Electrical and structural characteristics of thin buried oxides

Nuclear Instruments and Methods in Physics Research I3 84 (1994) 270-274 North-Holland

Electrical and structural characteristics

NOMB

Beam interactions with Materials&Atoms

of thin buried oxides

L. Meda, S. Bertoni and G.F. Cerofolini IGD-EniChem,

uia Fauser 4, 1-28100 Novara, Italy

H. Gassel Frawhofer-Institut

fiir Mikroclektronische Schalungen und Systeme, Finkenstrasse 61, D-4100 Duisburg I, Germany

In this paper a continuous thin BOX obtained by oxygen implantation into silicon at 200 keV is experimentally studied; the aim of this work is the correlation of BOX structural imperfections (inclusions and pinholes) as seen by transmission electron microscopy (TEM) with the electrical characteristics measured by capacitors with scaled areas. A method for obtaining information on the distribution of the BOX silicon inclusions from the breakdown voltage is proposed. The heterogeneity of these inclusions is found to affect the current-voltage characteristics.

1. Introduction The recently demonstrated feasibility of low fluence Buried Oxides (Boxes) opens new perspectives for microelectronic applications of silicon-on-insulator (SOI), although further improvements are necessary to produce an industrially exploitable material. The possibility of preparing thin BOXes in silicon, by implanting oxygen either at low or at high energies, is a relatively recent technological achievement [l-8]. This paper is devoted to deepen the understanding of the effect of inclusions in thin BOXes on electrical characteristics, so extending a previous analysis carried out by Badenes et al. [9]. The interest in thin BOXes is not only for decreasing the material cost [7], roughly proportional to the preparation time, but also for improving SO1 quality, as threading dislocations are naturally reduced by lowering the structural stress in SO1 [lo]. However, the presence of pinholes still represents an unsolved problem. As a consequence, thin BOXes will be a viable alternative only when the loss of dielectric insulation due to pinholes is avoided. The presence of inclusions in thin BOXes affects the electrical characteristics and, though it is possible to prepare thin BOXes without any inclusion [l], their characterization is certainly useful. From the applicative point of view, the most sensitive technique for characterizing a BOX layer is to measure the current-voltage (1-k’) characteristics on different area capacitors. The aim of this work is to make a comparison between the structural

and electrical characteristics inclusions.

of a thin BOX containing

2. Ex~~mentaI Oxygen ions were implanted into (100) silicon wafers of 4 in. diameter. The ion implanter was an Eaton NV-ZOO at the Fraunhofer Institut in Duisburg. The wafer temperature was maintained around 62O”C, and the energy was 200 keV. A fluence of 5.5 x 1017 cm-’ was implanted and the samples were annealed in a furnace at 1350°C for 6 h in argon with 1% oxygen to obtain an SO1 structure with a thin BOX. TEM observations were performed with a JEOL JEM 2010, operating at 200 kV, at IGD laboratories in Novara, on both plan and cross-sectioned samples to determine the structural features of the BOX and SO1 layers. Simple capacitors using the buried oxide as dielectric, with areas ranging from lo4 to 4.2 X lo7 wrn’, have been produced by aluminum metallization and plasma etching (see fig. 1). I(V) measurements were performed with a HP4145 parameter analyzer in the temperature range 20-300°C. The I(V) curves were obtained by raising the voltage from 0 to 40 V, with steps of 0.5 V and duration of approximately 0.3 s. Breakdown voltages were found to be in the range 15-30 V, thus measurement conditions were destructive to the buried oxide capacitors. A total area of about 300 cm’ was investigated.

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L. Meda et al, / Thin buried oxides

Fig. 1. Schematic section of the capacitor built with the BOX as a dielectric layer.

Fig. 2. TEM cross-section image showing different thickness inclusions and their position within the BOX.

Fig, 3. TEM plan-view image showing inclusion dimensions. An oxide precipitate within the SOI layer is not arrowed. III. SIMOX

212

L. hleda et al. / Thin buried oxides

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measured on many capacitors with three different areas: a, = 9X lo4 pm’s; a3 = 1.7x lo6 prn2_

az 5 2.4 X lo5 pm’;

L. Meda et al. / Thin buried oxides 3. Results 3.1. Microscopic observations

The whole implantation-plus-annealing process produced a (105 k 8) nm thick BOX, with an SO1 layer of (320 k 10) nm and a surface thermal oxide of (155 f 5) nm, over all the observed specimens of the batch (fig. 2). The interface waveness is about 3 nm within a period of 1 urn. Neither SO1 threading dislocations nor BOX pinholes were observed on the TEM explored area of about 50 urn’. Silicon inclusions were revealed in the BOX (fig. 2) with thicknesses ranging between 40 and 65 nm and with random position; they have a truncated octahedral shape and are almost perfectly aligned with the silicon bulk lattice. By plan-view analysis the average number per unit area is around (3 + 2) X 10’ cm-‘, with about half of them collected in clusters of 4-8 within an area of 6 urn*, leaving oxide regions free, of inclusions of about 10 urn*. In this view, the inclusions have a square or rectangular shape with a side ranging between 50 and 130 nm (fig. 3). An average surface of 0.3% is occupied by inclusions. 3.2. Electrical characterization Typical Z(V) curves of the devices described are shown in fig. 4. Their behaviour is almost identical to capacitors with thin thermal oxides. Up to a certain voltage (TV) the current is negligible and then increases almost exponentially up to the breakdown voltage (BV). The increasing current from TV to BV can be interpreted as a field-assisted tunneling current (Fowler-Nordheim current [11,12]) which is typical for thin oxides. Among the largest area capacitors of fig. 4(c) only one show typical resistive behaviour indicative of a pinhole in the buried oxide. The ratio of the number of capacitors with this behaviour to the total number of capacitors investigated gives the yield of good capacitors. This yield as a function of the area follows an exponential curve related to a Poisson distribution. From the slope of the curve the pinhole density can be calculated and was found to be (10.7 k 2) cm-*. This value is about one order of magnitude higher than that measured for thicker buried oxides prepared in the same facility. From the Z(V) measurements it can be inferred that neither BV nor TV and tunneling current were influenced by the substrate temperature. This behaviour supports the interpretation of the existence of a tunneling current through a thin oxide in the subbreakdown-voltage regime. 4. Discussion From an analysis of the Z-v characteristics of fig. 4 the BVs appear dispersed both with capacitor size and,

273

for an assigned size, from one capacitor to another. In particular, the highest BVs correspond to the smallest area. At voltages lower than BV, anomalous currents appear to be higher the lower the related BV. This behaviour is ascribed to an effective electric field higher than the nominal one, V/tBox, where tBox is the BOX thickness. In particular, the anomalous current increasing after TV can be ascribed to a field-assisted tunneling contribution, a typical mechanism of leakage through thin oxides. A locally higher electric field is the result of the presence in the BOX of silicon inclusions that act, in a first approximation, only by reducing the dielectric thickness. As a consequence, a decrease in BV and an increased tunneling current are obtained. As BV is determined by the thinnest dielectric region, only the biggest inclusion existing in the considered area is responsible for the observed BV, so that the statistical fluctuation related to the worst combination of thick inclusions and thin BOXes is seen as BV fluctuation. In this way the variations of BV within the same capacitor area are explained. From these fluctuations useful information on BOX effective thickness distribution, 4(teff), can be extracted. A statistical analysis of the existing BV data can, in principle, identify q5(teti) provided that it is an increasing function of teff. In fact, for three assigned capacitor areas the BV distribution functions, +(BV), have been calculated and the data have been normalized to their own areas, ai, so giving three comparable curves, +( BV)/a,, as shown in fig. 5 (left scale). Moreover, by assuming a value for the intrinsic breakdown field of 8 MV/cm, the abscissa of fig. 5 can be read as a scale of tcff (upper scale), and the data so referred to a thickness interval become 4(teff) (right scale). It can be observed that the three families of data, Ai, so calculated do not overlap. In fact, the capacitor family (A,), with largest area, shows two separate peaks; the family (A,), with smallest area, shows only the higher BV peak, while the family (A,), with intermediate area, shows a mixed situation. It is plausible to assume that the envelope of the higher BV peaks defines a region of the inclusion distribution function; in going from A, to A, the right peak is reduced by one order of magnitude because of the presence of rare and bigger inclusions that prevail, so giving lower BV. The existence of two peaks in the BV distribution can tentatively be ascribed to two differently generated series of inclusions: the first due to physical causes (higher BV) and the second due to the masking effect of dust particles on the surface (lower BV). It is worthwhile noticing that the most probable BV (26 k 2 V) obtained for smaller capacitors corresponds to a BOX effective thickness (33 f 3 nm) close to that found by cross-section TEM analysis when the thinnest oxide thickness is considered (teff = 105 - 8 - 65 nm = III. SIMOX

L. Meda et al. / Thin buried oxides

274

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sisted tunneling effect). This effect only acts by decreasing the BV but maintains good insulation in a certain voltage range. In this paper, an analysis of the Z-V characteristics from different-area capacitors, built on an SO1 structure with a thin BOX (105 nm), has been carried out. The major result has been the correlation between the dispersion of the electrical characteristics and the structural properties of the BOX. In particular, the measured BV have been ascribed to the BOX effective thickness due to the biggest inclusions. The distribution function c#~W), responsible for the BV in the range 15-30 V, has been found for each investigated capacitor area and a conversion to the effective thickness distribution function, c#J(~,_~~>, has been given. This last distribution is related to the inclusion distribution. Two different mechanisms resulting in the formation of different-size silicon inclusions in the BOX and hence causing BV in two different voltage regions have been advocated. Moreover, the voltage corresponding to the maximum of the BV distribution function compares well with the minimum BOX effective thickness as measured by TEM observations, thus relating electrical and structural analyses.

48

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Fig. 5. Three families of data related to the areas of fig. 4. On the left the distribution functions 4(BV)/ai vs BV are reported, while on the right the modified distribution functions 4(teff) vs teff arc reported. Two different peaks arc evidenced corresponding to two mechanisms of producing silicon inclusions in the BOX.

32 nm). As the number of inclusions found by TEM is at least three orders of magnitude higher than that resulting by BV measurements, it can be inferred that the inclusion distribution is a very steep function.

5. Conclusions The present generation of thin BOXes has sensitively decreased the structural defectiveness of the SO1 layers. However, these thin BOXes can have a higher pinhole density than thick ones, thus losing their insulation properties. In some special cases, the insulation properties of thin BOXes are not completely lost but only strongly affected by the presence of silicon inclusions. An inadequately accurate analysis can mistake pinholes for inclusions; this can be avoided by following the Z-I, characteristics to recognise resistance behaviour from conduction through an insulator (field-as-

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