Method, Substrate and Apparatus

Abstract

A substrate with a mask formed thereon is provided. The substrate is formed from a compound semiconductor material. A first plasma etch step is performed to anisotropically etch the substrate through the opening to produce a partially formed feature having a bottom surface comprising a peripheral region. A second plasma etch step is performed to anisotropically etch the bottom surface of the partially formed feature through the opening while depositing a passivation material onto the mask so as to reduce a dimension of the opening. The reduction of the dimension of the opening causes an attenuation in etching of the peripheral region thereby producing a fully formed feature having a bottom surface comprising a central region and an edge region. The central region is deeper than the edge region of the bottom surface of the fully formed feature.

Claims

1. A method of plasma etching a compound semiconductor substrate to form a feature, the method comprising the steps of: (a) providing a substrate with a mask formed thereon, the mask having an opening, wherein the substrate is formed from a compound semiconductor material; (b) performing a first plasma etch step to anisotropically etch the substrate through the opening to produce a partially formed feature having 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 mask so as to reduce a dimension of the opening, wherein the reduction of the dimension of the opening causes an attenuation in etching of the peripheral region thereby producing a fully formed feature having a bottom surface comprising a central region and an edge region, and wherein the central region is deeper than the edge region of the bottom surface of the fully formed feature.

2. The method according to claim 1, wherein the passivation material is deposited at a deposition rate that increases during the second plasma etch step.

3. The method according to claim 1, wherein the second etch step comprises varying a process parameter during the second plasma etch step.

4. The method according to claim 3, wherein the process parameter that is varied is ramped at an increasing rate of change.

5. The method according to claim 3, wherein varying the process parameter comprises varying a flow rate of a passivation material precursor during the second plasma etch step, and wherein the flow rate of the passivation material precursor is increased during the second plasma etch step.

6. The method according to claim 5, wherein the passivation material precursor comprises an oxygen-containing gas.

7. The method according to claim 3, wherein varying the process parameter comprises varying a bias power applied to the substrate during the second plasma etch step, and wherein the bias power applied to the substrate is decreased during the second plasma etch step.

8. The method according to claim 1, wherein the passivation material comprises a silicon oxide.

9. The method according to claim 1, wherein the passivation material and the mask are made of substantially the same material.

10. The method according to claim 1, wherein the second plasma etch step includes using an etch recipe comprising a chlorine-based etchant.

11. The method according to claim 10, wherein the chlorine-based etchant comprises Cl.sub.2 and/or SiCl.sub.4.

12. The method according to claim 1, wherein the bottom surface of the partially formed feature is substantially flat.

13. The method according to claim 1, wherein the feature is a trench.

14. The method according to claim 1, wherein the central region of the bottom surface of the fully formed feature is substantially flat.

15. The method according to claim 1, wherein the edge region of the bottom surface of the fully formed feature comprises a curved surface.

16. The method according to claim 15, wherein the edge region forms a rounded corner between the central region of the bottom surface and a sidewall of the fully formed feature.

17. The method according to claim 1, wherein the compound semiconductor substrate is a silicon carbide (SiC) wafer.

18. The method according to claim 1, further comprising: (d) selectively removing the passivation material from the substrate by wet etching.

19. The method according to claim 1, wherein steps (b) and (c) are performed using an inductively coupled plasma (ICP) etch apparatus.

20. A compound semiconductor substrate comprising a feature formed using the method according to claim 1, wherein the feature comprises a bottom surface, the bottom surface including a substantially flat central region and an edge region, wherein the central region is deeper than the edge region.

21. The compound semiconductor substrate according to claim 20, wherein the edge region of the bottom surface comprises a curved surface.

22. A plasma etch apparatus for plasma etching a substrate to form a feature using the method according to claim 1, the apparatus comprising: a chamber; a substrate support disposed within the chamber for supporting a substrate thereon; at least one gas inlet for introducing a gas or gas mixture into the chamber at a flow rate; a plasma generating means for sustaining a plasma in the chamber; a power supply for supplying a bias power to the substrate support; and a controller configured to switch from a first set of processing conditions to a second set of processing conditions, wherein the first set of processing conditions are configured to perform a first plasma etch step to anisotropically etch the substrate through an opening in a mask to produce a partially formed feature having a bottom surface comprising a peripheral region, and the second set of processing conditions are configured to perform a second plasma etch step to anisotropically etch the bottom surface of the partially formed feature through the opening whilst depositing a passivation material onto the mask so as to reduce a dimension of the opening, wherein the reduction of the dimension of the opening causes an attenuation in etching of the peripheral region thereby producing a fully formed feature having a bottom surface comprising a central region and an edge region, and wherein the central region is deeper than the edge region of the bottom surface of the fully formed feature.

Description

DESCRIPTION OF THE DRAWINGS

[0043] Embodiments of substrates and methods in accordance with the invention will now be described with reference to the accompanying drawings, in which:

[0044] FIG. 1 is a cross-sectional schematic view of a trench having a base with angled (square) corners (prior art);

[0045] FIG. 2 is a cross-sectional schematic view of a plasma etch apparatus suitable for performing a method according to an embodiment of the invention;

[0046] FIG. 3 is a schematic cross-sectional view of a substrate at subsequent stages of the present method;

[0047] FIG. 4 is a schematic cross-sectional view of an opening in a mask layer on a substrate;

[0048] FIG. 5 shows how the passivation thickness changes with a linear increase in flow rate of a passivation material precursor;

[0049] FIG. 6 shows a schematic cross-sectional view of a substrate comprising a feature having a base with tapered corners;

[0050] FIG. 7 shows how the passivation thickness changes with a non-linear increase in flow rate of a passivation material precursor;

[0051] FIG. 8 shows a schematic cross-sectional view of a substrate comprising a feature having a base with rounded corners; and

[0052] FIG. 9 shows an SEM image of a substrate comprising a feature having a base with rounded corners.

DETAILED DESCRIPTION OF THE DISCLOSURE

[0053] FIG. 2 shows a schematic representation of a plasma etch apparatus 20 suitable for performing methods according to embodiments of the present invention. A plasma etching tool suitable for performing the method of the present invention is the Omega® Synapse™ available from SPTS Technologies Limited of Newport, UK. Plasma etching of a substrate is typically performed using a plasma etch apparatus. The plasma etch apparatus can be an inductively coupled plasma (ICP) apparatus. However, etching can also be performed using other dry etch systems, such as helicon, RIE or microwave type apparatus. The operation of generating a plasma within such plasma etch apparatus is well-known in the art and will not be described here other than where necessary for an understanding of the present invention.

[0054] A plasma etch apparatus 20 typically comprises a substrate support (or platen) 22 disposed within a chamber 23 for supporting a substrate 25. A bias power can be supplied to the substrate by a RF power supply 250 via an impedance matching network 252. The chamber can comprise a chamber wall having a dielectric part 24. Process gases can be introduced into the chamber via one or more gas inlets 26. A plasma generating means 28, such as an inductive coil, can be used to generate and sustain a plasma within the chamber 23 as is known in the art (e.g. using a RF power supply 280 and impedance matching network 282). The gases can be removed from the chamber 23 via a pumping port 29.

[0055] FIG. 3 shows the stages of an exemplary method according to a first embodiment. In the first embodiment a compound semiconductor substrate 30 is etched to form a feature having rounded corners at the base of the feature. In the first embodiment, the feature is a trench, and the substrate 30 embodiment is a silicon carbide (SiC) wafer. However, other compound semiconductor substrates can alternatively be used. The substrate 30 includes a patterned mask layer 32, such as a silicon dioxide (SiO.sub.2) layer or other suitable resist layer. The mask layer 32 is more resistant to the plasma etch conditions that the bulk substrate material.

[0056] The substrate to be etched is positioned on the substrate support 22 in a plasma etch apparatus 20 with the face to be etched facing upwards. A pre-etch may (optionally) be performed to prepare the substrate 30 prior to the main etch cycle, for example to remove unwanted material from the open areas of the mask layer 32.

[0057] A first plasma etch step (i.e. a main etch) is performed to selectively etch the SiC substrate 30 so that a majority of the feature is formed. Numeral 34b represents the partially formed feature. The first plasma etch step corresponds to “Step 1” of FIG. 3. The first plasma etch step anisotropically etches the substrate through the opening. A bias power is applied to the platen 22 during the first plasma etch step. For example, the bias power applied to the platen can be in the range of about 100 W to 1400 W. This helps to impart a directionality to the species (e.g. ions) in the plasma so that the base of the feature is preferentially etched (rather than the sidewalls 38 of the feature). Consequently, the width of the partially formed feature substantially corresponds to the initial width of the open area in the mask layer 32.

[0058] During the first plasma etch step, the chamber pressure can be in the range of about 2 mTorr to about 20 mTorr. During the first plasma etch step, the plasma source power can be in the range of about 800 W to about 2000 W. Typically, the walls of the chamber 23 are cooled by water to about 55° C. By way of example only, the process gases used in the main etch step can comprise one or more of Cl.sub.2, SiCl.sub.4, O.sub.2, H.sub.2 and/or Ar gas. Fluorinated etchant gases, such as fluorocarbons, can also be used.

[0059] The first plasma etch step results in the formation of a partially formed feature 34b. The partially formed feature 34b comprises a flat base 36b that is approximately perpendicular to the sidewalls 38 of the trench 34b. The flat base 36b has a peripheral region (not labelled) proximate to the sidewall 38. If micro-trenching has occurred, the partially formed feature may comprise a substantially convex shape.

[0060] The substrate 30b is then subject to a second plasma etch step, shown as “Step 2” in FIG. 3, in which the feature is fully formed. The plasma processing parameters used during the second plasma etch step are different to those used during the first plasma etch step. The second plasma etch step is an anisotropic plasma etch step. That is, the species forming the plasma are generally directed towards the substrate with a high degree of directionality. In this example, the species forming the plasma generally bombard the substrate substantially perpendicularly to the substrate surface. Consequently, the bottom surface 36b of high aspect ratio features can be etched without significant etching of the sidewalls.

[0061] The second plasma etch step anisotropically etches the bottom surface 36b of the partially formed substrate. Simultaneously, a passivation material 40 is deposited onto a sidewall 42c of the mask 32 and also the sidewall 38 of the feature being etched (FIGS. 3 and 4). The passivation material 40 can be a silicon oxide, such as SiO.sub.2. The passivation material and the mask can be made from the same material. Without wishing to be bound by any theory or conjecture, it is believed that the thickness (t.sub.p) of the passivation material 40 gradually increases during the second plasma etch step, which has the effect of gradually reducing a dimension in the opening of the mask or feature. The dimension can be a critical dimension, and can correspond to the minimum width (w) of the opening in the mask. This dimension can substantially correspond to the width on the bottom surface of the feature that is etched. As the dimension of the opening is reduced, the etching at the peripheral region 43 is attenuated. The change in the dimension w translates to a reduction in the etched width at the base of the feature. Without wishing to be bound by any theory or conjecture, it is believed that the passivation material 40 overhangs the corners of the base of the feature and consequently shields the corners of the base of the feature from the highly directional anisotropic plasma etch process. This has the effect of reducing the etch rate at the peripheral region 43 of the bottom surface (compared to a central region 39 of the bottom surface). As a result, a feature can be formed whereby the base of the feature has rounded corners, a tapered profile, or a rounded profile in an edge region 44 and a substantially flat central region 39. By controlling the rate of deposition of the passivation material (and hence by controlling the dimension of the opening in the mask) it is possible to control the profile at the base of the feature as desired.

[0062] Once the feature is fully formed, a further deposition-stripping step (shown as “Dep strip” on FIG. 3) is performed to remove the passivation material from the side wall of the mask and feature. The removal of the passivation material can be performed using wet etching. For example, HF can be used as a wet etchant to remove the passivation material.

[0063] The resultant substrate 30d comprises a fully formed feature 34d which has a base having a substantially flat central region 39 and rounded or smoothed corners in an edge region 44 (or other shape as desired). A substrate which has a base or bottom surface with a smoothed profile (e.g. with rounded corners) can help to minimise field bunching. That is, the electric field is more uniformly dispersed about the feature. This can help to maximise breakdown voltages. Additionally, the present method helps to avoid the formation of micro-trenches at the corners of the feature being etched.

[0064] In order to control the profile of the base of the feature, it is necessary to control the rate of deposition of the passivation material 40. Typically, the second plasma etch step includes increasing the rate of deposition of the passivation material as the second plasma etch step proceeds. The rate of deposition of the passivation material can be controlled by varying one of the plasma process parameters. Any process parameter, including one or more of gas ratios, gas flow rates, etch time, plasma source power, platen power, and/or frequency of power applied, can be varied to control the deposition rate of the passivation material. For example, the deposition rate of the passivation material can be increased by gradually increasing (i.e. ramping) the rate of a gas that forms the passivation material (i.e. a passivation material precursor), such as an oxygen gas, introduced into the etch chamber during the second plasma etch step. In a further example, the rate of deposition of the passivation material can be increased by gradually reducing (i.e. ramping) the power applied to the platen during the second plasma etch step.

[0065] FIG. 5 shows how a linear increase in passivation material precursor gas flow rate during the second plasma etch step corresponds to a change in passivation thickness. The linear increase in passivation material precursor flow rate results in a trench having a base with a tapered profile, as shown in FIG. 6. That is, the slope at the corner of the feature (i.e. in the edge region 64) is substantially constant. The bottom surface of the feature comprises the tapered corners 64 and the substantially flat central region 62. Without wishing to be bound by any theory or conjecture, the parts of the feature that are covered by the passivation material (for example, where the passivation material overhangs the base of the feature) have a reduced etch rate. Consequently, as the passivation material thickness increases, this has the effect of reducing the dimension of the opening in the mask layer, and so the etchable width (w) at the base of the feature gradually decreases accordingly. This causes a tapered profile at the base of the feature.

[0066] FIG. 7 shows how a non-linear increase in passivation material precursor gas flow rate corresponds to a change in passivation thickness. More specifically, the gas flow of FIG. 7 is increased (i.e. ramped) during the second plasma etch step at an increasing rate of change. The non-linear ramped increase in passivation material precursor gas flow rate results in a trench having a bottom surface having rounded corners. That is, the slope of the bottom surface in the edge region 74 gradually changes from horizontal (at the central region 72) to substantially vertical (at the sidewall 78), as shown in FIG. 8.

[0067] FIG. 9 shows an SEM image of a feature 90 that has been formed in a silicon carbide substrate 92 using the method describe above. Process parameters for the first and second steps are shown in Table 1:

TABLE-US-00001 TABLE 1 Step 1 Step 2 Time (s) 90 115 Pressure (mTorr) 5 8 Source coil power (W) 950 1500 Platen power (W) 1400 250 .fwdarw.190 Helium back-side pressure (Torr) 10 10 O.sub.2 flow (sccm) 17 25 .fwdarw.98 H.sub.2 flow (sccm) 25 0 Cl.sub.2 flow (sccm) 130 0 SiCl.sub.4 flow (sccm) 19 160 Platen temperature (° C.) 20 20

[0068] FIG. 9 shows the passivation material 94 deposits on the sidewalls of the feature being formed, and also on the sidewall of the mask layer 96. The maximum thickness of the passivation material 94 was about 175 nm on each side of the feature 90. The etch rate during the first plasma etch step was about 400 nm/min.

[0069] By controlling the deposition rate of the passivation material, and hence by controlling the thickness of the passivation material, the dimension of the opening in the mask can be controlled. Control of this dimension allows the width of the etch at the base of the feature to be controlled, and can enable the shape of the bottom surface of the feature being etched to be controllably varied. The present method allows a feature, such as a trench, having a bottom surface with a controllable profile to be formed. Particular advantages are associated with a trench comprising a base with rounded corners. A rounded (or curved) corner can more uniformly distribute electric fields and therefore reduce field bunching. This can beneficially maximise breakdown voltages. Additionally, the present method can prevent the formation of micro-trenching at the corners of a feature because the etch rate is reduced in the corner of features during the second etch step.