MEASUREMENT OF THE SILICON DIOXIDE CONCENTRATION IN HAFNIUM SILICATE GATE DIELECTRICS WITH A TXRF

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1 Copyright JCPDS - International Centre for Diffraction Data 2005, Advances in X-ray Analysis, Volume MEASUREMENT OF THE SILICON DIOXIDE CONCENTRATION IN HAFNIUM SILICATE GATE DIELECTRICS WITH A TXRF ABSTRACT Chris M. Sparks 1, Patrick Lysaght 2, and Todd Rhoad 1 ATDF, Inc. 1 a wholly owned subsidiary of SEMATECH 2 Austin, Texas As complimentary metal oxide semiconductor (CMOS) devices continue to scale along the rapid pace of Moore s Law, gate dielectric materials with significantly higher dielectric constant (k = 10 25) are being evaluated as replacements for conventional silicon dioxide, SiO 2 (k = 3.9), and silicon oxynitride. This allows for the introduction of a physically thicker film with lower leakage current and with capacitance equivalent to a thinner (1.0 nm and below) SiO 2 layer [1-3]. Although binary metal oxide films such as HfO 2 and ZrO 2 exhibit higher permittivity than their corresponding silicates and aluminates, alloyed with various molecular percents of SiO 2 or Al 2 O 3, respectively, they are compromised by lower onset of crystallization temperature which contributes a higher degree of interfacial microroughness and increased gate leakage current due to dislocations and oxygen vacancies generated along grain boundaries. Accordingly, development of hafnium silicate has been the subject of intense investigation as an advanced gate dielectric thin film designed to meet the device manufacturing requirements of thermal stability in direct contact with substrate silicon and metal gate electrode materials. In this paper, we present results corresponding to the utilization of total reflection X-ray fluorescence spectroscopy (TXRF) as a quick, accurate, non-destructive technique for hafnium silicate composition determination based on detection of the Hf: Si ratio of (HfO 2 ) x (SiO 2 ) 1-x, where x varies over the range INTRODUCTION The effort to introduce a high-k gate dielectric thin film with equivalent electrical silicon oxide thickness (EOT) < 1 nm that meets required performance characteristics including low leakage current and high electron mobility, while demonstrating the capability of continued scaling, has evolved with the realization that the combination of many specific chemical treatments in conjunction with variations in Hf silicate (HfSiO) composition may be necessary. In addition, electrical performance has been further compromised by an unacceptably high density of interface states at the Si / dielectric interface and a high density of electron trap centers in the bulk film, giving rise to explorations of the process parameters during HfSiO deposition in order to optimize the resultant film quality. The frequency of experiments designed to improve the HfSiO film quality and the number of variable process parameters that may influence the resultant silicate composition is appreciable. To this end, it has been deemed essential to establish a means, other than the relatively slow, destructive technique of Rutherford Backscattering (RBS), to identify the HfSiO composition. Since TXRF is extremely sensitive to surface elemental composition, it was investigated as a method of measuring the percent SiO 2 of the HfSiO high-k films under development and as an means of monitoring the statistical process control of the deposition tool for consistent recipe reproduction over time.

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3 Copyright JCPDS - International Centre for Diffraction Data 2005, Advances in X-ray Analysis, Volume EXPERIMENTAL The TXRF used in this study was a Technos model 630T with a rotating tungsten anode (W-Lß 1 at 9.67 kev) and a silver tube anode (Ag-Kα at 22.1 kev). The Hf-Lα (7.898 kev) and the Si- Kα (1.74 kev) were the main lines monitored. Each wafer was analyzed at three spots per wafer at angles ranging from for the tungsten anode and for the silver anode. To reduce excessive dead time on the Si(Li) detector at higher incident angles, the tungsten anode was operated at 20 kv and 20 ma and the silver anode was operated at 40 kv and 15 ma. Counting times for all spectra in this paper were 100 seconds although recent experiments suggest this could be reduced to 10 seconds. Hafnium silicate films were deposited by atomic layer deposition (ALD) on 200 mm Si (100) wafers. The films were deposited with a wafer temperature of 330 C and a pressure of 1 Torr Three sets of five film compositions have been characterized; a five wafer set of 15 nm thick films deposited by the ALD tool supplier, and two complimentary five wafer sets of 15 nm and 3 nm thick films deposited in ATDF s fab. RESULTS AND DISCUSSION Figure 1 shows a plot of the ratio of Hf to Si counts taken from the TXRF for the series of 15 nm films deposited at the ALD tool supplier s site. The films ranged from a 0 film (a HfO 2 film) to an 80 film as determined by RBS. The data points plotted are an average of three sites across each wafer for a series of the incident radiation angles below the critical angle for this type of film and for both the tungsten and silver anode source X-rays ( ). The relative standard deviations (RSD s) for each point was < 5%. It was expected that this approach would indicate a linear relationship between Hf and Si in the films, and that the response could 10 9 Hf/Si (cps) 8 7 W anode Ag anode Figure 1. Ratio of Hf to Si counts against the mol. composition for 15 nm films measured below the critical angle

4 Copyright JCPDS - International Centre for Diffraction Data 2005, Advances in X-ray Analysis, Volume be used as a calibration curve for determining the mol. in unknown Hf silicate films. The tungsten anode does show a linear response from 20 to 80 mol. in the films. The loss of linearity below mol. 20% may indicate a limit of this approach. The Si signal component of the Hf to Si ratio of the HfO 2 film is most likely due to excitation of Si from the substrate below the HfO 2 film, an overlap from the Hf-M lines, or a contribution from Si in the Si(Li) detector. Based on the decrease in slope, the silver anode shows a decrease in sensitivity to composition relative to the tungsten anode. This may be due to the higher incident energy radiation either penetrating deeper into the sample exciting the Si substrate or having less efficient excitation of the lines of interest. Under total reflection, the estimated minimum penetration depth of the hafnium-based films is about 2 nm [4]. The results of the 3 nm HfSiO film measurements are shown in Figure 2. The films are too thin for RBS verification of the composition but they were deposited using the same process recipes used to produce the 15 nm thick film set (only the deposition time was adjusted to achieve the desired thickness) and it is expected that the compositions match. The loss of detection response linearity with composition is more pronounced for the Hf rich films and the Si substrate may be influencing the measurements. 7 Hf/Si (cps) 6 5 W anode Ag anode Figure 2. Ratio of Hf to Si counts against the mol. composition for 3 nm films measured below the critical angle Since an angle scan is a function of physical properties of a sample (for example, density) [5], we collected data from the HfSiO films by varying the incident radiation angle of the TXRF. An example of a Ag anode angle scan of the Hf to Si ratio from the 15 nm HfSiO films deposited at the tool supplier s site is shown in Figure 3. We can see that there is a distinct difference in the lines based on the mol. composition of the films. Taking the area under the curve for each of these lines, we get a plot as seen in Figure 4. The best-fit line that is shown has an R 2 correlation value of These points were averages of three measurements across a wafer and three repetitions over the course of two months. This best-fit line can be used as a calibration curve to quantify other films. Taking the area under the Hf to Si ratio angle scans for the 15 nm

5 Copyright JCPDS - International Centre for Diffraction Data 2005, Advances in X-ray Analysis, Volume films deposited in the ATDF s fab and plotting them on the line from Figure 4 gives the results seen in Table 1. The values calculated by plotting TXRF data on the line in Figure 4 agree within a few percent to the values determined by RBS except for the 33 where the TXRF calculated value is a higher 43%. ATDF s analytical characterization labs have also experimented with making this measurement by secondary ion mass spectrometry (SIMS) [6], and have reported on artifacts from analysis of high-k films with sputter-based techniques [7]. The SIMS data is closer in agreement to the target value for the low end of, and similar in variation to the target with the other concentrations calculated by TXRF Hf/Si (cps) 4 3 0% SiO2 20% SiO2 40% SiO2 60% SiO2 80% SiO angle Figure 3. Angle scans of the Hf to Si ratio of 15 nm HfSiO films using the Ag anode Hf/Si area Figure 4. Area under Hf to Si angle scans versus mol. in 15 nm HfSiO films using the Ag anode

6 Copyright JCPDS - International Centre for Diffraction Data 2005, Advances in X-ray Analysis, Volume mol. RBS values mol. TXRF calculated values Table 1. Comparison of calculated mol. concentrations to the RBS values for 15 nm films However, applying this method to measure the 3 nm HfSiO films is not as practical. We see plots similar to Figure 2 where there are poor line fits. Also, the repeatability of the area under the Hf to Si ratio angle scan is worse with RSD s around 20% for the 3 nm films. On the other hand, if we do not consider the ratio of Hf to Si and just plot the area under the angle scan of Hf using the Ag anode against the mol., we get the plot in Figure 5, remembering that the mol. values for the 3 nm HfSiO films are assumed from RBS results of the 15 nm films that were grown under similar conditions. For most points, the reproducibility is about 2% RSD Hf area Figure 5. Area under Hf signal angle scans versus mol. in 3 nm HfSiO films using Ag anode over three points per wafer and four measurements over the span of three months. The best-fit correlation of this line (R 2 ) is From Figure 5, the linear response of the area under the curve to mol. decreases at the highest value of mol. (67%). If we drop the 67% SiO 2 point, that would lead to an R 2 of for a line fit. It would be interesting to have HfSiO films in the range to verify linearity of this method at the low SiO 2 percentage range.

7 Copyright JCPDS - International Centre for Diffraction Data 2005, Advances in X-ray Analysis, Volume CONCLUSIONS TXRF has shown promise in quantifying the amount of SiO 2 in HfSiO films. This may lead to an in-fab and non-destructive metrology procedure. For thicker (15 nm) films a linear TXRF response to mol. can be obtained from either measuring the ratio of Hf to Si below the critical angle or from the area below the curve of the angle scan of the Hf to Si ratio. For most of the concentrations of SiO 2, these results agree with results obtained from RBS and SIMS. We also show a fairly linear TXRF response on the thinner (3 nm) HfSiO films by plotting the area under the Hf signal angle scans against the concentration of SiO 2. REFERENCES [1] D. G. Schlom and J. H. Haeni, MRS Bulletin Vol 27, p. 198, [2] G. D. Wilk, R. M. Wallace, and J. M. Anthony, J. Appl. Phys. 89, p. 5243, [3] A. I. Kingon, J-P. Maria, and S. K. Streiffer, Nature Vol 406, p. 1032, [4] R. Klockenkämper, Total-Reflection X-Ray Fluorescence Analysis, (edited by J. D. Winefordner, John Wiley & Sons, New York) p. 38, [5] Ibid., p 30. [6] J. Bennett, Determination of Si Content in HfSiO Films, 17th Annual SIMS Workshop, Westminster, Colorado, May 17-24, [7] C.F.H. Gondran, J. Bennett, M.R. Beebe Sputter Artifacts in Depth Profile Analysis of HfO 2 and HfSi x O y, AVS 49th International symposium, paper # AS-WeA2, Denver, Colorado, November 3-8, 2002.