Accurate detection of interface between SiO 2 film and Si substrate

Transcription

1 Applied Surface Science 253 (2007) Accurate detection of interface between SiO 2 film and Si substrate H.X. Qian a, W. Zhou a, *, X.M. Li b, J.M. Miao a, L.E.N. Lim a a Precision Engineering and Nanotechnology Centre, School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore , Singapore b WinTech Nano-Technology Services Pte. Ltd., 371 Beach Road, Singapore , Singapore Received 1 March 2006; received in revised form 20 September 2006; accepted 19 December 2006 Available online 23 January 2007 Abstract Accurate end point detection of interface for multilayers using focused ion beam (FIB) is important in nanofabrication and IC modification. Real-time end point graph shows sample absorbed current as a function of sputtering time during FIB milling process. It is found that sample absorbed current increases linearly with ion beam current for the same material and changes when ion beam is milling through a different material. Investigation by atomic force microscope (AFM) and FIB cross-sectioning shows that accurate SiO 2 /Si interface occurs to where the maximum sample absorbed current occurs. Since sample absorbed current can be real-time monitored in focused ion beam machine, the paper provides a viable and simple method for accurately determining the interface during FIB milling process for widely used SiO 2 /Si system. # 2007 Elsevier B.V. All rights reserved. PACS : Rf; Wk; Dz; Pb Keywords: Focused ion beam; End point; Sample absorbed current; Interface 1. Introduction Focused ion beam (FIB) is widely used in mask repairing and IC editing [1]. Ever-increasing applications of FIB in the field of nanomachining or nanopatterning require the ability to reliably and accurately detect the interface between different materials. Sample absorbed current, secondary electron and secondary ion signals that are material dependent can provide indications of end point [2 6]. However, exact end point is sometimes not easy to identify since transition from one layer to the other usually does not show any abrupt change of characteristic signals [4,7]; therefore, only rough monitoring of milling to just reach the interface can be achieved. In the study, sample absorbed current was used to provide an indication of end point. The relationship between ion beam current and sample absorbed current was investigated. The SiO 2 /Si interface was determined accurately from FIB end point graph, measurements by atomic force microscope (AFM) and FIB cross-sectioning. It is found that accurate interface * Corresponding author. Tel.: ; fax: address: WZhou@Cantab.Net (W. Zhou). occurs when the largest sample absorbed current is achieved during the milling process. 2. Experiment The 30 kev focused Ga + ions (FEI Quanta 3D) were used to irradiate SiO 2 films with various thicknesses on Si substrate. FIB dwell time and ion off-normal incident angle were fixed at 1 ms and 08, respectively. Various ion doses were used so that FIB milling can stop at various locations (i.e., within SiO 2 film, Si and the interface between the two materials). After FIB irradiation, optical microscope was used to observe the surface. The sputtering depth for different irradiation areas was characterized using atomic force microscope. In order to reveal the relationship between sputtering depth and film thickness directly, FIB sectioning was carried out. Pt strap with around 15 mm(l) 3 mm(w) 400 nm(h) in dimensions was first deposited over the milled area and surrounding unirradiated area to avoid charging and protect the top surface from sputtering. Regular cross-section and cleaning cross-section were carried out sequentially. Sectioned area was then titled and viewed with 30 pa FIB current at 458 incidence angle. Fig. 1 shows the tilted view of cross-section. Different /$ see front matter # 2007 Elsevier B.V. All rights reserved. doi: /j.apsusc

2 5512 H.X. Qian et al. / Applied Surface Science 253 (2007) Fig. 1. Schematic image showing how cross-section is prepared by focused ion beam. The areas within circles can clearly reveals the relationship between sputtering depth and film thickness. layers of materials can be clearly identified due to good material contrast in the ion-induced secondary electron image. The thickness of SiO 2 film and sputtering depth can be measured directly from the cross-section view by compensating the tilt angle. 3. Results and discussion The sample absorbed current was monitored continuously during FIB milling by connecting sample to an electrometer amplifier. End point detection is available through a sample current graph and a real-time monitor. Fig. 2 shows the end point graphs for areas irradiated with different ion beam currents. The vertical axis of the end point graph represents the sample absorbed current in the unit of pa and the horizontal axis represents the sputtering time in the unit of second. Little variation of ion beam current or uneven distribution of chemical composition may account for the noise of the curve. It is clearly shown that sample absorbed current increases with ion beam current used for ion milling by comparing the four graphs in Fig. 2. The quantitative relationship between sample absorbed current and ion beam current for different materials is plotted in Fig. 3. Sample absorbed current roughly follows a linear relationship with ion beam current for the same material. For different materials, semiconductor (Si) produces larger sample absorbed current than insulator (SiO 2 ) under the same irradiation condition. Sample absorbed current signal includes information from all the charged particles generated during milling process [2]: I sample ¼ I Ib þ I Se I Si þ (1) where I sample is the sample absorbed current, I Ib the incident ion beam current, I Se the current formed by ion-induced secondary electrons and I Si þ the current formed by ion-induced secondary ions. If we assume I Se ¼ K e I Ib (K e is material dependent) and I Si þ ¼ K i I Ib (K i is material dependent), then for the same material, sample absorbed current can be derived as I sample ¼ I Ib ð1 þ K e K i Þ (2) Fig. 2. End point graph showing sample absorbed current (in pa) as a function of sputtering time (in second) for films with 100 nm in thickness irradiated with the same ion dose of ions/cm 2 using different ion beam currents. (a) 30 pa; (b) 50 pa; (c) 100 pa; (d) 300 pa.

3 H.X. Qian et al. / Applied Surface Science 253 (2007) Fig. 3. Correlation between sample absorbed current and ion beam current for SiO 2 and Si, respectively. For ion milling carried out with very high ion beam current, change of absorbed current tends to be neglected due to short sputtering time, while ion milling with too small ion current will be time-consuming and the end point graph will be noisy due to low S/N ratio. Therefore, the selection of ion beam current for FIB milling is important for accurate end point monitoring. Fig. 4(a) shows areas of 10 mm 10 mm in size in SiO 2 film irradiated with ion doses varying from to ions/cm 2. Areas are alphabetically labeled to indicate the irradiation dose increases with alphabetical sequence (A, B, C, etc.). Fig. 4(b) shows sample absorbed current as a function of sputtering time. Sample absorbed current and sputtering time for different areas (A, B, C, etc.) are denoted in the end point monitor graph. For short sputtering time (Area A), FIB milling was carried out in SiO 2 film and absorbed current stabilized at 97 pa. When sputtering time was larger than 235 s (Area B), the absorbed current was characterized by a steady rise to the maximum value at sputtering time of 360 s (Area D), followed by a steady fall-off before leveling to a plateau with a constant value of 211 pa. There is color evolution with increasing sputtering time (ion dose) for different irradiated areas. For short sputtering time Fig. 5. (a) AFM image of Area D. (b) AFM line profile showing the sputtering depth of Area D. when SiO 2 was milled (e.g., in Area A), the color of irradiated area is bluish and close to the original film color. For very large sputtering time when Si was milled, the color is shown to be white (e.g., Area G). For sputtering times corresponding to the transition part from low to high stabilized current in Fig. 4(b), the color of irradiated area evolves from black (Areas B and C) through grey (Area D), then to white (Areas E and F), as shown in Fig. 4(a). The unique color for Area D might indicate sputtering time used for milling Area D is the exact interface Fig. 4. (a) Optical image showing color evolution for areas on SiO 2 /Si irradiated with 100 pa over different ion doses. (b) End point graph showing the relationship between sample absorbed current and sputtering time (ion dose).

4 5514 H.X. Qian et al. / Applied Surface Science 253 (2007) point between SiO 2 film and Si substrate. However, color perception is very complicated and dependent on many factors; therefore, color is not an ideal method for a quantitative study. AFM is a suitable tool to correlate sputtering depth to sample absorbed current. All the FIB irradiated areas were measured using AFM to determine the sputtering depth. The color and sputtering depth for different areas are listed in Table 1. Fig. 5 shows AFM measurement of Area D denoted in Fig. 4. The sputtering depth of Area D is 100 nm, equal to the known film thickness, which means the accurate end point at the interface is obtained at the highest sample absorbed current. One more SiO 2 /Si sample with a different thickness is used to further examine whether the accurate interface is exposed just when the highest sample absorbed current is achieved. Fig. 6(a) shows the change of sample absorbed current with increasing sputtering time. Cross-sections of three selected areas labeled as A, B and C are shown in Fig. 6(b d), respectively. Since material with higher average atomic number emits more secondary electrons, Pt strap displays the highest brightness in FIB cross-section images, followed by Si and SiO 2. For Area A at short sputtering time and low absorbed current denoted in Fig. 6(a), cross-section view shown in Fig. 6(b) reveals that only part of SiO 2 film has been milled. When sputtering stops at the point where the highest sample Table 1 Color and sputtering depth for different areas Area ID Color Sputtering depth (nm) A Bluish 26 B Black 60 C Black 70 D Grey 100 E White 110 F White 124 G White 204 absorbed current is achieved (i.e., B denoted in Fig. 6(a)), irradiated surface just reaches the SiO 2 /Si interface, as shown in Fig. 6(c). AFM measurement of Area B shows that the sputtering depth is 480 nm, equal to the film thickness measured from FIB cross-section. For Area C at saturated high absorbed current and long sputtering time, SiO 2 film is completely sputtered away and irradiated surface is deep into the Si substrate, as clearly shown in Fig. 6(d). AFM measurements and FIB cross-sections have shown that SiO 2 /Si interface is exposed when maximum sample absorbed current is reached. However, it should be noted that SiO 2 film subjected to ion beam irradiation cannot possibly be absolutely evenly etched away, so the accurate interface used here means that major films have been etched away. Fig. 6. (a) End point graph showing sample absorbed current as a function of sputtering time for film with a few hundred nanometer in thickness. (b) Cross-section of Area A denoted in (a). (c) Cross-section of Area B denoted in (a). (d) Cross-section of Area C denoted in (a).

5 H.X. Qian et al. / Applied Surface Science 253 (2007) Conclusions It is of significance to have accurate interface detection in IC modification and nanofabrication. The accurate SiO 2 /Si interface has been investigated using end point graph assisted by AFM measurements and FIB cross-sectioning. It is found that sample absorbed current increases linearly with ion beam current for the same material and changes with material for the same ion beam current used. The accurate SiO 2 /Si interface is achieved when the FIB milling reaches the highest sample absorbed current. Acknowledgment Two of the authors (Zhou and Lim) acknowledge the financial support from A*STAR (Agency for Science, Technology and Research), Singapore, through the Strategic Research Program on nanometrology for sustainable manufacturing growth. References [1] J. Melngailis, J. Vac. Sci. Technol. B 5 (1987) 469. [2] L.R. Harriott, A. Wagner, F. Fritz, J. Vac. Sci. Technol. B 4 (1986) 181. [3] B. Khamsehpour, S.T. Davies, Vacuum 45 (1994) [4] P.J. Heard, J.R.A. Cleaver, H. Ahmed, J. Vac. Sci. Technol. B 3 (1985) 87. [5] R. Hill, J.C. Morgan, R.G. Lee, T. Olson, Microelectron. Eng. 21 (1993) 201. [6] S.T. Davies, B. Khamsehpour, J. Vac. Sci. Technol. B 11 (1993) 263. [7] P.D. Prewett, G.L.R. Mair, Focused Ion Beams from Liquid Metal Ion Sources, Research Studies Press Ltd., England, 1991.