Method and apparatus for plasma etching substrate

Abstract

To provide an industry scalable dry plasma etch solution for GaN, which allows a rounded corner to be achieved in situ while a trench or other feature is dry etched.SOLUTION: A method includes the steps of: (a) providing a substrate with a mask formed thereon; (b) performing a first plasma etch step to anisotropically etch at least one GaN or GaN alloy layer through an opening to produce a partially formed feature having one or more side walls and a bottom surface comprising a peripheral region; and (c) performing a second plasma etch step to anisotropically etch the bottom surface of the partially formed feature through the opening while depositing a passivation material onto the walls.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method of plasma etching a substrate, and in particular to etching a substrate comprising at least one GaN or GaN alloy layer to form features. The present invention also relates to an associated apparatus for plasma etching a substrate and to a substrate comprising at least one GaN or GaN alloy layer having features formed therein. [Background technology]

[0002] Gallium nitride (GaN) is increasingly being used as a wide-bandgap, high-charge-mobility, and relatively low-dielectric-constant material for power semiconductor devices, particularly as gate trenches in MOSFET devices. These properties can be exploited in a wide range of devices and applications, including transistors with high switching speeds and low losses, and power converters with high power densities. Other applications include technologies for fast charging of electric vehicles via compact onboard chargers and HV DC-DC converters with twice the power density of alternative materials, highly efficient LED applications, always-on energy-intensive data centers with improved efficiency, high-power-density converters for photovoltaics and storage, and consumer electronics with higher power densities, such as next-generation chargers and PC DC-DC converters. Dry-etched GaN trenches with typical widths of 1–5 μm and typical depths of 1–3 μm are well suited for such applications.

[0003] In typical GaN power devices, such as MOSFET devices, the gate trench serves to control the width of the depletion region and, therefore, the conductivity of the source-drain channel. The precise geometry of the gate etch is itself important for many reasons, including device operation (sufficient penetration into the drift region, sufficient sidewall profile angle for good dielectric deposition coverage, etc.), and minimizing charge leakage, which is crucial for this application. Such leakage often occurs at the corners of the etched trench as a result of charge accumulation that creates a large electric field gradient across the channel-dielectric-metal interface. Charge leakage occurs when the resulting potential difference is greater than the height of the metal-insulator-semiconductor Schottky barrier at the interface. This causes a reduction in breakdown voltage and limits the maximum rectified power of the device. While first-generation GaN devices with breakdown voltages of approximately 100 V are already established and commercially available, 600 / 650 V platforms are currently of interest to the industry. However, global demand for devices exceeding 1200 V is increasing. Such devices, which are often vertically structured, suffer from additional charge losses and minimizing charge leakage becomes increasingly important. Therefore, trench formation is predicted to play a critical role in achieving high breakdown voltage GaN transistors, and corner rounding is critical.

[0004] Currently, the industry is focusing on reducing charge leakage by reducing microtrench formation at trench corners, where factors such as ion deflection, etching mechanism contrast, species transport and consumption at the etch front, and ion energy and wafer-level current density cause increased etching rates at trench corners compared to the trench center, inducing triangular spots. These spots act as sources of significant charge accumulation and leakage. Although trenches with flat bottoms can be successfully fabricated, breakdown voltages are still limited by discontinuities at flat trench corners. Therefore, there is an increasing demand from industry and academia for dry etching solutions to create rounded corners in GaN trenches. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2022-105265 Summary of the Invention [Problem to be solved by the invention]

[0006] Corner rounding during dry etching of GaN with a photoresist (PR) mask is not straightforward because GaN tends to form a highly anisotropic etch profile with sharp corners over most of the process window. Currently, corner rounding is possible through wet etching processes, such as the use of tetramethylammonium hydroxide. Although rounded base control can be achieved at much lower process temperatures (<100°C) than comparable wet processes used to round Si and SiC trenches (>1000°C with annealing), the additional process steps have significant drawbacks. These drawbacks include long process times (>70 minutes), the use of environmentally unfriendly chemicals, the difficulty of incorporating wet etching steps into existing process flows, the inherent difficulty in controlling the etch profile due to preferential etching of crystal planes, and the inability to easily set an endpoint. All of these make the provision of a viable alternative solution highly attractive. [Means for solving the problem]

[0007] The present invention, at least in some embodiments, addresses the above-mentioned problems, desires, and needs. In particular, the present invention, at least in some embodiments, proposes an industrially scalable dry plasma etching solution for GaN that allows corner rounding to be performed in situ during dry etching of trenches or other features. As a result, an additional non-plasma etching step is not required and does not significantly disrupt the process flow. The present invention is also applicable to etching GaN alloys such as AlGaN and InGaN.

[0008] According to a first aspect of the present invention, there is provided a method of plasma etching a substrate comprising at least one GaN or GaN alloy layer to form features, the method comprising: a. providing a substrate having a mask formed thereon, the mask having an opening, the substrate including at least one GaN or GaN alloy layer; b. performing a first plasma etching step to anisotropically etch the at least one GaN or GaN alloy layer through the opening to create a partially formed feature having one or more sidewalls and a bottom surface including a peripheral region, the first plasma etching step using an etch recipe including a chlorine-based etchant; c. performing a second plasma etching step to anisotropically etch a bottom surface of the partially formed feature through the opening while depositing passivation material on the walls, the second plasma etching step using an etch recipe including a chlorine-based etchant and a fluorocarbon-based passivation material precursor, wherein the deposition of the passivation material into the feature causes attenuation of etching of the peripheral region, thereby creating a fully formed feature having a bottom surface including a central region and an edge region, the edge region being curved and the central region being deeper than the edge region of the bottom surface of the fully formed feature. Includes.

[0009] In this way, GaN etch features with curved bottom corners can be created without using wet etching techniques. Instead, the curved corners are created using a dry etching solution that can be easily integrated into existing processes. While not wishing to be bound by any particular theory or speculation, it is believed that the passivation material that deposits on the sidewalls reduces the effective width of the partially formed feature and reduces the dimensions of the opening that leads to the bottom of the partially formed feature. This acts to protect the peripheral region from the distribution of species in the plasma during the second plasma etching step. As a result, the amount of material removed from the peripheral region during the second plasma etching step is less than the amount of material removed from the central region. By controlling the reduction in the dimensions of the opening during the second plasma etching step, the etch profile of the bottom of the feature can be controlled and formed into the desired shape. The GaN etch profile typically tracks the mask angle with a low degree of process control. Therefore, achieving a reduction in profile angle sufficient to create a curved corner by modifying process conditions alone is significant, surprising, and not trivial. The method allows features to have curved edge regions that can be rounded or otherwise smoothed (i.e., have a gradual change in slope). This can improve the distribution of the electric field within the feature and help minimize field concentration. As such, the method can help create features with high breakdown voltages. Additionally, the method can help avoid the formation of microtrenches at the edge regions (e.g., corners) of the bottom surface of the feature.

[0010] The passivation material may be deposited at an increasing deposition rate during the second plasma etching step. The deposition rate of the passivation material may increase at an increasing rate during the second plasma etching step. The effective width of the partially formed feature may decrease at the increasing rate. Increasing the deposition rate of the passivation material during the second plasma etching step can help create features with rounded corners at the bottom edge region. It has been found that this approach can avoid discontinuities that appear in curved etch profiles, particularly in the region where the edge region meets the central region. As a result, features with a generally flat-bottomed U-shaped profile can be produced.

[0011] At least one process parameter may be varied during the second plasma etch step. The varied process parameter may be ramped at an increasing rate of change. The varied process parameter may be ramped at an exponentially increasing rate of change. Alternatively, the varied process parameter may be ramped at a constant rate of change. The term "ramping" can mean either a systematic increase or a systematic decrease in value. Several ways in which the process parameters may be varied during the second plasma etch step are now described.

[0012] Varying the process parameters may include increasing the flow rate of the passivation material precursor. The flow rate (sccm) of the passivation material precursor at the end of the second plasma etching step may be at least 5, 10, or 50 times higher than the flow rate (sccm) of the passivation material precursor at the beginning of the second plasma etching step. The flow rate of the passivation material may be increased from about 0.5-5 sccm to about 80-120 sccm.

[0013] The etching recipe used in the second plasma etching step has an associated gas pressure. The variation of the process parameters may include increasing the gas pressure. The gas pressure at the end of the second plasma etching step may be at least 5, 10, or 50 times higher than the gas pressure at the beginning of the second plasma etching step. The gas pressure may be increased from about 1-5 mTorr to about 150-250 mTorr.

[0014] The second plasma etching step may use a plasma generated by a plasma generating device having an associated operating power. Varying the process parameters may include increasing the operating power. The operating power at the end of the second plasma etching step may be higher than the operating power at the beginning of the second plasma etching step by an amount in the range of about 300-600 W. The operating power at the beginning of the second plasma etching step may be in the range of about 150-250 W or about 200 W. The operating power at or toward the end of the second plasma etching step may be in the range of about 500-800 W, about 550-650 W, or about 600 W. The operating power during the second plasma etching step may be the same as or higher than the operating power during the first plasma etching step. The operating power during the first plasma etching step may be in the range of about 150-250 W or about 200 W.

[0015] The etching recipe used in the second plasma etching step may include an inert gas with an associated flow rate. The variation of the process parameters may include decreasing the flow rate of the inert gas. The flow rate (sccm) of the inert gas at the end of the second plasma etching step may be at least 20% or 30% lower than the flow rate (sccm) of the passivation material precursor at the beginning of the second plasma etching step. The flow rate of the passivation material may be decreased from about 75-125 sccm to about 50-75 sccm.

[0016] The passivation material precursor may be one or more of C4F8, C5F8, C4F6, and CF4, with C4F8 being particularly effective.

[0017] The etching recipe used in the second plasma etching step may include a chlorine-based etchant, a fluorocarbon-based passivation material precursor, and an inert gas. The chlorine-based etchant may be Cl2 or BCl3. The inert gas may be argon.

[0018] The etching recipe used in the first plasma etching step may include a chlorine-based etchant and an inert gas. The chlorine-based etchant may be Cl2 or BCl3. The inert gas may be argon. In principle, different chlorine-based etchants may be used in the first and second plasma etching steps. However, it is advantageous to use the same chlorine-based etchant for both etching steps.

[0019] The feature may be a trench. The feature may be a via.

[0020] A central region of the base of the fully formed feature may be substantially flat, and the edge regions may form rounded corners between the central region of the base and the sidewalls of the fully formed feature.

[0021] Without wishing to be bound by any particular theory or speculation, it is believed that the passivation material is an organic (carbon-based) material, preferably a polymer.

[0022] The mask may be of any suitable type. The mask may be a photoresist mask. The mask may be a hard mask, such as a SiO2 hard mask.

[0023] Steps (b) and (c) may be carried out using an inductively coupled plasma (ICP) etching device.

[0024] The fully formed features may have a depth in the range of 1.0 to 3.0 microns.The fully formed features may have a depth in the range of 1.0 to 5.0 microns.

[0025] A first plasma etching step can be performed to create partially formed features having a depth that is 60-80% of the depth of the fully formed features.

[0026] The method may further include selectively removing the passivation material from the substrate after performing the second plasma etching step. Any suitable removal technique may be used, and may be a dry or wet etching technique.

[0027] The GaN or GaN alloy may be present as a single layer or as two or more layers, for example as a stack of layers. Different GaN or GaN alloy layers with varying doping degrees may be provided.

[0028] The substrate may include additional layers. For example, the substrate may include one or more dielectric layers. The dielectric layer may be located directly below the mask. The dielectric layer may be silicon dioxide. The substrate may also include a support layer below the GaN or GaN alloy layer. The support layer may be formed of a material such as AlN that supports epitaxial growth of the GaN or GaN alloy.

[0029] According to a second aspect of the present invention, there is provided a substrate comprising at least one GaN or GaN alloy layer having a feature formed using a method according to the first aspect of the present invention, the feature comprising a bottom surface, the bottom surface comprising a central region and an edge region, the edge region being curved and the central region being deeper than the edge region of the bottom surface of the fully formed feature.

[0030] The feature may include substantially perpendicular sidewalls relative to a central region. The feature may have a base (or base) that includes rounded corners. An edge region of the base may include a curved surface. The edge region may provide a continuous change in slope from the central region of the feature to the substantially perpendicular sidewalls.

[0031] The feature may be a trench. The feature may be a via.

[0032] According to a third aspect of the present invention there is provided a plasma etching apparatus for plasma etching a substrate comprising at least one GaN or GaN alloy layer to form features using a method according to the first aspect of the present invention, the apparatus comprising: a chamber; a substrate support disposed within the chamber for supporting the substrate; at least one gas inlet for introducing a gas or gas mixture into the chamber at a flow rate; a plasma generating device for maintaining a plasma within the chamber; a power supply for supplying a bias electrical signal having an associated bias output to the substrate support; a controller configured to switch from a first set of process conditions to a second set of process conditions, the first set of process conditions configured to perform a first plasma etch step to anisotropically etch at least one GaN or GaN alloy layer through the opening to create a partially formed feature having one or more sidewalls and a bottom surface including a peripheral region, the first plasma etch step using an etch recipe including a chlorine-based etchant; and a second set of process conditions configured to perform a second plasma etch step to anisotropically etch a bottom surface of the partially formed feature through the opening while depositing passivation material on the walls, the second plasma etch step using an etch recipe including a chlorine-based etchant and a fluorocarbon-based passivation material precursor, the deposition of passivation material into the feature causing a decay of etching of the peripheral region, thereby creating a fully formed feature having a bottom surface including a central region and an edge region, the edge region being curved and the central region being deeper than the edge region of the bottom of the fully formed feature. Includes.

[0033] The plasma generating device may be an ICP etching device.

[0034] For the avoidance of doubt, whenever reference is made herein to "comprising" or "including" and similar terms, the invention is understood to also include more restrictive terms such as "consisting of" and "consisting essentially of."

[0035] The invention has been described above and extends to any inventive combination of the features set out above or in the following description, the accompanying drawings or the claims. Any feature disclosed in relation to one of the first, second and third aspects of the invention may be combined with any feature disclosed in relation to any other aspect of the invention, as appropriate.

[0036] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0037] [Figure 1] 1A-1C are cross-sectional schematic diagrams of stages in a GaN trench etching process: (a) before the start of the etching process, (b) after the first plasma etching step, and (c) at the completion of the process including the second plasma etching step. [Figure 2] 1 is a cross-sectional schematic diagram of a plasma etching apparatus suitable for practicing the present invention. [Figure 3] 1A-1D are cross-sectional schematic views of a substrate at various stages of a second plasma etching step; [Figure 4] FIG. 10 illustrates the ramping of C4F8 flow rate as a function of time during the second plasma etch step for two process schemes. DETAILED DESCRIPTION OF THE INVENTION

[0038] The present invention provides a two-step plasma etching process for etching GaN or GaN alloys to create etched features with curved bottom corners. The first plasma etching step is an anisotropic etch using a chlorine-based etchant. This etch step is the "bulk" or "main" etch and etches the majority of the feature's depth. The second plasma etching step also uses a chlorine-based etchant but also includes a fluorocarbon-based passivation material precursor, which allows deposition of passivation material on the feature's sidewalls as the second plasma etching step progresses. While not wishing to be bound by theory, it is believed that the deposition of passivation material acts to reduce the profile angle, subsequently causing a decline in etching of the bottom edge region of the feature.

[0039] FIG. 1 illustrates a simplified process flow showing the stages in which a substrate 10 undergoes etching. FIG. 1(a) shows the substrate before the start of the etching process. The substrate 10 includes a GaN device layer 13 and, optionally, an additional thin dielectric layer 12, such as SiO2, located on the GaN layer 13. A mask 11 is formed on the top surface of the substrate 10. Instead of a single layer of GaN, several GaN layers may be present. The substrate 10 may further include one or more additional base layers 14, such as an AlN layer, located below the GaN epitaxial layer. The additional layer 14 assists the epitaxial growth process. A first main anisotropic plasma etch step of the GaN layer 13 is performed to etch a partially formed trench, as shown in FIG. 1(b). If layer 12 is present in the substrate 10, one or more initial etch steps to open layer 12 may precede the first plasma etch step. The base of the partially formed trench includes an edge region 15 and a central region 16. The central region 16 is preferably flat, as can be seen in Figure 1(b). At this stage of the process, the lower corners of the trench edge region 15, i.e., where the trench sidewalls meet the base of the trench, are relatively sharp. This type of feature is undesirable because it can result in current leakage under certain operating conditions. It is advantageous to create rounded or otherwise curved corners with a second plasma etching step, while simultaneously slightly extending the depth of the feature. Figure 1(c) shows the fully formed trench after the second plasma etching step and a plasma strip process to remove the passivation material. The rounded corners 17 of the edge region can be seen. When a flat-base etch is used as the gate etch in vertical or semi-vertical GaN MOSFET devices, off-state leakage is significantly improved. With the rounded corners, reduced on-state leakage can be achieved, resulting in an increased breakdown voltage for the device.

[0040] FIG. 2 shows a schematic diagram of a plasma etching apparatus 20 suitable for practicing the present invention. Plasma etching of a substrate is typically performed using a plasma etching apparatus. The plasma etching apparatus may be an inductively coupled plasma (ICP) apparatus, such as an Omega Synapse™ tool available from SPTS Technologies Limited, Newport, UK. However, etching may also be performed using other dry etching systems, such as helicon, RIE, or microwave-based apparatus. The generation of plasma within such plasma etching apparatus is well known in the art and will not be described herein except as necessary for an understanding of the present invention.

[0041] The plasma etching apparatus 20 shown in FIG. 2 is an ICP apparatus and typically includes a substrate support (or platen) 22 disposed within a chamber 23 for supporting a substrate 25. A bias power may be supplied to the substrate by an RF power supply 250 via an impedance match network 252. The chamber may include chamber walls having a dielectric portion 24. Process gases may be introduced into the chamber through one or more gas inlets 26. A plasma generating device 28, such as an inductive coil, may be used to generate and maintain a plasma within the chamber 23 (e.g., using an RF power supply 280 and an impedance match network 282), as known in the art. Gases may be removed from the chamber 23 via a pump port 29.

[0042] The second plasma etching step will now be described in more detail. Once the etch front of the first plasma etching step has progressed to the desired endpoint, the second plasma etching step is initiated. The desired endpoint can be at any suitable depth. A depth of approximately 70% of the final feature depth is often appropriate, but the exact endpoint depth can be varied to suit the precise morphology of the resulting corner. In the second plasma etching step, a passivation material precursor gas such as CF8, used in combination with a chlorine-based etchant, also produces excellent results. When CF8 is added at a suitable constant flow level, a profile change has been observed due to the deposition of passivation on the feature sidewalls. As the second plasma etching step progresses, the profile changes away from the preceding profile angle and toward a more tapered, positive angle. The GaN etch profile typically follows the mask angle with a low degree of process control. Therefore, achieving such a reduction in profile angle, which is typically about 80° to 60°, through process modifications alone is not significant and straightforward.

[0043] It has been found that ramping one or more process parameters improves feature corner rounding. Figure 3 is a schematic diagram of stages in an idealized process. Figure 3(a) shows an early stage of a second plasma etching step performed at a relatively low total pressure, C4F8 gas flow, and source power to the plasma generating device. Under these conditions, a relatively thin conformal layer 31 of passivation material is deposited on the sidewalls of the etched feature. The feature is preferentially etched at its exposed bottom surface, resulting in a small decrease in the trench width Δw1 and depth Δh1. Figures 3(b) and 3(c) show further stages of the second plasma etching step in which the total pressure, C4F8 flow, and source power are progressively increased. A faster passivation deposition rate is achieved while still maintaining a stable plasma. Without wishing to be bound by theory, it is believed that higher pressure results in stronger scattering and an increase in the angular distribution of ions reaching the substrate. This is believed to subsequently increase the effective etch rate in regions closer to the top of the feature, such as 32 and 33, while simultaneously further etching the exposed surface at the base of the trench. Reducing the Ar flow rate is believed to reduce the ionic nature of the etch, leading to a higher chemical equilibrium favorable for the deposition of passivation material. As a result, the width of the feature is progressively and continuously reduced from Δw1 to Δw2 to Δwn, and the depth increases from Δh1 to Δh2 to Δhn, where n is the number of individual steps performed to reach the target value. After the second plasma etching step is completed, a passivation strip step is performed using known techniques, which may be wet or dry. This results in a fully formed feature with rounded corners 34, as shown in FIG. 3(d). The rate of change of Δw and Δh can be controlled by judicious selection of process parameters to achieve the desired corner profile.

[0044] A linear increase in the C4F8 flow rate during the second plasma etching step is believed to promote a progressive decrease in the profile angle as a function of time, resulting in a curved edge region profile. The curved morphology need not necessarily be circular; the curved profile may have a radius that varies as a function of feature depth. By introducing nonlinear ramping, a more constant radius can be created. An exponential ramping profile has been found to produce excellent results. Figure 4 shows two exponential ramping process schemes 40 and 42 for ramping the C4F8 flow rate, where the initial phase of the second plasma step results in minimal changes in the C4F8 flow rate, while the end of this step delivers a large flow of C4F8, achieving partially rounded corners. Process scheme 40 is particularly effective. The exponential process scheme is also advantageous in preventing abrupt changes in the profile where discontinuities exist. Other nonlinear ramping process schemes, such as logarithmic schemes, may also be contemplated. It was found that a substantial increase in the CF flow rate was necessary to achieve a favorably curved edge region profile. An increase from a low flow rate of less than 5 sccm, typically 1 sccm, to at least 40 sccm was used, although other flow rate regimes may be applicable for other etching equipment. While FIG. 4 illustrates ramping the CF flow rate, other process parameters may be ramped in addition to or instead of the CF flow rate. Indeed, it was found that large increases in pressure, plasma density (by increasing source power), and CF flow rate can reduce or even eliminate discontinuities and introduce a constant radius of curvature to achieve perfectly rounded corners or otherwise produce a gradual profile change in the edge region. It was found that progressively increasing the deposition rate of the passivation material in an exponential manner as etching progresses to a point where the curvature of the bottom of the feature flattens out is very effective. An exemplary process and parameters that achieved this result are described below.

[0045] Example Experimental studies were performed on epitaxial GaN layers on 200 mm silicon wafers with a photoresist mask (3 μm) and CDs of 2–4 μm. The trench depth was nominally approximately 1.5–2 μm. An Omega Synapse™ ICP etching tool (SPTS Technologies Limited, Newport, UK) was used to perform the first and second plasma etching steps. The first plasma etching step, with the conditions listed in Table 1, was used to achieve a flat-bottom trench, roughly as shown in Figure 1(b).

[0046] [Table 1]

[0047] A second plasma etching step was performed, followed by a passivation material strip step, according to the conditions shown in Table 2. Using an exponential ramping scheme, the chamber pressure, source power, and C4F8 flow rate were increased, while the Ar flow rate was decreased. The resulting trench was observed to have an excellent profile, with a flat center region and rounded edge regions without discontinuities between the regions.

[0048] [Table 2]

[0049] Those skilled in the art will recognize that many variations on the process illustrated above are possible. For example, other chlorine-based etchants, such as BCl3, may be used. Other passivation material precursors, such as C5F8, C4F6, or CF4, may be used. The principles described above for varying process conditions during the second plasma etching step can be easily adapted by those skilled in the art to suit the exact process environment and desired profile of the feature corners. In addition to etching through a photoresist mask, other masks, such as hard masks, may be used. [Explanation of symbols]

[0050] 10 substrate, 11 mask, 12 additional thin dielectric layer, 13 GaN layer, 14 base layer, 15 edge region, 16 central region, 17 rounded corners, 20 plasma etching apparatus, 22 substrate support, 23 chamber, 24 dielectric section, 25 substrate, 26 gas inlet, 28 plasma generating device, 29 pump inlet, 31 conformal layer of passivation material, 32 region, 33 region, 34 rounded corners, 40 exponential ramping process scheme, 42 exponential ramping process scheme, 250 RF power source, 252 impedance matching network, 280 RF power source, 282 impedance matching network.

Claims

1. 1. A method of plasma etching a substrate including at least one GaN or GaN alloy layer to form features, comprising: a. providing a substrate having a mask formed thereon, the mask having an opening, the substrate including at least one GaN or GaN alloy layer; b. performing a first plasma etching step to anisotropically etch the at least one GaN or GaN alloy layer through the opening to create a partially formed feature having one or more sidewalls and a bottom surface including a peripheral region, the first plasma etching step using an etch recipe including a chlorine-based etchant; performing a second plasma etching step to anisotropically etch the bottom surface of the partially formed feature through the opening while depositing passivation material on the walls, the second plasma etching step using an etch recipe including a chlorine-based etchant and a fluorocarbon-based passivation material precursor, the deposition of the passivation material into the feature causing attenuation of etching of the peripheral region, thereby creating a fully formed feature having a bottom surface including a central region and an edge region, the edge region being curved and the central region being deeper than the edge region of the bottom surface of the fully formed feature; A method comprising:

2. 10. The method of claim 1, The method of claim 1, wherein the passivation material is deposited at an increasing deposition rate during the second plasma etching step.

3. 3. The method of claim 1 or 2, The method of claim 1, wherein at least one process parameter is varied during the second plasma etching step.

4. 4. The method of claim 3, The method of claim 1, wherein the process parameter being varied is ramped at an increasing rate.

5. 5. The method of claim 4, The method of claim 1, wherein the process parameter being varied is ramped at an exponentially increasing rate.

6. 6. The method according to any one of claims 3 to 5, The method, wherein the variation of the process parameter comprises increasing a flow rate of the passivation material precursor.

7. 7. The method according to any one of claims 3 to 6, the etch recipe used in the second plasma etch step has an associated gas pressure, and the variation of the process parameter comprises increasing the gas pressure.

8. 8. The method according to any one of claims 3 to 7, 10. The method of claim 9, wherein the second plasma etching step uses a plasma generated by a plasma generating device having an associated operating power, and wherein the variation of the process parameter comprises increasing the operating power.

9. 9. The method according to any one of claims 3 to 8, the etch recipe used in the second plasma etch step includes an inert gas having an associated flow rate, and the variation of the process parameter includes decreasing the flow rate of the inert gas.

10. 10. The method of any one of claims 1 to 9, The passivation material precursor is C ４ F ８ , C ５ F ８ , C ４ F ６ , and C.F. ４ The method is characterized by one or more of the following:

11. 11. The method of any one of claims 1 to 10, the etching recipe used in the second plasma etching step includes the chlorine-based etchant, the fluorocarbon-based passivation material precursor, and an inert gas.

12. 12. The method of claim 11, The chlorine-based etchant is Cl ２ wherein the inert gas is argon.

13. 13. The method of any one of claims 1 to 12, The method, wherein the etching recipe used in the first plasma etching step includes the chlorine-based etchant and an inert gas.

14. 14. The method of claim 13, The chlorine-based etchant is Cl ２ wherein the inert gas is argon.

15. 15. The method of any one of claims 1 to 14, The method, wherein the feature is a trench.

16. 16. The method of any one of claims 1 to 15, the central region of the bottom surface of the fully formed feature being substantially flat.

17. 17. The method of claim 16, the edge regions forming rounded corners between the central region of the bottom surface and sidewalls of the fully formed feature.

18. 18. The method of any one of claims 1 to 17, The method, wherein the mask is a photoresist mask or a hard mask.

19. 19. The method of any one of claims 1 to 18, The method, wherein steps (b) and (c) are performed using an inductively coupled plasma (ICP) etcher.

20. 10. A substrate comprising at least one GaN or GaN alloy layer having features formed using the method of claim 1, comprising:

1. A substrate comprising: a feature including a bottom surface, the bottom surface including a central region and an edge region, the edge region being curved, and the central region being deeper than the edge region of the bottom surface of the fully formed feature.

21. 10. A plasma etching apparatus for plasma etching a substrate including at least one GaN or GaN alloy layer to form features using the method of claim 1, said apparatus comprising: a chamber; a substrate support disposed within the chamber for supporting a substrate; at least one gas inlet for introducing a gas or mixture of gases into said chamber at a flow rate; a plasma generating device for maintaining a plasma within the chamber; a power supply for supplying a bias electrical signal having an associated bias output to the substrate support; a controller configured to switch from a first set of process conditions to a second set of process conditions, the first set of process conditions configured to perform a first plasma etching step to anisotropically etch the at least one GaN or GaN alloy layer through the opening to create a partially formed feature having one or more sidewalls and a bottom surface including a peripheral region, the first plasma etching step using an etch recipe including a chlorine-based etchant; and the second set of process conditions configured to perform a second plasma etching step to anisotropically etch the bottom surface of the partially formed feature through the opening while depositing passivation material on the walls, the second plasma etching step using an etch recipe including a chlorine-based etchant and a fluorocarbon-based passivation material precursor, the deposition of the passivation material into the feature causing a decay of etching of the peripheral region, thereby creating a fully formed feature having a bottom surface including a central region and an edge region, the edge region being curved and the central region being deeper than the edge region of the bottom surface of the fully formed feature. A plasma etching apparatus comprising:

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