Currently shallow trench isolation (STI) and interlayer-dielectric (ILD) process recipes contain various steps accomplishing different objectives. End-of-polish conditions are extremely important to the final number of defects on the surface of the wafer after chemical mechanical planarization (CMP). Three conditions are theoretically necessary to leave behind slurry residues, pits, and scratches on the wafer surfaces after STI and ILD CMP. First, there must be colloidal particles present near the wafer surface. These particles are the source of residual particles on the wafer surface, they are the same particles that can agglomerate and cause microscratches and oxide pit defects. Second, physical force (i. e. high downforce) or velocity is necessary to overcome the energy barrier between a colloid (abrasive particle) and the wafer surface Both electrical repulsion and Van der Waals attraction combine to create the net energy barrier between the wafer surface and the colloid.

Third, in a high pH slurry system low pH reduces the electrical repulsion between the colloidal particles and makes the possibility of overcoming the energy barrier between the wafer surface and the colloidal particles more likely. Once the energy barrier is overcome, three types of destructive phenomenon can theoretically occur. First, colloidal particles begin to agglomerate. Second, colloidal particles and/or agglomerates of colloidal particles attach to the wafer surface. Third, larger agglomerates of colloidal particles scratch or pit the wafer surface, but do not adhere to the wafer surface.

Currently STI and ILD CMP process recipes contain various steps accomplishing different objectives. End-of-polish conditions are extremely important to final defectivity after CMP.

The final defectivity of the wafer determining the yield of the semiconductor fabrication process as is generally known to experts in the field of semiconductor fabrication.

What is needed is a method for reducing the number of defects during an end- of-polish process that removes the colloidal particles from the wafer environment that reduces the risk of breaching the energy barrier between a particle and the surface of the wafer by optimizing pH and downforce during the end-of-polish processing.

This object, and others, is satisfied by Applicant's present inventions disclosed herebelow.

The present invention is directed towards a system and method of reducing defects on a semiconductor wafer by polishing the semiconductor wafer while in contact with a high pH slurry, rinsing the slurry from said polishing pad and the surface of the wafer with a high pH solution sprayed on the polishing pad at high pressure and then spraying the polishing pad with deionized water to further clean the wafer. Additionally, it is preferred that the semiconductor wafer is biased against the polishing pad at low downforce while being rinsed with the high pH solution.

Related objects and advantages of the present invention will be apparent from the following description.

The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which: Figure 1 is a block diagram of a CMP machine.

Figure 2 is a side partial perspective view of a semiconductor wafer.

Figures 3-4 show a partial side perspective view of a currently used CMP machine during an end-of-polish cycle.

Figures 5-6 show a partial side perspective view of a CMP machine during an end-of-polish cycle in accordance with the present inventions.

Figure 7 is a flow diagram of the process for one particular preferred embodiment of a CMP polish cycle for reducing defects in accordance with the present inventions.

For the purposes of promoting an understanding of the principles of the inventions, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the inventions is thereby intended, such alterations and further modifications of the principles of the inventions as illustrated therein being contemplated as would normally occur to one skilled in the art to which the inventions relate.

Referring now to Figures 1 and 2, there is shown a block diagram of a CMP machine 100 and a side partial perspective view of a wafer 105 (Figure 2). The CMP machine 100 is fed wafers to be polished by an arm 101 and places them onto a rotating polishing pad 102. The polishing pad 102 is made of a resilient material and is textured, often with a plurality of predetermined grooves, to aid the polishing process. A conditioning arm 103 conditions the polishing pad. A wafer is held in place on the polishing pad 102 by the arm 101.

During polishing, the lower surface of the wafer 105 rests against the polishing pad 102. As the polishing pad 102 rotates, the arm 101 rotates the wafer 105 at a predetermined rate. The arm 101 forces the wafer 105 into the polishing pad 102 with a predetermined amount of down force. The CMP machine 100 also includes a slurry dispense tube 107, extending across the radius of the polishing pad 102. The slurry dispense tube 107 dispenses a flow of slurry 106 onto the polishing pad 102 from the slurry source 112. The slurry 106 is a mixture of deionized water and polishing agents designed to aid chemically the smooth and predictable planarization of the wafer. In a system using silica slurry the pH of the slurry is very high, typically having a pH of around 10 or 11. Additionally, during polishing a high downforce of between 4-5 psi is applied to the wafer by the wafer arm 101.

The rotating action of both the polishing pad 102 and the wafer 105, in conjunction with the polishing action of the slurry, combine to planarize, or polish, the wafer 105 at some nominal rate. The polishing action of the slurry is comprised of an abrasive frictional component and a chemical component. The abrasive frictional component is due to the friction between the surface of the polishing pad, the surface of the wafer, and abrasive particles suspended in the slurry. The chemical component is due to the presence in the slurry of polishing agents which chemically interact with the material of the dielectric or metal layer of the wafer. The chemical component of the slurry is used to soften the surface of the dielectric layer to be polished, while the frictional component removes material from the surface of the wafer.

After the slurry dispense process is terminated, deionized water is dispensed from the deionized water source 110 via the high-pressure spray nozzle 108 onto the pad.

High pressure spray nozzle 108 may be of the type disclosed in commonly assigned U. S.

Patent Application Serial No. 09/871,507.

Referring more particularly to Figure 3, using current end-of-polish recipes, once the slurry dispense is terminated the high pH slurry is mixed with deionized water rinse, which contributes to the destructive mechanisms. Additionally, slurry abrasive particles and agglomerations of abrasive particles, such as particles 106, are stuck to the surface of wafer 105 and are additionally stuck in the grooves 114 and in the fibers of the polishing pad 102.

In current pad cleaning methods, the pad is cleaned with high pressure deionized water spray 130 via the high-pressure spray nozzle. The wafer 105 floats on the deionized water during the pad cleaning step.

After the pad cleaning step, the wafer 105 is cleaned as part of the end-of- polish process. Referring now to Figure 4, in connection with the pad cleaning step shown in Figure 3, current end-of-polish processes clean the wafer by applying a downforce with the carrier ring to the wafer onto the clean pad and continuing the high pressure deionized water spray 130, via the high pressure spray nozzle 108. Using such a process, a number of abrasive particles 106 remain stuck to the wafer and in the grooves 114 of the pad 102. As such, these particles can cause wafer defects during the end-of-polish process while force is applied to the wafer in the presence of these abrasive particles in the low pH environment.

Referring now to Figures 5-7, there is shown a CMP system and method for optimizing the end-of-polish process which reduces the conditions that allow for defects to be generated on the semiconductor wafer surface.

All particulate matter develops an electrically charged thin layer when suspended in a liquid solution. This charge is known as the zeta potential and can be either negative or positive. The zeta potential appears at the outer surface of the particle such that a small charge field surrounds the particle. Silica particles in a basic aqueous solution having a pH of about 10 or more results in a negative zeta potential on the silica particles. In addition, the zeta potential of any other particles present, as well as that of the surfaces contacted by the solution, is negative at such a high pH. The silica particles are thus electrostatically repelled from the semiconductor wafer facilitating the removal of the slurry residue from the wafer surface. More importantly, the high pH solution at the surface prevents the silica particles from overcoming the electrical repulsion at the surface and reduces the Van der Waals attraction. When the pH at the surface of the wafer is lowered in the presence of silica

particles, colloids form and silica agglomeration occurs on the surface of the wafer. As such, any time the pH of the wafer surface is lowered, a higher defectivity environment exists in the presence of microscopic particles. Additionally, high downforce applied to the wafer can contribute to the abrasive particles overcoming the energy barrier between a colloid and the wafer surface, thus contributing to wafer defects. Defects generated include scratches on the wafer by slurry abrasive agglomerates and slurry abrasive (or any other particle) attaching to the wafer surface.

Knowing this, the end-of-process parameters desired for reduced defects are colloidal particles completely removed from the wafer environment, high pH and low downforce applied to the wafer (and hence, the colloid). Referring now to Figures 5 and 6, and the flowchart of Figure 7, there is shown one particular embodiment of the CMP system and process including the end-of-process optimization for reducing defects in accordance with the present inventions.

Referring now to Figures 5 and 7, the wafer is polished on the platen using a high pH slurry and a high downforce. Step 310. Next, the slurry is cleaned from the pad and wafer using a high pressure spray of a high pH solution, while the wafer is biased against the pad with a low downforce. Step 320. Referring more particularly to Figure 5, the system of the present embodiment includes a polishing pad 202 having grooves 214 thereon. Abrasive particles 206 are similarly adhered to the wafer surface and in the grooves 214 of the polishing pad 202. Wafer carrier arm 201 holds the surface of the semiconductor wafer 205 against the polishing pad 202. The carrier ring 216 applies a low downforce to the wafer 205. preferrably the downforce on the wafer 205 is from 1-3 psi. More preferably, the downforce on the wafer 205 during the pad and wafer cleaning step is around 2 psi.

Additionally, in the system and method of the present inventions the pad and wafer are rinsed with a high pH solution 225 from the pH adjusted solution source 212. The pad is rinsed by spraying the high pH solution 225 from the high pressure spray nozzle 208.

High pressure spray nozzle 208 directs the high pH solution 225 towards the pad 202 with a pressure of between 10 and 20 psi. More preferably, the pressure of the high pH solution spray is between 14 and 18 psi.

The high pH solution is directed into the spray nozzle by the valve 220. The high pH solution may be a solution such as dilute ammonia or NH40H and is preferably chosen to have a pH that approximates that of the slurry base and preferably has a pH of between 10 and 12. As such, the pH at the wafer 205 and pad 202 during the rinsing step is kept constantly high, maintaining the high electrostatic repulsion between the semiconductor

wafer and the abrasive particles. Thus, the majority of abrasive particles are rinsed from the pad and wafer, aided by the electrostatic repulsion of the high pH chemistry.

The use of valve 220 is not meant to be limiting, as other methods of providing a high pH solution to the high pressure spray nozzle 208 may be used. For example, it is contemplated that tank 212 could hold a concentrated high pH solution, and liquids from tank 210 and 212 can be mixed during use, when a high pH solution is desired at the polishing pad.

Referring now to Figures 6 and 7, once the pad and wafer have been thoroughly rinsed with the high pH solution such that a minimum amount of slurry abrasive particles 206 remain, the pH of the pad and wafer are lowered. Once the troublesome abrasive particles are removed, the pad and wafer are rinsed with deionized water. If desired, the deionized water may be high pressure deionized water spray 230 from the high pressure spray nozzle 208, however, this is not meant to be limiting as low pressure deionized water may also be used. Step 330 of Figure 7. If desired, the low downforce is maintained on the wafer 206 by the carrier film 216 of the wafer arm 201.

Thus, the present inventions reduce defects on the semiconductor wafer during the end-of-polish process by maintaining the net energy barrier between the defect causing abrasive and the semiconductor surface at a high level until the high pressure spray has a chance to clean the pad of the abrasive. Only after the abrasive has been removed from the pad is the pH of the wafer lowered.

While the inventions have been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiments have been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.