Ultra-thin TiO2 films by a novel method

Materials Science and Engineering, B13 (1992) 299-303

299

Ultra-thin TiO2 films by a novel method S. B. Desu Department of Materials Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 (USA) (Received September 29, 1991;in revised form October 25,1991)

Abstract Ultra-thin TiO z films were grown by a novel process--successive layer-wise chemisorption--on substrates such as fused quartz and single crystal silicon. Alternate chemisorption of easily hydrolysed TiCI 4 and water on a previously hydroxylated substrate surface permitted the synthesis of TiO 2 films with thickness precision down to a monolayer. The film thickness is insensitive to growth temperature in the region of 170-240 *C. The cycle thickness (growth rate per cycle) is around 0.27 nm under the growth conditions used. The films have anatase structure and the composition (Ti/O ratio) was shown to be 2. Refractive index of as-deposited films is 2.7 at 632.8 nm.

1. Introduction There has been a growing interest in titanium oxide (TiO2) films for applications such as memory cell capacitors, gate electrodes for MOS devices [1], high temperature optical filters, antireflection coatings [2], solar energy conversion [3], and high efficient catalysts [4]. Modern devices, such as ultra large scale integrated devices, Josephson tunnel junctions, high efficient optical filters (e.g. Rugate filters), require excellent quality TiO2 films of only a few nanometers thick. This requirement cannot be easily satisfied by the well established physical and chemical vapour deposition techniques, such as reactive evaporation [5], reactive sputtering [6], ion-assisted depositions [7], and low pressure chemical vapour deposition (including photoassisted and plasma assisted) techniques [8-11]. The preparation of thin films from the vapour deposition methods is usually accompanied by the formation of three-dimensional nuclei which significantly increases the critical thickness and prevents the formation of pore-free ultra-thin films. By exploiting the chemisorption properties of substrate surface functional groups, it is possible to modify the nucleation process from a three-dimensional to a two-dimensional nucleation, thereby facilitating the growth of good quality ultra-thin oxide films. Initial results of the successful development of such a growth process, the so-called successive layer-wise chemisorption (SLC) for TiO 2 thin films, are reported in this paper.

In the SLC method, TiO2 films on silica or on single crystal silicon are prepared by reacting TiCI4 with the surface functional groups (e.g. silanols) to form chlorine-containing functional groups (reaction 1 ). The subsequent hydrolysis of these chlorine-containing products (reaction 2) under appropriate conditions results in the formation of a monolayer of TiO2 on the surface. The repeated alteration of reactions 1 and 2 on a surface makes it possible to grow TiO: films of the required thickness. (---Si-OH)m +TIC14 ~

(-SiO)mTiC14_ m+ mHCI

(1)

(-=SiO)mTiC14- ,7 + (4 - m)H20 (-SiO)mTi(OH)4-

m +

(4 - m)HCI

(2)

In reactions 1 and 2, =Si represents a surface silicon atom, and m is an integer with a value from 1 to 3 [12]. The major difference between SLC and the traditional methods of film formation is the fact that the SLC process is conducted under conditions of irreversibility of the given reaction (substantially removed from equilibrium), which permits almost total utilization of all the surface functional groups. 2. Experimental procedure Figure 1 shows the growth chamber used in the present study. It comprises an inconel tube with an inner quartz tube as the main body of the structure, Elsevier Sequoia

300

S. B. Desu

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Ultra-thin TiO_,films by a novel method 15.0

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Fig. 2. Relationship between film thickness and number of SLC cycles.

MECHANICAL PUMP SCRUBBEF

Fig. 1. Deposition reactor.

which is connected on one side to two bubblers, consisting of reagents T i C I 4 and high purity water, and on the other side to a high vacuum system. The high vacuum system consists of a turbomolecular pump which is backed by a mechanical pump. The exhaust of the vacuum system is connected to a scrubber to remove unreacted reagents and product gases such as hydrogen chloride generated during the deposition process. The substrates are placed in the reaction zone through the opening at the end of the reactor tube. The substrate temperature is maintained with the aid of heater elements controlled by standard regulators. Three types of 50 mm diameter substrates were used in the present investigation; fused quartz, single crystal silicon (100) coated with 50 nm of thermal oxide, and single crystal silicon (100) with native oxide 2 nm thick. The thermal oxide (SIO2) on silicon was grown at 900 °C in pure, dry flowing 02. Before introducing the substrates into the reactor, they were subjected to a standard semiconductor clean. The substrates were first heated (dehydrated) in flowing dry argon to 600 °C for 2 h. The specimens were cooled to 100 °C, and rehydrated for 10 h by passing argon, saturated with water vapour, through the reactor. This dehydration and hydration process was shown to result in a known concentration of silanols, = Si-OH, (4.6 O H nm-2) on the surface of silica gel [13]. Subsequent to the rehydration process, the specimens were heated in dry argon to a predetermined growth temperature (Z~) between 100-350 °C. Before starting the growth process, the reactor was evacuated to a base pressure of 1 x 10 -6 Pa, and the

conditions were stabilized for 15 min. The deposition process was started by closing the high vacuum valve of the reactor and introducing TiCI4 vapour to a pressure of 200 Pa. The samples were exposed to 200 Pa of TiCI 4 for 20 rain, and then the reactor was pumped back to the base pressure. The specimens were subsequently exposed to water vapour of 200 Pa pressure for 20 min, after which the reactor was again pumped back to base pressure. This alternative chemisorption of TiCI4 and water on a previously hydroxylated surface completes one deposition cycle. Film thickness and properties were studied as a function of deposition conditions and number of deposition cycles (N). The thickness and refractive index of the resulting TiO 2 films were studied by an ellipsometer (Rudolph Auto El IV) with an incidence angle of 70 ° and at a wavelength of 632.8 nm. The crystallographic features of grown TiO2 films were examined using a thin film X-ray diffractometer (Rigaku TFD system). The composition was determined by electron spectroscopy for chemical analysis (ESCA) and Auger electron spectroscopy (AES). The film microstructure was examined by both optical microscopy and scanning electron microscopy.

3. Results and discussion

It was observed that, with fixed process parameters, such as deposition temperature, reagent pressure, residence time of the reagent, and residual partial pressure, the thickness of the titanium oxide film is a linear function of the number of deposition cycles (N). In this case, therefore, the SLC process can be characterized by a growth-rate constant (cycle thickness) do, i.e. the average increase of the thickness of the TiO2 layer per deposition cycle. Fig. 2 depicts the relationship between film thickness and number of SLC cycles for

S. B. Desu

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301

Ultra-thin TiO2films by a novel method 0,6

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0.4

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z _'-O k-r

/

_~5.0

0.2

Si/2 nm SiO 2

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p

~

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• 10

i f I 20 30 40 NUMBER OF SLC CYCLES

i 50

0.0 60

50

I 150

t 250

350

SUBSTRATE TEMPERATURE (°C)

Fig. 3. Relationship between film thickness and number of SLC cycles for Si/50 nm SiO2 and Si/2 nm SiO2 substrates.

Fig. 4. Variation of cycle thickness with substrate temperature.

the substrate fused quartz. The cycle thickness was observed to be 0.27 nm, which is very close to the size of the titanium-oxygen octahedron (0.28 nm), calculated from the length of the titanium-oxygen bond (0.201 nm). The relationship between film thickness and number of SLC cycles (N) for single crystal silicon substrates, with varying SiO 2 thickness 50 nm and 2 nm, is depicted in Fig. 3. Note that the deposition conditions for the data presented in both Figs. 2 and 3 are identical. As can be seen from Fig. 3, for substrate Si with thick oxide (50 nm), the relationship is linear and the cycle thickness is 0.27 nm, similar to fused quartz (Fig. 1 ). However, for the Si substrate with a very thin oxide (2 nm) film, a deviation from linearity can be observed (Fig. 3) in the initial stages of the synthesis. Furthermore, with these thin oxide substrates, the value of d o is much less than 0.27 nm (Fig. 3). This nonlinearity in the relationship between film thickness and N can be attributed to the influence of the crystalline substrate on the surface structure and properties of the oxide layer. It can be easily argued that reactivity of the surface silanol groups will play a key role in the formation of the first TiO2 layer on the substrate. The proton donor capacity of the silanol groups on the silica surface can be understood in terms of a collective activating effect of the surface Si-O bonds from a d z d = conjugation. A thin oxide layer on silicon can be represented as Six(SiO)y(SiO2)z-OH [14]. Depending on the value of z, the oxygen-deficient oxide can have a significant influence on the reactivity of surface silanols. If the thickness of SiO 2 is small, owing to a significant contribution of oxygen-deficient oxide, the shift of electron density away from the silanol group will be suppressed, and the proton donor properties (or reactivity to TiCl4) will be less marked. With increase in the thickness of the oxide film on Si, the properties of the surface approach more closely

those of the surface of fused quartz. Although this is one of the possible explanations, further experimentation is needed to justify the hypothesis. Figure 4 illustrates the effect of deposition temperature (substrate temperature, Ts) on the cycle thickness, d 0. Three characteristic regions can be seen from Fig. 4 in the relationship between Ts and do. In the temperature region 170-230 °C, the cycle thickness, do, is a constant with a value of 0.27 nm. It was also observed that in this temperature regime, the thickness uniformity within the sample was excellent ( < 1% over 50 mm diameter wafer). It is possible that in this temperature region, the film growth involves only surface functional groups. Below 170 °C, the cycle thickness increased as the substrate temperature decreased and further thickness nonuniformity is also increased. It was reported in the literature that temperatures of the order of 160 °C are needed to drive off all the molecularly-adsorbed water from the silica surface [15]. Based on the literature reports, it can be concluded that the presence of the molecularly-adsorbed water leads to an increase in the cycle thickness. It can also be seen from Fig. 4 that there is a sharp decrease in cycle thickness when the substrate temperature is raised above 230 °C. This decrease in do can be attributed to the decrease in concentration of surface silanol groups with increasing substrate temperature. In other words, with increasing Ts there is a breakdown of the continuity of the hydroxyl coverage. It has been well established [15] that the loss of the silanol groups is attributed to the formation of siloxane groups which can be represented as 2(--Si-OH) ~

(---Si-O-Si=-) + H 2 0

(3)

Appreciable removal of silanol groups from the surface of silicas (e.g. silica gel) begins at around 250 °C and continues up to 900-1000 °C. However, the siloxane groups formed below a certain temperature ( = 500 °C)

302

S. B. Desu

/

Ultra-thin TiO_,films by a novel method

are fairly active for rehydration. This would not contribute to the deposition of TiO2 because in the present procedure, the substrate is first exposed to TiC14 vapour after the equilibrium substrate temperature is reached. These results (Figs. 2 to 4) indicate that when /~= 200 °C, T d (dehydration temperature) is around 600 °C, reagent partial pressure is around 200 Pa, residual pressure is less than 1 Pa, and the reagent residence time is 20 min, a monolayer coverage of TiO 2 can be obtained. Under these conditions, the thickness is only a function of N. The required thickness for the film therefore can be synthesized, without any monitoring system, by a necessary number of repeating reaction cycles with a minimum discreteness in a single monolayer. X-ray diffraction studies of the films prepared under these conditions on fused quartz and Si (with 50 nm SIO2) indicate that the TiO2 has a polycrystaUine nature and an anatase structure. Neither optical nor scanning electron microscope investigations could resolve the grain structure indicating the small size of the grain size. To find the grain size, investigation by transmission electron microscopy is underway. The ellipsometric studies indicate that the refractive index is 2.7 at a wavelength of 632.8 nm; the value of the refractive index is not influenced by film thickness. Since the value of the refractive index is very close to that reported of bulk TiO2, one can expect that the films are of high density. It may be assumed that the nucleation phenomena in SLC are apparently modified to allow two-dimensional nucleation, ensuring very dense, uniform, ultra-thin layers of TiO2. Investigations by both AES and ESCA (Figs. 5 and 6) indicate that for the deposited films the ratio of Ti/O is 1/2 and, furthermore, no other elements (e.g. CI) were detected. The peak positions in Figs. 5 and 6 correspond very well with the literature values indicating the absence of any anomalies in the structure or composition.

4. Summary

A novel process (SLC), based on successive layerwise chemisorption of TiC14 and water on a previously hydroxylated surface, was reported for the fabrication of dense, uniform, ultra-thin films of TiO2. The SLC method can be used at comparatively low pressures (200 Pa) and low temperatures (200°C). Since this 10 8

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Fig. 6. (a) ESCA wide scan of TiO 2 film deposited by SLC; (b) ESCA spectra, O ls peak, of TiO2; (c) ESCA spectra, Ti 2p peak of TiO 2.

S. B. Desu /

Ultra-thinTiO2films by a novel method

method exploits the nature of surface functional groups, the preparative conditions of the substrate surface are very important. U n d e r ideal conditions, the cycle thickness of TiO2 is 0.27 nm. T h i c k e r films can be fabricated, with precision down to a monolayer, by a necessary n u m b e r of repeating reaction cycles. T h e TiO2 films are polycrystalline in nature, having anatase crystal structure. This m e t h o d may also p r o v e useful as a p r o b e to study the reactivity of surface functional groups.

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