Abstract:
A method including forming a transistor device having a channel region; implanting a first halo into the channel region; and implanting a second different halo into the channel region. An apparatus including a gate electrode formed on a substrate; a channel region formed in the substrate below the gate electrode and between contact points; a first halo implant comprising a first species in the channel region; and a second halo implant including a different second species in the channel region.

Description:
BACKGROUND  
         [0001]    1. Field  
           [0002]    Circuit devices and methods for forming circuit devices.  
           [0003]    2. Background  
           [0004]    The field effect transistor (FET) is a common element of an integrated circuit such as a multiprocessor or other circuit. The transistor typically includes a source and drain junction region formed in a semiconductor substrate and a gate electrode formed on a surface of the substrate. The gate length is generally the distance between the source and drain junction region. Within the substrate, the region of the substrate beneath the gate electrode and between the source and drain junctions is generally referred to as a channel with a channel length being the distance between the source and drain junctions.  
           [0005]    As noted above, many transistor devices are formed in a semiconductor substrate. To improve the conductivity of the semiconductor material of the substrate, dopants are introduced (e.g., implanted) into the substrate. Representatively, an N-type transistor device may have source and drain region (and gate electrode) doped with an N-type dopant such as arsenic. The N-type junction regions are formed in a well that has previously been formed as a P-type conductivity. A suitable P-type dopant is boron.  
           [0006]    A transistor device works generally in the following way. Carriers (e.g., electrons, holes) flow between source junction and drain junction by the establishment of contacts on the substrate to the source and drain junction. In order to establish the carrier flow, a sufficient voltage must be applied to the gate electrode to form an inversion layer of carriers in the channel. This minimum amount of voltage is generally referred to as a threshold voltage (V t ).  
           [0007]    In general, when fabricating multiple transistors of similar size, it is desired that a performance characteristic like threshold voltage be similar between devices. In general, the threshold voltage tends to decrease in response to reduced gate length. Of course, performance is often dictated by a reduction in transistor size (e.g., faster switching, more devices on a chip, etc.) that dominates the objectives of the semiconductor processing industry. As gate electrode lengths approach dimensions less than 100 nanometers (nm), what is seen is that the threshold voltage drops off or decreases rapidly. Therefore, even a small change in the gate electrode length (e.g., a 10 nanometer difference from a targeted length), can significantly affect the threshold voltage.  
           [0008]    Ideally, the threshold voltage should be constant over a range of gate lengths about a target gate length to account for manufacturing margins. To, in one aspect, promote a more constant threshold voltage over a range of acceptable gate lengths, locally implanted dopants (P-type in N-type metal oxide semiconductor FETS (NMOSFETS) and N-type dopants in P-type metal oxide semiconductor FETS (PMOSFETS) may be introduced under the gate edges. Such implants are referred to as “halo” implants. The implanted dopant tends to raise the doping concentration around the edges of the channel, thereby increasing the threshold voltage. One effect is to reduce the threshold voltage of the target size device while maintaining the threshold voltage of the worst case size device.  
           [0009]    Typical halo implants for NMOSFETS include boron (e.g., boron fluoride (BF 2 )) and indium (In). Halo implants for PMOSFETS include arsenic, antimony, and phosphorous. With respect to NMOSFETS, indium is a particularly preferred dopant because the channel of indium forms a retrograde profile from the surface of the device. Such a concentration profile with respect to indium, tends to decrease the threshold voltage required to meet a given leakage current (I off ) in the device relative to a boron dopant which does not have the same retrograde profile. One problem with indium is that indium achieves a state of solid solubility at a point below the concentration required to reach worst case leakage currents. Thus, to target small leakage currents (e.g., on the order of 40 nanoamps (na) at device sizes less than 100 nanometers (nm)), a halo implant of an indium species alone cannot reach such targets. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    The features, aspects, and advantages of the invention will become more thoroughly apparent from the following detailed description, appended claims, and accompanying drawings in which:  
         [0011]    [0011]FIG. 1 shows a cross-sectional side view of a portion of a circuit substrate including a transistor device having a first halo implant.  
         [0012]    [0012]FIG. 2 shows the device of FIG. 1 following a second halo implant.  
         [0013]    [0013]FIG. 3 shows a graphical representation of halo concentration in a substrate versus gate length for a selected leakage current.  
         [0014]    [0014]FIG. 4 shows the dopant concentration for P-type dopants in a silicon substrate.  
         [0015]    [0015]FIG. 5 shows a representative graph of threshold voltage versus P-type dopant concentration for a silicon substrate.  
         [0016]    [0016]FIG. 6 shows a graphical representation of leakage current versus threshold voltage for P-type dopants.  
         [0017]    [0017]FIG. 7 shows a graphical representation of threshold voltage versus gate length for an NMOSFET device.  
         [0018]    [0018]FIG. 8 shows a graphical representation of leakage current versus gate length for an NMOSFET.  
         [0019]    [0019]FIG. 9 shows a graphical representation of a number of devices on a substrate versus gate length.  
         [0020]    [0020]FIG. 10 shows a graphical representation of drive current versus gate length for a number of devices on a substrate.  
         [0021]    [0021]FIG. 11 shows a graphical representation of drive current versus leakage current for a transistor device. 
     
    
     DETAILED DESCRIPTION  
       [0022]    As noted above, indium is a preferred NMOSFET channel dopant (e.g., halo dopant) because its retrograde concentration profile results in lower threshold voltages and improved drive currents. However, for smaller devices, such as devices with target gate length at 60 nanometers or less, indium alone as a halo dopant is unacceptable because its solid solubility limit tends to prevent indium from doping an NMOSFET channel to a high enough level to maintain reasonable worst-case leakage currents.  
         [0023]    [0023]FIG. 1 shows a cross-sectional side view of a portion of a circuit substrate having a transistor device formed thereon. Structure  100  includes substrate  110  of, for example, a semiconductor material, representatively silicon. Formed in and on substrate  110  in FIG. 1 is a transistor device. Representatively, the transistor device is an NMOSFET, formed in P-type well  120 . The transistor device includes gate electrode  130  formed on the surface of substrate  110  having gate length  170 . The transistor device also includes source junction  140  and drain junction  150 . In an NMOSFET, source junction  140  and drain junction  150  are both N-type as typically is gate electrode  130 . Source junction  140  includes tip implant  145  formed, for example, as self-aligned to gate electrode  130  (by an implant prior to the formation of spacer portions  135 ). The bulk of source junction  140  is aligned to spacer portions  135  on gate electrode  130  (by an implant after spacer portions  135  are formed). Similarly, drain junction  150  includes tip implant  155  substantially aligned to gate electrode  130  (e.g., a lightly-doped drain). The bulk of drain junction  150  is aligned to spacers  135  on gate electrode  130 .  
         [0024]    [0024]FIG. 1 also shows a single halo implant in channel region  160  of substrate  110 . In an embodiment where the transistor device is an NMOSFET, first implant  180  is, for example, indium. Halo implants may be formed by introducing dopant ions, such as indium ions, into substrate  110  at a tilt angle of, for example, 25-30°. One way to introduce first halo  180  is an implanting operation after formation of the gate electrode (but before the spacers) so that the gate electrode acts as an aligned implant mask.  
         [0025]    [0025]FIG. 2 shows the structure of FIG. 1 following the introduction of second halo  190 . For an NMOSFET as shown, representatively, where first halo  180  is an indium species, second halo  190  is, for example, a boron species (e.g., boron diflouride). Second halo  190  may be introduced by implantation according to a similar technique as first halo  180 .  
         [0026]    In the example where first halo  180  is indium, and second halo  190  is boron or similar species, one technique involving multiple halos includes introducing first halo  180  into channel  120  of substrate  110  to a solid solubility of indium for silicon, generally 2E18 cm −3 . Following the implantation of an indium species to the indium solid solubility, a boron species is implanted as second halo  190  in an amount sufficient to achieve a target threshold voltage for a particular gate length device. It is appreciated that, having determined the appropriate amount of indium and boron dopants, the order by which either is introduced may vary.  
         [0027]    In the above embodiment, a first halo (e.g., first halo  180 ) of an indium species is introduced and a second halo (e.g., second halo  190 ) is introduced. Thus, structure  100  includes two halos introduced into channel  120 . The dopants described include indium and boron species. It is appreciated that other species may similarly be suitable for either NMOSFETS or PMOSFETS. In one example, indium is selected and introduced to its solid solubility in the context of reduced gate lengths (e.g., on the order of 70 nanometers or less) to achieve target threshold voltages, leakage currents and drive currents. FIG. 3 shows a graphical representation of halo concentration in a silicon substrate versus gate length for a selected leakage current (I off ) of, for example, 40 nA. FIG. 3 shows that as gate lengths are decreased beyond approximately 100 nm, indium saturates and cannot, alone, achieve the desired leakage current. FIG. 4 representatively shows halo concentration in a silicon substrate. FIG. 3 demonstrates that a concentration required to meet a leakage current requirement (e.g., 3E18 cm −3 ) is greater than the indium solid solubility (e.g., on the order of 2E18 cm −3 ).  
         [0028]    [0028]FIG. 5 shows a graphical representation of threshold voltage versus dopant concentration. FIG. 5 demonstrates that at its solid solubility, indium saturates. FIG. 6 shows the graphical representation of leakage current versus threshold voltage. FIG. 6 demonstrates that at its solid solubility, indium again saturates (e.g., on the order of 100 nanoamps/μm). Thus, with respect to achieving target threshold voltages and target leakage currents, an additional halo implant, in addition to a halo implant including an indium species, is used. As shown in FIG. 5 and FIG. 6, a second halo implant of a boron species may be used to achieve the target threshold voltage and target leakage current.  
         [0029]    Based on the above graphical representations, for example, where leakage current (I off ) for a worst-case gate length device is, for example, 100 nanoamps/μm, where a target gate length is, for example, 60 nanometers, an indium species may be introduced as a first halo to its solid solubility and a second halo of, for example, a boron species, may be introduced until a threshold voltage required to support the leakage current is established.  
         [0030]    [0030]FIGS. 7 and 8 show graphical representations associated with threshold voltages and leakage currents for a particular gate length device. The graphical representations illustrate the manufacturing tolerances associated with fabricating devices, particularly the acceptable variations in gate length. Representatively, for purposes of explanation, a target gate length is 70 nanometers (nm) with a worst-case gate length on the order of −10 nm. A halo implant, as illustrated in FIG. 7, tends to reduce the threshold voltage of the target size device while maintaining the threshold voltage of the worst case size device. However, the leakage current effects for various gate lengths are illustrated in FIG. 8. Representatively, for the prior art indium halo and boron well-type (indium halo/boron well) device, a worst-case gate length leakage current compared to a target gate length device, the difference between worst-case and target is on the order of a factor of 10. Thus, even though a worst-case device may dominate the total leakage current, a multiple halo device such as described tends to reduce the difference between the leakage current for a worst-case device and a target device by, representatively, a factor of two.  
         [0031]    Although a worst-case gate length device tends to dominate leakage current, the target devices tend to dominate drive current. FIGS. 9 and 10 illustrate a multiple halo device such as described above and a prior art single halo/boron well device. FIG. 9 shows a representation of devices formed on a substrate and their corresponding gate length. The devices adopt essentially a bell-shaped curve. FIG. 10 shows a typical drive current for the devices formed, in one case with a indium halo/boron well as in the prior art and, as multiple (indium and boron) halo devices. FIG. 10 shows that the multiple (indium and boron) halo devices tend to have higher drive currents at target gate lengths, because they have higher leakage currents at target gate lengths. FIG. 11 shows a graphical representation of drive current versus leakage current for a transistor device.  
         [0032]    In the preceding detailed description, specific embodiments are illustrated, including a dual halo device of separate indium and boron implants. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the claims. For example, indium and boron implants have been described for an N-type devices (P-type dopants). It is contemplated that other dopants for N-type devices may be introduced in a similar manner (e.g., multiple halo). Alternatively, for P-type devices, it is contemplated that N-type dopants such as arsenic and phosphorous may be introduced in a multiple halo process where effects such as, but not limited to, drive and leakage currents are to be optimized. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.