High Rate Chemical Mechanical Polishing of Boron-Doped Polycrystalline Silicon

Abstract

In this paper, the effect of lubrication on the polish rate and the surface quality of highly boron-doped polysilicon is presented. The mechanical effects of polishing were studied by altering the CMP process and lubrication regime. It was shown that increasing the polish velocity and slurry flow rate does not always increase the polish rate and that the lubrication behavior plays a dominant role in this process. Increasing the mechanical forces and altering the lubrication regime to boundary and dry lubrication were the key factors which resulted in a maximum polish rate. The maximum polish rate for the boron-doped polysilicon with 3.4 mΩ.cm−1 resistivity was ∼0.75μ/min and achieved in the dry lubrication regime. The coefficient of friction (COF) was also studied and agreed with the polish rate values where the dry lubrication polishing showed highest COF following the Stribeck curve. Finally the surface quality of the polished wafers were studied by AFM and it was shown that the lubrication behavior and friction force had minor effect on the surface roughness and all the wafers were perfectly smooth after polishing (Rq∼2Å).

Chemical mechanical polishing in the dry lubrication regime: Application to conductive polysilicon

“…This carrier concentration is ∼70% higher than the concentration in the samples in our prior work (Pirayesh and Cadien, 2014) and the polish rate is expected to be lower. As mentioned earlier, since the amount of material that has to be polished is >10 m, improving the polish rate is critical for throughput and manufacturability.…”

“…The curve has two regimes, between 80 and 90 rpm the COF increases rapidly, while at 100 rpm and above the COF increases slowly. In the boundary lubrication regime, it is expected that the COF would decrease with increasing velocity (Pirayesh and Cadien, 2014). This indicates that at low flow rates the polish is not in the boundary lubrication regime.…”

“…For moderate PRV cases, there is no dry lubrication and all the mechanical interaction occurs in the boundary lubrication regime. The results of low PRV were shown in Pirayesh and Cadien (2014) and it was suggested that polishing with low PRV occurs in the mixed or hydrodynamic lubrication regime. Finally, the combination of chemical and mechanical forces helps planarize the surface.…”

“…Liu et al (2001) theorized that the electrical interaction between semiconductor carriers (holes in boron doped films) and hydroxyl anions of the slurry solution cause this polish retardation (chemical factors of polishing). More recently, Pirayesh and Cadien (2014) studied the effect of lubrication regimes on the polish rate of boron-doped polysilicon (3.5 m cm resistivity). It was shown that in order to overcome the effects of boron dopant on the polish rate, polish conditions had to be changed so that the process always operated in the boundary lubrication regimes.…”

The Effect of Slurry Properties on the CMP Removal Rate of Boron Doped Polysilicon

“…Enhancement of polish rate can be achieved by optimizing the polish properties such as pressure, flow rate and velocity, as has been shown in our prior publications. 10,11 Polish rate can also be improved by enhancing the slurry chemistry and by understanding the basic properties of the slurry.Pietsch et al studied the influence of slurry pH and found that the maximum etch and polish rate of single crystal silicon (n-type Si (100) and (111) wafers with 150 cm resistivity) occurs at pH∼11. 8,[12][13][14] They showed that the presence of hydroxyl anions plays a major role in the chemical etching of the polysilicon surface since OH − anions weaken the Si-Si bonds by acting as a strong polarizer.…”