The present invention is related to a marking method for preparing a specimen, and more particularly to a method for distinguishing a specific region in a sample to be observed by a microscope applied in preparing an ultra-thin specimen.
Nowadays, due to the improvement in the manufacturing process of the microelectronic element and tendency of demanding a smaller line width, the semiconductor analysis is getting more and more difficult, particularly in the analysis technology of the finished dynamic random access memory (DRAM) having a size less than 0.25 xcexcm.
In the semiconductor industry, it is common to use a scanning electron microscopy (SEM) to observe the surface condition of a wafer and use a transmission electron microscopy (TEM) to examine the microstruture of a wafer to ensure that the fabricated microelectronic elements satisfy an expected standard. However, because the resolution of the scanning electron microscope (SEM) is not good enough to observe the detailed structure of the mirocelectronic elements, it is replaced by the transmission electron microscope (TEM) to make major failure analysis. As one may realize, most of the problems in TEM application are related to sample preparation, which is the most difficult part of TEM analysis. Traditionally, a sample is cut from a wafer to be examined. After thoroughly polished, the sample becomes ultra-thin and is ready to be examined by the transmission electron microscope (TEM) for determining the quality of the wafer. However, during the preparing process, an optical microscope and a suitable marking method for the sample must be employed so that an ultra-thin specimen for fixed-point failure analysis can be prepared easily.
At the present time, a conventional method for marking a specific region in a sample utilizes a laser technique. However, the laser cannot be focused further so that the width of the mark made by the laser technique is many times than 0.25 xcexcm. Due to the resolution limitation of the optical microscope, a single-bit failure analysis cannot be carried out by employing the laser technique to prepare the specimen. Hence, a focus ion beam technique (FIB) was designed to make TEM sample instead of the laser technique. The focus ion beam (FIB) technique utilizes focused high-energy gallium ions to remove materials from both sides of the desired region. The focus ion beam (FIB) technique can offer a reliable method to precisely obtain a cross section of the specific area, and the sample can be thined to less than 0.1 xcexcm. However, this method requires very expensive equipment and cost for preparing an ultra-thin specimen, which further limits the usage of this method. Therefore, it is not worthy to prepare an ultra-thin specimen directly by the focus ion beam (FIB) technique.
For the above reason, it is desirable to develop a low-cost and effective method for preparing an ultra-thin specimen of 0.25 xcexcm or less size dynamic random access memory (DRAM).
It is therefore a primary object of the present invention to provide a effective method for distinguishing a specific region in a sample to be observed by a microscope so that an ultra-thin specimen can be prepared easily.
Another object of the present invention is to provide a specific metallic mark xe2x80x9c==xe2x80x9d formed in the sample for distinguishing a specific region to be observed by a microscope during the preparing process so that the sample can be polished to a required thickness.
It is further another object of the present invention to provide an effective marking method for distinguishing a specific region in a sample by a focus ion beam technique so as to avoid unnecessary damage for the ultra-thin specimen and reduce the cost for preparing the ultra-thin specimen.
In order to accomplish the object of the present invention, a method for distinguishing a specific region in a sample to be observed by a microscope is provided. The method includes the steps of (a) forming a first concavity on a first side of the specific region by a focus ion beam (FIB) technique, (b) forming a second concavity on a second side of the specific region opposite to the first side by the focus ion beam technique, and (c) filling the first concavity and the second concavity with a first metallic packing and a second metallic packing respectively for defining the specific region to be observed. Preferably, the method further includes the steps of forming a third concavity on the first side of the specific region and a fourth concavity on the second side of the specific region, and then filling the third concavity and the fourth concavity with a third metallic packing and a fourth metallic packing respectively.
In accordance with one aspect of the present invention, the first concavity and the third concavity are positioned between the second concavity and the fourth concavity.
In accordance with another aspect of the present invention, the first concavity is positioned between the second concavity and the fourth concavity, and the fourth concavity is positioned between the first concavity and the third concavity.
In accordance with another aspect of the present invention, after the step (c), the method further includes a step of (d1) polishing the first side of the sample at a relatively higher polishing rate till the first metallic packing is partially polished. Then, after the step (d1), the method further includes a step of (d2) polishing the first side of the sample at a relatively lower polishing rate till the first metallic packing is completely polished.
In accordance with another aspect of the present invention, after the step (d2), the method further includes a step of (d3) stopping polishing the first side of the sample and then polishing the other side of the sample.
Preferably, the step (c) is executed by a sputtering technique.
Preferably, the microscope is one selected from the group consisting of an optical microscope (OM), a transmission electron microscope (TEM), and a scanning electron microscope (SEM).
Preferably, each of the metallic packings is made of one selected from Platinum (Pt) and Tungsten (W).
Preferably, each of the concavities is substantially a parallelepiped having a length larger than 2 xcexcm, a width ranged from 1 to 2 xcexcm, and a depth ranged from 3 to 5 xcexcm.
Preferably, each of the concavities is extended to a silicon substrate under the specific region of the sample.
Preferably, the distance between the first concavity and the third concavity is larger than 2 xcexcm.
Preferably, the specific region has a width of about 0.5 xcexcm.
A still further object of the present invention is to provide a method for distinguishing a specific region in a sample to be observed by a microscope. The method includes steps of (a) forming a first receptacle on a first side of a specific region, (b) forming a second receptacle on a second side of the specific region opposite to the first side, and (c) filling a first packing and a second packing respectively in the first receptacle and the second receptacle for defining the specific region to be observed.
The present invention may best be understood through the following description with reference to the accompanying drawings, in which: