JP4471668B2 - Capacitor and nonvolatile memory device using the capacitor - Google Patents

Description

  The present invention relates to an LSI capacitor, a ferroelectric nonvolatile memory, a piezoelectric pyroelectric sensor, a microactuator, a bypass capacitor, etc., and an element having a ferroelectric film as a component of the capacitor.

  As a typical ferroelectric film element, in recent years, a ferroelectric memory using a ferroelectric film as a capacitor film has been proposed in a nonvolatile memory element that retains stored contents even when drive power is cut off. .

However, a conventional ferroelectric film made of a ferroelectric material such as Pb (Zr, Ti) O ₃ (hereinafter abbreviated as PZT) or SrBi ₂ Ta ₂ O ₉ (hereinafter abbreviated as SBT) is used as a memory. The introduction of the capacitor film has problems such as requiring film formation or heat treatment at a high temperature for crystallization of the ferroelectric film and a large leak current.
Special table 2001-527281 Japanese Unexamined Patent Publication No. 5-259391 Japanese Patent Laid-Open No. 5-235525

  The present invention was created to solve the disadvantages of the prior art described above, and its purpose is to suppress a decrease in remanent polarization, to lower a crystallization temperature and to prevent an increase in leakage current, It is an object of the present invention to provide a capacitor ferroelectric film using a PZT film or an SBT film that improves the ferroelectric characteristics of the capacitor ferroelectric film and contributes to speeding up and capacity increase of the ferroelectric memory element.

  In order to solve the above-mentioned problems, the present inventors have conducted extensive research on this point, and as a result of various experiments, by adding silicon dioxide to a dielectric film such as a PZT film or an SBT film, capacitor insulation is achieved. It was confirmed that the dielectric properties of the film were improved, the crystallization temperature was reduced, and the leakage current was reduced, and the present invention was completed.

Invention of Claim 1 made | formed based on the said knowledge, The base, The 1st electrode film arrange | positioned on the said base, The insulating film arrange | positioned on the surface of said 1st electrode film, A second electrode film disposed on a surface of the insulating film, the insulating film being a capacitor mainly composed of a ferroelectric material, the insulating film containing silicon dioxide, and the insulating film Contains at least one ferroelectric material selected from the group consisting of Pb (Zr, Ti) O ₃ , SrBi ₂ Ta ₂ O ₉ and Bi ₃ Ti ₄ O ₁₂ In addition, the capacitor has an atomic concentration of silicon of 20% in the insulating film.
According to a second aspect of the invention, a capacitor of claim 1, wherein the thickness of the insulating film is a 1000nm or less of the capacitor more than 25 nm.
According to a third aspect of the invention, a capacitor of any one of claims 1 or claim 2, wherein the first, second electrodes, and Pt, and Ir, and Ru, and SrRuO _3, It is a capacitor containing any one type of conductive material selected from the group consisting of LaNiO ₃ .
According to a fourth aspect of the present invention, there is provided a non-volatile memory device comprising the capacitor according to any one of the first to third aspects, wherein the non-volatile memory device stores data by the electric charge accumulated in the capacitor. It is.

  The present invention is configured as described above. By adding silicon dioxide to the insulating film, smoothing of the insulating film is promoted, and crystallization of a ferroelectric material such as PZT or SBT is facilitated. Promoted. Accordingly, since the ferroelectric material is crystallized at a temperature lower than that of the prior art, the base is not easily damaged by heat, and the orientation of the crystal is improved, so that the spontaneous polarization of the insulating film is increased.

  Further, in the insulating film, at least silicon dioxide particles are present on the surface of the crystal grains of the ferroelectric material, and Si—O bonds are formed at the grain boundaries of the crystal grains. No slipping occurs, and the leakage current of the insulating film is reduced.

  When sputtering is performed using a target to which silicon dioxide is added and the atomic concentration of silicon is 5% or more and 25% or less and the atomic concentration of silicon is the same, silicon dioxide is contained and the atomic concentration of silicon is 5 % Or more and 25% or less of an insulating film can be obtained.

  According to the present invention, an increase in leakage current of an insulating film is suppressed by using a ferroelectric film prepared by adding silicon dioxide to a film of a ferroelectric material such as PZT or SBT. In addition, low temperature crystallization is achieved, and a memory element having good ferroelectric capacitor characteristics can be realized. Furthermore, the present invention can be applied to a film of a ferroelectric material manufactured by another film forming method such as a conventionally known MOCVD method, and its usefulness is tremendous.

A process of forming the insulating film 15 described above will be described. First, silicon dioxide (SiO ₂ ) powder is added to the metal oxide powder at a desired ratio so that the atomic concentration of silicon is 5% to 25%, and these powders are mixed by a ball mill to obtain a target material. . Here, lead oxide (PbO), zirconium oxide (ZrO ₂ ), and titanium oxide were used as metal oxides.

  After the target material is temporarily fired to remove moisture, it is finely pulverized and formed by a CIP (cold isostatic pressing) method, and then fired at a temperature higher than the temporary firing (main firing). Obtained.

  The target 5 produced by the manufacturing method described above is carried into the vacuum chamber 31 of the sputtering apparatus 3 shown in FIG. 1 and arranged so as to face the substrate holder 36. Next, after the first electrode film 14 made of a conductive film (here, a platinum film having a thickness of 100 nm) is formed on one surface of the base, which is the object to be processed, by another sputtering apparatus, the base is vacuumed. Carry into the tank 31.

  The base 19 has a substrate 11 made of silicon, a silicon oxide film 12 disposed on the surface of the substrate 11 and formed by heat treatment, and a titanium oxide film 13 formed on the surface of the silicon oxide film 12. The first electrode film 14 is formed on the surface of the titanium oxide film 13. The base 19 is held by the substrate holder 36 with the surface on which the first electrode film 14 is formed facing the target 5.

Sputtering gas is introduced into the vacuum chamber 31 from the sputtering gas introduction system 37 while the vacuum chamber 31 is evacuated by the evacuation system 39 connected to the vacuum chamber 31. When the power supply 35 connected to the target 5 is activated when the inside of the vacuum chamber 31 is stabilized at a predetermined pressure, the target 5 is sputtered, and at least the surface of the PZT (Pb (Zr, Ti) O ₃ ) particles has silicon dioxide. The insulating film 15 on which the particles are formed grows. Sputtering is stopped when the insulating film has grown to a predetermined thickness of 25 nm to 1000 nm (here, a thickness of 100 nm).

  The base 19 on which the insulating film is formed is carried from the sputtering apparatus 3 to another sputtering apparatus, and a platinum film (film thickness: 100 nm) as a conductive film is formed on the surface of the insulating film, and the second electrode film Form. When the insulating film 15 has PZT, when heated at a temperature of 450 ° C. or higher, the PZT in the insulating film 15 grows.

  Reference numeral 1 in FIG. 2 indicates the capacitor after the heat treatment, reference numeral 15 in FIG. 2 indicates an insulating film, and reference numeral 16 in FIG. 2 indicates a second electrode film. As described above, the insulating film 15 has PZT which is a ferroelectric material, and a voltage is applied between the first and second electrode films 14 and 16, and after the voltage application is finished, the insulating film 15 Charges are left inside.

  By containing silicon dioxide, the leakage current of the insulating film 15 is reduced, so that the charge in the insulating film 15 is maintained for a long time even after the voltage application is finished. Therefore, a nonvolatile memory using the capacitor 1 as a storage element can store a record for a long time.

For the capacitor 1 of the present invention, the following switching charge amount was measured.
<Switching charge amount>
Using seven types of targets 5 with different amounts of silicon dioxide added, sputtering is performed so that the silicon atomic concentration of each target is the same as the silicon atomic concentration of the insulating film, and seven types of capacitors 1 are produced. did. The silicon atomic concentration of each target 5 is 0%, 5%, 10%, 15%, 20%, 25%, 30%, and the silicon concentration of each insulating film 15 is 0%, 5%, 10%, 15 %, 20%, 25%, and 30%. Each second electrode film 16 was circular with a diameter of 5 mm.

Separately, except that strontium oxide, bismuth oxide and tantalum oxide were used as metal oxides, seven types of targets 5 having different silicon dioxide addition amounts were produced in the above-described steps, and these targets 5 were used. Then, sputtering was performed so that the silicon atomic concentration of each target 5 and the atomic concentration of the insulating film were the same, and seven types of capacitors 1 were produced. In the insulating film 15 of these capacitors 1, SrBi ₂ Ta ₂ O ₉ (SBT), which is a ferroelectric material, is formed.

  A voltage was applied between the first and second electrode films 14 and 16 of each capacitor 1, and the voltage application was completed. The charge per unit area remaining in the capacitor 1 after a lapse of a certain time from the end of voltage application was measured and used as the switching charge amount.

  The measurement results when the ferroelectric material of the insulating film 15 is PZT and SBT are shown in the graphs of FIGS. The vertical and horizontal axes in FIGS. 3 and 4 respectively indicate the switching charge amount and the atomic concentration of silicon in the insulating film 15.

  As can be seen from FIGS. 3 and 4, when the ferroelectric material of the insulating film 15 is either PZT or SBT, when the atomic concentration of silicon is 5% or more and 25% or less, silicon is not added (atomic concentration 0%) and the amount of switching charge was larger than when the atomic concentration of silicon was 30%, and the amount of switching charge was maximized when the silicon concentration was 20%.

  As the switching charge amount is larger, the spontaneous polarization of the insulating film 15 is higher. Therefore, when the ferroelectric material is both PZT and SBT, the silicon concentration in the insulating film 15 is higher. It can be seen that the capacitor 1 having high suitability as the storage element of the nonvolatile storage device can be obtained when the ratio is 5% or more and 25% or less.

Next, the “leakage current” shown below was measured using the 14 types of capacitors 1 described above.
<Leakage current>
The leakage current per unit area when a voltage of 5 V was applied between the first and second electrode films 14 and 16 of each capacitor 1 was measured. The measurement results when the ferroelectric material of the insulating film 15 is PZT and when it is SBT are shown in FIGS.

5 and 6, the vertical axis represents leakage current, and the horizontal axis represents the atomic concentration of silicon in the insulating film 15.
As is clear from FIGS. 5 and 6, the leakage current was lower when the silicon concentration of the insulating film 15 was 5% or more than when the silicon concentration was zero. The higher the silicon concentration, the lower the leakage current. However, when the silicon concentration exceeded 25%, no further reduction in the leakage current was observed. From these results, it was found that in both cases of PZT and SBT, if the silicon of the insulating film 15 is 5% or more, the leakage current is lowered.

<X-ray diffraction>
After forming an insulating film 15 having PZT and a silicon concentration of 20% on the surface of the platinum film, the insulating film 15 is subjected to heat treatment at temperatures of 450 ° C., 500 ° C., and 550 ° C. Test pieces were prepared, and X-ray diffraction of the insulating film 15 of each test piece was performed. Further, as a comparative example, an insulating film made of PZT and not added with silicon dioxide was formed. Similarly, after heat treatment, X-ray diffraction was performed.

  A chart in which no silicon dioxide is added and a chart in the case where silicon dioxide is added are shown in FIGS. 7 and 8, the horizontal axis indicates the diffraction angle, the vertical axis indicates the intensity, and the symbols a to c in FIGS. 7 and 8 indicate the temperature of the heat treatment, respectively.

  As is clear from FIG. 7, in the insulating film in which no silicon dioxide is added and the atomic concentration of silicon is zero, a peak on the (111) plane showing a PZT perovskite layer is observed when the heating temperature is less than 500 ° C. Although not shown in FIG. 8, in the insulating film 15 to which silicon was added, a peak on the (111) plane showing a PZT perovskite layer was observed even when the heating temperature was 450 ° C. In addition, when silicon dioxide was added, the peak intensity of the (111) plane showing the PZT perovskite layer was higher when silicon dioxide was added than when the silicon atomic concentration was zero. From these facts, it can be seen that if silicon dioxide is added, crystallization of PZT can be performed at a lower temperature and crystal growth is promoted.

  Next, after forming an insulating film 15 having SBT and having a silicon concentration of 20% on the surface of the platinum film, heat treatment is performed at each temperature of 650 ° C., 700 ° C., 750 ° C., and 800 ° C. to obtain four types These test pieces were prepared, and X-ray diffraction of the insulating film 15 of each test piece was performed. Further, as a comparative example, an insulating film made of SBT and not added with silicon dioxide was similarly heat-treated and X-ray diffraction was performed. FIGS. 9 and 10 show a chart when no silicon dioxide is added and a chart when silicon dioxide is added, respectively. 9 and 10 indicate the temperature of the heat treatment.

  It is known that when an SBT film is grown on a platinum film (first conductive film 14), a peak of a layered Bi structure is seen on the (115) plane. Referring to FIG. 9, when silicon dioxide is not added, the peak of the (115) plane does not appear when the heating temperature is less than 750 ° C., but when silicon dioxide is added, the heating temperature is 650 ° C. (115) plane peak was confirmed. From these, it can be seen that if silicon is added to SBT, crystallization of SBT proceeds at a lower temperature.

<Other examples>
The case where PZT and SBT are used as the ferroelectric material of the insulating film 15 has been described above. However, the type of the ferroelectric material is not particularly limited, and for example, Bi ₃ T ₄ O ₁₂ can be used.

In the case of forming the insulating film 15 having Bi ₃ Ti ₄ O ₁₂ , silicon dioxide is added to and mixed with bismuth oxide and titanium oxide, which are metal oxides, and a target material is prepared, as in the case of PZT. After the target material is molded, the target 5 is manufactured by firing.

  In the above, the case where the insulating film 15 is formed by the sputtering method has been described. However, the present invention is not limited to this. For example, silicon dioxide is added to and mixed with a ferroelectric material such as PZT or SBT. An insulating film material is prepared, and a coating solution in which the insulating film material is dispersed in a solvent is applied to the surface of the first electrode film 14 and dried, or MOCVD (Metal Organic Chemical Vapor Deposition) A film can also be formed by a phase deposition method.

  In the above embodiment, the film thicknesses of the first and second electrode films 14 and 16 are 100 nm, respectively, but the surface roughness is increased so long as it does not affect the characteristics of the PZT or SBT ferroelectric film. The film thickness is not particularly limited.

The constituent materials of the first and second electrode films 14 and 16 are not limited to platinum, and films made of metals such as Ir (indium), Ru (ruthenium), SrRuO ₃ , LaNiO ₃ , and alloys thereof, An oxide film conductor can also be applied to achieve a desired purpose. Also, the first and second electrode films 14 and 16 can be made of different types of conductors.

  In the above embodiment, the thickness of the insulating film 15 is 100 nm. However, this thickness is not particularly limited as long as it is an allowable value required for the structure of the ferroelectric memory element. However, if the film thickness of the insulating film 15 is less than 25 nm, a sufficient amount of switching charge cannot be obtained, and if the film thickness exceeds 1000 nm, excessive power is required to polarize the insulating film 15. The film 15 preferably has a thickness of 25 nm to 1000 nm.

Sectional drawing explaining an example of the sputtering device used for this invention Sectional drawing explaining an example of the capacitor | condenser of this invention A graph showing the relationship between the switching charge amount of an insulating film containing PZT and the atomic concentration of silicon The graph which shows the relationship between the switching charge amount of the insulating film containing SBT, and the atomic concentration of silicon The graph which shows the relationship between the leakage current of the insulating film containing PZT, and the atomic concentration of silicon The graph which shows the relationship between the leakage current of the insulating film containing SBT, and the atomic concentration of silicon X-ray diffraction chart of insulating film made of PZT X-ray diffraction chart of insulating film containing both PZT and silicon dioxide X-ray diffraction chart of insulating film made of SBT X-ray diffraction chart of insulating film containing both SBT and silicon dioxide

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Capacitor 3 ... Sputtering device 5 ... Target 11 ... Substrate 12 ... Silicon oxide film 13 ... Titanium oxide film 14 ... First electrode film 15 ... Insulating film 16 ... Second electrode film 19 …… Base 31 …… Vacuum chamber 35 …… Power supply 36 …… Substrate holder 37 …… Gas supply system 39 …… Vacuum exhaust system

Claims (4)

A base, a first electrode film disposed on the base, an insulating film disposed on a surface of the first electrode film, and a second electrode film disposed on a surface of the insulating film; And the insulating film is a capacitor mainly composed of a ferroelectric material,
The insulating film contains silicon dioxide ;
The insulating film is at least one ferroelectric material selected from the group consisting of Pb (Zr, Ti) O ₃ , SrBi ₂ Ta ₂ O ₉ and Bi ₃ Ti ₄ O _12. Containing materials,
A capacitor in which an atomic concentration of silicon in the insulating film is 20%. The capacitor according to claim 1 , wherein the insulating film has a thickness of 25 nm to 1000 nm. Wherein the first, the second electrode, Pt and, Ir and, Ru and a SrRuO _3, claim containing any one of a conductive material selected from the group consisting of LaNiO ₃ 1 or claim 2 The capacitor | condenser of any one of these. Claim 1 has a capacitor according to any one of claims 3, nonvolatile memory device for storing data by accumulating charges in the capacitor.

Images