Small Signal AC-Surface Photovoltage Technique for Non-Contact Monitoring of Near Surface Doping and Recombination-Gener
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225 Mat. Res. Soc. Symp. Proc. Vol. 591 02000 Materials Research Society
In 80's and 90's Kamieniecki [2] developed practical diagnostics methodology and corresponding wafer-scale diagnostics tools based on measurements of ac-SPV signal versus a DC bias applied to the pick-up electrode. The importance of interface traps in ac-SPV measurements has already been reported by Munakata and co-workers [3]. The present work further quantifies this effect. PHYSICAL MODEL A physical model of the Si/SiO 2 structure is shown in Figure 1. During the light on - light off cycle there is corresponding shrinking and expanding of the depletion layer width. A kinetic of the process is controlled by supply and removal of minority carriers, i.e. generation-recombination processes shown schematically on Figure 1 [4]. These mechanisms are: a) generationrecombination in the space charge region, b) carrier transport from surface inversion layer surrounding a test site, c) carrier diffusion from or into the substrate and d) carrier generation-
recombination by interface traps. From these four processes the first one - carrier generationrecombination in the space charge region - is the unique one because it provides information on generation-recombination centers located in the depletion layer. On the other hand the mechanisms (b) through (d) can be considered as parasitic processes, potentially interfering with the near surface doping measurements. In this paper we consider the effect of interface traps (process d) and the generationrecombination in the space charge region (process a). The corresponding equivalent circuit used in the numerical calculation is shown on Figure 2. Notations in that figure are listed in Table I. For low interface trap density, Cit --+ 0, the SPV at high light modulation frequencies has the simplified form: const , leff
VSV= 0)'CD()
From this equation the near surface doping is calculated as follows [5]: NAD
2 q= 0 qe
o
2
=(Vs,
( const .Ieff
__2___(2
w cV
)2
VsB and VsPv are the quantities determined from the experiment, while light modulation frequency and intensity are the preset variables. Probe Area - 6mm
E10, sin,
S'
}
surfs= Lawe
Fcui
utp
Figure 1. Processes contributing to minority carrier supply/removal to/from the depletion layer.
226
|
|
L-
A,
Space Charge
L
VNVR
)....J
RO _,
Wafer
TChuck
Figure 2. Equivalent circuit and corresponding wafer configuration. CB - wafer-chuck back side coupling capacitance; Cp - wafer-SPV pick-up electrode capacitance; RB - substrate resistance; Ispv - photogenerated ac-current in space charge region. The effect of interface traps on SPV signal depends on the density of interface traps, D5 '=Ci/q, and the time constant, 'r1 =Ct &.i.The SPV frequency characteristics calculated using Dit and Th as parameters are given in Fig.3 and Fig.4. The parameters of calculation are NA = 10' cm" 3; TR = 25 ms and Tit = 0.1ms for Fig.3; and CR = 5 ms and Dl = 2E12 q eV'1 cm"2 for Figure 4. 1) Density of interface traps, Di,. The ac-SPV frequency dispersi
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