Thin Solid Films, 81 (1981) 271-278
PREPARATION AND CHARACTERIZATION
A STUDY OF THE STRUCTURAL PROPERTIES OF ANODIZED ALUMINIUM FILMS K. S. CHARI* AND B. MATHUR
Department of Electrical Engineering, Indian Institute of Technology, Delhi, New Delhi (lndia) (Received September 16, 1980; accepted March 9, 1981)
The effects of the anodization conditions and of heat treatment on the structure of anodized aluminium films 30-500 nm thick were investigated by electron microscopy and electron diffraction techniques. These studies were carried out on films grown in a tartaric acid-ethylene glycol bath at constant current densities in the range 0.25-10 mA cm -2 and heat treated to temperatures in the range 300900 °C. The anodic films grown with normal current densities at or above 1 mA cm -2 and to thicknesses below 180 nm were found to be amorphous while thicker films consisted of mixed amorphous and polycrystalline 7-alumina phases. The films deposited at or below 0.25 mA cm- 2 were randomly oriented f.c.c. ~,'-alumina. The structure of the as-grown films underwent a transition from amorphous to randomly oriented polycrystalline 7'-alumina when they were heat treated to temperatures above 800 °C. The electrical performance characteristics of these films were investigated.
In the past few years there has been considerable interest in the use of anodized aluminium films in various device applications. Partially anodized aluminium films are used in coplanar charge-controlled devices (CCDs) and MOS field effect transistors (MOSFETs) 1'2 and as insulation in multilevel large scale integrated circuits 3 while completely anodized aluminium films are used in two-phase CCDs and MAOS devices 4' 5. In all these applications it is imperative to have high quality anodic A1203 films. In practice, anodic A120 3 films can be grown in a variety of aqueous and non-aqueous electrolyte baths. However, recent studies6'7 indicate that oxides grown in a tartaric acid-glycol bath are of superior quality compared with those grown in conventional electrolytes. In view of this the glycol bath oxides have the potential of yielding devices with better performance characteristics. The growth and electrical properties of these glycol bath oxides have been reported by Haden e t al. 6 and more recently by Chari s. However, no study of the structural properties of these films has yet been reported. In this paper the dependence of the * Present address: Department of Electronics, Government of India, New Delhi 110 003, India. 0040-6090/81/0000-0000/$02.50
© Elsevier Sequoia/Printed in The Netherlands
K.S. CHARI, B. MATHUR
oxide structure on the growth parameters, the thickness and the heat treatment are presented. During these studies the conditions necessary to obtain high quality anodized aluminium films were identified. 2. EXPERIMENTAL PROCEDURE Thin films of aluminium about 600 nm thick were prepared for anodization by vacuum deposition at a pressure of about 1 x 10 -s Torr onto oxidized silicon substrates. The deposited aluminium films were then oxidized in the electrolyte by polarizing them positively with respect to a platinum counterelectrode. The electrolyte bath consisted of 3% tartaric acid (pH, 6.0) and ethylene glycol in the ratio 1 : 2. All the specimens were partially oxidized at 25 °C and at a constant current density for various intervals of time. Current densities in the range of 0.2-10 mA era-2 were used in this study. During the growth runs the potential between the aluminium and platinum electrodes, which is referred to as forming voltage, was recorded and the growth process was discontinued when a predetermined forming voltage was attained: These forming voltages were subsequently calibrated and were used to monitor the oxide thickness during growth. These estimated thicknesses were later compared with the values obtained from the Talystep measurements. The agreement between the two methods was within - 3 nm. The as-grown films were also subjected to heat treatments over a temperature range 300-900 °C in a nitrogen ambient to determine whether these produced any structural changes. Films prepared under different conditions were studied by electron diffraction and transmission microscopy techniques using an AEI EM 802 electron microscope. In these studies, the A I 2 0 3 films were stripped from aluminium substrates by the well-known mercuric chloride method 9. For ease of stripping, part of the aluminium substrate layer near the edges was protected from the anodi~ation process. The protected substrate aluminium regions later served as the base for the mercuric chloride attack. The floated films were then picked up onto 200 mesh copper grids after a thorough rinse with distilled water. To facilitate viewing under the microscope, the thicker films were thinned in one of two types of thinning solutions depending on whether they were crystalline or amorphous. A mixture of 35 cm 3 of 85% phosphoric acid and 20 g of chromic acid per litre of solution was used at 85 °C for thinning the amorphous films. The etch rate of the oxide at this temperature was determined to be 2.7 A s- 1 and on the basis of this value the required thickness of the films was obtained prior to floating. Similarly the crystalline oxide films were thinned in dilute (109/o) phosphoric acid solution. During the thinning operation care was taken to protect the exposed aluminium regions from the acid attack. From the observed d spacings the structure of each analysed specimen was determined. Thin films of gold were used as a reference in determining the camera constant for the electron diffraction investigations. 3. RESULTS AND DISCUSSION
3.1. As.grown films On examination the anodic A12Os films showed three distinct phases: (1) amorphous, (2) crystalline and (3) mixed. The observed structure depended on the growth conditions and the thickness. These results are illustrated in Figs. 1-3.
STRUCTURAL PROPERTIES OF ANODIZED
Fig, 1. An electron diffraction pattern of a typical as-grown anodic AI203 film 100 nm thick (Jg = 1 mA cm-2). Halos in the pattern indicate the amorphous nature of the film.
Fig. 2. An electron micrograph and a diffraction pattern of a mixed phase A1203 film of thickness 200 nm (J~ = 1 mA cm -2).
Anodic A 1 2 0 3 films of thickness 30-180 nm showed electron diffraction patterns of the type depicted in Fig. 1. The presence of halos in the diffraction patterns of these oxides indicates that they are amorphous. These observations are in agreement with those reported by Hass 1° for anodic A120 3 films grown in an aqueous tartaric acid bath. These oxides usually exhibit a smooth surface with a grain size of 3-10 rim. During the thinning operation it was also observed that these thin amorphous oxide films dissolve completely in the mixture of chromic and phosphoric acids. In contrast, the films of intermediate thickness gave diffraction patterns which indicated the presence of crystalline as well as amorphous phases. Figs. 2(a) and 2(b) show representative examples of the electron micrographs and the diffraction patterns obtained for the 200 nm mixed phase films. The presefice of discrete rings as well as broad halos in the diffraction pattern clearly establishes the mixed nature of these films. It was observed that these films do not completely dissolve during thinning in the mixture of chromic and phosphoric acids. Since the amorphous phase was found to be readily soluble in this acid mixture, this means that the insoluble parts of these films are crystalline. Further studies showed that the undissolved portion of the oxide increased significantly with the film thickness. This
K.S. CHARI, B. MATHUR
indicates that the amount of the crystalline oxide increases with thickness. The diffraction patterns in Fig. 3 indicate this sequence.
I I 0.1 ~ n
(c) Fig. 3. The structure of thick as-grown anodic A1203 films grown at Jg = 1 mA cm-2: (a) electron diffraction pattern from a film 240 nm thick indicating the presence of polycrystalline y-AI203 (spinel); (b) electron mierograph and (c) diffraction pattern of a predominantly polycrystalline film 300 nm thick.
Figure 3(a) shows the electron diffraction pattern obtained from an anodic A120 3 film 240 nm thick. Table I gives the indexed reflections together with the estimated intensities for this pattern. These thicker oxides showed three prominent reflections corresponding to d spacings of 2.38, 1.97 and 1.39 A. These reflections agree very closely with the X-ray diffraction results of Rooksby 11 for chemically vapour-deposited A120 3 and indicate the presence of the polycrystalline ~,-A1203 phase in the as-grown anodic oxide. In addition, it is interesting to note that the results of Table I indicate the presence of some unknown new reflections. Prominent among these are the three reflections corresponding to the d spacings of 1.73, 1.24 and 1.07 A. These spacings do not correspond to any of the known reflections of y-A120 3. Anodic A120 3 films of thickness about 300 nm exhibited electron micrographs and diffraction patterns of the types illustrated in Figs. 3(b) and 3(c) respectively. These films gave only two strong reflections corresponding to d spacings of 1.97 and 1.39 A. These lines together with their sharp diffraction pattern indicate that the oxide is predominantly polycrystalline "f-A1203. From the electron micrograph of
S T R U C T U R A L P R O P E R T I E S OF A N O D I Z E D
TABLE I SPACINGS AND INTENSITIES FOR AN A I 2 0 3 FILM 240 n m THICK
1 2 3 4 5 6 7 8 9 10 11 12
Measured spacings and estimated intensities from Fig. 3(a)
Indexed spacings and intensities of ~-A1203 according to Rooksby 11
F MS M VS --
200 311 222 400 -511 440 -444 731 800 840 844 ----
2.790 2.385 2.271 1.973 --
1.397 1.238 1.140 1.069 -0.884 0.802 0.722 0.687 0.600
VS S VF S -VF MS VF VF VF
1,392 -1.138 1.025 0.987 0.882 0.804 ----
VS -F VF VF VF F ---
Reflecting planes (hkl)
K e y : S, s t r o n g ; MS, moderately strong; VS, v e r y s t r o n g ; F, faint; VF, v e r y faint.
Fig. 3(c), the grain size of these oxides was determined to be 40-180 nm. Films of thickness 450 nm showed similar results, except that they exhibited preferred orientation and a large grain size. We also investigated the effect of the anodization current density Jg on the film structure. As Jg was decreased the anodic film structure changed from an amorphous structure to a polycrystalline structure with random orientation and then to a preferentially oriented polycrystalline structure. Figure 4 shows a typical electron micrograph and a typical diffraction pattern obtained from a film 150 nm
Fig. 4. (a)An electron micrograph and (b) a diffraction pattern ofa typical oxide 150nm thick grownat a low current density (Jg = 0.25 m A c m -2). The diffraction pattern indexesas f.c.c. 7'-A1203.
K.S. CHARI, B. MATHUR
thick deposited a t J g - - - 0 . 2 5 mA cm --2. Table II gives the indexed reflections together with estimated intensities for this pattern. As can be seen from the table, these oxides exhibit three well-marked reflections corresponding to d spacings of 2.28, 1.97 and 1.39 A. These data agree closely with the X-ray diffraction results of Verwey 12 for anodic A120 a films and indicate the formation of y'-Al20 3. Furthermore the first three reflections indicate that the "/'-AI20 a is f.c.c, unlike the spinel structure of 7 - A 1 2 0 3. From the electron micrograph the grain size of these oxides was found to be in the range 20-80 nm. These oxides were insoluble in the mixture of phosphoric and chromic acids, showing that they are crystalline. TABLE II SPACINGSAND INTENSITIESFOR AN Al203 FILM 150 nm THICK
Measured spacings and estimated intensities from Fig. 4(b)
Indexed spacings and intensities of 7'-A1203 according to Verwe3/2
Reflecting planes (hkl)
1 2 3
2.280 1.969 1.396
F S VS
F VS VS
2.281 1,975 1,397 Not observed 1,140 0,988 Not observed 0.883 0.806
111 200 220 311 222 400 311 420 422
M MS MS S
From the etching results for the as-grown films we may draw some conclusions about the nature of these films. The oxides grown at normal current densities (above 1 mA cm- 2) always seem to consist of two types of oxides, namely an outer layer soluble in the mixture of chromic and phosphoric acids and an insoluble layer underneath. The proportion of the insoluble layer is markedly dependent on the film thickness and increases with the thickness. This indicates the predominantly crystalline nature of these films. In contrast with this double-layer arrangement, the oxides grown at lower current densities (typically below 0.25 mA cm-2) always appear to consist of only one type of oxide, namely an insoluble crystalline phase. The results obtained at normal current densities are in agreement with the observations of Altenpohl 13. In addition, these results also Suggest that the oxides grown at a constant voltage are likely to contain a double.layer arrangement unless the initial starting current density is below 0.25 mA c m ' 2.
3.2. Heat-treated films The effect of heat treatment on the structure of as-grown anodic films was studied over the temperature range 300-900 °C. Annealing the as-grown films at temperatures below 700 °C for 0.5 h did not cause any significant changes in the film structure. However, electron micrographs showed some grain growth during annealing. On aging at higher temperatures (above 800 °C), destruction of the as-
STRUCTURAL PROPERTIES OF ANODIZED
grown amorphous phase began, as was evident from the splitting of the halos in the diffraction pattern into a ring pattern, and a transition to the polycrystalline phase occurred. A typical electron micrograph and a typical electron diffraction pattern obtained from an as-grown film 150 nm thick annealed to 850 °C are depicted in Figs. 5(a) and 5(b) respectively. The sharp diffraction pattern indicates the predominant crystallinity of the heat-treated oxide. The pattern showed three wellmarked reflections corresponding to d spacings of 2.37, 1.97 and 1.39 A. These d values indicated that the heat-treated oxide comprised f.c.c. 7'-A1203. The heattreated oxides generally showed a larger crystallite size. From the transmission micrograph of Fig. 5(a), a grain size of 30-100 nm was deduced for these annealed oxide films 150 nm thick.
Fig. 5. (a) An electron micrograph and (b) a diffraction pattern of an as-grown amorphous film 150 nm thick after heat treatment in nitrogen at 850 °C for 0.5 h. The diffraction pattern clearly shows the transition to the polycrystalline form and indexes as f.c.c. 7'-AlzO 3.
Our investigation of the electrical properties of the as-grown oxides indicated that the resistivities of as-grown amorphous deposits are about 1017 f~ cm whereas those of the predominantly crystalline films are about 1015 fl cm. As-grown amorphous films which had been heat treated to temperatures below 700 °C showed resistivities in the range (2-6) x 1017 f2 cm. The slight improvement in the resistivity of these annealed films is possibly related to the gettering of defects in these oxides. The amorphous oxide films also exhibit a dielectric strength which is one order of magnitude higher than that of the crystalline film. Breakdown field strengths in the range (2--4) x 107 V cm- 1 were consistently obtained for amorphous films. 4. CONCLUSIONS It was shown that anodic AI20 a films formed in a tartaric acid-ethylene glycol bath at current densities of 1 mA cm - 2 or higher are amorphous up to 180 nm thick, fully crystalline above 250 nm thick and have mixed amorphous crystalline phases at intermediate thicknesses. Forming at current densities below 0.25 mA cm-2 always promotes crystalline oxide growth. The crystalline oxides grown at high current densities are of the 7-A120 a phase and those grown at lower current
K.S. CHARI, B. MATHUR
densities are of the 7'-A120 3 phase. At 800 °C the a s - g r o w n a m o r p h o u s phase u n d e r g o e s a t r a n s i t i o n to the crystalline y'-A120 3 phase. The a m o r p h o u s films always exhibited electrical properties which were superior to those of the crystalline films. REFERENCES 1 D.R. Collins, S. R. Shortes, W. R. McMahan, R. C. Bracken and T. C. Penn, J. Electrochem. Soc., 120 (1973) 521. 2 R.K. Raymond and M. B. Das, Solid-State Electron., 19 (1976) 181. 3 B.G. Carbajal and W. R. McMahon, Government Microcircuit Applications Conf., San Diego, CA, 1972, Digest of Technical Papers, pp. 327-331. 4 K.S. Chari, B. Mathur and J. Vasi, J. Microelectron., 6 (1979) 24-26. 5 K.S. Chari, unpublished, 1979. 6 C.R. Haden, J. L. Barret and J. L. Stone, IEEE J. Solid-State Circuits, 9 (1974) 118. 7 K.S. Chad and B. Mathur, Thin Solid Films, 75 (1981) 157. 8 K.S. Chari, Ph.D. Thesis, Indian Institute of Technology, Delhi, 1979. 9 H. Mahl, Metallwirtschaft, 19(1940) 1082. 10 G. Hass, J. Opt. Soc. Am., 39 (1949) 532. 11 H.P. Rooksby, The X-ray Identification and Structure of Clay Minerals, Mineralogical Society, London, 1957,p. 250. 12 E.J.W. Verwey, Z. Kristallogr., 91 (1935) 317. 13 D. Altenpohl, lRE Natl. Conv. Rec., Part lll (1954) 35.