Abstract:
An improved method for extracting and handling multiple samples for S/TEM analysis is disclosed. Preferred embodiments of the present invention make use of a micromanipulator that attaches multiple samples at one time in a stacked formation and a method of placing each of the samples onto a TEM grid. By using a method that allows for the processing of multiple samples, the throughput of sample prep in increased significantly.

Description:
This application claims priority from U.S. Prov. Appl. No. 61/794,816, filed Mar. 15, 2013 which is hereby incorporated by reference. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to the extraction and handling of samples for transmission electron microscopes and scanning transmission electron microscopes. 
     BACKGROUND OF THE INVENTION 
     Semiconductor manufacturing, such as the fabrication of integrated circuits, typically entails the use of photolithography. A semiconductor substrate on which circuits are being formed, usually a silicon wafer, is coated with a material, such as a photoresist, that changes solubility when exposed to radiation. A lithography tool, such as a mask or reticle, positioned between the radiation source and the semiconductor substrate casts a shadow to control which areas of the substrate are exposed to the radiation. After the exposure, the photoresist is removed from either the exposed or the unexposed areas, leaving a patterned layer of photoresist on the wafer that protects parts of the wafer during a subsequent etching or diffusion process. 
     The photolithography process allows multiple integrated circuit devices or electromechanical devices, often referred to as “chips,” to be formed on each wafer. The wafer is then cut up into individual dies, each including a single integrated circuit device or an electromechanical device. Ultimately, these dies are subjected to additional operations and packaged into individual integrated circuit chips or electromechanical devices. 
     During the manufacturing process, variations in exposure and focus require that the patterns developed by lithographic processes be continually monitored or measured to determine if the dimensions of the patterns are within acceptable ranges. The importance of such monitoring, often referred to as process control, increases considerably as pattern sizes become smaller, especially as minimum feature sizes approach the limits of resolution available by the lithographic process. In order to achieve ever-higher device density, smaller and smaller feature sizes are required. This may include the width and spacing of interconnecting lines, spacing and diameter of contact holes, and the surface geometry such as corners and edges of various features. Features on the wafer are three-dimensional structures and a complete characterization must describe not just a surface dimension, such as the top width of a line or trench, but a complete three-dimensional profile of the feature. Process engineers must be able to accurately measure the critical dimensions (CD) of such surface features to fine tune the fabrication process and assure a desired device geometry is obtained. 
     Typically, CD measurements are made using instruments such as a scanning electron microscope (SEM). In a scanning electron microscope (SEM), a primary electron beam is focused to a fine spot that scans the surface to be observed. Secondary electrons are emitted from the surface as it is impacted by the primary beam. The secondary electrons are detected, and an image is formed, with the brightness at each point of the image being determined by the number of secondary electrons detected when the beam impacts a corresponding spot on the surface. As features continue to get smaller and smaller, however, there comes a point where the features to be measured are too small for the resolution provided by an ordinary SEM. 
     Transmission electron microscopes (TEMs) allow observers to see extremely small features, on the order of nanometers. In contrast SEMs, which only image the surface of a material, TEM also allows analysis of the internal structure of a sample. In a TEM, a broad beam impacts the sample and electrons that are transmitted through the sample are focused to form an image of the sample. The sample must be sufficiently thin to allow many of the electrons in the primary beam to travel though the sample and exit on the opposite site. Samples, also referred to as lamellae, are typically less than 100 nm thick. 
     In a scanning transmission electron microscope (STEM), a primary electron beam is focused to a fine spot, and the spot is scanned across the sample surface. Electrons that are transmitted through the work piece are collected by an electron detector on the far side of the sample, and the intensity of each point on the image corresponds to the number of electrons collected as the primary beam impacts a corresponding point on the surface. 
     Because a sample must be very thin for viewing with transmission electron microscopy (whether TEM or STEM), preparation of the sample can be delicate, time-consuming work. The term “TEM” as used herein refers to a TEM or an STEM and references to preparing a sample for a TEM are to be understood to also include preparing a sample for viewing on an STEM. The term “S/TEM” as used herein also refers to both TEM and STEM. 
     Several techniques are known for preparing TEM specimens. These techniques may involve cleaving, chemical polishing, mechanical polishing, or broad beam low energy ion milling, or combining one or more of the above. The disadvantage to these techniques is that they are not site-specific and often require that the starting material be sectioned into smaller and smaller pieces, thereby destroying much of the original sample. 
     Other techniques generally referred to as “lift-out” techniques use focused ion beams to cut the sample from a substrate or bulk sample without destroying or damaging surrounding parts of the substrate. Such techniques are useful in analyzing the results of processes used in the fabrication of integrated circuits, as well as materials general to the physical or biological sciences. These techniques can be used to analyze samples in any orientation (e.g., either in cross-section or in plan view). Some techniques extract a sample sufficiently thin for use directly in a TEM; other techniques extract a “chunk” or large sample that requires additional thinning before observation. In addition, these “lift-out” specimens may also be directly analyzed by other analytical tools, other than TEM. Techniques where the sample is extracted from the substrate within the FIB system vacuum chamber are commonly referred to as “in-situ” techniques; sample removal outside the vacuum chamber (as when the entire wafer is transferred to another tool for sample removal) are call “ex-situ” techniques. 
     Samples which are sufficiently thinned prior to extraction are often transferred to and mounted on a metallic grid covered with a thin electron transparent film for viewing.  FIG. 1  shows a sample mounted onto a prior art TEM grid  10 . A typical TEM grid  10  is made of copper, nickel, or gold. Although dimensions can vary, a typical grid might have, for example, a diameter of 3.05 mm and have a middle portion  12  consisting of cells  14  of size 90×90 μm 2  and bars  13  with a width of 35 μm. The electrons in an impinging electron beam will be able to pass through the cells  14 , but will be blocked by the bars  13 . The middle portion  12  is surrounded by an edge portion  16 . The width of the edge portion is 0.225 mm. The edge portion  16  has no cells, with the exception of the orientation mark  18 . The thickness  15  of the thin electron transparent support film is uniform across the entire sample carrier, with a value of approximately 20 nm. TEM specimens to be analyzed are placed or mounted within cells  14 . 
     During the extraction process, the wafer containing the completed lamella is removed from the FIB and placed under an optical microscope equipped with a micromanipulator. A probe attached to the micromanipulator is positioned over the lamella and carefully lowered to contact it. Electrostatic forces will attract lamella to the probe tip. The probe tip with attached lamella is then typically moved to a TEM grid. Alternatively, the attachment of the lamella to the probe tip can be done using FIB deposition. 
     Samples which require additional thinning before observation are typically mounted directly to a TEM sample holder.  FIG. 2  shows a typical TEM sample holder  31 , which comprises a partly circular 3 mm ring. In some applications, a sample  30  is attached to a finger  32  of the TEM sample holder by ion beam deposition or an adhesive. The sample extends from the finger  32  so that in a TEM (not shown) an electron beam will have a free path through the sample  31  to a detector under the sample. The TEM sample is typically mounted horizontally onto a sample holder in the TEM with the plane of the TEM sample perpendicular to the electron beam, and the sample is observed. 
     Unfortunately, preparation of TEM samples using such prior art methods of sample extraction are time consuming. Conventional work flow usually has a user pick up one sample at a time and placed on the TEM sample holder. First, the sample is prepped. Using a micromanipulator, the sample is lifted out. The sample is then moved to a sample holder, positioned, and then lowered so the electrostatic forces will “drop off” the sample. The sample can also be removed and placed on the location of a TEM sample holder by physically severing the connection. CD metrology often requires multiple samples from different locations on a wafer to sufficiently characterize and qualify a specific process. In some circumstances, for example, it will be desirable to analyze from 15 to 50 TEM samples from a given wafer. When so many samples must be extracted and measured, using known methods the total time to process the samples from one wafer can be days or even weeks. Even though the information that can be discovered by TEM analysis can be very valuable, the entire process of creating and measuring TEM samples has historically been so labor intensive and time consuming that it has not been practical to use this type of analysis for manufacturing process control. For the user to prep and remove each of the TEM samples, the user must perform these steps repeatedly. In other words, the user repeats the steps of prepping another sample. The user then repeats the step of lifting the sample out. Then, the user moves the sample to the TEM sample holder one at a time. This current process for high volume TM lamella prep is performed serially, and the process is often time consuming and labor intensive. 
     Speeding up the process of sample extraction and transfer would provide significant advantages in both time and potential revenue by allowing a semiconductor wafer to be more rapidly returned to the production line. What is needed is an improved method for TEM sample analysis, including new ways of extracting more than one sample at a time. 
     SUMMARY OF THE INVENTION 
     An object of the invention, therefore, is to provide an improved method for TEM sample analysis that increases throughput for high volume sample preparation. Preferred embodiments of the present invention provide improved methods that allows for multiple pickups and drop off of samples that allows for efficient processing of the samples. The preferred embodiment of the present invention allows for a method to stack and multiple samples to each other to move the samples to the TEM sample holder with fewer steps. This process also minimizes stage motions, gas injection system (GIS) insert/retract action and manipulator insert/retract actions. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter. It should be appreciated by those skilled in the art that the conception and specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more thorough understanding of the present invention, and advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  shows a typical prior art TEM grid. 
         FIG. 2  shows a typical prior art TEM sample holder. 
         FIG. 3  shows multiple samples. 
         FIG. 4  shows one sample being picked up by the manipulator. 
         FIG. 5  shows 2 samples being picked up by the manipulator. 
         FIG. 6  shows 3 samples being picked up by the manipulator. 
         FIG. 7  shows 4 samples being picked up by the manipulator. 
         FIG. 8  shows 5 samples being picked up by the manipulator. 
         FIG. 9  shows 5 samples next to TEM sample holder. 
         FIG. 10  shows the last sample ( 105 ) being welded to the holder ( 110 ). 
         FIG. 11  shows sample ( 104 ) being attached to the grid. 
         FIG. 12  shows sample ( 103 ) being attached to the grid. 
         FIG. 13  shows sample ( 102 ) being attached to the grid. 
         FIGS. 14 &amp; 15  show graphs showing the average sample times for 5 samples using the conventional method and the methods in accordance with embodiments of the present invention. 
         FIG. 16  is a picture of a microprobe holding multiple samples in accordance with embodiments of the present invention. 
         FIG. 17  shows a picture of a microprobe during a sample drop in accordance with embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention provide a method of picking up and dropping of multiple samples onto a TEM sample holder, or TEM grid, which is done to increase throughput that will involve a less labor-intensive process. 
     A preferred method or apparatus of the present invention has many novel aspects, and because the invention can be embodied in different methods or apparatuses for different purposes, not every aspect need be present in every embodiment. Moreover, many of the aspects of the described embodiments may be separately patentable. 
     In a preferred embodiment of the present invention, one or more lamellae are first created on a wafer or other substrate.  FIG. 3  shows a number of lamellas  100  that have been milled or processed for preparation to be removed from the wafer. Preferably, a number of lamellas can be created using an automated ex-situ process where a lamella is thinned in place before removal as described in U.S. Provisional App. 60/853,183 by Blackwood et al. for “Method for S/TEM Sample Analysis” (which is hereby incorporated by reference). The sample milling process can be used to create one or more lamellae at different sites on a wafer or other substrate. This method could also be used for multiple chunk type extractions to a finger type grid. 
     Once the desired number of lamellas has been created and prepped, a micromanipulator is used to extract the lamellas. The list of all lamella sites, including the x-y coordinates for each lamella location, for each wafer can be transferred to the extraction tool from the FIB system used to mill the lamellae. As shown in  FIG. 4 , the micromanipulator  101  is an electrostatic/pressure manipulator where the electrostatic forces will attract the lamella  102  to a probe tip  150 . The lamella extraction process is preferably fully automated. Alternatively, the extraction process can be completely or partially controlled manually. 
     As shown in  FIG. 5 , the micromanipulator, or nano-manipulator, will then be moved to a different location containing the next lamella  103  that is ready for extraction. The micromanipulator  101  and the attached lamella  102  will be lowered so that lamella  102  is in contact with lamella  103 . Electrostatic forces will attract lamella  103  to lamella  102  attaching lamella  103  to  102  to for a stack of lamella. Alternatively, the attachment of the lamella to each other can be done by welding the lamellas using FIB deposition or electron beam, friction fitting, or wherein keyed type connections could be made, which is known in the industry. As shown in  FIG. 6 , the process is repeated with the next lamella  104 . Where the sample to be extracted has a vertical sample face, this results in the probe being also oriented at a 45 degree angle relative to the sample face. 
     As shown in  FIGS. 7 and 8 , the process is repeated so that each subsequent lamella is attached to the preceding lamella that was attached. Lamella  105  is attached to lamella  104 . Lamella  106  is attached to lamella  105 . The attachment area can comprise a small section that allows for the electrostatic forces to make the attachment and also allows for the electrostatic forces to be greater than the weight of the total number of lamellas intended to be attached. 
     A computer, or processor, with the appropriate software, can receive the x-y coordinates for the multiple lamellas to be extracted from the FIB system. The location of each lamella can then be matched with a corresponding TEM sample holder location once the samples are extracted and transferred to the TEM sample holder, or TEM grid (typically one lamella per cell). This allows for data traceability through the entire process so that the final TEM results can be automatically matched back to the particular sample site on the original wafer. 
     Because electrostatic attraction is used to adhere the sample to the probe tip, the angled bevel on the microprobe, along with the ability to rotate the probe tip 180 degrees around its long axis, allows the lamella to be precisely placed on the TEM sample holder. Special consideration of the angle is also made when the sample is welded to the probe tip or to each other using FIB deposition. 
     Once all of the lamellas are picked up in a stacked formation, the lamellas are removed to a TEM sample holder, or a TEM grid, for placement. As shown in  FIG. 9 , the lamellas are dropped off onto TEM sample holder  110 . The last lamella to be picked up, lamella  106  in  FIG. 10 , is lowered and positioned so that lamella  106  makes an attachment to TEM sample holder  110  at attachment spot  120 . The electrostatic forces from the TEM sample holder  110  are stronger than the electrostatic forces that attach lamella  106  and lamella  105 . Thus, the lamella  106  makes a willing attachment to TEM sample holder  110 . In the alternative, the samples can be physically detached by severing the connection and making an attachment by welding the sample to the TEM sample holder  110 . In accordance to  FIGS. 11 through 13 , the stacked lamellas are moved to a different location on the TEM sample holder  110  and removed one by one, wherein the last lamella extracted from the wafer is the first lamella to be lowered and attached to the TEM sample holder  110 . This is done successively until all of the lamellas are removed from the micromanipulator  101  and attached to TEM sample holder  110 . 
     The processing of the lamellas can be performed with an FIB system that can navigate to each additional sample site and repeat the process to prep each of the lamella. This may involve the process of milling each side of the desired sample locations. Because this method involves fewer motions between the bulk stage and the TEM sample holder, or TEM grid, the process minimizes stage motions, increases throughput, and minimizes the GIS insert/retract actions and manipulator insert retract actions. As automation of this process matures, a significant increase in throughput is expected. 
     As shown in the  FIGS. 14 and 15 , which shows the average sample time for 5 samples using a H450HP, H450ML, H450SF1NPC, and H450SF1 PC using a conventional methods of extraction of lamellas and the method in accordance with embodiments of the current invention, almost 2 minutes per sample was reduced with the current method. The total sample time that is saved ranges from a couple of minutes to more than 10 minutes for given samples of 5 lamellas. 
       FIG. 16  shows a micromanipulator that has successfully picked up four lamellas in accordance with embodiments of the current invention.  FIG. 17  shows an actual picture of a micromanipulator with the four lamellas that is intended for placement on the TEM sample holder. 
     The invention has broad applicability and can provide many benefits as described and shown in the examples above. The embodiments will vary greatly depending upon the specific application, and not every embodiment will provide all of the benefits and meet all of the objectives that are achievable by the invention. Further, although much of the previous description is directed at semiconductor wafers, the invention could be applied to any suitable substrate or surface. Also, although much of the previous description is directed at generally rectangular shaped lamellae which are less than 100 nm thick, the present invention could be used with lamellae of other thicknesses and with samples having other shapes. The accompanying drawings are intended to aid in understanding the present invention and, unless otherwise indicated, are not drawn to scale. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made to the embodiments described herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.