Abstract:
A method is provided for nondestructive measurement of minority carrier diffusion (L p ) length and accordingly minority carrier lifetime (Ó p ) in a semiconductor device. The method includes the steps of: reverse biasing a semiconductor device under test, scanning a focused beam of radiant energy along a length of the semiconductor device, detecting current induced in the DUT by the beam as it passes point-by-point along a length of the DUT, detecting current induced in the semiconductor device by the beam as it passes point-by-point along the scanned length of the semiconductor device to generate a signal waveform (Isignal), and determining from the Isignal waveform minority carrier diffusion length (L p ) and/or minority carrier lifetime (Ó p ) in the semiconductor device.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to and derived from Provisional Application Serial No. 60/079,716, filed on Mar. 27, 1998, in which the inventorship and assignee are the same as herein. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a method for measuring by means of an optical beam induced current (OBIC), or by an electron beam induced current (EBIC), minority carrier diffusion length and minority carrier lifetime in various semiconductor devices, such as lateral double diffused metal oxide semiconductors (LDMOS) which are intended for higher voltage (HV) applications, as well as other semiconductor devices including metal oxide semiconductor field effect transistors (MOSFET), and ultra-miniature dynamic random access memories (DRAM). 
     BACKGROUND OF THE INVENTION 
     It is well known that an induced current can be generated in a semiconductor having a p-n junction or Schottky barrier (metal-semiconductor rectifying contact) by shining a focused beam of radiation of above bandgap energy, either optical or electron beam, on the body of the semiconductor. Apparatus for generating such beams and for scanning them across a device under test (DUT) are commercially available. Where a DUT is small (e.g., smaller than a micron), a scanning electron microscope utilizing an electron beam and vacuum chamber is typically used to investigate the device. In the case of a large area device, such as a high-voltage HV LDMOS transistor (which typically is ten or more microns in length), it is convenient to use a laser beam shining through an optical microscope to illuminate and scan the device. Such laser-optical apparatus is also commercially available. But in either case, when a semiconductor with a p-n junction is illuminated by a radiant beam of appropriate wavelength and intensity, a small current is generated in the a semiconductor. In the case of an electron beam, current is generated by the “Compton effect”. For a laser beam, current is due to the photo effect. Both of these effects are well known. 
     A problem prior to this invention was how to quantitatively measure in a nondestructive way the degradation of materials of a semiconductor device caused by process-induced defects, such as dislocations, oxidation induced stacking faults (OSFS), thermal and stress induced slip, misfit, point defect agglomeration and precipitation, bulk micro defects (BMDs), etc. Minority carrier lifetime is a good measure of the overall quality of semiconductor material, such as a wafer of silicon (Si). After a number of wafer processing steps (e.g., a hundred or more steps) and thermal cyclings, such as during annealing at above 900° C. or so, process-induced defects may be nucleated and generated in devices being fabricated on the wafer. When this happens minority carrier lifetime in the devices will show a degradation to a greater or less degree. The recombination properties of minority carriers determine the basic electronic properties of Si and silicon-on-insulator (SOI) materials and control the performance of a variety of Si and SOI devices. It is thus desirable to be able to measure easily, accurately and in a nondestructive way the minority carrier recombination characteristics of such devices. It is highly important to be able to do so for the proper and rapid evaluation of new Si and SOI technologies, where novel composite material systems are used and which may have varying degrees of crystal lattice perfection and unknown defect content. 
     The present invention provides the ability for quick, accurate and nondestructive measurement of minority carrier diffusion length and minority carrier lifetime in semiconductor devices. Prior to the invention, so far as is known, no one previously utilized either an EBIC or OBIC scanning system for the measurement of minority carrier diffusion length and/or minority carrier lifetime in semiconductor devices. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a method for measurement of minority carrier diffusion length (L p ) and/or minority carrier lifetime (Ó p ) in a semiconductor device such as a high-voltage transistor having a p-n junction between a p-type conductivity region and an n-type conductivity type region. 
     In one aspect the present invention is directed to a method for measurement of minority carrier diffusion (L p ) length and accordingly minority carrier lifetime (Ó p ) in a semiconductor device. The method comprises the steps of reverse biasing the semiconductor device; scanning a focused beam of radiant energy along a length of the semiconductor device; detecting current induced in the semiconductor device by the beam as it passes point-by-point along the scanned length of the semiconductor device to generate a signal waveform (Isignal); and determining from the Isignal waveform minority carrier diffusion length (L p ) and/or minority carrier lifetime (Ó p ) in the semiconductor device. 
     From another aspect the present invention is directed to a method for nondestructive measurement of minority carrier diffusion (L p ) length in a semiconductor device having a p-n junction between a p-type conductivity region and an n-type conductivity region. The method comprises the steps of reverse biasing with a voltage the semiconductor device; scanning a focused beam of radiant energy along a distance “x” of a length of the semiconductor device over the p-n junction and into one region thereof; detecting current induced in the semiconductor device by the beam as it passes point-by-point along the scanned length of the semiconductor device to generate a signal waveform (Isignal) as a function of distance “x”; and determining from the Isignal waveform minority carrier diffusion length (L p ), and/or minority carrier lifetime (Ó p ) in the semiconductor device. 
     From still an other aspect the present invention is directed to a method for nondestructive measurement of minority carrier diffusion (L p ) length and/or minority carrier lifetime (Ó p ) in a semiconductor device, such as a high-voltage transistor having a p-n junction between a p-type conductivity region and an n-type conductivity region. The method comprises the steps of reverse biasing with a voltage a semiconductor device; scanning a focused laser beam along a distance “x” of a length of the semiconductor device over the p-n junction and into one region of the semiconductor device; detecting optically beam induced current (OBIC) in the semicondcutor device as the beam passes in the “x” direction along the scanned length of the semiconductor device to generate a signal waveform (Isignal) as a function of distance “x”; and determining from the Isignal waveform minority carrier diffusion length L p  and/or minority carrier lifetime Ó p  in the semiconductor device. 
     A better understanding of the invention together with a fuller appreciation of its many advantages will best be gained from a study of the following description and claims given in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG.  1 . is a schematic illustration partly in block diagram form of apparatus for scanning a semiconductor device under test (DUT) and for generating optical beam induced current in the DUT to determine a measurement of minority carrier diffusion length (Lp) in the DUT and thereby determine minority carrier lifetime (Ó p ) in accordance with a method of the present invention; 
     FIG. 2 is a circuit diagram showing how a semiconductor DUT, such as shown in FIG. 1, is reverse biased during measurement of diffusion length Lp; 
     FIG. 3 is a greatly enlarged schematic view showing the DUT;of FIG. 1; 
     FIG. 4 is a somewhat idealized graph of measured optical beam induced current (OBIC) versus distance in an “x” direction along a length of the DUT of FIG. 3, and shows a waveform of a signal (Isignal) obtained for a given supply voltage V as the optical beam scans in the “x” direction along the length of the DUT; 
     FIG. 5 shows actual Isignal waveforms for various supply voltages V, the waveforms being those projected on a CRT display unit of FIG. 1; and 
     FIG. 6 is a semi-logarithmic graph of measured values of Isignal versus distances along the DUT for a given voltage V. 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIG. 1, there is shown an optical beam scanning apparatus  10  utilized to measure the minority carrier depletion length Lp of a semiconductor device under test (DUT)  12  in accordance with a method of the present invention. The apparatus  10  comprises a laser  14  which emits a beam  16 , a polarizer  18 , a deflection mirror system  20 , a microscope  22  which focuses the beam  16  in a fine spot on the DUT  12 , a device power source  24 , a signal amplifier  26 , a signal mixer  28 , a raster generator  30 , a cathode ray tube (CRT) display  32 , and a personal computer (PC)  34 . While individual elements of the apparatus  10  are well known in the art, they are utilized in accordance with the method of the present invention in a unique way to measure nondestructively the diffusion length (Lp) of minority carriers in the DUT  12 , as will be explained in detail hereinafter. It is to be understood that the invention is not limited solely to use of optical beams but applies equally well to use of electron beams. 
     The laser  14  emits a beam  16  of light with a wavelength, for example, of 633 nanometers (nm) and of suitable intensity (e.g., several watts). The beam  16  passes through a polarizer  18  and into the deflection mirror system  20 . The mirror system  20  includes a plurality of moveable mirrors  40  and  42  which are driven back and forth by electrical signals received via a lead  44  from the raster generator  30 . Mechanical movement of the mirrors  40  and  42  deflects the beam  16  back-and-forth and side-to-side in synchronism with electrical signals from the generator  30  as is well known to synchronize it with the CRT display. The beam  16  passes from the deflection mirror system  20  and into the microscope  22  which focuses the beam  16  into a fine spot (e.g., about a micron in diameter) onto a surface of the DUT  12 . This results in optical beam induced current (OBIC) in the DUT  12 . The beam  16 , as it is being deflected by the deflection system  20 , thus scans line by line along a length of the DUT  12 . 
     During scanning by the beam  16 , the DUT  12  is reverse biased by a positive voltage (+V) from the device power source  24 , (a negative side of which is grounded), and a load resistor (R L )  46 . A photo-response current passes from the DUT  12  through a sensing resistor (R S )  48  to ground. A signal, termed “Isignal” is applied from the sensing resistor  48  via a lead  50  to an input of the amplifier  26 , an output of which is applied via a lead  52  to one input (S) of the signal mixer  28 . A raster signal is applied to another input (R) of the signal mixer  28  via a lead  54  from the raster generator  30 . Dual signals (R+S) from the signal mixer  28  are applied via a common connection  56  to an input of the CRT display  32  where the signals are displayed on a screen  58  as a waveform (to be discussed hereinafter) having the usual x and y coordinates. The dual R+S signals on the common connection  56  are also applied to the personal computer  32  where they are processed and the minority carrier diffusion length L p  and minority carrier lifetime (Ó p ) are obtained, as will be explained hereinafter. 
     Referring now to FIG. 2, there is shown a schematic circuit diagram  60  showing electrical connections to the DUT  12  of FIG.  1 . As seen in the diagram  60 , the DUT  12  is, by way of example, an LDMOS type transistor  62  having a drain  64 , a gate  65 , and a source  66 . It is to be understood, however, that the DUT  12  may be a semiconductor other than the transistor  62  shown here. The transistor  62 , while being scanned by the beam  16 , is reverse-biased with its drain  64  connected via the load resistor (R L )  46  (see FIG. 1) to a positive terminal  68  (+V) of the power supply  24  (not shown here but shown in FIG.  1 ). A negative side of the device power supply  24  (not shown here but shown in FIG. 1) is connected to a negative terminal  69  (−V) which is shown as ground. The gate  65  of the transistor  62  is connected directly to ground while the source  66  is connected via the sensing resistor (R S )  48  to ground. A voltage across the sensing resistor  48  is proportional to OBIC current as the DUT  12  is scanned by the beam  16 , as was previously explained. This voltage is applied to the lead  50  (also see FIG. 1) and is termed “Isignal” The output voltage +V of the supply  24  can be varied over a wide range until the onset of avalanche breakdown of the device. By way of illustration, the resistor (R L )  46  may have a value of 100 K ohms, the resistor (R S )  48  a value of 10 K ohms, and the resistance in reverse bias across the drain  64  and the source  66  of the transistor  62  may be approximately 100M ohms. 
     Referring now to FIG. 3, there is shown greatly enlarged and in highly schematic and simplified form of a lateral high voltage diode, the DUT  12 . Distance along the length of the DUT  12  is indicated at “x”; various locations of the light beam  16  as it is scanned to the right along the DUT  12  are as indicated, and a p-n junction  70  of the DUT  12  is indicated at x=0. A first shaded area of the DUT  12  represents a p+type conductivity body region  72 , an unshaded area on both sides of the p-n junction  70  represents an unsymmetrical space charge region  74 , and a second shaded area represents the remainder of the n-type conductivity drift region  76 . Electrical connections to the DUT  12  are as indicated (see also FIG.  2 ). 
     The unshaded area (space charge region  74 ) represents the depletion region adjacent to the p-n junction  70  of the DUT  12  where all of the laser induced photo-generated electron-hole pairs are separated and collected by the high local field applied by the reverse bias voltage, giving rise to the OBIC photocurrent. The second shaded area indicates a neutral n-type drift region  76  outside of the depletion region (space charge region  74 ) where the photo-generated carriers are not collected any more and no OBIC photocurrent is measured. 
     Within the space charge region  74  essentially all photo-generated electron-hole pairs are collected and yield a maximum signal which is applied to the lead  50  as “Isignal”. As the scanning laser beam  16  passes across an outer edge  78  of a depletion width, indicated at  79 , farther into the n-type drift region  76 , the collected OBIC (and Isignal) begins to decay. The actual depletion width, bounded by the edge  78 , is dependent on the voltage +V of the power source  24 . 
     It is known that the minority carrier current density (J p ) in a semiconductor as a function of distance (x) for the case of a reversed biased p-n junction of the semiconductor can be expressed by the following equation:              (         J   p          (   x   )       =       -     q     L   p              D   p          p   n                   x   n     -   x       L   p             )           Eq   .              1                                
     where J p (x) is minority carrier current density, 
     q is the elementary electronic charge, 
     L p  is the diffusion length, for minority hole carriers, 
     D p  is the diffusion constant for holes, 
     p n  is the equilibrium minority carrier concentration of holes in n-type material, and 
     where x n  is as shown at  78  in FIG. 3 and x is the measured distance to the right beyond the edge  78  of the depletion width  79 . 
     It can be shown by a mathematical derivation from Eq. 1 that an OBIC signal measured as Isignal (FIGS. 2 and 3) is proportional to the exponent of                    (       x   n     -   x     )       L   p                       or:          Isignal     ≈            (       x   n     -   x     )     /     L   p                 Eq   .              2                                
     or; 
     
       
           I signal≈ e   (xn−x)/Lp   Eq. 2 
       
     
     It can also be shown by a further mathematical derivation using the Einstein relationship that minority carrier lifetime (Ó p ) can be expressed as: 
     
       
           Ó   p   =L   p   2 /(μ p   kt/q )  Eq. 3 
       
     
     where kt/q at 300° K=2.586×10 −2  volts and μ p  is the mobility of holes. A closely similar equation defines the lifetime of electrons. 
     As was mentioned above (see Eq. 2), the OBIC photocurrent (and Isignal) beyond the edge  78  of the depletion width  79  in the n-type drift region  76  varies proportionally (or nearly so) to exponent (xn−x)/Lp. In other words, the exponential decay of the OBIC photocurrent (and Isignal) in the n-type drift region  76  within a certain number of diffusion lengths beyond the edge  78  of the depletion width is proportional to the minority carrier diffusion length Lp and consequently is a measure of the Lp of the semiconductor material from which the DUT  12  is built. This will be explained in greater detail hereinafter. 
     Referring now to FIG. 4, there is shown a graph  80  of a somewhat idealized waveform  82  of Isignal. The horizontal axis of the graph  80  represents distance in the x direction along the DUT  12  (FIG.  3 ). The vertical axis of the graph  80  represents the magnitude of Isignal with a normalized level of “ 1 . 0 ” representing the maximum measured value. A first vertical dashed line  84  of FIG.  4  and FIG. 3 designates the position of the p-n junction  70  at x=0. A second vertical dashed line  86  of FIG.  4  and FIG. 3 designates the position of the edge  78  of the depletion width  79  at x n , and a third vertical dashed line  88  of FIG.  4  and FIG. 3 represents the right end of the n-type drift region  76  of the DUT  12 . 
     The waveform  82  has a first, generally horizontal portion  90 , representing substantially constant OBIC (and Isignal) at the level 1.0 generated in the space charge region from x=0 to x=n. The waveform  82  then has a generally curved portion  92  beginning at x=n and decaying approximately exponentially from the level of 1.0 to 0 within a certain number of diffusion lengths as the beam  16  scans farther and farther along the n-type drift region  76 . Isignal falls to zero (0) well before the right end of the n-type drift region  76  is reached. The waveform  82 , which represents Isignal, is applied to the personal computer  34  (FIG. 1) which thereupon automatically computes from the measured values minority carrier diffusion length Lp and minority carrier lifetime (Ó p ) Computer programs for such computation are easily written by those skilled in the art. 
     Referring now to FIG. 5, there are shown a number of oscilloscope traces from the screen  58  of the CRT display unit  32  (FIG. 1) of Isignal waveforms  100 ,  101 ,  102 ,  103 ,  104 ,  105 ,  106 ,  107 ,  108 ,  109  and  110  for respective supply voltages  30 ,  40 ,  50 ,  60 ,  70 ,  80 ,  90 ,  100 ,  110 ,  120 , and  130  volts as indicated. The horizontal axis of the waveforms  100 -  110  represents distance x as the beam  16  is scanned in time along the length of the DUT  12  (FIG.  3 ), and the vertical axis represents the level (normalized) of Isignal (FIG.  4 ). The waveform  101  has been manually displaced a small amount vertically above the waveform  100  in order not to confuse one waveform with the other. And the remaining waveforms  102 - 110  are each similarly displaced one from the other for the same reason. 
     A vertical line  120  shows the beginning of each waveform  100 - 110  at x=0 (the P-n junction  70  of the DUT  12 ) as it is being scanned by the beam  16  (see also FIGS.  3  and  4 ). A dashed vertical line  124  indicates that the end of the DUT  12  has been reached (see the dashed vertical line  88  between FIGS.  3  and  4 ). Each waveform  100 - 110  has a generally horizontal portion, akin to the portion  90  in FIG. 4, and then a generally exponentially decaying portion, akin to the portion  92  in FIG. 4, which drops to zero (0) as the beam  16  scans farther and farther along the n-type drift region  76  of the DUT  12  (FIGS.  3  and  4 ). 
     An upwardly slanting dashed line  130  intersects each of the waveforms  100 - 110  at a point where the respective waveform begins to change from being generally horizontal (see the portion  90  of the waveform  82  of FIG. 4) to decreasing generally exponentially (see the portion  92  of the waveform  82  of FIG.  4 ). This dashed line  130  indicates that the width of the depletion region bounded by the edge  78  (FIG. 3) increases as the supply voltage V is increased from 30 V to 130 V as shown here. All of the waveforms  100 - 110  decay to zero (0) before the end (indicated by the vertical dashed line  124 ) of the DUT  12  is reached. 
     Referring now to FIG. 6, there is shown a semi-logarithmic graph  200  of actual measurements of respective values of Isignal versus distances in the “x” direction along the DUT  12  for a reverse bias voltage of 20V. The vertical axis of the graph  200  shows on a semi-logarithmic scale values of Isignal below a normalized value of “1.0”, and the horizontal axis shows linear values of distance “x” measured in microns. The respective measurements of Isignal versus distance are indicated at points  202  which, as plotted in the semi-logarithmic graph  200 , lie along a generally straight line  204 . It should be understood that the straight line  204  shown here is equivalent to (via mathematical transformation) the generally exponential portion  92  of the waveform  82  of FIG.  4 . As was mentioned previously, values of L p  and Ó p  are readily calculated from the data of graph  200 . Using the Equations 2 and 3 for the particular set of measurement points  202  shown here, a value of 12.66 microns is obtained for L p , and a value of 137.7 nanoseconds is obtained for Ó p . 
     The above description is intended in illustration and not in limitation of the invention. Various changes in the apparatus described and in the method of the invention as set forth may occur to those skilled in the art, and these changes may be made without departing from the spirit or scope of the invention as set forth in the accompanying claims. In particular, the invention is not limited solely to application with the transistor illustrated in FIGS. 2 and 3, but is applicable to other semiconductor devices. Nor is the invention limited solely to use with OBIC apparatus but includes any EBIC configuration in a scanning electron microscope [SEM].