Methods of fabricating nitride-based transistors with a cap layer and a recessed gate

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WO 2006/022874 PCT/US2005/010874

METHODS OF FABRICATING NITRIDE-BASED TRANSISTORS WITH A CAP

LAYER AND A RECESSED GATE

FIELD OF THE INVENTION

The present invention relates to semiconductor devices and, more particularly, to transistors that incorporate nitride-based active layers.

BACKGROUND

Materials such as silicon (Si) and gallium arsenide (GaAs) have found wide application in semiconductor devices for lower power and (in the case of Si) lower frequency applications. These, more familiar, semiconductor materials may not be well suited for higher power and/or high frequency applications, however, because of their relatively small bandgaps (e.g., 1.12 eV for Si and 1.42 for GaAs at room temperature) and/or relatively small breakdown voltages.

In light of the difficulties presented by Si and GaAs, interest in high power, high temperature and/or high frequency applications and devices has turned to wide bandgap semiconductor materials such as silicon carbide (2.996 eV for alpha SiC at room temperature) and the Group III nitrides (e.g., 3.36 eV for GaN at room temperature). These materials, typically, have higher electric field breakdown strengths and higher electron saturation velocities as compared to gallium arsenide and silicon.

A device of particular interest for high power and/or high frequency applications is the High Electron Mobility Transistor (HEMT), which, in certain cases, is also known as a modulation doped field effect transistor (MODFET). These devices may offer operational advantages under a number of circumstances because a two-dimensional electron gas (2DEG) is formed at the heterojunction of two semiconductor materials with different bandgap energies, and where the smaller bandgap material has a higher electron affinity. The 2DEG is an accumulation layer in the undoped (unintentionally doped), smaller bandgap material and can contain a very high sheet electron concentration in excess of, for example, 10¹³ carriers/cm². Additionally, electrons that originate in the wider-bandgap semiconductor transfer to

CA 02572244 2013-04-02 the 2DEG, allowing a high electron mobility due to reduced ionized impurity scattering.

This combination of high carrier concentration and high carrier mobility can give the HEMT a very large transconductance and may provide a strong performance advantage over metal-semiconductor field effect transistors (MESFETs) for highfrequency applications.

High electron mobility transistors fabricated in the gallium nitride/aluminum gallium nitride (GaN/ AlGaN) material system have the potential to generate large amounts of RF power because of the combination of material characteristics that includes the aforementioned high breakdown fields, their wide bandgaps, large conduction band offset, and/or high saturated electron drift velocity. A major portion of the electrons in the 2DEG is attributed to polarization in the AlGaN. HEMTs in the GaN/ AlGaN system have already been demonstrated. U.S. Patents 5,192,987 and 5,296,395 describe AlGaN/GaN HEMT structures and methods of manufacture. U.S. Patent No. 6,316,793, to Sheppard et al., describes a HEMT device having a semiinsulating silicon carbide substrate, an aluminum nitride buffer layer on the substrate, an insulating gallium nitride layer on the buffer layer, an aluminum gallium nitride barrier layer on the gallium nitride layer, and a passivation layer on the aluminum gallium nitride active structure.

One step in the fabrication of some nitride-based transistors is the formation of a gate contact in a recess. A thick cap structure of the transistor may be desirable in achieving high current capability and low dispersion. However, a gate recess through the cap layer may be desirable to achieve high breakdown voltage, low RF dispersion and/or high transconductance with concomitant high-frequency performance. On the other hand, an etching process used to form a recessed gate structure may damage the underlying barrier layer(s) from which electrons originate. United States Patent Application Serial No. 10/758,871, filed January 16, 2004 and entitled NITRIDEBASED TRANSISTORS WITH A CAP LAYER AND A LOW-DAMAGE RECESS AND METHODS OF FABRICATION THEREOF, describes devices and techniques to provide a low-damage recess through SiN passivation.

Other attempts to mitigate high leakage on etched GaN and AlGaN surfaces have included using a high temperature anneal after a Schottky metal is deposited.

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Such attempts have used anneal temperatures of 400-600 °C. Post-Schottky hightemperature annealing appears to improve a high leakage Schottky/AlGaN contact to a moderately leaky Schottky/AlGaN contact.

SUMMARY OF THE INVENTION

Some embodiments of the present invention provide methods of fabricating high electron mobility transistors (HEMTs), including forming a channel layer, forming a barrier layer on the channel layer, forming a cap layer on the barrier layer and forming a gate recess in the cap layer that extends to the barrier layer. The barrier layer, channel layer and cap layer with the gate recess are annealed and then a gate contact is formed in the annealed gate recess. The gate contact may be a Schottky contact.

In further embodiments of the present invention, annealing the barrier layer, channel layer and cap layer with the gate recess includes annealing the barrier layer, channel layer and cap layer with the gate recess at a temperature of at least about 700 °C. The annealing may be preceded by forming an encapsulation layer on the cap layer and in the gate recess. The annealing may be followed by removing the encapsulation layer and forming a Schottky gate contact may be preceded by removing the encapsulation layer. The encapsulation layer may include AIN.

In additional embodiments of the present invention, the methods further include forming ohmic contact recesses in the cap layer on opposite sides of the gate recess and forming ohmic contact material patterns in the ohmic contact recesses. In some embodiments, annealing the barrier layer, channel layer and cap layer with the gate recess includes annealing the barrier layer, channel layer, ohmic contact material patterns and cap layer with the gate recess. Furthermore, forming ohmic contact material patterns may be followed by, and annealing may be preceded by, forming an encapsulation layer on the cap layer, the ohmic contact material patterns and in the gate recess. The annealing may be followed by, and forming a gate contact may be preceded by removing the encapsulation layer. The encapsulation layer may include AIN.

In still further embodiments of the present invention, forming a gate recess includes patterning a mask layer on the cap layer to have an opening corresponding to the gate recess and etching the cap layer using the patterned mask layer as an etchmask to provide the gate recess. The annealing may be preceded by forming an

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WO 2006/022874 PCT/US2005/010874 encapsulation layer on the patterned mask layer and the gate recess. The encapsulation layer may include AIN. The annealing may be followed by removing the mask layer and the encapsulation layer utilizing a self-aligned lift-off technique so as to provide a portion of the encapsulation layer that remains in the gate recess. The portion of the encapsulation layer in the gate recess may be removed prior to forming the gate contact.

In some embodiments of the present invention, an insulating layer is formed on the cap layer and in the gate recess. The gate contact may be formed on the insulating layer in the gate recess. The gate contact may also extend onto the insulating layer on the cap layer.

In some embodiments of the present invention, forming a cap layer includes forming a GaN layer on the barrier layer. For example, the GaN layer may be an undoped GaN layer, an AlGaN layer graded to GaN, an AlGaN layer graded to GaN and a doped GaN layer and/or a doped GaN layer. Forming a cap layer may also include forming a GaN layer on the barrier layer and forming a SiN layer on the GaN layer. Furthermore, forming a gate recess may include forming a gate recess that extends through the cap layer and into but not through the barrier layer. In particular embodiments of the present invention, the cap layer includes a GaN based semiconductor material.

In additional embodiments of the present invention, methods of fabricating high electron mobility transistors include forming a first layer of GaN based semiconductor material on a. substrate, forming a second layer of AlGaN based semiconductor material on the first layer, the second layer being configured to induce a two-dimensional electron gas in a region proximate an interface between the first layer and the second layer and forming a third layer of GaN based semiconductor material on the second layer of GaN based semiconductor material. The methods further include forming a gate recess in the third layer that extends to the second layer, annealing the first layer, second layer and third layer with the gate recess and then forming a gate contact in the annealed gate recess. The gate contact may be a Schottky gate contact.

In further embodiments of the present invention, the annealing is preceded by forming an encapsulation layer on the third layer of GaN based semiconductor material and in the gate recess. The anneal may be followed by removing the

CA 02572244 2013-04-02 encapsulation layer. Forming a gate contact may be preceded by removing the encapsulation layer.

In still further embodiments of the present invention, the methods further include forming ohmic contact recesses in the third layer of GaN based semiconductor material on opposite sides of the gate recess and forming ohmic contact material patterns in the ohmic contact recesses. In such a case, annealing the first layer, second layer and third layer with the gate recess includes annealing the first layer, second layer, ohmic contact material patterns and third layer with the gate recess.

In additional embodiments of the present invention, forming a gate recess includes patterning a mask layer on the third layer of GaN based semiconductor material to have an opening corresponding to the gate recess and etching the third layer of GaN based semiconductor material using the patterned mask layer as an etch mask to provide the gate recess. In such a case, the annealing is preceded by forming an encapsulation layer on the patterned mask layer and the gate recess and is followed by removing the mask layer and the encapsulation layer utilizing a self-aligned lift-off technique so as to provide a portion of the encapsulation layer that remains in the gate recess. The encapsulation layer may include AIN, S1O2, and oxynitride, an ONO structure and/or SiN.

In some embodiments of the present invention, the portion of the encapsulation layer that remains in the gate recess is removed prior to forming the gate contact. Furthermore, an insulating layer may be formed on the third layer of GaN-based material and in the recess and the gate contact formed on the insulating layer in the gate recess. The gate contact may also be formed on the insulating layer on the third layer of GaN-based material.

In an embodiment, a method of fabricating a high electron mobility transistor (HEMT), includes: forming a barrier layer; forming a cap layer on the barrier layer; forming ohmic contact recesses in the cap layer; forming ohmic contact material patterns in the ohmic contact recesses; forming a gate recess in the cap layer that extends to the barrier layer; forming an encapsulation layer on the cap layer, the ohmic contact material patterns and in the gate recess; annealing the barrier layer, the ohmic contact material patterns and cap layer with the gate recess, the ohmic contact material patterns being annealed for a first time; removing the encapsulation layer; and then forming a gate contact in the annealed gate recess.

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In another embodiment, a method of fabricating a high electron mobility transistor (HEMT) includes: forming a barrier layer; forming a cap layer on the barrier layer; forming ohmic contact recesses in the cap layer; forming ohmic contact material patterns in the ohmic contact recesses; forming a gate recess in the cap layer that extends to the barrier layer; forming an encapsulation layer on the cap layer, the ohmic contact material patterns and in the gate recess; annealing the barrier layer, the ohmic contact material patterns and cap layer with the gate recess, the ohmic contact material patterns being annealed for a first time; and then forming a gate contact in the annealed gate recess; wherein forming a gate recess comprises: patterning a mask layer on the cap layer to have an opening corresponding to the gate recess; and etching the cap layer using the patterned mask layer as an etch mask to provide the gate recess; and wherein forming the encapsulation layer comprises forming the encapsulation layer on the patterned mask layer and the gate recess and wherein annealing is followed by removing the mask layer and the encapsulation layer utilizing a self-aligned lift-off technique so as to provide a portion of the encapsulation layer that remains in the gate recess.

In another embodiment, a method of fabricating a high electron mobility transistor includes: forming a first layer of GaN based semiconductor material on a substrate; forming a second layer of GaN based semiconductor material on the first layer, the second layer being configured to induce a two-dimensional electron gas in a region proximate an interface between the first layer and the second layer; forming a third layer of GaN based semiconductor material on the second layer of GaN based semiconductor material; forming ohmic contact recesses in the third layer of GaN based semiconductor material; forming ohmic contact material patterns in the ohmic contact recesses; forming a gate recess in the third layer that extends to the second layer; annealing the first layer, second layer ohmic contact material patterns and third layer with the gate recess, the ohmic contact material patterns being annealed for a first time; and then forming a gate contact in the annealed gate recess.

In another embodiment, a high electron mobility transistor (HEMT), includes: a barrier layer; a cap layer on the barrier layer; a gate recess in the cap layer that extends to the barrier layer; and a gate contact in the gate recess, wherein a sheet resistance of the barrier layer proximate the gate recess is the same after an anneal of the barrier layer, the cap layer and the gate recess as an as grown sheet resistance of the barrier layer.

5a

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BRIEF DESCRIPTION OF THE DRAWINGS

Figures 1A-1F are schematic drawings illustrating fabrication of transistors according to some embodiments of the present invention.

Figures 2A and 2B are schematic drawings illustrating fabrication of transistors according to further embodiments of the present invention. Figure 3 is a schematic drawing illustrating fabrication of transistors according to further embodiments of the present invention.

5b

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DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. However, this invention should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the thickness of layers and regions are exaggerated for clarity. Like numbers refer to like elements throughout. As used herein the term and/or includes any and all combinations of one or more of the associated listed items.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms a, an and the are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms comprises and/or comprising, when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

It will be understood that when an element such as a layer, region or substrate is referred to as being on or extending onto another element, it can be directly on or extend directly onto the other element or intervening elements may also be present. In contrast, when an element is referred to as being directly on or extending directly onto another element, there are no intervening elements present. It will also be understood that when an element is referred to as being connected or coupled to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being directly connected or directly coupled to another element, there are no intervening elements present. Like numbers refer to like elements throughout the specification.

It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another region, layer or section. Thus, a first element,

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WO 2006/022874 PCT/US2005/010874 component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the present invention.

Furthermore, relative terms, such as lower or bottom and upper or top, may be used herein to describe one element's relationship to another elements as illustrated in the Figures. It will be understood that relative terms are intended to encompass different orientations of the device in addition to the orientation depicted in the Figures. For example, if the device in the Figures is turned over, elements described as being on the lower side of other elements would then be oriented on upper sides of the other elements. The exemplary term lower, can therefore, encompasses both an orientation of lower and upper, depending of the particular orientation of the figure. Similarly, if the device in one of the figures is turned over, elements described as below or beneath other elements would then be oriented above the other elements. The exemplary terms below or beneath can, therefore, encompass both an orientation of above and below.

Embodiments of the present invention are described herein with reference to cross-section illustrations that are schematic illustrations of idealized embodiments of the present invention. As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, embodiments of the present invention should not be construed as limited to the particular shapes of regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. For example, an etched region illustrated as a rectangle will, typically, have tapered, rounded or curved features. Thus, the regions illustrated in the figures are schematic in nature and their shapes are not intended to illustrate the precise shape of a region of a device and are not intended to limit the scope of the present invention.

Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

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It will also be appreciated by those of skill in the art that references to a structure or feature that is disposed adjacent another feature may have portions that overlap or underlie the adjacent feature.

Some embodiments of the present invention utilize an anneal of a recess prior to formation of a Schottky contact to reduce gate leakage and/or provide a high quality Schottky contact in a semiconductor device, such as a transistor. In some embodiments of the present invention an anneal of a recess may also restore conductivity to the 2DEG channel under the etched recess area. The use of an encapsulation layer during the anneal may prevent further damage to the semiconductor in the gate recess of the transistor. The anneal may be provided, for example, by an anneal of ohmic contacts of the device. Thus, high quality gate and ohmic contacts may be provided with reduced degradation of the gate region that may result from providing a recessed gate structure as a result of etch damage in forming the recess.

Embodiments of the present invention may be particularly well suited for use in nitride-based HEMTs such as Group Ill-nitride based devices. As used herein, the term Group III nitride refers to those semiconducting compounds formed between nitrogen and the elements in Group III of the periodic table, usually aluminum (Al), gallium (Ga), and/or indium (hi). The term also refers to ternary and quaternary compounds such as AIGaN and AlInGaN. As is well understood by those in this art, the Group III elements can combine with nitrogen to form binary (e.g., GaN), ternary (e.g., AIGaN, AllnN), and quaternary (e.g., AlInGaN) compounds. These compounds all have empirical formulas in which one mole of nitrogen is combined with a total of one mole of the Group III elements. Accordingly, formulas such as Al_xGai_xN where 0 < x < 1 are often used to describe them.

Suitable structures for GaN-based HEMTs that may utilize embodiments of the present invention are described, for example, in commonly assigned U.S. Patent 6,316,793 and U.S. Patent Publication No. 2002/0066908A1 filed July 12, 2001 and published June 6, 2002, for ALUMINUM GALLIUM NITRJDE/GALLIUM NITRIDE HIGH ELECTRON MOBILITY TRANSISTORS HAVING A GATE CONTACT ON A GALLIUM NITRIDE BASED CAP SEGMENT AND METHODS OF FABRICATING SAME,

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U. S. Patent Publication No. 2002/0167023 Al to Smorchkova et ah, published November 14, 2002, entitled GROUP-ΠΙ NITRIDE BASED HIGH ELECTRON MOBILITY TRANSISTOR (HEMT) WITH BARRIER/SPACER LAYER, U.S. Patent Application Serial No. 10/617,843 filed July 11, 2003 for NITRIDE-BASED TRANSISTORS AND METHODS OF FABRICATION THEREOF USING NONETCHED CONTACT RECESSES, and U.S. Patent Publication No. 2003/0020092 filed July 23, 2002 and published January 30, 2003 for INSULATING GATE ALGAN/GAN HEMT.

Methods of fabrication according to some embodiments, of the present invention are schematically illustrated in Figures 1A-1F. As seen in Figure 1A, a substrate 10 is provided on which nitride based devices may be formed. In particular embodiments of the present invention, the substrate 10 maybe a semi-insulating silicon carbide (SiC) substrate that may be, for example, 4H polytype of silicon carbide. Other silicon carbide candidate polytypes include the 3C, 6H, and 15R polytypes. The term semi-insulating is used descriptively rather than in an absolute sense. In particular embodiments of the present invention, the silicon carbide bulk crystal has a resistivity equal to or higher than about lxlO⁵ Ω-cm at room temperature.

Optional buffer, nucleation and/or transition layers (not shown) may be provided on the substrate 10. For example, an AIN buffer layer may be provided to provide an appropriate crystal structure transition between the silicon carbide substrate and the remainder of the device. Additionally, strain balancing transition layer(s) may also be provided as described, for example, in commonly assigned U.S. Patent Publication No. 2003/0102482A1, filed July 19, 2002 and published June 5, 2003, and entitled STRAIN BALANCED NITRIDE HETROJUNCTION TRANSISTORS AND METHODS OF FABRICATING STRAIN BALANCED NITRIDE HETEROJUNCTION TRANSISTORS.

Appropriate SiC substrates are manufactured by, for example, Cree, Inc., of Durham, N.C., the assignee of the present invention, and methods for producing are

CA 02572244 2013-04-02 described, for example, in U. S. Patent Nos. Re. 34,861; 4,946,547; 5,200,022; and 6,218,680. Similarly, techniques for epitaxial growth of Group III nitrides have been described in, for example, U. S. Patent Nos. 5,210,051; 5,393,993; 5,523,589; and 5,592,501.

Although silicon carbide may be used as a substrate material, embodiments of the present invention may utilize any suitable substrate, such as sapphire, aluminum nitride, aluminum gallium .nitride, gallium nitride, silicon, GaAs, LGO, ZnO, LAO, InP and the like. In some embodiments, an appropriate buffer layer also may be formed.

Returning to Figure 1A, a channel layer 20 is provided on the substrate 10. The channel layer 20 may be deposited on the substrate 10 using buffer layers, transition layers, and/or nucleation layers as described above. The channel layer 20 may be under compressive strain. Furthermore, the channel layer and/or buffer nucleation and/or transition layers may be deposited by MOCVD or by other techniques known to those of skill in the art, such as MBE or HVPE .

In some embodiments of the present invention, the channel layer 20 is a Group Ill-nitride, such as Al_xGai__xN where 0 < x < 1, provided that the energy of the conduction band edge of the channel layer 20 is less than the energy of the conduction band edge of the barrier layer 22 at the interface between the channel and barrier layers. In certain embodiments of the present invention, x = 0, indicating that the channel layer 20 is GaN. The channel layer 20 may also be other Group Ill-nitrides such as InGaN, AlInGaN or the like. The channel layer 20 may be undoped (unintentionally doped) and may be grown to a thickness of greater than about 20 Â. The channel layer 20 may also be a multi-layer structure, such as a superlattice or combinations of GaN, AlGaN or the like.

A barrier layer 22 is provided on the channel layer 20. The channel layer 20 may have a bandgap that is less than the bandgap of the barrier layer 22 and the channel layer 20 may also have a larger electron affinity than the barrier layer 22. The barrier layer 22 may be deposited on the channel layer 20. In certain embodiments of the present invention, the barrier layer 22 is AIN, AllnN, AlGaN or AlInGaN with a thickness of between about 0.1 nm and about 40 nm. Examples of layers according to certain embodiments of the present invention are described in U.S.

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Patent Publication No. 2002/0167023 Al, to Smorchkova et al, entitled GROUP-III NITRIDE BASED HIGH ELECTRON MOBILITY TRANSISTOR (HEMT) WITH BARRIER/SPACER LAYER. In particular embodiments of the present invention, the barrier layer 22 is thick enough and has a high enough Al composition and doping to induce a significant carrier concentration at the interface between the channel layer 20 and the barrier layer 22 through polarization effects. Also, the barrier layer 22 should be thick enough to reduce or minimize scattering of electrons in the channel due to ionized impurities or imperfections deposited at the interface between the barrier layer 22 and the cap layer 24 (Fig. IB).

The barrier layer 22 may be a Group Ill-nitride and has a bandgap larger than that of the channel layer 20 and a smaller electron affinity than the channel layer 20. Accordingly, in certain embodiments of the present invention, the barrier layer 22 is AlGaN, AlInGaN and/or AIN or combinations of layers thereof. The barrier layer 22 may, for example, be from about 0.1 nm to about 40 nm thick, but is not so thick as to cause cracking or substantial defect formation therein. In certain embodiments of the present invention, the barrier layer 22 is undoped or doped with an n-type dopant to a concentration less than about 10 cm’ , hi some embodiments of the present invention, the barrier layer 22 is Al_xGai__xN where 0 < x < 1. In particular embodiments, the aluminum concentration is about 25%. However, in other embodiments of the present invention, the barrier layer 22 comprises AlGaN with an aluminum concentration of between about 5% and about 100%. In specific embodiments of the present invention, the aluminum concentration is greater than about 10%.

Figure IB illustrates formation of a relatively thick cap layer 24 on the barrier layer 22. The cap layer 24 may be a Group Ill-nitride, and, in some embodiments, a GaN based semiconductor material, such as GaN, A1GAN and/or InGaN. In particular embodiments of the present invention, the cap layer is GaN. Furthermore, the cap layer 24 may be a single layer or multiple layers of uniform and/or non-uniform composition and/or thickness. In some embodiments of the present invention, the cap layer 24 may be a graded AlGaN layer and a GaN layer as described in Shen et al, High-Power Polarization-Engineered GaN/AlGaN/GaN HEMTs Without Surface Passivation, IEEE Electron Device Letters, Vol. 25, No. 1, pp. 7-9, January 2004.

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For example, in some embodiments of the present invention, the cap layer 24 may be a GaN layer with a SiN layer on the GaN layer. The cap layer 24 moves the top surface of the device physically away from the channel, which may reduce the effect of the surface.

The cap layer 24 may be blanket formed on the barrier layer 22 and may be epitaxially grown and/or formed by deposition. For example, the cap layer may be formed by in-situ growth of SiN on a GaN cap layer or ex-situ PECVD of SiN or SiO₂on top of the GaN cap layer. Typically, the cap layer 24 may have a thickness of from about 2 nm to about 500 nm. For example, a cap layer 24 of SiN and GaN may have a thickness of about 300 nm. Examples of cap layers according to some embodiments of the present invention are described in U.S. Patent Publication No. 2002/0167023 Al, to Smorchkova et al, entitled GROUP-ΠΙ NITRIDE BASED HIGH ELECTRON MOBILITY TRANSISTOR (HEMT) WITH BARRIER/SPACER LAYER.

As illustrated in Figure 1C, windows are opened in a mask layer to provide a first mask pattern 40 for formation of ohmic contacts 30. In some embodiments of the present invention, the first mask layer may be a conventional photolithography mask material. In some embodiments of the present invention, the mask material may be SiN, SiO₂ or the like. The windows may be formed utilizing a patterned mask and an etch process to expose the underlying barrier layer 22. In some embodiments of the present invention, the etch may be a low damage etch. In some embodiments of the present invention the etch is a wet etch with a strong base, such as KOH with UV illumination. In other embodiments, the etch is a dry etch. Examples of low damage etch techniques for Group Ill-nitrides include etching techniques other than reactive ion etching, such as inductively coupled plasma using Cl₂, BCI3, CC1₂F₂ and/or other chlorinated species or electron cyclotron resonance (ECR) and/or downstream plasma etching with no DC component to the plasma. Examples of low damage etch techniques for SiN include etching techniques other than reactive ion etching, such as inductively coupled plasma using CF4/O2, NF3/O? and/or other fluorinated species or electron cyclotron resonance (ECR) and/or downstream plasma etching with no DC component to the plasma.

As is further illustrated in Figure 1C, for example, with a subsequent photolithography step and evaporation, ohmic metal is patterned to provide ohmic contact material patterns that when annealed provide the ohmic contacts 30. In some

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WO 2006/022874 PCT/US2005/010874 embodiments of the present invention utilizing SiN and/or S1O2 in the cap layer 24, the ohmic contacts 30 are spaced apart from the SiN and/or S1O2 portions of the cap layer 24 a distance sufficiently large to allow for misalignment tolerances in the formation and patterning of the ohmic contact metal. If the ohmic contact metal contacts the SiN and/or S1O2 portions of cap layer 24, the metal may diffuse into the cap layer 24 during subsequent heating steps that may result in a short between a gate contact and the ohmic contact(s) 30.

Figure ID illustrates the formation of a gate recess. As seen in Figure ID, a second mask pattern 42 is formed on the ohmic contacts and the cap layer 24 and patterned to form a window that exposes a portion of the cap layer 24. In some embodiments of the present invention, the second mask pattern 42 may be a conventional photolithography mask material. In some embodiments of the present invention, the mask material may be silicon nitride, silicon dioxide or the like. A recess 36 is then formed through the cap layer 24 to expose a portion of the barrier layer 22. In some embodiments of the present invention, the recess 36 is formed to extend into the barrier layer 22. The recess 36 may extend into the barrier layer 22 to, for example, adjust performance characteristics of the device such as threshold voltage, frequency performance, etc. The recess may be formed using the mask 42 and an etch process as described above. In particular embodiments where the ohmic contacts 30 provide source and drain contacts, the recess may be offset between the source and drain contacts such that the recess, and subsequently the gate contact 32, is closer to the source contact than the drain contact.

Figure IE illustrates the formation of an optional encapsulation layer 44 after removal of the second mask pattern 42. The encapsulation layer 44 may be formed on the cap layer 24 and in the recess 36. The encapsulation layer 44 may be silicon nitride (Si_xN_y), aluminum nitride (AIN), an ONO structure and/or other suitable protective material, such as silicon dioxide (S1O2) and/or an oxynitride. Other materials may also be utilized for the encapsulation layer 44 as long as the material may be removed without significantly damaging the underlying barrier layer 22. For example, the encapsulation layer 44 could also include magnesium oxide, scandium oxide, aluminum oxide and/or aluminum oxynitride. Furthermore, the encapsulation layer 44 may be a single layer or multiple layers of uniform and/or non-uniform composition.

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In particular embodiments of the present invention, the encapsulation layer 44 may be SiN, AIN, an ONO structure and/or SiO₂. The SiN, AIN and/or SiO₂ may be formed by PVD and/or CVD and may be non-stoichiometric. Suitable encapsulation materials are described in further detail with reference to the protective layer in U.S. Patent Application Serial No. 10/758,871, filed January 16, 2004 and entitled NITRIDE-BASED TRANSISTORS WITH A CAP LAYER AND A LOWDAMAGE RECESS AND METHODS OF FABRICATION THEREOF”.

The encapsulation layer 44 may be blanket formed on the cap layer 24 and into the recess 36 so as to be positioned on the barrier layer 22 in the recess 36 and may be formed by deposition. For example, a silicon nitride layer may be formed by high quality sputtering and/or PECVD. Typically, the encapsulation layer 44 may have a thickness of about 30 ran, however, other thickness layers may also be utilized. For example, the encapsulation layer should be sufficiently thick so as to protect the underlying layers during a subsequent anneal of ohmic contacts. Layers as thin as two or three monolayers may be sufficient for such purposes. However, in general, the encapsulation layer 44 may have a thickness of from about 10 ran to about 500nm. Also, a high quality SiN protective layer may be grown in-situ with the MOCVD growth of the group III nitride layers.

After formation of the gate recess 36 and the encapsulation layer 44 if present, but prior to formation of a gate contact, the ohmic contact material is annealed to provide the ohmic contacts 30. The anneal may be a high temperature anneal. For example, the anneal may be at a temperature of from about 700 °C to about 900 °C and in some embodiments, greater than about 900 °C. For example, an anneal may be carried out for from about 30 seconds to about five minutes in an environmental temperature of from about 700 °C to about 900 °C as measured using conventional measuring techniques in rapid thermal anneal systems, such as using a thermocouple embedded in a susceptor holding the wafer and/or optically by pyrometry. Through the use of an ohmic contact anneal, the resistance of the ohmic contacts may be reduced from a high resistance to less than about 1 Ω-mm. Thus, as used herein, the term ohmic contact refers to a non-rectifying contact that has a contact resistance of less than about 1 Ω-mm. The anneal of the etched gate recess 36 may remove damage to the barrier layer 22 that may result from fabrication of the gate recess 36. Thus, for example, the sheet resistance of the region of the barrier layer 22 proximate the gate

CA 02572244 2013-04-02 recess 36 after the high temperature ohmic contact anneal may be substantially the same as the sheet resistance of the region as if the remaining un-etched barrier layer were grown in the reactor.

As seen in Figure IF, the encapsulation layer 44 is removed if present and a gate contact 32 is formed in the recess and contacts the exposed portion of the barrier layer 22. The gate contact may be a T gate as illustrated in Figure IF and may be fabricated using conventional fabrication techniques. Suitable gate materials may depend on the composition of the barrier layer, however, in certain embodiments, conventional materials capable of making a Schottky contact to a nitride based semiconductor material may be used, such as Ni, Pt, NiSi_x, Cu, Pd, Cr, W and/or WSiN. It is possible that a small gap between the cap layer 24 and the gate contact 32 may arise as a result of, for example, anisotropy of the gate recess etch, resulting in an exposed surface of the barrier layer 22 between the protective layer 24 and the gate contact 32. This gap may be formed intentionally.

An optional passivation layer 45 may also be provided on the structure of Figure IF. The passivation layer 45 may be blanket deposited on the structure of Figure IF. In particular embodiments, the passivation layer 45 may be deposited so as to substantially fill the gap between the cap layer 24 and the ohmic contacts 30, if present, and also the gap between the cap layer 24 and the gate contact 32, if such gap exists. In certain embodiments of the present invention, the passivation layer 45may be silicon nitride, aluminum nitride, silicon dioxide, an ONO structure and/or an oxynitride. Furthermore, the passivation layer 45 may be a single or multiple layers of uniform and/or non-uniform composition.

Figures 2A and 2B illustrate fabrication of transistors according to further embodiments of the present invention. Fabrication of the transistors of Figures 2A and 2B may be provided in the same manner as described above with reference to Figures 1 A-ID. However, as seen in Figure 2A, rather than removing the second mask pattern 42, an encapsulation layer 46 is provided on the mask 42 and in the recess 36. The encapsulation layer 46 may, for example, be formed of the same materials as described above with reference to the encapsulation layer 44. In particular embodiments of the present invention, the encapsulation layer 46 is AIN. In further embodiments of the present invention, the encapsulation layer 46 may be any inert ceramic material. Furthermore, the second mask 42 may be SiN.

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In embodiments of the present invention where the second mask 42 is a photoresist or other material that may be substantially damaged by a subsequent anneal, the second mask 42 and portions of the encapsulation layer 46 are removed, for example, using a self-aligned lift-off technique, to provide a portion 26 of the encapsulation layer 46 in the gate recess. The structure of Figure 2 A is then annealed as described above. The encapsulation layer 46 may, in some embodiments of the present invention, have a thickness of from about 1 nm to about 500 nm.

As is seen in Figure 2B, the second mask 42 and portions of the encapsulation layer 46 are removed, for example, using a self-aligned lift-off technique, to provide a portion 26 of the encapsulation layer 46 in the gate recess. The gate contact 32 is then formed on the portion 26 remaining in the gate recess. In some embodiments, the portion 26 is removed prior to formation of the gate contact 32. In the embodiments illustrated in Figure 2B, the gate contact maybe formed of materials as described above.

Figure 3 illustrates formation of transistors according to further embodiments of the present invention. As seen in Figure 3, the structure of Figure 2B may have the portion 26 removed and an insulating layer 130 formed on the resulting structure including in the gate recess. The gate contact 32 may then be formed on the insulating layer 130. The insulating layer 130 may be one or more layers and may include, for example, SiN, AIN, SiO₂, and/or an ONO structure. Thus, in some embodiments of the present invention, an insulating gate HEMT may be provided, for example, as described in U.S. Patent Publication No. 2003/0020092 entitled INSULATING GATE ALGAN/GAN HEMT, to Parikh et al.

While embodiments of the present invention have been described with reference to a particular sequence of operations, some modification in the sequence may be made or other operations may be included while still benefiting from the teachings of the present invention. For example, the formation of the cap layer 24 prior to ohmic metal deposition may be omitted and the ohmic metal may be deposited and patterned on the barrier layer 22 to provide ohmic contact material regions 30 on the barrier layer 22. A cap layer could then be formed after deposition and patterning of the ohmic contact material. The cap layer may then be etched to provide the gate recess and the ohmic contact anneal carried out prior to formation of the gate contact.

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Operations described above as being provided in a single step may be provided in multiple steps and, likewise, operations described as multiple steps could be combined into a single step. For example, while the anneal to remove damage from the gate recess etch has been described as provided by the ohmic contact anneal, a separate anneal could also be provided. If the anneal is provided as a separate anneal, the anneal maybe carried out, for example, at a temperature of from about 600 °C to about 900 °C for from about 20 minutes to about thirty seconds. Additionally, the etch of the ohmic contact recess and the gate recess may be carried out using a single mask rather than two separate masks as described above.

While embodiments of the present invention have been described herein with reference to particular HEMT structures, the present invention should not be construed as limited to such structures. For example, additional layers may be included in the HEMT device while still benefiting from the teachings of the present invention. Such additional layers may include GaN cap layers, as for example, described in Yu et al., Schottky barrier engineering in III-V nitrides via the piezoelectric effect, Applied Physics Letters, Vol. 73, No. 13, 1998, or in U.S. Patent Publication No. 2002/0066908A1 filed July 12, 2001 and published June 6, 2002, for ALUMINUM GALLIUM NITRIDE/GALLIUM NITRIDE HIGH ELECTRON MOBILITY TRANSISTORS HAVING A GATE CONTACT ON A GALLIUM NITRIDE BASED CAP SEGMENT AND METHODS OF FABRICATING SAME. In some embodiments, insulating layers such as SiN, an ONO structure or relatively high quality AIN may be deposited for making a MISHEMT and/or passivating the surface. The additional layers may also include a compositionally graded transition layer or layers.

Furthermore, the barrier layer 22 may also be provided with multiple layers as described in U.S. Patent Publication No. 2002/0167023 Al, to Smorchkova et at, entitled GROUP-ΠΙ NITRIDE BASED HIGH ELECTRON MOBILITY TRANSISTOR (HEMT) WITH BARRIER/SPACER LAYER. Thus, embodiments of the present invention should not be construed as limiting the barrier layer to a single layer but may include, for example, barrier layers having combinations of GaN, AlGaN and/or AIN layers. For example, a GaN, AIN structure may be utilized to reduce or prevent alloy scattering. Thus, embodiments of the

CA 02572244 2013-04-02 present invention may include nitride based barrier layers, such nitride based barrier layers may include AlGaN based barrier layers, AIN based barrier layers and combinations thereof.

In the drawings and specification, there have been disclosed typical embodiments of the invention, and, although specific terms have been employed, they have been used in a generic and descriptive sense only and not for purposes of limitation.