ABRASIVE-FREE SELECTIVE CHEMO-MECHANICAL POLISH FOR TUNGSTEN
FTELD OF THE INVENTION
This invention relates to polishing processes and systems and, particularly, to tungsten polishing processes and systems.
BACKGROUND
Integrated circuit manufacturing processes often create topographic features (i.e. , rotrusions and depressions) on the surface in integrated circuit structures that reduce the accuracy of subsequent lithographic processes. For example, a process used to manufacture integrated circuits having tungsten structures results in the formation of the undesirable topographic features because the upper surface of the tungsten structure is lower than the surrounding structure.
Figures 1A-1C (prior art) show the formation of a tungsten via for use with a multiple metal layer interconnect structure 100, using a plasma etchbac process. Like reference numbers are used for like structures between drawings.
Figure 1A shows a structure 100 including a tungsten layer 110 deposited over an etchstop or barrier layer 120. Barrier layer 120 is typically made of Titanium ("Ti") or Titanium Nitride ("TiN") or both. Part of tungsten layer 110 forms a tungsten via 130 in a recess 140 in an insulating layer 150. Tungsten via 130 can be used to interconnect a layer 160 with a conductive layer, typically Aluminum ("Al"), subsequently deposited over tungsten via 130 and barrier layer 120 (after the plasma etchback process) . Figure IB shows tungsten via 130 and islands 170 and 180 formed over barrier layer 120 by a plasma etchback process. The plasma etch process removes the upper portion of tungsten layer 110 (i.e., the portion of tungsten layer 110 above the top surface 122 of barrier layer 120) . A top surface 132 of tungsten via 130 is approximately level with a top surface 122 of barrier layer 120. Islands 170 and 180 are undesired residual islands of tungsten leftover from tungsten layer 110 due to the initial roughness of the tungsten film which causes the etch rate to be greater in the opening of recess 140.
The plasma etchback process is allowed to continue (i.e., overetch) to remove islands 170 and 180 as shown in Figure 1C. However, due to continued exposure of tungsten via 130 to the etch chemistry, the top surface 132 of tungsten via 130 becomes lower than the top surface of 122 of barrier layer 120, thereby forming undesirable topographic features 190 in structure 100. Topographic features 190 will have a more pronounced detrimental effect as device sizes become smaller in the next generation process technologies.
Figure 2 (prior art) shows a chemo-mechanical system 200 having an abrasive slurry 210 for forming tungsten structures on a wafer 220. In system 200, abrasive slurry 210 is placed onto the surface of a pad 230, which is kept in contact with a surface 222 of wafer 220 by holder 240. Wafer 220 is held against pad 230 with a pressure in the range of 3-9 psi. Pad 230 is mounted on a table 250, and table 250 (and consequently, pad 230) is rotated by a motor (not shown) to polish the surface 222 of wafer 220. Slurry 210 flows between surface 222 and pad 230 during the polishing process to aid in removing tungsten from surface 222 of wafer 220. After polishing surface 222 of wafer 220 using system 200, the sizes of any topographic features are reduced; however, the abrasive slurry polishing process of system 200 is not selective to the barrier, and therefore degrades the integrity of a barrier layer surrounding the upper surface of the remaining tungsten. Thus, the abrasive slurry polishing process is continued into insulating layer 150, whereby portions of barrier layer 120 and insulating layer 150 are removed as illustrated in Figure 3A. As a result, top surface 132 of tungsten via 130 is substantially level with a top surface 152 of insulating layer 150. However, there is no barrier layer above insulating layer 150.
As stated above, tungsten via 130 is typically used to interconnect layer 160 with an Al layer deposited over tungsten via 130. However, a barrier layer 310 is deposited on top of insulating layer 150 and tungsten via 130, as illustrated in Figure 3B, to improve the conductive and electromigration properties of the subsequently deposited Al layer. Thus, an additional deposition is required after the abrasive slurry polishing process.
Further, (referring back to Figure 2) wafer 220 must be thoroughly cleaned to remove the abrasive particles and slurry medium from abrasive slurry 210 left behind on wafer 220. In addition, abrasive slurry 210 must be stirred during the polishing process to ensure homogeneity of slurry 210. Also, the slurry manufacturer must process slurry 210 to remove contaminants and ensure uniform particle size, increasing the cost of slurry 210.
SUMMARY
According to one embodiment of the present invention, an abrasive-free, selective chemo-mechanical process for polishing tungsten comprises the steps of applying hydrogen peroxide ("H202") to a polishing pad, and polishing the tungsten with the polishing pad and H202. This embodiment is used advantageously in applications such as polishing a structure with a tungsten layer deposited over a Ti/TiN barrier layer having recesses. The process is performed at room temperature using an approximately 30% H202 solution. The tungsten layer is polished to remove the upper portion of the tungsten layer so that the upper surfaces of the tungsten remaining in the recesses are substantially level with the upper surface of the
Ti/TiN layer, thereby substantially reducing the size of topographic features on the structure. Because the H202 is highly selective for reacting with tungsten relative to the Ti and TiN, this process selectively removes tungsten while leaving the Ti/TiN barrier layer intact. Thus, no additional barrier layer deposition is necessary. Further, no abrasive particles are left behind on the structure after polishing. Still further, H202 is relatively inexpensive compared to an abrasive slurry and need not be stirred during use.
BRIEF DESCRIPTION OF THE DRAWINGS
Figures 1A-1C (prior art) illustrate steps of a plasma etchback process for forming a tungsten via. Figure 2 (prior art) shows a chemo-mechanical polishing system using an abrasive slurry.
Figure 3A-3B (prior art) illustrate steps of forming a tungsten via using the abrasive slurry polishing system of Figure 2. Figure 4 shows an abrasive-free chemo-mechanical polishing system according to one embodiment of the present invention.
Figure 5 shows an abrasive-free chemo-mechanical polishing system according to another embodiment of the present invention.
Figure 6 illustrates a tungsten via resulting from a polishing process according to one embodiment of the present invention.
DETAILED DESCRIPTION Figure 4 shows an abrasive-free polishing system 400 according to one embodiment of the present invention, adapted for die polishing. System 400 is used to polish a die 410 having tungsten structures on the surface of die 410. For example, die 410 can include a tungsten layer, a barrier layer, and a tungsten via similar to tungsten layer 110, barrier layer 120, and tungsten via 130 shown in Figure 1A.
System 400 includes a motorized polishing wheel 420 with a polishing pad 422, such as models Poly et I or Ecomet I made by Bhueler, located at 41 Waukegan
Road, Lale Bluff, 111., 60044. A 30% H202 solution is applied to polishing pad 422. Although a 30% H202 solution is used in this embodiment, any concentration within the range 10-60% is suitable. The H202 reacts with the tungsten to form soluble tungsten oxides. Die 410 is fixed to a platen 430 and pyrex specimen mount centering ring 440 available from Gatan Inc., 780 Commonwealth Drive, Warrendale PA 15806. An adhesive such as crystal wax, also available from Gatan Inc., is used to fix die 410 to platen 430.
Polishing wheel 420 rotates at approximately 200 rpm, although any speed in the range of 0-300 rpm is suitable. The operator places platen 430 and centering ring 440 onto pad 422. The operator applies the surface of wafer 410 to pad 422 by pressing down on an end 432 of platen 430 with a light pressure to polish a surface 412 of die 410. The operator moves platen 430 and centering ring 440 along the surface of pad 422 in the opposite direction of the rotation of pad 422 for more uniform polishing. Die 410 is polished until the Ti/TiN barrier is visible over the entire surface (except, of course, the upper surface of the tungsten vias) of die 410. This process is performed at room temperature; however, any temperature in the range of 10-60°C is suitable. Generally, the polishing process is faster with higher temperature. At room temperature, this process takes approximately 5-240 seconds and results in a structure having the upper surface of the tungsten substantially level with the upper surface of the barrier layer. Because the H202 solution contains no abrasive particles, there are no abrasive particles to clean from the surface of polished die 410, as is necessary in system 200 (Figure 2) . Further, because the H202 solution is highly selective for tungsten relative to Ti and TiN, the Ti/TiN barrier layer of die 410 remains intact, thereby eliminating the need to redeposit the barrier layer as may be necessary when a plasma etchback system is used. Although a H202 solution is described, other solutions that are reactive with tungsten can be used, provided all of the reactants the solutions are relatively non-reactive with Ti and TiN. Figure 5 shows an abrasive-free polishing system 500 according to another embodiment of the present invention, adapted for wafer polishing. System 500 is used to polish a wafer 510 that includes a plurality of dice, each die being similar to die 410 shown in Figure 4. System 500 is similar to a conventional chemo- mechanical polishing system, except that system 500 includes a H202 solution 520 and does not include an abrasive slurry. For example, system 500 can include a polisher such as Model 6ds-sp made by Strausbaugh, located in San Luis Obispo, CA.
H202 solution 520 is stored in a dispenser 522 and applied onto the surface of a pad 530. Dispenser 522 controls the flow of H202 520 onto pad 530 to be approximately 100-200 ml/minute. Pad 530 is kept in contact with a surface 512 of wafer 510 by a holder 540. Wafer 510 is held against pad 530 and rotated by an overarm mechanism (not shown) coupled to holder 540 with a pad-to-wafer pressure of approximately 3-9 psi. Pad 530 is mounted on a table 550. Table 550 is rotated at approximately 30-40 rpm, in the same direction that wafer 410 is rotated, to polish the surface 512 of wafer 510. Although a rotation speed of 30-40 rpm is described, any speed in the range 0-100 rpm is suitable. H202 solution 520 "floods" pad 530, whereby H202 can selectively remove tungsten, aided by the mechanical contact of pad 530 on surface 512 of wafer 510. Wafer 510 is kept in contact with pad 530 until the upper surface of the tungsten is polished to the desired level.
Figure 6 illustrates a structure 600 having a tungsten via 610, resulting from the abrasive-free chemo-mechanical polishing processes described in conjunction with Figure 4, after being applied to a structure similar to structure 100 (Figure 1A) .
Although system 400 was used to form structure 600, system 500 could be used to form a structure substantially identical to structure 600. As shown in Figure 6, tungsten via 610 is formed in a recess 620 defined by a barrier layer 630. An upper surface 612 of tungsten via 610 is substantially level with an upper surface 632 of barrier layer 630, thereby reducing topographical features in the surface of structure 600. Further, barrier layer 630 remains intact and is suitable as an underlayer for a subsequent metal layer deposition.
The foregoing has described the principles and preferred embodiments of the present invention. However, the invention should not be construed as being limited to the particular embodiments described. For example, the H202 concentration, pad-to-wafer (or pad- to-die) interface pressure, pad rotation rate, and H202 solution temperature can be altered to achieve tungsten removal at different rates and/or efficiency. Further, polishing tools or fixtures different from those describe above can be used to obtain substantially similar results. Still further, other embodiments may be adapted for applications other than the formation of tungsten vias with barrier layers. In addition, solutions with reactants other than H202 can be used, provided the reactant is selective to react with tungsten relative to the barrier layer and other structures used in the application. Thus, above- described embodiments should be regarded as illustrative rather than restrictive. Variations can be made to those embodiments by workers skilled in the art without departing from the scope of the present invention as defined by the following claims.