Preparation Method for Lead-Zirconium-Titanium Based Thin Film

1. Field of the invention

The present invention relates, in general, to a novel organometallic zirconium precursor and its in-situ synthesis and, more particularly, to an organometallic zirconium precursor with an improvement in volatility. Also, the present invention is concerned with a lead-zirconium-titanium thin film using the same and with a preparation method for preparing the same.

2. Description of the Prior Art

Lead-zirconium-titanium (PbZr.sub.x Ti.sub.1-x O.sub.3) thin films or its related thin films (hereinafter referred to as "PZT thin films"), used as ferroelectric thin films for semiconductor capacitors, are prepared from organometallic precursor sources, e.g. lead source, zirconium source and titanium source. For production of the PZT thin films, there is generally used a metal-organic chemical vapor deposition process (hereinafter referred to as "MOCVD"), a technique for growing thin layers of compound semiconductors. In practice, MOCVD comprises preheating a liquid or solid of organometallic precursor to vaporize it and decomposing the vapor by heat or plasma, to deposit a thin film.

It is necessarily required that appropriate organometallic precursors be selected in order to obtain desired properties and quality of the PZT thin film. The following are the general conditions that organometallic precursors should have: first, organometaliic precursors should be able to be easily volatilized at low preheat temperatures, in addition to being stable at the preheat temperatures; second, the difference between the vaporization temperature and the decomposition temperature should be large enough; third, it is preferred that the organometallic precursors are not decomposed or changed by the moisture contained in air; finally, they should not be environmentally toxic.

Of the various organometallic precursors useful for preparing the PZT thin films, organometallic Ti precursors and organometallic Pb precursors are relatively well developed and may be volatilized at relatively low temperatures. By contrast, organometallic Zr precursors are disadvantageous in that they are very hygroscopic or difficult to volatilize.

Conventionally, it is tetra (2,2',6,6'-tetramethyl-3,5-heptanedione) zirconium (hereinafter referred to as "Zr(THD).sub.4 ") that is the most useful in providing a zirconium source for PZT thin films. Zr(THD).sub.4 exhibits outstanding thermal stability that it is not decomposed at up to 300.degree. C. In addition, Zr(THD).sub.4 is very resistant to moisture, unlike other Zr precursors. However, this Zr source for PZT thin film has a vaporization temperature that is much higher than those of the other sources, Pb and Ti sources. In other words, the gap in vaporizing point between the Zr source and the other sources is large. For example, Zr(THD).sub.4 is volatilized at a temperature of about 220.degree. to 240.degree. C. whereas Pb(THD).sub.2 and Ti(OCH.sub.2 CH.sub.3).sub.4 (hereinafter referred to as "Ti(OEt).sub.4 ") have a vaporizing point of about 140.degree. to 160.degree. C. and about 110.degree. to 120.degree. C., respectively.

Accordingly, it is difficult to control the temperature of a premixing chamber or preheat tube in which volatilized Pb, Ti and Zr precursors are mixed just before entering a reactor. For example, if the preheat temperature is controlled for the Pb or Ti precursor, the gas phase Zr(THD).sub.4 is highly apt to be solidified. On the other hand, if the preheat temperature is raised for the Zr precursor, the Pb and Ti precursors are decomposed or changed before entering the reactor. Further, preheating to the high temperature of around 250.degree. C. lays great restraint on selection of materials for bubbler or valve sealing. Consequently, in spite of superior thermal stability and moisture resistance, Zr(THD).sub.4 is disadvantageous because of its low volatility.

Therefore, a primary object of the present invention is to overcome the above problems encountered in the prior art and to provide a novel organometallic zirconium precursor, improved in volatility and in stability at its vaporizing point.

It is another object of the present invention to provide a method for synthesizing the organometallic zirconium precursor by a in-situ process.

It is a further object of the present invention to provide a lead-zirconium-titanium thin film made of an organometallic zirconium precursor, which is superior in reliability and reproducibility.

Based on intensive and through research and study by the present inventors, the above objects are accomplished by providing an organometallic zirconium precursor for ferroelectric lead-zirconium-titanium thin film, represented by the following Formula I:

wherein L is an electron donor ligand selected from the group consisting of NR.sub.3 (R.dbd.H, CH.sub.3) gas and Cl.sub.2 gas; THD denotes 2,2',6,6'-tetramethyl-3,5-heptanedione; and x is in the range of 0.3 to 1.5 with the proviso that L is NR.sub.3 (R.dbd.H, CH.sub.3) or in the range of 0.5 to 2 with the proviso that L is Cl.sub.2.

In accordance with another aspect of the invention, there is provided a method for the preparation of organometallic zirconium precursor, comprising flowing an electron donor in gas phase into a bubbler containing bis (2,2',6,6'-tetramethyl-3,5-heptanedione)Zr at a predetermined temperature, to synthesize, in-situ, an adduct represented by the above formula.

In accordance with a further aspect of the invention, there is provided a method for the deposition of lead-zirconium-titanium thin film, comprising the steps of: flowing an electron donor in the gas phase into a bubbler containing bis (2,2',6,6'-tetramethyl-3,5-heptanedione)Zr at a selected reaction temperature, to synthesize, in-situ, an adduct represented by the above formula; cooling the adduct to solidify it; heating the bubbler to a temperature lower than said reaction temperature, to volatilize the adduct; passing the adduct with a carrier gas into a metal-oxide vapor deposition reactor; and reacting the volatilized adduct with a titanium precursor source and a lead precursor source, at a high temperature and at reduced pressure in an oxidative atmosphere by metal-oxide chemical vapor deposition.

The above objects and other advantages of the present invention will become more apparent by describing in detail the preferred embodiment of the present invention with reference to the attached drawings in which:

FIG. 1 is a schematic longitudinal section showing bubbler equipment useful to synthesize an organometallic lead precursor of the present invention by an in-situ process; and

FIG. 2 is a schematic view showing an MOCVD apparatus useful for preparing a lead-titanium based thin film from the organometallic lead precursor of the present invention.

In a Zr(THD).sub.4 molecule, Zr(IV), the central atom, is coordinately bonded with THD, the ligands, by incorporation of the electrons on the carbonyl groups of the ligands into the bonding sphere of the metal. Generally, THD, which is a neutral ligand without nominal charge, cannot donate sufficient electrons to the Zr atom. Accordingly, the electrophilic Zr atom comes to attractively interact with the THD ligands of neighboring Zr(THD).sub.4 molecules. As a result, Zr(THD).sub.4 molecules form a stable solid structure at room temperature.

On the other hand, coordination bonding of the Zr atom with additional ligands capable of donating sufficient electrons results in weakening or breaking the intermolecular interaction in the solid structure. The coordinated Zr(THD).sub.4 molecules are able to volatilize at lower temperatures than original Zr(THD).sub.4.

In accordance with the present invention, NR.sub.3 (R.dbd.H, CH.sub.3) or Cl.sub.2 molecules are utilized as the additional electron donor. Since the NH.sub.3 or Cl.sub.2 molecule has lone electron pairs, it can supply sufficient electrons to the central atom. In addition, such additional electron donors are relatively small in size, so that they can easily contact with the Zr atom. Therefore, the NR.sub.3 (R.dbd.H, CH.sub.3) or Cl.sub.2 electron donor itself can be chemically adducted to Zr(THD).sub.4, as represented by the following reaction formulas:

wherein R is H or CH.sub.3, x is in the range of 0.3 to 1.5 and y is in the range of 0.5 to 2. In the NR.sub.3 (R.dbd.H, CH.sub.3) or Cl.sub.2 adduct, the additional electron donors break the intermolecular interaction among Zr(THD).sub.4 molecules, so that the adduct can be volatilized sufficiently at low temperatures, for example, about 110.degree. to about 170.degree. C., relative to the vaporizing point of Zr(THD).sub.4, about 220.degree. to 240.degree. C. Thus, the obtained adduct can be utilized as an organometallic zirconium precursor with a remarkable improvement in volatility.

In accordance with the present invention, the organometallic zirconium precursor is prepared by a characteristic method comprising flowing a gas phase electron donor into a bubbler containing Zr(THD).sub.4 at a predetermined temperature, to synthesize, in-situ, an adduct represented by the following formula:

wherein L is an electron donor ligand selected from a group

consisting of NR.sub.3 (R.dbd.H, CH.sub.3) and Cl.sub.2 ; and

x is in the range of 0.3 to 1.5 with the proviso that

L is NR.sub.3 (R.dbd.H, CH.sub.3), or x is in the range of 0.5 to 2 with the proviso that L is Cl.sub.2.

The gas phase electron donor used to prepare the organometallic lead precursors of the present invention is selected from the group consisting of NR.sub.3 (R.dbd.H, CH.sub.3) and Cl.sub.2.

A more detailed description for the preparation of the present organometallic zirconium precursors will be given as follows.

For the preparation of (NR.sub.3).sub.x.Zr(THD).sub.4 (R.dbd.H or CH.sub.3) by an in-situ process, solid phase Zr(THD).sub.4 is placed in the bubbler, and NR.sub.3 (R.dbd.H or CH.sub.3) gases are used as a carrier gas. The temperature of the bubbler is kept to 120.degree.-170.degree. C. Zr(THD).sub.4 in the bubbler reacts with NR.sub.3 (R.dbd.H or CH.sub.3) carrier gas, and (NR.sub.3).sub.x.Zr(THD).sub.4 (R.dbd.H or CH.sub.3, 0.3.ltoreq.x.ltoreq.1.5) is synthesized. The produced chemical species are so volatile that they are vaporized instantly at the bubbler temperature of 120.degree. to 170.degree. C. Vaporized (NR.sub.3).sub.x.Zr(THD).sub.4 is carried by unreacted electron donor gases (NH.sub.3 or N(CH.sub.3).sub.3), and can be used for the thin film deposition in the reactor. Consequently, by simply flowing NR.sub.3 (R.dbd.H or CH.sub.3) as carrier gas, Zr(THD).sub.4 precursor can be easily vaporized at a temperature of 120.degree. to 170.degree. C. This bubbling temperature is considerably lower than that of conventional process, which uses Zr(THD).sub.4 as precursor and Ar or N.sub.2 as carrier gas. Therefore, there is no decomposition of precursor during the bubbling process and a constant vapor pressure can be maintained over an elapsed bubbling time.

For preparation of solid phase (Cl.sub.2).sub.x.Zr(THD).sub.4, Zr(THD).sub.4 is placed in the silica boat as shown in FIG. 1 and then, an electron donor in the gas phase (Cl.sub.2) is supplied under the control of a Mass Flow controller. At an elevated temperature, an adduct between Zr(THD).sub.4 and Cl.sub.2 is synthesized in the gas phase. As for the reaction conditions, the temperature of Zone 1 is maintained in the range of about 200.degree. to about 240.degree. C. for (Cl.sub.2).sub.x.Zr(THD).sub.4, whereas the temperature of Zone 2 is maintained at room temperature. At this temperature ranges, Cl.sub.2 gas reacts with Zr(THD).sub.4 rapidly. The gas phase adducts ((Cl.sub.2).sub.x.Zr(THD).sub.4) resulting from the high temperature reaction are solidified in Zone 2 because the wall of the tube in Zone 2 is cooled to room temperature. Since Cl.sub.2 gases are instantly reacted with Zr(THD).sub.4 under these conditions, the amount of synthesized adduct can be calculated on the basis of the amount of charged Zr(THD).sub.4 in Zone 1. Meanwhile, unreacted electron donor gas is not passed into a reactor connected to the tube of Zone 2, but drained through a bypass and removed through reaction with an aqueous NaOH solution. Then, the carrier gas is replaced with N.sub.2 and the bubbler temperature is adjusted to 110.degree. to 160.degree. C., which is greatly lower than the temperature of the conventional process.

Referring now to FIG. 2, there is shown an MOCVD apparatus useful to deposit PZT thin films. As shown in this figure, this MOCVD apparatus, which is a kind of a low pressure CVD apparatus operating in a hot wall manner, is provided with bubbler equipment for precursor sources, e.g. a Pb source, a Zr source and a Ti source, a reactor and a cold trap.

For the deposition of a PZT thin film by using NR.sub.3 (R.dbd.H, CH.sub.3) adducted Zr(THD).sub.4, Zr(THD).sub.4 is first charged into a bubbler. Then, the temperature of bubbler is adjusted to 120.degree.-170.degree. C. and NR.sub.3 gas with a flow rate of 50-100 sccm is employed. The in-situ generated (NR.sub.3).sub.x.Zr(THD).sub.4 is carried to a reactor by the NR.sub.3, which is not consumed in the reaction with Zr(THD).sub.4. For the Ti source, Ti(OEt).sub.4 is typically used, and it is heated to 100 -125.degree. C., while 10-20 sccm of N.sub.2 gas is preferably used as a carrier gas. For the Pb source, Pb(THD).sub.2 is used, and it is heated to 140.degree.-160.degree. C., while 50 sccm of N.sub.2 gas is preferably used as a carrier gas.

For the deposition of a PZT thin film by using chlorine-adducted Zr(THD).sub.4, first, the prepared (Cl.sub.2).sub.x.Zr(THD).sub.4 in Zone 2 of the bubbler of FIG. 1 is first heated to 110.degree.-160.degree. C. In order to carry the vaporized chlorine-adducted Zr(THD).sub.4, N.sub.2 gas with a flow rate of 50-100 sccm is employed. In the same way, for the Ti source, Ti(OEt).sub.4 is typically used, and it is heated to 100.degree.-125.degree. C., while 10-20 sccm of N.sub.2 gas is preferably used as a carrier gas. For the Pb source, Pb(THD).sub.2 is used, and it is heated to 140.degree.-160.degree. C., while 50 sccm of N.sub.2 gas is preferably used as a carrier gas.

Thereafter, the vaporized precursor sources are decomposed in the reactor heated to high temperatures, for example, about 550.degree. to about 650.degree. C. and then, reacted with oxygen gas which is provided from another regulated route, for deposition of PZT thin film on a substrate. For PbZr.sub.x Ti.sub.1-x O.sub.3 thin film, the oxygen gas is flowed in a rate of 500 sccm. The cold trap which is connected with the reactor, in the meanwhile, removes unreacted precursor sources. A vacuum pump works to maintain a reduced pressure in the reactor.

The preferred embodiments of the present invention will now be further described with reference to specific examples.

The MOCVD apparatus of FIG. 2 was used to deposit a PZT thin film.

First, 1.5 g of Zr(THD).sub.4 was charged in a stainless tube of the bubbler and heated to about 160.degree. C. Then, when the bubbler temperature was stabilized, a dry ammonia gas was flowed in a rate of 100 sccm. The (NH.sub.3).sub.x.Zr(PHD).sub.4 adduct thus produced was carried into the reactor by unreacted NH.sub.3 gas.

Separately, 10 g of Ti(OEt).sub.4 and 1 g of Pb(THD).sub.2 were charged in respective bubblers and the temperatures in the Ti and Pb bubbler were raised to about 115.degree. C. and about 155.degree. C., respectively. The Ti source was supplied to the reactor by a carrier gas of N.sub.2 with a flow rate of about 15 sccm, whereas the Pb source was supplied by the same carrier gas of N.sub.2 with a flow rate of 50 sccm. Then, the reactor was heated to about 600.degree. C., to deposit the PZT thin film.

More detailed reaction conditions are reported in the following Table I.

Under the conditions given in Table I, the reactor was operated for 30 minutes, to obtain a PZT thin film of perovskite phase with a thickness of 300 nm.

(Cl.sub.2).sub.x.Zr(THD).sub.4 was synthesized using the bubbler equipment of FIG. 1.

For this, a quartz boat containing 1.5 g of Zr(THD).sub.4 was positioned in Zone 1. Zone 1 was heated to 220.degree. C. and Zone 2 was maintained in room temperature by circulating cold water around Zone 2. Dry chlorine gas flowed in a rate of 50 sccm for 20 minutes and was consumed by reaction with Zr(THD).sub.4 positioned in Zone 1. The gas phase adduct thus produced, (Cl.sub.2).sub.x.Zr(THD).sub.4, was solidified in Zone 2, cold region.

Thereafter, Example 1 was repeated using the deposition conditions given in the following Table II. Particularly, the carrier gas for the Zr source was replaced by nitrogen gas in this Example.

Under such conditions, the reactor was operated for 30 minutes, to obtain a PZT thin film of perovskite phase with a thickness of 300 nm.

The following are the advantages obtained when the organometallic zirconium precursors of the present invention, (NR.sub.3).sub.x.Zr(THD).sub.4 (R.dbd.H, CH.sub.3) and (Cl.sub.2).sub.x.Zr(THD).sub.4, are used for PZT thin films.

First, it is possible to obtain a PZT thin film superior in reliability and reproducibility, since the above precursors can be used repetitively in a bubbler without any decomposition.

Second, the precursors of the present invention are able to be vaporized at much lower temperatures than is Zr(THD).sub.4.

Third, the precursors of the present invention are very stable at the preheat temperatures for vaporization.

Fourth, since (NR.sub.3).sub.x.Zr(THD).sub.4 (R.dbd.H, CH.sub.3) and (Cl.sub.2).sub.x.Zr(THD).sub.4 are hygroscopic, there is a danger that these precursors decompose during treatment if they are prepared outside the bubbler and then delivered into the bubbler. In the present invention, the above danger is eliminated because the precursors are synthesized in the bubbler in-situ.

Finally, N.sub.2 gas, which is utilized as a carrier gas for (Cl.sub.2).sub.x.Zr(THD).sub.4 precursor prevents corrosion or damage to the reactor.

Other features, advantages and embodiments of the invention disclosed herein will be readily apparent to those exercising ordinary skill after reading the foregoing disclosures. In this regard, while specific embodiments of the invention have been described in considerable detail, variations and modifications of these embodiments can be effected without departing from the spirit and scope of the invention as described and claimed herein.

The expression "PZT thin films", or "lead-zirconium-titanium thin film", as used in the specification denotes Pb(ZrTi)O.sub.3 (PZT) and doped Pb(ZrTi)O.sub.3 (PZT), such as Pb(NbZrTi)O.sub.3 (PNZT), Pb(LaZrTi)O.sub.3 (PLZT), Pb(TaZrTi)O.sub.3 (PTZT) and Pb(ScZrTi)O.sub.3 (PSZT).