Semiconductor Device and Method for Preparing the Same

1. Field of the Invention

The invention relates to a semiconductor device and a method for preparing the same, and more particularly to a DRAM made of a material of high dielectric constant and including a capacitor using the material, and method for preparing the DRAM.

2. Description of the Related Art

A capacitor must keep its capacitance at a specified degree as cell sizes are reduced following development of DRAMs of higher density. A material of high dielectric constant and a capacitor construction using the material as well as a stacked capacitor of a larger surface area and a thin film dielectric have been researched in this regard. Ta.sub.2 O.sub.5 capacitor dielectric film has been given attention as a thin film dielectric of high dielectric constant. But, the Ta.sub.2 O.sub.5 dielectric film is provided with a lower electrode using a polysilicon film or silicide film (e.g., WSi.sub.2), so that it does naturally have SiO.sub.2 formed at an interface between Ta.sub.2 O.sub.5 and polysilicon, leading to deterioration of a practical dielectric constant of the capacitor.

Also, the Ta.sub.2 O.sub.5 film generally causes high leak current and is difficult to be employed by itself.

The present invention provides a semiconductor device comprising a capacitor having a lower electrode of a Si-containing material, an aluminum oxide film formed on the surface of the lower electrode, a dielectric film of Ta.sub.2 O.sub.5 formed on the aluminum oxide film and an upper electrode provided on the dielectric film.

Also, the present invention provides a method for preparing a semiconductor device which comprises;

(i) (a) forming an aluminum or aluminum oxide film on a lower electrode of a Si-containing material, followed by first annealing in a non-oxidative atmosphere, or (b) forming an aluminum or aluminum oxide film on a lower electrode of a Si-containing material upon while conducting the first annealing in non-oxidative atmosphere,

(ii) forming a dielectric film of Ta.sub.2 O.sub.5 on the resulting above mentioned aluminum oxide film, followed by second annealing in an oxidative atmosphere,

(iii) and then forming an upper electrode on the dielectric film of Ta.sub.2 O.sub.5.

FIG. 1 is a schematic diagram showing a structure of a semiconductor device provided in an example according to the present invention.

FIG. 2 is a schematic diagram showing a structure of a principal portion of a capacitor of the example.

FIG. 3 is a schematic diagram showing a structure of a capacitor of the example.

According to the present invention, a capacitor is formed on or over a silicon substrate which may provide a transistor element or other elements necessary for constituting a semiconductor device.

The capacitor of the present invention is composed of a lower electrode of a silicone containing material, an aluminum oxide film, a dielectric film of Ta.sub.2 O.sub.5 and an upper electrode.

The lower electrode can be formed by a conventional method and may be polysilicon, or silicide such as tungsten silicide.

The aluminum oxide film formed on the lower electrode is preferably 10 to 50 .ANG. in thickness. In the production of the semiconductor device according to the present invention, an aluminum (Al) film or aluminum oxide [A1.sub.2 O.sub.X (0<X<3)] film is initially formed and then converted into an aluminum oxide film [A1.sub.2 O.sub.Y (0<Y<3)]. The aluminum film may be formed by the conventional methods, for example, plasma CVD method using (CH.sub.3).sub.2 (Al)H gas or the like or preferably DC or RF sputtering method using Al target in argon gas since CVD method may restrain temperatures for growth of films. The Al.sub.2 O.sub.x film may be also provided by the conventional methods, such as thermal CVD method with Al(OC.sub.3 H.sub.7).sub.3 gas or the like or sputtering method using Al target.

The Al film or Al.sub.2 O.sub.x film can act to reduce a natural oxide formed through natural oxidation on the lower electrode of a Si-containing material when Al or Al.sub.2 O.sub.x changes to Al.sub.2 O.sub.Y during first annealing in a non-oxidative atmosphere as describe below.

Ta.sub.2 O.sub.5 for the dielectric film on the aluminum oxide film is preferably 50 to 250 .ANG. in thickness and may be formed by the conventional methods, such as thermal CVD method sing Ta(OC.sub.2 H.sub.5).sub.5 gas, or the like.

In the preparing method according to the present invention, on the lower electrode of a Si-containing material is formed Al or Al.sub.2 O.sub.x (0<x<3) film about 10 to 50 .ANG. in thickness, followed by first annealing in a non-oxidative atmosphere to convert the Al or Al.sub.2 O.sub.x (0<x<3) film into an aluminum oxide film, or the Al or Al.sub.2 O.sub.x film aluminum oxide film is formed about 10 to 50 .ANG. in thickness on the lower electrode of a Si-containing material while conducting the annealing in a non-oxidative atmosphere. The first annealing in a non-oxidative atmosphere is preferably carried out, for example, under nitrogen or hydrogen/nitrogen atmosphere at about 300.degree. to 450.degree. C. for about 10 to 100 mins. The first annealing may remove SiO.sub.2, the natural oxide formed on the lower electrode of the Si-containing material, by Al's reduction, resulting in that reaction "SiO.sub.2 to Si" by Al is facilitated.

Further, Ta.sub.2 O.sub.5 is formed about 50 to 250 .ANG. in thickness on the aluminum oxide film, followed by second annealing in an oxidative atmosphere to complete a layered structure of Ta.sub.2 O.sub.5 /Al.sub.2 O.sub.5 which is superior in insulation properties. The second annealing is preferably carried out, for example, at about 500.degree. to 800.degree. C. for about 10 to 100 mins in oxygen atmosphere, or at about 300.degree. to 600.degree. C. for about 10 to 100 mins in oxygen or ozone atmosphere, to improve quality of Ta.sub.2 O.sub.5 film.

Next, examples of the present invention will be detailed with referring to the accompanied drawings.

As shown in FIG. 1, the semiconductor device comprises a substrate 3 made of silicon and having thereon LOCOS 1 serving as an interlayer film and n+diffused layers 2, a word line 4 providing a gate, an interlayer dielectric film 6 having a contact hole 5 communicated with one of the n+diffused layers 2, a polysilicon film 7 filling the contact hole 5 and serving as a lower electrode for the capacitor, an aluminum oxide film 8 (FIG. 2), Ta.sub.2 O.sub.5 film 9 (FIG. 2), a polysilicon film 10 providing an upper electrode for the capacitor, and an interlayer dielectric film 11 separating a bit line 12 from the capacitor. The capacitor is composed of the polysilicon film 7, the Ta.sub.2 O.sub.5 film 9, the Al.sub.2 O.sub.x film 8 interposed therebetween and the polysilicon film 10, as shown. FIG. 3 shows a principal portion of the capacitor.

Next, a method for preparing the semiconductor device will be detailed.

The examples use the polysilicon film 7 for the lower electrode as shown in FIG. 1.

Al film 8 having thickness of 10 .ANG. was deposited on the polysilicon film 7 by plasma CVD method using (CH.sub.3).sub.2 (Al)H gas.

For the other example, (Al).sub.2 O.sub.x film 8 having thickness of 20 .ANG. was deposited on the polysilicon film 7 by thermal CVD method using Al(OC.sub.3 H.sub.7).sub.3 gas.

Al/SiO.sub.2 /polysilicon or (Al).sub.2 O.sub.x /SiO.sub.2 /polysilicon was then completed.

The polysilicon film 7 having the Al film 8 was subjected to the first annealing in N.sub.2 atmosphere at 400.degree. C. for 1 hr.

The another polysilicon film 7 having the (Al).sub.2 O.sub.x film 8 was subjected to the first annealing in N.sub.2 /H.sub.2 atmosphere at 400.degree. C. for 1 hr.

As a result, either case formed Al.sub.2 O.sub.3 /polysilicon.

Ta.sub.2 O.sub.5 film 9 was deposited 200 .ANG. on the Al film 8 and Al.sub.2 O.sub.3 film 8 by thermal CVD method using Ta(OC.sub.2 H.sub.5).sub.5 gas. Ta.sub.2 O.sub.5 /Al.sub.2 O.sub.3 /polysilicon was then completed.

The second annealing was carried out in oxygen atmosphere at 800.degree. C. for 1 hr to improve quality of the Ta.sub.2 O.sub.5 film 9.

With the temperature of wafer increased to 400.degree. C., Al film 8 and Al.sub.2 O.sub.x film 8 each having thickness of approximately 10 .ANG. were formed on respective polysilicon films 7, provided in the same manner as of the Examples 1 and 2, by RF sputtering method using Al target, and Ar gas or Ar+O.sub.2 gas, respectively. In this step, Al.sub.2 O.sub.3 /polysilicon appeared on the polysilicon films 7.

Ta.sub.2 O.sub.5 film 9 was deposited approximately 200 .ANG. on the Al film 8 and Al.sub.2 O.sub.x film 8 by thermal CVD method using Ta(OC.sub.2 H.sub.5).sub.5 gas. Ta.sub.2 O.sub.5 /Al.sub.2 O.sub.3 /polysilicon was then completed.

The second annealing was carried out in oxygen atmosphere at 800.degree. C. for 1 hr to improve quality of Ta.sub.2 O.sub.5 film 9. Ta.sub.2 O.sub.5 /Al.sub.2 O.sub.3 /polysilicon was then completed.

The examples used WSi.sub.2 for the lower electrode to form Ta.sub.2 O.sub.5 /Al.sub.2 O.sub.3 /polysilicon structure in the same manner as of the examples 1 and 2.

The examples used WSi.sub.2 for the lower electrode to form Ta.sub.2 O.sub.5 /Al.sub.2 O.sub.3 /polysilicon structure in the same manner as of the examples 3 and 4.

The above method for preparing a semiconductor device achieved a capacitor dielectric film which has low dielectric constant 4 or less and does not have SiO.sub.2 film. In detail, capacitance of Ta.sub.2 O.sub.5 /Al.sub.2 O.sub.3 structures of the capacitors provided by the methods of Examples 1 to 8 with 100 .ANG. of Ta.sub.2 O.sub.5 (dielectric constant [.epsilon.]=24) and 30 .ANG. of Al.sub.2 O.sub.3 (dielectric constant [.epsilon.]=8) is higher 1.4 times than that of Ta.sub.2 O.sub.5 /Al.sub.2 O.sub.3 /SiO.sub.2 structures with 100 .ANG. of Ta.sub.2 O.sub.5 (dielectric constant [.epsilon.]=24), 30 .ANG. of Al.sub.2 O.sub.3 (dielectric constant [.epsilon.]=8) and 20 .ANG.of SiO.sub.2 film (dielectric constant [.epsilon.]=4).

Also, Ta.sub.2 O.sub.5 which is usually likely to have higher leak current is provided in the form of a thin film of high dielectric properties, thereby enabling leak current to be reduced. Hence, Ta.sub.2 O.sub.5 together with Al.sub.2 O.sub.3 also of high dielectric properties provide the Ta.sub.2 O.sub.5 /Al.sub.2 O.sub.3 structure of excellent dielectric properties.