Metrology of Very Thin Silicon Epitaxial Films using Spectroscopic Ellipsometry
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the current measurement capability into the sub-0.5 jim regime. EXPERIMENT
Undoped epitaxial films of various thickness were CVD grown on heavily boron or arsenic doped substrates at 700 - 900'C using commercial ASM reactors. For comparison and process monitoring, a few epitaxial films were also grown on lightly doped substrates under exactly the same deposition conditions. The idea of using low deposition temperatures is to minimize dopant depletion, outdiffusion and autodoping so that a sharp dopant profile across the metallurgical junction between the undoped epitaxial film and the heavily doped substrate can be obtained. Cross-sectional transmission electron microscopy (XTEM) and SIMS were used to characterize some of the samples. Ellipsometric measurements were performed with a UVISEL spectroscopic phase modulated ellipsometer (SPME) made by Instrument SA, Inc. This ellipsometer is equipped with near infrared (NIR) extension and covers the spectral range 230 - 1700 nm. SPME measures the quantities I and I: [3] 1,
=
sin2osinA
(1)
I,
= sin2ocosA
(2)
The standard ellipsometry parameters 0 and A are defined by: =p-= tanoe'i•
(3)
where rp and r, are the Fresnel reflection coefficients for light polarized parallel and perpendicular to the plane of incidence, respectively. Silicon is semi-transparent below the direct band edge, resulting in A being near 180' or 00. Thus, for this particular application, SPME is superior to rotating analyzer ellipsometer (RAE) which measures the quantities tano and cosAŽ, but not sinA [4]., DATA ANALYSIS The model of the sample structure used in this study consists of air, native oxide, undoped silicon epitaxial film, and heavily doped silicon substrate. The dielectric function of heavily doped silicon is modeled with the Drude theory: 2 /m* ) (W) - 4xNe w2 + iw/
(4) 4)
where w is the photon angular frequency; E, is the dielectric function of undoped crystalline silicon; N is the free carrier concentration; m* is the free carrier effective mass, which is taken as 0.26m 0 for electrons and 0.36m 0 for holes, m 0 is the free electron mass; e is the electron charge; and r-is the free carrier relaxation time. Optical constants of silicon dioxide are taken from ref. 5. Optical constants of undoped crystalline silicon are taken from ref. 5 for photon energies below 1.5 eV, and from ref. 6 for photon energies above 1.5 eV. For photon energies above 0.7 eV, the dielectric function of heavily doped silicon is insensitive to the free carrier relaxation time T, thus, a value of 1.45x 10-14 s is used in the calculation. This value is calculated with a boron concentration of 1019 cm-3 using the formula given in ref. 7. The native oxide thickness, epitaxial film 494
1020
10 19
DuIu11
x
1018 0 -
17
o
18
10
0
Caritn
10
10L15
0.0
0.1
0.2
0.3
0.4
Depth (4~m)
Figure 1: SIMS profiles of an undoped epitaxial film grown at 900'C on a heavily boron doped substrate. thickness, and substrate free carrier concentration are then extracted from the measuredth and A spectra throu
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