Abstract:
A radiation-hardened transistor is formed in a p-type body. An active region is disposed within the p-type body and has a perimeter defined by a shallow-trench isolation region filled with a dielectric material. Spaced-apart source and drain regions are disposed in the active region, forming a channel therebetween. A polysilicon gate is disposed above, aligned with, and insulated from the channel region. A p-type isolation ring is disposed in the p-type body separating outer edges of at least one of the source and drain regions from the perimeter of the active region.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims the priority benefit of U.S. provisional application No. 61/651,689 filed May 25, 2012 and entitled “TID Hardened MOS Transistors and Fabrication Process,” the disclosure of which is incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to semiconductor technology, and specifically to MOS technology. More particularly, the present invention relates to radiation hardened MOS transistors and to methods for fabricating such transistors. 
         [0004]    2. The Prior Art 
         [0005]    The present invention is intended to solve the problem of transistor off-state leakage in n-channel MOS (NMOS) high-voltage (HV) transistors due to ionizing radiation. Ionizing radiation over time deposits positive charge in the insulating materials surrounding the transistor, causing NMOS devices to exhibit large parasitic drain-to-source leakages along the now inverted transistor sidewalls. These large leakage currents limit the usable lifetime of NMOS transistors in radiation environments. Due to the lower body doping of HV transistors, these devices are especially vulnerable to this failure mechanism. 
         [0006]    Total Ionizing Dose (TID) is a long-term degradation of electronics due to the cumulative energy deposited in a material. Typical effects include parametric failures, or degradations in device parameters such as increased leakage current, threshold voltage shifts, or functional failures. Major sources of TID exposure in the space environment include trapped electrons, trapped protons, and solar protons, as well as trapped charge in dielectrics caused by X-Rays and Gamma Rays and high energy ions. 
         [0007]    There are several transistor degradation modes caused as a result of ionization dose. One is a shift in threshold voltage V t . The V t  of NMOS and PMOS devices shift in a negative direction due to hole trapping in the gate oxide. Another is sidewall leakage. 
         [0008]    The V t  of parasitic isolation sidewall transistors also shifts in a negative direction. For NMOS transistors, as V t  becomes more negative, sidewall leakage increases exponentially as the parasitic transistor starts to turn on at a lower threshold voltage. This is the primary lifetime limitation for standard medium voltage (MV) and high-voltage (HV) NMOS devices. Shallow-trench isolation (STI) accumulates positive charge during irradiation. The positive charge turns on parasitic sidewall transistors at the STI edges, forming an uncontrolled conducting path from drain to source. 
         [0009]      FIGS. 1A through 1C  illustrate the effects of TID on a typical linear NMOS STI isolated transistor.  FIG. 1A  is an isometric view of the structure,  FIG. 1B  is a cross-sectional view of the left-most portion of the structure taken through the drain, and  FIG. 1C  is a side view of the edge of the structure at the inner boundary of the STI isolation trench. Positive charge built up in the STI oxide (shown as multiple “+” signs in  FIGS. 1A and 1B ) lowers the threshold of the transistor, causing leakage current to flow from the drain to source along the edge of the structure through a parasitic transistor that exists at the gate edge proximate to the STI boundary as shown by arrow  10  in  FIGS. 1A and 1C . 
         [0010]    Existing prior-art layout solutions to this problem include transistors formed using annular gate geometries in which there are no isolation sidewalls connecting the drain and source nodes, because the gate completely encircles the drain of the transistor. 
         [0011]      FIGS. 2A and 2B  are top and cross sectional views of an annular-gate transistor and illustrate an example layout of an existing annular-gate solution to the ionizing radiation problem for fabricating HV NMOS devices. The annular-gate transistor is fabricated within a boundary defined by a shallow trench isolation (STI) structure comprising a shallow trench filled with an insulating material such as a deposited silicon dioxide. An annular polysilicon gate is formed and defined in the center of the transistor region defined by the STI structure. An annular source region and a square-shaped drain region are then implanted by a self-aligned-gate process using the annular gate as an implant mask as is known in the art. The source comprises the region outside of the gate abutting the inner perimeter of the STI structure and the drain is formed through an aperture in the center of the gate. 
         [0012]    As may be seen from an examination of  FIGS. 2A and 2B , there is no drain edge at the inner STI periphery, since the annular source completely occupies the edge of the transistor structure. While this prevents the existence of a parasitic transistor at the gate edge at the STI region, since there is no gate edge at this location in the transistor, this solution to the problem is not entirely satisfactory. 
         [0013]    It is difficult to scale width and length for transistor design in such structures. For example, SPICE models cannot easily be used to determine effective widths and lengths of such devices. Curved and circular structures are not provided for in conventional simulation software to model transistors. In addition, as geometries shrink, the right-angle edges of the structures in the annular gate transistor become disallowed in design rules, creating a lower limit on the size of such transistors. For example below 65 nm, design rules prohibit 90° or even 45° angles on polysilicon over diffusion. 
         [0014]    Another prior art solution to the problem when using lateral transistors with STI isolation has been to add an additional p-type implant to the diffusion sidewall. This implant is performed after trench etch and before trench fill. This solution delays the onset of parasitic leakage, but does not eliminate it. In addition, the additional sidewall implant degrades junction breakdown, which is problematic in HV transistors. 
       BRIEF DESCRIPTION 
       [0015]    According to a first aspect of the present invention, a lateral n-p junction is created in the transistor to isolate the device channel from the sidewall of the STI isolation structure on both the source and drain regions of the transistor. Additional p-type implants may be added in this isolation ring to increase the parasitic V t  and improve TID immunity. The doping profile can be engineered so as not to degrade junction breakdown. In this embodiment of the invention, the drain of the transistor is isolated from STI by a lateral junction 
         [0016]    According to another aspect of the present invention, a lateral n-p junction is created in the transistor to isolate the device channel from the sidewall of the STI isolation structure on only the drain region of the transistor. 
         [0017]    According to another aspect of the present invention, additional P-type implants may be employed to increase TID immunity. The p-type implant may be separated from the edge of the N-type drain to preserve drain-body junction breakdown performance. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
         [0018]      FIGS. 1A   1 B, and  1 C are diagrams of an example layout of prior-art STI HV NMOS devices, illustrating the problems addressed by the present invention. 
           [0019]      FIGS. 2A and 2B  are diagrams of an example layout of an existing prior art annular gate transistor solution for constructing HV NMOS devices. 
           [0020]      FIGS. 3A ,  3 B, and  3 C are diagrams of an example layout of an STI isolated linear HV NMOS device according to one illustrative embodiment of the present invention. 
           [0021]      FIGS. 4A ,  4 B, and  4 C are diagrams of an example layout of an STI isolated linear HV NMOS device according to another illustrative embodiment of the present invention. 
           [0022]      FIGS. 5A ,  5 B, and  5 C are diagrams of an example layout of an STI isolated linear HV NMOS device according to another illustrative embodiment of the present invention. 
           [0023]      FIGS. 6A ,  6 B, and  6 C are diagrams of an example layout of an STI isolated linear HV NMOS device according to another illustrative embodiment of the present invention. 
           [0024]      FIGS. 7A and 7B  are diagrams of an example layout of an STI isolated linear HV NMOS device according to another illustrative embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    Persons of ordinary skill in the art will realize that the following description of the present invention is illustrative only and not in any way limiting. Other embodiments of the invention will readily suggest themselves to such skilled persons. 
         [0026]    Referring now to  FIGS. 3A ,  3 B, and  3 C, diagrams depict an illustrative embodiment of the present invention. According to this aspect of the present invention, the source and drain nodes of the NMOS transistor are electrically isolated from the trench sidewall by a lateral diode. This diode is junction engineered to provide isolation after exposure to ionizing radiation while maintaining the full junction breakdown performance of the original radiation-sensitive layout. 
         [0027]      FIG. 3A  is a three-dimensional drawing of the structure of a transistor  20  fabricated according to one aspect of the present invention.  FIG. 3B  is a cross-sectional view of the drain side of transistor  20  taken in a direction parallel to the channel.  FIG. 3C  is a top view of the transistor. 
         [0028]    Transistor  20  is formed in p-type body  22 , which may be a high-voltage triple well, including a body p-well in a deep-n-well in a p-substrate. Typical doping levels for such a body p-well are about 1×10 16  atoms/cm 3 . Transistor  20  is isolated by STI region  24  that surrounds the transistor. Source  26  and drain  28  define a channel region  30  disposed under a polysilicon gate  32 . A typical doping level for sources and drains is from about 1×10 19  atoms/cm 3  to about 1×10 19  atoms/cm 3 . The depth of a “deep-n-well” ranges from about 1 um to about 1.5 um in a process where a p-type body well junction depth ranges from 0.8 um to 1.5 um and an n-type well junction depth is from 0.8 um to 1.5 um. 
         [0029]    In this embodiment of the present invention, the source  26  and drain  28  of the NMOS transistor  20  are electrically isolated from the trench sidewall by a lateral diode. This may be thought of as effectively replacing the parasitic sidewall transistors which exist in parallel with the channel of the device with a series of parasitic transistors with progressively higher threshold voltages (VT). The leakage is determined by the highest VT device, which can potentially withstand many times higher radiation doses before the onset of undesired conduction. The lateral diode space thereby allows these higher Vt dopings without sacrificing the breakdown voltage. 
         [0030]    This lateral diode is formed by pulling the n-type source/drain implants back from the diffusion edge, leaving a region  34  of the p-type well or substrate doping. The perimeter of the diffusion is then implanted with additional p-type implant  36  to increase the parasitic threshold voltage and prevent punch-through to the inverted sidewall. P-type implant  36  is not shown at the front of the three-dimensional drawing of  FIG. 3A  in order to show the p-type body  22 , although it is present there as shown in the top view of  FIG. 3C . In a typical embodiment, the p-type implant can be at a level of about 1E16 atoms/cm 3 . In this device, the channel still exists under the gate, but it is disconnected from the transistor  20 . 
         [0031]    Referring now to  FIGS. 4A through 4C , another embodiment of the present invention is shown.  FIG. 4A  is a three-dimensional drawing of the structure of a transistor  20  fabricated according to this aspect of the present invention.  FIG. 4B  is a cross-sectional view of the drain side of transistor  20  taken in a direction parallel to the channel.  FIG. 4C  is a top view of the transistor  20 . 
         [0032]    The embodiment shown in  FIGS. 4A through 4C  is a variant of the embodiment described with reference to  FIGS. 3A through 3C . Instead of leaving a region  34  of the p-type well or substrate doping between the n-type source/drain implants and the diffusion edge, a lightly doped n-type region  38  is formed in that area. Region  38  is lightly doped n-type region, but is higher in doping than the P-type body  22 . In a typical embodiment, the n-type implant can be at a level of about 3×10 16  atoms/cm 3 . Because of the light doping, the BV will be high and can overlap the source/drain implants and the p-type implant  36 , making the alignment non critical. 
         [0033]    Referring now to  FIGS. 5A through 5C , another embodiment of the present invention is shown.  FIG. 5A  is a three-dimensional drawing of the structure of a transistor  40  fabricated according to this aspect of the present invention.  FIG. 5B  is a cross-sectional view of the drain side of transistor  40  taken in a direction parallel to the channel.  FIG. 5C  is a top view of the transistor  40 . 
         [0034]    Like transistor  20  of the previously-described embodiment, transistor  40  is formed in p-type body  42 , which may be a high-voltage triple well, including a body p-well in a deep-n-well in a p-substrate. Transistor  40  is isolated by STI region  44  that surrounds the transistor. Source  46  and drain  48  define a channel region  50  disposed under a polysilicon gate  52 . A typical doping level for sources and drains is from about 1×10 19  atoms/cm 3  to about 1×10 19  atoms/cm 3 . 
         [0035]    In the embodiment of the present invention shown in  FIGS. 5A ,  5 B, and  5 C, only the drain  48  of the NMOS transistor  40  is electrically isolated from the trench sidewall by a lateral diode. This lateral diode is formed by pulling the n-type drain implant back from the diffusion edge, leaving a region  54  of the p-type well or substrate doping. The perimeter of the diffusion is then implanted with additional p-type implant  56  to increase the parasitic threshold voltage and prevent punch-through to the inverted sidewall. P-type implant  56  is not shown at the front of the three-dimensional drawing of  FIG. 5A  in order to show the p-type body  42 , although it is present there as shown in the top view of  FIG. 5C . In a typical embodiment, the p-type implant can be at a level of about 1E16 atoms/cm 3 . 
         [0036]    Referring now to  FIGS. 6A through 6C , another embodiment of the present invention is shown.  FIG. 6A  is a three-dimensional drawing of the structure of a transistor  20  fabricated according to this aspect of the present invention.  FIG. 6B  is a cross-sectional view of the drain side of transistor  20  taken in a direction parallel to the channel.  FIG. 6C  is a top view of the transistor  20 . 
         [0037]    The embodiment shown in  FIGS. 6A through 6C  is a variant of the embodiment described with reference to  FIGS. 5A through 5C . Instead of leaving a region  54  of the p-type well or substrate doping between the n-type source/drain implants and the diffusion edge, a lightly doped n-type region  58  is formed in that area. Region  58  is lightly doped n-type region, but is higher in doping than the P-type body  42 . In a typical embodiment, the n-type implant can be at a level of about 3×10 16  atoms/cm 3 . Because of the light doping, the BV will be high and can overlap the source/drain implants and the p-type implant  56 , making the alignment non critical. 
         [0038]    Referring now to  FIGS. 7A and 7B , another embodiment of the present invention is shown.  FIG. 7A  is a top view of the structure of a transistor  60  fabricated according to this aspect of the present invention.  FIG. 7B  is a cross-sectional view of the drain side of transistor  40  taken in a direction parallel to the channel. 
         [0039]    The NMOS transistor  60  resides in a p-well  62 , which may be a high-voltage triple well, including a body p-well in a deep-n-well in a p-substrate. Transistor  60  is isolated by STI region  64  that surrounds the transistor. Source  66  and drain  68  define a channel region  70  disposed under a polysilicon gate  72 . A typical doping level for sources and drains is from about 1×10 19  atoms/cm 3  to about 1×10 19  atoms/cm 3 . 
         [0040]    The lateral diode in transistor  60  is formed by pulling the N+ source and drain implant back from the diffusion edge at STI region  64 , leaving only the body p-type well  62  (or substrate) doping. The source/drain junction is then graded by introducing a region  76  of lighter n-type lightly-doped-drain (NLDD) implant extending beyond the N+ source/drain regions. In an embodiment where the N+ source/drain diffusions have a doping level of about 1E19-1E20 atoms/cm 3 , the NLDD implant can have a level of about 1E18 atoms/cm 3 . The perimeter of the diffusion is then implanted with a P+ implant  78  to create a very high parasitic threshold voltage for ionizing radiation immunity. Typical doping levels for p-type implant  78  are about 1×10 19  to about 1×10 20  atoms/cm 3 . The P+ to p-well doping profile is graded by introducing a lighter p-type implant  80  encompassing the P+ region. Typical doping levels for p-type implant  80  are about 1×10 18  atoms/cm 3 . Finally, another p-type implant  82 , deeper than implant  80 , is added at the diffusion edge to increase the sidewall V T  and prevent punch-through to the transistor sidewall under high junction stresses. Typical doping levels for p-type implant  82  are about 1×10 18  atoms/cm 3 . All of these implants may be made using a species such as boron. 
         [0041]    In the embodiment of the invention illustrated in  FIGS. 7A and 7B , the p-type isolation is present only on the drain edges of the transistor. This allows for a reduction in the “x”-pitch of the transistor layout without any degradation of either the TID robustness or the junction breakdown. 
         [0042]    The present invention provides a significant total footprint reduction as compared to existing radiation-hardened layouts. It offers smaller source and drain junctions, reducing parasitic leakage and capacitance for better performance. The transistors also readily scalable in channel width and length, which is critical for efficient circuit design. This invention is implemented using a standard commercially available processes without need for modification, achieving radiation hardness solely via device layout. 
         [0043]    Persons of ordinary skill in the art will appreciate that the concepts of the present invention may be used to fabricate multiple transistors sharing a common central diffusion (e.g., a source region) with a pair of opposed drains extending in opposite directions from the central diffusion. 
         [0044]    The transistors of the present invention are easily fabricated using standard CMOS process modules. First, the trenches are formed. The radiation-hardening p-type implant to the trench walls is then performed. Next, polysilicon for the gates is deposited. The gates are then defined. A p-channel mask is applied for the p-type isolation rings. Then, if the transistors are to be high-voltage transistors an LDD implant is performed. Then an LDD mask is applied and the source/drain implants are performed. 
         [0045]    In this specification, the relative term “high-voltage” or “HV” is used with respect to transistors. Persons of ordinary skill in the art will appreciate that these terms are interchangeable. Such skilled persons will also appreciate that a high-voltage transistor is a transistor able to withstand more than 5V, usually higher than 10V. 
         [0046]    While embodiments and applications of this invention have been shown and described, it would be apparent to those skilled in the art that many more modifications than mentioned above are possible without departing from the inventive concepts herein. The invention, therefore, is not to be restricted except in the spirit of the appended claims.