Abstract:
A method of forming a self-aligned SOI diode, the method comprising depositing a protective structure over a substrate; implanting a plurality of diffusion regions of variable dopant types in an area between at least one pair of isolation regions in the substrate, the plurality of diffusion regions separated by a diode junction, wherein the implanting aligns an upper surface of the diode junction with the protective structure; and removing the protective structure. The method further comprises forming a silicide layer over the diffusion regions and aligned with the protective structure. The protective structure comprises a hard mask, wherein the hard mask comprises a silicon nitride layer. Alternatively, the protective structure comprises a polysilicon gate and insulating spacers on opposite sides of the gate. Furthermore, in the removing step, the spacers remain on the substrate.

Description:
BACKGROUND OF INVENTION 
   1. Field of the Invention 
   The invention generally relates to integrated circuit technologies, and more particularly to silicon on insulator, electrostatic discharge diodes incorporated in field effect transistor structures. 
   2. Description of the Related Art 
   Conventionally, diodes offered in silicon on insulator (SOI) technologies require the use of polysilicon to serve as a self-aligning mask for the anode/cathode and to block silicide formation, preventing shorting between the anode and cathode regions of the device. The standard practice is for the polysilicon gate to remain after processing is completed. However, this approach has two disadvantages; gate oxide breakdown limitations and increased capacitance loading. 
   An example of a conventional diode structure is illustrated in  FIG. 1 , wherein the diode is formed on a single area of silicon over a buried oxide insulator  10  over a substrate  5 , and in between the shallow trench isolation regions (STI)  15 . The polysilicon gate  30  is used to separate the anode and cathode and to provide silicide blocking. The anode connects to the P+ region  20  of the diode, while the cathode connects to the N+ region  27  of the diode. An N− region  25  (often referred to as the n-type body region), which is positioned under the gate  30 , separates the P+  20  and N+  27  regions from one another. A pair of insulating spacers  40  is attached to the sidewalls of the gate  30 , and a layer of silicide  35  is deposited over the upper surfaces of the gate  30 , P+ region  20 , and N+ region  27 . 
   A typical diode-based ESD protection circuit is provided in  FIG. 2 , with the diode structures shown in the encircled regions. As shown, the gate of the diode connects to the cathode. Also, an overlap capacitor is formed between the anode diffusion and the gate. During electrostatic discharge (ESD) events, large voltages can develop between the pad and ground, which can lead to gate oxide breakdown. As illustrated in  FIG. 2 , the diodes are used in the circuit to protect against the Human Body Model/Machine Model (HBM/MM) and Charged Device Model (CDM), which are well-known models of ESD events. resistors may be added in the circuit in order to have the correct output impedance for the desired I/O performance (impedance matching). 
   U.S. Patent Application No. 2003/0080386 published on May 1, 2003, the complete disclosure of which is herein incorporated by reference, describes creating an ungated SOI diode that has the PN junction formed between lightly doped body implant regions. &#39;386 publication describes using a MOS gate to form the diode as is known in the art for creating an SOI diode. in &#39;386, the P-well and N-well are implanted in separate processing steps and have to be aligned to each other, and the same is true for the P+ and N+ implants in the non-gated diode. 
   U.S. Pat. No. 6,589,823 issued on Jul. 8, 2003, the complete disclosure of which is herein incorporated by reference, describes adding a backside contact (“plug”) to SOI diodes in order to provide a path for heat dissipation. diode is not created in a self-aligned fashion as implant masks for N+ and P+ must be aligned to each other in separate lithographic steps, and the silicide blocking mask must also be aligned after the implants are complete. suggests that the resistance characteristics of the diode may suffer because the silicide region is not proximate to the diode junction. 
   Thus, there is an identified need for a process of manufacturing an SOI diode which solves gate oxide breakdown limitations, and includes decreased capacitance loading and optimally reduced on-resistance. While the conventional devices and methods are adequate for the purposes they were designed to solve, there remains a need for a novel method for processing a self-aligning low capacitance SOI ESD diode, which does not include a polysilicon gate in the final diode structure. 
   SUMMARY OF INVENTION 
   The invention provides a method of forming a non-gated silicon on insulator diode in a semiconductor substrate, the substrate including a plurality of isolation regions formed therein, the method comprising forming a first structure on an upper surface of the substrate in a region between at least one pair of the isolation regions; forming a first region of a first dopant type in the substrate, the first region comprising a first edge aligned to a first edge of the first structure; and removing the first structure. The method further comprises forming a second region of a second dopant type in the substrate, the second region comprising a second edge aligned to a second edge of the first structure. Moreover, the method further comprises forming a first suicide layer comprising a first silicide edge aligned to the first edge of the first structure. Additionally, the method further comprises forming a second silicide layer comprising a second silicide edge aligned to the second edge of the first structure. The first structure comprises a hard mask, wherein the hard mask comprises a silicon nitride layer. Alternatively, the first structure comprises a gate and insulating spacers on opposite sides of the gate. Furthermore, in the removing step, the spacers remain on the substrate. 
   The invention provides a method of forming a self-aligned SOI diode, the method comprising depositing a protective structure over a substrate; implanting a plurality of diffusion regions of variable dopant types in an area between at least one pair of isolation regions in the substrate, the plurality of diffusion regions separated by a diode junction, wherein the implanting aligns an upper surface of the diode junction with the protective structure; and removing the protective structure. The method further comprises forming a silicide layer over the diffusion regions and aligned with the protective structure. The protective structure comprises a hard mask, wherein the hard mask comprises a silicon nitride layer. Alternatively, the protective structure comprises a polysilicon gate and insulating spacers on opposite sides of the gate. Furthermore, in the removing step, the spacers remain on the substrate. 
   The invention provides a method of forming a self-aligned silicon over insulator diode, the method comprising implanting an N-well doping region in an implant region in between isolation regions in a semiconductor substrate; configuring a gate over the implant region; configuring a pair of sidewall spacers on opposite sides of the gate; using the gate to define P+ and N+ contact regions in the implant region; removing the gate; and using the sidewall spacers to align a silicide layer over the P+ and N+ contact regions. The method further comprises defining a diode junction region in between the P+ and N− regions. Moreover, the method further comprises removing the sidewall spacers. Furthermore, the method further comprises depositing the suicide layer over the N-well doping region. 
   The structure provided by the invention eliminates the requirement for polysilicon to be used in the final diode structure, and is not a cost adder to the technology if gate removal techniques are already being used. Eliminating the need for a polysilicon gate in the diode structure removes the gate oxide breakdown limitation and reduces the capacitance loading of the diode. The method provided by the invention removes the polysilicon gate in the ESD diode after implant processing is completed. The diode provided by the invention is formed simply between the highly doped P+ source/drain region and the lightly doped N− body. the invention provides a self-aligned ungated diode. 
   These, and other, aspects of the invention will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following description, while indicating preferred embodiments of the invention and numerous specific details thereof, is given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the invention without departing from the spirit thereof, and the invention includes all such modifications. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     The invention will be better understood from the following detailed description with reference to the drawings, in which: 
       FIG. 1  is a schematic cross-sectional diagram of a prior art diode structure; 
       FIG. 2  is a schematic circuit diagram of a prior art diode-based ESD protection circuit; 
       FIG. 3  is a schematic cross-sectional diagram of a partially completed diode structure according to a first embodiment of the invention; 
       FIG. 4  is a schematic cross-sectional diagram of a partially completed diode structure according to a first embodiment of the invention; 
       FIG. 5  is a schematic cross-sectional diagram of a partially completed diode structure according to a first embodiment of the invention; 
       FIG. 6  is a schematic cross-sectional diagram of a partially completed diode structure according to a first embodiment of the invention; 
       FIG. 7  is a schematic cross-sectional diagram of a partially completed diode structure according to a first embodiment of the invention; 
       FIG. 8  is a schematic cross-sectional diagram of a partially completed diode structure according to a first embodiment of the invention; 
       FIG. 9(   a ) is a schematic cross-sectional diagram of a completed diode structure according to a first embodiment of the invention; 
       FIG. 9(   b ) is a schematic cross-sectional diagram of a completed diode structure according to a second embodiment of the invention; 
       FIG. 9(   c ) is a schematic cross-sectional diagram of a completed diode structure according to a third embodiment of the invention; 
       FIG. 9(   d ) is a schematic cross-sectional diagram of a completed diode structure according to a fourth embodiment of the invention; and 
       FIG. 10  is a flow diagram illustrating a preferred method of the invention. 
   

   DETAILED DESCRIPTION 
   The invention and the various features and advantageous details thereof are explained more fully with reference to the non-limiting embodiments that are illustrated in the accompanying drawings and detailed in the following description. It should be noted that the features illustrated in the drawings are not necessarily drawn to scale. Descriptions of well-known components and processing techniques are omitted so as to not unnecessarily obscure the invention. The examples used herein are intended merely to facilitate an understanding of ways in which the invention may be practiced and to further enable those of skill in the art to practice the invention. Accordingly, the examples should not be construed as limiting the scope of the invention. 
   As previously mentioned, there is a need for a novel method for processing a self-aligned low capacitance SOI ESD diode, which does not include a polysilicon gate in the final diode structure. Referring now to the drawings, and more particularly to  FIGS. 3 through 10 , there are shown preferred embodiments of the invention. 
     FIGS. 3 through 10  illustrate the processing steps of a diode according to the invention.  FIG. 3  shows a standard SOI wafer including an insulating layer  100 , such as a buried silicon dioxide (BOX) layer over a bulk substrate  50 , such as a P− or P+ substrate. Standard processing techniques are used to create shallow trench isolation regions  150 , which isolates the diode from neighboring structures. An N-type silicon region  250  used for the PFET N-body is implanted over the insulating layer  100  using standard lithography and implantation processing. This implant could equally be the P-type used in an NFET P-body. 
   In  FIG. 4 , a polysilicon layer is deposited over the N− implant  250  and STI regions  150 , and using standard etching techniques is formed to create a polysilicon gate  300  over a portion of the N− implant region  250  to create a standard FET-like structure. Insulating spacers  400 , preferably comprising an oxide deposited by chemical vapor deposition (CVD) are formed on opposite sides of the gate  300  to isolate gate contact and to passivate the sidewalls of the gate stack  300 . Materials other than polysilicon could also be used to form this masking layer  300 . 
   As illustrated in  FIG. 5 , a photoresist mask  500  is deposited over one side of the diode structure and using standard lithography and implantation (represented by the arrows) techniques a P+ implant region  200  is formed on one side of the gate  300 , which is used to create the eventual P+ anode region of the diode. The P+ and N− interface (diode junction  205 ) is located underneath the gate spacer  400  closest to the P+ region  200 . 
   Next, as depicted in  FIG. 6 , another photoresist mask  500  is deposited over the first side (P+ side) of the diode structure and using standard lithography and implantation (represented by the arrows) techniques an N+ implant region  270  is formed on the other side of the gate  300 , which is used to create the eventual N+ cathode contact region of the diode. The N+ and N− contact interface is located underneath the gate spacer  400  closest to the N+ region  270 . Thus, as illustrated in  FIG. 6  there is a diode junction  205  located underneath the spacer  400 . Therefore, the implant area of the diode includes a P+, N−and N+ region,  200 ,  250 ,  270 , respectively, as illustrated in  FIG. 7 . 
   As shown in  FIG. 8 , the wafer is patterned using standard lithography techniques in areas where the diode is being formed. As indicated, the polysilicon gate  300  is selectively etched in the areas where the diodes are being formed, leaving the spacers  400  intact.  FIG. 9(   a ) illustrates a first embodiment of a completed diode structure according to the invention. In the first embodiment, presilicide cleaning processes are used to remove the remaining gate oxide from the device. Thereafter, a suicide layer  350  is deposited over the wafer using standard processes, with the spacers  400  remaining without suicide deposited thereon. The spacers  400  provide a break in the suicide  350  to prevent the shorting between the two diode terminals, P+ region  200  and the N+ region  270 , at the diode junction  205 . 
     FIG. 9(   b ) illustrates a second embodiment of a completed diode according to the invention, wherein the region in between the spacers  400 , where the gate  300  had previously existed is cleaned of remaining silicide  350 . Again, the spacers  400  prevent the silicide from shorting the two diode terminals. In  FIG. 9(   c ), a third embodiment of a completed diode according to the invention is illustrated, wherein the silicide  350  is etched (i.e., “pulled away”) from the spacers  400  to provide further prevention against shorting of the two diode terminals. This third embodiment includes retaining the spacers  400 . However, the spacers  400  can also be removed with the silicide “pulled away” from the diode junctions  205 , which would result in the completed device shown in the fourth embodiment of  FIG. 9(   d ). If the spacers  400  are removed, then CA-level inter-level dielectric (not shown) would preferably fill that unoccupied space. 
   As shown in  FIG. 10 , a flow diagram illustrating a preferred method of the invention is described. The method of forming a non-gated diode in a semiconductor substrate, wherein the substrate includes a plurality of isolation regions  150  formed therein, comprises the steps of forming  600  a first structure  300  on an upper surface  100  of a substrate  50  in a region between at least one pair of isolation regions  150 . The next step involves forming  602  a first region  200  of a first dopant type in the substrate, wherein the first region  200  comprising a first edge  205  aligned to a first edge  400  of the first structure  300 . Then, the invention forms  604  a second region  270  of a second dopant type in the substrate, wherein the second region  270  comprising a second edge  205  aligned to a second edge  400  of the first structure  300 . After this, the next step involves forming  606  a first silicide layer  350  comprising a first silicide edge  351  aligned to the first edge  400  of the first structure  300 . Thereafter, the subsequent step involves forming  608  a second silicide layer  350  comprising a second silicide edge  351  aligned to the second edge  400  of the first structure  300 . Steps  606  and  608  preferably occur simultaneously as indicated by the dotted line in  FIG. 10 . Finally, the invention removes  610  the first structure  300 . The step of removing  610  the first structure  300  may alternatively occur after the step of forming  604  the second region  270 , as indicated by the dashed line in  FIG. 10 . 
   The invention achieves several advantages. First, the invention eliminates the concern of diode gate oxide breakdown, which allows the diode pad to reach a higher voltage without damage during an ESD event, thereby reducing the size of the diode connected to V dd . In the case for stacked NFETs used in output devices, gate oxide breakdown in the diode is the limiting factor, and the invention overcomes this as indicated above. With the gate oxide breakdown eliminated, the area required for ESD could shrink, resulting in a 3% reduction in chip area in some cases. Capacitive loading could also be reduced by as much as 50%. 
   Again, the invention achieves these advantages by forming a diode on a single area of silicon between the STI regions  150 . The polysilicon gate  300  is removed from the structure to avoid gate oxide breakdown concerns. The P+ region  200  and N+ region  270  are separated from one another to reduce leakage across the diode. Moreover, suicide  350  is formed on the P+, N−, and N+ regions  200 ,  250 ,  270 , respectively; however spacers  400  provide a break in the suicide  350  to protect the underlying diode junction  205 , which prevents shorting the anode and cathode of the diode, or the spacers  400  are removed and the silicide  350  is pulled away from the diode junctions  205  to prevent shorting of the device. According to the invention, the entire diode gets an N-well doping, and then the gate edges define the P+ and N+ contact regions before the gate  300  is removed. invention self-aligns the silicide  350  to the diode, because the spacers  400  that are formed on the sides of the gate  300  cause a break in the silicide  350 . 
   Generally, the invention eliminates the requirement for polysilicon to be used in the final diode structure, and is not a cost adder to the technology if gate removal techniques are already being used. Eliminating the need for a polysilicon gate in the diode structure removes the gate oxide breakdown limitation and reduces the capacitance loading of the diode. Moreover, the invention removes the polysilicon gate in the ESD diode after implant processing is completed. The diode provided by the invention is formed simply between the highly doped P+ source/drain region and the lightly doped N-body. the invention provides a self-aligned, low capacitance, ungated diode. 
   The foregoing description of the specific embodiments will so fully reveal the general nature of the invention that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the invention has been described in terms of preferred embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the appended claims.