Abstract:
A transmission electron microscopy (TEM) or scanning electron microscopy (SEM) sample preparation method includes the steps of depositing a metal layer on top of a substrate, depositing a silicon nitride passivation layer on top of the metal layer, and cutting the substrate and the metal and passivation layers to expose their cross-sections for examination by electron microscopy. As a result, a TEM/SEM sample having sharp, well-defined boundaries is produced.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to electron microscopy and, more particularly, to an electron microscopy image sample having a silicon nitride (Si 3 N 4 ) passivation layer. 
     2. Description of the Related Art 
     Electron microscopy has been used to image semiconductor device cross-sections so that process characteristics, such as deposition rates, thicknesses, interfaces, via or step coverage, and other particular features, can be studied. 
     Either transmission electron microscopy (TEM) or scanning electron microscopy (SEM) has been used. TEM provides better resolution than SEM and its use has become more common as semiconductor device dimensions have become smaller. TEM is especially useful to study thin film process characteristics. However, TEM sample preparation is more complicated than SEM sample preparation. 
     In the conventional electron microscopy sample preparation, a spin-on-glass (SOG) layer is disposed on top of the semiconductor device being studied. Generally, the semiconductor device being studied includes at least a metal layer as the top layer and a silicon base. The SOG layer protects the metal layer from mechanical damage and functions to hold all of the layers together during the subsequent cutting process. The cutting process is necessary so that the cross-section of the sample can be exposed for imaging by the electron microscope. 
     Conventionally, the SOG layer is disposed on top of the study sample in the following manner. First, SOG is liquefied using a solvent. Second, the liquid SOG is dripped on top of the sample. Third, the sample is spun, forcing the liquid SOG to disperse evenly on top of the sample. Fourth, the liquid SOG is cured at temperatures above 400° C. to remove the solvent from the SOG. At this curing stage, the SOG is subject to high enough temperatures (&gt;400° C.) where it reacts with part or all of the underlying metal layer to produce low quality TEM/SEM samples. 
     SUMMARY OF THE INVENTION 
     An object of this invention is to provide a method for preparing an improved sample for examination using either TEM or SEM. 
     The sample preparation method according to the present invention includes the steps of depositing a metal layer on top of a substrate, depositing a passivation layer comprising silicon and nitrogen on top of the metal layer, and cutting the substrate and the metal and passivation layers to expose their cross-sections for examination by electron microscopy. 
     The above method produces a TEM/SEM sample having sharp, well-defined boundaries. Consequently, it facilitates the study of process characteristics, including deposition rates, thicknesses, metal-substrate interface, and via or trench sidewall and bottom coverage. The method according to the present invention is especially effective when studying process characteristics of thin and very thin metal films. 
     Another object of the invention is to provide an electron microscopy image sample having sharp, well-defined boundaries. The sample according to the invention includes a substrate, a metal layer disposed on top of the substrate, and a passivation layer comprising silicon and nitrogen disposed on top of the metal layer, and has its cross-section cut for imaging by either transmission or scanning electron microscopy. 
     In the invention, the substrate underlying the metal layer may be a semiconductor base, a superconductor base, or any other base on top of which thin metal films are deposited. The semiconductor base may be, for example, a silicon base or a silicon base having one or more layers, e.g., SiO 2 , Si 3 N 4 , or polysilicon layer, disposed on top of the silicon base. The superconductor base may be, for example, tantalum aluminide (TaAl 2 O 3 ). 
     The metal layer comprises one of the following layers: titanium (Ti), titanium nitride (TiN), tantalum (Ta), tantalum nitride (TaN), aluminum (Al), copper (Cu), tungsten (W), tungsten nitride (WN x ), and other layers conventionally used as metal layers in semiconductor process technology including other barrier-metal layers. A metal layer is considered to be “thin” when it is less than about 1000 angstroms. A metal layer is considered to be “very thin” when it is less than about 100 angstroms. 
     The passivation layer comprises silicon nitride. A TEM/SEM sample employing silicon nitride layer as the passivation layer has sharp, well-defined boundaries because the silicon nitride layer is less prone to react with the underlying metal layer than a SOG layer, especially when the silicon nitride layer is deposited at temperatures less than 400° C. 
     Preferably, the silicon nitride layer is deposited at low temperatures, between 300 and 350° C., to minimize any reactions between the silicon nitride layer and the underlying metal layer. To carry out the low temperature deposition, plasma-enhanced chemical vapor deposition (PECVD) is employed. 
     Additional objects, features and advantages of the invention will be set forth in the description of preferred embodiments which follows. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is described in detail herein with reference to the drawings in which: 
     FIG. 1 is a plan view of a semiconductor device; 
     FIG. 2 is a cross-section of the semiconductor device of FIG. 1; 
     FIGS. 3A to  3 E illustrate the electron microscopy sample preparation method according to the invention; and 
     FIG. 4 illustrates a cross-section of an electron microscopy sample produced using the method according to the invention. 
    
    
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred exemplary embodiments of the invention, and, together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The invention is illustrated using a semiconductor device  10 . FIG. 1 shows a plan view of the semiconductor device  10  and FIG. 2 shows the cross-sectional view of the semiconductor device  10  taken along line  2 — 2  of FIG.  1 . 
     The semiconductor device  10  includes a silicon base  20 , two active regions in the silicon base  20  formed by adding dopants, namely a source region  30  and a drain region  40 , and a polysilicon gate  50 . A gate oxide layer  60 , which is typically silicon oxide (SiO 2 ), is provided between the gate  50  and the silicon substrate  20 . An insulator layer  70 , which may be, for example, another SiO 2  layer or a Si 3 N 4  layer, covers the gate  50  and the gate oxide layer  60 . The insulator layer  70  is deposited onto the gate  50  and the gate oxide layer  60  by chemical vapor deposition (CVD). Etching is carried out to form openings  80  and  90  in the gate oxide layer  60  and the insulator layer  70 . 
     In the description that follows, the semiconductor device  10  illustrated in FIGS. 1 and 2 will be referred to as a “substrate.” In alternative embodiments of the invention, this “substrate” may include only the silicon base  20 , or a SiO 2  or Si 3 N 4  layer in addition to the silicon base  20 . In this regard, the term “substrate” is being used to generally define the elements or layers that underlie a layer of metal film. As noted earlier, the “substrate” may be a superconductor base or any other base on top of which a thin metal film is deposited. 
     FIGS. 3A to  3 E illustrate the method according to the invention. 
     First, a metal layer  110  is deposited on top of the substrate  100  to a thickness of about 100-1000 angstroms. The invention is illustrated here with a metal layer having a thickness of about 100-1000 angstroms but is applicable to other thickness ranges of the metal layer, e.g., 10-100 angstroms, 50-100 angstroms, 100-200 angstroms, etc. The resulting structure is illustrated in FIG.  3 A. Any conventional method for depositing metal may be used. The metal layer  110  comprises one of the following layers: Ti, TiN, Ta, TaN, Al, Cu, W, WN x  and other metal layers conventionally used in semiconductor process technology, including other barrier-metal layers. 
     Second, a Si 3 N 4  layer  120  is deposited on top of the metal layer  10 . The resulting structure is illustrated in FIG.  3 B. PECVD is employed and the deposition temperature is kept less than 400° C., preferably between 300° C. and 350° C. Other methods of depositing Si 3 N 4  may be employed so long as the deposition temperature is kept at similar low temperatures. Alternative to a Si 3 N 4  layer, a silicon oxynitride layer may be employed, so long as its deposition temperature is kept below the temperature where the silicon oxynitride layer is likely to react with the underlying metal film. 
     Third, the structure of FIG. 3C (FIG. 3C is a plan view of FIG. 3B) is cut along cross-section line A—A so that a cross-section of the SEM/TEM sample, like the one shown in FIG. 3B, is exposed for imaging. The cutting is performed by employing methods that are known in the art. 
     For example, the device illustrated in FIG. 3C may be cut along line A—A by a method known as cleave and polish. The resulting structure is illustrated in FIG. 3D for an SEM sample and in FIG. 3E for a TEM sample. (The cut portions of the device are illustrated in broken lines and the remaining portions of the device are illustrated in solid lines.) To obtain the SEM sample, polishing is carried out on only one side (i.e., the lower side  150  as illustrated in FIG. 3C) of the device. To obtain the TEM sample, polishing is carried out on both sides of the device (i.e., the lower side  150  and the upper side  155  as illustrated in FIG.  3 C). A very thin strip (e.g., depth “d” of the metal layer in the imaging direction B—B is about 50-100 angstroms) is required for TEM imaging because the electrons must be able to penetrate (i.e., transmitted through) the TEM sample. Polishing may be performed by mechanical abrasions or ion milling. 
     Alternatively, the device illustrated in FIG. 3C may be “dimpled.” This process includes the steps of: (1) polishing from the bottom surface  160  (see FIG. 3B) to the top surface  165  (see FIG. 3B) so that only about 99% of its thickness remains and the device becomes transparent from the top; (2) polishing one of the sides (either the lower side  150  or the upper side  155  of the device); and (3) ion milling a bowl on the top of the device. This final step is not necessary to prepare the SEM sample. It is necessary for the TEM sample because it cuts out a bowl on the top surface  155  of the device illustrated in FIG. 3D so that a thin wall is formed along line B—B. The thin wall has a thickness of about 50-100 angstroms so that TEM imaging may be performed. 
     FIG. 4 illustrates a cross-section of another electron microscopy sample produced in the above-described manner. This sample has a high aspect ratio (hole height/width &gt;3) contact hole  280 . Because of the high aspect ratio, the sidewall and bottom coverage of the metal layer within the contact hole  280  is poor. Typically, for a high aspect ratio contact hole, the sidewall coverage is less than 10% of the metal layer thickness at the top of the contact hole and the bottom coverage is about 12% of the metal layer thickness at the top of the contact hole. 
     The sample further includes a substrate  200 . A metal layer  210  having a thickness of about 100-1000 angstroms is disposed on top of the substrate  200 . The metal layer  210  comprises one of the following layers: Ti, TiN, Ta, TaN, Al, Cu, W, WN x , and other metal layers conventionally used in semiconductor process technology, including other barrier-metal layers. The top layer in FIG. 4 is a silicon nitride layer  220  having a thickness of about 2000-3000 angstroms. The silicon nitride layer  220  functions as the passivation layer. 
     The invention is particularly useful in imaging a sample having a high aspect ratio contact hole as in FIG. 4, because the silicon nitride layer  220 , unlike an SOG layer, suppresses reactions with the underlying metal layer. As a result, with the invention, the small amount of metal layer remaining on the sidewalls and the bottom of the contact hole retain their thicknesses to permit higher quality imaging of the metal layer by electron microscopy. 
     When the sample of FIG. 4 is used for TEM imaging, the unfilled portion of the contact hole  280  is filled in with an SOG layer (not shown). This SOG layer is isolated from the metal layer  210  by the passivation layer  220  and thus does not react with the metal layer as in conventional samples. As a result, the SOG layer provides the sample with an additional protective layer while maintaining the quality of the resulting TEM sample. 
     While particular embodiments according to the invention have been illustrated and described above, it will be clear that the invention can take a variety of forms and embodiments within the scope of the appended claims.