Abstract:
An electron microscope suitable for observing at least one sample is provided. The sample has at least one testing area, and a material of the sample on the testing area is semiconductive or conductive. The electron microscope includes a stage, an electron gun, and at least one probe. The stage is suitable for carrying the sample and the sample is not electrically grounded. The electron gun is suitable for generating an electron beam and accumulating charges on the sample. When the probe contacts with the testing area, the image contrast of the testing area will change. The current through the probe will also change upon contact. Methods have been provided based on these principles to determine “when” and “where” the probe starts to contact the sample surface inside an electron microscope.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims the priority benefit of Taiwan application serial no. 95116721, filed on May 11, 2006. All disclosure of the Taiwan application is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention is related to a method of determining the contact point and the contact of the probe, and more specifically, to a method of determining the contact of the probe with a sample surface in an electron microscope and a method to locate the contact point. 
   2. Description of Related Art 
   When using an electron microscope, one must take two measures at the same time in the current technique in order to manipulate the probe for making the probe tip in contact with the sample surface. First, a very thin and sharp probe is chosen to ensure that the ultimate contact point between the probe and the sample surface falls at the tip during the operation. Second, the probe must maintain an appropriate tilt angle while approaching the sample surface during the operation, allowing the user to monitor the position of the probe tip all the time. 
   However, since the image of the electron microscope cannot provide quantitative information of the perspective depth of field, whether the probe tip contacts with the sample surface during the operation is only determined by observing whether or not the probe tip begins to slide laterally. That is to say, the moment of time for the probe tip to contact with the sample surface cannot be obtained exactly. Thus, when the probe tip slides laterally, it is likely that the probe tip or the sample surface has been deformed or damaged due to the over-contact. In addition, as the probe with an extremely thin and sharp tip must be very expensive and liable to get damaged in usage, the maintenance cost of the conventional electron microscope with maneuverable probes is increased accordingly. Further, since the sample surface tends to be damaged by the probe, the overall cost for a typical measurement is also increased accordingly. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide an electron microscope to eliminate the high maintenance cost due to the damage of the conventional probe and the sample caused by the over-contact between them. 
   Another object of the present invention is to provide a method of determining the probe contact to avoid the over-contact between the probe and the sample. 
   Still another object of the present invention is to provide a method to determine the probe contact point for prolonging the service lifetime of the probe and expanding the ways of using the probe. 
   In order to achieve the above-mentioned or other objectives, the present invention provides an electron microscope suitable for observing at least one sample. The sample has at least one testing area, and the material of the sample on the testing area is semi-conductive or conductive. The electron microscope comprises: a stage, an electron gun and at least one probe. The stage is suitable for carrying the sample and the sample is not electrically grounded. The electron gun is suitable for generating an electron beam and accumulating charges on the sample. When the probe contacts with the testing area, the image contrast of the testing area changes. 
   In an embodiment of the present invention, the above-mentioned probe is electrically grounded. 
   In an embodiment of the present invention, the above-mentioned electron microscope further comprises an ammeter, and the probe is electrically grounded via the ammeter. 
   In an embodiment of the present invention, the above-mentioned electron microscope further comprises an ammeter electrically connected to the probe. 
   In order to achieve the above-mentioned or other objectives, the present invention further provides a method to determine the probe contact, which is suitable for use in an electron microscope having at least one probe. The method to determine the probe contact comprises: providing at least one sample, wherein the sample has at least one testing area, and the material of the sample on the testing area is semi-conductive or conductive; disposing the sample in the electron microscope, wherein the sample is not electrically grounded; accumulating charges on the sample; and contacting the probe with the testing area, the image contrast of which will change upon contact. 
   In an embodiment of the present invention, the above-mentioned electron microscope has an electron gun for accumulating charges on the sample. 
   In an embodiment of the present invention, the above-mentioned probe is electrically grounded. 
   In an embodiment of the present invention, the above-mentioned electron microscope has an ammeter, and the probe is electrically grounded via the ammeter. 
   In an embodiment of the present invention, the above-mentioned electron microscope has an ammeter electrically connected to the probe. 
   In order to achieve the above-mentioned or other objectives, the present invention further provides a method to determine the probe contact point, which is suitable for use in a probe of an electron microscope. The method to determine the probe contact point comprises: providing a sample, wherein the sample has two sets of parallel semi-conductive or conductive strip-shaped testing areas, the common direction of which in one set is different from that in the other by an angle; disposing the sample within the electron microscope, wherein the sample is not electrically grounded; accumulating charges on the sample; contacting the probe with a strip-shaped testing area in the first set, wherein the image contrast of the corresponding strip-shaped testing area in the first set changes upon contact, a first axis at the probe is defined; and contacting the probe with a strip-shaped testing area in the second set, wherein the image contrast of the corresponding strip-shaped testing area in the second set changes upon contact, a second axis at the probe is defined. The crossing point between the second axis and the first axis is the contact point of the probe. 
   In an embodiment of the present invention, the above-mentioned electron microscope has an electron gun for accumulating charges on the sample. 
   In an embodiment of the present invention, the above-mentioned probe is electrically grounded. 
   In an embodiment of the present invention, the above-mentioned electron microscope has an ammeter, and the probe is electrically grounded via the ammeter. 
   In an embodiment of the present invention, the above-mentioned electron microscope has an ammeter electrically connected to the probe. 
   As the present invention utilizes the probe to contact with the testing area of the sample which is not electrically grounded, the image contrast of the testing area changes, thereby achieving the object of determining the contact. Therefore, the conventional phenomenon of causing damages to the probe tip and the sample surface due to the over-contact may be eliminated. Besides, the present invention further utilizes the above method for locating the probe contact point without requiring a very thin and sharp probe to be used together with the electron microscope. Therefore, the cost of the probe is reduced and its service lifetime is also prolonged. 
   In order to make the aforementioned and other objects, features and advantages of the present invention in comprehensible, preferred embodiments accompanied with figures are described in detail below. 
   It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
       FIGS. 1A-1B  are flow charts of the operation of a probe in an electron microscope according to an embodiment of the present invention. 
       FIG. 2  is a reference graph of the current through the probe to the ground terminal as a function of time for two events of contact. 
       FIGS. 3A-3C  are flow charts for locating the probe contact point according to an embodiment of the present invention. 
   

   DESCRIPTION OF EMBODIMENTS 
     FIGS. 1A-1B  are flow charts of the operation of a probe in an electron microscope according to an embodiment of the present invention. Referring to  FIGS. 1A-1B , the electron microscope  100  is suitable for observing at least one sample  110 . The sample  110  has a plurality of testing areas  112 ,  114 ,  116 , and the material of the sample  110  on the testing areas  112 ,  114 ,  116  is semi-conductive or conductive. The electron microscope  100  comprises: a stage  120 , an electron gun  130  and at least one probe  140 . The stage  120  is suitable for carrying the sample  110 , and the sample  110  is not electrically grounded. In addition, the electron gun  130  is suitable for generating an electron beam  132  to accumulate charges on the sample  110 . In this embodiment, the electron microscope  100  further comprises an ammeter  150 . The probe  140  is electrically connected to the ammeter  150  and electrically grounded via the ammeter  150 . 
   During the operation, first, the electron gun  130  is used to generate an electron beam  132  to accumulate charges on the testing areas  112 ,  114 ,  116  of the sample  110 . Next, the probe  140  contacts with one testing area  112  of the sample  110 , thus the charges accumulated on the testing area  112  are transferred to the ground terminal via the probe  140  and the ammeter  150  in sequence. Thus, it can be observed via the electron microscope  100  that, the image contrasts of the testing area  112  is different from that of the testing areas  114 ,  116  (having no contact with the probe  140 ) due to the different densities of charges accumulated thereon, as shown in  FIG. 1B . Therefore, the present invention may make qualitative determination of the moment of time when the probe  140  contacts with the surface of the sample  110  according to the change of image contrast. 
     FIG. 2  is a schematic view of the current through the probe to the ground terminal as a function of time for two events of contact. The Y-coordinate is the magnitude of current through the probe to the ground terminal with pico-ampere as the unit, which is equal to 10 −12  ampere; the X-coordinate is the time with second as the unit. Referring to both  FIG. 1B  and  FIG. 2 , when the probe  140  contacts with the testing area  112  of the sample  110 , the charges accumulated on the testing area  112  are grounded via the probe  140  and the ammeter  150  in sequence. Consequently, when the charges pass through the ammeter  150 , a rising waveform  210  is formed for the reading of the ammeter  150  due to the sudden increase of current, as shown in  FIG. 2 . While the charges accumulated on the testing area  112  have been gradually reduced due to being grounded, the electron gun  130  still generate the electron beam  132  continuously, resulting in a continuous charge transfer from the testing area  112  to the ground terminal via the probe  140  and the ammeter  150 . In other words, as shown in  FIG. 2 , the magnitude of the current shown on the ammeter  150  has gradually reduced to a stable current  220  as time elapsed. Finally, when the probe  140  is removed from the testing area  112 , the reading of the ammeter  150  is also suddenly reduced to form a descending waveform  230  as shown in  FIG. 2 . Therefore, as shown in  FIG. 1B , the present invention not only makes qualitative determination of the time for the probe  140  to contact with the sample  110  through the image contrasts of the testing areas  112 ,  114 ,  116 , but also makes quantitative determination of the time through the change of the reading on the ammeter  150 . 
   In the above-mentioned embodiment, the probe  140  is electrically grounded via the ammeter  150 . However, the present invention is not limited to this embodiment. For example, the ammeter  150  may be replaced by other measuring meters, such as voltmeter, also having the function of quantitative determination. In another embodiment, the probe  140  may be electrically grounded directly without connecting to the ammeter  150 , and this embodiment may also utilize the image contrasts of the testing areas  112 ,  114 , and  116  as shown in  FIG. 1B  to make qualitative determination of the time when the probe  140  contacts with the sample  110 . In another embodiment, the probe  140  may not be electrically grounded, but only the changes of the image contrast when the probe  140  contacts with one of the testing areas  112 ,  114 ,  116  are utilized to make qualitative determination of the time when the probe  140  contacts with the sample  110 . In addition, although this embodiment takes a single sample  110  as an example for illustration, the electron microscope  100  may also be used for observing a set of conductive samples without being electrically connected to each other. Similarly, although this embodiment takes a single probe  140  as an example for illustration, the electron microscope  100  may also be used for observing the contact between a set of probes and the sample at the same time. In addition, although the sample  110  of this embodiment takes a set of testing areas  112 ,  114 ,  116  as an example for illustration, the sample  110  having a single testing area may also be used in this embodiment. In addition, the method to determine the probe contact in this embodiment may also be used to measure the contact point of the probe, which is illustrated below in details. 
     FIGS. 3A-3C  are flow charts for locating the probe contact point according to an embodiment of the present invention. Referring to  FIG. 3A , a sample  310  is provided, and the sample  310  has the first set of parallel strip-shaped testing areas  312  and the second set of parallel strip-shaped testing areas  314 . The material of the sample  310  on the first set of strip-shaped testing areas  312  and the second set of strip-shaped testing areas  314  is semi-conductive or conductive, and the conductive material is preferred. Each strip-shaped testing area in the first set  312  is tilted from each strip-shaped testing area in the second set  314  by an angle, which is 90 degrees in this embodiment, but other degrees of angles is also possible. At this time, the probe  320  does not contact with the sample  310 . The sample  310  is first disposed within the electron microscope (not shown), and the sample  310  is not electrically grounded. Then, charges are accumulated on the sample  310  through the above method. Referring to  FIG. 3B , when the probe  320  contacts with a strip-shaped conductive part  312   a  in the first set of strip-shaped testing areas  312 , the image contrast of the strip-shaped conductive part  312   a  changes, allowing us to define the first axis  330   a  at the probe  320 . Then, referring to  FIG. 3C , when the probe  320  contacts with a strip-shaped conductive part  314   a  in the second set of strip-shaped testing areas  314 , the image contrast of the strip-shaped conductive part  314   a  changes, allowing us to define the second axis  330   b  at the probe  320 . The crossing point(s) between the second axis  330   b  and the first axis  330   a  is the contact point(s) of the probe  320  with the sample  310 . The conventional electron microscope employs a probe with a very thin and sharp tip, which is not only used for determining the contact time, but also used for locating the contact point. However, as the present invention defines the contact point between the probe and the sample, the selection of the probe is no longer limited to the very expensive probe with extremely thin and sharp tip, thereby reducing the cost. 
   In summary, as the present invention uses the probe to contact with the testing area on the sample, the charges accumulated on the testing areas are transferred to the probe, resulting in a difference between the charges accumulated on this testing area and that accumulated on other testing areas, thereby the image contrasts are also different, allowing us to judge whether the probe contacts with the sample. In other words, not only the use of the very expensive probes with extremely thin and sharp tips is avoided, but also the over-contact between the probe and the sample is avoided, thereby preventing the probe or the sample surface from being damaged. Meanwhile, the present invention further determines whether or not the probe contacts with the sample by measuring the current passing through the probe. Therefore, the present invention is capable of implementing qualitative determination through the image contrast and quantitative determination through the current change at the same time, allowing the user to determine more precisely the moment of time when the probe contacts with the sample. 
   Besides, the user may locate the contact point of the probe beforehand by using the changes of the image contrast when the probe contacts with a specially patterned sample. Therefore, whether or not the probe is sharp is no longer a problem for determining the contact point, so that the user need not use the very expensive probe with extremely thin and sharp tip, and the service lifetime of the probe is also prolonged. Therefore, the present invention not only reduces the cost of the probe, but also facilitates its performance and maintenance. 
   It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention covers modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.