Abstract:
Described here is a method of forming a thin-film portion for allowing electrons produced from a transmission electron microscope to pass therethrough at a portion to be observed of a semiconductor and effecting a predetermined etching process on the thin-film portion thereby to create a semiconductor sample for the transmission electron microscope. Prior to the execution of the etching process, grooves for reducing a stress introduced into the thin-film portion by the etching process are defined in the thin-film portion.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a semiconductor sample suitable for a transmission electron microscope (hereinafter called simply “TEM”), which is to be observed by the TEM and a method of manufacturing the semiconductor sample. 
     2. Description of the Related Art 
     It is of importance to recognize in detail the state of distribution of impurities in an impurity diffusion region for semiconductor devices like MOS transistors incorporated into a semiconductor substrate as components of a semiconductor integrated circuit in order to evaluate and analyze the semiconductor integrated circuit. 
     A TEM is known as one of means for observing the impurity diffusion region, etc. in such a semiconductor substrate. 
     In order to observe a semiconductor by the TEM, a thin-film portion for allowing an electron beam to pass therethrough is formed in the semiconductor to be observed. The thin-film portion is formed at an edge portion of the semiconductor by beam-based grinding processing using a focused ion beam device (hereinafter called simply as “FIB”), for example. 
     Prior to the observation of the thin-film portion by the TEM, the entire semiconductor including the surface of the thin-film portion is subjected to a selective etching process using an etchant. The selective etching process makes it possible to selectively remove a lattice defective portion produced due to impurities in the thin-film portion, etc., whereby an impurity distribution can be observed by the TEM for the first time. 
     Meanwhile, according to the above-described conventional method of manufacturing the sample, the thin-film portion formed integrally with the edge portion of the semiconductor at the edge portion thereof is provided in continuation with a main body of the semiconductor in a region excluding the edge portion. Therefore, since a large stress is concentrated on the thin-film portion when the entire semiconductor including the thin-film portion is subjected to the selective etching process, bending distortion has occurred in the thin-film portion due to the stress. 
     The bending distortion of the thin-film portion results in the creation of stripe-like contrast in an image observed by the TEM, thus causing a hindrance to a good-quality TEM image. 
     SUMMARY OF THE INVENTION 
     Thus, an object of the present invention is to provide a semiconductor sample free from the introduction of bending distortion into a thin-film portion owing to selective etching corresponding to pre-treatment, and a method of manufacturing the semiconductor sample. 
     According to one aspect of this invention, there is provided a semiconductor sample manufacturing method, comprising the following steps of: 
     forming a thin-film portion for allowing electrons produced from a transmission electron microscope to pass therethrough at a portion to be observed of a semiconductor; 
     thereafter subjecting the thin-film portion to a predetermined etching process thereby to manufacture a semiconductor sample for the transmission electron microscope; and 
     defining grooves for reducing a stress introduced into the thin-film portion by the etching process in the thin-film portion prior to the execution of the etching process. 
     According to the present invention, even if the conventional stress is produced in the semiconductor including the thin-film portion by the etching process, the grooves defined in the thin-film portion substantially absorb a stress that acts on the thin-film portion. Therefore, the conventional concentration of stress on the thin-film portion is prevented from occurring, so that the thin-film portion is prevented from being deformed due to the stress concentration. 
     Typical ones of various inventions of the present application have been shown in brief. However, the various inventions of the present application and specific configurations of these inventions will be understood from the following description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention, it is believed that the invention, the objects and features of the invention and further objects, features and advantages thereof will be better understood from the following description taken in connection with the accompanying drawings in which: 
     FIG. 1 is a perspective view showing one process step of a method of manufacturing a semiconductor sample according to the present invention; 
     FIG. 2 is a perspective view, partly in enlarged form illustrating a process step for forming a thin-film portion of the semiconductor sample according to the present invention; 
     FIG. 3 is a view similar to FIG. 2, depicting a process step for defining grooves in the thin-film portion of the semiconductor sample according to the present invention; 
     FIG. 4 is a perspective view showing a process step for separating a sample region of the semiconductor sample according to the present invention; 
     FIG. 5 is a perspective view illustrating a state of completion of the semiconductor sample according to the present invention; 
     FIG. 6 is a perspective view depicting one process step of another method of manufacturing a semiconductor sample according to the present invention; 
     FIG. 7 is a view similar to FIG. 2, showing a process step for forming a thin-film portion of the semiconductor sample shown in FIG. 6; 
     FIG. 8 is a view similar to FIG. 3, illustrating a process step for defining grooves in the thin-film portion shown in FIG. 7; and 
     FIG. 9 is a perspective view showing a semiconductor sample subjected to the definition of the grooves shown in FIG.  8  and prior to being subjected to a selective etching process. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will hereinafter be described in detail by illustrated preferred embodiments. 
     SPECIFIC EXAMPLE 1 
     FIG. 1 shows one process step of a method for manufacturing a sample for a transmission electron microscope, according to the present invention. 
     In the illustrated example, for example, a silicon semiconductor substrate  10  is used which has a longitudinal dimension Y of about 11 mm, a transverse dimension X of about 3 mm and a thickness dimension T of about 0.5 mm and is shaped in the form of a rectangular parallelepiped over its entirety. 
     A separating slit  11  extending along the transverse direction (X) of the silicon semiconductor substrate  10  is defined in the surface of the silicon semiconductor substrate  10 . Owing to the slit  11 , the silicon semiconductor substrate  10  is partitioned into a sample region  13  with a rising edge portion  12  having an impurity region, for example, corresponding to a portion to be inspected, and a dummy region  14  for making it easy to treat the semiconductor substrate  10  including the sample region  13 . 
     A thickness dimension t 1  of a thin-wall portion  15  of the semiconductor substrate  10 , which has left behind owing to the provision of the slit  11 , may preferably be set to approximately 0.2 mm to facilitate separation resultant from a cleavage developed between the sample region  13  and the dummy region  14  to be described later. 
     The sample region  13  has a width dimension W 1  of about 0.5 mm. Further, the sample region  13  has the rising edge portion  12  provided on one side thereof in the transverse direction thereof so as to extend in the longitudinal direction of the sample region  13  with a width dimension W 2  of 40 μm, for example. 
     A length dimension of the dummy region  14  along the longitudinal dimension Y may preferably be set to approximately 10 mm to facilitate the handling of the semiconductor substrate  10 . 
     FIG. 2 shows a process step for forming a thin-film portion in the sample region  13 . 
     As shown in FIG. 2, a thin-film portion  16  having a thickness dimension t 2  of 0.2 μm, for example, is formed in the sample region  13 . 
     As has conventionally been well known, the thin-film portion  16  can be formed by applying convergent or focused beams  17  each composed of, for example, a Ga ion produced from an unillustrated FIB device from the upper edge side of the rising edge portion  12  to both sides thereof. 
     Owing to the application of the focused beams  17  to the rising edge portion  12 , a pair of rectangular concave portions  18  and  18  with the upper edge of the rising edge portion  12  as one side is defined in both sides of the rising edge portion  12  as viewed along the transverse direction (W 2 ) of the rising edge portion as has conventionally been well known. Thus, the thin-film portion  16  having a thickness dimension t 2  which is an approximately uniform thickness dimension and extends in the direction coincident with the transverse direction (W 2 ) of the rising edge portion  12 , is defined between both the concave portions  18 . 
     The thin-film portion  16  has bases  19   a  and a pair of sides  19   b  connected to the sample region  13  excluding the thin-film portion  16 , i.e., a sample body ( 13 ). The two sides  19   b  extend toward their corresponding bases  19   a  substantially at right angles to an outer edge  12   a  of the rising edge portion  12  from the top face  12   a  of the rising edge portion  12 , i.e., the outer edge  12   a  thereof. 
     Means other than the focused beams  17  produced from the above-described FIB device can be utilized to form the thin-film portion  16 . It is however desirable to use the focused beams  17  produced from the FIB device with a view toward forming a thin-film portion  16  having a desired thickness dimension t 2  with high accuracy. 
     Upon attachment of the semiconductor substrate  10  to the FIB device, the semiconductor substrate  10  is placed on a conductive sample table with an auxiliary material such as silver paste for increasing conductivity interposed therebetween in order to prevent the semiconductor substrate  10  from being charged up due to the application of the focused beams  17 . The application of the auxiliary material to the dummy region  14  upon application of the auxiliary material to the semiconductor substrate  10  makes its possible to prevent the sample region  13  from being contaminated due to the auxiliary material. 
     The thin-film portion  16  can be formed prior to the formation of the slit  11 . The front and back of its procedure can be suitably determined. 
     FIG. 3 shows a process step for introducing grooves into the thin-film portion  16 . 
     After the formation of the thin-film portion  16 , stress-relieving grooves  20  are defined in the thin-film portion  16  along their corresponding sides  19   b  in an example illustrated in FIG.  3 . The grooves  20  can be defined by using the focused beams  17  produced from the FIB device. 
     The pair of grooves  20  is interposed between the sample body ( 13 ) and the thin-film portion  16  by extending the grooves  20  along the sides  19   b  continuously connected to the sample body ( 13 ) of the thin-film portion  16 , whereby additional side edge portions  19   b′  of the thin-film portion  16  are defined. 
     As a result, both side edge portions  19   b′  of the thin-film portion  16  are sectioned from the sample body ( 13 ) by the pair of grooves  20 . Further, the thin-film portion  16  is provided in continuation with the sample region  13 , i.e., the sample body ( 13 ) only by the bases  19   a  paired with one sides coincident with the outer edge  12   a  of the rising edge portion  12 . 
     In order to selectively remove an impurity diffusion region of the thin-film portion  16  to be observed after the formation of the grooves  20  in the thin-film portion  16 , the sample body ( 13 ) including the thin-film portion  16 , i.e., the sample region  13  is immersed into a selective etchant over its entirety. 
     Upon the handling of the semiconductor substrate  10  for this selective etching process, the dummy region  14  of the semiconductor substrate  10  can be nipped with a pincette or the like, for example. The nipping of the dummy region  14  by the pincette makes it possible to prevent the sample region  13  including the observed portion from being damaged due to its handling and execute a reliable and easy etching process. 
     Since a metal portion other than the semiconductor substrate  10  does not touch the etchant upon the etching process, the sample region can be assuredly prevented from being contaminated due to the etching of the metal portion. 
     Such a strong stress similar to the conventional example as to produce a bend in the thin-film portion  16  is developed in the sample body ( 13 ) including the thin-film portion  16  while the etching process is being executed. 
     In the method according to the invention of the present application, however, since the thin-film portion  16  subjected to the selective etching process is not restrained by or locked to the sample body ( 13 ) with the bases  19   a  and the pair of sides  19   b  as in the conventional example and is provided in continuation with or connected to the sample body ( 13 ) by the bases  19   a  alone, edge portions extending in three directions, including the pair of sides  19   b′  serve as free edge portions free of restraint. 
     Therefore, even if the strong stress acts on the sample body ( 13 ), it is not transferred to the thin-film portion  16  as in the conventional example. Further, the stress that acts on the thin-film portion  16 , is relieved due to temporary deformation of the thin-film portion  16 . Therefore, no strong stress remains in the thin-film portion  16  and strong bending deformation due to the remaining stress is no longer produced. 
     Thus, the conventional strong bending deformation is not produced in the thin-film portion  16  owing to the above-described selective etching process. 
     In the semiconductor substrate  10  including the sample body ( 13 ) subjected to the selective etching process, the sample body ( 13 ) including the thin-film portion  16  is separated from the semiconductor substrate  10  by the cleavage using the slit  11  as shown in FIG.  4 . The separated sample body ( 13 ) is placed on a metal holder  21  composed of a metallic mesh as shown in FIG.  5 . 
     As indicated by an arrow  22  in FIG. 5, an electron beam is transmitted through the thin-film portion  16  of the sample body ( 13 ) placed in an unillustrated TEM via the metal holder  21 . 
     Since the conventional large bending deformation due to the stress is not produced in the thin-film portion  16  while image contrast is obtained due to a diffraction effect of the transmitted light as a principal, interference fringes incident to the bending deformation do not occur in the image contrast. 
     Thus, according to the method of the present invention, a semiconductor sample, which causes no interference fringes due to the bending deformation of the thin-film portion  16 , can be formed with relative ease. According to the semiconductor sample related to the present invention as well, a clear TEM image excluding such interference fringes can be obtained. 
     The embodiment described above shows as an example the case in which the grooves  20  for relieving the stress are defined along the pair of sides  19   b  to set the three-direction edge portions of the thin-film portion  16  as the free edges free from restraint of the sample body ( 13 ). As an alternative to this example, however, stress-relieving grooves ( 20 ) can be defined along both edges of one side  19   b  and the base  19   a  respectively. 
     Further, one groove ( 20 ) can be formed along the edge of either one side  19   b  or the side  19   b . According to this, two-direction edge portions of the thin-film portion  16  can be formed as free edges. It is however desirable that the three-direction edge portions of the thin-film portion  16  are formed as the free edges as described above to improve the effect of reducing the stress applied to the thin-film portion  16  by the selective etching. 
     When the thin-film portion  16  is subjected to the beams produced from the FIB device in the process step for forming the thin-film portion  16  and the process step for defining the grooves  20 , an amorphous layer indicative of resistance to the subsequent selective etching might be formed on the surface of the thin-film portion  16 . 
     The amorphous layer will result in a reduction in the efficiency of the subsequent selective etching process. Further, etching characteristics vary due to such variations in the etching efficiency. It is therefore desired that the amorphous layer formed on the surface of the thin-film portion  16  is removed in advance by ion-milling using an ion such as Ar or the like prior to the selective etching process with a view toward improving the efficiency of the selective etching process and obtaining uniform etching characteristics. 
     SPECIFIC EXAMPLE 2 
     FIGS. 6 through 9 respectively show a sample manufacturing method according to the present invention, which makes no use of the dummy region referred to above. 
     As shown in FIG. 6, a sample body  13  is formed which comprises, for example, a silicon semiconductor which has a rising edge portion  12  similar to the above, having, for example, a transverse dimension W 2  of 40 μm and a longitudinal dimension X of 3 mm and which is shaped in the form of a rectangular parallelopiped over its entirety. A transverse dimension W 1  of the sample body  13  can be set to 0.5 mm, for example. 
     As shown in FIG. 7, a pair of concave portions  18  is defined in the rising edge portion  12  of the sample body  13  by means of focused beams  17  produced from an FIB device similar to one described in line with the specific example  1 , whereby a thin-film portion  16  is defined between both concave portions  18 . 
     A thickness dimension t 2  of the thin-film portion  16  can be set to 0.2 μm, for example in a manner similar to the specific example 1. 
     After the formation of the thin-film portion  16 , stress-relieving grooves  20  similar to ones referred to above are defined in the thin-film portion  16  along a pair of sides  19   b  by means of the focused beams  17  similar to the above as shown in FIG.  8 . 
     Owing to the formation of the grooves  20 , both sides  19   b  of the thin-film portion  16  are separated from the sample body  13  so as to serve as free edges  19   b′  free from restraint of the sample body  13 . 
     While the thin-film portion  16  is subjected to a selective etching process similar to the above, the sample body  13  with the grooves  20  defined therein is attached to a metal holder  21  as shown in FIG. 9 to facilitate the handling of the sample body  13  in the etching process. 
     Since the selective etching process can be effected on the thin-film portion  16  of the sample body  13  attached to the metal holder  21  according to the handling of the metal holder  21 , the sample body  13  including the thin-film portion  16  serving as a portion to be observed can be assuredly prevented from being subjected to damage resulting from the handling of the sample body. 
     When the metal holder  21  touches a selective etchant upon the selective etching process of the sample body  13 , it is subjected to erosion. When the metal holder  21  is subjected to erosion and the metallic material subjected to erosion is attached to the thin-film portion  16 , the thin-film portion  16  corresponding to the portion to be observed might be contaminated by such an adherent. 
     It is considered that the metal holder  21  is formed by a resin material exhibitive of an etching resistant characteristic to reliably prevent such contamination. 
     Since, however, the resin material having the etching resistant characteristic normally exhibits electrical insulating properties, means for solving charge-up similar to that developed in the FIB device is needed when the sample body  13  is incorporated into a TEM by using such a holder. 
     While the metal holder  21  is used as shown in FIGS. 1 through 5 from these points of view, the optimal one is the method described in the specific example 1 using the dummy region  14  which does not bring about contamination of the sample body  13  due to the erosion of the metal holder  21 . 
     While the sample for the TEM, for observing the distribution of impurity diffusion of the silicon semiconductor has been described above, the invention of the present application can be applied to various observations by other semiconductor TEM. 
     According to the manufacturing method related to the present invention, the stress that will act on the thin-film portion with the selective etching process for pretreatment, is substantially absorbed by the grooves defined in the thin-film portion as described above. Thus, the concentration of stress on the thin-film portion as is the case in the conventional example is prevented from occurring and the thin-film portion is prevented from deforming due to the stress concentration. As a result, a semiconductor sample free of the introduction of bending distortion into the thin-film portion can be manufactured with relative ease. 
     Further, according to the semiconductor sample related to the present invention, since the bending distortion due to pre-treatment is not introduced into the thin-film portion to be observed by the TEM, a good-quality TEM image free from the occurrence of stripe-like contrast due to the bending distortion of the thin-film portion can be obtained. 
     While the present invention has been described with reference to the illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to those skilled in the art on reference to this description. It is therefore contemplated that the appended claims will cover any such modifications or embodiments as fall within the true scope of the invention.