溶膠凝膠中獨(dú)立的厚膜

1、Self-Standing Thick FilmsINTRODUCTIONOne of the most important technological aspects of solgel processing is the preparation of thin films by common techniques such as dip- or spin-coating. A wide variety of materials with special functions of optical and opto-electronic importance have been produce

2、d to date by the application of these coating methods. However, the dip- and spin-coating methods include the experimental experience that inorganic films thicker than 1 m are virtually impossible to dry without cracks regardless of the drying rate (Brinker and Scherer, 1990).On the other hand, ther

3、e are various economical demands for oxide films with a thickness of the order of several micrometers formed on substrate materials in the field of electrical, optical and opto-electronic devices (Kawachi, 1990; Moilanen, 1994). The formation of such oxide films have been made by repetitive spin-coa

4、ting and rapid thermal annealing (Syms and Holmes, 1994), flame hydrolysis (Kawachi, 1983), plasma CVD (Grand, 1990), screen printing (Fernandes, 1995; Futakuchi, 1999; Akiyama, 1999), hydrothermal process (Shimomura, 1991), gas deposition process (Ichiki, 1997), electrophoresis (Nishimori, 1995), e

5、tc. But each process has drawback inherent to the method. Some of them are time-consuming and less economical. Others require high sintering temperature and sometimes give the films of low density or poor flatness.Recently a new technique of forming oxide gel films corresponding to oxide films of se

6、veral to 20 m in thickness based on interfacial polymerization was developed by a research group of Tokyo Institute of Technology (Yamane, 1994). The technique originally developed aiming the formation of a thick silica glass film on a silicon substrate for the purpose of the fabrication of a planer

7、 wave guide enlarged the restricted film thickness related to conventional solgel processing and was extended to the formation of thick PZT films used for piezoelectric devices in MEMS such as micro-actuators, sensors, ultrasonic transducers, ultrasonic motors and so on (Tsurumi, 2003).The principle

8、 of the interfacial polymerization method is based on the hydrolysis and polycondensation of precursors at the interface of two immiscible liquids without direct contact with the substrate surface. The evaporation of the liquid in the upper phase and the subsequent careful drainage of the liquid in

9、the bottom phase causes a gentle placement of the free standing gel film onto the substrate surface without formation of chemical bonding, which is of great advantage over spin- or dip-coating in avoiding cracks during drying.In this article, the outline of the method, effects of various reaction pa

10、rameters on the gel film properties will be introduced in the case of both thick silica glass film and PZT film on silicon substrates.OUTLINE OF GEL FILM FORMATION BY AN INTERFACIAL POLYMERIZATIONThe formation of a free-standing gel film by an interfacial polymerization is carried out within a cylin

11、drical container having a drainpipe at its bottom. First, a substrate material on which the formed gel film is placed is set near the bottom of the cylindrical container. Then water for the hydrolysis of an alkoxide is poured in the container to cover the substrate to the level several millimeter ab

12、ove the surface. Next, the precursor solution prepared by dissolving the alkoxide in an organic solvent is gently pored onto water.Figure 16-1. Schematic illustration of gel film formation process by an interfacial polymerization method.The hydrolysis and polycondensation of the alkoxide takes place

13、 at the interface formed between two immiscible liquids. The reaction proceeds until the introduced alkoxide is spent and turns into a gel film. The formed gel film separates from the container wall by the capillary force induced by the evaporation of the organic solvent and shrinks to some extent w

14、ithout restriction from the substrate. After the complete evaporation of the organic solvent, the formed gel film is placed on the substrate by draining water from the bottom of the container and subjected to drying in an ambient atmosphere. The schematic illustration of the process is given in Figu

15、re 16-1.The composition and the amount of the precursor solution and the diameter of the container determine the properties of the film such as porous structure, thickness, and so on. The precursor solutions containing proper amount of fine particles, as well as alkoxides, are sometimes used in orde

16、r to reduce the shrinkage during drying. Water for the hydrolysis of the alkoxide usually contains appropriate catalyst.PREPARATION OF A SILICA FILMPrecursor SolutionThe preparation of the precursor solution begins with the dissolution of silicon alkoxide or its derivatives in an organic solvent lik

17、e hexane that meets the conditions: (1) immiscible with water, (2) lower density than water, and (3) relatively high vapor pressure. Among commercially available silicon alkoxides and their derivatives, ethyl silicate 40 (E-40), partially hydrolyzed tetraethoxysilane (TEOS) comprising the 5-membered

18、 oligomers, turned out to be the most suitable by the survey of various materials including TEOS, tetramethoxysilane (TMOS) and its derivative methyl silicate 51 (Yamane, 1994).Figure 16-2. Dependence of film thickness on the concentration of E-40 in hexane (reaction with ammonia water of pH = 11 fo

19、r 24 h).The advantage of using E-40 rather than TMOS or TEOS is attributed to the low hydrolysis rate of silicon alkoxide. As it is widely known, the hydrolysis of TMOS or TEOS is enhanced by acidic catalyst, while the polycondensation rather proceeds under base-catalyzed circumstance. On the other

20、hand, the catalyst that can be added in the water for the interfacial polymerization reaction is limited to either acid or base only. Then, the use of E-40 which already had been hydrolyzed and does not need the assistance of acid catalyst is obviously advantageous over the employment of TMOS or TEO

21、S.In the film formation from E-40, its concentration in the precursor solution is obviously one of the important parameters determining the reaction time to obtain a gel film of desired thickness. Figure 16-2 shows the relation between the concentration of E-40 and the weight of gel film per square

22、centimeter obtained by the reaction with ammonia water of pH = 11 for 24 h. It is known from the figure that a film of about 2 mg/cm2, which corresponds to the thickness of about 10 m in terms of the eventual silica glass film, is obtained from the precursor of the concentration 1.21.5 mol/l under g

23、iven experimental conditions.Catalyst to be Used for the ReactionThe effects of catalysts on the gel film formation are different depending on the type of catalyst and its concentration, i.e., pH of water (Shulze-Bergkamen, 1995, Yamane, 1997). For example, a gel film is obtainable from E-40 by the

24、reaction with ammonia water of pH 10. The thickness of the film increases with the increase in pH of water as it is seen in Figure 16-3. It is also possible to form a film in the region pH 10 or pH Na2CO3 NaOH, the differences in the effects shown in the table is attributed to the difference in the

25、concentration of the catalysts in the upper organic phase. This was confirmed by the experiment carried out by directly dissolving tri-methylamine in hexane. The rate of gel film formation was dramatically increased compared with ammonia-catalyzed reaction, as it is known from Table 16-2. Only 4 h o

26、f reaction time was enough to obtain a gel film corresponding to a glass film of 10 mm in thickness, i.e., 12 mg/cm2, even with much more diluted precursor solution. This shows that the reaction proceeds quite efficiently if a catalyst is contained in the upper organic phase along with E-40.TABLE 16

27、-1. EFFECTS OF VARIOUS BASE CATALYSTS ON FILM FORMATION AND THEIR SOLUBILITY DATA (YAMANE, 1997)CatalystFilm thickness (g cm2)aGel time (h)bAmmonia40 10 1045060NaOH2 0.5 1045060Na2CO310 2 1045060Solubility parameter (cal/cm3)2Solubility in hexane (mole fraction)Solubility in water (mole fraction)H2O

28、23.82.55 101C6H147.254.41 1027NH3134.89 1013.49 102NaOHc4.90 101Na2CO31.21 102apH = 10.6; CE40 = 200 ml l1; T= 30C; t= 20 h.bE-40/MeOh/H2O =1/1/1 (ml); pH = 9.5; T = 60C.cNo available data.Figure 16-3. Dependence of film thickness on the pH of ammonia water (concentration of E-40 in hexane; 1.2 mol/

29、l, reaction time; 24 h) (Yamane et al., 1994).TABLE 16-2. COMPARISON OF THE EFFECTS OF TRI-ETHYLAMINE AND AMMONIA ON FILM FORMATION (YAMANE, 1997)CatalystCE40 (ml l1)Reaction time (h)Film thickness (g cm2)(C2H5)3N33414NH32002012Figure 16-4. Increase in the film thickness with reaction time (concentr

30、ation of E-40 in hexane; 1.2 mol/l, ammonia water of pH = 11).Reaction Time and Rate-determining StepThe reaction time is another parameter to control the thickness of gel film formed at the interface of two immiscible liquids. As it is shown in Figure 16-4, the film thickness increases approximatel

31、y with the square root of reaction time, suggesting that the reaction is a diffusion-controlled process. From the difference in the catalytic effects of various electrolytes and the dependence on their concentration mentioned in the above, the diffusion of a catalyst as well as water from the lower

32、inorganic phase to the upper organic phase across the interface is the most plausible rate-determining step. If this is the case, the gel film formed at the interface is considered to increase its thickness toward the upper organic phase, as it is schematically illustrated in Figure 16-5.Microstruct

33、ure of FilmsThe gel films formed at the interface between hexane and ammonia water did not shrink upon drying and remained intact through out the process until completely dried on a silica glass substrate. They were translucent or white in appearance depending on the pH of ammonia water and the conc

34、entration of E-40 in the precursor solutions. Some examples of gel film obtained by the ammonia-catalyzed reaction of E-40 are shown in Figure 16-6.Figure 16-5. Schematic illustration of the film formation mechanism (Yamane, 1997).The microstructure of the gel film observed by a scanning electron mi

35、croscope, SEM, was quite different from that expected for the aggregate of silica particles but rather close to the mixture of flakes of a few to 20 m in size, as it is shown in Figure 16-7. Although the high porosity and the peculiar shape of the flakes are advantageous in hindering the fracture of

36、 gel film during drying, they are rather drawbacks for the densification of the film to a bubble free silica glass through viscous sintering. In such a case, an additional treatment for the reduction of pore volume is one of the ways to complete the densification without damaging substrate materials

37、.Figure 16-8 shows the photographs obtained by such a treatment for the enhancement of the densification. A silica gel film formed by an interfacial polymerization and dried on a silica glass substrate was soaked in an alcoholic solution of boric acid so that the pores of the gel film was reduced by

38、 the deposition of boric acid after the evaporation of alcohol. After the complete removal of alcohol, the gel film was subjected to heat treatment for densification at 1250C for 2 h. The thickness of the finally obtained transparent borosilicate glass film was about 8 m.Figure 16-6. Appearance of g

39、el films obtained by the ammonia catalyzed reaction of E-40.PREPARATION OF PZT FILMPrecursor SolutionIn the preparation of PZT film from precursor solution consist of organometal compounds of Pb, Zr, Ti dissolved in acetic ester, it is necessary to take it into considerations that the reactivity of

40、organometal compounds with water is very high and the hydrolysis of such compounds often results in the precipitation of ultra fine powders. A gel film formed by an interfacial polymerization of organometal compounds using hexane as a solvent consists of such ultra fine powders. The film shows a lar

41、ge shrinkage by the capillary force induced by the evaporation of hexane, resulting in the fracture even during free-standing on water. This is the big difference from the silica gel film prepared by the similar process.One of the ways to hinder such a large shrinkage is the introduction of fine PZT

42、 powders of the size a few tenths of a micrometer in diameter in the organic phase. The powders of this size will precipitate to the interface of two immiscible phases in a short time and remain there, which allows water for the hydrolysis of organometal compounds to diffuse from the lower phase onl

43、y through the pores formed by the precipitated particles. Then the hydrolysis reaction resulting in the formation of ultra fine powders of PZT will proceed only inside the pores so that the influence of capillary force attributed to the reaction product is hindered.An example of the preparation of a

44、 precursor solution for a thick PZT film based on this considerations is seen in the report by Tsurumi et al. (2003). It begins with the dissolution of PZT (Zr/Ti = 53/47) alkoxide solution in hexane along with ethanol, followed by adding PZT fine powders of about 0.2 m in diameter and a surfactant

45、(sorbitan monooleate) to give a good dispersion of the powders. The PZT fine powders to be introduced are coated in advance with Pb5Ge5O11 in order to enhance the sintering of the eventual gel film without influencing the dielectric and piezoelectric properties of the final product.Figure 16-7. Scan

46、ning electron micrographs of gel films prepared from E-40.Figure 16-8. Photographs of thick glass film obtained by reducing the pore volume by soaking the gel film in an alcoholic solution of boric acid: before densification (left), after densification (right).Typical amounts of respective materials

47、 to be introduced in the container of 45 mm inside diameter for the film of about 25 m in thickness are, precursor solution; 800 l, hexane; 10 ml, ethanol; 25 l, PZT fine powders; 0.1 g, surfactant; 1.0 104 mol, respectively. No particular catalyst for the hydrolysis and condensation is necessary be

48、cause of the high reactivity of the alkoxide.Microstructure and Properties of PZT FilmThe PZT gel film formed at the interface between two immiscible phases from the precursor solution of prescribed composition has a structure as if the particles of the introduced powder are bonded to each other thr

49、ough the reaction products of organometal compounds. It is flexible rather than fragile and can be settled without fracture even on a substrate material with a curved surface.By a careful control of humidity of atmosphere and heat cycle throughout the process from room temperature to 950C, the gel f

50、ilm can be dried and sintered to a uniform and dense PZT film of the order of 20 m in thickness with good adhesion between a surface-modified Si substrate of Pt/IrO2/SiO2/Si structure.According to the report by the authors, the remanent polarization of thus prepared PZT thick film was 33.1 C/cm2. Th

51、e piezoelectric d33 constant measured with a Mach-Zehnder interferometer was 22 pm/V and was independent over the frequency range from 0.2 to 3 kHz.ReferencesAkiyama Y., Yamanaka K., Fujisawa E., Kawata Y. Development of lead zirconate titanate family thick films on various substrate. Jpn J. Appl. P

52、hys. 1999; 38: 55245527Brinker, Scherer, SolGel Science. San Diego: Academic Press, 1990Fernandes J.F, Nieto E., Moure C., Duran P., Newnham R.E. Processing and microstructure of porous and dense PZT thick films on Al2O3. J. Mater. Sci. 1995; 30: 53995404Futakuchi T., Matsui Y., Adachi M. Preparatio

53、n of PhZrO3PbTiO3Pb(Mg1/3Nb2/3)O3 thick films by screen printing. Jpn J. Appl. Phys. 1999; 38: 55285530Grand G., Jadot J.P., Denis H., Vallette S., Fournier A., Grovillet A.M. Low-loss PECVD silica channel waveguides for optical communications. Electron. Lett. 1990; 26: 21352137Ichiki M., Akedo J., S