Non-Destructive Characterization of Porous Silicon Using X-Ray Reflectivity
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E. CHASON, T.R. GUILINGER, M.J. KELLY, T.J. HEADLEY and A.J. HOWARD
Sandia National Laboratories, Albuquerque, NM 87185 ABSTRACT Understanding the evolution of porous silicon (PS) layers at the early stages of growth is important for determining the mechanism of PS film growth and controlling the film properties.
We have used X-ray reflectivity (XRR) to determine the evolution of layer thickness and interfacial roughness during the growth of thin PS layers (< 200 nm) prepared by electrochemical
anodization. The porous layer grows at a constant rate for films as thin as 15 nm indicating a very short incubation period during which the surface may be electropolished before the PS structure
begins to form. Interface roughness measurements indicate that the top surface of the film remains relatively smooth during growth while the roughness of the PS/silicon interface increases
only slightly with film thickness. The XRR results are compared with results obtained from the same films by cross-sectional transmission electron microscopy (XTEM), atomic force microscopy (AFM) and gravimetry. INTRODUCTION
Porous silicon (PS) films have properties that make them interesting for a variety of technological applications, e.g., as insulating layers in microelectronics [1] and for micromachining applications [2]. The discovery of visible photoluminescence by Canham [3] has generated further interest because of the possibility of creating light emitting devices in Si-based materials. Although there has been significant interest in utilizing PS, there is still not a complete understanding of the porous layer formation process or the resulting microstructure. Part of the reason for this is the difficulty of characterizing the porous structure accurately. Many of the available techniques are time-consuming and sample preparation can be difficult. In this paper, we report on the use of X-ray reflectivity (XRR) to study the evolution of the PS film structure. XRR is a sensitive probe of thin film layer thickness and surface and interface roughness. The technique is non-destructive, requires minimal sample preparation and, because X-rays are highly penetrating, it is possible to study buried interfaces. We have concentrated primarily on the evolution of the PS film structure during the very early stages of film formation by measuring a series of samples prepared under identical condition for different anodization times. For comparison of the resolution of XRR with other techniques, some of the samples were also measured using gravimetry, XTEM and AFM. The porous silicon (PS) was formed by electrochemical anodization of 0.5-1.0 0)-cm, boron-doped, (100) p-Si in a dual tank electrochemical cell. Details of the design and testing of this cell are given elsewhere [4]. The PS samples were formed in 5 wt% HF at 0.58 mA/cm 2 for times ranging from 5 to 376 s. The total area of PS formed was 45.8 cm2 .
321 Mat. Res. Soc. Symp. Proc. Vol. 358 0 1995 Materials Research Society
X-RAY REFLECTIVITY (XRR) X-ray reflectivity is a technique for me
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