Abstract:
Replacement metal gates well suited for self-aligned contact formation are made by replacing the dummy gate with a recessed polysilicon layer and then effecting an aluminum-polysilicon substitution. The resulting upper polysilicon layer is easily removed from the recessed aluminum layer, which can then be protected with a protective dielectric layer for subsequent formation of a source or drain contact hole.

Description:
BACKGROUND 
       [0001]    The invention relates to semiconductor devices with self-aligned source or drain contacts and methods for making the same. 
         [0002]    As the scale of transistors continues to decrease, it has become increasingly difficult to form a source or drain contact hole that exposes only the intended source or drain region, and does not also expose part of the gate electrode. If the source or drain contact reaches not only the source or drain but also the gate electrode, an electrical short is created and the transistor does not function. 
         [0003]    A protective dielectric film such as SiN is therefore formed on the gate electrode and as gate sidewalls prior to contact hole formation, which protects the gate electrode during contact hole formation by reactive ion etching (RIE), since the RIE process is selective to the silicon dioxide interlayer dielectric in which the contact hole is to be formed, but selective against the SiN protective layer. 
         [0004]    However, the conventional techniques and structures for protecting the gate electrode during contact hole formation, especially in the case of forming self-aligned source and drain contacts in replacement metal gate (RMG) transistors, are attended by several difficulties. Selective formation of a dielectric cap has been found to provide insufficient selectivity with respect to the region to be protected. Another proposal has been to oxidize and passivate the upper surface of the metal gate film; however, in that technique it is difficult to control the extent of oxidation of the metal, which can result in a resistivity increase at the gate electrode and increased variation in threshold voltage of the transistor. Yet another proposal has been to recess the metal electrode and cover it with a dielectric cap; however, conventional techniques for recessing a metal gate electrode involve RIE, which is difficult to perform in terms of processing time and selectivity. 
       SUMMARY 
       [0005]    The present inventors have discovered that replacement metal gates well suited for self-aligned contact formation are made by replacing the dummy gate with a recessed polysilicon layer and then effecting an aluminum-polysilicon substitution. The resulting upper polysilicon layer is more easily removed from the recessed aluminum layer, which can then be protected with a protective dielectric layer for subsequent formation of a source or drain contact hole. 
         [0006]    Thus, the present invention in one aspect relates to a semiconductor device comprising a semiconductor substrate and a transistor formed on the semiconductor substrate, the transistor comprising a gate electrode and a gate dielectric film located between the gate electrode and the semiconductor substrate. The gate electrode comprises a metal substituted layer and a barrier conductive layer, the barrier conductive layer being located between the gate dielectric film and the metal substituted layer. Gate sidewalls are formed of a dielectric material and extend along the gate electrode on a source and drain side thereof. A protective dielectric layer overlies the gate electrode, and a contact extends to a source or drain region of the transistor, the contact filling a contact hole that partially overlaps the gate electrode. The barrier conductive layer is recessed relative to the sidewalls such that the barrier conductive layer covers lower surfaces of the sidewalls while exposing upper surfaces of the sidewalls. 
         [0007]    In preferred embodiments of the semiconductor device according to the present invention, the protective dielectric layer comprises silicon nitride. 
         [0008]    In preferred embodiments of the semiconductor device according to the present invention, the protective dielectric layer provides stress for a channel region of the transistor. 
         [0009]    In preferred embodiments of the semiconductor device according to the present invention, the barrier conductive layer contains nitrogen. 
         [0010]    In preferred embodiments of the semiconductor device according to the present invention, the transistor is an N-channel transistor and the device further comprises another transistor that is a P-channel transistor, the P-channel transistor also being formed on the semiconductor substrate, and wherein the barrier conductive layer for the N-channel transistor has a thickness that is different from a thickness of the barrier conductive layer for the P-channel transistor. 
         [0011]    In preferred embodiments of the semiconductor device according to the present invention, the metal substituted layer has a silicon concentration in a range from 0.1 to 10 atomic %. 
         [0012]    In preferred embodiments of the semiconductor device according to the present invention, the metal substituted layer comprises at least one of Al, Cu, Ag, Pt and W. 
         [0013]    In preferred embodiments of the semiconductor device according to the present invention, the transistor has a gate length of 20 nm or less. 
         [0014]    In preferred embodiments of the semiconductor device according to the present invention, the transistor has a gate length of 10-20 nm. 
         [0015]    In preferred embodiments of the semiconductor device according to the present invention, the transistor has a gate height of 60 nm or less. 
         [0016]    In preferred embodiments of the semiconductor device according to the present invention, the transistor has a gate height of 40-60 nm. 
         [0017]    In preferred embodiments of the semiconductor device according to the present invention, the gate electrode is recessed relative to the sidewalls such that the gate electrode covers lower surfaces of the sidewalls while exposing upper surfaces of the sidewalls. 
         [0018]    In another aspect, the present invention relates to a method of making a semiconductor device, comprising forming a dielectric layer over a dummy gate electrode overlying a semiconductor substrate, and over side walls adjacent to the dummy gate electrode and source and drain regions adjacent the sidewalls. An upper surface of the dummy gate electrode is exposed and the dummy gate electrode is removed, thereby forming a trench between the sidewalls. Next, a gate dielectric layer, a barrier conductive layer and a silicon layer are formed in the trench. The silicon layer is etched such that an upper surface of the silicon layer is recessed from upper surfaces of the sidewalls, and a metal is deposited on the silicon layer and between the sidewalls, the metal one that is capable of undergoing a substitution reaction with polysilicon. The metal is annealed so as to effect a substitution of the metal for the silicon layer, followed by removal of the silicon layer that is formed overlying the metal following the substitution reaction. A protective dielectric layer is formed overlying the metal and between the sidewalls, the protective dielectric layer protecting the metal during contact hole formation. 
         [0019]    In preferred embodiments of the method according to the present invention, the protective dielectric layer comprises silicon nitride. 
         [0020]    In preferred embodiments of the method according to the present invention, the barrier conductive layer is a nitrogen-containing film. 
         [0021]    In preferred embodiments of the method according to the present invention, an N-channel transistor and a P-channel transistor are formed in a same semiconductor substrate, and a thickness of the barrier conductive layer for the N-channel transistor is different from a thickness of the barrier conductive layer for the P-channel transistor. 
         [0022]    In preferred embodiments of the method according to the present invention, the annealing is conducted at a temperature of 400° C. to 500° C. 
         [0023]    In preferred embodiments of the method according to the present invention, after the annealing step the metal has a silicon content of 0.1 to 10 atomic %. 
         [0024]    In preferred embodiments of the method according to the present invention, the metal is at least one of Al, Cu, Ag, Pt and W. 
         [0025]    In preferred embodiments of the method according to the present invention, the sidewalls are spaced from one another by a length of 20 nm or less. 
         [0026]    In preferred embodiments of the method according to the present invention, the sidewalls are spaced from one another by a length of 10-20 nm. 
         [0027]    In preferred embodiments of the method according to the present invention, the trench has a depth of 60 nm or less. 
         [0028]    In preferred embodiments of the method according to the present invention, the trench has a depth of 40-60 nm or less. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    The invention will be more fully understood from the following detailed description of various non-limiting examples thereof, taken with reference to the accompanying drawings, in which: 
           [0030]      FIG. 1  shows a first stage of processing of a semiconductor device according to an embodiment of the present invention, as well as of a method according to an embodiment of the present invention; 
           [0031]      FIG. 2  shows a subsequent stage of processing of a semiconductor device according to an embodiment of the present invention, as well as of a method according to an embodiment of the present invention; 
           [0032]      FIG. 3  shows a subsequent stage of processing of a semiconductor device according to an embodiment of the present invention, as well as of a method according to an embodiment of the present invention; 
           [0033]      FIG. 4  shows a subsequent stage of processing of a semiconductor device according to an embodiment of the present invention, as well as of a method according to an embodiment of the present invention; 
           [0034]      FIG. 5  shows a subsequent stage of processing of a semiconductor device according to an embodiment of the present invention, as well as of a method according to an embodiment of the present invention; 
           [0035]      FIG. 6  shows a subsequent stage of processing of a semiconductor device according to an embodiment of the present invention, as well as of a method according to an embodiment of the present invention; 
           [0036]      FIG. 7  shows a subsequent stage of processing of a semiconductor device according to an embodiment of the present invention, as well as of a method according to an embodiment of the present invention; 
           [0037]      FIG. 8  shows a subsequent stage of processing of a semiconductor device according to an embodiment of the present invention, as well as of a method according to an embodiment of the present invention; 
           [0038]      FIG. 9  shows a subsequent stage of processing of a semiconductor device according to an embodiment of the present invention, as well as of a method according to an embodiment of the present invention; 
           [0039]      FIG. 10  shows a subsequent stage of processing of a semiconductor device according to an embodiment of the present invention, as well as of a method according to an embodiment of the present invention; 
           [0040]      FIG. 11  shows a subsequent stage of processing of a semiconductor device according to an embodiment of the present invention, as well as of a method according to an embodiment of the present invention; and 
           [0041]      FIG. 12  shows a device according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0042]    In  FIG. 1 , certain conventional aspects of replacement metal gate (RMG) transistor formation have already been performed. In particular, a dummy gate dielectric and a dummy gate electrode were formed over semiconductor substrate  10 , but as these layers have already been removed they are not shown in  FIG. 1 . A pair of side wall spacers  12  was then formed on the substrate  10 , at the both sides of the dummy gate electrode. 
         [0043]    Sidewall spacers  12  may be formed of materials known for that purpose, for example, silicon nitride. Source  14  and drain  16  regions were then formed in substrate  10  by diffusion. Although in this embodiment the source and drain regions  14 ,  16  are recessed within substrate  10  they may in other embodiments of the invention be elevated in relation to substrate  10 . 
         [0044]    Dielectric layer  18 , for example of silicon dioxide, was then formed over the dummy gate electrode, side walls  12 , and source and drain regions  14 ,  16 . An upper surface of the dummy gate electrode was then exposed through dielectric layer  18 , and the dummy gate electrode and dummy gate dielectric layer were removed so as to form trench  20  between side walls  12 . As is known to those skilled in this art, trench  20  is typically elongated in the direction perpendicular to the plane of the page in  FIG. 1 , and the gate that will be formed in that trench may serve plural transistors arrayed along the trench. 
         [0045]    As shown in  FIG. 1 , a gate dielectric layer  22  is then formed, which covers the trench  20 , which is to say the inner surfaces of side walls  12  and the surface of substrate  10  exposed between the side walls  12 . Gate dielectric layer  22  as formed also covers the dielectric film  18 . Gate dielectric layer  22  is preferably a high-k (HK) material that is suitable for current and future generation device dimensions. Examples of suitable high-k gate dielectrics are hafnium-based gate dielectrics, including hafnium silicate materials and dielectrics composed of layers of materials having different chemical compositions. Gate dielectric layer  22  is preferably deposited by atomic layer deposition (ALD) to a thickness in the range of 0.1 to 10 nm. 
         [0046]    Next, as shown in  FIG. 2 , a barrier conductive layer  24  is formed on the gate dielectric layer  22 . Barrier conductive layer  24  is preferably a work function metal such as titanium nitride (TiN). Barrier conductive layer  24  acts as a stopper during the metal-silicon substitution reaction that will be described hereinafter. Barrier conductive layer also permits adjusting the threshold voltage of the transistor. For example, when N-channel and P-channel transistors are both formed on substrate  10 , then it is preferred that barrier conductive layer  24  have a different thickness in the N-channel transistors than it does in the P-channel transistors, in particular a greater thickness for the PMOS devices than for the NMOS devices. This permits the different threshold voltages that are necessary for PMOS and NMOS devices to be achieved solely by varying the thickness of the barrier conductive layer. 
         [0047]    Barrier conductive layer  24  may be formed for example by chemical vapor deposition (CVD) or by atomic layer deposition (ALD), and is preferably formed to a thickness in the range from 0.1 to 10 nm. 
         [0048]    Next, as shown in  FIG. 3 , trench  20  is filled with polysilicon  26 , for example deposited by CVD, and excess polysilicon  26  along with layers  22  and  24  are removed from dielectric layer  18  and from the upper surfaces of sidewalls  12 , for example by chemical mechanical polishing (CMP). Thus, polysilicon layer and layers  22  and  24  remain only within the trench  20  that has now been filled by those layers. 
         [0049]    Then, as shown in  FIG. 4 , the upper part of polysilicon layer  26  is removed, such that the remaining polysilicon is recessed relative to sidewalls  12  and layers  22  and  24  that remain covering the inner surfaces of those sidewalls. A shallower trench  28  is thereby created, defined between sidewalls  12  and from below by polysilicon layer  26 . 
         [0050]    This partial removal of polysilicon  26  is preferably performed by reactive ion etching (RIE). As those skilled the art are aware, polysilicon is relatively easy to remove by RIE, in contrast to, for example, aluminum, and this is a significant benefit of the devices and methods according to preferred embodiments of the present invention. 
         [0051]    Next, as shown in  FIG. 5 , a layer of metal  30  is deposited for example by sputtering or CVD so as to fill the trench  30 . Excess metal  30  will in practice typically also cover dielectric layer  18  and the top of sidewalls  12 , in which case the excess metal  30  is removed for example by CMP, such that the metal  30  remains only between the sidewalls  12 , as shown in  FIG. 6 . 
         [0052]    Metal  30  is a metal that is capable of undergoing a substitution reaction with silicon, especially polysilicon, under the influence of heat. Examples of such materials include aluminum, copper, silver, platinum and tungsten. Mixtures of such metals may also be used. Aluminum is most preferred among such metals at present. 
         [0053]    The structure depicted in  FIG. 6  is then subjected to annealing at a temperature in the range of 400° C. to 500° C., and more preferably 400° C. to 450° C. Such annealing causes the aluminum  30  and polysilicon  26  to undergo a substitution reaction as is described for example by Fukuda et. al., “A New Aluminum Pattern Formation Using Substitution Reaction of Aluminum for Polysilicon and Its Application to MOS Device Fabrication,”  IEEE Trans. Electr. Dev.,  vol. ED-31, no. 6, pp. 828-832 (June 1984). 
         [0054]    The replacement of polysilicon by aluminum and vice-versa will be nearly complete in such a substitution reaction; however, a small residual quantity of silicon will remain in the now underlying aluminum layer, and may serve as a forensic indicator of the substitution reaction having been performed. In particular, the metal layer  30  after it is relocated by the substitution reaction preferably has a silicon concentration in a range from 0.1 to 10 atomic %. 
         [0055]    As shown in  FIG. 8 , the relocated polysilicon layer  26  is then removed from above the metal layer  30 , to create a new trench  32 . Polysilicon layer  26  is preferably removed by RIP, which again is a significant benefit in relation to conventional techniques in view of the comparative ease with which polysilicon can be removed by RIE. It will be noted that the RIE serves also to remove the portions of barrier conductive layer  24  that had been above the metal layer  30 , such that the barrier conductive layer  24  is now recessed relative to sidewalls  12  to approximately the same extent as is metal layer  30 . High-k layer  22  on the other hand is less susceptible to RIE and therefore remains covering all or substantially all of the inner surfaces of sidewalls  12 . 
         [0056]    Turning now to  FIG. 9 , the newly-formed trench  32  is next filled with a protective dielectric layer  34 , which in this embodiment is silicon nitride (SiN), the same material utilized for side walls  12 . In  FIG. 9  the protective dielectric layer  34  is shown only between the sidewalls  12 , indicating that excess dielectric material deposited on the layer  18  and sidewall  12  upper surfaces has been removed by CMP. 
         [0057]    Next, as shown in  FIG. 10 , an interlayer dielectric layer  36  is formed, which may be for example silicon dioxide. Then, as shown in  FIG. 11 , a contact hole  38  is formed by RIE of layers  36  and  18 , so as to expose the drain region  16 . Although contact hole  38  in this embodiment partially overlaps the gate electrode, the contact hole  38  nevertheless does not expose the metal layer  30 , thanks to the protective layer  34  that is not attacked by RIE. 
         [0058]    Lastly, as shown in  FIG. 12 , contact hole  38  is filled with a conductive material to form a drain contact  40 . In this embodiment, contact  40  is copper and is formed by a damascene technique in which the larger pattern of interconnects is formed using trenches and vias. Contact  40  is preferably although not necessarily a material that is different from that of the metal layer  30 . In this embodiment, metal layer  30  is aluminum and contact  40  is copper. Contact  40  could alternatively be formed, for example, of tungsten. 
         [0059]      FIG. 12  also illustrates that the devices and methods of the present invention provide an advantage to the formation of N-channel and P-channel transistors on a common semiconductor substrate  10 . In  FIG. 12 , the substrate  10  is suitably doped so as to have a P-channel region  42  and an N-channel region  44 , such that the left-side transistor in  FIG. 12  is a PMOS transistor and the right side transistor in  FIG. 12  is an NMOS transistor. As is known to those skilled in the art, different threshold voltages are necessary for PMOS and NMOS devices. In  FIG. 12 , barrier conductive layer  24 - 1  for the PMOS transistor is formed to a greater thickness than barrier conductive layer  24 - 2  for the NMOS transistor. This permits the different threshold voltages that are necessary for PMOS and NMOS devices to be achieved solely by varying the thickness of the barrier conductive layer. 
         [0060]    As discussed above, the embodiments and examples discussed herein are non-limiting, and various constitutions other than those described above can also be adopted. 
         [0061]    For example, amorphous silicon may be used instead of polysilicon. Amorphous silicon is advantageous for certain applications as it can be formed at lower temperatures than polysilicon, which can provide better gap fill characteristics and/or prevent possible variations in the threshold voltage of the transistors. 
         [0062]    It is furthermore apparent that the present invention may be variously modified without departing from the scope and spirit of the present invention as set forth in the accompanying claims.