SEMICONDUCTOR DEVICE

Abstract

A semiconductor device including a contact plug connected to a diffusion layer of a cell transistor, a heater electrode connected to a phase-change film, and a buffer plug connecting between the contact plug and the heater electrode.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a circuit diagram of a phase-change memory cell;

FIG. 1B is another circuit diagram of a phase-change memory cell;

FIG. 2 is a cross-sectional view of a phase-change memory cell according to a first prior art;

FIG. 3 is a cross-sectional view of a phase-change memory cell according to a second prior art;

FIG. 4 is a cross-sectional view of a phase-change memory cell according to the present invention; and

FIG. 5 is a cross-sectional view of another phase-change memory cell according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A semiconductor device according to the present invention will now be described in detail with reference to FIGS. 4 and 5. FIG. 4 is a cross-sectional view of a phase-change memory cell of the present invention. FIG. 5 is a cross-sectional view of another phase-change memory cell of the present invention. The semiconductor device of the present invention includes a buffer plug 9. Specifically, a three-stage structure composed of a heater electrode 31, the buffer plug 9, and a contact plug 8 connects between a phase-change film 3 and a drain diffusion layer 10.

The memory cell shown in FIG. 4 includes a cell transistor, a contact plug 8, a buffer plug 9, a heater electrode 31, a phase-change film 3, an upper electrode 4, and a GND wiring 7. The cell transistor is composed of a drain diffusion layer 10, a source diffusion layer 6, and a gate electrode 5. The source diffusion layer 6 is connected to the GND wiring 7 via a plug. The gate electrode 5 is connected to a word-line. The drain diffusion layer 10 is connected to the heater electrode 31 via the contact plug 8 and the buffer plug 9. Furthermore, the phase-change film 3 is disposed on the upper face of the heater electrode 31, and the upper electrode 4 is disposed on the upper face of the phase-change film 3. The phase of the phase-change film 3 is changed around the interface with the heater electrode 31 by applying a voltage between the upper electrode 4 and the GND wiring 7. Thereby, the series electric resistance is changed. In this event, the area where the phase-change occurs is referred to as a phase-change region 2.

In the present invention, the heater electrode 31 is disposed on the top of the buffer plug 9. The buffer plug 9 is disposed on the top of the contact plug 8. Further, a layer of the phase-change film 3 lies on the upper face of the heater electrode 31, and the upper electrode 4 is disposed on the phase-change film 3. By applying a pulse voltage to the upper electrode 4, a current flows from the upper electrode 4 to the GND wiring 7 through the phase-change film 3, the heater electrode 31, the buffer plug 9, the contact plug 8, and the cell transistor. In this case, heat is generated in the interface between the heater electrode 31 and the phase-change film 3 to produce the phase change of the phase-change film 3 at this area so that a change in the series electric resistance is created.

The contact plug 8, the buffer plug 9, and the heater electrode 31 are separately disposed in the respective interlayer insulating films and are stacked in the direction perpendicular to a semiconductor substrate in this order from the bottom so as to have the centers at approximately the same position vertically. In the order of the contact plug 8, the buffer plug 9, and the heater electrode 31, the heater electrode 31 has the highest specific resistance and the smallest diameter. The contact plug 8 forms an ohmic contact with the diffusion layer by being composed of, for example, a deposition of Ti, a deposition of TiN serving as a barrier metal, and a deposition of W for embedding. Thus, the contact plug 8 is made of a material having a very low resistance.

The buffer plug 9 is formed of a material, such as TiN, having a resistance higher than that of the contact plug 8. In general, a material having a high resistance has a low heat conductivity. Since the buffer plug 9 has a diameter smaller than that of the contact plug 8 and has a resistance higher than that of the contact plug 8, the heat generated in the heater electrode 31 hardly diffuses to the lower direction. Further, the heater electrode 31 has a diameter smaller than that of the buffer plug 9 and has a resistance higher than that of the buffer plug 9. Thus, a structure having a high current density and a high heat generation efficiency is obtained by decreasing the diameter and increasing the resistance.

Thus, with a multistage structure including the buffer plug 9 intervening between the contact plug 8 and the heater electrode 31, the aspect ratios of holes opening to the respective interlayer insulating films can be reduced so that optimum diameters can be selected. In addition, the resistances can be adjusted to the respective optimum values. The buffer plug 9 lying at the middle has a medium diameter and a medium specific resistance. Since the buffer plug 9 has a low heat conductivity, the heat generated in the interface between the heater electrode 31 and the phase-change film 3 hardly diffuses to the contact plug 8 disposed at the lower side. Consequently, the heat is transmitted to the phase-change film 3 disposed at the upper side. Accordingly, the thermal efficiency is improved and the current necessary for rewriting can be reduced, compared to those in the related structure. As a result, data in a memory cell can be rewritten even if the current capability of the cell transistor is low. In addition, the cell transistor size is small and thereby the cell size can be reduced, resulting in an improvement in the cost performance of the memory.

Next, a method of producing the heater electrode according to the present invention will be described. A cell transistor and a GND wiring 7 are formed by ordinary processes, and then a first interlayer insulating film 41 is formed. A contact hole is formed in the interlayer insulating film 41 so that a drain diffusion layer 10 is exposed. The contact hole is filled with an electrically conductive film to form a contact plug 8. The contact plug 8 forms an ohmic contact with a diffusion layer by being composed of, for example, a deposition of Ti (titanium), a deposition of TiN (titanium nitride) serving as a barrier metal, and a deposition of W (tungsten) for embedding. The deposited electrically conductive films are planarized by CMP (Chemical Mechanical Polishing).

Thus, the contact plug 8 is formed so as to have low reactivity with the diffusion layer and to have a very low resistance value. For example, the specific resistance of W, which is a main material of the contact plug 8, is 7 μΩ·cm. The total specific resistance of the contact plug 8, which includes Ti and TiN in addition to W, is about 20 μΩ·cm. In a contact plug having a diameter of 200 nm and a depth of 600 nm, the resistance of the contact plug is 3.8Ω. A contact plug 8 having a lower resistance value, specifically, 10Ω or less, is preferred.

Then, a second interlayer insulating film 42 is formed, and a contact hole is formed in this second interlayer insulating film 42 so that the upper face of the contact plug 8 is exposed. The contact hole is filled with an electrically conductive film to form a buffer plug 9 so that the buffer plug 9 has the center at the position approximately corresponding to the center of the contact plug 8. The buffer plug 9 has a diameter smaller than that of the contact plug 8 and has a specific resistance higher than that of the contact plug 8.

The buffer plug 9 is formed of, for example, TiN. In general, a material having a high resistance has a low heat conductivity. A structure in which the heat generated in the heater electrode 31 hardly diffuses to the lower direction can be given by forming the buffer plug 9 by a material with a resistance higher than that of the contact plug 8. For example, a TiN buffer plug formed by a usual CVD (Chemical Vapor Deposition) method has a specific resistance of 200 to 500 μΩ·cm. In a buffer plug 9 having a diameter of 100 to 120 nm and a depth of 200 nm, the resistance of the buffer plug 9 is 35 to 127Ω. The resistance of the buffer plug 9 is adjusted to 10Ω or more and 200Ω or less in order to reduce the heat conductivity.

Subsequently, a third interlayer insulating film 43 is formed, and a contact hole is formed in this third interlayer insulating film 43 so that the upper face of the buffer plug 9 is exposed. The contact hole is filled with an electrically conductive film with a high specific resistance to form a heater electrode 31 serving as a heating element so that the heater electrode 31 has the center at the position approximately corresponding to the centers of the contact plug 8 and the buffer plug 9. Further, the heater electrode 31 has a diameter smaller than that of the buffer plug 9. With this smaller diameter, the current density flowing in the heater electrode 31 can be increased. The heater electrode 31 is formed of a material with a specific resistance higher than that of the buffer plug 9.

Examples of the material with a high resistance used for the heater electrode 31 include TiN (titanium nitride), TiSiN (titanium silicon nitride), TIAIN (titanium aluminum nitride), C (carbon), CN (carbon nitride), MoN (molybdenum nitride), TaN (tantalum nitride), Ptlr (platinum irridium), TiCN (titanium carbon nitride), and TiSiC (titanium silicon carbon).

Herein, the buffer plug 9 and the heater electrode 31 are formed of the same TiN, but the specific resistances of the both are different from that of each other by varying deposition conditions of TiN. In general, the specific resistance of Ti is about 42 μΩ·cm and the specific resistance of TiN is about 200 μΩ·cm, but these specific resistances can be increased by varying deposition conditions. For example, TiN formed by a CVD (Chemical Vapor Deposition) method using a TiCl₄ (titanium tetrachloride) gas can have a specific resistance of 200 to 500 μΩ·cm. Furthermore, TiN formed by a MO-CVD (Metal Organic Chemical Vapor Deposition) method using a Ti(N(CH₃)₂)₄ (tetrakis(dimethylamino) titanium: TDMAT) gas can have a further high specific resistance of about 4500 μΩ·cm.

The heater electrode 31 is made of TiN having a specific resistance of 1000 μΩ·cm or more. In a heater electrode 31 having a diameter of 50 to 70 nm, a depth of 100 to 130 nm, and a specific resistance of 1000 μΩ·cm, the resistance of the heater electrode 31 is 260 to 660Ω. When the resistance is designated by a specific resistance, for example, the contact plug 8 is formed of a material having a specific resistance of 50 μΩ·cm or less, the buffer plug 9 is formed of a material having a specific resistance of 100 μΩ·cm or more, and the heater electrode 31 is formed of a material having a specific resistance of 1000 μΩ·cm or more. When the contact plug 8, the buffer plug 9, and the heater electrode 31 are each composed of a plurality of films, the specific resistance value is the average specific resistance value obtained according to the thickness of each laminated film.

Then, a phase-change film 3 is deposited, and then an upper electrode 4 is deposited thereon. The phase-change film 3 may be made of, for example, a material containing at least any two of germanium (Ge), antimony (Sb), tellurium (Te), selenium (Se), gallium (Ga), and indium (In). Examples of such a material include gallium antimonide (GaSb), indium antimonide (InSb), indium selenide (InSe), antimony telluride (Sb₂Te₃), germanium telluride (GeTe), Ge₂Sb₂Te₅, InSbTe, GaSeTe, SnSb₂Te₄, and InSbGe.

Next, a structure of another memory cell will be described with reference to FIG. 5. The memory cell shown in FIG. 5 is different from that of FIG. 4 in that a high-resistance portion 32 having a resistance higher than that of the heater electrode 31 is formed at a part of the top of the heater electrode 31. In the heater electrode 31, the area of contact of a heater electrode 31 and a phase-change film 3 is desired to most efficiently generate heat. Therefore, the high-resistance portion 32 is formed by further increasing the specific resistance of the top of the heater electrode 31 at the area being brought into contact with the phase-change film 3. For example, a heater electrode 31 is formed, and then the specific resistance of the top of the heater electrode 31 can be further increased by implanting, for example, nitrogen ions into the heater electrode 31 from the upper face. The specific resistance may be increased by implanting ions of nitrogen (N), oxygen (O), carbon (C), or silicon (Si). In addition, the specific resistance may be increased by supplying nitrogen or oxygen from plasma or by thermal oxidation. The high-resistance portion 32 may be formed in a part of the top of the heater electrode 31 as shown in FIG. 5 or may be formed in the entire heater electrode 31.

The phase-change memory of the present invention has a multistage structure composed of the contact plug 8, the buffer plug 9, and the heater electrode 31 between the diffusion layer 10 of the cell transistor and the phase-change film 3. The contact plug 8, the buffer plug 9, and the heater electrode 31 are separately disposed in the respective interlayer insulating films 41,42 and 43 and are stacked in the direction perpendicular to a semiconductor substrate in this order from the semiconductor substrate side so as to have the centers at approximately the same position vertically. In the order of the contact plug 8, the buffer plug 9, and the heater electrode 31, the heater electrode 31 has the highest specific resistance and the smallest diameter. Since the heater electrode 31 has a small diameter and a high resistance, the current density is large and the heat generation efficiency is high. In addition, the heat diffusion can be reduced by slightly increasing the resistance of the buffer plug 9. Thereby, the heat generation efficiency can be enhanced, and the rewriting current necessary for rewriting data (phase change) can be reduced. Consequently, the cell transistor and the cell can be miniaturized in size. In this manner, a semiconductor device including a phase-change memory which is small and can efficiently perform the rewriting process can be obtained.

The present invention is specifically described based on the embodiments above, but is not limited to these embodiments. The present invention can be variously modified without departing from the scope of the present invention, and such modifications are included in the present invention.

Claims

1. A semiconductor device, comprising: a cell transistor having a diffusion layer;a phase-change film;a contact plug connected to the diffusion layer;a heater electrode connected to the phase-change film; anda buffer plug connected between the contact plug and the heater electrode.

2. The semiconductor device according to claim 1, wherein: the contact plug, the buffer plug, and the heater electrode are separately disposed in respective interlayer insulating films and are stacked in a direction perpendicular to a semiconductor substrate in this order from the semiconductor substrate side so as to have centers at substantially the same position vertically.

3. The semiconductor device according to claim 2, wherein: the buffer plug has a diameter smaller than that of the contact plug, and the heater electrode has a diameter smaller than that of the buffer plug.

4. The semiconductor device according to claim 2, wherein: the buffer plug has a specific resistance higher than that of the contact plug, and the heater electrode has a specific resistance higher than that of the buffer plug.

5. The semiconductor device according to claim 4, wherein: the contact plug contains tungsten.

6. The semiconductor device according to claim 4, wherein: the buffer plug contains titanium nitride deposited by a Chemical Vapor Deposition method.

7. The semiconductor device according to claim 4, wherein: the heater electrode contains any one selected from the group consisting of TiN (titanium nitride), TiSiN (titanium silicon nitride), TiAIN (titanium aluminum nitride), C (carbon), CN (carbon nitride), MoN (molybdenum nitride), TaN (tantalum nitride), Ptlr (platinum irridium), TiCN (titanium carbon nitride), and TiSiC (titanium silicon carbon).

8. The semiconductor device according to claim 7, wherein: a top of the heater electrode has a portion with a higher specific resistance formed by implantation of oxygen, nitrogen, carbon, or silicon ions from an upper face.