Over-polishing

The baking soda serves as a conductive ionic solution that permits electrons to move from the aluminum atoms to the silver ions. What is the advantage of this approach over polishing the silver with an abrasive paste ... 

Unlike W plasma etch back process, the typical W CMP process usually removes the adhesion layer such as Ti/TiN or TiN during the primary polish. As a result, during the over polish step there is some oxide loss. Since the oxide deposition, planarization CMP (oxide CMP), and tungsten CMP steps are subsequent to each other, the oxide thickness profile could become worse further into the process flow. Therefore, the across-wafer non-uniformity of the oxide loss during W CMP process is one of the very important process parameters needs to be optimized. To determine the effect of the process and hardware parameters on the polish rate and the across-wafer uniformity, designed experiments were run and trends were determined using analysis of variance techniques. Table speed, wafer carrier speed, down force, back pressure, blocked hole pattern, and carrier types were examined for their effects on polish rate and across-wafer uniformity. The variable ranges encompassed by the experiments used in this study are summarized in Table I. 

Fig.3. oxide erosion versus feature size obtained from a preliminaiy experiment. The data collected on the same wafer but different dies is represented by the same symbols linked with a line. The erosion obviously increases with the over-polish lime despite of the data scatter. 

Fig.4. a schematic comparison between the proposed and the traditional process. The bold lines represent tlie film height measured from any given reference plane. A and B refer to any two points on a wafer, witli the assumption that removal rate sees its minimum at point B. In tlie proposed process, the over-polish is greatly reduced because tlie onset of the oxide exposure is delayed due to the protection provided by barrier layers. 

In other words, the oxide is protected by the barrier layer until aluminum is completely removed at point B. The barrier layer is then cleared by switching the slurry from Zone 1 to Zone 2, By this way, the over-polish at point A is reduced and the erosion is suppressed. 

Figure 4 Slurry comparison made by subtraction of the post-CMP remaining nitride thickness (HSS-LSS) corresponding to different density features and over-polish conditions.

Pattern density dependency, arising for system-on-a-chip design, is characterized with CMP process optimization, slurry selectivity, over-polish extent and dummy pattern structure. For low density devices, dummy structure is desirable under conditions that the supportive dense structure could exceed a threshold value for reduction of CMP erosioa Physical and electrical evaluations help to identify the process window for different density structure. The characterized SIT process feature will allow designers to print circuit patterns more efficiently and cram more fiinctions onto the silicon. 

Figure 7 Normalized nitride thickness at different density features for wafers without dummy structure. The pattern density dependency is very sensitive to over-polish sequence. 

Controlled wafers 15% over-polish 30% over-polish... 

The elastic tool holder avoids the hard components and washes out the softer parts. The longer elastic polish agent carriers are used the coarser the crystal structure, the greater the risk of over-polishing, also known as the orange peel effect because of the appearance. Figure 4.71 shows a polish lapping step. 

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