the present invention relates generally to a method and system for detecting or reviewing defective contacts in the fabrication of semi - conductor devices by using a scanning electron microscope ( sem ). the following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements . various modifications to the preferred embodiments and the generic principles and features described herein will be readily apparent to those skilled in the art . thus , the present invention is not intended to be limited to the embodiments shown , but is to be accorded the widest scope consistent with the principles and features described herein . a system and method in accordance with the present invention overcomes the above described problem and provides a method to identify open contacts on dram or the like structure , in which source and / or drain contacts constitute an array and share one or multiple common control gates . defective contacts can be identified with the sem utilizing a positive ( extraction field ) mode or a negative ( retarding field ) mode . to describe these modes in more detail refer to the following discussion in conjunction with the accompanying figures . a first conventional embodiment uses the positive mode with the landing energy ( fig2 ) le in between e 1 and e 2 . since a modern low landing energy sem tends to negatively bias the wafer ( refers to 310 of fig3 ), or stage ( not show ) with respected to the final objective lens 311 to reduce the landing energy , there is naturally an extraction field imposed between the wafer 302 and sem column 311 . if the primary electron beam operates in a positive mode region , the sample surface tends to be charged up positively while scanning . open contacts hold positive charges which attract ses , thus making the contact appear dark in the voltage contrast image . fig3 illustrates different effects of positive mode on nmos and pmos transistors 302 a and 302 b . for the nmos transistor 302 a , a positive surface charging imposes a reverse biased voltage on the associated pn - junction by primary beam scanning , which switches off the pn - junction . for normal contact 308 a and open contact 309 a , no significant different in signal can be made as indicated by 304 and 305 . for the pmos transistor 302 b , a positive surface charge voltage (& gt ; 0 . 7v ) is induced to switch on the associated pn - junction , excessive charges on normal contacts will be released to the n - well , which contributes more se signals 306 from the surface . contrary to the normal contact 308 b , an open contact 309 b retains secondary electrons , as indicated by 307 , thus making the contact appear dark in the voltage contrast image . instead of achieving a positive surface charging , a second conventional embodiment operates in a negative mode with a landing energy greater than e 2 . as a result , negative charging is induced on the surface . if the charging cannot be released in time as in the case of the open contact , more ses are generated and repelled in the proximity towards the detector , and therefore the open contact appears brighter than normal . fig4 , which illustrates the second conventional embodiment , depicts the effects of a negative mode on nmos and pmos transistors 402 a and 402 b . in the case of an nmos transistor 402 a , the negative charging voltage on the n + plug can be strong enough to offset the global extraction field and forward bias the associated pn - junction . excessive charges on the normal contact 408 a will be released via the pn - junction resulting in less se signals as indicated by 404 . this differentiates open contact 409 a from normal contact 408 a in a nmos transistor 402 a . in the case of a pmos transistor 402 b , negative mode simply sets the corresponding pn - junction to reverse bias , and therefore signals 406 and 407 from normal contact 409 b and open contact 408 b respectively cannot be differentiated effectively . u . s . pat . no . 4 , 843 , 330 introduced a grid at a proximal distance above the wafer and which can be independently biased positive or negatively with respect to the wafer substrate or stage . this makes it possible to locally create an extraction or retarding field above the wafer to affect the collection of secondary electrons and the charging of wafer surface . postek in “ critical issues in scanning electron microscope metrology ”, j . res . natl . inst . stand . technol . 99 , 641 ( 1994 ), pp . 662 , reported a similar approach by hitachi using an electrode to control the local field , enhancing the collection of se . paul ( u . s . pat . no . 6 , 720 , 779 ) claimed a method which utilizes a much higher extraction field to trigger the reverse biased pn - junction to junction breakdown . fig5 shows a third conventional embodiment which illustrates the application of an extraction field mode to nmos and pmos transistors 502 a and 502 b . by increasing the positive bias voltage 510 on the grid 512 located between the objective lens 511 and the wafer 502 , an extraction field is imposed on wafer surface . as it increases , the pn - junction of nmos transistor 502 a is set in close proximity to reverse breakdown . excessive positive charges on the normal contact 508 a will trigger the junction leak , thus the contact no longer accumulates charge . this lack of accumulation differentiates the signals 505 of an open contact 509 a from signals 504 of a normal contact 508 a for nmos . with this embodiment , the open contacts 509 a and 509 b of both nmos transistor 502 a and pmos transistor 502 b can be identified as dark voltage contrast images due to their relatively weak se signals 505 and 507 compared to normal signals 504 and 506 . the third conventional embodiment overcomes the difficulty of the second conventional embodiment . however , there is a high risk of electrical arcing over the rough surface due to the strong extraction field , which can reach voltages up to and beyond 1000v / mm . also , the pn - junction sometimes may be over stressed causing permanent damage . a fourth conventional embodiment as shown in fig6 biases the grid 612 at a negative voltage 610 with respect to the wafer 602 . retarding field helps provide a negative charge to the wafer surface and contacts , resulting in a forward bias voltage on the pn - junctions of nmos 602 a . the pn - junction is turned on and drains away excessive charges , resulting less se signals 604 from a normal contact 608 a . for an open contact 609 a , charges accumulate continuously to reduce the retarding field , and as a result more se 605 can escape from the surface and show as bright vc . this embodiment is not sensitive enough for the detecting or reviewing of open contacts to pmos devices . the negative mode in the second conventional embodiment of fig4 and the retarding field mode in the fourth conventional embodiment of fig6 both rely on the primary electron beam as the source for surface negative charging . simultaneously the difference of surface charging is also sensed by the same primary electron beam . the multiple tasks imposed on the primary beam sometimes cannot be fully accomplished at a given sampling time , and therefore multiple scans must be utilized resulting in a trade off in throughput . u . s . pat . no . 4 , 417 , 203 describes a second source of electron beam , called a flood gun , to create a negative charging on the wafer surface in advance prior to imaging , as indicated by 713 in fig7 . this patent is consistent with the second conventional embodiment of fig4 and the fourth conventional embodiment of fig6 in realizing a negative charged surface . if combined together with properly optimized conditions , the detecting or reviewing of the open contact to the nmos transistor will be more effective and sensitive . fig7 shows the combination as a fifth conventional embodiment . similar to the second conventional embodiment of fig4 , the fourth and fifth conventional embodiments of fig6 and 7 provide a better indication of an open contact to an nmos device . but there are other issues which arise from utilizing a strong retarding field : se signals are significantly suppressed and signal to noise ratio is relative low . furthermore , the developing of negative surface charging is difficult to control and maintain in a uniform and stable state . fig8 illustrates a memory cell utilized in a dram . a memory cell comprises a nmos transistor 802 and a capacitor 804 . for a deep trench dram , the capacitor and gate are formed at the early process steps , thus they are buried in a dielectric layer . if the relevant pn - junction leak current 914 is minimal , fig9 illustrates the difficulties in producing difference between signal 903 from a normal bit contact 909 and 905 from an open bit contact 908 . a method and system in accordance with the present invention overcoming the above - described problems and provides a method and system for detecting or reviewing open contacts on a semiconductor device such as a dram . in general for the method and system , two steps of scanning are involved with either one or two sources of beams : the first source is of primary electron beam for imaging ; the second source is an accessory beam can be either an electron beam , ion beam , or laser beam for pre - charging the device into expected status . in one embodiment , a method is disclosed which comprises scanning a specific area of common gate contacts of mos devices with second source of beam a plurality of times , until the gate is positively charged enough over a threshold voltage . then scanning the associated source and / or drain contacts with first beam source in order to excite se signals and form a voltage contrast image . the image is then analyzed to observe the grey level of the contact to reveal the difference in electrical properties , or the scanned image is compared with a reference image to detect contacts with an abnormal grey level as defective contacts . in another embodiment , a method is disclosed which comprises scanning the gate contact area and source and / or drain contact area simultaneously with an accessory beam a plurality of times . then scanning the source and / or drain contacts with the first beam source in order to excite se signals and form a voltage contrast image . the image is then analyzed to observe the grey level of the contact to reveal the electrical properties difference quantitatively , or the scanned image is compared with a reference image to detect contacts which display an abnormal grey level as defective cells . to describe the features of the present invention in more detail , refer now to the following discussion in conjunction with the accompanying figures . memory cells are arranged into rows and columns to form a memory array . fig1 shows the layout 1000 of a cell array in folded architecture , as well as an example of cross - sections 1002 at the manufacturing step of the bit contact layer . word lines 1003 connect the columns of the memory array , and bit lines 1004 connect the rows of bit contact of the memory array . a word line decoder ( not shown ) is connected to the word lines for selecting one of the word lines . a sense amplifier ( not shown ) is connected to the bit lines to sense a voltage change on the bit line in an array . fig1 illustrates complete sub - array circuitry architecture 1100 . a dram operates by sending a charge through the appropriate contacts to the world line to activate the transistor at each bit . when writing , the bit lines contain the state that the capacitor should hold . when reading , the sense - amplifier determines the level of charge in the capacitor . during deep trench dram manufacturing , when the manufacturing process reaches the bit contact layer step , both the world line and the cell capacitor have been already fabricated and buried in the dielectric . from the top layer , an sem image can only resolve the bit contacts and other contacts of control circuitry . fig1 shows the top view of cell arrays and word line decoder circuitry . the gate of cells along a word line can only be accessed via word line contacts . as defined hereinbefore , conventional approaches focus on ways to manipulate the associated pn - junctions into different kinds of mode or status . they all exhibit ineffectiveness in identifying open bit contacts at the bit contact layer . for a deep trench dram , the buried capacitor causes the source and / or drain of the nmos cell to have an unbalanced connection . when a scanning process is carried out utilizing an electron beam , charging over the source and / or drain will result in a potential drop between them . if the there is way to turn the nmos cell into expected status , it will create an opportunity for detecting or reviewing the open bit contacts . if the cell gates are turned off , each bit contact is isolated from its cell capacitor . when scanning the cell array solely by primary electron beam , the charges induced on the cells , with reference to fig9 , cannot drain to the cell capacitors even if the contact is normal . this makes an open bit contact 909 indistinguishable from a normal bit contact 908 . if a cell gate is turned on , i . e ., the word line 1307 , as shown in fig1 , is positively charged over vdd , an inversion layer 1306 is created below the gate , wherein electrons are the dominant carrier in the narrow band . by scanning the cell array with a primary beam of proper landing energy , positive charging can be generated on the bit contacts . for a normal contact 1308 , positive charges drain to their capacitors via the inversion layer 1306 , while for open contact 1309 , charges accumulate . positive charging results in a relatively weak se signal 1303 compared to 1305 , thus the open contact 1309 should appear dark from a voltage contrast image . accordingly , it is desirable to image bit contacts with gate turned on . detecting or reviewing open bit contacts at bit contact layer generally involves two separate steps for a complete sequence as shown in fig1 . in a first step 1402 , the gate contacts are precharged to introduce a positive charge on the word lines which are high enough to turn on the transistor . unlike the cell capacitors which leak the charging in pico - seconds , the resulting positive charging on word lines can be held for a significant period of time , for instance tens of minutes or hours at vacuum . the second step 1404 is to scan the relevant bit contacts over the cell array with a primary electron beam to simulate the process of writing and capture an se image simultaneously . it is necessary to use a proper landing energy between e 1 and e 2 for the bit contact material used , so that positive charges can be induced . this is equivalent to charging up the cell for writing . if a bit contact is normal , the charging will be written to its corresponding cell capacitor . since the bit contact capacitor is so large with respect to the charging current , or the leakage is so significant that capacitor cannot be charged to full capacity within given scan rate or pixel dwelling time , the contact will remain at low voltage . if there is an open contact , the charge on the contact cannot release to the cell capacitor , thus makes the open contact appears relatively darker than normal . fig1 a and 15 b show the images of the cell array an open bit contact before ( 1502 a ) and after ( 1502 b ) word lines are activated . as is seen , the normal contacts 1504 are turned from dark 1504 b to bright 1504 a after activating gate , making the open contact 1506 stand out apparently as a dark vc . in one embodiment the dram or the like structure can be part of a test structure in which the source and / or drain contacts constitute an array and share one or multiple common gates . utilizing a system and method in accordance with the present invention allows for a more efficient detecting or reviewing of the electrical defects occurring in the relevant process . there are multiple embodiments which may be utilized to achieve the pre - charge of the word line over the word line decoder circuitry . according to one embodiment , a simple way is to use the same primary electron beam as is utilized in the sem for imaging , with the same beam condition or not , to pre - scan the gate contacts at either the level of decoder circuitry or the entire wafer . the goal is to accumulate enough positive charge voltage on the word line to turn on the gate of each cell . due to the limited primary beam current , the pre - scan may be carried out one or a plurality of times to establish the designated charging voltage . according to a second embodiment , an electron beam originated from a second electron source is used to induce the positive charging voltage on the word line to turn on the turn on the gate of each cell . this electron beam from the secondary source can be controlled with much flexibility , so that the buried word lines can be charged to positive more effectively and efficiently . it can be controlled to scan or irradiate the word line decoder circuitry , the entire sub - array , or the entire wafer , multiple times or a single time . this turns on the gate of each cell and sets up the device condition for the next step of voltage contrast imaging with the primary electron beam . according to a third embodiment , a positive ion beam is used to achieve a positive charge on the buried word lines either by scanning or irradiating gate contacts over word line decoder circuitry , or simply over the entire wafer . this also turns on the gate of each cell and sets up the device condition for the next step of voltage contrast imaging with the primary electron beam . according to a fourth embodiment , an uv light , instead of charged particles , is used to induce a positive charge on the buried word lines . irradiating the gate contacts of decoder or the entire wafer with light rays of wave lengths ranging from 300 nm to 1200 nm , also excites the se , and leaves positive charges behind . the level of positive charging can be controlled by the duration or power of the light . this also turns on the gate of each cell and sets up the device condition for a next step of voltage contrast imaging with the primary electron beam . the electron beam of the secondary source , the ion beam , and the uv light are generally referred as accessory beams hereafter . although the present invention has been described in accordance with the embodiments shown , one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention . accordingly , many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims .