Thermodynamical study and elaboration of conductive films (Ti-N-C, W-N-C) by PEALD (Plasma Enhanced Atomic Layer Deposition) for Metal/Isolant/Metal capacitors

Author
Rym Benaboud
Year
2009
Language of the thesis
French
Thesis name in original language
Etude thermodynamique et élaboration de dépôts métalliques (W-N-C, Ti-N-C) par PEALD (Plasma Enhanced Atomic Layer Deposition) pour la réalisation d'électrodes de capacités Métal/Isolant/Métal dans les circuits intégrés.
Abstract & Cover

We are interested in the development of processes for depositing thin films of TiN-C and W-N-C by PEALD (Plasma Enhanced Atomic Layer Deposition) for the production of Metal/Insulator/Metal (MIM) capacitor electrodes MIM capacitors are used in many applications such as DRAM memories, analog/digital converters, RF filtering or decoupling. In order to meet the required performance, the Ta2O5 material was chosen as the dielectric. Good voltage linearity and high dielectric permittivity make it an excellent material for this application. In the case of electrodes, the choice was made for the Ti-N-C and W-N-C ternary systems which have low resistivity, high work function and good thermodynamic stability with Ta2O5. The PEALD method was chosen as the deposition method because it allows conformal deposition in the trenches and the deposition temperatures are compatible with the processes for developing integrated circuits (less than 400°C). We started the analysis of Ti-N-C and W-N-C deposits by performing thermodynamic simulations of the ALD deposit from the precursors TDMAT and BTBMW, respective precursors of Ti-N-C and W-N-C deposition. Regarding the Ti-N-C system we obtain the following results: the film obtained is composed of a solid solution Ti(C,N) + Cgraphite. the amount of carbon in the solid solution increases as the temperature increases and the pressure decreases. For the W-N-C system, we obtain a WN-WC mixture but as soon as the temperature increases and/or the pressure decreases, WN being unstable, the nitrogen disappears from the composition of the solid. We then showed that the PEALD deposits of our films meet the ALD deposit criteria. the growth rate is independent of the temperature in the “ALD window” the saturation of the surface of the substrate is obtained when the duration of injection of the precursor increases Studies of the influence of the deposition parameters on the properties of Ti films -N-C and W-N-C have shown that the temperature and the plasma play a very important role on the properties of the deposited films.


Indeed, the increase in temperature makes it possible to reduce the resistivity of the deposited films. The plasma remains the most important parameter. First of all, by its nature, in fact a nitrogen plasma will lead to depositing films of high resistivity, whereas a hydrogen plasma tends to reduce this resistivity by eliminating the carbon impurities and by promoting the bonds carbon-metal which lead to less resistive films than films containing many C-H bonds. Nitrogen plasma promotes C-H bonds. Then, the increase in the power or the duration of the plasma promotes the decrease in the resistivity by modifying the type of majority bonds. Indeed, at high power the formation of metal-carbon bonds (Ti-C or W-C) is favored. This change in linkage with the increase in power will also lead to a decrease in work output. Indeed, nitrogen being more electronegative than carbon, the film deposited at low power will have a higher work function. The microstructure of the deposited films also depends on the deposition power. Ti-N-C films are composed of a solid solution between TiC and TiN: Ti(N,C), as shown by thermodynamic simulations. When the deposition power is increased, the percentage of TiC in Ti(N,C) increases. As for W-N-C films, these are composed at low power of the solid solution W(N,C) whose composition limits are W2N and WC1-x. At high power, the films obtained are composed of WC1-x. We then proposed PEALD growth mechanisms for Ti-N-C deposits as well as for W-N-C deposits. In the case of Ti-N-C deposition from TDMAT, the mechanism first involves a transamination step. Then, the surface compound reacts with hydrogen radicals from the plasma. At low power, the dimethylamino –N(CH3)2 species will gradually accumulate in the Ti-N-C film after each cycle and lead to a low density of the deposited film. This facilitates the oxidation of the TiN film under exposure to air and increases its resistivity.


At high power, on the other hand, a rearrangement of carbon and nitrogen atoms, or a transposition reaction will occur. This reaction will tend to promote the formation of Ti—C bonds, which leads to a decrease in the resistivity of the films. Regarding W-N-C deposition from BTBMW, two growth mechanisms are likely to occur. In the first case, the mechanism is similar to TDMAT, it first includes a transamination step where the amine radicals are released from the BBTBMW by reaction with hydrogen, forming Me2C=CH2. Then the surface compound will react with the hydrogen radicals from the plasma. This makes it possible to create the W-N and/or W-C links. In the second hypothesis, the molecule decomposes before adsorption, radicals will be generated and the compound WC being thermodynamically more stable than WN, it will have a greater tendency to form. Finally, we studied the electrical characteristics of MIM capacitors integrating Ti-NC and W-N-C as an electrode. From a morphological point of view, the PEALD deposits of Ti-N-C are consistent in the trenches and homogeneous in thickness and composition. The low PEALD deposition temperature compared to MOCVD leads to low leakage currents because the dielectric is less degraded. The results obtained with W-N-C films, integrated as electrodes in MIM capacitance structures, are as follows: the capacitance values are higher than those obtained with TiN alone. This can be attributed to a thinner parasitic interface layer in the case of W-N-C. The voltage linearity is degraded. This could be due to the hydrogen plasma effect which generates defects in the dielectric, and degrades the linearity by trapping charges for example. In negative polarization the leakage currents are degraded. This is probably due to the plasma effect, as it degrades the electrode/dielectric interface. On the other hand, the leakage currents have been improved in positive polarization thanks to the higher work function of W-N-C compared to TiN.


The W-N-C resistivity is stronger than the resistivity of TiN, which can generate a resistance in series with the capacitance, and degrade the performance of MIMs It would be interesting later, to study in more detail the interface between the W-N-C electrodes (or Ti-N-C) and the dielectric in order to understand the improvement of the capacitance value and the degradation of the linearity. The TEM images showed an absence of interface layer between the dielectric and the W-N-C electrodes, however these results must be coupled with XPS and XRR analyses. We have shown that the electrical characteristics such as resistivity and work function are a function of the properties of the deposited material, and that these depend on the deposition parameters. It would then be interesting to use another precursor to compare the physico-chemical and electrical properties of the films obtained. It appears after this study that electrodes made from Ti-N-C or W-NC ternary compounds improve the electrical performance of MIM capacitors compared to TiN usually used in microelectronics. On the other hand, the PEALD deposition process is not suitable for the deposition of electrodes, especially the upper electrode. In fact, the plasma degrades the properties of the dielectric and therefore degrades the electrical performance of the capacitors. It would be interesting to use a process with a remote (or indirect) plasma that does not directly affect the substrate.
 

Source of Information
http://plasma-ald.com/theses.php
University
Science et Ingénierie des Matériaux et Procédés
(Grenoble, France)
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