CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is related to U.S. Patent Application Serial No. 13/456,039, filed 4/25/2012. This application also claims priority to and the benefit of U.S. Patent Application Serial No. 14/041,667 filed 9/30/2013 and U.S. Patent

Application Serial No. 14/290,029 filed 5/29/201 , which are hereby incorporated by reference in their entirety.

STATEMENT REGARDING FEDERAL FUNDING

[0002] This invention was made under U.S. Government contract DE-AR-0000117. The U.S. Government has certain rights in this invention .

TECHNICAL FIELD

[0003] This disclosure relates to III-Nitride field effect transistors (FETs) and in particular to insulated gates for FETs. Otherwise stated, this writing pertains to III-Nitride insulating-gate transistors with passivation.

BACKGROUND

[0004] Ill-nitride transistors are promising for high-speed and high-power applications, such as power switches, which may be used for motor drivers and power supplies, among other applications .

[0005] Many of these applications require the transistor to operate in normally-off mode. Normally-off mode operation can be realized by a number of approaches, but typically at the penalty of higher on-resistance and lower output-current.

[0006] U.S. Patent Application No. 13/456,039, filed April 25, 2012 describes a normally-off III-Nitride field-effect transistor and a method for making a normally-off FET.

[0007 ] U.S. Patent Application No. 14/041,667, filed

9/30/2013 describes normally-off Ill-nitride transistors with high threshold-voltage and low on-resistance.

[ 0008 ] High-power applications with normally-off Ill-nitride transistors need an insulated gate to achieve low leakage current, and an effective passivation dielectric to achieve minimal trapping effects.

[ 0009 ] The best-suited gate insulator and the best-suited passivation dielectric are usually different materials, which may cause processing compatibility problems. For example, plasma-enhanced chemical vapor deposition (PECVD) SiN film is a known good passivation material, while metal organic chemical vapor deposition (MOCVD) A1N is a known good gate insulator material .

[ 0010 ] Unfortunately, the process of forming MOCVD A1N can degrade a PECVD SiN film that is already deposited on the semiconductor .

[ 0011 ] What is needed is a device structure and method of making the device that resolves this process incompatibility and that has a high breakdown voltage and low on resistance. The embodiments of the present writing consider these and other needs .

SUMMARY

[ 0012 ] In a first embodiment disclosed herein, a field-effect transistor (FET) comprises a plurality of semiconductor layers, a source electrode contacting at least one of the semiconductor layers, a drain electrode contacting at least one of the semiconductor layers, a first dielectric layer covering a portion of semiconductor top surface between the source

electrode and the drain electrode, a first trench extending through the first dielectric layer and having a bottom located on a top surface of the semiconductor layers or within one of the semiconductor layers, a second dielectric layer lining the first trench and covering a portion of the first dielectric layer, a third dielectric layer over the semiconductor layers, the first dielectric layer, and the second dielectric layer, a second trench extending through the third dielectric layer and having a bottom located in the first trench on the surface of or within the second dielectric layer, and extending over a portion of the second dielectric on the first dielectric, and a gate electrode filling the second trench.

[ 0013 ] In another embodiment disclosed herein, a method of fabricating a field-effect transistor (FET) comprises forming a plurality of semiconductor layers, forming a source electrode contacting at least one of the semiconductor layers, forming a drain electrode contacting at least one of the semiconductor layers, forming a first dielectric layer covering a portion of semiconductor top surface between the source electrode and the drain electrode, forming a first trench extending through the first dielectric layer and having a bottom located on a top surface of the semiconductor layers or within one of the semiconductor layers, forming a second dielectric layer lining the first trench and covering a portion of the first dielectric layer, forming a third dielectric layer over the semiconductor layers, the first dielectric layer, and the second dielectric layer, forming a second trench extending through the third dielectric layer and having a bottom located in the first trench on the surface of or within the second dielectric layer, and extending over a portion of the second dielectric on the first dielectric, and forming a gate electrode filling the second trench .

[ 0014 ] These and other features and advantages will become further apparent from the detailed description and accompanying figures that follow. In the figures and description, numerals indicate the various features, like numerals referring to like features throughout both the drawings and the description.

BRIEF DESCRIPTION OF THE DRAWINGS

[ 0015 ] FIG. 1 shows a diagram of Ill-nitride field effect transistor in accordance with the present disclosure;

[ 0016] FIG. 2 shows a typical off-state current voltage (IV) characteristic of a FET in accordance with the present

disclosure;

[ 0017 ] FIG. 3 shows a typical dynamic current voltage (IV) characteristic of a FET in accordance with the present

disclosure;

[0018] FIG. 4 shows a diagram of another field effect transistor in accordance with the present disclosure;

[ 0019 ] FIG. 5 shows a diagram of yet another field effect transistor in accordance with the present disclosure;

[ 0020 ] FIG. 6 shows a diagram of still another field effect transistor in accordance with the present disclosure; and

[ 0021 ] FIG. 7 shows a diagram of a gate insulator stack in accordance with the present disclosure.

DETAILED DESCRIPTION

[ 0022 ] In the following description, numerous specific details are set forth to clearly describe various specific embodiments disclosed herein. One skilled in the art, however, will understand that the presently claimed invention may be practiced without all of the specific details discussed below. In other instances, well known features have not been described so as not to obscure the invention.

[ 0023 ] FIG. 1 shows a diagram of Ill-nitride field effect transistor (FET) in accordance with the present disclosure. The FET has a buffer layer 14 formed on a substrate 12. A channel layer 16 is formed on the buffer layer 14 and a barrier layer 18, is formed on the channel layer 16.

[0024 ] The substrate 12 material may be silicon (Si), silicon carbide (SiC) , sapphire (A1 ₂ 0 ₃ ) , gallium nitride (GaN) , or aluminum nitride (A1N) .

[ 0025 ] The buffer layer 14 may be a stack of III-Nitride materials grown on the substrate 12 by chemical vapor deposition or molecular beam epitaxy.

[0026 ] The channel layer 16 may be a III-Nitride material, such as GaN, grown on the buffer layer 14 by chemical vapor deposition or molecular beam epitaxy. Typically the channel layer 16 is an undoped GaN layer with the thickness ranging from 5 nanometers to a few micrometers.

[ 0027 ] The barrier layer 18 may be 1-30 nanometers thick and may typically be only 5 nm thick. The barrier layer 18 may be AlGaN, with a 25% Al composition.

[0028 ] A source electrode 20 and a drain electrode 22 are in contact with the channel layer 16 and extend through the the barrier layer 18. The source electrode 20 and drain electrode 22 are on opposite ends of the channel layer 16.

[ 0029 ] A dielectric layer 30, which may be lnm to lOOnm thick and is typically lOnm thick, and which may be SiN, is deposited by metal organic chemical vapor deposition (MOCVD) on top of the AlGaN barrier layer 18. In a preferred embodiment the

dielectric layer 30 is deposited by MOCVD at a temperature higher than 600 degrees C, and typically at 900 degrees C.

[ 0030 ] The dielectric layer 30 is patterned to be on top of the AlGaN barrier layer 18 in a gate area for a distance of Ls2, Lsl, Lg, Ldl and Ld2, as shown in FIG. 1, between the source 20 and drain 22. In the embodiment of FIG. 1, the dielectric layer 30 is not in contact with either the source 20 or the drain 22.

[ 0031 ] A first gate trench 32 with a length of Lg, as shown in FIG. 1, is formed through the dielectric layer 30 and the barrier layer 18. The bottom 38 of the gate trench 32 is located within the channel layer 16, and extends below the barrier layer 18 and into the channel layer 16 by a vertical distance 36. This vertical distance 36 is between an interface of the barrier layer 18 and channel layer 16 and the bottom 38 of the gate trench 32, and is typically between 0 and 10 nanometers (nm) . The vertical distance 36 needs to be equal or greater than 0 nm for normally-off operation, and needs to be as small as possible to in order to minimize the on-resistance .

[ 0032 ] A gate insulator 33 is formed in the gate trench 32 and over the dielectric layer 30. As shown in FIG. 7, the gate insulator 33 may include a stack of: a layer of single- crystalline AIN 104 at the bottom of the gate trench 32, which may be up to 2nm thick and typically lnm thick; a layer of polycrystalline AIN 102 on the single crystalline AIN layer, which is lnm to 50nm thick and typically lOnm thick; and an insulating layer of SiN 100, which may be lnm to 50nm thick and typically lOnm thick, formed on the polycrystalline AIN layer.

[ 0033 ] The single crystalline AIN 104 is preferably grown at a temperature greater than 600C, and less than 1100C. A preferred temperature for growing the single crystalline AIN 104 is 900C. The poly crystalline AIN 102 is preferably grown at a temperature greater than 300C, and less than 900C, and a preferred temperature is 600C.

[ 0034 ] The gate insulator 33 stack makes the FET a normally off FET. Under a positive gate bias the FET has a very low gate leakage, and a high-mobility electron channel is formed at the interface between the barrier layer 18 and the channel layer 16.

[0035 ] The single-crystalline AIN layer 104 of the gate insulator stack 33 provides a high-quality interface for electron transport in the channel layer 16. Furthermore, the single crystalline AIN layer 104 provides an energy barrier to prevent electron trapping into the poly-crystalline AIN layer 102. The thickness of the single crystalline AIN layer 104 is chosen to be thin enough, typically below 2nm, to avoid

accumulation of channel electrons in absence of a positive gate bias .

[ 0036] The SiN layer 100 serves as a blocking layer to leakage paths through grain boundaries of the polycrystalline AIN layer 102.

[ 0037 ] The gate insulator 33 is formed in the trench 32 and over the dielectric layer 30. The gate insulator 33 and the dielectric layer 30 are removed in regions beyond the gate area of Ls2, Lsl, Lg, Ldl and Ld2, as shown in FIG. 1.

[0038 ] A passivation dielectric 34, which may be SiN and have a thickness of lOnm to 500nm with a typical thickness of lOOnm, is deposited by plasma-enhanced chemical vapor deposition

(PECVD) over the barrier layer 18 between the source 20 and the drain 22, over the gate insulator 33 in the trench 32, and over the gate insulator 33 on the dielectric layer 30. In a

preferred embodiment the passivation dielectric 34 is deposited by PECVD at a temperature lower than 500 degrees C, and

typically at 300 degrees C.

[ 0039 ] A second gate trench 40 is formed in passivation dielectric 34 by etching and may have a length of the sum of Lg, Lsl and Ldl, as shown in FIG. 1. The second gate trench 40 extends to the gate insulator 33 in the gate trench 32 and overlaps the gate insulator 33 on the dielectric layer 30 by a distance Lsl and Ldl, as shown in FIG. 1. The gate insulator 33 on the dielectric layer 30 for distance Ls2 and Ld2 on either side of Lsl and Ldl, as shown in FIG. 1, remains covered by the passivation layer 34.

[ 0040 ] A gate electrode 24 is formed within the second gate trench 40 and may extend over the passivation layer 34 partially toward the source electrode 20 by a distance Ls3, as shown in FIG. 1, and partially toward the drain electrode 22 by a distance Ld3, as shown in FIG. 1, to form an integrated gate field-plate. The gate electrode 24 may be any suitable metal.

[ 0041 ] As shown in FIG. 1, two types of dielectric are in contact with the AlGaN barrier layer 18 in the gate to drain region. The two types of dielectric are: a dielectric layer 30, which may be a SiN layer deposited by metal organic chemical vapor deposition (MOCVD) , and a passivation dielectric layer 34, which may be a SiN layer deposited by plasma-enhanced chemical vapor deposition (PECVD) .

[ 0042 ] Dielectric layer 30 is deposited prior to the

deposition of the gate insulator layer stack 33. The dielectric layer 30 serves as a etch stop layer for the patterning of the gate insulator layer stack 33, and dielectric layer 30 can survive subsequent high-temperature steps, such as the

deposition of gate insulator layer stack 33 and the alloying of source 20 and drain 22 contacts.

[ 0043 ] Dielectric layer 34 serves the purpose of mitigating trapping behaviors. Dielectric layer 34 is deposited after the deposition of gate insulator layer stack 33, to avoid the impact of high-temperature processing on the properties of dielectric layer 34.

[ 0044 ] FIG. 2 shows a typical off-state current voltage (IV) characteristic of a FET in accordance with the present

disclosure. As shown in FIG. 2, the off-state current is very low even at 600 volts, demonstrating the breakdown voltage is greater than 600 volts.

[ 0045 ] FIG. 3 shows a typical dynamic current voltage (IV) characteristic of a FET in accordance with the present disclosure, and the graph demonstrates that the on-resistance for a FET is only minimally degraded.

[ 0046] FIG. 4 shows a diagram of another field effect transistor in accordance with the present disclosure. The embodiment of FIG. 4 is similar to the embodiment of FIG. 1. However, in the embodiment of FIG. 4 the dielectric layer 30, which may be lnm to lOOnm thick and is typically lOnm thick, and which may be SiN, is deposited by metal organic chemical vapor deposition (MOCVD) on top of the AlGaN barrier layer 18 and extends from the source 20 to the drain 22, as shown in FIG. 4, rather than just in the gate area as shown in FIG. 1.

[ 0047 ] FIG. 5 shows a diagram of yet another field effect transistor in accordance with the present disclosure. The embodiment of FIG. 5 is similar to the embodiment of FIG. 4. However, in the embodiment of FIG. 5 the gate insulator stack 33 extends from the source 20 to the drain 22, as shown in FIG. 5, rather than just in the gate area as shown in FIG. 4.

[ 0048 ] FIG. 6 shows a diagram of still another field effect transistor in accordance with the present disclosure. The embodiment of FIG. 6 is similar to the embodiment of FIG. 1. However, in the embodiment of FIG. 6 the bottom 38 of the gate trench 32 is located within the barrier layer 18, and does not extend below the barrier layer 18 into the channel layer 16. The gate trench 32 may also be only to the top surface of the barrier layer 18. Variations of the embodiment of FIG. 6 may also include an embodiment where the dielectric layer 30 extends from the source 20 to the drain 22, and another embodiment where both the dielectric layer 30 and the gate insulator stand extend from the source 20 to the drain 22.

[ 0049 ] Having now described the invention in accordance with the requirements of the patent statutes, those skilled in this art will understand how to make changes and modifications to the present invention to meet their specific requirements or conditions. Such changes and modifications may be made without departing from the scope of the invention as disclosed herein.

[0050 ] The foregoing Detailed Description of exemplary and preferred embodiments is presented for purposes of illustration and disclosure in accordance with the requirements of the law. It is not intended to be exhaustive nor to limit the invention to the precise form(s) described, but only to enable others skilled in the art to understand how the invention may be suited for a particular use or implementation. The possibility of modifications and variations will be apparent to practitioners skilled in the art. No limitation is intended by the

description of exemplary embodiments which may have included tolerances, feature dimensions, specific operating conditions, engineering specifications, or the like, and which may vary between implementations or with changes to the state of the art, and no limitation should be implied therefrom. Applicant has made this disclosure with respect to the current state of the art, but also contemplates advancements and that adaptations in the future may take into consideration of those advancements, namely in accordance with the then current state of the art. It is intended that the scope of the invention be defined by the Claims as written and equivalents as applicable. Reference to a claim element in the singular is not intended to mean "one and only one" unless explicitly so stated. Moreover, no element, component, nor method or process step in this disclosure is intended to be dedicated to the public regardless of whether the element, component, or step is explicitly recited in the Claims. No claim element herein is to be construed under the provisions of 35 U.S.C. Sec. 112, sixth paragraph, unless the element is expressly recited using the phrase "means for. . ." and no method or process step herein is to be construed under those provisions unless the step, or steps, are expressly recited using the phrase "comprising the step(s) of. . .."

[ 0051 ] All elements, parts and steps described herein are preferably included. It is to be understood that any of these elements, parts and steps may be replaced by other elements, parts and steps or deleted altogether as will be obvious to those skilled in the art.

[ 0052 ] Broadly, this writing discloses at least the

following :

A field-effect transistor (FET) includes a plurality of semiconductor layers, a source electrode and a drain electrode contacting one of the semiconductor layers, a first dielectric layer on a portion of a top semiconductor surface between the source and drain electrodes, a first trench extending through the first dielectric layer and having a bottom located on a top surface or within one of the semiconductor layers, a second dielectric layer lining the first trench and covering a portion of the first dielectric layer, a third dielectric layer over the semiconductor layers, the first dielectric layer, and the second dielectric layer, a second trench extending through the third dielectric layer and having a bottom located in the first trench on the second dielectric layer and extending over a portion of the second dielectric, and a gate electrode filling the second trench .

[ 0053 ] CONCEPTS

This writing also presents at least the following concepts.

Concept 1. A field-effect transistor (FET) comprising:

a plurality of semiconductor layers;

a source electrode contacting at least one of the semiconductor layers;

a drain electrode contacting at least one of the

semiconductor layers;

a first dielectric layer covering a portion of

semiconductor top surface between the source electrode and the drain electrode;

a first trench extending through the first dielectric layer and having a bottom located on a top surface of the

semiconductor layers or within one of the semiconductor layers; a second dielectric layer lining the first trench and covering a portion of the first dielectric layer;

a third dielectric layer over the semiconductor layers, the first dielectric layer, and the second dielectric layer;

a second trench extending through the third dielectric layer and having a bottom located in the first trench on the surface of or within the second dielectric layer, and extending over a portion of the second dielectric on the first dielectric; and

a gate electrode filling the second trench.

Concept 2. The FET of concept 1 further comprising:

a substrate comprising silicon (Si), silicon carbide (SiC), sapphire (AI2O3) , gallium nitride (GaN) , or aluminum nitride (A1N) ;

wherein the plurality of semiconductor layers comprise: a Ill-nitride channel layer; and

a barrier layer over the channel layer.

Concept 3. The FET of concept 2 wherein:

the channel layer comprises GaN and has a thickness ranging from 5 nanometers to a few micrometers; and

the barrier layer comprises AlGaN having a thickness ranging from 1-30 nanometers and having a 25% Al composition.

Concept 4. The FET of concept 2 wherein:

a distance between an interface of the channel layer and the barrier layer and the bottom of the first trench is equal to or greater than 0 nanometers and less than or equal to 10 nanometers .

Concept 5. The FET of concept 1, 2, 3 or 4 wherein:

the first dielectric layer is deposited by metal organic chemical vapor deposition; and

the third dielectric layer is deposited by plasma-enhanced chemical vapor deposition.

Concept 6. The FET of concept 1, 2, 3 or 4 wherein:

the first dielectric layer comprises SiN deposited at a temperature greater than 600 degrees centigrade by metal organic chemical vapor deposition; and

the third dielectric layer comprises SiN deposited at a temperature lower than 500 degrees centigrade by plasma-enhanced chemical vapor deposition.

Concept 7. The FET of concept 1, 2 , 3 or 4 wherein:

the gate electrode extends over the third dielectric partially toward the source electrode and the drain electrode to form an integrated gate field-plate.

Concept 8. The FET of concept 1, 2, 3 or 4 wherein:

the first dielectric layer comprises SiN having a thickness of lnm to lOOnm; and

the third dielectric layer comprises SiN having a thickness of lOnm to 500nm. Concept 9. The FET of concept 1, 2, 3 or 4 wherein the second dielectric comprises:

a single-crystalline AIN layer at the bottom of the gate trench;

a polycrystalline AIN layer on the single crystalline AIN layer; and

an insulating layer comprising SiN on the polycrystalline AIN layer.

Concept 10. The FET of concept 9 wherein:

the single crystalline AIN is grown at a temperature greater than 600C and less than 1100C;

the poly crystalline AIN is grown at a temperature greater than 300C and less than 900C.

Concept 11. The FET of concept 9 wherein:

the single crystalline AIN layer is up to 2nm thick;

the polycrystalline AIN layer is lnm to 50nm thick; and the insulating layer is lnm to 50nm thick.

Concept 12. The FET of concept 1, 2, 3 or 4 wherein:

the first dielectric is only in a gate area and the second dielectric covers the first dielectric in the gate area; or the first dielectric extends from the source electrode to the drain electrode and the second dielectric covers the first dielectric only in the gate area; or

the first dielectric extends from the source electrode to the drain electrode and the second dielectric covers the first dielectric and extends from the source electrode to the drain electrode . Concept 13. A method of fabricating a field-effect transistor (FET) comprising:

forming a plurality of semiconductor layers;

forming a source electrode contacting at least one of the semiconductor layers;

forming a drain electrode contacting at least one of the semiconductor layers;

forming a first dielectric layer covering a portion of semiconductor top surface between the source electrode and the drain electrode;

forming a first trench extending through the first

dielectric layer and having a bottom located on a top surface of the semiconductor layers or within one of the semiconductor layers ;

forming a second dielectric layer lining the first trench and covering a portion of the first dielectric layer;

forming a third dielectric layer over the semiconductor layers, the first dielectric layer, and the second dielectric layer;

forming a second trench extending through the third dielectric layer and having a bottom located in the first trench on the surface of or within the second dielectric layer, and extending over a portion of the second dielectric on the first dielectric; and

forming a gate electrode filling the second trench.

Concept 14. The method of concept 13 further comprising:

providing a substrate comprising silicon (Si), silicon carbide (SiC) , sapphire (AI ₂ O ₃ ) , gallium nitride (GaN) , or aluminum nitride (A1N) ;

wherein the plurality of semiconductor layers comprise: a Ill-nitride channel layer; and a barrier layer over the channel layer.

Concept 15. The method of concept 14 wherein:

the channel layer comprises GaN and has a thickness ranging from 5 nanometers to a few micrometers; and

the barrier layer comprises AlGaN having a thickness ranging from 1-30 nanometers and having a 25% Al composition.

Concept 16. The method of concept 14 wherein:

a distance between an interface of the channel layer and the barrier layer and the bottom of the first trench is equal to or greater than 0 nanometers and less than or equal to 10 nanometers .

Concept 17. The method of concept 13, 14, 15 or 16 wherein: forming the first dielectric layer comprises depositing the first dielectric layer by metal organic chemical vapor

deposition; and

forming the third dielectric layer comprises depositing the third dielectric layer by plasma-enhanced chemical vapor deposition .

Concept 18. The method of concept 13, 14, 15 or 16 wherein: the first dielectric layer comprises SiN deposited at a temperature greater than 600 degrees centigrade by metal organic chemical vapor deposition; and

the third dielectric layer comprises SiN deposited at a temperature lower than 500 degrees centigrade by plasma-enhanced chemical vapor deposition.

Concept 19. The method of concept 13, 14, 15 or 16 further comprising : forming the gate electrode to extend over the third dielectric partially toward the source electrode and the drain electrode to form an integrated gate field-plate.

Concept 20. The method of concept 13, 14, 15 or 16 wherein: the first dielectric layer comprises SiN having a thickness of lnm to lOOnm; and

the third dielectric layer comprises SiN having a thickness of lOnm to 500nm.

Concept 21. The method of concept 13, 14, 15 or 16 wherein the forming the second dielectric comprises:

forming a single-crystalline AlN layer at the bottom of the gate trench;

forming a polycrystalline AlN layer on the single

crystalline AlN layer; and

forming an insulating layer comprising SiN on the

polycrystalline AlN layer.

Concept 22. The method of concept 21 wherein:

the single crystalline AlN is grown at a temperature greater than 600C and less than 1100C;

the poly crystalline AlN is grown at a temperature greater than 300C and less than 900C.

Concept 23. The method of concept 21 wherein:

the single crystalline AlN layer is up to 2nm thick;

the polycrystalline AlN layer is lnm to 50nm thick; and the insulating layer is lnm to 50nm thick.

Concept 24. The method of concept 13, 14, 15 or 16 wherein: the first dielectric is only formed in a gate area and the second dielectric covers the first dielectric in the gate area; or

the first dielectric is formed to extend from the source electrode to the drain electrode and the second dielectric covers the first dielectric only in the gate area; or

the first dielectric is formed to extend from the source electrode to the drain electrode and the second dielectric covers the first dielectric and extends from the source electrode to the drain electrode.

Concept 25. The method of concept 13, 14, 15 or 16 wherein: the first dielectric layer provides an etch stop for patterning of the second dielectric layer.

Concept 26. The method of concept 13, 14, 15 or 16 wherein: forming the first trench comprises etching; and

forming the third trench comprises etching and the second dielectric provides an etch stop.