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sur face science ELSEVIER Applied Surface Science 92 (1996) 621 - 625

Formation of bottom oxides in porous silicon films by anodic oxidation

C.H. Lee a, C.C. Yeh a, C.H. Lee b, K.Y.J. Hsu a,. a Semiconductor Technology and Applications Research Group, Department of Electrical Engineering, National Tsing Hun University,

Hsinchu, Taiwan, R OC b Department of Materials Science, National Tsing Hua University, Hsinchu, Taiwan, ROC

Received 12 December 1994; accepted for publication 4 March 1995

Abstract

The formation of bottom oxide by electrochemical oxidation in porous silicon layers is studied. A technique of controlling the oxide layer thickness is developed. It is shown that the design of the anodization current level and of the porous silicon texture is an effective method for oxide formation control.

I. Introduction

Porous silicon (PS) was once studied for forming thick silicon dioxide layers in silicon wafers [1,2]. It has again been intensively investigated in recent years because of its interesting visible luminescence property and the potential in industrial application. The development of efficient electro-luminescent de- vices of this material is now an obvious target and, if successful, it is expected to pose great impact onto the semiconductor optoelectronics industry. There- fore, both the optical and electrical properties of porous silicon have to be thoroughly studied. It has been uneasy to study the electrical properties due to the high resistivity of porous silicon. When the properties of a thin PS layer incorporated in the active region of silicon wafers are desired, high

* Corresponding author. Fax: +886 35 715971.

leakage currents usually make the measurements im- possible. It has been shown that an anodically oxi- dized layer at the bottom region of a PS layer can effectively reduce leakage currents so that the elec- trical properties of the PS layer can be measured [3]. The formation of the bottom oxide in the PS layer by electrochemical oxidation has another potential of forming full-wafer scale silicon-on-insulator sub- trates [4]. Besides, the electrochemical oxidation is a low temperature process, which is an advantage. For the applications mentioned above, it is essential to design the thickness and the quality of bottom oxide layers as well as the residual silicon layers above. However, although the mechanism of the anodic oxidation has been studied [5], there was not sys- temic investigation on controlling the technology for applications.

In this work, the control of the formation of bottom oxide layers in PS has been studies in detail. By varying various factors such as the porosity of

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622 C.H. Lee et al. /Applied Surface Science 92 (1996) 621-625

PS, the oxidizing current during the anodization process, etc., different bottom oxide thickness can be obtained.

2. Experimental

In the experiment, a porous silicon layer was formed by standard electrochemical anodization mostly on (100), 1-3 f l . cm P- wafers. The sur- face of the wafer was exposed in the etching solution composed of 25-37.5% HF and in the dark during anodization. Back-side deposition of gold and sinter- ing ensured good ohmic contacts to the wafer. The constant current density of anodization was set to be within the range of 6 to 8 m A / c m 2, except for a few special cases, and the etching time was varied be- tween 2 and l0 min to result in a porous silicon layer (PSL) depth of 1-6 /~m and a porosity of 50-60%. Immediately following the formation of the PSL, the HF solution was replaced by 1M HCl solution and the anodization process was then continued at a current density lower than 1.6 m A / c m 2, which led to the formation of an oxide layer mainly at the bottom of the just formed PSL [3-5]. The current level utilized in this work in this work was not the best one for the purpose of oxidation control. How- ever, it was usually low enough for preventing full oxidation. The sample surface was carefully treated during the change of solutions so that it was not exposed to air thus no native oxide was grown before anodic oxidation, which is crucial for prevent-

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Fig. 2. Dependence of the exchanged charge on the HF concentra- tion of PS formation in a P - wafer. (Anodic oxidation current is 0.78 mA/cm2.)

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Fig. 1. Dependence of the exchanged charge on the anodic oxidation current in a typical P - PS layer formed in 37.5% HF solution.

Fig. 3. Bright-field STEM images of two PS samples with PS layers made at 7.8 m A / c m 2 and using different I-IF concentra- tions. (a) 37.5%, (b) 25%.

C.H. Lee et a l . / Applied Surface Science 92 (1996) 621-625 623

ing the oxidation of the whole PSL. The anodic oxidation was stopped when the monitored voltage drop between the electrodes suddenly rose up to 3 V. At this point, a continuous oxide layer at the PSL-Si interface was formed. The sample was then rinsed in deionized water (DI) water for cleaning. It is in-

evitable that a thin native oxide was grown at the surface of each silicon grain in the porous silicon region during this process, which is not harmful since the native oxide provides stabilization to the

• whole structure. To explore the techniques of controlling the hot-

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Fig. 4. Bright-field STEM image of a multi-texture PS sample with the presence of bottom oxide. Selected area electron diffraction patterns from the various regions are also shown. (The PS layer is formed at 7.8 m A / e m 2 with HF concentration varying from 37.5% to 25%, and the bottom oxide is formed at 0.78 m A / e m 2 in HCI solution.)

624 C.H. Lee et aL / Applied Surface Science 92 (1996) 621-625

the bottom oxide layer was complete. The bottom oxide became a good blockade to the vertical current path.

The resultant samples were mainly characterized by using scanning transmission electron microscopy (STEM) operating at 200 kV to identify their struc- tare parameters and quality, supplemented by other tools like Fourier transform infrared spectroscopy (FI'IR) and electrical measurements.

Fig. 5. Bright-field STEM image of a multi-texture PS sample prepared by varying the anodization current from 0.4 to 7.8 mA/cm 2 in 37.5% HF solution.

tom oxide, some samples with multi-texture silicon layers were formed prior to the anodic oxidation. The multi-texture samples were made either by changing the concentration of HF solution or by changing the anodization current during the forma- tion of PS. Furthermore, in some samples, the gal- vanostatic condition for oxidation was switched to the potentiostatic condition for a period of time after the oxidation potential reached the set value (3 V). Under this condition, the oxidizing current gradually diminished and the formation of the continuum of

3. Results and discussion

For anodic oxidation to occur, the crystalline sub- strate must provide holes to the PS layer. Thus the oxidation is most likely to occur at the bottom of the PS layer, as long as the oxidation current is not too high. Fig. 1 shows the dependence of the exchanged charge on the anodization current during the anodic oxidation process in a typical PS layer on a P- wafer. As the oxidizing current increases, the ex- changed charge also increases, which implies that a higher current results in a higher degree of oxidation. Under a higher current, because more holes are supplied to the PS layer, the oxide layer extends more from the bottom region toward the upper re- gion. Experiments of etching off the silicon layer on

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Fig. 6. FTIR spectra before and after the removal of the residual PS layer.

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C.H. Lee et a l . / Applied Surface Science 92 (1996) 621-625 625

top of the bottom oxide by NaOH solution revealed that higher oxidation currents did leave thinner sili- con layers above the oxide layers. Therefore, one can in principle adjust the oxidation current to obtain a different degree of oxidation. However, this does not guarantee that absolutely no oxidation occurs in the upper region of the PS layer. Actually, there is still some oxide located in the upper layer even when the oxidation current is low, except that the amount is less.

Fig. 2 shows the dependence of the exchanged charge of oxide formation on the HF concentration of PS formation in a P - wafer. The anodic oxidation current and the anodization current during PS forma- tion were both kept constant for this case. It is well known that a lower HF concentration results in a finer silicon texture in the PS layer which is less tree-like. Fig. 3 shows bright-field STEM images of two PS samples made by using different HF concen- trations. The difference in texture can be clearly seen. Correlated with Fig. 2, it can be suggested that the finer PS texture confines the oxidation more at the bottom region and thus the oxidation extent is less than that in the PS layers with a coarser texture. It was also found from dark-field images that the sample with the finer texture has a thinner bottom oxide layer. Therefore, tuning the texture provides another clue for controlling the position where oxida- tion takes place. By fabricating a multi-texture PS layer, the thickness of the bottom oxide could be tuned more precisely. For example, with the fine texture layer lying below a coarse texture layer, the bottom oxide formation could be concentrated in the lower layer because it would be more difficult for holes to move through this lower layer to upper layer,

The PS texture can also be adjusted by varying the anodization current during PS formation. A lower current produces finer structures. Therefore, multi- texture PS layers can also be formed for oxide formation control in this way.

Fig. 4 shows an example of controlling the bot- tom oxide thickness via texture control. It is clear that the finer texture layer is almost fully oxidized while the upper coarse layer still has silicon grains,

as seen from the diffraction patterns. Fig. 5 shows a bright-field STEM image of another multi-texture PS sample prepared by varying the anodization current. A clear layered structure is observed, which shows the feasibility of controlling the texture by control- ling the anodization current.

The existence of bottom oxide in the samples was confirmed by observing the FTIR spectra, as demon- strated in Fig. 6. In addition to electron diffraction patterns, electrical measurements of the vertical cur- rent showed that the bottom oxide can sustain volt- ages as high as several volts. Further densification of the oxide could be obtained by thermal treatment. To retain the structural stability of the upper porous silicon layer, rapid thermal annealing should be a preferred choice.

In summary, the technique of controlling the for- mation of bottom oxide in porous silicon layers is explored in this work. The current change during anodic oxidation and the texture design of the PS layer facilitate the control of the oxide thickness. The bottom oxide provides isolation to the upper PS layer and has the potential for other applications like full-wafer SOI fabrication.

Acknowledgements

The authors would like to thank Professor B.H. Tseng and Mr. G.L. Gu at the Sun Yat-Sen Univer- sity, and Professor T.R. Yew and Mr. K.L. Chen at the Tsing Hua University for the assistance in TEM experiments. The financial support from the National Science Council of ROC is acknowledged.

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