Patent Document

FIELD OF THE INVENTION 
     The present invention relates to the field of stress-induced defect detection in semiconductor devices; more specifically, it relates to a system of devices and test methodologies for detecting stress-induced defects and to the use of particular of these devices as antifuses. 
     BACKGROUND OF THE INVENTION 
     The fabrication processes for silicon chips often lead to the formation of small stress-induced silicon defects that may coalesce into dislocations or stacking faults that degrade the product functionality, yield and reliability. Examples of such processes include ion implantation, trench isolation and other dielectric isolation processes, trench capacitor processes, oxidation processes in general and film deposition processes. Results of stress-induced defects include gate and capacitor dielectric leakage, which may be yield or reliability defects. 
     Semiconductor silicon substrates, being crystalline are subject to shearing of one portion of the crystal with respect to another portion of the crystal along a specific crystal plane. Dislocations, which are postulated as crystalline defects, occur in different types including: edge dislocations, screw dislocations and declinations. 
     In dynamic random access memory (DRAM) technologies employing deep trench storage capacitors, the leakage requirements for the capacitor are very stringent, and monitor systems are introduced for the detection of process induced defects in the active area of the DRAM deep trench storage capacitors. 
     While methods exists for monitoring processes for defects and other methods exist for detecting stress during processes development, an efficient and sensitive monitoring systems for detecting stress-induced defects that could be used for both development and routine monitoring in manufacturing is limited. Therefore, a method is needed to detect the formation of silicon defects that is sensitive, simple, applicable to process monitoring and process development and applicable to logic and DRAM technologies. 
     SUMMARY OF THE INVENTION 
     A first aspect of the present invention is a method for detecting semiconductor process stress-induced defects comprising: providing a polysilicon-bounded test diode, the polysilicon-bounded test diode comprising a diffused first region within an upper portion of a second region of a silicon substrate, the second region of an opposite dopant type from the diffused first region, the diffused first region surrounded by a peripheral dielectric isolation and a peripheral polysilicon gate comprising a polysilicon layer over a dielectric layer and the polysilicon gate overlapping a peripheral portion of the diffused first region; stresser the polysilicon-bounded test diode; and monitoring the stressed polysilicon-bounded test diode for spikes in gate current during the stress. 
     A second aspect of the present invention is a method for detecting semiconductor process stress-induced defects comprising: providing one or more polysilicon-bounded test diodes, each polysilicon-bounded test diodes comprising a diffused first region within an upper portion of a second region of a silicon substrate, the second region of an opposite dopant type from the diffused first region, the diffused first region surrounded by a peripheral dielectric isolation and a peripheral polysilicon gate comprising a polysilicon layer over a dielectric layer, the polysilicon gate overlapping a peripheral portion of the diffused first region; stressing each the polysilicon-bounded test diode; measuring during the stressing for each the polysilicon-bounded test diode, the current through the first region as a function of a forward bias voltage applied between the first and second regions at at least a predetermined forward bias voltage; and determining the frequency distribution of the slope of the forward bias voltage versus the first region current at the pre-selected forward bias voltage for the one or more polysilicon-bounded test diodes. 
     A third aspect of the present invention is a method for detecting semiconductor process stress-induced defects comprising: providing one or more polysilicon-bounded test diodes, each polysilicon-bounded test diode comprising a diffused first region within an upper portion of a second region of a silicon substrate, the second region of an opposite dopant type from the diffused first region, the diffused first region surrounded by a peripheral dielectric isolation, a peripheral polysilicon gate comprising a polysilicon layer over a dielectric layer, the polysilicon gate overlapping a peripheral portion of the diffused first region; stressing each the polysilicon-bounded test diode for a pre-determined amount of time; and monitoring, after the stressing, each the polysilicon-bounded test diode for soft breakdown. 
     A fourth aspect of the present invention is a method for detecting semiconductor process stress-induced defects comprising: providing a test DRAM, the test DRAM having a transfer device comprising a channel region between first and second P+ regions formed in a N-well in a silicon substrate and a gate formed over the channel region, the second P+ region electrically connected to a conductive core of a deep trench capacitor, the substrate acting as a second plate of the deep trench capacitor; stressing the test DRAM; and monitoring the stressed test DRAM for spikes in first P+ region current during the stressing. 
     A fifth aspect of the present invention is a method for detecting semiconductor process stress-induced defects comprising: providing a test DRAM, the test DRAM having a transfer device comprising a channel region between first and second P+ regions formed in a N-well in a silicon substrate and a gate formed over the channel region, the second P+ region electrically connected to a conductive core of a deep trench capacitor, the substrate acting as a second plate of the deep trench capacitor; stressing the test DRAM; and monitoring the stressed test DRAM for spikes in gate current during the stressing. 
     A sixth aspect of the present invention is a method for detecting semiconductor process stress-induced defects comprising: providing a test DRAM, the test DRAM comprising a transfer device comprising a channel region between first and second P+ regions formed in a N-well in a silicon substrate and a gate formed over the channel region, the second P+ region electrically connected to a conductive core of a deep trench capacitor, the substrate acting as a second plate of the deep trench capacitor; stressing each the test DRAM; measuring during the stressing, for the test DRAM, the current through the first P+ region as a function of a forward bias voltage applied between the first P+ region and the N-well at at least a pre-selected forward bias voltage; and determining the frequency distribution of the slope of the forward bias voltage versus the first P+ region current at the pre-selected forward bias voltage for the one or more test DRAMs. 
     A seventh aspect of the present invention is a method for detecting semiconductor process stress-induced defects comprising: providing a test DRAM, the test DRAM comprising a transfer device comprising a channel region between first and second P+ regions formed in a N-well in a silicon substrate and a gate formed over the channel region, the second P+ region electrically connected to a conductive core of a deep trench capacitor, the substrate acting as a second plate of the deep trench capacitor; stressing the test DRAM for a pre-determined amount of time; and monitoring, after the stressing, each the test DRAM for soft breakdown. 
     An eighth aspect of the present invention is a method of fabricating an antifuse comprising: providing a silicon substrate having a surface; forming a ring of shallow trench isolation having an inner and an outer perimeter in the substrate extending from the surface of the substrate into the substrate; forming a polysilicon gate overlapping the inner perimeter of the shallow trench isolation on the surface of the substrate, the polysilicon gate comprising a dielectric layer between the surface of the substrate and a polysilicon layer, the polysilicon gate having an inner and outer perimeter; damaging the dielectric layer in a region along the inner perimeter of the polysilicon gate with a heavy ion specie implant to lower the breakdown voltage of the damaged dielectric layer in the region compared to the breakdown voltage in undamaged dielectric regions; and forming a diffused region in the silicon substrate within the inner perimeter of the shallow trench isolation, the diffused region extending from the surface of the substrate into the substrate a depth not exceeding a depth of the shallow trench isolation. 
     A ninth aspect of the present invention is an antifuse comprising: a silicon substrate having a surface; a ring of shallow trench isolation having an inner an outer perimeter in the substrate extending from the surface of the substrate into the substrate; a polysilicon gate overlapping the inner edge of the shallow trench isolation on the surface of the substrate, the polysilicon gate comprising a dielectric layer between the surface of the substrate and a polysilicon layer, the polysilicon gate having an inner and outer perimeter; a damaged region of the dielectric layer, the damaged region along the inner perimeter of the polysilicon gate, the damaged region damaged with a heavy ion specie implant and having a lower breakdown voltage than undamaged regions of the dielectric layer; and a diffused region in the silicon substrate within the inner perimeter of the shallow trench isolation, the diffused region extending from the surface of the substrate into the substrate a depth not exceeding a depth of the shallow trench isolation. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The features of the invention are set forth in the appended claims. The invention itself, however, will be best understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a top view of a polysilicon-bounded test mi diode for use in a test system for detecting and monitoring stress-induced defects in semiconductor devices according to the present invention; 
     FIG. 2 is a partial cross-sectional view through  2 — 2  of the polysilicon-bounded test diode of FIG. 1 according to the present invention; 
     FIG. 3 is a top view of a STI-bounded reference diode for use in a test system in conjunction with the polysilicon-bounded test diode of FIG. 1, for detecting and monitoring stress-induced defects in semiconductor devices, according to the present invention; 
     FIG. 4 is a partial cross-sectional view through  4 — 4  of the STI-bounded reference diode of FIG. 3 according to the present invention; 
     FIG. 5 is a partial cross-sectional view of a test DRAM device adapted for use in a test system for detecting and monitoring stress-induced defects in semiconductor devices to the present invention; 
     FIG. 6 is:a partial cross-sectional view of a reference device adapted for use in a test system in conjunction with the test DRAM of FIG. 5, for detecting and monitoring stress-induced defects in semiconductor devices, according to the present invention; 
     FIGS. 7A through 7C are flowcharts illustrating first, second and third test methodologies respectively, according to a first embodiment of the present invention; 
     FIG. 8 is a plot of P+ diffusion and gate currents versus diffusion reverse bias voltage for the polysilicon-bounded test diode of FIG. 1 having no stress-induced defects; 
     FIG. 9 is a plot of P+ diffusion and gate currents versus diffusion reverse bias voltage for the polysilicon-bounded test diode of FIG. 1 having stress-induced defects; 
     FIGS. 10A and 10B are flowcharts illustrating fourth and fifth test methodologies respectively according to a second embodiment of the present invention; 
     FIG. 11 is a plot of the forward bias current versus forward bias voltage for three different polysilicon-bounded test diodes of FIG. 1, each having different quantities of stress-induced defects; 
     FIG. 12 is a histogram of the distribution of the slope, in mV/decade of current versus the forward bias current-voltage characteristics of polysilicon-bounded test diodes of FIG.  1  and STI-bounded reference diodes of FIG. 3; 
     FIGS. 13A,  13 B and  13 C are flowcharts illustrating sixth, seventh and eighth test methodologies respectively, according to a third embodiment of the present invention; 
     FIG. 14 is a plot of the polysilicon gate current versus stress time for polysilicon-bounded test diodes of FIG. 1 with and without stress-induced defects; 
     FIGS. 15A through 15K are partial cross-sectional views illustrating fabrication of an antifuse according to the present invention; 
     FIG. 16 is a Weibull distribution for Time-to-Fail (T BD ) and Charge-to-Breakdown (Q BD )) for a polysilicon-bounded test diode of FIG. 1 used as an antifuse, with and without stress-induced defects; 
     FIG. 17 is a plot of the dielectric breakdown field at 30° C. versus germanium implantation dose of the antifuse of FIG. 15K; 
     FIG. 18 is a plot of dielectric breakdown voltage versus the inverse of absolute temperature of the antifuses of FIG. 15K, fabricated with two thickness of dielectric; and 
     FIG. 19 is a plot of dielectric breakdown voltage versus germanium implantation dose of the antifuse of FIG. 15K at three temperatures. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description of the present invention the term stress-induced defect when used in conjunction with silicon substrates is intended to mean dislocations, stacking faults and other silicon crystal plane defects. 
     FIG. 1 is a top view of a polysilicon-bounded test diode for use in a test system for detecting and monitoring stress-induced defects in semiconductor devices according to the present invention. In FIG. 1, polysilicon-bounded test diode  100  is formed in a silicon substrate  105 . Polysilicon-bounded test-diode  100  includes a P+ diffusion region  110  having a length “L D ” and a width “W D ” formed over an N-well region  115 . P+ diffusion region  110  is bounded by a peripheral shallow trench isolation (STI) region  120 . A peripheral polysilicon gate  125  overlaps the entire STI/P+ diffusion region interface  130  giving polysilicon-bounded test diode  100  a high perimeter to area ratio. Polysilicon gate  125  has a width “W P ” and overlaps P+ diffusion region  110  by a distance “O P .” Polysilicon-bounded test diode  100  also includes a first probe pad  135  electrically connected to P+ diffusion region  110  by a first contact  140 , a second probe pad  145  electrically connected to polysilicon gate  125  by a second contact  150  and a third probe pad  155  connected to an N+ diffusion region  160  by a third contact  165 . N+ diffusion region  160  provides low resistance electrical connection to N-well  115 . Both STI  120  and N+ diffusion region  160  are formed in the shapes of rings, the N+ diffusion region surrounding the STI. 
     FIG. 2 is a partial cross-sectional view through  2 — 2  of the polysilicon-bounded test diode of FIG. 1 according to the present invention. In FIG. 2, polysilicon gate  125  includes a dielectric layer  170  formed on a top surface  175  of silicon substrate  105  and a polysilicon layer formed on top of the gate dielectric layer. P+ diffusion region  110  does not extend below a bottom surface  185  of STI  120 . Also, the overlap of polysilicon gate  125  of STI  120  and P+diffusion region  110  is clearly illustrated. 
     In one example, “L D ” is about 50 to 500 microns, W D ) is about 2 to 10 microns, “W P ” is about 0.5 to 1.5 microns and “O P ” is about 0.1 to 0.6 microns. Gate dielectric layer  170  may be thermal oxide about 1 to 10 nm thick. 
     The lower limit of “L D ” is chosen so as not to impact the sensitivity of the measurement to be performed and the upper limit is constrained by silicon real estate concerns. That is, large: devices consume valuable silicon area and small devices are subject to noise. The lower limit of “W P ” is limited by technology ground rules and process equipment limitations (i.e. photolithography and etching.) The upper limit must be high enough to provide low noise to signal ratios for the measurement being performed, narrow devices being noisier than wider devices. The upper and lower limits of “O P ” are primarily driven by technology ground rules and process equipment limitations. 
     Polysilicon-bounded diode  100  comprises a first portion of a defect test system, the test device. A second portion of the defect test system comprises a control or calibration device and is illustrated in FIGS. 3 and 4 and described below. 
     FIG. 3 is a top view of a STI-bounded reference diode for use in a test system in conjunction with the polysilicon-bounded test diode of FIG. 1, for detecting and monitoring stress-induced defects in semiconductor devices, according to the present invention. In FIG. 3, STI-bounded reference diode  200  is formed in silicon substrate  105 . STI-bounded reference diode  200  includes a P+ diffusion region  210  having a length “L D ” and a width “W D ” formed over N-well region  215 . P+ diffusion region  210  is bounded by a peripheral STI  220 . STI-bounded reference diode  200  has a high perimeter to area ratio. STI-bounded reference diode  200  also includes a first probe pad  235  electrically connected to P+ diffusion region  210  by a first contact  240  and a second probe pad  255  connected to an N+ diffusion region  260  by a second contact  265 . N+ diffusion region  260  provides low resistance electrical connection to N-well  215 . Both STI  220  and N+ diffusion region  260  are formed in the shapes of rings, the N+ diffusion region surrounding the STI. 
     FIG. 4 is a partial cross-sectional view through  4 — 4  of the STI-bounded reference diode of FIG. 3 according to the present invention. In FIG. 4, P+ diffusion region  210  does not extend below a bottom surface  285  of STI  220 . 
     In one example, “L D ” is about 50 to 500 microns and “W D ” is about 2 to 10 microns. In practice, “L D ” and “W P ” of polysilicon-bounded test diode  100  would be the same as the “L D ” and “W P ” of STI-bounded reference diode  200 . If more than one size of polysilicon-bounded test diode  100  is used, then corresponding sizes of STI-bounded reference diode  200  are used. Both polysilicon-bounded test diode  100  and STI-bounded reference diode  200  are fabricated simultaneously and the STI, N-well, P+ diffusion and N+ diffusion processes would be common to both devices. 
     For both polysilicon-bounded test diode  100  and STI-bounded reference diode  200  reverse polarity diodes may be used. P+ diffusion regions would be replaced by N+ diffusions, N+ diffusions by P+ diffusions and N-well by P-well. While STI technology has been illustrated other types of isolation such as local oxidation of silicon (LOCOS.) 
     The test and reference structures of the first embodiment of the present invention are suitable for both testing both Logic devices/processes using complimentary metal-oxide-silicon (CMOS) and DRAM technologies. The test and reference structures of the second embodiment are more suited to testing DRAM technology devices/processes and are illustrated in FIGS. 5 and 6 and described below. 
     FIG. 5 is a partial cross-sectional view of a test DRAM device adapted for use in a test system for detecting and monitoring stress-induced defects in semiconductor devices according to the present invention. In FIG. 5, a test DRAM device  300  is formed in a P+ silicon substrate  305  and in a P− epitaxial layer  310  grown on the P+ silicon substrate. Formed in P epitaxial layer  310  is a N-well  315 . A P− region  320  of P− epitaxial layer  310  remains P type doped between N-well  315  and P+ substrate  305 . An N+ diffusion  325  provides low resistance electrical connection to N-well  315 . Also formed in N-well  315  is STI  330 . STI  330  does not extend into P− region  320 . Further formed in N-well  315  is a deep trench capacitor  335 . Deep trench capacitor  335  extends through N-well  315 , P− region  320  and into P+ substrate  305 . Deep trench capacitor  335  comprises a polysilicon core  340  surrounded by a dielectric liner  342 . Formed in N-well  315 , between STI  330  and deep trench capacitor  335  is a PFET transfer device  345 . PFET transfer device  345  comprises a first P+ diffusion  350 A adjacent to STI  330 ,a second P+ diffusion region  350 B adjacent to deep trench capacitor  335 , channel region  355  and a polysilicon gate  360 . First P+ diffusion  350 A and second P+ diffusion  350 B are separated by a channel region  355  of N-well  315 . Polysilicon gate  360  is formed over channel region  355  and aligned to first and second P+ diffusions  350 A and  350 B. Polysilicon gate  360  comprises a gate dielectric portion  365  formed over channel region  355  and a polysilicon portion  370  formed on top of the gate dielectric portion. A metal strap  375  electrically connects deep trench capacitor  335  to second P+ diffusion  350 B. 
     Schematically illustrated in FIG. 5 is a substrate contact  380  to P+ substrate  305 , an N-well contact  385  to N+ diffusion  325 , a bit-line contact  390  to first P+ diffusion  350 A and a word line contact  395  to polysilicon gate  360 . 
     Test DRAM device  300  comprises a first portion of a defect test system, the test device. A second portion of the defect test system comprises a control or calibration device and is illustrated in FIG.  6  and described below. 
     FIG. 6 is a partial cross-sectional view of a reference device adapted for use in a test system in conjunction with the test DRAM of FIG. 5, for detecting and monitoring stress-induced defects in semiconductor devices, according to the present invention. In FIG. 6, a reference device  400  is formed in a P+ silicon substrate  305  and in a P− epitaxial layer  310  grown on the P+ silicon substrate. Formed in P− epitaxial layer  310  is an N-well  415 . A P− region  320  of P− epitaxial layer  310  remains P type doped between N-well  415  and P+ substrate  305 . An N+ diffusion  425  provides low resistance electrical connection to N-well  415 . Also formed in N-well is STI  430 . STI  430  does not extend into P− region  320 . Further formed in N-well  415  is a deep trench capacitor  435 . Deep trench capacitor  435  extends through N-well  415 , P− region  320  and into P+ substrate  305 . Deep trench capacitor  435  comprises a polysilicon core  440  surrounded by a dielectric liner  442 . Formed in N-well  415 , between STI  430  and deep trench capacitor  435  is a P+ diffusion  450 . A metal strap  475  electrically connects deep trench capacitor  435  to P+ diffusion  450 . 
     Schematically illustrated in FIG. 6 is a substrate contact  480  to P+ substrate  305 , an N-well contact  485  to N+ diffusion  425  and a P+ diffusion contact  495  to P+ diffusion  450 . 
     While a PFET transfer device has been illustrated for test DRAM device  300  and a P+ diffusion for reference device  400 , the present invention is equally applicable to a test DRAM device using an NFET transfer device in conjunction with a reference device using a N+ diffusion. 
     When used for semiconductor process development or product testing and/or screening, multiplicities of polysilicon-bounded test diodes  100  with or without STI-bounded reference diodes  200  and/or test DRAM devices  300  with/or without reference devices  400  may, in one example, be formed in the kerf areas of chips on semiconductor wafers during chip fabrication and tested at appropriate points in the process. Sets of polysilicon-bounded test diodes  100 , STI-bounded reference diodes  200  of varying dimension “W D ”, “L D ”, “O P ” and “W P ” may be used. 
     It should be noted that whenever a test methodology uses a test DRAM device  300  (see FIG. 5) the terms “bit line contact ( 390 )” and “first P+ diffusion region( 350 A)” are interchangeable, the terms “word line contact( 395 )” and “gate ( 360 )” are interchangeable, the terms “N-well contact ( 385 )” and “N-well ( 315 )” are interchangeable and the terms “substrate contact ( 380 )” and “substrate ( 305 )” are interchangeable. 
     It should be noted that whenever a test methodology uses a reference device  400  (see FIG. 6) the terms “P+ diffusion contact ( 495 )” and “P+ diffusion ( 450 )” are interchangeable and the terms “substrate contact ( 480 )” and “substrate ( 305 )” are interchangeable. 
     FIGS. 7A through 7C are flowcharts illustrating first, second and third test methodologies respectively, according to a first embodiment of the present invention. Referring to FIG. 7A, in step  500 , a polysilicon-bounded test diode  100  (see FIGS.  1  and  2 )is selected. In step  505 , polysilicon-bounded test diode  100  is maintained at a pre-selected temperature. In one example, the pre-selected temperature is 180° C. However, any temperature in the range of about 100 to 200° C. may be used. In step  510 , polysilicon gate  125 , N-well  115  and substrate  105  (see FIG. 1) are held at ground potential. In one example, ground potential is about 0 volts. In step  515 , P+ diffusion region  110  (see FIG. 1) is ramped from about 0 volts to about −6 volts. In step  520 , the current through polysilicon gate  125  (see FIG. 1) is monitored for current spikes. An example is illustrated in FIGS. 8 and 9 and described below. 
     Referring.to FIG. 7B, in step  525 , a test DRAM device  300  (see FIG. 5) is selected. In step  530 , test DRAM device  300  is maintained at a pre-selected temperature. In one example, the pre-selected temperature is 180° C. However, any temperature in the range of about 100 to 200° C. may be used. In step  535 , N-well contact  385 , and bit line contact  390  are held at ground potential and word line contact  395  is held at a voltage sufficient to turn on transfer device  345  (see FIG. 5) In one example, ground potential is about 0 volts and the turn on voltage is about −2 volts. In step  540 , substrate contact  380  (see FIG. 5) is ramped from about 0 volts to about −6 volts. In step  545 , the current through bit line contact  390  (see FIG. 5) is monitored for current spikes. 
     Referring to FIG. 7C, in step  550 , a test DRAM device  300  (see FIG.  5 )is selected. In step  555 , test DRAM device  300  is maintained at a pre-selected temperature. In one example, the pre-selected temperature is 180° C. However, any temperature in the range of about 100 to 200° C. may be used. In step  560 , N-well contact  385 , by substrate contact  380  and wordline contact  395  (see FIG.  5 )are held at ground potential. In-one example, ground potential is about 0 volts. In step  565 , bit line contact  390  (see FIG. 5) is ramped from about 0 volts to about −6 volts. In step  570 , the current through word line contact  395  (see FIG. 5) is monitored for current spikes. 
     FIG. 8 is a plot of P+ diffusion and gate currents versus diffusion reverse bias voltage for the polysilicon-bounded test diode of FIG. 1 having no stress-induced defects and FIG. 9 is a plot of P+ diffusion and gate currents versus diffusion reverse bias voltage for the polysilicon-bounded test diode of FIG. 1 having stress-induced defects. While FIGS. 8 and 9 are for polysilicon-bounded test diodes having a gate dielectric of five nm of thermal oxide, similar plots would be obtained for the test DRAM device of FIG.  3 . 
     It is clear from FIGS. 8 and 9, that the diffusion reverse bias leakage is higher for a polysilicon-bounded diode with stress-induced defects then for a polysilicon-bounded diode without stress-induced defects. Comparing FIGS. 8 and 9, it may be seen that the gate current for a polysilicon-bounded diode with stress-induced defects exhibits spiking or sudden increases by as much as ten times more than the background gate leakage, as the P+ diffusion reverse bias voltage is changed from 0 to about −4V. This behavior is not present for the polysilicon-bounded test diodes without stress-induced defects. 
     For reverse bias voltages more negative than −4 V, the gate current increases exponentially due to Fowler-Nordhein tunneling, and the gate current becomes more significant than the spiking due to the stress-induced defects. The spiking in gate current occurs because of carrier generation at the site of the stress-induced defects, which act as carrier-generation sites. 
     In the case of a test DRAM device, the processing of the deep trench could cause stress-induced defects to be generated in the P+ substrate very close to the outer surface of the thin insulator of the deep trench. Under the second test methodology the presence of stress-induced defects causes spiking in the current flowing through the thin insulator of the deep trench, which then flows from the polysilicon filling the deep trench, through the channel of the transfer device and can be measured at the diffusion terminal. Under the third test methodology, stress-induced defects in the N-well/P+ diffusion close to the thin gate dielectric of the transfer device are detected. 
     When polysilicon-bounded test diodes  100  and test DRAM devices  300  are used in testing for stress-induced defects under the first, second and third test methodologies, the screen or fail limit for gate current spiking due to presence of stress-induced defects is about a three times increase in gate current over the background value. This increase in gate current can be observed by any of several techniques known in the art, such as connecting an oscilloscope to the polysilicon gate terminal. 
     FIGS. 10A and 10B are flowcharts illustrating fourth and fifth test methodologies respectively, according to a second embodiment of the present invention. Referring to FIG. 10A, in step  575 , one or more polysilicon-bounded test diodes  100  (see FIGS.  1  and  2 )is selected. In step  580 , for each polysilicon-bounded test diode  100 , polysilicon gate  125 , N-well  115  and substrate  105  (see FIG. 1) are held at ground potential. In one example, ground potential is about 0 volts. In step  585 , for each polysilicon-bounded test diode  100 , P+ diffusion region  110  (see FIG. 1) is ramped from about 0 volts to about 0.85 volts. In step  590 , for each polysilicon-bounded test diode  100 , the current through P+ diffusion region  110  (see FIG. 1) is measured as a function of voltage and a frequency distribution analysis of the slope of the forward bias voltage/P+ diffusion current at a pre-selected forward bias voltage is performed. In step  595 , one or more STI-bounded reference diodes  200  (see FIGS.  3  and  4 )is selected. In step  600 , for each STI-bounded reference diode  200 , N-well  215 , and substrate  105  (see FIG. 3) are held at ground potential. In one example, ground potential is about 0 volts. In step  605 , for each STI-bounded reference diode  200 , P+ diffusion  210  (see FIG.  3 ) is ramped from about 0 volts to about 0.85 volts. In step  610 , the current through P+ diffusion region  210  (see FIG. 3) is measured as a function of voltage and a frequency distribution analysis of the slope of forward bias voltage/P+ diffusion current at the pre-selected forward bias voltage is performed. In step  615 , the frequency distributions of the slope of the forward bias voltage/P+ diffusion current at the pre-selected voltage value for polysilicon-bounded diodes  100  and STI-bounded reference diodes  200  are compared. An example forward bias voltage versus P+ diffusion current and of a frequency distribution analysis are illustrated in FIGS. 11 and 12 and described below. 
     Referring to FIG  10 B, in step  620 , one or more test DRAM devices  300  (see FIG. 5) is selected. In step  625 , for each test DRAM devices  300 , N-well contact  385  and substrate contact  380  are held at ground potential and in step  630 , word line contact  395  is held at a voltage sufficient to turn off transfer device  345  (see FIG. 5.) In one example, ground potential is about 0 volts and the turn off voltage is about 2 volts. In step  635 , for each test DRAM devices  300 , bit line contact  390  (see FIG. 5) is ramped from about 0 volts to about 0.85 volts. In step  640 , for each test DRAM devices  300 , the current through bit line contact  390  (see FIG. 5) is measured as a function of voltage and a frequency distribution analysis of the slope of forward bias voltage/bit line current at a pre-selected forward bias voltage is performed. In step  645 , one or more reference devices  400  is selected. In step  650 , for each reference device  400 , N-well contact  485  and substrate contacts  490  are held at ground potential. In one example, ground potential is about 0 volts. In step  655 , for each reference device  400 , P+ diffusion contact  495  (see FIG. 6) is ramped from about 0 volts to about 0.85 volts. In step  660 , for each reference device  400 , the current through P+ diffusion contact  495  (see FIG. 6) is measured as a function of voltage and a frequency distribution analysis of the slope of forward bias voltage/bit line current at the pre-selected forward bias voltage is performed. In step  665 , the frequency distributions of the slope of the forward bias voltage/bit line current at the pre-selected voltage value for the test DRAM  300  and reference device  400  are compared. 
     FIG. 11 is a plot of the forward bias current versus forward bias voltage for three different polysilicon-bounded test diodes of FIG. 1, each having different quantities of stress-induced defects and FIG. 12 is a histogram of the distribution of the slope, in mV/decade of current versus the forward bias current voltage characteristics of polysilicon-bounded test diodes of FIG.  1  and STI-bounded reference diodes of FIG.  3 . While FIGS. 11 and 12 are for polysilicon-bounded test diodes and STI-bounded reference diodes, similar plots would be obtained for the test DRAM device of FIG.  5  and the reference device of FIG.  6 . 
     The forward bias slope of forward bias current versus bias voltage is defined by the amount of forward bias voltage/decade of diode current. This slope has a value of 59.4 mV/Decade at room temperature (27° C.) for a silicon diode without stress-induced defects. Using equation (1) the value of the forward bias slope may be calculated to be 59.4 mV/Decade at room temperature (27° C.) 
     
       
           S =Ln(10)× KT/q   (1) 
       
     
     Where: 
     S is forward bias voltage/decade of diode current; 
     Ln is the natural logarithm; 
     K is Boltzmann&#39;s constant; 
     T is absolute temperature in degrees Kelvin; and 
     q is the electron charge. 
     Diodes with stress-induced defects show forward bias slopes higher than 59.4 mV/Decade. FIG. 11 indicates that the increase in the slope becomes more significant as the density of dislocations increases from none to low to high. 
     In FIG. 11, measurements on a set of polysilicon-bounded test diodes with no stress-induced defects, a low level of stress-induced defects, a medium level of stress-induced defects and a high level of stress-induced defects are plotted. The level of stress-induced defects was verified by transmission electron microscopy (TEM.) 
     FIG. 12 is a histogram of the distribution of the slope, in mV/decade of current versus the forward bias current-voltage characteristics of polysilicon-bounded test diodes of FIG.  1  and STI-bounded reference diodes of FIG.  3 . In FIG. 12, the distribution of forward bias voltage versus current slopes is plotted as a histogram for one or more polysilicon-bounded test diodes and one or more STI-bounded reference diodes at a predetermined forward bias voltage (in this example, 0.45 volts.) STI-bounded reference diodes have no stress-induced defects (see below.) The dimensions for both polysilicon-bounded and STI-bounded diodes was, in this example, “W P ”=0.5 microns and “L D ”=of 100 microns (see FIG. 1.) FIG. 12 illustrates that for diodes with stress-induced defects, the forward bias versus current slopes at the pre-determined forward bias voltage have values well in excess of 59.4 mV/decade of current, reaching as high as 112 mV/decade of current. 
     When this test methodology is used for testing the screen or fail limit for the forward bias slope (at a pre-determined voltage), indicating presence of stress-induced defects may be set, in one example, at 64 mV/Decade, which is about 8% above the target value of 59.4 mV/decade for the forward bias slope of diodes without dislocations. This 8% tolerance allows for variations in measurement sensitivity. 
     Experiments performed with STI-bounded reference diodes, showed normal forward bias slope with no indication of stress-induced defects indicating STI-bounded reference diodes are suitable for use as control devices. The presence of stress-induced defects (in one example, dislocations) in polysilicon-bounded test diodes and lack of stress-induced defects (dislocations) in STI-bounded reference diodes was verified by transverse electron microscope (TEM) analysis. Determination of forward bias voltage versus current slope at about 0.4 to 0.5 volts of forward bias is optimum for this test methodology. Use of about 0.4 to 0.5 volts of forward bias voltage, with semiconductor stress-induced defects, results in the maximum increase in forward bias versus current slope with the presence of stress-induced defects, resulting in high sensitivity for the detection and characterization of stress-induced defects. 
     FIGS. 13A,  13 B and  13 C are flowcharts illustrating sixth, seventh and eighth test methodologies respectively, according to a third embodiment of the present invention. Referring to FIG. 13A, in step  670 , one or more polysilicon-bounded test diodes  100  (see FIGS. 1 and 2) is selected. In step  675 , each polysilicon-bounded test diode  100  is maintained at a pre-selected temperature. In one example, the pre-selected temperature is 160° C. However, any temperature in the range of about 100 to 200° C. may be used. In step  680 , for each polysilicon-bounded test diode  100 , N-well  115 , substrate  105  and polysilicon gate  125  (see FIG. 1) are held at ground potential. In one example, ground potential is about 0 volts. In step  685 , for each polysilicon-bounded test diode  100 , a pre-determined voltage is applied to P+ diffusion region  110  (see FIG. 1) for at least a pre-determined time. In one example, the predetermined voltage is about −6.3 volts or less and the pre-determined time is about 0.5 hours or more. In step  690 , for each polysilicon-bounded test diode  100 , the current through polysilicon gate  125  (see FIG. 1) is monitored for “soft” breakdown. 
     “Soft” breakdown is defined as an increase in gate current of about 10 to 50 times the breakdown current of an unstressed gate. “Hard” breakdown is defined as an increase in gate current greater than about 50 times the breakdown current of an unstressed gate. (In the present example, −6.3 volts for 0.5 hours are the stress conditions.) 
     Referring to FIG. 13B, in step  700 , one or more test DRAM devices  300  is selected. In step  705 , each test DRAM device  300  is maintained at a pre-selected temperature. In one example, the pre-selected temperature is 160° C. 
     However, any temperature in the range of about 100 to 200 ° C. may be used. In step  710 , for each test DRAM device  300 , N-well contact  385  and bit line contact  390  (see FIG. 5) are held at ground potential. In one example, ground potential is about 0 volts. In step  715 , for each test DRAM device  300 , word line contact  395  is held at a voltage sufficient to turn on transfer device  345  (see FIG. 5.) In step  72 G, for each test DRAM device  300 , a pre-determined voltage is applied to substrate contact  380  (see FIG. 5) for at least a pre-determined time. In one example, the predetermined voltage is about −5.0 volts or less and the pre-determined time is about 0.5 hours or more. In step  725 , for each test DRAM device  300 , the current through bit line contact  390  (see FIG. 5) is monitored for “soft” breakdown. 
     Referring to FIG. 13C, in step  730 , one or more test DRAM devices  300  (see FIG.  5 )is selected. In step  735 , each test DRAM device  300  is maintained at a pre-selected Hi temperature. In one example, the pre-selected temperature is 160° C. However, any temperature in the range of about 100 to 200° C. may be used. In step  740 , for each test DRAM device  300 , N-well contact  385 , substrate contact  380  and word line contact  395 (see FIG. 5) are held at ground potential. In one example, ground potential is about 0 volts. In step  745 , for each test DRAM device  300 , a pre-determined voltage is applied to bit line contact  390  (see FIG. 5) for at least a pre-determined time. In one example, the predetermined voltage is about −6.3 volts or less and the pre-determined time is about 0.5 hours or more. In step  750 , for each test DRAM device  300 , the current through word line contact  395  (see FIG. 5) is monitored for “soft” breakdown. FIG. 14 is a plot of the polysilicon gate current versus stress time for polysilicon-bounded test diodes of FIG. 1 with and without stress-induced defects. The data plotted in FIG. 14 was obtained from a polysilicon-bounded test diode having 5 nm of thermal oxide gate dielectric. The stress conditions were −6.3 volts at 160° C. for about 1.5E5 seconds. 
     It may be readily seen from FIG. 14 that the gate current prior to breakdown (prior to about 2.8E4 seconds) is about the same for diodes with and without stress-induced defects. FIG. 14 clearly illustrates that polysilicon-bounded test diodes with stress-induced defects show earlier breakdown than polysilicon-bounded test diodes without stress-induced defects. The earlier gate breakdown in polysilicon-bounded test diodes having stress-induced defects is attributed to the stress-induced defects causing spikes in the gate current which in turn stresses the gate dielectric causing it to breakdown. 
     FIG. 14 also clearly illustrates polysilicon-bounded diodes with stress-induced defects exhibit “soft,” limited, breakdown as defined above. In “hard” breakdown, the increase in gate current is limited only by the external circuit resistance, with almost no resistance contribution due to the gate oxide. While FIG. 14 is for polysilicon-bounded test diodes, similar plots would be obtained for the test DRAM device of FIG.  5 . 
     The seventh test methodology (illustrated in FIG.  13 B and described above) is particularly suited to detect stress-induced defects in the substrate near the deep trench capacitor. The eighth test methodology (illustrated in FIG.  13 C and described above) is particularly suited to detect stress-induced defects in the P+ diffusion/N-well interface near the gate dielectric of the transfer device. 
     It should be noted that the optimization of the polysilicon-bounded test diode/STI-bounded reference diode test system for the detection and characterization of the semiconductor stress-induced defects is a strong function of the perimeter-to-area ratio of polysilicon gate  125  of polysilicon-bounded test diode  100  of FIG.  1 . The sensitivity of stress-induced defect detection using the polysilicon-bounded test diode/STI-bounded reference diode test system increases as the gate perimeter to area ratio increases. A polysilicon gate perimeter-to-area ratio of 1.48/microns has been found to give satisfactory sensitivity. 
     It should also be noted that that the optimization of the polysilicon-bounded test diode/STI-bounded reference diode test system for the detection and characterization of the semiconductor stress-induced defects is also a function of the overlap space of polysilicon gate  125  with P+ diffusion region  110  (“O P ” in FIG. 1) An “O P ” value of about 0.26 microns has been found to give satisfactory sensitivity. 
     The structures and the test methodologies of the present invention may be used to monitor formation of stress-induced defects during fabrication of semiconductor devices providing a powerful tool for improving those processes in order to lower the number of stress-induced defects those processes cause. By use of the structures and the test methodologies of the present invention, processes and tools that contribute stress-induced defects can be more easily identified and corrected. 
     It has been found because of the sensitivity of polysilicon-bounded test diode  100  (see FIG.  1  and  2 ), such a device having intentionally created dielectric defects will function as an antifuse. We now turn our attention to this embodiment of the present invention. A diode (or antifuse) having a P+ diffusion region in an N-well is defined as a PN diode (or PN antifuse.) A diode (or antifuse) having a N+ diffusion region in a P-well is defined as an NP diode (or antifuse.) 
     FIGS. 15A through 15K are partial cross-sectional views illustrating fabrication of an antifuse according to the present invention. A top view is illustrated in FIG.  1  and the sections illustrated in FIGS. 15A through 15K are taken through line  2 — 2  of FIG.  1 . FIGS. 15A through 15K illustrate formation of a PN diode. An NP diode may be formed in a similar manner. Only the processes illustrated in FIG.  1 SE and described below differ from the processes that may be used to fabricate polysilicon-bounded test diode  100 . In the case of an antifuse the following dimensions are applicable (see FIG.  1 ): “L D ” is about 1 to 500 microns, “W D ” is about 1 to 10 microns, “W P ” is about 0.5 to 1.5 microns and “O P ” is about 0.1 to 0.6 microns. 
     In FIG. 15A, a silicon substrate  800  is provided. A ring of STI  805  is formed in silicon substrate  800  by, for example, well known trench etch and chemical-mechanical-polish (CMP) processes. STI  805  has an inner perimeter  807  and an outer perimeter  808 . In one example, silicon substrate  800  is doped P− with boron (B) at a concentration of 5E15 atoms/cm 2 . 
     In FIG. 15B, an N-well  810  is formed by ion implantation of phosphorus (P.) In one example, multiple phosphorous implants are performed, a first P implant at an energy of 650 Kev and a dose of 2.4E13 atoms/cm 2 , a second P implant at an energy of 300 Kev and a dose of 5E12 atoms/cm 2  and a third P implant at an energy of 35 Kev and a dose of 1E12 atoms/cm 2 . For a NP diode, boron would be implanted to form a P-well instead of an N-well. In one example, for an NP diode, the first implant is B at an energy of 260 Kev and a dose of 2.2E13 atoms/cm 2 , the second implant is B at an energy of 130 Kev and a dose of 6E12 atoms/cm 2  and the third implant is BF 2  at an energy of 35 Kev and dose of 1E12 atoms/cm 2 . N-well  810  extend below a bottom  815  of STI  805 . 
     In FIG. 15C, a gate dielectric layer  820  is formed on a top surface  825  of silicon substrate  800  and a polysilicon layer  830  is formed on a top surface  835  of the dielectric layer. In one example, dielectric layer  820  is thermal oxide about 10 to 120 Å thick and polysilicon layer  830  is about 1200 to 2000 Å thick formed by well known low pressure chemical vapor deposition (LPCVD) processes. 
     In FIG. 15D, dielectric layer  820  and polysilicon layer  830  are selectively removed by well know photolithographic and reactive ion etch (RIE) to form polysilicon gate  840 . Polysilicon gate  840  has an inner perimeter  842  and an outer perimeter  843 . Polysilicon gate  840  overlaps inner perimeter  807  of STI  805 . 
     In FIG. 15E, a protective layer  845  is formed on top surface  825  of substrate  800 . A photoresist layer  850  is formed and patterned (by well known photolithographic processes) on top of protective layer  845 . Protective layer  845  is exposed only inside of antifuse area  855 . In one, example, protective layer  845  is about 60 Å of thermal oxide. A heavy ion specie implant is performed in order to create defects in the inner perimeter  860  of gate dielectric  820 . In one example, the heavy ion specie is germanium (Ge) implanted at an energy of 40 Kev, a dose of 3E15 atoms/cm 2  and an angle of 7 degrees. In a second example, the heavy ion specie is arsenic (As) implanted at an energy of 45 Kev, a dose of 5E15 atoms/cm 2  and an angle of 7 degrees. The higher the atomic weight of the heavy ion specie, the lower the implantation dose and energy required in order to induce the desired damage in inner perimeter  860  of gate dielectric  820 . Then photoresist layer  850  is removed. 
     In FIG. 15F, protective layer  845  of FIG. 15E is removed. First silicon nitride spacers  865  are formed by well-known processes, on sidewalls  870  of polysilicon gates  840 . In one example first silicon nitride spacers  865  are formed from about a 125 Å thick film of silicon nitride. Then an angled halo ion implant is performed. In the case of a PN diode, the halo implant includes a relatively low energy and low dose implant(s) selected from the group consisting of germanium, arsenic, indium, boron and combinations thereof. In the case of a NP diode, the halo implant includes a relatively low energy and low dose implant(s) selected from the group consisting of germanium, arsenic, boron (as BF 2 ) and combinations thereof. 
     In FIG. 15G, second silicon nitride spacers  875  are formed over first silicon nitride spacers  865  by well-known processes. In one example second silicon nitride spacers  875  are formed from about a 800 Å thick film of silicon nitride. 
     In FIG. 15H, a photoresist layer  880  is formed and patterned (by well known photolithographic processes) on top surface  825  of silicon substrate  800 . An ion implant is performed to form N+ N-well contacts  885 . The ion implant includes relatively low energy and low to high dose implant(s) selected from the group consisting of germanium, phosphorous and combinations thereof. In the case of a NP diode, the ion implant includes a relatively low energy and low to high dose implant(s) selected from the group consisting of germanium, boron and combinations thereof. Photoresist layer  880  is then removed. 
     In FIG. 15I, a photoresist layer  885  is formed and patterned (by well known photolithographic processes) on top surface  825  of silicon substrate  800 . An ion implant is performed to form a P+ diffusion region  890  in N-well  810  between STI  815 . The ion implant includes relatively low energy and low to high dose implant(s) selected from the group consisting of germanium, boron and combinations thereof. In the case of a NP diode, the ion implant includes a relatively low energy and low to high dose implant(s) selected from the group consisting of germanium, phosphorus and combinations thereof to form an N+ diffusion region. Photoresist layer  885  is then removed. 
     In FIG. 15J, a silicide layer  895  is formed by well-known processes on N-well contact  885 , P+ diffusion region  890  and on top of gates  840 . In one example, silicide layer is cobalt silicide, titanium silicide or combinations thereof. 
     In FIG. 15K, a dielectric layer  900  is formed on top lo, surface  825  of substrate  800 . A multiplicity of stud contacts  905  are formed in dielectric layer  900  to make electrical contact to N-well contacts  905  and thence to N-well  810 , gates  840  and P+ diffusion region  890 . 
     FIG. 16 is a Weibull distribution for Time-to-Fail (T BD ) and Charge-to-Breakdown (Q BD ) for a polysilicon-bounded test diode of FIG. 1 used as an antifuse, with and without stress-induced defects. FIG. 16 compares a PN antifuse represented having a gate dielectric thickness of 5 nm of thermal oxide and having received a 7 degree angled germanium ion implant of 3E14 atoms/cm 2  at an energy of 40 Kev as illustrated in FIG.  15 E and described above to a PN antifuse without the germanium implant. Stress conditions were polysilicon gate and N-well at ground, P+ diffusion at −6.3 volts and temperature at 160° C. 
     FIG. 17 is a plot of the dielectric breakdown field at 30° C. versus germanium implantation dose of the antifuse of FIG.  15 K. For an antifuse to be reliably programmed a current density sufficient to induce a breakdown of the gate dielectric must flow through the dielectric. The voltage applied to the gate to obtain breakdown of the gate dielectric is the programming voltage. Implantation of heavy ion specie degrades the gate dielectric quality, effectively allowing more current through the gate dielectric for a given gate voltage then would occur without the implant thus causing more damage to the dielectric for a given voltage. Prior to heavy ion specie implantation, the gate dielectric breakdown electric field for dielectric thickness at or below 12 nm is about 14 MV/cm. FIG. 17 clearly shows that electric field required for breakdown of the gate dielectric is very sensitive to heavy ion specie implant dose and is significantly lowered even at relatively low implant doses. 
     FIG. 18 is a plot of dielectric breakdown voltage versus the inverse of absolute temperature of the antifuses of FIG. 15K, fabricated with two thickness of dielectric. FIG. 18 indicates that the breakdown voltage drop with increasing temperature has an activation energy of about 0.0124 eV. From the required energy to induce the required gate dielectric breakdown, it was determined that the minimum required programming current is under 2 micro amperes to be applied for a duration of about 0.05 seconds or a PN antifuse fabricated as illustrated in FIGS. 15A through 15G and described above and having a “L D ”=1.5 cm and a “W D ”=1.0 μm. 
     FIG. 19 is a plot of dielectric breakdown voltage versus germanium implantation dose of the antifuse of FIG. 15K at three temperatures. Similar plots can be directly obtained for any other gate dielectric thickness using equation (2): 
     
       
           V   P   =EBD×T   OX   ×AT   (2) 
       
     
     Where: 
     V P  is the required programming voltage; 
     EBD is the electric field required for breakdown; 
     T OX  is the effective electrical thickness of the gate dielectric taking into account any polysilicon depletion or surface depletion effects; and 
     AT is the temperature acceleration factor given by equation (3): 
     
       
           AT =exp{(Δ H/K )×[(1 /T )−(1 /TR )]}  (3) 
       
     
     Where ΔH is the activation energy (0.0124 eV from FIG. 18 ); 
     K is Boltzmann&#39;s constant; 
     T is the programming temperature in Kelvin; and 
     TR is the reference temperature in Kelvin (30° C.=303° K) 
     The area of the damaged edge of the gate dielectric following germanium implantation is very small (much less than one square micrometer.) Thus, the area of the antifuse fuse of the present invention may be very small, for example, as small as about 1-2 square micrometers. In fact, the area of the antifuse is limited only by the minimum critical dimension of the photolithographic system used to fabricate the antifuse. 
     The gate dielectric resistance prior to breakdown is in excess of 10 10  ohms at one volt. The gate dielectric resistance following breakdown is equal to or less than 1000 ohms, at one volt. Thus, the ratio of the dielectric resistance prior to breakdown, to that following breakdown, at one volt, is greater than 10 7 . 
     The plots illustrated in FIGS. 17,  18  and  19  may be used to determine the required implantation dose of germanium necessary to induce breakdown at any desired voltage and temperature. In one example: for a polysilicon-bounded PN diode used as an antifuse, having a gate dielectric thickness of 5 nm and receiving a 7 degree germanium implantation at an energy of 40 Kev and a dose of 3E15 atom/cm 2 , the required breakdown voltage is about 3.3V at 30° C. 
     Thus, it has been shown that the programming voltage of antifuse of the present invention is controlled by or tunable by heavy ion implant dose, gate dielectric thickness and temperature. It has further been shown that the programming voltage of antifuse of the present invention is dependent upon:the area of the region of the gate dielectric damaged by heavy ion specie implant and independent of the total area of the antifuse. 
     The description of the embodiments of the present invention is given above for the understanding of the present invention. It will be understood that the invention is not limited to the particular embodiments described herein, but is capable of various modifications, rearrangements and substitutions as will now become apparent to those skilled in the art without departing from the scope of the invention. Therefore it is intended that the following claims cover all such modifications and changes as fall within the true spirit and scope of the invention.

Technology Category: 5