JP3514940B2 - Method of forming ferroelectric thin film - Google Patents

Description

Description: BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The present invention relates to a method for forming a ferroelectric thin film applied to a semiconductor device or the like.
More specifically, the present invention relates to a method for forming a perovskite-based bismuth layered structure compound ferroelectric thin film containing strontium, bismuth, and tantalum. 2. Description of the Related Art As a new use of a dielectric material, a study for forming a nonvolatile memory on a semiconductor substrate by using ferroelectricity has been conducted. As an example, there is a method of forming a memory cell using one transistor and one ferroelectric capacitor. The stored information is detected as a difference in the amount of current flowing corresponding to the polarization inversion and non-inversion of the ferroelectric capacitor.
Another method uses a ferroelectric material for the gate insulating film of the field-effect transistor, and detects the difference in drain current by changing the channel resistance of the transistor depending on the polarization state. In these applications, Pb (Zr, Ti) O ₃ and SrBi ₂ Ta ₂ O ₉ have been studied and developed as ferroelectric materials. In the prior art, one of the perovskite-based SrBi ₂ containing strontium, bismuth and tantalum is used.
When a Ta ₂ O ₉ thin film is used, after the thin film is deposited, a high-temperature treatment at 800 ° C. is usually performed in oxygen gas in order to develop ferroelectricity required for application. This temperature is S
In the step before depositing the rBi ₂ Ta ₂ O ₉ thin film, severe thermal history is added to the transistors and peripheral circuit elements formed in advance on the wafer. In particular, when the thin film is applied to a memory capacitor, a conductor structure of tungsten, platinum, polycrystalline silicon, or the like is formed by the high-temperature heat treatment at a connection between the field-effect transistor formed in a previous process and the conductor of the capacitor. It has been known that mutual diffusion of constituent atoms occurs at an interface between objects, causing a problem that the conductivity of the connection portion is deteriorated. As a countermeasure, a method of inserting various materials such as a titanium nitride thin film into the connection interface to prevent diffusion of constituent atoms has been studied, but it has a drawback that the process becomes complicated and the effect is not sufficient. The most reliable method is to lower the high temperature heat treatment. However, conventionally, this method has a problem that when the temperature is lowered, the residual polarization value of the ferroelectric cannot be sufficiently obtained. [0004] What has been recently announced as an attempt low temperature, the residual polarization is also a high-temperature heat treatment in oxygen gas at a heat treatment temperature that the 600 ° C. performed in a reduced pressure environment of 5Torr is the 2P _r = 16μC / cm ^2, a heat treatment The same value as in the case of a temperature of 800 ° C. can be obtained (Ushikubo, Yokoyama, Ito, Matsunaga, Atsugi, Yonezawa, Ogi: Proceedings of the 43rd Joint Lecture on Applied Physics, 2nd volume, p. 45
8, March 1996). However, in order to perform a high-temperature heat treatment in a reduced-pressure atmosphere, it is necessary to prepare a reduced-pressure oxidation furnace different from the conventional one, and there is a drawback that the capital investment is increased. Also, in order to exhaust high-pressure oxygen gas under reduced pressure, it is necessary to devise measures to prevent high-temperature oxygen gas from coming into contact with oil for vacuum sealing such as an oil rotary pump for explosion prevention. There was a problem that it was difficult to become. SUMMARY OF THE INVENTION An object of the present invention is to modify the high-temperature heat treatment step in the production of a ferroelectric thin film of a perovskite-based bismuth layer structure compound containing strontium, bismuth and tantalum. Accordingly, an object of the present invention is to solve the problem of lowering the process temperature at low cost while maintaining the remanent polarization characteristics to a necessary degree. SUMMARY OF THE INVENTION The present invention provides a method for forming a ferroelectric thin film of a perovskite-based bismuth layer structure compound containing strontium, bismuth, and tantalum at 450 ° C. in an oxidizing gas atmosphere. High-temperature heat treatment process at a temperature exceeding 45 ° C in a nitrogen or rare gas atmosphere.
A heat treatment step at a temperature exceeding 0 ° C. and within 200 seconds is added. The difference between this means and the prior art is that
This is to add a high-temperature short-time heat treatment step in a non-oxidizing gas atmosphere different from the conventional oxidizing gas atmosphere, for example, a nitrogen or rare gas atmosphere. By the above means, the temperature of the high-temperature heat treatment can be lowered within a temperature range of several tens of degrees Celsius or more while maintaining the remanent polarization characteristic at a sufficient level as compared with the conventional method. In addition, there is an advantage that it is possible to reduce the heat history applied to elements such as transistors and peripheral circuit elements formed on the wafer, and to reduce deterioration of conductor connection parts such as wiring. In addition, there is an advantage that the conventional manufacturing equipment can be used as it is, and an improvement effect can be obtained at a low cost such that no new equipment investment is required only by adding one new heat treatment step in which the type of gas is changed. . FIG. 1 shows a part of a main part of a process when a ferroelectric capacitor is manufactured as a test sample. This step is not different from a case where a practical capacitor is formed alone or a case where it is formed on a semiconductor device as an element step. By preparing a substrate 1 made of Si, to form an amorphous insulating layer 2 made of SiO ₂ by thermal oxidation in oxygen gas (Figure 1 (a)). A lower electrode 5 is formed by continuously depositing a lower Ti layer 3 having a thickness of 0.03 μm and a lower Pt layer 4 having a thickness of 0.2 μm on the upper surface of the amorphous insulator layer 2 by using an electron beam evaporation method. (FIG. 1B). SrBi ₂ Ta ₂ O _{9 was} repeated four times by spin coating using a metal alkoxide as a starting material by a sol-gel method, desolvating at 150 ° C. for 15 minutes, and calcining at 450 ° C. for 15 minutes four times. A ferroelectric thin film 6 having a composition of 0.2 .mu.m thick is formed as shown in FIG.
(C)). This is followed by a high-temperature short-time treatment in argon gas according to the present invention. The gas pressure is atmospheric pressure, the temperature is 700 ° C., and the heat treatment time is 5 to 6 using a lamp heating mechanism.
It is a short time of 0 second. After that, as in the conventional case, the first baking is performed following the high-temperature short-time heat treatment and changing only the gas type without lowering the temperature in the same furnace. condition is,
700 ° C., oxygen gas, atmospheric pressure, 30 minutes. next,
An upper electrode 7 made of platinum having a thickness of 0.2 μm and a diameter of 100 μm is formed thereon by a vapor deposition method through a metal shadow mask (FIG. 1D). Further, the sample is subjected to a second firing under the same conditions as the first firing. Thus, a capacitor structure is formed. Note that the composition of the film constituent elements can take various values deviating from the atomic composition ratio in the range of ± 20% in atomic ratio. Further, the same effect can be obtained even if the order of the high-temperature and short-time heat treatment according to the present invention is reversed from that of the conventional heat treatment in an oxidizing gas. [0009] Figure 2, the capacitor of the test samples of the two terminals having a three-layer structure of metal / ferroelectric / metal formed in this embodiment, the characteristics showing the relationship between the residual polarization 2P _r and the applied voltage FIG. In this case, as the value of the remanent polarization, a change in localization due to polarization reversal, that is, twice the one-way polarization (P _r ) is used. The processing temperature is set to 700 ° C. compared to the high temperature processing of 800 ° C. in oxygen in the prior art.
It is shown that the value of the remanent polarization decreases to about 2/3 when the value is reduced to. When a memory cell is formed from a film formed under these conditions, it is necessary to take measures such as further increasing the applied voltage. However, an excessive increase causes other problems such as an increase in leakage current of the element. In effect, it makes application difficult. On the other hand, when a high-temperature and short-time heat treatment at 700 ° C. for 5 to 60 seconds is applied in the argon gas according to the present invention, an increase in remanent polarization is obtained. It is possible to recover to a value close to the case. When the applied voltage is not changed, when a memory cell is formed by one transistor and one ferroelectric capacitor, a circuit between "0" and "1" signals corresponding to a decrease in remanent polarization. Although the margin is reduced, this cell can be used as a storage cell. If the heat treatment time is further increased,
It is shown that the effect of the heat treatment is reversed after 200 seconds, and that the effect of the present invention is obtained when the temperature is increased to 240 seconds, and the remanent polarization is reduced as compared with the conventional case of 700 ° C. I can't. As described above, according to this embodiment,
It has been shown that the heat treatment temperature can be reduced from 800 ° C. in the related art to 700 ° C. in the present example without causing significant deterioration of the characteristics. In this embodiment, the high-temperature short-time heat treatment is performed for 7 minutes.
Although the temperature was set at 00 ° C. for 5 to 60 seconds, a temperature of about 450 ° C. was conventionally required in the film deposition step before the heat treatment, and the effect of the heat treatment step cannot be obtained unless the temperature is higher than 450 ° C. . Further, as shown in the embodiment of FIG. 2, if the time exceeds 200 seconds, the remanent polarization characteristic is conversely deteriorated, and the effect of the present invention cannot be obtained. The heat treatment atmosphere gas is argon, but the gas may be non-oxidizing, and the effect of the present invention can be obtained by using other rare gas or nitrogen gas. In short, by heat treatment in a non-oxidizing gas atmosphere for the constituent atoms centered on bismuth in the thin film, the bond with the oxygen atoms was temporarily weakened, and the constituent atoms were arranged at more stable lattice positions. Thereafter, by sufficiently bonding again with oxygen atoms, crystal defects such as unpaired electrons and interstitial atoms are reduced, and the ferroelectric characteristics sensitively reflecting the crystal structure are improved. In this embodiment, an application example in which a memory cell is constituted by one transistor and one ferroelectric capacitor has been described. However, the present invention can be applied to a gate insulating film of a field effect transistor. . Further, application to a scanning probe type recording apparatus using a polarization domain is also possible. As described above, when a thin film of a perovskite-based bismuth layer structure compound containing strontium, bismuth, and tantalum, which is applied to a semiconductor device by the method according to the present invention, is formed, a conventional heat treatment is achieved. It is possible to lower the temperature by several tens of degrees Celsius or more and to reduce the heat history applied to elements such as transistors and peripheral circuit elements formed on the wafer before the formation of the ferroelectric thin film. There is an advantage that the deterioration of the portion can be reduced. Also, since only one new heat treatment step with a different type of gas is added, the conventional manufacturing equipment can be used as it is, and the effect of lowering the temperature can be obtained at low cost, such as no new capital investment is required. There is. Although a Si wafer is used as a substrate in the embodiment, the present invention is not limited to a substrate material,
It is apparent that the present invention can be applied to a compound semiconductor such as aAs, a ceramic plate, a polymer material substrate for mounting a semiconductor device, and the like. In the above embodiment, SrBi ₂ Ta ₂ O
_The film with the composition of ₉ was described, but the point is that Sr and Bi
The effect of the present invention can be obtained by using a thin film of a perovskite-based bismuth layered structure compound containing Ti and Ta as main components, and the present invention is not limited to SrBi ₂ Ta ₂ O ₉ .

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view showing a main part of a process of forming a capacitor as a test sample using the present invention. 2 is a characteristic diagram showing the relationship between the residual polarization 2P _r and the applied voltage of the test sample with a prior art or capacitor structure of the ferroelectric thin film fabricated by the method of the present invention. [Description of Signs] 1 ... Substrate, 2 ... Amorphous insulator layer, 3 ... Lower Ti layer, 4 ...
Lower Pt layer, lower electrode composed of 5 ... 3 and 4, 6 ... ferroelectric thin film, 7 ... upper electrode.

──────────────────────────────────────────────────続 き Continuation of the front page (51) Int.Cl. ⁷ identification code FI H01L 27/108 H01L 29/78 301G 29/78 (58) Investigated field (Int.Cl. ⁷ , DB name) C01G 35/00 H01L 27/10 451

Claims (1)

(57) [Claim 1] A method for forming a perovskite-based bismuth layered compound ferroelectric thin film containing strontium, bismuth, and tantalum, wherein the thin film is formed in a non-oxidizing gas atmosphere. A method for forming a ferroelectric thin film, comprising: heating the film to a temperature exceeding 450 ° C. within 200 seconds, and heating the thin film to a temperature exceeding 450 ° C. in an oxidizing gas atmosphere.