PCA Characterization of Residual Subsurface Damage After Silicon Wafer Mirror Polishing and Its Removal
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Mat. Res. Soc. Symp. Proc. Vol. 566 ©2000 Materials Research Society
PCA TECHNIQUE The ultra violet laser-pulse having such a short pulsewidth Tw as ins and such a short wavelength as 337.1nm irradiates the silicon surface to excite excess carriers as shown in the bottom left figure of Fig. 1.The generated carriers are confined within the subsurface since extremely short penetration depth of 200A. An amplitude (or intensity) of the photoconductivity
25 20 :15 (U
0
10 5 00.001
01
IW
W1
0.01
0.1
Tb1
(Is)
#I" A
1
10
Fig. 1 Calculated PCA as a function of Tb 1. Two layer model for calculation (Bottom left). Definition of PCA (Bottom right).
Fig. 2 UV/millimeter wave PCD maesurement system.
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transient induced is defined as a photocnductivity amplitude (PCA) as shown in bottom right drawing in the figure. Therefore, PCA detects carrier recombination interacting with imperfection within the subsurface. Using a two-layer wafer model consists of both subsurface and bulk layers as shown in inset of Fig. 1, calculated PCA signals are shown in Fig. 1 as a function of lifetime thi within a subsurface depth wl of 1 jtm which stands
for imperfection in the subsurface, under parameters of bulk lifetime Trb(500 [ts) and surface recombination So (104 cm/s). PCA is responsible to tb1 less than round of 0.1 [Ls. The measurement system employed to observe PCA signals is shown in Fig. 2. The N2 laser with a pulse width of ins and a spot size of 3x5 mm 2 irradiated the surface of the silicon wafer sample to generate excess carriers. The sample placed apart from the aperture of the WG-10 waveguide by several millimeter was irradiated by the mm-wave of 100 GHz. PCA signals as photoconductivity variation detected with the diode from the reflected mm-wave were observed directly using the fast digitalizing oscilloscope (Lecroy, 6120) with 5G-sampling and 350 MHz bandwidth. EXPERIMENTAL
10
3, 5,9 times S
8
cleaning
A
Oc-cennfcm Once SC1 cleaning
> -
No SC cleaning
•44 2•
,10
SPolished 0
0
100
CZ wafer (PW) , 500 400 300 200 Time (ns)
Fig. 3 PCA signals measured through 9 times SC1 cleaning of CZ wafers polished,
Boron (B)-doped (100) plane p-type CZ-wafers (PW) with 10 in resistivity and 150 mm in a diameter were chemomechanicaly polished in commercial use. A n-type (100) plane epitaxial wafer (EW) with Q2 cm and 150 mm was also prepared as the reference sample. Both wafers were treated with 9 times SC1 and SC2 cleaning. The respective cleaning was made for 6 minutes at 80'c using the SC1 solution with the composition of NH 4 OH:H202: H20=1:1:8.
10
The removal depth was about 70 _n per once SC1 cleaning. The EN samples with 1. 3. 5. 9 times SC1 cleaning through 9 "-a 6"C times cleaning were employed for experiments in this study. "M 4•the Gate oxide integrity (GOI) was evaluated for MOS diodes with a a. 2 phosphorous doped polysilicon "Epitaxialwafer with an area of 16 gate-electrode 1 0 mm 2 , and an oxide thickness of 400 500 100 200 300 0 100 A produced by the wetTime (ns) oxygen oxidation at 8
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