F. J. Cadieu*, I. Vander, Y. Rong, and R. W. Zuneska, Physics Department, Queens College of CUNY, Flushing, NY

Transcription

1 Copyright JCPDS-International Centre for Diffraction Data 2012 ISSN X-Ray Measurements of Nanometer Thick Ta x O 1-x and Hf x O 1-x Films on Silicon Substrates for Thickness and Composition Determination F. J. Cadieu*, I. Vander, Y. Rong, and R. W. Zuneska, Physics Department, Queens College of CUNY, Flushing, NY ABSTRACT Tantalum oxide, Ta x O 1 x, and hafnium oxide, Hf x O 1 x, films were prepared by magnetron sputtering Ta and Hf in oxygen onto heated silicon (100) substrates. The film thicknesses were measured by three different techniques in order to obtain thickness as well as an oxygen concentration. The first method was by x ray reflectivity which yields a thickness value independent of the film composition. The second method of thickness determination uses the simultaneous measurement of Ta and Hf fluorescence counts. For these less than 200 nm thick films there is very little matrix effects so that the Ta and Hf fluorescence counts are expected to, and are observed to, increase linearly with the film thickness. The third method was by measuring the attenuation of the Si Kα x ray line from the underlying Si excited by a glancing incidence x ray beam. For the sputtering conditions employed the tantalum oxide and hafnium oxide films were observed to grow in an initial mode characterized by a high mass absorption times density product and then grow as characterized by a lower mass absorption times density product. For the hafnium oxide films this change over occurred at a film thickness of 13 nm. For the tantalum oxide films the change over occurred at a film thickness of 23 nm. Pure Ta and Hf films were also made by magnetron sputtering from Ta and Hf targets in argon. All x ray measurements, including the reflectivity measurements, were made, with the addition of an x ray fluorescence detector, using a Panalytical MRD system. INTRODUCTION Tantalum oxide and hafnium oxide films have proven very useful as boundary layers to allow the deposition of strongly adherent metallic films onto Si semiconductor device wafers. Recently hafnium oxide films in which the hafnium oxide exhibits an exceptionally high dielectric constant have proven enabling for the fabrication of 45 nm versus otherwise 65 nm linewidth semiconductor circuits.(royal Society of Chemistry 2007) Ta, Hf, and Ta x O 1 x, Hf x O 1 x, films have been made by sputtering the respective element in either argon or oxygen onto heated Si device wafers. The thicknesses of the films have been measured by three methods. Method 1 is X ray reflectivity of the films as deposited onto Si device wafers. This method gives a thickness measurement which is independent of the film composition. Method 2 uses the simultaneous X ray fluorescence count accumulation of the Ta or Hf elements. The film layers investigated are relatively thin so that matrix effects are minimal. Method 3 uses the fact that

2 This document was presented at the Denver X-ray Conference (DXC) on Applications of X-ray Analysis. Sponsored by the International Centre for Diffraction Data (ICDD). This document is provided by ICDD in cooperation with the authors and presenters of the DXC for the express purpose of educating the scientific community. All copyrights for the document are retained by ICDD. Usage is restricted for the purposes of education and scientific research. DXC Website ICDD Website -

3 This document was presented at the Denver X-ray Conference (DXC) on Applications of X-ray Analysis. Sponsored by the International Centre for Diffraction Data (ICDD). This document is provided by ICDD in cooperation with the authors and presenters of the DXC for the express purpose of educating the scientific community. All copyrights for the document are retained by ICDD. Usage is restricted for the purposes of education and scientific research. DXC Website ICDD Website -

4 Copyright JCPDS-International Centre for Diffraction Data 2012 ISSN films of these materials are relatively good absorbers of the Si Kα X ray fluorescence line as excited from the underlying Si substrate. EXPERIMENT X Ray reflectivity data has been collected using a Panalytical Materials Research Diffractometer equipped with a Cu tube operated at 45 KV, 40 ma. A 4 bounce hybrid X ray mirror was used as an incident beam monochrometer. A parallel plate diffracted beam optic was used for data collection using Ω 2θ scans. The geometry for the reflectivity scans is illustrated in Fig. 1. A key aspect of these studies is that glancing incidence X ray irradiation using a Panalytical Materials Research Diffractiometer equipped with a programmable divergence slit has been used to precisely collimate the X ray fluorescence excitation source. In these studies the glancing angle Ω was set to 2. A Cu anode X ray tube with a Ni Cu KΒ filter operated at 45 KV and 5 ma was used for fluorescence excitation. A Peltier cooled Amptek XR-100T energy dispersive X ray fluorescence detector placed 7 mm directly in front of the Si substrate in normal atmospheric air has been used for fluorescence count data collection. The geometry used for the fluorescence measurements is illustrated in Fig. 2. The Si substrates are polished (100) p type semiconductor wafers which are very smooth and flat so that the geometry was well defined without hitting the fluorescence detector with the primary excitation beam. The films deposited onto the Si substrates then attenuate by one perpendicular film thickness the Si Kα flux excited by the primary beam in the substrate. If one knows the mass absorption coefficient and density of the film layer then the film thickness can in principle be precisely measured by a measurement of the Si Ka attenuation by the film. The Ta and Hf containing films were magnetron sputtered using a 3 MightyMak US Inc. sputtering source. The elemental films of Ta and Hf were sputtered using 12 mtorr of Ar at 3.0 KVDC onto 350 C Si substrates. The oxygen containing films were sputtered using 36 mtorr of pure O 2 at 3.0 KVDC onto 350 C Si substrates.

5 Copyright JCPDS-International Centre for Diffraction Data 2012 ISSN Fig. 1. The Panalytical MRD X ray optics used for the reflectivity measurements is indicated. Fig. 2. The X ray geometry used for the fluorescence measurements is indicated.

6 Copyright JCPDS-International Centre for Diffraction Data 2012 ISSN RESULTS AND DISCUSSION For these studies boundary layers of TaO and HfO on Si (100) device wafers have been used as substrates for the deposition of SmCo based permanent magnet films. An example of a SmCo based film deposited onto a Si (100) substrate precoated with a 27 nm thick Ta 2 O 5 boundary layer is shown in Fig. 3. The inplane intrinsic coercivity in this case is about 6 koe which is sufficient for many thin film device applications. The development of crystal texturing in the sputtered permanent magnet films has been previously reported.(cadieu 1992, 2000, 2011) In this paper the primary focus is on the boundary layer films of TaO and HfO that facilitate the growth of the magnetic films onto the Si single crystal substrates Inplane ESmCo Perp. Plane ESmCo emu x nm TaO Applied Field (koe) Fig. 3. Magnetic hysteresis loops measured in the film plane and perpendicular to the film plane are shown for a SmCo based film deposited onto a Si substrate was sputter coated with a 27 nm thick TaO film layer. This film was sputtered from a set of SmCo targets enriched in Sm so that the Sm content in the resultant film was in the desired range.

7 Copyright JCPDS-International Centre for Diffraction Data 2012 ISSN An example of a reflectivity measurement for a Ta x O 1 x film made by sputtering Ta in 36 mtorr of O 2 for a Si substrate temperature of 350 C is shown in Fig. 4. The thickness of the TaO film layer was determined using the Period function of Panalytical Wingixa. In this case the use of a period over 20 fringes gives an estimated thickness of 54.2±0.1 nm. Fig. 5 shows the reflectivity fringing for a Ta film deposited in 12 mtorr Ar for a Si substrate temperature of 350 C. Fig. 6 shows the reflectivity fringing for a Hf x O 1 x film made by sputtering Hf in 36 mtorr O 2 on Si at 350 C. Each of these films were made by magnetron sputtering with an applied voltage of 3.0 KVDC TaO 60 minutes, 350 C, on Si (100) t = 54.2 nm Intensity Omega 2 Theta Scan Angle Omega degrees Fig. 4. A X ray reflectivity scan for a TaO film made by sputtering Ta in 36 mtorr of pure O 2 onto a Si (100) substrate is shown.

8 Copyright JCPDS-International Centre for Diffraction Data 2012 ISSN Ta 2 minute, 21.8 nm on Si (100) Intensity Angle Omega degrees Fig. 5. A X ray reflectivity scan for a Ta film made by sputtering Ta in 12 mtorr pure Ar is shown HfO 30 minutes onto Si (100) p type Thickness = 12.9 nm Counts Angle Omega degrees Fig. 6. A X ray reflectivity scan for a HfO film made by sputtering Hf in 36 mtorr pure O 2 is shown.

9 Copyright JCPDS-International Centre for Diffraction Data 2012 ISSN Table 1 shows the relative growth rate for Ta, Hf, and TaO and HfO films. The oxide films show a decided change in film accumulation rate at a thickness of 23 nm for the TaO films and at a thickness of 13 nm for the HfO films. Whether this change is due to a different oxide formation or due to a transition from an amorphous to more crystalline deposit has not yet been determined. It should be noted that the relative accumulation rates for pure Ta and Hf films are exactly as expected from the relative sputtering coefficients for Ta and Hf. Table1. The film deposition rate for Ta and Hf films grown at 12 mtorr Ar, 3.0 KVDC magnetron sputtering, onto 350 C Si, and for TaO and HfO films grown at 36 mtorr O 2, 3.0 KVDC, onto 350 C Si are indicated. Deposition Rate (nm/minute) Ta 10.9 Hf 17.3 Ta + O for t < 23 nm Ta + O for t > 23 nm Hf + O for t < 13 nm Hf + O for t > 13 nm Fig. 7 shows X ray fluorescence counts collected from the Ta and Hf bulk sputtering targets and from a bare Si substrate. The use of a Ni Cu Kβ filter in the primary beam allowed the Ta and Hf Lβ to be present without any overlap from other peaks. The Ta and Hf M lines do overlap with the Si Kα peaks but are relatively weak because the Ta and Hf M lines can only escape from a relatively thin surface layer of these bulk targets. Fig. 8 shows X ray fluorescence counts collected from HfO thin films that were sputtered in O 2 for times of 0 (bare Si), 15, 30, and 60 minutes. The Hf Lβ has no overlap with any peaks from the bare Si substrate and the relative counts for this peak are expected to be proportional to the film thickness for sufficiently thin films. Fig. 9 shows the film thicknesses as determined from the reflectivity measurements on the left axis versus film sputtering time. The right vertical axis shows the Hf Lβ fluorescence counts versus sputtering time. The thickness values determined from the X ray reflectivity measurements and the Hf Lβ fluorescence are precisely scalable. This shows that once the fluorescence counts are calibrated, the reflectivity measurements and the fluorescence counts can equally be used to measure film thickness values.

10 8 Counts 1800 s deadtime corrected Copyright JCPDS-International Centre for Diffraction Data 2012 ISSN Hf target Si substrate Ta target X Ray Energy (KeV) Fig. 7. X Ray fluorescence data collections from a bare Si substrate and from Hf and Ta sputtering targets as excited in glancing incidence using a Cu tube with a Ni Cu Kβ filter operated at 45 KV, 5 ma, are shown. HfO fluorescence vs Sputtering Time Si Kα Lα Lβ Counts Lγ Energy (kev) Fig. 8. X Ray fluorescence data collections for HfO films sputtered for 0, 15, 30, and 60 minutes onto Si using the same excitation as in Fig. 7 are shown.

11 Copyright JCPDS-International Centre for Diffraction Data 2012 ISSN Thickness Reflectivity (nm) HfO refl TaO HfO fluor Sputtering Time in 36 mtorr O2 at 350 C Hf Fluorescence Counts Fig. 9. Film thicknesses as measured by X ray reflectivity versus sputtering time are shown on the left vertical axis for HfO and TaO films. The right vertical axis shows Hf fluorescence counts versus sputtering time. The film thickness values measured by X ray reflectivity are precisely scalable with the accumulated Hf fluorescence counts. In the glancing incidence fluorescence excitation employed here the primary X ray beam excites Si Kα fluorescence which is then attenuated by the film thickness before reaching the X ray fluorescence detector. The distance from the film surface to the detector has by design been made very short so that atmospheric absorption of the Si Kα radiation can be neglected. The formula for this attenuation is particularly simple and just involves one perpendicular transmission through the film deposited onto the Si substrate. μ ρ where μ is the mass absorption coefficient, mac, of the film for Si Kα radiation, ρ is the film density, and t is the film thickness. In principle if the mac for Si Kα and the film density are known, the film thickness can be determined from a simple measurement of the relative Si Kα attenuation by the film that was deposited onto the Si substrate. For the thin films considered here the relative excitation of the Si Kα occurs over a greater substrate depth than that of the film thickness so that the relative excitation of the Ta or Hf M radiation should be small compared to the Si Kα excitation yield. Unfortunately two complications arise in applying this simple formula. One is that the mac values for Si Kα radiation for Ta and Hf obtained from different sources differ greatly. The second is that the Ta and

12 Copyright JCPDS-International Centre for Diffraction Data 2012 ISSN Hf M lines have energies very close to that of Si Kα so that some intensity corrections although believed small for thin films, become increasingly important for slightly thicker films. Table 2 indicates mac values obtained from two different sources as indicated in the table caption. Table 2. The mass absorption coefficients, mac, for Si Kα radiation for Fe, Cu, Hf, and Ta as obtained from two different sources are indicated. SEM table values are from Goldstein et al., Panalytical values were obtained using Panalytical High Score X Ray Analysis Software V2.1.0, which are identical to those given by V3. mac (cm 2 /g) for Si Ka SEM tables Panalytical. 100% High Score values V2.1.0, V3 % difference Fe Cu Hf Ta The mac values from these two sources for the commonly analyzed elements Fe and Cu agree to within less than 10%, but the respective values for Hf and Ta differ by a factor of 2. The relevant factor for absorption is the product mac * ρ(density). Figure 10 illustrates that the product mac*ρ can be expected to vary over a wide range of about a factor of 3 for the different known Ta oxides. To construct Fig. 10 values of mac for the different Ta oxides were computed using the mac calculator tool in Panalytical High Score. The respective densities of the known Ta oxides were obtained from the respective PDF records for the different oxides. Even though the exact values of mac obtained from High Score may be unconfirmed, the total variation of mac*ρ over a factor of about 3 for the different oxides is probably reasonable. In principle then measurements of the Si Kα attenuation versus film thickness can be used to determine a mac*ρ product and hence a level of oxygenation of the films.

13 Copyright JCPDS-International Centre for Diffraction Data 2012 ISSN mac*ρ for Si Kα radiation mac Si *density Ta (1/cm) x atomic fraction Ta in TaO Fig. 10. This graph shows the calculated mac for Si Kα times the respective density for Ta x O 1 x systems. A value of 60,000/cm roughly corresponds to pure Ta films, while a value of 30,000/cm corresponds to Ta 2 O 5. The mac values for this figure were obtained using the Panalytical High Score X Ray Analysis software V The densities for the different TaO oxides were obtained from the densities reported on the respective TaO PDF records. Measurements of the Si Kα radiation from the substrate attenuation ratios are shown in Fig. 11 for a series of TaO films with different thicknesses. Films less than 23 nm in thickness can best be fit by a relatively large mac*ρ = 6 x 10 3 /nm = 6 x 10 4 /cm. Films with thicknesses greater than 23 nm to at least 125 nm thickness can best be fit by mac*ρ = 2.7 x 10 3 /nm = 2.7 x 10 4 /cm. Comparing these results to Fig. 10 indicates that the thinner films are mostly metallic Ta and do not fully oxidize until the films reaches a thickness greater than 23 nm. Figure 12 shows the attenuation of the Si Kα radiation for a series of metallic Hf films made by sputtering Hf in 12 mtorr of pure Ar. The very rapid decrease in the Si Kα intensity ratio indicates that the mac value for Hf assuming a density of 13.3 g/cm 3 is at least as large as that given by Goldstein 2003, but certainly larger than that given by Panalytical High Score.

14 Copyright JCPDS-International Centre for Diffraction Data 2012 ISSN exp curve Si Ka atten. Ratio mac*ρ = 6e 3/nm mac*ρ = 2.7e 3/nm TaO Thickness (nm) data fit Fig. 11. The Si Kα attenuation ratio for a series of TaO films of different thicknesses is shown. The dashed curve shown a composite fit for two exponential regions. Si Kα ratio Hf metal Si Ka atten ratio vs thickness mca*rho =68,000/cm measured thickness (nm) Fig. 12. The Si Kα attenuation ratio for a series of Hf films of different thicknesses is shown. The dashed curve shows the expected attenuation ratio for a mac*ρ value of 6.8 x 10 4 /cm.

15 Copyright JCPDS-International Centre for Diffraction Data 2012 ISSN CONCLUSIONS X Ray reflectivity and X ray fluorescence counts have both been shown to give precisely scalable film thickness values for TaO and HfO for thicknesses from a few to at least 125 nm thickness range. X Ray reflectivity although relatively expensive at about $300K gives thickness values that are independent of the chemical composition. A drawback is that in general smooth single crystal substrates are required to give reasonable interference fringes. X Ray fluorescence requires thickness standards but has two advantages over X ray reflectivity. One is that a fluorescence detector and counting system can be obtained relatively cheaply for about $10K. The second is that fluorescence counting can be done for cheaper substrates such as glass and metal substrates. It was also shown that the initial growth of TaO and HfO proceeds at a different rate than the subsequent growth for the same sputtering conditions. For the sputtering conditions used here of 36 mtorr O 2 and a substrate temperature of 350 C, the change over occurred at a film thickness of 13 nm for HfO films and at 23 nm for TaO films. The change over thickness is expected to be temperature dependent but this aspect has yet to be shown. The initial growth mode corresponds to a greater mac value than the subsequent growth. Mass absorption values reported for Ta and Hf differ greatly as reported by different methods. Some of this difference may be due to measurements being made on thin films in the initial growth mode versus values for slightly thicker films. Our measurements indicate that the mac value for Hf is substantially greater than that for Ta films. This is consistent with the values reported in Goldstein. REFERENCES *contact author: cadieu@qc.edu Cadieu, F. J. (1992). Permanent Magnet Thin Films in Physics of Thin Films, Vol. 16, (Academic Press, San Diego). Cadieu, F. J. (2000). Permanent Magnet Films for Applications, Chapter 1, Magnetic Film Devices volume of the Handbook on Thin Film Devices Technology and Applications, editors: J. Douglas Adam and Maurice H. Francombe, (Academic Press, San Diego). Cadieu, F. J., Vander, I., Rong, Y., and Zuneska, (2011), Glancing XRD and XRF for the Study of Texture Development in SmCo Based Films Sputtered onto Silicon Substrates, Advances in X Ray Analysis, Vol. 54. Goldstein, Joseph, Newbury, Dale E., Joy, David C., Lyman, Charles E., Echin, Patrick, Lifshin, Eric, Sawyer, Linda, and Michael, J. R. (2003), Scanning Electron Microscopy and X Ray Microanalysis, 3 rd Ed. (Springer, New York). Royal Society of Chemistry 2007, Hafnium oxide helps make chips smaller and faster, rsc.org/chemistry world/issues/2007/march.