Patent Application
20090289328
1. Field of the Invention
The present invention relates to an insulation film for a capacitor element, a capacitor element, and a semiconductor device.
Priority is claimed on Japanese Patent Application No. 2008-134015, filed May 22, 2008, the content of which is incorporated herein by reference.
2. Description of Related Art
In conjunction with progress toward miniaturization and higher integration of DRAM, the size of the capacitor elements that configure memory cells has also contracted, thereby making it difficult to ensure a sufficient amount of stored charge. In order to ensure the amount of stored charge, development is progressing with respect to the application to capacitor elements of insulation film having high permittivity. As regards capacitor elements that configure DRAM memory cells, it is important not only to have high permittivity of the insulation film, but also to inhibit leakage current from the insulation film.
Among the variety of high-permittivity films for capacitor elements, niobium pentoxide (Nb2O5) film is one promising candidate. With respect to the relative permittivity of niobium pentoxide film, very large values are obtained on the order of 50 in an amorphous state, and on the order of 135 in a crystalline state. Thus, while niobium pentoxide enables easy formation of film having high permittivity, inhibition of leakage current is difficult, and application to DRAM capacitor elements is problematic.
In order to inhibit leakage current while putting to good use the high permittivity of niobium pentoxide, a method has been proposed which combines niobium pentoxide and another material to form the insulation film of the capacitor element (see, Japanese Unexamined Patent Application, First Publication No. 2002-164516 and Japanese Unexamined Patent Application, First Publication No. 2004-327607, which are hereinafter referred to as “Patent Documents 1 and 2, respectively). The insulation film disclosed in Patent Documents 1 and 2 is formed by crystallization of a combination of niobium pentoxide and tantalum pentoxide (Ta2O5).
However, although Patent Document 1 discloses the heat treatment temperatures for formation of the insulation film, and evaluations of the permittivity of the insulation film, it discloses nothing regarding leakage current values of the insulation film. The present inventors conducted an evaluation of leakage current values using completely crystallized insulation film identical to the insulation film disclosed in Patent Document 1. As a result, it was found with respect to use as a capacitor element in DRAM memory cells that leakage current is large, and that inhibition of leakage current is required.
Patent Document 2 discloses insulation film in which amorphous oxides exist in the crystal grain boundary areas of oxides in a crystalline state. However, it is extremely difficult to stably manufacture insulation film with this type of structure merely by temperature control. That is, it is difficult to have amorphous material exist uniformly at all crystal grain boundary areas, with the result that it is difficult to form insulation film having the property of a uniform leakage current value regardless of location.
In one embodiment, there is provided an insulation film that includes niobium, oxygen and a metal element, and the insulation film having a band gap width of larger than 4.2 eV, and at least a portion of the insulation film including an amorphous structure.
The above features and advantages of the present invention will be more apparent from the following description of certain preferred embodiments taken in conjunction with the accompanying drawings, in which:
The invention will be now described herein with reference to illustrative embodiments. Those skilled in the art will recognize that many alternative embodiments can be accomplished using the teachings of the present invention and that the invention is not limited to the embodiments illustrated for explanatory purposes.
The band gap of niobium pentoxide (Nb2O5) film alone is 4.2 eV in terms of energy level. The insulation film 3 according to the present embodiment is formed by adding to niobium pentoxide a metal element which composes a metal oxide that has a large band gap and a high relative permittivity relative to niobium pentoxide. Therefore, the band gap of the insulation film 3 is larger than that of niobium pentoxide of 4.2 eV.
Specifically, the insulation film 3 is formed by adding one or more of the elements selected from among yttrium (Y), aluminum (Al), tantalum (Ta), and the lanthanoid elements—which are metal elements that compose metal oxides having a large band gap and high relative permittivity with respect to niobium pentoxide (NB2O5)—to niobium pentoxide Here, “lanthanoid elements” signify the elements from lanthanum (La) with an atomic number of 57 to lutetium (Lu) with an atomic number of 71. The band gap of yttrium oxide (Y2O3) is 6.0 eV. The band gap of aluminum oxide (Al2O3) is 8.0 eV. The band gap of tantalum oxide (Ta2O5) is 5.3 eV. The band gap of lanthanum oxide (La2O3) is 5.4 eV. Each of yttrium oxide, aluminum oxide, tantalum oxide, and lanthanum oxide has a larger band gap than that of niobium pentoxide of 4.2 eV. Among these, aluminum oxide has the largest band gap value. Namely, the band gaps of these metal oxides are in the range of larger than 4.2 eV and not larger than 8.0 eV. Therefore, the band gap of the insulation film 3 including niobium oxide and the above metal oxide is in the range of larger than 4.2 eV and less than 8.0 eV.
It is preferable that the insulation film 3 have at least partially an amorphous structure, and that it not be completely crystallized. The meaning of a “not completely crystallized” state extends to a state in which the entire structure is in a completely amorphous state, and the structure has a short-range order.
Even when the insulation film 3 according to the present embodiment is in a state that is not completely crystallized, it is possible to satisfy the required electrical properties of the capacitor element.
The permittivity and leakage current properties of the insulation film 3 vary according to the concentration of the aforementioned elements that are added to niobium pentoxide. That is, the relative permittivity of the insulation film 3 increases as the concentration of the aforementioned elements that are added to niobium pentoxide decreases, and the leakage withstand voltage of the insulation film 3 increases as the concentration of the aforementioned elements that are added increases.
In the present embodiment, as the metal elements that are added to niobium pentoxide, it is acceptable to have one or more selected from among yttrium, aluminum, tantalum, and the lanthanoid elements. There are no problems with respect to the properties of the insulation film 3 even if two or more of these metal elements are added to niobium pentoxide, but when considered in terms of mass productivity in the manufacturing process, it is preferable that only one of these be added.
The addition ratio of the metal element(s) to be added can be expressed by the concentration ratio relative to the entirety obtained by joining the niobium (Nb) and the added element(s). For example, in the case where yttrium (Y) is added to niobium pentoxide, the addition ratio of Y(Y/(Y+Nb)) is 10%-50%. In this way, an insulation film is obtained which balances both leakage withstand voltage and relative permittivity, supposing the case of application of a capacitor element as a DRAM memory cell.
In the case where tantalum (Ta) is added to niobium pentoxide, the addition ratio of Ta(Ta/(Ta+Nb)) is 40%-80%. In this way, an insulation film is obtained which balances both the leakage withstand voltage and relative permittivity of the capacitor element.
Even in cases where aluminum or a lathanoid element is used as the additive element, an appropriate amount of the element is added to niobium pentoxide so as to obtain the desired capacitor properties.
With respect to the insulation film 3 according to the present embodiment, even if a small amount of the respective element is added to niobium pentoxide, it is possible to obtain better properties as insulation film of a capacitor element than with insulation film composed only of niobium pentoxide. Accordingly, in the case where, for example, yttrium is selected as the additive element, its addition ratio is not limited to within 10%-50%, and it is also possible to add 10% or less, or 50% or more. The recommended concentrations of these additive elements also vary according to film thickness and film formation method.
The insulation film 3 according to the present embodiment can be formed by the sputtering method, normal CVD (chemical vapor deposition) method, ALD (atomic layer deposition), and so on using an ordinary semiconductor manufacturing apparatus.
The method for manufacturing the insulation film 3 by adding yttrium to niobium pentoxide using the sputtering method is described below.
In
In the case where an insulation film which is composed of niobium pentoxide to which yttrium has been added is formed according to the above method, the additive amount of yttrium in the niobium pentoxide is proportional to the amounts of the raw materials constituting the insulation film that are supplied to the surface of the lower electrode 1 from each target. That is, adjustment of the amounts of the raw materials constituting the insulation film that are supplied from the targets is conducted by a method for controlling RF power, enabling formation of niobium pentoxide films with different additive amounts of yttrium.
The amount of the raw materials constituting the insulation film that are supplied from the target to the lower electrode surface can also be varied by a method for changing the yttrium content that is contained in the target. Accordingly, it is also possible to change the additive amount of yttrium in the niobium pentoxide by changing the yttrium content that is contained in the target that is used when forming the insulation film.
It is also acceptable to control the additive amount by controlling the amounts of the raw materials constituting the insulation film that are supplied to the film formation face from the targets using a combination of the method for controlling the RF power that is supplied to the targets and the method for changing the yttrium content that is contained in the target.
It is also acceptable to similarly control the additive amount of yttrium using a target that is formed from material containing yttrium other than Y2O3.
In the foregoing example, a method was described for forming insulation film composed of niobium pentoxide to which yttrium is added In cases where insulation film is formed which is composed of niobium pentoxide to which elements other than yttrium are added, it is similarly possible to control the additive amount of the element in the niobium pentoxide by the method for changing the RF power that is supplied to the targets and/or the method for changing the content of the additive element contained in the target.
After forming the insulation film 3 on top of the lower electrode 1, annealing (heat treatment) is conducted for 1 minute to 10 minutes in an oxygen atmosphere at a temperature of 500° C.-700° C. The objective of this annealing is not to completely crystallize the insulation film 3, but to improve film quality. In the case where the insulation film 3 is formed by the sputtering method at around 300° C., it may happen that defects occur on the film, or that oxidation of the insulation film 3 is insufficient, and thereby the leakage properties of the insulation film 3 may be impaired. By conducting annealing after film formation, film defects originating in low temperature formation of the insulation film 3 can be ameliorated, and leakage properties can be further improved. The temperature and time of this annealing can be determined according to the film formation method of the insulation film 3 and the leakage properties that are required in the insulation film 3. With respect to the apparatus used for annealing, either a furnace apparatus or a lamp annealing apparatus is acceptable.
The occurrence of film defects can be inhibited during film formation of the insulation film 3 in the case where a sophisticated sputtering technology such as one that employs oxidants is used when forming the insulation film 3 by the sputtering method, or in the case where the insulation film 3 is formed by a film formation method such as the CVD method. Consequently, in such cases, one may dispense with conduct of an annealing process after film formation.
That is, with respect to the method of formation of the insulation film 3 according to this embodiment, post-film-formation annealing is not an indispensable process. Whether or not to conduct annealing may be determined according to the properties that are ultimately to be obtained by application to semiconductor devices such as DRAM. Even when annealing is conducted, it is possible to vary conditions such as temperature and time according to the desired properties of the insulation film 3.
Moreover, the insulation film 3 according to the present embodiment can obtain the desired capacitor properties even in a state where the film is not completely crystallized. Accordingly, when the insulation film 3 is used in an amorphous state, there is no occurrence of the problem of surface roughness that stems from crystallization.
In the case where the insulation film 3 is a completely crystallized film, as it is affected by the crystallinity of the material that composes the lower electrode 1 during crystallization, the quality of the insulation film 3 is highly dependent on the material and film quality of the lower electrode 1, resulting in major practical limitations and difficulties. On the other hand, as the insulation film 3 according to the present embodiment does not need to be completely crystallized, and as it may be used even in an amorphous state, it is possible to offer a fixed quality that is independent of the quality of the lower electrode 1. Consequently, formation of the lower electrode 1 and insulation film 3 is easy, and options pertaining to the material and method of formation of the lower electrode 1 can be increased.
Moreover, as there is no need to control the substance so as to be segregated at the crystal grain boundary of the niobium pentoxide, and as the element that is added to the niobium exists in a mixed state, it is possible to easily manufacture an insulation film with uniform properties regardless of location.
After conducting formation and, if necessary, annealing of the insulation film 3, the capacitor element is completed upon formation of the upper electrode 2.
The capacitor element according to the present embodiment may be applied not only in planar form, but also when the electrodes have a three-dimensional structure. A description is given of a capacitor element having a three-dimensional structure with reference to
As shown in
Below, as a specific example of a semiconductor device using the capacitor element according to the present embodiment, the case of application to a DRAM memory cell is described
As shown in
A plurality of gate electrodes 206 are disposed on the semiconductor substrate so as to cross the active regions 204. The gate electrodes 206 function as DRAM word lines. In the portion of the active region 204 that is not covered with the gate electrode 206, an impurity such as phosphorus is ion injected to form an N-type diffusion layer region. This N-type diffusion layer region functions as the source drain region of a transistor.
In
At the center of each active region 204, a contact plug 207 is provided, and contacts the N-type diffusion layer region on the surface of the active region 204. Contact plugs 208 and 209 are provided at both ends of each active region 204, and these contact the N-type diffusion layer region on the surface of the active region 204. The contact plugs 207, 208 and 209 can be simultaneously formed.
In the layout shown in
The capacitor element 217 is composed of a lower electrode 217a, upper electrode 217b, and insulation film 217c that is interposed between the lower electrode 217a and the upper electrode 217b. The insulation film 217c is composed of the same material as the insulation films 3, 5, and 8 that were described above. The capacitor element 217 shown in
Furthermore,
By turning on the MOS-type transistor 201, it is possible to determine whether or not electric charge has accumulated in the capacitor element 217 via the bit line (wiring layer 212), and to conduct operations as a DRAM memory cell that is capable of conducting information storage operations.
With respect to the capacitor element according to the present embodiment, not only is the permittivity of the insulation film high, but leakage current can also be inhibited, thereby enabling easy formation of memory cells that have excellent charge retention properties (refresh properties) Accordingly, it is possible to easily manufacture high-performance DRAM.
The capacitor element according to the present embodiment can also be used apart from DRAM memory cells and may also be applied, for example, to common semiconductor devices such as logic that do not have memory cells, so long as it is a device that uses a capacitor element.
An insulation film for a capacitor element and a capacitor element according to an embodiment of the present invention enable coexistence of high permittivity and lea age current inhibition, and have an excellent uniformity of properties. Moreover, a semiconductor device according to an embodiment of the present invention has excellent charge retention properties (refresh properties).
As example 1, an evaluation of the properties of a capacitor element which uses insulation film that is formed by adding yttrium to niobium pentoxide film is stated below.
First, the method of manufacture of the capacitor element used for evaluation is described As shown in
Next, as insulation film 13, niobium pentoxide film was formed with addition of yttrium using a multi-source sputtering apparatus. During fabrication, RF discharges were simultaneously conducted inside the chamber of the sputtering apparatus using targets of Nb2O5 and Y2O3. Here, the semiconductor substrate 10 on which the platinum film 12 (lower electrode) is deposited was disposed at the center of a position that is opposite the targets, and was rotated to form a uniform film thickness. The substrate temperature was set at 300° C., argon and oxygen (O2) gases were simultaneously supplied at the same flow rate (10 sccm), and pressure inside the chamber was set to 0.5 Pa.
As the adhesion amounts on the platinum film 12 that serves as the lower electrode from the Nb2O5 and Y2O3 targets is proportionate to the RF power values that are given to the respective targets, the compositional ratio of niobium and yttrium is obtained by appropriately setting the respective RF power. The film thickness of the insulation film 13 was formed to 40 nm.
Next, a platinum film 14 that serves as an upper electrode was formed to approximately 30 nm under the same conditions as the platinum film 12 of the lower electrode, and the insulation film 13 was subjected to annealing treatment for 10 minutes at 600° C. by a furnace apparatus using oxygen gas, thereby forming the capacitor element.
From
As example 2, evaluation of the properties of a capacitor element which uses an insulation film that is formed by the addition of tantalum (Ta) to niobium pentoxide film is stated below.
With the exception of the insulation film, the capacitor element was formed by the same method as example 1, and platinum film was used as the lower electrode of the capacitor element. The insulation film was formed by the sputtering method, and Nb2O5 and Ta2O5 were used as the targets during film formation. The insulation film was subjected to annealing treatment for 10 minutes at 600° C. by a furnace apparatus using oxygen gas. After this annealing, the insulation film was in a state that was not completely crystallized. By controlling the RF power supplied to the respective targets, the addition ratio of tantalum relative to niobium (Ta/(Ta+Nb)) was varied in a range of 0-100%.
Table 1 shows the results of measurement of the ratio of tantalum and niobium in the formed insulation film measured by the RBS (Rutherford Backscattering Spectrometry) method.
From
It is apparent that the present invention is not limited to the above embodiments, but may be modified and changed without departing from the scope and spirit of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2008-134015 | May 2008 | JP | national |
