Electronic device with a gate insulating film and a cap layer of silicon nitride having crystallinity

A semiconductor wafer is provided, which includes a wafer; a nitride crystal layer formed of one or more crystal layers of group III nitride; and a cap layer; the wafer, the nitride crystal layer and the cap layer are positioned in an order of the wafer, the nitride crystal layer and the cap layer, and the cap layer is a silicon nitride layer having crystallinity and has a thickness of 5 nm or more. Also, a semiconductor wafer is provided, where a layer that is of the nitride crystal layer and that is in contact with the cap layer, and a layer near the layer function as active layers of a field-effect transistor, the cap layer is a silicon nitride layer having crystallinity and a thickness that is equal to or larger than a thickness in which a gate of the field-effect transistor can be embedded.

BACKGROUND

1. Technical Field

The present invention relates to a semiconductor wafer and an electronic device.

2. Related Art

For example, Patent Document 1 has disclosed a method of manufacturing a semiconductor device, to stably and effectively perform thermal treatment at a high temperature while preventing nitrogen leakage from a nitride-based semiconductor layer. In the document, it has been described that after an n-AlxGa1-xN layer12is formed on an n-GaN wafer11, impurities are doped in the n-AlxGa1-yN layer12, and a first cap layer2aformed of AlyGa1-y)N and a second cap layer2bformed of AlxGa1-xN that are epitaxial films are sequentially formed on a surface of the n-AlxGa1-xN layer12by an MOCVD method to form a treated wafer2. However, an Al composition ratio y is larger than an Al composition ratio x and is also larger than an Al composition ratio z.

For example, in the Patent Document 2, a semiconductor device and a method of manufacturing the semiconductor device to obtain a semiconductor device that realizes both high blocking voltage and large current and has an excellent heterojunction characteristic has been disclosed. In the document, it has been described that the semiconductor device includes an electron channel layer3that is formed of GaN; an electron supplying layer4that is formed of AlGaN, is provided on the electron channel layer3and supplies electrons to 2DEG; a first gap layer5that is formed of an undoped semiconductor, is provided on the electron supplying layer4, and has an opening that is opened so as to surround a gate electrode11; and a second gap layer6that is formed of an n type semiconductor, is provided only on the first gap layer5, and has an opening being opened on a downstream side of a gate electrode in a flowing direction of electrons of 2DEG and having a larger opening width than the opening of the first gap layer5in the flowing direction of electrons of 2DEG, where an electron channel layer formed of GaN, an electron supplying layer formed of AlGaN, a first gap layer and a second gap layer are generated according to a continuous epitaxial growth process.

In these Patent Documents 1 and 2, to reduce defects caused by annealing and the like during a semiconductor device manufacturing process or to adjust band potential in the semiconductor device, a cap layer formed of AlGaN is formed on an electron supplying layer (in the cited document 1, the n-AlxGa1-xN layer12, and in the cited document 2, the electron supplying layer4).

PRIOR ART LITERATURE

Patent Document

SUMMARY

The inventors of the present application figured out that the presence of the above-described cap layer not only realizes the purposes described in the Patent Documents 1 and 2 but also contributes to protection of channel forming layers (the n-GaN wafer11and the n-AlxGa1-xN layer12in the cited document 1 and the electron channel layer3formed of GaN and the electron supplying layer4formed of AlGaN in the cited document 2) during the epitaxial growth process.

That is, the group III nitride crystal layer, such as a GaN, AlGaN, and AlN layer, is crystal-grown with defect repair by hydrogen etching at the time of epitaxial growth by a Metal Organic Chemical Vapor Deposition (MOCVD) Method. A growth temperature for the epitaxial growth is generally a high temperature of approximately 1000° C., and after the epitaxial growth is ended, in a step in which the temperature is decreased to approximately 600° C., an epitaxially grown surface is exposed in hydrogen atmosphere, and defects may occur in the epitaxial wafer due to the etching by the atmosphere. From the view point of preventing defects from occurring due to such an etching, it is meaningful to form a cap layer on the uppermost surface of an epitaxially grown layer. When the cap layer is GaN or AlGaN, a solution to form the cap layer or the like is considered by taking into account in advance that the cap layer should have a thickness that can be decreased by the etching into a desired thickness.

However, when the cap layer is GaN or AlGaN, as also described in Patent Document 1 and the like, the thickness or composition affects characteristic of a semiconductor device, such as a transistor, which is to be formed later. For this reason, it is preferable to control the thickness, the composition or the like of the cap layer as precisely as possible. However, even in the method of forming the cap layer having a larger thickness by taking into account in advance that the etching that is performed after the epitaxial growth is ended, it is difficult to perform a precise thickness control on the cap layer.

Also, to use a cap layer for a control of band potential in a semiconductor device such as a transistor, uniformity of the thickness within a wafer surface is a crucial factor for enhancing a yield of the semiconductor device.

Further, it is preferable to have a high degree of freedom in designing for development of a semiconductor device such as a transistor, and for that reason, it is desirable to provide various cap layers to respond to a request of design diversity.

The present invention is to provide a technology of a cap layer that is not affected or is slightly affected by the etching by hydrogen atmosphere in an epitaxial growth ending step. Also, the present invention is to provide a technology of a cap layer that can respond to the request for a high degree of freedom in designing. Further, the present invention is to provide a technology of a cap layer that can realize high film thickness uniformity within a wafer surface.

To solve the above-described issue, in a first aspect of the present invention, a semiconductor wafer is provided, which includes a wafer, a nitride crystal layer formed of one or more crystal layers of group III nitride, and a cap layer, where the wafer, the nitride crystal layer and the cap layer are positioned in an order of the wafer, the nitride crystal layer and the cap layer, and the cap layer is a silicon nitride layer having crystallinity and has a thickness of 5 nm or more.

Also, in a second aspect of the present invention, a semiconductor wafer is provided, which includes a wafer, a nitride crystal layer formed of one or more crystal layers of group III nitride, and a cap layer, where the wafer, the nitride crystal layer and the cap layer are positioned in an order of the wafer, the nitride crystal layer and the cap layer, a layer that is of the nitride crystal layer and that is in contact with the cap layer, and a layer near the layer function as active layers of a field-effect transistor. The cap layer is a silicon nitride layer having crystallinity, and the cap layer has a thickness that is equal to or larger than a thickness in which a gate of the field-effect transistor is to be embedded.

A thickness of the cap layer can be made in a range from 5 nm to 550 nm, preferably can be 10 nm or more, more preferably can be 20 nm or more, and further preferably, can be 40 nm or more. The concept “having crystallinity” includes, in the first place, a case where an atom or a molecule has an exact periodicity in a three-dimensional space, as a single crystal or polycrystal, and also include a case where an atom or a molecule shows a certain degree of a periodicity even if the atom or the molecule does not have an exact periodicity. For example, a case where a spot pattern (an ordered spatial structure) is observed in an electron beam diffraction image is included. On the contrary, a disordered state (an amorphous state) in which only a halo pattern is observed in an electron beam diffraction image is not included in the concept “having crystallinity”.

The nitride crystal layer may include an element forming layer that is in contact with the cap layer, the element forming layer may include a first crystal layer and a second crystal layer that has a larger band gap than the first crystal layer, and two-dimensional carrier gas may be generated near a hetero interface between the first crystal layer and the second crystal layer. The nitride crystal layer may include a buffer layer that is positioned closer to the wafer than the element forming layer, and stress generated in the buffer layer may cancel stress between the element forming layer and the wafer. The wafer may be a silicon wafer, and the nitride crystal layer may include a reaction suppressing layer that suppresses a reaction between a silicon atom and a group III atom.

In a third aspect of the present invention, an electronic device in which a semiconductor wafer is used is provided, where the electronic device includes a gate structure of a field-effect transistor or a wiring structure that is connected to the nitride crystal layer, and a height of the gate structure or the wiring structure is smaller than a thickness of the cap layer.

As the above-described wafer, a silicon wafer, a sapphire wafer, a GaAs wafer and the like can be given, and as an electronic device, a High Electron Mobility Transistor (HEMT), a Heterojunction Bipolar Transistor (HBT), a Light Emitting Diode (LED) and the like can be given as examples.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1shows a cross-sectional view of a semiconductor wafer100that is used in an inspection method of the present embodiment. The semiconductor wafer100includes a wafer102, a nitride crystal layer120and a cap layer140, and the wafer102, the nitride crystal layer120and the cap layer140are positioned in an order of the wafer102, the nitride crystal layer120and the cap layer140. The nitride crystal layer120is formed of one or more crystal layers of group III nitride. The nitride crystal layer120includes, for example, a reaction suppressing layer104, a buffer layer106and an element forming layer108.

The wafer102is a support wafer that supports the nitride crystal layer120and the cap layer140. Although a material of the wafer102is preferably silicon, the material is not limited to this. As a material of the wafer102, sapphire, GaAs crystal and the like can be given as examples. When silicon wafer is used as the wafer102, material cost can be reduced, and a semiconductor manufacturing apparatus that has been used during a conventional silicon process can be utilized. Accordingly, cost competitiveness can be improved. Further, by using a silicon wafer as the wafer102, a large-size wafer having a diameter of 150 mm or more can be industrially utilized at a cheap price.

When the wafer102is a silicon wafer, the reaction suppressing layer104suppresses a reaction between a silicon atom contained in the silicon wafer and a group III atom contained in the buffer layer106and the like. When the nitride crystal layer in an upper layer of the reaction suppressing layer104is a GaN-based semiconductor layer such as AlGaN and GaN, alloying of a Ga atom and a silicon atom contained in the GaN-based semiconductor layer and a silicon atom contained in the silicon wafer can be prevented. As the reaction suppressing layer104, AlzGa1-zN (0.9≤z≤1) can be given, and an AlN layer can be given as a representative example. According to the reaction suppressing layer104, the surface of the wafer102can be protected, thereby surely supporting the upper layer. Also, the reaction suppressing layer104can form an initial nucleus of a crystal layer that is formed on the wafer102. A thickness of the reaction suppressing layer104can be made to be 30 nm or more and 300 nm or less.

The buffer layer106is positioned closer to the wafer102than the element forming layer108, and generates stress that cancels stress between the element forming layer108and the wafer102. The buffer layer106has, for example, a multi-layer stack structure in which a plurality of two-layer stack structures106care repeatedly stacked, each two-layer stack structure106cbeing formed of a first layer106aand a second layer106b. According to such a multi-layer stack structure, compressive stress is generated, and as a result, the buffer layer106functions as a stress generating layer that reduces warping of the entire semiconductor wafer100. The buffer layer106also functions as an insulating layer that electrically insulates the wafer102and the element forming layer108.

The first layer106ais formed of a group III nitride crystal having a lattice constant, a1, in a bulk crystal, for example, and the second layer106bis formed of a group III nitride crystal having a lattice constant, a2 (a1<a2), in a bulk crystal, for example. The repeated number of two-layer stack structures106ccan be made to be 2 to 500, for example. By stacking a large number of the two-layer stack structures106c, the compressive stress generated in the buffer layer106can be made large. Also, the magnitude of the compressive stress generated in the buffer layer106can be easily controlled by the number of the stacks of the two-layer stack structures106c. Further, by stacking a large number of the two-layer stack structures106c, the improvement of the blocking voltage by the first layer106acan be further enhanced.

In the present embodiment, although it is described, as an example, that the buffer layer106is formed by repeatedly stacking a plurality of the two-layer stack structures106c, a plurality of two-layer stack structures106cmay not be repeatedly stacked, and in this case, the buffer layer106is formed of a single two-layer stack structure106c. The buffer layer106may also have a structure formed of a three-layer stack including, in addition to the first layer106aand the second layer106b, a third crystal layer having a lattice constant, a3 (a2<a3) in a bulk crystal. Alternatively, the buffer layer106may also be a graded crystal layer having a lattice constant in a bulk crystal that increases in a continuous or stepwise manner in a direction away from the wafer102. Further, the buffer layer106may also be a multi-layer stack structure in which a plurality of three-layer stacks or graded crystal layers are repeatedly stacked.

As the first layer106a, AlqGa1-qN (0.9≤q≤1) can be given as an example, and as the second layer106b, AlpGa1-pN (0≤p≤0.3) can be given as an example. A thickness of the first layer106acan be made to be 1 nm or more and 20 nm or less, and preferably, can be made to be more than 5.0 nm and less than 20 nm. A thickness of the second layer106bcan be made to be 5 nm or more and 300 nm or less, and preferably, can be 10 nm or more and 300 nm or less.

The element forming layer108is in contact with the cap layer140, and includes, for example, a first crystal layer112and a second crystal layer114. In the element forming layer108, any device, such as a transistor or a light emitting diode (LED), can be formed. A band gap of the second crystal layer114may also be larger than a band gap of the first crystal layer112, and in this case, two-dimensional carrier gas is generated near a hetero interface between the first crystal layer112and the second crystal layer114. In this case, a High Electron Mobility Transistor (HEMT) in which the two-dimensional carrier (electron) gas on the hetero interface is taken as a channel can be formed. The second crystal layer114may be in contact with the first crystal layer112and also be lattice-matched or pseudo-lattice-matched with the first crystal layer112.

The first crystal layer112is, for example, an AlxGa1-xN (0≤x<1) layer, and specifically, a GaN layer can be given as an example. A thickness of the first crystal layer112can be selected within a range from 200 nm to 2000 nm, and can be made to be 800 nm, for example.

The second crystal layer114is, for example, an AlyGa1-yN (0<y≤1, x<y) layer, and specifically, an AlyGa1-yN (0.1<y≤0.3) layer, and for example, Al0.25Ga0.75N can be given as an example. A thickness of the second crystal layer114can be selected within a range from 10 nm to 100 nm, and can be made to be 25 nm, for example.

The cap layer140is a silicon nitride layer having crystallinity and has a thickness of 5 nm or more. The thickness of the cap layer140can be set within a range from 5 nm to 550 nm, and preferably, can be made to be 10 nm or more, more preferably, can be made to be 20 nm or more, and further preferably, can be made to be 40 nm or more.

By making the cap layer140as a silicon nitride layer, an influence of hydrogen atmosphere etching in the epitaxial growth ending step can be avoided or the influence can be made slight. Also, by making the cap layer140as a silicon nitride layer, various cap layers can be provided in addition to the GaN-based cap layer, and can increase a degree of freedom in device designing. Also, by making the cap layer140to have crystallinity and have a thickness of 5 nm or more, the uniformity of the thickness of the cap layer140within the wafer surface can be enhanced.

Note that the presence of the crystallinity in the cap layer140means that a crystal structure of the cap layer140is well ordered, and according to this, it is presumed that the film growth process of the cap layer140is in a thermal equilibrium state. Different from an amorphous film that grows in a thermal nonequilibrium state, the film growth in a thermal equilibrium state is insensitive to a growth parameter and uniformity of the film growth is easily maintained. For that reason, the within-surface uniformity of the thickness of the cap layer140is enhanced, and as a result, a preferable effect that enhances yield during a device manufacturing process, and the like is expectable.

Here, the concept “having crystallinity” includes, in the first place, a case where an atom or a molecule has an exact periodicity in a three-dimensional space, as a single crystal or polycrystal, and also include a case where an atom or a molecule shows a certain degree of a periodicity even if the atom or the molecule does not have an exact periodicity. For example, a case where a spot pattern (an ordered spatial structure) is observed in an electron beam diffraction image is included. On the contrary, a disordered state (an amorphous state) in which only a halo pattern is observed in an electron beam diffraction image is not included in the concept “having crystallinity”.

The nitride crystal layer120such as the reaction suppressing layer104, the buffer layer106and the element forming layer108can be formed using a general Metal Organic Chemical Vapor Deposition (MOCVD) method. For example, when a layer formed using the MOCVD method is an AlN layer, an AlGaN layer and a GaN layer, trimethylaluminum (Al(CH3)3) and trimethylgallium (Ga(CH3)3) can be used as the group III source gas, and ammonia (NH3) can be used as nitrogen source material gas. The growth temperature can be selected within a range from 1100° C. to 1260° C., and a V/III ratio, that is, a flow rate ratio of the group V source material gas to the group III source gas can be selected within a range from 160 to 5000. Regarding a thickness of the layer to be formed, a growth time period corresponding to a designed thickness can be calculated from a growth speed obtained by a preliminary experiment, for example, and the thickness can be controlled by the growth time period.

The cap layer140can be formed using a Thermal Chemical Vapor Deposition (thermal CVD) method, for example. For example, silane (SiH4) or silane (Si2H6) can be used as the Si source material gas, and ammonia (NH3) can be used as the nitrogen source material gas. The growth temperature can be selected within a range from 1000° C. to 1260° C. Regarding the thickness of the layer to be formed, a growth time period corresponding to a designed thickness can be calculated from a growth speed obtained by a preliminary experiment, for example, and the thickness can be controlled by the growth time period. In the above-described MOCVD or thermal CVD, in addition to the source material gas, carrier gas can be used, and as the carrier gas, hydrogen gas or nitrogen gas can be given as examples.

When a field-effect transistor is formed in the element forming layer108that is in contact with the cap layer140, that is, the two-dimensional carrier gas is formed on a hetero interface between the first crystal layer112and the second crystal layer114, and when these crystal layers function as active layers of a transistor, the cap layer140can be made to be a silicon nitride layer having crystallinity and having a thickness that is equal to or more than a thickness in which a gate of the field-effect transistor can be embedded.

FIG. 2shows a cross-sectional view of an electronic device200in which a field-effect transistor is formed on the semiconductor wafer100described in the Embodiment 1. The electronic device200is formed in a semiconductor wafer including the wafer102, the buffer layer106, the first crystal layer112, the second crystal layer114and the cap layer140, and has a gate structure of a field-effect transistor or a wiring structure that is connected to the nitride crystal layer120(in particular, the first crystal layer112on which a two-dimensional electron gas202is formed, and the second crystal layer114, and an interface therebetween), where a height of the gate structure or the wiring structure is smaller than the thickness of the cap layer140. Note that the connection includes not only mechanical connection and physical connection but also electrical connection.

That is, the field-effect transistor of the electronic device200includes a gate insulating film204, a gate electrode206, a gate cap layer208, a side wall210and a source/drain region212, and a total height h of the gate insulating film204and the gate electrode206that are the gate structure is smaller than the thickness of the cap layer140. This is relatively defined by comparing the thickness of the cap layer140to the gate structure, and makes a similar effect to the Embodiment 1. Also, in the present embodiment, by making the thickness of the cap layer140equal to or more than the height h of the gate structure, the cap layer140functions as the inter-layer insulating layer, and flattening of inter-layer films after the transistor is formed can be easily performed. As a result, it is an expectable effect that a wiring layer, a second inter-layer insulating layer, a third wiring layer, and the like to be formed on an upper layer can be easily formed. In a case where miniaturization is progressed, it is considered that the effect is more significant and the importance increases.

FIG. 3toFIG. 5respectively show a cross-sectional view for describing a manufacturing process of the electronic device200. As shown inFIG. 3, using a similar method to the method described in the Embodiment 1, a semiconductor wafer including a wafer102, a buffer layer106, a first crystal layer112, a second crystal layer114, and a cap layer140is formed. Here, the thickness of the cap layer140is d. Two-dimensional electron gas202is generated on an interface between the first crystal layer112and the second crystal layer114.

As shown inFIG. 4, a groove300is formed in the cap layer140, and after a coating film that is the gate electrode206and the gate cap layer208is formed, the coating film is patterned to form the gate electrode206and the gate cap layer208.

As shown inFIG. 5, the gate cap layer208is used in a mask to etch the cap layer140of a bottom of the groove300, and after a coating film that is the side wall210is formed, the coating film is anisotropically etched to form the side wall210.

The gate cap layer208and the side wall210are used for a mask to form the source/drain region212on the first crystal layer112and the second crystal layer114of the bottom of the groove300, by an ion implantation method, for example. In this way, the electronic device200ofFIG. 2is formed.

Example

A Si wafer of a diameter of 150 mm having a (111) plane as a main surface was used as the wafer102to form the reaction suppressing layer104, the buffer layer106and the element forming layer108. As the reaction suppressing layer104, an AlN layer having a designed thickness of 150 nm to 160 nm was formed. As the buffer layer106, a plurality of AlN/AlGaN stack structures (two-layer stack structures106c) each formed of an AlN layer (first layer106a) having a designed thickness of 5 nm and an AlGaN layer (second layer106b) having a designed thickness of 28 nm were repeatedly stacked and formed. As the element forming layer108, a GaN layer (first crystal layer112) having a designed thickness of 800 nm and an AlGaN layer (second crystal layer114) having a designed thickness of 25 nm were formed. An Al composition of the AlGaN layer (second crystal layer114) was set to be 0.25. Further, as the cap layer140, a Si3N4layer having a thickness 110 nm was formed.

For the forming of the reaction suppressing layer104, the buffer layer106and the element forming layer108(AlN layer, AlGaN layer and GaN layer), the MOCVD method was used, trimethylaluminum and trimethylgallium were used as the group III source gas, and ammonia was used as the nitrogen source material gas. The growth temperature was set within a range of 1100° C. to 1260° C. The V/III ratio, that is, the flow rate ratio of the group V source material gas to the group III source gas was selected within a range from 160 to 3700. By controlling the thickness of each layer by the growth time period calculated from the growth speed obtained by the preliminary experiment, the actual thickness of each layer is different from the designed thickness.

The thermal CVD method was used for forming the cap layer140(Si3N4layer), and silane was used as the Si source material gas, and ammonia was used as the nitrogen source material gas. The growth temperature was set within a range from 1000° C. to 1260° C.

FIG. 6shows a film thickness distribution of the cap layer140within the wafer surface. Over the entire surface of the wafer having a diameter of 150 mm, the thickness was within a range of 108.6±10.9 nm, and it can be learned that the film thickness uniformity is good.

FIG. 7shows an electron beam diffraction image of the cap layer140. Because the halo pattern cannot be observed and lots of spots (bright spots) were observed, the periodicity of the structure can be confirmed and it is known that the ordering is progressing.

EXPLANATION OF REFERENCES