FIELD OF THE INVENTION
The invention relates to a selective barrier slurry comprising abrasive particles, a barrier removal chemical, and a pH adjusting component. The invention also relates to methods of preparing the composition and methods of its application.
Conventional semiconductor devices generally include a semiconductor substrate, usually a silicon substrate, and a plurality of sequentially formed dielectric interlayers such as silicon dioxide and conductive paths or interconnects made of conductive materials. Copper and copper alloys have recently received considerable attention as interconnect conductor because of their superior electromigration and low resistivity characteristics. The interconnects are usually formed by filling copper in features or cavities etched into the dielectric interlayers by a metallization process. The preferred method of copper metallization process is electroplating. In an integrated circuit, multiple levels of interconnect networks laterally extend with respect to the substrate surface. Interconnects formed in sequential interlayers can be electrically connected using vias or contacts.
In a typical process, first an insulating interlayer is formed on the semiconductor substrate. Patterning and etching processes are performed to form features such as trenches, vias or dual damascene structures in the insulating layer. Then, copper is electroplated to fill all the features. However, the plating process results in a copper layer within the features, as well as on the substrate surface. The excess copper overburden on the surface is then removed before the subsequent processing step.
Conventionally, after patterning and etching, the insulation layer is first coated with a barrier layer, typically, a tantalum or tantalum/tantalum nitride composite layer. The barrier layer coats the vias and the trenches as well as the top surface of the insulation layer to ensure good adhesion and acts as a barrier material to prevent diffusion of the copper into the semiconductor devices and through the insulation layer. Next a seed (conductive) layer, which is often a copper layer, is deposited on the barrier layer. The seed layer forms a conductive material base for copper film growth during the subsequent copper deposition. As the copper film is electroplated, the copper layer quickly fills the vias but coats the wide trench and the surface in a conformal manner. When the deposition process is continued to ensure that the trench is also filled, a thick copper layer or overburden is formed on the substrate. Conventionally, after the copper plating, a CMP (Chemical Mechanical Polishing) process is employed to globally planarize and reduce the thickness of the copper layer down to the level of the surface of the barrier layer. To electrically isolate the copper-filled features, the barrier layer is then removed by another CMP step.
CMP of materials for VLSI and ULSI applications has important and broad application in the semiconductor industry. CMP is a semiconductor wafer polishing and planarizing process that combines chemical removal of layers such as insulators, metals, and photoresists with mechanical polishing or buffering of a wafer layer surface. CMP is generally used to flatten surfaces during the wafer fabrication process, and is a process that provides global planarization of the wafer surface. For example, during the wafer fabrication process, CMP is often used to flatten the topography that builds up in multilevel metal interconnection schemes. Achieving the desired flatness of the wafer surface must take place without contaminating the desired surface. Also, the CMP process must avoid polishing away portions of the functioning circuit parts
Conventional systems for the chemical mechanical polishing of semiconductor wafers will now be described. One conventional CMP process requires positioning a wafer on a holder rotating about a first axis and lowered onto a polishing pad rotating about a second axis. The wafer holder presses the wafer against the polishing pad during the planarization process. A polishing agent or slurry is typically applied to the polishing pad to polish the wafer. The content of the polishing solution depends on the nature of the material to be removed during the CMP. For example, if the material is metallic, the polishing solution may be composed any one or more of abrasives, oxidizers, complexing reagents (etching chemicals), inhibitors and/or surfactants. The oxidizer in the polishing solution oxidizes the surface of the metallic material while the oxidized metallic material is removed chemically and mechanically by abrasion due to the friction with the pad or the abrasive particles or both. Etching chemicals can be used to increase the polishing rate of the metallic material.
In another conventional CMP process, a wafer holder positions and presses a wafer against a belt-shaped polishing pad while the pad is moved continuously in the same linear direction relative to the wafer. The so-called belt-shaped polishing pad is movable in one continuous path during this polishing process. These conventional polishing processes may further include a conditioning station positioned in the path of the polishing pad for conditioning the pad during polishing. Factors that need to be controlled to achieve the desired flatness and planarity may include polishing time, pressure between the wafer and pad, speed, slurry particle size, polishing solution feed rate, the chemistry of the polishing solution, and pad material.
Recent developments in CMP include devices that cause the polishing pad to move in a bi-directional linear fashion. For example, such devices may include a polishing pad or belt secured to a mechanism that allows the pad or belt to move in a reciprocating manner, i.e. in both forward and reverse directions, at high speeds. The constant bidirectional movement of the polishing pad or belt as it polishes the wafer provides superior planarity and uniformity across the wafer surface. When a fresh portion of the pad is required, the pad is moved through a drive system containing rollers, such that the rollers only touch a back side of the pad, thereby minimizing sources of friction other than the wafer that is being polished from the polishing side of the pad, and maximizing the lifetime of the polishing pad. Further discussion on devices that cause such bidirectional movement of the pads can be seen in, for example, U.S. application Ser. No. 09/880,730 filed Jun. 12, 2001, titled “Polishing Apparatus and Method with Belt Drive System Adapted to Extend the Lifetime of a Refreshing Polishing Belt Provided Therein,” and U.S. application Ser. No. 10/126,464 filed Apr. 18, 2002 titled “Pad Tensioning Method and System in a Bi-Directional Linear Polisher” which provide for polishing a semiconductor wafer to a high degree of planarity and uniformity using pads at high bi-directional linear or reciprocating speeds. Achieving an efficient and stable CMP performance requires a good combination of the consumables with the right CMP tools. The former includes pad materials and construction, and slurry (particle types, particle size, chemistry, pH, etc); the latter includes pressure control and stability, speed of the head and pad.
Although the CMP processes described above are widely used and accepted in the semiconductor industry, challenges remain. Due to the differences in the properties of materials used for copper, barrier and other layers which need to be subjected to polishing, conventional systems often include different polishing solutions and different types of polishing pads for the differing layers. Specifically, the art of polishing has attempted to solve the problem of polishing a copper layer above a barrier layer of tantalum. It should be noted that all descriptions made with reference to Ta barrier are applicable to a TaN barrier and to a Ta/TaN stack.
In the CMP technology field, attempts to remove copper and tantalum (Ta) barrier with the desired removal rate and selectivity present a challenging problem, for both fixed abrasive pads and polymeric pads. Typically in industrial applications, both the copper and Ta barrier are required to be removed on the same pad at a high throughput. For example, there are some abrasive pads that have shown a high copper removal rate with a large overpolishing window and low dishing. However, the available commercial slurries have a low Ta removal rate at a low down force (around 1 psi or less) on the fixed abrasive pads. Some slurries exist which are formulated from hydroxylamine for Ta removal at acidic pH. One drawback of these slurries is that the Ta removal rate is still below 200 A/min at 1 psi on the fixed abrasive pads. Accordingly, there exists a need for a slurry or polishing solution that provides a high Ta removal rate at low down force on a fixed abrasive pad as well as on a polymeric pad.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention relates to a selective barrier slurry comprising abrasive particles, a barrier removal chemical, and a pH adjusting agent useful for adjusting the pH of the composition. In copper interconnect technology for IC manufacturing, CMP is the enabling technology to planarize the surface by removing copper and then the barrier layer. In particular, the inventive slurry comprises abrasive particles, a barrier removal chemical, and a component useful for adjusting the pH of the composition. The content of the abrasive particles comprise about 0.1% to about 5% of the slurry, preferably from about 1% to about 2%. The pH adjusting component can be any component that is able to adjust the pH of the slurry to the desired range. The final pH of the slurry is alkaline and is preferably in the range of about 7 to about 12, and more preferably in the range of about 9 to about 10. The barrier removal chemical can be any chemical that allows for active and efficient removal of the barrier. Preferably, the slurry further comprises a corrosion inhibitor. The slurry may also contain additives or surfactants, which can be used to increase the stability of the silica. For the purposes of the invention, “slurry”, “polishing solution”, and “polishing agent” are used interchangeably.
FIG. 1 shows a graph comparing Cu and Ta removal rates with different formulation containing TMAH or HDA.
FIG. 2 shows a graph of the Ta removal rate versus abrasive content with the same chemistry.
FIG. 3 shows a graph of the Ta removal rate versus abrasive content and different concentrations of HDA.
FIG. 4 shows a graph of the Ta removal rate with and without abrasive under the same chemistry.
The invention relates to a selective barrier slurry comprising abrasive particles, a barrier removal chemical, and a pH adjusting agent usefull for adjusting the pH of the composition. In copper interconnect technology for IC manufacturing, CMP is the enabling technology to planarize the surface by removing copper and then the barrier layer. The barrier is usually tantalum or tantalum nitride, although other barrier materials such as titanium or titanium nitride have also been used. The dilemma for the barrier is that the material should be chemically inert and mechanically stable and reliable, and yet able to be polished in the CMP process. Tantalum is a very inert material unless it is exposed to strong acids or HF. The prior art has shown barriers polished by slurries with high solid content (5% to 15%) under higher pressure (3-4 psi). The nature of this process is more mechanically driven rather than chemically etching. However, as the dielectric materials are evolving from silicon oxide (a hard and strong material) to more fragile low k materials such as carbon doped organic silicate glass or even porous low k, the down force in CMP has to be lower (0.5 psi to 1.5 psi). In addition, high solid content in the slurry has caused high dishing and especially high erosion and probably higher tendency of scratches from abrasive particles agglomeration. All of these challenges call for a barrier slurry with low solid or no solid content with more chemical interactions with the barrier layer. For the purposes of the invention, “slurry” and “polishing solution” are used interchangeably.
The invention provides such a slurry. In particular, the inventive slurry comprises abrasive particles, a barrier removal chemical, and a component useful for adjusting the pH of the composition. The content of the abrasive particles comprise about 0.1% to about 5% of the slurry, preferably from about 1% to about 2%. Preferably, the abrasive particles are silica or cerium based particles, having a diameter of about 50 to about 100 nm, more preferably 50-70 nm, and are preferably dispersed in alkaline solution to maintain the stability of the particles. More preferably, the abrasives are selected from the group consisting of fumed silica, colloidal silica or ceria. The abrasive particles, albeit typically in a much higher concentration, are commercially available, such as 1501-50 from Rodel in Delaware, or SS-12 from Cabot in Illinois.
The pH adjusting component can be any component that is able to adjust the pH of the slurry to the desired range. Examples of such adjusting components include KOH, NaOH, NH4OH and TMAH (tetramethylammonium hydroxide). The final pH of the slurry is alkaline and is preferably in the range of about 7 to about 12, and more preferably in the range of about 9 to about 10. The final pH is most preferably around 9.
The barrier removal chemical can be any chemical that allows for active and efficient removal of the barrier. For example, when the barrier is tantalum, examples of barrier removal chemical include hydroxylamine and derivatives therefrom. Such derivatives include, for example, hydroxylamine hydrochloride, hydroxylamine nitrate, hydroxylamine phosphate, and hydroxylamine sulfate. Preferably, the concentration of the barrier removal chemical in relation to the slurry is from about 0.1% to about 2%, more preferably from about 0.1% to about 0.5%, and most preferably from about 0.2% to about 0.5%.
The slurry may also contain additives or surfactants, which can be used to increase the stability of the silica. This is preferable when using colloidal silica particles. Preferably the content of the additive is no more than about 1% of the content of the slurry. An example of additives that may be used in the invention include polyethylene glycol and any derivatives therefrom.
Preferably the slurry further comprises a corrosion inhibitor. For example, it may be preferable to protect the copper layer while removing the tantalum. In such cases, a copper corrosion inhibitor may be added to the slurry. Examples of such corrosion inhibitors include benzotriazole (BTA) and derivatives therefrom, such as methyl-benzotriazole. However, any nitrogen containing compound that can be used as a corrosion inhibitor may be used. Preferably the concentration of the corrosion inhibitor is in the range of about 0.001% to about 0.1%.
The invention can be used with a compact CMP module in which both copper and Ta barrier are removed on the same pad at a high throughput. For example, some polishing pads have shown high Cu removal rate with large overpolishing window and low dishing. An example of a polishing pad that contains these characteristics is the fixed abrasive pad such as MWR66 marketed by 3M company that is 6.7 mils (0.0067 inches) thick and has a density of 1.18 g/cm3. Such polishing pads are made of a flexible material, such as a polymer, that are typically within the range of only 4-15 mils thick. The environment that the pad is used in, such as whether a linear, bi-linear, or non-constant velocity environment will allow other pads to be used, although not necessarily with the same effectiveness. It has been determined, further, that pads having a construction that has a low weight per cm2 of the pad, such as less than 0.5 g/cm2, coupled with the type of flexibility that a polymeric pad achieves, also can be acceptable.
Another consideration with respect to the pad is its width with respect to the diameter of the wafer being polished, which width can substantially correspond to the width of the wafer, or be greater or less than the width of the wafer.
In addition, the pad is preferably substantially optically transparent at some wavelength, so that a continuous pad, without any cut-out windows, can allow for detection of the removal of a material layer (end point detection) from the front surface of the wafer that is being polished, and the implementation of a feedback loop based upon the detected signals in order to ensure that the polishing that is performed results in a wafer that has all of its various regions polished to the desired extent.
However, all the commercially available slurries that were tested have a low Ta removal rate at low down force (1 psi or less) on the fixed abrasive pads. Other known slurries include hydroxylamine based slurries for copper CMP and tantalum removal at acidic pH with silica abrasives. However, the Ta removal rate is still limited to below 200 A/min at 1 psi on the fixed abrasive pads. One of the advantages of the invention is that it provides a barrier slurry at neutral to basic pH with a high Ta removal rate at low down force on the fixed abrasive pads.
A preferred embodiment of the inventive slurry uses abrasive 1501-50 from Rodel and the chemical solution containing tetramethylammonium hydroxide (TMAH) or hydroxylamine (HDA) and benzotriazole (BTA) as shown in FIG. 1. Hydroxylamine seems more effective than TMAH in removing Ta. The slurry is preferably applied after the wafer has undergone excess Cu removal with a Cu removal solution. In the preferred embodiment, when the slurry is applied, less than about 200 A/min of Cu is removed while about 100 A/min to about 500 A/min of Ta is removed.
The abrasive alone had certain removal rate on blanket wafers. However, the abrasive alone (e.g. 6%) had more Ta residue left in the dense arrays of patterned wafers, suggesting that some chemical etching is needed to take care of Ta residue. On the other hand, the chemical solution without any abrasive also had a tendency of Ta residue. A preferred combination is 0.1 to 0.5% hydroxylamine and 1-3% silica abrasive from Rodel's 1501 slurry. To prevent Cu corrosion, 0.001 to 0.1% BTA may be added to the slurry. Cu removal rate can be adjusted by adding other ingredients such as organic acids or hydroxylamine nitrate (HAN). For instance, when 0.1% of HAN was added to the above formulation, Cu removal rate increased to around 600 A/min and Ta removal rate maintained at 500-800 A/min.
FIG. 2 shows the Ta removal rate versus abrasive content with the same chemistry. FIG. 3 shows the Ta removal rate versus abrasive content and different concentration of HDA. Moreover, FIG. 3 shows that further increasing abrasive content did not increase the Ta removal rate It appears that only about 6% abrasive alone could remove Ta but that about 6% abrasive alone did not clear Ta efficient as shown in Table 1.
|TABLE 1 |
|Formulation ||Time to Clear Ta on SKW CMP test Wafers |
|0.25% HDA only ||> 1 min |
| 6% abrasive only ||>> 1 min |
|0.25% HDA + 3% abrasive ||≦ 1 min |
|0.25% HDA + 1% abrasive ||≦ 1 min |
FIG. 4 shows the Ta removal rate with and without abrasive under the same chemistry. This batch of wafers showed higher removal rate than usual but the trend is still valid. However, although FIG. 4 shows that abrasive did not make much difference in removing Ta on blanket wafers, much difference was observed on patterned wafers as shown in Table 1. Thus, a preferable formulation is low abrasive content (e.g. around 1%) and low concentration of HDA with BTA in the range of 0.002% to about 0.005% for preventing Cu corrosion.
For the purposes of clarity, the specific details of the invention will be illustrated with reference to especially preferred embodiments. However, it should be appreciated that these embodiments and examples are for the purposes of illustration only and are not intended to limit the scope of the invention. Accordingly, having disclosed exemplary embodiments and the best mode, modifications and variations may be made to the disclosed embodiments while remaining within the subject and spirit of the invention as defined by the following claims.
Formulation of Tantalum slurry: 0.25% hydroxylamine, 1% silica abrasive, 0.005% BTA, ph 9. At 1 psi on fixed abrasive pad from 3M with this formulation, Ta removal rate was 200-300 A/min, Cu removal rate was around 100 A/min, and oxide removal rate was 50 A/min or less. The above slurry formulation has a high selectivity in polishing Ta versus oxide and copper.