1. Field of the Invention
The present inventions pertain to semiconductor fabrication processing. More particularly, the present inventions relate to a system and method for reducing defects during short polishes of semiconductor wafers by raising the pH of the wafer surface in the presence of abrasives.
2. Description of the Prior Art
Currently semiconductor wafers are reworked on a regular basis. Wafers are reworked when the final thickness of the wafer after the polish is too thick to send on to the next step in the line. The causes of a wafer being underpolished are tool interruptions, incorrect process recipes, improper tool setup and bad consumables.
Wafer rework recipes are typically shortened wafer polishing product recipes. Rework recipes, like polish recipes, typically include two or more linear or rotary tables that do the planarizing and a third table that does a water buff on a softer pad.
Referring now to FIGS. 1 and 2, there is shown a block diagram of a CMP machine 100 including a rotary process table and a side partial perspective view of a wafer 105 (FIG. 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 with a predetermined amount of down force.
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 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. Typically, the polishing pad 102 is primed with slurry 106 for about 8 seconds. The slurry 106 is a mixture of deionized water and polishing agents designed to aid chemically the smooth and predictable planarization of the wafer. 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. In current systems using silica slurry the pH of the slurry is very high, typically having a pH of around 10 or 11.
After the slurry dispense process is terminated, deionized water is dispensed from the deionized water source 110 via the water dispense tube 108 onto the pad. The wafer substrate is then rid of the slurry.
Referring now to FIG. 3, there is shown a block diagram of one example of a CMP process 200 which is typically used to rework wafers having a final thickness too thick to send on to the next step in the line. Input/output station mechanism 210 is used to load and unload the wafers and to transfer the wafer to polishing platen 220, where a high pH slurry polish is followed by an automatic rinse of deionized water, once the polish is complete. The wafer is then transferred to secondary polishing platen 230, where a second high pH slurry polish is again followed by a deionized water rinse, when the secondary polish is complete. The wafer is then transferred to a third, softer pad, where it is buffed with deionized water. The above three platens are included on the same multiple platen CMP machine 205. The processing that occurs on the platens defined by CMP machine 205 is referred to herein collectively as the xe2x80x9cCMP polish processingxe2x80x9d.
The wafer 105 may then be then unloaded manually or may be unloaded using the input station mechanism 210. The wafer then undergoes post CMP cleaning. If desired, the wafer 105 may be transferred to brush stations 250 and/or 255 where the wafer is brushed with a scrub solution spray which, typically, has a high pH. Finally, the wafer 105 is transferred to the spin rinse and dry station 260, where the wafer is rinsed with deionized water and then dried. The processing that occurs at the brush stations and the drying station 260 is referred to herein collectively as the xe2x80x9cpost-CMP polish processingxe2x80x9d.
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. 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. Defects generated include scratches on the wafer by slurry abrasive agglomerates and slurry abrasive (or any other particle) attaching to the wafer surface. If the pH of the liquid in contact with the wafer surface is not maintained at a high pH, the combination of downforce and abrasive particles will lead to high defects.
Three conditions are theoretically necessary to leave behind slurry residues, pits, and scratches on the wafer surfaces. First, there must be colloidal particles present on 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, high downforce 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 silica based 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 used short polish methods do not add defects when the polish time on each platen is greater then twenty seconds. However, when the planarization or oxide removal time on each platen decreases to less than about 15 seconds per platen, defects, especially slurry residues, become an issue. This minimum amount of total polish time forces a minimum film removal allowed for a short polish or rework. This minimum film removal is sometimes more than the final thickness specifications allow. The limitations of short polishes then, create a tradeoff between correct final wafer thickness and low post-CMP defectivity.
Referring now to FIG. 4, there is shown a first polishing platen 220 and a second polishing platen 230, such as those of FIG. 3. Once the slurry polish is completed on platen 220, the wafer 410 is kept wet with deionized water until the wafer 415 on platen 230 is finished being processed. Deionized water on platen 220 is carried on the surface of wafer 410 to platen 230. After the end-of-polish clean of wafer 415 is completed on platen 230, the platen 230 is primed with slurry, typically for about 8 seconds, to prepare for incoming wafer 410 from platen 220. Wafer 410 is placed on the pad. The net result is low pH at the start of the process and in the presence of abrasive. As discussed above, low pH at the wafer surface in the presence of abrasive causes agglomeration of particles, sticking of particles and agglomerates to the surface of the wafer and pitting and scratching of the wafer surface by large agglomerates. Additionally, surface uniformity is impacted. Referring now to FIG. 5, there is shown a graph of oxide removed vs. time on platen 2 and added defects vs. time on platen 2 for two platen 1 polish conditions. In the experiment used to generate FIG. 5, the two platen 1 polish conditions used were (1) the wafer dechucked on platen 1, but having no downforce on the wafer (xe2x80x9cthe first conditionxe2x80x9d); and (2) the wafer was polished normally for 5 seconds on platen 1 (xe2x80x9cthe second conditionxe2x80x9d). FIG. 5 was generated from data acquired from wafers polished for varying times on platen 2 for the above two conditions.
Lines 510 and 512 of FIG. 5 represent the added defects vs. time on platen 2 for the first condition and for the second condition, respectively. Lines 514 and 516 represent the oxide removed vs. time on platen 2 for the first condition and for the second condition, respectively. Note that for both platen 1 conditions, defects added to the wafer peaked between 6 and 9 seconds of platen 2 polish times. Also, the data shows that the wafers must be polished for at least 15 seconds to restore defects to an acceptable level. It is believed that the defects caused early on in the platen 2 polish cycle, as shown by lines 510 and 512 of FIG. 5, are the result of interaction between the processes of platens 1 and 2. The low pH at the surface of the wafer when first transferred from platen 1 to platen 2 makes the conditions ripe for and actually introduces defects, which must then be removed by longer time spent on platen 2.
What is needed is a method for providing a short polish of a wafer without the defectivity issues of current wafer rework methods. What is additionally needed is a process and system for raising the pH of the wafer surface to be short polished/reworked on platen 2 prior to polishing on platen 2.
These objects, and others, is satisfied by Applicant""s present inventions disclosed herebelow.
The present inventions are directed towards a system and method of reworking a semiconductor wafer on a CMP machine ensuring for a short period of time, for example, less than 15 seconds, while improving defectivity. A high pH is maintained on the surface of the wafer when placed on a polishing pad. Additionally, in some embodiments downforce is optimized to further improve defectivity.
Related objects and advantages of the present invention will be apparent from the following description.