PATENT ABSTRACT
A deep, buried n -  channel blanket implant beneath both n -  channel and p-channel devices in MOS integrated circuits, whether complementary MOS (CMOS) or not. It is known to use deep, lightly-doped n -  channel implant to improve the characteristics of p-channel (PMOS) devices, although one skilled in the art would expect such an n -  implant to be detrimental to n-channel (NMOS) devices. It has been discovered that such implants not only do not degrade the NMOS devices, but in fact improve their performance, with respect to body effect and junction capacitance.

PATENT DESCRIPTION
FIELD OF THE INVENTION 
     The invention relates to the fabrication of NMOS transistors, and, more particularly, relates to the use of buried doped implants to enhance the properties of and simplify the fabrication of NMOS transistors particularly when used in complementary MOS (CMOS) integrated circuits. 
     BACKGROUND OF THE INVENTION 
     In the ever-continuing effort to improve the performance of metal-oxide-silicon (MOS) devices it has been discovered that the current leakage properties of a short p-channel MOS (PMOS) transistor may be improved by introducing a deep arsenic implant underneath the shallow boron channel implant. See, for example, S. Chiang, et al. &#34;Optimization of Sub-micron p-Channel FET Structure,&#34; IEEE International Electronic Devices Meeting Papers, Vol. 24.6 1983, pp. 534-537. The use of a deep, n -   channel implant under p-channel devices does not cause a process sequence problem if all of the devices on the integrated circuit under construction are PMOS, however, if this technique were used to enhance the characteristics of p-channel devices in CMOS, one skilled in the art would expect that a mask step would be necessary to prevent the deep, buried n -   channel implant from appearing under the n-channel devices present. The expectation is that an n -   channel implant under an NMOS device would create punchthrough problems. 
     It would be desirable to discover a process by which the deep, buried n -   channel implant of PMOS devices could be applied to CMOS integrated circuits without the addition of an extra mask step. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a technique by which deep, n -   channel implant may be used to enhance NMOS devices whether used in an NMOS integrated circuit or a CMOS integrated circuit. 
     Another object of the invention is to provide a means by which a deep, n -   channel buried implant may be employed on CMOS transistors without the need for an extra mask step. 
     In carrying out these and other objects of the present invention, there is provided, in one form an n-channel MOS device made by performing a deep, lightly-doped n -   donor implant in a p acceptor doped region, then forming a thin gate oxide layer, performing boron channel implants, subsequently forming a heavily-doped n +   donor gate, and finally implanting heavily doped n +   donor source/drain regions. Optionally, the gate may be formed in two steps, one before and one after the boron channel implants. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-section of an n-channel MOS transistor under construction showing the blanket implantation of the deep, buried n -   implant through a sacrificial oxide layer; and 
     FIG. 2 is a cross-section of the n-channel MOS transistor of FIG. 1 in completed form. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     It has been surprisingly discovered that a deep, buried n -   channel implant beneath an NMOS device does not degrade transistor performance, particularly with regard to punch-through, and in fact benefits the NMOS device characteristics. It is expected that the principal application of this technique is in a CMOS process flow, where both NMOS and PMOS transistors are present. By virtue of this discovery, a mask need not be used in the formation of the deep n -   implant as it may be laid beneath both n-channel and p-channel devices in a CMOS integrated circuit. 
     Shown in FIG. 1 is an NMOS transistor 10 under construction. Fabrication taken place in a p-doped region 12, which can either be a p substrate or a p-well in an n-doped substrate; the present invention would work well in either situation. The deep n -   implant 14 is introduced into the channel region of both n- and p-channel transistors. This implant 14 is formed by the blanket or unmasked ion implantation of arsenic or phosphorus. The implant conducted as shown in FIG. 1 compensates back the p-type region 12 and p-channel region 17 with n-type dopant but does not bring them to a fully compensated or n -   state. The exact depth of the n -   implant is not critical; for example, it may be positioned at approximately 2000 angstroms below the surface as in the example described below. Preferably, the peak of the deep n -   channel implant 14 is at or below the n +  /p junction of the eventual source 20 and drain 22 regions, as seen in FIG. 2. This implant 14 is dependent upon the other device parameters for its exact depth placement. The downward pointing arrows in FIG. 1 indicate the ion implantation step. 
     Since the implant must be done at fairly high energies, it may be preferred to implant through a thin implant screen, which in many cases will be silicon dioxide as represented at 16 in FIG. 1. Typically, this implant screen will be a sacrificial oxide layer 16. In the example, a 400 angstrom layer was used. Its exact thickness is, of course, dependent on the nature of the devices being built. 
     As noted above, it is well known that buried implant 14 improves the punchthrough properties of p-channel devices. But it is surprising to find that implant 14 also improves the performance of the final n-channel transistor 18 shown in FIG. 2. Finished NMOS transistor 18 has an n -  channel implant 14 buried in p-doped region 12, and also possesses p -   channel region 17, source region 20, drain region 22 and gate region 24 on thin dielectric layer 26. For an NMOS transistor 18, source 20, drain 22 and gate 24 are heavily n +   doped regions of silicon. Typically, gate region 24 is polycrystalline silicon (polysilicon) and the thin gate dielectric layer 26 is silicon dioxide, although the invention is not limited by these suggestions of possible materials. 
     As will be shown, both the variations of threshold voltage with substrate bias, called the &#34;body effect&#34;, and n +  /p source/drain junction capacitance are reduced by a factor of 1.5 to 2. The punchthrough properties of the n-channel device exhibit no degradation due to the n -   implant 14. The invention will be further illustrated with reference to the following example. 
     EXAMPLE 1 
     This experiment focused on improving the short channel leakage properties of PMOS devices by introducing a deep arsenic implant below the boron channel implant normally employed. The effects of the blanket arsenic implant on NMOS devices were also studied which led to the discovery of this invention. In an earlier experiment it was found that light doses of arsenic (less than or equal to 1×10 11  atoms/cm 2 ) were insufficient to have much effect. Therefore, relatively heavier doses (3, 5 and 7×10 11 , commonly expressed as 7E11, for example) were used in the present experiment. The arsenic ion implantation was performed through 400 angstroms of sacrificial gate oxide. The boron ion implanation was performed through 500 angstroms of polysilicon and 250 angstroms of gate oxide. The results are summarized in the tables below. 
     
                       TABLE I______________________________________p-Channel ResultsBlanket  Blanket             V.sub.T atB.sup.+  dose  As.sup.+  dose            V.sub.TLCH                      L.sub.eff =                             I.sub.L at L.sub.eff =(30 keV)  (360 keV) (V).sup.a 0.7 um.sup.b                             0.7 um.sup.c______________________________________6.5 E11  3 E11     -1.05 V   -0.53 V                             &gt;1 E-8 A/um  8 E11  5 E11     -1.04 V   -0.57 V                              2.5 E-11 A/um  9 E11  7 E11     -1.05 V   -0.65 V                              5 E-12 A/um4.5 E11  None      -1.00 V   -0.32 V                             &gt;1 E-8 A/um  (control)______________________________________ .sup.a V.sub.BB = 0 V, V.sub.TLCH = Long channel (L = 50 um) threshold voltage. .sup.b V.sub.DS = -5 V, V.sub.BB = 0 V. .sup.c Leakage current at V.sub.DS = -5 V, V.sub.GS = -0.3 V, V.sub.BB = V. 
    
     The results show an improvement in short channel behavior as the arsenic dose is increased. The reduction in threshold voltage V T  with shorter L eff  is minimized and the leakage current is reduced, for devices with L eff  greater than or equal to 0.7 μm. 
     
                                           TABLE II__________________________________________________________________________n-Channel ResultsNet Enh.As.sup.+ V.sub.T at              ΔV.sub.TB.sup.+  DoseDose V.sub.TLCH         L.sub.eff =              0-5 V                  n.sup.+ /p Cj.sup.c30 keV360 keV     (V).sup.a         0.8 um.sup.b              V.sub.BB                  0 V                     5 V                        I.sub.L at L.sub.eff = 0.8__________________________________________________________________________                        um.sup.d 2.9 E127 E11     1.07 V         0.87 V              0.63 V                  0.2                     .073                        3 E-12 A/um2.45 E12None 0.94 V         0.90 V              1.15 V                  0.3                     .10                        3 E-12 A/um(control)__________________________________________________________________________ .sup.a Long channel (L = 50 um) threshold voltage, V.sub.BB = 0 V. .sup.b V.sub.DS = 5 V, V.sub.BB = 0 V. .sup.c Junction capacitance in fF/um.sup.2 .sup.d Leakage current at V.sub.DS = 5 V, V.sub.GS = 0.3 V, V.sub.BB =  0 V 
    
     The results for the n-channel devices show that a substantial reduction in body effect (the change in V TH  as V BB  varies from 0 to -5 V) and junction capacitance (C J ) as compared with the control, may be obtained using a deep n -   implant. Punchthrough effects, indicated by I L , show no degradation.