Nonvolatile semiconductor memory device and method of manufacturing the same

According to one embodiment, a nonvolatile semiconductor memory device includes a magnetoresistive element formed on a semiconductor substrate, a first contact plug which extends through an interlayer dielectric film formed on the semiconductor substrate and immediately below the magnetoresistive element, has a bottom surface in contact with an upper surface of the semiconductor substrate, and is adjacent to the magnetoresistive element, and an insulating film formed between the magnetoresistive element and the first contact plug and on the interlayer dielectric film, wherein the insulating film includes a first region positioned on a side of the interlayer dielectric film, and a second region positioned in the insulating film and on an upper surface of the first region, the insulating film is made of SiN, and the first region is a nitrogen rich film compared to the second region.

FIELD

Embodiments described herein relate generally to a nonvolatile semiconductor memory device and a method of manufacturing the same.

BACKGROUND

A spin transfer torque MRAM (Magnetic Random Access Memory) including a magnetoresistive element containing a ferromagnetic material as a memory element has been proposed (this memory will be called an MRAM hereinafter). This MRAM is a memory that stores information by controlling the electrical resistance of the magnetoresistive element in two states, i.e., a high-resistance state/low-resistance state by changing the magnetization direction in a magnetic layer by an electric current to be injected into the magnetoresistive element.

The magnetoresistive element includes a memory layer as a ferromagnetic layer having a variable magnetization direction, a reference layer as a ferromagnetic layer having an invariable magnetization direction, and a tunnel barrier layer as a nonmagnetic layer formed between them.

DETAILED DESCRIPTION

In general, according to one embodiment, a nonvolatile semiconductor memory device includes a magnetoresistive element formed on a semiconductor substrate, a first contact plug which extends through an interlayer dielectric film formed on the semiconductor substrate and immediately below the magnetoresistive element, has a bottom surface in contact with an upper surface of the semiconductor substrate, and is adjacent to the magnetoresistive element, and an insulating film formed between the magnetoresistive element and the first contact plug and on the interlayer dielectric film, wherein the insulating film includes a first region positioned on a side of the interlayer dielectric film, and a second region positioned in the insulating film and on an upper surface of the first region, the insulating film is made of SiN, and the first region is a nitrogen-rich film compared to the insulating film in the second region.

This embodiment will be explained below with reference to the accompanying drawings. In this explanation, the same reference numerals denote the same arrangements throughout the drawings. However, it should be noted that the drawings are exemplary views, so the relationship between the thickness and the planar dimension, the ratio of the thicknesses of the respective layers, and the like are different from actual ones. Accordingly, practical thicknesses and dimensions should be judged by referring to the following explanation. Also, the individual drawings of course include portions having different dimensional relationships and different ratios.

First Embodiment

In the first embodiment, it is possible to deposit an insulating film that covers a magnetoresistive element and has a uniform thickness, and, by depositing this insulating film, suppress a leakage current generated between the magnetoresistive element and a contact plug CP connected to a source line.

An overall configuration example of a nonvolatile semiconductor memory device according to this embodiment will be explained with reference toFIGS. 1,2, and3. The nonvolatile semiconductor memory device according to this embodiment includes an MRAM.

FIG. 1is a circuit diagram showing a memory cell array of the MRAM according to this embodiment.

As shown inFIG. 1, a memory cell in a memory cell array MA includes a series circuit of a magnetoresistive element33and a switching element (e.g., an FET) T.

One terminal of the series circuit (one terminal of the magnetoresistive element33) is electrically connected to a bit line BL, and the other terminal of the series circuit (one terminal of the switching element T) is electrically connected to a source line SL.

The control terminal of the switching element T, e.g., the gate electrode of the FET is electrically connected to a word line WL.

A first control circuit1controls the potential of the word line WL. A second control circuit2controls the potentials of the bit line BL and source line SL.

The structure of the MRAM will be explained in detail with reference toFIGS. 2 and 3.

FIG. 2is a plan view showing an arrayed arrangement (to be called a memory cell array hereinafter) of the MRAM according to this embodiment.FIG. 3is a sectional view taken along a line3-3′ inFIG. 2. Note thatFIG. 3shows a section of a source line contact35, in addition to a section of the magnetoresistive element33.

As shown inFIGS. 2 and 3, the memory cell array MA includes, for example, a plurality of word lines WL and a plurality of dummy word lines DWL running in the Y direction, and a plurality of bit lines BL and a plurality of source lines SL running in the X direction. Note that the X direction is perpendicular to the Y direction.

Sets each including two word lines WL and one dummy word line DWL are alternately arranged along the X direction.

Also, the bit lines BL and source lines SL are arranged on an active area AA, and are alternately arranged along the Y direction.

An element isolation region49is buried between adjacent active areas AA. That is, the element isolation region49and active area AA are alternately formed along the Y direction.

The element isolation insulating layer49is formed by, e.g., STI (Shallow Trench Isolation). As the element isolation insulating layer49, an insulating material having a high filling characteristic such as silicon nitride (SiN) is used.

The sectional view will now be explained.

In the memory cell array MA as shown inFIG. 3, an element isolation insulating layer is formed in the surface region of a p-type semiconductor substrate (e.g., a silicon substrate)21, and this region functions as the element isolation region49.

In the semiconductor substrate21, a selection transistor T using, e.g., an n-channel MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is formed as the switching element T. The selection transistor T has a structure obtained by forming a recess in the semiconductor substrate21, and burying a gate electrode20containing, e.g., polysilicon in this recess.

More specifically, the selection transistor T includes a gate insulating layer22, the gate electrode20, and two diffusion layers25aand25b(a drain-side diffusion layer and source-side diffusion layer).

The gate insulating layer22is formed on the inner surface of the lower portion of the recess formed in the surface of the semiconductor substrate21and extending in the Y direction.

The gate electrode20is formed on the inner surface of the gate insulating layer22so as to fill the lower portion of the recess. The gate electrode20corresponds to the word line WL. An insulating layer24made of, e.g., SiN is formed on the upper surfaces of the gate insulating layer22and gate electrode20so as to fill the upper portion of the recess.

The upper surface of the insulating layer24has a height equal to that of the upper surface of the semiconductor substrate21(the upper surfaces of the diffusion layers25aand25bto be described below).

The two diffusion layers25aand25bare formed in the surface of the semiconductor substrate21so as to sandwich the gate insulating layer22, gate electrode20, and insulating layer24.

Also, as shown inFIG. 3, the element isolation regions49are arranged on the two ends of the diffusion layers25aand25b.

Furthermore, an interlayer insulating layer30is formed on the semiconductor substrate21(on the insulating layer24and diffusion layers25aand25b).

A contact plug CP2is formed in the interlayer dielectric layer30on the diffusion layer25a. The contact plug CP2will be called a BEC (Bottom Electronic Contact) hereinafter.

The BEC is formed in contact with a part of the upper surface of the diffusion layer25aand a part of the upper surface of the insulating layer24.

In other words, the BEC and diffusion layer25apartially overlap each other in a plane. This is so because the processing methods of the BEC and the diffusion layer25a(the recess) are different. The planar shape of the interlayer dielectric layer30is, e.g., a square. The BEC contains, e.g., TiN, but the material is not limited to this.

On the diffusion layer25b, a contact plug CP1having a bottom surface in contact with the diffusion layer25bis formed to extend through the interlayer dielectric layer30. The contact plug CP1also extends through an interlayer dielectric layer31(to be described later), and has an upper surface in contact with a source line (55b(bBL) inFIG. 3).

In addition, the magnetoresistive element (to be referred to as an MTJ hereinafter)33electrically connected to the BEC is formed in the interlayer dielectric layer31.

The MTJ is formed in contact with the upper surface of a lower electrode10. The MTJ has, e.g., a circular planar shape, and is formed into a pillar shape. Note that in this embodiment, the planar area of the magnetoresistive element33and that of the lower electrode10have the same value. However, the planar area of the MTJ is desirably smaller than that of the lower electrode10. This makes it possible to bring the entire lower surface of the MTJ into contact with the upper surface of the lower electrode10, and reduce the contact resistance between them.

This MTJ includes a memory layer11, a tunnel barrier layer12, and a reference layer13in this order from below. That is, the memory layer11is formed on the lower electrode10formed on the upper surface of the BEC.

The memory layer11is a ferromagnetic layer in which the magnetization direction is variable, and has perpendicular magnetic anisotropy perpendicular to or almost perpendicular to the film surfaces (upper surface/lower surface). “The magnetization direction is variable” herein mentioned means that the magnetization direction changes with respect to a predetermined write current. Also, “almost perpendicular” means that the direction of the residual magnetization falls within the range of 45°<θ≦90° with respect to the film surfaces.

The tunnel barrier layer12is formed on the memory layer11, and the reference layer13is formed on the tunnel barrier layer12.

The tunnel barrier layer12is a nonmagnetic layer, and contains a nonmagnetic material such as MgO. However, the material is not limited to this, and the tunnel barrier layer12may also contain a metal oxide such as Al2O3, MgAlO, ZnO, or TiO.

The reference layer13is a ferromagnetic layer in which the magnetization direction is invariable, and has perpendicular magnetic anisotropy perpendicular to or almost perpendicular to the film surfaces. “The magnetization direction is invariable” herein mentioned means that the magnetization direction does not change with respect to a predetermined write current. That is, the magnetization-direction switching energy barrier of the reference layer13is larger than that of the memory layer11.

Furthermore, a plurality of insulating films (to be also called a multilayered film hereinafter) are formed along the upper surface and sidewalls (side surfaces) of the MTJ, and along the surfaces (upper surfaces) of the BEC and interlayer dielectric layer30. A practical arrangement of the insulating films will be described below with reference to an enlarged view inFIG. 3. This enlarged view is a sectional view in the direction of3-3′, and shows the periphery of the MTJ in an enlarged scale.

An insulating film14is formed along the sidewalls of the lower electrode10, memory layer11, tunnel barrier layer12, and reference layer13. The insulating film14is made of silicon-rich SiN. More specifically, x and y in SixNyare set such that the ratio of silicon to nitrogen is higher than or close to a reference composition ratio. Practical values of x and y are x=1 and y=1.1.

Also, the thickness of the insulating film14is about 1 to 2 nm.

In addition, an insulating film15is formed along the sidewalls of an upper electrode17(to be described later) and the insulating film14, and along the BEC and interlayer dielectric layer30.

The insulating film15is made of nitrogen-rich SiN. That is, when the reference ratio of x to y in SixNyis 3:4, the ratio of nitrogen to silicon is sometimes close to this reference ratio, and sometimes higher than the reference composition ratio.

The insulating film15is made of a low-conductivity material. That is, the insulating film15having a low conductivity can be formed by using nitrogen-rich SiN.

Furthermore, the thickness of the insulating film15on the sidewall differs from that of the insulating film15on the bottom surface and upper surface. More specifically, the thickness is about 1 to 2 nm on the sidewall. When formed along the interlayer dielectric layer30and BEC, the thickness of the insulating film15is about 3 to 4 nm.

Also, the composition ratio of nitrogen in the SiN film changes in accordance with this thickness difference. More specifically, the ratio of x to y on the upper surface and bottom surface is x=1 to y=1.5, and the ratio of x to y on the sidewall is x=1 to y=1.3.

As will be described later, this nitrogen-rich SiN can be obtained by performing plasma nitridation on silicon-rich SiN.

In addition, as shown inFIG. 3, the upper surface of the insulating film15formed on the sidewall of the MTJ is higher than the upper surface of the reference layer13. However, the arrangement is not limited to this.

For example, the upper surface of the insulating film15formed on the sidewall of the MTJ and the upper surface of the reference layer13may also be leveled with each other. Note that the upper surface of the insulating film15is sometimes higher than that of the reference layer13because the reference layer13is also etched when an insulating film16and the insulating film15are etched by RIE in Step. 5 (to be described later).

Furthermore, an insulating layer16is formed to cover the whole MTJ, i.e., along the sidewalls of an upper electrode17and the insulating layer15.

Like the above-mentioned insulating layer14, the insulating layer16is made of silicon-rich SiN. That is, the composition ratio of x to y can also be the same as that in the insulating layer14.

Also, the thickness of the insulating film16on the sidewall of the MTJ differs from that of the insulating film16on the bottom surface and upper surface of the MTJ. More specifically, the thickness is about 12 nm on the sidewall. When formed along the interlayer dielectric layer30and BEC, the thickness of the insulating film16is about 16 to 17 nm.

InFIG. 3, therefore, the value of tsw1 is about 20 nm, and the value of tsw2 is about 14 nm.

In the interlayer dielectric layer31as described above, the nitrogen-rich insulating film15and silicon-rich insulating film16are sequentially stacked on the interlayer dielectric layer30with respect to the semiconductor substrate21.

Note that the multilayered film including the insulating films15and16need not clearly be separated. For example, in the multilayered film, a first region in which the content of nitrogen is higher than that of silicon may exist on the side of the semiconductor substrate21, and a second region in which the content of silicon is higher than that of nitrogen may exist on the side of the interlayer dielectric layer31.

That is, a region where the composition ratio of nitrogen is higher than that of silicon need only exist at least along a path in which the MTJ and contact plug CP1are electrically connected.

The explanation of the arrangement shown inFIG. 3will be continued.

An upper electrode17is formed on the upper surface of the reference layer13, and a contact plug CP3(Top Electronic Contact, to be called a TEC hereinafter) having a bottom surface in contact with the upper surface of the upper electrode17and an upper surface in contact with the bit line BL is formed.

As described above, the contact plug CP1extends through the interlayer dielectric layers30and31and insulating films15and16, and has a bottom surface in contact with the upper surface of the diffusion layer25b. The upper surface of the contact plug CP1is connected to the source line (55b(bBL) inFIG. 3).

Note that of three gate electrodes20adjacent to each other in the X direction as shown inFIG. 2, two gate electrodes20are electrically connected to the magnetoresistive elements33and function as the word lines WL, and one gate electrode20is not electrically connected to the magnetoresistive element33and functions as the dummy word line DWL.

The manufacturing steps of the MRAM according to the first embodiment will be explained below with reference toFIGS. 4,5,6, and7. In the following manufacturing steps, the insulating films14to16formed to cover the MTJ will be noted.

First, layers are formed up to a BEC buried in an interlayer dielectric layer30by using the well-known techniques.

After that, a metal layer10, magnetic layer11, insulating layer12, and reference layer13are sequentially stacked on the interlayer dielectric layer30and BEC, and a resist film formed after that is partially left behind.

Then, the reference layer13, insulating layer12, magnetic layer11, and metal layer10are etched by using the resist film as a mask, thereby forming a gate structure shown inFIG. 4. In this step, the corners of the reference layer13of the gate structure are rounded.

Subsequently, while the ambient temperature is set at about 300° C., an SiN film50is formed to cover the upper surfaces of the interlayer dielectric layer30and BEC and the upper surface and sidewalls of the gate structure by using TSA (Trisilylamine) in RLSA plasma CVD.

Consequently, a silicon-rich SiN film50having a uniform thickness is formed to cover the interlayer dielectric layer30, BEC, and gate structure.

Note that the ambient temperature is set at about 300° C. because if the SiN film50is formed at a temperature higher than 300° C., the metal layer10, magnetic layer11, and the like may be damaged.

In this embodiment as shown inFIG. 4, the SiN film50has a thickness equal to a height “h1” on the upper surface of the gate structure and on the interlayer dielectric layer30and EEC, and a thickness equal to a width “w1” on the sidewalls of the gate structure.

Although the values of “h1” and “w1” are different, the SiN film50is a flat film on the sidewalls, upper surface, and bottom surface, i.e., the SiN film50is an unbiased film on the film growth surfaces with respect to the structure. This is so because the SiN film is deposited at a low temperature of 300° C.

After that, the SiN film50as an insulating film50is changed (modified) into a nitrogen-rich SiN film15by a plasma nitriding process. That is, in the composition ratio of x to y described above, the ratio of nitrogen increases compared to silicon. More specifically, the ratio of nitrogen rises to approximately 3:4, or SiN is made of nitrogen more than this reference value, as described previously.

In this step, the sidewalls are not entirely nitrided, and the silicon-rich SiN film remains. This film is the insulating film14described above.

Note that in this plasma nitriding process, the height “h” becomes about 3 to 4 nm, and the width “w” becomes about 1 to 2 nm. As described previously, the ratio of nitrogen to silicon in the insulating layer15formed on the side surfaces of the gate structure is lower than that of the insulating film15formed on the upper surface of the gate structure and on the BEC and interlayer dielectric layer30.

In addition, an insulating film16is deposited to cover the insulating film15by RSLA plasma CVD. Like the insulating film14, the insulating film16is a silicon-rich SiN film.

Then, the insulating films15and16as prospective formation regions of an upper electrode17are etched by RIE, and a metal film (that becomes an upper electrode17later) is buried. An upper electrode17is formed by polishing this metal film by CMP.

Furthermore, after an interlayer dielectric layer31shown inFIG. 3is stacked, a through hole extending through the interlayer dielectric layer31, insulating film16, insulating film15, and interlayer dielectric layer30and reaching the diffusion layer25b, and a prospective TEC formation region are formed by, e.g., RIE.

After that, a contact plug CP1and TEC are formed by burying metal layers in the through hole and prospective TEC region. As a consequence, the structure shown inFIG. 3can be obtained.

As shown inFIG. 7, a distance “1” between the contact plug CP1and MTJ is about 8 to 10 nm.

The distance “1” is smaller than the distance between this MTJ and an adjacent MTJ (not shown).

<Effects of First Embodiment>

The MRAM according to the first embodiment can achieve effects (1) to (4) below.

(1) The insulating film15having a uniform thickness can be deposited.

This will be explained by taking a comparative example in order to facilitate understanding. This comparative example is a structure in which a nitrogen-rich insulating film15having a large thickness covering the gate structure is deposited in one step instead of Steps. 1 to 3 described above. Note that the same reference numerals denote the same structures.

The characteristic of the nitrogen-rich insulating film15has the problem that a uniform thickness is difficult to obtain on, e.g., the upper surface and sidewalls of the gate structure. For example, the insulating film15is difficult to deposit on a corner formed by the side surface of the metal layer10and the upper surface of the BEC, and a domed insulating film15like a swelled film is deposited on the interlayer dielectric layer30and on the upper surface of the gate structure.

By contrast, the insulating film15having a uniform thickness can be deposited in the MRAM according to the first embodiment.

This is so because the embodiment uses the characteristic that a silicon-rich SiN film has a uniform thickness as described above. Accordingly, a silicon-rich SiN film need only be deposited as the insulating film15, but this film allows an easy flow of an electric current (has conductivity). This characteristic poses a problem to be explained in effect (2) below.

In the first embodiment, therefore, a film having a uniform thickness is first formed by depositing the silicon-rich SiN film50, and then a plasma nitriding process is performed. This makes it possible to deposit the nitrogen-rich insulating film15having a uniform thickness, although the values of “h” and “w” are different.

(2) A leakage current flowing between the MTJ and contact plug PC1can be reduced.

This feature will also be explained by taking a comparative example in order to facilitate understanding.

This comparative example is a structure in which a single-layered, silicon-rich SiN film is deposited on the upper surface and sidewalls of the gate structure, on the BEC, and on the interlayer dielectric layer30as described above.

As described previously, the deposited insulating film has a uniform thickness.

However, the distance “1” between the MTJ and contact plug CP1is as short as about 8 to 10 nm, and a silicon-rich SiN film has conductivity as described above.

In the structure of the comparative example, therefore, when executing, e.g., a data read or write operation, a potential difference is produced between the MTJ and contact plug CP1, so an electric current flowing through the MTJ flows into the contact plug CP1through the path between the interlayer dielectric layer30and silicon-rich SiN film. This causes an operation error of the nonvolatile semiconductor memory device.

By contrast, in the nonvolatile semiconductor memory device according to the first embodiment, the insulating film15is formed between the MTJ and contact plug CP1as shown inFIGS. 3 and 7.

The insulating film15has a low conductivity as described previously, and hence can suppress a leakage current such as that generated in the comparative example. More specifically, the current value can be reduced by about two orders of magnitude from that in the structure of the comparative example. That is, it is possible to prevent an operation error of the nonvolatile semiconductor memory device, and improve the reliability of the device.

(3) Deterioration of the MTJ can be prevented.

In the nonvolatile semiconductor memory device according to the first embodiment, the value of the thickness “w” of the nitrogen-rich SiN film on the sidewalls is smaller than that of the thickness “h” of the nitrogen-rich SiN film formed on, e.g., the upper surface of the MTJ as mentioned earlier. However, this thickness difference can achieve the effect of preventing deterioration of the MTJ during the nitriding process.

The reasons for this are:

(i) Since the nitriding process has a high energy, the MTJ readily deteriorates if the nitriding process is performed on the sidewalls of the MTJ for a long time period.

(ii) Although the suppression of the leakage current is explained in above-mentioned effect (2), the insulating film15can be deposited at least on the interlayer dielectric layer30and between the MTJ and contact plug CP1where a leakage current is readily generated.

For reasons (i) and (ii) above, deterioration of the MTJ can be prevented more when the value of the thickness “w” of the insulating film15formed on the sidewalls is smaller than that of “h”.

(4) The parasitic capacitance can be reduced.

In the nonvolatile semiconductor memory device according to the first embodiment, the parasitic capacitance between adjacent MTJs can be reduced.

The dielectric constant of a nitrogen-rich SiN film is higher than that of a silicon-rich SiN film. That is, if the thickness “w” on the sidewalls of adjacent MTJs is large, the parasitic capacitance increases between these adjacent MTJs. This may change data held in these MTJs.

By contrast, in the nonvolatile semiconductor memory device according to the first embodiment, the parasitic capacitance between adjacent MTJs can be reduced because the thickness “w” on the sidewalls is small.

Note that the same effect can be achieved for the parasitic capacitance between the MTJ and contact plug CP1.

Next, a nonvolatile semiconductor memory device according to a modification of the first embodiment will be explained with reference toFIGS. 8,9, and10.

This modification is a structure in which the insulating film14is excluded from the above-mentioned first embodiment. Only an arrangement different from the first embodiment will be explained below.

FIG. 8shows the section of a memory cell array according to the modification.FIG. 8is a sectional view taken along a line3-3′ inFIG. 2.

As shown inFIG. 8, an insulating film15is directly formed along an upper electrode17, reference layer13, tunnel barrier layer12, memory layer11, lower electrode10, BEC, and interlayer dielectric layer30.

The insulating layer15has a thickness of about 1 to 2 nm on the sidewalls, and a thickness of about 3 to 4 nm on the BEC and interlayer dielectric layer30.

In addition, an insulating film16is formed to cover the insulating film15.

Manufacturing steps according to the modification will now be explained with reference toFIGS. 9 and 10. Note that an explanation of the same steps as those of the nonvolatile semiconductor memory device according to the first embodiment will be omitted.

First, as shown inFIG. 9, an insulating film15made of nitrogen-rich SiN is deposited along an upper electrode17, reference layer13, tunnel barrier layer12, memory layer11, lower electrode10, BEC, and interlayer dielectric layer30by using RSLA plasma CVD.

Note that the insulating film15is deposited in one step in the nonvolatile semiconductor memory device according to the modification, but the film thickness differs from the thickness of the insulating film15of the comparative example described in effect (1) of the first embodiment. That is, since the insulating film15of the modification has a thickness equivalent to that of the first embodiment, so the problem explained in the comparative example of effect (1) does not occur in this modification.

After that, an insulating film16is deposited as shown inFIG. 10, and the structure shown inFIG. 8is obtained through the step explained with reference toFIG. 7.

Even the nonvolatile semiconductor memory device according to this modification can achieve effects (1) to (4) of the above-mentioned first embodiment, and can also achieve effect (5) below.

(5) The number of manufacturing steps can be reduced.

When manufacturing the nonvolatile semiconductor memory device according to the modification, Steps. 2 and 3 described previously can be omitted. That is, it is possible to reduce the manufacturing cost while obtaining effects (1) to (4).

Note that as explained in the first embodiment and its modification, it is desirable to uniformly deposit the insulating film15on the entire surface of the MTJ in order to suppress the leakage current between the MTJ and contact plug CP1.