Patent Application
20060267113
This invention relates to integrated circuits, and more particularly to forming a gate dielectric for transistors of the integrated circuit.
Gate dielectrics have historically been silicon oxide, but as gate dielectric thickness has been decreased, current leakage from the gate to the channel has increased. In order to overcome this leakage problem, other materials have been developed for the gate dielectric. The materials are preferably high k dielectrics so they can be sufficiently thick to prevent the excessive current leakage while retaining sufficient electrical coupling between the gate and the channel for effective transistor operation. A variety of possible materials have been developed for this material, particularly metal oxides. One problem of this type of material is that it has been found to be a poor barrier to oxygen diffusion, which is important in avoiding excessive silicon oxide growth underneath the metal oxide. Another problem is that defect states in the metal oxide trap charge that leads to variable transistor threshold voltages making-circuits operate inconsistently.
Other materials, such as silicon, aluminum, and nitrogen, have been added to the metal oxide to overcome these and other problems with metal oxide. These tend to add problems that may be just as bad as the problem they are solving. For example, the addition of nitrogen tends to suppress the formation of extra silicon oxide growth but also tends to degrade mobility and shift threshold voltage from the desired values. Similarly, the addition of aluminum tends to reduce oxygen diffusion but tends to degrade mobility. The addition of silicon also tends to slow down oxygen diffusion and improve mobility but lowers the dielectric constant.
Thus there is a need for a gate dielectric that overcomes or reduces one or more of these problems.
The foregoing and further and more specific objects and advantages of the invention will become readily apparent to those skilled in the art from the following detailed description of a preferred embodiment thereof taken in conjunction with the following drawings:
In one aspect a gate dielectric for a transistor is made using a plurality of alternating layers of a first type and second type. The first type comprises a metal oxide and the second type comprises a metal layer that comprises a metal and at least one of nitrogen and carbon. Layers of the first type separate a layer or layers of the second type from the substrate and also the gate. Layers of the second type may have the effect of providing the beneficial effect of adding nitrogen by reducing oxygen diffusion but avoiding the adverse effect of reducing mobility and varying threshold voltage. A subsequent introduction of oxygen converts the second type of layer from a conductor to a dielectric. This is better understood by reference to the drawings and the following description.
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Hafnium oxide layer 16 is preferably in the range of 5 to 30 Angstroms thick and preferably deposited on silicon oxide layer 14 by chemical vapor deposition (CVD), but could also be formed by plasma-enhanced CVD (PECVD), atomic layer deposition (ALD) or sputtering, or some other technique. The layer is preferably free of impurities, particularly carbon and chlorine. Titanium nitride layer 18 is deposited on hafnium oxide layer 16 to a thickness of preferably 5-10 Angstroms preferably by sputtering but could also be done by PECVD, CVD, or ALD. The atomic concentration of nitrogen and titanium in layer 18 is preferably 1 to 1. A different concentration would be more permeable and less desirable as a barrier. Hafnium oxide layer 20 is preferably deposited on titanium nitride layer 18 in the same manner and to the same thickness range as titanium nitride layer 16 was deposited. Hafnium oxide layer 20 could also be deposited using a different technique than for hafnium oxide layer 16. For example, sputtering may be more desirable for the case in which titanium nitride layer 18 is deposited by sputtering if possible to avoid removing device structure 10 from one tool and taking it to another. An exemplary alternative to hafnium oxide is hafnium zirconium oxide. Also ALD may be preferable for all three layers 16, 18, and 20, especially in manufacturing because it would be particularly effective in precisely controlling the thickness and being able to perform all of the depositions in a single tool. Being able to perform all of the depositions in a single tool is particularly helpful in avoiding contamination at the interface between layers that is difficult to avoid when a surface is removed from a tool.
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In a typical hafnium oxide deposition, such as the deposition of hafnium oxide layers 16 and 20, there is a post-deposition anneal that densities the hafnium oxide and after source/drain formation, such formation of source/drains 28 and 30, there are a high temperature anneals. These anneals normally also have the effect of driving oxygen from the hafnium oxide and to the substrate to form a thicker and thus less desirable silicon oxide layer. Titanium nitride layer 19 is useful in collecting the oxygen that is diffusing. The titanium nitride layer 19 effectively provides a magnet for the diffusing oxygen due to the large free energy formation of titanium oxide compared to titanium nitride. Thus, the diffusing oxygen will diffuse toward titanium nitride layer 19 rather than toward substrate 12. Similarly, the oxygen diffusing in hafnium oxide layer 20 will go toward titanium nitride layer 16 rather than gate 24. Gate stack 22 has shown to result in a more reliable transistor than using hafnium oxide alone.
Other barriers may be effective in addition to titanium nitride. The barrier may be considered generally to be a metal in combination with one of carbon or nitrogen. Titanium carbide (TiC) for example may be effective. Nitrogen is particularly attractive for use because it has relatively small adverse effect as contaminant in small quantities. For example, a small amount of nitrogen may diffuse to the interface with silicon but will cause minimal impact. To the extent it does, it decreases mobility and shifts threshold voltage. If the effect is small, this may be acceptable. On the other hand, excess carbon in the presence of silicon may form silicon carbide which can cause device failure. Similarly, another metal than titanium may be effective in combination with nitrogen or carbon. One example of such a metal is tantalum. In the case of tantalum, silicon and nitrogen (TaSiN) can be included in the combination as the barrier. TaSiN, which is amorphous, is a better barrier than titanium nitride but is less of an attractor of diffusing oxygen.
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The plurality of alternating layers provides for a plurality of interfaces making an additional impediment for diffusing oxygen. This also provides for improved immunity to electrical failure for a given thickness. The dielectric constant also has less variability across gate dielectric stack 56. The materials of titanium nitride and hafnium oxide may be varied as described for device structure 10.
An alternative to the oxide anneal shown in
Various changes and modifications to the embodiments herein chosen for purposes of illustration will readily occur to those skilled in the art. For example, other metal oxides than hafnium oxide and hafnium zirconium oxide may be useful. To the extent that such modifications and variations do not depart from the spirit of the invention, they are intended to be included within the scope thereof which is assessed only by a fair interpretation of the following claims.
