Method for sample separation and lift-out

Description of equivalent WO 02095378 A1

METHOD FOR SAMPLE SEPARATION AND LIFT-OUT

0002TECHNICAL FIELD OF THE INVENTION The invention relates to methods for separating and manipulating microscopic samples from objects within electron-beam or ion-beam instruments.

0003BACKGROUND OF THE INVENTION The present invention relates to a method for separating a sample and a method for preparing the separated sample for analysis, in cases where analysis is desired; and particularly relates to a method for separating a minute sample region from a substrate such as a semiconductor wafer. This application describes embodiments in which a sample is cut out of a semiconductor wafer or other object by use of a focused ion beam ("FIB") and analyzed, if desired, through a transmission electron microscope ("TEM"), or by other means. In a conventional method for TEM sample preparation, a chip, or ribbon, having a length of several mm and a width of 100-500 μm is cut out from a semiconductor integrated circuit wafer by use of a diamond wafering saw. The chip is mounted on a standard TEM grid. Then the chip is formed into a thin sample (typically 50 μm) by the FIB. The thin film sample is irradiated with an electron beam and observed by use of the TEM. In conventional TEM observations a sample is thinned by polishing, while being observed. With this method, it is difficult to set the place of observation and direction of the sample desirably and precisely. It is necessary to carry out a step in which a region having a length of several mm and a width of 100-500 μm, and including a portion to be analyzed, is mechanically separated from the chip of an integrated circuit or semiconductor wafer. When a wafer is a sample substrate, it is necessary to divide the wafer for observation. It is difficult to process a sample to have a thickness less than 100 μm through mechanical processing by means of a diamond wafering saw or the like, because it is difficult to maintain accuracy and avoid damage. Portions which could not be cut to be thin enough through mechanical processing, would then have to be thinned with the FIB, which adds to processing time. In another conventional method, the sample is cleaved first. The plane of cleavage is then observed by a scanning-electron microscope. This method however, makes it difficult to specify a desired portion precisely, and makes it difficult to observe the section cleaved. All of the foregoing conventional techniques have disadvantages because it is difficult to make a section even substantially flat and parallel to the sample surface for observation, and it is impossible to observe, for example, a horizontal section of a contact hole. Another conventional method uses the FIB to cut a sample from a wafer by cutting the sample from at least two different angles after a probe has been attached to the sample. The probe then removes the sample for analysis. This method has the disadvantage that the wafer must be moved to a second angle for the second cut to be effected. These techniques typically rely on a break in electrical conductivity or trial and error to tell when the sample is isolated. There is a need for a method of sample separation which can produce precise samples, but which does not need multiple cuts. There is a particular need for a sample- separation method which will allow the operator to see the operation in process and to clearly see when the sample separates, without the need for electrical contact tests, or the like. SUMMARY OF THE INVENTION The invention is a method for sample separation and lift-out within a FIB instrument. The preferred embodiment comprises the steps of, first, cleaving a wafer, having an area of interest, or target, to be removed, so as to place the target near the edge of the cleaved wafer. Then, the tip of a micromanipulator probe is fixed to the wafer by ion-beam metal deposition. The FIB ion-beam is positioned at an angle of approximately 50 degrees to the plane of the wafer, and the ion beam cuts a U-shaped path on the surface of the wafer, so that the path surrounds the target and the fixed probe, and completely released the sample from the wafer. Next, the stage of the FIB is lowered slightly, so that the wafer is lowered beneath the released sample, now attached to the probe. The probe with the sample fixed to it is moved to a TEM grid. The TEM grid is preferably cut across its plane so as to provide a V-shape open above. The opening allows easy access to the V-shape for placement of the sample and further operations with the ion beam. The sample is then fixed to the TEM grid by ion-beam metal deposition, and the probe is then cut by the ion-beam and detached from the fixed sample. The sample may now be thinned by the ion beam for TEM inspection, or inspected by other means. The reader should note that with the method just described, only one ion-beam cut at one angle is necessary to release the sample from the wafer and allow it to be lifted out.

0004BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a typical wafer with an area of interest shown. The wafer is cut to create an edge near the area of interest. Figure 2 shows the attachment of a probe to the wafer. Figure 3 shows the FIB cutting out a sample bearing the area of interest, or target. Figure 4 shows removal of the sample from the wafer. Figures 5, 6, 7, and 8 show the attachment of the sample to a TEM grid, and detachment of the probe. 1

2 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

00063 Figure 1 shows a semiconductor wafer (100) which has been cleaved or sawed so

00074 the area of interest or "target" (110) for lift-out is located as close as possible to newly

00085 formed edge (115). "Cleaving" may be done by sawing The further from the edge, the

00096 longer (in y direction) and the deeper (in z direction), is the cut required to remove the

00107 sample. A longer and deeper cut will significantly add time to the operation. The

00118 operator inserts the wafer in FIB. In this application, we describe the object under

00129 examination as a "wafer." The "wafer" (100) need not be a semiconductor device. It

001310 may, for example be a micromechanical device, or any substance whatever requiring

001411 TEM or SEM analysis, such as particles, granules, biological materials, or thin films.

001512 The FIB may be either a single-beam model, or a dual-beam model. Typical FIB

001613 instruments are those manufactured by FEI Company of Hillsboro, Oregon, as models

001714 200, 820, 830, or 835.

001815 After the cleaved wafer (100) is placed in the FIB instrument, the eucentric

001916 position of the ion beam (160) should be set on the target (110). The sample should rest

002017 at an angle α depicted in Figure 3 to the ion beam (160). The angle α can be any angle

002118 less than ninety degrees to the plane of the wafer (100), although an angle of 45-60

002219 degrees is preferable. Typically, the wafer (100) will be perpendicular to the electron 0 beam, if the FIB has an electron beam in addition to the ion beam (160). 1 Figure 2 shows a probe (120) that has made contact with the wafer (100) near 2 the target (110). The probe (120) is a component of a conventional micro-manipulator 3 tool which is attached to the FIB instrument with vacuum feed-through. A typical such 4 micro-manipulator tool is the Model 100 by Omniprobe, Inc. of Dallas, Texas. The 5 probe (120) is moved until its tip makes physical contact with the wafer (100), as shown 6 in Figure 2. The operator then preferably uses ion-beam metal deposition to form a weld 7 (130) that fixes the tip of the probe (120) to the wafer (100). Other methods may be 8 used to fix the probe (120) to the wafer (100), such as electrostatic attraction or 9 adhesives.

002330 Next, the operator uses the layout functions of the FIB instrument to specify a

002431 pattern on the wafer (100) surrounding the target (110), and preferably extending to the

002532 edge (115) of the wafer (100) in the y-direction, as shown in Figure 3. The pattern is preferably U-shaped around the target (110), although a rectangular area, or an arbitrary shape enclosing the target (110) could also be used. This area defines the sample area to be milled by the ion-beam (160). The milling cut (150) should be as thick as necessary to prevent redeposited material from filling the cut. This will depend on the properties of the wafer (100). The specified pattern is milled to completely cut out the sample (140). Gas-assisted etching may also be used, if desired. This operation is illustrated in Figure 3. By viewing the image as the operation proceeds, it will be possible to see when the sample (140) is completely detached. After the sample (140) is completely cut from the wafer (100), the operator will increase the distance between the probe (120) (now holding the sample (140) and target (110)), and the remaining wafer (100), preferably by lowering the FIB stage height by at least 5-10 μm to drop the wafer (100). The operator withdraws the probe (120) holding the target (110) so the sample (140) will be clear of any obstacles when the wafer (100) is exchanged for the TEM grid (200). This operation is shown in Figure 4. Of course, the sample holder need not be a TEM grid (200) as shown, but may be another type of holder or device suitable for the intended inspection. When the probe (120) and the sample (140) are completely clear of any potential obstruction, the operator removes the wafer (100) to exchange it for the TEM grid (200) holder. Separation of the cut sample (140) from the wafer (100) is preferably assisted by slightly lowering the wafer (100); typically by lowering the stage in the FIB instrument. The TEM grid (200) is a standard grid, such as an uncoated 100 or 200 square mesh Cu grid. For the purposes of the method disclosed, the TEM grid (200) is cut across so that the cut bars form "V's" open above (in the z-axis) (see Figure 5). Using the probe (120) the TEM grid (200) is mounted in the FIB with the cut side up, or with its plane parallel to the z-axis. When the TEM grid (200) is inserted into the FIB, the eucentric position should be set on the cut edge of the TEM grid (200). The operator then moves the probe (120) with the sample (140) attached as shown in Figure 5. The probe (120) is moved until both ends of the sample (140) contact the bars of the TEM grid (200) as shown in Figure 5 and the following figures. Using FIB metal deposition, the operator welds a free edge of the sample (140) to a first bar (220) of the TEM grid (200) forming a first weld (210). The operator should choose an edge which will allow the ion beam (160) to contact the location of the first weld (210) without interference of the probe (120). Figure 6 shows the probe (120) detached from the sample weld (130) by use of the ion beam (160). The probe (120) may now be retracted. The operator then uses FIB metal deposition to weld the newly freed edge of sample (140) to a second bar (230) of the TEM grid (200), forming a second weld (240), as shown in Figures 7 and 8. The sample can now be thinned by the FIB to a thickness suitable for the TEM application. In another embodiment of the invention, the method may be used generally to remove a portion of a wafer (considered generally). The probe is attached as described above to the sample of the wafer desired to be removed. Then, the ion beam is positioned to make a single cut that releases the sample. The sample is held attached to the probe, which may be moved away from the wafer and inspected or simply disposed of, if the object is to shape the remaining portion of the wafer. Thus the method is applicable to such applications as micromachining or fabrication of atomic-force- microscope tips. Since those skilled in the art can modify the specific embodiments described above, we intend that the claims be interpreted to cover such modifications and equivalents.

0026CLAIMS We claim: