Abstract:
A method for preparing a sample includes separating a portion of substrate from a sample, performing focused ion beam milling, and removing additional sample material using an etchant.

Description:
TECHNICAL FIELD OF THE INVENTION  
       [0001]     This invention relates generally to the field of sample preparation and more specifically to a method for preparing a sample for transmission electron microscopy imaging and analysis.  
       BACKGROUND OF THE INVENTION  
       [0002]     Preparing samples for plan view transmission electron microscopy generally requires the use of focused ion beam (FIB) or standard chemical etching. Standard FIB sample preparation is site specific and allows a single component of a film stack within a semiconductor device to be selected for analysis, but it suffers from sample thickness variations which limit the viewing area of the sample. Standard chemical etching is only slightly site specific, and has thickness variations that are not as severe as FIB sample preparation. With standard chemical etching, however, components from film stacks within a semiconductor device are difficult to separate, and the region surrounding the sample area is often very thin leading to mechanical support problems and a possible loss of the sample. Both standard FIB milling and standard chemical etching allow for analysis of a very limited area of the sample. Accordingly, current techniques for sample preparation for transmission electron microscopy may be unsatisfactory in many applications.  
       SUMMARY OF THE INVENTION  
       [0003]     In accordance with the present invention, disadvantages and problems associated with previous techniques for sample preparation for plan view transmission electron microscopy may be reduced or eliminated. According to one embodiment, a sample preparation technique includes thinning the silicon substrate beneath the area for plan view analysis using a focused ion beam, followed by a second step of using a silicon specific etch to remove the remaining substrate below the site of interest for plan view analysis.  
         [0004]     An advantage of an embodiment of the invention includes allowing an analysis of a site of interest ten times larger than currently provided by known methods of sample preparation. Another advantage is greater structural and mechanical support for the sample during analysis. Yet another advantage includes the ability to analyze specific film layers within a semiconductor device with greater clarity and detail.  
         [0005]     Certain embodiments of the invention may include none, some, or all of the above advantages. One or more other advantages may be readily apparent to one skilled in the art from the figures, descriptions, and claims included herein.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]     For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings:  
         [0007]      FIG. 1  is a flow chart illustrating a method according to an embodiment of the present invention;  
         [0008]      FIG. 2A  is a three-dimensional view of a semiconductor device containing a site of interest therein in accordance with an embodiment of the present invention;  
         [0009]      FIG. 2B  is a view of a sample of the semiconductor device in  FIG. 2A ;  
         [0010]      FIGS. 3A and 3B  are three-dimensional and cross-sectional views of plan view of a transmission electron microscopy sample prepared in accordance with an embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0011]     Analysis of materials using transmission electron microscopes requires that the sample analyzed should be thin enough to allow electrons to pass through it. For silicon, electron transparency is generally achieved at a thickness of approximately 0.1 μm. Semiconductor devices often contain multiple layers of materials including, for example, one or more oxide layers disposed on or between multiple polysilicon layers, all disposed on a silicon substrate. Typical semiconductors may have a thickness of 0.1 mm to 2 mm or more. Thus, significant thinning of the semiconductor device must be performed to allow for a sample of the device to be analyzed appropriately with a transmission electron microscope.  
         [0012]     Under current techniques, samples for analysis with transmission electron microscopes (TEMs) are prepared by either thinning the area surrounding the site of interest in the sample using focused ion beam (FIB) milling or performing an etch on the site of interest to thin the area sufficiently to allow for TEM analysis. Both FIB milling and chemical or plasma etching typically result in either a very small area that is sufficiently thin for analysis or a structurally unsound sample that is very fragile and difficult to analyze, or both. Additionally, chemical or plasma etching is often insufficiently accurate to enable analysis of a specific film within a site of interest.  
         [0013]     According to various embodiments of the present invention, a semiconductor sample may be prepared in such a way that a larger surface area of the device may be analyzed, with increased specificity of the area to be analyzed, and increased structural stability of the sample. Standard FIB sample preparation suffers from sample thickness variations from the FIB in what is known as the classic “V” shape. Current techniques of standard FIB sample preparation result in an increased slope from the edge of the material to be analyzed, leaving an analysis region of only about 2 μm from the edge of the sample. This phenomenon occurs because of the extremely small area that can be milled and the difficulty of maintaining a consistent milling depth. The result is that when a thick layer is milled, an area of interest of the sample may be milled to the proper thickness, but areas of the sample further from the area of interest become progressively thicker, thus resulting in a cross-sectional view resembling a “V”. In other words, only a limited portion of the extreme edge is thin enough for analysis.  
         [0014]     Typical chemical etching or dimpling techniques result in concavity near the site of interest on the sample which may result in an increased dimension of analysis at the site of interest of approximately 5 μm. However, it becomes difficult to view a specific film due to the concave shape of the etch. An additional method of sample preparation known as tripod polishing also gives a uniformly increasing sample thickness similar to FIB milling, thus limiting the analysis sample size to approximately 2 μm. Additionally, this method can be quite time consuming, and components from film stacks on the semiconductor device are difficult to separate and may be polished away or may be outside the normal analysis width. Another problem with tripod polishing may include thin areas on either side of the site of interest that may reduce mechanical support rendering the sample unusable.  
         [0015]     Referring to  FIG. 1 , at step  10  a site of interest is marked for easy identification on a semiconductor device using an optical microscope. Marking the site with the aid of an optical microscope may have the advantage of insuring that the sample cut from the semiconductor device or wafer is preserved throughout the sample preparation process. At step  12  the sample may be cut from the semiconductor device or wafer preferably using a dicing saw or wafer saw. In a particular embodiment the sample has a length of approximately 3 mm and a width of approximately 1 mm, wherein the site of interest is near the edge of the 3 mm side. Though the width and length of the sample is given as 3 mm and 1 mm respectively, any suitable dimensions may be used that allow for TEM analysis. Step  14  represents a first thinning step whereby the sample is thinned by removing some of the substrate from a first side with an additional cut. This first side is preferably the side which is opposite of the plan view side. In typical plan view analysis, and in accordance with an embodiment of the present invention, the term “plan view” refers to an orientation normal to the planes defined by the multiple film layers and silicon substrate of the semiconductor device.  
         [0016]     In a particular embodiment, step  14  may reduce the thickness of the about 100 μm to 160 μm, though samples prepared according to the present invention may have thicknesses which are greater than 160 μm or less than 100 μm without exceeding the scope of the present invention. Step  14  will preferably be performed by using a wafer saw or other suitable device to mechanically separate thin layers from a semiconductor device or sample. At step  16 , a second thinning step is performed so that an additional portion of the silicon substrate is cut from the sample to create a narrow strip of the silicon substrate along the edge of the sample containing the site of interest. In a particular embodiment this second thinning step  16  results in a sample having a terraced, or “step” effect wherein the thickness of the sample in the area subject to the cut of step  16  may be approximately 20 μm to 35 μm thick. Again, it should be understood that though one embodiment of the present method may result in a sample with thicknesses of 160 μm for step  14  and 35 μm after step  16 , any suitable thicknesses obtained by these steps  14  and  16  may be used without exceeding the scope of the present invention. For example, the thickness after step  14  may be approximately 100 μm and the thickness after step  16  may be approximately 20 μm.  
         [0017]     At step  18 , a portion of the remaining substrate at the site of interest is removed using a focused ion beam (FIB) milling technique, wherein the remaining substrate is approximately 2 μm in a particular embodiment. At step  20 , all or substantially all of the remaining substrate at the area of interest at the side opposite of the plan view side is removed using an etch. The etch of step  20  may be performed using a wet etch or a plasma etch. In a particular embodiment, the sample is etched in a solution of approximately 0.5% hydrofluoric (HF) acid for approximately three minutes to remove any native oxide. Following this initial etch, the sample is etched in choline for approximately one hour. It should be noted that in any particular embodiment, any silicon-specific etch may be used to remove the remaining silicon substrate on the first side of the sample at the site of interest. Examples of silicon-specific etches include choline, HF with nitric acid, and Tetramethyl Ammonium Hydroxide (TAMH), among others. It should also be noted that in the present embodiment the sample is still over 20 μm thick outside of the site of interest, which enhances mechanical stability. In step  22 , the FIB is used to further thin the sample on the second side by removing any oxides or polysilicon layers on the plan view side of the site of interest. It should also be noted that step  22  is not required for some embodiments, depending on the particular site of interest that is to be analyzed.  
         [0018]     Referring to  FIG. 2A , device  100  is a device containing a site of interest for transmission electron microscope analysis. Device  100  may have layers  110 ,  120  and  130 , which may represent layers of oxides, polysilicon, and silicon substrate, respectively. First side  140  of device  100  is the side opposite of the plan view side of device  100  and represents a view normal to layers  110 ,  120  and  130  of device  100 . Second side  142  represents the plan view side of device  100 . Sample  200  of device  100  represents a portion of device  100  containing a site of interest for analysis by TEM. It should be noted that layers  110 ,  120  and  130  may contain single layers of oxides, polysilicon, and silicon substrate, multiple layers of oxides, polysilicons, and silicon substrates, or any combination of such layers and other various materials commonly known to those of ordinary skill in the art.  
         [0019]     Referring to  FIG. 2B , sample  200  has been cut from device  100  using a dicing or wafer saw, or any other method suitable for removing the sample  200  from the device  100 . Sample  200 , by definition, contains the same characteristics of device  100  including layers  210 ,  220  and  230 , which correspond to layers  110 ,  120  and  130  of device  100 . Additionally, first side  240  of sample  200  is the side opposite of the plan view side of sample  200 , and second side  242  is the plan view side of sample  200 . Additionally, sample  200  contains site of interest  250  which may be located in any layer  210 ,  220  or  230  or sample  200 .  
         [0020]     Referring to  FIGS. 3A and 3B , sample  300  is a sample prepared according to an embodiment of the present invention. Accordingly, layers  310 ,  320  and  330  correspond to layers  210 ,  220  and  230 , of sample  200  of  FIG. 2B . Additionally, first side  340  is the side opposite of the plan view side of sample  300  and second side  342  is the plan view side  340 . First side  340  is created by cutting off a portion of the silicon substrate  330 , so that the overall thickness less than the original thickness of the sample  200 , and is approximately 100 μm to 160 μm. After forming first side  340 , a first thinned surface  360  is formed by cutting or mask etching a portion of layer  330  to result in sample  300  having a thickness B which is less than thickness A (e.g., approximately 35 μm) in the area of sample  300  near the site of interest  350 . In the present embodiment, site of interest  350  is within layer  320  of sample  300 , but in other embodiments, site  350  may be in another layer or other layers of sample  300 .  
         [0021]     Second thinned surface  370  is created by two consecutive steps designed to further thin site of interest  350  to a thickness C which is less than thickness B. In the first step, focused ion beam milling is used to thin site of interest  350  at first thinned surface  360  to a roughly uniform thickness of approximately 2 μm. It is not of uniform thicknes due to the well known slope variation generated by FIB milling from the edge of a sample to the thicker portions of a sample. After thinning site of interest  350  to approximately 2 μm, a silicon-specific etch is performed at site of interest  350  on the first side  340  to further reduce the thickness of the site of interest  350  in a uniform manner. In a particular embodiment, a silicon-specific etch may be used which allows a polysilicon layer or an oxide layer to remain intact while all of the remaining silicon substrate is removed. In a particular embodiment, sample  350  may be ready for TEM analysis after creating side  370  on sample  300 . In another embodiment, a third thinned surface  380  is created by a second FIB milling on the first side  342  at site of interest  350 . This second FIB milling may be necessary when site of interest  350  is located in a layer of sample  300  that is disposed a distance from second side  342  that is greater than electron transparency.  
         [0022]     In one embodiment of the present invention, thickness A, defining the distance from first side  340  to second side  342  may be 160 μm. Also, in a particular embodiment, thickness B defining the substantially normal distance from first thinned surface  360  to second side  342  may be 35 μm. In these particular embodiments as well as others, site of interest  350  may have electron transparency for a region having a dimension of up to 50 μm or more as measured along the x-axis defined on  FIGS. 3A and 3B .  
         [0023]     Although the present invention has been described in detail, it should be understood that various changes, substitutions, and alterations may be made, without departing from the spirit and scope of the present invention as defined by the claims. For example, the sample material may be gallium arsenide, indium oxide, a suitable metal, metal alloy, or any other solid material that is capable of analysis or imaging through transmission electron microscopy.