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The complex refractive index of SiGe alloys at 632.8 nm has been measured as a function of the Ge concentration using in situ ellipsometry while the material is slowly removed from a silicon substrate using reactive-ion etching (RIE). Homogeneous, strained epitaxial SiGe films on silicon substrates were used. The Ge concentration was obtained by Rutherford backscattering. If an unknown SiGe structure is etched with RIE, in situ ellipsometry yields combinations of the ellipsometric angles ¿ and ¿ with time. Starting at the Si substrate, these points are, on a point-to-point basis, converted into combinations of complex refractive index and depth in a numerical procedure. For this inversion of the ellipsometry equations, the known relation between the real and the imaginary part of the refractive index of SiGe is used. Finally the refractive indices are converted into Ge concentrations. Thus the depth profile of the Ge concentration in an unknown epitaxial SiGe structure can be inferred from an in situ ellipsometric measurement during RIE of the unknown structure. The obtained resolutions in depth and Ge concentration are 0.3 nm and 0.3%, respectively.

Reactive‐ion etching (RIE) of epitaxial, strained Si1−xGex alloys, x≤0.20, in fluorine‐chlorine‐, and bromine‐based low‐pressure plasmas has been investigated. The reactive‐ion etch rates of the Si1−xGex alloys increase with the Ge content of the alloy for most etching gases. This effect is most pronounced for fluorine (CF4 and SF6) plasmas where the etch rate of a Si80Ge20 alloy is increased by a factor of ≂2 over that of Si. The etch rate enhancement is of reduced importance for chlorine (CF2Cl2) and bromine (HBr) plasmas, where the etch rate increases by less than 50% for a Si80Ge20 alloy relative to that of elemental Si. Analysis shows that the etch rate increase is not accounted for by the greater rate of gasification of Ge atoms alone but that the presence of Ge atoms in the SiGe alloy increases the Si etch product formation rate. Directional SiGe profiles are observed for CF2Cl2 and HBr plasmas which are identical to those obtained with Si. After etching in CF4 or SF6 plasmas the Ge surface concent...

The complex refractive index at a wavelength of 632.8 nm of strained epitaxial SiGe layers on silicon substrates has been determined as a function of the germanium content using in situ ellipsometry during reactive ion etching. The germanium concentration was obtained from Rutherford backscattering. These index values are used to invert the ellipsometry equations. Using this principle, the Ge concentration depth profile of an unknown SiGe structure can be determined from an in situ ellipsometry measurement sequence that is taken while the unknown sample is being etched

Reactive ion etching of SiGe alloys is desirable for pattern transfer in the fabrication of heterojunction bipolar transistors using a SiGe base region. Such devices are of rapidly increasing technological importance, as it has been demonstrated that they may be used at speeds previously unattainable by Si-based devices. * Of particular technolog- ical importance are strained epitaxial SiGe alloys with a proportion of Ge of 20% or less.' Here, the reactive ion etching of SiGe, Si, and Ge in HBr plasmas has been stud- ied. HBr etching plasmas have previously been examined for Si etching,3-5 and offer anisotropic etching which may be very selective with respect to an oxide mask. Another attractive feature of HBr etching processes on Si is the very thin reaction layer which is formed-about one monolayer thick.5*6 ing reactor by the atmosphere. Patterned SiGe, Si, and Ge samples employing photoresist or oxide masks were used to examine the profile obtained using HBr RIE. Figure 1 shows reactive ion etch rates of SiGe alloy and elemental Si and Ge films as a function of the Ge content of the layer. The etch rate of the SiGe alloys is seen to increase as the percentage of Ge in the alloy is raised. The etch rate increase is consistent with the -3 times greater etch rate of elemental Ge as compared to that of Si. The inset shows the SiGe/Si etch rate ratio which is achieved under these conditions as a function of Ge con- tent, It is seen that for 15% Ge in the SiGe alloy the etch rate is -40% greater than that of Si. Etching of SiGe alloys under these conditions is highly directional, and near-vertical sidewalls are formed. This is demonstrated by the scanning electron micrograph shown in Fig. 2 which displays a shallow trench etched through a 800-nm-thick Sis,GetS alloy film into the Si substrate. An oxide mask (which was removed prior to taking the scanning electron microscopy (SEM) image) and a gas mixture of 98 seem HBr/2 seem CF4 were employed for this experiment. A near-vertical etch profile all the way from the top of the SiGe into the Si substrate is obtained, and no difference in directionality between the SissGe,, alloy and the Si sub- strate is noticeable. The width of the SiGe base layer of heterojunction bipolar transistors is of the order of only a few tens of nanometers and a precisely controlled Ge profile is re- quired for optimum electrical performance.' If, as a result of reactive ion etching significant enrichment or depletion of Ge at the surface occurred, the device performance could be degraded. The bromides of Ge are less volatile than those of Silo and therefore a Ge-rich reaction layer on the surface of the dry-etched SiGe surface may be ex- pected. The composition and extent of the surface reaction layer formed as a result of HBr RIE was examined using x-ray photoelectron spectroscopy (XPS ) . Blanket (unpat- terned) SiGe films etched for 5 min were used for the XPS studies. Immediately after etching they were transferred from the etching chamber to the analysis chamber (at a pressure of about 10 - lo Torr) using a vacuum transfer mechanism. XPS measurements were made using a Vac- uum Generators ESCALAB MkII system, with a non- monochromatized Al Ko x-ray source. Measurements were taken at two different electron emission angles: Nor

rates of single;crystal Si and Gem are shown. The etch rates of the SiGe alloys increase with the Ge content for each etching gas which is qualitatively consistent with the greater etch rate of Ge as compared to Si. The dashed lines which connect the SiGe alloy nata with the elemental Ge etch rate data are intended to help in the comparison of SiGe and Ge-etch rates, but it is not implied that the actual etch rates would show a depen-m dence on Ge content given by the dashed lines. The SiGe etch rate shows the strong@ increase with Ge content for the CF, plasma. Analysis of profiles etched in photoresist- or oxidemasked SiGe films has shown that mask undercutting takes place for fluorine-rich discharges, e.g., SF,. No mask undercutting was observed with CF&& and HBr plasmas. These etch profile characteristics indicate that fijr Si, -t;GeX alloys F-based etching is ion assisted with a strong chemical etching component; but that Cl- and Brbased etching require ion bombardment. The behavior of Sil -XGeX alloys, xQl.20, is therefore in this respect the same as that of Si.’ A more detailed discussion of the processing parameters and-etch profile results will