Effect of Polish By-Products on Copper Chemical Mechanical Polishing Behavior

Abstract

The stains on the pad caused by polishing by-products can be observed in a copper chemical and mechanical polishing ͑CMP͒ process. In this study, the effects of stains on CMP performance such as erosion, dishing, and nonuniformity were evaluated as a function of the degree of stains accumulated on the pad. The stains on the pad deteriorate the nonuniformity of removal rate and result in the increase in erosion and dishing. CMP by-products adhere on both pores and grooves of the pad and block the flow of slurry through the grooves, resulting in the deterioration of nonuniformity. The selectivity ͑ratio of removal rate, Cu to TaN or dielectric film͒ is important in order to minimize erosion. When wafers were polished on a stained pad, the removal rate of Cu decreased ϳ30% due to the poor slurry distribution and the selectivity decreased more than 40% because the mechanical abrasion was enhanced by the presence of by-products on pad surfaces. The lower the selectivity, the higher the level of erosion on the polished patterned wafers. The higher frictional force on a stained pad results in higher temperature and etch rate of Cu which might be the reason for recess and dishing of Cu lines. The rapid scaling of metal-oxide-semiconductor field-effect transistors ͑MOSFETs͒ has been successfully continued for more than three decades since the early 1970s.1,2 In the past, these improvements were achieved mainly from the front-end process, such as reducing the transistor dimension as well as the thickness of gate oxide. Little attention was paid to the back-end of line ͑BEOL͒ multilevel interconnects. Because of the scaling of BEOL, resistance and capacitance ͑RC͒ delay cannot catch up with the improvement of transistors, and multilevel interconnects are rapidly becoming a bottleneck to overall performance. 3 The use of new materials for interconnects will reduce the RC delay. Copper application for interconnection offers faster and more reliable integrated circuit ͑IC͒ device due to its low electrical resistivity and superior electromigration resistance compared to aluminum. By replacing Al with Cu, there is a 40% reduction in resistance. 4 For Cu interconnection, a damascene process using chemical mechanical polishing ͑CMP͒ has been introduced and Cu CMP has been actively investigated in recent years.5 CMP is a process of polishing assisted by chemical and mechanical reactions. Especially, the performance of Cu CMP is strongly dependent on the slurry chemistry because Cu easily corrodes and does not form a thick Cu oxide on Cu surface. 6During Cu CMP process, polish by-products are usually found on the polishing pad. The polish by-products are generated by the chemical interaction between the Cu film on the wafer surface and the slurry. The polish by-products cannot be easily removed by simple pad conditioning using a diamond disk, and the color of the pad is changed to brown after Cu CMP. Many semiconductor manufacturers have been using a special pad-cleaning solution to eliminate the stains on the pad, but it cannot re...

Pad surface management as a strategy to reduce the cost of ownership for CMP

“…As previously reported in Ref. [5], pad staining due to accumulation of Cu CMP by-products can significantly impact MRR, WIWNU, dishing, erosion, and selectivity. They also noted that conventional DI water rinsing and pad conditioning had little to no effect on pad staining [5].…”

“…[5], pad staining due to accumulation of Cu CMP by-products can significantly impact MRR, WIWNU, dishing, erosion, and selectivity. They also noted that conventional DI water rinsing and pad conditioning had little to no effect on pad staining [5]. Specific pad cleaning products (e.g., citric or oxalic acid solutions) have been sold to help reduce pad staining, but not eliminate it, which add additional process cost and can complicate waste disposal.…”

“…Nonetheless, removing >90% of Cu CMP by-products from the pad should provide a noticeable increase in pad life with reduced or eliminated pad staining during Cu CMP. It is anticipated that elimination of Cu pad staining will result in vastly improved process consistency across a wafer run [5].…”

“…The high concentration of agglomerated slurry particles and pad debris results in micro scratches and chatter marks, some of which lower yield and some decrease device reliability [4]. A buildup of Cu reaction products, leading to pad staining, has been shown to have a deleterious effect on MRR, WIWNU, dishing, erosion, and selectivity [5]. Removing process waste (i.e., spent slurry, reaction products, pad debris, etc.)…”

Micro-scratch reduction of replacement metal gate aluminum chemical mechanical polishing at 28nm technology node

“…Pad stained by polish by-products induced decrease o f removal rate, increase o f non-uniformity, and change of selectivity [10]. Hence, pad cleaning was necessary for CMP.…”