FIB Lift-Out for Defect Analysis

Lucille A. Giannuzzi and Brian W. Kempshall University of Central Florida, Orlando, FL

Shawn D. Anderson and Brenda I. Prenitzer

Agere Systems, Orlando, FL

Thomas M. Moore Omniprobe, Dallas, TX

Abstract

The focused ion beam (FIB) lift-out (LO) technique has been exploited for site-specific transmission electron microscopy (TEM) and other analytical usage. In this paper we summarize the methods used for the ex-situ lift-out (EXLO) and in-situ lift-out (INLO) techniques. Specific examples for FIB LO applications are demonstrated using TEM techniques such as energy filtering TEM, electron holography, lattice imaging, and Z-contrast STEM imaging.

Introduction The use of focused ion beam (FIB) instruments has become a mainstay for the preparation of specimens for subsequent analysis by any number of analytical tools. The versatility of the FIB makes it advantageous for numerous applications. In particular, FIB milling specimen preparation techniques are generally quite fast (on the order of ~ 0.5 to 1 hour for many applications). FIB preparation for transmission electron microscopy (TEM) analysis can take longer (e.g., from 0.5 to 4 hours). In addition, because of the small spot size achievable by new FIB instruments (e.g., < 10 nm), FIB specimen preparation techniques presently offer the best spatial resolution where site specificity is required. Since FIB milling procedures for specimen preparation are often performed at glancing angles to the surface of interest (an incident angle of approximately 90o from the sample surface normal), high quality specimens are routinely obtained from multi-layered bulk materials by successive milling through the different layers. This is particularly useful in microelectronic and other applications where multiple thin films, phases, or layers with very different sputtering behaviors, may be stacked together. A primary advantage to the “lift-out” (LO) technique over other FIB specimen preparation methods is that specimens may be prepared from the starting bulk sample with little or no initial specimen preparation. This is particularly useful when performing failure or

defect analysis, since destructive preparation of the original wafer or bulk sample may be avoided. Thus, the lift-out technique may be defined as a series of FIB milling steps and removal schemes used to produce a site-specific specimen for subsequent analytical observation. The FIB specimen may be thinned to its ultimate thickness either prior to LO (for ex-situ LO), or after LO (as for in-situ LO). For conventional TEM analysis, the FIB specimen is milled to electron transparency (e.g., ~ 100 nm). Using the LO technique requires the addition of either a micromanipulator station for ex-situ LO (EXLO), or a vacuum compatible micromanipulator probe assembly for in-situ LO (INLO). Lift-out specimens described herein were prepared using either an FEI DB235 Dual Beam or FEI 200TEM FIB instrument. Ex-situ micromanipulation was performed with a Micro-Optics system, while in-situ lift-out was performed with an Omniprobe system. All TEM micrographs were obtained using a Philips/FEI Tecnai F30 instrument operating at 300 keV unless otherwise noted.

Ex-Situ Lift-Out The concept of EXLO was first described in 1993 and, was then followed by other references to the technique.1-4 A detailed description of the EXLO technique was given in 1997.5 The EXLO technique requires that the specimen to be analyzed be completely FIB-milled prior to removal from the bulk. A series of FIB cuts are performed to create a free-standing membrane. The cutting procedures may be automated for the preparation of numerous specimens.6 The bulk sample containing the FIB prepared specimen is then removed from the FIB and positioned on a light optical microscope bench whereby a glass rod attached to a hydraulic micromanipulator is used to remove the LO specimen from the FIB trench. The EXLO specimen is then moved to a suitable specimen holder. For TEM analysis, the EXLO specimen is usually positioned onto a carbon coated 400 mesh TEM grid.

Microelectronic Failure Analysis Desk Reference 2002 Supplement. Copyright© 2002. ASM International®

Alternatively, an EXLO specimen may be positioned such that no carbon or support film hinders TEM observation. One report has shown that the specimen may be micromanipulated directly onto a smaller mesh grid that does not contain a carbon coating.7 The advantage to this technique is that the specimen may be broad beam Ar ion milled to remove any FIB damage.

EXLO for EFTEM Elemental Analysis Another alternative approach to EXLO is discussed below. Figure 1 shows a low magnification bright field TEM image of an EXLO specimen that was positioned onto a carbon coated TEM grid. It was observed that the specimen quality was improved in the specimen shown in Figure 1 by performing a final FIB polish at a lower beam energy and lower beam current to reduce FIB damage and redeposition artifacts. Note that the center square piece of the carbon support film has been FIB milled away prior to micromanipulation of the specimen. Thus, the specimen has been positioned such that the carbon film supports the specimen while the region of interest does not contain any carbon below it. The removal of the carbon film is particularly advantageous for electron holography, energy filtered imaging, and higher quality high-resolution imaging.

Figure 1. An EXLO specimen positioned on a carbon coated TEM grid with the center portion of the carbon FIB milled away. A region containing a W window contact (W1) at transistor level shown in figure 1 was further studied using electron energy loss energy filtered mapping (EFTEM). Figure 2 shows EFTEM elemental maps of Sik, Ok, TiL, Nk, respectively. The EFTEM maps were obtained on the Tecnai F30 at 300 keV using a

Gatan GIF100. Note that the FIB specimen preparation allows for elemental identification in a predetermined site-specific region of interest. The bright contrast in the Si map shows some diffraction effects within the Si substrate. Slight deviations in the Si contrast are observed within the dielectric material indicating small deviations in composition between the layers. The O map shows slight variations in contrast in the dielectric material that are consistent with the Si map. In addition, O is also observed at the interfaces bordering the gate structures. The Ti map confirms the elemental presence of the Ti typically used in semiconductor diffusion barrier layers. The presence of N is observed by the N EFTEM map as indicated by the presence of SiN spacers adjacent to the gate structures. Figure 2. EFTEM maps of Si, O, Ti and N obtained from a specimen prepared by EXLO.

In-Situ Lift-Out The concept of INLO was first described by researchers at Hitachi.8 INLO may be performed in either a single beam FIB or a dual beam (SEM + FIB) instrument. In the case of INLO, first a large section of material containing the region of interest is completely FIB milled free. Typical dimensions of the section of material may be ~ 10-30 µm in length, ~ 10 µm in width, and 5-10 µm in depth. Samples greater than 100 µm in length have been lifted-out and thinned for TEM analysis.9 Removal of this piece of material is then performed using a micromanipulator assembly located inside of the vacuum chamber in conjunction with the ion beam assisted chemical vapor deposition (CVD) process available with the FIB tool. There are various

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strategies for removing the area of interest using in-situ lift-out, one of which follows. The probe is positioned to just touch the FIB milled piece of material. A CVD Pt deposition process is then used to affix the probe to the specimen. Using the probe, as seen in the ion beam induced secondary electron image in Figure 3, the section is lifted out of the FIB trench. The probe/sample assembly is then retracted away from the sample stage. A suitable sample holder is then positioned in the field of view and the probe is used to lower the attached sample onto the holder (e.g., TEM grid). The CVD Pt is then used to secure the sample to the holder as shown in the ion beam induced secondary electron image in Figure 4. Once attached to the TEM grid, the probe is then FIB-milled free from the sample. The sample may then be milled using conventional FIB milling steps to prepare a specimen for TEM or other analysis. A specimen that has been thinned for TEM analysis using a dual beam instrument is shown in the SEM image in Figure 5. (Note that an analogously- obtained ion beam image of a TEM thinned specimen would incur large amounts of ion irradiation damage to the specimen and render it useless for many advanced TEM techniques.)

Figure 3. An INLO sample removed using a probe inside of a FIB instrument.

Figure 4. An INLO sample mounted on a TEM grid.

Figure 5. An SEM image of a thinned INLO TEM specimen FIB milled using a dual beam instrument.

INLO For 2D Dopant Profiles

Electron holography has been used to show the distribution of dopant atoms in pn junctions of semiconductor devices.10,11 In order to collect an electron hologram, the field of view defined by the biprism fringes must contain both the area of interest within the specimen as well as a region of vacuum (e.g., a hole in the specimen). In addition, the specimen must be of uniform thickness in the 200-400 nm range. It has been shown that a dual beam FIB/SEM instrument may be used for the preparation of specimens suitable for electron holography.12 Figure 6 shows an example of INLO specimen prepared in a single beam FIB instrument for the

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application of electron holography. The specimen was thinned to approximately 1 µm after the section was lifted out from the bulk and attached to a TEM grid. Then the specimen was tilted to 45o and a region just above the gate was FIB milled away to create a hole close to the gate as shown in Figure 6. This hole provides the vacuum region necessary to obtain a reference hologram. The specimen was then thinned to ~ 300 nm using conventional FIB milling methods.

Figure 6. A FIB image showing an INLO specimen prepared for electron holography applications.

Figure 7. TEM image of a gate structure prepared by INLO. Figure 7 shows a TEM image of the gate region used for subsequent electron holography analysis. Damage in the single crystal Si that resulted from the dopant implantation is clearly evident. Note the bright region above the gate structure denoting the region of vacuum (e.g., a hole). Figure 8 shows a reconstructed phase image obtained from an electron hologram of the region shown in Figure 7. Note that the curtaining effect of FIB milling is readily observed in the phase image as indicated by the light band of contrast directly below the gate region. Curtaining may occur when a low sputter-yield material partially blocks the FIB beam from reaching a faster sputter- yield material that may be below it. This has the effect of causing the underlying region that is partially blocked to be

slightly thicker than the surrounding regions (note the mass/thickness contrast in the bright field TEM image of Figure 1). Thus, it may be inferred from the reconstructed phase image that the bright band below the gate indicates a thicker portion of the specimen. Quantification of this thickness difference is possible.13 In addition to thickness variation effects, the reconstructed phase image in Figure 8 also shows a difference in contrast in 2-dimensions. This difference is related to the dopant concentration at the pn junction within the Si substrate. For example, the contrast directly under the gate starts out dark and gradually lightens until it reaches the contrast of the un-doped Si substrate. Using electron holography to determine 2D dopant profiles will be increasingly important as gate sizes shrink. The corresponding electrical properties of the pn junction may also be determined readily using this technique.11 Figure 8. Reconstructed phase image obtained from

an electron hologram from the region in Figure 7.

INLO for Interface Analysis FIB LO is particularly useful for isolating individual interfaces and/or grain boundaries for defect analysis. An example of a Fourier filtered high resolution TEM image of a 45o/[100] Cu twist grain boundary prepared by FIB INLO is shown in figure 9. The [011] and [001] grains are shown in the image. The arrows in the image point to the approximate location of the grain boundary. A Cu grain boundary that had been segregated with Bi was analyzed using high angular annular dark field (HAADF) scanning transmission electron microscopy (STEM) and is shown in figure 10. The bright band of contrast running diagonally across the image indicates that the presence of Bi is limited to the grain boundary region. Other isolated regions of bright contrast within the grains are due to diffraction effects. Thus, individual planar defects (e.g., grain boundaries) may be chosen for site-specific analytical observation using the FIB INLO technique.

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Figure 9. Fourier filtered lattice image of a Cu 45o/[001] twist boundary prepared by FIB INLO.

Figure 10. HAADF STEM image of Bi segregation to a Cu grain boundary.

INLO for MEMS Device

An example of the use of INLO is the inspection of MEMS devices such as the TI Digital Micromirror Device™ (DMD™). The DMD™ is a micromachined array of 1.3 million aluminum mirrors fabricated on top of CMOS circuitry. The array is processed in wafer form and fabricated by standard semiconductor manufacturing methods. An electrostatic field that is digitally generated controls each mirror. Projected images or light signals are formed by reflecting an external bright light from the mirrors. A schematic diagram of a portion of the mirror array on a DMD™ is shown in Figure 11.

Figure 11. Schematic diagram of a TI DMD™ (courtesy Texas Instruments DLPTM Products).14 Figure 12 illustrates the application of INLO to assemble or disassemble MEMS components. This capability is especially useful as part of reliability testing performed during product development. The in situ micromanipulator probe enables the MEMS designer to expose and examine portions of the MEMS structure that would otherwise be very difficult to access without creating a distracting level of damage. These exposed surfaces can be studied for wear, corrosion and contamination, and can also be prepared for TEM inspection to reveal microstructural changes. For MEMS product development, the micromanipulator allows MEMS designers the go beyond the limits of two-dimensional etch technology to create three-dimensional structures, or structures with materials that do not lend themselves to typical semiconductor processing. In this example, the ion beam has been used to release (e.g., mill) a single mirror from its yoke to enable lift-out by the in-situ micromanipulator. Subsequent inspection of both the components beneath the mirror as well as the backside of the mirror is then easily performed.

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Figure 12. FIB INLO of a MEMS device. (Courtesy Texas Instruments DLPTM Products).

Comparison Between EXLO and INLO INLO and EXLO offer different methods for producing TEM specimens within the FIB. Both methods are effective for this purpose and offer unique advantages. The primary advantages of EXLO are that EXLO can be faster than INLO, and the procedure for transferring the thin section to the TEM grid is performed outside the FIB, thus reducing the time and expense of FIB usage. Future automation of the INLO process offers the potential of reducing the process time and allowing unattended overnight processing of samples. Because the EXLO grids are membrane grids, and INLO grids are "half grids", the robustness and storage requirements for the two grid types may differ. The long-term storage performance of the two grid types is not yet known. INLO offers several advantages for TEM specimen preparation, including sample protection, the ability to re-FIB samples, and cleaner samples. In contrast to the EXLO technique, where the final step involves potentially damaging mechanical manipulation of the thin membrane, the final step using the INLO technique is sample thinning. Thus, when using INLO, the region of interest for TEM analysis remains protected. Further, since the sample never leaves the FIB during INLO, the location on the sample where the TEM section is being created is easy to find. In addition, because the sample is mechanically bonded to the metal TEM grid during INLO, sample modification and image degradation due to TEM heating and charging effects are greatly reduced, i.e., better thermal and electrical grounding of an INLO specimen is available in the TEM. The INLO specimen may be re-thinned easily by FIB or broad beam Ar ion milling. Also, the ability to lift-

out a specimen within the FIB may be easier in samples with a great deal of surface topography. EXLO is more difficult in samples with surface topography due to the poor depth of field that exists for light optical microscopes. In addition, the amount of FIB-implanted Ga observed within a TEM specimen may be reduced in INLO specimens since redeposition artifacts are less likely to be problematic due to the large escape volume that is available in an INLO geometry. However, excellent quality EXLO specimens devoid of redeposition artifacts may be prepared with use of careful FIB procedures (e.g., see Figures 1 and 2 and ref. 15).

Reasons For Using EXLO and INLO There are three primary reasons for producing TEM specimens in the FIB that are common to both INLO and EXLO. These include material issues, site- specific requirements, and success rate. Material issues may make conventional TEM specimen prep (e.g., wet lab prep) impractical. This is true for materials that do not lend themselves well to grinding, polishing, and etching due to preferential thinning of multilayers, deformation of soft materials, or slow removal rates as a result of exceptional hardness. The requirement to perform one or more site-specific TEM inspections, possibly at different angles, from one sample, makes FIB specimen preparation preferred over conventional methods. And finally, FIB-based TEM specimen preparation methods offer the potential for a much higher success rate, which can be critical with one-of-a-kind samples.

Conclusions

The FIB LO techniques have been summarized. Both EXLO and INLO may be used routinely to prepare high quality site-specific specimens for subsequent analysis by TEM as well as other analytical techniques. INLO enables disassembly of MEMs devices for further inspection. Lastly, both methods enable the ability to perform failure analysis, defect analysis, and routine microstructural analyses using a variety of TEM techniques including conventional TEM imaging, HAADF/STEM, EELS, XEDS, HREM, and electron holography.

Acknowledgements

The help from Molly McCartney in acquiring the electron hologram is gratefully acknowledged. Samples in Figures 1,2,6-8, were obtained courtesy of Agere Systems. Support was provided by AMPAC, Agere Systems, Lissa Magel (Texas Instruments DLPTM Products), Cheryl Hartfield (Texas Instruments), Omniprobe, and NSF DMR#9703281.

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