Crystallization behavior of thin ALD-Al O films

Thin Solid Films 425 (2003) 216–220

Crystallization behavior of thin ALD-Al2O3 films Stefan Jakschika,b,*, Uwe Schroedera, Thomas Hechta, Martin Gutschea, Harald Seidla, Johann W. Barthab a

Infineon Technologies Dresden GmbH & Co OHG, Manfred-von-Ardenne-Ring 20, D-01099 Dresden, Germany b Dresden University of Technology IHM, D-01062 Dresden, Germany Received 26 November 2002; received in revised form 6 December 2002; accepted 10 December 2002

Abstract Integration of materials with a high dielectric constant into storage or gate capacitor applications requires a detailed understanding of the crystallization behavior. The dependence of crystallinity on annealing temperature and time was studied for thin atomiclayer-deposited (ALD) Al2O3 films of varying thickness, using grazing-incidence X-ray diffraction. The correlation between dielectric constant and annealing condition was investigated and an increase in dielectric constant due to annealing was observed. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Al2O3; Crystallization; Atomic layer deposition (ALD); Dielectric constant

1. Introduction Continuous miniaturization is necessary in integrated circuit technology to keep up with the progress in technical and economic performance. Over the years, device shrinkage could follow Dennard’s rules w1x, thereby reducing the lateral dimensions and junction profiles, as well as the thickness of the gate and capacitor dielectric material. However, plain shrinking of short channel devices causes changes in electrical parameters, e.g. parasitic capacitance, leakage current, threshold voltage, etc., which lead to a non-operational chip. A major problem in this respect is the appearance of a significant tunnel current through the gate and capacitor oxide. To maintain the shrinkage speed, a number of innovations have recently been brought into industry use w2–4x. Integration of high-dielectric-constant materials instead of SiO2 or silicon oxide–silicon nitride–silicon oxide (ONO) has been considered for logic and dynamic random access memory (DRAM) applications, since they allow thicker films for similar capacitance. One candidate of the so-called high-k materials is Al2O3 w5– 8x. The dielectric material has to withstand the thermal *Corresponding author. Tel.: q49-351-886-6056; fax: q49-351886-7752. E-mail address: [email protected] (S. Jakschik).

budget of subsequent process steps. Hence, a clear understanding of the crystallization process and the effect of thermal treatment on electrical properties is essential. Previous investigations by Chang et al. w9x were carried out on samples with an Al2O3 thickness of 12 nm and addressed the interface dependence of the crystallization point. However, dielectric-film thickness dependence and changes in electrical parameters such as permittivity have not been investigated so far. We studied the impact of thermal treatment on structural modifications and electrical properties of thin Al2O3 films of thickness as used in final devices. The thickness and thermal budget (anneal time and temperature) dependence of crystallization and the dielectric constant are the main points of focus of this report. 2. Experiments To analyze the crystallization behavior of thin Al2O3 films, the material was deposited on p-type silicon wafers by atomic layer deposition (ALD). ALD is a process in which single precursors are supplied into a reaction chamber in a multitude of alternating absorption and reaction steps. Thus, the material is deposited in a self-limited layer-by-layer growth mode and exhibits superior homogeneity, as well as nearly perfect uniformity.

0040-6090/03/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 2 . 0 1 2 6 2 - 2

S. Jakschik et al. / Thin Solid Films 425 (2003) 216–220

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Fig. 1. First appearance of long-range order in Al2 O3 films as detected by XRD analysis. The inset shows XRD spectra for 5.0 nm Al2O3 layers annealed for 60 s at different temperatures.

For the current experiments, trimethylaluminum (TMA) and water were used as precursors w10,11x. Prior to the ALD process step, the native oxide of the Si substrate was removed and a well-defined chemical silicon oxide with a thickness of dSiO2f1.0 nm was grown by wet processing. Caused by the deposition, ALD-Al2O3 films contain OHy, carbon and hydrogen, identified by some papers as methyl groups w12x. Immediately after deposition the wafers were annealed on a rapid thermal processing (RTP) tool in a pure nitrogen atmosphere. As required by the RTP tool, 10% oxygen was added for temperatures above 1000 8C. Experiments were performed on specimens with an Al2O3 film thickness of 3.5, 5.0 and 8.0 nm. The thermal budget (anneal temperature and time) was varied with a heating temperature of 300–1100 8C and a heating time of 60–1800 s. Samples with an Al2O3 thickness of 8.0 nm were prepared in two different ALD reactors. Since there was no significant difference in the specific results, average values of these samples are presented here. Crystallization was observed by grazing-incidence Xray diffraction (XRD) (Bruker AXS D8 Advance) w13x. For the X-ray analysis, CuKa radiation at a grazing angle of 1–28 was utilized. Electrical data were obtained with two different types of samples. In a first set-up a corona-charge oxide semiconductor (COS) capacitor was measured with a Kelvin probe w14x prior to XRD analysis. The second type of specimen was stacked up with a platinum electrode after the physical observation without applying a relevant additional thermal budget on the samples.

Modifications of the elemental depth distribution inside the films, caused by thermal treatment, were studied by time-of-flight secondary-ion mass spectroscopy (ToFSIMS). 3. Results 3.1. Results of X-ray diffraction analysis After ALD preparation, the films are X-ray-amorphous. The first appearance of long-range order in the films, indicated by a diffraction spot on the 2u plot, is displayed in Fig. 1 as a function of anneal temperature and time. The inset in Fig. 1 shows XRD results obtained for samples with 5.0 nm Al2O3 annealed at different temperatures for 60 s. An anneal temperature of 900 8C results in a peak at an angle of 2uf678. This angle is assigned to a reflection plane which appears for different crystallization phases of Al2O3. Comparing samples of different thickness shows that decreasing thickness leads to a higher crystallization temperature. Similar results were previously shown for ZrO2 by Zhao et al. w15x. Furthermore, films annealed at lower temperature need a longer period of time in order to crystallize. Although the XRD signal became weaker with decreasing film thickness, peak broadening was observed for smaller film thickness, as shown in Fig. 2. Since the layers are polycrystalline, this relates to a reduction in grain size in the direction of the X-ray beam. By utilizing the Scherrer formula w16x: Ds

KØlØ57.3 HBØcosu

(1)

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Fig. 2. XRD spectra of samples with different Al2O3 thickness. The resulting grain size is approximately the length of the grain in the direction of the X-ray beam.

with grain size D, wavelength l, peak broadening HB and constant K (0.89«1.39) the proportionality of grain size and film thickness is evident. 3.2. Results of corona charge measurement The impact of crystallization on the equivalent oxide thickness (Tox) is illustrated in Fig. 3 for samples with an Al2O3 thickness of 5.0 and 8.0 nm. Here, Tox is the thickness determined by Kelvin probe measurement with the QUANTOX tool by assuming a relative dielectric constant of ks3.9, equivalent to SiO2. Tox is derived from the capacitance measured and includes depletion effects. With increasing anneal temperature, Tox decreas-

Fig. 4. ToF-SIMS spectra of an annealed and non-annealed Al2O3 specimen. The intensity of the spectra is comparable, since they are taken under the same conditions.

es. Ellipsometer measurements of annealed Al2O3 films show a thickness loss, which is depicted as well. For temperatures up to 700 8C the layer shrinks by ;15%. However, the thickness decrease above 700 8C is not sufficient to explain the reduction in equivalent oxide thickness completely, which differs from results obtained by Chang et al. w9x. The reasons for further improvement in equivalent oxide thickness are: 1. Crystallization of the film affects the dielectric constant, since a clear step to smaller Tox is observed above crystallization; and 2. The film composition changes due to annealing and this enhances the electric polarizability.

Fig. 3. Impact of crystallization of Al2O3 films on equivalent oxide thickness as determined by Kelvin probe measurement of COS capacitors (left axis). The physical thickness was measured with an ellipsometer, indicating thickness shrinkage due to densification (right axis). Data were obtained for the 60 s anneal time.


Fig. 5. Relative dielectric constant values of Al2O3 films for different annealing temperatures for the 60 s anneal time. Data were obtained with a gradient of linearly fitted ToxMIS values over the physical film thickness (dAl2O3: 3.5, 5.0 and 8.0 nm).

As an example of the latter, the film composition of an annealed and an as-prepared specimen was studied by ToF-SIMS (Fig. 4). Obviously, the hydrogen content close to the Al2O3 –Si interface is reduced due to annealing. Further changes in composition are possible. To give one example, Parson et al. w17x reported oxygen diffusion into Al2O3 layers at higher anneal temperatures. Improvements in Tox below crystallization are attributed to film composition, while the relatively abrupt change at the crystallization temperature is an indication of grain or lattice effects on the dielectric constant, although further changes in film composition on hightemperature annealing are also possible (e.g. diffusion). 3.3. Results obtained with metal insulator semiconductor capacitors Fig. 5 summarizes the results obtained with metal insulator semiconductor (MIS) samples. With the measured capacitance taken from capacitance–voltage (C– V) curves at a bias voltage of VBsy1 V, which corresponds to accumulation, the equivalent oxide thickness is computed. Values of the relative dielectric constant are obtained by a linear fit of the equivalent oxide thickness vs. the physical Al2O3 thickness in the range between 3.5 and 8.0 nm. Changes in the dielectric constant as a function of Al2O3 film thickness are not taken in account. However, since the measurement was performed using the same conditions for every specimen, the relative relation can be observed. A first increase in the dielectric constant by a factor of 1.3 is observed at an anneal temperature range of 700–800 8C. The data point at 900 8C in Fig. 5 includes

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electrical measurements taken on amorphous 3.5 nm thick layers and crystalline layers with a thickness of 5.0 and 8.0 nm. This difference in film structure results in a larger error bar, obtained from linear fitting. The dielectric constant increases further in the case when all three films are crystalline, which holds for anneal temperatures above 1000 8C. Thus, the MIS measurements confirm the COS results, which showed an increase in dielectric constant of Al2O3 films due to annealing at temperatures higher than 700 8C and a second increase at the 950 8C anneal temperature. This enhancement of the dielectric constant is not stepwise; rather, it undergoes a transition to higher values at approximately the crystallization temperature regime. Relative dielectric constant values of Al2O3 previously reported vary between ks8.5 w18x for an anneal temperature of 900 8C and ks11 w19x for 1000 8C. Our results are consistent with these previous data. 4. Summary By comparing the crystallization behavior of Al2O3 films with a thickness of 3.5, 5.0 and 8.0 nm, it is observed that thinner films form smaller grains than thicker ones because the grain size is limited by film thickness. The thermal budget required to form crystalline grains is higher in thinner layers, which can be explained by the higher probability of nucleation seeds in thicker films. The study of ALD Al2O3 films showed correlated changes in structural and electrical properties with thermal treatment. As-deposited films are amorphous and consist of aluminum and oxygen, as well as carbon and hydrogen, which results in a dielectric constant lower than the bulk value. Annealing at temperatures up to crystallization of the film increases the dielectric constant, although the film remains amorphous. This is attributed to densification and a change in the composition of the film. Raising the temperature above the crystallization point of the aluminum oxide increases the dielectric constant, since granular films have a better ability to be polarized. Both heating effects are relevant for the electrical behavior of MOS devices. If leakage current through the dielectric film is well controlled, crystalline Al2O3 films have better electrical properties with respect to capacitance than amorphous ones. Acknowledgments The authors would like to thank Herbert Goebel and Manfred Schuster (Siemens AG, Corporate Technology, Munich, Germany) for the XRD analysis, as well as for the interesting, beneficial discussions, and Rolf Treichler (Siemens AG, Corporate Technology, Munich, Germany) who carried out the ToF-SIMS analysis.

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