Depth Profile Analysis of Metals Gettered By Bulk Micro-Defects (BMDs) in Silicon Substrates
Authors: Saga Koichiro , Ohno Rikiichi 
Published: February 2020

As semiconductor devices continue to shrink, metallic contamination on silicon surfaces becomes to have a detrimental impact on ULSI device performance and yield [1]. Several gettering techniques have been proposed, and one of the intrinsic gettering techniques is gettering using BMDs (bulk micro defects, oxygen precipitates) in silicon. In this technique, the total surface area of the BMDs is known to affect the gettering efficiency [2, 3]. However, the depth profiles of metals gettered by the BMDs have not been reported. We have experimentally demonstrated the depth profiles of metals gettered by BMDs in silicon wafers with different densities and sizes of BMDs.

CZ-Si (100) wafers A, B, and C with different densities and sizes of BMDs were prepared by C- or N-doping and annealing. These wafers were quantitatively contaminated with Cr, Fe, and Cu. The surface concentrations were 2 × 1015 atoms/cm2. The wafers were then annealed at 1000 ºC for 30 min. The surface of the wafers was finally cleaned to remove residual metals. The BMDs in the wafers were measured by laser scattering tomography. The metal concentration at the depths from the surface to 50 μm of the wafers was analyzed using secondary ion mass spectrometry (SIMS).

50-70 nm diameter BMDs are uniformly distributed at a depth of more than 10 μm in wafer A. The deviation of the diameter of BMDs changes from 50-60 nm to 120nm at a depth of more than 13 μm in wafer B. The densities of the BMDs of wafer A and B are more than 6.2 × 109
cm-3
and 3.5 × 109
cm-3
saturated at depths of 10 μm and 13 μm, respectively. Figure 1 shows a sharp rise in the total surface areas of BMDs produced by the densities and sizes at a depth of around 10 μm and 13 μm and then the total surface area of the BMDs is saturated for wafer A, while it slightly rises with increasing depth. This means that the denuded zones of wafer A and B are 10 μm and 13 μm, respectively.

The SIMS depth profiles of the metals in the wafers show that Cr, Fe, and Cu concentrations are extremely low at the depth from the surface to 10 μm and 13 μm, while the concentrations are highest at around the depth where the total surface area of the BMDs sharply rises, and the concentrations then decrease with increasing depth, regardless of the BMD size and density profiles, as shown in Fig. 1. The critical surface area of the BMD (S
= 4πr2

NBMD

d) for effective gettering was observed to be approximately 2 x 10-2, where 4π

r2

is each BMD surface area, NBMD
is the BMD density, and d is the wafer thickness. This value agrees well with an example in the literature [2, 3]. The total surface area of the BMDs of wafer C at the depth from the surface to 50μm is less than 2 x 10-2, and no metals were detected using SIMS. Modified wafer A with a denuded zone of 2 μm instead of 10 μm also gives the peak of Cr concentration at the depth where the surface area of BMD sharply rises and the Cr concentration at the peak gives approximately a fifth of that of wafer A.

It should be noted that although the size or density of the BMDs is dispersed uniformly through the wafers or slightly increases with depth, more metals are gettered at around the depth where the total surface area of the BMDs sharply increases. It is considered that metals diffused in the deeper regions are dispersedly gettered by each BMD in deeper regions during cooling, while metals diffused in the denuded zone (DZ) are gettered by BMDs near the DZ during cooling. For effective gettering by the BMDs, sufficient total surface area of the BMDs should be close to the DZ.

 

Reference

 

[1] K. Saga, et.al, ECS Journal of Solid State Science and Technology, 4 (5) P131-P136 (2015).

[2] K. Sueoka, ECS Journal of The Electrochemical Society,
152 (10) G731-G735 (2005).

[3] D. Kot, et, al, ECS Transactions, vol.16 no.6, 207-218 (2008).

Figure 1