GB2338962A - Thin film formation method - Google Patents

Description

2338962 THIN FILM FORMATION METHOD BACKGROUND OF THE INVENTION 1. FIELD OF

THE INVENTION 5 The present inven ion relaes to a method :t of manufacturing a thin film used for an electronic component, a semiconductor integrated circuit, and the like.

2. DESCRIPTION OF THE PRIOR ART

At present, thin film formation techniques are indis pensable to techniques of manufacturing electronic compo nents and, particularly, semiconductor integrated circuits.

These techniques are more important for memory elements or memory integrated circuits using ferroelectrics or high dielectrics as a capacitor film material.

In a conventional Si-LSI process, a single-crystal Si substrate is used as a substrate. An Si oxide film and an Si nitride film are formed as ferroelectric films on the substrate, and a poly-Si film is stacked as an electrode film. These film formation techniques realize precise film thickness control and high-speed deposition.

At present, deposition techniques such as CVD and sputtering used in the Si-LSI process have been developed for ferroelectric and high-dielectric thin films which receive a great deal of attention as new capacitor dielec- tric films. Well-known solid materials exhibiting ferroelectric properties are composite metal compounds having a perovskite crystal structure and a layered perovskite crystal structure. As the composite metal compound having the perovskite crystal structure, lead zirconate titanate (PZT), BaTi03, SrTi03, and the like are known. As the composite metal compound having the layered perovskite crystal structure, SrB'2Ta2o9, PbB'2Ta2o9, and the like are known.

Many examples employing the above-described CVD and sputtering, and the sol-gel method as the deposition method have been reported. One example is disclosed in Japanese Unexamined Patent Publication No. 6-57411, and its technique will be described with reference to Fig. 1.

As shown in Fig. 1, a conductive film 123 is formed on a substrate 121 directly or through a buffer layer 122- A dielectric underlayer 124 is formed on the conductive film 123, and a perovskite oxide dielectric thin film 125 is formed on the dielectric underlayer 124. A metal film formed on the thin film 125 is processed to form an upper electrode 126.

C.K. Kwock et al. deposited a PZT thin film on a Pt underlayer by RF sputtering at a substrate temperature of about 2000C (Material. Research Society Symposium Proceed- ings vol. 200. 1990. p. 83).After the PZT thin film was sputter-deposited on the Pt underlayer heated to a certain temperature, the resultant structure was annealed in a furnace to obtain perovskite crystals. In this case, it was reported that lead diffused outward from the film in high- temperature annealing, and the percent in lead loss from the film was proportional to the annealing temperature (see Fig. 2).

The present inventors also confirmed by experiments that constituent elements diffused into an underlayer even when SrBi2Ta2O, (to be referred to as SBTO hereinafter) receiving a great deal of attention as a dielectric film material for a ferroelectric memory was deposited. Figs. 3A to 3D show the results of energy spectral analyses by energy dispersive X-ray spectroscopy (EDX) when an SBTO film was formed on a Pt/Ti film by magnetron RF sputtering without heating a substrate, and the resultant structure was annealed in oxygen at 8006C.

In this case, the deposition film thickness was about nm. The deposition underlayer was constituted such that a 500-nm thick Si oxide film was formed on an Si substrate, and a 20-nm thick Ti film and a 200-nm thick Pt film were formed on this Si oxide film.

Analyses were performed for an annealed structure and a structure not subjected to annealing. Analyzed portions were two portions, i.e., the SBTO film and underlayer (metal 4 film) of each structure.

In deposition, no peak representing the presence of Bi element contained in the SBTO f ilm was observed in the underlayer (metal film), as shown in Fig. 3B.

After annealing at 8000C, the peak of the Bi element contained in the SBTO film appeared in the underlayer (metal film), as shown in Fig. 3D, which represented that the Bi element diffused from the SBTO film to the underlayer upon annealing.

In a film structure shown in Fig. 3A, a film immediate ly after deposition without heating a substrate consisted of crystals including many amorphous crystals or defects, so it did not exhibit ferroelectric properties or its properties were very poor. In Fig. 3C, crystallization was sufficient ly perf ormed upon heating to obtain desired f erroelectric properties. Therefore, the annealing process is necessary.

However, constituent elements contained in the dielec tric thin film also diffuse during crystallization. Atoms which do not contribute to the crystal growth are precipi tated in the film or diffuse outside it. "Elements which do not contribute to the crystal growth" means both "elements excessive in constituting crystals" and "elements which diffuse before being entrapped in crystals". When an SBTO film is used as a dielectric thin film on a Pt/Ti underlay er, it is observed that Bi greatly diffuses to the underlay- er.

Fig. 4 shows an example of measuring ferroelectric properties obtained when an SBTO f ilm is sputter- depos i ted on a Pt/Ti underlayer on an Si oxide f ilm and annealed in the oxygen atmosphere at 8000C, and then an upper Pt elec trode is arranged to form a capacitor structure. The composition ratios of Bi to Sr before and after annealing which are examined by the ICP (Induced Coupled Plasma) analysis are 2.1 and 2.0, respectively, which are almost stoichiometric composition ratios.

The polarization characteristics, however, are almost 10% the reported value (2Pt = 15 to 20 pC/cm2), and are not preferable. This is because many Bi components diffuse at the interface region with the underlayer before being entrapped in crystals in crystal growth, resulting in the Bi loss at the interface with the underlayer, or because Bi necessary for crystallization decreases upon diffusion of Bi to the underlayer, resulting in a state with many defects or a region in a state wherein a desired crystal structure does not satisfactorily grow.

As described above, in the method of depositing a composite metal oxide as a dielectric thin film, when the film is deposited without heating a substrate to a high temperature, the constituent elements of the dielectric thin film diffuse outward or into an underlayer. As a result, the composition ratio of the elements of the dielectric thin film crystallized by annealing undesirably changes. In particular, the distribution at the interface between the surface region of the film and the underlayer is disadvanta geously greatly influenced.

When the film is deposited while heating the substrate, if an element having a high vapor pressure is contained as a constituent element in film formation, a crystalline thin film is difficult to grow, while ensuring the stoichiometric composition ratio. Especially, the composition disadvanta geously shifts at the interface region with the underlayer.

In addition, the constituent elements of crystals are lost to interfere with the film growth. Even if a dielectric thin film serving as a buffer layer and having dielectric properties and the like which hardly influence target properties is deposited as an intermediate layer on an underlayer on which the dielectric thin film having neces sary properties should be deposited, the film quality is degraded during deposition unless the respective deposition temperatures are properly controlled.

In Japanese Unexamined Patent Publication No. 6-57411 shown in Fig. 1, as the dielectric underlayer 124 of the perovskite dielectric thin film 125 consisting of AB03 (A and B are perovskite crystals at A and B sites as atom positions generally designated), a dielectric thin film consisting of 7 A'B'03 or B'03 using the same constituent elements is effective.

If this underlayer is formed of a dielectric, the composition ratio of the elements of the formed film must exhibit dielectric properties. That is, when a dielectric layer is formed on the dielectric underlayer 124 so as to contact the dielectric underlayer 124 deposited with a determined composition as a dielectric, if the formation temperature is high, or if energy to promote generation of point defects is applied to promote diffusion of elements, diffusion of the elements of the underlayer to the lower conductive film 123 (lower electrode) is difficult to suppress, resulting in poor film properties.

In Japanese Unexamined Patent Publication No. 6-57411, the buffer layer 122 is arranged between the conductive film 123 and the substrate 121 to prevent mutual diffusion between the substrate and the dielectric film. Even with this arrangement, diffusion of the elements of the dielec tric layer to the conductive film 123 is difficult to prevent.

SUMMARY OF THE INVENTION

The invention is defined in the independent claims below, to which references should now be made. Advantageous features are set forth in the appendant claims.

The invention will now be described in more detail, by way of example, with reference to the drawings, in which:

Fig. 1 is a sectional view showing main part of a prior art;

Fig. 2 is a graph for explaining the loss of Pb in annealing a PZT film; Figs. 3a to 3d are graphs, respectively, showing the results of energy spectral analyses upon annealing an SBTO film, in which existence or non-existence of B1 diffusion is shown in only a grown film immediately after film growth (Fig. 3a), only a metal underlayer immediately after film growth (Fig. 3b), only the grown film after annealing (Fig.3c), and only the metal underlayer after annealing (Fig. 3d).

Fig. 4 is a graph showing polarization characteristics obtained when an SBTO film is deposited by a conventional sputtering deposition technique to form a capacitor element; Figs. 5a to 5d are sectional views, respectively, showing manufacturing steps according to a first thin film formation method; Fig. 6 is a sectional view showing main part of a second thin film formation method; Fig. 7 is a sectional view showing a modification of the second method shown in Fig. 6; Fig. 8 is a graph showing the composition ratios of Bi to Sr in SBTO films when the films are deposited using Ar and Ar/O, reaction gases at various substrate temperatures; and Fig. 9 is a graph showing polarisation characteristics obtained when an SBTO film is deposited by the method of the first thin film formation method to form a capacitor element.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present inventors have found that the elements of a dielectric thin film diffused at the interface region with an underlayer in the crystal growth of the dielectric thin film before being entrapped in the underlayer. On the basis of the findings, the amounts of elements to be diffused are increased to change the composition ration of the dielectric thin film before crystallization, thereby compensation for diffused elements.

Crystallisation by annealing is performed after depositing the thin films, or crystallization by annealing is performed in depositing the thin films. At least one of the first and second layer thin films is formed by sputtering.

1 0 - Further, in the described embodiments, there is provided a thin film formation method comprising the first layer deposition step, the crystallization step, and the second layer deposition step. In the first layer deposition step, raw material particles serving as film formation nuclei are supplied onto an underlayer, and the raw material particles are grown to form the first-layer thin film. In the crystallization step, the first-layer thin film is crystallized by annealing. In the second layer deposition step, the second-layer thin film is formed on the first-layer thin film by using one of raw material particles serving as film formation nuclei and a thin film layer, while heating the underlayer and the crystallized first-layer thin film.

Annealing performed after depositing the first-layer thin film is performed at a temperature set higher than the deposition temperature of the first- or second-layer thin film.

The dielectric thin film may be constituted by an SrBi2Ta.09 film by sputtering. If so, the constituent element in the first-layer thin film having the composition larger than the stoichiometric composition in consideration of diffusion outside the film is Bi.

When the dielectric thin film is constituted by the film by sputtering, the first-layer thin film is SrBi2Ta.09 deposited using a gas mixture of argon and oxygen as a plasma reaction gas, and the second-layer thin film is sputter- deposited using only argon as a plasma reaction gas.

in this case, the second-layer thin film is deposited at a deposition temperature set higher than the deposition temperature of the first-layer thin film.

When the dielectric thin film is constituted by the SrBi2Ta2o9 film by sputtering, the first-layer thin film may be deposited using the gas mixture of argon and oxygen as-.a plasma reaction gas, and the second-layer thin film may be deposited using the gas mixture of argon and oxygen as a plasma reaction gas at a deposition temperature set higher than the deposition temperature of the first-layer thin film.

In the thin film tormation method described a capacitor insulating film for a capacitor element formed through an insulating film of, e.g., Si oxide is formed on a semicon ductor substrate or an integrated circuit formed on the semiconductor substrate. According to the thin film formation method,... the capacitor element is constituted such that firstand second-layer thin films consisting of many elements are deposited to a total thickness of about 300 nm or less on a metal thin film (underlayer) serving as a lower electrode of, e.g, Pt about 50 to 200 nm thick, these films are crystallized by annealing at a temperature of 900C or less to form a dielectric thin film, zind an upper electrode is formed on the dielectric thin film. In this case, it is important that at least one kind of element of the constitu ent elements of the first-layer thin film before crystalli zation has a composition larger than the stoichlometric composition (than composition ratios of remaining elements) in consideration of diffusion outside the film (including the underlayer). The thicknesses of the first- and second-layer thin films are preferably determined based on the excessive amount of the element excessively contained in the first-layer thin film. With this setting, the flatness can be improved.

It may be possible that the first-layer thin film is crystallized by annealing or the like upon deposition, and the second-layer thin film is deposited on the crystallized first-layer thin film. If the first-layer thin film has a thickness of about 20 nm or less, it may change f rom a consecutive film to a surface shape having scattered island-like structures upon annealing before depositing the second-layer thin film. In this case, the island-like structures serve as f ilm growth nuclei for depositing the second-layer thin film.

Examples of t he dielectric material are perovskite structure crystal films including Pb, other layered crystal oxides including, e.g., PZT and Bi, SrBi2Ta2Oq, Bi.Ti 3012, PbBi.Ta2o,, and the like. The deposition is preferably performed by sputtering, and the conditions of the reaction gas and the deposition substrate temperature must be properly selected.

As the deposition method, MOCVD (Metal-Organic Chemical Vapor Deposition) using an organic metal raw material can also be employed. However, sputtering is safer and more convenient because it does not use a metal gas (e.g., an organic metal gas) which is toxic to the human body and difficult to handle.

Next, the first thin fiim tormation method will be described. Fig. 8 shows the composition ratio of the Bi element to the Sr element constituting an SBTO film serving as a dielectric thin film when the SBTO film serving as the dielectric thin film is deposited on a Pt/Ti film formed on an Si oxide film by sputtering using only Ar gas or an Ar/02 gas mixture as a reaction gas, while variously changing the substrate heating temperature. A target used for sputtering shown in Fig. 8 is a sintered ceramic target.

In Fig. 8, bullets and hollow bullets represent the results of crystallization by annealing in the oxygen atmosphere at 800C in and after film deposition. The Bi/Sr ratio hardly depends on the substrate temperature by using the Ar/02 gas mixture as a reaction gas. In the deposition 4 method as described a dielectric thin film having desired dielectric properties can be f ormedby variously combining the conditions of the s puttering reaction gas used to deposit f irst- and second-layer thin films for the dielectric thin film, and the substrate temperature in f ilm deposition by sputtering on the basis of the data.

The method in which a dielectric thin film is contituted by SBTO films on the basis of the data shown in Fig. 8 will be described with reference to Figs. 5A to 5D. As shown in Fig. 5A, a Pt/Ti film 102 is deposited as an underlayer on an Si substrate 101 having an Si oxide film 103 deposited thereon. In this case, of the Pt/Ti film 102, the Pt film has a thickness of 200 nm, and the Ti film has a thickness of,20 run.

As shown in Fig. 5B, an SBTO film 104 about 20 nm thick is sputter- depo sited as a first-layer thin film on the Pt/Ti film 102 at a substrate temperature of 350C using an Ar/02 reaction gas. A target used in this case is a single sintered ceramic target of SBTO (sputtering is not performed for a plurality of targets simultaneously or sequentially), and has the same target composition as the data in Fig. 8.

Then, an SBTO film 105 serving as a second-layer thin film is sputterdeposited on the SBTO film 104 serving as the first-layer thin film. The sputter-deposition is performed using a reaction gas of only Ar at a substrate temperature of 2000C in thin film deposition in a chamber wherein the SBTO film 104 serving as the first-layer thin film has been sputter-deposited.

The SBTO film 104 serving as the first-layer thin film and the SBTO film 105 serving as the second-layer thin film are crystallized by performing annealing in the oxygen atmosphere at 800C, thereby integrating them into a dielec tric thin film 106. The film deposition temperature at this time is preferably 200C or more for a target having the data in Fig. B. The annealing temperature in the oxygen atmosphere is desirably 6000C or more. At a lower tempera ture, annealing must be performed for a longer time.

Identical targets are used for deposition of the SBTO film 104 serving as the first-layer thin film and the SBTO film 105 serving as the second-layer thin film. Sputtering is radio-frequency (RF) sputtering, and its applied RF power is 1 kW.

According to the above-described thin film forrhation method it is considered that crystal nuclei for growing desired crystals in annealing for crystallization are easily produced at the interface region between the dielectric thin film and the underlayer, because there is no degradation of the stoichiometric composition of film constituent elements at the interface region. This is also 16 important f or control of the crystal orientation of the formed dielectric thin film (crystalline film) with respect to the underlayer.

Although the above method exemplif ies the case using sputtering for film deposition, another method other than sputtering can be employed. It is also effective to crystalize the deposited SSTO film 104 serving as the first-layer thin film and the deposited SBTO film 105 serving as the second-layer thin film by performing anneal ing at 600C or more, thereby integrating them into the dielectric thin film 106.

When the SBTO film 104 serving as the first-layer thin film is deposited using the reaction gas of only Ar, the substrate is not heated, or the deposition temperature is set low, e.g., 200C or less with reference to Fig. S. With this setting, the Bi element contained in the SBTO film 104 in an excessive amount is made to be entrapped in the Pt/Ti film 102 serving as the underlayer. The SBTO f ilm 105 serving as the second-layer thin f ilm is deposited using the reaction gas of only Ar at a relatively high deposition substrate temperature of, e.g., 200C with reference to Fig. S.

When the SBTO film 104 serving as the first-layer thin film and the SBTO film 105 serving as the second-layer thin film are to be deposited by sputtering using the Ar/O. gas 17 mixture as a reaction gas, it is preferable that the SBTO film 104 serving as the first-layer thin film be deposited at a deposition substrate temperature of about 3000C (according to Fig. 8, the temperature may be about 500C when the substrate is not heated), and then the SBTO film serving as the second-layer thin film be deposited at a higher deposition substrate temperature of 800C. That is, the deposition temperature for the second-layer film is higher that the deposition temperature for the first-layer film.

In this case, the deposition temperature for the second-layer thin film depends on the target composition.

For the Ar/O 2 reaction gas, the Bi element in the first-layer thin film in an excessive amount tends to be easily en trapped in the underlayer. For this reason, a decrease in Bi composition of the target is effective. In this case, since crystallization is sufficiently performed in deposi tion, no annealing is required upon film deposition.

It is also possible that the SBTO film 104 serving as the first-layer thin film is deposited using the Ar gas or the Ar/O. gas mixture as a reaction gas, and crystallized by annealing at 8000C, and then the SBTO film 105 serving as the second-layer thin film is deposited using the reaction gas of only Ar (or the Ar/02 gas mixture). The SBTO film 105 serving as the second-layer thin film may be deposited at 200C with reference to Fig. 8 and crystallized into a ferroelectric thin film by annealing, or deposited at a deposition substrate temperature (e.g., 800C or 600C or more) sufficient for crystallization. Alternatively, after the substrate is temporarily extracted from the deposition chamber, annealing for crystallization can be performed in another chamber or an annealing furnace.

The target for sputter deposition in the above embodi ment may be an alloy target of Sr. Bi, and Ta. In use of a metal alloy target, a DC voltage can be used as plasma generation energy. Although the Ar gas or the Ar/O. gas mixture is used as a plasma reaction gas, Xe can be used instead of Ar. When a film is deposited using the metal alloy target in a low-temperature atmosphere like a liquid nitrogen or liquid helium atmosphere, processing of crystal lizing the first- and second-layer thin films is effectively performed in oxyger-.

1:5 Next, a further thin film formation method will be described-with reference to Fig. 6. Fig- 6 is a sectional view showing a memory element having a ferroelectric capacitor element using a ferroelectric thin film deposited by the deposition method of the first embodiment shown in Figs. 5A to 5D. A structure shown in Fig. 6 is constituted by a switching transistor, a capacitor element, a selection word line, a bit line, and a plate line. In Fig. 6. reference numeral 107 denotes an Si substrate; 108, a gate electrode (word line); 109, a source/drain diffusion layer; 110, an upper electrode (plate 19 line); 111, a dielectric thin film consisting of an SBTO film; 112, a lower electrode; 113, a bit line (WSi.); 114, an Al interconnection for a bit line; 115, an interlevel insulating film; and 116, an Al interconnection for a plate line.

The capacitor element shown in Fig. 6 is constituted by the SBTO film (dielectric thin film) 111 interposed between the upper electrode 110 and the lower electrode 112. The upper electrode 110 is connected as the plate line to the Al interconnection 116. The dielectric thin film 111 and the lower electrode 112, other than the upper electrode 110, are processed by photolithography using the same mask in the same step. The lower electrode 112 can be used as a plate line. In this case, the dielectric thin film 111 and the upper electrode 110, other than the lower electrode 112, may be processed by photolithography using the same mask in the same step.

By CVD, or sputter deposition with a high sputtering efficiency of sputter particles on the side surface of a used cylindrical target, a ferroelectric film (dielectric thin film) 118 is deposited on the upper and side surfaces of a lower electrode 117 and used as a capacitor, as shown in Fig. 7.

in a modification shown in Fig. 7, the ferroelectric film 118 consisting of SBTO is deposited by MOCVD on the 500-nm lower electrode 117 processed into a block shape, e.g., a Pt film. The material of the lower electrode 117 may be a conductive metal oxide such as Ru oxide. An upper Pt electrode 119 is deposited to 100 nm on the ferroelectric film 118, and processed using a mask. An interlevel insulating film is deposited, a contact hole is formed, and an Al interconnection 120 for a plate line is arranged.

Fig. 9 is a graph showing polarization characteristics obtained when a dielectric- thin f ilm consisting of SBTO is deposited using the first thin film formation method. and used as a capacitor element. It is apparent from Fig. 9 that the polarization values are larger than the characteristic values in the prior art shown in Fig. 4, and the character istics are improved.

Since the capacitor film is sputter-deposited, the undulating surface reflecting the corrugation shape of the lower layer can be f ormed af ter depositing the interlevel insulating film without performing polishing or the like before forming the capacitor element so as to planarize the entire surf ace of the substrate in the memory element as 'he semiconductor memory device shown in the slecond method In the modif ication of the nethOd shown in Fig. 7, since the ferroelectric thin film is formed on the side surface of the lower electrode by CVD, a larger amount of electric charge can be accumulated on the same substrate area, and a larger number of memory cells can be arranged on the same area.

Claims (7)

1. A thin film formation method comprising a first layer thin film deposition step, a first-layer thin film crystallization step, and a second-layer thin film deposition step, wherein the first-layer thin film deposition step is a process of supplying raw material particles serving as film formation nuclei onto an underlayer, and growing said raw material particles to form said first-layer thin film, the crystallization step is a process of crystallizing said first-layer thin film by annealing, and the second-layer thin film deposition step is a process of forming said second-layer thin film on said first-layer thin film by using one of raw material particles serving as film formation nuclei and a thin film layer, while heating said underlayer and said crystallized first-layer thin film.

2. A method according to claim 1, wherein annealing in the crystallization step performed after depositing said first-layer thin film is performed at a temperature set higher than a film deposition temperature of said first or second layer thin film.

3. A method according to claim 1, wherein said first layer thin film is deposited using a gas mixture of argon and oxygen as a plasma reaction gas, and said second-layer thin film is sputter-deposited using only argon as a plasma reaction gas.

4. A method according to claim 1, wherein said second layer thin film is deposited at a deposition temperature set higher than a deposition temperature of said first-layer thin film.

5. A method according to claim 1, wherein said first layer thin film is deposited using a gas mixture of argon and oxygen as a plasma reaction gas, and said second-layer thin film is deposited using the gas mixture of argon and oxygen as a plasma reaction gas at a deposition temperature set higher than a deposition temperature of said first-layer thin film.

6. A thin film formation method comprising a first layer thin film deposition step, a first-layer thin film crystallization step and a second-layer thin film deposition step in order to form a dielectric thin film consisting of many elements, wherein:

the first-layer thin film deposition step is a process of supplying raw material particles serving as film forma-Lion nuclei onto an underlayer, and growing said raw material particles to form a first-layer thin film, said first-layer thin film closer to said underlayer is deposited with a larger composition of at least one kind of film constituent eletnent constituting said first-layer thin film that a stoichiometric composition in order to prevent degradation of the stoichiometric composition of said at least one kind of constituent element caused by diffusion outside said thin film at a time when heat treatment is applied thereto; the crystallization step is a process of crystallizing said first-layer thin film by annealing; and the second-layer thin film deposition step is a process of forming a second-layer thin film on said first-layer thin film by using one of raw material particles serving as film formation nuclei or a thin film layer, while heating said underlayer and said crystallizing first-layer thin films.

7. A semiconductor device incorporating a thin film made by the method of any preceding claim.