Recessed metal lines for protective enclosure in integrated circuits

Encapsulating areas of metallization in a liner material, such as Tantalum, Tantalum Nitride, Silicon Carbide allows aggressive or harsh processing steps to be used. These aggresive processing steps offer the possibility of fabricating new device architectures. In addition, by encapsulating the areas of metallization, metal ion migration and electromigration can be prevented. Further, the encapsulated areas of metallization can serve as a self-aligning etch mask. Thus, vias etched between adjacent areas of metallization allow the area of the substrate allocated to the via to be significantly reduced without increasing the possibility of electrical shorts to the adjacent areas of metallization.

TECHNICAL FIELD

The present invention relates generally to the manufacture of semiconductor devices and more particularly to a method of encasing areas of metallization to produce an intermediate structure that allows aggressive processing steps, prevents metal ion migration or electromigration, and increases packing density by using the encased areas of metallization as a self-aligned mask for etching cavities such as trenches or vias from an upper level of metallization through an intermediate dielectric to lower level circuits or metallization. The encapsulation and planarization after encapsulation provides a smoother surface for subsequent processing and enables the use of more effective and/or aggressive processing techniques such as chlorine-based RIE (Reactive Ion Etching) and consequently enables the creation of new device architectures.

BACKGROUND

As will be appreciated by those skilled in the art, most presently-used integrated circuit processing includes polishing of Damascene deposited copper or tungsten metal lines and vias. The resultant exposed copper or tungsten lines and vias are particularly susceptible to corrosion resulting from subsequent processing steps. Consequently, many of the more effective processing steps are simply too harsh to be used with such exposed copper or tungsten lines. Such limits on the available etching techniques and other processing requires costly modifications to a less expensive process flow that might otherwise be used. Therefore, it would be advantageous if the more effective, yet harsher, processing steps could be used on substrates containing copper or tungsten Damascene type integrated structures.

The present invention not only allows the use of very harsh processing steps, including chlorine-based etching steps, to be used with a copper or tungsten Damascene structures, but also controls out-diffusion or migration of harmful metal ions and atoms to adjacent sensitive circuits and may be used to provide a self-aligning mask for increasing the packing density of devices on a semiconductor chip.

For example, as will be appreciated by those skilled in the art, most semiconductor devices have several layers of circuits interconnected by vias etched through insulating and/or dielectric materials separating the two levels of circuits and filled with a conductive material such as, for example only, copper or tungsten. To avoid electrical shorts, it is very important that these vias filled with conductive metals do not unintentionally come into contact with other conductive lines and/or devices. Since electrical circuits and devices in an integrated chip are very small, a via that does successfully connect two levels of circuits together, but is misaligned by only a few tenths of microns may cause shorts and render a full wafer of devices useless. As will be appreciated by those skilled in the art, most misaligned vias are the result of a misaligned etching mask. Therefore, it is important that precautions be taken to be sure minor misalignment will not cause shorts. At present, one of the common ways to avoid such destructive electrical shorts is to increase the area that is allocated for the via etch. That is, increase the separation between circuits, or electrical conductive lines, and the location where the via is etched from an upper level to a lower level. This is, of course, a simple and effective solution. Unfortunately, since each of such multi-layer devices will typically include several vias, and since each wafer includes hundreds of devices, increasing the area for each via is also wasteful and decreases yield.

SUMMARY OF THE INVENTION

These and other problems are generally solved or corrected, and technical advantages are generally achieved, by embodiments of the present invention which provides methods and apparatus in a semiconductor structure to enhance further processing of the semiconductor. The methods and apparatus comprise a substrate with a dielectric having a top surface that defines at least one area of metallization having an exposed top surface such as is provided by a typical copper or tungsten Damascene process. For many applications, the area of metallization formed by the Damascene process will also include a protective liner or barrier covering the sides and bottom of the cavity, trench, or via before the copper or other metal is deposited. Suitable materials for the protective liner include, but are not necessarily limited to, Tantalum (Ta), Tantalum Nitride (TaN), Titanium (Ti), Titanium Nitride (TiN), Silicon Nitride (SiN), and Silicon Carbide (SiC). The top surface of the exposed copper or metallization is then recessed by any suitable processing step such as wet etching, RIE (Reactive Ion Etching) with, e.g., CO NH3 plasma for etching copper, IBE (Ion Beam Etching),or modified CMP (chemical-mechanical polishing). A layer of protective liner material, such as Ta, TaN, Ti, TiN, SiN, or SiC, is then deposited over the recessed top surface of the area of metallization to encase or encapsulate the area of metallization and may be planarized by a CMP (Chemical Mechanical Polish) to provide a very smooth top surface. The planarized encapsulated or protected areas of metallization provide a smoother surface for successive processing steps and also allow more aggressive or harsh processing steps. As a first example, a stack of magnetic films can be deposited over the dielectric layer and the protected areas of metallization. The magnetic stack of films can then be pattern etched with a chlorine-based RIE (Reactive Ion Etch), which is simply too corrosive to be used with exposed copper or tungsten lines. According to a second example, two neighboring encased areas of metallization can be used as the mask for etching a via from a top surface through the intermediate layer if the liner and encapsulation encasing the areas of metallization is a dielectric. A conductive metal, such as copper with metal liner, or tungsten, can then be deposited in the via in direct contact with material encasing the lines without short-circuiting the encased areas of metallization. According to a third use, the material encasing the areas of metallization will act as a barrier to migration or electromigration of metal ions or atoms, such as copper for example, into the vicinity of sensitive circuit components. Lastly, the encapsulation of the areas of metallization provides an effective adhesion promoter and protection against oxidation or corrosion so that a wider range of dielectrics can be utilized as films to be deposited on the areas of metallization. For example, one could use silicon oxide directly on the area of metallization without an intermediate silicon nitride layer, and thus reduce significantly the effective dielectric constant and capacitance of the structure.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Referring now to the prior art FIG. 1A , there is shown a cross-section of a substrate with areas of metallization (e.g., trenches and/or vias) formed in a dielectric. Typically, the areas of metallization will be formed by the Damascene process but could be formed by any other suitable technique. As shown, a substrate 10 is comprised of a first level of dielectric 12 which may include a multiplicity of various types of selected circuits. For example, the connecting pads 14 a and 14 b may represent connections to areas of metallization or terminals of various circuits such as the bit line of memory cells located in dielectric layer 12 . Thus, it should be understood that the substrate 10 could represent a single layer of circuits or metallization lines on top of a silicon wafer or, alternatively, the term substrate may be used to represent multiple layers of interconnecting circuits. In any event, there is shown a layer of dielectric material 16 defining a multiplicity of trenches 18 a , 18 b and 18 c etched therein. The sidewalls and bottom of the trenches as well as the top surface of dielectric layer 16 are typically covered by a barrier layer or material 20 , such as for example, Ta, TaN, Ti, TiN, SiN, or SiC that may be deposited such as by a PVD (plasma vapor deposition) or CVD (chemical vapor deposition) process. After the barrier layer 20 is deposited, a suitable conductive material 22 , such as copper, is deposited to fill the trench and cover the top surface of the dielectric layer 16 .

The excess copper or conductive material 22 and those portions of the barrier layer or material 20 covering the top surface of the dielectric material 16 are then removed to obtain the strips of metallization 22 a , 22 b and 22 c defined in the trenches 18 a , 18 b and 18 c lined with barrier material 20 as shown in FIG. 1 C. The configuration of FIG. 1C may be achieved, for example, by a two step CMP (chemical-mechanical polishing) process. The first polishing step would use a polishing chemical selective to the material chosen as the barrier layer 20 so as to remove the copper or other metallization down to the barrier layer 20 as shown in FIG. 1 B. The second polishing step would then use a chemical selective to the dielectric 16 to remove those portions of the barrier layer or material 20 that are on top of the dielectric 16 so as to result in the structure of FIG. 1 C. It will be appreciated by those skilled in the art that to this point, traditional Damascene processing steps have been used.

The use of a barrier layer 20 on the sides and bottom of the trenches, encases the areas of metallization so as to enhance adhesion while preventing the migration of metal atoms or ions, such as for example, copper, into the surrounding dielectric. In addition, as is discussed hereinafter, fully encasing (including the top surface) or encapsulating the metal lines with a material such as Ta, TaN, Ti, TiN, SiN, or SiC protects the metal lines (such as copper or tungsten) so that very harsh or aggressive processing steps or methods can be employed in future steps. The ability to use such aggressive processing, such as for example chlorine-based RIE, will allow the fabrication of new device architectures.

Referring now to FIG. 2 A and FIG. 2B , there are described initial processing steps according to one embodiment of the present invention. As shown in FIG. 2A , the surface of the area of metallization 22 a , 22 b and 22 c has been recessed by a process step that is chosen to be selective to the dielectric layer 16 , and can also (but not necessarily) be selective to the barrier layer 20 . An effective process method for recessing the areas of metallization is to use further CMP in a way that removes the top portion of copper or metallization lines 22 a , 22 b and 22 c but is selective to the dielectric layer 16 . Other process steps that may be suitable include a metal RIE plasma etch (e.g., with CO NH3 plasma to etch copper), an ion milling (sputter) etch, or a wet etch that readily removes the metal and is also selective to the dielectric layer 16 . No matter the process step selected, the result as shown in FIG. 2A is that the top surface 24 of the areas of metallization (copper or tungsten, for example) 22 a , 22 b and 22 c is recessed between about 10 nm and 100 nm below the top surface 26 of the dielectric material 16 .

After the metal has been recessed, a capping liner or barrier layer 28 of a selected material is deposited over the dielectric material 16 and the recessed metal lines of FIG. 2 A. Thus, the copper (or other metal) lines are completely encased or encapsulated by liner, such as for example, Ta, TaN, Ti, TiN, SiN, or SiC as shown in FIG. 2 B. In one embodiment, the encapsulating layer 28 will be chosen to be the same material used to line the trenches 18 a , 18 b and 18 c . However, the use of the same material is not necessary, and other materials suitable for protecting the copper lines may be used.

Referring now to FIG. 2C , the liner material 28 located on the dielectric layer 16 is planarized by any suitable process, such as for example, by CMP down to the top surface 26 of dielectric layer 16 . This results in the areas of metallization 22 a , 22 b and 22 c being completely encapsulated by the barrier liner 20 that lines the trenches and the capping layer or material 28 a , 28 b and 28 c as shown in FIG. 2 C. The planarization in this embodiment will also isolate the areas of metallization from neighboring areas of metallization, and provide a very smooth surface for subsequent processing steps. Alternately, the layer of barrier material 28 may be deposited with sufficient thickness that it can be polished down so that it not only encases the areas of metallization but also leaves a planarized layer of the barrier material that covers both the areas of metallization and the dielectric layer 16 . As discussed below, encapsulating the areas of metallization in a suitable material provides the opportunity for additional aggressive and harsh processing steps.

As will also be appreciated by those skilled in the art, additional layers of electronic elements of lines of metallization may be formed above the encapsulated lines or areas of metallization such as shown in FIG. 2 D. For example, another dielectric layer 29 may be deposited directly on top of protective encapsulant material 28 a , 28 b and 28 c and the dielectric layer 16 . The encapsulating material 28 allows the dielectric layer 29 to be deposited directly without first forming a silicon nitride or silicon carbide layer as was typically required by prior art processes. Consequently, if silicon oxide or a low-K material is used as the dielectric layer 29 in close contact with the areas of metallization of copper and tungsten the capacitance between electronic elements and/or areas of metallization can be reduced relative to the prior art which requires a higher-K material to be used as an adhesion promoter and diffusion inhibitor.

Referring now to FIGS. 3A and 3B , and 4 A, 4 B and 4 C, there are illustrated examples of processing methods enabled by this invention. As shown in FIG. 3A , an MRAM or Magnetic Random Access Memory may be fabricated by depositing a stack 30 of magnetic films on top of the planarized structure of FIG. 2C as shown in FIG. 3A. A photoresist or alternative hard mask 32 is then deposited and patterned as also shown in FIG. 3 A. As will be appreciated by those skilled in the art, to etch the magnetic stack 30 , harsh or aggressive processing steps are often required. For example, because of the protective encapsulation of the areas of metallization, the magnetic stack 30 may now be etched with a chlorine-based RIE (Reactive Ion Etch). Prior art structures with unencapsulated copper (or tungsten) areas of metallization can be very difficult to reactive etch with chlorine-based chemicals since practical endpoints result in exposed copper (or tungsten) with unacceptable corrosion of the copper (or tungsten) due to the chlorine. However, according to this invention, encapsulating the copper lines with a material such as Ta, TaN, Ti, TiN, SiN, or SiC gives adequate protection for a substantial over-etch of the magnetic stack 30 without causing corrosion of the copper (or tungsten) lines. The resulting structure is shown in FIG. 3 B.

FIGS. 4A , 4 B, 4 C and 4 D illustrate an alternate embodiment for using aggressive or harsh processing steps. According to this embodiment, the prior art processing steps discussed above will typically be followed to arrive at the structure illustrated in FIG. 1 B. Then, unlike the previous embodiment, portions of the protective liner material 20 covering dielectric layer 16 are not removed as shown in FIG. 1C , but are left in place. The metal in the trench is again recessed as illustrated in FIG. 4A , according to one of the processes used as discussed above with respect to FIG. 2A. A suitable barrier or protective encapsulating material 28 a , 28 b and 28 c , such as Ta, TaN, Ti, TiN, SiN, or SiC, is then deposited over the top surface 24 of the copper lines 22 a , 22 b and 22 c so as to fill the recesses above the copper or tungsten lines by a process such as was discussed with respect to FIG. 2 B. The barrier or liner material 28 will not only be deposited in the recesses above the copper lines, but will also be deposited over the existing liner material 20 used to line the trenches. The structure is then CMP (chemical-mechanical polished) to planarize the encapsulating material 28 as shown in FIG. 4B , with the option of polishing away the liner material 20 atop the dielectric 16 . For example, according to one embodiment, 40 nm of TaN is deposited over the structure and then polished to leave a 20 nm coating. A stack 30 of magnetic films is then deposited over the planarized structure followed by a mask 32 as was discussed above and as also shown in FIG. 4 C. The magnetic stack 30 can then be etched with a harsh or aggressive etching step, such as a chlorine-based RIE, to produce the structure shown in FIG. 4 D.

After the patterning of the magnetic film stack 30 , a layer of suitable material such as Silicon Nitride (SiN) or Silicon Oxide (SiOx), can be deposited over the patterned structure and CMP performed in a manner suitable for proceeding with still another level of metallization or circuitry. It will also be appreciated that by selecting a liner material such as Ta, TaN, Ti, TiN, SiN, or SiC, the encapsulated metal lines can be used as a hard mask during an etching step.

Therefore, referring now to FIG. 2C along with FIGS. 5A and 5B , there is illustrated a second advantage of the present invention. As shown, the barrier material 20 and 28 encapsulating the metal lines 22 a and 22 b , is suitable for use as an etch hard mask. Thus, as shown in FIG. 5A , the dielectric 16 between the encapsulated metal lines 22 a and 22 b may be etched away leaving self-aligned vias 34 A and 34 B. Also as shown, a suitable conductive metal can be used to fill vias 34 A and 34 B to form conductive plugs 35 A and 35 B that are only slightly separated from the areas of metallization 22 a , 22 b , and 22 c by the barrier material 20 but still is not in electrical contact with these areas of metallization. This is particularly simple to implement if the liner 20 and encapsulant 28 are chosen to be dielectrics such as SiN or SiC, in which case the etched vias are naturally insulated from the adjacent metal lines 22 a and 22 b.

Referring now to FIG. 5B , there is shown an example of a top view of a structure that could be fabricated according to the teachings of this invention by depositing resist strips 36 a , 36 b and 36 c located perpendicular to the areas of metallization 22 a , 22 b and 22 c . Because vias can be precisely placed, significant space can be saved, which in turn can increase yield.

Previous processes that did not use the self-aligning mask process of the present invention typically required twice the distance between two parallel areas of metallization to minimize electrical shorts and other affects of this alignment.

It will also be appreciated that encapsulating the areas of metallization, as discussed above, will also effectively prevent the migration of metal ions, such as copper, into surrounding or adjacent sensitive electronic elements or components. The use of encapsulants such as Ta, TaN, Ti, TiN, SiN, or SiC on trenches and vias can also inhibit surface diffusion (e.g., along the sides of trenches) as seen in electromigration, and can be very beneficial as a barrier to diffusion (e.g., due to electromigration in vias).