- Email: [email protected]
Precise nanofabrication with multiple ion beams for advanced circuit edit
Microelectronics Reliability 54 (2014) 1779–1784
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Precise nanofabrication with multiple ion beams for advanced circuit edit Huimeng Wu ⇑, David Ferranti, Lewis Stern Carl Zeiss Microscopy, LLC, One Corporation Way, Peabody, MA 01960, USA
a r t i c l e
i n f o
Article history: Received 31 July 2014 Accepted 1 August 2014 Available online 10 September 2014 Keywords: Nanofabrication Helium ion beam microscope Beam chemistry Metal deposition Milling Endpoint detection
a b s t r a c t Gallium focused ion beam (Ga-FIB) systems have been used historically in the semiconductor industry for circuit edit. Significant efforts have been invested to improve the performance of Ga-FIB. However, as the dimensions of integrated circuits continue to shrink, Ga-FIB induced processes are being driven to their physical limits. A helium ion beam offers high spatial resolution imaging as well as precise ion machining and sub-10 nm nanofabrication capabilities because the probe size can be brought to as small as 0.25 nm. However, it is limited by its relatively low material removal rate. Recently, the new Zeiss Orion-NanoFab microscope provides multiple ion beams (He, Ne and Ga as an option) into one platform and promotes the further studies of He and Ne induced deposition and etching processes to compare with a Ga ion beam. Because of the mass difference between He, Ne and Ga ions, the interactions of ions with sample surface and precursor molecules result in different sputtering rates, implantation and deposition yields. This presentation gives an overview of our current studies using this new platform to deposit or mill nanostructures for circuit edit. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction According to Moores’ law, the number of transistors has doubled every 18 months through the size reduction of transistor components since 1965. Moore’s law will continue for several chip generations. The next step from the current 22 nm node is 14 nm, which should be in production this year. In 2015, 10 nm transistors should be achieved, followed by 7 nm in 2017 and 5 nm in 2019. Such rapid progress of integrated circuit development requires advancing current techniques or developing entirely new techniques to address future circuit edit challenges. The new techniques should have: (1) Ultra small beam size for imaging tiny features and subtle defects in the complex IC physical structure and patterning structures well below 30 nm. (2) Precise beam placement accuracy for milling or depositing at well localized sites to match the critical geometry requirement. (3) Beam chemistry for developing high quality metal, dielectric and etch processes. (4) High sensitivity detection for high signal to noise ratio (SNR) to amplification and display of image and endpoint signal. Gallium focused ion beam (Ga-FIB) systems have been used historically in the semiconductor industry for circuit edit. Ga-FIB
⇑ Corresponding author. Tel.: +1 (978) 968 6860; fax: +1 (978) 532 2501. E-mail address: [email protected] (H. Wu). http://dx.doi.org/10.1016/j.microrel.2014.08.003 0026-2714/Ó 2014 Elsevier Ltd. All rights reserved.
systems have beam size and invasiveness issues [1]. Significant efforts have been invested to improve the performance of Ga-FIB. However, as the dimensions of integrated circuits continue to shrink, Ga-FIB induced processes are being driven to their physical limits. A helium ion beam offers sub-nanometer spatial resolution imaging as well as precise ion machining and nanofabrication capabilities because the probe size can be brought to as small as 0.25 nm [2]. Although a He ion beam has demonstrated fabrication of sub-10 nm structures, it is limited by its relatively low material removal rate. Recently, the new Zeiss Orion-NanoFab microscope provides multiple ion beams (He, Ne and Ga as an option) into one platform and promotes the further studies of He and Ne ion beam induced deposition and etching processes to compare with a Ga ion beam. Because of the mass difference between He, Ne and Ga ions, the interactions of ions with sample surface and precursor molecules result in different sputtering rates, implantation and deposition yields [3]. Significantly smaller probe sizes (He: 0.5 nm; Ne: 1.9 nm) and ten times lower energy spread (<1 eV FWHM) than the liquid metal ion sources (LMIS) provide an alternative solution for the future circuit edit. This paper gives an overview of our current studies using the Zeiss Orion-NanoFab platform to mill and deposit nanostructures for advanced circuit edit.
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2. Experiments All chemicals were delivered by an Omniprobe gas injection system (GIS). A Fibics NPVE pattern generator was used to control the scanning parameters. To deposit or mill fine lines, a single line scan mode was used. To optimize the test results using either a He or Ne ion beam, various beam conditions and precursors have been explored For He or Ne ion beam milling tests, no chemicals was used. For metal deposition, three metal precursors have been tested: dicobalt octacarbonyl, (trimethyl) methylcyclo pentadienyl platinum (IV) (C9H16Pt) and tungsten carbonyl (W(CO)6). The deposition conditions can be found in Ref. [4–6]. The metal lines were deposited on electrical test structures for electrical measurements to calculate their resistivity. The resulting metal deposits were directly inspected for their geometric dimensions by helium ion microscopy (HIM) because of its high imaging resolution. For dielectric deposition, two precursors have been tested: penta methyl cyclo penta siloxane (PMCPS) and tetraethoxysilane (TEOS). Xenon difluoride (XeF2) was used for chemical enhanced etching tests.
3. Results and discussion 3.1. Milling by a He or Ne ion beam Because of the ultra-small probe size, a He ion beam has demonstrated milling of sub-10 nm nanostructures with well-defined shapes. A Ne ion beam also has a very small probe size, but a much faster material removal rate than a He ion beam. Previous reports related to Ne ion beams focused on the beam stability [7] and ion beam damage [8]. But few have studied the milling resolution of a Ne ion beam. Fig. 1 shows the nano-channels made by He (Fig. 1a) and Ne (Fig. 1b) ion beams. These nano-channels were made by milling through the thin gold on glass (100 nm thickness. Image J was used to calculate the FWHM width of the channels. It should also be noted that the milling results are dependent not only on the substrate material, but also on many processing parameters, such as ion beam energy, angle of incidence and scanning procedures. To determine the leakage current caused by the ion beam damage, two ion beams were used to cut the connec tion between the two gold pads and then their leakage current vs voltage plots were measured directly (see the inset in Fig. 2a). Here the thick ness of gold test structure is 100 nm on 5 nm Cr layer on 500 nm SiO2. Since the gold milling rate of a He ion beam is much slower than
Fig. 2. I–V curves (a) for both cuts using Ga and Ne ion beams and (b) for the cut made by a Ne ion beam. The inset shows the two probe measurement [4].
that of Ne and Ga, here we chose Ne to compare to Ga with respect to implantation. For a fair com parison, the same size boxes were milled by Ga and Ne ion beams, respectively. Fig. 2a is typical I– V curves measured by the two-probe method after Ga and Ne ion beam cut the connection between the two gold pads. Clearly, at the same external voltage, the leakage current from the Ga ion beam cut is significantly larger than that from the Ne ion beam cut. At 10 V, the calculated resistance of Ne ion cut is 30,000 times higher than that of the Ga ion cut. Because the gold milling rate of Ga ion beam is faster than that of Ne ion beam, the total dose amount using a Ga ion beam is about 1=4 of that using Ne ion beam. This test confirms that a Ga ion beam can significantly affect/mod ify the electrical property, while a Ne ion beam has less modifi cation.
Fig. 1. HIM images of the nano-channels made by He (a) and Ne ion beams (b). The inset red curves are the plot profiles of the channels, simulated by Image J. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
H. Wu et al. / Microelectronics Reliability 54 (2014) 1779–1784
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Fig. 3. PMCPS deposits induced by a He ion beam. 45° titled HIM images of (a) 1 lm 1 lm PMCPS boxes deposited on a gold pad at variable dose amounts, (b) 5 lm 5 lm, (c) 10 lm 10 lm, (d) 30 lm 30 lm. (e) A top-view optical image.
3.2. Dielectric deposition As determined above, the Ga implantation can cause more leakage current than the Ne implantation. In order to reduce the amount of Ga implantation, precursors with high deposition yields are performed. The highest resistivity using Ga-FIB induced dielectric deposits has been reported is on the order of 1011 X cm, which was achieved using penta methyl cyclo penta siloxane (PMCPS) as the precursor with a very high deposition yield (1 lm3/nC) [9]. But the resistivity is still 3–4 orders of magnitude lower than that of bulk silicon oxide, which is 1014–1016 X cm. The contribution from Ga implantation is one of major reasons for the decreased resistivity. Using a He or Ne ion beam to deposit the PMCPS [10] yields 0.4–0.5 lm3/nC and 0.7–1 lm3/nC respectively, depending on precursor flow rate, substrate, scanning parameters, beam current, landing energy, dwell time, pixel spacing, nozzle position and so on. The resistivity is 1013–1014 lX lcm, 2–3 orders of magnitude higher than the best Ga-FIB induced deposition [9] and 1–2 orders of magnitude lower than that of the bulk material.
Fig. 3a shows a 45° titled HIM image of PMCPS depositions on a gold pad, induced by a He ion beam. The significant contrast difference between the background and the deposit (Fig. 3a–c) indicates that the deposition should be a high quality insulator. The thickness of the deposits can be modified by varying dose amount (see Fig. 3a). Fig. 3b–d are 45° tilted HIM images of PMCPS boxes using similar gas flow and beam parameters with various pattern sizes. Fig. 3e is an optical image of a 30 lm 30 lm dielectric deposit covering the lower gold pad with an 18 lm 18 lm smaller gold pad on the top of the dielectric layer. This parallel plate capacitor test structure was used to measure the resistivity of the dielectric deposition. The dielectric was first deposited with a He ion beam on an existing gold pad, and then the top electrode (gold) layer was produced by e-beam lithography. The thickness of this insulator deposit is 80 nm. The gold pad structure underneath the dielectric deposit is clearly seen. This optical transparency also indicates that the deposition should be a high quality insulator. A two point probing method was used for I–V measurements. For the detailed information about the electrical measurement, please refer to the Ref. [10].
Fig. 4. 45° tilted HIM images of multiple Co line deposits with 11 nm line width and 50 nm pitch. The inset is a 3D ‘‘Image J’’ image generated from the blue box area. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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3.3. Metal deposition We have demonstrated the feasibility of depositing high-quality cobalt lines by a He ion beam with metallic conduction within an order of magnitude of the bulk value [5]. Fig. 4 shows multiple cobalt lines with 50 nm pitch and the average width of 11 nm. The high magnification image (Fig. 4b) shows that the lines have almost no overspray around the deposits with good step coverage. Besides cobalt, we also investigated other metal precursors. Platinum and tungsten are the most common choices for the metal precursor gases in FIB or e-beam induced deposition. The mass difference between He and Ne ions results in significant differences in the beam energy density. A Ne ion beam has a smaller ion range and is dominated by nuclear stop ping; while a He ion beam has a larger range and is dominated by electronic stopping. Therefore, energy density of a Ne ion beam is much greater than that of a He ion beam at the same beam landing energy. The He ion beam deposited Pt has smaller platinum grain size and much high er resistivity (P3 104 lX-cm) than Ne ion deposited material
Table 1 Summary of metal depositions with He/Ne ion beam [11]. Metal
Precursor
Ion species
Sizemin (nm)
Resistivity (lX-cm)
Co
Co2(CO)8
He
10
50 150
Pt
PtC9H16
He Ne
13[12] 35
P3 104 P600
W
W(CO)6
He Ne
20 35
P3000 150 250
(P600 lX-cm) [6]. Similarly, the He ion beam deposited W has higher resistivity (P3000 lX-cm), while the resistivity of W induced by Ne ion beam is 150–250 lX cm (see Table 1 for the summary) [11].
3.4. Chemically enhanced etching A He ion beam has been used as a fine and precise nanomachining tool. But it is less efficient for removing a relatively large amount of material. A Ne ion beam has a much higher sputter rate than a He ion beam. But the milling rate of a Ne ion beam is still relatively low compared with a Ga-FIB. To improve the material removal rate, XeF2 was tested as chemical enhancement agent. The XeF2 etching enhancement is dependent on many factors including, but not limited to, XeF2 flow rates, sample materials, scanning parameters, beam conditions and nozzle positions. Two methods have been used to test the XeF2 enhancement factor. One is to use the same ion beam conditions (including beam current, landing energy, dose amount, dwell time, pixel spacing and nozzle position. . .) and compare the depths of milled boxes with and without XeF2 to calculate the enhancement factor. The depths of the milled boxes can be directly measured from the tilted HIM images. Fig. 5a and b shows 45° tilted HIM images of boxes milled in SiO2 substrates with and without XeF2. The result shows the box depth, milled using the XeF2 gas assisted Ne ion beam, is about 7 times deeper than that of the straight Ne ion beam sputtering on SiO2. Another way to test the enhancement factor is to compare the dose amounts required to mill through a known thickness layer
Fig. 5. (a and b) 45° tilted Ne ion microscope images of Ne ion beam milled 600 600 nm boxes on a SiO2 substrate. (a) without XeF2; (b) the box etched with XeF2 gas flow at 1.2E-5 Torr of chamber pressure. The red arrows indicate the bottom of the milled boxes. The depths are (a) 94 nm, (b) 660 nm. The enhancement in material removal rate due to XeF2 is about 7 times [10]. (c and d) endpoint plots using a Ne ion beam to mill through 1 lm SiO2 without (c) and with XeF2 gas flow at 1.5 E-5 Torr of chamber pressure (d). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 6. (a) A typical endpoint plot during Ne ion beam milling through the test structure. Six EPs are shown here. The inset is a schematic structure of the test sample. The green arrows show the milling progress at each EP. (b) A 45° tilted HIM image of the cross-section of a large trench (150 nm 200 nm) milled through all layers. (c) M1 exposure by stopping the milling at EP5. (d) Small trench milling (50 nm 200 nm) stopped at EP4, between EP4 and EP5, EP5 respectively.
with and without XeF2, instead of comparing the depths of milled boxes at the same amount of ion beam dose. Secondary electron signal (endpoint detection) was monitored to detect material change during the milling process. The sample for this test was a small piece of 500 nm thick SiO2 on silicon. From the endpoint plots, we should be able to tell the dose amount of the ion beam required to mill through the oxide layer without imaging the cross-section of vias. Fig. 5c and d are two end-point plots without and with XeF2. The result shows that the dose amount to mill through 500 nm oxide layer, with XeF2 is about 10 times less than that of the straight Ne ion beam.
3.5. Endpoint detection using He and Ne ion beams The critical steps for successful circuit edit include positioning the beam accurately over the target, milling through many layers and then stopping at the target structure without affecting neighboring structures. As the via aspect ratio increases and the feature size shrinks, precise endpoint detection during the milling procedure becomes very difficult because the secondary electron signal becomes weak and eventually can reach undetectable level compared with background noise levels. In addition, a Ga-FIB suffers the limitations due to large probe size, Ga implantation and high re-deposition rate [13]. A new method with a decreased beam probe size, improved signal collection, controlled material removal at nano scale and reduced re-deposition is needed. A He/Ne ion beam has a smaller probe size and lower mass than a Ga ion beam, which allows precise and controlled machining at sub 10 nm scale and ensures reliable, accurate end point detection during the milling procedure. Fig. 6 shows the test results of endpoint detection using a 5 keV Ne ion beam to mill through the test sample. Fig. 5a is an endpoint
plot collected by milling a large trench 150 200 nm. Fig. 5b is a 45° tilted HIM image of the cross session of the trench. From the endpoint plot, in situ imaging during milling and the cross section image, we can clearly determine the milling progress at each endpoint. For this test structure, there are 6 endpoints (EPs). EP1 is the endpoint where the ion beam has reached the deposit liner/seed layer (D-liner) by fully milling through the metal 2 layer (M2); EP2 is the endpoint in low-k (SiCOH) through the etch stop layer (ESL); EP3 is the endpoint in ESL through low-k; EP4 is the endpoint in the metal 1 layer (M1) through ESL; EP5 is the endpoint in the ESL through M1 and EP6 is the endpoint in TEOS through ESL. After fully understanding where each endpoint signal, we can precisely control the milling process and accurately stop the milling at target layers. For example, we can expose M1 by stopping at EP4. The copper layer (M1) is clearly shown at the bottom of the trench (Fig. 6c). The endpoint detection of smaller trenches (50 200 nm) was also tested. Fig. 6d is a 45° tilted HIM image of the cross-section view of the three trenches stopped at different endpoints, respectively. Trench #1 was stopped at EP4 to expose the M1. Trench #2 was stopped in the M1 (between EP4 and EP5). Trench 3 was stopped at EP5, milling through the M1. It should also be noted that the trench width at the bottom is only 5–7 nm, which allows us to mill at well localized sites to match the critical geometry requirement. Table 2 summarizes the FWHM, depth and aspect ratio for each trench. Table 2 Summary of the geometry of the small trenches. via#
FWHM (nm)
Depth (nm)
Aspect ratio
1 2 3
36 51 54
300 330 430
8.3 6.5 8.0
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4. Conclusions
Acknowledgements
In summary, we have demonstrated that both He and Ne ion beams are capable of milling sub-10 nm holes and channels. The leakage current from the Ga contamination is significantly higher than that from Ne implantation. We have shown the feasibility of depositing high-quality dielectric and metal materials using He and Ne ion beams. For dielectric deposition, ultra high resistivity (on the order of 1013 X-cm) has been achieved. For metal deposition, both 10 nm wide single lines and multiple metal lines at high density have been achieved with minimal overspray around the deposits. The metallic conduction of these deposits is only an order of magnitude higher than their bulk value. To improve the milling rate, XeF2 gas assisted etching was tested. Preliminary test results show a 7–10 times etching enhancement factor in terms of material removal rate of SiO2 using the Ne ion beam with XeF2 gas compared to a straight Ne ion beam sputtering on SiO2. Finally, we have demonstrated that we can precisely control the milling process by monitoring the endpoint signal and accurately stop at the target layer with small trench and high aspect ratio. The above results confirm that He and Ne ion beams represent an attractive approach for precise nanofabrication and may be well suited for future FIB circuit edits at 14 nm and beyond.
The authors wish to thank IARPA and AFRL for supporting this work under contract/order no. FA8650-11-C-7100. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
Harriott LR, Wagner A, Fritz F. J Vac Sci Technol B 1986;4:181–4. Ward BW, Notte JA, Economou NP. J Vac Sci Technol B 2006;24:2871–4. Notte JA. Microsc Today 2012;20:16–22. Wu HM, Ferranti D, Stern L et al. Carl Zeiss White paper; 2014. Wu HM, Stern L, Xia D, Ferranti D, Thompson B, Klein K, et al. J Mater Sci: Mater Electron 2013:1–9. Wu HM, Stern L, Chen JH, Huth M, Schwalb CH, Winhold H, et al. Nanotechnology 2013:175302–6. Rahman FHM, McVey S, Farkas L, Notte JA, Tan SD, Livengood RH. Scanning 2012;34:129–34. Livengood RH, Tan SD, Hallstein R, Notte J, McVey S, Rahman FHMF. Nucl Instrum Meth A 2011;645:136–40. Edinger K, Melngailis J, Orloff J. J Vac Sci Technol B 1998;16:3311–4. Wu HM, Ferranti D, Stern LA et al. ISTFA; 2013. Thomason W, Ferranti D, Wu HM et al. ISTFA; 2012. Scipioni L, Sanford C, Maas D, van Veldhoven E. Microsc Today 2011;19: 22–5. Rajsiri S, Kempshall B, Schwarz S, Giannuzzi L. Microsc Microanal 2002;8:50–1.
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Precise nanofabrication with multiple ion beams for advanced circuit edit Huimeng Wu ⇑, David Ferranti, Lewis Stern Carl Zeiss Microscopy, LLC, One Corporation Way, Peabody, MA 01960, USA
a r t i c l e
i n f o
Article history: Received 31 July 2014 Accepted 1 August 2014 Available online 10 September 2014 Keywords: Nanofabrication Helium ion beam microscope Beam chemistry Metal deposition Milling Endpoint detection
a b s t r a c t Gallium focused ion beam (Ga-FIB) systems have been used historically in the semiconductor industry for circuit edit. Significant efforts have been invested to improve the performance of Ga-FIB. However, as the dimensions of integrated circuits continue to shrink, Ga-FIB induced processes are being driven to their physical limits. A helium ion beam offers high spatial resolution imaging as well as precise ion machining and sub-10 nm nanofabrication capabilities because the probe size can be brought to as small as 0.25 nm. However, it is limited by its relatively low material removal rate. Recently, the new Zeiss Orion-NanoFab microscope provides multiple ion beams (He, Ne and Ga as an option) into one platform and promotes the further studies of He and Ne induced deposition and etching processes to compare with a Ga ion beam. Because of the mass difference between He, Ne and Ga ions, the interactions of ions with sample surface and precursor molecules result in different sputtering rates, implantation and deposition yields. This presentation gives an overview of our current studies using this new platform to deposit or mill nanostructures for circuit edit. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction According to Moores’ law, the number of transistors has doubled every 18 months through the size reduction of transistor components since 1965. Moore’s law will continue for several chip generations. The next step from the current 22 nm node is 14 nm, which should be in production this year. In 2015, 10 nm transistors should be achieved, followed by 7 nm in 2017 and 5 nm in 2019. Such rapid progress of integrated circuit development requires advancing current techniques or developing entirely new techniques to address future circuit edit challenges. The new techniques should have: (1) Ultra small beam size for imaging tiny features and subtle defects in the complex IC physical structure and patterning structures well below 30 nm. (2) Precise beam placement accuracy for milling or depositing at well localized sites to match the critical geometry requirement. (3) Beam chemistry for developing high quality metal, dielectric and etch processes. (4) High sensitivity detection for high signal to noise ratio (SNR) to amplification and display of image and endpoint signal. Gallium focused ion beam (Ga-FIB) systems have been used historically in the semiconductor industry for circuit edit. Ga-FIB
⇑ Corresponding author. Tel.: +1 (978) 968 6860; fax: +1 (978) 532 2501. E-mail address: [email protected] (H. Wu). http://dx.doi.org/10.1016/j.microrel.2014.08.003 0026-2714/Ó 2014 Elsevier Ltd. All rights reserved.
systems have beam size and invasiveness issues [1]. Significant efforts have been invested to improve the performance of Ga-FIB. However, as the dimensions of integrated circuits continue to shrink, Ga-FIB induced processes are being driven to their physical limits. A helium ion beam offers sub-nanometer spatial resolution imaging as well as precise ion machining and nanofabrication capabilities because the probe size can be brought to as small as 0.25 nm [2]. Although a He ion beam has demonstrated fabrication of sub-10 nm structures, it is limited by its relatively low material removal rate. Recently, the new Zeiss Orion-NanoFab microscope provides multiple ion beams (He, Ne and Ga as an option) into one platform and promotes the further studies of He and Ne ion beam induced deposition and etching processes to compare with a Ga ion beam. Because of the mass difference between He, Ne and Ga ions, the interactions of ions with sample surface and precursor molecules result in different sputtering rates, implantation and deposition yields [3]. Significantly smaller probe sizes (He: 0.5 nm; Ne: 1.9 nm) and ten times lower energy spread (<1 eV FWHM) than the liquid metal ion sources (LMIS) provide an alternative solution for the future circuit edit. This paper gives an overview of our current studies using the Zeiss Orion-NanoFab platform to mill and deposit nanostructures for advanced circuit edit.
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2. Experiments All chemicals were delivered by an Omniprobe gas injection system (GIS). A Fibics NPVE pattern generator was used to control the scanning parameters. To deposit or mill fine lines, a single line scan mode was used. To optimize the test results using either a He or Ne ion beam, various beam conditions and precursors have been explored For He or Ne ion beam milling tests, no chemicals was used. For metal deposition, three metal precursors have been tested: dicobalt octacarbonyl, (trimethyl) methylcyclo pentadienyl platinum (IV) (C9H16Pt) and tungsten carbonyl (W(CO)6). The deposition conditions can be found in Ref. [4–6]. The metal lines were deposited on electrical test structures for electrical measurements to calculate their resistivity. The resulting metal deposits were directly inspected for their geometric dimensions by helium ion microscopy (HIM) because of its high imaging resolution. For dielectric deposition, two precursors have been tested: penta methyl cyclo penta siloxane (PMCPS) and tetraethoxysilane (TEOS). Xenon difluoride (XeF2) was used for chemical enhanced etching tests.
3. Results and discussion 3.1. Milling by a He or Ne ion beam Because of the ultra-small probe size, a He ion beam has demonstrated milling of sub-10 nm nanostructures with well-defined shapes. A Ne ion beam also has a very small probe size, but a much faster material removal rate than a He ion beam. Previous reports related to Ne ion beams focused on the beam stability [7] and ion beam damage [8]. But few have studied the milling resolution of a Ne ion beam. Fig. 1 shows the nano-channels made by He (Fig. 1a) and Ne (Fig. 1b) ion beams. These nano-channels were made by milling through the thin gold on glass (100 nm thickness. Image J was used to calculate the FWHM width of the channels. It should also be noted that the milling results are dependent not only on the substrate material, but also on many processing parameters, such as ion beam energy, angle of incidence and scanning procedures. To determine the leakage current caused by the ion beam damage, two ion beams were used to cut the connec tion between the two gold pads and then their leakage current vs voltage plots were measured directly (see the inset in Fig. 2a). Here the thick ness of gold test structure is 100 nm on 5 nm Cr layer on 500 nm SiO2. Since the gold milling rate of a He ion beam is much slower than
Fig. 2. I–V curves (a) for both cuts using Ga and Ne ion beams and (b) for the cut made by a Ne ion beam. The inset shows the two probe measurement [4].
that of Ne and Ga, here we chose Ne to compare to Ga with respect to implantation. For a fair com parison, the same size boxes were milled by Ga and Ne ion beams, respectively. Fig. 2a is typical I– V curves measured by the two-probe method after Ga and Ne ion beam cut the connection between the two gold pads. Clearly, at the same external voltage, the leakage current from the Ga ion beam cut is significantly larger than that from the Ne ion beam cut. At 10 V, the calculated resistance of Ne ion cut is 30,000 times higher than that of the Ga ion cut. Because the gold milling rate of Ga ion beam is faster than that of Ne ion beam, the total dose amount using a Ga ion beam is about 1=4 of that using Ne ion beam. This test confirms that a Ga ion beam can significantly affect/mod ify the electrical property, while a Ne ion beam has less modifi cation.
Fig. 1. HIM images of the nano-channels made by He (a) and Ne ion beams (b). The inset red curves are the plot profiles of the channels, simulated by Image J. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
H. Wu et al. / Microelectronics Reliability 54 (2014) 1779–1784
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Fig. 3. PMCPS deposits induced by a He ion beam. 45° titled HIM images of (a) 1 lm 1 lm PMCPS boxes deposited on a gold pad at variable dose amounts, (b) 5 lm 5 lm, (c) 10 lm 10 lm, (d) 30 lm 30 lm. (e) A top-view optical image.
3.2. Dielectric deposition As determined above, the Ga implantation can cause more leakage current than the Ne implantation. In order to reduce the amount of Ga implantation, precursors with high deposition yields are performed. The highest resistivity using Ga-FIB induced dielectric deposits has been reported is on the order of 1011 X cm, which was achieved using penta methyl cyclo penta siloxane (PMCPS) as the precursor with a very high deposition yield (1 lm3/nC) [9]. But the resistivity is still 3–4 orders of magnitude lower than that of bulk silicon oxide, which is 1014–1016 X cm. The contribution from Ga implantation is one of major reasons for the decreased resistivity. Using a He or Ne ion beam to deposit the PMCPS [10] yields 0.4–0.5 lm3/nC and 0.7–1 lm3/nC respectively, depending on precursor flow rate, substrate, scanning parameters, beam current, landing energy, dwell time, pixel spacing, nozzle position and so on. The resistivity is 1013–1014 lX lcm, 2–3 orders of magnitude higher than the best Ga-FIB induced deposition [9] and 1–2 orders of magnitude lower than that of the bulk material.
Fig. 3a shows a 45° titled HIM image of PMCPS depositions on a gold pad, induced by a He ion beam. The significant contrast difference between the background and the deposit (Fig. 3a–c) indicates that the deposition should be a high quality insulator. The thickness of the deposits can be modified by varying dose amount (see Fig. 3a). Fig. 3b–d are 45° tilted HIM images of PMCPS boxes using similar gas flow and beam parameters with various pattern sizes. Fig. 3e is an optical image of a 30 lm 30 lm dielectric deposit covering the lower gold pad with an 18 lm 18 lm smaller gold pad on the top of the dielectric layer. This parallel plate capacitor test structure was used to measure the resistivity of the dielectric deposition. The dielectric was first deposited with a He ion beam on an existing gold pad, and then the top electrode (gold) layer was produced by e-beam lithography. The thickness of this insulator deposit is 80 nm. The gold pad structure underneath the dielectric deposit is clearly seen. This optical transparency also indicates that the deposition should be a high quality insulator. A two point probing method was used for I–V measurements. For the detailed information about the electrical measurement, please refer to the Ref. [10].
Fig. 4. 45° tilted HIM images of multiple Co line deposits with 11 nm line width and 50 nm pitch. The inset is a 3D ‘‘Image J’’ image generated from the blue box area. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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3.3. Metal deposition We have demonstrated the feasibility of depositing high-quality cobalt lines by a He ion beam with metallic conduction within an order of magnitude of the bulk value [5]. Fig. 4 shows multiple cobalt lines with 50 nm pitch and the average width of 11 nm. The high magnification image (Fig. 4b) shows that the lines have almost no overspray around the deposits with good step coverage. Besides cobalt, we also investigated other metal precursors. Platinum and tungsten are the most common choices for the metal precursor gases in FIB or e-beam induced deposition. The mass difference between He and Ne ions results in significant differences in the beam energy density. A Ne ion beam has a smaller ion range and is dominated by nuclear stop ping; while a He ion beam has a larger range and is dominated by electronic stopping. Therefore, energy density of a Ne ion beam is much greater than that of a He ion beam at the same beam landing energy. The He ion beam deposited Pt has smaller platinum grain size and much high er resistivity (P3 104 lX-cm) than Ne ion deposited material
Table 1 Summary of metal depositions with He/Ne ion beam [11]. Metal
Precursor
Ion species
Sizemin (nm)
Resistivity (lX-cm)
Co
Co2(CO)8
He
10
50 150
Pt
PtC9H16
He Ne
13[12] 35
P3 104 P600
W
W(CO)6
He Ne
20 35
P3000 150 250
(P600 lX-cm) [6]. Similarly, the He ion beam deposited W has higher resistivity (P3000 lX-cm), while the resistivity of W induced by Ne ion beam is 150–250 lX cm (see Table 1 for the summary) [11].
3.4. Chemically enhanced etching A He ion beam has been used as a fine and precise nanomachining tool. But it is less efficient for removing a relatively large amount of material. A Ne ion beam has a much higher sputter rate than a He ion beam. But the milling rate of a Ne ion beam is still relatively low compared with a Ga-FIB. To improve the material removal rate, XeF2 was tested as chemical enhancement agent. The XeF2 etching enhancement is dependent on many factors including, but not limited to, XeF2 flow rates, sample materials, scanning parameters, beam conditions and nozzle positions. Two methods have been used to test the XeF2 enhancement factor. One is to use the same ion beam conditions (including beam current, landing energy, dose amount, dwell time, pixel spacing and nozzle position. . .) and compare the depths of milled boxes with and without XeF2 to calculate the enhancement factor. The depths of the milled boxes can be directly measured from the tilted HIM images. Fig. 5a and b shows 45° tilted HIM images of boxes milled in SiO2 substrates with and without XeF2. The result shows the box depth, milled using the XeF2 gas assisted Ne ion beam, is about 7 times deeper than that of the straight Ne ion beam sputtering on SiO2. Another way to test the enhancement factor is to compare the dose amounts required to mill through a known thickness layer
Fig. 5. (a and b) 45° tilted Ne ion microscope images of Ne ion beam milled 600 600 nm boxes on a SiO2 substrate. (a) without XeF2; (b) the box etched with XeF2 gas flow at 1.2E-5 Torr of chamber pressure. The red arrows indicate the bottom of the milled boxes. The depths are (a) 94 nm, (b) 660 nm. The enhancement in material removal rate due to XeF2 is about 7 times [10]. (c and d) endpoint plots using a Ne ion beam to mill through 1 lm SiO2 without (c) and with XeF2 gas flow at 1.5 E-5 Torr of chamber pressure (d). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 6. (a) A typical endpoint plot during Ne ion beam milling through the test structure. Six EPs are shown here. The inset is a schematic structure of the test sample. The green arrows show the milling progress at each EP. (b) A 45° tilted HIM image of the cross-section of a large trench (150 nm 200 nm) milled through all layers. (c) M1 exposure by stopping the milling at EP5. (d) Small trench milling (50 nm 200 nm) stopped at EP4, between EP4 and EP5, EP5 respectively.
with and without XeF2, instead of comparing the depths of milled boxes at the same amount of ion beam dose. Secondary electron signal (endpoint detection) was monitored to detect material change during the milling process. The sample for this test was a small piece of 500 nm thick SiO2 on silicon. From the endpoint plots, we should be able to tell the dose amount of the ion beam required to mill through the oxide layer without imaging the cross-section of vias. Fig. 5c and d are two end-point plots without and with XeF2. The result shows that the dose amount to mill through 500 nm oxide layer, with XeF2 is about 10 times less than that of the straight Ne ion beam.
3.5. Endpoint detection using He and Ne ion beams The critical steps for successful circuit edit include positioning the beam accurately over the target, milling through many layers and then stopping at the target structure without affecting neighboring structures. As the via aspect ratio increases and the feature size shrinks, precise endpoint detection during the milling procedure becomes very difficult because the secondary electron signal becomes weak and eventually can reach undetectable level compared with background noise levels. In addition, a Ga-FIB suffers the limitations due to large probe size, Ga implantation and high re-deposition rate [13]. A new method with a decreased beam probe size, improved signal collection, controlled material removal at nano scale and reduced re-deposition is needed. A He/Ne ion beam has a smaller probe size and lower mass than a Ga ion beam, which allows precise and controlled machining at sub 10 nm scale and ensures reliable, accurate end point detection during the milling procedure. Fig. 6 shows the test results of endpoint detection using a 5 keV Ne ion beam to mill through the test sample. Fig. 5a is an endpoint
plot collected by milling a large trench 150 200 nm. Fig. 5b is a 45° tilted HIM image of the cross session of the trench. From the endpoint plot, in situ imaging during milling and the cross section image, we can clearly determine the milling progress at each endpoint. For this test structure, there are 6 endpoints (EPs). EP1 is the endpoint where the ion beam has reached the deposit liner/seed layer (D-liner) by fully milling through the metal 2 layer (M2); EP2 is the endpoint in low-k (SiCOH) through the etch stop layer (ESL); EP3 is the endpoint in ESL through low-k; EP4 is the endpoint in the metal 1 layer (M1) through ESL; EP5 is the endpoint in the ESL through M1 and EP6 is the endpoint in TEOS through ESL. After fully understanding where each endpoint signal, we can precisely control the milling process and accurately stop the milling at target layers. For example, we can expose M1 by stopping at EP4. The copper layer (M1) is clearly shown at the bottom of the trench (Fig. 6c). The endpoint detection of smaller trenches (50 200 nm) was also tested. Fig. 6d is a 45° tilted HIM image of the cross-section view of the three trenches stopped at different endpoints, respectively. Trench #1 was stopped at EP4 to expose the M1. Trench #2 was stopped in the M1 (between EP4 and EP5). Trench 3 was stopped at EP5, milling through the M1. It should also be noted that the trench width at the bottom is only 5–7 nm, which allows us to mill at well localized sites to match the critical geometry requirement. Table 2 summarizes the FWHM, depth and aspect ratio for each trench. Table 2 Summary of the geometry of the small trenches. via#
FWHM (nm)
Depth (nm)
Aspect ratio
1 2 3
36 51 54
300 330 430
8.3 6.5 8.0
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4. Conclusions
Acknowledgements
In summary, we have demonstrated that both He and Ne ion beams are capable of milling sub-10 nm holes and channels. The leakage current from the Ga contamination is significantly higher than that from Ne implantation. We have shown the feasibility of depositing high-quality dielectric and metal materials using He and Ne ion beams. For dielectric deposition, ultra high resistivity (on the order of 1013 X-cm) has been achieved. For metal deposition, both 10 nm wide single lines and multiple metal lines at high density have been achieved with minimal overspray around the deposits. The metallic conduction of these deposits is only an order of magnitude higher than their bulk value. To improve the milling rate, XeF2 gas assisted etching was tested. Preliminary test results show a 7–10 times etching enhancement factor in terms of material removal rate of SiO2 using the Ne ion beam with XeF2 gas compared to a straight Ne ion beam sputtering on SiO2. Finally, we have demonstrated that we can precisely control the milling process by monitoring the endpoint signal and accurately stop at the target layer with small trench and high aspect ratio. The above results confirm that He and Ne ion beams represent an attractive approach for precise nanofabrication and may be well suited for future FIB circuit edits at 14 nm and beyond.
The authors wish to thank IARPA and AFRL for supporting this work under contract/order no. FA8650-11-C-7100. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
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