quite costly. 
     Hence, for reasons of manufacturability, multi-layer gate architectures have been proposed, such as a metal-inserted polysilicon (MIPS) gate, in which a thin (5-10 nm) metal layer such as a TiN, TaN, W or MoON layer, is covered by a thick (100 nm) polysilicon layer which is partly converted into silicide. However, such gate architectures can suffer from an increased gate resistance compared to poly-Si gates. This causes a different problem, because the gate resistance is well-known to degrade performance at high transistor operating frequencies, for instance radio frequency (RF), where a substantial gate resistance can affect RF figures of merit. RF CMOS transistors typically operate in the 100 GHz frequency range. CMOS scaling has also pushed the digital clock speed into the GHz domain, which implies that individual transistors switch at frequencies well in excess of 100 GHz. Indeed, typical ring oscillator delays per stage are in the range of 10 ps, equivalent to 100 GHz. Therefore, it can be expected that gate resistance will degrade digital switching speed. 
     The gate resistance R gate  is in nature a distributed quantity, containing gate layer material parameters and dimensions of the gate layers between the gate contact and the gate dielectric. A good approximation is given by the following formula: 
     
       
         
           
             
               R 
               gate 
             
             = 
             
               
                 
                   
                     ρ 
                     silicide 
                   
                   · 
                   W 
                 
                 
                   12 
                    
                   
                       
                   
                    
                   L 
                 
               
               + 
               
                 
                   
                     ∑ 
                     interfaces 
                   
                    
                   
                     ρ 
                     c 
                   
                 
                 
                   W 
                   · 
                   L 
                 
               
             
           
         
       
         
         where L and W are the length and width of a gate line, ρ c  is the contact resistance between different layers in the gate electrode and ρ silicide  is the silicide sheet resistance. 
       
    
     For a poly-Si gate stack, consisting of about 100 nm heavily doped polysilicon which is partly converted into silicide such as CoSi or NiSi, typical parameter values are ρ silicide =6Ω/square and ρ c =3Ω·μm 2  for the NiSi to polysilicon interface. For transistor dimensions of L=25 nm and W=0.4 μm, which are typical transistor dimensions in a 32 nm CMOS technology, this results in a gate resistance R POLY ≈300Ω. 
     For an advanced MIPS gate stack consisting of a thin metal layer covered by a thick layer of polysilicon which is partly converted into NiSi, an additional contact resistance ρ c =20Ω·μm 2  for the polysilicon to gate metal interface must be accounted for. For a transistor of L=25 nm and W=0.4 μm, this results in a much higher gate resistance R MG ≈2.3 kΩ. Hence, it can be seen that although such MIPS gates in combination with high-k dielectrics may successfully address the tunneling problem associated with poly-Si gates on SiO 2 , the MIPS gates are likely to exhibit serious performance issues at GHz operating frequencies of the transistor comprising one or more of such gates. 
     In the paper ‘Metal Inserted Poly-Si (MIPS) and FUSI Dual Metal (TaN and NiSi) CMOS integration’ by R. Singamalla et al. in 2008 Institution of Engineering and Technology, April 2007, pages 45-46, a CMOS device is disclosed in which the n-type field-effect transistor (FET) comprises a gate stack of a TaN metal portion covered by a poly-Si portion that has been fully silicided using Ni as the silicidation metal. The metallic nature of the silicide reduces the additional contact resistance of the metal/poly-Si interface, which improves the high-frequency characteristics of the transistor. 
     However, it has been found that it is very difficult to avoid the formation of a thin oxide layer at the metal/poly-Si or metal/silicide interface, which causes the metal-silicide gate stack to act as a metal-insulator-metal capacitor, and which introduces a undesirable contact resistance with the metal portion and the silicide portion respectively. 
     The present invention seeks to provide a method of manufacturing an IC that can operate in a GHz range. 
     The present invention further seeks to provide an IC that can operate in the GHz range. 
     According to an aspect of the present invention, there is provided a method of manufacturing a transistor, comprising providing a substrate comprising a source and drain region separated by a channel region, said channel region being covered by a gate stack separated from the channel region by a dielectric layer, the gate stack comprising a metal portion over the dielectric layer and a polysilicon portion over the metal portion; implanting an oxide reducing dopant into the polysilicon portion; depositing a silicidation metal over the implanted polysilicon portion; and converting the implanted polysilicon portion into a silicide portion. 
     The introduction of an oxide-reducing dopant, such as Al, Ti or Yb prior to the silicidation step ensures that the oxide-reducing dopant is driven through the poly-Si during the silicidation process. This is also known as the snow-plough effect. Hence, by fully siliciding the poly-Si, the oxide reducing dopant is pushed to the thin oxide layer at the interface between the poly-Si portion and the metal portion, where it reacts with the thin oxide layer, thus reducing the contact resistance between the silicide portion and the metal portion of the gate stack. 
     The source and drain region may be protected from silicidation. This may for instance be achieved by depositing a masking layer over the source region and the drain region prior to said implanting step, and wherein the step of depositing the silicidation metal comprises depositing the silicidation metal over the polysilicon portion and the masking layer, the method further comprising removing unreacted silicidation metal following the converting step. 
     In an embodiment, the masking layer is deposited by means of spin-coating. Because spin-coating allows for excellent control of the thickness of the deposited layer, the making layer may be deposited without covering the poly-Si portion of the gate stack, thus obviating the need for further process steps such as a planarization step to expose the poly-Si portion. 
     The source and drain regions may also be silicided. This may be done in a separate silicidation step, in which case the method may comprise providing a mask over the polysilicon portion; siliciding the source region and the drain region; and removing said mask prior to said implanting step. 
     Alternatively, the source and drain regions may be silicided at the same time as the poly-Si portion of the gate stack. To this end, the method may comprise removing the masking layer following said implanting step, and wherein said depositing step comprises depositing the silicidation metal over the polysilicon portion, the source region and the drain region, and wherein said converting step comprises simultaneously converting the polysilicon portion into a silicide portion, the source region into a silicide source region and the drain region into a silicide drain region. 
     If necessary, the thickness of the poly-Si layer portion may be reduced prior to the removal of the masking layer. This reduces the duration of the subsequent silicidation step due to the fact that less poly-Si has to be silicidized. 
     The method of the present invention may be applied to both planar transistors and non-planar transistors, e.g. fin-shaped transistors such as FinFETs, and may be applied to single gate and multiple gate transistors. 
     According to a further aspect of the present invention, there is provided an integrated circuit comprising a plurality of transistors, each transistor comprising a channel region connecting a source region to a drain region, the channel region being covered by a dielectric layer and a gate stack comprising a metal portion and a silicide portion, wherein the interface between the metal portion and the silicide portion has been chemically altered by an implanted species, thereby lowering the resistance of the interface. The transistors of such an IC are typically characterized by the accumulation of an oxide-reducing dopant near said interface. 
     Such an IC, which may be integrated in a suitable electronic device, has transistors that can be operated at radio frequencies, e.g. 100 GHz. 
    
    
     
       Embodiments of the invention are described in more detail and by way of non-limiting examples with reference to the accompanying drawings, wherein 
         FIG. 1   a - d  schematically depict the inventive concept of the present invention; 
         FIG. 2   a - f  schematically depict an embodiment of the method of the present invention; 
         FIG. 3   a - e  schematically depict another embodiment of the method of the present invention; and 
         FIG. 4   a - f  schematically depict yet another embodiment of the method of the present invention. 
     
    
    
     It should be understood that the Figures are merely schematic and are not drawn to scale. It should also be understood that the same reference numerals are used throughout the Figures to indicate the same or similar parts. 
       FIG. 1   a  depicts a cross-section of a MIPS gate stack on a dielectric layer  110 . The gate stack comprises a thin metal layer  112 , which for instance may be several nanometers, e.g. 5-10 nm, thick. The metal layer  112  is covered by a poly-Si layer  116 , which may be an order thicker than metal layer  112 , e.g. several tens of nanometers, e.g. 50-100 nm, thick. The manufacturing of such a MIPS gate stack is well-known to the skilled person and will therefore not be discussed in further detail for reasons of brevity. In the formation of the MIPS gate stack, it is very difficult to avoid the formation of a thin oxide layer  114  between the metal layer  112  and the poly-Si layer  116 . The oxide may be formed by the partial oxidation of the metal layer  112  and/or the poly-Si layer  116 . 
     This oxide layer  114  increases the contact resistance between the metal layer  112  and the poly-Si layer  116 , which impairs the high-frequency operation of a transistor controlled by the MIPS gate. In accordance with an embodiment of the present invention, a dopant  130  is implanted into the poly-Si layer  116 , as shown in  FIG. 1   b . This dopant is chosen such that it can react with the oxide layer  114 , thereby converting the oxide layer into a further layer having a lower resistance than the oxide layer  114 . The exact chemical reaction leading to the lower interface resistance is experimentally very difficult to establish. However, it is likely that the oxide layer  114  contains a large number of Si-O bonds, i.e. is SiO 2  like. By implanting a metallic dopant  130  such as Al, Ti, Y or other suitable dopants, the SiO x  layer  114  is converted by the reaction: 
       M+SiO x   MO y +Si     The converted layer has a significantly lower resistance. This may be because MO y  has a lower resistivity than SiO x , or because the reaction causes the agglomeration of the reaction product into islands, thus leaving a large fraction of the interface essentially oxide-free.   
     The dopant  130  can be migrated towards the oxide layer  114  at the interface between the metal layer  112  and the poly-Si layer  116  using the snow-plough effect of a silicidation conversion of the poly-Si layer  116 , as shown in  FIG. 1   c . To this end, a silicidation metal  140  is deposited over the poly-Si layer  116 , after which the gate stack is subjected to a thermal budget, i.e. an elevated temperature for a predefined period of time. During the silicidation reaction, the silicidation front in the poly-Si layer progresses from the silicidation metal  140  towards the oxide layer  114 , thereby pushing the dopant  130  forward. 
     The thermal budget is chosen such that the whole poly-Si layer  116  is converted into a silicide, which ensures that the dopant  130  reaches the oxide layer  114 . At the oxide layer  114 , the dopant  130  reacts with the oxide as previously explained, yielding a metal-silicide gate stack as shown in  FIG. 1D , where the metal-silicide interface is substantially free of oxide, thus yielding a gate that can be operated at radio frequencies. 
     The above principle may be applied to any suitable MIPS gate stack, such as a planar gate stack or a non-planar gate stack such as the gate stack of a FinFET.  FIG. 2   a - f  depict an embodiment of the method of the present invention, in which the above principle is applied to a planar gate stack. 
       FIG. 2   a  shows a cross-section of an intermediate structure in an IC manufacturing process. A substrate  100 , which may be any suitable substrate such as a bulk-Si wafer or a silicon on insulator (SOI) wafer. The substrate  100  comprises a source region  102 , a drain region  104  and a channel region  106 . A gate stack as shown in  FIG. 1   a  is formed over the channel region  106 , comprising a dielectric layer  110 , a metal portion  112  and a poly-Si portion  116 . It should be understood that the oxide layer  114  is not shown for reasons of clarity only. 
     The dielectric layer  110  may for instance comprise SiO 2 , SiON or any suitable high-k dielectric material. The metal portion  112  may for instance comprise TiN, TaN, W, MoON or any other suitable metal. Since it is well-known to the skilled person how to manufacture the intermediate structure in  FIG. 1   a , this will not be explained in further detail for reasons of brevity only. 
     In an embodiment, the source region  102  and the drain region  104  may be silicided. To this end, a mask  118  may be formed over the poly-Si portion to facilitate the selective silicidation of the source region  102  and the drain region  104 . The source region  102  and the drain region  104  are subsequently silicided, as shown in  FIG. 2   b . The deposition of the silicidizing metal prior to the silicidation of these regions is not shown. 
     Next, as shown in  FIG. 2   c , a protective layer  120  is deposited over the substrate  100 . This layer may for instance be a SiO 2  layer. The deposition of the protective layer  120  may be followed by a planarization step (not shown) to etch-back the protective layer  120  such that the poly-Si portion  116  is exposed. The planarization step may be performed using any suitable technique, e.g. chemical mechanical planarization (CMP). In an alternative embodiment, the protective layer  120  is spin-coated onto the substrate  100 . The protective layer may be a polymer, e.g. polyimide or may be a SiO 2  layer formed by means of a spin-on-glass technique. Spin-coating facilitates selective deposition of the protective layer  120  such that the poly-Si portion  116  of the gate stack will not be covered by the protective layer  120 , thus obviating the need for a subsequent etch-back step. 
     In a next step, shown in  FIG. 2D , the dopant  130  is implanted into the poly-Si portion  116 , after which the silicidation metal  140  is deposited over the protective layer  120  and the poly-Si portion  116 , as shown in  FIG. 2   e . Any suitable metal, e.g. Ni, Co, Pt or Ti may be used as the silicidation metal  140 . The stack is subsequently exposed to a thermal budget ensuring that the poly-Si portion is fully silicided, such that the dopant  130  reaches the interface between the metal portion  112  and the poly-Si portion  116 , where it reacts with the unwanted oxide layer (not shown), as previously explained. Any unreacted silicidation metal  140  is subsequently removed from the substrate stack. 
     Finally, the protective layer  120  is removed as shown in  FIG. 2   f  to yield a transistor having a gate stack in accordance with an embodiment of the present invention. The protective layer  120  may be removed in any suitable way, e.g. by means of a selective etch step. 
     The IC manufacturing process may be completed in any suitable way. Since this is not relevant to the present invention, and since subsequent process steps will be apparent to the skilled person, these steps will not be discussed in detail for reasons of brevity only. 
     In  FIG. 2 , the poly-Si portion  116  and the source and drain regions  102 ,  104  have been converted into a silicide in separate steps. However, it is equally feasible to simultaneously convert the poly-Si portion  116  and the source and drain regions  102 ,  104  into a silicide in a self-aligned process by altering the sequence of the process steps of  FIG. 2 . An example of such a process in shown in  FIG. 3 . 
     In  FIG. 3   a , the substrate  100  is covered by the protective layer  120  such that the source region  102  and the drain region  104  are protected by the protective layer  120  and the poly-Si portion  116  is still exposed. 
     The poly-Si portion  116  is subsequently implanted with the dopant  130 , as shown in  FIG. 3   b.    
     Optionally, the poly-Si portion  116  may be etched back to reduce the thickness of the poly-Si portion  116 , as shown in  FIG. 3   c . The implantation of the dopant  130  may be performed before or after etching back the poly-Si portion  116 . Obviously, in case of the dopant  130  being implanted before the etch-back step, the dopant must be implanted deep enough not to be removed by the etch-back. 
     In a next step, the protective layer  120  is removed and the silicidation metal  140  is deposited over the poly-Si portion  116  and the source and drain regions  102 ,  104 , as shown in  FIG. 3   d . The intermediate device is subsequently exposed to the thermal budget for fully converting the poly-Si portion  116 , as well as the source region  102  and the drain region  104 , into a silicide, thereby pushing the dopant  130  towards the interface between the metal portion  112  and the poly-Si portion  116 . Any unreacted silicidation metal  140  is subsequently removed, thus yielding the intermediate device in  FIG. 3   e , which again may be completed using conventional (back-end) processing steps. 
     The above methods are not limited to planar transistors, but may also be applied to a non-planar transistor such as a FinFET. 
       FIG. 4   a  depicts a cross-section an intermediate structure in the manufacturing of a FinFET device. The substrate  100  carries a fin  406  which is covered by a dielectric layer  110  and a gate stack comprising a metal layer  112  and a poly-Si layer  116 . The thin oxide layer between the metal layer  112  and a poly-Si layer  116  is not shown for reasons of clarity only. The source and drain regions of this intermediate structure are also not shown in  FIG. 4   a . The source and drain regions may already have been silicided during which the poly-Si layer  116  has been protected by e.g. a poly-Si mask, as previously explained. 
     In a next step, the poly-Si layer  116  is covered by the protection layer  116  such that only the top of the poly-Si layer  116  is exposed. If required, the protection layer  116  may be etched back to expose the top of the poly-Si layer  116  as previously explained. 
     The top of the poly-Si layer  116  is subsequently implanted with the dopant  130 , as shown in  FIG. 4   c , after which the silicidation metal  140  is deposited over the protective layer  120  and the top of the poly-Si layer  116 . The intermediate device is subsequently subjected to a thermal budget such that the poly-Si layer surrounding the fin  406  is fully converted to a silicide. This silicidation step causes the dopant  130  to ‘snow-plough’ towards the thin oxide layer between the metal layer  112  and the poly-Si layer  116 , where it reacts with the oxide as previously explained. The thermal budget may be chosen such that the silicidation process extends laterally into portions  116 ′ of the poly-Si layer  116 . Any unreacted silicidation metal  140  and the protective layer  120  are subsequently removed to yield the intermediate device shown in  FIG. 4   f . This device may be completed using conventional processing steps. 
     At this point, it is emphasized that in the context of this application, ‘fully converted to a silicide’ does not necessarily imply that the silicidized poly-Si is uniformly silicidized, and is also intended to cover embodiments in which a silicidation gradient is present in the poly-Si. For instance, at the top of the poly-Si, the conversion degree may be higher than near the interface with the metal layer  112 . Also, in case of e.g. a FinFET device, the degree of conversion may be higher in the poly-Si on top of the fin compared to the poly-Si laterally to the fin. 
     It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word “comprising” does not exclude the presence of elements or steps other than those listed in a claim. The word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.