JP3205666B2 - Method for synthesizing CeO2 epitaxial single crystal thin film on Si single crystal substrate - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for synthesizing an insulating CeO ₂ thin film on a semiconductor substrate by epitaxial growth.

[0002]

2. Description of the Related Art Formed on a semiconductor crystal substrate represented by Si
As an insulating layer to be formed, a conventional SiO 2 _Two Ca instead of thin film
F_Two , ZrO_Two The use of ceramic thin films such as
ing. Ceramic thin films are deposited, sputtered,
It is formed on a crystal substrate by CVD or the like. Above all,
CeO_Two The crystal substrate on which is epitaxially grown is Ce
O_Two Focus on the fact that acts as an inert buffer layer at high temperatures,
SOI (silicon-on-insulato)
r), stacking memory, FET device, DRA
The development as M memory etc. is expected. Spatter
Oxidation-reactive evaporation using ring method, CVD method, metal evaporation source
Generally, 10^-FiveHigh pressure gas exceeding torr
The substrate is a single crystal silicon
The effect of surface cleanliness is significant.

In a vapor deposition method using a ceramic material as an evaporation source, electron beam heating is employed as an evaporating means since the ceramic material is generally a high melting point material. At this time, since the ceramic material is always kept in a molten state, for example, in the case of oxide ceramics,
Oxygen is easily released selectively from the molten raw material into a high vacuum atmosphere. The released oxygen instantaneously covers the surface of the crystal substrate and adversely affects the crystallinity of the film grown on the substrate surface. That is, at a normal film forming temperature exceeding 400 ° C., the clean surface of the Si crystal is easily oxidized by desorbed oxygen,
An SiO _x layer is formed. The SiO _x layer has an amorphous state with a random structure. Therefore, the ceramic layer to be formed thereon is likely to be a polycrystalline film that does not reflect the structure of the substrate crystal or a random preferential alignment film in a plane. Further, with the release of oxygen, the amount of oxygen of the raw material becomes smaller than the stoichiometric composition, and the quality of the ceramic material layer is deteriorated.

[0004] In order to solve the problems accompanying the evaporation of the ceramic material and obtain a high quality epitaxially grown layer, various methods have been conventionally studied. For example, in J.I. A
ppl. Phys. 69 (12), 15 June 1
991, pp8313-8315, (111) Si
The temperature dependence of a CeO ₂ layer epitaxially grown on a substrate has been reported. In this report, the influence of the substrate temperature on the crystallinity of the CeO ₂ thin film is investigated while maintaining the substrate in the temperature range of 200 to 850 ° C. in the molecular beam epitaxy method. It is disclosed that when vapor deposition is performed in an ultra-high vacuum, a sufficient epitaxial layer can be formed even at a low substrate temperature of 200 ° C. or lower. Also, Jpn.
J. Appl. Phys. Vol. 33 (1994) p
In P270-274, discloses that the formation and oxidation of the metal Ce layer prior to the reactive sputtering is effective in epitaxial growth of the CeO ₂ layer.

[0005]

In the molecular beam epitaxy method, although the substrate temperature can be lowered as compared with the conventional method, polycrystal is easily generated when the substrate temperature is around 150 ° C. To obtain a single crystal having good crystallinity, a heating temperature still exceeding 200 ° C. is required. On the other hand, in the reactive sputtering method,
If not heated, a single crystal cannot be obtained, and if the temperature reaches 250 ° C., a polycrystal is formed. Japanese Patent Application Laid-Open No. 3-232550 discloses a method in which a ceramic material is subjected to laser abrasion in a high vacuum to epitaxially grow a ceramic thin film on a clean surface of a semiconductor substrate.
According to this method, since the ceramic material layer is epitaxially grown only by the interaction between the substrate surface and the growth film that are kept clean, there is no adverse effect due to impurities, and an epitaxial layer having good crystal characteristics can be obtained. However, also in this case, it is required to maintain the substrate temperature in the range of 600 to 800 ° C.

In each of the conventional methods, a ceramic material layer is epitaxially grown on a crystal substrate kept at a high temperature. When the substrate surface is at a high temperature, atoms and molecules on the surface are activated, and the growth interface is disturbed. Since epitaxial growth proceeds thereon, the resulting ceramic material layer has reduced crystal characteristics due to disorder of the growth interface. As a result, the leak current may increase due to an increase in the interface defect density, or the dielectric constant of the insulating film may decrease due to the deterioration of the crystallinity. The present invention has been devised in order to solve such a problem, and it has been made possible to synthesize a CeO ₂ thin film having good crystal properties and uniformity by enabling epitaxial growth at room temperature without causing a disorder at the growth interface. Aim.

[0007]

In order to achieve the object, a method for synthesizing a CeO ₂ epitaxial single crystal thin film of the present invention comprises placing a Si substrate having a cleaned surface in a high vacuum atmosphere of 10 ^-7 Torr or less. Vapor generated by laser ablation of a CeO ₂ target is vapor-deposited on the surface of the Si substrate, and before the thickness of the vapor-deposited film reaches 5 nm, the oxygen potential in the vacuum atmosphere is increased to 10 ⁻⁶ Torr or more, and laser is continuously applied. It is characterized by ablation deposition. With respect to the Si substrate maintained at room temperature, C
When eO ₂ is deposited, an adverse effect due to disorder of the growth interface is eliminated, and an epitaxially grown thin film having excellent crystal characteristics can be obtained. The oxygen potential of the vacuum atmosphere in the later stage of growth is adjusted by introducing an oxidizing gas such as oxygen gas, ozone, or oxygen radical. As the Si substrate to be vapor-deposited, it is desirable to use a substrate having a surface terminated with H by NH ₄ F treatment after HF treatment. Hydrogen termination improves the oxidation resistance of the Si substrate surface,
The formation of an amorphous silicon oxide film on the Si substrate surface is suppressed even after being left in air for several hours. Further, since vapor deposition at room temperature is possible, a CeO ₂ single crystal thin film can be similarly epitaxially grown on a Si substrate on which an organic resist film weak to heat is formed.

[0008]

When laser ablation deposition is performed using an oxide as a target, elements constituting an oxide film such as oxygen and metal are simultaneously supplied onto the substrate. At this time, if the substrate temperature is maintained at room temperature, disturbance due to uneven heat distribution does not occur at the deposition interface. When the deposition proceeds in this state, even in a deposition atmosphere with a very high degree of vacuum, oxygen does not escape into the atmosphere, and the amount of oxygen necessary to form a predetermined oxide thin film is included together with the metal element. It is supplied to the substrate surface. As a result, an oxide thin film imitating the crystal orientation of the substrate surface grows epitaxially on the substrate surface. Further, since vapor deposition by laser ablation in which a laser is introduced through a window of a film formation chamber is employed, there is no mixing of an oxygen element or an impurity element other than an oxide target. Therefore, the oxygen of the evaporation source is consumed as it is as a deposited film, and the composition stability of the generated epitaxially grown film is improved.

Since the formed thin film grows epitaxially while being affected by the crystal orientation of the outermost surface of the substrate, the influence of the substrate is reduced as the growth proceeds, and oxygen defects are likely to occur due to disorder of the crystal. If there is an oxygen vacancy, the resistance value at that portion will decrease, causing current leakage. Therefore, the oxygen potential of the atmosphere is raised in the latter half of the growth to take in oxygen from the atmosphere. Thereby, a necessary amount of oxygen is supplied to the growth interface throughout the entire growth process, and an oxide thin film having excellent crystal characteristics can be obtained. The atmosphere in the early growth period is maintained at a high vacuum of 1 × 10 ⁻⁷ Torr or less. This degree of vacuum is an important factor in transporting Ce and O _{2 in} amounts corresponding to the CeO ₂ composition evaporated from the target to the Si substrate surface. When the degree of vacuum exceeds 1 × 10 ⁻⁷ Torr, excess oxygen is taken in as compared with the CeO ₂ composition, and the appearance of crystal phases other than CeO ₂ and the formation of non-epitaxial phases such as polycrystals that do not match the substrate crystal orientation are generated. And other defects are likely to occur.

At this high vacuum, a CeO ₂ thin film is epitaxially grown on the surface of the Si substrate to a thickness of about 50 °. When the CeO ₂ thin film reaches such a thickness, element evaporation from the Si substrate is suppressed, and crystal defects caused by surface irregularities are not brought into the CeO ₂ thin film. CeO ₂
When the thin film has grown to about 50 °, the oxygen potential of the atmosphere is raised to 1 × 10 ⁻⁶ Torr or more by introducing oxygen gas, ozone, oxygen radicals, and the like. Ce is
Since oxygen is an element having a very high oxygen affinity, oxygen, which tends to be insufficient, is taken in from the atmosphere, and C having a stoichiometric composition
Grow an eO ₂ thin film.

[0011]

EXAMPLE As a Si substrate, 5-inch Si (111) was used.
It was used. However, the present invention is not limited to this, and Si (110), Si (100), and the like can be used. Further, since the substrate is grown epitaxially without heating, there is no restriction on the size of the substrate. The Si substrate is ultrasonically cleaned using an organic solvent such as acetone or ethanol, and 35% HC maintained at a temperature of 80 ° C.
The substrate was immersed in a 1-31% H ₂ O ₂ aqueous solution for 10 minutes to oxidize the surface of the Si substrate. Then 5% with a 10% HF solution at room temperature.
After that, the oxide film was removed from the surface of the Si substrate. However, the surface of the HF-treated Si substrate became severe when observed with an atomic force microscope. Then, the oxide film was further removed by performing treatment with a 40% NH ₄ F solution at room temperature for 5 minutes, and the surface of the Si substrate was terminated with hydrogen. The Si substrate after the NH ₄ F treatment exhibited a flat surface with reduced surface irregularities, and had excellent stability in air due to hydrogen termination.

The Si substrate whose surface was prepared in this manner was set in a film forming room in a clean room. The film forming chamber was evacuated until the internal pressure became 10 ⁻⁷ Torr or less.
A pulse excimer laser having a wavelength of 248 nm was focused on a CeO ₂ target maintained in a vacuum atmosphere via a focusing lens. At this time, irradiation conditions were set so that the laser energy density on the CeO ₂ target was 1 Joule / cm ² . Further, the Si substrate was maintained at room temperature without heating. When deposition was performed under these conditions, the film formation rate was 0.1 ° / pulse. Film thickness 5
When the temperature reaches 0 ° (5 nm), the oxygen potential becomes 1
Oxygen gas was introduced into the film forming chamber so as to be at least 10 ^-6 Torr. Then, film formation was continued in an atmosphere with an increased oxygen potential. When synthesizing the CeO ₂ thin film in the above steps, the atmosphere conditions were changed as shown in Table 1, and the influence of the atmosphere conditions on the crystal characteristics of the CeO ₂ thin film was investigated.

[0013]

[Table 1]

The obtained CeO ₂ thin film was diffracted by monochromatic X-rays (CuKα rays), and the spectral distribution was examined. As a result, as shown in FIG. 1 (a), in the case of vapor deposition in a vacuum atmosphere of the same degree of vacuum of 1 × 10 ⁻⁹ throughout the entire period, the diffraction angle θ = ± 1 degree in a relatively wide range , A peak of the CeO ₂ thin film was observed. This indicates that the resulting CeO ₂ thin film has a different crystal characteristic mixed therein. The spread of the diffraction angle θ could be suppressed as shown in FIG. 1B by increasing the oxygen potential throughout the entire period to 1 × 10 ⁻⁶ . However, in this case, a plurality of peaks not matching the crystal orientation of the Si substrate were observed, and CeO ₂ (200) and CeO ₂ (2
20), contamination of CeO ₂ (311) and the like was confirmed. As a result, the obtained thin film had a portion that was not epitaxially grown, and had poor crystal characteristics.

On the other hand, in the ^cases (c) and (d), the atmosphere at the start of growth was maintained at a vacuum degree of 1 × 10 ⁻⁹ Torr and the oxygen potential of the atmosphere was increased after 10 minutes. Also, as shown in FIGS. 1C and 1D, the variation in the diffraction angle θ was suppressed. That is,
It was confirmed that CeO _{2 having} a different crystal orientation was prevented from being mixed and a CeO ₂ thin film having no oxygen defect was formed. With respect to the CeO ₂ thin film obtained under the conditions (b) and (d), a reflection high-energy electron diffraction (RHEED) pattern was examined.
The survey results are shown in comparison with FIG. As shown in FIG. 2A, a ring pattern indicating the presence of polycrystal was detected in the thin film obtained under the condition (b). The thin film obtained under the condition (d) has a linear pattern as shown in FIG.
Epitaxial growth of a CeO ₂ thin film on the surface of the Si substrate was confirmed.

In FIG. 1, CeO _{2 of} X-ray diffraction is shown.
(222) The peak intensity of the CeO ₂ thin film in the process (c) is 8,000, whereas the peak intensity is 8,000 in the process (d).
Shows a high value of 25,000 for the CeO ₂ thin film. From this, it can be said that the process (d) having a high oxygen potential after 10 minutes is more suitable for obtaining a thin film having more excellent crystal characteristics. FIG. 3 shows a spectrum result of the CeO ₂ thin film by the process (d) in which Rutherford backscattering (RBS) measurement was performed using He ions accelerated to 3 MeV. In FIG. 3, the upper line is a random spectrum when He ions are incident from a random direction with respect to the crystal orientation, and the lower line is an aligned spectrum when the He ions are incident with the crystal orientation aligned. The smaller the intensity ratio of the aligned spectrum line to the random spectrum, the better the crystallinity.

The intensity ratio between the random spectrum and the aligned spectrum is minimum at Ce at χ _min = 28.
%, Indicating that CeO ₂ is completely epitaxially grown on the Si substrate. The difference between the random spectrum and the aligned spectrum is not different in a polycrystalline sample, and χ _min = 100%, and 、 _min does not fall below 50% in a sample having poor crystallinity. In this regard, RBS
If χ _min ≦ 50% in the spectrum, it is evaluated that sufficient epitaxy is maintained, and the thin film of χ _min = 28% obtained according to the present invention has very excellent crystal characteristics. You can see that.

[0018]

As described above, in the present invention, by employing laser ablation vapor deposition,
Enables CeO ₂ thin film epitaxial growth to have been Si substrate surface maintained at room temperature, CeO ₂ thin film crystal defects caused by the disturbance of the growth interface is not brought to obtain.
In the latter stage of growth, the oxygen potential of the atmosphere is raised, and oxygen, which tends to be insufficient, is taken in from the atmosphere by utilizing the high oxygen affinity of Ce itself.
O ₂ thin film is synthesized. Ce obtained in this way
O ₂ thin film has good lattice matching to Si substrate,
It is a thin film that makes full use of the advantages of CeO ₂ itself, which has high insulating properties, and is used as an insulating film for various semiconductor devices.

[Brief description of the drawings]

FIG. 1 X-ray diffraction pattern showing the effect of oxygen potential in the atmosphere on the crystal structure of a thin film

FIG. 2 shows an electron diffraction (RHEE) showing the crystallinity of a thin film.
D) Pattern

FIG. 3. Rutherford backscattering (RBS) spectrum showing epitaxy of thin films

Continuation of the front page (58) Fields surveyed (Int. Cl. ⁷ , DB name) C30B 1/00-35/00 CA (STN) JICST file (JOIS)

Claims (5)

(57) [Claims] 1. A Si substrate whose surface has been cleaned is placed in a high vacuum atmosphere of 10 ⁻⁷ Torr or less, and vapor generated by laser ablation of a CeO ₂ target is deposited on the surface of the Si substrate. A method for synthesizing a CeO ₂ epitaxial single crystal thin film, wherein the oxygen potential in the vacuum atmosphere is increased to 10 ⁻⁶ Torr or more before the temperature reaches 5 nm, and laser ablation deposition is continuously performed. 2. The method according to claim 1, wherein the Si substrate is kept at room temperature and CeO ₂ is deposited. 3. The method according to claim 1, wherein the oxygen potential in a vacuum atmosphere is increased by introducing an oxidizing gas such as oxygen gas, ozone, or oxygen radical. 4. The method according to claim 1, wherein after the HF treatment, a Si substrate whose surface is terminated with H by NH ₄ F treatment is used. 5. An organic resist film is provided on a part of the surface of the Si substrate according to claim 1, and an epitaxial single crystal thin film of CeO ₂ is grown on a surface of the Si substrate which is not covered with the organic resist film. Synthesis method