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The present invention relates to the formation of metal interconnect stacks. More particularly, the present invention relates to the formation of titanium metal during the sputtering of the metal interconnect stack to improve metallization performance and reliability.
A further problem introduced by the miniaturization of metal interconnect lines is stress induced by grain boundary "pinning." This pinning effect can lead to an increase in stress and further electromigration, along with an increase in film resistivity. Pinning occurs with polycrystalline materials such as titanium, which are formed of a microscopic grain structure. Since titanium has a much smaller grain structure than aluminum, when aluminum is deposited on to the titanium a much larger grain results on the surface, with much smaller grains being trapped underneath. During the final anneal the underlying layer shrinks and forms voids.
In one attempt to eliminate void formation, the aluminum is mixed with another metal to form an aluminum alloy. For example, copper has been added to aluminum. In turn, the copper appears to increase the energy required to cause the voids to form in the metal interconnect line. This remedy, however, is only partial, as void formation does occur over time, especially as the size of the metal interconnect line decreases. Titanium is also frequently deposited together with the aluminum and is alloyed to the aluminum with a high temperature anneal step.
Voids also form during post metal deposition anneals, which are typically conducted at temperatures of about 425° C. and for times of about 100 minutes. When Al is deposited on Titanium, the Titanium and Aluminum react to form TiAl x . As a result, the stress in the metal line increases due to a volume loss that occurs because of the density change which occurs during the reaction of converting Titanium and Aluminum to TiAl x . As a consequence, voids form to relieve the stress in the metal line.
One method used in the prior art for forming metal interconnect stacks with titanium and aluminum comprises first depositing a titanium layer, then depositing an overlying aluminum film, after which a titanium nitride layer is deposited above the aluminum and titanium layer. Finally, an anneal is conducted in a furnace at about 425° C. for about 100 minutes to alloy the titanium pipe and aluminum. The titanium-aluminum alloy is used in order to retain certain properties such as electromigration resistance for preventing migration failures. Nevertheless, the smaller grain size of the titanium-aluminum alloy results in a lower conductivity, and a volume loss is sustained during the anneal, accompanied by an increase in the tensile stress of the titanium-aluminum alloy that in some cases exceeds the yield strength of the alloy. This stress can result in voiding during the cooling that occurs after the anneal.
To Learn About Titanium:
1.Read up on Latin roots and history. Learn that titanium got its name from the Latin word "Titan," the first son of Gaia in Greek mythology. Be sure to look for facts about how Martin Heinrich Klaproth, who named titanium, also named the planet Uranus after one of the Titans.
2.Learn that the discovery of titanium is credited to the Reverend William Gregor of England in 1791. Read up on scientific history to find out that titanium was independently discovered several years later by Martin Heinrich Klaproth, who confirmed it as a new element.
3.Find out that titanium metal is not found free in nature but does occur in numerous minerals, with the main sources being rutile and ilmenite. There are significant deposits in Australia, Malaysia, North America and Scandinavia. Through mining and chemical manufacturing techniques, titanium metal is commercially produced by the Kroll process.
The present invention seeks to resolve the above and other problems that have been experienced in the art. More particularly, the present invention constitutes an advancement in the art by providing a method for in situ formation of titanium aluminide which achieves each of the objects listed below.
It is an object of the present invention to provide a method for in situ formation of titanium aluminide which can be effectively used to form metal interconnect stacks with a high degree of miniaturization.
It is also an object of the present invention to provide a method for in situ formation of titanium aluminide which decreases the stress on the metal interconnect stack formed thereby, thus reducing void formation.
It is further an object of the present invention to provide such a method for in situ formation of titanium sponge which eliminates the effects of pinning to further reduce void formation.
These and other objects, features, and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.
To achieve the foregoing objects, and in accordance with the invention as embodied and broadly described herein, a method is provided for in situ formation of titanium aluminide which overcomes the problems in the art of void formation in titanium-aluminum metal interconnect stacks, thereby improving metallization performance and reliability.
The first step of the method of the present invention comprises providing a silicon substrate upon which the metal interconnect stack is to be formed. This typically comprises forming a P-type silicon substrate on an in-process integrated circuit wafer. Under the method of the present invention, a passivation layer is then formed on the silicon substrate. The passivation layer typically comprises silicon dioxide. Next, a layer of titanium manufacturer is deposited over the passivation layer. An aluminum film is then deposited over the titanium layer with a thickness of between about 2.5 to 3 times the thickness of the titanium layer. Preferably, the aluminum film has a thickness of approximately 2.8 times the thickness of the titanium layer.
As a next step, the wafer is annealed in an anneal chamber in situ in a cluster tool or as part of the chamber wherein the deposition is taking place. This intermediate step anneal is typically conducted at a temperature of greater than about 450° C. and for a time of about 4 to 6 minutes, depending on the thickness of the titanium layer, to completely (e.g. substantially) react the titanium and aluminum and thereby form a titanium aluminide layer. This intermediate anneal step and pre-forming of the titanium aluminide layer avoids the problems in the prior art of pinning of the underlying aluminum and titanium during the final step anneal of the prior art.
The next step under the method of the present invention comprises the deposition of an overlying conducting material which typically comprises the remainder of the required aluminum to form the remainder of the metal interconnect stack. This is typically conducted with a sputtering process. As a result of the prior formation of the titanium aluminide layer with the intermediate anneal, the overlying conducting material, when comprising aluminum, does not substantially react with the titanium pipes .
The next step comprises the deposition of an ARC (anti-reflective coating) layer above the overlying conducting material. This is also typically conducted with a sputtering process. Finally, the wafer undergoes an anneal process such as that of the prior art, which is conducted in a furnace at conventional temperatures and for a conventional amount of time. This helps to adhere the layers to each other and cure defects within the layers.
Thus, a method is provided for forming metal interconnect stacks which overcomes the problems of the prior art. Specifically, the method of the present invention overcomes the problems of void formation due to volume loss and stress within the layers of the metal interconnect stack by pre-forming the titanium aluminide prior to aluminum deposition. Furthermore, the overlying conducting layer, when formed of aluminum is not substantially reacted with the titanium and maintains its large grain size, providing further resistance to electromigration, greater conductivity, and greater reliability. source:bloggumtitanium
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