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1. INTRODUCTION 1 Overview
1.5 Basics and principle of CMP
By definition, CMP is a process where the collaborative effect of the chemical and mechanical actions improvises the material removal rate (MRR) to give a near perfect planar surface on which layers of integrated circuitry are to be assembled. The unwanted materials on the wafer are removed by using a chemical slurry in complement with the mechanical impacts.
The three salient elements in the CMP process are: a) entity to be polished b) polishing pad and c) polishing slurry. The sample/metal/dielectric coated wafer is mounted on the holder and the
surface to be polished is pressed upon a polymer-based pad. Slurry containing abrasives and chemical reagents are allowed to flow between the sample surface and the pad. The simultaneous rotation of the sample holder and the polishing pad (usually circular) on its own axis causes removal of the material. The polishing pad is attached on the platen and a membrane like object is mounted on the sample holder to apply appropriate downforce on the sample. The applied force usually lies in the range of 1-10 psi (Oliver, 2004). The planarization ability of the CMP process is measured by calculating the removal rate by weight loss method given by
Removal rate (RR)(nm πππ)
= Weight loss (gm) Γ 107 Density (ππ
ππ3) Γ Time (πππ) Γ Surface area(cm2)
[1.1]
The typical CMP equipment that are commercially available in the industries can be segregated into four section (a) rotary-type polisher where the sample carrier and the platen move in reciprocation; (b) rotary-type polisher with an oscillating carrier; (c) orbital-type polisher with orbital rotating platen; and (d) linear-type polisher where the polishing pad is a linear motion belt. It is to be noted that the table diameter for most of the commercial CMP machines are in the range of 20-26"(Oliver, 2004). Such CMP machines which are commercially used, usually polish one wafer at a time.
A typical CMP process is displayed in Figure1.1. As shown, the slurry comprising of abrasives and different chemical reagents are allowed to dispense from a tube to the pad and the surface of the sample to be polished. As the platen and the sample rotates, the slurry envelops into the groves of the polishing pad and comes in proximity with the surface of the sample to be
on the sample will cause the required removal of material. The sample moves relatively to the pad and the material removal rate is governed by Prestonβs law (Qin, Moudgil and Park, 2004):
Removal Rate (RR) = KpΓ P Γ v [1.2]
Here, Kp stands for Prestonβs coefficient (constant), P stands for pressure applied on the sample surface and v for relative velocity between the sample holder and pad respectively.
Figure 1. 1 A Schematic diagram of Chemical Mechanical Polishing Process 1.6 Factors/Variables/Parameters Governing the CMP
The output variables such surface smoothness, the uniformity, the material removal rate etc determine the efficiency of a CMP process/machine. However, the factors or input variables relating to sample-pad interaction play a very prominent role in achieving this efficiency. The ultimate polishing rates obtained is the synergetic action of the input variables. Some of the input variables and output variables (Li, 2007; Baklanov, Ho and Zschech, 2012) for the CMP process are as follows:
1.6.1 Input Variables (Li, 2007)
β’ Process parameters: Down force, linear velocity, flow rate of the slurry.
β’ Slurry chemistry: Abrasives, oxidizers, complexing agents, inhibitors, surfactants, pH tuners, pH.
β’ Pad properties: Pad uniformity, conditioning, mechanical strength.
β’ Substrate properties: Wafer size, wafer layer and its strength, size to density ratio.
1.6.2 Output Variables(Li, 2007)
β’ Polishing rate
β’ Polishing efficiency
β’ Surface texture
β’ Selectivity between metals
β’ Uniformity amongst multiple wafer
β’ Surface Uniformity
β’ Dishing and erosion
A brief illustration on the major parameters such as slurry chemistry, machine parameter, and material removal rate are as follows:
β’ Slurry chemistry: The slurry comprising of abrasives and other chemical reagents such as oxidizer, complexing agent, inhibitor, surfactant etc. in DI water is the most complex component consumed in a CMP process (Zhao and Lu, 2013). The pH value of the chemical slurry along with the particle size, concentration of the chemicals and abrasives used play an important role in providing excellent local and global planarization in a CMP process. An ideal CMP slurry should have the capability to provide uniformity with good surface quality, desired removal rate, desired selectivity, less failure along with reduced corrosion. The slurry being the most complex
and the wafer/samples are the most unexplored part in a semiconductor fabrication process.
β’ Machine parameters: As mentioned above, the CMP process involves both chemical and mechanical actions. This collaboration has been the concern of numerous investigations; however, focus was mainly on the mechanical impacts as understanding the chemical mechanism was complicated. However, mechanical impacts can't alone affect the global planarity and diminish the micro scale roughness of IC fabrication as the contribution of the chemical reagents also has a prime role on it. The variations in different machine parameters are the initial adjustments made in the CMP process to acquire desirable results (Zantye, Kumar and Sikder, 2004).
β’ Removal rate: Removal rate of a particular metal to be polished is highly dependent on the slurry chemistry used in the CMP process. Different components used in a slurry have different impact on the removal rate. The oxidizer accelerates passivation on the metal surface (due to anodic shift of corrosion potential) (Ein-Eli and Starosvetsky, 2007). Complexing agents are added to form soluble compounds and to get an enhanced etch rate and a desired removal rate (Ein-Eli and Starosvetsky, 2007). Inhibitors plays an key role in controlling the corrosion damage on the metal caused due to exposure of various chemicals during CMP process (Ein-Eli and Starosvetsky, 2007). The slurry pH has a significant role as the oxidation state of the metal along with the compounds formed on its interaction with the chemical reagents are highly reliant on the pH value of the system.
β’ Selectivity: The selectivity is one of the most important criteria in formulating a CMP slurry. The ratio of removal rate of a given material as compared to the other material on using the same slurry gives the selectivity. Selectivity is important when a slurry
need to remove the materials at a time, example: the interconnect and the barrier metal or the underlying dielectric materials.