Abstract:
A method of determining electron tunneling values at various locations in a capacitor structure having a first and a second conductive plate with a dielectric material disposed there between, wherein each plate has first and second ends, including the steps of: determining the nominal tunneling voltage of the dielectric material at its thickness to provide a target voltage. Applying a first voltage level equally across the first plate. Applying a second voltage level to the first end of the second plate which together with the voltage applied to the first plate establishes a positive offset voltage with respect to the target voltage. Applying incrementally changing voltage levels to the second end of the second plate, which varying voltage levels change the voltage at the second end of the second plate of each set to vary the length of the capacitive structure above the target voltage.

Description:
BACKGROUND OF INVENTION 
     1. Field of the Invention 
     This invention relates generally to testing of capacitor structures and, more particularly, to the testing of capacitor structures for electron tunneling. In even more particular aspects, this invention relates to testing of the dielectric material in capacitor structures for variations in electron tunneling at various locations in the dielectric material. 
     2. Background Information 
     In capacitor structures wherein a film of dielectric material is disposed between two plates of conductive material, electron tunneling occurs above some voltage differential between the plates, less than breakdown voltage. The voltage at which tunneling starts to occur (sometimes referred to as the tunneling voltage) is dependent upon the characteristics of the dielectric material. In an ideal situation, the dielectric would be entirely uniform in all regions, and thus electron tunneling would occur uniformly at the same voltage at all regions of the capacitor structure for the same thickness of dielectric material. However, such uniformity cannot be achieved in production, so tunneling can, and often does, occur at different voltages at different regions of the capacitor structure. This difference is especially important in low voltage (e.g. about 2 volts) thin film capacitors of the type used in microelectronics, such as integrated circuit (I/C) devices. 
     Present conventional test methods for testing thin film capacitors test only the entire capacitor structure, not any particular region. This will give an average value of the electron tunneling voltage across the entire structure, but it does not show any variations in tunneling voltage from region to region. However, a technique is desired to check for variations in voltage required for electron tunneling at various regions of the capacitor structure. 
     SUMMARY OF INVENTION 
     A method of determining electron tunneling values at various locations in a capacitor structure having a first and a second conductive plate with a dielectric material disposed there between, wherein each plate has first and second ends comprising the steps of; determining the nominal tunneling voltage of the dielectric material at its thickness to provide a target voltage. 
     Applying a first voltage level equally across the first plate. Applying a second voltage level to the first end of the second plate which together with the voltage applied to the first plate establishes a positive offset voltage with respect to the target voltage. Applying incrementally changing voltage levels to the second end of the second plate, which varying voltage levels change the voltage at the second end of said second plate of each set to vary the length of the capacitive structure above the target voltage. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view of a thin film capacitive structure for testing according to this invention with various test voltages shown in conjunction therewith; 
         FIGS. 2–4  are views similar to  FIG. 1  showing different test voltages applied; 
         FIG. 5  is a plan view of the top plate of a capacitive device test structure suitable for both transverse and orthogonal testing of scanning; 
         FIG. 6  is a sectional view taken substantially along the plane designated by the line  6 — 6  of  FIG. 1 ; and 
         FIG. 7  depicts another embodiment for testing a capacitive structure according to this invention. 
     
    
    
     DETAILED DESCRIPTION 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings and for the present to  FIG. 1 , a thin film capacitor  10 , structured for testing according to this invention, is shown. It is to be understood that this invention, as disclosed, is primarily for the purpose of research testing, and is somewhat different from a structure used in production, although such use is not precluded. 
     The capacitor structure  10  is comprised of top or upper plate  12  having a plurality of individual electrically conductive sections  14  joined by a central conductor  16 . (As used herein, the term conductor means electrically conducting, unless otherwise noted, and, hence, can refer to either electrical conductors or semiconductors.) The structure of individual sections  14  is to facilitate isolating various sections of the capacitor structure  10  during testing as will become apparent presently. A conductive bottom or lower plate  20  is provided. A dielectric insulating material  22  separates the upper plate  12  and lower plate  20  to form the capacitor structure  10 . In the disclosed embodiment, which is for thin film capacitors, the sections  14  of the upper plate  12  are polysilicon about 0.15 microns thick, the central conductor  16  preferably is polysilicon about 0.15 microns thick, the lower plate  20  is silicon about 700 microns thick, and the dielectric insulating material  22  is oxinitride less than 4 nanometers thick. On the top plate, on the right side as seen in the figures, is an electrical contact  26  at the end of central conductor  16 . Also, a contact  28  is on the left side of the central conductor  16 , and a contact  29  is provided on the bottom plate  20 . A sectional view of the structure is shown in  FIG. 6 . As will be described presently, by applying various voltages to the different contacts, the uniformity, or lack thereof, of the dielectric material  22  can be determined. 
     Before referring specifically to the drawings, and a detailed description of the technique, an overview of this technique will be given. First, a nominal value of the minimum voltage for tunneling to occur in the dielectric material  22  is determined, and this is referred to as the target value or target voltage  30 . One plate, e.g., bottom plate  20 , is set at a given voltage, i.e. ground or 0, and one end, e.g. contact  26  of the top plate  12 , is set at a voltage value that, with reference to the bottom plate  20 , is greater than or off set to the target value  30 . The opposite end of the top plate  12 , e.g. contact  28 , is set at a lower value such that, with reference to the bottom plate  20 , the voltage at that end of the top plate is below the target value; i.e. there is a voltage drop from one end of the top plate, e.g. contact  26  to the other end e.g. contact  28 . Hence, some portion of the capacitive structure  10 , e.g., that to the right, is above the target voltage  30 , and electron tunneling will occur. The other portion of the capacitive structure  10 , e.g. that to the left, will be below the target voltage  30  and tunneling will not occur. The relative lengths of the two portions will depend upon the relative voltages of the contacts  26  and  28 , and the portion of the capacitive structure in which tunneling is occurring can be determined. By raising the voltage incrementally at contact  28 , length to the right of the capacitive structure at which tunneling occurs is increased and the length of the capacitive structure where tunneling does not occur is decreased. By measuring the increase in tunneling, sometimes referred to as scanning, the tunneling voltage at various locations can be calculated, as will be described presently. This technique is demonstrated in  FIGS. 1–4 . 
     In  FIG. 1 , the bottom plate  20  is maintained at a given voltage; in this case,  0  or ground Voltage, and the entire top plate is maintained at a given voltage, i.e. 2 volts so that the voltage between the bottom plate  20  and the top plate  12  is an offset  32  above the target voltage  30  across the entire capacitive structure, as shown in the shaded area  34  in  FIG. 1 . In this case, the tunneling effect will be equal to the average tunneling value across the entire capacitive structure  10 , irrespective of variations in the dielectric  22  at various locations along the capacitive structure. 
     Referring now to  FIG. 2 , there is depicted the situation wherein the left side of the top plate  12  has the voltage maintained at a level which is substantially below the target voltage  30 , while the right side is maintained at the 2 volt level. With the voltage drop from right to left on the top plate, the voltage across the plates  20  and  12  drops below the target voltage at the location  36 ′. Thus, only the shaded portion  34 ′ is above the target voltage and, hence, tunneling is occurring only in this section of the capacitive structure corresponding to the shaded portions  34 ′, and not across the entire capacitive structure  10 . The amount of tunneling is measured in this section. 
     Referring now to  FIG. 3 , the voltage has been raised an increment at contact  28  so that it is higher than the voltage of  FIG. 2 , but less than the target voltage. This will move the position of location  36 ′ where the voltage drops below the target voltage  30 , to the left, so that the shaded area  34 ″, which is a larger area than shaded area  34 ′, represents the corresponding section of the capacitive structure  10  in which tunneling is occurring and is measured. The increase in tunneling as a function of the increase in the length of the capacitive structure is determative of the relative homogeneity of the dielectric material  22  between the section  34 ′ measured in  FIG. 2  and the increase in section  34 ″ shown in  FIG. 3 . It should be noted that while the objective is to change the voltage at the left hand side of  FIG. 2  to that of  FIG. 3 , the most precise way is not to directly control the voltage at location  28 , but to control the current flowing between the contact  26  and contact  28  since current can be more precisely controlled, and since there is a voltage differential between contact  26  and  28 , current will flow of course, since E=IR. 
       FIG. 4  shows yet another incremental increase of the voltage at the left side of upper plate  12  above that shown in  FIG. 3 , but still below the target voltage  30 . This will move the location  36 ′″ to the left of location  36 ″ shown in  FIG. 3 , again increasing the shaded area  36 ′″, thus increasing the area of the capacitive structure in which tunneling occurs. It is to be understood that the method has been described by starting with a voltage at the left side of the plate at a voltage below the target voltage, and then incrementally increasing the voltage. However the method can also start with the left hand side having a voltage equal to or greater than the target voltage, and then incrementally decreasing the voltage. Also it is to be understood that the terms “right” and “left” and “top” and “bottom” merely designate the locations and orientations in the drawings as depicted, and the locations and orientations could be reversed or otherwise changed. 
     As indicated earlier, for test purposes, it is preferred that the top plate  12  be formed in discrete sections since this allows for better incremental discrete measurements of tunneling. Indeed, the width of the sections  14  in large measure determines the increments used to vary the voltage. 
     If desired, the entire process can be repeated going in the opposite direction, e.g. keeping a constant voltage on the contact  28 , and varying the voltage on contact  26 , and also scanned orthogonally if desired.  FIG. 5  shows a preferred top plate  12 ′ construction for longitudinal and orthogonal scanning. In this embodiment the top plate is formed in sections  14 ′ with perpendicular conductors  16 ′. This construction allows both transverse and orthogonal scans to be made. 
     If a location is identified as having electron tunneling at a lower voltage than the target voltage, this region can then be subjected to failure analysis tests. 
     Also, it is possible to start both contacts  26  and  28  at the same offset voltage, and then reducing one incrementally, rather than the opposite. In this case the starting voltage would look like  FIG. 1 , and the last incremental scan would look like  FIG. 2 . 
     Various formulae to calculate tunneling effects are as follows: Current is forced for constant dX/dl across structure after establishing target voltage and Vbias above target voltage (bias where defective tunneling current is measurable during normal voltage ramp testing and additional offset above target voltage, respectively). 
     The measured tunneling current of the structure on the preceding page should have discernable steps where the tunneling contribution of each capacitor leg adds to the total as X (point along the structure where target voltage is attained) is swept across the structure. 
     Formulae where Vtarget=Target bias, Voffset=Offset bias, Vtotal=Target bias+offset bias (total voltage applied to the right side), V′o=Resulting Voltage in forcing current referenced to Vtotal, L=Length of the structure, R=Resistance of the structure, X′=initial position along L (position when Vforced=lower plate voltage; or V′o=Vtotal), X=position along L from the Vtotal side, Iforced =forced Current, X=(Voffset/Vtotal)*L, I=V′o/R=(Voffset/R)*(L/X), V forced=Vtotal−V′o. A slightly modified embodiment, the initial set voltage at contact  26  is set at about the target voltage, and the voltage at contact  28  again is set below target voltage and increased incrementally. This will determine if any locations of the capacitive structure have dielectric material  22  that tunnels below the target voltage. 
     Referring now to  FIG. 6  another embodiment of the invention is shown. In this embodiment each segment of the upper plate  14  is tested against another segment of the upper plate for leakage through the dielectric material there under between the bottom plate  20 . As shown in  FIG. 6 , one top segment, designated as  14   a  is tested against another segment designated as  14   b.  In this case the bottom plate is  20  is divided into individual segments designated  20   a  and  20   b  under plate segments  14   a  and  14   b  respectively. The segments  14   a  and  20   a  and  14   b  and  29 b are separated by insulating sheet not shown in this Figure. In this embodiment top segment  14   a  has a pair of spaced contacts  40   a  and  42   a,  and top segment  14   b  has a pair of spaced contacts  40   b  and  42   b.  The bottom segments  20   a  and  20   b  are connected to opposite sides of a differential amplifier  46  to amplify the difference between sensed current or voltage at  20   a  and  20   b.    
     In operation, each of the plate segments  20   a  and  20   b  is maintained at a given voltage below the target voltage, e.g. at ground. The contacts e.g.  40   a  and  40   b  at one end of each segment  14   a  and  14  b are maintained at the target voltage, as described above and the contacts e.g.  42   a  and  42   b  are maintained at a voltage level below target value and the electron tunneling at each section  20   a  and  20   b  is measured. If for any given voltage level there is any significant difference in the sensed level, then this is an indication that there needs to be failure analysis of the insulating dielectric material.