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Recent advances in application of focused ion beam technology
Microelectronic Engineering 21 (1993) 179-186 Elsevier
179
Recent advances in application of focused ion beam technology. J.M. Lindquista, R.J. Youngb and M.C. Jaehniga
a FE1 Company, 19500 N.W. Gibbs Drive, Suite 100, Beaverton, Oregon, U.S.A. b FEI-Europe, Ltd., Brookfield Business Center, Cottenham, Cambridge, CB4 4PS, England
Abstract Focused ion beam (FIB) technology has seen application in a wide variety of disciplines including microelectronic and optoelectronic device fabrication, modification, lithographic mask repair, and materials analysis. Ongoing technological advances in these fields as well as increasing utilization of FIB within them has naturally led to new and novel application of focused ion beams. Recent advances in FIB applications include selective and enhanced material removal in microelectronic devices, three-dimensional patterning capabilities, and coupling with high resolution scanning electron microscopy and secondary ion mass spectrometry. This manuscript will provide background to application of focused ion beams and specific examples of new application areas.
1. INTRODUCTION Focused ion beam (FIB) technology is now a commonly accepted tool for integrated circuit failure analysis, device modification, and a variety of other ap lications (see for example [ 1 I). Spot sizes ~50 nm and current densities greater than 1 A/cm ? allow precision beam placement as well as efficient material removal and deposition. “Via drilling” and FIB-induced metal deposition associated with circuit modification typically have little difficulty coping with device geometries seen today at 0.8 pm or less. While the application of FIB technology appears to meet current failure analysis and device engineering demands, it is still widely accepted that focused ion beams have not reached their limit of utility in the IC industry. Advances in device fabrication technologies and methods consistently increase the demands for more precise cross sections, accurate beam placement and improved image resolution. In order to meet these demands, FIB technology has improved in current/spot size characteristics and also new applications developed by coupling with a variety of other techniques. This manuscript presents a variety of recent advances in the application of focused ion beams. Most involve integrated circuit devices, either related to analysis or modification. Examples are presented which provide some detailed insight to the concepts behind these applications.
0167-9317/93/$06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved.
J.M. Lindquist et al.
180
2. ENHANCED
MATERIAL
REMOVAL
Application of FIB technology generally encompasses three fundamental types of processes: material removal, material deposition and collection of secondary particles. While a mechanism for material removal at ion current densities in the range of 1 A/cm2 may not be completely understood, it is generally accepted that the collision cascade model reasonably predicts the removal of material by a FIB. Material removal thus may be envisioned as a process where momentum is transferred from the ion to the surface with subsequent ejection of surface and near-surface species in a cosine distribution about the surface normal. Under many conditions, it is important to recognize that the ejected material may not necessarily leave the local area irradiated by the ion beam. For example, rastering of the ion beam over a small area will remove material and form a hole which may easily attain proportions where the depth exceeds the width. Structures such as this with an aspect ratio >l typically have some sputtered material redeposited on the side walls of the hole. In this situation, side walls of the hole become slanted as shown in Figure 1. Material redeposition shapes and ultimately limits the aspect ratio and thus the depth of a crater.
ion dose -
-
__o
Figure 1. Cosine distribution of sputtered material (upper right) and its effect on crater shape when milling high aspect ratio structures. Control of redeposition is critical to a variety of FIB operations such as via formation and severing conductive lines.
As presented by Young, etal., [ 2 1, the presence of chlorine or other halogen gases may chemically react with some materials such that the removal rate under ion beam irradiation becomes enhanced. As shown in equation 1, reaction of the irradiated surface species with absorbed halogen produces volatile species which, for the discussion here, is advantageous because redeposition of surface material is essentially quenched. Figure 2 presents an example of application of chemically enhanced material removal for cut of a circuit conductor. In addition, as the nature of this enhanced removal is chemically oriented, the “enhancement As applied to FIB, this means that under factor” varies from one material to another. conditions of chemical enhancement, significant differences in material rate are experienced. This material-dependent enhancement, or material selectivity, may be utilized as shown in Figure 3 in conjunction with FIB cross sectioning to add to topographical contract to the surface.
Substrate
FIB + Halogen + Substrate
x Halogen
(r>
(1)
Application
of focused
ion beam technology
Figure 2. Comparison of line cut without (lower) and with (upper) chemically enhanced FIB etching. Note the presence of bright redeposited aluminum in the lower cut as well as the lack of redeposition and removal of carbon overlayer in the cut where enhanced etching was employed.
Figure 3. Comparison of SEM images without (left) and with (right) chemically enhanced etching to “stain” or selectively remove materials from the face of a device cross section.
181
J.M. Lindquist et al.
182
3. ANALYTICAL
METHODS
In addition to material removal, ion beams are associated with a wide range of techniques for materials analysis. Methods such as ion scattering spectroscopy (ISS), Rutherford backscattering (RBS) and secondary neutral/ion mass spectrometry (SNMS and SIMS, respectively) yield qualitative and quantitative information regarding surface composition and material structure. Focused ion beams have been used with SIMS analysis since the mid-1980’s [ 3 ] primarily for surface composition and endpoint detection. In addition, Levi-Setti [ 4 ] has shown great promise for high resolution SIMS imaging by coupling a FIB with a quadrupole mass analyzer. More recently, Crow, et. al., has coupled SIMS analysis with a FIB primadly for the purpose of microcircuit analysis [ 5 1. Figures 4, 5 and 6 illustrate two examples of FIB-SIMS applied to analysis of integrated circuits. Figure 4 shows a SIMS depth profile which Al+, Si+, B+ were monitored as a function of time as a subsurface Al trace was severed by the ion beam patterned over a 1 l.rn? area. Note the appearance of trace Al at the SiON/SiO interface, indicating a minor process deviation. In addition, although the crater which ultimately severed the Al trace was nearly 5 l.trn deep, the profile shows acceptable depth resolution and delineation of each material layer in the microcircuit under analysis.
7ol 60
-1
_._._ Si
---Al ..-.. B
l-0
w
io
Time (mid
Figure 4. FIB-SIMS depth profile and line cut as presented delineation of device layers throughout the device.
by Crow [ 5 1.
Note the
Figures 5 and 6 present two examples of high spatial resolution SIMS imaging used for material identification and location. Figure 5 shows how this technique was used to localize circuit contamination by sodium, a somewhat common yet detrimental species of integrated circuits. Figure 6 illustrates an example of material identification in a device which was previously cross sectioned by a FIB. Note the delineation of Al and Ti layers. Of interest also is the lack of intensity from poly-Si layers and bulk Si in this image, owing to the predominance of oxygen enhanced secondary ion generation in the device.
Application of focused ion beam technology
183
Figure 5. Use of FIB-SIMS to localize Na contamination in a device. High spatial resolution of the FIB and high secondary ion yield of Na allow elemental mapping of Na at concentrations below 1% over areas of a few square microns.
Figure 6. FIB-SIMS elemental map of a device cross section localizing various materials within the device (bottom) as compared with FIB generated secondary electron image (top). In the elemental map orange indicates .&3+ intensity, purple indicates +l+ intensity.
184
J.M. Lindquist et al.
4. INTEGRATED
SEWFIB
TECHNOLOGIES
Although present day characteristics of focused ion beams provide image resolution below 30 nm and beam placement accuracy in the 100 nm range, analysis of integrated circuit devices often requires even greater performance. Integration of SEM and FIB technologies in a single instrument offers the possibility of providing high resolution (4 nm) SEM imaging together with precision material removal by a FIB.
convergence point
Figure 7. Example configuration of integrated SEM/FIB system. Orientation of the electron and ion focusing columns in this manner allows convergence of the two beams at a single point on the specimen. This configuration allows a variety of unique applications for FIB and SEM technologies as outlined in Table 1.
Integration of focused ion and electron beams is a significant advance in application of both electron and ion technologies. Although examples of this utility are in their infancy, it appears appropriate to propose that this combination may provide benefits and application areas not possible with separate beams. For example, one implementation of integrated SEM and FIB technologies (Figure 7) yields a single point of conveyance of the two beams. This allows the placement of the beams at one location on the sample which allows applications which would not be possible with separated beams. Table 1 lists a variety of benefits associated with such an approach.
Table 1. Benefits of coupled FIB and SEM technologies. .
Navigation once for FIB and SEM work.
l
Improved cross section accuracy by viewing sections with SEM as it is prepared
l
Neutralization of FIB or SEM induced charging.
l
Electron beam induced deposition of insulating materials.
Note: Most of the applications above are possible only with convergence the sample surface.
of the two beams on
Application of focused ion beam technology
185
5. CONCLUSIONS Focused ion beam technology has encountered rapid acceptance within the integrated circuit industry over the past several years. Improvements in device fabrication methods yield ever decreasing line geometries which ultimately push the limits of focused ion beam application performance. A variety of advances in application of focused ion beams including chemically enhanced etching, FIB-SIMS, and coupled SEM/FIB technologies allow a wide range of benefits to integrated circuit analysis and modification.
6. REFERENCES 1 2 3 4
R.A.D. MacKenzie and G.D.W. Smith, Phys. A. 39 (1990) 183. R.J. Young, J.R.A. Cleaver, and H. Ahmed, Microelectronic Engineering, 11 (1990) 409. L.R. Harriott and M.J. Vasile, J. Vat. Sci. Tech. B, 7 (1989)181. R. Levi-Set& J.M. Chabala, P. Hallegot and Y.-L. Wang, Microelectronic Engineering, 9 (1989) 391. 5 G.A. Crow, Proc. 17th Int. Symp. for Test and Failure Analysis (1991).
179
Recent advances in application of focused ion beam technology. J.M. Lindquista, R.J. Youngb and M.C. Jaehniga
a FE1 Company, 19500 N.W. Gibbs Drive, Suite 100, Beaverton, Oregon, U.S.A. b FEI-Europe, Ltd., Brookfield Business Center, Cottenham, Cambridge, CB4 4PS, England
Abstract Focused ion beam (FIB) technology has seen application in a wide variety of disciplines including microelectronic and optoelectronic device fabrication, modification, lithographic mask repair, and materials analysis. Ongoing technological advances in these fields as well as increasing utilization of FIB within them has naturally led to new and novel application of focused ion beams. Recent advances in FIB applications include selective and enhanced material removal in microelectronic devices, three-dimensional patterning capabilities, and coupling with high resolution scanning electron microscopy and secondary ion mass spectrometry. This manuscript will provide background to application of focused ion beams and specific examples of new application areas.
1. INTRODUCTION Focused ion beam (FIB) technology is now a commonly accepted tool for integrated circuit failure analysis, device modification, and a variety of other ap lications (see for example [ 1 I). Spot sizes ~50 nm and current densities greater than 1 A/cm ? allow precision beam placement as well as efficient material removal and deposition. “Via drilling” and FIB-induced metal deposition associated with circuit modification typically have little difficulty coping with device geometries seen today at 0.8 pm or less. While the application of FIB technology appears to meet current failure analysis and device engineering demands, it is still widely accepted that focused ion beams have not reached their limit of utility in the IC industry. Advances in device fabrication technologies and methods consistently increase the demands for more precise cross sections, accurate beam placement and improved image resolution. In order to meet these demands, FIB technology has improved in current/spot size characteristics and also new applications developed by coupling with a variety of other techniques. This manuscript presents a variety of recent advances in the application of focused ion beams. Most involve integrated circuit devices, either related to analysis or modification. Examples are presented which provide some detailed insight to the concepts behind these applications.
0167-9317/93/$06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved.
J.M. Lindquist et al.
180
2. ENHANCED
MATERIAL
REMOVAL
Application of FIB technology generally encompasses three fundamental types of processes: material removal, material deposition and collection of secondary particles. While a mechanism for material removal at ion current densities in the range of 1 A/cm2 may not be completely understood, it is generally accepted that the collision cascade model reasonably predicts the removal of material by a FIB. Material removal thus may be envisioned as a process where momentum is transferred from the ion to the surface with subsequent ejection of surface and near-surface species in a cosine distribution about the surface normal. Under many conditions, it is important to recognize that the ejected material may not necessarily leave the local area irradiated by the ion beam. For example, rastering of the ion beam over a small area will remove material and form a hole which may easily attain proportions where the depth exceeds the width. Structures such as this with an aspect ratio >l typically have some sputtered material redeposited on the side walls of the hole. In this situation, side walls of the hole become slanted as shown in Figure 1. Material redeposition shapes and ultimately limits the aspect ratio and thus the depth of a crater.
ion dose -
-
__o
Figure 1. Cosine distribution of sputtered material (upper right) and its effect on crater shape when milling high aspect ratio structures. Control of redeposition is critical to a variety of FIB operations such as via formation and severing conductive lines.
As presented by Young, etal., [ 2 1, the presence of chlorine or other halogen gases may chemically react with some materials such that the removal rate under ion beam irradiation becomes enhanced. As shown in equation 1, reaction of the irradiated surface species with absorbed halogen produces volatile species which, for the discussion here, is advantageous because redeposition of surface material is essentially quenched. Figure 2 presents an example of application of chemically enhanced material removal for cut of a circuit conductor. In addition, as the nature of this enhanced removal is chemically oriented, the “enhancement As applied to FIB, this means that under factor” varies from one material to another. conditions of chemical enhancement, significant differences in material rate are experienced. This material-dependent enhancement, or material selectivity, may be utilized as shown in Figure 3 in conjunction with FIB cross sectioning to add to topographical contract to the surface.
Substrate
FIB + Halogen + Substrate
x Halogen
(r>
(1)
Application
of focused
ion beam technology
Figure 2. Comparison of line cut without (lower) and with (upper) chemically enhanced FIB etching. Note the presence of bright redeposited aluminum in the lower cut as well as the lack of redeposition and removal of carbon overlayer in the cut where enhanced etching was employed.
Figure 3. Comparison of SEM images without (left) and with (right) chemically enhanced etching to “stain” or selectively remove materials from the face of a device cross section.
181
J.M. Lindquist et al.
182
3. ANALYTICAL
METHODS
In addition to material removal, ion beams are associated with a wide range of techniques for materials analysis. Methods such as ion scattering spectroscopy (ISS), Rutherford backscattering (RBS) and secondary neutral/ion mass spectrometry (SNMS and SIMS, respectively) yield qualitative and quantitative information regarding surface composition and material structure. Focused ion beams have been used with SIMS analysis since the mid-1980’s [ 3 ] primarily for surface composition and endpoint detection. In addition, Levi-Setti [ 4 ] has shown great promise for high resolution SIMS imaging by coupling a FIB with a quadrupole mass analyzer. More recently, Crow, et. al., has coupled SIMS analysis with a FIB primadly for the purpose of microcircuit analysis [ 5 1. Figures 4, 5 and 6 illustrate two examples of FIB-SIMS applied to analysis of integrated circuits. Figure 4 shows a SIMS depth profile which Al+, Si+, B+ were monitored as a function of time as a subsurface Al trace was severed by the ion beam patterned over a 1 l.rn? area. Note the appearance of trace Al at the SiON/SiO interface, indicating a minor process deviation. In addition, although the crater which ultimately severed the Al trace was nearly 5 l.trn deep, the profile shows acceptable depth resolution and delineation of each material layer in the microcircuit under analysis.
7ol 60
-1
_._._ Si
---Al ..-.. B
l-0
w
io
Time (mid
Figure 4. FIB-SIMS depth profile and line cut as presented delineation of device layers throughout the device.
by Crow [ 5 1.
Note the
Figures 5 and 6 present two examples of high spatial resolution SIMS imaging used for material identification and location. Figure 5 shows how this technique was used to localize circuit contamination by sodium, a somewhat common yet detrimental species of integrated circuits. Figure 6 illustrates an example of material identification in a device which was previously cross sectioned by a FIB. Note the delineation of Al and Ti layers. Of interest also is the lack of intensity from poly-Si layers and bulk Si in this image, owing to the predominance of oxygen enhanced secondary ion generation in the device.
Application of focused ion beam technology
183
Figure 5. Use of FIB-SIMS to localize Na contamination in a device. High spatial resolution of the FIB and high secondary ion yield of Na allow elemental mapping of Na at concentrations below 1% over areas of a few square microns.
Figure 6. FIB-SIMS elemental map of a device cross section localizing various materials within the device (bottom) as compared with FIB generated secondary electron image (top). In the elemental map orange indicates .&3+ intensity, purple indicates +l+ intensity.
184
J.M. Lindquist et al.
4. INTEGRATED
SEWFIB
TECHNOLOGIES
Although present day characteristics of focused ion beams provide image resolution below 30 nm and beam placement accuracy in the 100 nm range, analysis of integrated circuit devices often requires even greater performance. Integration of SEM and FIB technologies in a single instrument offers the possibility of providing high resolution (4 nm) SEM imaging together with precision material removal by a FIB.
convergence point
Figure 7. Example configuration of integrated SEM/FIB system. Orientation of the electron and ion focusing columns in this manner allows convergence of the two beams at a single point on the specimen. This configuration allows a variety of unique applications for FIB and SEM technologies as outlined in Table 1.
Integration of focused ion and electron beams is a significant advance in application of both electron and ion technologies. Although examples of this utility are in their infancy, it appears appropriate to propose that this combination may provide benefits and application areas not possible with separate beams. For example, one implementation of integrated SEM and FIB technologies (Figure 7) yields a single point of conveyance of the two beams. This allows the placement of the beams at one location on the sample which allows applications which would not be possible with separated beams. Table 1 lists a variety of benefits associated with such an approach.
Table 1. Benefits of coupled FIB and SEM technologies. .
Navigation once for FIB and SEM work.
l
Improved cross section accuracy by viewing sections with SEM as it is prepared
l
Neutralization of FIB or SEM induced charging.
l
Electron beam induced deposition of insulating materials.
Note: Most of the applications above are possible only with convergence the sample surface.
of the two beams on
Application of focused ion beam technology
185
5. CONCLUSIONS Focused ion beam technology has encountered rapid acceptance within the integrated circuit industry over the past several years. Improvements in device fabrication methods yield ever decreasing line geometries which ultimately push the limits of focused ion beam application performance. A variety of advances in application of focused ion beams including chemically enhanced etching, FIB-SIMS, and coupled SEM/FIB technologies allow a wide range of benefits to integrated circuit analysis and modification.
6. REFERENCES 1 2 3 4
R.A.D. MacKenzie and G.D.W. Smith, Phys. A. 39 (1990) 183. R.J. Young, J.R.A. Cleaver, and H. Ahmed, Microelectronic Engineering, 11 (1990) 409. L.R. Harriott and M.J. Vasile, J. Vat. Sci. Tech. B, 7 (1989)181. R. Levi-Set& J.M. Chabala, P. Hallegot and Y.-L. Wang, Microelectronic Engineering, 9 (1989) 391. 5 G.A. Crow, Proc. 17th Int. Symp. for Test and Failure Analysis (1991).