Abstract:
Transistors having self-aligned dielectric layers under the source/drain contacts are formed by constructing transistors up to the LDD implant; etching STI oxide selective to Si and nitride to form a self-aligned contact recess; depositing an insulating layer in the bottom of the contact recess; recessing the insulating layer to leave room for a conductive contact layer; depositing the contact layer to make contact on a vertical surface to the Si underneath the gate sidewalls; recessing the contact layer; forming interlayer dielectric and interconnect to complete the circuit.

Description:
FIELD OF THE INVENTION 
     The field of the invention is integrated circuit processing, in particular low-capacitance high speed circuits. 
     BACKGROUND OF THE INVENTION 
     It is well known in the field that junction capacitance between sources and drains (S/D) and the substrate is an important limiting factor in circuit performance. Also, leakage current between S/D and substrate results in power consumption without benefit. 
     Silicon on insulator technology has less junction capacitance than bulk technology because the buried insulator reduces the capacitance, but is more expensive. 
     It is desirable to develop a low-capacitance transistor structure for bulk silicon integrated circuits that is economical to manufacture. 
     SUMMARY OF THE INVENTION 
     The invention relates to a transistor structure in which a layer of insulator is placed under the transistor contacts to reduce capacitance between the source/drain and the silicon substrate. 
     A feature of the invention is the formation of a self-aligned contact pad electrically connected to the portion of the source and drain normally found under the sidewall spacers adjacent to the gate sidewalls. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows the result of the inventive process. 
     FIGS. 2-6 show intermediate steps in the process. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 2, there is shown in cross section, a p-type silicon substrate  10  containing an n-well  20 . Another substrate material, such as silicon-germanium (SiGe) may be used. An NFET  100  is to be constructed on the left of the figure, and a PFET  200  is to be constructed on the right. In referring to the two FETs, the same numerals will be used for the same elements in both transistors and numerals differing by 100 (105, 205) will be used for elements that are similar. 
     At the upper left of FIG. 2, shallow trench isolation (STI) members  30  isolate the transistors (and other elements), defining a set of transistor areas that will contain the transistors. In this embodiment, STI members  30  are formed conventionally from deposited oxide, planarized by chemical-mechanical polishing (CMP) to the level of the silicon surface. 
     Transistor  100  includes a gate stack comprising gate electrode  105  above gate dielectric  102 , topped with dielectric (nitride, Si 3 N 4 ) gate cap layer  120 . Conventional sidewalls  110  partly cover a region  115  in which whatever S/D engineering is needed for the devices, e.g. low-doped or extensions, has been implemented. The S/D has undergone the low-level doping step in standard transistor formation before the sidewall formation, but not the high-level doping step. Transistor  200  is the same, except for standard doping changes (source/drains  215  are p + ) and optional change in gate polarity. Up to this point, the processing has been conventional, except that gate cap  120  is about 100 nm of nitride instead of the more typical thickness of about 20-70 nm of oxide. Conductive member  217  on the right has been implanted at the same time as the LDD implant dose that is primarily directed at S/D  115 . In the illustrative case, member  217  will be used as a well contact. Member  217  is also implanted n-type. 
     FIG. 3 shows the result of a first etching step to form a self-aligned recess that will contain the transistor contacts. A Si etching process selective to nitride and oxide (SiO 2 ), illustratively HBr, Cl, or SF 6  chemistry, has etched contact recess apertures  32  on opposite sides of the gate stack down into the Si to an illustrative depth of about 120 nm (for a contact thickness of about 60 nm). Note that the recesses are self-aligned to the gate stack and the STI because of the etch selectivity. During this etching step, interconnect member  217  was protected by conventional resist to prevent etching. Arrow  150  denotes the exposed area that is etched. Advantageously, alignment of the resist is noncritical because the amount of the STI members exposed does not matter. 
     Next, FIG. 4 shows the result of depositing a layer of isolation dielectric  40 , illustratively the same TEOS oxide as is in the STI, in the contact recess apertures and planarizing it in a CMP operation, stopping on the gate cap dielectric  120 . A layer of resist  330  has been deposited and patterned to protect portions of layer  40  above STI  30 . 
     FIG. 5 shows the result after the next step, in which layer  40  has been etched (selective to nitride and Si) down to the substrate and below it, forming contact recesses  32  on opposite sides of each transistor and self-aligned between the transistor and the STI. The contact recesses have a depth such that there is a thick enough layer of oxide  34  to form an isolation member to reduce capacitance, and there is a large enough vertical junction contact area  35  on the vertical wall of the silicon underneath the gate sidewall to provide a good contact between the lightly doped (LDD) portion of the silicon under the sidewall ( 115 ,  215 ) and the junction contact member that will be deposited in the contact recess. Optionally, a conventional cleaning step plus a light oxide etch can be used to ensure provide good contact between the contact and the LDD source/drain. When this etching step is carried out between two diffusion regions separated by STI, it provides a recess region in the STI that will subsequently be filled with conductive material, providing a local interconnect between diffusions. 
     Next, in FIG. 6 is shown the result of depositing a conductive contact material  50  such as polysilicon (or amorphous Si , or low temperature, ultra-high vacuum SiGe), performing a CMP to the level of the gate cap and etching the deposited material down to slightly above the STI surface. The result is a set of contact members  142 ,  242  that are electrically connected to the remainder of S/D  115  and  215  and are isolated from the substrate by the oxide isolation members  34 . This etch should be selective to oxide and nitride, illustratively HBR, Cl or SF 6  chemistry. The amount of conductive material  50  remaining above the nominal STI surface will be sufficient to allow for process variation and to leave a layer embedded in the STI regions to provide an interconnect. The nitride gate cap can be removed to allow doping of the gate electrode in the next step. This could result in spacer removal, which may require a second spacer formation process prior to silicide formation. 
     The contact members  42 , interconnect and well contact  217  (and potential gate electrode) are implanted with an N +  or p +  dose as in conventional S/D formation. The nitride gate cap  120  is stripped (which may require a later spacer formation step) and the exposed silicon (the top of the gate electrode, the contact members and the interconnect and well contacts) are silicided in an optional step. 
     Referring now to FIG. 1, there is shown the final structure, in which transistor  100  has gate  105  over gate dielectric  102 , sidewalls  110 , silicide cap  55 , reduced S/D  115  and contact members  142  covered with silicide  55 . The reduced S/D members are directly over the silicon substrate, but the contact members are placed above dielectric. Both the contact members and the silicide on top of them fill the space between the sidewalls and the STI. Transistor  200  has gate  105  over gate dielectric  102 , sidewalls  110 , silicide cap  55 , S/D  215  and contact members  242  covered with silicide  55 . Contact members  142  and  242  are structurally equivalent to raised sources and drains, with the added benefit of reduced capacitance and leakage to the substrate. 
     Conventional processing then completes the circuit by adding interconnects to other devices and interlayer dielectrics, denoted schematically by a box labeled  300 . 
     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. Various materials can be used for the substrate and for the conductive materials. Not all the polishing steps are required and some designers may decide to eliminate one or more of them, using etching to planarize and recess. The invention can be practiced on SOI substrates where the upper semiconductor device layer is required for some reason to be thicker than the depth of the sources and drains.