Abstract:
A contact between a source/drain and a gate is made by making a selected portion of the gate dielectric conductive by an implant into that selected portion of the gate dielectric. The gate material is in a layer over the entire integrated circuit. Areas where gates are to connect to source/drains are indentified and the gate dielectric at those identified locations is implanted to make it conductive. The source/drains are formed so that they extend under these areas of conductive gate dielectric so that at these locations the implanted gate dielectric shorts the gate to the source/drain. This saves area on the integrated circuit, reduces the need for interconnect layers, and avoids the problems associated with depositing and etching polysilicon on an exposed silicon substrate.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to integrated circuits and more particularly to making contact between interconnect and substrate. 
     2. Related Art 
     Integrated circuits comprise transistors formed in active regions that are interconnected by interconnect layers. Typically, these interconnect layers are polysilicon or metal that are made in layers above the substrate. Active regions are formed in the substrate itself. A relatively simple connection that is commonly required in the integrated circuit is a source/drain of one transistor connected to a gate of another transistor. Typically, this occurs by providing an interconnect layer for making the connection from a layer that is above the gate. This requires area on the integrated circuit and thus is a factor in the overall size of the integrated circuit. Another type of contact that has been utilized is known as a buried contact in which polysilicon, which is the typical gate material, is in direct contact with the substrate for making contact between a source/drain and gate. 
     One of the difficulties and problems associated with buried contacts is that the substrate tends to be excessively etched in the area immediately adjacent to or at the edge of the polysilicon that was making contact to the substrate. This occurs because the opening to the substrate, and thus to the source/drain, must be made before the polysilicon is deposited. Thus the etch of the polysilicon that is used for the contact can create a substrate overetch problem. 
     Thus there is a need for contacts that do not require as much space as those of upper level interconnect and do not have the problems associated with buried contacts. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram of a circuit known in the prior art; 
     FIG. 2 is a top view of the circuit of FIG. 1 made according to an embodiment of the present invention; 
     FIGS. 3-6 are sequential cross-sections in processing of a portion of the circuit of FIG. 2; and 
     FIGS. 7-9 are sequential cross-sections in processing of another portion of the circuit of FIG.  2 . 
    
    
     DESCRIPTION OF THE INVENTION 
     A standard connection of a drain to a gate is achieved by an implant into gate dielectric material to achieve a connection between a source/drain formed in a substrate and a gate of another transistor. The connection between the gate material and the substrate, and thus source/drain, is achieved by implanting into the gate material in that area so that the gate dielectric becomes conductive in that location. This is better understood by reference to the figures and the following description. 
     Shown in FIG. 1 is a circuit  10  that is known in the prior art and is very common in integrated circuits but which is made differently than that of the prior art and configured differently than that of the prior art. Circuit  10  comprises a transistor  12  and a transistor  14 . Transistor  12  has a gate  16 , a source/drain  18 , and a source/drain  20 . As used herein a source/drain is the doped region adjacent to a gate that can be a source or a drain, depending how it is used in a given circuit. Transistor  14  has a source/drain  22  and a source/drain  24 . Source/drains  18 ,  22  and  24  are shown as not being connected but in a completed circuit would have connections to either other circuitry or to power supply terminals. Similarly, gate  16  would be connected to other circuitry or a reference or signal. Source/drain  20  is connected to gate  26  of transistor  14 . A source/drain to gate connection is very common in integrated circuits. 
     Shown in FIG. 2 is circuit  10  in a layout according to an embodiment of the invention. Circuit  10  comprises an active region  28  and an active region  30 . Active region  28  is for forming transistor  12  and active region  30  is for forming transistor  14 . Gate  26  overlies active region  30  and is connected to source/drain region  20  of transistor  14  at a contact  32 . Active region  28  has gate  16  overlying it and contains source/drain regions  18  and  20 . Active region  30  contains source/drain regions  22  and  24 . Contact  32  is achieved by having at least a portion of that region implanted so that gate dielectric material becomes conductive in that area. Thus, gate  26 , which extends over active region  18 , makes contact with drain  20  through contact  32 . 
     Shown in FIG. 3 is a cross-section showing transistors  12  and  14  at a relatively early stage in processing. This stage in processing is conventional. Circuit  10  comprises a substrate  38 , which is P-, having a P-well  34  and an N-well  36 . Substrate  38  is shown as having conductive material  50  over P-well  34  and N-well  36  and overlying gate dielectric material  46  and  48 . P-well  34  is between isolation regions  40  and  42 . N-well  36  is between isolation regions  42  and  44  as shown in FIG.  3 . Overlying FIG. 3 between isolation regions  40  and  42  is gate dielectric material  46  and between isolation regions  42  and  44  is gate dielectric material  48 . Overlying gate dielectric material  46  and  48  is polysilicon layer  50 . Substrate  38  may also be silicon on insulator (SOI) in which case there would be a semiconductor layer over an in insulator. Wells  34  and  36  would be in the semiconductor layer and isolation regions  40 ,  42 , and  44  would extend to the insulator. 
     Shown in FIG. 4 is circuit  10  after photoresist  52  has been deposited and patterned so that there is an opening at a location for contact  32  in active region  28 . This shows an implant that has its center depth at dielectric material  46  in contact area  32 . The result is a doped region  33  in P-well  34 , an implanted gate dielectric  35 , and an implanted polysilicon region  37 . An effective dopant material is boron for the purpose of causing dielectric material  46  at contact  32  to be highly conductive. Gate dielectric material may be other materials and instead of polysilicon the gate electrode may be formed of other materials as well. In such cases it may be desirable to provide a different dopant than boron to form the needed conduction and short circuit between the gate dielectric material and the overlying gate material. One such dopant may be aluminum, and another may be phosphorus. It may be advantageous to use phosphorus, which causes N-type conductivity, when implanting into the P-well. A gate dielectric material other than silicon oxide may be hafnium oxide. Other materials may be chosen as well. Thus, a short circuit is formed to the polysilicon without the polysilicon having to have been etched prior to forming the short circuit. In conventional buried contacts it is necessary to open the well or area to be contacted followed by a polysilicon deposition. The subsequent etching is not only cumbersome in terms of the sequence but also can form the etched out areas of the substrate adjacent to the area where the polysilicon is etched after the substrate has been exposed. 
     Shown in FIG. 5 is polysilicon  50  and gate materials  46  after a patterned etch to leave gate  16  over gate dielectric  54  formed of gate dielectric material  46 . Also, a source/drain extension  56  and a source/drain extension  58  are formed using gate  16  as a mask. Source/drain extensions  56  and  58  are N-type formed by implanting a combination of arsenic and phosphorus. The particular species and combination of species for implanting may vary as needed. Also left is gate  26  that extends to contact area  32 . Thus, gate  26  overlaps the region of gate dielectric material that was implanted. The portion, of the gate dielectric that was implanted is the portion that ultimately forms the contact between gate  26  and drain  20  so some of this implanted gate material must remain after the etch of polysilicon  50 . It may be desirable to have a combination boron and indium halo implant before extensions  56  and  58 . This halo implant is not shown because halo implants are well understood in the art and for not unnecessarily complicating the drawings. 
     Shown in FIG. 6 is a circuit  10  after sidewall formation and a combination arsenic and phosphorus implant to form heavily doped source/drain regions  60  and  62 . Gate  16  has sidewall spacers  64  and  66 . Gate  26  has sidewall spacers  68  and  70 . This shows a completed transistor  12  of the N-channel type and completed transistor  14  of the P-channel type. Gate  26  is connected to drain  20  at contact  32 . Drain  20  is made up of heavily doped region  62  and extension region  58 . If contact area  32  is implanted with phosphorus, the contact area may extend past source/drain extension  58  as shown in FIG.  6 . If, however, contact area  32  is doped with boron, which forms P-type regions, then contact area  32  should not extend laterally past source/drain extension  58 . The doped region of the gate dielectric material is conductive and thus will make contact with the doped region under it. If this doped gate dielectric is only in contact with the source/drain, then it will only short gate  26  to drain  20  and not to well  34 . If the doped portion of the gate dielectric extends laterally past drain  20 , then it should be in contact with a doped region of a conductivity type opposite to that of well  34  and thus the implant of the gate dielectric as shown in FIG. 6 should form N-type regions. 
     Shown in FIG. 7 is transistor  14  at the same stage in processing as that shown in FIG.  3 . In this cross section, FIG. 7 shows N-well  36  and isolation regions  70  and  72  as well as gate dielectric material  48 . 
     Shown in FIG. 8 is transistor  14  after polysilicon  50  has been etched to form gate  74  and source/drain extensions  76  and  78 . These extensions are formed in a manner analogous to that shown in FIG. 5 but at a different process step because the doping type is different for transistor  12  than for transistor  14 . These source/drain extensions are formed by implanting a combination of arsenic and boron. The steps required for FIG. 8 may be either before or after those for FIG.  5 . 
     Shown in FIG. 9 is a completed transistor  14  after formation of sidewall spacers  80  and  82  that provide a mask for formation of heavily doped source/drain regions  84  and  86  to complete formation of source/drain region  22  and source/drain region  24  formed by implants using a combination of boron. 
     Thus is it seen that a completed circuit  10 , as shown in cross-sections  3 - 6  and cross-sections  7 - 9 , show a completed transistor  12  and a completed transistor  14  having a drain of transistor  12  connected to the gate of transistor  14  without having to apply a conductive material over a source/drain in order to make that contact. This avoids the problems associated with buried contacts and provides very compact interconnect between the drain of one transistor and the gate of another. This has the effect of reducing the need for higher level interconnect thus reducing complexity of processing and opportunities for defects. Thus, by having the reduced area, the chip area is reduced overall providing an opportunity to provide a lower cost without significant process complications. 
     Also a similar benefit may be achieved by a circuit feature other than a transistor such as a resistor. Another circuit feature may have an electrode, such as polysilicon gate  26 , extending over a source/drain and making electrical contact thereto by way of a conductive implanted dielectric such as region  35  shown in FIGS. 4,  5 , and  6 . 
     Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature or element of any or all the claims. As used herein, the terms “comprises,” “comprising,” or any other, variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus.