[0001] This relates generally to semiconductor devices, and more particularly to III-N field effect transistors in semiconductor devices.

BACKGROUND

[0002] An enhancement mode gallium nitride field effect transistor (GaN FET) includes a recessed gate extending into a stressor layer and barrier layer, and vertically separated from a low-doped gallium nitride (GaN) layer. Forming the gate recess by etching to have a desired vertical separation from the low-doped GaN layer is problematic. Timed etching results in unacceptable variation in the separation from the low-doped GaN layer. Forming the gate recess using etch-blocking layers produces defects in the barrier layer and/or the stressor layer.

SUMMARY

[0003] In described examples, a semiconductor device containing an enhancement mode GaN FET on a III-N layer stack includes a low-doped GaN layer, a barrier layer including aluminum disposed over the low-doped GaN layer, a stressor layer including indium disposed over the barrier layer, and a cap layer including aluminum disposed over the stressor layer. A gate recess of the enhancement mode GaN FET extends through the cap layer and the stressor layer, but not through the barrier layer. A gate dielectric layer is disposed in the gate recess, and a gate is disposed on the gate dielectric layer.

[0004] The semiconductor device is formed by forming the barrier layer with a high temperature metal organic chemical vapor deposition (MOCVD) process, forming the stressor layer with a low temperature MOCVD process, and forming the cap layer with a low temperature

MOCVD process. The gate recess is formed by a two-step etch process including a first etch step to remove the cap layer, and a second etch step to remove the stressor layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005] FIG. 1 is a cross section of an example semiconductor device.

[0006] FIG. 2A through FIG. 21 are cross sections of the semiconductor device of FIG. 1 depicted in successive stages of an example fabrication sequence.

[0007] FIG. 3 A and FIG. 3B are cross sections of the semiconductor device of FIG. 1 depicted in an alternative process sequence for forming the gate recess.

DETAILED DESCRIPTION OF EXAMPLE EMB ODEVIENT S

[0008] The figures are not necessarily drawn to scale. In this disclosure, some acts or events may occur in different orders and/or concurrently with other acts or events, and some illustrated acts or events are optional.

[0009] For the purposes of this description, the term "III-N material" refers to semiconductor materials in which group III elements (aluminum, gallium and indium, and possibly boron) provide a portion of the atoms in the semiconductor material, and nitrogen atoms provide the remainder of the atoms in the semiconductor material. Examples of III-N semiconductor materials are gallium nitride, boron gallium nitride, aluminum gallium nitride, indium nitride, and indium aluminum gallium nitride. Terms such as aluminum gallium nitride describing elemental compositions of materials do not imply a particular stoichiometry of the elements. For the purposes of this description, the term GaN FET refers to a field effect transistor that includes III-N semiconductor materials.

[0010] FIG. 1 is a cross section of an example semiconductor device. The semiconductor device 100 includes an enhancement mode GaN FET 102 and a depletion mode GaN FET 104. The semiconductor device 100 includes a substrate 106, which may be a wafer of silicon or other semiconductor material. A buffer layer 108 of III-N material is disposed over the substrate 106. For example, the buffer layer 108 may include 100 to 300 nanometers of aluminum nitride on the substrate 106 and 1 to 7 microns of graded layers of Al _x Gai _-x N which is aluminum rich at a bottom surface, on the aluminum nitride, and gallium rich at a top surface of the buffer layer (108). An electrical isolation layer (110) is disposed on the buffer layer (108). For example, the electrical isolation layer (110) may be 300 to 2000 nanometers of semi-insulating gallium nitride. Also, for example, the electrical isolation layer (110) may be semi-insulating to provide a desired level of electrical isolation between layers below the electrical isolation layer (110) and layers above the electrical isolation layer (110). Alternatively, the electrical isolation layer (110) may be doped with n-type or p-type dopants to reduce undesired effects of charge trapping on current density in the semiconductor device (100). A low-doped layer (112) is disposed on the electrical isolation layer (110). For example, the low-doped layer (112) may be 25 to 1000 nanometers of gallium nitride. The low-doped layer (112) may be formed to minimize crystal defects, which may have an adverse effect on electron mobility. The method of formation of the low-doped layer (112) may result in the low-doped layer (112) being doped with carbon, iron or other dopant species, such as with a net doping density less than 10 ¹⁷ cm ^"3 .

[0011] A barrier layer 114 is disposed over the low-doped layer 112. The barrier layer 114 may be primarily aluminum gallium nitride, with less than 1 atomic percent indium. The barrier layer 114 may have a stoichiometry of Al _0. ioGao.9oN to Alo.30Gao.70N, and a thickness of 1 nanometers to 5 nanometers. A minimum thickness of the barrier layer 114 may be selected to provide ease and reproducibility of fabrication. A maximum thickness may be selected to provide a desired off-state current in the enhancement mode GaN FET 102, where increasing the thickness of the barrier layer 114 increases the off-state current. The thickness may depend on a stoichiometry of the barrier layer 114. For example, an instance of the barrier layer 114 with a stoichiometry of Al _0. ioGao.9oN to Alo.30Gao.70N may have a thickness of 1.5 nanometers to 2.0 nanometers.

[0012] A stressor layer 116 is disposed over the barrier layer 114. The stressor layer 116 is primarily indium aluminum nitride, with a stoichiometry of Ino.05Alo.95N to In ₀ .3oAl _0.7 oN, and a thickness of 1 nanometers to 5 nanometers. In one version of this example, the stressor layer 116 may have a stoichiometry of In ₀ .i6Al ₀ .84N to Ino.i8Alo.82N and a thickness of 3.5 nanometers to 4.5 nanometers, which may provide a desired balance between providing a desired charge density in a two-dimensional electron gas (2DEG), which decreases with indium content, and providing a desired etch selectivity to the underlying barrier layer 114, which increases with indium content. The stoichiometry of In ₀ .i6Al ₀ .84N to Ino.i8Alo.82N may also provide a desired lattice match to the low-doped layer 112.

[0013] A cap layer 118 is disposed over the stressor layer 116. The cap layer 118 has less than 1 atomic percent indium, and may be primarily aluminum gallium nitride. A thickness of the cap layer is selected to prevent oxidation of the stressor layer 116 during subsequent fabrication steps. An example cap layer 118 may have a stoichiometry of Alo.05Gao.95N to Alo.30Gao.70N, and a thickness of 4 nanometers to 20 nanometers. The cap layer 118 advantageously prevents oxidation of the indium in the stressor layer 116.

[0014] A gate recess 120 extends through the cap layer 118 and the stressor layer 116 in the enhancement mode GaN FET 102. The gate recess 120 may extend completely through the stressor layer 116 and not extend into the barrier layer 114, as depicted in FIG. 1. Alternatively, the gate recess 120 may extend partway into the barrier layer 114, or may extend only partway through the stressor layer 116 and stop short of the barrier layer 114.

[0015] An enhancement mode gate dielectric layer 122 is disposed in the gate recess 120 in the enhancement mode GaN FET 102. A depletion mode gate dielectric layer 124 is disposed over the cap layer in the depletion mode GaN FET 104. The enhancement mode gate dielectric layer 122 and the depletion mode gate dielectric layer 124 may be 5 nanometers to 50 nanometers thick and may include one or more layers of silicon dioxide, silicon nitride and/or aluminum oxide. In one version of this example, the enhancement mode gate dielectric layer 122 and the depletion mode gate dielectric layer 124 may have substantially equal thicknesses and compositions, possibly as a result of being formed concurrently. In an alternative version, the enhancement mode gate dielectric layer 122 and the depletion mode gate dielectric layer 124 may have different thicknesses and compositions to separately optimize performance of the enhancement mode GaN FET 102 and the depletion mode GaN FET 104.

[0016] A field plate dielectric layer 126 may optionally be disposed over the cap layer 118 and under the enhancement mode gate dielectric layer 122 adjacent to the gate recess 120 and under the depletion mode gate dielectric layer 124 adjacent to a gate area in the depletion mode GaN FET 104. For example, the field plate dielectric layer 126 may include one or more layers of silicon dioxide and/or silicon nitride, and may be 10 nanometers to 100 nanometers thick. In an alternative version of this example, the field plate dielectric layer 126 may be disposed over the enhancement mode gate dielectric layer 122 and the depletion mode gate dielectric layer 124.

[0017] An enhancement mode gate 128 is disposed over the enhancement mode gate dielectric layer 122 in the gate recess 120. The enhancement mode gate 128 may overlap the field plate dielectric layer 126 in the enhancement mode GaN FET 102, as depicted in FIG. 1. A depletion mode gate 130 is disposed over the depletion mode gate dielectric layer 124 in the gate area of the depletion mode GaN FET 104 and may overlap the field plate dielectric layer 126 in the depletion mode GaN FET 104, as depicted in FIG. 1. The enhancement mode gate 128 and the depletion mode gate 130 may have substantially equal compositions, possibly as a result of being formed concurrently. Accordingly, the gate dielectric layer and the planar gate of the depletion mode GaN FET 104 may be formed concurrently with the gate dielectric layer and the gate of the enhancement mode GaN FET 102.

[0018] Dielectric isolation structures 132 extend through the cap layer 118, the stressor layer 116 and the barrier layer 114 and possibly through the low-doped layer (1 12), in order to laterally isolate the enhancement mode GaN FET (102) and the depletion mode GaN FET (104). For example, the dielectric isolation structures 132 may include silicon dioxide and/or silicon nitride.

[0019] A source contact 134 and a drain contact 136 provide electrical connections to a 2DEG in the enhancement mode GaN FET 102. A source contact 138 and a drain contact 140 provide electrical connections to a 2DEG in the depletion mode GaN FET 104.

[0020] During operation of the semiconductor device 100, the barrier layer 114 advantageously provides a low carrier density in the 2DEG of the enhancement mode GaN FET 102 under the gate recess 120, in order to provide a desired off-state current. The stressor layer 116 advantageously provides a desired high carrier density in the 2DEG of the enhancement mode GaN FET 102 in the access regions between the gate recess 120 and the source contact 134 and the drain contact 136, in order to provide a desired on-state current. The configuration of the gate recess 120 extending through the stressor layer 116 advantageously contributes to the low carrier density in the 2DEG of the enhancement mode GaN FET 102 under the gate recess 120. The stressor layer 116 extending under the depletion mode gate 130 advantageously provides a desired on-state current in the depletion mode GaN FET 104.

[0021] FIG. 2A through FIG. 21 are cross sections of the semiconductor device of FIG. 1 depicted in successive stages of an example fabrication sequence. Referring to FIG. 2A, the buffer layer 108 is formed over the substrate 106. The electrical isolation layer (110) is formed over the buffer layer (108), and the low-doped layer (112) is formed over the electrical isolation layer (110). For example, the buffer layer 108, the electrical isolation layer (110) and the low-doped layer (112) may be formed by a series of MOCVD processes.

[0022] In this example, process parameters will be described for a case in which the substrate 106 is a 150 millimeter substrate. The substrate 106 is placed on a susceptor 142, possibly of graphite, in an MOCVD chamber 144. For example, the susceptor 142 is heated by heating coils, to a temperature of 900 °C to 1100 °C. A carrier gas (such as hydrogen (H ₂ ) as indicated in FIG. 2A) is flowed into the MOCVD chamber 144 at a flow rate of 80 standard liters per minute (slm) to 120 slm, and a nitrogen source (such as ammonia (NH3) as indicated in FIG. 2 A) is flowed into the MOCVD chamber 144 at a flow rate of 5 slm to 30 slm. An aluminum precursor (such as trimethylaluminum (TMA1) as indicated in FIG. 2A, or triethylaluminum) is flowed into the MOCVD chamber 144 at a rate of 80 standard cubic centimeters per minute (seem) to 130 seem, and a gallium precursor (such as trimethylgallium (TMGa) as indicated in FIG. 2A, or triethylgallium) is flowed into the MOCVD chamber 144 at a rate of 40 seem to 60 seem. A pressure in the MOCVD chamber 144 is maintained at 50 torr to 200 torn The nitrogen source, the aluminum precursor and the gallium precursor react at the existing surface of the semiconductor device 100 to form the barrier layer 114 over the low-doped layer 112 in the areas for the enhancement mode GaN FET 102 and the depletion mode GaN FET 104. Forming the barrier layer 114 at a temperature of 900 °C to 1100 °C advantageously provides fewer defects and therefore higher reliability for the semiconductor device 100, compared to a barrier layer formed at a lower temperature. In this example, substantially no indium precursor is flowed into the MOCVD chamber 144 while the barrier layer 114 is formed. In an alternative version of this example, the barrier layer 114 may include a quaternary III-N material, so it may include another element in addition to aluminum, gallium and nitrogen. The barrier layer 114 may be formed in situ after the low-doped layer (112) to advantageously reduce defects in the semiconductor device (100).

[0023] Referring to FIG. 2B, the substrate 106 remains on the susceptor 142 in the MOCVD chamber 144. The susceptor 142 is heated to a temperature of 700 °C to 850 °C. A carrier gas, indicated in FIG. 2B as nitrogen (N ₂ ), is flowed into the MOCVD chamber 144 at a flow rate of 60 slm to 100 slm. Also, a nitrogen source, indicated in FIG. 2B as ammonia (NH ₃ ), is flowed into the MOCVD chamber 144 at a flow rate of 5 slm to 40 slm. An aluminum precursor, indicated in FIG. 2B as trimethylaluminum (TMAl), is flowed into the MOCVD chamber 144 at a rate of 80 seem to 130 seem. An indium precursor (such as trimethylindium (TMIn) as indicated in FIG. 2B, or triethylindium) is flowed into the MOCVD chamber 144 at a rate of 100 seem to 300 seem. A pressure in the MOCVD chamber 144 is maintained at 100 torr to 400 torr. The nitrogen source, the aluminum precursor and the indium precursor react at the existing surface of the semiconductor device 100 to form the stressor layer 116 over the barrier layer 114 in the areas for the enhancement mode GaN FET 102 and the depletion mode GaN FET 104. Forming the stressor layer 116 at a minimum temperature of 700 °C may advantageously enable a desired concentration of indium and uniform distribution of indium in the stressor layer 116, compared to forming at a lower temperature. Forming the stressor layer 116 at a maximum temperature 850 °C may advantageously reduce indium diffusion into the barrier layer 114, compared to forming at a higher temperature. In this example, substantially no aluminum precursor is flowed into the MOCVD chamber 144 while the stressor layer 116 is formed. In an alternative version of this example, the stressor layer 116 may include a quaternary III-N material. Forming the stressor layer 116 in situ with the barrier layer 114 may advantageously reduce defects in the semiconductor device (100).

[0024] Referring to FIG. 2C, the substrate 106 remains on the susceptor 142 in the MOCVD chamber 144. The susceptor 142 is heated to a temperature of 750 °C to 900 °C. A carrier gas, indicated in FIG. 2C as hydrogen (H ₂ ), is flowed into the MOCVD chamber 144 at a flow rate of 80 slm to 120 slm. Also, a nitrogen source, indicated in FIG. 2C as ammonia (NH ₃ ), is flowed into the MOCVD chamber 144 at a flow rate of 5 slm to 35 slm. An aluminum precursor, indicated in FIG. 2C as trimethylaluminum (TMAl), is flowed into the MOCVD chamber 144 at a rate of 80 seem to 130 seem. A gallium precursor, indicated in FIG. 2C as trimethylgallium (TMGa), is flowed into the MOCVD chamber 144 at a rate of 40 seem to 60 seem. A pressure in the MOCVD chamber 144 is maintained at 50 torr to 200 torn The nitrogen source, the aluminum precursor and the gallium precursor react at the existing surface of the semiconductor device 100 to form the cap layer 118 over the stressor layer 116 in the areas for the enhancement mode GaN FET 102 and the depletion mode GaN FET 104. Forming the cap layer 118 at a maximum temperature of 900 °C may advantageously reduce indium diffusion into the barrier layer 114 and the cap layer 118, compared to forming at a higher temperature. In this example, substantially no indium precursor is flowed into the MOCVD chamber 144 while the cap layer 118 is formed. The cap layer 118 may be formed in situ after the stressor layer (116) to advantageously reduce defects in the semiconductor device (100).

[0025] Referring to FIG. 2D, the field plate dielectric layer 126 is formed over the cap layer 118. For example, the field plate dielectric layer 126 may be formed by forming a layer of dielectric material containing silicon dioxide and/or silicon nitride over the cap layer by a plasma enhanced chemical vapor deposition (PECVD) process. A field plate mask 146 is formed over the layer of dielectric material to expose gate areas for the enhancement mode GaN FET 102 and the depletion mode GaN FET 104. The layer of dielectric material is removed where exposed by the field plate mask 146 by an etch process, such as a plasma etch process at over 100 torr, forming the field plate dielectric layer 126 with sloped sides as depicted in FIG 2D.

[0026] Referring to FIG. 2E, a recess mask 148 is formed over the cap layer 118 to expose an area in the enhancement mode GaN FET 102 for the gate recess 120. The recess mask 148 may include photoresist and may be formed by a photolithographic process. The recess mask 148 may further include an antireflection layer, such as an organic bottom antireflection coating (BARC) and/or a hard mask layer such as silicon dioxide or silicon nitride. The recess mask 148 covers the area for the depletion mode GaN FET 104.

[0027] A first etch process 150 such as a plasma etch process using chlorine radicals removes the cap layer 118 in the area exposed by the recess mask 148 to form a portion of the gate recess 120. The indium in the stressor layer 116 has a lower etch rate in the first etch process 150 than the cap layer 118, so at least a portion of the stressor layer 116 remains in the area for the gate recess 120 after the first etch process 150 is completed. For example, the first etch process 150 may be an inductively-coupled plasma reactive ion etch (ICP-R E) process using chlorine (Cl ₂ ) gas sulfur hexafluoride (SF ₆ ) gas, which has been demonstrated to desirably provide an etch selectivity of gallium aluminum nitride to indium aluminum nitride greater than 1.0. Forming the cap layer 118 at a maximum temperature of 900 °C, in combination with the indium content in the stressor layer 116, may advantageously increase the etch selectivity for the first etch process 150 to reduce the amount (if any) of the stressor layer 116 removed by the first etch process 150.

[0028] Referring to FIG. 2F, a second etch process 152 removes the stressor layer 116 in the gate recess 120 to form the complete gate recess 120. The second etch process 152 has a different chemistry than the first etch process 150 of FIG. 2E. The barrier layer 114 has a lower etch rate in the second etch process 152 than the stressor layer 116, so at least a portion (and possibly all) of the barrier layer 114 remains under the gate recess 120 after the second etch process 152 is completed. For example, the second etch process 152 may include a wet etch process using a 1 molar aqueous solution of 1,2 diaminoethane, which has been demonstrated to desirably provide an etch selectivity of indium aluminum nitride to gallium aluminum nitride greater than 1.0 at room temperature. The first etch process 150 may provide a desirably rough surface on the exposed stressor layer 116, which may advantageously provide a more uniform initial etch rate for the second etch process 152.

[0029] Referring to FIG. 2G, a remaining portion 154 of the stressor layer 116 may be in the gate recess 120, possibly a transition layer 154 that includes elements of the underlying barrier layer 114. An oxidizing liquid 156 oxidizes the remaining portion 154 of the stressor layer 116 in the gate recess 120. The remaining portion 154 of the stressor layer 116 may be oxidized by an anodic oxidation process in which electrical current is passed through the oxidizing liquid 156. For example, the oxidizing liquid 156 may be an aqueous solution of nitriloacetic acid and 0.3 molar potassium hydroxide (KOH) with a pH value of 8.5. The electrical current may have a value of about 20 microamperes per square centimeter of exposed stressor layer 116. The oxidized remaining portion 154 may be subsequently removed, such as by a wet etch process using a dilute aqueous acidic solution, such as a dilute nitric acid solution or a citric acid solution. The recess mask 148 is removed, possibly after the wet etch process 152 of FIG. 2F is completed, or possibly earlier. Accordingly, the stressor layer may be oxidized by the anodic oxidation process in the gate recess to facilitate removal by the second etch step.

[0030] Referring to FIG. 2H, a layer of gate dielectric material 158 is formed over the field plate dielectric layer 126, extending into the gate recess 120 and overlying the barrier layer 114 at a bottom of the gate recess 120. In this example, the layer of gate dielectric material 158 extends over the cap layer 118 in the depletion mode GaN FET 104. The layer of gate dielectric material 158 may include one or more layers of silicon dioxide and/or silicon nitride, formed such as by PECVD processes. A layer of gate material 160 is formed over the layer of gate dielectric material 158. For example, the layer of gate material 160 may include gallium nitride or other III-N material, or may include polycrystalline silicon, referred to as polysilicon, or may include metal. In this example, the layer of gate material 160 is formed in the areas for the gates of both the enhancement mode GaN FET 102 and the depletion mode GaN FET 104.

[0031] Referring to FIG. 21, the layer of gate material 160 of FIG. 2H is patterned to concurrently form the enhancement mode gate 128 and the depletion mode gate 130. The enhancement mode gate 128 and the depletion mode gate 130 may be formed by an etch process including: forming an etch mask over the layer of gate material 160, which covers area for the enhancement mode gate 128 and the depletion mode gate 130, and subsequently removing the layer of gate material 160 where exposed by the etch mask. Alternatively, the enhancement mode gate 128 and the depletion mode gate 130 may be formed by a liftoff process including: forming a liftoff mask of solvent-soluble organic material (such as photoresist), which exposes the layer of gate dielectric material 158 in the areas for the enhancement mode gate 128 and the depletion mode gate 130, forming the layer of gate material 160 over the liftoff mask, and subsequently removing the liftoff mask and the overlying layer of gate material 160, leaving the layer of gate material 160 in the areas exposed by the liftoff mask to provide the enhancement mode gate 128 and the depletion mode gate 130. Forming the enhancement mode gate 128 and the depletion mode gate 130 concurrently may advantageously reduce fabrication cost and complexity of the semiconductor device 100. In an alternative version of this example, the enhancement mode gate 128 and the depletion mode gate 130 may be formed separately, of materials with different work functions, to increase performance of both the enhancement mode GaN FET 102 and the depletion mode GaN FET 104. After forming the enhancement mode GaN FET 102 and the depletion mode GaN FET 104, fabrication is continued to provide the structure of FIG. 1.

[0032] FIG. 3 A and FIG. 3B are cross sections of the semiconductor device of FIG. 1 depicted in an alternative process sequence for forming the gate recess. Referring to FIG. 3A, the recess mask 148 is formed over the cap layer 118. The cap layer 118 is removed in the area exposed by the recess mask 148 to form a portion of the gate recess 120, as described in reference to FIG. 2E. An oxidizing liquid 162, such as an anodizing aqueous solution containing an aqueous solution of nitriloacetic acid and 0.3 molar KOH with a pH value of 8.5 with an electrical current of about 20 microamperes per square centimeter of exposed stressor layer 116, oxidizes the stressor layer 116 where exposed by the cap layer 118 in the gate recess 120 to form an oxidized stressor layer 164 which includes indium oxide. In this example, the barrier layer 114 may include a layer of gallium nitride (GaN) 1 nanometer to 3 nanometers thick immediately below the stressor layer 116 to prevent oxidation of the aluminum gallium nitride in the barrier layer 114. At least a portion of the barrier layer 114 under the stressor layer 116 in the gate recess 120 is not oxidized.

[0033] Referring to FIG. 3B, a second etch process 166 removes the oxidized stressor layer 164 of FIG. 3 A to form the gate recess 120, while leaving at least a portion (and possibly all) of the barrier layer 114 under the gate recess 120. For example, the second etch process 166 may include a dilute aqueous solution of nitric acid, phosphoric acid, and/or hydrochloric acid, or an aqueous solution of an organic acid such as citric acid. The oxidation process described in reference to FIG. 3A and the second etch process of FIG. 3B may be repeated to completely remove the stressor layer 116 from the gate recess 120. The recess mask 148 is removed, and fabrication is continued as described in reference to FIG. 2G.

[0034] Modifications are possible in the described embodiments, and other embodiments are possible, within the scope of the claims.