Scratch Level on STI Patterned Wafers

Scratch Level on STI Patterned Wafers

Material Name: No data
Record No.: 48
Primary Chemical Element in Material: No data
Sample Type: Wafer
Uses: Polishing
Etchant Name: None
Etching Method: Polishing
Etchant (Electrolyte) Composition: No data
Procedure (Condition): No data
Note: During the CMP process, the pad surface can undergo plastic deformation and the surface becomes smoother as the pores are filled with the pad materials. Using a glazed pad causes the removal rate to drop significantly. Polishing pads were conditioned with a diamond conditioner to provide consistent performance and to prevent the glazing effect. Usually, diamond grits used for pad conditioning are attached to an alloy substrate using electrochemical deposition methods. Yang et al. investigated the CMP process based on material removal rate and scratch defects by studying the pad interaction and conditioner effect using two types of polishing pads: a porous pad and a solid pad with micro holes (Fig. 24). When a solid pad with micro holes was used with a fumed silica slurry and a 180 µm diamond grit conditioner, the material removal rate decreased by approximately 10% compared with the porous pad. However, the scratch defects were reduced when compared with the porous pad which is shown in Fig. 25. In order to increase the removal rate obtained using a solid pad with micro holes to a level comparable to a regular porous pad, various diamond conditioners with diamond size ranging from 70 to 130 µm were adopted. Also, pad surface roughness and contact area were analyzed to understand the removal rate and the scratch generation. Figure 26 shows the effect of diamond size of conditioner on the removal rate and scratch generation. It was found that the micro holes in the pad acted as a defect source or coarse particle reservoir to prevent micro scratching during the process. They reported optimized results of solid pads with micro holes using the hole depth control procedure to reduce the defects.

As mentioned earlier, pad debris can be generated due to tearing of the pad by the conditioner. Prasad et al. studied the generation of pad debris and its characterization. They reported that pad debris could act as a main scratch source, resulting in scratches with several size ranges with irregular shapes, mostly in agglomerated form. It was also proposed that the surface properties were changed by their adsorption with abrasive particles. Figure 27 shows FESEM images of fresh pad particles and pad debris generated using DI water and silica abrasive particles. Park’s group also investigated the scratch number using the three different scratch source (vis., pad debris, dried particles, and diamond particles) on scratch formation comprehensively with their classification. Figure 28 shows the material removal rate and generated scratch number as a function of scratch source. A small amount of impurity in slurry did not affect the MRR. However, scratch number was affected by the kind of scratch sources. Figure 29 shows the distribution of scratchess formed by adding different scratch sources during polishing. Borken chatter type of scratches was easily formed when dry slurry paritcles were added but group chatter when pad debris were added. Yang et al. measured the pad surface hardening phenomenon based on force–distance (F–D) curves. It was found that the interaction between abrasive particle and polyurethane pad under tribo-mechanical action could change the pad surface hardness. Benner et al. used a vacuum cleaner to remove the pad debris and agglomerated large particles from the pad; they dubbed this process the pad surface manager (PSM). Figure 30 contains a plot of light-point defects measured using a Tencor 6220 on polished oxide wafers using different levels of PSM vacuum. The data were normalized to that observed without vacuum. As the PSM vacuum level was increased, CMP induced wafer defects decreased. Approximately a 50% reduction in light-point defects was observed using the PSM technique.
Reference: Tae-Young KWON, Manivannan RAMACHANDRAN, Jin-Goo PARK, Scratch formation and its mechanism in chemical mechanical planarization (CMP), Friction 1(4): 279–305 (2013).

Figure 24: SEM micrographs (top) and schematics (bottom) of (a) porous pads and (b) solid pads.

Figure 25: Scratch level on STI patterned wafers generated by porous and solid pads with 180 µm diamond conditioner.

Figure 26: The effect of diamond size on (a) removal rate and (b) scratch generation.

Figure 27: SEM image and EDX analysis of (a) fresh pad, (b) pad debris with only DI water, and (c) pad debris with silica slurry.

Figure 28: (a) Material removal rate, non-uniformity and (b) the variation of number of scratches formed with addition of different scratch sources.

Figure 29: (a) Effect of addition of pad debris, dried slurry particle and (b) diamond particles on distribution of scratch shapes formed on oxide surface after CMP process with silica slurry.

Figure 30: A plot of the dependence of light-point defect counts, measured with a Tencor 6220 on oxide wafers, as a function of PSM vacuum level. A reduction of nearly 50% was observed as a PSM vacuum.