Alkali-induced oxidation of silicon

Surface Science 189/190 (1987) 245-251 North-Holland, Amsterdam

245

ALKALI-INDUCED OXIDATION OF SILICON E.G. MICHEL, E.M. OELLIG, M.C. ASENSIO * and R. MIRANDA Departamento de Fisica de la Materia Condensada C-HI, Facultad de Ciencias, Universidad Aut6noma de Madrid, Cantoblanco, 28049 Madrid, Spain Received 31 March 1987; accepted for publication 16 April 1987

Photoelectron and high resolution Auger spectroscopy indicate that multilayers of K deposited on Si(100)2 x 1 promote the oxidation of Si under subsequent exposure to 02. The oxygen uptake is enhanced by four orders of magnitude. The alkali can be completely desorbed from the surface at 900 K leaving behind a layer of SiO 2, whose thickness is proportional to the amount of K deposited. The process can be further repeated. We propose a mechanism for this promoted oxidation supported by spectroscopic evidence.

The oxidation of Si is a process of enormous technological relevance in the field of microelectronics. Thus, the 300 A thick gate oxide in MOS devices is formed by thermal oxidation of single-crystal Si wafers. The oxidation process is usually carried out at elevated temperatures ( - 1300 K) and high pressures of O 2 (1 atm) because of the difficulty in producing a thick-enough homogeneous film of SiO 2 [1]. Recently, proposed gate oxide dimensions have shrunk to 50-100 ,~ in order to achieve superscaled MOS devices [2]. Therefore it is highly desirable to oxidize Si at lower temperatures to avoid the enhanced diffusion of dopants and metal contacts taking place during high-temperature processing of devices. This could be achieved by means of a catalyst. Previous attempts to find a "catalyst" for Si oxidation involved Au [3], Ag [4], Cu, Pd [5], Cr [6] and Ce overlayers [7]. All of them increase the Si oxidation rate when deposited on the Si surface. Unfortunately, these metals and rare earths react rather strongly with Si and form stable compounds (e.g. cerium silicates [7]) upon oxidation which are hard to remove from the surface. Therefore they do not act as real catalysts. Very recently an enhancement in the oxygen uptake has been also found for monolayer amounts of Cs, Na [8,9] and K [10] on Si surfaces. In this case, the "catalyst" can be easily removed for the surface by mild annealing to - 900 K. However, in refs. [3,9] the amount of oxide produced does not exceed 2-3 layers. This fact suggests that the * Permanent address: Instituto de Investigaciones Fisicoquimicas Te6ricas y Aplicadas, Universidad Nacional de La Plata, Sucursal 4, Casilla de Correo 16, 1900 La Plata, Argentina.

0039-6028/87/$03.50 9 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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E.G. Michel et al. / Alkali-induced oxidation of Si

behaviour of these metal overlayers is not strictly catalytic because a true catalyst is renewable, i.e. it should produce reaction products in quantities exceeding many times the stoichiometry. A number of models have been proposed to explain the enhancement in the oxygen adsorption rate by these metal overlayers [3-9]. Most of them assume that the role of the "catalyst" is to facilitate the dissociation of 02 molecules. With this idea in mind only metal coverages up to one monolayer (ML) have been used [3-9]. In the work at hand we will show that oxides with a thickness useful for device applications can be obtained by depositing multilayers of K on a Si(100) surface, exposing the sample to oxygen and annealing to 900 K. We will provide experimental evidence that the so-called catalytic effect is related to the thermal decomposition of three-dimensional potassium oxides. The thickness of the silicon dioxide produced is strictly proportional to the amount of K predeposited on the Si surface. The experiments have been carried out in two UHV chambers. The first chamber allows us to study the sample using energy electron diffraction (LEED), Auger spectroscopy (AES), and Kelvin probe (A4~) facilities. In the other chamber photoelectron spectroscopy (XPS, UPS) experiments have been carried out. Extreme care has to be taken in order to ensure a water-free residual atmosphere, for water adsorption in alkali metals can strongly influence oxygen adsorption. K was evaporated from a commercial dispenser (SAES Getter) onto Si(100)2 x 1 surfaces (50 ~2 cm, n-type). The amount of K deposited and the oxygen uptake were calibrated from their peak-to-peak intensity and from the area of the K 2p or O ls levels in AES and XPS, respectively. Further details on equipment and experimental procedures will be published elsewhere [10]. Here we will concentrate on photoemission results for multilayer adsorption. It has been shown that K grows layer by layer on Si(100)2 x 1 [11]. One monolayer of K is defined as 3.39 x 10 ~4 a t o m s / c m 2, i.e., 1 K atom per 2 Si atoms in the (100) Si surface. The work function decreases by 3.0 eV at completion of the first layer. Multilayer growth results in a 0.2 V increase in the work function. The XPS spectra of a thick, metallic film of K taken at 110 K compare well with previous data by Bonzel et al. [12] and Petersson and Karlsson [13]. In particular, strong plasma losses at 3.8 eV are observed in the K 2p and Si 2p core levels. The UPS (He I) spectrum shows a M23VV Auger transition at 3.8 eV below the Fermi level (EF) and two plasma losses 6 and 10 eV below E F. The Auger transition is due to a hole in the K 3p core level at 18.7 eV and appears as a rise in intensity from 2.5 eV below E F when photons of 21:2 eV are used. Oxygen adsorption at 110 K on multilayer K films results in the formation of a three-dimensional compounds of K and O. Its electronic structure at 110 K and the results of thermal processing are shown in fig. 1 and 2. Fig. 1 depicts the angle-integrated UPS spectra of the valence band region. Fig. 2

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BINDING ENERGY (eV) Fig. 1. UPS photoelectron spectra of a Si(100)2x1 surface covered with 13.8 ML of K and exposed to 104 L of oxygen. The spectra depict the evolution of the valence band region upon thermal annealing at the indicated temperature.

displays the corresponding XPS O ls and Si 2p core levels. In this particular case the K coverage is 13.8 ML and the oxygen exposure 104 L (1 L = 10 -6 Torr s). The plasma losses of the K film disappear in the early stages of oxygen exposure indicating volumic oxidation of the metallic film. The K 2p core line does not shift noticeably upon oxidation. The area below the oxidised K 2p line decrease by - 25% with respect to the clean spectrum due to a smaller photoelectron mean free path in potassium oxides [13]. The O l s shows a doublet with peaks at 531.6 and 535.1 eV. The growth of the peak at 531.6 mimics the disappearance of the plasmons. Therefore it reflects the formation of a volumic potassium oxide. The most important information for the Si oxidation process is the unchanged lineshape of the Si 2p level that indicates that substantial oxidation of the Si substrate has not yet taken place. Therefore, we conclude that the majority of the adsorbed oxygen at 100 K is bonded to K and not to Si. The formation of a K - O volumic compound with well-defined stoichiometry is further supported by studies of the oxygen uptake versus oxygen exposure as a function of K coverage [10] where it has

E.G. Michel et al. / Alkali-induced oxidation of Si

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been shown that the oxygen uptake at saturation is linear with the K coverage. A unique identification of the K - O compound formed by sequential adsorption at 110 K is difficult. A number of bulk potassium oxides (KO2, K 202, K 203, K 20) are known to exist but photoelectron spectra for identified species have been reported only for K 2 0 [13]. K 2 0 is supposed to be characterised by an O ls peak at 528.6 eV. The UPS data in fig. 1 show five well resolved features for the spectrum at 110 K. Their energy position is 4.0, 5.9, 7.2, 10.0 and 12.0 eV below E F. Tentatively one might attribute the peak at 4.0 eV to the K Auger transition. In order to assign the remaining four peaks it should be noted that four peaks in this energy region are characteristic of molecular oxygen either in the gas phase or as 02 containing species. They correspond to the l~rg, 1% and 3Og orbitals, the multiplet splitting of the latter giving rise to two peaks. Experi-

E.G. Michel et al. / Alkali-induced oxidation of Si

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mental data for gas-phase O z [14] and 02 condensed below 30 K on AI and Ga [15] indicate an energetic separation between the four peaks of 4.4, 1.5 and 2.0 eV clearly different from ours. O z containing molecules, such as HzOz, also show four peaks in UPS when measured in the gas phase, but now the energetic separation is 1.0, 2.6 and 2.1 eV [16] almost identical to our values of 1.3 and 2.8 and 2.1 eV. Ab initio calculations for LiOzH and LizO 2 [17] also suggest that increased charge transfer to the O2 molecule results in a relative shift in energy of the O - O orbitals in the sense of our observations. Four peaks have also been found with He I by Wijers et al. [18] for 10 L exposure of 02 on bulk Cs, K and Rb samples and were interpreted as reflecting the formation of Oz-containing species. We believe that our data could be interpreted as corresponding to the formation of a potassium oxide of the type Me~ + O2a-. Upon thermal treatment the K - O compound decomposes a n d / o r reacts with Si transferring oxygen to the Si substrate and resulting in an efficient oxidation of the Si crystal. The annealing was performed by heating the sample to a given temperature for a fixed period of time (3 min), followed by cooling to 110 K in order to freeze intermediate species. The spectra were always taken at 110 K. The oxidation process is reflected in the Si 2p spectra of fig. 2 where intensity begins to grow at the higher binding energy side until a peak chemically shifted 4.5 eV is clearly resolved. A complex evolution is observed in both O ls and UPS spectra. A detailed identification of the intermediate species is not essential at this stage because the details of the thermal evolution differ from sequential deposition to coadsorption and even for coadsorption experiments the species formed depend on the experimental parameters, e.g. oxygen pressure. However, some of the products of the decomposition can be identified. Thus, at 300 K the O ls spectrum shows the appearance of a peak at a BE of 528.6 eV, that was identified as KaO by Petersson and Karlsson [13]. The UPS indicates three peaks at 3.7, 6.6 and 11.1 eV below E v. The formation of KzO indicates that the 02 containing species has partially decomposed at 300 K, e.g. some of the O=-O bonds are broken at 300 K. This is supported by the corresponding UPS spectrum (see fig. 1) which also indicates that KzO is mainly concentrated at the surface of the potassium oxide film. Further heating to 400 K results in the disappearance of the KzO species and the initiation of the silicon oxidation (see fig. 2). During the reaction process, K is desorbed from the surface, its amount decreasing almost linearly with increasing T. At 900 K no signal of potassium is detected. The final oxide is characterised by an UPS spectrum with three peaks at 7.6, 11.5 and 15.0 eV below Ev, a single O ls level at 532.9 eV and a chemically shifted Si 2p level. The UPS is in agreement with previous data for SiO 2 [19]. The value of the Si 2p chemical shift is 4.5 eV, in agreement with bulk SiO2 [20].

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The thickness of the oxide produced after the process described above can be evaluated by using standard techniques. The ratio of the areas Si 2p oxide/Si 2p substrate when plotted as a function of the Si 2p substrate attenuation follows an universal curve [19]. Thus, it can be translated into oxide thickness (in units of the inelastic mean free path). One needs only to assume a value for the inelastic mean free path, ~, in order to obtain the thickness of the oxide in ]k. As a rule of the thumb of ratio Si 2p oxide/Si 2p substrate = 1 (0.3) corresponds to an oxide thickness d = 0.8 (0.35)h. Fig. 3 shows the thickness of the resulting oxides as a function of the porevious K coverage. The thickness scale has been obtained by taking ~ = 30 A [19]. In view of the uncertainties involved in the determination of ~, absolute values of the oxide thickness are only accurate within 30%. The SiO 2 thickness is l i n e a r with the K coverage suggesting that the alkali film is not acting as a true catalyst able to produce reaction products, i.e. oxides in quantities much larger than the number of catalyst atoms. On the contrary, these data strongly indicate that the production of SiO 2 in strict proportionality to the alkali coverage is the result of formation and decomposition of a potassium oxide. The role of the alkali film is to capture very efficiently 0 2 molecules from the gas phase, ensuring, through their transfer to Si, a massive presence of oxygen at the Si interface. This conclusion is reinforced by coadsorption experiments (evaporation of K onto Si in 5 • 10 -5 Torr of 02)

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that indicate that the maximum efficiency of the overall oxidation process is achieved for coadsorption at 500 K. In this way, oxides of thickness - 30 A, potentially useful for superscaled MOS devices have been grown [10]. We acknowledge the experimental aid of E. Conejero and J. Ferrrn. We are indebted to S. Ferrer, E. Artacho. E. Martinez and Frlix Yndurfiin for illuminating discussions. Financial support of this work by the US-Spain Joint Committee and the CAICyT through grants CC84/063 and 0.387-84 respectively is gratefully acknowledged. One of us (M.C.A.) thanks the Consejo Nacional de Investigaciones Cientificas y Trcnicas de la Repfiblica Argentina for a fellowship.

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