Abstract:
A non-critical block mask exposes one of the source and drain in an SOI FET, which is implanted with a leakage implant that increases the leakage in the exposed element, thus providing a conductive path to draw away holes from the transistor body.

Description:
FIELD OF THE INVENTION 
     The field of the invention is that of forming a body contact in SOI integrated circuits. 
     BACKGROUND OF THE INVENTION 
     The need for a body contact in SOI FETs is well known. Many schemes have been proposed to provide a conductive path to ground to draw holes away from the transistor body. A straightforward approach is to increase the active area within the isolation dielectric to provide room to place a contact on the surface and an implant below the surface to provide a low-resistance path from the body to the contact. Such an approach, of course, takes up valuable silicon area. 
     Additionally, as the silicon device layer becomes thinner, it becomes increasingly more difficult to contact the body without incurring a large series resistance in the traditional approach. 
     SUMMARY OF THE INVENTION 
     The invention relates to a body contact that employs a leaky p-n junction (diode) in one of the source and drain, so that a conductive path is formed from the body through the leaky p-n junction to the transistor terminal. The other p-n junction in the FET has standard properties, so that there is no excessive leakage through the transistor. 
     A feature of the invention is the use of a non-critical block mask for one or more leakage implants, together with an angled leakage implant that penetrates under the gate to deliver a higher concentration of leakage ions at the p-n junction. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows in cross section a transistor being implanted according to the invention. 
     FIG. 2 shows in cross section a transistor being implanted in a way that avoids implants where they are not wanted. 
     FIG. 3 shows in cross section a transistor being implanted with a misaligned mask. 
     FIG. 4 shows a plan view of a set of transistors oriented at different angles. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1, there is shown in cross section a transistor constructed according to the invention. A conventional substrate  10  with an insulator layer  20 , illustratively a buried oxide (BOX) layer formed by implantation of oxygen into substrate  10  supports silicon device layer  100 . Device layer  100  contains an NFET including source  112 , drain  114  on either side of body  116 , the body being below gate  110 . A p-n source junction  115  is formed between the N+ source  112  and p-type body  116 . The transistor is formed by conventional processes. The transistor is surrounded by dielectric isolation  40 . 
     A non-critical blocking mask  50  has been put down and patterned, illustratively forming an aperture having one edge on the gate and the other over the isolation. 
     An ion implant is shown as being implanted at an angle, so that a higher concentration of ions reaches junction  115  than would be the case if the implant were vertical. Illustratively, the ion species may be Indium, Germanium, Carbon or other implanted species. The term “leakage implant” will be used herein to mean an implant the primary effect of which is to increase the leakage current across the p-n junction. Boron or Phosphorous, for example, would not normally be leakage implants because they primarily change the characteristics of the p-n junction. 
     A typical dose would be in the range of 1×10 12 /cm 2  to 1×10 15 /cm 2 . The voltage will be set according to the thickness of the device layer and the implant species, typically in the range from about 5 to 80 keV. It is an advantageous feature of the invention that the body tie extends along the full length of the source, thus providing low resistance without any area penalty. For convenience, FIG. 1 will be referred to as looking North, so the implant is coming in from the East. The leakage implant is preferably not annealed for long periods of time or at high temperatures. 
     Referring to FIG. 2, there is shown a case where the implant comes from the other side of the transistor (the West, where the same North-looking orientation is assumed). In that case, the resist and/or gate blocks the ions, so that the area close to the gate edge does not receive a direct implant. Those skilled in the art will appreciate that, when the implant dose is set to apply an optimum ion concentration to sources exposed as in FIG. 1, the embodiment of FIG. 2 will not receive an adequate dose. 
     Referring to FIG. 4, there is shown a plan view of a portion of a circuit. In an area denoted with the numeral 200, there are six transistors oriented in three different directions. Transistor  110 , referred to as being disposed along a first axis, is oriented along the E-W direction, with source  112  on the East. Transistor  120 , referred to as being disposed along a second axis perpendicular to the first axis, is oriented along the N-S direction, with source  112  on the North. Transistor  130 , referred to as being disposed along a third axis at an acute angle with respect to the first axis, is oriented along a NE-SW direction, with source  132  on the North-East end. Transistors  110 ′,  120 ′ and  130 ′ are the complementary set, aligned along the same axes, but in the opposite sense. If the circuit designer has chosen to have some E-W transistors with the source on the East and also some with the source on the West, then implants from both directions will be required to cover both the set and the complementary set. 
     Referring now to FIG. 3, there is shown a transistor and implant as in FIG. 2, but with a gap  36  between the mask and the gate. With the implant orientation shown, the area within gap  36  will not be significantly implanted because of the shadowing effect of mask  50 . The same applies if the implant is oriented as in FIG. 1, because of shadowing by the gate. If the implant comes from the North or South, however, then a significant number of ions may be implanted, depending on the width of the gap, the magnitude of the dose and the ease of diffusion of the ions. Thus, the mask alignment of FIG. 1 or  2  is preferable. 
     In the most general case, there will be six implant orientations for the cases illustrated in FIG.  4 . There need be only one mask, since the total dose is the sum of all the angled implants. 
     The invention applies as well to PFETs. In that case, the drain receives the leakage implant. The ions used are typically the same species for both NFETs and PFETs, but this does not have to be the case. If different ions are used for NFETs and PFETs, then there will be appropriate changes in the number and locations of masks. A CMOS circuit will have both NFETs and PFETS with this leakage implant. 
     The invention may be practiced with bonded SOI wafers and with SiGe substrates, as well as with implanted wafers and silicon substrates. While the invention has been described in terms of a single preferred embodiment, those skilled in the art will recognize that the invention can be practiced in various versions within the spirit and scope of the following claims.